the effects of diagenesis on geochemical paleoredox...

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Geochimica et Cosmochimica Acta 232 (2018) 265–287

The effects of diagenesis on geochemical paleoredox proxiesin sedimentary carbonates

Ashleigh v.S. Hood a,b,⇑, Noah J. Planavsky a, Malcolm W. Wallace b,Xiangli Wang a,c,d

aYale University, Department of Geology and Geophysics, New Haven, CT 06511, USAbThe University of Melbourne, School of Earth Sciences, Parkville, VIC 3010, Australia

cUniversity of South Alabama, Department of Marine Sciences, Mobile, Alabama 36688, USAdDauphin Island Sea Lab, Dauphin Island, Alabama 36528, USA

Received 3 June 2017; accepted in revised form 19 April 2018; available online 28 April 2018

Abstract

Metal and metal isotope records in carbonates have the potential to provide novel insights into ancient ocean–atmosphereredox conditions, paleoenvironmental conditions, and biogeochemical cycling. However, trace element geochemical signa-tures in carbonates can record either diagenetic or depositional signatures. Here we explore the variability in uranium iso-topes, trace metal and rare earth element + yttrium (REY) concentrations in carbonate successions that have undergoneseveral common types of diagenetic alteration. Case studies include the Cryogenian Balcanoona Reef, Australia (marinedolomitization, neomorphism); the Devonian Canning Basin reefs, Australia (burial dolomitization, karstification); the Pale-ozoic of the Great Basin, USA (high temperature and regional burial dolomitization) and the Carboniferous WaulsortianLimestone, Ireland (Pb-Zn mineralization). In all of the examined cases there are significant heavier and/or lighter shifts inU isotope values between the most petrographically pristine marine depositional components and altered or late-stage diage-netic phases. Although we also found that REY patterns can be overprinted during diagenesis, normalized REY profilesappear to be commonly retained through diagenetic recrystallization, consistent with previous studies. The direction and mag-nitude of the change in metal isotope systems during diagenesis cannot be generalized between case studies, consistent withcarbonate alteration being controlled by a wide range of factors including the composition and source of the alteration fluids.In this light, we build on framework developed from traditional isotope systems and support the view that future work onmetal and metal isotope compositions of carbonates should include integrated sedimentological and petrographic analysis.Petrographic work not only represents a screening procedure for sedimentary geochemical work, but can also significantlyenhance the paleo-environmental interpretation of metal isotope data.� 2018 Elsevier Ltd. All rights reserved.

Keywords: Carbonate geochemistry; Diagenesis; Metal isotopes; Carbonate diagenesis; Sedimentary geochemistry; Paleo-redox proxies

1. INTRODUCTION

Non-traditional isotope systems and trace metal geo-chemistry of marine sediments have been used for a variety

https://doi.org/10.1016/j.gca.2018.04.022

0016-7037/� 2018 Elsevier Ltd. All rights reserved.

⇑ Corresponding author.E-mail address: [email protected] (A.S. Hood).

of broad-scale paleoenvironmental applications (e.g. Webband Kamber, 2000; Anbar and Rouxel, 2007; Frei et al.,2009; Montoya-Pino et al., 2010; Crowe et al., 2013;Romaniello et al., 2013; Planavsky et al., 2014; John andConway, 2014; Lyons et al., 2014; Kendall et al., 2015;Tostevin et al., 2016). This toolset has been explored in awide range of sedimentary archives and has played a critical


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266 A.S. Hood et al. /Geochimica et Cosmochimica Acta 232 (2018) 265–287

role in developing a more complex understanding of thecoevolution of surface conditions and life.

Trace metal and metal isotope studies have focused onshales, iron formations, phosphates, sedimentary pyrite,and carbonates. Marine carbonates are potentially idealtargets for trace metal work as they are common primarymarine precipitates, often with limited detrital influence.Additionally, carbonates generally preserve a recognisablepetrographic and geochemical record of alteration and lithi-fication during diagenesis, allowing identification of basicsample preservation. Geochemical analysis of the carbonatearchive (e.g. microfossil records, deep sea sediments, reefsystems) contributes to our understanding of climatechange, ocean chemistry and biological evolution in thePhanerozoic (e.g. Veizer, 1989; Zachos et al., 2001;Brand, 2004; Misra and Froelich, 2012; Gothmann et al.,2015). Further, in the pre-Cenozoic record there are strati-graphically continuous carbonate records that can be corre-lated by biostratigraphy and chemostratigraphy. Forexample, the Phanerozoic seawater 87Sr/86Sr isotoperecord, as preserved in petrographically and geochemicallyscreened marine carbonates is one of the most importantchemostratigraphic tools as well as a proxy parameter forEarth’s tectonic activity (Burke et al., 1982; Veizer, 1989;Halverson et al., 2007).

Despite the utility of carbonates as geochemical archives,like all sedimentary rocks, carbonates can be altered duringdiagenesis. Geochemical and petrographic changes in car-bonates during diagenesis have been analysed and refinedover the last �70 years (e.g. Ginsburg, 1957; Gross, 1964;Land, 1970, 1980; Bathurst, 1971, 1983, 1993; Brand andVeizer, 1980, 1981; for recent review see Swart, 2015). Inparticular, the use of traditional stable isotopes (d13C andd18O) and trace metal concentrations in combination withpetrographic techniques including cathodoluminescencemicroscopy has provided a more complete understandingof carbonate diagenesis (e.g. Gross, 1964; Barnaby andRimstidt, 1989; Swart, 2015). The recognition of the signif-icance of diagenetic processes in carbonates has meant thatfor many years, studies of ancient marine carbonate chem-istry have usually incorporated an extensive diagenetic anal-ysis (e.g. Hurley and Lohmann, 1989).

It is only by recognizing the effects of later meteoric andburial diagenetic processes that the original marine andenvironmental chemistry can be deciphered from the rock.If the carbonate is subject to a high water-rock ratio duringdiagenesis (fluid-buffered), then the carbonate may be sub-ject to multiple episodes of recrystallization, dissolutionand re-precipitation (cementation). For example, duringopen-system diagenesis in fluid-buffered systems, the chem-istry of the carbonate end-products will likely dominantlyreflect the chemistry of a diagenetic, non-marine fluid com-position (e.g. meteoric or basinal) (e.g. Land, 1970; Brandand Veizer, 1980; Veizer, 1983; Banner and Hanson,1990). This is particularly true for chemical constituents ofcarbonates that are in low abundances in the rock comparedto common diagenetic fluids (e.g. Sr, Mn, trace metals).

Here we explore the effects of a variety of diagenetic pro-cesses on trace metal concentrations and metal isotopes onmarine carbonates. Specifically, we target component-

specific and bulk-rock U isotopes, trace metals and rareearth element + yttrium (REY) concentrations in carbon-ates that have undergone different diagenetic histories. Arange of case studies have been used to highlight commondiagenetic processes such as: early diagenetic dolomitiza-tion; recrystallization, fluid-rich saddle dolomitization;and epigenetic Pb-Zn mineralization (e.g. Mississippi ValleyType mineralisation). Case studies span the late Precam-brian to Paleozoic from Australia (Cryogenian BalcanoonaReef, Devonian Canning Basin reefs), North America(Paleozoic of the Great Basin) and Ireland (CarboniferousWaulsortian mounds). Case studies are based on previousor new petrographic and sedimentological work on a setof marine carbonate samples where the stratigraphic settingis well-understood. Each case-study is presented as anexample of how to characterize the effects of diagenesison marine carbonates using a comparative petrographicand geochemical analysis between marine and later-diagenetic components.

We found that significant variation in geochemical sig-natures exists between carbonate lithologies that haveundergone early and late diagenetic processes both spatiallyand temporally. Therefore, we make a case that a coupledpetrographic-geochemical screening for sample preserva-tion is required on a case-by-case basis in future work priorto using carbonates to constrain paleo-environmental con-ditions. The importance of petrographic work in carbonategeochemical studies is by no means a new suggestion, andthis study builds on earlier work to establish the importanceof petrographic analysis with non-traditional as well as tra-ditional isotope systems. Building from well-establishedconcepts in carbonate petrography and insights from ourpreliminary work on metal isotope diagenesis, we suggestthat petrographic work will also enhance our understand-ing of paleo-environmental and diagenetic conditions.

2. URANIUM ISOTOPES AND RARE EARTH

ELEMENTS

Uranium isotopes (U238/U235:d238U) have been increas-ingly used as a marine paleo-redox proxy (e.g. Stirlinget al., 2007; Weyer et al., 2008; Montoya-Pino et al.,2010; Brennecka et al., 2011a; Asael et al., 2013; Dahlet al., 2014; Kendall et al., 2013, 2015; Azmy et al., 2015;Wang et al., 2016; Hood et al., 2016; Andersen et al.,2014, 2016; Lau et al., 2016, 2017a; Jost et al., 2017; Songet al., 2017). There are several proposed uranium isotopemass balances, and marine U burial fluxes are reasonablywell understood (Dunk et al., 2002; Schauble, 2007;Tissot and Dauphas, 2015; Andersen et al., 2016). U iso-tope fractionations are primarily controlled by the nuclearvolume effect and the largest fractionation occurs duringU(VI) reduction (Stirling et al., 2007; Schauble, 2007,Weyer et al., 2008, Andersen et al., 2014; Stylo et al.,2015). Under high enough atmospheric oxygen values(e.g. post-Archean), crustal U (d238U: �0.29 ± 0.03‰,Tissot and Dauphas, 2015) is weathered and transportedby rivers into the oceans, where it is taken up by carbon-ates, ferro-manganese crusts and sediments under reducingbottom waters (Dunk et al., 2002). The U isotope value of

A.S. Hood et al. /Geochimica et Cosmochimica Acta 232 (2018) 265–287 267

modern seawater (�0.39‰ ± 0.02 2SD, Tissot andDauphas, 2015; Noordmann et al., 2015) is globallyhomogenous due to a long residence time for U (�400kyr) relative to �1 kyr for ocean mixing (Dunk et al.,2002; Ku et al., 1977).

