Source Material And Melting Condition of Nakhlite Magmas

The shergottite, nakhlite and chassignite (SNC) meteorites are widely accepted as being of martian origin (McSween, 1994), and have been studied extensively in relation to martian magmatism. Much research has therefore been conducted on the SNC meteorites in order to elucidate the nature of martian magmatism. Although the parent magma compositions and crystallization histories of Martian magmas have been well examined (e.g., Wadhwa and Crozaz, 1995), the source materials of Martian magmas are not yet fully understood. Consequently, the nature and the origin of the source materials are still ambiguous. Recently, Shimoda et al. (2003) have proposed a model that constrains the geochemical characters of source materials of martian magmas. Although this model can successfully explain the origin of martian magmas and the relation among the SNC meteorites, there remain two questions for nakhlite production processes. One is the process that responsible for high CaO and low Al2O3 concentrations of nakhlites and the other is degree of partial melting of nakhlites. The latter is rather problematic, because the major element composition of nakhlites suggests very high degree of partial melting, however, the trace element, in particular REE, composition and the depleted isotopic signatures of nakhlites suggests very small degree of partial melting. In this study, we propose improved model for nakhlite production processes based on the plume model proposed by Shimoda et al. (2003). According to this model, the depleted isotopic signatures of nakhlites were produced by existence of residual majorite during the early differentiation processes at the bottom of martian mantle. The nakhlite production processes were probably occurred at the middle of martian and this high-pressure melting would be responsible for the high CaO and low Al2O3 concentrations of nakhlites due to the absence of residual clinopyroxene. The existence of residual garnet during the melting can reproduce the REE composition of nakhlite with realistic degree of partial melting (5 %). The estimated major element and REE compositions of nakhlite parent magma are well agree with those of previously reported nakhlite parent magmas, suggesting the validity of the model.