Olivine hydration in the deep upper mantle: Effects of temperature and silica activity

Abstract

Abstract. Although water controls the biology and geology of the surface, hydrogen is perhaps the most poorly constrained compositional variable in the bulk Earth. Its concentration in the upper mantle appears to be controlled by its solubility as hydroxyl in the nominally anhydrous silicate phases, olivine, pyroxene, garnet, wadsleyite, and ringwoodite. Here we describe a series of experiments showing that the solubility of H2O in olivine at 12 GPa increases with temperature to 8900 ppm by weight at 1250°C and decreases at higher temperature with the onset of melting. Sample characterization by infrared spectroscopy indicates that the primary hydration mechanism is the substitution of 2H+ for Mg2+. Similar results obtained from samples coexisting with clinohumite (low-silica) and with clinoenstatite (high-silica) indicate that silica activity has minimal effect on hydration under these conditions. Single-crystal X-ray diffraction measurements constrain the volume of hydration and indicate significant M-site vacancies. Hydrogen thus appears to become a geochemically compatible element as depths approach 400km.

Crystal structural features of hydrous forsterite: Effect of Fe on the M-site vacancies, possible hydrogen positions and variation of the unit cell dimensions

“…Then, the steric facility of variarion is in the order, Table 3 ( Fujino et al, 1981;Takeuchi et al, 1984). This is also consistent to the experimental results that the variations of unit cell constants with pressure (Hazen, 1976;Kudoh and Takeuchi, 1985), temperature (Takeuchi et al, 1984) and with composition of hydrous forsterite in Table 3 ( Kudoh et al, 2006;Smyth et al, 2006) are in the order, ∂b…”

“…Kudoh et al (2006Kudoh et al ( , 2007a reported the result of single crystal x -ray structural analysis of Fe -free hydrous forsterite in which the cation vacancies predominantly occur at the M1 site. Smyth et al (2006) reported the very strong absorbance features in the polarized FTIR spectra of Fe -free and Fe -bearing hydrous forsterite which appear to correspond to protonation of octahedral edges of M1. Kudoh et al (2007b) reported the result of the structural refinement of synthetic Fe -bearing hydrous forsterite in which the cation vacancies at the octahedral M sites predominantly occur at the M2 site, in contrast to the case of Fe -free hydrous forsterite.…”

High‐resolution transition zone structures of the Gorda Slab beneath the western United States: Implication for deep water subduction

“…If the transition zone outside of the slab were also water saturated [e.g., Smyth et al, 2006], the ∼35 km depression of the 660 would have to be completely contributed by the temperature anomaly of as much as ∼700°K. This looks impossible with reference to the mantle geotherms [e.g., Akaogi et al, 1989;Litasov et al, 2005b].…”

“…[4] Although geochemical evidence indicates that the transition zone should be extremely dry (<0.00075 wt%) [Dixon et al, 2002;Rupke et al, 2006], more and more experimental evidence [e.g., Kohlstedt et al, 1996;Locke et al, 2002;Hirschmann et al, 2005;Smyth et al, 2006] and seismological observations [e.g., Revenaugh and Sipkin, 1994;van der Meijde et al, 2003;Song et al, 2004;Blum and Shen, 2004] suggest the existence of substantial deep water (>0.1 wt%) in the transition zone. The water solubility of olivine increases a hundred times relative to the ambient conditions at pressure above 10 GPa (∼300 km), and wadsleyite and ringwoodite can incorporate up to ∼2.9 wt% water in the transition zone [Kohlstedt et al, 1996;Mosenfelder et al, 2006].…”

Nickel Extraction from Olivine: Effect of Carbonation Pre-Treatment

“…Olivine is solid-solution of iron-and magnesium-silicates containing relatively small amounts of nickel and chromium, and is the precursor of weathered lateritic ores. Olivine (also known as dunite, an ore containing at least 90% olivine [6]) is abundantly present in the Earth's upper mantle [7], and intrudes in some locations into the Earth's crust, most notably in the Fjordane Complex of Norway, which contains the largest ore body (approx. two billion metric tons) under commercial exploitation [6].…”