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We investigated exterior and interior subsamples from the Martian shergottite meteorites Allan Hills ( ALH) A77005 and Roberts Massif ( RBT) 04261 for secondary minerals, oxygen isotopes, Ar-Ar, and noble gas signatures. Electron microprobe investigations revealed that RBT 04261 does not contain any visible alteration even in its most exterior fractures, whereas fracture fillings in ALHA77005 penetrate into the meteorite up to 300 μm, beyond which the fractures are devoid of secondary minerals. Light noble gases seem to be almost unaffected by terrestrially induced alteration in both meteorites. Thus, a shock metamorphic overprint of 30-35 GPa can be deduced from the helium measurements in RBT 04261. Oxygen isotopes also seem unaffected by terrestrially weathering and variations can easily be reconciled with the differences in modal mineralogy of the exterior and interior subsamples. The measurements on irradiated samples (Ar-Ar) showed a clear Martian atmospheric contribution in ALHA77005, but this is less apparent in our sample of RBT 04261. Exterior and interior subsamples show slight differences in apparent ages, but the overall results are very similar between the two. In contrast, krypton and xenon are severely affected by terrestrial contamination, demonstrating the ubiquitous presence of elementally fractionated air in RBT 04261. Although seemingly contradictory, our results indicate that RBT 04261 was more affected by contamination than ALHA77005. We conclude that irrespective of on which planet the alteration occurred, exposure of Martian rocks to atmosphere (or brine) introduces noble gases with signatures elementally fractionated relative to the respective atmospheric composition into the rock, and relationships of that process with oxygen isotopes or mineralogical observations are not straightforward. [ABSTRACT FROM AUTHOR]

Provides information on a study that discussed the geological setting and biofacies of, and presented argon-argon ages for, two glaciomarine sedimentary deposits that crop out at James Ross Island, Antarctica. Experimental procedure; Results and discussion on the study; Conclusions.

New SHRIMP U–Pb zircon, Rb–Sr whole-rock, and 40Ar–39Ar data are presented for the Jurassic silicic volcanic rocks and related granitoids of Patagonia and the Antarctic Peninsula. U–Pb is the only reliable method for dating crystallization in these rocks; Rb–Sr is prone to hydrothermal resetting and Ar–Ar is additionally affected by initial excess 40Ar. Volcanism spanned more than 30 My, but three episodes are defined on the basis of peak activity: V1 (188–178 Ma), V2 (172–162 Ma) and V3 (157–153 Ma). The first essentially coincides with the Karoo–Ferrar mafic magmatism of South Africa, Antarctica and Tasmania. The silicic products of V1 are lower-crustal melts that have incorporated upper-crustal material. The geochemistry of V2 and V3 ignimbrites is more characteristic of destructive plate margins, but the presence of inherited zircon still points to a crustal source. The pattern of volcanism corresponds in space and in time to migration away from the Karoo mantle plume towards the proto-Pacific margin of Gondwana during rifting and break-up. The heat required to initiate bulk crustal fusion may have been supplied by the spreading plume-head, but thinning of the crust during continental dispersion would also have facilitated anatexis. [ABSTRACT FROM PUBLISHER]