The thermal history of ascending magma diapirs and the thermal and physical evolution of magmatic conduits

Abstract

If, over millions of years, the positions of island arc volcanoes remain fixed relative to their source regions, magma bodies must repeatedly traverse a path through the lithospheric mantle. The repeated passage of such bodies thermally and chemically modifies the adjacent mantle. For diapirs with radii of 3–6 km, ascending at 4 × 10−8 to 2 × 10−7 m/s, 60,000 to ∼900,000 years are required to establish a magmatic conduit to crustal depths. During this period, wall-rock temperatures increase 200–400°C below 60 km and 400–750°C between 60 and 20 km. The solidification of large volumes of magma within the mantle also provides a mechanism for the introduction of kilometer-scale chemical and isotopic heterogeneities into the lithospheric mantle. Because the crystallization sequences of magmas ascending at different stages of conduit development vary, conduit thermal and physical evolution also influences magmatic differentiation. These differences in crystallization sequence result from the different P·T trajectories followed by individual magmatic diapirs. Crystal fractionation of successive diapirs, with or without concurrent wall-rock assimilation can, therefore, generate different liquid lines of descent. As successive magma bodies chemically react with their wall rock and solidify at various depths, the mantle also changes in composition from peridotite toward basalt. Depending on the stage of conduit development, assimilation of wall rock could, therefore, yield variable magmatic differentiation trends. For successive magma bodies, the degree of contamination may decline as the conduit and its margin evolve physically. Lithospheric conduit evolution has several important ramifications for geochemical modelling. First, since magma ascent from source regions to crustal depths is unlikely to be isochemical, radiogenic and incompatible-element ratios may not always reflect characteristics of the magma source region. Second, the earliest stage of arc magmatism, e.g. submarine volcanism, is unlikely to reflect the processes of arc magma generation. Rather, this phase of volcanism records the interaction between ascending magma and unmodified mantle. The characteristics of primary arc magmas are likely to be understood only through examining more highly developed volcanic centers supplied by mature magmatic conduits.