The Role of Recycled Oceanic Crust in Mantle Plumes -Revisited

The role of recycled material in mantle plumes is difficult to quantify on the basis of incompatible trace elements and isotopes because of the great variability of subducted material. Another approach is to use major elements and compatible trace elements because these are more uniform in the mantle and are strongly controlled by the phase petrology of melting. Subducted crustal lithologies invariably differ from mantle peridotite, and this introduces olivine-free lithologies such as pyroxenites and eclogites into the mantle. Our massive study of olivine phenocrysts and trapped melt inclusions shows unusually high Ni and Si contents in many recent primary Hawaiian magmas. Similar compositions are found in the Canary Islands, W. Greenland, and the Siberian flood basalts. These magmas are not in equilibrium with an olivine bearing source under thick lithosphere (more than 100 km) typical of these localities, because an olivine-pyroxene assemblage would buffer both Ni and Si at lower levels. In contrast, magmas from plumes located under thin lithosphere, such as Iceland or Azores show no significant Si and Ni excess, and they could be in equilibrium with a shallow, olivine-bearing source. High-Si magmas can be produced by melting of eclogite, but this does not yield high Ni contents. Therefore, the eclogite-derived melt must acquire high Ni by converting surrounding peridotite to a solid pyroxenite, which ultimately melts a shallower level. Because unreacted peridotite may also begin to melt at shallow depths, this results in mixed melts derived from (secondary) pyroxenite and peridotite. In settings of thick lithosphere, the amount of peridotite-derived melt will be relatively small. Therefore, the recycled component represented by pyroxenite-derived melt may dominate. In settings of shallow melting, the peridotite will melt more extensively, and the signal from the recycled component will be diluted. Quantitative modeling shows that over half of the Hawaiian magma volume formed during the last 1 Myr came from secondary pyroxenite representing the recycled oceanic crust. The results are consistent with a plume with potential temperature of 1600 deg.C containing about 20 percent of recycled oceanic crust in the central part. These results are also consistent with estimates of volcano volumes, magma volume flux, and seismological observations. In the context of this model, the recent increase in Hawaiian magma flux is produced by an unusually high proportion of recycled crustal material in this part of the plume.