Initial Pb Isotopes of Some SNC Meteorites

Details Initial Pb Isotopes of Some SNC Meteorites Initial Pb Isotopes of Some SNC Meteorites

Dec 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.p51a..03j&link_type=abstract

American Geophysical Union, Fall Meeting 2001, abstract #P51A-03

Physics

6215 Extraterrestrial Materials, 6225 Mars, 6240 Meteorites And Tektites

Scientific paper

Terrestrial Pb, which is difficult to remove even with intense leaching procedures, overwhelms most SNC meteorites exposed to post-fall weathering. However, a pure plagioclase separate leached by dilute HF can give a reliable "initial" Pb isotopic composition (i.e. the time of recent mineral disturbance). Here we discuss the relationship between this initial Pb and the Rb-Sr isotopic systematics of some SNC meteorites. The Pb isotopes behave in a qualitatively similar manner to the Sr isotopes. On a 207Pb/204Pb vs. 206Pb/204Pb diagram the initial Pb plots close to the 4.5Ga Pb-Pb geochron. This implies that the early differentiation indicated by the Sr isotopes also pertains to the U-Pb systematics. The first group of SNC meteorites, represented by Shergotty and Los Angeles, defines a reservoir with elevated U/Pb, which parallels its high Rb/Sr ratio. The second group identifies a U-depleted reservoir as indicated by the less radiogenic initial Pb of SaU 005 and Nakhla. The only representative of the third group, Y-793605, has a slightly more radiogenic Pb than Los Angeles, but our limited sampling cannot define the limits or possible overlap of the isotopic fields. Also, the possibility exists for some remaining terrestrial contamination of the plagioclase. While the initial Pb of only 5 SNC meteorites has been measured so far the general picture seems to be consistent with the Rb-Sr systematics. The SNC meteorites appear to be derived from a remarkably primitive parent body recording only an early magmatic differentiation. The young disturbance of their mineral systems lacks the chemical characteristics of a magmatic differentiation, and may be more appropriately related to the impact excavation and/or ejection of the rock. If the SNC meteorites have indeed originated from Mars, as is generally assumed, from our limited sampling this planet would seem to be very primitive.

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