SUBDUCTION INITIATION PERIDOTITES OF THE WESTERN PACIFIC AND IMPLICATIONS FOR BONINITE GENESIS: RESULTS FROM IODP EXPEDITION 366

Peridotites that melt and re-equilibrate during subduction initiation are commonly preserved as lithospheric mantle that underlies the leading edge of the newly formed upper plate, and become trapped in the forearc once stable subduction ensues. IODP Expedition 366 returned peridotite samples from 3 large serpentine mud volcanoes in the Mariana forearc. Peridotite is dominantly harzburgite with less common dunite; geochemical data indicate a range of compositions that reflect varying amounts of melt depletion. This progressive melt depletion is reflected in the mineral chemistry, with Mg# up to 92.6 in olivine and enstatite, and up to 95.7 in diopside. Cr# in spinel ranges from 20 to 84, with most clustering between 35 and 60.

There are three main textural groups: porphyroclastic, protogranular, and reactive porous flow textures superimposed on protogranular textures. Porphyroclastic textures contain large highly strained porphyroclasts in a matrix of less strained olivine and pyroxene; granular aggregates of unstrained neoblasts are common. Spinel typically occurs as interstitial grains with holly leaf shapes or as subhedral prisms. This texture grades into tabular equigranular as porphyroclasts decrease in size and abundance. Protogranular textures are characterized by subequant olivine and enstatite, less intense strain, and vermicular intergrowths of spinel with pyroxene that form by decompression breakdown of garnet. Reactive porous flow textures are confined to samples with vermicular spinel reflecting an original protogranular texture. These harzburgites are characterized by resorbed enstatites, with lobate olivine embayments and apices of enstatite separating olivine grains. Flat facets on olivine suggest growth from a melt phase, as do rare euhedral olivine inclusions in enstatite. These textures resemble cumulate textures with post-cumulus enstatite. These textures are interpreted to reflect diffuse reactive porous flow that dissolves enstatite and precipitates olivine. This results in a silica-understaturated reactive melt that becomes progressively more silica-rich. We suggest that this reactive porous flow is related to the formation of low-silica boninite, which is a characteristic product of subduction initiation.