Water in mantle eclogite: Origin, distribution and physical consequences
Final Report Abstract
Water in Earth’s interior exists mostly as OH- anion in trace abundances in nominally anhydrous minerals, yet exerts a strong influence on mantle rheology and on fundamental Earth processes, such as the operation of plate tectonics. Clinopyroxene is the main water carrier in the mantle, and eclogite, which forms a subordinate part of the cratonic lithosphere, contains ~50% clinopyroxene, making this potentially an important H2O reservoir that could affect lithospheric stability. Deeply buried 3.0-1.9 Ga palaeo-oceanic crust is sampled as eclogite fragments (xenoliths), which are quickly exhumed by kimberlite volcanism from depths >100 km. Some of these xenoliths are remarkably pristine, allowing to investigate the extent to which H2O was retained in ancient downgoing oceanic plates, after their dehydration and metamorphism to eclogite. High Al2O3 contents, characteristic of eclogites with deep crustal, plagioclase-rich cumulate protoliths, are associated with high H2O contents in clinopyroxene. Because clinopyroxene is rich in Al only at high pressure, where the crust is already dehydrated, but the underlying seawater-altered mantle begins to liberate fluids, interaction with this fluid may explain the Al-H2O association. The eclogite-based H2O estimate for ancient crust is similar to that for modern crust, suggesting that deep water cycling in the crustal part of subducting slabs changed little in the last 3 billion years. Eclogite samples showing evidence for overprint (metasomatism) by a volatile-rich kimberlite-like melt (addition of highly incompatible elements, e.g. Ce, ± phlogopite and/or amphibole) were used to estimate the effect of mantle metasomatism. Surprisingly, metasomatised eclogites do not have systematically higher H2O abundances than pristine ones, which would be expected if H2O was added along with similarly incompatible components, such as Ce. This apparent paradox is resolved when considering crystal chemical effects of mantle metasomatism on eclogite, which decreases the jadeite component in clinopyroxene and the grossular component in garnet, both of which are known to aid H2O incorporation. As a consequence, equilibrium with a hydrous metasomatic agent has little positive net effect on H2O abundances in eclogite, which can explain the often-reported lack of correlation of H2O content with typical indicators of mantle metasomatism. Correlations of the distribution of some elements between amphibole and phlogopite in eclogite and pyroxenite xenoliths with temperature unequivocally demonstrates that these minerals crystallised at mantle depth rather than during ascent. Consistent with this, phlogopite Ar-Ar dating and garnet U-Pb in amphibole-bearing xenoliths shows that it was added or isotopically reset during metasomatism within a few 100 Ma of xenolith entrainment. These minerals will significantly affect the solidus and geophysical signature of eclogite and pyroxenite in the cratonic mantle lithosphere.
Publications
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Fe3+ Distribution and Fe3+/ΣFe-Oxygen Fugacity Variations in Kimberlite-Borne Eclogite Xenoliths, with Comments on Clinopyroxene-Garnet Oxy-Thermobarometry. Journal of Petrology, 63(8).
Aulbach, Sonja; Woodland, Alan B.; Stagno, Vincenzo; Korsakov, Andrey V.; Mikhailenko, Denis & Golovin, Alexander
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Mineral Inclusions in Lithospheric Diamonds. Reviews in Mineralogy and Geochemistry, 88(1), 307-391.
Stachel, Thomas; Aulbach, Sonja & Harris, Jeffrey W.
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H2O analyses of clinopyroxene and garnet (FTIR) and oxygen isotope analysis of garnet (SIMS) in eclogite xenoliths from Udachnaya (Siberian craton), Version 1.0. Dataset, Interdisciplinary Earth Data Alliance (IEDA)
Aulbach S., Stalder R. & Stern R.A.
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Obtaining and interpreting lithospheric mantle ages. Gordon Research Conference on Timing, Tempo and Drivers of Lithospheric Evolution, Mount Snow, USA
Aulbach S.
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Water in Omphacite and Garnet From Pristine Xenolithic Eclogite: T‐X‐fO2 Controls, Retentivity, and Implications for Electrical Conductivity and Deep H2O Recycling. Geochemistry, Geophysics, Geosystems, 24(12).
Aulbach, Sonja; Stalder, Roland; Massuyeau, Malcolm; Stern, Richard A.; Ionov, Dmitri A. & Korsakov, Andrey V.
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Water in omphacite and garnet from unmetasomatised xenolithic eclogite: T-X-ƒO2 controls, and implications for conductivity and the deep H2O cycle. Abstract, 33nd V. M. Goldschmidt Conference, Lyon, France
Aulbach S., Stalder R., Massuyeau M., Stern R.A., Ionov D.A., Heaman L.M. & Korsakov A.V.
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Origins and petrogenetic significance of amphibole and phlogopite in kimberlite-borne eclogite xenoliths. Short Abstract, 11th International Kimberlite Conference, Yellowknife, Canada
Aulbach S., Kuiper K., Tinguely C., Jacob D., Wijbrans J. & Le Roex A.
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Water in cratonic eclogites and peridotites from the Superior and Sask Archean Cratons: impacts of tectonothermal events on mantle lithosphere evolution and dynamics. Short Abstract, 11th International Kimberlite Conference, Yellowknife, Canada
Davies R.M., Aulbach S. & Stachel T.
