Source lithology of the Galápagos plume

We have measured the contents of Ni, Ca, and Mn in olivine phenocrysts from volcanoes in the Galápagos archipelago to infer the mantle source lithologies. Results show that peridotite is the dominant source lithology for Fernandina, Floreana, Genovesa, Marchena, Pinta, Wolf Island, and Darwin Island. These volcanoes largely characterize the PLUME, WD, FLO and DUM Nd, Sr, and Pb isotopic endmembers of Harpp and White (2001). Only a minor pyroxenite component contributes to Fernandina and Floreana. Peridotite is also the dominant source lithology for Volcan Wolf, Alcedo, and Cerro Azul, and that these have isotopic compositions that can be defined by mixing of the 4 endmembers. Peridotite is therefore the dominant source lithology of the Galápagos plume. However, pyroxenite melting is significant in two spatially separated domains which are also isotopically distinct: Roca Redonda, Volcan Ecuador, Sierra Negra in the enriched western part of the archipelago and Volcan Darwin, Santiago, Santa Cruz, and Santa Fe in the depleted east. An implication is that the western and eastern pyroxenite domains likely represent two separate bodies of recycled crust within the Galápagos mantle plume. Isotopically enriched and depleted domains of the archipelago melted from both peridotite and pyroxenite, and there is no relationship between source lithology and its isotopic characteristics. The identification of peridotite source melting in volcanoes with isotopic characteristics that have been attributed to recycled crust points to the importance of mixing in OIB genesis, in agreement with studies on the Canary Islands.M.S.Includes bibliographical referencesby Christopher Allen Vidit

Abrupt transition from fractional crystallization to magma mixing at Gorely volcano (Kamchatka) after caldera collapse

A series of large caldera-forming eruptions (361–38 ka) transformed Gorely volcano, southern Kamchatka Peninsula, from a shield-type system dominated by fractional crystallization processes to a composite volcanic center, exhibiting geochemical evidence of magma mixing. Old Gorely, an early shield volcano (700–361 ka), was followed by Young Gorely eruptions. Calc-alkaline high magnesium basalt to rhyolite lavas have been erupted from Gorely volcano since the Pleistocene. Fractional crystallization dominated evolution of the Old Gorely magmas, whereas magma mixing is more prominent in the Young Gorely eruptive products. The role of rechargeevacuation processes in Gorely magma evolution is negligible (a closed magmatic system); however, crustal rock assimilation plays a significant role for the evolved magmas. Most Gorely magmas differentiate in a shallow magmatic system at pressures up to 300 MPa, ∼3 wt% H2O, and oxygen fugacity of ∼QFM + 1.5 log units. Magma temperatures of 1123–1218 °C were measured using aluminum distribution between olivine and spinel in Old and Young Gorely basalts. The crystallization sequence of major minerals for Old Gorely was as follows: olivine and spinel (Ol + Sp) for mafic compositions (more than 5 wt% of MgO); clinopyroxene and plagioclase crystallized at ∼5 wt% of MgO (Ol +Cpx + Plag) and magnetite at ∼3.5 wt% of MgO (Ol + Cpx + Plag +Mt). We show that the shallow magma chamber evolution of Old Gorely occurs under conditions of decompression and degassing. We find that the caldera-forming eruption(s) modified the magma plumbing geometry. This led to a change in the dominant magma evolution process from fractional crystallization to magma mixing. We further suggest that disruption of the magma chamber and accompanying change in differentiation process have the potential to transform a shield volcanic system to that of composite cone on a global scale