μxanes study of iron redox state in serpentine during oceanic serpentinization
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Lithos 178 (2013) 70–83
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μXANES study of iron redox state in serpentine during oceanic serpentinization
M. Andreani a,⁎, M. Muñoz b,c, C. Marcaillou b, A. Delacour d,e
a Laboratoire de Géologie de Lyon, UMR 5276, ENS-Université Lyon 1, Franceb Institut des Sciences de la Terre, Université Grenoble 1, Francec European Synchrotron Radiation Facility, Grenoble, Franced Géosciences Environnement Toulouse, Université Toulouse 3, Francee Laboratoire Magmas et Volcans, UMR 6524, Université St Etienne, France
⁎ Corresponding author.E-mail addresses: [email protected] (M
[email protected] (M. Muñoz), adelie.dela(A. Delacour).
0024-4937/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.lithos.2013.04.008
a b s t r a c t
a r t i c l e i n f oArticle history:Received 28 August 2012Accepted 11 April 2013Available online 20 April 2013
Keywords:SerpentineIronRedoxHydrogenW/RMid-ocean ridge
Serpentinization of ultramafic rocks at mid-ocean ridges generates significant amounts of H2, CH4, andsupports specific biological communities. The abiotic H2 production is attributed to the reduction of H2Oduring serpentinization, which balances oxidation of ferrous iron contained in primary minerals (mainlyolivines and pyroxenes) to ferric iron contained in secondary minerals (mainly serpentines and magnetite).Magnetite has thus far been considered as the sole Fe3+-carrier for estimating bulk H2 production, notablybecause the valence of iron in serpentine minerals and its relationship with both magnetite abundance andserpentinization degree are usually not determined. We show that the serpentine contribution to the Fe andFe3+ budget has a significant effect on H2 production. We performed μ-XANES analysis at the Fe K-edge onthin sections of peridotites with various degrees of serpentinization from ODP Leg 153 (MARK region, 23°N).Fe3+/FeTot in oceanic serpentines is highly variable (from ~0.2 to 1) at the thin section scale, and it is relatednon-linearly to the local degree of serpentinization. A typical value of 0.7 is observed above 60% serpentinization.The highest values of Fe3+/FeTot observed within or close to late veins suggest that the Fe3+/FeTot in serpentinerecord the local water–rock (W/R) ratio, as previously proposed from thermodynamic modeling. We estimatethat the (W/R) ratio increased from ~0.6 to 25 during serpentinization at MARK, and locally reached ~100 inveins. Mass balance calculations combining all mineral and bulk rock analyses provide the distribution of Feand Fe3+ as serpentinization progresses. Serpentine dominates the Fe3+ budget of the rock over magnetiteduring the first 75% of serpentinization, contributing up to 80% of the total Fe3+. At later stages, serpentinecontribution to the Fe3+ budget decreases down to ~20%, while magnetite formation exponentially increases.Iron transfer from serpentine to magnetite balances the bulk Fe3+ content of the rock that increases almostlinearly with the advance of the reaction. Formation of serpentine accounts for the majority of Fe3+ and H2
production at early stages of serpentinization at a depth >2 km at MARK where the concentration of H2 canreach more than 100 mM according to the low W/R. H2 production values and depths can vary from one siteto another, depending on the evolution of the temperature, W/R ratio, inlet fluid composition, and favoredformation of serpentine vs. magnetite. At MARK, Fe3+ in serpentine represents 15–27% of the total Fe containedin a rock serpentinized to more than 80%, and accounts for 25% of the total H2 production that is estimated at325–335 mmol/kg of rock. The absence of magnetite does not necessarily mean a negligible H2 production,even at low T conditions (b150–200°C) under which the Fe- and Fe3+-richest serpentines have been observed.Serpentine minerals are important Fe3+-carrier in the altered ocean lithosphere, and may affect mantle redoxstate while dehydrating at depth in subduction zones.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Serpentinization is a hydration and redox reaction that transformsmantle-forming minerals (olivine and pyroxenes) into hydrousphyllosilicates (serpentine +/− brucite), iron oxides and hydrogen. Inthe oceanic lithosphere, serpentinization has been widely documented
. Andreani),[email protected]
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at slow- and ultraslow-spreading ridges where partially to fullyserpentinized mantle peridotites represent up to 50% of the lithologiesalong some ridge segments (Cannat et al., 1995a; Escartin et al., 2008;Lagabrielle et al., 1998; Smith et al., 2006). Oceanic serpentinization islocally associated with active hydrothermal vents known to releasesignificant amounts of abiotic H2, CH4, and possibly other complexcarbon molecules (Charlou et al., 2002; Holm and Charlou, 2001; Langet al., 2010; Proskurowski et al., 2008).
Production of large amounts ofH2 during serpentinization has raisedan increasing interest in the last decade as it is a source of energy thatcould be optimized and industrially exploited, and it constitutes a
71M. Andreani et al. / Lithos 178 (2013) 70–83
metabolic substrate for microorganisms that develop away from photo-synthetic energy supply (Schrenk et al., 2011; Takai et al., 2004).Numerous experimental studies have demonstrated the efficientproduction of H2 by bulk rock serpentinization of olivine or peridotitesbetween 200–400 °C and 300–500 bars (Allen and Seyfried, 2003;Berndt et al., 1996; Malvoisin et al., 2012; Marcaillou et al., 2011;McCollom and Seewald, 2001; Seyfried et al., 2007). Thermodynamiccalculations suggest that H2 production is most favored at 200–315 °Cfor a pressure of 350 bar (McCollom and Bach, 2009).
The abiotic production of hydrogen is attributed to the reduction ofH2O during oxidation of the ferrous component of primary minerals.The 1:1 linear correlation between hydrogen production and the totalamount of ferric iron in product minerals has also been verified experi-mentally (Marcaillou et al., 2011). The raw amount of hydrogen formedis thus directly linked to the amount and oxidation state of iron incorpo-rated into product minerals, mainly magnetite and serpentine. Usually,magnetite is assumed to be the only Fe3+-carrier considered for relatingthe bulk H2 production to mineralogical changes in natural, experimen-tal or theoretical systems (Cannat et al., 2010; Malvoisin et al., 2012;McCollom and Bach, 2009). Actually, the serpentinization reaction isoften further simplified in the literature by considering that serpentinecontains no iron. When iron in serpentine is taken into account, it isusually assumed to be ferrous (Cannat et al., 2010; Malvoisin et al,2012; McCollom and Bach, 2009; Sleep et al., 2004). Hence, the ferriccomponent of serpentine is neglected, although several authors havedemonstrated that a significant amount of ferric iron can be incorpo-rated in the serpentine structure, with a Fe3+/FeTot ranging between ~0and 1 in both natural (Andreani et al., 2008; Fuchs et al., 1998; Kleinet al., 2009; O'Hanley and Dyar, 1993) and experimental samples(Marcaillou et al., 2011; Seyfried et al., 2007). Trivalent cation is incor-porated into the trioctahedral serpentine structure by a ferri-Tschermack's substitution (toward a cronstedtite end-member) or byan octahedral-vacancy model (Evans, 2008; Wiks and Plant, 1979). Inany case, serpentine can play an important role in Fe3+ storage thathas been disregarded when estimating H2 production.
Klein et al. (2009) have considered the possible Fe3+ uptake inserpentine in reaction path modeling of serpentinization. In theirequilibrium model, the temperature (T), water–rock ratio (W/R), andinitial modal rock composition control hydrogen production via ironspeciation and iron partitioning between secondary phases. However,natural serpentinizing systems are kinetically controlled and opened
Table 1Bulk rock (BR) characteristics of the selected MARK samples.
L7
Selected samples, ODP 153-920B-3R187–92 cm
Given name MARK7Depth (mbsf) 24Density 2.65Porosity 0.10BR serpentinization degree1 % 83.4BR serpentinization degree2 % 86.8BR chemical analyses
SiO2 wt 37.90Al2O3 wt 0.95Fe2O3Tot wt 7.77MnO wt 0.14MgO wt 38.26CaO wt 0.33Na2O wt b L.D.K2O wt b L.D.TiO2 wt 0.01P2O5 wt b L.D.LOI wt 13.79Tot wt 99.14FeO (Fe2+ titration) wt 2.19Fe3+/FeTot 0.69
to external fluid inputs. In addition, T and W/R may vary with timeand space, depending on the geological setting, and lead to variableH2 fluxes.
