Experimental constraints on the origin and evolution of mildly alkalic basalts from the Kerguelen Archipelago, Southeast Indian Ocean |
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| Authors: | James S Scoates Mauro Lo Cascio Dominique Weis Donald H Lindsley |
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| Institution: | (1) Department of Earth & Environmental Sciences (DSTE), Université Libre de Bruxelles CP160/02, Avenue F.D. Roosevelt 50, Brussels, Belgium;(2) Department of Geosciences, State University of New York at Stony Brook, Stony Brook, NY, USA;(3) Present address: Department of Earth & Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, Canada;(4) Present address: Department of Geological Sciences, Brown University, 324 Brook St – Box 1846, Providence, RI, USA;(5) Present address: Department of Earth & Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, Canada |
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| Abstract: | This experimental study examines the role of clinopyroxene fractionation on major element trends and alkalinity variations in mildly alkalic basalts from the Kerguelen Archipelago, Southeast Indian Ocean. Equilibrium crystallization experiments were carried out on a natural basalt (MgO=5 wt.%, alkalinity index=0.10) over a range of pressures (0–1.43 GPa) and water contents (nominally dry to hydrous, 1.2 wt.% H2O) under relatively oxidizing conditions (Δlog FMQ=+1 to +2) at 0 GPa and relatively reducing conditions (Δlog FMQ=0 to –2) at all higher pressures. The hydrous experiments at 0.93 GPa closely reproduce most of the compositional variations in the 24–25 Ma mildly alkalic lavas from the archipelago, which supports a major role for high-Al clinopyroxene fractionation (5–9 wt.% Al2O3) at pressures corresponding to the base of the Northern Kerguelen Plateau (15–20 km). However, clinopyroxene fractionation at depth fails to produce important changes in the alkalinity of the residual melts. The transition from tholeiitic to mildly alkalic basalts on the Kerguelen Archipelago thus reflects primarily changes in melting conditions (lower extents of partial melting at higher pressures), which is related to crustal and lithospheric thickening as distance from the Southeast Indian Ridge increased over time from 43 to 24 Ma. |
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