impact-related ir anomaly in the middle ordovician lockne impact structure, jämtland, sweden

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This article was downloaded by: [McMaster University] On: 08 December 2014, At: 12:18 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK GFF Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/sgff20 Impact-related Ir anomaly in the Middle Ordovician Lockne impact structure, Jämtland, Sweden Erik F.F. Sturkell a a Dept. of Geology and Geochemistry , Stockholm University , SE-106 91, Stockholm, Sweden E-mail: Published online: 06 Aug 2009. To cite this article: Erik F.F. Sturkell (1998) Impact-related Ir anomaly in the Middle Ordovician Lockne impact structure, Jämtland, Sweden, GFF, 120:4, 333-336, DOI: 10.1080/11035899801204333 To link to this article: http://dx.doi.org/10.1080/11035899801204333 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions

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Page 1: Impact-related Ir anomaly in the Middle Ordovician Lockne impact structure, Jämtland, Sweden

This article was downloaded by: [McMaster University]On: 08 December 2014, At: 12:18Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: MortimerHouse, 37-41 Mortimer Street, London W1T 3JH, UK

GFFPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/sgff20

Impact-related Ir anomaly in the Middle OrdovicianLockne impact structure, Jämtland, SwedenErik F.F. Sturkell aa Dept. of Geology and Geochemistry , Stockholm University , SE-106 91, Stockholm,Sweden E-mail:Published online: 06 Aug 2009.

To cite this article: Erik F.F. Sturkell (1998) Impact-related Ir anomaly in the Middle Ordovician Lockne impact structure,Jämtland, Sweden, GFF, 120:4, 333-336, DOI: 10.1080/11035899801204333

To link to this article: http://dx.doi.org/10.1080/11035899801204333

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) containedin the publications on our platform. However, Taylor & Francis, our agents, and our licensors make norepresentations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose ofthe Content. Any opinions and views expressed in this publication are the opinions and views of the authors,and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be reliedupon and should be independently verified with primary sources of information. Taylor and Francis shallnot be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and otherliabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to orarising out of the use of the Content.

This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in anyform to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

Page 2: Impact-related Ir anomaly in the Middle Ordovician Lockne impact structure, Jämtland, Sweden

Sturkell, E.F.F., 1998: Impact-related Ir anomaly in the Middle Ordovi-cian Lockne impact structure, Jämtland, Sweden. GFF, Vol. 120 (Pt. 4, December), pp. 333–336. Stockholm. ISSN 1103-5897.

Abstract: A distinct Ir anomaly with an Ir content of up to 4.5 ppb is found in resurge deposits tied to the Middle Ordovician Lockne impact structure, Jämtland, Sweden. The impact took place in a marine environ-ment and the sea water played an important role in the impact pro-cess and in the aftermath. The samples yielding the Ir anomaly also contain abundant shocked quartz, which proves that the structure was formed by the impact of an extraterrestrial body. The mid-Ordovician volcan-ism close to Scandinavia did not generate any distinct Ir content. About 20% of the meteorite Ir remained in the resurge deposit around the cra-ter. The Ir from the impact-related deposits in Lockne correlates well with Cr. There is a weaker correlation between Ir and chalcophile and siderophile (e.g., Fe, Sb, Zn, and Ni, Co) elements. The low-tempera-ture hydrothermal system, which acted after the impact event, mobilised some of the chalcophile and siderophile elements and, thus, resulted in their weaker correlation with Ir in some samples.Keywords: Impact, iridum, chromium, gold, chalcophile elements, Middle Ordovician, Lockne, Jämtland, Sweden.

E.F.F. Sturkell, Dept. of Geology and Geochemistry, Stockholm Uni-versity, SE-106 91 Stockholm, Sweden, email [email protected]. Present email [email protected]. Manuscript received 2 April 1997. Re-vised manuscript accepted 20 August 1998.

Platinum-group elements are depleted in the Earthʼs crust in com-parison with their cosmic abundance (cf. Goldschmidt 1954). An Ir anomaly induced by an extraterrestrial body has been shown in the Cretaceous/Tertiary (K/T) boundary bed world wide (Alvarez et al. 1980). The finding of shocked quartz in the boundary bed and the discovery of the 65 m.y. old and 180 to 300 km diameter Chicxulub crater are generally regarded as valid confirmation that the Ir anomaly is of extraterrestrial origin (e.g. Hildebrand et al. 1991). However, some types of volcanism can generate Ir anomalies; Schmitz & Asaro (1996) found an anomaly of up to 0.750 ppb Ir in tholeiitic ash layers.

