Armalcolite-bearing, Ti-rich metasomatic assemblages in harzburgitic xenoliths from the Kerguelen Islands: Implications for the oceanic mantle budgetof high-field strength elements

Some mantle clinopyroxene- and spinel- harzburgite xenoliths from the Kergu elen Islands contain an unusual metasomatic mineral association consisting of feldspar, olivine II, chromite II and Ti-oxides (rutile, ilmenite and ar malcolite). These metasomatic minerals occur in veins along the grain bound aries of the olivine, orthopyroxene and clinopyroxene, but may cut across b oth these anhydrous silicates and pre-existing metasomatic amphibole and ph logopite. To our knowledge, this is the first time that such a mineral asso ciation, representing a type of mantle metasomatism that is distinct from t hose commonly attributed to potassium-rich hydrous fluids in cratonic mantl e lithosphere or to carbonated Fe-Ti-rich silicate melts in non-cratonic ma ntle lithosphere, is reported from peridotite samples in the oceanic mantle . The vein-forming minerals are inferred to have crystallized at a minimum pressure of about 1.3 GPa and 1150-1200 degrees C from a strongly alkaline magma, with low water activity buffered by the wall-rock harzburgites. The oxygen fugacity is well-constrained at NNO -2.5 to -3.0 log units. Exchange reactions of thr parental melt with the peridotite wall-rock released sili ca (from orthopyroxene), chromium (from clinopyroxene and original Cr-Al sp inel) and magnesium (from orthopyroxene and clinopyroxene); this element ex change was essential to form the armalcolite paragenesis. Such Ti-rich vein s cause strong local enrichment in some incompatible trace elements, includ ing LILE, that are largely concentrated in feldspar (e.g., Ba, Sr, LREE) an d HFSE (Ti, Nb, Zr) in rutile and armalcolite, even though these comprise a pproximately only 0.05 wt% of the rock. Copyright (C) 2000 Elsevier Science Ltd.