Chemical Alterations in Martian Meteorites from Cold and Hot Deserts

Details Chemical Alterations in Martian Meteorites from Cold and Hot Deserts Chemical Alterations in Martian Meteorites from Cold and Hot Deserts

Apr 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja....10642d&link_type=abstract

EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #10642

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Martian meteorites (SNC) provide evidence of the magmatic fractionation processes of their parent body. As 23 of the total of 27 meteorites are finds, the influence of chemical alterations during their residence time on Earth must be considered, when interpreting the mineralogical, chemical, or isotopic features. Many basaltic shergottites and nakhlites were collected both in the cold desert, Antarctica, and in hot deserts of North Africa and Asia. To detect alteration processes in the finds we have to compare their analytical data with those obtained from the very scarce falls. In this way, we find an overabundance of iodine in meteorites from Antarctica. The iodine contamination is caused by aerosols adhering to the ice. Therefore, iodine can penetrate into the meteorite during its residence in Antarctica. The iodine content measured in the Antarctic shergottites varies from 0.060 to 4.6 ppm and seems to depend on their residence time on ice. The paired Yamato nakhlites Y-000593 and Y-000749 recently discovered in Antarctica also reveal an iodine overabundance compared to Nakhla, which is the only fall among the nakhlites. However, in Nakhla we have another problem of alteration. Nakhla has unusually high Br and Cl concentrations which could originate from terrestrial or parent body alterations. As Cl and Br are readily extracted during water leaching experiments we favor a terrestrial contamination. A Br overabundance was also found in many olivine-rich shergottite finds from hot deserts, DaG 476, Dhofar 019, and SaU 005. However, in the basaltic shergottite Dhofar 378 and in the nakhlite NWA 817 [1] no Br contamination was observed. The olivine phases of the shergottites seem to be preferably attacked by weathering reactions in the hot deserts. In the shergottites from hot deserts, the subchondritic La/U ratios are remarkable, indicating a U contamination. All these meteorites are covered with an evaporation product, caliche. Caliche has a high content of typical alteration elements like C, Sr, Br and U. Uranium, which was enriched in the meteorite by weathering, might influence the Pb isotope systematics. Thus, we did not expect any valid internal age relation using the U-Pb isotopes on these heavily altered shergottites. In the Sm/Nd isotope systematics all acid leachate (carbonates, phosphates) of the hot desert SNC meteorites plot along a terrestrial contamination mixing line whereas the residues and mafic minerals are uncontaminated. This may obscure an internal or leach-residue isochron. Ref.: [1] Sautter V. et al. (2002) EPSL 195, 223-238.

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