Cumulate clasts in the Bellecombe Ash Member, Piton de la Fournaise, Reunion Island, and their bearing on cumulative processes in the petrogenesis ofthe Reunion lavas

Among the ejecta from a phreatomagmatic eruption on Piton de la Fournaise v olcano are wehrlitic and gabbroic cumulate clasts. These fresh samples comp rise olivine (Fo(85.0) to Fo(69.0)), augite (En(47.1)Wo(44.6)Wo(44.6) to En (41.1)Fs(15.4)Wo(43.5)) and plagioclase (An(82.8)Ab(16.8)Or(0.4) to An(53.8 )Ab(44.8)Or(1.4)), with spinel (Ti-rich ferrichromites to Ti-magnetite) and some ilmenite. A few percent of highly vesicular, interstitial glass is co mmonly present. Textures indicate that the cumulates were not wholly crysta lline at the time of eruption and that the glass represents a residual inte rcumulus melt that varies from basaltic to mugearitic, Sulphide globules, t ypically spherical and up to 40 mum diameter, represent immiscible melts wi thin the glasses. The sulphides contain relatively high contents of Cu and Ni and appear to be high-temperature homogeneous pyrrhotite-rich compositio ns. Rank order of element abundance in the sulphides is Fe, S, Cu, Ni, Co a nd Zn (wt.%s), although in one sample Ni is more abundant than Cu. Lack of correlation between the sulphide globule compositions and their host glasse s is probably due to complex partitioning between the silicate melts, silic ate and oxide crystals, sulphide melt and separating volatile phase. Wherea s the Cu/(Cu + Ni) (at.) values of the sulphides are similar to those of th e sulphide ore bodies from major gabbroic complexes, the Ni/(Hi + Fe) value s are notably high with regard to the low MgO content of their host glasses . Oxide pairs, regarded as reflecting equilibria prior to eruption, indicat e temperatures of 1017-897 degreesC and -log f(O2) values of 9.6-12.7. Comp action of analogous cumulate mushes, with extraction of relatively low-dens ity basaltic to mugearitic land ultimately trachytic) residual melts is inf erred to have yielded the Differentiated Series lavas and ash-flows of the older volcano on Reunion, Piton des Neiges. The sequence of crystallisation , with augite as the second silicate phase (rather than plagioclase as is n ormal in the lava sequences) suggests crystallisation at depths >5 km. The Bellecombe ash cumulate clasts show more evolved assemblages than most othe r xenolith suites on the volcano tin which dunites predominate) and are pro bably products of stagnated magma batches, possibly arrested in sill-like i ntrusions in the deeper crust. The cumulates are recent components in what is probably a large (ca. 70 km across) layered complex beneath the volcanic edifices, accreted during the build up of Reunion. (C) 2000 Elsevier Scien ce B.V. All rights reserved.