Noble gases and chemical composition of Shergotty mineral fractions, Chassigny, and Yamato 793605: The trapped argon‐40/argon‐36 ratio and ejection times of Martian meteorites

Terribilini, D.; Eugster, O.; Burger, Mario; Jakob, A.; Krähenbühl, Urs

doi:10.1111/j.1945-5100.1998.tb01673.x

Cited by 32 publications

(42 citation statements)

References 35 publications

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“…The CRE age for the paired meteorite NWA 480, from analyses of both cosmogenic noble gases (Mathew et al, 2003) and 10 Be (Nishiizumi and Caffee, 2006), is similar to, if not identical to, CRE ages for ''enriched" basaltic shergottites Zagami, Shergotty, and Los Angeles (Eugster et al, 1997(Eugster et al, , 2002. Several analyses of Shergotty and Zagami in various laboratories gave a range in 36 Ar cos concentrations of 1.77-2.49 Â 10 À9 cm 3 /g for whole rock samples and 1.50-2.71 Â 10 À9 cm 3 /g for feldspar separates (Eugster et al, 1997;Terribilini et al, 1998;Park and Nagao, 2003;Park, 2005;Schwenzer et al, 2007). Variations in these 36 Ar cos values are to be expected because of variable sample composition and space shielding among individual samples.…”

Section: Ar-ar Isochron Agementioning

confidence: 90%

Concordant Rb–Sr, Sm–Nd, and Ar–Ar ages for Northwest Africa 1460: A 346Ma old basaltic shergottite related to “lherzolitic” shergottites

Nyquist¹,

Bogard²,

Shih³

et al. 2009

Geochimica et Cosmochimica Acta

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Multiple lines of evidence show that the Rb-Sr, Sm-Nd, and Ar-Ar isotopic systems individually give robust crystallization ages for basaltic (or diabasic) shergottite Northwest Africa (NWA) 1460. In contrast to other shergottites, NWA 1460 exhibits minimal evidence of excess 40 Ar, thus allowing an unambiguous determination of its Ar-Ar age. The concordant RbSr, Sm-Nd, and Ar-Ar results for NWA 1460 define its crystallization age to be 346 ± 17 Ma (2r). In combination with petrographic and trace element data for this specimen and paired meteorite NWA 480, these results strongly refute the suggestion by others that the shergottites are $4.1 Ga old. Current crystallization and cosmic-ray exposure (CRE) age data permit identification of a maximum of nine ejection events for Martian meteorites (numbering more than 50 unpaired specimens as of 2008) and plausibly as few as five such events. Although recent high resolution imaging of the Martian surface has identified limited areas of sparsely cratered terrains, the meteorite data suggest that either these areas are representative of larger areas from which the meteorites might come, or that the cratering chronology needs recalibration. Time-averaged 87 Rb/ 86 Sr = 0.16 for the mantle source of the parent magma of NWA 1460/480 over the $4.56 Ga age of the planet is consistent with previously estimated values for bulk silicate Mars in the range 0.13-0.16, and similar to values of $0.18 for the ''lherzolitic" shergottites. Initial e Nd for NWA 1460/480 at 350 ± 16 Ma ago was +10.6 ± 0.5, which implies a time-averaged 147 Sm/ 144 Nd of 0.217 in the Martian mantle prior to mafic melt extraction, similar to values of 0.211-0.216 for the ''lherzolitic" shergottites. These time-averaged values do not imply a simple two-stage mantle/melt evolution, but must result from multiple episodes of melt extractions from the source regions. Much higher ''late-stage" e Nd values for the depleted shergottites imply similar processes carried to a greater degree. Thus, NWA 1460/480, the ''lherzolitic" shergottites and perhaps EET 79001 give the best (albeit imperfect) estimate of the Sr-and Nd-isotopic characteristics of bulk silicate Mars. Published by Elsevier Ltd.

