科研之友
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摘要
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 degrees C. Most mid-ocean ridge basalts (MORB) have slightly higher FeOliq than at Siqueiros; if Fo(max) (=91.5) and Fe2+-Mg exchange at Siqueiros apply globally, then upper mantle T-p is closer to 1466 +/- 59 degrees 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 degrees C this value is used to calculate T-ex. Global MORB variations in FeOliq indicate that 95% of the sub-MORB mantle has a global T range of +/- 140 degrees C; 68% of this range (1 sigma) exhibits temperature variations of +/- 34 degrees 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 1 sigma estimates derived from phase transitions at the 4 10 km and 670 kin seismic discontinuities. Mantle potential temperatures at Hawaii and Samoa are identical at 1722 degrees C and at Iceland is 1616 degrees C hence T-ex is approximate to 268 degrees C at Hawaii and Samoa and 162 degrees C at Iceland. Furthermore, T-p estimates at Hawaii and Samoa exceed maximum T-p estimates at MORs by > 100 degrees 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 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.
- 出版日期2007-7-15
