Plagioclase Ultraphyric Basalts (PUB) from the Northeastern (NE) Pacific Ridge System: Mineral Textures and Geochemical Characteristics

Plagioclase crystals in plagioclase ultraphyric basalts (PUB) record changes in the major, trace, and isotopic compositions of evolving magmas. Those changes represent the record of specific processes in the crust and upper mantle that impart distinct crystal textures onto which chemical signatures are written. We report on the textural and compositional characteristics of PUB lavas from the Northeastern (NE) Pacific Ridge System (Juan de Fuca Ridge, Blanco Fracture Zone, and Gorda Ridge). The distribution of textural and compositional characteristics (e.g., single mode, bimodal, or trimodal distribution) are consistent with the presence of more than one parental magma in some samples. Specifically, we found that the populations of different textural types of plagioclase megacrysts are not compositionally distinct. We interpret the disconnect between texture and composition to be indicative of a system characterized by mixing of similar batches of magma resulting in populations of plagioclase megacrysts dominated by homogeneity or small degrees of normal/reverse zoning in most but not all cases.