An Ir anomaly in resurge deposits generated in connection with a marine impact event has not been previously established at any such structure. As the Lockne impact structure formed in a marine environment resurge deposits were formed which are not represented at on-shore impacts. The resurge deposits formed immediately after the impact as the water entered the newly formed crater. It is in the fall-back ejecta and ejecta blanket that an Ir anomaly is to be expected for bodies with a diameter ex-ceeding 50–100 m as they are vaporised at impact. In the marine Lockne impact the ejecta material is assumed to be blended in the resurge deposits.

The fine-grained resurge deposits formed in connection with the Middle Ordovician Lockne impact structure were sampled and analysed (Fig. 1); the co-ordinates for each sample are given in Table 1. Elements determined were: Ir, Co, Cr, Fe, Ni, Sb, Zn, and Au (Table 1).

GFF volume 120 (1998), pp. 333–336. Note

Impact-related Ir anomaly in the Middle Ordovician Lockne impact structure, Jämtland, SwedenERIK F.F. STURKELL

The Lockne impactThe Lockne impact structure is situated about 20 km south of Östersund, central Sweden (Fig. 1). The structure was formed in the Middle Ordovician sea at c. 455 Ma, and consists of an inner crater, 7.5 km in diameter, in the Proterozoic basement sur-rounded by an outer crater, 13.5 km in diameter (Lindström et al. 1996). Today the Lockne area is situated at the erosional Cal-edonian front, within a narrow belt of autochthonous Cambrian and Ordovician strata. The crater itself and the crater filling are tectonically autochthonous.

The impact-generated breccia is called the Tandsbyn Brec-cia. It is by definition monomict and authigenic in the sense of Stöffler & Grieve (1994). The resurge deposits formed when the water rushed back into the newly excavated crater (Lindström et al. 1996). Such deposits can be divided into two main types, the first of which is a clast-supported breccia (Lockne Breccia) with clast sizes ranging from centimetres to tens of metres. The Lockne Breccia is overlain by sandstone and siltstone (Loftar-stone) representing the final stage of the resurge. The two types of resurge deposit are petrologically alike, however, the Loftar-stone has a larger proportion of material originating from the deepest excavated part of the crater.

Impacts on land leave a recognisable ejecta blanket around the crater. In marine impact craters the ejecta deposits are forth-with reworked by the resurging water mass and included in the re-surge deposits. Resurge deposits are found in the immediate surroundings of the Lockne impact structure as far as 40–45 km from the center, and along the Caledonian front in central Jämt-land (Fig. 1).

The Loftarstone has a thickness of at least 47 m in the inner crater, as indicated from boreholes. The thickness of the Loftar-stone in the outer crater is only a few metres except in the re-surge gullies which cut the outer crater radially. The outcropping thickness of the Loftarstone deposited in the gullies appear to be greatest in the area of samples FF1 and FF6 (Fig. 1), where the thickness is less than 10 m.

Petrology of the resurge depositsThe Lockne Breccia is altogether dominated by orthoceratite limestone. It is only in its most fine-grained parts that the amount of crystalline components increases to any significant quantity (Fig. 2A). The transition from Lockne Breccia to Loftarstone is often gradual. However, sharp boundaries are not uncom-mon where the flow has eroded down into the already deposited Lockne Breccia. The Loftarstone (Fig. 2B) is composed of ma-terial originating from orthoceratite limestone, crystalline base-ment, Cambrian claystone, and melt particles. The grain size of the Loftarstone ranges from coarse sand to silt, in a fining upward sequence. The fining upward reflects the diminishing energy of the resurge. The rock is dominated by limestone clasts, which often are rounded, either because of abrasion, or because they originated as limestone nodules. The rock is well lithified.

