Hydrogen incorporation in olivine: Insight from minor, trace and ultra-traces atomic impurities (Invited)

Description

Perfect natural mineral does not exist, they all contain various amount of defects at the atomic scale. Indeed, refractory minerals of the Earth upper mantle can incorporate a range of atomic impurities covering the entire chemical classification; from heavy (U, Th) to light elements (H, Li, and He) even if the concentration remains very low (ppb to ppm level). Here, the chemical compositions of 17 olivine crystals from 10 different localities are determined: (i) Major (by μXRF), minor, trace and ultra-trace element concentrations (by LA-ICP-MS) were obtained; (ii) H concentration was quantified using unpolarized and polarized (FTIR) on the very same olivine grains. The Fo contents range from 83.2 to 94.1, assessing the geological diversity of the selected crystals. H concentrations range from 0 to 54 ppm H2O by wt (based on mineral-specific calibration). Total minor element concentrations range from 3487 ppm to 6493 ppm and are dominated by Ni, Mn and Ca or B for metamorphic/hydrothermal olivines. The total amount of rare Earth elements and other incompatible elements (e.g., U, Th, Sr, Zr, Nb, etc) is low (<1 ppm) as expected. The most-enriched olivine in atomic impurities has a magmatic origin and the poorest a metamorphic origin. Mantle-derived olivines have concentrations between these two extremes. Not all the selected olivines have detectable OH bands by FTIR, however for the H-bearing crystals, the variety of OH bands detected by FTIR is important. The H concentration inversely correlates with di-, tri-, tetra- or penta-valent impurities, as expected from competition between vacancy site occupancy. Notably, there is no positive correlation between the total amount of H and Ti4+ or Al3+ concentration. Nevertheless, a scattered positive correlation between the two OH bands at 3575 and 3525 cm-1 and the Ti concentration is observed, confirming the existence of this complex associated hydrogenated point defect. However, Ti does not appear as the exclusive impurities controlling bulk H incorporation in olivine as other atomic impurities have similar effect and thus dilutes Ti effect. Therefore H emerges as an incompatible and opportunist element, entering preferentially trace elements poor olivine (where vacancies remain), generating a negative correlation between H and minor/trace element concentrations.

Period	13 Dec 2019
Event title	AGU Fall Meeting 2019
Event type	Conference
Location	San Francisco, United States, California
Degree of Recognition	International