Petrology of ultramafic xenoliths from Loihi Seamount, Hawaii

Ultramafic xenoliths were recovered in four alkalic lava flows from Loihi Seamount at depths between 2200 and 1400m. No xenolith bearing flows were sampled near the summit despite a concentrated dredge program. The flows, three of alkalic basalt and one of basanite, contain common olivine megacrysts and small xenoliths of dunite, rarer harzburgite, and a single wehrlite. Olivine megacrysts as large as 8 mm are Fo_{84–88 6} and contain magnesiochromite inclusions with 1·1–3·5 wt.% TiO₂ Dunite contains Fo_{83 5–88·5} olivine, magnesiochromite with l·5–6·9 wt.% TiO₂ (avg. 3·2 wt.%), and extremely rare chrome-rich diopside. The wehrlite contains euhedral Fo_{85 9} olivine and magnesiochromite with 1·9–4·7 wt.% TiO₂ poikilitically enclosed in chrome-rich diopside (Wo_{45 4}En_{48 0}Fs_6·6).Most of the olivine megacrysts, dunite, and the wehrlite are cumulates of Loihi alkalic lavas that accumulated in a magma storage zone located at least 16km below sea level. The rarity of dunite related to tholeiitic magmas supports the interpretation that the alkalic lavas at Loihi generally predate the tholeiitic lavas. The harzburgite xenoliths have cataclastic textures and contain Fo_{89 5–92 6} olivine, enstatite (Wo_{2 0–2·7}En_{90·0–88 7}Fe_8·0–8·6), Cr-rich endiopside (Wo_{43 4–44 5}En_{52 0–50 0}Fs_{4 6–4 5}), and translucent red-brown magnesiochromite. The harzburgite xenoliths, which have 2-pyroxene temperatures of 1066 ± 35°C, originated in the uppermost mantle in a region of high strain rate, probably near the boundary between the mantle and the overlying ocean crust. The presence of upper mantle xenoliths indicates that the magma storage zone is located below the base of the ocean crust within the uppermost mantle.