Melt, fluid and crystal inclusions in olivine phenocrysts from Kerguelen plume-derived picritic basalts: evidence for interaction with the Kerguelen Plateau lithosphere

Abstract

Melt, fluid and crystal inclusions have been studied in olivine phenocrysts from a suite of picritic basalts dredged during the “Marion Dufresne” MD 109 cruise (Dredge 6) from a seamount located between the Kerguelen Archipelago and Heard Island (Southern Indian Ocean). A two-stage polybaric crystallisation of the trapped melt is indicated by compositions of the host olivine+spinel assemblage and by the density of coexisting fluid inclusions. The melt trapped in early olivine phenocrysts (Fo 82–85.5) crystallised under pressures from 2 to 4 kbar, whereas the melt trapped in late olivine phenocrysts (Fo 78.7–80) crystallised at shallower depths (<1 kbar). There is a wide range of incompatible elements in the homogenised melt inclusions (P 2O 5=0.4–1.6 wt.%, K 2O=1.4–2.4 wt.%) at a relatively restricted MgO content (7.0–9.6 wt.%). The small amounts of P 2O 5-rich melt trapped in the early olivine phenocrysts are remarkably similar to the contemporaneous basalt suite (Dredge 5) from the same seamount. The chemistry of the host dredge 6 basalts requires a significant contribution (more than 80%) of richer in K 2O and poorer in P 2O 5 subalkaline melt (K 2O>2.5 wt.%, P 2O 5<0.34 wt.%). The contribution of such subalkaline melt with slightly unradiogenic Pb–Nd isotope ratios and major and trace element composition, similar to the low 206Pb/ 204Pb– 143Nd/ 144Nd trachybasaltic suite of the Aphanasey Nikitin Rise, may explain the striking Nb–Ta depletion, Pb enrichment and isotopic compositions of the dredge 6 basalts. The petrogenesis of the picritic basalts from dredge 6 is controlled by two-component mixing between a P 2O 5-rich Kerguelen plume-derived melt and a subalkaline melt derived from the Kerguelen Plateau lithosphere and provides more evidence for the involvement of low 206Pb/ 204Pb– 143Nd/ 144Nd lithospheric material into young Kerguelen plume-derived basaltic magmas.