Understanding of the serpentinization dynamics at natural sitesrequires the knowledge of iron content and redox state in naturalserpentine minerals, and its relationship with both magnetite abun-dance, serpentinization degree, and the regional setting. We performedμ-XANES analyses at the iron K-edge (ID24 beamline, ESRF; France) fordetermining the redox state of serpentine minerals selected fromserpentinized peridotites drilled during ODP Leg 153, at 23°N alongthe Mid-Atlantic Ridge. This site is well characterized, notably for thetemperature (Agrinier and Cannat, 1997) and depth of serpentinization(Canales et al., 2000), and its petrographic and tectonic evolution(Andreani et al., 2007). Combined with bulk rock analyses and magne-tite content (Oufi et al., 2002), we built a model of the evolution of ironspeciation and hydrogen production associatedwith serpentinization ofoceanic peridotite.
2. Geological setting
Serpentinites were sampled in the cores recovered from Holes 920B (126.4 m.b.s.f., meter below sea floor) and D (200.8 m.b.s.f.). Theseholes were drilled during ODP Leg 153 within the western fault scarpbounding the Mid-Atlantic Ridge valley at 23°20.31′N, 30 km south ofits intersection with the Kane transform fault (MARK area). Thefull-spreading rate in this area is ~2.7 cm/years, and the drilledserpentinites are part of a b1 m.y. old oceanic crust exposed underthe footwall of a major detachment fault (Cannat et al., 1995b). Inter-pretation of the seismic velocity structure at ODP Site 920 shows acontinuous velocity gradient that increases to reach mantle velocities(~8 km/s) at ~3–4 km b.s.f., suggesting a progressive decrease ofserpentinization with depth (Canales et al., 2000) and a serpentinite-dominated lithology below Site 920.
Holes 920 B and D have predominantly recovered partially to fullyserpentinized harzburgites with an average mode of 82% olivine, 15%orthopyroxene, 2% clinopyroxene and 1% of spinel (Cannat et al.,1995b). The degree of serpentinization along both holes is estimatedbetween 50% and 100% from mean relic mineral abundances atthe thin section scale. No periodicity or gradient of serpentinizationwith depth is observed along the alteration profiles in these holes. The
L9 L11 L24
153-920D-5R22–9 cm
153-920D-14R291–94 cm
153-920D-14R319–23 cm
MARK9 MARK11 MARK2438 116 1172.66 2.88 2.830.10 0.02 0.0481.0 54.1 59.984.5 57.2 63.1
38.59 41.02 39.321.29 1.30 1.207.28 8.38 8.530.09 0.13 0.1238.85 39.47 40.69b L.D. 1.90 1.35b L.D. b L.D. 0.00b L.D. b L.D. 0.020.02 0.02 0.01b L.D. b L.D. 0.0013.32 7.72 9.2299.44 99.93 100.462.21 4.96 4.430.66 0.34 0.42
72 M. Andreani et al. / Lithos 178 (2013) 70–83
degree of serpentinization varies from 50 to 100% over a few tens ofcentimeters.
Detailed petrostructural study of MARK serpentinites indicatesthat serpentinization is accompanied by abundant veining of differentgenerations (V1 to V4), characteristic of different mechanisms of trans-fer, and of tectonic control of the advancement of serpentinization reac-tion during exhumation (Andreani et al., 2007). Two main stages ofserpentinization have been identified: firstly, a diffusion-dominatedstage occurring deeper than 2 km b.s.f. at T b 350 °C that accounts for40–50% of the serpentinization (V1 to V3); secondly, an advection-dominated serpentinization stage at shallower levels (V4). Oxygenand hydrogen isotope compositions show that serpentinization resultsessentially from interaction of seawater with peridotites at tempera-tures ranging from 350–400 °C to less than 200 °C (Agrinier andCannat, 1997; Hebert et al., 1990). Water–rock ratios of 0.5 to 1 werecalculated for T = 400 °C during the first stage of serpentinization.Sulfur mineralogy, abundance, and isotopic compositions suggest thatseawater most probably reacted with mafic units before peridotitehydration at amaximum temperature of 400 °C (Alt and Shanks, 2003).
3. Analytical methods
Bulk Rock (BR) major element concentrations were determined byInductively Coupled Plasma Optical Emission Spectrometry (ICP-OES)at the SARM-CRPG (Nancy, France) after fusion with LiBO2 and disso-lution with HNO3. Separate analyses of Fe2+(BR) contents were alsoperformed by titration with potassium dichromate after dissolution ofthe sample in a HF/H2SO4 mixture in order to calculate Fe3+/FeTot(BR)ratios.
Density and porosity have been measured by the triple weighingmethod (e.g. Dullien, 1992). Each sample was first dried in an ovenuntil constant weight was attained (Mdry = dry weight). This wasfollowed by a water-saturated weighing in the air (Msat = saturated
A
C
MeshTxt 1
Txt 2
MeshTxt 1
Fig. 1. Evolution and heterogeneity of serpentinization textures at the thin section scaledisplaying Txt 1 (in A and B) and Txt 1 + Txt 2 (in C). D) Fully serpentinized sample (MAR
weight) and finally by a weighing in water (Mim = immersed weight;Archimedes push method). Then, porosity (ϕ) and density (ρ in kg/m3)are calculated using the following formula, with ρw the density of water:
ϕ ¼ Msat−Mdry
Msat−Mimρ ¼ ρwMdry
1−ϕð Þ Msat−Mimð Þ :
FEG-SEM images have been made on broken fragments of rockspreviously cleaned in an ultrasonic bath. They were then glued on acarbon tape before being observed with a FIB Zeiss NVision 40 atthe CLYM (Lyon, France).
Raman measurements were performed with a Horiba Jobin-YvonLabRAM HR800 Raman spectrometer at the ENS (Lyon, France)using green laser at 514 cm−1 with a beam size of ~3 μm on polishedthin sections. Raman regions corresponding to both lattice and internalvibrationmode (low-frequency: 200–1200 cm−1) and to the stretchingvibrations of the OH groups (high-frequency: 3600–3800 cm−1) ofserpentine were analyzed. The resolution of the detection system is ofapproximately 1 cm−1.
TEM observations were performed on a JEOL 2000FX high-resolution transmission electron microscope under a 200 kV at theCINaM (Marseille, France). Single-hole copper TEM grids were gluedon selected areas of a thin section, andextracted from the glass substrateby heating the resin. The specimens were then thinned by ion-beammilling (Precision Ion Polishing System, Gatan 690) and carbon-coated.
Major element composition of minerals has been measured by in-situ electronmicroprobe analyses using a Cameca SX100 at GeosciencesMontpellier (France).
Local measurements of iron redox state in serpentine minerals hasbeen extracted from the pre-edge region of Fe K-edge XANES (X-ray ab-sorption near-edge structure) spectra of serpentine. Fe K-edge XANESspectroscopic measurements were performed at the ID24 beamline ofthe ESRF (European Synchrotron Radiation Facility; Grenoble, France),
B
D
MeshTxt 2
V4
BstTxt 1
MeshTxt 1
Opx
Ol.
under cross-polarized light. A–B–C) Partially serpentinized peridotites (MARK 24)K 9) displaying a strong recrystallization of Txt 1 to Txt 2.
73M. Andreani et al. / Lithos 178 (2013) 70–83
using a dispersive Si(311) monochromator. Only one of the threeundulators was used to prevent heat-load of the optics. Point analysisand hyperspectral XANES micro-mapping were collected using a pin-diode fluorescence detector positioned at 90° from the incident X-raybeam (see Muñoz et al., 2006, 2008 for details), and in the Turbo-XASsetup (Pascarelli et al., 1999, 2006). The position of the sample, perpen-dicular to the X-ray beam, allows minimizing self-absorption (Pfalzeret al., 1999). 30 μm-thick polished thin sections were used. The beam-size at the sample surface was 6 by 6 μm at FWHM; i.e. much largerthan the grain size of powdered standards and of serpentine mineralsin the studied rocks where crystals can be considered either randomly-oriented or seldom “poorly-oriented”, depending on the local textureof serpentinites (see Section 4.2 on serpentine textures). Thus, potentialcrystal orientation effects on FeK-edge spectra are avoided. In the case ofserpentines, the maximum Fe3+/FeTot variations due to single-crystalorientation only reaches 0.15 (Muñoz et al., accepted for publication)but it is most likely close to 0 here.
