Laser argon dating of melt breccias from the Siljan …...Laser argon dating of melt breccias from...

Meteoritics amp Planetary Science 40 Nr 4 591ndash607 (2005)Abstract available online at httpmeteoriticsorg

591 copy The Meteoritical Society 2005 Printed in USA

Laser argon dating of melt breccias from the Siljan impact structure Sweden Implications for a possible relationship to Late Devonian extinction events

Wolf U REIMOLD1 Simon P KELLEY2 Sarah C SHERLOCK2 Herbert HENKEL3 and Christian KOEBERL4

1Impact Cratering Research Group School of Geosciences University of the Witwatersrand Private Bag 3 P O Wits 2050 Johannesburg South Africa

2Department of Earth Sciences Open University Walton Hall Milton Keynes MK7 6AA UK3Department of Land and Water Resources Engineering Division of Engineering Geology and Geophysics

Royal Institute of Technology Teknikringen 72 SE 100-44 Stockholm Sweden4Department of Geological Sciences University of Vienna Althanstrasse 14 A-1090 Vienna Austria

Corresponding author E-mail reimoldwgeoscienceswitsacza

(Received 12 July 2004 revision accepted 08 February 2005)

AbstractndashIn earlier studies the 65ndash75 km diameter Siljan impact structure in Sweden has been linkedto the Late Devonian mass extinction event The Siljan impact event has previously been dated by K-Ar and Ar-Ar chronology at 342ndash368 Ma with the commonly quoted age being 3627 plusmn 22 Ma (2 σrecalculated using currently accepted decay constants) Until recently the accepted age for theFrasnianFamennian boundary and associated extinction event was 364 Ma which is within errorlimits of this earlier Siljan age Here we report new Ar-Ar ages extracted by laser spot and laser stepheating techniques for several melt breccia samples from Siljan (interpreted to be impact meltbreccia) The analytical results show some scatter which is greater in samples with more extensivealteration these samples generally yield younger ages The two samples with the least alteration yieldthe most reproducible weighted mean ages one yielded a laser spot age of 3772 plusmn 25 Ma (95confidence limits) and the other yielded both a laser spot age of 3761 plusmn 28 Ma (95 confidencelimits) and a laser stepped heating plateau age over 706 39Ar release of 3775 plusmn 24 Ma (2 σ) Ourconservative estimate for the age of Siljan is 377 plusmn 2 Ma (95 confidence limits) which issignificantly different from both the previously accepted age for the FrasnianFamennian (FF)boundary and the previously quoted age of Siljan However the age of the FF boundary has recentlybeen revised to 3745 plusmn 26 Ma by the International Commission for Stratigraphy which is withinerror the same as our new age However the currently available age data are not proof that there wasa connection between the Siljan impact event and the FF boundary extinction This new resulthighlights the dual problems of dating meteorite impacts where fine-grained melt rocks are often allthat can be isotopically dated and constraining the absolute age of biostratigraphic boundaries whichcan only be constrained by age extrapolation Further work is required to develop and improve theterrestrial impact age record and test whether or not the terrestrial impact flux increased significantlyat certain times perhaps resulting in major extinction events in Earthrsquos biostratigraphic record

INTRODUCTION

The Siljan impact structure (Fredriksson and Wickman1963 Wickman et al 1963 Svensson 1971 1973 Rondot1975 BodEgraven and Eriksson 1988 Juhlin and Pedersen 1987Kenkmann and von Dalwigk 2000 Henkel and Aaro 2005) islocated in the Dalarna region of south-central Swedencentered at 61deg02primeN14deg52primeE (Fig 1) Siljan is the largest

known impact structure in Europe Its diameter was originallyestimated at 52 km (Grieve 1988) but Von Dalwigk andKenkmann (1999) and Kenkmann and Von Dalwigk (2000)made a case for a larger diameter of at least 65 km on the basisof structural geological considerations and by applying theempirical morphometric scaling laws provided by Therriaultet al (1997) In contrast Henkel and Aaro (2005) observe a75 km wide current topographic expression and estimate that

592 W U Reimold et al

the pre-erosional crater diameter could have been as much as85 km

The evidence for impact at Siljan includes the presenceof shatter cones and planar deformation features (PDFs) inquartz Exposure is poor in much of the Siljan structure andbona fide impact melt rock has to date remained elusive

Consequently very little material that could be used to obtaina reliable age for this impact structure has been producedOnly ldquoallochthonous breccia small and narrow breccia dikesand float of melt brecciardquo were reported by Rondot (1975)Svensson (1971 1973) suggested a post-Silurian age for theimpact event

Fig 1 The geology of the Siljan impact structure The inset shows the location of Siljan in Scandinavia

Laser Ar dating of melt breccias from Siljan 593

In the 1980s Siljan experienced an ldquoimpact explorationboomrdquo in the wake of Goldrsquos proposal (reviewed in Gold andSoter 1980 Gold 1987 1988) that the structure could provideaccess to significant mantle-derived hydrocarbon resourcesthat might have infiltrated into the impact-deformed basementof the structure Accordingly Siljan was extensively anddeeply drilled (BodEgraven and Eriksson 1988) but no economicpotential could be established Some impact-relatedhydrothermal Pb-Zn mineralization does however occur andhas been mined locally (eg Reimold et al 2005 andreferences therein Hode et al 2002)

Bottomley et al (1978) referred to an outcrop with aldquosmall dikelet [of melt breccia] at shatter cone locality 3 ofSvensson (1973)rdquo Two samples from this site were describedas containing 30ndash25 inclusions predominantly quartz thatoccasionally shows shock deformation but also with clasts offeldspar and brecciated granite with incipient recrystallizationand rare inclusions of sandstone Melt matrix wasmicrocrystalline and granular Interstitial devitrified glass didalso occur One sample yielded a humped spectrum with amaximum age of 380 Ma but no plateau The second yieldeda similar pattern but with a three-step plateau comprising 92of release but only because the analytical error on the middlestep was 45 (around 10 times the two adjacent plateausteps) Re-analysis of the Bottomley et al (1978) data usingISOPLOT (Ludwig 1999) yields an age of 3583 plusmn 48 Ma(2 σ) In addition the final age quoted by Bottomley et al(1978) was quoted at the 1 σ level and was not the plateau agebut an integrated total fusion age in effect a K-Ar age It iswell documented that fine-grained whole rock samplesshowing younger ages in the highest temperature releaseresult from 39Ar recoil (McDougall and Harrison 1999) andare more likely obtained from altered samples Ar-Aranalyses presented below demonstrate that the hydrothermalalteration around Siljan has led to alteration of many meltsamples and it is likely that the age of 3627 plusmn 22 Masignificantly underestimates the true age of the impact atSiljan In fact a later publication by Bottomley et al (1990)quotes an age of 368 plusmn 11 Ma apparently from the samedataset although no explanation was given for the difference

A K-Ar age of 349 Ma for ldquoshock meltrdquo from anotherlocality was cited by Aringberg and Bollmark (1985) Juhlin et al(1991) cite a 40Arndash39Ar date for a ldquogranitic pseudotachyliterdquoof 359 plusmn 4 Ma as well as two K-Ar ages for ldquodoleriticpseudotachyliterdquo of 342 plusmn 3 Ma and 349 plusmn 2 Ma (whereby itis assumed that these breccias would have been formed by theimpact event) The published constraints on the age for thelarge Siljan impact event thus clearly define a post-Silurianage but are themselves not tightly constrained The limiteddata available demand further dedicated chronological workespecially in the light of the widely quoted possibility of acausal link between the Siljan impact and environmentalcatastrophe in the Late Devonian

The early ages for the impact event seemed to fall into an

intriguing part of the Late Devonian during which severalimportant events occurred (geological time scale of Harlandet al 1989) In this scheme the Devonian was placed between417 and 354 Ma with the Givetian stage between 380 and370 Ma the Frasnian from 370ndash364 Ma and the Famennianfrom 364 to 354 Ma It should also be noted that Ellwoodet al (2003) suggested the presence of evidence for impact atthe EifelianGivetian stage boundary of the mid-Devonian asuggestion that has remained controversial (Racki andKoeberl 2004) As reviewed by Sandberg et al (2002) theLate Frasnian mass extinction occured just prior to 364 Ma(within 20000 years) representing a major extinction eventthat decimated most groups of marine organisms This eventhas been associated with alleged impact evidence including aweak iridium enrichment found at the FrasnianFamennianboundary in southern China (Wang et al 1991) and in a cross-boundary section in the state of New York (Over et al 1997)In addition the presence of so-called ldquomicrotektite-like glassrdquoat a locality in Belgium was reported by Claeys and Casier(1994) and discussed by Sandberg et al (1988) The Siljanimpact structure was proposed by Claeys and Casier as thepossible source for these Belgian microtektites Furthermorethe Amˆnau catastrophic event of central Germany has alsobeen tentatively linked with impact (Sandberg et al 20002002) although no bona fide impact evidence has beenreported for this event The Amˆnau event was placed at theGivetianFrasnian boundary at 370 Ma Thebiochronologically dated Alamo impact breccia of southernNevada (eg Warme et al 2002) occurred in the earlyFrasnian punctata zone at about 367 Ma (Sandberg andWarme 1993 Sandberg and Morrow 1998 Sandberg et al2002) The latter authors proposed links between this Alamoimpact breccia and the Siljan and Flynn Creek impactstructures and suggested that these events could have resultedfrom a comet shower Such a link is however highly unlikelyas the geometry of the distribution of the Alamo breccia ratherindicates a nearby source crater in Nevada A further massextinction close to the DevonianCarboniferous boundary wasplaced at 357 Ma

