Evolution of water reservoirs in the early solar system through their oxygen isotopic composition

• Main Author: Baker, Lee
• Published: Open University 2001
• Subjects: 520
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.523916

A new technique has been developed to enable analyses of δ¹⁷0 and δ¹⁸0 from small water samples extracted from meteorites. Isotopic measurements are made using a continuous-flow, isotope ratio mass spectrometer (Delta C, Finnigan Mat) utilising a helium carrier gas. Oxygen is extracted from water using the powdered, solid fluorinating agent CoF₃, and is purified using a GC column (PLOT, 5Å). Reproducibility of the method, determined using solid standards and reference waters is ±.30%c for δ¹⁸O and +0.14%c for Δ^I7O (1σ in each case). The technique has been applied to three suites of meteorites: Carbonaceous chondrites (CI and CM), SNC meteorites (four meteorites) and a selection of Antarctic samples, three eucrites and one ordinary chondrite. Results were used to constrain models of the origin of Solar System water and its subsequent interaction on the meteorite parent bodies. Results from carbonaceous chondrites suggest that water accreting to bodies in the early Solar System was enriched in both ¹⁷O and ¹⁸O and had a Δ¹⁷0 of at least +2%c The isotopic composition of water evolved during hydrothermal alteration on parent bodies toward lower Δ¹⁷O values, more similar to that of the host rocks. The complex release profile indicate that these meteorites experienced several periods of hydrothermal activity. SNC sample results have suggested the presence of two isotopically distinct reservoirs, the silicate crust and the hydrosphere each possessing distinct Δ¹⁷0 values. A third component with a large ¹⁷O excess (+4%) was detected in ALH84001. The magnitude of this anomaly indicates an origin due to heterogeneous accretion or late stage veneer and cannot be produced by hydrodynamic escape. Results from the eucrites and ordinary chondrite also suggested isotopically distinct parent body silicate and hydrous reservoirs and may also be consistent with widespread late input of isotopically distinct water in the Solar System.