Simon (1987) presented a component analysis of the Loftar-

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334 Sturkell: Impact-related Ir anomaly in the Lockne impact structure GFF 120 (1998)

stone. The following petrographical description is based on Simonʼs work, with a few complementary new observations. Simon (1987) differentiated between carbonate clasts and fos-sil fragments. Because the vast majority of the fossil fragments originated from the orthoceratite limestone the two groups may be treated as one. The combined groups make up 30–40% of the Loftarstone. The fine-grained Loftarstone is dominated by ma-trix. The matrix consists of carbonates, claystones, totally frag-mented crystalline rock, and probably melt. The amount of lithic crystalline fragments decreases with grain size while the pro-portion of quartz and feldspar monomineral particles increases with diminishing grain size. These particles together represent approximately 30% of the components of Loftarstone regardless of grain size. Simon (1987) reported pyroclastic material in the Loftarstone, material which now is interpreted as impact gener-ated melt. Up to 20% of the fragments are classified as melt. The Cambrian claystone occurs as black shale fragments (Fig. 2B), being of coarse sand size to components of the matrix. The shale makes up 10–15% of the Loftarstone. Shocked quartz is abundant in the Loftarstone. A study of the microscopic planar deformation features (PDF) in quartz grains from the Grubban locality (sample FF6) was presented by Therriault & Lindström (1995).

Material and methodsThe five Loftarstone (the fine-grained resurge deposits) samples analysed were taken in outcrops in the direct surrounding of the

Lockne structure (Fig. 1). All samples were taken within the im-pact crater. The samples FF3 and FF4 were taken in a <50 m wide outcrop (not shown in Fig. 1). The samples FF2 and FF3 originate from the outcropping resurge deposits at the edge of the inner crater, with a distance of approximately 3.5 km from the impact center. Sample FF4 was taken at the edge of the inner crater in the mouth of a small resurge gully, at approximately 3.5 km from the center. The samples FF1 and FF6 were taken in two different resurge gullies that cut through the outer crater, with a distance of 4 and 5 km from the center, respectively. All samples are poorly sorted. The most abundant PDF observed in sample FF6 correspond to those found at known impact craters (Therriault & Lindström 1995). At the edge of the inner crater the post-impact sediments (Dalby Limestone) are frequently in direct contact with Tandsbyn Breccia or shattered basement rock. Thus, only a narrow band of Loftarstone can be mapped. The circumstance that post-impact sediment rests either directly on the Tandsbyn Breccia or on shattered basement shows that no or a very thin layer of resurge material was deposited in the outer parts of the inner crater.

The five bulk samples of the Loftarstone (Table 1) were ana-lysed at the Lawrence Berkeley Laboratory in Berkeley, Cali-fornia. The analyses include iridium and six additional trace elements with the Luis W. Alvarez-Iridium Coincidence Spec-trometer (LWA-ICS). This instrument measures the coincident emission of two gamma rays (316.5 keV of 192Ir and 468.1 keV of 192Ir) after neutron irradiation of the samples. Through this technique Ir-related gamma rays can be discriminated from background radiation more efficiently than by single gamma-ray counting (Schmitz & Asaro 1996). The standard DINO-1 (Schmitz & Asaro 1996) was used for the Ir calibration and Standard Pottery (Perlman & Asaro 1969) for all other elements. For a detailed description of the analytic procedure and analytic errors, see Schmitz & Asaro (1996).

The Au analyses of the same samples were performed at

Fig. 1. Geological map of the Lockne impact structure showing localities of the five samples.

Fig. 2. Resurge deposits from the

drillcore LOC2. The core-diameter

is 42 mm. A. From the 206.9 m level,

fine-grained Lockne Breccia with ortho-

ceratite limestone and crystalline

clasts. B. Loftarstone in a typical appear-

ance from the 196.3 m level. It is com-

posed of limestone, crystalline (lithic and monomineral

grains), shale (black spots) and melt frag-ments. Photographs

by Uno Samuelsson.

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GFF 120 (1998) Sturkell: Impact-related Ir anomaly in the Lockne impact structure 335

SGAB in Luleå, Sweden, using the Fire Assay method. In this procedure 20 g of sample is mixed with PbO. The PbO is then reduced by heating the sample, resulting in a pearl of precious metal remains. This pearl is dissolved in aqua regia and then ana-lysed in the ICP-AES, using the South African SARM7-standard for calibration.

Results and discussionThe result of the analyses included a large Ir anomaly, up to 4.5 ppb, in the resurge deposits. The Proterozoic basement in the Lockne area is dominated by granitoids and has not been sampled for Ir analysis. Instead average values of Ir content of continental crust have been used. The average concentration of Ir in continental crust is 0.05 ppb (Wedepohl 1995). In the work by Schmidt et al. (1997), data of average content of Ir from con-tinental crust (Nördlinger Ries) vary from 0.002 to 0.043 ppb. This is consistent with previous analyses which indicate a low Ir content in silicic igneous and volcanic rocks (e.g., Greenland 1971; Greenland et al. 1974; Orth et al. 1990; Schmitz & Asaro 1996). The Ir enrichment factor in the Loftarstone is 15–90 times relative to the average crust value (Wedepohl 1995). The asso-ciation of a proven impact structure and abundant shocked quartz in a deposit which yielded a clear Ir anomaly strongly suggests an extraterrestrial origin of the Ir.

