HYDROXYL IN CLINOPYROXENE FROM THE DEEP SUBDUCTED CRUST: EVIDENCE FOR H2O TRANSPORT INTO THE MANTLE

Abstract

Clinopyroxenes in eclogites from the deep subducted crust in the Kokchetav, Kazakhstan, ultra-high-pressure metamorphic terrane were found by infrared spectroscopy to contain up to 3020 ppm OH. These rocks were recrystallized at pressures over 60 kbar and temperatures over 1000 °C, where no hydrous minerals are stable. Infrared spectra of the clinopyroxenes show three hydroxyl absorption bands in the regions 3440–3460 cm−1, 3500–3530 cm−1, and 3600–3625 cm−1. The hydroxyl absorbance increases with recrystallization pressure of the eclogites and with vacancy concentration in the pyroxene structure (Ca-Eskola component). Clinopyroxenes represent about 40–50 vol% of eclogites, such that the bulk eclogites contain approximately 1300 ppm hydroxyl at depths greater than 150 km. Thus, subducted oceanic crust can transport trace amounts of H2O into the upper mantle, even in the absence of nominally hydrous minerals.