Plagioclase Ultra-Phyric Basalts (PUBs): Implications for the nature of the plumbing system at ultraslow-spreading ridges

Plagioclase ultra-phyric pillow basalts (PUBs) of the Southwest Indian Ridge contain up to 35% anorthitic plagioclase crystals. Plagioclase crystals exhibit compositional and textural heterogeneities reflecting a history of complex crystallization conditions. Phenocrysts and megacrysts (> 0.5 mm) that range in composition from An 85 to An94 are too calcic to be in equilibrium with the host glass. Microlite (< 0.1 mm) compositions range between An 65 and 78 and are in equilibrium with the host glass. Primitive (Mg# > 82) clinopyroxenes are found as inclusions in plagioclase and as heavily resorbed, isolated phenocrysts. Phase equilibrium simulations with MELTs using variable pressure conditions and magmatic H2O, CaO, and Na2O compositions demonstrate that high anorthite plagioclase can crystallize at crustal pressures from a hydrated melt (0.5 < wt% H2O) containing low Na2O. Crystal size distributions (CSDs) for plagioclase phenocrysts and megacrysts show linear population densities, consistent with steady crystal nucleation and growth and are similar for samples from different ridge segments. Crystal residence times are less than several hundred years based on crystal growth rates of 10-10 mm/sec. Plagioclase size populations, textures, and chemical results are interpreted to reflect open-system processing in the production of plagioclase ultra-phyric basalts. Primitive magmas emplaced into the upper crust (~1-4 km) may become hydrated by reaction with older, hydrothermally altered country rock. A magma chamber with gradients in temperature and composition can produce crystals that display variations in textures and morphology, including zoning, resorption, dissolution, and skeletal growth features. However, the comparable CSD results from along the ridge axis suggest the overall thermal environment of crystal growth is broadly similar between ridge segments.