Water in Omphacite and Garnet From Pristine Xenolithic Eclogite: T ‐ X ‐ f O 2 Controls, Retentivity, and Implications for Electrical Conductivity and Deep H 2 O Recycling

• Published in: Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2023-12, Vol.24 (12)
• Main Authors: Aulbach, Sonja, Stalder, Roland, Massuyeau, Malcolm, Stern, Richard A., Ionov, Dmitri A., Korsakov, Andrey V.
• Format: Article
• Language: English
• Subjects: Sciences of the Universe
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2023GC011170

Kimberlite‐borne eclogite xenoliths having Precambrian oceanic crustal protoliths and entrained from ≥100 km depth can retain pristine geochemical features despite extended residence in the cratonic lithospheric mantle, making them valuable archives of deep chemical cycling including that of water. We determined, by Fourier Transform Infrared Spectroscopy, structural OH contents in clinopyroxene and garnet from 15 unmetasomatized eclogite xenoliths. Calculated total c(H 2 O) is 100–510 wt.ppm for clinopyroxene and below detection (∼2 wt.ppm) to 200 wt.ppm for garnet, while garnet δ 18 O, determined by Secondary Ion Mass Spectrometry, ranges from +5.0‰ to +7.3‰, (similar to high‐ and low‐temperature seawater‐altered oceanic crust). Estimated electrical conductivity in pristine eclogites increases with temperature (i.e., depth for conductive geotherms), while clinopyroxene‐garnet H 2 O partition coefficients decrease with increasing temperature and garnet grossular component (i.e., Ca#), similar to other incompatible components. Various considerations suggest the retention of primary H 2 O in the samples, likely occurring in km‐sized pods of coarse‐grained eclogite. High Al 2 O 3 in clinopyroxene as omphacite component, stabilized during high‐pressure metamorphism, facilitates H 2 O uptake. Therefore, the high bulk c(H 2 O) estimated for samples with plagioclase‐rich, deep crustal protoliths (median 290 wt.ppm) may indicate an interaction with fluids expelled at depth from serpentinites. The c(H 2 O) of ancient and modern subducted bulk oceanic crust (∼220–240 wt.ppm) are similar, suggesting constant mantle ingassing since at least 3 Ga ago. This places constraints on factors, such as mantle temperatures, that determine the efficiency of deep water cycling. Water in Earth's interior exists mostly as OH − anion in trace abundances in nominally anhydrous minerals. Despite these low concentrations, deep water exerts a strong influence on fundamental Earth processes, such as partial melting of the mantle and the operation of plate tectonics. However, the extent to which the loss of water via volcanism has been compensated over time by retention in downgoing oceanic plates, after their dehydration and metamorphism to eclogite, remains poorly known. Deeply buried Archean and Paleoproterozoic oceanic crust is sampled as remarkably pristine eclogite fragments quickly exhumed by volcanism from depths >100 km. High Al 2 O 3 contents, characteristic of deep crustal pla