Sr isotope systematics during melt generation by intrusion of basalt into continental crust

We have studied the Sr isotopic composition of partial melts of biotite granite generated experimentally and by intrusion of basalt into the Sierra Nevada Batholith. The experiments employed large, 3-cm cubes of granite to duplicate natural grain-boundary textures and were performed in air over the temperature interval 1000-1250 degrees C, to simulate basalt-induced wall rock and xenolith melting in the shallow crust. In both the experimental and natural analogs, fusion of plagioclase + alkali-feldspar + quartz and biotite + plagioclase + quartz results in the formation of colorless and brown melt (quenched to glass) respectively. Accordant with this melting behavior, brown glasses are enriched in radiogenic Sr and MgO, FeO, CaO, and TiO2 relative to colorless glasses. These results support recent studies indicating that the isotopic compositions of crustal melts can reflect the relative contributions of mineral phases entering the melt, rather than the isotopic composition of the bulk source rock. In addition, we show that at shallow-crustal conditions preferential breakdown of biotite leads to initial high-Sr-87/Sr-86, low-Sr concentration melts. However, as the degree of melting increases, melts become less radiogenic yet are more enriched in elemental Sr due to loss of biotite from the restite and increased consumption of feldspars. Our results therefore suggest, if partial melts of granitic crust segregate rapidly during episodic magmatic underplating, successive melt batches can evolve from high-Sr-87/Sr-86 to low-Sr-87/Sr-86 liquids as melting progresses.