Undepleted mantle composition beneath Hawaii

Abstract

Compositions of undepleted “Mauna Loa” mantle have been calculated assuming derivative: residuum relations between a tholeiitic picrite from the 1852 eruption of Mauna Loa and mantle residua whose compositions are delineated by experimental data and the compositions of the most refractory picrite phases, namely Mg-rich olivine and Cr-rich spinel. The preferred calculated composition based on 40% picrite: 60% residuum composed of olivine, orthopyroxene and Cr-spinel has a lower 100 Mg(Mg+ ΣFe) ratio (84.1) than the ratios of pyrolite-like undepleted mantle compositions (88–90) and compared with many such compositions it also has significantly higher abundances of TiO2, Al2O3, CaO, Na2O, K2O and P2O5. The higher abundances of incompatibles have implications for the volumes and character of potential derivative melts. A calculated mode which assumes that a titaniferous pargasitic amphibole is a component of the undepleted lherzolite indicates that amphibole could be a significant primary phase. At temperatures slightly above the lherzolite solidus amphibole would be eliminated on incongruent melting and its breakdown products (mainly clinopyroxene plus minor olivine) would become part of a re-equilibrated anhydrous peridotitic mineral assemblage. The lower 100 Mg(Mg+ ΣFe) ratio of the calculated composition would permit the generation of primary basaltic melts with relatively low 100 Mg(Mg+Fe2+) ratios at lower (< 40%) degrees of melting. Such lower ratios are usually attributed to olivine fractionation of more Mg-rich primary melts from pyrolite-like mantle sources.