With the dispersive setup we used for X-ray absorption, spectraldistortions occur when the sample is heterogeneous. Thus we madesure that point XANES spectra were acquired only in the homogeneouspart of the samples and especially avoidedmagnetite bearing areas. TheTurbo-XAS detection presents the advantage to strongly reduce thephoton flux at the sample surface compared to the standard dispersivemeasurements. In parallel, the acquisition time remained relativelyshort since a full XANES spectrum is recorded in about 1 min. Nomodification of the spectra is observed after acquisition of 2 to 4 spectraat the same point demonstrating the absence of significant beamdamage. Therefore, theuse of Turbo-XAS setup prevents for any possiblephoto-oxidation (or photo-reduction) or damage of the samples duringmeasurements. Fe K-edge XANES spectra for metallic iron were collect-ed for calibrating the system.
μ-XANES spectra andmaps were analyzed, respectively, with Athena(Ravel and Newville, 2005) and XasMap (Muñoz et al., 2006). The ironK-edge position is used as a qualitative proxy of Fe3+/FeTot(serp) onmapbecause this strong signal is less affected bynoise than the relativelyweak pre-edge peak. Quantitative Fe3+/FeTot(serp) was obtained afteraveraging at least four spectra acquired locally, within a homogeneousregion of the thin section, in order to increase the signal-to-noise ratioof the pre-edge region. Accurate Fe-K pre-edge analysis was based onthe background subtraction, normalization and fitting procedure de-scribed by Wilke et al. (2001). Pre-edge centroid energy and integratedintensity were fitted with a final uncertainty of 0.05 eV and 0.001 eV,respectively, resulting in uncertainties (σ) on Fe3+/FeTot ratio of lessthan 0.05 (Table 1). Four powdered standard samples were used to per-form such quantification (Wilke et al., 2001), namely olivine, andradite,staurolite and sanidine, which were already used in the study of Muñozet al. (accepted for publication).
4. Results
Sample investigations have been performed at two different length-scales: a) the decimeter-scale, which corresponds to the size of handspecimens and of bulk rock (BR) analyses; and b) the millimeter- andmicrometer-scales which correspond, respectively, to the scale oftextural heterogeneity and point analyses.
4.1. Bulk rock characteristics of the selected samples
Four representative serpentinite samples were selected for thisstudy from the suite studied by Andreani et al. (2007), on the basis oftheir macroscopic and petrographic properties. The selected sampleshave similar primary modes, close to the mean value of the site, anddifferent degrees of serpentinization. Table 1 summarizes the resultsof bulk rock (BR) analyses for these 4 samples. Density measurementswere used to calculate the BR serpentinization degree following twodifferent laws, for comparison. The two values obtained for the BR
serpentinization degree are given in Table 1 as “BR serpentinizationdegree 1” and “BR serpentinization degree2”. BR serpentinizationdegree 1 is given by the empirical law of Miller and Christensen(1997): ρ = 3.3–0.785 S, and BR serpentinization degree 2 is calculatedwith the theoretical formula calculated from mineral density for atypical residual harzburgitic composition (Oufi et al., 2002): ρ = 3.32–0.777 S; with ρ the measured density and S the BR serpentinizationdegree of the sample in modal %.
Fe3+/FeTot(BR), calculated using the measured Fe2O3Tot
(BR) andFe2 + Tot
(BR), shows a linear increase with the BR % of serpentinization,similar to the one observed by Evans (2008) from a large compilationof data on natural serpentinites. Fe2O3
Tot(BR) content after conversion
to anhydrous proportions varies little from 8.46 wt.% to 9.35 wt.%,with a mean value of 9.00 wt.%. These variations most probably reflectthe slight variations in primary modes. It has been shown that theserpentinite major element composition, in particular the iron content,is mainly controlled by the primarymineralogy, with no obvious gain orloss during serpentinization. Indeed, the anhydrous proportions ofelements are classically used to study peridotite compositions and asso-ciated magmatic processes (Godard et al., 2008; Paulick et al., 2006).Therefore, we assume in this study that the BR iron content remainsconstant during serpentinization and close to an initial content ofperidotite of Fe2O3
Tot(BR) = 9.00 wt.%, corresponding to a content of
7.8 wt.% for a fully serpentinized peridotites containing 13.5 wt.% ofvolatiles in average (Table 1).
4.2. Textural evolution of serpentinites during serpentinization
Serpentinites from Holes 920 B and D have in common the pseudo-morphic mesh (Mesh) and bastite (Bst) textures of serpentinization,replacing olivine and orthopyroxene respectively (Fig. 1). This textureis typical of a static serpentinization process, and characteristic ofoceanic serpentinization (Mevel, 2003). Clinopyroxenes, when present,are almost not altered.Most of the time, partially serpentinized samplesdisplay a heterogeneous degree of serpentinization at the thin sectionscale (mm-scale). This means that for a given serpentinization degreeestimated by bulk rock analyses (Table 1), it is possible to identifymillimetric regions of the thin section where the degree ofserpentinization is much higher or lower. For example, areas displayedin Figs. 1A, B, C and 3A come from the same thin section (sample MARK24) and clearly show different reaction progress. This provides anopportunity to locally explore the early serpentinization history, inac-cessible from BR analyses. Image analyses of millimetric areas (2 ×2 mm) selected for μXANES measurements have provided an estima-tion of the “local serpentinization degree” (Table 2), by opposition tothe “BR serpentinization degree” estimatedwith densitymeasurements(Table 1). Binary images have been created using a threshold in order toobtain modal analyses of primary mineral fractions remaining in eachzone. This rough estimation allows classifying the analyzed zonesaccording to their relative degree of serpentinization.
Two main pseudomorphic serpentinization textures can be distin-guished using petrographic microscope under cross-polarized light.The first texture (Txt 1) is better observed in the least serpentinizedregions of the thin sections where serpentines form light-gray toyellowish veinlets around relict olivine grains (Fig. 1A, B and C), and itcan also be partially preserved in more serpentinized samples (Fig. 3A).Serpentine in Txt 1 has been systematically identified as lizardite byRaman spectroscopy, in agreement with the detailed investigation ofsimilar textures by Boudier et al. (2009). These authors showed thatthis type of texture is made of columnar lizardite growing non-topotactically at the expense of olivine; (001) plane of lizardite beingparallel to margins of the veinlet crosscutting olivines. They alsoemphasize the presence of voids between lizardite columns and theabsence of brucite, even at the scale of TEM observations. SEM imagingof olivine-serpentine contact in MARK samples (Fig. 2C) also shows thedevelopment of columnar lizardite close to the interface, sometimes
Table 2Electron microprobe and μXANES analyses in serpentine minerals.
Sample MARK 7 MARK 9 MARK 9 MARK 9 MARK 9 MARK 9 MARK 9 MARK 11 MARK 11 MARK 11
Petrographic characteristicsTexture Txt 2 Txt 2 Txt 2 Txt 1 Txt 1 Txt 1
Vein (V4) Mesh rim Mesh rim Mesh core Vein (V4) Vein (V4) Vein (V4) Mesh rim bst bstLocal serpentinization degree (%) 100 100 100 100 100 100 100 79 55 55Local magnetite content (%) n.d. 7.0 7.0 7.0 n.d. n.d. n.d. 3.0 n.d. n.d.
