Storage, fractionation and melt-crust interaction of basaltic magmas at oceanic and continental settings

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2016-08

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Gao, Ruohan

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This study uses phenocrysts and xenoliths to examine storage, fractionation and melt-crust interaction of basaltic magmas. Gabbroic xenoliths from Hualalai Volcano, Hawaii include fragments of lower oceanic crust (LOC) cumulates. Oxygen and Sr isotope compositions of these gabbros indicate minimal hydrothermal alteration. Magmas from fast ridges fractionate on average at shallower and less variable depths and undergo more homogenization than those from fast ridges. These features suggest a long-lived shallow magma lens exists at fast ridges, which limits the penetration of hydrothermal circulation into the LOC. Anorthitic plagioclases in these LOC gabbros therefore unlikely derive from hydrous melting or hydrothermal replacement. The strongly positive correlation between plagioclase anorthite content and whole rock Re concentration of Hualalai LOC gabbros may place further constraints on the origin of anorthitic plagioclase at mid-ocean ridges. Most Hualalai xenoliths represent Hualalai melt-derived cumulates. MELTS modeling and equilibration temperatures suggest Hualalai shield-stage-related gabbros crystallized within local LOC. Therefore, a deep magma reservoir existed within or at the base of the LOC during the shield stage of Hualalai Volcano. Melt–crust interaction between Hawaiian melts and Pacific crust partially overprinted Sr, Nd, and Pb isotope compositions of LOC-derived gabbros. The modified isotope compositions of Pacific LOC (and likely lithospheric mantle) are similar to Hawaiian rejuvenated-stage lavas. Although minor assimilation of Pacific crust by Hawaiian melts cannot be excluded, the range of oxygen isotope compositions recorded in Hawaiian magmas cannot be generated by assimilation of the in situ LOC. The Papoose Canyon (PC) monogenetic eruption sequence in the Big Pine volcanic field, California displays temporal-compositional variations indicating mixing of two distinct melts. PC phenocrysts and xenoliths derive from melt that is more fractionated and enriched than PC lavas. Pressure constraints suggest these phenocrysts and xenoliths crystallized at mid-crust depths. PC lavas also show evidence of crustal contamination. Therefore, PC phenocrysts and xenoliths likely derive from early PC melts that ponded, fractionated and assimilated continental crust in mid-crustal sills, which were mixed with more primitive melts as the eruption began. The temporal-compositional trends thus reflect gradual exhaustion of these sills over time.

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