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Section: Ar-ar Isochron Agementioning

confidence: 90%

Concordant Rb–Sr, Sm–Nd, and Ar–Ar ages for Northwest Africa 1460: A 346Ma old basaltic shergottite related to “lherzolitic” shergottites

Nyquist¹,

Bogard²,

Shih³

et al. 2009

Geochimica et Cosmochimica Acta

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“…On the other hand, Treiman et al (1994) found from the radionuclide activities of LEW88516 that most of its cosmic-ray exposure probably occurred as an individual meteoroid. Furthermore, the cosmogenic 22 Ne/ 21 Ne ratios are 1.227 ± 0.035 for LEW88516 (Eugster et al, 1997a) and 1.207 ± 0.020 for Y793605 (Terribilini et al, 1998), indicating irradiation as small bodies for the total duration of exposure. A complicated exposure history beginning with excavation of LEW88516 and Y793605, followed by ejection within 1-2 Myr, or reburial of these two rocks between their initial excavation and ejection, seems improbable.…”

Section: Crystallization and Ejection Agesmentioning

confidence: 98%

“…We begin our discussion with early Mars, working towards the present day. References: a1 Geiss and Hess (1958), recalculated to the K-decay constants by Steiger and Jäger (1977); a Eugster et al (1997a); b Bogard et al (1979); c Bogard and Garrison (1999); d Nyquist et al (1979a); e Jagoutz and Wänke (1986); f Shih et al (1982); g Chen and Wasserburg (1986); h Sano et al (2000); i Nyquist et al (1995); j Nyquist et al (2000); k Nyquist et al (1986); l Nyquist et al (2001); m Borg et al (1997); n Borg et al (2000); o , Jagoutz and Jotter (2000); p Miura et al (1995); q Jagoutz (1989); r Borg et al (2001b); s,t Borg et al (1998aBorg et al ( , 1998b; u Chen and Wasserburg (1993); v Misawa et al (1997); w Terribilini et al (1998).…”

Section: Radiometric Agesmentioning

confidence: 99%

Ages and Geologic Histories of Martian Meteorites

Nyquist

Bogard

Shih

et al. 2001

Space Sciences Series of ISSI

380

558

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Abstract.We review the radiometric ages of the 16 currently known Martian meteorites, classified as 11 shergottites (8 basaltic and 3 lherzolitic), 3 nakhlites (clinopyroxenites), Chassigny (a dunite), and the orthopyroxenite ALH84001. The basaltic shergottites represent surface lava flows, the others magmas that solidified at depth. Shock effects correlate with these compositional types, and, in each case, they can be attributed to a single shock event, most likely the meteorite's ejection from Mars. Peak pressures in the range 15 − 45 GPa appear to be a "launch window": shergottites experienced ∼30 − 45 GPa, nakhlites ∼20 ± 5 GPa, Chassigny ∼35 GPa, and ALH84001 ∼35 − 40 GPa. Two meteorites, lherzolitic shergottite Y-793605 and orthopyroxenite ALH84001, are monomict breccias, indicating a two-phase shock history in toto: monomict brecciation at depth in a first impact and later shock metamorphism in a second impact, probably the ejection event.Crystallization ages of shergottites show only two pronounced groups designated S 1 (∼175 Myr), including 4 of 6 dated basalts and all 3 lherzolites, and S 2 (330 − 475 Myr), including two basaltic shergottites and probably a third according to preliminary data. Ejection ages of shergottites, defined as the sum of their cosmic ray exposure ages and their terrestrial residence ages, range from the oldest (∼20 Myr) to the youngest (∼0.7 Myr) values for Martian meteorites. Five groups are distinguished and designated S Dho (one basalt, ∼20 Myr), S L (two lherzolites of overlapping ejection ages, 3.94 ± 0.40 Myr and 4.70 ± 0.50 Myr), S (four basalts and one lherzolite, ∼2.7 − 3.1 Myr), S DaG (two basalts, ∼1.25 Myr), and S E (the youngest basalt, 0.73 ± 0.15 Myr). Consequently, crystallization age group S 1 includes ejection age groups S L , S E and 4 of the 5 members of S, whereas S 2 includes the remaining member of S and one of the two members of S DaG . Shock effects are different for basalts and lherzolites in group S/S 1 . Similarities to the dated meteorite DaG476 suggest that the two shergottites that are not dated yet belong to group S 2 . Whether or not S 2 is a single group is unclear at present. If crystallization age group S 1 represents a single ejection event, pre-exposure on the Martian surface is required to account for ejection ages of S L that are greater than ejection ages of S, whereas secondary breakup in space is required to account for ejection ages of S E less than those of S. Because one member of crystallization age group S 2 belongs to ejection group S, the maximum number of shergottite ejection events is 6, whereas the minimum number is 2.Crystallization ages of nakhlites and Chassigny are concordant at ∼1.3 Gyr. These meteorites also have concordant ejection ages, i.e., they were ejected together in a single event (NC). Shock effects vary within group NC between the nakhlites and Chassigny.The orthopyroxenite ALH84001 is characterized by the oldest crystallization age of ∼4.5 Gyr. Its secondary carbonates are ∼3.9 Gyr old, an age corresponding t...

