The chlorine isotope composition of chondrites and Earth

The chlorine isotope composition of chondrites provides information about isotopic reservoirs that were present in the solar nebula and the effects of secondary processes on their isotopic composition. We analyzed chlorine isotope ratios of 19 chondrites that included both unaltered (type 3) and altered (types 1,2 and 4–6) chondrites from the carbonaceous, ordinary, and enstatite classes as well as an enstatite chondrite impact melt breccia and a partial melt residue. Chlorine isotope compositions (δ37Cl, ‰ vs. SMOC) and 1σ uncertainty () are as follows (excluding melt residue and anomalous Parnallee):MaterialCarbonaceous chondritesOrdinary chondritesEnstatite chondritesStructurally-bound Cl−0.3(0.6)−0.4(0.6)0.4(0.4)Water-soluble Cl−0.9(0.9)0.3(2.1)0.4(0.6)Bulk (total) Cl−0.2(0.6)−0.4(0.7)0.4(0.3)Full-size tableTable optionsView in workspaceDownload as CSVPetrologic type 3 chondrites are the least equilibrated and are those most likely to retain primary nebular signatures. Type 3 carbonaceous chondrites have bulk δ37Cl values that average −0.3 ± 0.5‰ and structurally-bound Cl values of −0.3 ± 0.3‰. These values are indistinguishable from the δ37Cl values of the bulk Earth (−0.2‰) and close to the lowest measured value of the Moon (−0.7‰). From this similarity, we conclude that the inner regions of the early solar nebula had a homogeneous chlorine isotope reservoir. For samples that have undergone secondary processing, δ37Clbulk values range from −1.2‰ to 0.8‰ for metamorphosed samples (average 0.0‰) and −0.3‰ to 0.3‰ (average 0.1‰) for aqueously-altered samples (Orgueil, Ivuna, Murray, and Murchison). Most of the data can be explained in terms of fractionation processes similar to those that occur on the Earth, such as interaction with pore waters. No apparent correlations exist with Cl concentration data. Ordinary chondrites have the lowest and most restricted range of Cl concentration, but the largest range in δ37Cl values. Carbonaceous and enstatite chondrites have wide ranges of Cl concentration and more restricted ranges of δ37Cl values.We have analyzed additional terrestrial mantle-derived materials of the Earth in order to better constrain the δ37Cl value of this reservoir and compare with the chondrite data. The mantle data cluster at −0.3‰ to −0.1‰, and support the idea that the most pristine type C chondrites and the bulk Earth have the same δ37Cl value, indicating that no fractionation by evaporative processes occurred during the formation of the Earth. The discrepancy between our data and previously published results is discussed.