Microscale distribution and behavior of Th and K in late-stage melts and shock melts in olivine-phyric sher- gottites: Implications for the interpretation of remote sensing and in situ measurements

Introduction: Remotely sensed geochemical data provide extremely valuable information concerning the global distribution of elements on a planetary surface, and are valuable in the identification of major crustal terranes. Planetary geochemical maps of Mars for several elements such as Th, K, and Fe have been or are currently being prepared and have yielded significant insights into large-scale planetary processes [e.g., 1,2,3,4]. The most precise chemical information obtained by remote sensing thus far has been collected by Mars Odyssey, which characterized the Fe, Th and K concentrations across the entire surface of the planet. The Odyssey gamma-ray spectrometer (GRS) measures the abundances in the upper 10's of centimeters of the martian surface, which consists of soils and bed-rock. It is critical to understand how surface processes affect the behavior of Th, REE, and K when interpreting these data. An important observation from the Mars Odyssey and various in situ missions (Pathfinder, MER) is that the Th and K in many of the Mars surface lithologies are not reflected in the martian meteorite collection (Figure 1). If the martian meteorites represent the crustal rocks on Mars then an enrichment mechanism is needed to obtain the higher abundances measured by the GRS. This variation has been attributed partially to surface and hydrothermal alteration of magmatic lithologies. One possible explanation of the dichotomy between meteorites and surface collected data is that K and Th are enriched into the mesostasis, followed by the preferential alteration and erosion of this mesosta-sis, releasing the enriched elements into the soils. In this study, we combine electron and ion microprobe techniques in the analysis of Sm, Th, K, and major elements in both late stage-melt (mesostasis, melt inclusions) and shock-melt assemblages in martian meteorites. The intent of this study is to examine the variation in K and Th in components of martian meteorites that should be enriched in such incompatible elements and are more susceptible to sur-face/hydrothermal alteration. These data sets will contribute to a more accurate interpretation of remotely collected planetary data obtained through orbital or rover missions. Analytical Methods: The initial samples analyzed in this study of martian meteorites are olivine-phyric shergottites: EETA79001, Y980459, and SaU005. Shock melt glass (lithology C in EETA79001,