Clinoenstatite in boninites from the Bonin Islands, Japan

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 region^1-5 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.