Contrasting compositional trends of rocks and olivine-hosted melt inclusions from CerroNegro volcano (Central America): Implications for decompression-driven fractionation ofhydrous magmas

Melt inclusions in olivine Fo83-72 from tephras of 1867, 1971 and 1992 eruptions of Cerro Negro volcano represent a series of basaltic to andesitic melts of narrow range of MgO (5.6-8 wt%) formed by ~46 wt% fractional crystallization of olivine (~6 wt%), plagioclase (~27 wt%), pyroxene (~13 wt%) and magnetite (<1 wt%) from primitive basaltic melt (average SiO2=49 wt%, MgO=7.6 wt%, H2O=6 wt%) as it ascended to the surface from the depth of about 14 km. The crystallization occurred at increasing liquidus temperature from 1050 to 1090 oC in the pressure range from 400 to 50 MPa and was induced by release of mixed H2O-CO2 fluid from the melt at decreasing pressure. Matrix glass compositions fall at the high-Si end of the melt inclusion trend and represent the final stage of melt crystallization during and after eruption. The bulk compositions of erupted Cerro Negro magmas (tephras and lavas) range from high- to low MgO (3-10 wt%) basalts, which form a compositional array crossing the trend of melt inclusions so that virtually no rock from Cerro Negro has composition akin to true melt represented by the inclusions. The variations of the bulk magma (rocks) and melt (melt inclusions) compositions can be generated in a dyke connecting deep primitive magma reservoir with the Cerro Negro edifice. While the melt inclusions represent the compositional trend of instantaneous melts along the magma pathway at decreasing pressure and H2O content, occurrence of low-Mg to high-Mg basalts reflect the process of phenocryst re-distribution in progressively evolving melt. The crystallization scenario is anticipated to operate everywhere in dykes feeding basaltic volcanoes and can explain the predominance of plagioclase-rich high-Al basalts in island arc as well as typical compositional variations of magmas during single eruptions.