Résumé
The role of carbon-rich or carbonatitic melts as an important metasomatizing agent in the Earth's mantle is supported by direct and indirect evidence of their involvement, ranging from the presence of erupted carbonatitic lavas to metasomatic reactions documented in minerals and melt inclusions from mantle xenoliths. Carbonatite metasomatism in hot-spot settings, and more particularly in the mantle sources of rejuvenated Hawaiian lavas, has long been suspected. However, an unequivocal geochemical tracer of carbonated mantle sources in alkaline volcanic suites is still missing. We here examine high-quality major- and trace-element compositions of similar to 400 primitive Hawaiian lavas (MgO = 8.5-21 wt%, SiO2 = 37-50 wt%) and associated xenoliths, focusing on those erupted during rejuvenated stages of activity of the Hawaiian hot spot, Pacific Ocean. The rejuvenated-stage alkaline lavas are the most enriched in volatile elements among the four-stage Hawaiian lavas. Our compilation shows that these rejuvenated-stage lavas range from melilite/nephelinite to transitional basalts and are characterized by low-Si and high-Na, -K and -Ca contents, along with the enrichment of REE, Th and Ba relative to K, Hf, Zr, Ti and Nb. Their trace-element systematics argues against derivation from a homogeneous lherzolitic or pyroxenitic source, regardless of the involvement of residual garnet or hydrous phases. Based on a comprehensive review of natural and experimental constraints on partitioning between carbonatites and mantle minerals and numerical simulations of open-system melting, we show that it is rather consistent with carbonatite metasomatism in their source. Variations in SiO2, CaO, alkali contents and trace-element proxies such as Hf/Sm also specify temporal variations in the depth of melting and/or the respective contribution of lherzolites and pyroxenites in a hybrid (probably asthenospheric) source fluxed by carbonatitic melts. We suggest that this episode took place <= 4.2 Ma at temperatures and pressures in excess of 1100 degrees C and 2 GPa. The low-solidus carbonatite melts were likely generated following a time lag which allowed for cooling of the plume and likely derived from an ancient (>1Ga), recycled mantle, or lower mantle, source in the Hawaiian plume, in good agreement with Sr-Nd-Hf-Os isotope systematics and other chemical and mineralogical features of Hawaiian rejuvenated lavas and xenoliths. The identification of a deep carbonated mantle source for Hawaiian rejuvenated volcanic series is also in line with noble gas and light stable isotope systematics and suggests that the interaction between carbonatite melts and peridotites/pyroxenites may be a critical process explaining the compositional variability of many oceanic island magmas.