The Use of Crystallization Modelling Software for the Approximation of Multiple Saturation Points for Lunar and Martian Basalt Compositions

Abstract:

Determining the high P-T conditions of liquidus multiple saturation points (MSPs) for planetary basalts, which approximate the conditions where a magma may have last equilibrated with the residual mantle phases during partial melting, is typically done through crystallization experiments. However, petrologic modelling software that can calculate phase equilibrium at high P-T conditions present an attractive potential alternative to this type of experiments. In order to explore how effectively software such as pMELTS and MAGPOX can approximate experimental MSP results for lunar and martian basalt compositions, we assessed seven lunar picritic glasses or mare basalt compositions, and four martian basalt compositions used in previous crystallization experiments and carried out a series of crystallization calculations using pMELTS (5.6.1 version). The compositions reported in the literature, which consisted of four martian basalt compositions with experimentally determined MSPs and a subset of lunar compositions spanning the ranges of TiO₂ contents and MSP depths, were investigated at redox conditions of IW for the lunar compositions and between IW +0.5 to +2 for the martian compositions. Multiple iterations for batch crystallization were carried out from 1000°C to 1700°C and 0.25 to 3 GPa.

Experimentally determined MSPs occurred between 0.6 GPa to 2.5 GPa with temperatures usually above 1500°C. The pMELTS results reproduce olivine-pyroxene MSP’s for all compositions, consistent with experimental results. However, the MSP P-T conditions showed little variation, particularly in pressure, compared to experimental results, regardless of composition. All the pMELTS MSP’s occurred between 1 and 1.3 GPa and between 1300 and 1400 °C, except for a high-Ti mare basalt composition (~0.7 GPa). Similar results had already been obtained for the martian compositions by a previous research (Balta and McSween, 2013, JGR-Planets).The lack of variation despite the compositional differences may be a product of the model calibration, which has focused on melting of peridotitic terrestrial mantle compositions. At present, pMELTS is not able to approximate MSP conditions for planetary basalts with high fidelity. A more thorough analysis using additional compositions and the MAGPOX software is currently underway.