Mineral chemistry of mildly alkalic basalts from the 25 Ma Mont Crozier section, Kerguelen Archipelago: Constraints on phenocryst crystallization environments

Phenocryst compositions and mineral-melt equilibria in the mildly alkalic basalts from the 25 Ma Mont Crozier section on the Kerguelen Archipelago are used to estimate the depths at which magmas stalled and crystallized and to constrain the role of crustal structure in the evolution of magmas produced by the Kerguelen mantle plume. The Crozier section, of nearly 1000 m height, consists of variably porphyritic flows (up to 21 vol. % phenocrysts), dominated by plagioclase +/- clinopyroxene +/- olivine +/- Fe-Ti oxides. Feldspars show an extreme range of compositions from high-Ca plagioclase (An(88)) to sanidine and variable textures that are related to extensive fractionation, degassing, and mixing in relatively low-pressure (sub-volcanic) magma chambers. Although clinopyroxene is a minor phenocryst type (0-3 vol. %), its non-quadrilateral components, principally Al (1.9-8.6 wt % Al2O3), vary widely. The results of clinopyroxene-liquid thermobarometry and clinopyroxene structural barometry indicate that the Crozier magmas crystallized at pressures ranging from similar to1 kbar to 11-12 kbar with high-Al clinopyroxene recording the highest pressures of crystallization. High-Al clinopyroxene-rich cumulates may represent an important component of the seismic crust-to-mantle transition zone at 14-16 km depth. High-pressure, high-Al clinopyroxene crystallization became important in the mildly alkalic basaltic magmas from the Kerguelen Archipelago as ascending magmas stalled and fractionated at or near the crust-mantle interface, which became deeper as a result of progressive crustal thickening as the archipelago moved from a ridge-centered setting at similar to40 Ma to an intraplate position by 25 Ma.