Enrichment of Ir by influx of extraterrestrial material is de-scribed by, e.g., Alvarez et al. (1980), but also volcanism can generate an enrichment of Ir, especially basaltic volcanism (Schmitz & Asaro 1996). Extensive mid-Caradocian volcanism generated the Big Bentonite in Baltoscandia. Huff et al. (1992) suggested that this bentonite correlates with the North Ameri-can Millbrig K-bentonite. The mid-Caradocian volcanic episode generating the bentonites was one of the most extensive during the Phanerozoic. The Ir content in the Big Bentonite is, however, very low: less than 0.04 ppb (Schmitz & Asaro 1996). An aver-age limestone has an Ir content of about 0.005–0.010 ppb. The Lower and Middle Ordovician orthoceratite limestone, however, yields an Ir content of up to 0.067 ppb (Schmitz et al. 1996). The Ir content in the Loftarstone is considerably larger than the anomaly in the orthoceratite limestone. No volcanic activity is known from Jämtland during the Ordovician, and the exten-sive volcanic activity responsible for the Big Bentonite, with its source outside Scandinavia did not generate any Ir anomaly. The Loftarstone deposit is structurally and stratigraphically tied to

the Lockne impact, and there is a clear Ir anomaly in it.Sturkell & Lindström (1996) estimated 13 km3 of the original

quantity of resurge deposit within a radius of 50 km from the impact centre. The average Ir content in the samples is 2.12 ppb, with a density of the resurge rock of 2.7 (Törnberg & Sturkell in prep.). From these figures one obtains a total of 74.4 tonnes of Ir. This quantity is then compared with a 800 m diameter mete-orite with a density of 3.7 (average chondrite density from Was-son 1974, pp. 175–176). With an Ir content of 400 ppb, the total amount will be 400 tonnes. Thus, about 20% of the orginal Ir amount in the meteorite occurs in the resurge deposits.

The scatter diagram in Fig. 3 gives a good correlation between Cr and Ir for all five samples. Schmitz (1992) showed that the chalcophile elements in the K/T boundary bed do not correlate with Ir, while Ir and Cr correlate well. The good correlation of Cr and Ir at the K/T boundary bed indicates a common origin and a similar behaviour. This relationship can also be inferred in the case of the Loftarstone samples.

With the one sample excluded (see below) there are relatively good Co–Ir, Fe–Ir, Ni–Ir and Au–Ir correlations. Between Sb and Ir there is a weak correlation, and between Zn and Ir there is no correlation.

There is a high Au content in all samples. For most chondrites and iron meteorites (Table 1) the Ir content is twice or more the Au content. The Au/Ir value in the three samples with the high-est Ir content is between 1.78 and 3. This makes a comparison to chondrites and iron meteorites difficult. The consistency of the Au/Ir value in most of the samples (see below) suggests an origi-nal signature of the meteorite. This ratio differs from most of the meteorites listed in Table 1 except the enstatite chondrite.

An influx of Au among other elements from a different source to, especially, sample FF3 might be possible. A whole-rock anal-ysis of the basaltic rock in the southern part of Lake Lockne (Fig. 1) yielded an Au content of 8 ppb. The volcanic rock suite in the southern part of the lake is intersected by several small shear zones trending NNW–SSE. Mansfeld et al. (in prep.), in a study of the basic volcanic rocks containing a shear zone which hosts a quartz vein sulphide deposit, suggest a mobilisation of solutions along the shear zone during the Lockne event or the Caledonian orogeny. Fluid inclusion compositions and temperatures point to the Lockne event (Mansfeld et el. in prep.). The shear zone trends towards locality FF3 and further to another small sulphide prospecting pit NNW of FF3 (Fig. 1). An enrichment of Au and other easily mobilised elements along this shear zone is therefore

Table 1. Element concentration in bulk Loftarstone samples and the composition of average crust and the average Ir and Au content for chondrites and iron meteorites.

