Plate Tectonics Constrained by Evidence-Based Magmatic Temperatures and Phase Relations of Fertile Lherzolite (Invited)

In order to understand Earth’s plate tectonics we must interpret the most direct probes for mantle composition and temperature distribution i.e. the primitive basaltic magmas and peridotites representing partial melts and mantle residues. An evidence-based approach to identification of parental magmas and determination of their temperatures requires glass and phenocryst compositions and experimentally calibrated Fe/Mg partitioning between olivine and melt. We have compared magmatic crystallization temperatures between ‘hot-spot’(proposed to be plume-related) and normal mid-ocean ridge basalt (MORB) parental liquids, by examining three representative magmatic suites from both ocean island (Hawaii, Iceland, and Réunion) and mid-ocean ridge settings (Cocos-Nazca, East Pacific Rise, and Mid-Atlantic Ridge). We have glass and olivine phenocryst compositions, including volatile (H2O) contents, and have calculated parental liquid compositions at 0.2GPa by incrementally adding olivine back into the glass compositions until a liquid in equilibrium with the most-magnesian olivine phenocryst composition is obtained. The results of these calculations demonstrate that there is very little difference (maximum of ~20°C) between the ranges of crystallization temperatures of the parental liquids (MORB:1243-1351°C versus OIB:1286-1372°C) when volatile contents are taken into account. However while lacking temperature contrast, the source regions for ‘hot-spot’ parental magmas contain geochemical signatures of old subducted crust/lithosphere. The mantle depths of origin determined for both the MORB and OIB suites are similar (MORB:1-2 GPa; OIB:1-2.5 GPa). Calculations of mantle potential temperatures (Tp) are model dependent, particularly to melt fraction from an inferred source. Assuming similar fertile lherzolite sources, the differences in Tp values between the hottest MORB and the hottest ocean island tholeiite sources are ~80°C. These differences disappear if the hotspot magmas are derived by smaller melt fraction from a refertilized refractory source. In the plate tectonics paradigm, intraplate volcanic chains associated with broad swells are due to upper mantle compositional heterogeneity and consequent buoyancy contrasts and are not a consequence of deep mantle thermal plumes A new experimental study has determined the solidus and melting behaviour of model fertile lherzolite (MORB source) between 1.5 and 6 GPa, and with water contents from ~500 ppm to 14.5 wt % H2O, i.e. from water in nominally anhydrous minerals (NAMs) to vapour-leaching conditions. The lithosphere/asthenosphere boundary is attributed to a change in the water-storage capacity of fertile lherzolite from 2000-4000 ppm at <3GPa to ~200 ppm at >3 GPa, due to the high pressure instability of pargasite. The consequent appearance of silicate melt along an oceanic geotherm at depths >3GPa causes the rheological change characterising thin plate tectonics. The upper asthenosphere becomes chemically enriched (intraplate magma source) and lower asthenosphere depleted in incompatible elements (MORB source, including ~200 ppm H2O in NAMs) by movement of an incipient melt fraction at the water-saturated solidus.