Biologically-mediated reduction of U in sediments undermarine sub-oxic or euxinic bottom waters represents thebiggest sink for uranium, and also imparts a large U isotopefractionation, enriching sediments in the heavier isotopeand resulting in a relatively lighter seawater d238U compo-sition (e.g. Dunk et al., 2002; Weyer et al., 2008;Andersen et al., 2014; Stylo et al., 2015). In contrast, bioticmarine carbonates, the second largest sink for U (Dunket al., 2002), incorporate seawater d238U with only a rela-tively minor fractionation (mean of recent Bahaman car-bonates �0.37 ± 0.12‰, Romaniello et al., 2013). Metaloxides represent the other major U burial flux, and buryisotopically light uranium (Brennecka et al., 2011b; Gotoet al., 2014; Wang et al., 2016).

Recent experimental work suggests that abiotic marinecarbonates are not significantly fractionated from seawaterd238U at modern pH, but may be 0.11 ± 0.02‰ heavierthan seawater at higher pH (Chen et al., 2016). With theseconditions in mind, carbonates have been used as anarchive for paleo-redox conditions in ancient oceans, witha d238U value approximately reflecting the U isotope com-position of seawater (e.g. Brennecka et al., 2011a; Dahlet al., 2014; Hood et al., 2016; Lau et al., 2016, 2017a).However, marine and diagenetic carbonate componentswithin a single carbonate lithology may have very disparateU isotope compositions (dependant on diagenetic fluidcomposition) and care must be taken to sample only well-preserved marine components (Hood et al., 2016; Chenet al., 2016).

The chemical behaviour and distribution of rare earthelements and yttrium (REY) is well understood in the mod-ern Earth’s surface system given sustained work on the sys-tem for over 40 years (Elderfield and Greaves, 1982; DeBaar et al., 1985). The distribution of REY within the mar-ine, sedimentary geological record has been used to trackmarine processes through Earth’s history (e.g. Webb andKamber, 2000; Kamber and Webb, 2001; Nothdurftet al., 2004; Planavsky et al., 2010; Hood and Wallace,2015; Della Porta et al., 2015; Tostevin et al., 2016;Wallace et al., 2017). Modern oceanic REY concentrations,(shown normalised to a shale composite: PAAS,McLennan, 1989), are controlled largely by particulatescavenging and carbonate complexation in seawater andhave a LREE depleted shape with a strong depletion in cer-ium (e.g. Elderfield and Greaves, 1982; German andElderfield, 1990). The relative amount of cerium in marinecarbonates (Ce anomaly) compared to the other light REE(LREE) has been well established as a paleo-redox proxy(German and Elderfield, 1990; Bau and Dulski, 1996; calcu-lation from Lawrence et al., 2006). Cerium is redox-sensitive and when oxidised, is removed from solution(around the MnO2/Mn2+ redox transition) (German andElderfield, 1990; Moffett, 1994). REY concentrations inmarine carbonates (e.g. microbialites, marine cements,ooids) are higher than in seawater, but generally reflect

the marine REY composition (Webb and Kamber, 2000;Nothdurft et al., 2004). Modern, oxic seawater is also char-acterised by a superchondritic Y/Ho ratio. However inanoxic water or during redox cycling the dissolution ofFe-Mn oxides causes this ratio to become approach chon-dritic values (Bau et al., 1997). In contrast to metal iso-topes, there have been several studies on the effects ofdiagenesis on carbonate REY patterns and there is evidencethat carbonates are commonly rock-buffered during diage-nesis and dolomitization (Banner et al., 1988; Nothdurftet al., 2004; Webb et al., 2009).

3. METHODS

Fieldwork for this study included detailed measuring ofstratigraphic sections at a decimetre to metre scale fromoutcrop and core in Ireland, Australia and the USA. Dolo-mite and limestone samples were chosen to represent vari-ous states of preservation (e.g. limestone to fabric-destructive dolomite), when possible within in a recogniz-able stratigraphic horizon. Thin sections were cut andexamined using standard petrographic techniques andcathodoluminescence microscopy. Cathodoluminescencephotomicrographs were taken on a Nuclide ELM2BCathodoluminoscope at the University of Melbourne anda Reliotron Cathodoluminoscope at Yale University, bothattached to Wild M400 Photomacroscope, under an accel-erating voltage of �8 kV and a beam current of �0.5 mA.

Geochemical analysis consisted of bulk and component-specific solution analysis, coupled with laser ablation anal-ysis. Where possible, each technique was used on the samesample and same components, to target geochemical vari-ability at a range of scales from outcrop to thin section.Laser ablation work was able to target more specific areasof each sample than solution work, due to differences inthe scale of analysis, but component-specific and bulk solu-tion analysis were also used to show variation in geochem-ical composition at different scales within hand samples.Uranium isotope analysis was undertaken at the YaleMetal Geochemistry Center. For component-specific analy-sis, samples were microdrilled (with a tungsten carbide drillbit). For bulk-sampling, samples were cut and crushed in anagate mill to take representative whole rock samples. Pow-ders were leached in 0.5 M HCl or 1 M HCl for three 30min steps, (with sufficient acid volume to account for dou-ble the mass of powder, to ensure carbonate dissolution).For sample-specific details, see Supplementary Tables 1–5). Trace metal concentrations were measured at YaleUniversity from splits from each digest on a ThemoFinni-gan Element XR magnetic sector ICP-MS. Element mea-surement precision was better than 5% for duplicatesamples.

For U isotopes, the 236U-233U double spike was addedand samples were prepared using the UTEVA columnmethod (Weyer et al., 2008; Wang et al., 2016; Hoodet al., 2016) (Supplementary Table 1). Uranium isotopes(d238U) were measured on a ThemoFinnigan NeptunePlusmulti-collector ICP-MS at Yale University at low mass res-olution, with a blank U level less than 50 pg, which is neg-ligible compared to a 50–100 ng U sample. The standard


CRM-112a was measured after every three samples to mon-itor instrumental stability. d238U values are reported rela-tive to CRM-112a (0.0‰). External reproducibility wasbetter than 0.07‰ (2SD) based on full protocol duplicatesof the geostandard NOD-A-1 (Supplementary Table 1a).However, with some small sample sizes, some sampleshad larger errors (2r internal error ranged from <0.04 to0.18‰). Although some errors were significantly largerthan minimum possible values, the error was still far lessthan the observed range of d238U values between samples.234U/238U values for some samples deviated from unity(secular equilibrium), which may have been caused by dis-solution techniques (partial leaching of samples to targetthe carbonate phase), although recent (<1 Ma) weatheringprocesses cannot be fully excluded.

Carbon and oxygen isotopes were measured on aThermo MAT 253 with a KIEL IV Carbonate Device atthe Yale Stable Isotope Center on �100 lg of microdrilledpowder. Samples were normalised to the VPDB (ViennaPee Dee Belemnite) delta notation. Standard 1-sigma errorsfor d13C and d18O were 0.08‰ and 0.10‰ respectively.

Laser ablation ICP-MS was undertaken on polishedthick sections with a Helex 193 nm ArF excimer laser abla-tion system connected to an Agilent 7700x quadrupole ICP-MS at the School of Earth Sciences, the University of Mel-bourne. Operating conditions included a laser repetitionrate of 64 Hz and a 40 to 100 mm spot size, bracketed bythe NIST SRM612. Data was reduced by Iolite softwareusing the Trace Elements data scheme with Ca as an inter-nal standard (Woodhead et al., 2007; Paton et al., 2011).Outliers were rejected at the ±3 sd level for spot analyses.Average limits of detection for spots were at ppb level fortrace metals (Supplementary Table 1c). Internal precision(2-sigma error) for samples was generally less than ±10%.External reproducibility is reported as less than ±0.3%(2SD, n = 29) based on repeated NIST SRM612 standardmeasurements. Laser spot data was reduced to avoid sam-pling large Al spikes in downhole ablation time series.However, laser scan data has not been filtered and has onlybeen standardized to calcium.

4. CASE STUDIES

4.1. Balcanoona Reef-marine dolomite precipitation

4.1.1. Geological setting, petrography and preservation

The Balcanoona reef complex is a series of mid-Cryogenian (�650 Ma) carbonate platforms with steepreefal margins, in the northern Flinders Ranges, South Aus-tralia (Giddings et al., 2009; Hood and Wallace, 2012;Wallace et al., 2015) (Fig. 1A) (Supplementary Table 6).These reefs provide one of the best examples of mimeticdolomitization and dolomite precipitation during marinediagenesis (Hood et al., 2011; Hood and Wallace, 2012).Marine or early diagenetic dolomitization has increasinglybeen recognised as a widespread diagenetic process in manyPrecambrian carbonates (e.g. Tucker, 1982, 1983; Corsettiet al., 2006; Hood et al., 2011; Shuster et al., in press)and therefore has been highlighted here as a case study.Component-specific and laser ablation analysis methods

have been used in this case study to determine the differencein metal incorporation and response to diagenesis betweenlimestone, dolomitised and primary marine dolomite reefalcomponents.

Reefal platforms consist of dolomitic peritidal redbedsof the Angepena Fm. (Fromhold and Wallace, 2011), andplatform and margin facies of the Balcanoona Fm. (sheetcavity networks, ooid shoals and two reef frameworks;Giddings et al., 2009; Wallace et al., 2015) (Fig. 1). Reefslope deposits consist of reef talus and allochthonous mate-rial interbedded with basinal shales (Giddings et al., 2009).The reefal platforms were mimetically dolomitised by sea-water during syn-sedimentary diagenesis, and marinecements are dominantly primary fibrous dolomite (Hoodet al., 2011) (Fig. 1B–D, Supplementary Table 6). Dolomitemarine cements are petrographically the best-preservedcomponents, as are some dolomitised reefal components,whereas calcitic allochthonous material generally showspoor depositional fabric preservation and abundant recrys-tallization (Fig. 1E).

