Oxygen Isotope Compositions of Mineral Separates from Snc Meteorites: Constraints on Snc Parental Magmas

Introduction: Martian (SNC) meteorites are igneous rocks from young (except for ALH 84001, which is much older), thick lava flows or shallow intrusions [1]. The shergottites (‘S’ of SNC) can be categorized according to LREE depletion into highly, moderately, and slightly depleted groups [1]. The La/Lu ratio (and other depletion indicators) is positively correlated with oxygen fugacity, and this has been interpreted as evidence of mixing of two source components [1-4]. The inferred end members could be distinct mantle components (e.g., enriched, oxidized domains entrained in the ambient depleted, reduced mantle), perhaps residual to early planetary differentiation [3,5]; alternatively, depleted, reduced magmas could be partial melts of a reduced peridotitic mantle, whereas enriched, oxidized magmas might contain large fractions of oxidized, aqueously altered crustal rocks [2, 6]. Whole-rock SNC oxygen isotope ratios appear to correlate with oxygen fugacity and degree of depletion [3]. This relationship has been interpreted as evidence that martian magmas formed by melting of a depleted, reduced mantle followed by variable amounts of contamination by oxidized, 18 O-rich, aqueously altered crust rather than by mixing of melts from two mantle domains. On Earth, oxygen isotope ratios of mantle peridotites differ systematically from crustal materials (mainly due to aqueous alteration in the shallow crust) [7, and references therein]. This contrast in oxygen isotope composition has been used to constrain crustal assimilation processes and source heterogeneity in terrestrial magmas [8-9]. If the martian crust is similarly enriched in 18 O due to aqueous alteration, this approach could also be applied to quantifying crustal assimilation on Mars. This might be the case if phyllosilicates such as those detected on Noachian crust by OMEGA [10-11] are volumetrically significant components of the crust. Subtle variations in ! 17 O in the SNC's (even within a single meteorite) have also been interpreted as evidence of SNC magma source heterogeneity, possibly from crustal assimilation [12]. Previously available oxygen isotope data may not adequately constrain the " 18 O values of martian magmas [13]: (1) Many SNC's contain high-" 18 O alteration phases from martian and/or terrestrial weathering; and, (2) many SNC's are cumulates that likely differ in " 18 O from the magmas from which they accumulated. Furthermore, the variation recently found in ! 17 O by [12] Table 1. Data obtained by CO2 and O2 analyses. All errors reported on an average (numbers in parentheses are the number of analyses that were averaged) are standard errors. All errors reported on single measurements are 1# of the UWG-2 standard ran on the same day as the sample. Px, pyroxene; Msk, maskelynite; Ol, olivine; Cpx, clinopyroxene; Opx, orthopyroxene.