Hydrogen Zoning in Olivine from Kimberlites Based on Coupled FTIR and SIMS Analyses: Significance for H₂O Distribution in the Lithospheric Mantle and H₂O Concentrations in Kimberlite Melts

Kimberlite melts are widely considered to be enriched in volatiles, both CO₂ and H₂O. Yet, estimated H₂O concentrations in primitive kimberlites vary between 3.0 and 12 wt %, and it is unclear whether these variations are a true reflection of variable H₂O in their mantle sources. The main problem rests with the origin of serpentine, the main H₂O host in kimberlite rocks, and specifically, whether it derives from magmatic and/or crustal fluids. To obtain estimates of primary H₂O contents in kimberlite melts, we have examined the systematics of proton incorporation in olivine point defects (referred as H) in grains from eight representative kimberlites from Africa, Canada and Greenland. These kimberlites show highly variable groundmass and, therefore, melt compositions and include varieties enriched in serpentine, carbonate and/or phlogopite. Olivine grains are strongly zoned in major and minor elements based on BSE images and EPMA analysis and include mantle-derived xenocrystic cores and magmatic rims. FTIR maps and profiles show that the olivine cores are zoned with H₂O decreasing outward due to diffusive loss most likely triggered by decompression and related H₂O loss from the transporting kimberlite melt after xenocryst entrainment. The central portions of the cores exhibit homogeneous H₂O contents representative of mantle values. A combination of Al-in-olivine thermometry with appropriate geothermal gradients and H₂O determinations from SIMS analyses (from 9 to 241 µg/g across the entire sample set) shows similar systematics to those of olivine in mantle xenoliths from the same localities. H₂O analyses of olivine cores in kimberlites are hence valuable to systematically examine the vertical distribution of H₂O in the lithospheric mantle traversed by kimberlites. The magmatic rims examined by SIMS invariably show low H₂O contents (<50 µg/g but mostly $\leq$ 20 µg/g) with very limited within-sample variation regardless of olivine major-minor element chemistry and groundmass composition. Experimentally derived hydrogen partition coefficients yield $\leq$ 1 wt % of H₂O in the corresponding melts, values that probably reflect post-emplacement re-equilibration in the upper crust rather than equilibrium conditions during crystallisation. These low H₂O contents reflect, at least in part, exsolution of C-O-H fluids during kimberlite ascent. The implication is that kimberlite melts contain insufficient H₂O to crystallise the large amount (up to 50 vol %) of serpentine commonly observed in kimberlites. Serpentine rather requires contributions by crustal fluids and/or deeply exsolved kimberlite-related fluids that percolate upwards along the pipe- or dike-like emplacement structures, modifying the composition of previously crystallised kimberlites.

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