H 2O [sbnd]CO 2 fluids supplied in alpine-type mantle peridotites: electron petrology of relic fluid inclusions in olivines

Abstract

Fluids supplied in alpine-type mantle peridotites and trapped as fluid inclusions in olivines have been fixed by low-temperature reactions, and their CO 2/H 2O ratios can be deduced from the minerals in the inclusions. Relic fluid inclusions were commonly observed by the optical microscope in olivines from almost all examined solid intrusive ultramafic complexes (Papua, Oman, Troodos and eleven alpine-type complexes of Japan). Such complexes were emplaced into the crust in a solid state. Electron microscopic studies of olivines from three complexes, Higashiakaishi, Horoman and Iwanai-dake, showed that relic fluid inclusions in these olivines have distinctive mineral parageneses: serpentine + magnesite + talc, serpentine + magnesite + brucite, and serpentine + brucite, respectively, depending on the CO 2/(H 2O+CO 2) ratio of the trapped fluid. It is deduced that the fluids had been supplied to peridotites, at least partly, but almost wholly in some case, when the peridotites were still hot, probably at the upper mantle for the following reasons: (1) the curved surfaces along which the inclusions are distributed are cut by post-emplacement serpentine veins; (2) for the Higashiakaishi dunite, the relic fluid inclusions are exclusively found in porphyroclast olivines and are totally absent in matrix olivines recrystallized during the Sanbagawa metamorphism. Recent models on the derivation of ophiolitic or some alpine-type peridotites favor the island-arc or fore-arc settings. Dehydration of the descending oceanic slab may supply H 2O CO 2 vapor to the overlying mantle wedge. Fluid inclusions trapped in such mantle wedge may abound in H 2O component. H 2O-bearing fluid inclusions may, therefore, be important H 2O containers in the upper mantle, especially near the edge of the mantle wedge above downgoing oceanic slabs.