The influence of variable oxygen fugacity on the source depths of lunar high-titanium ultramafic glasses

Abstract

We present the results of high pressure, high temperature multiple saturation experiments at variable oxygen fugacity (fO2) conditions (IW+1.5 and IW−2.1) on three lunar high titanium ultramafic glasses: the Apollo 17 Orange glass (A17O, 9.1 wt% TiO2), the Apollo 15 Red glass (A15R, 13.8 wt% TiO2), and the Apollo 14 Black glass (A14B, 16.4 wt% TiO2). We performed experiments in graphite (fO2 = IW+1.5) and iron (fO2 = IW−2.1) capsules. The experimentally determined multiple saturation points (MSPs) in graphite capsules are 2.5 GPa and ∼1530 °C (A17O), 1.3 GPa and ∼1350 °C (A15R), and 1.55 GPa and ∼1425 °C (A14B). In iron, we found MSPs of 3.3 GPa and ∼1565 °C (A17O), 2.8 GPa and ∼1490 °C (A15R), and 4.0 GPa and ∼1540 °C (A14B). These results, when combined with previous experiments on the lunar ultramafic glasses, indicate that the increase in the pressure of multiple saturation is linearly proportional to the TiO2 content of the melt (ΔMSPGPa=0.14×TiO2(wt%)-0.15, R2 = 0.93, RMSE = 0.2 GPa). The high depths of melting correlated with the lowest fO2 conditions are hard to reconcile with buoyancy constraints on these iron and titanium rich magmas. In addition, measurements of fO2 on the orange glass as well as the presence of iron blebs in the glasses suggest that the glasses were reduced during eruption. To reconcile buoyancy constraints with fO2 estimates, we present a model in which the high titanium magmas experienced higher fO2 conditions at their source, but underwent subsequent reduction at shallow depths (4–52 km) just prior to their eruption. In this model, we can then further bracket the depth of melting to be from the minimum multiple saturation pressure in graphite to the deepest depth at which the magmas are buoyant: assuming the Hess and Parmentier (1995) post overturn cumulate mantle, the depths of melting range from ∼550–770 km for the A17O glass, ∼260–490 km for the A15R glass, and ∼320–350 km for the A14B glass.