4.1.2. Geochemical results

In earlier work, a series of carbonate components(including ooids, marine cements, burial cements andmicrobialites) from both dolomitic Balcanoona Reef faciesand calcitic allochthonous blocks were sampled for ura-nium isotope work (Hood et al., 2016). We found signifi-cant variation in d238U composition between depositional,early and late-stage diagenetic components, encompassingthe range of compositions seen in the modern Earth systemwithin a single hand sample (Hood et al., 2016). Uraniumisotope composition did not change significantly when thesame samples were dissolved in weaker 1 M HCl digestionsversus 6 M HCl, although some trace metal concentrations(e.g. Th, Fe) increased with stronger dissolution techniques(Hood et al., 2016, Data Repository). Here, the samecomponent-specific samples have been characterised withcarbon and oxygen isotopes to address the variation in car-bonate chemical composition during early dolomitizationor limestone recrystallization (Fig. 2, SupplementaryTable 2). In addition, laser ablation transects through dif-ferent components have been used to address intricatechanges in trace metal chemistry between marine and latercarbonate components (Fig. 3).

Carbon isotope values are broadly similar betweenmimetically dolomitised depositional components, dolo-mite marine cements and depositional limestone from theBalcanoona reefs (3.1‰ ± 1.4 2SD, Fig. 2A). However,there is a clear difference in oxygen isotope compositionbetween the limestone blocks (mean limestone d18O:�14.71‰) and dolomitised reef components (originallyderived from the same reef facies, mean d18O: �3.83‰)(also discussed in Hood and Wallace, 2012). Late-stage fer-roan dolomite burial cements have a much wider range ofcarbon isotope compositions over a more tightly con-strained range in d18O. Carbon and uranium isotopes arewell correlated in burial dolomite cements (r2 value ford13C and d238U is 0.94, p � 0.01) but not significantlybetween other components (Fig. 2B). The lowest d238Uvalue is found in a recrystallized limestone allochthonous

Fig. 1. (A) Stratigraphic diagram of the Cryogenian sediments of the northern Flinders Ranges showing the position of the Balcanoona andAngepena formations. Modified after Giddings et al., 2009 and Wallace et al., 2015. (B–E) Thin section photomicrographs of the Balcanoona,Angepena and Yankaninna formations. (B) Peritidal peloid-ooid grainstones of the Angepena-Balcanoona transitional beds with Fe-oxide-rich depositional dolomite (D), dolomite marine cemented sheet cavities (M) and coarse, ferroan burial dolomite cements (B). (C)Cathodoluminescence photomicrograph of dolomite marine cements in the Angepena Fm. showing well-preserved non- and bright-luminescent zonation. (D) Dolomite marine cements (M), burial dolomite spar (B) and microbial frameworks (MF) of the Balcanoona Fm.(E) Cathodoluminescence photomicrograph of poorly-preserved fabrics of the Balcanoona reef framework in basinal allochthonous reefmaterial of the Yankaninna Fm. (non-bright-dull luminescent calcite after aragonite).


Fig. 2. (A) d18O vs. d13C and (B) d238U vs. d13C of carbonate components of the Cryogenian Balcanoona Fm. (d238U values from Hood et al.,2016). Limestone samples have much lighter oxygen isotope values, and dolomite burial cements have a broad range of d13C values but aconsistent oxygen isotope value. In burial dolomite cements, d13C and d238U are correlated with depleted d13C showing more negative d238U.


block (Fig. 2B), and is associated with the most negatived18O (Fig. 2A and B). Depositional components have thehighest average 234U/238U values, and the largest range ofvalues (mean: 1.168 ± 0.290 2SD) (Supplementary Table 2).

In addition to variations in isotope geochemistry, semi-quantitative laser ablation transects through depositionaland diagenetic phases highlight the variation in trace metalchemistry between components of the Balcanoona Reef atmuch smaller scales. Fig. 3 demonstrates the change in tracemetal and REY chemistry between depositional finely-crystalline carbonate, marine cements and late-stage fer-roan dolomites in a sheet cavity sample from the AngepenaFm. The finely-crystalline carbonate shows high levels ofAl, Cr and rare earth elements (represented in Fig. 3 byCe) compared to marine cements. When normalized tothe length of the transect, approximately 77% of the Crmeasured in this carbonate is found in depositional micrite(which has abundant fine-grained, non-carbonate phasesincluding Fe-oxides and clays). Assuming an average crus-tal Cr/Al ratio for detrital material (Rudnick and Gao,2003; although it should be noted that detrital values varysignificantly from this average: Cole et al., 2017), 37% ofthis Cr could be derived from detrital contamination, or28% of the total Cr in the rock (as represented by this tran-sect). Burial cements are significantly enriched in Fe (83%total Fe, normalized to length) and rare earth elements.In this transect, U is highest in the earliest phases (deposi-tional and early marine cements, together accounting for79% of total U, normalized to the transect), but lowest inlate-stage cements. Uranium, and other trace metals arehighly heterogeneously distributed in these samples. Mnlevels vary significantly through this cement transect andcorrespond to brightness in CL zonation (also see Hoodand Wallace, 2015). Shale-normalised REY profiles varysignificantly through this cement crust, with increasing con-centration, LREE depletion then MREE enrichment withdistance through the crust (Fig. 3).

4.1.3. Interpretation

While it is encouraging that marine cements and deposi-tional components were not completely overprinted by anepisode of this fluid flow during burial diagenesis (homog-enizing metal and metal isotope geochemistry towards dia-genetic compositions), there is a high spatial variability inmetal concentrations and d238U compositions (Figs. 2 and3). Depositional fabrics are generally well-preserved inmicrocrystalline dolomite (Hood and Wallace, 2012)(Fig. 1), but micritic sediments have visible non-carbonateinclusions, significantly elevated and irregular Cr and othertrace metal concentrations, all suggestive of significantdetrital contamination (Figs. 2 and 3). In peritidal redbeds,large portions of metals are likely to have been derivedfrom Fe-Mn-oxide phases (Hood and Wallace, 2012)(Fig. 3).

In comparison, dolomite marine cements are petrograph-ically the best-preserved components (i.e. retained primarycathodoluminescent growth zonation suggests no mobilityof Fe or Mn during subsequent diagenesis), and have lowdetrital contents (Hood and Wallace, 2015). Geochemically,these marine cements are similar in carbon and oxygen iso-tope and REY composition to dolomitized depositionalcomponents (Fig. 2; Hood and Wallace, 2012; 2015). Incombination with sedimentological evidence for marinedolomitization (Hood andWallace, 2012), these results rein-force that dolomitization and marine dolomite cement pre-cipitation were derived from the same (marine) fluidcomposition. However variation in uranium isotope compo-sition is evident between reefal components (mean: �0.30‰± 0.30 2SD), suggesting marine cements, micrite, and micro-bialites may either incorporate U differently or are influ-enced by differing amounts of U isotope resetting during:carbonate precipitation and stabilisation (e.g. Chen et al.,2016); marine dolomitization; and/or later diagenesis(Fig. 2) (Hood et al., 2016). While further work is neededto highlight the specific differences in U incorporation into

Fig. 3. Example of using a semi-quantitative laser profile to determine the variation in trace metal chemistry between depositionalcomponents (D.C.), marine cements (including a late, bright cement generation) and burial cement (B.C.) diagenetic phases, Angepena Fm.,sample AQJ. Transect length is approximately 2 mm. Fe and REY are very high in burial cements in comparison to marine cements. Cr andAl are high in depositional components- e.g. have a higher potential for being related to detrital (oxide) contamination. Mn and Fe correlatewith cathodoluminescent zonation. Within marine cements, U is in high concentrations in bright-luminescent early and late cement zones, andis not homogenously distributed. Lower panel shows (quantitative) average of laser ablation PAAS normalised REY analyses across eachcomponent, with similar profiles between depositional micrite and marine cements, and progressive enrichment of REY towards burialcements, co-occurring with MREE enrichment and LREE depletion.


these precipitates and their responses to alteration, it is clearthat U isotopes are not homogeneous in carbonates.

Overall, a less reactive dolomite mineralogy for reefalcomponents appears less susceptible to the effects of later-stage burial diagenesis than depositional components pre-served in limestone. Originally aragonitic components fromallochthonous reefal limestone blocks are destructivelyrecrystallised (Fig. 1) and have very light oxygen isotopecompositions suggesting that the aragonite neomorphosedto calcite in contact with either meteoric fluids (possiblyrelated to subaerial exposure and karstification during ter-minal reef growth, Giddings et al., 2009) or at higher tem-peratures during burial diagenesis (Hood et al., 2011; Hoodand Wallace, 2012). Although further work is needed, pre-liminary work shows that these blocks have a significantrange in U isotope values (�0.72 to �0.24‰), and together

with very light oxygen isotope compositions, suggests thatneomorphism in meteoric and/or high temperature (high-T) basinal fluids can significantly affect d238U inlimestones- possibly resulting in lighter U isotope composi-tions than equivalent marine dolomites (Fig. 2) (Hoodet al., 2016).

In laser transects of the reef microfacies, burial dolomitecements (ferroan carbonates) had high REY concentrationswith MREE enriched profiles compared to marine cements,both characteristic of diagenetic carbonates precipitatingunder conditions of Fe reduction (e.g. Haley et al., 2004;Hood and Wallace, 2015) (Fig. 3). A highly variable carbonisotope composition, a strong correlation between d238Uand d13C and low U concentrations in ferroan, burial dolo-mite spar suggests this spar may have precipitated fromconnate (anoxic) seawater fluids towards more reducing


burial conditions (with marine values apparent for each sys-tem, and then a spread of correlative lighter isotope compo-sitions in both systems) (Figs. 1–3).

4.2. Paleozoic of Nevada-hydrothermal dolomitization


The Great Basin of Nevada hosts a number of Paleozoiccarbonate platforms that have undergone burial dolomiti-zation including occurrences of zebra textures, or‘‘Hydrothermal Zebra Dolomite” (HZD) associatedcarbonate-hosted mineralisation (Diehl et al., 2010;Wallace and Hood, in press) (Supplementary Table 6).Stratabound and fault-related saddle dolomites are widelydistributed within Ediacaran to early Carboniferous plat-formal limestone units. Dissolution of carbonate and theprecipitation of saddle dolomite is thought to be associatedwith the descending, basin-ward flow of evaporative brinesdeveloped on shallow platforms in late Paleozoic, arid,equatorial settings (Diehl et al., 2010).

