Petrogenesis of Plagioclase Ultraphyric Basalts (PUB) From the Northeastern (NE) Pacific Ridge System: Evidence From Mineral Textures and Geochemical Characteristics

Abstract

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. The extent to which the type and abundance of plagioclase textures and chemical signatures identified in one tectonic setting are found in others is still unclear. Here 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 NE Pacific Ridge system exhibits a variety of populations of plagioclase megacrysts. 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. In PUB lava from the NE Pacific, we found that the populations of different textural types of plagioclase megacrysts are not compositionally distinct. Specifically, in any individual sample, there is no correlation of texture with major and trace element composition. 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. Furthermore, the populations of megacrysts and glomerocrysts are distinct in each sample and not correlated with their setting (on-axis, off-axis, or intra-transform basin).