The Grande Ronde Basalt lavas constitute ~63% of the Columbia River Basalt Group, a large igneous province in the NW United States. The lavas are aphyric or contain less than 5% phenocrysts of plagioclase, augite, pigeonite, and olivine (altered). Plagioclase hygrometry shows that the erupted lavas generally contained less than 0.3% dissolved H 2 O; however, the presence of rare disequilibrium An 96 plagioclase phenocrysts suggests that some magmas may have originally had 4.5 wt% dissolved H 2 O at depth, but they all degassed during ascent and eruption. The size of plagioclase phenocrysts suggests an average plagioclase phenocryst residence time in the magmas of 160 yr. Ignoring hiatuses between eruptions, we estimate that the ~110 flows of the Grande Ronde Basalt erupted over a cumulative time of 17,600 yr, with an average eruption rate of ~8.6 km 3 /yr. The average interval between eruptions is estimated to be 3658 yr. It is envisaged that a shallow intrusive dike-sill complex, rather than large kilometer-sized magma chamber(s), fed the Grande Ronde basalt lavas. We performed model simulations using the COMAGMAT software to retrace the pre-eruption histories of the Grande Ronde Basalt lavas. Based on such simulations and petrological reasoning, we find that the primary melts could have been generated from a spinel peridotite source at 1.5 GPa, perhaps under hydrous conditions. Extensive melting of lithospheric eclogite may have played an important role as well; however, this is not constrained by our simulations. All lavas were contaminated by the crust, and they were last processed (mixing, crystallization) during their short residence within shallow (6 km) intrusive rocks prior to eruption. Our petrologic conclusions lead us to present a petrotectonic model that supports the hypothesis that the Columbia River Basalt Group magma generation was greatly aided by a thinned lithosphere and H 2 O that may have come off the asthenospheric wedge.

Basaltic lavas of Nyos volcano (Cameroon) mostly contain mantle peridotite xenoliths consisting of spinel-bearing lherzolites and harzburgites. Based on the trace-element patterns, especially rare earth element (REE) patterns, two groups of samples have been distinguished: group 1 samples are characterized by spoon-shaped REE patterns, and group 2 samples show light (L) REE–enriched patterns. Mineralogical characteristics together with major- and trace-element compositions point to a low degree of partial melting (less than 5%) and metasomatic processes. The latter mechanism explains in particular the LREE content of bulk rocks and clinopyroxenes and the occurrence of hydrous minerals in some samples. All the metasomatic features observed in both groups of samples are related to more or less alkaline—and carbonated—mafic silicate melts. These melts are related to the magmatic activity of the Cameroon volcanic line, leading in particular to the eruption of the host lava xenoliths.