Ambient and excess mantle temperatures, olivine thermometry, and active vs. passive upwelling

Mantle temperatures provide a key test of the mantle plume hypothesis, and olivine-liquid equilibria provide perhaps the most certain means of estimating mantle temperatures. Here, we review mantle temperature estimates and olivine thermometers, and calculate a new convective geotherm for the upper mantle. The convective geotherm is determined from estimates of sub-mid-ocean ridge (MOR) mantle potential temperatures (Tp is the T the mantle would have if it rose adiabatically without melting, and provides a reference for measuring excess temperatures at volcanic hot spots; Tex = Tphot spot − TpMOR). The Siqueiros Transform has high MgO glass compositions that have been affected only by olivine fractionation, and yields TpSiqueiros = 1441 ± 63 °C. Most mid-ocean ridge basalts (MORB) have slightly higher FeOliq than at Siqueiros; if Fomax (= 91.5) and Fe2+–Mg exchange at Siqueiros apply globally, then upper mantle Tp is closer to1466 ± 59 °C. Since our global MORB database was not filtered for hot spots besides Iceland, Siqueiros may in fact be representative of ambient mantle, so we average these estimates to obtain TpMOR = 1454 ± 81 °C; this value is used to calculate Tex. Global MORB variations in FeOliq indicate that 95% of the sub-MORB mantle has a global T range of ± 140 °C; 68% of this range (1σ) exhibits temperature variations of ± 34 °C. Our estimate for TpMOR defines the convective mantle geotherm; this estimate is consistent with T estimates from sea floor bathymetry, and overlaps within 1σ estimates derived from phase transitions at the 410 km and 670 km seismic discontinuities. Mantle potential temperatures at Hawaii and Samoa are identical at 1722 °C and at Iceland is 1616 °C; hence Tex is ≈ 268 °C at Hawaii and Samoa and 162 °C at Iceland. Furthermore, Tp estimates at Hawaii and Samoa exceed maximum Tp estimates at MORs by > 100 °C. Our Tex estimates agree with estimates based on excess topography and dynamic models of mantle flow and melt generation. Rayleigh number calculations further show that if our values for Tex extend to depths as small as 135 km, thermally driven, active upwellings will ensue. Hawaii, Samoa and Iceland thus almost assuredly result from thermally driven active upwellings, or mantle plumes. Estimates of Tex account for generalized differences in H2O contents between ocean islands and MORs, and are robust against variations in CO2, and major element components, and thus cannot be explained away by the presence of volatiles or more fusible source materials. However, our temperature variations at MORs do not account for H2O variations within the MORB source region.