PERIDOTITES FROM THE SOUTHERN MARIANA FORE-ARC - HETEROGENEOUS FLUID SUPPLY IN MANTLE WEDGE

Detailed petrological work was carried, out on serpentinized peridotit e dredged and sampled by submersible fi om the southern part of the Ma riana Trench to reveal the nature of the mantle wedge in the southern Mariana forearc. The southern part of the Mariana Trench is important in that we should expect to find a transect of a typical island are st ructure; that is, from east to west, the Mariana forearc, the Mariana are proper, the Mariana Trough (active back-are spreading center), and the West Mariana Ridge (remnant arc). The most striking feature of pe ridotites from the southern part of the trench is that primary hornble nde is a major constituent mineral in many specimens. Thus, the perido tite samples are divided into anhydrous (A-type), hydrous (H-type) and intermediate (I-type) groups. Petrological data suggest that each typ e of peridotite is a residue of extensive partial melting in the upper mantle. It is argued here that the I- and H-type peridotites were mod ified from 'proto-A-type peridotite' by fluid infiltration. The fluid was enriched in Al, Ti, Fe, and alkalis, and may have caused changes i n mineral and bulk chemical compositions of the peridotites. A-type pe ridotite derives from the 'proto-A-type peridotite' directly, without any fluid contamination. After the formation of the 'proto-A-, I-, and I-I-type peridotites', lower-temperature fluids, probably of seawater origin, produced retrograde metamorphism and alteration including ser pentinization. The mantle wedge in the southern Mariana forearc was he terogeneous in fluid supply.