Orthopyroxenite ALH84001 and shergottite ALH77005: Additional evidence for a martian origin from noble gases

102

Abstract-We analyzed noble gases from 18 samples of weathering products ("iddingsite") from the Lafayette meteorite. Potassium-argon ages of 12 samples range from near zero to 670 * 91 Ma. These ages confirm the martian origin of the iddingsite, but it is not clear whether any or all of the ages represent iddingsite formation as opposed to later alteration or incorporation of martian atmospheric 40Ar. In any case, because iddingsite formation requires liquid water, this data requires the presence of liquid water near the surface of Mars at least as recently as 1300 Ma ago, and probably as recently as 650 Ma ago. Krypton and Xe analysis of a single 34 p g sample indicates the presence of fractionated martian atmosphere within the iddingsite. This also confirms the martian origin of the iddingsite. The mechanism of incorporation could either be through interaction with liquid water during iddingsite formation or a result of shock implantation of adsorbed atmospheric gas.

Section: Discussion Krypton and Xenonmentioning

confidence: 99%

Noble gases in iddingsite from the Lafayette meteorite: Evidence for liquid water on Mars in the last few hundred million years

Swindle

Treiman

Lindstrom

et al. 2000

102

“…For two other ALH 84001 samples, Ar data were reported: both Miura et al (1995) and Swindle et al (1995) found indications of a substantial amount of Martian atmospheric Ar. On the other hand, Chassigny appears to be inhomogeneous with respect to its trapped Ar content.…”

Section: Allan Hills 84001 and Chassignymentioning

confidence: 99%

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

Eugster

Burger

et al. 1998

Abstract-We report the elemental and isotopic composition of the noble gases as well as the chemical abundances in pyroxene, maskelynite/mesostasis glass, and bulk material of Shergotty and of bulk samples from Chassigny and Yamato 793605. The 4oK-4oAr isochron for the Shergotty minerals yields a gas retention age of 196 Ma, which is, within errors, in agreement with previously determined Rb-Sr internal isochron ages. Argon that was trapped at this time has a 40Ar136Ar ratio of 1100. For Chassigny and Y-793605, we obtain trapped 40Ar/36Ar ratios of 1380 and 950, respectively. Using these results and literature data, we show that the three shergottites, Shergotty, Zagami, and QUE 94001; the lherzolites ALH 77005, LEW 88516, and Y-793605; as well as Chassigny and ALH 84001 contain a mixture of Martian mantle and atmospheric Ar; whereas, the trapped 40Ar/36Ar ratio of the nakhlites, Nakhla, Lafayette, and Governador Valadares cannot be determined with the present data. We show that Martian atmospheric trapped Ar in Martian meteorites is correlated with the shock pressure that they experienced. Hence, we conclude that the Martian atmospheric gases were introduced by shock into the meteoritic material. For the Shergotty minerals, we obtain 3He-, 21Ne-, and 38Ar-based cosmic-ray exposure ages of 3.0 Ma, and for the lherzolite Y-793605,4.0 Ma, which confirms our earlier conclusion that the lherzolites were ejected from Mars -1 Ma before the shergottites. Chassigny yields the previously known ejection age of 1 I .6 Ma.

“…From these we calculate limits to the 36Arc,, concentration of 0.63 and 1.46 x 1 0-9 cm3 STP/g, respectively. Prior determinations of galactic cosmic-ray-produced 36Arc0s concentrations in ALH 77005 gave 1.G1.3 x cm3 STP/g (Bogard et al, 1984;Miura et al, 1995;Garrison et al, 1995), but three unirradiated samples of ALH 77005 reported in Appendix 2 (discussed later) suggest lower concentrations of -0.9 x cm3 STP/g. We adopt 38Arc0s = 1.1 x cm3 STP/g, an amount that is only -15% of the total 36Ar in our ALH 77005 sample.…”

Section: Irradiated Shergottite Samplesmentioning

confidence: 99%

Argon‐39‐argon‐40 “ages” and trapped argon in Martian shergottites, Chassigny, and Allan Hills 84001

Bogard

Garrison

1999

105

100

Abstract-Argon-isotopic abundances were measured in neutron-irradiated samples of Martian meteorites Chassigny, Allan Hills (ALH) 84001, ALH 77005, Elephant Moraine (EET) 79001, Yamato (Y) 793605, Shergotty, Zagami, and Queen Alexandra Range (QUE) 94201, and in unirradiated samples of ALH 77005. Chassigny gives a 39Ar-40Ar age of 1.32 2 0.07 Ga, which is similar to radiometric ages of the nakhlites. Argon-39-Argon-40 data for ALH 84001 indicate ages between 3.9 and 4.3 Ga. A more precise definition of this age requires detailed characterization of the multiple trapped Ar components in ALH 84001 and of 39Ar recoil distribution. All six shergottite samples show apparent 39Ar-40Ar ages substantially older than the -165-200 Ma range in ages given by other isotope dating techniques. Shergottites appear to contain ubiquitous Ar components acquired fkom the Martian atmosphere, the Martian mantle, and commonly terrestrial atmospheric contamination. Zagami feldspar also suggests inherited radiogenic 4oAr. These data analyses indicate that the recent Martian atmospheric component trapped in shergottites has a 40ArP6Ar ratio possibly as low as -1750 and no greater than -1900. These ratios are less than the value of 3000 2 500 reported by Viking. The 40Ar/36Ar ratio for the Martian mantle component is probably <500 but is poorly constrained. The correlation between trapped 40Ar/36Ar and 129Xe/132Xe ratios in shergottite impact glasses and unirradiated samples of ALH 77005 shows considerable scatter and suggests that the 36Ar/132Xe ratio in the Martian components may vary. Resolution of Martian atmospheric 40Ar/36Ar ratio at different time periods ( i e . , at -4.0 and 0.2 Ga) is also difficult without an understanding of the composition of various trapped components.