The global set of samples investigated here were produced at various degrees of partial melting and in-

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a b s t r a c t a r t i c l e i n f oMost geochemical variability in MOR basalts is consistent with low-to moderate-pressure fractional crystallization of various mantle-derived parental melts. However, our geochemical data from MOR highsilica glasses, including new volatile and oxygen isotope data, suggest that assimilation of altered crustal material plays a significant role in the petrogenesis of dacites and may be important in the formation of basaltic lavas at MOR in general. MOR high-silica andesites and dacites from diverse areas show remarkably similar major element trends, incompatible trace element enrichments, and isotopic signatures suggesting similar processes control their chemistry. In particular, very high Cl and elevated H 2 O concentrations and relatively light oxygen isotope ratios (~5.8‰ vs. expected values of~6.8‰) in fresh dacite glasses can be explained by contamination of magmas from a component of ocean crust altered by hydrothermal fluids. Crystallization of silicate phases and Fe-oxides causes an increase in δ 18 O in residual magma, but assimilation of material initially altered at high temperatures results in lower δ 18 O values. The observed geochemical signatures can be explained by extreme fractional crystallization of a MOR basalt parent combined with partial melting and assimilation (AFC) of amphibole-bearing altered oceanic crust. The MOR dacitic lavas do not appear to be simply the extrusive equivalent of oceanic plagiogranites. The combination of partial melting and assimilation produces a distinct geochemical signature that includes higher incompatible trace element abundances and distinct trace element ratios relative to those observed in plagiogranites.

Understanding how and where magmas are generated, transported, and focused in the mantle before erupting at mid-ocean ridges (MORs) and seamounts is fundamental to determining how the majority of the Earth's oceanic crust is formed. Most investigations of mid-ocean ridge basalt (MORB) geochemistry occur

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