Electron microprobe analysesOxide wt%Al2O3 4.22 0.47 0.72 1.14 1.04 1.23 1.30 0.32 1.95 2.05SiO2 39.65 42.79 42.14 41.17 42.49 42.54 42.01 40.88 39.70 39.15Cr2O3 0.02 0.01 0.08 0.00 0.00 0.05 0.00 0.00 0.90 0.87TiO2 0.03 0.01 0.02 0.01 0.01 0.00 0.00 0.00 0.02 0.05Na2O 0.03 0.02 0.03 0.04 0.00 0.00 0.02 0.01 0.02 0.00MgO 38.64 37.49 39.09 38.18 38.98 38.63 39.03 37.94 33.20 33.52MnO 0.07 0.03 0.03 0.03 0.00 0.04 0.00 0.08 0.25 0.24FeO 1.68 1.73 1.75 1.98 1.64 1.46 1.75 4.10 4.92 5.49K2O 0.01 0.02 0.02 0.02 0.00 0.00 0.02 0.01 0.01 0.01CaO 0.12 0.03 0.03 0.04 0.00 0.00 0.02 0.03 0.46 0.51NiO 0.00 0.17 0.24 0.06 0.08 0.02 0.00 0.37 0.06 0.07SO2 0.04 0.08 0.12 0.18 0.00 0.05 0.08 0.08 0.28 0.27Total 84.57 82.86 84.32 82.89 84.48 84.05 84.26 83.83 82.49 82.22
Structural formula (a.p.f.u)Al 0.25 0.03 0.04 0.07 0.06 0.07 0.07 0.02 0.11 0.12Si 1.96 2.11 2.08 2.03 2.03 2.03 2.01 2.02 1.96 1.93Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.04 0.03Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Na 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Mg 2.85 2.76 2.88 2.81 2.78 2.75 2.78 2.79 2.45 2.47Mn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01Fe 0.07 0.07 0.07 0.08 0.07 0.06 0.07 0.17 0.20 0.23K 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Ca 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.03Ni 0.00 0.01 0.01 0.00 0.00 0.00 0.00 0.01 0.00 0.00S 0 00 0.00 0 00 0 01 0 00 0 00 0 00 0 00 0 01 0 01Total 5.14 4.99 5.10 5.01 4.94 4.92 4.94 5.03 4.81 4.83
μXANES measurementsPre-edge centroid energy(±0.05 eV)
7114.00 7114.27 7114.40 7114.01 7114.30 7114.40 7114.40 7114.00 7114.10 7114.00
Integrated pre-edge intensity(±0.001)
0.100 0.213 0.190 0.158 0.128 0.146 0.147 0.097 0.161 0.123
Fe3+/FeTot (±) 0.61 (0.04) 0.81 (0.05) 1.00 (0.05) 0.54 (0.04) 0.88 (0.05) 1.00 (0.05) 1.00 (0.05) 0.62 (0.04) 0.63 (0.04) 0.58 (0.04)Fe3+/FeTot error (±) 0.04 0.05 0.05 0.04 0.05 0.05 0.05 0.04 0.04 0.04Fe3+/FeTot error (±%) 3.77 4.65 5.00 3.65 4.87 5.00 5.00 3.77 3.77 3.71
0.97614979 0.97474581 0.97546154 0.97177494 0.97699782 0.97930848 0.9754569 0.94285615 0.9231913 0.91591212
74 M. Andreani et al. / Lithos 178 (2013) 70–83
separated from olivine by a ~100 nm-thick layer of iron-rich brucite(Fig. 2C and F).
The second texture (Txt 2) is almost isotropic under cross-polarizedlight and clearly overprints the previous ones (Figs. 1C, D and 2). It isheterogeneously distributed as it mainly progresses along ~50 μm-thickveinlets crosscutting the previous textures (Fig. 1C). Txt 2 is locallymoredeveloped close to larger veins of several hundreds of μm in thicknesse.g. Figs. 1D and 3A; type V4 veins in Andreani et al. (2007). Txt 2 isobserved in both partially and fully serpentinized samples. It can formfrom olivine (Fig. 2A) or from the previously formed lizardite (Figs. 1D,2A and 3A). In the later case, columnar lizardite formed at the olivine-serpentine interface is replaced by an assemblage of fibrous serpentine(chrysotile and polygonal serpentine; Fig. 2D) that develops fromthe center of serpentine veinlet toward the lizardite–olivine contact(Figs. 1C and 2C). This replacement is expected to increase the porositybecause of the tubular nature of fibrous serpentines that have centralholes of ~200 nm in diameter over several microns in length (Fig. 2E).At the macroscopic scale, this is marked by a change in porosityfrom 0.02 in the least serpentinized sample MARK11 to 0.1 in fullyserpentinized samplesMARK7 andMARK9. According to textural obser-vations and cross-cutting criteria, Txt 1 and Txt 2 can be associated withthe two main episodes of serpentinization deduced from the detailed
vein study of Andreani et al. (2007): stage 1 forming V1–V3 and Txt1,and stage 2 forming V3–V4 with Txt 2.
A third texture (Txt 3) is occasionally observed in MARK samples(Fig. 3). It presents a green-brown color and forms the central coreof mesh with or without relict olivine (Fig. 3B). It is located withinzones of high local serpentinization degree, where Txt 2 is alreadywell developed and it seems to exclusively replace the rare olivine rel-icts. This suggests that Txt 3 is a late stage of olivine alteration. Ramanspectroscopy (Fig. 3D) and electronmicroprobe analyses (Table 2) indi-cate that this phase is an iron-rich serpentine, most probably lizardite.This type of brown texture made of an iron-rich serpentine is verysimilar to the one described by Evans et al. (2009) and in which theyestimated by stoichiometry a high ferric component of up to Fe3+/FeTot(serp) = 0.7 using electron microprobe data.
Magnetite is the othermainmineral composing serpentinites. It prin-cipally forms within the pseudomorphic mesh texture after olivine andis seldom observed in bastite and in the late stage veins studied here.Analyses ofmesh texture images have been used to quantify themagne-tite content called “local magnetite content” in Table 2. This “local mag-netite content” can be related to the degree of serpentinization (Fig. 4A),and compared with the BR magnetite content derived from magneticmeasurements from the same drill Hole 920 (Oufi et al., 2002). They
MARK 24 MARK 24 MARK 24 MARK 24 MARK 24 MARK 24 MARK 24 MARK 24 MARK 24 MARK 24 MARK 24 MARK 24
Spot 5 mapa Spot 4 mapa Spot 3 mapa Spot 2 mapa
Txt 1 Txt 1 Txt 1 Txt 1 Txt 1 Txt1 Txt 2 Txt 2 Txt 3 Txt 3 Txt 1 Txt 1Mesh rim ol Mesh rim ol Mesh rim ol Mesh rim ol Mesh rim ol Mesh rim Mesh rim Mesh core Mesh core Mesh core bst bst17 35 35 35 35 98 98 98 98.00 98.00 17 170.0 1.5 1.5 1.5 1.5 5.0 5.0 5.0 5.00 5.00 0.0 0.0
1.28 0.75 0.70 0.41 0.57 0.75 0.60 0.75 0.38 0.05 1.15 1.3841.21 41.71 40.94 39.15 40.01 41.53 41.23 39.67 40.16 37.06 41.17 40.840.00 0.01 0.01 0.00 0.00 0.01 0.01 0.01 0.00 0.00 0.60 0.720.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.010.02 0.04 0.03 0.05 0.02 0.02 0.02 0.01 0.02 0.02 0.00 0.0236.20 36.59 36.58 37.45 36.66 36.32 38.28 38.20 35.69 38.66 34.75 33.840.20 0.17 0.16 0.11 0.11 0.10 0.08 0.10 0.16 0.11 0.23 0.275.58 5.72 5.69 5.43 5.09 3.89 2.96 3.42 7.60 6.65 6.20 6.670.01 0.00 0.01 0.03 0.03 0.02 0.02 0.01 0.01 0.02 0.00 0.010.09 0.06 0.07 0.07 0.07 0.08 0.06 0.07 0.02 0.05 0.10 0.080.23 0.19 0.25 0.39 0.43 0.09 0.32 0.41 0.51 0.29 0.00 0.000.10 0.16 0.12 0.09 0.09 0.17 0.08 0.10 0.16 0.10 0.17 0.1684.93 85.42 84.56 83.18 83.07 82.98 83.66 82.76 84.71 83.03 84.39 84.00
0.07 0.04 0.04 0.04 0.03 0.04 0.04 0.04 0.02 0.00 0.07 0.082.01 2.02 2.02 2.02 1.91 2.05 2.04 1.97 1.99 1.83 1.97 1.950.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.030.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.000.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.002.59 2.69 2.69 2.69 2.61 2.68 2.82 2.82 2.63 2.85 2.48 2.410.01 0.01 0.01 0.01 0.00 0.00 0.00 0.00 0.01 0.00 0.01 0.010.25 0.24 0.24 0.24 0.20 0.16 0.12 0.14 0.31 0.28 0.25 0.270.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.000.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.000.01 0.01 0.01 0.01 0.02 0.00 0.01 0.02 0.02 0.01 0.00 0.000 00 0 00 0.00 0.00 0.00 0.01 0.00 0.00 0.01 0.00 0.01 0.014.95 5.02 5.02 5.02 4.79 4.95 5.04 5.01 5.00 4.99 4.80 4.76
7113.60 7113.90 7113.90 7113.80 7114.00 7113.90 7114.15 7114.15 7114.21 7114.20 7113.40 7113.70
0.105 0.156 0.153 0.105 0.158 0.099 0.182 0.163 0.159 0.129 0.092 0.117
0.30 (0.03) 0.41 (0.04) 0.44 (0.04) 0.45 (0.03) 0.52 (0.04) 0.52 (0.04) 0.67 (0.04) 0.70 (0.04) 0.75 (0.05) 0.77 (0.05) 0.21 (0.03) 0.34 (0.03)0.03 0.04 0.04 0.03 0.04 0.04 0.04 0.04 0.05 0.05 0.03 0.033.14 3.65 3.65 3.24 3.65 3.65 3.765 3.765 4.55 4.55 2.89 3.240.91130069 0.91971533 0.91971533 0.91971533 0.92768379 0.94338196 0.95844265 0.95221589 0.89331256 0.91196507 0.90902076 0.90048031
75M. Andreani et al. / Lithos 178 (2013) 70–83
are in good agreement for highly serpentinized regions and providequalitative complementary data for low serpentinization stages. Alto-gether, these data confirm the non-linear increase of magnetite contentas a function of serpentinization degree at MARK.
