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

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 (P2O5=0.4-1.6 wt.%, K2O=1.4-2.4 wt.%) at a relatively restricted MgO content (7.0-9.6 wt.%). The small amounts of P2O5-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 K2O and poorer in P2O5 subalkaline melt (K2O > 2.5 wt.%, P2O5<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 Pb-206/Pb-204-Nd-143/Nd-144 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 P2O5-rich Kerguelen plume-derived melt and a subalkaline melt derived from the Kerguelen Plateau lithosphere and provides more evidence for the involvement of low Pb-206/Pb-204-Nd-143/Nd-144 lithospheric material into young Kerguelen plume-derived basaltic magmas. (C) 2002 Elsevier Science B.V. All rights reserved.