Recently the International Commission on Stratigraphy(ICS) proposed a revised geological time scale (Gradsteinet al 2004 Ogg 2004 Gradstein and Ogg 2004) resulting in ashift of the ages for the Givetian Frasnian and Famennianstages of the Devonian period from 370ndash380 to 3853ndash392364ndash370 to 3745ndash3853 and 3592ndash3745 Ma (errors at 25ndash27 Ma) respectively This means that the Alamo impactevent historically dated at about 367 Ma and the Siljan impactas dated previously move both into the Famennian stage

The FrasnianFamennian event(s) is (are) of globalimportance and represent(s) one of the five most significantmass extinction events in the Phanerozoic (McGhee 1996)Sandberg et al (2002) suggested that several subcriticaloceanic impacts could best explain the evidence from anumber of widely separated regions in the world


While it is still debated what magnitude impact isrequired to cause a significant global extinction event to dateonly one mass extinctionmdashthe KT boundary eventmdashhasbeen unambiguously linked with a large impact event ie theabout 180 km wide 65 Ma old Chicxulub impact structure inMexico

Clearly it is important to investigate whether the Siljanimpact event is indeed coeval with any of the Late Devoniancatastrophic events especially in light of the recent changesof the geological time scale and with regard to the relativelyhigh uncertainty on the Siljan impact age (361ndash368 Ma)Improvement of the terrestrial impact cratering record whichis still far from complete is also required with special regardto the possibility of periodic increase in cratering activity andpossible relationship to terrestrial mass extinction events asdiscussed extensively in recent years (eg Rampino 2002 andreferences therein) To this effect we have carried out laserspot argon as well as laser stepheating (40Arndash39Ar) dating onseveral samples of melt breccia that have recently beenretrieved from the Siljan structure

GEOLOGY OF THE SILJAN IMPACT STRUCTURE

The Siljan structure (Fig 1) was formed in Svecokareliancrystalline basement overlain by supracrustals of Ordovicianand Silurian age Juhlin et al (1991) provided a variety of40Arndash39Ar mineral ages and U-Pb zircon as well as titaniteages for granitic lithologies ranging from 1436 Ma to1702 Ma Argon chronology on a number of dolerite samplesyielded ages between 789 and 1098 Ma An undefined ldquomeltrdquoindicated an age of 1163ndash1193 Ma It is obvious that the post-Silurian impact event is chronologically well separated fromthese various target rockbasement ages

The 28ndash30 km diameter central part of the Siljancomplex impact structure (Fig 1) comprises shocked andbrecciated granites (the so-called Dala granites) that havemostly been related to the Svecokarelian but that alsoinclude several younger intrusives (compare above) Thisarea represents a topographic high that is surrounded by arelatively depressed ring-shaped zone (up to a diameter ofapproximately 44 km) which is partly covered by lakes andin which predominantly sedimentary strata of Ordovician andSilurian age occur These strata include downfaultedOrdovician conglomerate and limestone and Silurian shaleand sandstone Several authors have commented that thecentral area could represent either a central uplift structure orthe remnant of a peak-ring structure (BodEgraven and Eriksson1988 Grieve 1988 Kenkmann and Von Dalwigk 2000) Thelatter authors also presented a detailed structural analysis ofthe Siljan structure BodEgraven and Eriksson (1988) reported thatthe sedimentary strata occur in part as chaotically arrangedmega-blocks

Shatter cones have been observed throughout the upliftedcentral part of the structure and at some peripheral locations

(Juhlin et al 1991) Planar deformation features (PDFs) inquartz from the central region of the basement complexindicate shock pressures between 12 and 17 GPa (Svensson1973 Tamminen and Wickman 1980 Grieve 1984 Aringbergand Bollmark 1985) Apparent melt veinlets from smallexposures and from drill core have been compared topseudotachylite Pseudotachylitic breccias (Reimold et al2005) of both doleritic and granitic composition have beenreported (Collini 1988) According to BodEgraven and Eriksson asample of the former was used by Bottomley et al (1978) fordating Two of us (WUR HH) observed millimeter-thickmelt-like covers on slickensides as well as veinlets ofpseudotachylitic breccia in a limestone quarry near Kallholenin the northwestern part of the ring of Paleozoic strataMicroscopic analysis of these breccia veinlets revealed thatall investigated occurrences from this quarry represent purecataclastic breccia devoid of any evidence of melting(frictional or other) Juhlin et al (1991) stated that ldquotrueimpact melts have not so far been foundrdquo They proceededhowever to refer to two localities where according to Aringberget al (1988) impact melt had been suspected but resolvedthat it was unlikely that this material represented impact meltrock In the course of recent fieldwork a number of melt rockoccurrences were identified at the locations shown in Fig 2(coordinates are listed in Table 1) These samples provide thebasis for this chronological investigation

SAMPLES

Sample Si-1 originates from a dyke- or pod-like exposureapproximately 1 m wide and more than 10 m long atTrollberget near the center of the impact structure (Fig 2)This melt rock cuts across granite as well as a mafic dike Onthe basis of published geographic information it would not beimpossible that this melt rock could represent the samematerial dated by Bottomley et al (1978 1990) It comprisesan extremely fine-grained matrix that in reflected lightappears fully crystalline (Figs 3a and 3b) The mode includesquartz feldspar and pyroxene or amphibole as well as anopaque phase (either magnetite or ilmenite) Clasts in excessof the matrix grain size amount to about 15 vol and includealkali feldspar and granite-derived (quartz plus feldspars)lithic clasts Most clasts of 03 mm grain size or larger are atleast partially annealed Clast shapes are generally angular tosubrounded but a small number of plastically deformed andwell-rounded to folded clasts are also present The matrixappears altered in places where reddish patches of tinycrystallites of hematite occur Some of these patches can berecognized as loci of felsic ghost clasts Other clasts displayreaction rims that are also strongly hematite-stained Besidesthe obvious thermal overprint on clasts no shock deformationcould be discerned

In contrast however Bottomley et al (1978) reported thepresence of planar shock deformation features in quartz in


their samples which could indicate that our sample does notnecessarily represent the same material analyzed by theseauthors A single 25 mm wide strongly altered clast with asubophitic texture of laths that likely originally representedfeldspar could represent an inclusion derived from an igneousprecursor rock or of crystalline impact melt

Sample Si-2 is a granitoid that is locally transected by adense network of millimeter-wide breccia veinlets Theseveinlets are generally thinner than 3 mm and enclose orinfiltrate into cm- to dm-sized host rock clasts The brecciatedparts of the sample are strongly impregnated with secondarycalcite feldspar in such areas is strongly altered to carbonateThe sample originates from Stumsnpermils near the edge of thecentral uplift (Fig 2) It is impossible to ascertain whether thebreccia represents a pure cataclasite or if locally melting may

have occurred The brecciated parts of our thin section showextensive aggregates of euhedral medium-grained galena andespecially sphalerite plus trace amounts of chalcopyrite Noindication of shock deformation was noted in quartz orfeldspar

Sample Si-3 (Figs 3c and 3d) was obtained from theMuseum of Natural History in Stockholm where only theapproximate locality of origin information was available(compare Table 1) The sample is derived from a melt brecciawith granitic clasts from a locality close to that shown inFig 2 on the central uplift The sample is a fluidal-texturedmelt rock with a matrix that optically appears glassy (locally)to crypto-crystalline Matrix seems to flow around stronglydeformed (brecciated partially annealed and locally melted)clasts most of which are granite-derived Locally the glass is

Fig 2 The locations in the Siljan structure where samples for this study were taken


oxidized mostly where it carries remnants of a mafic (gabbroor amphibolitic) precursor rock Aggregates of tiny crystals ofhematite lend these patches a reddish color At least 30 ofall clasts are completely annealed and many display plasticdeformation in the form of folded shapes It thus appearslikely that such clasts were melted and recrystallized Other

clasts display only cataclasis Matrix also contains some tinyeuhedral crystals of rutile Planar deformation features(PDFs) have been observed in several quartz and feldspargrains within granitic clasts

Sample Si-4 is from a boulder in the northwest part of thestructure (Fig 2) It comprises a relatively clast-rich breccia(Figs 4a and 4b) in which internally brecciated clasts areprominent The clast distribution is quite heterogeneous andit was obviously attempted to separate relatively clast-poorermaterial for the dating experiments The matrix is essentiallyclastic but contains some hematite-bearing patches thatoptically appear as glass They are characterized by thepresence of numerous tiny quartz clasts There are alsofragments of melt some of which are strongly extended andform stringers or schlieren Larger granitic clasts arebrecciated and partially annealed and locally even melted NoPDFs were observed in quartz or feldspar

Sample Si-5 is also from a boulder near the center of the

Fig 3 Photomicrographs of the analyzed breccia samples from Siljan in plane-polarized light all widths of view are 35 mm a) and b) showsample Si-1 which is a clast-poor aphanitic to microcrystalline melt breccia with most clasts clearly granitoid derived c) and d) show sampleSi-3 which is a fluidal-textured and variegated melt rock with a significant clast component Again most clasts are derived from granitoidprecursors Many clasts display evidence for plasticity and have been at least partially melted

Table 1 Geographic coordinates of sample locations Note that the location for sample Si-3 which was provided by the Stockholm Museum of Natural History is not as precise as the others

