The problem of inherited 40Ar* in dating impact glass by the 40Ar/ 39Ar method: Evidence from the...

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Geochimica et Cosmochimica Acta 71 (2007) 1214–1231

The problem of inherited 40Ar* in dating impact glass by the 40Ar/39Armethod: Evidence from the Tswaing impact crater (South Africa)

F. Jourdan a,b,*, P.R. Renne a,b, W.U. Reimold c

a Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA94709, USAb Department of Earth and Planetary Science, University of California, Berkeley, CA94720, USA

c Museum f. Natural History (Mineralogy), Humboldt-University, Invalidenstrasse 43, 10115 Berlin, Germany

Received 3 July 2006; accepted in revised form 8 November 2006

Abstract

The Tswaing meteorite impact crater is a 1.13 km diameter structure located in the 2.05 Ga Nebo granite of the Bushveld Complex.The impact age had previously been determined by fission track dating to 220 ± 104 ka. 40Ar/39Ar step-heating and total fusion exper-iments performed on single- and multi-grain impact glass aliquots gave apparent ages ranging from 1.0 ± 0.3 Ma to 204 ± 6 Ma. These‘‘ages’’ indicate that the radiogenic Ar derived from the target rocks has not been completely degassed as a result of the impact process,despite fusion of the target material. Results of step-heating experiments imply that the 40Ar�inherited trapped within the glass is located intwo distinct reservoirs thought to be the glass matrix and fluid/vapor inclusions (or un-melted residual clasts). Calculations assuming anage of 0.2 ± 0.1 Ma for Tswaing (fission track data) reveal that the amount of inherited 40Ar*(40Ar�inherited) relative to the pre-impactconcentration varies from 0.015% to 4.15%. The spread defined by 40Ar�inherited likely reflects the various quench rates experienced bythe glass, most certainly due to the pre-impact position of the sample relative to the center of the crater. We compare the influenceof 40Ar�inherited on the apparent 40Ar/39Ar age determination of five impact structures. Our calculations show that the main characteristiccontrolling the age offset (for a given proportion of 40Ar�inherited) is the age difference between the impact and the target rocks (i.e., the40Ar* concentration in the target rock). The buffer effect for a given crater structure can be predicted knowing the age of the basementand having a rough estimation of the age of the crater structure itself. The occurrence of 40Ar�inherited is likely influenced by (1) the degreeof polymerization (i.e., silicate structure complexity) of the target rock and presumably related to the diffusivity of Ar in the melt andglass, (2) the Ar partial pressure at the grain boundary, (3) the quantity of energy involved in the impact, and (4) the porosity of the targetrocks. For glass that inevitably suffers inherited and/or excess 40Ar*, the use of the inverse isochron technique can be appropriate butshould be applied with careful statistical treatment.� 2006 Elsevier Inc. All rights reserved.

1. Introduction

Meteorite impact was the most fundamental process forplanet formation in the early solar system and for planetarysurface modification (Shoemaker, 1977). The impact fluxhas drastically decreased since the early history of theearth, but impacts have nevertheless played a crucial rolethroughout earth history. Impacts are sudden and cata-strophic events compared to most geological events thattake place on the time scale of millions of years. Major

0016-7037/$ - see front matter � 2006 Elsevier Inc. All rights reserved.

doi:10.1016/j.gca.2006.11.013

* Corresponding author. Fax: +1 510 644 9201.E-mail address: [email protected] (F. Jourdan).

impacts have the potential to alter the environment and cli-mate on the global scale and, as a consequence, to triggermajor mass extinctions (e.g., the K–T crisis and the Chic-xulub impact at 65 Ma; Hildebrand et al., 1991; Swisheret al., 1992). They have also been proposed to trigger tec-tono-magmatic activity such as formation of large igneousprovinces (e.g., Ingle and Coffin, 2004; Elkins-Tanton andHager, 2005) and are held responsible for the genesis ofmajor ore deposits such as in the Sudbury and Vredefortstructures (e.g., Grieve, 2005; Reimold et al., 2005). On amore modest scale, smaller impacts (<10 km) can triggerregional environmental disruption (Haines et al., 2004)and potentially local disruption of civilization (Chapman

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The problem of inherited 40Ar* in dating impact glass 1215

and Morrison, 1994). In more recent well-preserved struc-tures (including the Tswaing crater), the crater is generallyfilled with continental sediments that can provide informa-tion about the paleo-environmental evolution in a givenarea.

Currently, the global earth impact crater databaseincludes 176 craters (see http://www.unb.ca/passc/Impact-Database/index.html). Among them, only 25 craters haveages known with a precision better than 2%. Furthermore,it has recently been shown (e.g., Reimold et al., 2005) thateven apparently precise ages reported for impact structuresmay be wrong or only approximations of a true age.Improvement of the crater age database is crucial in (1) cor-relating cause and effects on the bio- and geosphere for thesecatastrophic processes, (2) better constraining the impactorflux through geological time (e.g., Grieve and Schoemaker,1994) and evaluation of potential impact periodicity (Alva-rez and Muller, 1984), (3) calibrating the absolute strati-graphic time scale (e.g., Deutsch and Scharer, 1994), and(4) calibrating the age of within-crater continental sedimen-tary deposits (e.g., for regional paleo-climatic analysis; Par-tridge, 1999). Thus, it is of crucial importance to understandthe behavior of the potential radiometric chronometers dur-ing these extremely brief events.

A very versatile and powerful chronometer is the40Ar/39Ar method, because of the availability of internalreliability criteria such as age plateaux and/or isochrons,as well as the possibility to obtain compositional parame-ters (i.e., Ca/K, K/Cl and 40Ar*). In principle, 40Ar* accu-mulated in the target rock since the closure of the system isreset by the impact process because of the extremely highenergy transmitted by the projectile. Previous studies ofshocked (unmelted) minerals from target rocks showedthat complete degassing by volume diffusion is notachieved (e.g., Stephan and Jessberger, 1992; Deutschand Scharer, 1994). Therefore, the most suitable materialsfor 40Ar/39Ar dating are impact-generated melts and glass(fragments in impact breccia, impact melt bodies or veins,and pseudotachylitic breccia), as they all result from melt-ing of target rock. The melting process provides a higherchance for complete degassing of inherited 40Ar*(40Ar�inherited).

Here, we investigate the age of a 1.13 km diameter qua-ternary impact crater emplaced in the �2.05 Ga Nebogranite of the Bushveld Complex. A previous fission-trackinvestigation (Storzer et al., 1999) yielded an age of220 ± 104 ka1 for the Tswaing (Pretoria Saltpan) crater.We attempted to date sub-millimeter, 3 wt%-K2O glassfragments using 40Ar/39Ar geochronology in order to im-prove the precision on the age of the Tswaing crater.Though we failed to obtain a geologically concordantand meaningful age, this study provides interesting possi-bilities for the investigation of the behavior of the Ar chro-nometer during the impact process.

1 All ages in this paper are given at 2r.

2. Geological setting

The Tswaing crater structure is a 1.13-km diameter, wellpreserved impact crater (Fig. 1). It is located in South Afri-ca, within the 2.05 Ga (Walraven et al., 1990) Nebo graniteof the Lebowa granite suite, of the Bushveld Complex.Extensive description has been given elsewhere (Reimoldet al., 1992, 1999) and will be only summarized here. TheTswaing crater is a sub-circular structure with rim eleva-tion of 119 m relative to the crater floor. The crater wasdrilled and shown to be filled with a 90 m-thick sequenceof quaternary lacustrine sediments (see Partridge, 1999)underlain by a 60 m-thick unit of unconsolidated, granitic,suevitic (i.e., melt fragment bearing) breccia above frac-tured and locally brecciated Nebo granite. The sedimentarysequence has been used for paleoenvironmental (Partridge,1999) and hydrological (McCaffrey and Harris, 1996)reconstructions.

