PETROLOGICAL MODELS OF MAGMA EVOLUTION AND DEEP CRUSTAL STRUCTURE BENEATH HOTSPOTS AND FLOOD BASALT PROVINCES

Abstract

Seismic experiments imaging the deep crustal structure beneath hotspot tracks and oceanic plateaus indicate unusually high seismic velocities (VP = 7.3–7.8 km/s) at the base of the crust. These high-velocity ‘layers’, up to 10 km thick, are generally interpreted as large igneous intrusions. In this paper we investigate the extent to which plume magmatism affects the crust, and propose a model for the origin and composition of the high-velocity layers. We use the petrological code MELTS [1] to quantify the evolution of primary mantle melts as they ascend to the surface along an imposed pressure-temperature path. Crystal fractionation is an important process for plume volcanism, as suggested by the discrepancy between the composition of the erupted lavas, predominantly tholeiitic basalts, and the expected picritic composition of the parental mantle melts. In our model the initial liquid compositions are taken from melting experiments on spinel lherzolites at 10–30 kbar pressure and span reasonable compositional differences over a depth range of melting. MELTS rigorously calculates the amount and composition of the fractionated solid phases and the major oxide concentration of the residual liquid. For picritic initial liquid compositions (i.e., those formed at 30 and 20 kbar pressure) our petrological model predicts a distinct crustal structure: at the base of the crust (e.g., at P = 3 kbar) the fractionated cumulates are olivine and augitic pyroxene, with variable amounts of plagioclase. The calculated compressional wave velocities of the cumulates are 7.5–7.9 km/s. In the upper crust (P = 1 kbar) intrusions are olivine gabbros (VP ∼ 7.0 km/s). We also find that the coexistence of high-velocity layers at the base of the crust, with high upper crustal velocities is diagnostic of deep (30-20 kbar pressure) melting. Our results indicate that the lower-crustal high-velocity layers beneath oceanic plateaus and hotspot tracks represent fractionated cumulates from picritic mantle melts and are therefore an integral part of plume volcanism. This conclusion suggests that high-velocity layers should be present also beneath continental flood basalt provinces.