Congratulations to Xiaofei Pu who defended her dissertation on March 9, 2018

Advisor: Rebecca Lange

Abstract

Erupted basalts are windows into the deep Earth. This dissertation provides evidence that basalts grow their phenocrysts during rapid ascent to the surface, and that the most Mg-rich olivine in erupted samples approximates the first mineral to crystallize from a liquid with a composition of the whole rock.  This presents opportunities to constrain the temperature, oxidation state and water content of these basalts on the basis of olivine-melt equilibrium. 

Chapter II develops a new olivine-melt thermometer based on the partitioning of Nickel (DNiol/liq), and provides evidence that it is far less sensitive to the effects of pressure and dissolved H2O in the melt than DMg(ol/liq), which is the basis of most olivine-melt thermometers in the literature. The application of both thermometers to a set of subduction-zone basalts allows the depression of the olivine liquidus due to the effect of dissolved water to be determined based on the different temperatures calculated from the two thermometers; a minimum H2O content in the melt at the onset of olivine crystallization can be determined. 

Chapter III investigates the sensitivity of DNi(ol/liq) to dissolved H2O in basaltic melts through a series of olivine-melt equilibrium experiments. Four 1-bar experiments, one anhydrous experiment at 0.5GPa, and five hydrous experiments at 0.5GPa are presented. The Ni-thermometer developed in Chapter II recovers the experimental temperature for all ten experiments within 14˚C on average, including those where the melt contained at least 4.4 wt% H2O. In contrast, the Mg-thermometer (Chapter II) recovers the T(expt) of the anhydrous experiments within error (±26˚C), but overestimates T(expt) by 88-141˚C for the hydrous experiments. The results shows a negligible dependence of DNi(ol/liq) on pressure and dissolved water under crustal conditions, which confirms that the olivine-melt Ni-thermometer can be applied to hydrous basalts at <1 GPa without corrections for H2O content in the melt and pressure.   

Chapter IV applies the new Ni-based olivine-melt thermometer developed and tested in Chapter II and III to a set of mantle-derived, high-K melts that erupted within the Colima rift in western Mexico, where a mid-ocean spreading ridge is interacting with a subduction zone.  Application of the Ni-thermometer, together with phase-equilibrium experiments on phlogopite lherzolite from the literature, shows that the K-rich Colima melts segregated near the base of the relatively thick lithosphere of the Jalisco block at relatively high pressures (~2.5 GPa) compared to most subduction-zone melts (~1.5 GPa).  These temperatures and pressures of melt segregation provide key constraints on geodynamic models of this complex tectonic setting.

Chapter V investigates the cause of the relatively high oxygen fugacity (fO2) of the pristine K-rich Colima lavas (Chapter IV). The pre-eruptive oxidation state (Fe3+/Fe2+ ratio), derived from olivine-melt Fe2+ - Mg exchange equilibrium, indicate fO2 values that are ~2 log units lower than the post-eruptive value; yet they are still significantly higher than those found in most terrestrial basalts. This elevated pre-eruptive fO2 leads to higher solubility of sulfate in the melt, which degasses as SO2 and H2S, and drives melt oxidation. This is the first documentation of sulfur degassing-induced oxidation observed in natural samples.