Sample E–W N–S Dominating aΔT Ir Co Cr Fe Ni Sb Zn Au Au/Irno. co-ord. co-ord. grain size (min) (ppb) (ppm) (ppm) (%) (ppm) (ppm) (ppm) (ppb)

FF1 1450960 6984890 very coarse 43.2 4.5±0.3 24.28 96.3±1.0 4.29 148±18 0.45±0.11 25.6±0.8 8 1.8FF2 1446990 6986640 very coarse 53.2 2.5±0.2 21.23 72.3±0.9 3.29 131±17 0.41±0.11 37.3±0.8 5 2.0FF3 1448205 6991850 coarse 64.2 0.8±0.1 31.47 48.2±0.8 5.12 118±17 0.47±0.11 234 11 13.8FF4 1447580 6991185 coarse 51.2 2.0±0.2 16.45 65.1±0.9 3.18 100±16 0.21±0.10 32.1±0.7 6 3.0FF6 1444920 6987190 medium 53.2 0.8±0.1 16.87 48.3±0.9 1.68 73±15 0.27±0.10 32.7±0.8 4 5.0Average continental crust (Wedepohl 1995, pp. 1224–1225) 0.05 24 126 4.39 56 – 65 2.5 50Average Ir and Au contents in chondrites and iron meteorites (Wedepohl 1978) chondrites class Cc 590 170 0.29 Ce 220 340 1.55 Cl 380 180 0.47 CH 480 220 0.46iron meteorites class H 8800 740 0.08 Og & Ogg 3900 & 4400 1270* Om 4800 1440 0.30 Of & Off 1500 & 450 1320* D 7100 1330 0.19

aCounting time for iridium (ICS) analysis; *Au given with one value for two classes.

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336 Sturkell: Impact-related Ir anomaly in the Lockne impact structure GFF 120 (1998)

suggested. The difference in Fe content of the samples may be caused by the presence of clasts of local Proterozoic volcanics in the Loftarstone. The volcanic rocks in the Lockne area contain up to 10.8 wt% Fe2O3

tot (Mansfeld et al. in prep.) while the grani-toids range between 1 and 5.5 wt% Fe2O3

tot.The low-temperature hydrothermal system acting in the cool-

ing stage after the impact caused an enrichment of chalcophile elements at some places in the structure (Sturkell et al. 1998). The Tandsbyn Breccia and the shattered basement rock were ini-tially rich in open cavities. Cavity-grown minerals are, in order of decreasing abundance, calcite, quartz, chalcopyrite, pyrite and minor occurrences of galena. In one borehole in the structure, a sphalerite-rich level occurs which has a Zn content of 2.36%. The S precipitated in the hydrothermal system, which was active shortly after the impact, was mainly leached from the Protero-zoic basement (Sturkell et al. 1998).

It is suggested that sample FF3 has been subjected to some hydrothermal alteration as referred to above. This sample seems to be chemically disturbed relative to the other samples. It does not follow the general trend of the other samples, except in the case of Cr. It is enriched relatively to the other samples in the elements Co, Fe, Sb, and Zn; also an enrichment occurs of the Pt-group element Au. Nickel does not show any particular en-richment (Fig. 4).

Fig. 4. Scatter diagrams for five Loftarstone samples. Sample FF3 is excluded in the regression. Correlation values (r2), with statistical sig-nificane at 95% confidence level.

Fig. 3. Scatter diagram for five Loftarstone samples. Correlation values (r2), with statistical sig-nificane at 95% confidence level.

ConclusionsThe Loftarstone contains shocked quartz and an Ir anomaly sup-porting an extraterrestrial origin of the Lockne crater. A volcanic source is unlikely, because the volcanic activity in the Middle Ordovician which generated the Big Bentonite in Baltoscandia and the Millbrig K-bentonite in North America did not yield any Ir anomaly. About 20% of the total Ir content of the meteorite ended up in the resurge deposit. The Ir in all samples correlates very well with Cr, and Ir also shows a relatively good correla-tion with Co, Fe, Ni, and Au. An exception is provided by one sample (FF3) which shows an enrichment of chalcophile and siderophile elements. Directly after the impact a low-tempera-ture hydrothermal system was active in which chalcophile ele-ments were leached and enriched, as demonstrated by Sturkell et al. (1998). This low temperature system apparently enriched the chalco-phile and siderophile elements in sample FF3, but did not affect the Ir/Cr relation.Acknowledgements. – I am grateful for fruitful discussions with, and critical reviews by, Maurits Lindström, Birger Schmitz and Curt Broman. I also express my gratitude to Birger Schmitz and Frank Asaro for providing the Ir analysis.

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