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

Some mantle clinopyroxene- and spinel- harzburgite xenoliths from the Kerguelen Islands contain an unusual metasomatic mineral association consisting of feldspar, olivine II, chromite II and Ti-oxides (rutile, ilmenite and armalcolite). These metasomatic minerals occur in veins along the grain boundaries of the olivine, orthopyroxene and clinopyroxene, but may cut across both these anhydrous silicates and pre-existing metasomatic amphibole and phlogopite. To our knowledge, this is the first time that such a mineral association, representing a type of mantle metasomatism that is distinct from those commonly attributed to potassium-rich hydrous fluids in cratonic mantle lithosphere or to carbonated Fe-Ti-rich silicate melts in non-cratonic mantle 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°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 the parental melt with the peridotite wall-rock released silica (from orthopyroxene), chromium (from clinopyroxene and original Cr-Al spinel) and magnesium (from orthopyroxene and clinopyroxene); this element exchange was essential to form the armalcolite paragenesis. Such Ti-rich veins cause strong local enrichment in some incompatible trace elements, including LILE, that are largely concentrated in feldspar (e.g., Ba, Sr, LREE) and HFSE (Ti, Nb, Zr) in rutile and armalcolite, even though these comprise approximately only 0.05 wt% of the rock.