The suevitic breccia unit contains mineral particlesexhibiting evidence of shock metamorphism including (1)shocked quartz, plagioclase and K-feldspar with planardeformation features (PDFs), and (2) partially or com-pletely (diaplectic glass) isotropized quartz and feldspargrains. Melt fragments have been recovered from the sandybreccia and show a variety of shapes (varying from brokenshards to spherules) and colors (from light brown to black).The glass clasts are, on average, close to Nebo granite com-position, though slightly enriched in mainly Mg, Fe, Cr,Co, Ni and Ir and somewhat depleted in Ca and alkalic ele-ments (Reimold et al., 1992, 1999). The 187Os/188Os ratio ofthe breccia (0.205) is close to the meteorite array and dis-tinct from the Nebo granite (0.72; Koeberl et al., 1994).Some pyritic spherules, either pure sulfide or sulfide enclos-ing silicate cores, have also been identified. Koeberl et al.(1994) proposed, on the basis of geochemical comparisonbetween the impact glass and the granite, that the projectilemay have been of CI–chondrite or enstatite–chondritecomposition.

2.1. Previous geochronology

The age of the Tswaing crater has been approachedby several methods. Fission track, K/Ar and Rb/Sr agesobtained on various intrusive rocks found at the craterrim mostly ranged from ca. 1.3 to 1.6 Ga (e.g., Miltonand Naeser, 1971 and Brandt et al., 1996) and indicatethat the target was affected by the ca. 1.2 Ga regionalPienaars River magmatic phase (Brandt and Reimold,1999). These ages are clearly at odds with the excellentpreservation state of the crater. 14C dating has been per-formed on algal debris recovered from the upper 20 m ofthe lacustrine sediments. By extrapolation of the sedi-mentation rate an approximate age of 200 ka was derived(Partridge et al., 1993; Partridge, 1999). Fission-trackdating was performed on 456 glass fragments recoveredfrom the suevitic breccia unit (Storzer et al., 1999) andyielded a relatively imprecise mean age of 220 ± 104 ka,

http://www.unb.ca/passc/ImpactDatabase/index.html


Spherules

Dark grains

Brown grains

Alkaline intrusions

Lebowa Granite Suite

Rashoop Granophyre Suite

Rustenburg Layered Suite

Rooiberg Group

Transvaal Supergroup

N 100 Km

26˚ 28˚ 30˚

26˚

25˚

Pretoria

Johannesburg

Warmbaths

Rustenburg

Africa

TSWAINGCRATER

Fig. 1. Simplified geological map of the Bushveld complex, South Africa and approximate location of the Tswaing impact crater. Modified afterBuchanan et al. (2004).

1216 F. Jourdan et al. 71 (2007) 1214–1231

however in good agreement with the age estimated fromthe sedimentation rate provided by the 14C age. Compar-ison with the morphology of the ca. 50 ka-old MeteorCrater (Arizona) shows that Tswaing has suffered com-paratively significant degradation, with reduction of rimelevation (measured from rim top to crater floor) andbroadening of the eroded crater rim. Nevertheless, theTswaing crater morphology is still prominent and sugges-tive of a young Holocene age. A 50% error on the pre-vious best age estimate, however, warrants a furtherattempt to improve on this.

1mm

Fig. 2. Selected glass particles classified as dark and brown fragments andsmooth spherules. Scale bar is 1 mm. (For interpretation of colormentioned in this figure the reader is referred to the web version of thearticle.)

3. Sample descriptions and analytical methods

We investigated �0.5 to 1.5 millimeter sized glass parti-cles from the suevitic breccia package with relatively highK2O content (average = 3.6 ± 0.2 wt%; compare Reimoldet al., 1999) making them suitable for 40Ar/39Ar dating.The handpicked glass samples were recovered from 90 mdepth in the borehole section. The shapes of the grainsrange from broken shards to sub-spheres (Fig. 2). A previ-ous description of a thin section of a melt fragment re-vealed the presence of vesicles and mineral clasts(Reimold et al., 1999). About one-third of the samplesdid not require acid treatment because of their excellentfreshness and complete freedom from adhering particles.The other two-thirds showed variable amounts of whitish,superficial alteration products (probably due to post-im-pact hydrothermal circulation) and, consequently, wereleached in diluted (2 N) HF for 10 minutes and then thor-

oughly rinsed in distilled water. We then carefully selectedthe freshest grains under a binocular microscope. Glassparticles were classified (and analyzed) according to theircolor and shape. We distinguished clean (no acid treat-ment) and HF leached samples. In each of these two cate-gories, we classified the glass as dark and brown fragments,respectively, and as dark spherules.


One irradiation of 30 min duration was performed in theCd-shielded (to minimize undesirable isotopic interferencereactions) CLICIT facility of the TRIGA reactor at Ore-gon State University, USA. Samples were loaded into 2wells within one aluminum disc of 1.9 cm diameter and0.3 cm depth. Alder Creek sanidine (ACs-2) was used asneutron fluence monitor and was loaded into the samepit as the samples. We calculated J-values relative to anage of ACs-2 of 1.193 Ma (Nomade et al., 2005) and usingthe decay constants of Steiger and Jager (1977). The J-val-ues are given in electronic annex as weighted means andstandard deviations of J-values for the sample wells. Thecorrection factors for interfering isotopes correspond tothe weighted mean of 10 years of measurements of K–Feand CaSi2 glasses and CaF2 fluorite in the TRIGA reactor.They are (39Ar/37Ar)Ca = (7.60 ± 0.09) · 10�4, (36Ar/37Ar)Ca = (2.70 ± 0.02) · 10�4 and (40Ar/39Ar)K = (7.30 ±0.90) · 10�4.

40Ar/39Ar analyses were performed at the BerkeleyGeochronology Center. Either single grain or multi-grainsamples were degassed using a CO2 laser with focused orbeam-integrator lens, respectively. The gas was purified ina stainless steel extraction line using two C-50 getters anda cryogenic condensation trap. Argon isotopes were mea-sured in static mode using a MAP 215-50 mass spectrome-ter with a Balzers electron multiplier, mostly using 10 cyclesof peak-hopping. A more complete description of the massspectrometer and extraction line is given by Renne et al.(1998). Blank measurements were generally obtained afterevery three sample runs. Mass discrimination was moni-tored several times a day (every 9 steps) and provided amean value of 1.00653 ± 0.00298 per atomic mass unit.Ar isotopic data corrected for blank, mass discriminationand radioactive decay are given in electronic annex.Individual errors in electronic annex are given at the 1rlevel.

Our criteria for the determination of plateau andmini-plateau ages are as follows: plateaus and mini-plateaus must include at least 70% and 50% of 39Ar re-leased, respectively, distributed over a minimum of 3consecutive steps and satisfying a probability of fit ofat least 0.05. In this study they are both given the samevalidity. Plateau and mini-plateau ages are given at the2r level and are calculated using the mean of all the pla-teau steps, each weighted by the inverse variance of theirindividual analytical error. Integrated ages (2r) are calcu-lated using the total gas released for each Ar isotope.Inverse isochrons include the maximum number of con-secutive steps for which a probability of fit P0.05 wasobtained (see also discussion hereafter). The uncertaintieson the 40Ar*/39Ar ratios of the monitors are included inthe calculation of the integrated and plateau age uncer-tainties, but not the errors on the age of the monitorand on the decay constants (internal errors only, seediscussion in Min et al., 2000). 40Ar/39Ar results areshown in Table 1 and Electronic annex Table 2 and Figs.3 and 4.

4. Results

4.1. Single grain total fusion analysis

Single-grain total fusion analysis was performed on 14glass particles. Results show a large range of apparent ages,mostly between 0.98 ± 0.34 and 32.55 ± 0.23 Ma, but withtwo distinctly older ages of 123.1 ± 1.7 and142.1 ± 1.8 Ma.

4.2. Step-heating analysis

Step-heated single grains (8 analyses; Fig. 4a) yieldedvariable integrated ages ranging from 6.3 ± 1.0 to204 ± 6 Ma. Three grains gave mini-plateau ages of4 ± 2 Ma (#58075-13), 5 ± 1 Ma (#58075-10) and50.2 ± 1.8 Ma (#58075-15). Multi-grain populations (7analyses; Fig. 7b) were initially analyzed to ensure highAr ion beam and to achieve more precise ages. They yield-ed integrated ages ranging from 16.2 ± 1.6 to24.8 ± 0.7 Ma. We obtained one weighted-mean (36% of39Ar released) age at 10.0 ± 0.3 Ma (#58075-20; Table 1and Electronic annex).