4.3. Iron content and redox state in serpentine during serpentinization
Electron microprobe analyses of primary phases (olivine Fo90,enstatite, diopside and spinel) and serpentines from MARK Andreaniet al. (2007) were complemented by new data on the serpentine min-erals investigated by μXANES in this study (Table 2). Structural formulaeare calculated on the basis of 7 anhydrous oxygens. The total ironcontent in serpentine (FeOTot
(serp)) ranges from 1.4 wt.% in late veins,to 7.6 wt.% in mesh of Txt 3. Fig. 4B shows that FeOTot
(serp) globallydecreases when the local degree of serpentinization increases (Fig. 4B).This trend had also been observed by Oufi et al. (2002) on a larger seriesof analyses. The decrease is slow before 60% of serpentinization and thensteepens, especially over 80% of serpentinization. The abrupt drop inFeOTot
(serp) recorded by the lowest values of FeOTot(serp) in the last 10%
of serpentinization coincides with the occurrence of serpentine vein V4in the samples. Mesh textured serpentine close to the veins Fig. 1D(MARK 9) has a low iron content (1.73–1.98 wt.%) similar to those of
the neighboring veins (1.46–1.75 wt.%). Iron-rich serpentines formingTxt 3 are out of the main trend, they present the highest FeOTot
(serp) =7.6 wt.%, and formed around olivine relicts in highly serpentinizedzones (Fig. 3A and B).
Characteristic XANES spectra obtained in serpentines and primaryphases at the iron K-edge are shown in Fig. 5A. The difference ofabsorbance after and before the edge, or edge jump, is proportional tothe amount of iron in the mineral and can be used to map the relativeiron content of the phases within a thin section region as the oneof Fig. 6C. This illustrates the contrasted iron content between Txt 1, 2and 3 in an almost fully serpentinized region of sample MARK 24(Fig. 6A and B).
Quantification of Fe3+/FeTot(serp) in serpentine minerals fromμXANES data can be obtained using the measured standards to consti-tute the calibration grid of Fig. 5B (see Wilke et al., 2001 for details).The measured energy positions and integrated intensities of XANESpre-edges are reported in Table 2 and on Fig. 5B. The variations in pre-edge centroid energy directly depend on the iron redox state, and thevariations in pre-edge intensity may be attributed to octahedral sitedistortions or to the occurrence of tetrahedral iron. The entire dataset plots within the lower-right region of the variogram betweenthe VIFe2+–VIFe3+ and VIFe2+–IVFe3+ joins (Fig. 5B). As expected
76 M. Andreani et al. / Lithos 178 (2013) 70–83
from the general crystallochemical behavior of iron and from previousdata on serpentine (e.g. Fuchs et al., 1998; O'Hanley and Dyar, 1993),this confirms the absence of tetra-coordinated Fe2+ in serpentine struc-ture and the occurrence of tetra-coordinated ferric iron in serpentines.Hence, we chose to attribute the change in pre-edge intensity to theoccurrence of tetrahedral ferric iron, rather than to distortions. Iffurther studies demonstrate that distortions can also affect the pre-edge intensity of serpentine minerals, the quantification of ferric ironwill lie on the VIFe2+–VIFe3+ joins of the variogram, resulting in slightlyhigher ferric iron content for some points (+0.08 at most). The Fe3+/FeTot can be quantified at ± 0.03 to 0.05 (Table 2) according to theerrors on pre-edge fitting parameters. Fe3+/FeTot(serp) ranges between0.21 and 1 (Table 2), which is consistent with the wide range of valuespublished in literature using Mössbauer spectrometry on powders orcalculations using stoichiometric constraints (Evans et al., 2009; Fuchset al., 1998; Klein et al., 2009; O'Hanley and Dyar, 1993).
B
Liz.
Bru.
A
2 µm
100 µm
C
B
Olivine
Liz.
Liz.
1
Liz+Ctl + PS
Fig. 2. Replacement of Txt 1 by Txt 2 through the partial recrystallization of lizardite (Liz.)image of Txt 2 replacing Txt 1 under cross-polarized light. B) SEM image under back-scatheavy elements, correlated to iron enrichment here. C) SEM images under secondary electrotwo olivine grains in a partially serpentinized rock (MARK 11). D) TEM image of the central punder SEM secondary electron mode of the central part of a mesh. The texture is dominatedimage, under secondary electron mode, of an olivine-serpentine contact displaying the loca
No systematic variations of Fe3+/FeTot(serp) with the main types oftextures can be evidenced since mesh, bastite and veins all display alarge range of variations (Fig. 7A). However, bastites and mesh rims oftype Txt 1 have the lowest Fe3+/FeTot(serp) values below 0.6. In contrast,other mesh textures of type Txt 2 and 3, and late veins, have systemat-ically higher Fe3+/FeTot(serp). This is illustrated on the mapping of theiron K-edge position in the heterogeneously serpentinized sampleMARK 24 (Fig. 6D). The ironK-edge position shifts toward higher valueswith the increasing Fe3+/FeTot(serp) ratio in the mineral (see spot quan-tifications in Table 2). The best preserved regions of Txt 1-serpentinedisplay lower edge position (blue regions, spot 5, Fe3+/FeTot(serp) =0.52) than the one of Txt 2 (yellow regions, spot 3 and 4, Fe3+/FeTot(serp) = 0.70 and 0.67), which is also lower than the one of Txt 3(orange regions, spot 2, Fe3+/FeTot(serp) = 0.75). Olivine (dark blueregion, spot 1) presents the lowest edge position of the mapped regionand provides a zero reference for Fe3+/FeTot(serp). Red spots represent
Liz.
Ctl + PS
D
µm
Bru.
Olivine
Liz+Ctl + PS
Liz.
F
E
to fibrous serpentines chrysotile (Ctl) and polygonal serpentine (PS). A) Petrographictered mode of coexisting Txt 1 and Txt 2. Contrasts show that the former is richer inn mode illustrating the lateral evolution of the serpentine filling mesh texture betweenart of a mesh showing a partial replacement of lizardite by fibrous serpentine. E) Zoomby fibrous serpentines that intrinsically display a high nano- to micro-porosity. F) SEMl occurrence of a Fe-brucite nano-layer.
77M. Andreani et al. / Lithos 178 (2013) 70–83
iron oxides. In the fully serpentinized sampleMARK9,mesh rim of Txt 2displays the highest measured values of Fe3+/FeTot(serp), tending tothose of late veins, when they are close to them, while mesh coreretains intermediate values. All these observations suggest the role oflocal advancement of serpentinization reaction on the Fe3+/FeTot(serp).This is confirmed by Fig. 7Bwhich shows that the Fe3+/FeTot(serp) in ser-pentine increases non-linearlywith the local degree of serpentinization,even at the thin section scale. Mesh and veins from sample MARK 9have the highest Fe3+/FeTot(serp) values among those obtained on100% serpentinized zones that, otherwise, tend to a mean value of ~0.7.
5. Discussion
5.1. Mass balance and iron distribution during serpentinization at MARK
As discussed in Section 4.1, we will consider iron as a conservativeelement at the sample scale (constant BR content) during theserpentinization reaction. Therefore the iron initially contained inprimary minerals has to be progressively distributed duringserpentinization between the major iron-bearing secondary phases,i.e. serpentine and magnetite. Toft et al. (1990) already stressed thatproduction of a Fe-enriched serpentine is required in addition tomagnetite during serpentinization, in order to balance the reactionfor iron. Indeed, the BR amounts of magnetite deduced frommagneticsusceptibility measurements in serpentinites were lower than theones predicted from theoretical reaction equations using Fe-freeserpentines. A common characteristic of natural serpentinites isthe exponential increase of modal magnetite content as a functionof serpentinization degree in bulk rock (Bach et al., 2006; Oufi et al.,2002; Toft et al., 1990). The available data on BR magnetite content
A
200 400 600
Inte
nsi
ty (
a.u
.) 689
385232
Txt 3Txt
1 2
Ol.
