1. Magmatic water content in HIMU basalts from the Cook-Austral Islands: constraints on degassing processes and source composition from clinopyroxene phenocrysts.
- Author
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Bruce-Etzel, Leslie A., Marshall, Edward W., and Lassiter, John C.
- Abstract
The amount of water recycled during subduction is unclear. Water/Ce estimates in HIMU (high-
238 U/204 Pb) basalts are variable, ranging from < 100 to > 250 in glasses and melt inclusions. Because clinopyroxene (cpx) is a common early liquidus phase and the cpx/melt partitioning of water is well constrained, cpx phenocrysts provide an additional constraint on magmatic water contents. We present water and trace element concentrations in HIMU-basalt-hosted cpx phenocrysts from the Austral Islands. Calculated melt [H2 O] (up to 4.3 wt.%) and H2 O/Ce ratios (22–825) are higher than in olivine-hosted inclusions, as are estimated equilibration pressures. Correlation between estimated melt [H2 O] and crystallization or entrapment pressure suggests significant water loss during magma ascent. Most ocean island basalts (OIBs) span a limited range in H2 O (~ 1–1.5 wt.%), and low (< 100) H2 O/Ce ratios are primarily observed in melts with unusually high [Ce] (up to 350 ppm). Additionally, [H2 O] and H2 O/Ce in some suites correlate with entrapment pressure despite having quench pressures high enough to prevent significant water loss from open- or closed-system degassing (< 100 MPa). Polybaric "sparging", whereby low-P melts re-equilibrate with CO2 -rich fluids exsolved at higher pressure, may result in water loss at pressures less than CO2 saturation. This may more accurately describe OIB degassing processes than open or closed system degassing. After correcting for degassing, primary Australs melts likely have H2 O/Ce of ~ 300–600. If applicable to OIB sources in general, this limits the total water budget of the mantle, including the mantle transition zone, to < 2.4 ocean masses. [ABSTRACT FROM AUTHOR]- Published
- 2024
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