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

Abstract

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-midocean ridge (MOR) mantle potential temperatures (T p 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; T ex = T p hot spot − T p MOR ). The Siqueiros Transform has high MgO glass compositions that have been affected only by olivine fractionation, and yields T p Siqueiros = 1441 ± 63°C. Most midocean ridge basalts (MORB) have slightly higher FeO liq than at Siqueiros; if Fo max (= 91.5) and Fe 2+ -Mg exchange at Siqueiros apply globally, then upper mantle T p 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 T p MOR = 1454 ± 81°C; this value is used to calculate T ex . Global MORB variations in FeO liq 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 T p MOR 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 T ex is ≈ 268°C at Hawaii and Samoa and 162°C at Iceland. Furthermore, T p estimates at Hawaii and Samoa exceed maximum T p estimates at MORs by N 100°C. Our T ex 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 T ex 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 T ex account for generalized differences in H 2 O contents between ocean islands and MORs, and are robust against variations in CO 2 , 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 H 2 O variations within the MORB source region.

High-K ankaramitic melt inclusions and lavas in the Upper Cretaceous Eastern Srednogorie continental arc, Bulgaria: Implications for the genesis of arc shoshonites

“…This assumption is consistent with the fact the those olivine-liquid geothermometers resulted the lowest temperatures that are known to best reproduce experimental results for hydrous magmas (i.e. method of Langmuir et al, 1992 andPutirka et al, 2007).…”

“…For this purpose, we used bulk melt inclusion compositions that were already corrected for the post-entrapment crystallization of the host mineral on the walls of the inclusions. The resulting temperatures for high-Fo core-melt inclusion pairs from sample SG12 range between 1146 and 1266°C with the lowest temperatures obtained with the Langmuir et al (1992) and Putirka et al (2007) thermometers, and the highest temperatures obtained with those of Putirka (1997) and Beattie (1993), respectively. A pressure of 0.5 and 0.7 GPa is assumed for these calculations (see below), but all methods are fairly insensitive, giving an offset of~5°C/ 0.1 GPa.…”

Temperature and rheological properties of the mantle beneath the North American craton from an analysis of heat flux and seismic data

“…Poor constraints on the water content of mantle minerals add further uncertainties. In a similar fashion, current knowledge of the mantle average potential temperature is good to at best ±50 K [e.g., Katsura et al, 2004;Putirka et al, 2007;Lee et al, 2009]. Thus, the goal of predicting the true mantle viscosity from first principles is still far removed.…”

The 2011 eruption of Nabro volcano, Eritrea: perspectives on magmatic processes from melt inclusions

“…According to this model, almost all the inclusions Table 6 Intrinsic variables calculated using a range of models (Andersen and Lindsley 1988;Beattie 1993;Ghiorso and Evans 2008;Harrison and Watson 1984;Papale et al 2006;Putirka 2005;Putirka et al 2003Putirka et al , 2007Sisson and Grove 1993;Spencer and Lindsley 1981) Oxide formulae recalculated according to Stormer (1983). Ilmenite-magnetite was calculated for a single touching pair.…”

“…References T (°C) P (kbar) fO 2 Apatite Harrison and Watson (1984) 980-1020 Olivine-melt (incl) Beattie (1993) 1066-1120 Putirka et al (2007) 1037-1084 Sisson and Grove (1993) 992-1051 Olivine-melt (matrix) Beattie (1993) 1120-1146Putirka et al (2007 1091-1126 Sisson and Grove (1993) 1029-1059 Plagioclase-melt (inclusions)…”