HR: 1330h
AN: V52A-0421    [PDF]
TI: The Role of Shock in the Redistribution of Volatile Elements in Basalts. Implications for the Interpretation of Martian Magmatism.
AU: * Shearer, C K
EM: cshearer@unm.edu
AF: Institute of Meteoritics, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 871131 United States
AB: The mineralogy and geochemistry of martian basalts have been used to estimate the amount of water in martian magmas and to model the martian hydrologic cycle. The mineralogy of martian basalts and the composition of associated melt inclusions have been interpreted as indicating that martian basaltic magmas had very little water (less than 0.1% H2O). On the other hand, the behavior of geochemical tracers (Li and B) has been interpreted as indicating that this low water content was the product of extensive magmatic outgassing and that martian basalts initially contained significant water. This interpretation is based on the observation that Li and B decrease in the rims of pyroxenes in basaltic shergottites resulting from the partitioning of these incompatible elements into a fluid phase during degassing. In that many martian basalts experienced substantial shock (20-40 GPa), it is possible that the magmatic volatile record preserved in martian basalts has been disturbed. To better understand the possible effects of shock on this volatile record, we are studying the redistribution of volatile elements in naturally and experimentally shocked basalts. The initial study of tholeiitic basalts associated with the Lonar impact structure in India illustrates one of the possible effects of shock on the redistribution of volatile elements such as Li and B. High-Ca pyroxene in the unshocked Lonar basalt ranges in composition from approximately Fs23 in the core to Fs41 at the rim. As expected in a basaltic system, incompatible trace elements Ce, Be, Li and B increase from pyroxene core to rim. Relationships among incompatible elements are characteristic of basaltic systems (i.e. Li/Be decreases with increasing Li). The shocked basalt examined in this study is characterized by the conversion of plagioclase laths to maskelynite. This corresponds to shock pressures between 20-40 GPa. The high-Ca pyroxene in this basalt has similar major element characteristics as the unshocked basalt and both Ce and Be increase from core to rim. In contrast to the unshocked basalt, the pyroxene exhibits a decrease in Li, Li/Be and Li/Ce in the high-Fe rim. These initial observations suggest that shock may produce the decrease of volatile trace elements in pyroxenes that had been attributed to degassing.
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
DE: 1065 Trace elements (3670)
DE: 3640 Igneous petrology
DE: 3662 Meteorites
DE: 3672 Planetary mineralogy and petrology (5410)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting