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

Among the ejecta from a phreatomagmatic eruption on Piton de la Fournaise volcano are wehrlitic and gabbroic cumulate clasts. These fresh samples comprise olivine (Fo85.0 to Fo69.0), augite (En47.1Wo44.6Wo44.6 to En41.1Fs15.4Wo43.5) and plagioclase (An82.8Ab16.8Or0.4 to An53.8Ab44.8Or1.4), with spinel (Ti-rich ferrichromites to Ti-magnetite) and some ilmenite. A few percent of highly vesicular, interstitial glass is commonly present. Textures indicate that the cumulates were not wholly crystalline at the time of eruption and that the glass represents a residual intercumulus melt that varies from basaltic to mugearitic. Sulphide globules, typically spherical and up to 40 μm diameter, represent immiscible melts within the glasses. The sulphides contain relatively high contents of Cu and Ni and appear to be high-temperature homogeneous pyrrhotite-rich compositions. Rank order of element abundance in the sulphides is Fe, S, Cu, Ni, Co and Zn (wt.%s), although in one sample Ni is more abundant than Cu. Lack of correlation between the sulphide globule compositions and their host glasses is probably due to complex partitioning between the silicate melts, silicate and oxide crystals, sulphide melt and separating volatile phase. Whereas the Cu/(Cu+Ni) (at.) values of the sulphides are similar to those of the sulphide ore bodies from major gabbroic complexes, the Ni/(Ni+Fe) values are notably high with regard to the low MgO content of their host glasses. Oxide pairs, regarded as reflecting equilibria prior to eruption, indicate temperatures of 1017–897°C and −log fO2 values of 9.6–12.7. Compaction of analogous cumulate mushes, with extraction of relatively low-density basaltic to mugearitic (and ultimately trachytic) residual melts is inferred to have yielded the Differentiated Series lavas and ash-flows of the older volcano on Réunion, Piton des Neiges. The sequence of crystallisation, with augite as the second silicate phase (rather than plagioclase as is normal in the lava sequences) suggests crystallisation at depths >5 km. The Bellecombe ash cumulate clasts show more evolved assemblages than most other xenolith suites on the volcano (in which dunites predominate) and are probably products of stagnated magma batches, possibly arrested in sill-like intrusions 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 Réunion.