High pressure experimental study of eclogite with varying H2O contents

Abstract:

Given the strong influence of volatiles on mantle melting processes, it is critical to understand the behaviour of volatiles (such as H₂O) in subducted oceanic crustal material (eclogite) during subduction and subsequent recycling and mantle melting processes, and their impacts on volcanism.

As natural samples from subduction zones from the deep Earth’s interior are largely inaccessible, the only way to determine the H₂O content of eclogite is to simulate high pressure (P) and temperature (T) conditions equivalent to conditions of the Earth’s interior using high-P experimental facilities.

A particular interest is to determine the H₂O content of eclogitic nominally anhydrous minerals (NAMs; such as garnet, clinopyroxene) at the conditions where hydrous phases (such as phengite) are breaking down to release H₂O that would then leave the slab.

As a starting material, we use average oceanic basalt (GA1, representative of recycled oceanic crust [1]) with varying bulk %H₂O (≤7 wt.%). We conducted experiments using GA1 at different P’s (6-10 GPa), T’s (850-1500°C) and bulk %H₂O (up to 7 wt.%) using multi anvil apparatuses.

The run products at each P, T, and bulk H₂O contents show well-equilibrated eclogitic phase assemblages of garnet ± clinopyroxene ± coesite/stishovite ± rutile ± phengite ± melt ± vapour. Runs (>0.5 wt.% H₂O) at 6 GPa and up to ~950°C, and at 8-9 GPa and up to ~1050°C are subsolidus, while towards higher T small melt fractions appear.

Similar to previous studies [e.g. 2-6], the stability of phengite varies as a function of P, T, buffering mineral paragenesis and bulk H₂O concentration. Phengite breaks down >9 GPa. Eclogitic NAMs and phengite also break down at subsolidus conditions in the presence of excess of hydrous fluids. For instance, K₂O in phengite and clinopyroxene decrease with increasing bulk H₂O content at subsolidus conditions at given P, T, suggesting a leaching role of K₂O by a vapour-rich fluid.

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