Variations in magma transport recorded by plagioclase ultraphyric basalts: Preliminary results from SWIR, Blanco and Juan de Fuca

Plagioclase ultraphyric basalts (PUBs) have been sampled at slow to intermediate spreading oceanic centers worldwide. PUBs contain >15% (often anorthitic) plagioclase phenocrysts, with plagioclase making up > 90% of the phenocryst mode. The petrogenesis of PUBs has traditionally been attributed to inclusion of plagioclase from the crystal mush below spreading ridges. However, the conditions under which this occurs and the process of crystal sorting remain an enigma. To build a more complete model for PUB formation, we compiled published major and trace element data from the literature and PetDB for MORBs containing >15% plagioclase phenocrysts. While there is a clear connection between spreading rate and the occurrence of PUBs (more common at slow and intermediate rate ridges), we find that PUBs have no preferred erupted glass compositions. Therefore, we conclude that the generation of PUBs is dominantly a physical process rather than related to a specific magma type. One of our primary goals is to use information from the population of plagioclase phenocrysts to understand PUB magma differentiation and transport processes. In situ major and trace element and 87Sr/86Sr isotopic data were collected for plagioclase and its host glass from samples at the Southwest Indian Ridge (SWIR), Blanco Transform, and the Juan de Fuca Ridge in order to understand the relationship between the phenocrysts and their host lava suite. The plagioclase megacrysts record contrasting magma storage and transport conditions at different spreading ridges. Crystals from a single sample are often isotopically distinct from the magma they reside in and may or may not be distinct from other crystals in the same sample. Lavas from the East Blanco depression contain plagioclase phenocrysts that are more radiogenic than their host glass. Additionally, plagioclase-hosted melt inclusions have chemical signals that are more evolved than the ambient glass. This demonstrates that the plagioclase megacrysts at Blanco crystallized from a more enriched magma and are xenocrystic with respect to their current host liquid. Conversely, at SWIR, at least one sample contains plagioclase phenocrysts that are much less radiogenic than their host glass. While another dredged sample from SWIR has plagioclase phenocrysts that are mostly in isotopic equilibrium with their host liquid. In all samples, in situ trace elements in plagioclase exhibit a wide range of distinctive chemical characteristics. This suggests diverse magma transport conditions prior to being aggregated. Our findings indicate that PUBs often do not crystallize from simple genetic suites, but instead reflect the amalgamation of diverse melts and complex storage systems that exist below mid-ocean ridges. The presence of PUBs may be a distinctive characteristic of specific magma transport conditions, however, the boundary conditions for their formation is as yet unclear, and the subject of continued work.