Sample 1 Longitudelatitude

Si-1 14deg502prime61deg030Si-2 14deg497prime60deg532Si-3 sim15deg61degSi-4 15deg50prime61deg05rsquoSi-5 14deg503prime61deg03Si-6 14deg501prime61deg04rsquo


structure close to the Si-1 locality This sample resembles Si-4 but contains significantly less clastic componentNevertheless it still is a clast-rich melt breccia (Fig 4c) Therock is strongly hematite-stained The clast content isgenerally granite-derived PDFs occur in quartz of lithic clastsas well as in several feldspar clasts In some patches thematrix is glassy or cryptocrystalline in others incipientdevitrification in the form of tiny microlites of feldspar is seenLocally microlites form dense aggregates indicating flowFlow directions are not uniform which is interpreted as thisbreccia representing an agglomeration of different meltfragments or as a result of turbulent flow Shocked plagioclase(diaplectic glass in alternate lamellae of polysyntheticallytwinned crystals) and fused feldspar and quartz (as identifiedon the basis of rosettes and spherulitic aggregates ofmicrocrystals in granite-derived clasts) are distinct

Sample Si-6 was taken from a local boulder in thenorthwest part of the central uplift near Hpermilttberg close to a50 times 50 m large partly excavated outcrop of granite in whichshatter cones are prominent This sample represents a narrow(lt10 cm wide) melt dikelet The sample has a variegatedfluidal-textured matrix with several narrow bands that

represent strongly extended (schlieren) granitic clasts (Fig4d) The overall appearance could suggest that this matrixwas melt The matrix is locally altered Clasts are stronglybrecciated and annealed Several larger clasts have stronglysericitized feldspar Several large brecciated granitic clastsare impregnated with secondary carbonate Locally patchesof strongly altered melt matrix have remnants of smallfeldspar laths Shocked feldspar clasts with alternate twinlamellae converted to maskelynite are noted and a number ofdiaplectic quartz or feldspar glass clasts occur they onlydisplay limited alteration

With the general lack of field control on the occurrencesof these breccias it is basically impossible to evaluatewhether they represent impact melt injections orpseudotachylitic breccia formed locally within the basementof the central uplift The generally moderate degree of shockdeformation (12ndash17 GPa) reported for basement at the currentlevel of exposure favors the origin of the breccias in situ aspseudotachylite or other pseudotachylitic breccia (for detailon such breccias refer to eg Gibson and Reimold [2005] orDressler and Reimold [2004])

Based on the above descriptions samples Si-3 and Si-5

Fig 4 Photomicrographs of the analyzed breccia samples from Siljan in plane-polarized light all widths of field of view are 35 mm (a) and(b) show sample Si-4 which is an aphanitic melt rock with locally very variable clast content (Fig 4b shows a very clast poor area) and localhematite staining The rare large granitoid clasts have been partially melted or locally annealed c) Sample Si-5 which is a melt rock that isvery similar to Si-3 but does not exhibit fluidal texture to the same degree Plastic deformation and evidence of melting in clasts is howeververy evident d) Sample Si-6 with several partially assimilated clasts in an aphanitic locally microlithic melt matrix


have the largest amounts of relatively fresh melt material andit was anticipated that they would present the best chances forobtaining argon chronological results The presence of bonafide shock deformation (PDFs and diaplectic glass) in boththese samples forms a direct link between melt brecciaformation and the impact event

Analytical Methods

The six rock samples were powdered and analyzed formajor element abundances in the X-ray fluorescencelaboratory of the School of Geosciences University of theWitwatersrand Johannesburg A range of international and

Table 2 Chemical compositions of Siljan samples All Fe as Fe2O3Wt Si-1 Si-2 Si-3 Si-4 Si-5 Si-6

SiO2 5406 5438 5946 6383 6146 5585TiO2 176 011 068 090 056 140Al2O3 1490 464 1681 1368 1687 1626Fe2O3 913 267 517 569 386 848MnO 018 014 005 009 003 016MgO 353 033 115 141 047 397CaO 404 1218 130 194 095 224Na2O 287 099 312 261 199 488K2O 622 355 926 699 1148 231P2O5 034 144 018 019 010 027LOI 266 967 208 158 132 380Total 9969 9010 9926 9891 9909 9962

ppmSc 143 090 709 127 114 124V 121 11 43 72 15 116Cr 198 251 129 245 61 165Co 233 107 404 851 156 150Ni 20 50 12 20 10 28Cu 13 186 24 26 8 14Zn 350 58100 87 75 178 150As 015 200 020 025 050 105Se 015 027 065 03 06 05Br 03 32 03 15 05 04Rb 167 156 196 296 348 105Sr 410 233 357 295 206 386Y 37 63 46 45 43 39Zr 290 90 570 280 555 300Nb 18 lt3 23 23 25 21Sb 005 649 004 013 006 017Cs 112 178 142 770 246 109Ba 830 720 1420 1650 690 520La 775 369 902 160 365 462Ce 988 861 118 253 466 918Nd 491 458 603 955 247 456Sm 848 102 802 117 258 781Eu 203 138 158 157 206 126Gd 685 811 645 86 147 727Tb 101 126 085 110 181 094Tm 049 049 044 061 092 054Yb 302 263 321 402 574 397Lu 047 035 050 067 088 062Hf 661 071 110 767 138 832Ta 062 030 034 101 065 094W 02 02 01 07 14 05Ir (ppb) lt06 lt08 lt03 lt02 lt04 lt06Au (ppb) 03 lt05 02 125 21 25Th 459 315 602 402 155 137U 071 303 134 323 172 249


SARM reference materials were analyzed for calibrationpurposes Accuracies from duplicate analyses are similar tothose reported by Reimold et al (1994) The samples werealso analyzed for 35 trace elements by instrumental neutronactivation analysis at the Department of Geological SciencesUniversity of Vienna (for details on the methodologyincluding information on instrumentation standards datareduction accuracy and precision see Koeberl [1993]) Theresults are listed in Table 2

Samples for argon chronology were prepared initially assquare 5 mm thick slabs from which 100ndash300 microm thickpolished sections were prepared Sections selected to containfew clasts were released from the glass slide andultrasonically cleaned using methanol and deionized waterSample Si-6 is clast-rich and thus the area exhibiting mostmelt was selected Specimens were wrapped in aluminumfoils and irradiated at the McMaster Nuclear Reactor Canadatogether with biotite standard GA1550 (9879 plusmn 096 Ma)(Renne et al 1998) to monitor neutron flux The samples werepacked adjacent to each other and represented a package only3 mm long sandwiched by standards The J values calculatedfrom the two GA1550 standards were within 02 and thus asingle J value is assigned to all samples with a 05 errorSamples were analyzed using techniques outlined in Kelleyand Gurov (2002) The individual laser spot data are given inTable 3 and stepped heating data in Table 4 Twelve to fifteenpoints were analyzed on each sample except Si-2 where justfive points were analyzed Final weighted mean ages werecalculated using ISOPLOT-Ex after Ludwig (1999) whichenhances the errors using the sum of students lsquotrsquo and squareroot of the MSWD

RESULTS

Chemical Composition

The major element data indicate significant chemicalvariability within this sample suite Samples haveintermediate SiO2 concentrations (54 to 64 wt) withrelatively high Al2O3 Fe2O3 and alkali element contentsThese compositions are strongly suggestive of mixingbetween relatively more felsic (granite) and more maficprecursor materials Sample Si-2 is characterized by low totaland elevated loss on ignition concomitant with relativeenrichment in CaO in accordance with petrographicobservations of secondary carbonate and presence ofsignificant amounts of sulfide The chemical compositionsalone do not allow identification of the true nature of thesesamples as either impact melt injections into basement orlocal formations of pseudotachylitic breccia in the centraluplift Notably samples Si-3 and Si-5 have high K2Ocontents which favor these samples for argon datingattempts but could be an indication of secondary alteration(compare petrographic descriptions)

Trace element data are also quite variable and generallyin keeping with concentrations that one would expect forgranitoid dominated materials The somewhat elevated CuCo Ni and As values as well as the very high Zn content forsample Si-2 are in line with the presence of secondarysulfides in particular sphalerite in this sample Iridiumconcentrations in all six samples are below the detection limit(05ndash1 ppb) indicating a maximum chondritic contribution tothe melt rocks of less than 05 The rare earth element(REE) patterns for this suite of samples are all very similarThey are relatively enriched in the light REE (LREE) withrelatively high concentrations as expected for felsic crustalrocks (chondrite-normalized La abundances betweenapproximately 100 and 1000) The LREE patterns are flatnegative Eu anomalies are prominent but somewhat variableOverall the trace element characteristics of these samples areconsistent with their derivation from mainly granitic materialwith a limited but significant contribution from maficmaterial (see petrographic descriptions)

Argon Chronology

Sample Si-1 yielded a range of Ar-Ar laser spot agesfrom 3506 plusmn 83 Ma to 3759 plusmn 48 Ma (Fig 5a) The datahave an average of 29 atmospheric contamination butexhibit little correlation between age and 36Ar39Ar Themajority of the data points form a vertical array on the 36Ar40Ar versus 39Ar40Ar diagram similar to those seen in glassyvolcanic rocks containing devitrified glass (eg Turner et al1994) The scatter of data points is insufficient to form anisochron

Sample Si-2 (not shown in Fig 5) yielded rather scatteredages ranging from 588 plusmn 6 Ma to 788 plusmn 32 Ma Although anattempt was made to target breccia veinlets the resulting ageswere strongly variable and reflected mainly Ar extracted frompartially reset host rock grains

Sample Si-3 yielded ages ranging from 3717 plusmn 18 Ma to3844 plusmn 27 Ma neglecting two points (not shown in Fig 5)which fell more than 4 sigma below the mean value Theindividual spot ages form a very tight cluster close to the 39Ar40Ar axis (Fig 5b) and an average atmospheric contaminationof only 03 (considerably less than for example Si-1) andthus do not form an isochron The data yield a weighted mean(Ludwig 1999) of 3772 plusmn 25 Ma

