The bulk valence state of Fe and the origin of water in chondrites

There is abundant petrologic evidence for the oxidation of Fe during the aqueous alteration of chondrites, and water must have been the oxidant for this process. The H2 lost from the chondrite parent bodies as a result of Fe oxidation would have been isotopically very light, enriching any residual water in D. The extents of the D enrichments will have depended on the fractions of water consumed and the temperatures during Fe oxidation. Here we have estimated the likely ranges of water consumed by Fe oxidation in the CI, CM, CR and LL parent bodies, as well as the likely range of changes in water H isotopic compositions this would have produced. We first used Fe XANES to determine the Fe valences of bulk meteorite powders in Orgueil (CI1), a number of CMs and CRs that experienced varying degrees of alteration, and Semarkona (LL3.00). The total ranges of bulk Fe valences we obtained were: Orgueil 2.77, CMs 2.40-2.63, CRs 1.46-2.54, and Semarkona 2.10. Combining previous estimates of the present water/OH contents of our samples with the present bulk Fe valences and an estimated range of initial bulk Fe valences, we estimate the likely ranges of fractional water losses to have been: Orgueil 15-26%, Semarkona 73-83%, CMs 23-48%, and CRs 39-62%. The associated maximum and minimum changes in the H isotopic compositions of the remaining water were estimated assuming the equilibrium H-2-H2O isotopic fractionation factor, Rayleigh fractionation of the H2, and oxidation temperatures of 0-200 degrees C. Using previous estimates of the water H isotopic compositions in the chondrites, the ranges of estimated dD values for the initial chondritic waters are: Orgueil -672% to -422%, CMs -676% to -493%, CRs -527% to -56%, and Semarkona -527% to 154%. The CI, CM, CR and ordinary chondrites all accreted water with similar H isotopic compositions that were distinct from the compositions of comets or Saturn's moon Enceladus. Thus, the carbonaceous chondrites are unlikely to have come from comets or from bodies that were scattered into the Asteroid Belt from comet forming regions by orbital migration of the giant planets. If the carbonaceous chondrites did form in the outer Solar System, as some models predict, it was probably not beyond 7 AU. However, based on water isotopic compositions at present it is equally plausible that the carbonaceous chondrites formed in the inner Solar System. (C) 2017 Elsevier Ltd. All rights reserved.