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Journal of Petrology | Volume 43 | Number 7 | Pages 1389-1413 | 2002
© Oxford University Press 2002
Mineral Chemistry of Mildly Alkalic Basalts from the 25 Ma Mont Crozier Section, Kerguelen Archipelago: Constraints on Phenocryst Crystallization Environments
1DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES, UNIVERSITÉ LIBRE DE BRUXELLES CP 160/02, AVENUE F. D. ROOSEVELT 50, B-1050, BRUSSELS, BELGIUM
2DEPARTMENT OF EARTH, ATMOSPHERIC AND PLANETARY SCIENCES, MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MA 02139, USA
3LABORATOIRE DE GÉOLOGIE–PÉTROLOGIE, UNIVERSITÉ JEAN MONNET, CNRS–UMR 6524, SAINT-ÉTIENNE, FRANCE
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 (An88) 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 
1 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 40 Ma to an intraplate position by 25 Ma.
KEY WORDS: Kerguelen Archipelago; Mont Crozier; mildly alkalic basalts; phenocrysts; high-Al clinopyroxene
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