Devonian and Cambrian limestone lithologies of south-central Nevada were sampled from stratigraphic horizonsthat have undergone variable amounts of dolomitization

Fig. 4. Thin-section photomicrographs of Paleozoic carbonates of Nev(lowermost stromatolites of the Yellow slope forming member), showinoriginal depositional fabrics (samples ET-2, ET-3 and ET-1 from left topreserved in the Cambrian Whipple Cave Fm. Nevada, showing (lowerdolomite. Sample SPA-480, location: 38� 300 49.06200 N, 114� 580 4.2600

saddle dolomite with no preservation of original texture. Porosity has been(lower right) during late-stage meteoric diagenesis/karstification.

(Fig. 4, Supplementary Table 6). Samples were collectedfrom the Upper Cambrian Whipple Cave Formation westof Shingle Peak, in the upper unit at the transition fromlimestone to dolomite. Dolomitization is extensive in thissection and is thought to be associated with a normal faultat Shingle Pass (Kellogg, 1963). In outcrop, limestone iscommonly grey to dark grey and preserves depositional tex-tures (e.g. crinoidal wackestone and microbialite) whereasdolomite is white, coarsely crystalline, fabric destructiveand may develop zebra textures. Late-stage meteoric disso-lution and karstification is also evident in this locality, withsome samples (e.g. SPA-483) showing fibrous cave calciteprecipitates (pendant cements) in dissolution cavities. Addi-tionally, we sampled the Mid-Late Devonian transgressiveslope carbonates of the Guilmette Fm. west of HandcockSummit, Nevada, in the lowermost ‘‘Yellow slope-formingmember” (YSF) (Sandberg et al., 1997; Warme et al.,2008). In outcrop, several beds of columnar and branchingstromatolites are evident at the base of this Late Givetian(conodont zone 20; Sandberg et al., 1997) lower member,and range from limestone to dolomite with some preserva-tion of depositional textures, to fabric-destructive zebratextures.

ada. (A–C) Thin section photomicrographs of the Guilmette Fmg the effects of progressive dolomitization on the preservation ofright with increasing dolomite content). (D) Dolomitization front) crinoidal wackestone limestone to (upper) euhedral replacementW. E) Dolomitized Whipple Cave Fm., showing extensive coarse,filled by pendant calcite cements and possible microbial carbonates


In each example, three samples were taken from onestratigraphic unit (several metres in thickness) representing(1) relatively pure limestone, (2) partially- and (3) fully-dolomitised samples. In thin section, the limestone samplesshow good preservation of a microbial texture (GuilmetteFm.: Fig. 4A) or crionoidal limestone (Whipple CaveFm.: Fig. 4D) with minor dolomite (Fig. 4). However,coarse, clear to brown saddle dolomite crystals overprintand progressively destroy these depositional textures inmore dolomitised samples (Fig. 4). Stratigraphically equiv-alent bulk samples were taken from these three stages ofdolomitization in order to characterise the effect of theamount of dolomite present on metal isotope compositions.Samples were then characterised by laser ablation work tolook at the REY and trace metal composition of specificcalcite and dolomite phases.


Uranium, carbon, and oxygen isotopes were analysed inwhole rock samples and show considerable variabilitybetween limestone and dolomite compositions (Fig. 5, Sup-plementary Tables 3a and 3b). For Whipple Cave Fm. sam-ples, uranium isotopes change significantly betweenlimestone and dolomite samples, with the purest limestonesample slightly lighter (�0.52 ± 0.08‰ d238U) than themodern seawater value (�0.39 ± 0.2‰ d238U; Tissot andDauphas, 2015), while partially dolomitized limestoneshows lighter (�0.83 ± 0.03‰ d238U), and saddle dolomite,heavier U isotope compositions (�0.25 ± 0.06‰ d238U)(Fig. 4A). Carbon and oxygen isotopes became slightlyheavier through dolomitization (d13C from 0.24 to 0.66‰;d18O from �11.62 to �10.33) (Fig. 5B).

Devonian samples that have undergone progressivedolomitization show significant changes in U and O iso-topes (Fig. 5). Oxygen isotopes are altered with partialdolomitization, and change from �6.71‰ in limestone to�8.90‰ and �8.88‰ in partially and fully dolomitisedsamples. Uranium isotopes again change significantly, fromalmost modern seawater-like values in limestones (e.g. �0.34 ± 0.13‰) to much heavier values in dolomitised samples

Fig. 5. Variability in U, C, and O isotope composition between bulk samPass, and the Guilmette Fm., Handcock Summit West. Colours darken

(0.12 ± 0.13‰ and 0.06 ± 0.23‰) (Fig. 5A). While thesevalues have high errors, it is important to note that thechange in d238U is larger than the error, and is on the scaleof the variation in the U isotope mass balance for the mod-ern Earth. In comparison, d13C isotopes do not change con-siderably with partial dolomitization but are over 5‰heavier in fully dolomitised samples (Fig. 5B).

Accompanying this, rare-earth element and yttrium(REY) concentrations were measured on the same samplesusing bulk-rock solution methods and component-specificlaser ablation (Fig. 6). REY concentrations progressivelyincreased with a higher proportion of dolomite in samples.In addition, REY profile shape (normalised to PAAS,McLennan, 1989) changed considerably with dolomitiza-tion. Devonian limestone samples show a relatively flat pro-file with no depletion in cerium, but samples composedlargely of saddle dolomite have a light rare earth element(LREE) enriched profile and develop a strong positive Euanomaly (usually associated with hydrothermally-derivedfluids, Bau and Dulski, 1999). This same pattern wasobserved via laser ablation in dolomitized Cambrian sam-ples, with increasing dolomitization linked to higher REYconcentrations, more LREE enrichment, reduced Y/Hoand a negative Eu anomaly. The Ce anomaly did notchange significantly between limestone and dolomite sam-ples from either formation.


In the Paleozoic stratigraphy of Nevada, the influence ofdolomitizing fluids on marine limestones during burial dia-genesis appears to significantly overprint their depositionalfabrics and some geochemical signatures (Figs. 4–6). Theseformations have been influenced by episodic developmentand circulation of (high-T) basinal brines associated withevaporite dissolution and dolomitization from the Ordovi-cian to Early Carboniferous (Diehl et al., 2010). The effectof fluid–rich alteration on Cambrian and Devonian shallowmarine carbonates is represented by the amount of saddledolomite present in bulk samples (Fig. 5). The differentd238U and REY compositions between Cambrian and

ples of limestone and dolomite of the Whipple Cave Fm., Shinglewith proportion of dolomite in samples.

Fig. 6. Rare earth element and yttrium concentrations in limestone and dolomite samples of the Guilmette Fm., Handcock Summit West, andthe Whipple Cave Fm., Shingle Pass (normalised to PAAS, McLennan, 1989). Colours darken with proportion of dolomite in samples. (A)Bulk dissolution of whole rock samples of the Guilmette Fm. The limestone sample shows a relatively flat profile, and lower REYconcentrations, while saddle dolomite samples are more enriched in REY, particularly LREE, and show a strong Eu anomaly. (B) Laserablation analysis of Guilmette Fm. samples (scale is the same as A). Ablated samples show similar changes in REY with dolomitization as thebulk dissolution, although with less of an Eu anomaly. (C) Laser ablation analysis of Whipple Cave Fm. samples. Progressive dolomitizationincreases REY concentrations, but reduces Y/Ho and the Eu anomaly.


Devonian dolomitised limestones may be a factor of differ-ent: facies or lithologies (i.e. microbialite interbedded withsiltstone vs. crinoid wackestone); original marine geochem-ical signature; depth in the basin during dolomitization; andmost likely is related to the composition and flow path ofdolomitizing fluids.

The REY composition of moderately well-preservedlimestone stromatolites from the Devonian Guilmette Fm.is dissimilar to modern microbialites and seawater (Webband Kamber, 2000; Elderfield and Greaves, 1982). This dif-ference might be attributed to a REY composition of Pale-ozoic seawater not resembling that of modern seawater (e.g.Wallace et al., 2017), or could be the result of alteration dur-ing episodes of burial dolomite precipitation. Saddle dolo-mite samples from the same stratigraphic horizon showLREE enriched, normalised REY profiles, with significantpositive Eu anomalies (Fig. 6A), characteristic of high-T,reducing fluids (e.g. Bau and Dulski, 1999). This is also con-sistent with fluid inclusion and oxygen isotope compositionsof saddle dolomites, suggesting formation temperaturesfrom �70 to 150 �C (Diehl et al., 2010). Bulk dissolved sam-ples show a more prominent positive Eu anomaly than laserablation analyses of specific dolomite and dolomite cementcomponents (Fig. 6A and B), suggesting whole rock sam-pling may incorporate more significant secondary carbonatephases with diagenetic chemical compositions. NormalisedREY profiles from the Whipple Cave Fm. are LREEenriched and depleted in Y with increased dolomitization(typical of hydrothermal fluids: Bau and Dulski, 1999),but show a slightly negative Eu anomaly. This could suggestprecipitation from either fluids with lower temperature thanDevonian examples, or chemical complexation mechanismswhich extend the stability field of Eu3+ to lower oxygenfugacities, perhaps preventing the reduction to Eu2+, andtherefore lessening the Eu anomaly (Bau, 1991).

Carbon and uranium isotopes appear to change consider-ably in bulk samples with the amount of saddle dolomite pre-sent (Figs. 5 and 6). Oxygen isotopes are uniformly lightacross both formations and d238U is heaviest in fully dolomi-tised samples. Combined with REY data and previous work(Diehl et al., 2010) suggesting that dolomitising fluids are

pervasive, reduced, high-T basinal brines, it appears thatthese fluids can transport the heavy uranium fraction, thoughwith some variability. The original marine d238U value of theshallow seawater where these limestones formed may havebeen lighter than presently preserved (if interaction with bri-nes has preferentially taken up heavy U isotopes). However,U concentrations are generally lowest in dolomites, and thesefluids are thought to be reducing when they interacted withthe limestones (suggesting only limited solubility of ura-nium). It is also possible that the uranium in the saddle dolo-mite may have been sourced from evaporative strata or otherrocks when they dissolved in (oxic) fluids (to create the bri-nes), and that the limestone values remain similar to the orig-inal marine values after depositional stabilisation.

Regional or basinal high-T brine migration associatedwith dissolution during diagenesis therefore has the poten-tial for significantly localized and disparate effects on car-bonate metal concentrations and isotope compositions,even in broadly similar lithologies.

