38 Economic Geology, Igneous and Metamorphic Petrology, Mineralogy (Posters)

Tuesday, 17 April 2018
8:00 AM - 5:30 PM
First Floor Lobby

IMPLICATIONS OF BIMODAL OLIVINE COMPOSITIONS IN VHK BASALTS

Paper number: 38-15
Program: Theme Sessions
Time: 8:00 AM – 5:30 PM

Authors

Abstract

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 (FoMgOlivine > 60, FoFeOlivine < 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.

Geological Society of America Abstracts with Programs. Vol. 50, No. 4

doi: 10.1130/abs/2018NC-313048

© Copyright 2018 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.