High pressure experimental study of eclogite with varying H2O contents

Monday, 15 December 2014
Anja Rosenthal1, Daniel J Frost1, Sylvain Petitgirard1, Gregory Mark Yaxley2, Andrew Berry3, Alan B Woodland4, Zsanett Pinter1, Prokopiy Vasilyev3, Dmitri A Ionov5, Dorrit E Jacob6, Graham D Pearson7, Istvan Kovacs8 and Alberto Padron-Navarta5, (1)Bayerisches Geoinstitut, Universitaet Bayreuth, Bayreuth, Germany, (2)Australian National University, Research School of Earth Sciences, Canberra, ACT, Australia, (3)Australian National University, Research School of Earth Sciences, Canberra, Australia, (4)Univ Frankfurt, Frankfurt, Germany, (5)University of Montpellier II, Montpellier Cedex 05, France, (6)Macquarie University, Sydney, Australia, (7)University of Alberta, Earth & Atmospheric Sciences, Edmonton, AB, Canada, (8)Geological and Geophysical Institute of Hungary, Laboratory Department, Budapest, Hungary
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.

[1] Yaxley, G. M. & Green, D. H. Earth Planet Sci Lett 128, 313-325 (1994). [2] Hermann, J. Green, D. H. Earth Planet Sci Lett 188, 149-168 (2001). [3] Hermann J. & Spandler, C. J. J Petrol 49, 717-740 (2008). [4] Schmidt, M. W. Science 272, 1927-1930 (1996). [5] Schmidt, M. W. & Poli, S. Earth Planet Sci Lett 163, 361-379 (1998). [6] Schmidt, M. W. et al. Earth Planet Sci Lett 228, 65-84 (2004).