Origin of Water in Mantle Eclogites from the V. Grib Kimberlite Pipe, NW Russia

Abstract

The water content in the garnet and clinopyroxene in the mantle eclogites from the V. Grib kimberlite pipe (Arkhangelsk Diamondiferous Province, NW Russia) was analysed using Fourier transform infrared spectrometry. The results show that all clinopyroxene grains contained structural water at concentrations of 39 to 247 ppm, whereas two garnet samples contained detectable water at concentrations of 211 and 337 ppm. The low-MgO eclogites with oceanic gabbro precursors contained significantly higher water concentrations in the omphacites (70–247 ppm) and whole rock (35–224 ppm) compared to those with oceanic basalt protoliths (49–73 ppm and 20–36 ppm, respectively). The incorporation of water into the clinopyroxene may be associated with vacancies at the M2 site, Al in the tetrahedral position, and the elements that filled the M2 site (mostly Na and Ca). The highest water content in the omphacite was detected in a nonmetasomatised sample and was assumed to represent residual water that survived during subduction. Other eclogite samples showed signs of modal and/or cryptic metasomatism and contained less water in the omphacites compared to the nonmetasomatised sample. The water content was heterogeneous within the eclogite section of the sampled lithospheric mantle. The lack of distinct and uniform correlations between the indices of eclogite modification and their water content indicated that the saturation with water was disturbed during their residence within the lithospheric mantle.

Water in Omphacite and Garnet From Pristine Xenolithic Eclogite: T‐X‐fO2 Controls, Retentivity, and Implications for Electrical Conductivity and Deep H2O Recycling

“…Results for c(H 2 O) along the refractive indices α , β , and γ in omphacite as well as total contents for both omphacite and garnet are reported in Table 1. Concentrations for α and β were found to be indistinguishable, as in previous studies (Agasheva et al., 2021; Koch‐Müller et al., 2004; Kolesnichenko et al., 2018; Radu et al., 2022).…”

“…Similarly, Agasheva et al. (2021) report c(H 2 O) of 35–225 wt.ppm for eclogite xenoliths with gabbroic protoliths from the Grib kimberlite (Kola craton), compared to 20–36 wt.ppm for those with basaltic protoliths.…”

“…Water contents in eclogitic garnet are typically low and often reported as below detection in eclogite xenoliths from the Siberian craton (e.g., Agasheva et al., 2021; Kolesnichenko et al., 2018). In this study, both clinopyroxene and garnet H 2 O abundances are fully quantified, allowing us to investigate the partitioning (or distribution D ) of water between clinopyroxene and garnet as a function of various parameters.…”

“…Several studies have delineated the crystal-chemical controls on OH incorporation in garnet and pyroxenes, which are complex due to extensive solid solutions and multiplicity of hydrogen-associated point defects (e.g., Ingrin & Skogby, 2000;Lu & Keppler, 1997;Mookherjee & Karato, 2010;Reynes et al, 2020;Stalder, 2004;Stalder & Ludwig, 2007). As contributions for polarization parallel to α and β are indistinguishable (Agasheva et al, 2021;Koch-Müller et al, 2004;Kolesnichenko et al, 2018;Radu et al, 2022; this study), the orientation of the OH dipole is reflected by the ratio of γ to α component.…”