Clinopyroxene in postshield Haleakala ankaramite: 1. Efficacy of thermobarometry

Magma storage depth is a fundamental aspect of a volcano's magmatic plumbing system that may be resolved using mineral-melt thermobarometry, assuming crystal growth occurs at near-equilibrium conditions. We acquire major and minor element compositional analyses of whole rock, groundmass separates, and clinopyroxene in ankaramite erupted ca. 214 ka at Haleakala volcano to evaluate the efficacy of thermobarometry. Using various thermometer and barometer combinations, we obtain values of crystallization pressure (60-1500 MPa) that are generally consistent with those of previous studies, but find that the models most successful at recovering the conditions of relevant equilibrium experiments yield values at the low end of this range (<= 950 MPa). We use quantitative EPMA spot analyses to transform X-ray element intensity maps into metal oxide concentrations maps and to produce qualitative pressure maps of whole crystals. The spatial context provided by this procedure reveals two compositionally distinct domain types not evident in the spot analysis data set, with median Na2O contents differing by up to 26 % between domains. Na-rich domains represent putative crystallization pressures that are up to 365 MPa higher than Na-poor domains, within individual crystals. The presence of Na-rich domains associated with euhedral facets in contact with matrix is not consistent with concentric growth at near-equilibrium conditions of decreasing pressure, but rather co-crystallization of both domains under conditions of partial disequilibrium. Conservatively assuming that low-Na regions are less prone to kinetic partitioning, crystallization pressures for the Haleakala ankaramite correspond to crustal levels. We conclude that the reservoir supplying postshield eruptions at Haleakala has not deepened into the mantle, as was reported in a previous application of clinopyroxene thermobarometry to Haleakala's postshield magma (Chatterjee et al. 2005).