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Primary magmas and mantle temperatures

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

^* E-mail: director.rses{at}anu.edu.au

This paper was presented at the EMPG VIII meeting in Bergamo, Italy (April 2000)

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 pressure) 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 temperatures 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 partial 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 temperature contrast of 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 Hawaiian source is attributed to interaction between old subducted lithospheric slabs, buoyant or suspended in the upper mantle, and surrounding ambient mantle at T_p = 1430°C.

Key-words: mantle melting, mantle potential temperature, plumes, primary magmas.

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