COUPLED 63CU AND 16O EXCESSES IN CHONDRITES

Abstract

Recent developments in multiple-collector magnetic-sector ICP-MS (inductively coupled plasma-mass spectrometry) have permitted the relative abundances of the two isotopes 63 and 65 of copper to be measured with unprecedented precision (40 ppm). Here, we report Cu isotopic variations among eight carbonaceous chondrites (CCs) from the CI, CM, CO, and CV groups and the presently ungrouped Tagish Lake, and 10 ordinary chondrites (OCs) from the H, L, and LL groups. The widest isotopic range of ~0.8%% per a.m.u. is observed for the carbonaceous chondrites. Copper in carbonaceous chondrites becomes isotopically lighter with petrologic type in the order 1 to 3 but seems extremely homogeneous for each type. The Cu isotopic composition of Tagish Lake confirms its other characteristics that are intermediate between CI and CM. In three of the groups (CI-CM-CO), as well as for Tagish Lake, 63Cu excess over terrestrial mantle abundances correlates well with 16O excess. For all four groups, 63Cu excess also correlates remarkably well with elemental refractory/volatile ratios (e.g., Ca/Mn). For ordinary chondrites, small differences exist between the H, L, and LL groups, with Cu becoming isotopically heavier in that order. Equilibrated and unequilibrated samples, however, exhibit the same Cu isotopic signature within each group. Although the range of Cu isotopic compositions in ordinary chondrites is smaller than in carbonaceous chondrites, 63Cu excesses still correlate with 16O excesses. The observed trends of isotopic variation seem incompatible with a single-stage fractionation process by either volatilization or low-temperature metamorphism. The correlations between 63Cu excesses and 16O excesses suggest the presence of at least two and perhaps three isotopically distinct Cu reservoirs in the early Solar System: (1) an Earth-like reservoir common to the CI and LL probably representing the main Cu stock of the inner Solar System, (2) a reservoir present in all carbonaceous chondrites, but most abundant in CV, with large 63Cu and 16O excesses (this reservoir is probably hosted in refractory material), and (3) possibly a third reservoir present in ordinary chondrites. The OC trend may also be explained as a mixture of the first two Cu reservoirs if its oxygen was first equilibrated with nebular gas. The coexistence of 63Cu and 16O excesses in the same component raises the issue of how volatile Cu was preserved in refractory material. A strong correlation between 63Cu/65Cu and Ni/Cu ratios suggests that 63Cu excess may have originated as more refractory 63Ni (T1/2 = 100 yr) upon irradiation of refractory grains by electromagnetic flares and particle bursts during the T-Tauri phase of the Sun.