50 µm
C
+-
T 1
Fig. 3. Characteristics of the late serpentine texture Txt 3. A) Petrographic image under crossobserved. B) Zoom of figure A, under natural light. C) SEM images, under electron backscatteis enriched in heavy elements, iron here, compared to Txt 1 ± 2. D) Characteristic Raman s
in serpentinites from MARK (Site 920) are displayed in Fig. 4A.The best fit obtained for these data is described by the followingequation:
m ¼ −0:15þ 0:15 e S=27ð Þ R2 ¼ 0:72 ð1Þ
with m the magnetite content and S the serpentinization degree in %.This empirical relation is slightly different from the one proposed byOufi et al. (2002) for ODP site 895, located 2°N in the Eastern PacificOcean (Hess Deep valley). Higher magnetite content is observed atMARK, in particular in the less serpentinized zones in agreement withthe discussion of Oufi et al. (2002) who reported a higher productionrate of magnetite at MARK than at Hess Deep. Eq. (1) reproduces the0% of magnetite before serpentinization initiates. It gives a maximumof 6 wt.% of magnetite with serpentine minerals containing 1.5 wt.%of FeOTot
(serp) (Table 2), within a serpentinite rock containing 7.8 wt.%of Fe2O3
ToT (see Section 4.1). This maximum amount also fits well withiron mass balance for a characteristic, fully serpentinized rock, fromMARK. For an iron-conservative system, the exponential increasein magnetite content as a function of serpentinization degree has tobe correlated with a non-linear decrease in iron content in othersecondary phases, i.e. serpentine in this case. This is consistent within-situ FeOTot
(serp) analyses of serpentine minerals (Fig. 4B). We havecalculated the required theoretical evolution of FeOTot
(serp) (blackcurve, Fig. 4B) that balances the total iron mass at each reaction stepwith the hypothesis that iron is progressively consumed by secondaryminerals (i.e. the formation of 10 wt.% of serpentinites consumes10 wt.% of the final iron in the serpentinite rock). The calculatedFeOTot
(serp) fits reasonably well the observed serpentine compositionconsidering their intrinsic heterogeneity.
B
3600 3700800 1000 1200
Raman shift (cm-1)
Txt 3
Txt 1 2+-
Txt 3
D
xt 2+-
-polarized light of a highly serpentinized region of thin section where Txt 3 is typicallyred mode, on Txt 3 forming at the expense of relict olivine. Contrasts indicate that Txt 3pectra of serpentines forming Txt 1 ± 2 and Txt 3.
0
2
4
6
8Txt 3
FeO
To
t in
serp
enti
ne
(wt%
)
Local serpentinization degree (%)
MeshBstVein V4
A
B
0 20 40 60 80 100
0 20 40 60 80 100
0
2
4
6
8M
agn
etit
e co
nte
nt
(%)
Serpentinization degree (%)
Oufi et al.,2002MARK (this study)
Fig. 4. Evolution of (A)magnetite content and (B) iron content in serpentine as a functionof serpentinization degree.
0,0
0,1
0,2
0,3
0,4
0,5
0,2
Inte
gra
ted
pre
-ed
ge
inte
nsi
ty
Olivine
Pyroxene
Mesh
Vein
Bastite
No
rmal
ized
ab
sorb
ance
Energy (eV)
7120 7140 7160 7180 7200
A
Fig. 5. μXANES results on serpentine minerals. A) Characteristic XANES spectra of mantleFe3+/FeTot(serp) based on the empirical method proposed by Wilke et al. (2001).
78 M. Andreani et al. / Lithos 178 (2013) 70–83
Considering now the ferric iron budget, our BR analyses and theones compiled by Evans (2008) suggest that Fe3+/FeTot(BR) increasesalmost linearly with the hydration degree (used as a proxy of thedegree of serpentinization) to reach a final value ranging between~0.5 and 1. This is not compatible with the exponential increase ofmagnetite if Fe3+ is exclusively in magnetite. Without consideringFe3+ in serpentine, the Fe3+/FeTot(BR) of serpentinites should followthe magnetite evolution trend with little Fe3+ during the first halfof reaction, and a final increase to Fe3+ / FeTot(BR) = 0.53 (calculatedwith a constant Fe3+ / FeTot(Mgt) = 2/3 inmagnetite). Thismay be trueat some other sites where such an evolution of magnetite content iscorrelated with Fe-brucite production instead of serpentine in thebeginning of reaction (Bach et al., 2004; Klein et al., 2009). In the caseof MARK, the calculated Fe3+/FeTot(BR) ratio is 0.53 while the measuredvalues, in the most serpentinized samples, range around 0.66–0.69(Table 1). This observation and our XANES data indicate that Fe3+
content in serpentine has to be included to balance the BR Fe3+.Mass balance calculation from BR data shows that the Fe3+ in
serpentine should represent 72 wt.% of the total Fe3+(BR) for a degreeof serpentinization S = 55% (MARK 11) and ~20–23 wt.% of the finalFe3+(BR) budget of MARK serpentinites (S = 81–87%; MARK 7 and 9).Therefore, a significant error is made on the total Fe3+(BR) contentwhen using magnetite as the sole Fe3+-carrier in serpentinites. Thisleads to an underestimation of H2 production in natural, experimentalor theoretical studies. In the present case, the formation of 1 kg ofserpentinites (i.e. serpentinization of ~865 g of peridotite) produces atotal amount of 260 mmol of H2 if Fe3+ is only considered inmagnetite,whereas it increases by more than 25% (325–335 mmol) if Fe3+ inserpentine is included.
In-situ μXANES measurements allow going a step further into thesystem dynamics by quantifying the Fe3+ content in serpentines, andits contribution to the H2 production in the course of serpentinizationreaction. The latter information cannot be easily deduced fromBR measurements alone because of the general lack of weaklyserpentinized rocks to measure the Fe3+/FeTot(BR) for 0 b S b ~50%,and because of the large scatter of Fe3+/FeTot(BR) values available inliterature that prevents extracting a precise evolution trend from S =0 to 100%. In-situ measurements of Fe3+/FeTot(serp) in serpentine
Pre-edge centroid energy (eV)
[6] Fe(II) Ol.
.dinaS)III(eF]4[
[6] Fe(III) Andr.
[4] Fe(II) Stau.
StandardsSerpentines
Wilke et al., (2001)
% Fe3+/FeTot
20 %
40 %
60 %
80 %
1%00
10 %30 %
50 %
70 %
90 %0 %
7113,0 7113,5 7114,0 7114,5
B
primary minerals and replacing serpentines. B) Calibration variogram for quantifying
Edge jump Edge position (eV)
5
1
2
Txt 2
Txt 3A B
C D
Txt 1
Txt 3
Txt 2
Txt 1
Ol.
Ol.
Txt 1
Txt 1
Txt 3
Txt 3
Txt 3
Txt 3
Ox.
Txt 1 + Ox.
Txt 3
Txt 3
Fig. 6. μXANES mapping of iron content and iron oxidation state in one region of sample MARK 24 showing olivine (Ol.), oxides (Ox.) and serpentines of Txt 1, Txt 2 and Txt 3. A) andB) are images of the thin section region selected for μXANES mapping, in natural and cross-polarized lights, respectively. C) and D) are maps of the iron-K edge jump, and iron-Kedge position illustrating, respectively, the relative iron content and relative iron oxidation degree of minerals.