Sample Si-4 yielded ages in the range 3632 plusmn 49 Ma to3785 plusmn 38 Ma neglecting one point Analyses of this sampleyielded an average atmospheric contamination of 58(Fig 5c) and data scatter along a regression line whichcorresponds to an age of 3663 plusmn 90 Ma with a 40Ar36Arintercept of 343 plusmn 150 and an MSWD of 64

Sample Si-5 yields ages in the range 3672 plusmn 29 Ma to3844 plusmn 18 Ma Like Si-3 the analyses contained very lowatmospheric contamination with an average of just 05 Thedata form a cluster close to the 39Ar40Ar axis (Fig 5d) but do


Table 3 Argon chronological data Summary of laser spot data (amounts of 39Ar in cc STP times 10minus12)J value = 0001189 plusmn 0000055Siljan 1 40Ar39Ar 38Ar39Ar 37Ar39Ar 36Ar39Ar 39Ar 40Ar39Ar Age (Ma) +minusSpot 1 19691 00113 0445 minus000012 160 19726 3801 271Spot 2 19702 00124 0418 000075 976 19481 3759 48Spot 3 19460 00120 0352 000186 1506 18911 3659 34Spot 4 19307 00125 0380 000174 1144 18794 3639 46Spot 5 19248 00126 0389 000278 1166 18427 3574 47Spot 6 19159 00134 0375 minus000010 1000 19188 3708 52Spot 7 19212 00114 0389 000134 876 18817 3643 54Spot 8 19172 00127 0407 000365 782 18092 3515 66Spot 9 19486 00123 0358 000256 791 18729 3627 59Spot 10 19429 00111 0305 000164 1464 18944 3665 36Spot 11 19180 00092 0352 000273 596 18375 3565 78Spot 12 19268 00111 0353 000333 490 18283 3549 94Spot 13 18497 00126 0412 000093 805 18221 3538 58Spot 14 18907 00146 0444 000293 551 18041 3506 83

J value = 0001190 plusmn 0000055Siljan 2 40Ar39Ar 38Ar39Ar 37Ar39Ar 36Ar39Ar 39Ar 40Ar39Ar Age (Ma) +minusSpot 1 51766 00240 14105 001956 216 45987 7875 166Spot 2 38130 00139 1703 000488 4411 36687 6536 30Spot 3 32730 00116 0111 000107 2666 32412 5885 29Spot 4 33928 00121 0054 minus000073 1175 34144 6151 43Spot 5 43621 00121 0145 000431 5992 42348 7362 39

J value = 0001192 plusmn 0000055Siljan 3 40Ar39Ar 38Ar39Ar 37Ar39Ar 36Ar39Ar 39Ar 40Ar39Ar Age (Ma) +minusSpot 1 19486 00117 0055 minus000001 2966 19491 3769 23Spot 2 19465 00115 0049 000013 2869 19425 3757 23Spot 3 19236 00116 0057 000014 8065 19194 3717 18Spot 4 19372 00113 0065 000029 4114 19286 3733 20Spot 5 18399 00111 0037 000032 5091 18305 3561 19Spot 6 19443 00104 0043 000011 3417 19410 3749 44Spot 7 19877 00099 0035 minus000001 5012 19879 3831 24Spot 8 19412 00080 0041 000031 3952 19321 3734 36Spot 9 18438 00100 0000 000014 5840 18395 3571 19Spot 10 19566 00101 0042 000008 4703 19543 3772 22Spot 11 19941 00096 0046 000037 3328 19833 3823 23Spot 12 19907 00098 0039 minus000016 2833 19954 3844 27Spot 13 20007 00088 0046 000079 3679 19775 3813 21Spot 14 19577 00107 0048 000055 5261 19416 3750 23Spot 15 19597 00102 0049 000019 10410 19541 3772 18Weighted mean of 13 points (95 confidence limit) 3772 25

MSWD 130

J value = 0001192 plusmn 0000055Siljan 4 40Ar39Ar 38Ar39Ar 37Ar39Ar 36Ar39Ar 39Ar 40Ar39Ar Age (Ma) +minusSpot 1 19558 00110 0175 000146 6978 19127 3705 23Spot 2 19928 00108 0160 000241 4627 19216 3721 20Spot 3 19743 00102 0143 000242 2739 19029 3688 24Spot 4 19967 00100 0135 000422 3059 18720 3634 24Spot 5 19815 00100 0177 000292 3395 18951 3674 23Spot 6 19967 00100 0148 000425 3821 18711 3632 21Spot 7 20512 00116 0265 000395 2196 19346 3744 31Spot 8 20289 00096 0124 000320 1429 19344 3743 35Spot 9 20397 00101 0131 000509 984 18893 3664 52Spot 10 19357 00104 0162 000418 995 18123 3529 49Spot 11 21228 00113 0130 000677 1482 19229 3723 34Spot 12 21836 00112 0150 000762 1646 19585 3785 38


not allow an isochron to be constructed The data yield aweighted mean (Ludwig 1999) age of 3761 plusmn 28 Ma

Sample Si-6 yielded ages in the range 3551 plusmn 37 Ma to412 plusmn 44 Ma for an average atmospheric contamination of44 (Fig 5e) Again there was too much scatter to constructan isochron

Figure 5f shows the variability of the laser spot data inparticular highlighting the difference in atmosphericcontamination between the low contamination samples Si-3and Si-5 and the others Si-3 and Si-5 are also the least alteredsamples and thus alteration is the most likely cause of thehigher atmospheric contamination and less reproducible Arisotope data for the other samples

The spot data indicate that old ages resulting frominherited argon released from clasts which were not degassedor only partly degassed in the impact event are not a majorproblem in these melt samples This observation iscorroborated by the annealed nature of many of the clasts Theonly ldquohighrdquo clast ages were those determined for sample Si-2which appears to have undergone only cataclasticdeformation and no melting and several higher ages obtainedfor clast-rich sample Si-6 In view of the low clastcontribution we decided to test the difference between alteredand less altered samples by step-heating fragments of one

sample with lower atmospheric contamination (Si-5) and onewith higher atmospheric contamination (Si-4)

Sample Si-4 yielded a relatively flat release spectrum butno plateau The total gas age was 410 plusmn 58 Ma (Fig 6a)which is older than any of the individual laser spot ages forthe same sample The high total gas age is caused almostentirely by step three (4388 plusmn 33 Ma) without which the agewould have been within error of the weighted mean spot ageSample Si-5 yielded a plateau over 706 of the 39Ar releasewith an age of 3775 plusmn 41 Ma (Fig 6b) which is within errorsidentical to the total gas age of 3807 plusmn 40 Ma and theweighted mean laser spot age of 3761 plusmn 28 Ma for the samesample The low CaK ratios of both samples are reflected inlow 37Ar39Ar ratios throughout gas release (Figs 6c and 6d)Si-4 exhibits slowly falling 37Ar39Ar ratios indicating someCa contamination in low temperature phases whereas Si-5exhibits near zero 37Ar39Ar concentrations The differencebetween atmospheric contents of Si-4 and Si-5 in the spot datais again mirrored in the stepped heating data

In summary the samples which are least altered yieldedthe most reliable age data Weighted mean ages for thesamples with the lowest contamination are 3772 plusmn 25 Ma(Si-3) and 3761 plusmn 28 Ma (Si-5) and the stepped heatingplateau age for Si-5 falls within errors at 3775 plusmn 41 Ma It

J value = 0001192 plusmn 0000055Siljan 5 40Ar39Ar 38Ar39Ar 37Ar39Ar 36Ar39Ar 39Ar 40Ar39Ar Age (Ma) +minusSpot 1 19431 00091 0025 000054 3813 19270 3730 31Spot 2 19510 00097 0026 000029 12856 19426 3758 20Spot 3 19629 00098 0026 000018 9068 19576 3784 19Spot 4 19456 00098 0032 000025 3214 19381 3750 22Spot 5 19609 00100 0021 000054 3056 19448 3761 23Spot 6 19235 00103 0025 000022 3681 19170 3713 21Spot 7 19674 00091 0032 000023 12452 19606 3789 18Spot 8 20019 00082 0037 000033 8544 19920 3844 18Spot 9 19064 00100 0033 000042 8760 18939 3672 29Spot 10 19408 00107 0029 000031 7849 19316 3738 20Spot 11 19647 00106 0020 minus000015 2894 19692 3804 48Spot 12 19431 00099 0043 000052 5508 19276 3731 19Weighted mean of 11 points (95 confidence limit) 3761 28

MSWD 160


Table 3 Continued Argon chronological data Summary of laser spot data (amounts of 39Ar in cc STP times 10minus12)

