Age of a eucrite clast from the Bholghati howardite

Abstract

39Ar- 40Ar, Rb-Sr, and Sm-Nd investigations of the “large” eucrite clast (,1) studied by the Bholghati Consortium show that it crystallized from a melt ~ 4.51 Ga ago. The crystallization age is given most precisely by a Sm-Nd internal isochron of 4.510 ± 0.034 Ga (2σ error limits) for λ( 147Sm) = 0.00654 (Ga) −1. 142Nd 144Nd measurements show that the clast contained live 146Sm at the time of crystallization. A solar system initial 146 Sm 144Sm 0 = 0.00431 +0.0039 −0.0026 at 4.555 Ga ago can be calculated from a 146Sm- 142Nd isochron and the age of the clast. Alternatively, using ( 146Sm 144Sm ) 0 = 0.0045 as previously reported in the literature, a 146 Sm 142 Nd formation interval of 0.05 +0.12 −0.08 Ga can be calculated, which is concordant with the 147Sm- 143Nd isochron age and in agreement with Pu-U-Xe and Pu-Nd-Xe formation intervals reported in a companion Consortium paper. The initial 143Nd 144 Nd ( I Nd ) of the clast was I Nd = 0.505958 ± 50 (2σ error limits), normalized to 146Nd/ 144Nd = 0.724140 and is consistent with the CHUR reference value for evolution in a source having chondritic relative REE abundances prior to magma generation. However, when interlaboratory bias is considered, the data are also permissive of a LREE depleted source as suggested in a companion Consortium paper. A primary Rb-Sr isochron yields an age of 4.54 ± 0.06 Ga (2σ error limits) for λ( 87Rb) = 0.01402 (Ga) −1, in agreement with the more precise Sm-Nd age. The initial 87Sr 86Sr (I Sr) of the clast was I Sr = 0.699006 ± 21 and was the same within error limits as I Sr previously determined for a pristine eucrite clast from polymict eucrite Y75011; compatible with a common parent body. The 39Ar- 40Ar data and some of the Rb-Sr data show that Bholghati was affected by a major thermal event ~ 2–3 Ga ago and also provide weak evidence for a minor thermal event ≤ 1 Ga ago. The age and isotopic systematics of the clast are consistent with those of other eucrites and support the widely held assumption of a common parent body for howardites and eucrites. The isotopic investigations of polymict eucrites and howardites point to a prolonged cratering record on the howardite-eucrite-diogenite (HED) parent body and suggest that it survived the “cataclysm” affecting the Moon ~ 3.8-4.0 Ga ago.