Hornblende, cummingtonite, and biotite hydrogen isotopes: Direct evidence of slab-derived fluid flux in silicic magmas of the Taupo Volcanic Zone, New Zealand

Abstract

Hornblende, cummingtonite, and biotite δD values from rhyolitic and mingled/mixed mafic rocks from the Central Taupo Volcanic Zone were determined in an effort to characterize the isotopic composition of the dissolved fluid of magmas in this predominantly rhyolitic volcanic zone. Hand samples were obtained from pyroclastic flow, airfall, and lava flow deposits as well as co-eruptive cognate plutonic blocks. Mineral δD values range from −88 to −35‰ SMOW (Standard Mean Ocean Water). Cummingtonite phenocryst δD overlaps with hornblende from the same rocks, while biotite values are consistently ~7‰ more negative than hornblende phenocrysts from the same hand sample. Calculated volcanic steam δD values (ca. −46 to −25‰) are consistent with an island arc crustal source (ca. −40 to −15‰), with a negligible to absent mantle (ca. −100 to −60‰) component.Earlier researchers suggested that the Taupo Volcanic Zone fluid flux included dilute concentrations of slab-derived phases (i.e. CO2, Cl) similar to back-arc volcanic systems at other convergent margins. However, the island arc type fluid composition estimated from the minerals we studied is similar to hornblende and biotite compositions from pyroclastic deposits and volcanic fumaroles from the Kamchatka and Japanese arcs. The silicic rocks from the 2.0–1.0Ma Mangakino caldera, the most inland caldera system we studied, have the lowest δD hornblende and biotite, suggesting that silicic volcanism, distal to the subduction trench, was derived from a back-arc like (i.e. mixture of mantle and slab-fluid). However, we found that the currently active more proximal locus of silicic volcanism reflects typical arc-like fluid compositions. Our results further suggest that the composition of volcanic steam migrating through the upper crust was similar in composition to regional meteoric waters (ca. −45 to −23‰).