Effects of magma chamber processes on water and H2O/Ce ratios in HIMU magmas from the Cook-Austral Islands : new insights from clinopyroxene phenocrysts

The HIMU mantle end member is characterized by a radiogenic Pb-isotopic composition and is thought to represent recycled oceanic crust. Therefore, the H2O content of HIMU mantle sources can provide constraints on the amount of H2O recycled into the deep mantle via subduction. Low H2O/Ce ratios (<100) have been reported for EM-type submarine quenched glasses, suggesting a relatively “dry” source [Dixon et al., 2002]. Olivine-hosted melt inclusions in HIMU lavas from Mangaia, Cook Islands have H2O/Ce ratios up to 245, suggesting a damp source [Cabral et al., 2014]. On the other hand, Jackson et al. [2015] reported low H2O/Ce ratios (<100) in submarine glasses from Tuvalu seamount with HIMU-like isotopic signatures. Hydrogen diffusion is rapid in olivine and could result in H2O loss or gain in olivine-hosted inclusions after entrapment. Hydrogen diffusion is slower in clinopyroxene than in olivine. Therefore, we measured H2O, major and trace elements of clinopyroxene phenocrysts in a suite of lavas from the Cook-Austral Islands. Calculated H2O concentrations and H2O/Ce ratios of melts in equilibrium with pyroxene phenocrysts range from 0.26 to 3.1 wt% and 78 to 304, respectively. H2O concentrations of melts in equilibrium with pyroxene phenocrysts are systematically higher than most olivine-hosted melt inclusions from the same samples. These observations suggest that olivine-hosted melt inclusions often experience post-entrapment H2O loss. Additionally, negative global correlations between H2O/Ce and Ce show that low H2O/Ce ratios are likely the result of high Ce rather than low H2O. Although open and closed system degassing models show little H2O loss at pressures greater than 10 to 50 MPa, there is a strong correlation between H2O/Ce and entrapment pressure in global OIB data. We propose H2O may be lost via “sparging” where CO2-rich vapors generated at high-pressures percolate through magmas at lower pressures, resulting in degassing trends that approach isopleths of constant CO2/H2O. Because most samples previously used to constrain H2O/Ce in different OIB mantle sources have low quench or entrapment pressures, previous estimates of H2O/Ce ratios in EM and HIMU mantle may be too low.