4.3. Devonian reef complexes of the Canning Basin- early

burial dolomitization and late-stage meteoric diagenesis


Devonian reef complexes of the Canning Basin, WesternAustralia, are one of the best-preserved examples of ancientreef ecosystems (Playford, 1980). Moreover, these com-plexes are well-characterised in terms of their marine anddiagenetic histories (e.g. Kerans et al., 1986; Hurley andLohmann, 1989; Carpenter et al., 1991; Wallace et al.,1991) (Supplementary Table 6). Reefal carbonates of theCanning Basin have undergone multiple stages of marineand burial diagenesis, as well as late-stage karstificationand meteoric diagenesis from the Upper Devonian to Ceno-zoic (Hurley and Lohmann, 1989; Wallace et al., 1991).Abundant fibrous calcite marine cements were precipitatedin reef and slope cavities during early marine diagenesis,followed by porosity-occluding equant calcite spar in plat-form facies (Kerans et al., 1986; Hurley and Lohmann,1989; Wallace et al., 1991). Dolomitization also occurredduring early burial diagenesis and was particularly evident


in platform facies near Precambrian basement (Hurley andLohmann, 1989; Wallace et al., 1991). Later, minor (fer-roan) calcite precipitation associated with karstificationoccurred during subaerial exposure in the Late Carbonifer-ous and reburial during the Permian to Cenozoic. This dia-genetic history appears complex, but both the complexityand the type of diagenetic events from the well-studied Can-ning Basin are not uncommon in the geological record.

Bedded fenestral limestones of the back reef facies of thePillara Lst. were partially dolomitized during early burialdiagenesis (e.g. Hurley and Lohmann, 1989) (Figs. 7A, 8).Dolomitized beds alternate with limestone in this facies,where the grainsize of the depositional limestone in eachbed affected the amount of dolomite present (with finergrained micrite more likely to have been dolomitized)(Wallace et al., 1991) (Figs. 7A, 8). Fenestral beds are lar-gely preserved as limestone. Replacement dolomite is gener-ally <200 lm in crystal size, with cloudy centres and clear

Fig. 7. Field photographs of Canning Basin carbonates with diagenetic pfenestral, bedded limestones (L) which have been selectively dolomitised a7 cm wide. (B) Photograph of late-stage, ferroan calcite cavity fill in Gedating marine cements (white) in cavity in fore-reef slope limestones, So

dolomite cement rims (mottled to bright-luminescent undercathodoluminescence, Fig. 8B). Marine cements appear tobe petrographically unaffected by dolomitization. Lime-stone porosity in back reef settings is subsequently filledwith clear calcite cements, which are dominantly non-luminescent with late bright-luminescent banding (Wallaceet al., 1991) (Fig. 8A).

Karstification and calcite cementation of remainingreefal porosity began during late diagenetic subaerial expo-sure of the Devonian reefs (Hurley and Lohmann, 1989).Minor calcite cementation continued through a second epi-sode of burial diagenesis from the Permian to Cenozoic(Wallace et al., 1991). Dull-luminescent calcite cementsfrom this later cementation that formed in solution cavitiesmay be up to 40 cm in length (Fig. 7B). Similar brown, dull-luminescent calcite spar is found in reef margin and fore-reef cavities that were not fully-occluded by earlier cementgenerations, and in cross-cutting veins (Figs. 7C, 8C).

recipitates present. (A) Platform carbonates of the Pillara Fm. withlong bedding planes (D). Arrow indicates late-stage veins, lens cap isike Gorge. (C) Field photograph of brown, late-stage calcite post-uth Oscar Range.

Fig. 8. Photomicrographs of early burial dolomite in platformal Pillara Lst., Canning Basin. Field of view is 6.5 mm. (A) CL image offenestral limestone from the platform facies, Dingo Gap (Sample PL) showing peloidal micritic calcite and early diagenetic, non-luminescentcalcite spar (with occasional bright-luminescent bands). (B) Sample from similar stratigraphic horizon showing complete dolomitization withbright and non-luminescent dolomite rhombs overprinting any original texture (Sample PD). (C) Non-luminescent calcite spar with brightzones filling in vein structure in dolomitized sample (Sample PD-2). Dolomite rhombs have bright-luminescent cement rims. Laser spot inlower dolomite rhomb. (D and E) Paired plane light and CL images of early burial dolomite replacement (cloudy) and cement (clear) showingnon- and bright-luminescent zonation.


In this study, limestone samples from the platform andslope facies of the Canning Basin Devonian reefs have beenanalysed to determine the effects of various stages of diage-nesis on their trace metal concentrations and U isotopecomposition. Platformal carbonates of the Frasnian PillaraLimestone, which have undergone various amounts of earlyburial dolomitization, were sampled from Dingo Gap, Wes-tern Australia. These samples were analysed using bulkrock techniques, but veins were microdrilled to target morecertain diagenetic features, and the same samples were anal-ysed using laser ablation to target specific components. Weadditionally sampled late-stage dull-luminescent, ferroancalcite from (drillcore) solution cavities to test the tracemetal and isotopic composition of late-stage meteoric andburial diagenetic fluids.


The uranium isotope composition of a platform fenes-tral limestone sample is �0.03 ± 0.17‰, whereas partiallydolomitised and dolomite samples have lighter d238U values(�0.33 ± 0.16‰ in partial dolomites, �0.34 and �0.84 ±0.18‰, in dolomite samples) (Fig. 9A, SupplementaryTable 4a). Within a single dolomitised hand sample,(DGA-PD2), variability between replacement dolomiteand late-stage vein calcite is particularly high (�0.84‰ to0.20‰). In bulk-leached samples, dolomitized lithologiesmay have significantly higher metal concentrations thanthe equivalent limestones, particularly in regards to Ni, V,Nd and Pb concentrations (Supplementary Table 4a). Rareearth element and yttrium spider diagrams (from laser abla-tion analysis) show that depositional limestones are similarto dolomitized samples in terms of normalized REY pro-files and Ce anomalies, but may vary in concentration, withthe former slightly more LREE enriched (Fig. 10A). Burial

calcite spar has more pronounced negative Ce anomaliesand lower REY concentrations. Laser ablation of platfor-mal carbonate components suggests that burial cements(late-calcite) have Cr, U and Al concentrations (mean Cr:0.52 ppm, U: 0.02 ppm, Al: 19 ppm across all burial cementsamples) (Fig. 10B, Supplementary Table 4b). In contrast,both early burial dolomite and original micritic or peloidallimestones have significantly higher Cr and Al concentra-tions, with variable enrichments in U (mean Cr: 2.23ppm, U: 0.076 ppm, Al: 1741 ppm from dolomite, andmean Cr: 3.43 ppm, U: 0.116 ppm, Al: 3225 ppm fromdepositional limestones) (Fig. 10B).

Bulk dissolution techniques on late-stage clear calcitespar showed very low concentrations of uranium fromonly 3 to 16 ppb, (Fig. 9B, Supplementary Table 4a). Incontrast, these cements have much higher Mn concentra-tions than equivalent slope limestones (mean is 4288ppm in three calcite samples vs. 80 ppm for 51 slopeand platform limestone samples) (Fig. 9B). They also havesignificantly lower Fe and Al concentrations than reeflimestones (mean Al: 16 ppm vs. 246 ppm; Fe 29 ppm vs.243 ppm).

Previous stable isotope work on these carbonates showthat the inferred marine d13C value of Late Devonian sea-water is preserved in reef slope marine cements (+2.0 to+3.0‰ d13C PDB; Carpenter et al., 1991) and reefal marinecements of the Oscar Range (d13C of 2.0 ± 0.5‰ PDB,d18O �4.5 ± 0.5‰ PDB; Hurley and Lohmann, 1989).Early diagenetic dolomite from Geike Gorge has a similarstable isotope composition to marine cements, clusteringaround 1.5‰ d13C and �2.5‰ d18O; but late-stage dull-luminescent calcite has a depleted and more variable iso-tope composition (�8.7 to 2.6‰ d13C; �16.3 to �11.0‰d18O) (Wallace et al., 1991).

Fig. 9. Uranium isotope and trace metal geochemistry of the Canning Basin carbonates. (A) Uranium isotopes from limestone platformalsamples, to partially dolomitised to dolomitised samples, including most visually altered dolomite samples. The limestone sample is �0.03 ±0.17‰, which is heavier than modern seawater. (B) Component-specific uranium and manganese concentrations for marine and diageneticphases of the Devonian reefs. Late-stage, dull-luminescent calcite (associated with solution cavities) has high Mn, and low U concentrations incomparison to marine limestone components of the platform and slope lithologies.

Fig. 10. (A) REY normalised spider plot (PAAS, McLennan, 1989) and (B) U vs. Cr concentrations of platformal carbonates of the PillaraLimestone, Dingo Gap. Non-luminescent calcite cements (PPD, PL and PD2) have generally lower U and Cr concentrations and a moresignificant Ce anomaly. Depositional limestone micrite is similar to burial dolomite in terms of U and REY concentrations (with slight LREEenrichment in the former).




Although the Canning Basin is generally considered anexample of an exceptionally preserved carbonate platform,even a well-preserved carbonate succession can undergocomplex interaction with diagenetic fluids. Devonian car-bonates (which formed as low- or high-Mg calcite) gener-ally retain fine-scale preservation of depositional texturesin limestone, but may be affected by fabric-destructive,early-burial dolomitization as well as late-stage diagenesis(Figs. 7 and 8). The presence of overprinting early-burialdolomite in platform limestones of the Pillara Lst. appearsto affect whole-rock U isotope and some trace metal geo-chemistry (e.g. U, Ni, Pb, Cr), but does not significantlyalter the limestone stable isotope composition from thatof Devonian seawater (from marine cements, Hurley andLohmann, 1989).

