Small time differences recorded in differentiated meteorites

Details Small time differences recorded in differentiated meteorites Small time differences recorded in differentiated meteorites

Jul 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29r.493l&link_type=abstract

Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 493-494

Computer Science

7

Breccia, Meteoritic Composition, Planetary Evolution, Radioactive Age Determination, Chromium Isotopes, Manganese Isotopes, Temperature Effects

Scientific paper

Our study of the (53)Mn-(53)Cr system in the angrites Angra dos Reis and LEW 86010 has shown that the (53)MN/(55)Mn ratio at their Pb-Pb age of 4.558 Ga was (1.29 +/- 0.07) x 106. If the closure of the (53)Mn-(53)Cr system in these meteorites occurred contemporaneously with that of the U-Pb system, then the Pb-Pb age serves as an absolute time marker for the (53)MN/(55)Mn ratio in the inner solar system. The assumption here is that at least the inner solar system was homogeneous in terms of (53)Mn/(55)Mn. In more recent work we began to measure the (53)Mn-(53)Cr system in eucrites. The first results were obtained on the relatively unbrecciated noncumulate eucrite Chervony Kut (CK). From our measurements on two bulk rocks, and chromite, pyroxene, and plagioclase separates from this meteorite we obtained a good Mn-Cr isochron whose slope corresponds to a (53)Mn/(55)Mn ratio of (3.6 +/- 0.5) x 10-6 for the time when CK cooled below the closure temperature of the (53)Mn-(53)Cr isotope system. If the assumption of Mn-53 homogeneity is valid, then an absolute age for CK can be calculated using the angrite Pb-Pb age and the relative Mn-Cr age of CK. This 'absolute' age then is a surprisingly old 4.563 +/- 0.001 Ga. Taking into account all known uncertainties, this age for CK implies that it has taken only less than 6 m.y. from the time Allende CAIs were formed to fully accrete and differentiate the eucrite parent body and solidify basalts on its surface. In order to test this result we have measured bulk rock samples and mineral separates from the brecciated eucrite Juvinas (JUV). Again we were able to contain a reasonably good Mn-Cr isochron, although the range in Mn/Cr ratios is slightly smaller.

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