Zoisite in cratonic eclogite xenoliths - Implications for water in the upper mantle

• Published in: Lithos Vol. 418-419; p. 106681
• Main Authors: Radu, I.B., Moine, B.N., Bolfan-Casanova, N., Ionov, D.A., Devidal, J.L., Deloule, E., Korsakov, A.V., Golovin, A.V., Oleinikov, O.B., Cottin, J.Y.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01-06-2022; Elsevier
• Subjects: Den föränderliga jorden; Earth Sciences; Eclogite xenoliths; Eclogite xenoliths Zoisite Exsolution Omphacite Water Nominally anhydrous minerals NAMs; Exsolution; Nominally anhydrous minerals NAMs; Omphacite; Sciences of the Universe; The changing Earth; Water; Zoisite; Exsolution; Water; Nominally anhydrous minerals NAMs; Zoisite; Eclogite xenoliths; Omphacite
• ISSN: 0024-4937; 1872-6143; 1872-6143
• DOI: 10.1016/j.lithos.2022.106681

A significant portion of the Earth's water is found at trace levels in nominally anhydrous rock forming minerals in the crust and mantle. Its distribution has far-reaching implications in melt-generating processes, mantle rheology and the stability of the lithosphere. We report petrographic, major and trace element, Fourier transform infrared spectroscopy and secondary ion mass spectrometry data on four eclogite xenoliths from the Obnazhennaya kimberlite, Siberia that contain hydrous minerals at upper mantle conditions. Al-rich (>9.9 wt% Al2O3) omphacitic clinopyroxene, a major mineral of eclogites, contains crystallographically controlled exsolutions of CaO-rich (up to 23.6 wt%) garnet (coronae, lenses), garnet-zoisite (lamellae) or amphiboles (needle/blade-like exsolutions). The omphacite hosting the exsolved lamellae shows enrichment or depletion in major (Al, Si, Mg) and trace (e.g., HREE) elements compatible with garnet and zoisite, proportional to the abundance of exsolutions.
We argue that zoisite likely formed concomitantly with garnet exsolution from a water- and Al-rich precursory omphacite, without any fluid addition. The compositional gradients are consistent with diffusion-controlled partitioning in a closed system, and their preservation indicates the exsolution likely took place shortly prior to eclogite entrapment by the kimberlite. Large, non-exsolved, omphacite grains contain minimum ~870–1500 ppm wt. H2O (determined as OH). We infer that intertwined lamellae of hydrous garnet (600–960 ppm wt. H2O) and minor (<1%) zoisite (2 wt% H2O) formed at mantle conditions due to progressive exsolution, without external fluids. Recalculated whole rock water contents range from ~320 to 970 ppm wt. H2O. These estimates exceed by far those for the surrounding peridotitic mantle and suggest that, although a minor component, eclogites may locally be an important water reservoir in the cratonic mantle.
•Rare eclogites from Obnazhennaya kimberlite contain water-rich exsolutions.•Zoisite stability at mantle conditions is demonstrated for the first time in natural samples.•Bulk water concentrations of eclogites exceed those of the surrounding peridotites.•Eclogites may be unique carriers and storage for water in the lithospheric mantle.