A Reappraisal of Redox Melting in the Earth’s Mantle as a Function of Tectonic Setting and Time

Abstract

Redox melting refers to any process by which melt is generated by the contact of a rock with a fluid or melt with a contrasting oxidation state. It was originally applied to melting owing to the oxidation of reduced CH4- and H2-bearing fluids in contact with more oxidized blocks in the mantle, particularly recycled crustal blocks. This oxidation mechanism causes an increase in the activity of H2O by the reaction of CH4 with O2, and the increased aH2O causes a rapid drop in the solidus temperature, and is here termed hydrous redox melting (HRM). Recently, a second redox melting mechanism (carbonate redox melting; CRM) has been discovered that operates in more oxidized conditions, and may post-date the first mechanism in the same geographical area, explaining the sequence of igneous rock types from lamproites to ultramafic lamprophyres that occurs during the development of rifts through cratons. The CRM mechanism relies on the oxidation of solid carbon as graphite or diamond that has accumulated in the lithosphere over time. The solidus temperature for rocks with both CO2 and H2O is lower than in conditions with H2O alone; it does not occur at depths less than 65 km, but has recently been confirmed experimentally to depths of at least 200 km. Melts produced by HRM are not SiO2-undersaturated, even at depths of 200 km, and may often resemble lamproites or SiO2-rich picrites, whereas melts produced by CRM are always SiO2-undersaturated and range from carbonatitic to ultramafic lamprophyric or melilititic with increasing degree of melting. The operation of redox melting may be more common than has been recognized because the oxidation state of the upper mantle is not uniform as a function of depth, geodynamic setting or geological time. The general decrease of oxygen fugacity (fO2) of c. 0·7 log units per 1 GPa pressure increase dictates that rapidly subducted oceanic lithosphere will be considerably more oxidized than ambient mantle peridotite at depths of 200–300 km. Hydrothermal alteration (serpentinization), addition of continental or carbonate sediments, and dehydration reactions during subduction all contribute to the heterogeneity of oxidation states in the subducted slab, which may vary over 6 log units; this raises the potential for redox reactions on local and regional scales. The oceanic lithosphere has a lower average fO2 than either continental or cratonic mantle lithosphere at a given depth, so that the HRM mechanism dominates in recycled blocks and at the base of the continental lithosphere. The higher thermal gradients dictate that HRM is more common in the modern Earth beneath ocean islands and in upwelling mantle currents than in subduction zones. The oxidation state of the mantle is often described as having been constant since 3·5 Ga, but this overlooks the bias towards continental samples. Redox melting of oxidized recycled blocks (at approximately the fayalite–magnetite–quartz buffer) in the mantle was not important in the Hadean and Archaean, as it had to await the gradual oxidation of the mantle and the establishment of the subduction process, as well as the stabilization of the continents. The lack of CRM explains the lack of carbonatites before 2·7 Ga. However, the lower fO2 of the Archaean asthenosphere and higher volatile contents caused more prevalent HRM in the Hadean and Archaean mantle. Degassing is controlled by solubility of volatile species in melts, which are H2O-rich but C-poor in reducing conditions. Silicate melts under reduced conditions contain much less carbon but more nitrogen than melts in the modern mantle, arguing for a nitrogen-rich, CO2-poor early atmosphere.