The oxidation state of deeply subducted, altered oceanic crust: an experimental study and the evidence from natural samples

Abstract

Eclogitic xenoliths from kimberlites are occasionally diamond-bearing, and are often interpreted as having an origin as subducted oceanic crust. The existence of diamonds in these rocks constrains equilibrium temperatures and pressures of some eclogites to the upper mantle. However the additional critical parameter controlling the stability of diamonds, oxygen fugacity (fO₂), is poorly constrained in eclogitic assemblages.

A series of piston-cylinder experiments were conducted using model carbonate and kyanite bearing eclogite assemblages to determine the oxygen fugacity of the limiting reaction for carbonate vs. graphite/diamond: Dolomite + 2Coesite = Diopside + 2Graphite/Diamond + 2Oxygen as a function of pressure (P=3.5-6 GPa) and temperature (T=900-1300 degree C). The oxygen fugacity in the experiments was determined using Fe-Ir alloy ƒO₂ sensors and a newly developed Fe-Pd-based redox sensor for high-pressure experiments. The experimental data allowed calibration of two redox reactions (involving garnet-clinopyroxene and garnet-kyanite) as oxybarometers to determine ƒO₂ of eclogitic rocks. Both reactions can be used to evaluate the ƒO₂ of UHP metamorphic eclogites and eclogite xenoliths from kimberlites.

The accuracy of the calculated ƒO₂ is highly dependent on precision of the garnet Fe³⁺/ΣFe measurements, which were obtained using the flank method and the synchrotron based Fe K-edge XANES method. Both reactions were calibrated and used to estimate ƒO₂ of diamond, kyanite and coesite bearing eclogite xenoliths from Udachnaya kimberlite pipe, Yakutia, Russia. The relatively high ƒO₂ of diamond stability in eclogite relative to peridotite at the upper mantle PT conditions may explain the higher abundance of diamonds in eclogite xenoliths and constrains the mechanism of transport of carbon to the deep mantle.