Single- and multi-grain analyses show strongly per-turbed age spectra even for those samples that yieldedmini-plateau ages. A striking feature common to most ofthe samples is the sudden increase of the age over a singlestep suggesting degassing of distinct Ar reservoirs (analysis#58-075-13; 58075-18; 58075-19; 58075-20; 58075-21;58075-22; Fig. 4). Step-heated samples show Ar degassingcurves (relative argon released as a function of increaseof laser power (Ar/dE) against the laser power; not shown)ranging from single- to multi-peak spectra, with no obviousrelationship between spectrum type and sample category orage spectrum behavior.

The Ca/K (derived from the 37ArCa/39ArK) ratios of theglass aliquots mostly vary between 0.01 and 0.2. Apparentages do not show a correlation with the Ca/K ratios but doco-vary with the relative proportion of atmospheric 40Arreleased from the samples.

4.3. Isochron results

All analyses have been plotted on an ‘‘inverse’’ isochroncorrelation diagram (i.e., 36Ar/40Ar vs. 39Ar/40Ar; Fig. 5).Isochrons have been proven a useful technique to obtainages when homogeneous excess or inherited 40Ar* istrapped in a sample (e.g., Roddick, 1978; Heizler and Har-rison, 1988, and discussion hereafter). Isochron age resultsfor these Tswaing data range from 5 to 43 Ma, are general-ly poorly defined, and show huge uncertainties (from ±1.5to ±400) on the non-radiogenic 40Ar/36Ar intercept. Agesderived from the isochron rarely agree with the respectiveintegrated ages. 40Ar/36Ar ratio intercepts suggest largeamounts of 40Ar�inherited, with most values ranging from300 ± 2 to 1700 ± 400, in comparison to the atmosphericratio of 295.5 (Nier, 1950). All the samples yielded data

Table 1Summary table indicating integrated, plateau/mini-plateau and isochron ages for the Tswaing impact glass samples

Sample no. Description Leaching Numberof grain

Integratedage(Ma, ±2r)

Plateau/mini-plateau* age(Ma, ±2r)

Total 39Arreleased(%)

MSWD(plateau)

Isochron age(Ma, ±2r)

n

(isochron)

40Ar/36Arintercept(±1r)

MSWD(isochron)

F ð40Ar�inheritedÞnot corrected(%)

F ð40Ar�inheritedÞcorrected from Kvaporisation (%)

Total fusion age

58074–05 Brownshard

N 1 0.98 ± 0.34 — — — — — — — 0.02 0.015


N 1 2.05 ± 0.51 — — — — — — — 0.05 0.04


N 1 2.18 ± 0.54 — — — — — — — 0.05 0.04


N 1 9.45 ± 0.19 — — — — — — — 0.24 0.18

58075–02 Darkshard

N 1 11.14 ± 0.19 — — — — — — — 0.29 0.21


Y 1 12.75 ± 0.44 — — — — — — — 0.33 0.24


Y 1 14.30 ± 0.19 — — — — — — — 0.37 0.27


Y 1 17.65 ± 0.29 — — — — — — — 0.46 0.34


Y 1 27.85 ± 0.64 — — — — — — — 0.73 0.54


Y 1 27.97 ± 0.48 — — — — — — — 0.73 0.54


Y 1 29.40 ± 0.52 — — — — — — — 0.77 0.57


Y 1 32.55 ± 0.33 — — — — — — — 0.85 0.63


Y 1 123.1 ± 1.7 — — — — — — — 3.33 2.45


Y 1 142.1 ± 1.8 — — — — — — — 3.87 2.84

Step-heating (single grain)


Y 1 8 ± 10 4 ± 2 100 0.97 — — — — 0.10 0.07


Y 1 6.3 ± 1 5 ± 1* 56 0.85 9.1 ± 1.3 24 250 ± 30 1.4 0.23 0.17


Y 1 13.6 ± 1.2 — — — 5 ± 2 5 350 ± 40 1.2 0.13 0.09


Y 1 28.3 ± 1.5 — — — 33 ± 5 8 940 ± 40 1.1 0.87 0.64


Y 1 40.2 ± 1.6 — — — 5.3 ± 1.9 7 860 ± 30 0.38 0.13 0.10


Y 1 63 ± 2 50.2 ± 1.8* 53 0.27 43.1 ± 0.9 6 430 ± 5 0.48 1.14 0.84


Y 1 92 ± 3 — — — — — — — 2.47 1.81


Y 1 204 ± 6 — — — — — — — 5.65 4.15

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Step-heating (multi-grains)

0

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2

3

4

5

6

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Ap

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(Ma)

b

Fig. 3. (a) Age vs. rank for Tswaing glass fragments (n = 29). Black circle:total fusion age on single grain; white circle: step-heating age of singlegrains and gray circle: step-heating age of multi-grains aliquot. (b)Zoomed area between 0 and 8 Ma.


that define clusters rather than extending along a regressionline; thus, the isochron results will be referred as pseudo-isochrons. Yet, ages obtained from pseudo-isochrons cannot be considered valid. If all the data are plotted into a‘‘global’’ isochron, a huge number of analyses are excluded(275 of 298 data) from the calculation due to the large scat-ter of the data. The remaining 23 data define a pseudo-iso-chron with an apparent age of 4.3 ± 0.7 Ma(MSWD = 1.5; p = 0.06) and an 40Ar/36Ar intercept of319 ± 9. This pseudo-isochron is clearly geologically andstatistically (because of the high number of analysesexcluded) meaningless.

The failure of isochrons to define a consistent age forglasses is not surprising. The definition of an inverse iso-chron (binary mixing line) requires two distinct reservoirswith distinct Ar release kinetics. Glasses, lacking differentcrystal-chemical environments arising from an orderlylattice, presumably host both inherited and radiogenic Arin energetically equivalent atomic environments. Thus,isochrons for glasses should only be feasible if the inheritedargon is trapped in lacunae. We return to this topic in a lat-er Section 5.4.

5. Discussion

5.1. Age of the Tswaing crater

The 40Ar/39Ar results from the various Tswaing meltfragments and spherules are strongly heterogeneous,

0

40

80

58075-10

-10

0

10

20

30

40

5.0 ± 1.0 Ma (MSWD = 0.85, p = 0.58)

Integrated Age = 6.3 ± 1.0 Ma

Age

(Ma)

Ca/

K%

atm

Ar

0

40

80

58075-12

0

0.1

0.2

0

50

100

150

200

250

300

350

400

Integrated Age = 204 ± 6 Ma

Age

(Ma)

Ca/

K%

atm

Ar

0

40

80

58075-13

0

0.1

0.2

0 10 20 30 40 50 60 70 80 90 100

0

20

40

60

80

100


Age

(Ma)

Ca/

K%

atm

Ar

Cumulative % 39Ar Released

0

40

80

58075-14

-10

0

10

20

30

40

50


Age

(Ma)

Ca/

K%

atm

Ar

0

40

80 58075-15

0

0.1

0.2

0

20

40

60

80

100

120

140

50.2 ± 1.8 Ma(MSWD = 0.27, p = 0.90)


Age

(Ma)

Ca/

K%

atm

Ar

0

40

8058075-16

0

0.1

0.2

0

50

100

150

200

250

300


Age

(Ma)

Ca/

K%

atm

Ar

0

40

8058075-17

0

0.1

0.2

0 10 20 30 40 50 60 70 80 90 100

0

20

40

60

80

100


Age

(Ma)

Ca/

K%

atm

Ar


0

40

80

58075-11

-10

0

10

20

30

404 ± 2 Ma (MSWD = 0.97, p = 0.48)


Age

(Ma)

Ca/

K%

atm

Ar

-1

0

1

2

0

1

2

-1

-1

0

1

2

a

Fig. 4. 40Ar/39Ar apparent age, Ca/K ratios, and percentage of atmospheric 40Ar spectra. (a) Step-heating spectra of multi-grain aliquots; (b) step-heatingspectra of single grain. Note the ‘‘burst’’ reflecting the degassing of isolated reservoir of 40Ar�inherited. Integrated- and plateau-ages are provided at the 2rconfidence level.

