Highly magnesian andesitic lavas were considered a rare and unusual rock type, before boninites proved to be so extensive among the Tertiary volcanic rocks in the Benin-Mariana region1-5 and some ophiolitic basalts 6,7. Protoenstatite is also a rare mineral and its terrestrial occurrence has been confined to the high-Mg andesites from Cape Vogel, eastern Papua8, and the Mariana Trench3. Some boninites from the Bonin Islands were found to contain abundant multiply-twinned clinoenstatite (inverted protoenstatite) with or without olivine. The clinoenstatite, coexisting with olivine with a similar forsterite content to that in the mantle, shows that protoenstatite crystallized later than the olivine, whereas in the olivine-free, clinoenstatite-bearing rock which has more SiO2 than the clinoenstatite-bearing or -free boninites with olivine, the protoenstatite may have been the liquidus mineral. We show here that the presence of inverted protoenstatite in the Bonin Islands confirms our earlier suggestion 1,9,10 that boninites may have been formed by extensive partial melting of hydrous peridotite at relatively low pressures and rapid quenching from high temperatures at shallow depths.
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