602 W U Reimold et alTa

ble

4 A

r geo

chro

nolo

gy d

ata

for l

aser

ste

p-he

ated

sam

ples

of S

iljan

mel

t bre

ccia

s T

he s

teps

indi

cate

incr

easi

ng la

ser p

ower

thou

gh n

o te

mpe

ratu

re

mea

sure

men

ts w

ere

poss

ible

(err

ors

are

2 σ

and

incl

ude

J er

ror o

f 05

J

val

ues

as s

how

n in

Tab

le 3

)Si

ljan

4La

ser s

tep

40A

r39A

r38

Ar39

Ar

37A

r39A

r36

Ar39

Ar

39A

r (

)40

Ar 39

Ar

Age

(Ma)

plusmn2 σ

Step

123

132

000

720

0083

000

596

148

213

7137

31

26

Step

223

248

000

650

0134

000

243

207

225

3039

13

37

Step

326

502

000

620

0078

000

300

306

256

1743

88

33

Step

422

444

000

590

0055

000

180

377

219

1138

16

27

Step

520

440

000

650

0107

000

174

398

199

2535

01

91

Step

620

519

000

690

0044

000

139

509

201

0935

31

39

Step

721

707

000

550

0032

000

106

519

213

9337

34

122

Step

820

121

000

690

0062

000

085

609

198

6934

92

47

Step

920

018

000

660

0037

000

134

723

196

2134

53

22

Step

10

218

960

0064

000

380

0016

681

621

405

373

62

5St

ep 1

121

499

000

700

0013

000

199

841

209

1236

58

67

Step

12

222

710

0070

000

150

0025

099

721

531

375

612

0St

ep 1

321

575

001

41minus0

021

20

0000

110

00

215

7337

63

400

No

plat

eau

Silja

n 5

Lase

r ste

p40

Ar39

Ar

38A

r39A

r37

Ar39

Ar

36A

r39A

r39

Ar

40A

r 39A

rA

ge (M

a)plusmn2

σSt

ep 1

188

990

0107

000

010

0008

710

418

641

362

02

0St

ep 2

197

130

0095

000

000

0001

213

919

679

380

23

7St

ep 3

194

150

0098

000

060

0002

025

619

357

374

51

6St

ep 4

195

400

0106

000

070

0002

744

619

459

376

31

6St

ep 5

200

600

0118

000

000

0008

348

019

816

382

63

0St

ep 6

195

610

0088

000

080

0001

351

019

521

377

45

4St

ep 7

196

250

0097

000

070

0002

754

219

545

377

82

3St

ep 8

195

030

0078

000

070

0000

063

819

503

377

11

8St

ep 1

019

838

000

350

0015

000

043

810

197

1038

07

19

Step

11

203

380

0063

000

220

0005

288

920

186

389

09

3St

ep 1

224

988

000

570

0051

000

261

892

242

1745

75

218

Step

13

204

100

0055

000

340

0001

091

320

381

392

47

0St

ep 1

424

936

001

000

0008

000

081

946

246

9646

55

98

Step

15

198

570

0067

000

160

0003

910

00

197

4138

12

72

Plat

eau

age

over

70

6 o

f 39A

r rel

ease

377

54

1


Fig 5 Inverse isochron diagrams illustrating laser spot data for samples Si-1 to Si-6 Note that all of the samples plot close to the 39Ar40Araxis and are scaled accordingly a) Sample Si-1 b) sample Si-3 c) sample Si-4 The regression line represents a fit obtained using ISOPLOTwhich yields an age of 3663 plusmn 90 Ma with a 40Ar36Ar intercept of 342 plusmn 150 and an MSWD of 64 This is within error of the mean spot agesand plateau age obtained from other samples though with poorer precision d) Sample Si-5 e) sample Si-6 f) all samples plotted showingvariation in atmospheric contamination open symbols are Si-1 Si-4 Si-6 exhibiting higher atmospheric contamination closed symbols areSi-3 and Si-5 symbols and the dashed lines represent 1 and 10 atmospheric contamination


seems likely that the larger scatter on ages of the othersamples is the result of devitrification alteration andincomplete degassing of lithic and mineral clasts We find thatthe best estimate of the age for the formation of the Siljancrater is a combination of the best ages in a weighted meanallowing for geological scatter by multiplying the error bystudents lsquotrsquo multiplied by square root of MSWD yielding anage of 377 plusmn 2 Ma (95 confidence limit)

DISCUSSION AND CONCLUSIONS

The commonly quoted Ar-Ar age for Siljan (Bottomleyet al 1978) is an integrated age (an age calculated bysumming all gas released equivalent to a K-Ar age) from twosamples one of which yielded a plateau (3583 plusmn 48 Ma 2

errors) and one that did not form a plateau The analyses werealso affected by 39Ar recoil during irradiation indicating thepresence of a component of fine-grained potassium-bearingphyllosilicate that grew during post-impact hydrothermalalteration The presence of a fine-grained phyllosilicatecomponent indicates that the resulting age could be anunderestimate of the true age of the Siljan impact New laserargon spot data and step-heating data presented above showsome scatter but this scatter is correlated with the alterationstate of the samples We have illustrated all data in order toemphasize the correlation which appears to result in slightlylower ages for more altered samples The two least alteredsamples yield consistent ages for laser spot and steppedheating and support a revised age for this impact event of 377plusmn 2 Ma (95 confidence limits) Thus the Siljan case

Fig 6 Results for stepped heating on selected whole rock chips (a) shows sample Si-4 which did not yield a plateau age (b) shows sampleSi-5 showing a plateau over 706 of 39Ar release and an age of 3775 plusmn 41 Ma (c) shows 37Ar39Ar release spectrum for sample Si-4 and(d) shows 37Ar39Ar release spectrum for sample Si-5


provides further evidence for the need to cautiously interpretexisting geochronological results on impact breccias The keyto obtaining good age data for impact melt rocks is detailedpetrographic and chemical characterization of samples In theSiljan samples characterization of clast content and state ofalteration were crucially combined with Ar-Ar analysis of asuite of samples which also assessed the effects of alterationand likelihood of clast-derived extraneous argon Howeverwell-preserved impact melt samples are rare and dating theseimportant terrestrial events continues to provide a challengeto isotope geochronology

The revised Siljan age (377 plusmn 2 Ma) does not correspondwith the previously accepted stratigraphic age for theFrasnianFamennian boundary (364 Ma Gradstein and Ogg1996) Thus any discussion of whether or not this impactevent can be correlated with any of the known catastrophicevents in the Late Devonian period (Sandberg et al 2002)would be rendered invalid However the recent revision ofthe geological time scale (Gradstein et al 2004 Gradstein andOgg 2004) has resulted in the curious situation that the newSiljan age falls within errors of the newly recommended agefor the FrasnianFamennian boundary at 3745 plusmn 26 Ma Thenew boundary is based partly on a reappraisal of the Devoniantime scale using new U-Pb zircon ages from the DevonianAppalachian Basin in the USA (Tucker et al 1998) This caseshows the extreme difficulty in tying absolute ages andbiostratigraphic boundaries particularly in older events (egDeutsch and Schpermilrer 1994) Achieving the close controlwhich has been achieved for the KT boundary may simplynot be possible in older sequences Reliance will have to beplaced more upon obtaining short term climate changesignals

Siljan may have originally been as large as 85 kmdiameter (Henkel and Aaro 2005) but could Siljan havegenerated detectable global catastrophe and mass extinctionReimold and Koeberl (2002) discussed evidence that a strongrelationship between a large impact and global environmentalextinction event only exists for the CretaceousTertiaryboundary event at Chicxulub an impact structure thatmeasures approximately 180 km in diameter Several impactstructures with diameters around or just below 100 kmincluding the Chesapeake Bay structure (85 km age 355 Ma)at the eastern seaboard of the United States (Poag et al 2004)Manicouagan in Canada (100 km age 214 Ma) and Popigaiin Siberia (100 km age 357 Ma) have not been related tomajor global extinction events

Based on the currently defined impact flux for thePhanerozoic (eg Hughes 2000 Schmitz and Peucker-Ehrenbrink 2001) an impact event of comparable magnitude(producing craters in the 65ndash85 km diameter range) wouldhave taken place at a likely rate of 1 per 10ndash20 million yearsThus the presently known cratering record of the LateDevonian and Early Silurian period is clearly incompleteAlthough there appears to be a cluster of impacts during the

Late Eocene (including Chesapeake Bay and Popigai) it willbe much more difficult to obtain the same level of constrainton impact structures suggested to be of similar age to Siljan(eg Charlevoix 357 plusmn 15 Ma Woodleigh 364 plusmn 20 Ma andFlynn Creek 360 plusmn 20 Ma) Craters of Devonian age oftenhave poorly constrained ages more detailed work is requiredto improve the geochronology before we can determine ifthey form a significant cluster In addition it is unlikely thatthese events of relatively minor magnitude even if they hadoccurred as a cluster of events would have resulted in a majorglobal extinction event such as that at the FrasnianFamennian boundary

AcknowledgmentsndashSharon Turner carried out the XRFanalyses and Lyn Whitfield and Henja Czekanowskaprovided expert drafting and photographic support SampleSi-3 was kindly provided by Dr Jan Olov Nystrˆm of theMuseum of Natural History Stockholm CK is supported bythe Austrian Science Foundation (FWF) SCS acknowledgesNERC fellowship NERIS200200692 and SPKacknowledges funding from the Leverhulme Trust Criticalreviews by Philippe Claeys and Birger Schmitz as well aseditorial comments from Alex Deutsch are muchappreciated This is University of the Witwatersrand ImpactCratering Research Group Contribution No 85

Editorial HandlingmdashDr Alexander Deutsch

REFERENCES

Aringberg G and Bollmark B 1985 Retention of U and Pb in zirconsfrom shocked granite in the Siljan impact structure SwedenEarth and Planetary Science Letters 74347ndash349

BodEgraven A and Eriksson K G editors 1988 Deep drilling incrystalline bedrock volume 1 The deep gas drilling in the Siljanimpact structure Sweden and astroblemes Berlin Springer-Verlag 364 p

Bottomley R J York D and Grieve R A F 1978 40Ar-39Ar agesof Scandinavian impact structures I Mien and SiljanContributions to Mineralogy and Petrology 6879ndash84