1220 F. Jourdan et al. 71 (2007) 1214–1231

0

40

80 58074-06

0

0.1

0.2

0 10 20 30 40 50 60 70 80 90 100-10

0

10

20

30

40


Age

(Ma)

Ca/

K%

atm

Ar


0

40

80

58075-19

0

0.1

0.2

-10

0

10

20

30

40

50

60


Age

(Ma)

Ca/

K%

atm

Ar

0

40

80

58075-20

0

0.1

0.2

0

20

40

60

80

100

10.0 ± 0.3 Ma(MSWD = 1.73, p = 0.05)


Age

(Ma)

Ca/

K%

atm

Ar

0

40

80 58075-21

0

0.1

0.2

0 10 20 30 40 50 60 70 80 90 100

0

20

40

60

80

100


Age

(Ma)

Ca/

K%

atm

Ar

Cumulative % 39 Ar Released

0

40

80 58075-23

0

0.1

0.2

0

20

40

60

80

100


Age

(Ma)

Ca/

K%

atm

Ar

0

40

80 58075-22

0

0.1

0.2

0

20

40

60

80

100


Age

(Ma)

Ca/

K%

atm

Ar

0

40

80

58075-18

0

0.1

0.2

-10

0

10

20

30

40

50

60


Age

(Ma)

Ca/

K%

atm

Ar

b

Fig. 4 (continued)


varying from 1 to 204 Ma (Fig. 3). The oldest results aresystematically associated with 40Ar/36Ar intercepts thatare significantly higher than the atmospheric value, asshown by the isochron calculation (electronic annex).

Although the isochrons are not considered significant dueto their excess scatter, these results qualitatively indicatethat the melted rocks have not been satisfactorily resetand liberated of their 40Ar�inherited.

0 0.004 0.008 0.012 0.016 0.020 0.0240

0.0005

0.0010

0.0015

0.0020

0.0025

0.0030

0.0035

0.0040

Age = 9.1 ± 1.3 Ma40Ar/

36Ar Int. = 250 ± 30

MSWD = 1.4, P = 0.08, n = 24

58075-10

39 40Ar/ Ar

3640

Ar/

Ar

Fig. 5. Inverse correlation isochron plot of 36Ar/40Ar vs. 39Ar/40Ar for a selected (and representative) step-heated sample.

1222 F. Jourdan et al. 71 (2007) 1214–1231

In the absence of other evidence, our best estimate ofthe Tswaing impact age would have to be the single graintotal fusion age of 0.98 ± 0.34 Ma because of its youngerage, probably less affected by 40Ar�inherited. However, thedata represent only a single analysis and do not allowconfident interpretation of this ‘‘age.’’ In addition, thisdate is significantly older than the age of �200 ka ob-tained by both extrapolation of the sedimentation rate(calibrated with 14C age data) and fission track dating,thus indicating that even this sample could be affectedby presence of some inherited Ar. Clearly, the 40Ar/39Artechnique failed to provide a reliable age for the Tswaingimpact crater. In our opinion, the best absolute age ob-tained so far for the crater is therefore the fission trackage of 200 ± 104 ka (Storzer et al., 1999), although itshould be mentioned that this technique may be sensitiveto alteration processes (i.e., give a spuriously young age;e.g., Deutsch and Scharer, 1994).

5.2. Un-degassed target rocks

Several studies have shown that only a very smallfraction of the impact products is likely to have experi-enced significant loss of its 40Ar�inherited (e.g., Bogardet al., 1988, 1995). Impact melt and impact-generatedmelt phases (i.e., tektites, glass fragments, impact meltbodies, and pseudotachylitic breccia) are considered thebest material for 40Ar/39Ar dating of impact structures(Deutsch and Scharer, 1994), essentially because theyare the results of melting and related degassing of targetrock. Shock compression in the central part of the im-pact structure results in pressure extremes in excess of60 GPa (e.g., Koeberl et al., 1994), and upon decompres-sion leads to melting of minerals and even bulk rock,

thereby achieving transgression of Ar closure tempera-tures. These extreme conditions could entirely reset theAr chronometer (i.e., degas the 40Ar* previously accumu-lated in the target rock) if sustained for a sufficientlylong time. However, the short time sale of these process-es violates the precepts of the closure temperature con-cept (Dodson, 1979) and is clearly not applicable forthe Tswaing melt fragments.

We can estimate the amount (F) of 40Ar* inherited fromthe target rock relative to the total 40Ar* accumulated inthe Nebo granite (40Ar�ðTRÞ) before the impact. This rela-tionship is given by the following equation:

F ð40Ar�inheritedÞ ¼40Ar�inherited

40Ar�ðTRiÞð1Þ

¼ ð40Ar�Þm � ð40Ar�Þcð40Ar�ÞTRtot

� ð40Ar�Þcð2Þ

If we consider the K composition as identical in the targetrock and the glass, then Eq. (2) can be approximated by:

F ð40Ar�inheritedÞ �40Ar�

40K

� �m� 40Ar�

40K

� �c

40Ar�40K

� �TRtot� 40Ar�

40K

� �c

ð3Þ

As

40Ar�

40K¼ ke

k� e k�tð Þ � 1� �

ð4Þ

then Eq. (3) can be simplified to:

F ð40Ar�inheritedÞ �eðk�tðmÞÞ � eðk�tðCÞÞ

eðk�tðTRtotÞÞ � eðk�tðCÞÞð5Þ

where (40Ar*)m and (t)m are, respectively, the radiogenic ar-gon (40Ar*) or the apparent age (t) measured in the glass.

0.01

0.1

1

10

100

1000

0 1000 2000 3000

Age of the basement (Ma)

Ag

em

easu

red

(Ma)

0.01

0.1

1

10

100

0% 0.2% 0.4% 0.6% 0.8% 1.0%

% inherited Ar*40

Ag

e m

easu

red

(M

a)

Tswaing emp. age

Tswaing emp. age

0.015%

4.15%

Impactglass

Fig. 6. (a) Graph showing the influence of the age of the target rocks onthe measured age for 0.015% (black line) and 4.15% (dashed line) of40Ar�inherited. (b) Graph showing the influence of the percentage of40Ar�inherited on the measured age for target rocks with an age of 2.05 Ga.Curves are calculated using Eq. (1). Gray area represents the fission trackage for the Tswaing crater (Storzer et al., 1999).


Similar notation is used where c stands for the theoretical(calculated) radiogenic argon or age of the glass. TRi

stands for target rock at the time of the impact. TRtot istarget rock at present. k and ke are, respectively, the totaldecay constant and the electron capture partial decay con-stant of 40K, respectively.

In Eq. (3) we assume that (1) no initial excess 40Ar*was present in the target rock. (2) no 40Ar�inherited is trans-mitted from the �4.5 Ga-old meteorite projectile, (3) theK composition (and thus the 39ArK) of the target rocksand impact melt is identical, and (4) no K has been lostby vaporization or fractionated isotopically as a conse-quence of the impact. (1) can be tested by checking theAr isotopic composition of the target rock, but may behard to identify if the excess 40Ar* resides in non-datableminerals such as quartz (e.g., Baxter, 2003), unless per-forming whole rock Ar analysis. If excess Ar is present,then it can exacerbate the effect of 40Ar�inherited; i.e., makethe apparent age of the target rock older. In this case;when assuming no excess Ar, our equation provides onlya minimum F ð40Ar�inheritedÞ value. Similarly, a maximumvalue would be produced in the case of Ar loss. Theassumption of K-conservation (3) is not exactly correctas the composition of the Nebo granite target rock(K2O = 4.9 wt%) is noticeably different from the composi-tion of the Tswaing impact glass (K2O = 3.6 wt%; e.g.,Reimold et al., 1999), likely due to K vaporization. Sim-ilarly, the K content will not be conserved in the case ofpartial melting of the target rock (i.e., in contrast to bulkmelting). Therefore, although the error introduced in thecalculation do not significantly alter our estimation ofF(40Ar�inherited), the calculated values represent only upperlimit estimates. A more accurate solution takes into ac-count the non-conservation of K and is given by:

F ð40Ar�inheritedÞ ¼eðk�tðmÞÞ � eðk�tðCÞÞ

ðeðk�tðTRtotÞÞ � 1Þ � ðK2OÞTR

ðK2OÞgls� ðeðk�tðCÞÞ � 1Þ

ð6Þ

Which requires the K2O concentration of both the targetrock and the glass (gls) to be known.

