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.
Lithos, 2022, 418-419, 106681
Voir en ligne : https://doi.org/10.1016/j.lithos.2022.106681
Abstract :
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.