Paper No. 95-4
Presentation Time: 9:00 AM-1:00 PM
TEXTURAL AND COMPOSITIONAL FEATURES OF PLAGIOCLASE AND MAFIC PHASES OF PLAGIOCLASE ULTRAPHYRIC BASALTS (PUB) FROM THE NE PACIFIC RIDGE SYSTEM
Plagioclase compositional and textural profiles have long been used as recorders of the conditions and processes occurring in mafic and intermediate volcanic rocks. Despite the fact that plagioclase is an abundant phase in the lavas from many tectonic settings (continental arcs, ocean islands, and mid-ocean ridges), there have been few studies focusing on textural characteristics of plagioclase ultraphyric basalts (PUB) from mid-ocean ridge (MOR) systems. Two such investigations are Bennett et al. (2019) on the Gakkel Ridge and Li et al. (2020) on the Southwest Indian Ridge (SWIR) both settings with ultra-slow spreading rates and low magma supply. Therefore, the extent to which associated textures and chemical signatures vary from one tectonic setting to another is still unclear and there are no such studies of MOR systems with faster spreading rates or higher magma supply. Here, we report the results of a study of PUB lavas from the NE Pacific ridge system (Blanco Fracture Zone (BFZ), West Valley of Juan de Fuca Ridge (JdF), Endeavor segment of JdF, and the Gorda Ridge) using the textural features (crystal morphology, size, phase assemblage, zoning) and major element characteristics of plagioclase and associated mafic phases used for the Gakkel and SWIR PUB lavas. The NE Pacific ridge system is characterized by a range of settings that include intermediate spreading rates as well as intra-transform basins with a range of magma supply rates. Major element analyses of the plagioclase megacryst populations in lavas from the NE Pacific Ridge system has revealed that the megacryst populations in each sample are distinct from one another with respect to how anorthite (An) content correlates with MgO, TiO2, and FeO. Some samples are characterized by modes that correlate with textures, while others do not. In a simple example, some samples are bimodal in that there are distinct high-An and low-An populations, while other samples have distinct but subparallel (in An vs MgO) populations. We observe that in some samples, the megacryst rim compositions are all in the lower-An mode while the core compositions are in both modes. There are also samples rim and core compositions are present in both modes. Understanding why there is this difference will inform what, how and when processes such as mixing and fractionation are happening.