Similarly, some isotopic fractionation of K is expectedduring vaporization (Humayun and Clayton, 1995) butthe effects are negligible for present purposes. For the cal-culation, we adopt an age of 220 ka for the Tswaing crater(Storzer et al., 1999) and 2.05 Ga for the Nebo granite(Walraven et al., 1990).

F results (obtained using Eq. 6) are listed in Table 1 andrange from 0.015% (1 Ma) to 4.15% (204 Ma) 40Ar�inherited.Particularly striking is the strong age-bias induced by only0.015% of 40Ar�inherited retained. Fig. 6 shows the variationof the ages measured according to the age of the target rocksfor 0.015% and 4.15% of 40Ar�inherited (Fig. 6a) and relative tothe percentage of 40Ar�inherited (Fig. 6b). Fig. 6a demonstratesthat the influence of the 40Ar�inherited on the age is stronglydependent on the age of the target rock, especially whenthe contamination is significant. For an age of 2.05 Ga forthe target rocks (i.e., Nebo granite), the Ar clock is very sen-

sitive to 40Ar�inherited, as even a tiny contamination of 0.004%multiplies the apparent age of the crater by two (Fig. 6b).

The large range of F(40Ar�inherited) values displayed by theTswaing impact glass (0.015–4.15%) implies that degassingof the melt was heterogeneous. Various parameters mayinfluence the Ar diffusivity efficiency during the thermalhistory of the samples. Maybe the most significant onesare heterogeneous melt temperatures and time–tempera-ture histories experienced by the melt/glass fragments.Temperature/pressure conditions are likely to show a sig-nificant lateral gradient during the impact event, with thehighest temperature–pressure conditions occurring nearthe center of the crater (e.g., Bogard et al., 1988). This isin agreement with recent hydrocode-based simulation ofthe post-impact temperature front gradient computed forsmall-sized craters (Ormo and Lepinette, 2006). The meanAr diffusion coefficient at a given temperature can also bestrongly controlled by the chemical composition/viscosityof the melt (e.g., Nowak et al., 2004, and discussion here-after). Finally, length of residence in hot impact ejectacan result in effective degassing—a process that clearlywas not important in the case of an impact structure assmall as Tswaing where relatively little melt is producedand a correspondingly thin and quickly cooled ejecta layeris formed. In the case of the Tswaing crater, chemical het-erogeneity does not seem to be responsible for the observed40Ar�inherited heterogeneity for two reasons: (1) the colorvariation of the glass, which can be considered as a good

1224 F. Jourdan et al. 71 (2007) 1214–1231

proxy for their composition (e.g., Culler et al., 2000), doesnot show any correlation with the F ð40Ar�inheritedÞ (Table 1),and (2) melted fragments actually analyzed show a verynarrow range of composition (Reimold et al., 1999), pre-sumably due to the chemically isotropic nature of the Nebogranite and little admixture from other target rockcomponents.

The location and nature of the 40Ar reservoirs in the glassseem to be multiple. The significant fraction of atmospheric36Ar (and associated 40Ar) homogeneously distributed inthe glass can be due either to excess Ar already present inthe target rock or to incorporation of atmospheric Ar atatmospheric overpressure during the melting stage. Accord-ing to shock experiments (Bogard et al., 1986; Wiens andPepin, 1988), a shock-induced melt incorporates a strongatmospheric component with little or no isotopic fraction-ation. Therefore, it is quite likely that the large amount of36Ar observed in the Tswaing glass has an atmospheric ori-gin. However, we do not exclude that some excess 40Ar*may have been originally present in the target rock. Theage spectra display generally a roughly flat age-‘‘baseline’’,which is, however, punctuated by several single older-age‘‘bursts’’ (Fig. 4). Close inspection shows that the age-base-lines generally display old ages throughout the Ar spectra(e.g., samples # 58075-19 and 58075-15) arguing for a moreor less uniform distribution of the 40Ar�inherited in the glassmatrix compared to the bursts (but not uniform enoughto yield isochron ages). The age spectra do not show alog-shaped Fickian diffusion profile typically found in par-tially reset minerals (e.g., Stephan and Jessberger, 1992).More striking is the abrupt age increase mostly occurringover one step. This reservoir can be represented by eithermineral clasts not completely melted and with extremelynarrow activation energy, or more likely by fluid/vaporinclusions entrapped in vesicles (Wiens, 1988) that candecrepitate in sudden ‘‘burst’’ when the heating temperatureis high enough. If the fluid/vapor inclusion explanation iscorrect, then the ‘‘burst’’ problem can be theoretically over-come by crushing of the samples before step-heating. Thishas been successfully accomplished in many thermochrono-logical studies involving metamorphic minerals with signif-icant excess 40Ar* trapped in fluid-inclusions (e.g., Qiu andWijbrans, 2006). Another tool from the 40Ar/39Ar tech-nique repertory is the ultraviolet laserprobe (e.g., Kelley,2002) that would allow avoiding tapping the inclusionsdue to the high spatial resolution of the laser beam duringgas extraction. However, in the case of the Tswaing glass,the high F ð40Ar�inheritedÞ heterogeneously trapped in the ma-trix (the age-‘‘baseline’’) would most certainly render bothoperations inappropriate.

5.3. On the cause and influence of inherited 40Ar*

5.3.1. Comparison with other impact structures

In addition to the Tswaing crater, we chose to studythe theoretical influence of 40Ar�inherited and degassing prop-erty of melt samples from four impact structures of vari-

ous ages and basement ages and lithologies. The fourstructures are: (1) the 18 km-diameter El’gygytgyn crater(Russia) which is a well preserved structure that has beendated at 3.58 ± 0.04 Ma (Layer, 2000). The target rocksare of Cretaceous (89–65 Ma; in Layer, 2000) age andare mostly dense rhyo-dacitic lava-flows (Gurov et al.,2005); (2) the notorious �180 km diameter (Morganet al., 2006) Chicxulub crater (Yucatan) dated at65.0 ± 0.1 Ma (e.g., Swisher et al., 1992) and resultingfrom impact into Pan-African and Cretaceous sedimenta-ry and crystalline rocks (e.g., Tuchscherer et al., 2005); (3)the �25 km diameter Rochechouart crater (France) datedat 214 ± 8 Ma and located in upper Paleozoic (�260 Ma)igneous and metamorphic rocks (Kelley and Spray, 1997);and (4) the Strangways impact structure (Australia; 9-11 km diameter) dated at 646 ± 42 Ma and located inMezoproterozoic quartzite–shale–siltstone–sandstonebasement (�1450 Ma; Spray et al., 1999). The age ob-tained on the Rochechouart crater is different in the sensethat it has been measured on pseudotachylitic breccia in-stead of impact melt (although it is not impossible that insome impact structures where such breccias have been de-scribed), these materials could actually represent shockmelt (e.g., Reimold, 1998; Reimold and Gibson, 2005).Pseudotachylitic breccia samples that represent frictionmelt may be more prone to include unmelted/undegassedmineral clasts compared to impact melt/glass. The Roc-hechouart age has been measured using a high-spatial res-olution ultraviolet laser probe on glassy matrix, thusavoiding heating unmelted clasts (Kelley and Spray,1997). Therefore, the 40Ar�inherited will be modeled as entire-ly provided by the glass matrix (i.e., equivalent to thebulk impact melts of the other crater structures).

A rearranged Eq. (5) has been used to investigate theinfluence of (1) the age of the basement, (2) the age differ-ence between the impact event and the target rocks, and (3)the amount of inherited 40Ar* on the 40Ar/39Ar age mea-sured on melt samples. We use Eq. (5) as the compositionof the target rocks and/or the impact glass analyzed sam-ples are not necessarily available, preventing the use ofthe more accurate Eq. (6). However, as stated before, themodification of the calculated estimates is negligible forthe purpose of this exercise and particularly with regardto the large range of values covered by the five crater struc-tures. Henceforth, we distinguish between the age mea-sured relative to the age of the crater (t(m)/t(C)) and theabsolute age difference (in Myr) between the age measuredand the age of the crater (t(m) � t(C)).