Bottomley R J York D and Grieve R A F 1990 40Argon-39Argondating of impact craters Proceedings 20th Lunar and PlanetaryScience Conference pp 421ndash431

Claeys P and Casier J-G 1994 Microtektite-like glass associatedwith the Frasnian-Famennian boundary mass extinction Earthand Planetary Science Letters 122303ndash315

Collini B 1988 Geological setting of the Siljan ring structure Deepdrilling in crystalline bedrock volume 1 The deep gas drilling inthe Siljan impact structure Sweden and astroblemes edited byBodEgraven A and Eriksson K G Berlin Springer-Verlag 364 p

Deutsch A and Schpermilrer U 1994 Dating terrestrial impact eventsMeteoritics 29301ndash322

Ellwood B B Benoist S L El Hassani A Wheeler C Crick R E2003 Impact ejecta layer from the mid-Devonian Possibleconnection to global mass extinctions Science 3001734ndash1737

Fredriksson K and Wickman F E 1963 Meteoriter In Svensknaturvetenskap edited by Lundholm B Stockholm SwedishNatural Science Research Council pp 121ndash157

Gibson R L and Reimold W U 2005 Shock pressure distribution


in the Vredefort impact structure South Africa In Largemeteorite impacts III Boulder Colorado Geological Society ofAmerica pp 329ndash350

Gold T 1987 Power from the Earth London J M Dent amp Sons Ltd208 p

Gold T 1988 The deep earth gas theory with respect to the resultsfrom the Gravberg-1 well In Deep drilling in crystallinebedrock volume 1 The deep gas drilling in the Siljan impactstructure Sweden and astroblemes edited by BodEgraven A andEriksson K G Berlin Springer-Verlag pp 18ndash27

Gold T and Soter S 1980 The deep-earth gas hypothesis ScientificAmerican 242154ndash161

Gradstein F M and Ogg J G 1996 A Phanerozoic time scaleEpisodes 193ndash4

Gradstein F M and Ogg J G 2004 Geologic Time Scale 2004mdashWhy how and where next Lethaia 37175ndash181

Gradstein F M Ogg J G Smith A G Bleeker W and Lourens L J2004 A new geologic time scale with special reference toPrecambrian and Neogene Episodes 2783ndash100

Grieve R A F 1988 The formation of large impact structures andconstraints on the nature of Siljan In Deep drilling in crystallinebedrock volume 1 The deep gas drilling in the Siljan impactstructure Sweden and astroblemes edited by BodEgraven A andEriksson K G Berlin Springer-Verlag pp 328ndash348

Hallam A and Wignall P B 1997 Mass extinctions and theiraftermath Oxford Oxford University Press 320 p

Henkel H and Aaro S 2005 Geophysical investigations of the Siljanimpact structure A review In Impact tectonics edited byKoeberl C and Henkel H Berlin Springer-Verlag pp 247ndash283

Hode T von Dalwigk I and Broman C 2002 A hydrothermalsystem associated with the Siljan impact structure SwedenmdashImplications for the search for fossil life on Mars Astrobiology3271ndash289

Hughes D W 2000 A new approach to the calculation of thecratering record of the Earth over the last 125 plusmn 20 Myr MonthlyNotices of the Royal Astronomical Society 317429ndash437

Juhlin C and Pedersen L B 1987 Reflection seismic investigationsof the Siljan impact structure Sweden Journal of GeophysicalResearch 9214113ndash14122

Juhlin C 1991 Scientific summary report of the Deep Gas DrillingProject in the Siljan ring structure Swedish State Power BoardU(G) 199114 357 p

Kelley S P and Gurov E 2002 Boltysh another end-Cretaceousimpact Meteoritics amp Planetary Science 371031ndash1043

Kenkmann T and von Dalwigk I 2000 Radial transpression ridgesA new structural feature of complex impact craters Meteoriticsamp Planetary Science 351189ndash1201

Koeberl C 1993 Instrumental neutron activation analysis ofgeochemical and cosmochemical samples A fast and provenmethod for small sample analysis Journal of Radioanalyticaland Nuclear Chemistry 16847ndash60

McGhee G R Jr 1996 The Late Devonian mass extinction TheFrasnianFamennian crisis New York Columbia UniversityPress 303 p

Ogg J G 2004 Staus of divisions of the international geologic timescale Lethaia 37183ndash199

Over D J Conaway C A Katz D J Goodfellow W D andGregoire D C 1997 Platinum group element enrichments andpossible chondritic RuIr across the Frasnian-Famennianboundary western New York State PalaeogeographyPalaeoclimatology Palaeoecology 132399ndash410

Racki G and Koeberl C 2004 Comment on ldquoImpact ejecta layerfrom the mid-Devonian Possible connection to global massextinctionsrdquo Science 303471

Rampino M R 2002 Role of the galaxy in periodic impacts and

mass extinctions on the Earth In Catastrophic events and massextinctions Impacts and beyond edited by Koeberl C andMacLeod K G Boulder Colorado Geological Society ofAmerica pp 667ndash678

Reimold W U and Koeberl C 2002 Petrography and geochemistryof a deep drill core from the edge of the Morokweng impactstructure South Africa In Impact markers in the stratigraphicrecord edited by Koeberl C and Martinez-Ruiz F HeidelbergSpringer-Verlag pp 271ndash292

Reimold W U Gibson R L Koeberl C and Dressler B O 2005Economic ore deposits in impact structures and their geologicalsetting In Impact tectonics edited by Koeberl C and Henkel HBerlin Springer-Verlag pp 479ndash552

Renne P R Swisher C C Deino A L Karner D B Owens T Land DePaolo D J 1998 Intercalibration of standards absoluteages and uncertainties in 40Ar39Ar dating Chemical Geology145117ndash152

Renne P R Reimold W U Koeberl C Hough R and Claeys P2002 Critical comment on lsquoK-Ar evidence from illitic clays of aLate Devonian age for the 120 km diameter Woodleigh impactstructure Southern Carnarvon Basin Western Australiarsquo by I TUysal et al Earth and Planetary Science Letters 201221ndash232

Rondot J 1975 Comparaison entre les astroblemes de Siljan Suegravedeet de Charlevoix Quebec Bulletin of the Geological Institutionsof the University of Uppsala 685ndash92 In French

Sandberg C A and Morrow J R 1988 Role of conodonts indeciphering and dating Late Devonian Alamo impactmegabreccia southeastern Nevada USA (abstract)Proceedings Seventh International Conodont Symposium pp93ndash94

Sandberg C A and Warme J E 1993 Conodont dating biofaciesand catastrophic origin of Late Devonian (early Frasnian) Alamobreccia southern Nevada (abstract) Geological Society ofAmerica Abstracts with Programs 2577

Sandberg C A Ziegler W Dreesen R and Butler J L 1988 LateFrasnian mass extinction Conodont event stratigraphy globalchanges and possible causes Proceedings First InternationalSenckenberg Conference and 5th European ConodontSymposium pp 263ndash307

Sandberg C A Morrow J R and Ziegler W 2000 Possible impactorigin of the enigmatic early Late Devonian Amˆnau brecciaRheinisches Schiefergebirge Germany (abstract 3020)International Conference on Catastrophic Events and MassExtinctions Impacts and Beyond

Sandberg C A Morrow J R and Ziegler W 2002 Late Devoniansea-level changes catastrophic events and mass extinctions InCatastrophic events and mass extinctions Impacts and beyondedited by Koeberl C and MacLeod K G Boulder ColoradoGeological Society of America pp 473ndash487

Schmitz B and Peucker-Ehrenbrink B editors 2001 Accretion ofextraterrestrial matter throughout Earthrsquos history New YorkKluwer AcademicPlenum Publishers 492 pp

Schmitz B Haggstrom T and Tassinari M 2003 Sediment-dispersed extraterrestrial chromite traces a major asteroiddisruption event Science 300961ndash964

Steiger R J and Jpermilger E 1977 Subcommission on geochronologyConvention on the use of decay constants in geo- andcosmochronology Earth and Planetary Science Letters 36359ndash362

Svensson N B 1971 Probable meteorite impact crater in centralSweden Nature 22990ndash92

Svensson N B 1973 Shatter cones from the Siljan structure centralSweden Geologiska Foreningens I Stockholm Forhendlingar95139ndash143

Therriault A M Grieve R A F and Reimold W U 1997 Original


size of the Vredefort structure Implications for the geologicalevolution of the Witwatersrand Basin Meteoritics amp PlanetaryScience 3271ndash77

Tucker R D Bradley D C Straeten C A V Harris A G EbertJ R and McCutcheon S R 1998 New U-Pb zircon ages and theduration and division of Devonian time Earth and PlanetaryScience Letters 158175ndash186

Turner S P Kelley S P Hawkesworth C J and Mantovani M1994 Magmatism and continental breakup in the South AtlanticHigh precision 40Ar-39Ar geochronology Earth and PlanetaryScience Letters 121333ndash348

Uysal I T Golding S D Glikson A Y Mory A J and Glikson M2002 K-Ar evidence from illitic clays of a Late Devonian age forthe 120 km diameter Woodleigh impact structure centralCarnarvon Basin western Australia Earth and PlanetaryScience Letters 192281ndash189

Von Dalwigk I and Kenkmann T 1999 The Siljan impact structureNew constraints for a diameter reconstruction (abstract)Proceedings 23rd Nordic Geological Winter Meeting p 24

Wang K Orth C J Attrep M A Jr Chatterton B D E Hou Hand Geldsetzer H H J 1991 Geochemical evidence for acatastrophic biotic event at the FrasnianFamennian boundary inSouth China Geology 10776ndash779