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“…In principle, it is also possible to obtain Ar-isochron ages, which date the rock formation, from Martian meteorites by plotting the ratio of trapped plus radiogenic 40 Ar to trapped 36 Ar ( 40 Ar tr + rad / 36 Ar tr ) versus K/ 36 Ar tr , the ratio of potassium to trapped 36 Ar (Terribilini et al 1998). However, it is unlikely that these agree with ages from undisturbed (by shock) isotopic systems.…”

Section: Argonmentioning

confidence: 99%

“…Ar is given in units of 10 −6 cc STP/g, Xe in 10 −12 cc STP/g. Stöffler et al (1986): average of all given analyses (min, max); b Terribilini et al (1998); c Smith and Hervig (1979); d Hale et al (1999); e Stolper and McSween (1979); f Bogard and Garrison (1999); g Shih et al 1982;h Treiman and Sutton (1991): range of all given analyses; i Steele and Smith (1982): range of all given analyses. Estimates of radiogenic 40 Ar are given in units of 10 −6 cc STP/g.…”

Section: Martian Interior Argonmentioning

confidence: 99%

Noble gases in mineral separates from three shergottites: Shergotty, Zagami, and EETA79001

Schwenzer¹,

Herrmann²,

Mohapatra

et al. 2007

Meteorit & Planetary Scien

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Abstract-This study provides a complete data set of all five noble gases for bulk samples and mineral separates from three Martian shergottites: Shergotty (bulk, pyroxene, maskelynite), Zagami (bulk, pyroxene, maskelynite), and Elephant Moraine (EET) A79001, lithology A (bulk, pyroxene). We also give a compilation of all noble gas and nitrogen studies performed on these meteorites. Our mean values for cosmic-ray exposure ages from 3 He, 21 Ne, and 38 Ar are 2.48 Myr for Shergotty, 2.73 Myr for Zagami, and 0.65 Myr for EETA79001 lith. A. Serious loss of radiogenic 4 He due to shock is observed. Cosmogenic neon results for bulk samples from 13 Martian meteorites (new data and literature data) are used in addition to the mineral separates of this study in a new approach to explore evidence of solar cosmic-ray effects. While a contribution of this low-energy irradiation is strongly indicated for all of the shergottites, spallation Ne in Chassigny, Allan Hills (ALH) 84001, and the nakhlites is fully explained by galactic cosmic-ray spallation. Implanted Martian atmospheric gases are present in all mineral separates and the thermal release indicates a near-surface siting. We derive an estimate for the 40 Ar/ 36 Ar ratio of the Martian interior component by subtracting from measured Ar in the (K-poor) pyroxenes the (small) radiogenic component as well as the implanted atmospheric component as indicated from 129 Xe * excesses. Unless compromised by the presence of additional components, a high ratio of ~2000 is indicated for Martian interior argon, similar to that in the Martian atmosphere. Since much lower ratios have been inferred for Chassigny and ALH 84001, the result may indicate spatial and/or temporal variations of 40 Ar/ 36 Ar in the Martian mantle.

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Noble gases and chemical composition of Shergotty mineral fractions, Chassigny, and Yamato 793605: The trapped argon‐40/argon‐36 ratio and ejection times of Martian meteorites

Cited by 32 publications

References 35 publications

Concordant Rb–Sr, Sm–Nd, and Ar–Ar ages for Northwest Africa 1460: A 346Ma old basaltic shergottite related to “lherzolitic” shergottites

Concordant Rb–Sr, Sm–Nd, and Ar–Ar ages for Northwest Africa 1460: A 346Ma old basaltic shergottite related to “lherzolitic” shergottites

Ages and Geologic Histories of Martian Meteorites

Noble gases in mineral separates from three shergottites: Shergotty, Zagami, and EETA79001

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