Green, 2001 Primary magmas and mantle temperatures

Green, 2001 Primary magmas and mantle temperatures - Eur....

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Introduction The variation of temperature with depth in the Earth’s crust and mantle cannot be directly mea- sured and must be inferred. One of the more direct indications of interior temperatures is provided by the eruption temperatures of volcanoes and the matching of those temperatures with knowledge of the depth of origin and magma/source relation- ship. In the ‘Plate Tectonics’ model of the modern Earth, it is commonly inferred that Mid-Ocean Ridge (MOR) volcanism is an expression of con- vective upwelling from relatively shallow mantle depths. To a first approximation, MOR volcanism is characterized by globally similar primary or parental magma compositions, with the most mag- nesian quenched glass, observed in all young ocean basins, having eruption temperatures around Eur. J. Mineral. 2001, 13 , 437-451 Primary magmas and mantle temperatures D AVID H. GREEN 1, * ), T REVOR J. FALLOON 1,2) , S TEPHEN M. EGGINS 1) and G REGORY M. YAXLEY 1) 1) Research School of Earth Sciences, Australian National University, Canberra ACT, Australia 0200 2) School of Earth Sciences, University of Tasmania, Hobart, Tasmania, Australia 7005 Abstract: The composition of olivine phenocrysts in Hawaiian tholeiitic picrites and in Mid-Ocean Ridge picrites vary up to Mg # 91.3 and Mg # 92.1 respectively. The compositions and liquidus temperatures of the magmas crystallizing the most magnesian phenocrysts can be estimated and we find that anhydrous liquidus temperatures (at 1 bar pres- sure) of Hawaiian tholeiitic picrites average 1365°C, for E-MOR picrites average 1355°C, and for N-MOR picrites average 1335°C. Water contents of the magmas decrease in the order Hawaiian picrites, E-MOR picrites to N-MOR picrites, and consideration of liquidus depression by these water contents leads to the conclusion that magma tem- peratures for all types were approximately 1325°C at ~ 1 bar. The data from parental or primary magmas suggests that the temperature contrast between ‘Hot-Spot’ and MOR magmas is £ 20°C. Application of information from par- tial melting studies of lherzolites and liquidus studies of the Hot-Spot and MOR picrites leads to the conclusion that both ‘Hot-Spot’ and MOR primary basalts are derived from mantle with potential temperature T p ~ 1430°C. Insofar as primitive magmas may be used to infer the potential temperature of their sources, there is no evidence for a tem- perature contrast of D T p = 100-250°C between ‘Hot-Spot’ or ‘Deep Mantle Plume’ sources and ambient (MOR source) asthenospheric mantle. Although magma temperatures are similar, the residual mantle compositions for Hawaiian picrites are refractory harzburgites, more refractory (including Cr/Cr+Al ratio) than the lherzolite to harzburgite residue from MOR picrite extraction. It is argued that the buoyancy plume and geophysically anomalous mantle beneath the Hawaiian Arch is due to compositional and not temperature contrasts in the upper mantle. The four-component mixing identified in the
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Green, 2001 Primary magmas and mantle temperatures - Eur....

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