RAPID CRYSTAL GROWTH IN THE SOLAR SYSTEM’S SLOWEST COOLED MAGMA? INVESTIGATING THE PARADOX OF OLIVINE P-ENRICHMENT IN TROCTOLITE 76535 PARENT MELT WITH LABORATORY CRYSTALLIZATION EXPERIMENTS

Lunar troctolite 76535 is a cumulate of geochemical significance for the evolution of the lunar interior; it retains textural and compositional evidence of extremely slow cooling. The paradox posed by recent discovery of mm-scale dendritic P-zoning in olivine crystals of 76535 (Hammer et al., 2017, LPSC), signaling rapid initial olivine crystal growth, motivates our experimental study of olivine nucleation and growth kinetics. Guided by compositional constraints (Sonzogni et al., 2015, and O’Sullivan and Neal, 2013) and MELTS (Gualda and Ghiorso, 2012, Journal of Petrology) modeling, we synthesized a starting composition from reagents and performed one high-temperature quench and two dynamic cooling experiments in a 1-atm furnace utilizing a CO₂-H₂ gas mixture to control oxygen fugacity at 2 log units below the FMQ buffer and Re wire loops to suspend samples in the furnace hot spot. In the crystallization runs, samples were first superheated (held 24 h at 100 and 50 °C above the MELTS calculated liquidus, 1293 °C) and then undercooled (to 75 °C below the liquidus and held 24 h). The quench experiment contains only glass, and the crystallization experiments contain glass, forsteritic olivine (Fo₈₈), and anorthitic plagioclase (An_96-97). The first of the latter experiments was compromised by Fe-loss to a nearby Pt wire. The olivine and plagioclase compositions in the crystallization experiments are similar (except as related with Fe-loss) and match those in 76535, affirming a suitable starting melt. In an unexpected texture, we observed plagioclase laths with internal cavities that are either unfilled (only glass) or filled with olivine. Olivine crystals occurred almost exclusively in contact with plagioclase, not distributed homogeneously throughout the sample; the crystals attached to the edges of plagioclase laths and in contact with glass are polyhedral and lesser in Mg than those in the plagioclase. We interpret this texture as indicating that both phases grew rapidly and that one or the other nucleated heterogeneously on the first-to-form phase. Our experiments were not intended to explicitly mimic the thermal history of 76535 parent magma, but the high superheating and undercooling ensured we induced rapid crystal growth and could begin to explore those influences on olivine growth kinetics.