K_2O-rich trapped melt in olivine in the Nakhla meteorite: Implications for petrogenesis of nakhlites and evolution of the Martian mantle

摘要

We used new analytical and theoretical methods to determine the major and minor element compositions of the primary trapped liquid (PTLs) represented by melt inclusions in olivine and augite in the Martian clinopyroxenite, Nakhla, for comparison with previously proposed compositions for the Nakhla (or nakhlite) parent magma. We particularly focused on obtaining accurate K_2O contents, and on testing whether high K_2O contents and K_2O/ Na_2O ratios obtained in previous studies of melt inclusions in olivine in Nakhla could have een due to unrepresentative sampling, systematic errors arising from electron microprobe techniques, late alteration of the inclusions, and/or boundary layer effects. Based on analyses of 35 melt inclusions in olivine cores, the PTL in olivine, PTL_(oliv), contained (by wt) approximately 47% SiO_2, 6.3% Al_2O_3, 9.6% CaO, 1.8% K_2O, and 0.9% Na_2O, with K_2O/ Na_2O = 2.0. We infer that the high K_2O content of PTL_(oliv) is not due to boundary layer effects and represents a real property of the melt from which the host olivine crystallized. This melt was cosaturated with olivine and augite. Its mg# is model-dependent and is constrained only to be ≥19 (equilibrium Fo = 40). Based on analyses of 91 melt inclusions in augite cores, the PTL in augite, PTL_(aug), contained (by wt) 53-54% SiO_2, 7-8% Al_2O_3, 0.8-1.1% K_2O, and 1.1-1.4% Na_2O, with K_2O/Na_2O = 0.7-0.8. This K_2O content and K_2O/Na_2O ratio are significantly higher than inferred in studies of melt inclusions in augite in Nakhla by experimental rehomogenization. PTL_(aug) was saturated only with augite, and in equilibrium with augite cores of mg# 62. PTL_(aug) represents the Nakhla parent magma, and does not evolve to PTL_(oliv) by fractional crystallization. We therefore conclude that olivine cores in Nakhla (and, by extension, other nakhlites) are xenocrystic. We propose that PTL_(oliv) and PTL_(aug) were generated from the same source region. PTL_(oliv) was generated first and emplaced to form olivine-rich cumulate rocks. Shortly thereafter, PTL_(aug) was generated and ascended through these olivine-rich cumulates, incorporating fragments of wallrock that became the xenocrystic olivine cores in Nakhla. The Nakhla (nakhlite) mantle source region was pyroxenitic with some olivine, and could have become enriched in K relative to Na via metasomatism. A high degree of melting of this source produced the silica-poor, alkali-rich magma PTL_(oliv). Further ascension and decompression of the source led to generation of the silica-rich, relatively alkali-poor magma PTL_(aug). Potassium-rich magmas like those involved in the formation of the nakhlites represent an important part of the diversity of Martian igneous rocks.