High-temperature water–olivine interaction and hydrogen liberation in the subarc mantle

Oxidized fluids in the subduction zone may convert polyvalent elements in the mantle to their higher valence states. The most abundant polyvalent element in the mantle is Fe, a significant part of which is contained in olivine as Fe 2+ . Results of the study of arc mantle xenoliths, in lab high-pressure–high-temperature experiments, and thermodynamic modeling have shown that at pressures of 50–2000 MPa and temperatures of 1000–1250 °C, well above the serpentine stability field, Fe 2+ from olivine reacts with free aqueous fluid according to the following simplified reaction: 3Fe 2 SiO 4 + 2H 2 O ⇆ 3SiO 2 + 2Fe 3 O 4 + 2H 2 . The resulting ferric iron is preserved in spinel of a certain composition, ( Mg,Fe_^2 + )Fe_2^3 + O_4 , whereas new high-Mg olivine, with magnesium number up to 96 in natural samples and 99.9 in experiments, forms in the reaction zone. SiO 2 produced in the reaction either dissolves in the fluid or, with a small amount of water, reacts with olivine to form orthopyroxene as follows: (Mg,Fe) 2 SiO 4 + SiO 2 = (Mg,Fe) 2 Si 2 O 6 . The released H 2 may decrease the oxidation state of polyvalent elements present in the fluid (e.g., S 4+ , S 6+ ). Traces of high-temperature water–olivine interaction appear as swarms of fluid-spinel inclusions and are ubiquitous in olivine from ultramafic arc xenoliths. The described process is similar to serpentinization but occurs at higher pressure and temperature conditions and yields different reaction products. The reducing capacity of olivine is relatively low; however, given the large volume of mantle (and crustal) peridotites, the overall effect may be significant.