Warme J E Morgan M and Kuehner H 2002 Impact-generatedcarbonate accretion lapilli in the Late Devonian Alamo brecciaIn Catastrophic events and mass extinctions Impacts andbeyond edited by Koeberl C and MacLeod K G BoulderColorado Geological Society of America pp 489ndash504

Wickman F E Blomqvist N G Geijer P Parwel A V Ubisch Hand Welin E 1963 Isotopic constitution of ore lead in SwedenArkiv foumlr Mineralogi och Geologi 3193ndash257


the pre-erosional crater diameter could have been as much as85 km

The evidence for impact at Siljan includes the presenceof shatter cones and planar deformation features (PDFs) inquartz Exposure is poor in much of the Siljan structure andbona fide impact melt rock has to date remained elusive

Consequently very little material that could be used to obtaina reliable age for this impact structure has been producedOnly ldquoallochthonous breccia small and narrow breccia dikesand float of melt brecciardquo were reported by Rondot (1975)Svensson (1971 1973) suggested a post-Silurian age for theimpact event

Fig 1 The geology of the Siljan impact structure The inset shows the location of Siljan in Scandinavia

















SAMPLES



























Analytical Methods








RESULTS




Argon Chronology










MSWD 130











MSWD 160




ble

4 A

r geo

chro

nolo

gy d

ata

for l

aser

ste

p-he

ated

sam

ples

of S

iljan

mel

t bre

ccia

s T

he s

teps

indi

cate

incr

easi

ng la

ser p

ower

thou

gh n

o te

mpe

ratu

re

mea

sure

men

ts w

ere

poss

ible

(err

ors

are

2 σ

and

incl

ude

J er

ror o

f 05

J

val

ues

as s

how

n in

Tab

le 3

)Si

ljan

4La

ser s

tep

40A

r39A

r38

Ar39

Ar

37A

r39A

r36

Ar39

Ar

39A

r (

)40

Ar 39

Ar

Age

(Ma)

plusmn2 σ

Step

123

132

000

720

0083

000

596

148

213

7137

31

26

Step

223

248

000

650

0134

000

243

207

225

3039

13

37

Step

326

502

000

620

0078

000

300

306

256

1743

88

33

Step

422

444

000

590

0055

000

180

377

219

1138

16

27

Step

520

440

000

650

0107

000

174

398

199

2535

01

91

Step

620

519

000

690

0044

000

139

509

201

0935

31

39

Step

721

707

000

550

0032

000

106

519

213

9337

34

122

Step

820

121

000

690

0062

000

085

609

198

6934

92

47

Step

920

018

000

660

0037

000

134

723

196

2134

53

22

Step

10

218

960

0064

000

380

0016

681

621

405

373

62

5St

ep 1

121

499

000

700

0013

000

199

841

209

1236

58

67

Step

12

222

710

0070

000

150

0025

099

721

531

375

612

0St

ep 1

321

575

001

41minus0

021

20

0000

110

00

215

7337

63

400

No

plat

eau

Silja

n 5

Lase

r ste

p40

Ar39

Ar

38A

r39A

r37

Ar39

Ar

36A

r39A

r39

Ar

40A

r 39A

rA

ge (M

a)plusmn2

σSt

ep 1

188

990

0107

000

010

0008

710

418

641

362

02

0St

ep 2

197

130

0095

000

000

0001

213

919

679

380

23

7St

ep 3

194

150

0098

000

060

0002

025

619

357

374

51

6St

ep 4

195

400

0106

000

070

0002

744

619

459

376

31

6St

ep 5

200

600

0118

000

000

0008

348

019

816

382

63

0St

ep 6

195

610

0088

000

080

0001

351

019

521

377

45

4St

ep 7

196

250

0097

000

070

0002

754

219

545

377

82

3St

ep 8

195

030

0078

000

070

0000

063

819

503

377

11

8St

ep 1

019

838

000

350

0015

000

043

810

197

1038

07

19

Step

11

203

380

0063

000

220

0005

288

920

186

389

09

3St

ep 1

224

988

000

570

0051

000

261

892

242

1745

75

218

Step

13

204

100

0055

000

340

0001

091

320

381

392

47

0St

ep 1

424

936

001

000

0008

000

081

946

246

9646

55

98

Step

15

198

570

0067

000

160

0003

910

00

197

4138

12

72

Plat

eau

age

over

70

6 o

f 39A

r rel

ease

377

54

1

















REFERENCES












































































SAMPLES



























Analytical Methods








RESULTS




Argon Chronology










MSWD 130











MSWD 160




ble

4 A

r geo

chro

nolo

gy d

ata

for l

aser

ste

p-he

ated

sam

ples

of S

iljan

mel

t bre

ccia

s T

he s

teps

indi

cate

incr

easi

ng la

ser p

ower

thou

gh n

o te

mpe

ratu

re

mea

sure

men

ts w

ere

poss

ible

(err

ors

are

2 σ

and

incl

ude

J er

ror o

f 05

J

val

ues

as s

how

n in

Tab

le 3

)Si

ljan

4La

ser s

tep

40A

r39A

r38

Ar39

Ar

37A

r39A

r36

Ar39

Ar

39A

r (

)40

Ar 39

Ar

Age

(Ma)

plusmn2 σ

Step

123

132

000

720

0083

000

596

148

213

7137

31

26

Step

223

248

000

650

0134

000

243

207

225

3039

13

37

Step

326

502

000

620

0078

000

300

306

256

1743

88

33

Step

422

444

000

590

0055

000

180

377

219

1138

16

27

Step

520

440

000

650

0107

000

174

398

199

2535

01

91

Step

620

519

000

690

0044

000

139

509

201

0935

31

39

Step

721

707

000

550

0032

000

106

519

213

9337

34

122

Step

820

121

000

690

0062

000

085

609

198

6934

92

47

Step

920

018

000

660

0037

000

134

723

196

2134

53

22

Step

10

218

960

0064

000

380

0016

681

621

405

373

62

5St

ep 1

121

499

000

700

0013

000

199

841

209

1236

58

67

Step

12

222

710

0070

000

150

0025

099

721

531

375

612

0St

ep 1

321

575

001

41minus0

021

20

0000

110

00

215

7337

63

400

No

plat

eau

Silja

n 5

Lase

r ste

p40

Ar39

Ar

38A

r39A

r37

Ar39

Ar

36A

r39A

r39

Ar

40A

r 39A

rA

ge (M

a)plusmn2

σSt

ep 1

188

990

0107

000

010

0008

710

418

641

362

02

0St

ep 2

197

130

0095

000

000

0001

213

919

679

380

23

7St

ep 3

194

150

0098

000

060

0002

025

619

357

374

51

6St

ep 4

195

400

0106

000

070

0002

744

619

459

376

31

6St

ep 5

200

600

0118

000

000

0008

348

019

816

382

63

0St

ep 6

195

610

0088

000

080

0001

351

019

521

377

45

4St

ep 7

196

250

0097

000

070

0002

754

219

545

377

82

3St

ep 8

195

030

0078

000

070

0000

063

819

503

377

11

8St

ep 1

019

838

000

350

0015

000

043

810

197

1038

07

19

Step

11

203

380

0063

000

220

0005

288

920

186

389

09

3St

ep 1

224

988

000

570

0051

000

261

892

242

1745

75

218

Step

13

204

100

0055

000

340

0001

091

320

381

392

47

0St

ep 1

424

936

001

000

0008

000

081

946

246

9646

55

98

Step

15

198

570

0067

000

160

0003

910

00

197

4138

12

72

Plat

eau

age

over

70

6 o

f 39A

r rel

ease

377

54

1

















REFERENCES




















































































Analytical Methods








RESULTS




Argon Chronology










MSWD 130











MSWD 160




ble

4 A

r geo

chro

nolo

gy d

ata

for l

aser

ste

p-he

ated

sam

ples

of S

iljan

mel

t bre

ccia

s T

he s

teps

indi

cate

incr

easi

ng la

ser p

ower

thou

gh n

o te

mpe

ratu

re

mea

sure

men

ts w

ere

poss

ible

(err

ors

are

2 σ

and

incl

ude

J er

ror o

f 05

J

val

ues

as s

how

n in

Tab

le 3

)Si

ljan

4La

ser s

tep

40A

r39A

r38

Ar39

Ar

37A

r39A

r36

Ar39

Ar

39A

r (

)40

Ar 39

Ar

Age

(Ma)

