Apatite as a Key to Evaluating the Volatile Budget of Martian Magmas: Implications from the Chassigny Meteorite

Introduction: Apatite has been extensively studied on Earth because of its geological, geobiological, and materials importance [1], and it is one of the most common mineralogical reservoirs for phosphorus on Earth. Apatite is also present in each of the three types of SNC meteorites, implying the same may also be true for Mars. In addition to phosphorus, apatite commonly contains some combination of fluorine, chlorine, and hydroxyl. Because F, Cl, and OH are three of the most important magmatic volatiles, apatite is potentially an excellent tool for assessing volatiles in magmatic systems. However, interpreting magma budgets from apatite volatile contents is not a straightforward process because apatite volatile chemistry almost always represents the conditions to which the apatite was last exposed ([2],[3]). This problem may be overcome in instances where chemical isolation took place during various stages of crystallization. In such cases, very useful information can be obtained from the change in apatite chemistry ([2],[3]). The work described here focuses on the Chassigny meteorite because it contains chemically isolated apatite that may have preserved information of at least two stages of the magmatic history. The Chassigny meteorite is a cumulate dunite with cumulate olivine and chromite, some intercumulus pyroxene, and late-stage interstitial phases such as maskelynite, apatite, and sulfides. Some of the cumulate olivines contain polyphase melt inclusions consisting of augite, low-Ca pyroxene, kaersutite, pyrrhotite, chromite, pentlandite, Ti-biotite, apatite, and rhyolitic glass [4]. Apatite is also found in the interstitial maskelynite. Apatite in the Chassigny meteorite has been previously reported as chlorapatite ([4],[5]) by default, because of the lack of fluorine analyses. Furthermore, no distinction was made in previous work between apatite found in melt inclusions and that found in the maskelynite. We have analyzed both melt inclusion and maskelynite-hosted apatites with special emphasis on F and Cl in order to identify any possible chemical variations that may have preserved information on magmatic volatile history. Results: Microprobe analyses of Chassigny apatites are given in Table 1 along with respective structural formulas based on 13 anions. These apatite analyses reflect halogen concentrations obtained along the a-axis in order to circumvent the problem of over-counts of halogens (because of electron-beam induced migration of halogens to the surface of the analyzed spot [6]) as can occur in analyses along the c-axis. However, the apatite crystals within the olivine-hosted melt inclusions are extremely small, and the effects of a highly magnified microprobe beam may bias the analysis.