Oxygen isotope evidence for the formation of silicic Kermadec island arc and Havre–Lau backarc magmas by fractional crystallisation

Silicic magmas in subduction-related settings are believed to form either by extreme crystal fractionation or by partial melting of crustal rocks, particularly hydrothermally metamorphosed mafic rocks. Oxygen isotopes in combination with Cl concentrations and ratios in fresh volcanic glasses provide a means to evaluate and discriminate the relative contributions from hydrothermally altered basalt and sedimentary material. Andesitic to rhyolitic samples from two Kermadec island arc volcanoes and basalts to dacites from the Lau and Havre backarc rifts yield δ18O between 5.4 and 6.5‰ VSMOW, i.e. very close to the composition of mantle-derived magmas. Partial melting of high-temperature hydrothermally metamorphosed rocks in the island arc crust or the backarc crust can be ruled out because such rocks typically are depleted in 18O and have high Cl/Nb ratios. Marine sediments have δ18O > 10‰ and sedimentary melts are therefore unlikely to contribute significantly to the formation of the silicic magmas in the arc and backarc region. Neither the island arc dacites and rhyolites nor the backarc andesites and dacites indicate a significant involvement of the crust in their petrogenesis. The narrow range of δ18O in the arc and backarc silicic melts is due to crystal fractionation processes and contributions of hydrous fluids from subducted altered basalt/serpentinite with little sediments.

► Silicic magmas in island arc and backarcs form by extensive crystal fractionation.
► Assimilation of hydrothermally altered crust and sediments is insignificant.
► Magmas stagnate deeper than the hydrothermal convection reaches in the crust.