Pre-bombardment crystallization ages of basaltic clasts from Antarctic howardites EET87503 and EET87513

Details Pre-bombardment crystallization ages of basaltic clasts from Antarctic howardites EET87503 and EET87513 Pre-bombardment crystallization ages of basaltic clasts from Antarctic howardites EET87503 and EET87513

Mar 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994lpi....25.1015n&link_type=abstract

In Lunar and Planetary Inst., The Twenty-Fifth Lunar and Planetary Science Conference. Part 2: H-O p 1015-1016 (SEE N94-36136 11

Computer Science

1

Achondrites, Basalt, Breccia, Crystallization, Meteoritic Composition, Antarctic Regions, Homogenizing, Metamorphism (Geology), Pyroxenes, Sedimentary Rocks

Scientific paper

Igneous clasts of basaltic eucrites are found in both howardites and polymict eucrites. We have studied the Rb-Sr and Sm-Nd isotopic systematics of a number of such clasts, of metamorphic grades 1-6, using the classification of Takeda and Graham. Here, we report Rb-Sr, (147)Sm-(143)Nd, and (145)Sm-(142)Nd studies of clast, 53 from Antarctic howardite EET87503. Although there is no evidence of disturbance of trace element systematics by Antarctic weathering, the Rb-Sr and conventional Sm-Nd isotopic systematics are severely disturbed, which we ascribe to thermal metamorphism. The Ar-Ar age spectrum shows ages ranging from approximately 3.85-3.55 Ga in an unusual 'down stairstep'. The (146)Sm-(142)Nd systematics, however, show the presence of live (146)Sm(t1/2 = 103 Ma), with (146)Sm/(144)Sm = 0.0061 +/- 0.0007 at the time of crystallization. This result is very similar to that previously obtained for basaltic clast, 18 from howardite EET87513 (paired with EET87503), which has concordant Rb-Sr and Sm-Nd ages of approximately 4.5 Ga. Thus, the two clasts are nearly the same age, and we conclude further than the EET87503,53 clast crystallized within 33 +/- 19 Ma of the LEW86010 angrite by comparing initial (146)Sm/(144)Sm to that of the angrite. We suggest that disturbances in the isotopic systematics of EET87503,53 are consanguineous with pyroxene homogenization.

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