plusmn2 σ

Step

123

132

000

720

0083

000

596

148

213

7137

31

26

Step

223

248

000

650

0134

000

243

207

225

3039

13

37

Step

326

502

000

620

0078

000

300

306

256

1743

88

33

Step

422

444

000

590

0055

000

180

377

219

1138

16

27

Step

520

440

000

650

0107

000

174

398

199

2535

01

91

Step

620

519

000

690

0044

000

139

509

201

0935

31

39

Step

721

707

000

550

0032

000

106

519

213

9337

34

122

Step

820

121

000

690

0062

000

085

609

198

6934

92

47

Step

920

018

000

660

0037

000

134

723

196

2134

53

22

Step

10

218

960

0064

000

380

0016

681

621

405

373

62

5St

ep 1

121

499

000

700

0013

000

199

841

209

1236

58

67

Step

12

222

710

0070

000

150

0025

099

721

531

375

612

0St

ep 1

321

575

001

41minus0

021

20

0000

110

00

215

7337

63

400

No

plat

eau

Silja

n 5

Lase

r ste

p40

Ar39

Ar

38A

r39A

r37

Ar39

Ar

36A

r39A

r39

Ar

40A

r 39A

rA

ge (M

a)plusmn2

σSt

ep 1

188

990

0107

000

010

0008

710

418

641

362

02

0St

ep 2

197

130

0095

000

000

0001

213

919

679

380

23

7St

ep 3

194

150

0098

000

060

0002

025

619

357

374

51

6St

ep 4

195

400

0106

000

070

0002

744

619

459

376

31

6St

ep 5

200

600

0118

000

000

0008

348

019

816

382

63

0St

ep 6

195

610

0088

000

080

0001

351

019

521

377

45

4St

ep 7

196

250

0097

000

070

0002

754

219

545

377

82

3St

ep 8

195

030

0078

000

070

0000

063

819

503

377

11

8St

ep 1

019

838

000

350

0015

000

043

810

197

1038

07

19

Step

11

203

380

0063

000

220

0005

288

920

186

389

09

3St

ep 1

224

988

000

570

0051

000

261

892

242

1745

75

218

Step

13

204

100

0055

000

340

0001

091

320

381

392

47

0St

ep 1

424

936

001

000

0008

000

081

946

246

9646

55

98

Step

15

198

570

0067

000

160

0003

910

00

197

4138

12

72

Plat

eau

age

over

70

6 o

f 39A

r rel

ease

377

54

1

















REFERENCES































































RESULTS




Argon Chronology










MSWD 130











MSWD 160




ble

4 A

r geo

chro

nolo

gy d

ata

for l

aser

ste

p-he

ated

sam

ples

of S

iljan

mel

t bre

ccia

s T

he s

teps

indi

cate

incr

easi

ng la

ser p

ower

thou

gh n

o te

mpe

ratu

re

mea

sure

men

ts w

ere

poss

ible

(err

ors

are

2 σ

and

incl

ude

J er

ror o

f 05

J

val

ues

as s

how

n in

Tab

le 3

)Si

ljan

4La

ser s

tep

40A

r39A

r38

Ar39

Ar

37A

r39A

r36

Ar39

Ar

39A

r (

)40

Ar 39

Ar

Age

(Ma)

plusmn2 σ

Step

123

132

000

720

0083

000

596

148

213

7137

31

26

Step

223

248

000

650

0134

000

243

207

225

3039

13

37

Step

326

502

000

620

0078

000

300

306

256

1743

88

33

Step

422

444

000

590

0055

000

180

377

219

1138

16

27

Step

520

440

000

650

0107

000

174

398

199

2535

01

91

Step

620

519

000

690

0044

000

139

509

201

0935

31

39

Step

721

707

000

550

0032

000

106

519

213

9337

34

122

Step

820

121

000

690

0062

000

085

609

198

6934

92

47

Step

920

018

000

660

0037

000

134

723

196

2134

53

22

Step

10

218

960

0064

000

380

0016

681

621

405

373

62

5St

ep 1

121

499

000

700

0013

000

199

841

209

1236

58

67

Step

12

222

710

0070

000

150

0025

099

721

531

375

612

0St

ep 1

321

575

001

41minus0

021

20

0000

110

00

215

7337

63

400

No

plat

eau

Silja

n 5

Lase

r ste

p40

Ar39

Ar

38A

r39A

r37

Ar39

Ar

36A

r39A

r39

Ar

40A

r 39A

rA

ge (M

a)plusmn2

σSt

ep 1

188

990

0107

000

010

0008

710

418

641

362

02

0St

ep 2

197

130

0095

000

000

0001

213

919

679

380

23

7St

ep 3

194

150

0098

000

060

0002

025

619

357

374

51

6St

ep 4

195

400

0106

000

070

0002

744

619

459

376

31

6St

ep 5

200

600

0118

000

000

0008

348

019

816

382

63

0St

ep 6

195

610

0088

000

080

0001

351

019

521

377

45

4St

ep 7

196

250

0097

000

070

0002

754

219

545

377

82

3St

ep 8

195

030

0078

000

070

0000

063

819

503

377

11

8St

ep 1

019

838

000

350

0015

000

043

810

197

1038

07

19

Step

11

203

380

0063

000

220

0005

288

920

186

389

09

3St

ep 1

224

988

000

570

0051

000

261

892

242

1745

75

218

Step

13

204

100

0055

000

340

0001

091

320

381

392

47

0St

ep 1

424

936

001

000

0008

000

081

946

246

9646

55

98

Step

15

198

570

0067

000

160

0003

910

00

197

4138

12

72

Plat

eau

age

over

70

6 o

f 39A

r rel

ease

377

54

1

















REFERENCES
































































MSWD 130











MSWD 160




ble

4 A

r geo

chro

nolo

gy d

ata

for l

aser

ste

p-he

ated

sam

ples

of S

iljan

mel

t bre

ccia

s T

he s

teps

indi

cate

incr

easi

ng la

ser p

ower

thou

gh n

o te

mpe

ratu

re

mea

sure

men

ts w

ere

poss

ible

(err

ors

are

2 σ

and

incl

ude

J er

ror o

f 05

J

val

ues

as s

how

n in

Tab

le 3

)Si

ljan

4La

ser s

tep

40A

r39A

r38

Ar39

Ar

37A

r39A

r36

Ar39

Ar

39A

r (

)40

Ar 39

Ar

Age

(Ma)

plusmn2 σ

Step

123

132

000

720

0083

000

596

148

213

7137

31

26

Step

223

248

000

650

0134

000

243

207

225

3039

13

37

Step

326

502

000

620

0078

000

300

306

256

1743

88

33

Step

422

444

000

590

0055

000

180

377

219

1138

16

27

Step

520

440

000

650

0107

000

174

398

199

2535

01

91

Step

620

519

000

690

0044

000

139

509

201

0935

31

39

Step

721

707

000

550

0032

000

106

519

213

9337

34

122

Step

820

121

000

690

0062

000

085

609

198

6934

92

47

Step

920

018

000

660

0037

000

134

723

196

2134

53

22

Step

10

218

960

0064

000

380

0016

681

621

405

373

62

5St

ep 1

121

499

000

700

0013

000

199

841

209

1236

58

67

Step

12

222

710

0070

000

150

0025

099

721

531

375

612

0St

ep 1

321

575

001

41minus0

021

20

0000

110

00

215

7337

63

400

No

plat

eau

Silja

n 5

Lase

r ste

p40

Ar39

Ar

38A

r39A

r37

Ar39

Ar

36A

r39A

r39

Ar

40A

r 39A

rA

ge (M

a)plusmn2

σSt

ep 1

188

990

0107

000

010

0008

710

418

641

362

02

0St

ep 2

197

130

0095

000

000

0001

213

919

679

380

23

7St

ep 3

194

150

0098

000

060

0002

025

619

357

374

51

6St

ep 4

195

400

0106

000

070

0002

744

619

459

376

31

6St

ep 5

200

600

0118

000

000

0008

348

019

816

382

63

0St

ep 6

195

610

0088

000

080

0001

351

019

521

377

45

4St

ep 7

196

250

0097

000

070

0002

754

219

545

377

82

3St

ep 8

195

030

0078

000

070

0000

063

819

503

377

11

8St

ep 1

019

838

000

350

0015

000

043

810

197

1038

07

19

Step

11

203

380

0063

000

220

0005

288

920

186

389

09

3St

ep 1

224

988

000

570

0051

000

261

892

242

1745

75

218

Step

13

204

100

0055

000

340

0001

091

320

381

392

47

0St

ep 1

424

936

001

000

0008

000

081

946

246

9646

55

98

Step

15

198

570

0067

000

160

0003

910

00

197

4138

12

72

Plat

eau

age

over

70

6 o

f 39A

r rel

ease

377

54

1

















REFERENCES





































































MSWD 160




ble

4 A

r geo

chro

nolo

gy d

ata

for l

aser

ste

p-he

ated

sam

ples

of S

iljan

mel

t bre

ccia

s T

he s

teps

indi

cate

incr

easi

ng la

ser p

ower

thou

gh n

o te

mpe

ratu

re

mea

sure

men

ts w

ere

poss

ible

(err

ors

are

2 σ

and

incl

ude

J er

ror o

f 05

J

val

ues

as s

how

n in

Tab

le 3

)Si

ljan

4La

ser s

tep

40A

r39A

r38

Ar39

Ar

37A

r39A

r36

Ar39

Ar

39A

r (

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Ar

Age

(Ma)

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Step

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Step

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000

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Step

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570

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12

72

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f 39A

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54

1

















REFERENCES





























































ble

4 A

r geo

chro

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poss

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are

2 σ

and

incl

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ror o

f 05

J

val

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as s

how

n in

Tab

le 3

)Si

ljan

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ser s

tep

40A

r39A

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Ar

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r (

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Step

123

132

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26

Step

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248

000

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92

47

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53

22

Step

10

218

960

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621

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373

62

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121

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Step

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612

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No

plat

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Silja

n 5

Lase

r ste

p40

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3St

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11

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350

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203

380

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220

0005

288

920

186

389

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3St

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224

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000

570

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000

261

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242

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218

Step

13

204

100

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000

340

0001

091

320

381

392

47

0St

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001

000

0008

000

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946

246

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55

98

Step

15

198

570

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000

160

0003

910

00

197

4138

12

72

Plat

eau

age

over

70

6 o

f 39A

r rel

ease

377

54

1

















REFERENCES












































































REFERENCES




























































Laser argon dating of melt breccias from the Siljan …...Laser argon dating of melt breccias from...

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Transcript of Laser argon dating of melt breccias from the Siljan …...Laser argon dating of melt breccias from...