Primary magmas and mantle temperatures

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(#) respective ly. 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 a verage 1365 degreesC, for E-MOR picrites average 1355 degreesC, and for N-M OR picrites average 1335 degreesC. Water contents of the magmas decrease in the order Hawaiian picrites, E-MOR picrites to N-MOR picrites, and conside ration of liquidus depression by these water contents leads to the conclusi on that magma temperatures for all types were approximately 1325 degreesC a t similar to 1 bar. The data from parental or primary magmas suggests that the temperature contrast between 'Hot-Spot' and MOR magmas is less than or equal to 20 degreesC. Application of information from partial melting studi es of Iherzolites and liquidus studies of the Hot-Spot and MOR picrites lea ds to the conclusion that both 'Hot-Spot' and MOR primary basalts are deriv ed from mantle with potential temperature T-p similar to 1430 degreesC. Ins ofar as primitive magmas may be used to infer the potential temperature of their sources, there is no evidence for a temperature contrast of Delta T-p = 100-250 degreesC between 'Hot-Spot' or 'Deep Mantle Plume' sources and a mbient (MOR source) asthenospheric mantle. Although magma temperatures are similar, the residual mantle compositions f or Hawaiian picrites are refractory harzburgites, more refractory (includin g Cr/Cr+Al ratio) than the Iherzolite to harzburgite residue from MOR picri te extraction. It is argued that the buoyancy plume and geophysically anoma lous mantle beneath the Hawaiian Arch is due to compositional and not tempe rature contrasts in the upper mantle. The four-component mixing identified in the Hawaiian source is attributed to interaction between old subducted l ithospheric slabs, buoyant or suspended in the upper mantle, and surroundin g ambient mantle at T-p = 1430 degreesC.