Mathematics – Logic
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p32b..02b&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P32B-02
Mathematics
Logic
[1000] Geochemistry, [1027] Geochemistry / Composition Of The Planets
Scientific paper
The return of samples from Mars has been a long standing NASA objective that, if achieved, would dramatically change our view of the formation and evolution of the terrestrial planets, as well as the physical and chemical conditions in the solar system at the time when life formed on Earth. Significant insight into Mars has been gleaned from the >50 martian meteorites that have landed on Earth. These meteorites fall into 3 categories: First, 165 to 570 Ma basalts and basaltic cumulates, second 1.3 Ga clinopyroxenites and dunites, and third a >4 Ga orthopyroxenite. The basaltic meteorites form a continuum between those with geochemical and isotopic characteristics indicating derivation from depleted and enriched sources. This is most easily explained as mixing between a depleted mantle reservoir and a poorly constrained enriched reservoir in the mantle or crust. Like some basalts. the clinopyroxenites/dunites are slightly enriched in incompatible elements relative to chondrites, but cannot be related to the basaltic meteorites by any simple process. This is most clearly supported by their vastly different short-lived isotopic compositions (e.g. 182W and 142Nd) compared to the basalts. Although all samples have undergone some low temperature alteration on Mars, the orthopyroxenite has undergone significantly more. These observations led to several fundamental conclusions: (1) the young ages indicate the planet is geologically active at present, (2) non-chondritic 182W and 142Nd suggest core formation and silicate differentiation occurred <5 Ma and <25Ma after CAIs, respectively, (3) preservation of isotopic anomalies in short-lived isotopic systems requires geochemical reservoirs on Mars to remain isolated for much of Mars history, (4) basalt geochemical relationships indicate these reservoirs mixed recently during basalt genesis and that these samples are closely related, and (5) water based alteration present in the meteorites occurred throughout martian history, but decreased with time. It is important to temper these conclusions with the understanding that close petrogenetic relationships between the basalts, as well as the clinopyroxenites/dunites, suggests that only a few martian localities have been sampled, imperiling simple extrapolations of planetary scale evolutionary processes discerned from the meteorites. This problem would be overcome by obtaining igneous samples from different martian locations than those represented in the meteorite collection.
Borg Lars E.
Symes Steve
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