Wade, J.A., T. Plank, E.H. Hauri, K. Roggensack, K. Kelley (2006) Prediction of arc magma water contents via measurement of H2O in clinopyroxene. Eos. Trans. AGU, 87(52) Fall Meet. Suppl., Abstract V53E-07.
Water is fundamental to arc magma genesis, evolution, and eruption, but most models make predictions that have yet to be tested against actual pre-eruptive H2O measurements. This is because few direct measurements of H2O have been made in arc volcanoes, as rocks found at the surface have extensively degassed upon eruption. We present here a new method for calculating pre-eruptive magmatic H2O, which corresponds closely to water contents measured directly in olivine-hosted melt inclusions.
We picked clinopyroxene (cpx) phenocrysts from ash/lapilli samples from 4 volcanoes whose magmatic H2O contents span the global range: Galunggung (Java), Irazu (Costa Rica), Arenal (Costa Rica), and Cerro Negro (Nicaragua). Nominally anhydrous minerals (NAM’s) such as pyroxene and olivine can incorporate non-trivial amounts of water into their crystal structures, and water contents in these arc cpx range from ~ 40–900 ppm (measured by SIMS ion microprobe). Transects across several grains reveal high H2O in the cores, which decreases towards the rim along with Mg#, Al, and Ca. For example, Irazu cpx range from > 300 ppm H2O at Mg# 88 to < 150 ppm wt% H2O at Mg# 72. These features are most likely the result of growth zoning that develops during degassing and fractionation, giving insight into magmatic evolution during ascent that neither whole rocks nor melt inclusions typically yield.
In order to quantify water concentrations of liquids in equilibrium with the cpx, we have used the Al(IV)-dependent partitioning relationship described by Hauri et al. (2006). In all 4 volcanoes, calculated maximum magmatic H2O contents agree remarkably well with melt inclusion data from the same samples: 0.45, 3.4, 4.3, and 6.5 wt% H2O in Galunggung, Irazu, Arenal, and Cerro Negro, respectively. In the Central American samples, water contents also correlate with Sr/Nd in the cpx (measured by LA-ICP-MS), as predicted by models for slab fluid compositions. The H2O-Sr/Nd trend preserved in the cpx's, however, requires higher Sr/Nd than that predicted by most published estimates of slab fluid compositions, consistent with the high Sr/Nd carbonate-rich sediments subducting at the Central American trench. Therefore cpx phenocrysts not only track magmatic history during ascent, but also may record conditions of the mantle source.
Reference: Hauri, E.H., Gaetani, G.A., Green, T.H. (2006) Partitioning of water during melting of Earth’s upper mantle at H2O-undersaturated conditions. EPSL 248(3-4) p. 715-734.