Normalised REY profiles for dolomitised limestones arerelatively similar to calcite micritic components (thoughwith slightly flattened profiles and reduced concentrations)(Fig. 10). This result reinforces that fabric destructivedolomitization does not systematically reset the REY com-position of marine limestones (Banner et al., 1988), thoughsome alteration has taken place (Fig. 10) (Nothdurft et al.,2004). However, the uranium isotope composition of thesecarbonates shows a progression to significantly lighter Uisotopic composition with increased amounts of dolomitepresent in samples (Fig. 9). Very light d238U early burialfluid compositions (e.g., represented by the dolomite d238Ucomposition) may be related to reducing conditions, wherebiotic reduction of U resulted in the removal of heavy ura-nium from the dolomitizing fluid (e.g. Stylo et al., 2015).Reducing conditions are also implicated by the lack of a sig-nificant Ce anomaly in dolomite samples (Fig. 10) and bythe bright-luminescence of the dolomite rims (suggestingMn2+ in solution). However, the similarity in stable isotopecomposition between both well-preserved marine carbon-ates (i.e. marine cements) and dolomite samples with com-plete fabric destruction suggests the dolomitising fluid wasderived from connate seawater (e.g. Wallace et al., 1991).

Clear, non-luminescent calcite, which may be porosity-occluding or a vein-filling spar, is not rich in trace metals(e.g. very low Al, Fe, Mn, Cr and U: SupplementaryTable 4b) but has a very different REY composition todepositional limestone and burial dolomite (Fig. 10). Thesecements are generally restricted to coarse-grained platfor-mal lithologies, and their distribution as well as their stableisotope composition suggests precipitation from connatemarine or meteoric fluids in a shallow burial setting(Wallace et al., 1991). Additionally, these cements arenon-luminescent and have more significant negative Ceanomalies than marine and dolomite phases, further sup-porting precipitation from an oxidised diagenetic fluid(Supplementary Table 4b). The relatively heavy and vari-able d238U of these cements (�0.25 and 0.20‰) may be off-set from original depositional limestone values and also arenot similar to modern seawater (e.g. Tissot and Dauphas,2015). While the U isotope composition of Devonian sea-water is not known, these variable d238U values (limestone,burial dolomite, burial calcite, Fig. 9) suggest that interac-tion with oxic fluids during shallow burial has the potential

to incorporate non-depositional d238U into multi-phase car-bonate rocks via cementation and recrystallization. There-fore, sampling cemented grainstones (i.e. many shallow-water carbonates from the geological record) should beapproached by integrated petrographic-geochemical meth-ods and not necessarily simply by bulk rock sampling.

Dull-luminescent, ferroan calcite solution-fill precipi-tates have very low U concentrations and high Mn concen-trations (Fig. 9B), as well as depleted d18O values,potentially suggesting higher temperatures of precipitation(i.e. during late-stage reburial) (Wallace et al., 1991). How-ever, post-Carboniferous exhumation, and late-stage mete-oric and basinal fluid flow associated with this calcitecementation of solution cavities did not broadly recrystal-lize nor significantly effect the chemical composition ofthe surrounding marine calcites (except around cave mar-gins and diagenetic fractures- Wallace et al., 1991) suggest-ing some late stage diagenetic processes may have onlylocalised effects on carbonate geochemistry.

4.4. Carboniferous Waulsortian Mounds of Ireland- Pb-Zn

mineralisation and burial diagenesis


Carboniferous mud mounds from the WaulsortianLimestone (Feltrim Fm.) of Ireland contain MississippiValley Type (MVT) Pb-Zn mineralisation precipitated dur-ing burial diagenesis (e.g. Hitzman and Beaty, 1996; Reedand Wallace, 2004) (Supplementary Table 6). Precipitationof Pb-Zn minerals in carbonates during burial diagenesis(e.g. non-economic, MVT mineralization) is relatively com-mon, and minor epigenetic Pb-Zn mineralization is wide-spread in sedimentary basins through most of thegeological record (Leach et al., 2005).

A detailed analysis of burial cementation includingstable isotope work from equivalent carbonates from theSilvermines district, Ireland, describes a complex cementstratigraphy of multiple different mineral phases (Reedand Wallace, 2004). In this study we target different marineand diagenetic calcite phases, identified from cathodolumi-nescence petrography. Crinoid ossicles are generally well-preserved in non-luminescent calcite (Fig. 11). Marinecements are composed of cloudy radiaxial fibrous calciteand is succeeded by equant calcite spar. Burial calcite sparis clear under plane light but zoned into a classic non-,bright-, dull-luminescent burial sequence under cathodolu-minescence (e.g. after Barnaby and Rimstidt, 1989) (Figs. 11and 12). Samples were selected from exploration coresdrilled around Ballinalack, and analysed by laser ablationICP-MS spot and line scan analysis for trace metal andREY concentrations. Quantitative spot analyses (n = 40,Supplementary Table 5) from well-preserved marine com-ponents (calcite marine cements, crinoid ossicles) and cal-cite burial cements from the Waulsortian Limestone(Feltrim Fm.) were selected based on petrography andcathodoluminescence microscopy (Fig. 11). Spot analyseswere separated into four categories based on petrography:marine, non-, bright- and dull-luminescent calcite cements.Metal isotope work was not undertaken as componentswere not of a sufficient size to microdrill.

Fig. 11. Cathodoluminescence photomicrograph of a crinoid andcalcite overgrowth from the Waulsortian Limestone from Balli-nalack, Ireland. Crinoid ossicle (centre) is composed of, mottled,non-luminescent calcite. Burial cement sequence comprises non-,bright- then mottled dull-luminescent zoned calcite. Average Crand U values are included for each component (based on anaverage of all spot samples, crinoid values are the same as, andintegrated with marine cements) and the location of a laser scan(Fig. 12A) is shown by the dashed white line (approximately1.5 mm in length).



Large changes between marine cements and later diage-netic cement zones are evident in many different trace metalconcentrations and REY profiles (Figs. 11 and 12). Theclearest examples of this are Mn and Fe, which are well-correlated to CL brightness (e.g. after Barnaby andRimstidt, 1989). In these semi-quantitative line scans, Mnis highest in bright-luminescent cements and Fe is high indull-luminescent calcite, with some variation (Fig. 12).

Fig. 12. (A) Semi-quantitative laser profile through crinoid ossicle, nLimestone, with trace metal concentrations. Transect length is approximrunning average, Cr is smoothed to a 20 point running average. This tranLaser profile through marine, bright (Br.) and dull cements of sample 481panel shows Ce smoothed to a 5 point moving average, U to a 10 pointshows Fe and Mn with a 20 point moving average. Marine cements haconcentrations and dull-luminescent cements have high Fe/Mn ratios. Rspike in the bright cements represents <5 laser data points.

Uranium concentrations from averaged (quantitative) laserspot data increase from a mean of 0.05 ppm in marinecements, to 0.79 ppm in late dull-luminescent cements(Fig. 11, see also semi-quantitative variation in Fig. 12).Laser spot analyses show that Cr concentrations are lowestin marine cements and components (mean = 0.34 ppm) andremain low in non-luminescent cements (Fig. 11). Cr con-centrations increase significantly in bright-luminescentcements (mean: 0.78 ppm), with correlative, large increasesin Mn; and remain high in dull-luminescent cements(mean: 0.54 ppm). Rare earth element concentrations arehighest and more variable in bright- and dull-luminescentcalcite cements (e.g., Ce, Fig. 12). Trace metal concentra-tions are lowest in non-luminescent calcite cements. REYprofiles change from typical LREE depleted modern-seawater-like profiles with negative Ce anomalies (e.g.German and Elderfield, 1990), to progressively moreMREE enriched profiles with no Ce anomaly through theburial cement sequence (Fig. 13). From marine to late-stage diagenetic calcite, the Eu anomaly becomes more sig-nificantly pronounced and the Y/Ho ratio decreases.


Laser scans from Waulsortian mound facies, which havebeen regionally subject to mineralizing fluids, show largechanges in REY profiles and trace metal composition overdistances of less than a millimetre from marine to burial cal-cite cements (Fig. 12). The earliest formed marine compo-nents show REY profiles very similar to modern seawater(Fig. 13), suggesting precipitation from an oxic, marinefluid (Wallace et al., 2017). Progressive reduction of marineand burial fluids resulted in a non-bright-dull-luminescenceCL sequence in cements (Barnaby and Rimstidt, 1989),where Mn and Fe are increasingly reduced and dissolved(i.e. are incorporated into cements, Fig. 12). Correlating

on, bright (Br.) and dull cements of sample 4805, Waulsortianately 1.5 mm. Element concentrations are smoothed by a 10 pointsect is the same as indicated by the dashed white line in Fig. 11. (B)2, Waulsortian Limestone, with trace metal concentrations. Uppermoving average and Cr to a 20 point moving average. Lower panelve low metal concentrations. Bright cements have the highest MnEY concentrations are highest in dull-luminescent cements. The U-

Fig. 13. Rare earth element and yttrium concentrations of marine and burial diagenetic carbonate phases (normalised to PAAS, McLennan,1989). Averages; 40 spots analysed.


with these petrographic (cathodoluminescence) observa-tions, normalised REY profiles become more MREEenriched and have a lessened Ce anomaly as cements pre-cipitate from increasingly reduced burial fluids (Fig. 13).This petrographic and geochemical sequence is typical ofmany Phanerozoic carbonates, where seawater is oxic,and connate seawater or burial fluids become progressivelyreduced during burial (Barnaby and Rimstidt, 1989).

Significantly, the modern-seawater-like REY signaturefrom Waulsortian marine components (also see marinecements analyses in Wallace et al., 2017) has not been resetby fluids during burial diagenesis, even including mineralis-ing basinal brines with increased REY concentrations andsignificantly different REY compositions (Fig. 13). Thisreinforces the suggestion that REY in marine carbonatescan be robust through burial diagenesis including mineral-ising fluid interaction (e.g. Banner et al., 1988; Nothdurftet al., 2004, Webb et al., 2009). However, as with laser scansof Neoproterozoic cements (Fig. 3), trace metal concentra-tions vary significantly through the cement stratigraphy,often with lowest concentrations in marine and early burial(non-luminescent) cements, and large spikes in metalsthrough bright- and dull-luminescent burial cements(Fig. 12). Although these cement crusts were too thin tobe sampled for isotope work, this trace metal work suggestsfor isotope systems where redox reactions control isotopecompositions (i.e. Cr, U), these burial cements should beavoided during (bulk) sampling.

