IMPLICATIONS OF BIMODAL OLIVINE COMPOSITIONS IN VHK BASALTS

The petrogenetic pathway of very high potassium (VHK) basalts remains a mystery - they are hypothesized to be derived from high alumina (HA) basalts, but the source of potassium within them remains uncertain. Three hypotheses have been proposed to explain VHK evolution: granite assimilation by a HA magma, combined KREEP and granite assimilation by a HA magma, or impact melt infiltration. There appears to be a correlation between the infiltration of potassium-rich material and the stabilization of Fayalitic olivine as most VHK basalt samples contain both Fayalitic and Forsteritic olivine crystals, which could signify changing conditions in the lunar magma. To investigate conditions that stabilized iron-rich olivine in VHK basalts whole samples were analyzed for major and minor element compositions using a CAMECA SX-50 electron microprobe, and select crystals were further analyzed for trace elements using LA-ICP-MS via a New Wave Research UP213 laser and a Thermo Finnigan Element 2 high resolution inductively coupled plasma mass spectrometer. Elemental data gathered revealed that Mg-rich olivine crystals were chemically distinct from Fe-rich ones (Fo_MgOlivine > 60, Fo_FeOlivine < 40), and that K-feldspar generally surrounds Fe-rich olivines, but does not touch magnesian ones. LA-ICP-MS analyses revealed that Fe-rich olivines are enriched in heavy rare earth elements as well as incompatible elements such as Sc and Ti, while Mg-rich olivines exhibit higher compatible element content (i.e. Co, V, Cr). These two groups do not overlap which suggests that Forsteritic olivine crystals were insulated from the effects of infiltrating alkali material that stabilized Fayalitic olivine. This melt appears to have been highly fractionated, but more work remains to see if olivine compositions are indicative of material assimilation or impact melt infiltration.