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The isotopic composition of chlorine in apatite from eucrites

Introduction: Chlorine is an important element being incompatible in nearly all silicates, extremely volatile and strongly hydrophilic [1]. The Cl isotope composition (Cl/Cl) of meteorites, therefore, can provide useful information regarding the isotopic reservoirs that were present in the early Solar System and any secondary processes that may have occurred on different parent bodies [2, 3]. There is relatively a narrow range in the Cl isotope compositions of terrestrial rocks, and martian and chondritic meteorites (δCl ~ 4 to +1 ‰) [2-4], with an exception to this rule being the Moon. It has recently been demonstrated that lunar samples display a significant range in Cl isotopic composition, especially lunar apatite [Ca5(PO4)3(F,Cl,OH)], which is a common accessory mineral that can contain abundant Cl, and which shows δCl values ranging from typical terrestrial values of ~ -4 ± 2 ‰ to extremely enriched values of +81 ± 2 ‰ [5-11]. The Howardite-Eucrite-Diogenite (HED) suite of meteorites represents the largest suite of crustal and sub-crustal rocks available from a differentiated basaltic asteroid (most probably 4 Vesta) [12-13]. The eucrites are also some of the oldest igneous rocks in the Solar System, typically with crystallization ages from 4547 Ma to 4559 Ma [14-15]. Therefore, characterizing the volatile inventories of eucrites can help us understand the processes operating on asteroid parent bodies early in the Solar System history. In order to investigate the history of volatiles in eucrites, we have measured Cl isotopic compositon of apatite from the cumulate eucrite Moore County and the basaltic eucrite Dar al Gani 945 (DaG 945). Apatite grains in these two samples were previously analyzed for their H isotopic composition [16]. Methods: The Cl content and isotopic compositions were measured using the Cameca NanoSIMS 50L at The Open University in multi-collection mode based upon the protocol of Tartese et al. [9] and Barnes et al. [10], acquiring negative secondary ions of OH, O, Cl, Cl and CaF, simultaneously on electron multipliers. A primary current of ~50 pA (in Cs+ mode) was used for the analyses. Typically a 3 × 3 μm area was analyzed, however for some larger grains this area was increased to 5 × 5 μm. No electronic gating was used. Results: A total of 12 measurements from nine individual apatite grains were obtained. Apatite in both samples is typically located within mesostasis regions (Fig. 1).