Rb‐Sr and Sm‐Nd internal isochron ages of a subophitic basalt clast and a matrix sample from the Y75011 eucrite

Abstract

The Yamato polymict eucrite Y75011 contains several coarse‐grained mesostasis‐rich basaltic clasts with subophitic texture. The preservation of extensive Mg‐Fe chemical zoning in pyroxenes and the presence of a dark mesostasis shows that these clasts are representative of pristine lava‐like basalts extruded on the surface of the Howardite‐Eucrite‐Diogenite (HED) parent body. Rubidium‐strontium analyses of a large clast, Y75011,84B, yielded a precise internal isochron age of 4.60 ± 0.05 b.y. for λ(87Rb) = 0.0139 (b.y.)−1 or 4.50 ± 0.05 b.y. for λ(87Rb) = 0.0142 (b.y.)−1. The Rb‐Sr systematics of a matrix sample are nearly identical to those of the pristine clast, showing that the matrix is dominantly comminuted mesostasis‐rich basalt. The Rb‐Sr internal isochron age of the matrix is 4.56 ± 0.06 b.y. for λ(87Rb) = 0.0139 (b.y.)−1 or 4.46 ± 0.06 b.y. for λ(87Rb) = 0.0142 (b.y.)−1. These ages are within error limits of the Rb‐Sr whole rock age of chondrites and are in agreement with 206Pb/207Pb and Sm‐Nd ages of ordinary monomict eucrites for λ(87Rb) = 0.01402 (b.y.)−1 based on a comparison of the Rb‐Sr and U‐Th‐Pb ages of chondrites (Minster et al., 1982). The good precision obtained for the Rb‐Sr ages of both clast and matrix is due to the presence of a mesostasis phase in which the Rb/Sr ratio is enriched 3–4 times above that of any eucrite phase previously analysed. The initial 87Sr/86Sr ratios of clast and matrix are 0.69894 ± 2 and 0.69896 ± 3, respectively, and are in satisfactory agreement with the BABI value (Papanastassiou and Wasserburg, 1969). There is evidence of isotopic disturbance in both the Rb‐Sr and Sm‐Nd systems of the most magnetic separate analysed. A Sm‐Nd internal isochron of 4.55 ± 0.14 b.y. for the combined data from both clast and matrix exclusive of the most magnetic separate is in agreement with the more precise Rb‐Sr age. The corresponding initial 143Nd/144Nd = 0.50587 ± 19, normalized to 148Nd/144Nd = 0.24308. The clast data alone, exclusive of the most magnetic separate, define an isochron age of 4.52 ± 0.16 b.y. and initial 143Nd/144Nd = 0.50593 ± 19. The isochron error limits quoted here for the Sm‐Nd data are 2σ from the York (1966) program and include the factor [S/(N−2)]½ used in that program to compensate for “geological error.” However, this factor is not appropriate for the Rb‐Sr isochrons and has been omitted from the quoted error limits. The initial 143Nd/144Nd of the pristine clast at 4.56 b.y. ago is 0.505877 ± 25, in agreement within error limits with the CHUR value (Jacobsen and Wasserburg, 1984). Clast Y75011,84 represents the oldest pristine, unambiguously basaltic lava as yet recovered from a solar system object. Rare earth element abundance measurements (Shimizu and Masuda, 1985) show it to be one of the most differentiated eucrites. Thus moderately evolved basaltic lavas were produced very early in the history of the HED parent body. Their preservation as clasts in the polymict eucrites suggests derivation from a different geologic setting on the parent body than that occupied by the more thermally metamorphosed monomict eucrites. We suggest that the preservation of pristine clasts in polymict eucrites may be due to their impact excavation to the surface of the parent body away from heat sources concentrated within impact craters. The monomict eucrites may be derived from the floors of craters where they were thermally metamorphosed by shock heating and by the presence of overlying impact melts and nearly molten breccias.