Mixing and hybridization in basalt-to-rhyolite magmatism on an intermediate spreading mid-ocean ridge, Alarcon Rise, EPR.

Abstract:

Disequilibria between crystal cargoes and their host melts (glasses) provide insight into magmatic assimilation and fractional crystallization (AFC) processes and the space and time scales on which they operate. MBARI-led expeditions in 2012 and 2015 to the ~50km long Alarcon Rise, the northernmost spreading segment of the East Pacific Rise, collected 1-m bathymetry of the entire neovolcanic zone and portions of the adjacent fracture zones at a width of ~1.8-8km. The ROV sample collection includes >400 compositionally diverse lavas and >120 ~0.1-1m long sediment pushcores. Basalts are abundant in the neovolcanic zone. More evolved lavas are limited to the northern ~10km of segment and the curved intersection with adjacent Pescadero Fracture Zone. Isotopic studies show that lavas are sourced from a Pacific MORB-like mantle with negligible input from continental or altered oceanic material.

The hallmarks of open-system magmatism are amplified due to the eruption of such compositionally dissimilar lava. Evidence of mixing between mafic and evolved components include widespread chemical disequilibrium between melts and their crystal cargoes (particularly in clinopyroxene and plagioclase), ranges in crystal compositions that generally increase with host melt differentiation, common mafic xenoclasts in dacites and rhyolites, and many “hybrid” basaltic andesites and andesites. Thus, crystal cargoes and host melts preserve different aspects of a shared magmatic system.

Radiocarbon dates from foraminifera extracted from basal sediment provide minimum eruption ages for underlying dacitic to basaltic lava flows. These data are combined with maximum eruption ages determined from U-Th ages of zircons and Ar-Ar ages of plagioclases in rhyolite to constrain the local eruption chronology and hence the pace of AFC processes. Rhyolites erupted ~19±3ky, were preceded by basalts, and were followed by a period of dacite, andesite, and basaltic andesite eruptions, and finally basalt during the last few kyr. Compositional variability recorded in relatively few closely-spaced eruptions means that the magma reservoir was stratified and that during an eruptive episode, stored magma was mobilized by, partially mixed with, and expelled by newly injected melt.