For a given F(40Ar�inherited), Fig. 7 suggests that the abso-lute age-bias, t(m) � t(C), is dependent on the age of the tar-get rocks and, in particular, the time span between thetarget rock emplacement and the cratering event, but thatit is dependent neither on the age of the crater itself noron the K content (as suggested by the superimposition ofthe five curves in Fig. 7a). However, it is apparent that a1 Ma offset does not have the same significance forQuaternary impacts such as Tswaing as it would have for

0

2

4

6

8

10A

ge

mea

sure

d -

age

crat

er (

Ma)

0.1% Ar*40inherited

0

1

2

3

4

5

0 1000 2000 3000

Ag

e m

easu

red

/ ag

e cr

ater

0.1% Ar*40inherited

Tswaing (0.2Ma)

El'gygytgyn (3.58 Ma)

Chicxulub (65 Ma)

Rochechouart (214 Ma)

Strangways (646 Ma)

t(TR)- t(c) (Ma)

b

Fig. 7. Model curves for five impact structures (Tswaing, El’gygytgyn,Chicxulub, Rochechouart and Strangways structures) having differentages and target rock ages. (a) Plot showing the absolute age differencebetween the measured age and age of the crater (Ma) vs. the age differencebetween the target rock and the impact age for 0.1% of 40Ar�inherited,showing that the absolute age bias is not dependent of the age of the craterand K content of the target rock. (b) Relative age difference between themeasured age and age of the crater structure vs. the age of the targetbasement for 0.1% of 40Ar�inherited, showing the strong effect of 40Ar�inherited

on young glass.

0.1

1

10

100

1000

0% 2% 4% 6% 8% 10%

% inherited Ar*40

Ag

e m

easu

red

- a

ge

crat

er (

Ma)

2050 Ma

804 Ma

72 Ma

56 Ma

5 Ma?

0

1

10

100

1000

10000

0% 2% 4% 6% 8% 10%

% inherited Ar*40

Ag

e m

easu

red

/ ag

e cr

ater

Tswaing (0.2Ma)

El'gygytgyn (3.58 Ma)

Chicxulub (65 Ma)

Rochechouart (214 Ma)

Strangways (646 Ma)

804 Ma56 Ma

5 Ma?

2050 Ma

72 Ma

b

Fig. 8. Plots showing the effect of the proportion of 40Ar�inherited on the agemeasured for 5 craters (cf. text). Numbers on the curves indicate the agedifferences between the target rocks and the impact structure. Note thecorrelation between the age difference between the crater structure and thetarget rocks and the buffering ability of the glass with respect to40Ar�inherited.


Proterozoic craters. This is particularly striking when the 5craters are plotted in a t(m)/t(C) vs. t(TR) � t(c) graph (Fig. 7b).

We tested the influence of 40Ar�inherited on t(m) � t(C) andt(m)/t(C) according to the various ages of the 5 crater struc-tures relative to their respective target rocks. It is apparentfrom Fig. 8 that the main characteristic influencing the40Ar�inherited-induced age bias is again the age difference be-tween the target rocks and the impact event. For example,the Chixculub tektites can ‘‘afford’’ to contain 2%40Ar�inherited, as this changes the measured age by only0.1 Ma (0.15%). In contrast, the same amount would shiftthe age of the Strangways crater by 20 Ma (3%). This canbe intuitively understood, as the concentration of 40Ar*accumulated in the target rock is proportional to its age.For example, 2% of 1. · 10�12 mol/g of 40Ar* expectedfor old target rocks will not have the same significance as2% of 1.0 · 10�14 mol/g for young target rocks. An impor-tant consequence is that a given amount of 40Ar�inherited doesnot equally affect the 40Ar/39Ar age measured on a meltedrock, according to the crater-basement age difference (i.e.,40Ar* initial concentration).

5.3.2. Influence of the composition of the melt on Ar diffusion

As previously mentioned, the composition and viscosityof the target rocks both play an important role in the Ar dif-fusion in a melt specimen (Nowak et al., 2004). Most diffu-sion experiments are based on solid phases (e.g.,

McDougall and Harrison, 1999). For instance, shock-in-duced Ar loss by volume diffusion has been shown to be moreefficient for K-feldspar than for biotite, as exemplified byminerals from polymict impact breccia from the Haughtonstructure, Canada (Stephan and Jessberger, 1992). However,empirically, K/Ar laboratory experiments have demonstrat-ed that it is much more difficult to extract all the 40Ar* fromviscous sanidine melt compared to melt of mafic minerals(Webb and McDougall, 1967). More recent experimentsregarding Ar diffusion were performed by Nowak et al.(2004) on iron free synthetic melt with compositions rangingfrom hawaiite (i.e., similar to alkali basalt) to rhyolite. Re-sults showed that Ar diffusion at 1623 to 1773 K (i.e., duringmelting) increases exponentially with the degree of depoly-merization (i.e., the complexity of the silicate structure, relat-ed to the melt viscosity; Nowak et al., 2004). In other words,granitic melt will tend to retain 40Ar�inherited, whereas poorlypolymerized basic (and by extrapolation, silica-poor sedi-mentary) melts would allow a largely more efficient40Ar�inherited diffusion rate. Concerning the impact glass, sub-sequent 40Ar�inherited loss during the cooling stage is dependenton the cooling rate, but we note that this process is likely to benegligible in melt clasts in suevitic impact breccia (and partic-ularly the spherules), as they likely experienced a very fastcooling rate after the impact (e.g., Deutsch and Scharer,1994).

1000

3000

5000

7000

9000

0.1 1 10 100Time (s)

A-T-t path-1

A-T-t path-2

F=

0%

0.01%1%

5%

10%

1000

3000

5000

7000

9000

Tem

per

atu

re(K

)

RhyoliteHawaiite

Rhyolite

a

b

Fig. 9. A–T–t diagrams showing the degassing evolution of a meltedsample as a function of the temperature of the melt and the time. (a) Plotof time–temperature conditions required to entirely degas spherical(r = 300 lm) melted sample for rhyolitic and hawaiitic melts. Frequencyfactor and activation energy values are from Nowak et al. (2004) and arelog D0 = 5.23 (with D in m2 s�1) and Ea = 182 kJ mol�1 for a rhyoliticmelt and logD0 = 2.35 and Ea = 257 kJ mol�1. (b) Plot showing time–temperature conditions in order to obtain different amount of inherited40Ar* (black line) and two possible A–T–t path. Path 1 (dotted gray line):T0 = 4500 K and cooling rate = 300 K s�1. Path 2 (dashed gray line):T0 = 3000 K and cooling rate = 800 K s�1.

1226 F. Jourdan et al. 71 (2007) 1214–1231

The fraction of the 40Ar* retained in the sample duringthe melting stage can be expressed as a function of diffusionof the Ar relative to the temperature and the time spent atthis temperature. When F 6 0.15 (e.g., McDougall andHarrison, 1999; Crank, 1975), the fraction of Ar retainedin a spherical sample is given by:

F ðsphereÞ ¼ 1� f ¼ ð6=p2Þ � eð�p2 �D�t

r2 Þ ð7Þ

where f is the fraction of argon degassed, r is the radius ofthe sphere and t is the time required to achieve a givenamount of Ar degassing. D is the diffusivity, whose temper-ature dependence is given by the Arrhenius relation:

D ¼ D0 � eð�EaR�T Þ ð8Þ

where Do is the frequency factor and Ea is the activationenergy for a given chemical composition and at a givenpressure. R is the gas constant and T is the absolute max-imum temperature of the melt or glass.

The time required to achieve the degassing of a certainamount of 40Ar* for a constant temperature is given bycombining Eqs. (7) and (8) as follows:

t ¼ � r2

p2 � D0 � eð�EaR�T Þ� ln p2

6� F ðsphereÞ

� ð9Þ

We applied this equation to investigate the behavior of vis-cous (rhyolite) and non-viscous (Hawaiite) silicate melts. Inour calculation we use Ea and D0 as determined by Nowaket al. (2004) at 500 MPa in diffusion experiments on iron-free synthetic melts. For a rhyolitic melt: logD0 = 5.23(with D in m2 s�1) and Ea = 182 kJ mol�1, and for aHawaiitic melt: logD0 = 2.35 (with D in m2 s�1) andEa = 257 kJ mol�1.

Temperatures have been plotted as a function of thetime required to achieve total degassing of the 40Ar�inherited

for a sphere with a radius of 300 lm (Fig. 9a). This calcu-lation illustrates clearly how the diffusion coefficient (D) isdependent on the viscosity of the melt. For instance, a tem-perature of 3000 K allows rapid degassing of a Hawaiiticmelt (2 s) whereas it would takes more than 90 s for a rhy-olitic melt to totally degas.