5. DISCUSSION

5.1. Case studies: effects of diagenesis on carbonate

U isotope, trace metal and REY geochemistry

At the most basic level, geochemical variability in car-bonate bulk-rock samples can be traced to contaminationof samples by non-marine components (e.g. detrital mate-rial and diagenetic precipitates). On top of this, diageneticfluids may also alter the chemical composition of existing

phases. The varying propensity for alteration between geo-chemical systems (e.g. trace metals, metal isotopes and tra-ditional stable isotopes) is a result of the water to rock ratioof diagenetic processes, as well as how open the system isduring diagenesis (Bathurst, 1971; Brand and Veizer,1980; Veizer, 1983; Banner and Hanson, 1990; also seerecent review for Ca isotopes in Lau et al., 2017b). Thedegree of alteration depends on the composition of the flu-ids interacting with the original carbonate lithology duringdiagenesis, as well as how well-buffered the element and itsisotopes are to ‘resetting’. For example, elements such as Caand C that are in high concentrations in carbonates are lesslikely to be altered during fluid-rich alteration, but elementsthat have trace concentrations in carbonates (e.g., Mn, Pb,U, Cr, etc.) are more likely to be altered (e.g. Brand andVeizer, 1980; Banner and Hanson, 1990).

These case studies highlight this potential for significantvariability in trace metal enrichments and metal isotope val-ues within carbonates with different diagenetic histories. Inparticular, these examples emphasise that the incorporationof non-marine carbonate components (e.g. burial cements)in samples can have large effects on the preservation of adepositional geochemical REY and U isotope signatureusing bulk sampling techniques. This is in contrast to dis-cussion by Zhang et al. (2018), who considered the micro-scale redistribution of U isotopes in samples, but not theintroduction of isotopically different uranium from diage-netic cementation. Secondly, although most of the exam-ined cases of fluid interaction (cementation,recrystallization, etc.) occur at a temperature above averagesurface conditions, there is evidence that these processes areaccompanied by significant shifts in whole-rock sedimen-tary uranium isotopes values. Therefore, in contrast to car-bonate carbon isotopes and REY compositions (whichappear relatively robust through diagenesis), uranium iso-topes (similar to oxygen isotopes and trace metal composi-tions) can be affected by post-depositional diageneticprocesses in modified marine fluids or by late-stage fluidinteraction.


In each diagenetic scenario presented here, petrographicanalysis has been used as an independent test for the effectsof diagenetic processes in marine carbonates. Samples andmarine carbonate components that are well-preserved(e.g. retain their original depositional fabrics, show no signsof recrystallization, veining or non-carbonate inclusions inthin section and under CL) are generally considered to pre-serve the least-diagenetically modified geochemical compo-sition. This reasoning is possible in carbonates specifically,as their petrographic expression is easily influenced by post-depositional processes, so that evidence of fluid interactionin the vast majority of cases can be observed in thin section(Figs. 1,4,7,8,11). While this does not strictly mean that thebest petrographically preserved components will record aseawater geochemical composition, this reasoning repre-sents a strong starting point for justifying paleo-environmental geochemical work.

5.2. Diagenetic processes

5.2.1. The effects of dolomitization

These case studies highlight that the broader geologicalcontext for samples is important to consider when under-taking sedimentary geochemical work. In the most generalsense, carbonates formed in oceans with a different majorion chemistry and/or redox state appear to have undergonedifferent marine and diagenetic histories. For example, Pre-cambrian carbonates are more commonly dolomitized thanPhanerozoic carbonates, but this dolomitization is morelikely to have been early in (marine) diagenesis, and depo-sitional fabrics are often well-preserved (Tucker, 1982,1983; Corsetti et al., 2006; Hood and Wallace, 2012;Shuster et al., in press). In comparison, Phanerozoic car-bonates are more commonly dolomitized during open-system, burial diagenesis and are generally destructivelyrecrystallized (e.g. Banner et al., 1988). However, this broaddifference in the style of dolomitization through time is nota strict separation of processes but highlights the impor-tance of well-developed dolomitization model.

The timing of dolomitization in marine, early or lateburial diagenesis appears to have disparate effects on thepreservation of the original limestone precipitates’ geo-chemical composition. This is illustrated in a comparisonof the effects of marine, mimetic dolomitization (Cryoge-nian Balcanoona Fm., Fig. 2), early diagenetic dolomitiza-tion (Devonian Canning reefs, Fig. 9) and high-Tdolomitization (Devonian Guilmette Fm., Fig. 5). In thefirst two cases, the rock-fluid system appears to have beenmore closed during early diagenesis (i.e. more restrictedfluid evolution with burial), where as open-system diagene-sis in the latter case could explain large-scale modificationof depositional fabrics and chemical composition.

Marine-dominated dolomitising fluids appear to be ableto more faithfully preserve a depositional geochemical sig-nature with respect to some geochemical systems (e.g. car-bon isotopes, REY) in some carbonate components (e.g.Balcanoona Reef, Figs. 1–3). In this case study, early ormarine dolomite precipitates, rather than formerly arago-nitic lithologies, appear to better retain an original deposi-tional chemical composition with respect to uranium and

oxygen isotopes, even through progressive interaction withhigh-T, meteoric and dolomitising burial fluids (Figs. 2 and3) (Hood et al., 2016). However, large variability in U iso-topes in reefal components may indicate that microbialites,micrite and marine cements incorporate metals (and metalisotopes) with variable partitioning and fractionation(Fig. 2). Further, significant chemical heterogeneity in sam-ples suggests component-specific work may be better thanwhole-rock sampling for deriving a depositional and burialgeochemical history from shallow marine carbonates(Figs. 2, 3 and 9–12).

In contrast, carbonates that have experienced fabric-destructive dolomitization during early burial (e.g. Devo-nian Canning reefs, Figs. 7–10) appear to have significantlydifferent U isotope compositions between well-preservedand fully recrystallised bulk samples (Fig. 9). This dataimplies that the development of reducing connate marinefluids during burial (Wallace et al., 1991) has the possibilityto produce a relatively light U isotope composition in bulk-sampled, dolomitised limestones compared to un-dolomitised samples (Fig. 9). However, the dolomitisedmicrite samples show almost the same REY compositionas the depositional calcite micrite (though with somereduced concentrations), reinforcing the suggestion thatREY may not be as significantly affected by early burialdolomitization (Fig. 10) (Banner et al., 1988). In contrast,higher temperature, reducing, basinal dolomitizing fluidsunder advective fluid flow regimes (e.g. Paleozoic ofNevada, Figs. 4–6) have more significant effects on over-printing the original limestone geochemistry with respectto U isotopes and REY concentrations. In these lithologies,bulk sampling may not capture the depositional limestonechemical composition if even minor amounts of saddledolomite are present. However, in all of these cases, petro-graphic analysis prior to geochemical work is a simplemethod to effectively screen samples with respect to theextent and type of dolomitization.

5.2.2. The effects of facies specific diagenesis

The effects of spatially and temporally variable interac-tion with diagenetic fluids can be significantly diverse withina single formation or carbonate platform. These effects arehighlighted using the example of the Great Basin(Figs. 4–6), where the effects of burial fluid interaction onthe geochemical composition of the Cambrian WhippleCave Fm. and the Devonian Guilmette Fm. are signifi-cantly different between U, O and C isotopes (Fig. 5). Sim-ilar variations in the original (geographic and temporal)location, lithology and facies may affect how the carbonateis exposed to diagenetic fluids (e.g. Wallace et al., 1991;Melim and Scholle, 1999). A classic example of this is reefcomplexes, which are often heavily-cemented at the reefmargin during marine diagenesis (e.g. the Canning Basinreefs, Playford, 1980; Kerans et al., 1986; Wallace et al.,1991; and the Permian Capitan reef, Melim and Scholle,1999). In general, the difference between facies across a car-bonate platform can lead to very different intra-formationaldiagenetic histories, where extensively marine-cementedfacies can be relatively impervious to late-stage diageneticfluid-flow due to a lack of porosity and permeability. In


the Canning Basin reefs, marine cements occlude virtuallyall porosity at the reef margin, and are largely petrograph-ically well-preserved (e.g. Kerans et al., 1986). In contrast,platform lithologies are more finely crystalline, and areoften the first to be altered and/or dolomitised by fluidsdue to their highly-porous structure (Wallace et al., 1991)(Figs. 9 and 10). While fine-grained carbonates may bemore susceptible to recrystallization, coarser grainedlithologies (with higher depositional porosity and littlematrix) may be more heterogeneous as they will incorporatemore burial diagenetic precipitates. This highlights againthat geological context and petrographic work are key inproviding a framework for sedimentary geochemical work.

5.2.3. The effects of marine carbonate mineralogy and

meteoric diagenesis

The primary mineralogy of carbonates is another criticalfactor to consider when sampling carbonates for geochem-ical analysis. This is because calcite and aragonite precipi-tates incorporate metals differently and also responddifferently during diagenesis (e.g., for uranium isotopes,Chen et al., 2016). Aragonite is metastable in non-marinefluids and is usually neomorphosed to calcite or micritizedduring meteoric, marine or burial diagenesis (e.g. Folk;1965; Mazzullo, 1980; Reid and Macintyre, 1998). So theeffects of early fluid-rich diagenesis on an aragonitic lithol-ogy may be more significant than those on calcite (which ismore stable particularly in a low-Mg form: Bathurst, 1971;Marshall and Ashton, 1980; Brand, 2004; see BalcanoonaFm. vs. Devonian Canning Basin case studies). SignificantU isotope fractionation has been observed associated withthe appearance of dolomite in modern Bahaman tidalponds, which is possibly related to early meteoric conver-sion of aragonite (Romaniello et al., 2013). Oxidised, burialfluids in the Canning basin platformal carbonates (possiblymeteoric or connate marine fluids; Wallace et al., 1991,Fig. 9a) have similar to significantly heavier U isotopescompositions than equivalent bulk-sampled limestones,suggesting interaction with oxic fluids may influence thepreservation of depositional U isotope compositions inwhole-rock samples even in calcitic mineralogies (Fig. 9).Further work is needed in both modern and ancient settingsto more quantitatively determine the effects of early mete-oric diagenesis on metal isotopes in carbonates.

