Stable Chlorine Isotope Reservoirs in Chondrites

Introduction: By analyzing the stable Cl-isotopes of halite from the Zag H-breccia and comparing the results with those from other chondritic materials we can provide new information about isotopes in the early Solar System. In particular Cl-isotope work may complement O-isotope studies. We show for the first time that substantial Cl/Cl fractionation exists between chondrite components. Zag halite crystallized from an evaporating brine at << 100C on the H-parent body [1,2]. The absence of clay alteration in olivine shows the rapid nature of this process which could have been started through melting of ice emplaced onto the planetesimal surface [1]. Techniques: Water soluble extracts were prepared from halite and H4 matrix from Zag (extract was primarily dissolved halite). Analysis of Cl/Cl was carried out by TIMS. To further characterise brine compositions halogen values were determined by ion chromatography [3]. Extracts were also analysed from the CM2 fall Murchison (Br/Cl ratio only) and H5 find Plains. δCl is relative to SMOC. Results and discussion: Zag halite and the silicate extract have light Cl-isotopic values compared to bulk carbonaceous chondrites [4] (Fig. 1). The Br/Cl ratios of the halite samples are slightly less than that of Orgueil. Evaporation fractionates Cl-isotopes by ≤ 0.4‰ relative to a parent brine [5] suggesting that Zag isotope ratios are close in value to an initial reservoir. Pore fluid fractionation and crystallization of clays can also act to fractionate Cl-isotopes towards light values [5] but there is no sign of such extensive fluid activity in Zag [1]. The higher Br/Cl ratios of extracts from the Plains find suggest that terrestrial adsorption of halogens acts to increase Br/Cl ratios. This might also be expected to alter the δCl values but adsorption of aerosols tends to increase δCl [6], so the influence of terrestrial alteration is currently uncertain. However, the Zag halite was sampled pure so terrestrial contamination can be ruled out for it [1,2]. In the absence of any obvious mechanism to fractionate the Cl-isotopes during fluid activity, the difference in δCl between whole carbonaceous chondrites and our water soluble extracts suggests that there were at least 2 distinct reservoirs with δCl ~ -1 to –2‰ (ice/parent body brine) and δCl ~ 2.5 to 4‰ (chondrite silicate solids). These reservoirs might be related to O-poor fluids and O-rich solids on parent bodies [7]. Fig. 1. Br/Cl (wt) vs. δCl (‰). Water soluble extracts (this study): Zag halite (2 samples from 1 grain); Zag (H4 matrix); Plains (H5 find). Terrestrial seawater [3]. Whole chondrite pyrolysis, Br/Cl ratios for 3 carbonaceous chondrites [4,8].