In practice, the system is in constant evolution from themelting stage to glass with temperature decreasing as timeincreases (i.e., the cooling rate). Such evolution would leadto a kind of 40Ar�inherited–temperature–time path (‘‘A–T–t

path’’ hereafter). Fig. 9b shows two possible A–T–t pathsfor rhyolitic melt such as the Tswaing glass particles withvarious values of F and cooling rate. We note that in thecase of the spherules, the time as melt should be on the or-der of a few seconds (i.e., during travel in the air) in orderto maintain a spherical shape. Fig. 9b also illustrates thatgiven the relatively low initial temperature of the melt fromthe Tswaing crater (because of the small size of the crater)and the high viscosity of the melt, complete degassing ofthe glass is almost impossible to achieve. Even at a constanttemperature of 3000 K, complete degassing will take threetime longer than degassing that yields 0.01% of 40Ar�inherited.

A more accurate solution for F that accounts for varyingtemperature and is given by

F ðtÞðsphereÞ ¼ ð6=p2Þ � exp�p2 � D0 � e

�EaR�ðT 0�T ðtÞÞ

� �� t

r2

0B@

1CA

8><>:

9>=>;ð10Þ

where T(t) is the evolution of the melt temperature as afunction of time, defined by

T ðtÞ ¼ dTdt� t ð11Þ

T0 is the initial temperature of the melt, dT/dt is the cool-ing rate (K s�1), and t is time. However in practice (1) it isextremely hard to confidently estimate the temperature,melting state duration, and cooling rate for each particle,especially because of the dependence of these values uponthe distance from the center of the crater (where peak melt-ing temperatures are achieved), and (2) 40Ar�inherited can bedegassed by convection and bubble transportation, whichis likely more efficient processes for a melt than volumediffusion.

The characteristic cooling time would be substantiallylonger for a melt sheet if the diffusion length scale iscomparable to the melt sheet dimensions. However, the


complexity of a melt sheet of a given size that can containmillimeter- to meter-size host-rock clasts that can be partlymelted, micrometer to millimeter-size mineral fragmentsand neo-formed crystals limit the use of our model in thiscase.

5.3.3. Other factorsTwo further factors can possibly influence the 40Ar�inherited

diffusion rate and are given by Fick’s second law: (1) thegradient of Ar concentration at the grain boundary and(2) the temperature of the melt. Factor (1) is controlledby the Ar partial pressure of the environment surroundingthe melt. Although, this condition is hard to estimate, itseems likely that Ar partial pressure is more elevated dur-ing the formation of pseudotachylitic breccia (i.e., byhigh-pressure rock friction) than for the formation of tek-tites that travel at atmospheric pressure at �1% Ar duringtheir flight. Similarly, this is different for spherules formedon planetary bodies like the moon or asteroids, where noatmosphere is present. In the latter case and according toFick’s second law, it is expected to find a much faster Ardiffusion rate at a 0-concentration boundary. For example,several studies have attempted to estimate the cratering his-tory and rate on the moon using 40Ar/39Ar on high-K glassspherules (e.g., Ryder et al., 1996; Culler et al., 2000; Le-vine et al., 2005). Despite a 0-concentration boundary, re-sults suggest that heterogeneously distributed 40Ar�inherited

is also present in some of the dated spherules as indicatedby data scattering in isochron plots (Levine et al., 2005).We can speculate that faster cooling in vacuum might limitthe total diffusion of 40Ar�inherited. To our knowledge, noexperiment has been performed to estimate the Ar diffusionrate (in and out) relative to the Ar ambient pressure in sil-icate melt.

Factor (2) is mainly controlled by the size and velocityof the projectile but also by the position of the meltedmaterial relative to the crater center (e.g., Bogard et al.,1988). In addition, and as a counter-effect of the polymer-ization law (cf. above), the high porosity and poor cohesionof ‘‘soft’’ target rock such as (low-polymerized) wet sedi-ments may reduce melting by strongly absorbing theshock-wave (by comparison to ‘‘hard’’ target rocks). Thisis similar to the so-called ‘‘gravity’’ and ‘‘strength’’ regimemodels of the asteroid impact processes (Asphaug et al.,1996). This effect would therefore significantly hamperthe Ar degassing efficiency.

5.4. Dating impact structures by inverse isochron

It is unlikely that all the melt of a specific crater experi-ences complete degassing of 40Ar�inherited due to the variableP–T condition history of each specimen/clast (e.g., a vari-able location relative to the center of the crater; Bogardet al., 1988). This is particularly illustrated by the signifi-cant age variations displayed by four of five of the craterstructures described above (except for Chicxulub): Tswaing(1–240 Ma; this study), El’gygytgyn (3.38–4.38 Ma; Layer,

2000), Rochechouart (231–1400 Ma, Kelley and Spray,1997), and Strangways (784–5010 Ma; Spray et al., 1999).The Yucatan tektites do not show such an age spread(Swisher et al., 1992). We note that 40Ar�inherited can be pres-ent in these samples but is masked due to the low age dif-ference between the target rock and the tektites (i.e., thelow concentration of initial 40Ar* in the target rock).

In the Rochechouart and Strangways cases, despite theobvious presence of 40Ar�inherited, the ages of the impactstructures have been successfully determined by using aninverse isochron technique. Ar isotopic compositions weremeasured by single-spot laser fusion on single melt clasts.In these results, the inherited 40Ar* is largely evidencedby the non-atmospheric 40Ar/36Ar intercept (e.g.,557 ± 30 for the Strangways crater) but it appears thatthe 40Ar is a simple mixture between two reservoirs. Thisis shown by the data defining a relatively low MSWD iso-chron (i.e., low data scatter). In this case, the inverse iso-chron can be interpreted as homogeneous mixing betweenthe inherited 40Ar* and the 40Ar* produced after the im-pact. When appropriate conditions are met (see below),this technique has been shown to yield valid ages evenfor non-atmospheric trapped 40Ar/36Ar (e.g., Merrihueand Turner, 1966; Roddick, 1978; Heizler and Harrison,1988; Renne et al., 1997; Sharp and Renne, 2005) andseems promising for dating impact glass that suffers from40Ar�inherited.

However, careful statistical treatment should be appliedto ensure age-accuracy and realistic propagation of the er-rors in the final result. We recommend the following crite-ria: An isochron should (1) include the maximum numberof consecutive steps (except obvious outliers) with a prob-ability of fit P0.05. This value corresponds to the measureof the goodness of fit of points on a best-fit line determinedusing the chi-square distribution; (2) show a 40Ar/36Arintercept P295.5 as values below atmospheric ratio aremostly interpreted as the signature of recoil-induced 39Arredistribution in the samples, and (3) define a clear mixingline and do not represent an agglutination of points (e.g.,as seen in Fig. 5). Specifically, if the scatter is so great thatit exceeds the uncertainties of each analysis (probability offit 60.15), then the classical age error calculation should beexpanded by student’s t and the square root of the MSWDas follows:

rAge ¼ trwm

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiMSWDp

ð12Þ

where student’s t takes into account the number of observa-tions, rwm is the error propagated from the weighted mean,and MSWD is the mean square of the weighted deviates(McIntyre et al., 1966) that compares observed scatter withthat expected from analytical uncertainties alone. This isequivalent to adjusting the average uncertainty of the datapoint to be consistent with MSWD = 1. This approach isvalid only if a case can be made that the uncertainties ofthe individual observations have been underestimated; itshould not be regarded as a panacea for excessively scattered

1228 F. Jourdan et al. 71 (2007) 1214–1231

data if the scatter is due to geological causes (e.g., alteration,undegassed mineral with their own excess/inherited Ar reser-voirs; excess Ar not homogenized with inherited Ar), inwhich case the isochron ‘‘age’’, even with uncertaintyexpanded as in Eq. (12), may be meaningless.

