Petrology of Ultramafic Xenoliths from Loihi Seamount, Hawaii |
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| Authors: | CLAGUE DAVID A |
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| Institution: | U.S. Geological Survey Menlo Park, CA 94025 |
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| Abstract: | Ultramafic xenoliths were recovered in four alkalic lava flowsfrom Loihi Seamount at depths between 2200 and 1400m. No xenolithbearing flows were sampled near the summit despite a concentrateddredge program. The flows, three of alkalic basalt and one ofbasanite, contain common olivine megacrysts and small xenolithsof dunite, rarer harzburgite, and a single wehrlite. Olivinemegacrysts as large as 8 mm are Fo84–88 6 and containmagnesiochromite inclusions with 1?1–3?5 wt.% TiO2 Dunitecontains Fo83 5–88?5 olivine, magnesiochromite with l?5–6?9wt.% TiO2 (avg. 3?2 wt.%), and extremely rare chrome-rich diopside.The wehrlite contains euhedral Fo85 9 olivine and magnesiochromitewith 1?9–4?7 wt.% TiO2 poikilitically enclosed in chrome-richdiopside (Wo45 4En48 0Fs6?6).Most of the olivine megacrysts,dunite, and the wehrlite are cumulates of Loihi alkalic lavasthat accumulated in a magma storage zone located at least 16kmbelow sea level. The rarity of dunite related to tholeiiticmagmas supports the interpretation that the alkalic lavas atLoihi generally predate the tholeiitic lavas. The harzburgitexenoliths have cataclastic textures and contain Fo89 5–926 olivine, enstatite (Wo2 0–2?7En90?0–88 7Fe8?0–8?6),Cr-rich endiopside (Wo43 4–44 5En52 0–50 0Fs4 6–45), and translucent red-brown magnesiochromite. The harzburgitexenoliths, which have 2-pyroxene temperatures of 1066 ? 35?C,originated in the uppermost mantle in a region of high strainrate, probably near the boundary between the mantle and theoverlying ocean crust. The presence of upper mantle xenolithsindicates that the magma storage zone is located below the baseof the ocean crust within the uppermost mantle. |
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