ZONED CLINOPYROXENES IN ALKALIC BASALT - CLUES TO FRACTIONATION AND MAGMA-MIXING HISTORIES FOR SEEMINGLY PRIMITIVE MAGMAS

Tertiary alkalic basalts in northeastern Brazil have > 12 wt.% MgO, ul tramafic xenoliths, olivine xenocrysts, and zoned clinopyroxene phenoc rysts. We use clinopyroxene compositions to assess whether these basal ts represent mantle-equilibrium melts, as MgO suggests, or fractionate d magmas with evolved compositions obscured by contributions to MgO fr om xenocrystic peridotitic olivine. Our approach is to analyze the cli nopyroxene compositional zones by electron microprobe, and to obtain b ackscattered-electron imagery. Most phenocrysts have cores and mantles , but are diopside in composition throughout, consistent with cognate origins. Mg#s range 86-73, where cores have highest Mg#s. The range in Mg# corresponds to melts in which fractionation increased original Fe O/MgO ratios by a factor of similar to 2, the equivalent to crystalliz ing Fo(90-80). Diopside Al-4/Al-6 ratios, evaluated together with the sieved-core textures of some grains and the Mg#s of individual zones, suggest that diopside with the higher Mg#s represents crystallization under a higher mantle pressure than diopside with the lower Mg#s, and that the evolved diopside represents mixing of primitive and evolved m elts in the lower pressure environment. Our study shows that detailed documentation of clinopyroxene zonations can disclose magma pre-erupti on histories, such as mixing of melts and multiple storage and crystal lization regimes, otherwise masked by artificially high MgO. We also o bserve that some clinopyroxene phenocrysts have cores of xenocrystic C r-diopside that represent mantle peridotite, and cores of green, Fe-ri ch diopside that are probably fragments of metasomatized mantle.