79M. Andreani et al. / Lithos 178 (2013) 70–83
(Fig. 7B), excluding veins that are local structures, can be fittedwith thefollowing empirical relation:
Fe3þ=FeTot serpð Þ ¼ A 1−BS� �
R2 ¼ 0:87 ð2Þ
with S the local degree of serpentinization in %, and A = 0.72 (±0.02)and B = 0.970 (±0.002). The errors on these fitting parametersderive from the uncertainty on the Fe3+/ FeTot(serp) quantificationand are propagated through the following mass balance calculations.Combining the Fe3+/FeTot(serp) evolution to the FeOTot
(serp) andmagnetitecontent evolutions (Fig. 4), it is nowpossible to plot the actual cumulativeevolution of iron distribution and redox state along the serpentinizationadvancement at MARK (model 2, Fig. 8C and D). Model 1 (Fig. 8A andB) shows the calculation for a simple system in which Fe3+ goes exclu-sively into magnetite. Model 2 includes ferric iron in serpentine. Forboth models, iron is represented in moles for 1 kg of serpentinite, i.e.the serpentinization of ~865 g of peridotite considering a finalserpentinite that contains a total 54.6 g of FeTot(BR) (i.e. Fe2O3
ToT(BR) =
7.8 wt.%; FeToT(BR) = 0.975 mol) and 13.5 wt.% of water (seeSection 4.1). The validity of our results for model 2 is supported by thegood agreement between the two independent calculations of iron distri-bution for S = 55% and S = 100% using BR data on one hand (see previ-ous paragraph), and in-situ analyses on the other hand (Fig. 8, Model 2).The latter predicts a Fe3+/FeTot(BR) =0.33 (±0.02) with Fe3+serp(BR)/Fe3+Tot(BR) = 0.72 (±0.02) for S = 55%, and a Fe3+/FeTot(BR) = 0.67(±0.01) with Fe3+serp(BR)/Fe3+Tot(BR) = 0.21 (±0.01) for S = 100%; i.every close to the one inferred from BR data on MARK11 and MARK7–9,
respectively. Our results confirm that Fe3+/FeTot(BR) increases almost lin-early during serpentinization despite the non-linearity of serpentine andmagnetite evolutions (Fig. 8C).
Three different stages can be identified from Fig. 8B and can becorrelated to textural observations. The first one lies between 0 and~40% of reaction. It is characterized by a dominant distribution of ironin serpentine, mainly as Fe2+, that can be related to the iron-rich Txt1-serpentine (Fig. 1A and B). It is worth noting that the lower Fe3+/FeTot(serp) recorded during this stage is not due to a low Fe3+(serp)
content in serpentine but rather to a high FeTot(serp) which explainsthe important contribution of serpentine to the Fe3+ budget (74–78%). Then, during the second stage, for 40% ≤ S ≤ 80%, serpentineforms with a decreasing FeTot because Fe2+(serp) stagnates while mag-netite proportion starts increasing significantly. However, serpentinestill dominates the Fe3+ budget (54–74%) as Fe3+(serp) keeps increasing.This step is characterized by enhanced mass transfers from primaryphases and serpentine toward magnetite. It is illustrated at S ~70%,where a part of the Fe2+(serp) gets partially oxidized and used to formmagnetite. Such transfers correspond to the progressive replacementof Fe-rich Txt 1 by Fe-poorer Txt 2 serpentine (Figs. 1C and 2A). At S>80%, third stage, the rapid increase in magnetite content cannot beaccommodated with the iron released from primary phases alone, anda transfer of both Fe2+(serp) and Fe3+(serp) from serpentine tomagnetiteis required. The iron content thus drops in serpentine. This stage allowscompletion of the reaction by the development of Txt 2, helped locallyby veining (Figs. 1D and 3A). Txt 3 is not represented in this evolution-ary scheme because it is poorly developed at MARK. However, it is veryinformative. As Txt 3 is characterized by a Fe- and Fe3+-rich serpentine
Local serpentinization degree (%)
0,0
0,2
0,4
0,6
0,8
1,0
Mesh rimTxt 2
BstVein V4
Mesh coreTxt 2,3
Fe3+
/FeT
ot
0,0
0,2
0,4
0,6
0,8
1,0
Fe3+
/FeT
ot
Mesh rimTxt 1
A
B
MARK 7 MARK 7 MARK 9 MARK 11 MARK 24
MARK 24
MARK 9 MARK 11
Vein
Vein
Vein MARK 7
0 20 40 60 80 100
Fig. 7. Influence of texture and serpentinization degree on the Fe3+/FeTot in serpentine.
80 M. Andreani et al. / Lithos 178 (2013) 70–83
with little associated magnetite, it is the trace of a late iron oxidationstage, possibly accompanied by H2 production, but exclusively accom-modated by serpentine. The depth, T and W/R conditions at which theidentified serpentinization stages occurred are discussed below.
5.2. Relation with the evolution of serpentinization conditions at MARK
Isotopic analyses on MARK serpentinites indicate a large tempera-ture range for serpentinization, starting at high T of 350–400 °C witha W/R of 0.5–1.5 and possibly going down to T b 200 °C (Agrinier andCannat, 1997). The present petrographic observations show that thefirst stage of hydration is characterized by Txt 1-serpentine, affectingboth olivine and orthopyroxene. Clinopyroxenes, when present, arenot altered to tremolite, and orthopyroxenes are not transformed totalc. The later assemblage should occur only if the temperature ofharburgite hydration is higher than ~350–400 °C because tremoliteis unstable below 390 °C and the increasing stability of olivine vs.orthopyroxene at T > 350 °C considerably limits serpentine formationand favors more siliceous phases like talc (Bach et al., 2004; Martinand Fyfe, 1970). Consequently, if the hydrothermal alteration was initi-ated at 400 °C at MARK, the main serpentinization event occurred atT b 350°C. The last texture of serpentinization, Txt 3, is characterizedby a very iron-rich serpentine and a nearly-absence of magnetite thatcan be related to a serpentinization T b 150–200 °C (Klein et al., 2009;Seyfried et al., 2007).
According to reaction path modeling of Klein et al. (2009), Fe3+/FeTot in serpentine is mainly controlled by the W/R ratio for
T b 350°C. This is supported by the constant Fe3+/FeTot(serp) ~0.2attained by serpentine in experiments run by Marcaillou et al. (2011)under a constant W/R of 1.5. While in experiments the oxygen fugacitymay be externally buffered, redox conditions in natural systems can belocally buffered or just transient, leading to variable redox conditions inspace or in time. Our μXANES measurements record a progressive in-crease of the Fe3+/FeTot(serp) with the serpentinization reaction ad-vancement (Fig. 7B) that can be easily correlated here with anincrease of the W/R ratio. This change of the W/R ratio can be eithertemporal, related to the change in deformation mode during tectonicunroofing of peridotites at MARK (Andreani et al., 2007), or spatial, be-cause of a heterogeneous permeability, e.g. around fault zones of frac-tures. The lowest Fe3+/FeTot(serp) values, obtained during the firststages of reaction, are of 0.2–0.3. These values are not expected at350 °C but correspond to a W/R of 0.6–1.5 for T = 300 °C (Klein et al.,2009), i.e. close to the temperatures proposed from isotopic analyses(Agrinier and Cannat, 1997). This also confirms that the firstserpentinization stage started at T b 350°C. The Fe3+/FeTot(serp) pro-gressively increases during the first half of serpentinization reaction(stage 1), and tends to a mean value of 0.6–0.7 corresponding to a W/Rof ~17–25 at the end of reaction. Fully serpentinized samples display awide range of Fe3+/FeTot(serp), varying from a sample to the other (andevenwithin a sample), evidencing the high heterogeneity of fluid circula-tion. MARK 9, for example, displays the highest Fe3+/FeTot(serp) values inlate stage veins (V4) and in surrounding mesh textures, correspondingto a local W/R > 100. Far from these localized zones of focused fluidflow, the serpentinization reaction is achieved with a minimum W/R of10. Fig. 9A shows the whole evolution of the W/R ratio duringserpentinization at MARK as deduced from Fe3+/FeTot(serp) measure-ments andmodeling of Klein et al. (2009). Taking into account the uncer-tainty of the Fe3+/FeTot(serp) quantification, maximum and minimumvalues are provided for the W/R ratio.
