| Summary: | To compare magmatic crystallization temperatures between ocean island basalt (OIB) proposed to be plume-related and normal mid-ocean ridge basalt (MORB) parental liquids, we have examined and compared in detail 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). For each suite we have good data on both glass and olivine phenocryst compositions, including volatile (H2O) contents. For each suite we have calculated parental liquid compositions at 0.2 GPa 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 (a maximum of ∼20 °C) between the crystallization temperatures of the parental liquids (MORB 1243-1351 °C versus OIB 1286-1372 °C) when volatile contents are taken into account. To constrain the depths of origin in the mantle for the parental liquid compositions, we have performed experimental peridotite-reaction experiments at 1.8 and 2.0 GPa, using the most magnesian of the calculated parental MORB liquids (Cocos-Nazca), and compared the others with relevant experimental data utilizing projections within the normative basalt tetrahedron. The mantle depths of origin determined for both the MORB and OIB suites are similar (MORB 1-2 GPa; OIB 1-2.5 GPa) using this approach. Calculations of mantle potential temperatures (TP) are sensitive to assumed source compositions and the consequent degree of partial melting. For fertile lherzolite sources, TP for MORB sources ranges from 1318 to 1488 °C, whereas TP for ocean island tholeiite sources (Hawaii, Iceland, and Réunion) ranges from 1502 °C (Réunion) to 1565 °C (Hawaii). The differences in TP values between the hottest MORB and ocean island tholeiite sources are ∼80 °C, significantly less than predicted by the thermally driven mantle plume hypothesis. These differences disappear if the hotspot magmas are derived by smaller degrees of partial melting of a refertilized refractory source. Consequently the results of this study do not support the existence of thermally driven mantle plumes originating from the core-mantle boundary as the cause of ocean island magmatism.
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