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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.

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Pressure is one of the key variables that controls magmatic phase equilibria. However, estimating magma storage pressures from erupted products can be challenging. Various barometers have been developed over the past two decades that exploit the pressure-sensitive incorporation of jadeite (Jd) into clinopyroxene. These Jd-in-clinopyroxene barometers have been applied to rift zone magmas from Iceland, where published estimates of magma storage depths span the full thickness of the crust, and and whole-rock compositions using an iterative scheme because most clinopyroxene analyses were too primitive to be in equilibrium with their host glasses. High-Mg# clinopyroxenes from the highly primitive Borgarhraun eruption in north Iceland record a mean pressure in the lower crust (4.8 kbar). All other eruptions considered record mean pressures in the mid-crust, with primitive clinopyroxene populations recording slightly higher pressures (3.5-3.7 kbar) than evolved populations (2.5-2.8 kbar). Thus, while some magma processing takes place in the shallow crust immediately beneath Iceland's central volcanoes, magma evolution under the island's neovolcanic rift zones is dominated by mid-crustal processes.

OVERVIEWNumerous models have been proposed to explain the formation of magma chambers. Hypotheses fall roughly into two categories. In the first one, principles of fracture mechanics, states of stress, or mechanical properties of the lithosphere are invoked as controlling factors (e.g.,

[1] Temperature differences between lavas erupted at ocean islands and mid-ocean ridges are crucial to documenting the existence of mantle plumes. Olivines are useful for T estimation because they provide a less homogenized account of the melting process compared to glass and whole rock samples. Olivineliquid equilibria, and olivine phenocrysts from Hawaii, Iceland, and several mid-ocean ridge localities, indicate higher melting temperatures compared to mid-ocean ridges (MORs) by at least $250 ± 52°C and 165 ± 62°C, respectively. When translated to differences in mantle potential temperature, DT p , Hawaii and Iceland potential temperatures are hotter than ambient MORs by 213-235 and 162-184°C, respectively, similar to estimates required by geodynamic models for mantle thermal upwellings. Absolute mantle potential temperatures are more uncertain as they depend on estimates of melt fraction and depth of equilibration, but olivine-liquid equilibria support the following estimates: T p Hawaii = 1688°C; T p Iceland = 1637°C; T p MORs = 1453-1475°C. All of these estimates include the effects of H 2 O, Na 2 O + K 2 O, and SiO 2 on olivine-melt equilibria and are robust against other variations in source or liquid composition, such as FeO and CO 2 . These estimates show that at least at Iceland and Hawaii, volcanism is driven by large temperature anomalies whose magnitudes are consistent with the existence of thermally driven mantle plumes.Components: 9374 words, 8 figures, 1 table.

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