If experimental data and equilibrium thermodynamic modelsprovide valuable constraints on extreme serpentinization conditionsat MARK, they fail to reproduce the intermediate evolution of irondistribution, i.e for 150–200 °C b T b 350 °C. The exponential pro-duction of magnetite has not been reproduced yet. In the opposite,a linear evolution is recorded during the course of the reaction in ex-periments (Malvoisin et al., 2012). Equilibrium models predict nei-ther the observed high FeTot(serp) and Fe3+/FeTot(serp) in serpentine,nor the decrease in FeTot(serp) with magnetite increase, for any T–W/R evolution. In addition, experimental works seldom provide theevolution of FeTot(serp), Fe3+/FeTot(serp) and magnetite together forcomparison with the present data. When available, they record theprogressive achievement of equilibrium under constant T–W/R con-ditions and for a closed system (Marcaillou et al., 2011) because ofexperimental constraints. When T decreases during the reaction, asexpected at MARK, experiments and modeling both show an in-crease in FeTot in hydrous phases while magnetite production de-creases, slightly for T b 330 °C, and more importantlyfor T b 250 °C, down to a T ~150–200 °C at which magnetite disap-pears from assemblages (Klein et al., 2009; Malvoisin et al., 2012;Seyfried et al., 2007). Thus the evolution of iron distribution fromstage 1 (Txt 1: high FeTot(serp)—low magnetite) to stages 2 and 3 (Txt2: low FeTot(serp)—high magnetite) is better explained by a kinetically-controlled return to equilibrium of the system, under conditionsfavoring the assemblage Mg-serpentine + magnetite, rather than by asuccession of equilibrium steps due to T change. Since Mg-serpentine +magnetite are stable over a wide range of temperatures, transition fromTxt 1 to Txt 2 can occur either under similar T conditions or in a coolingsystem from 330 °C to ~200–250 °C. The absence of brucite in mosttextures suggests that the silica activity was relatively high, contraryto predictions for a closed system (e.g. Klein et al., 2009). Brucite isonly observed in the vicinity of olivine (Fig. 2C and F), where it mayformmetastably (Beard et al., 2009). A silica input, at least near olivines,is thus required during the whole alteration history. For T ≥ 330 °C,
Fe2+Prim
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Fe3+TotFeTot
FeTot Bulk Rock = Cste
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A
C D
B
Fig. 8. Distribution and redox state of iron in secondary phases during the serpentinization progress at MARK. The system is considered mass-conservative for iron. The iron releasedby primary phases is exclusively distributed within serpentine and magnetite, according to petrographic observations. Mass balance is calculated using the trends shown onFigs. 4A, B and 7B. A–B) Model 1: Fe3+ occurs only in magnetite (Mgt), and C–D) model 2: Fe3+ occurs both in magnetite and serpentine (Serp). A) and C) show the evolutionof Fe distribution in secondary phases during the progressive replacement of primary phases (Prim). B) and D) represents the Fe distribution in secondary phases normalized tothe total amount of Fe into the serpentinized rock (FeTot).
81M. Andreani et al. / Lithos 178 (2013) 70–83
reaction path modeling of harzburgite hydration (Klein et al., 2009)shows that silica can be provided by the dissolving orthopyroxenethrough the following reaction:
Olivine þ Orthopyroxene þ H2O→serpentine þ magnetite þ H2ðaqÞ:ð3Þ
The simultaneoushydration of olivine and orthopyroxene is efficientin a T range of 250–350 °C (Martin and Fyfe, 1970). Below this T range,olivine serpentinization largely dominates and equilibrium modelingpredicts the increasing formation of brucite. In the case of MARK, aperennial source of silica can also be provided by modified seawater(Alt and Shanks, 2003) to allow reaction (4) and favor serpentine vs.brucite at all stages:
Olivine þ SiO2ðaqÞ þ H2O→serpentine þmagnetite þ H2ðaqÞ: ð4ÞExternal SiO2 input, and local iron transfer during Txt 1 → Txt 2,
can be both facilitated during the second stage (S > 40%, Fig. 8B) bythe shift from a diffusion-dominated to an advection-dominatedmode of transfer that has been documented near the middle of theserpentinization advancement at MARK (Andreani et al., 2007).
The calculated evolution of the W/R and the Fe3+(BR) productionallows evaluating the H2 concentration (mM) of a fluid issued fromthe formation of 1 kg of serpentinites at a given stage of theserpentinization reaction, i.e. for a given iron distribution and W/R.The calculated H2 concentrations (Fig. 9B) hypothesize a stagnant fluid,H2 staying on the site of production, and provides an instantaneousimage of the expected H2 gradient between regions of different reactionadvancement. It can correspond to a vertical gradient due to a progres-sive decrease of serpentinization degree with depth, or to a lateralgradient between regions of contrasted permeabilities. The maximumand minimum values of H2 concentration due to the uncertainty ofthe Fe3+/FeTot(serp) quantification are also provided. The highest H2
concentration, creating the strongest reducing conditions, is observednear the reaction front (stage 1), where both FeTot(serp) and Fe3+(serp)
are high, and W/R is low. Concentration rapidly decreases whenserpentinization increases, i.e. at shallower levels and/or close to fluidpaths since the W/R increases faster than the Fe3+ content of therock. The actual gradient of H2 concentration should of course differfrom the one of Fig. 9B because the H2-enriched fluid formed at thefront will certainly migrate upward or toward main fluid paths andmix with fluids at shallower levels, resulting in lower H2 concentration
0 20 40 60 80 1000
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Fig. 9. Evolution of (A) the W/R and (B) the H2 concentration in fluid duringserpentinization of MARK peridotites. Dotted and dashed curves corresponds tomaximum and minimum estimates of Fe3+/FeTot.
82 M. Andreani et al. / Lithos 178 (2013) 70–83
at the exhaust of hydrothermal systems. This simple model highlightsthe potential high heterogeneities in redox conditions at all scaleswithin a serpentinizing system that probably affects the location ofH2-related processes like ecosystems development, or CO2 reductionreactions. Part of the H2 produced can be heterogeneously stored inthe rock (e.g. by adsorption) or partially consumed by other reactionslike abiotic methanization or metabolic activity. In the simplified caseof a homogeneous and linearmixing of fluidwith an increasing reactionadvancement, a final concentration of 15 to 20 mM is expected at levelswhere S > 70%. Note that this value is very close to the ones typicallymeasured in fluids venting from active, ultramafic-hosted hydrothermalsites (10–20 mM: Charlou et al., 2002; Kelley et al., 2005).
6. Conclusions
Bulk rock and detailed in-situ analyses of secondary phases innatural serpentinites from MARK (23°N, MAR) allowed unraveling theevolution of iron distribution and redox state during serpentinizationadvancement. These data have been comparedwith available thermody-namicmodels to provide constraints on the evolution of serpentinizationconditions (T, W/R), and resulting hydrogen production, at MARK.
Results show that serpentinization started at T b 350 °C and endedat T b 150 °C under a W/R ratio evolving from 0.6 to 25, and reaching~100 locally, in large open veins. During the whole process, serpentineminerals incorporate important amount of Fe3+ in their structure,
whatever the structural type (planar or fibrous). Hence they are respon-sible for a significant part of the H2 production, reaching 80% duringthe first half of reaction (S b 50%), decreasing to 22–40% over 90% ofreaction. Quantifying H2 production issued from serpentinization usingmagnetite content only, results in an underestimation of 25% on fullyserpentinized rocks, this error increasing as the degree ofserpentinization decreases.
This study also shows that in natural settings, an open system canextend the T conditions under which Fe- and Fe3+-rich serpentineforms, instead of the Fe2+-rich brucite classically predicted forT b 330°C. Notably, Fe- and Fe3+-rich serpentine has been observedin the last stage of serpentinization here, with very little magnetiteassociated. It suggests that the serpentine alone may account for H2
production. Introducing these new data in models should enlarge therange of temperatures over which H2 is efficiently produced, in particu-lar at low temperatures (b150 °C) where magnetite formation is notthermodynamically favored.
The measured Fe3+ contents in natural oceanic serpentine alsoprovide a reference value for their contribution to the input budget insubduction zones. This has important implications for estimating therole of serpentinites on the anomalous oxidation state of arc magmas,and it has to be completed by a study of the behavior of iron in serpen-tine during subduction.
Finally, we show that the different serpentinization stages observedat MARK are not simply due to a succession of equilibrium at differentconditions. The system openness, involving heterogeneous fluid trans-fer, input of elements, and local chemical gradients, also plays amajor role. It enhances kinetic effects that are not predicted frombatch experiments or modeling. The spatial and temporal evolution ofthe permeability network, controlling fluid composition and renewal,hence appears determinant in natural systems. All these non-equilibrium effects are expected to vary from one site to another,depending on the regional setting. Hence, generalizing observationson the serpentinization process from one natural site to the othermay not be trivial. The heterogeneity of transport-reaction processesmakes the accurate estimation of the evolving P–T, W/R and redoxconditions from macroscopic or bulk rock analyses more complex. In-situ analyses are thus required to improve our understanding of naturalserpentinizing systems that can display strong physico-chemical gradi-ents at all scales.
Acknowledgments
The authors thank Sakura Pascarelli for the support duringbeamline adjustment, Bertrand Van de Moortele for SEM imagingand Gilles Montagnac for assistance on Raman spectrometers. Wethank Catherine Mével, Marguerite Godard, Bruno Reynard, and twoanonymous reviewers for their fruitful comments on the manuscript.This work has been funded by the “Programme InterdisciplinaireEnergie” of the CNRS through the HY-GEO project. Raman spectroscopyfacility at the Ecole Normale Supérieure (ENS) de Lyon is supported bythe Institut National des Sciences de l'Univers. Electron microscopy inLyon is supported by the CLYM.
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