5.3. Broader implications

Overall, the results of this work suggest that combinedpetrographic-geochemical techniques are an effective wayto characterise a samples’ potential for preserving a deposi-tional geochemical signature. While depositional processesincluding the precipitation and early stabilisation of marinecarbonate minerals may influence the preservation of amarine geochemical signature in carbonates (e.g. for U iso-topes, Romaniello et al., 2013; Chen et al., 2016), petro-graphic evidence (including cathodoluminescencecharacter) can help identify the least-modified and least-contaminated carbonate phases. Additionally, there is arange of other information that can be gained from petro-graphic work to aid geochemical interpretations (e.g. the

spatial distribution of cements reflecting fluid sources orfacies- or phase-specific geochemical signatures). Further-more, this combination of petrography and metal isotopework opens up a whole new way to develop our under-standing of diagenetic processes, which are also intimatelylinked to Earth’s surface conditions.

We have shown that carbonates can reliably preserve arange of U isotope and REY geochemical signaturesthrough a long history of diagenetic processes withoutbroad-scale geochemical resetting or petrographic over-printing. However, it is clear that bulk sampling withoutprior petrographic work can lead to the incorporation ofconsiderable amounts of diagenetic precipitates and detritalphases in samples, disguising the depositional carbonategeochemical signature. One of the most important conclu-sions of this case study work is that it is fundamentallyimportant to isolate marine carbonate components fromdiagenetic precipitates prior to analysis. Therefore,although much attention in geochemical literature has beenfocused on the specific effects of certain types of diageneticalteration on carbonates, it is perhaps more important tofocus simply on first sampling the best-preserved (marine)components and sediments.

Our results provide a new look at the effects of diagene-sis on emerging metal isotopes. However, a fair case couldbe made that the conclusions drawn from these case studiesare not novel. For example, our central conclusion—thatcare is required to prove that samples are well-preservedprior to geochemical analyses—has been a large part of car-bonate geochemical work since the seminal work on diage-nesis of Bathurst (1971). It has also been demonstrated thatsome trace metals (e.g. Mn, Sr, Fe) in carbonates arestrongly affected by diagenetic alteration (Brand andVeizer, 1980; Veizer, 1983; Banner and Hanson, 1990;review in Swart, 2015). While we focused on U isotopes,this study has significant implications for the response ofa wide range of metal isotope systems (e.g., Cr, B, Fe,Mo, Zn) to a variety of common diagenetic processes.Based on the variability in d238U, and in other systems(e.g. Garrecht Metzger and Fike, 2013; Present et al.,2015; Fike et al., 2015; Zhang et al., 2017) it is likely thatmany trace metals and their isotopes may be similarlyaffected.

5.4. Future directions for sedimentary metal isotope work

While integrated geochemical-petrographic techniquesare not unfamiliar, the fundamental need for their applica-tion in future work needs to be highlighted. For example,the majority of sedimentary metal isotope studies over thelast several years have not included any or only had minorpetrographic work (Supplementary Table 7), but all carbon-ates throughout the geological record have been affected insome way through lithification and burial. Despite theircomplexity, the case studies presented here demonstrate asimple point: the effects of diagenesis on carbonates needsto be tested on a case-by-case basis, with consideration toboth the proxy and the geological context (also see Swart,2015, for a similar conclusion). This does not detract fromthe use of carbonates as paleoenvironmental archives, but


rather suggests that with some attention to detail, thepetrology of carbonates lithologies is advantageous in geo-chemical work.

There are a variety of petrographic and geochemicalprocedures that have been used to determine the preserva-tion of marine chemistry in carbonates. While these screen-ing procedures may identify poorly-preserved carbonates,no series of tests will fully guarantee the preservation of amarine chemical signature in carbonate samples. However,overall, it appears that the best petrographically preservedcomponents, with low detrital contents and low amountsof burial diagenetic phases present are a good starting pointfor determining a depositional geochemistry in future metalisotope work. Some general considerations from integratedgeochemical-petrographic work include.

5.4.1. Stratigraphic context

If a marine geochemical signature is being targeted, thensediments should be chosen to represent open marine con-ditions, rather than restricted or estuarine facies, condensedsequences or settings influenced strongly by hydrothermalactivity or riverine input. For example, nodular carbonatescan form during early diagenesis, but commonly capturethe chemical signatures of microbial reduction zones oper-ating the sediment (Mozley and Burns, 1993; Raiswelland Fisher, 2000). Carbonates forming hardgrounds canalso show chemical compositions more related seafloor pro-cesses (e.g. bioturbation, microbially-induced carbonateprecipitation, Fe-Mn crust formation, clay formation;Marshall and Ashton, 1980; Christ et al., 2015). Further,the composition of carbonate grainstones –whether grainsare biologically derived or non-skeletal– may also influencebulk geochemical signatures. Multiple stratigraphic sectionsare one way to test for localised diagenesis (e.g. fault-related) or facies-controlled diagenesis (e.g. Lau et al.,2017b).

5.4.2. Sample preservation

Beyond outcrop observations, thin section petrographycan be a simple way to determine primary carbonate miner-alogy and state of preservation. Using plane light micro-scopy, samples can be rapidly characterised and rejectedif they contain too much visible detrital contamination orare significantly recrystallized (e.g. including patchy orcomplete destruction of original depositional textures; vein-ing; staining; the presence of coarse, euhedral overprintingdolomite or calcite spar. Cathodoluminescence microscopycan be used to visually inspect the dominant chemical com-position of carbonates and their chemical preservation.Some evidence of good preservation may include fine-scale CL zonation in marine cements (or pristine non-luminescence in Phanerozoic marine carbonate compo-nents), as well as component-specific CL brightness’s andcolours rather than uniform sample luminescence.

5.4.3. Chemical heterogeneity

In addition to cathodoluminescence microscopy, ele-mental mapping techniques and component-specific geo-chemical work can be used to determine the distributionof metals in depositional, marine and later diagenetic car-

bonate components. Although further work is needed totest diffusional processes in carbonates, these techniquesmay be used to identify open versus closed system diagene-sis, and could also aid in determining a sampling strategyfor geochemical work. For example, bulk sampling mayonly be appropriate where samples are relatively homoge-neous, with limited diagenetic or detrital phases present.

5.4.4. Geochemical screening

There is a wealth of literature on screening proceduresfor sample preservation from a geochemical perspective(e.g. Brand and Veizer, 1980; Fairchild and Spiro, 1987;Banner and Hanson, 1990; Kaufman and Knoll, 1995,Banner, 1995; Brand, 2004; for review see Swart, 2015;Lau et al., 2017b). These tests are generally focused onthe use of stable isotope work and element ratios e.g.d13C, d18O, Sr87/Sr86, Mg/Ca, Mn/Sr at various cutoff levelsor correlations to indicate diagenetic overprinting on sam-ples. In particular, Sr87/Sr86 ratios can be a powerful tech-nique in identifying diagenetically altered carbonates (e.g.Banner, 1995), with significant increases in Sr87/Sr86 withminor amounts of alteration, if diagenetic fluids interactedwith any clastic or crustal material. Changes in carbon andoxygen isotope composition can similarly be used toaddress the influence of diagenetic fluids, with carbon iso-topes (in carbonates) being generally relatively robust dur-ing diagenesis.

While these geochemical cutoffs are important in deter-mining the best-preserved samples for geochemical work,assumptions are required for the values and ranges of thesechemical proxies that are not necessarily reflective of adynamic Earth system. Therefore, it must be emphasizedthat some criteria that may be applicable in Phanerozoiccarbonates, may not necessarily be applicable to carbonatesthat precipitated in an anoxic water column (i.e. likely mostPrecambrian carbonates). For example, samples with highMn/Sr may have been affected by reducing diagenetic fluids(with dissolved Mn in solution), or could have formed inseawater that had a redox state below the Mn redox bound-ary (and therefore preserved hydrothermal metal concen-trations e.g. Huang et al., 2011; Hood and Wallace, 2015).

6. CONCLUSIONS

Metal isotope and trace metal geochemistry of marinecarbonates can provide new insights into the paleo-environmental evolution of the Earth. However, the effectsof diagenesis on carbonates, and the incorporation on dia-genetic and detrital phases can result in the variable preser-vation of a depositional geochemical signal, depending onthe samples’ paragenetic history. Nevertheless, while car-bonates can be altered by a variety of diagenetic processes,petrographic analysis can be used as an initial screeningprocedure to identify the preservation of samples. Whencombined with sedimentology and stratigraphy, prelimi-nary geochemical analysis of the best preserved marinephases compared to diagenetic, recrystallized and/ordetritally-contaminated components can be used to testthe effects of diagenesis and contamination on sample sets.Using case studies, this method has been demonstrated for


a variety of diagenetic scenarios from the Precambrian toPaleozoic. The results show that the effects of burial diage-nesis and dolomitization on carbonates are highly variableand must be defined on a case-by-case basis. This work is acall to revisit the seminal sedimentological-geochemicalwork of the 1980s and 1990s, where well-preserved compo-nents are selected from the sedimentary archive for geo-chemical analysis only after careful petrographic analysis.This type of work enhances geochemical results and shouldbe a fundamental part of future geochemical analysis of thesedimentary geological archive.

ACKNOWLEDGEMENTS

We would like to thank the editor Stefan Weyer; StephenRomaniello, Huyue Song, Rachel Wood and three anonymousreviewers for their helpful comments which significantly improvedthe manuscript. This work was supported by the U.S. NationalScience Foundation Earth-Life Transitions Program, and theAlternative Earths NASA Astrobiology Institute (NAI). A.v.S.H.acknowledges the support of a NAI Postdoctoral Fellowship andthe Elizabeth and Vernon Puzey Fellowship at the University ofMelbourne. M.W.W. acknowledges support from an AustralianResearch Council Discovery Grant DP130102240. AdditionallyA.v.S.H. would like to thank E. Bellefroid and B. Kalderon-Asael for assistance in the lab, and C. Reed at Teck Ltd. fordiscussion.

APPENDIX A. SUPPLEMENTARY MATERIAL

Supplementary data associated with this article can befound, in the online version, at https://doi.org/10.1016/j.gca.2018.04.022.

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