Why can some impact structures be dated by 40Ar/39Areven with evidence of 40Ar�inherited (e.g., Rochechouart andStrangways), whereas others cannot (e.g., Tswaing)? Asmentioned above, the inverse isochron should reflect mix-ing between a unique trapped Ar component (i.e.,40Ar�inherited) and the radiogenic 40Ar component. If the dis-tribution of the trapped Ar is not homogeneous at thewithin-sample scale, i.e., the various components of argonare distributed in different physical locations, then no iso-chron will be produced (e.g., Reddy et al., 1997). Becauseall the melt clasts are likely to experience different ther-mal/degassing histories (and thus have different degreesof isotopic equilibration between trapped and ambient,i.e., atmospheric argon), then only single grains should beinvestigated (by step-heating or laser spot techniques) wheninherited Ar is present. Furthermore, when a sample con-tains visible mineral clasts with their own 40Ar�inherited com-ponent, then the use of an ultraviolet high spatialresolution laserprobe is required. In the case of the Tswa-ing crater, the multiple reservoirs of trapped Ar even atthe single grain scale invalidated the use of the inverse iso-chron approach as well. Relative scatter of the data ob-tained for the Rochechouart and Strangways structuressuggest that either imperfect homogenization of the40Ar�inherited within samples was achieved or that crypticun-degassed mineral with their own 40Ar�inherited componenthas been heated. The most direct consequence is that theprecision of the age determined is quite low (e.g.,646 ± 42 Ma for the Strangways impact structure; Sprayet al., 1999). Finally, we stress that great care should betaken to avoid interpreting potentially false inverse isoch-rons, which can lead to meaningless ages when the40Ar�inherited is heterogeneously distributed.

5.5. Consequence for geochronology of impact structures

A large number of impact structures remain to be datedon earth (e.g., http://www.unb.ca/passc/ImpactDatabase/index.html). 40Ar/39Ar geochronology is a promising toolto investigate the age of these structures (Deutsch andScharer, 1994). However, and as far as the 40Ar/39Ar tech-nique is concerned, some structures seem to be easier tar-gets than others with regard to the 40Ar�inherited problem.Not all the melt samples from different impact structureswill retain their 40Ar�inherited in the same way (dependingon the target composition; Fig. 9a) and a given amountof 40Ar�inherited will not equally affect the age measured of im-pact structures (depending on the age difference betweenthe impact event and the target rocks; e.g., Fig. 8). Typical-ly, if our goal is to constrain the age of an impact structureat the ±1 Ma level (Deutsch and Scharer, 1994), then thetask will be uneven depending on the characteristics of the

structures. The most interesting candidate structures tominimize the occurrence of 40Ar�inherited and/or its influenceon the age measured would be (1) craters that have been em-placed in relatively recent target rocks relative to the age ofthe impact, (2) relatively old crater structures that are rela-tively insensitive to 40Ar�inherited, (3) large structures that haveexperienced higher temperature and slower cooling historyallowing a more complete diffusion of 40Ar�inherited, and (4)structures emplaced in low-polymerized and low-porositytarget rocks such as basalt/gabbro or silica-poor sedimenta-ry rocks. These observations suggest that the (almost) worstcase scenario for 40Ar/39Ar dating would be a small Quater-nary impact occurring in a Proterozoic granitic basement,as nicely illustrated by the Tswaing case.

An interesting example that would help to decipher thevarious influences of these parameters would be the 1.8 kmdiameter Lonar impact crater. This structure is located inthe 65 Ma-old Deccan basaltic traps, India (Osae et al.,2005). The age of the impact is not well constrained butis estimated to be around 30–50 ka (Osae et al., 2005,and references therein). According to our modeling, theimportant age difference between the target rocks and theimpact event would render a melt/glass age very sensitiveto 40Ar�inherited. For instance, only 0.05% of un-degassed40Ar�inherited would double the apparent age of a melt sample.The relatively small size of the structure (i.e., low-energyimpact) would argue for a very small volume (if any) ofwell degassed glass. On the other hand, impacting low-po-lymerized basaltic target rocks should allow for rather effi-cient degassing of 40Ar�inherited, relative, for instance, togranitic rock. This could also allow a more homogeneousdistribution of the residual 40Ar�inherited.

For the worst case-scenario, where even the inverse iso-chron technique failed, a solution to overcome the problemof the 40Ar�inherited would be to investigate neo-formed miner-als (e.g., biotite and zircon in the Morokweng impact struc-ture, Hart et al., 1997; Koeberl et al., 1997), but these areoften not easy to find and Ar-datable material can poten-tially suffer from inherited and/or excess Ar. Also of interestfor young crater structures is the promising U–Th/He tech-nique (e.g., Farley, 2002) applied to melt rock, mainly be-cause of the fast diffusion rate of 4He relative to 40Ar and,thus, the lower susceptibility to inherited 4He*.

Finally, although we voluntarily focused this study on theproblem of 40Ar�inherited, additional complicating factors needto be taken into account when dating impact glasses. Themore important ones are (1) the presence of un-meltedrefractory target-rock minerals in the glass that can stronglyhamper any age determination, as these minerals can containtheir own reservoir of inherited and/or excess 40Ar*; (2) thepresence of an appreciable amount of excess 40Ar* in the tar-get rock that will contribute to the total concentration of40Ar* and possibly constitute a distinct 40Ar* reservoir; (3)hydrothermal alteration of the glass that tends to remobilizeK and Ar within the sample and crystallize younger mineralphases; and (4) crater structures with superimposed tectonichistory that can partially reset the Ar system.




6. Conclusions

The 40Ar/39Ar technique performed on impact glassfailed to provide an absolute and reliable age for the Tswa-ing impact glass and cannot help to improve the fission-track age of 0.2 ± 0.1 Ma. Based on our data, the bestage estimate would be a single grain total fusion age of1.0 ± 0.3 Ma. However, this ‘‘age’’ clearly suffers from40Ar* inherited (40Ar�inherited) from the 2.05 Ga target rocks.Assuming an age of 0.2 Ma, our calculations suggest thatthe glass fragments retain a range of 40Ar�inherited amountsbetween 0.015% and 4.15%. The spread of 40Ar�inherited val-ues likely reflects the various P–T conditions experiencedby the glass samples, most certainly due to the pre-impactposition of the sample relative to the center of the crater,and the variable subsequent cooling histories of the sam-ples. Apparent age spectra of the impact glasses show that40Ar�inherited is located in two distinct reservoirs including (1)a heterogeneous distribution in the glass matrix and (2)fluid/vapor inclusions or un-melted residual clasts.

We compared the influence of 40Ar�inherited on the40Ar/39Ar apparent age determination of five impact struc-tures. Calculations show that the age difference between theimpact and the target rock emplacement (i.e., the mean40Ar�inherited concentration in the target rock) is the mainparameter that controls the influence of 40Ar�inherited on themeasured age. In other words, a young impact structureemplaced in an old basement would be much more sensitiveto a given amount of 40Ar�inherited than a crater structure withroughly the same age as the basement.

Calculations show that a factor that can improve the Ardiffusion during melting is a low-degree of polymerizationof the impact melts produced from the target rocks. Otherfactors are (1) a zero-concentration of Ar at the grainboundary (e.g., lunar spherules), (2) a sufficiently large im-pact with very high shock energy, and (3) relatively lowporosity target rocks to avoid too much shock absorption.

For glass that suffers from the presence of 40Ar�inherited,investigation by single grain step-heating or laser ablationcan give good results, but we emphasize that careful statis-tical treatment should be applied in order to avoid mean-ingless ages. Additional experiments on impact glass fromselected craters would help to better constrain the influenceof 40Ar�inherited on 40Ar/39Ar age dating but also the param-eters influencing its occurrence.

Finally, we would like to stress that much work remainsto be done concerning the geochronology (multi-tech-niques) of impact structures considering that as of 176 suchstructures recorded in 2006, only 25 structures have ageswith reported precision better than 2%.

Acknowledgments

We thank The Ann and Getty Foundation for fundingsupport and T.A. Becker for mass spectrometry assistance.The Council for Geoscience, Pretoria, provided the sample,and Joseph Aphane (University of the Witwatersrand,

Johannesburg) assisted with sample preparation. We thankC. Blitz and D. Shuster for discussion, and S. Sherlock andtwo anonymous reviewers for constructive reviews of themanuscript.

Associate editor: Rainer Wieler

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at doi:10.1016/j.gca.2006.11.013.

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The problem of inherited 40Ar* in dating impact glass by the 40Ar/ 39Ar method: Evidence from the...

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