Plagioclase Compositional Profiles from Plagioclase Ultraphryic Basalts

Plagioclase compositional profiles are used for interpretation of processes within arc environments, but less commonly within mid-ocean ridge (MOR) systems. Our study focuses on using combined major and trace element profiles of plagioclase crystals (≥1mm) from plagioclase ultra phyric basalt (PUB) to quantify important system parameters such as residence time, number of mixing components, provenance of crystals and magma transport variables. Plagioclase megacrysts from PUB likely represent components of a lower crustal mush zone underlying mid-ocean ridges entrained during magma influx (e.g. Hansen and Grönvold, 2000). This mush zone is an important site of primary melt modification. Evidence of modification processes should be recorded in PUB plagioclase zonation. We have examined plagioclase megacryst profiles from PUB located at four different oceanic spreading centers (Southeast Indian Ridge, Southwest Indian Ridge, Axial Seamount and West Valley Segment of the Juan de Fuca Ridge) with the purpose of observing compositional profile variations within and between samples that represent a number of different ridge characteristics. A central goal of our work is to develop a methodology to distinguish between the patterns expected of fractionation, mixing and diffusive equilibration. Towards that end, we will use information on elements characterized by a range of partition and diffusion coefficients - comparing elements with similar Ds and different diffusion rates (e.g. Mg and Ti), with those with different Ds and diffusion rates (e.g. Sr, Ba). Our results document PUB plagioclase (An94-80) to exhibit a relatively narrow range of individual crystal anorthite content (±8%) and a negative correlation of anorthite and MgO wt%. Partitioning models (e.g. Bindeman et al., 1998) suggest MgO (0.17-0.27 wt%) is often in equilibrium with crystal anorthite while elements characterized by lower rates of diffusion and different partitioning behavior (Sr, Ti, Ba) exhibit a range of profiles in a single sample. Profiles can exhibit core to rim decreases in Ti (40-50 ppm) and increases in Sr (10-20 ppm), potentially a record of fractional crystallization. Less commonly, profiles display homogenous core and rim compositions (Ti ~120 ppm, Ba ~10 ppm) with a depleted crystal mantle (Ti~40 ppm, Ba ~4 ppm), likely the product of melt mixing. Such profiles suggest DBa>DTi and provide an original zoning profile permitting the calculation of minimum residence time. Few profiles show Ba increasing towards crystal rim (+6 ppm), often coincident with Sr (+50 ppm), when other trace elements are equilibrium with anorthite content. We believe such inter-phenocryst variations in profile behavior within a single sample can be interpreted to illustrate the range of magmatic histories represented by plagioclase megacrysts in any individual PUB; while differences between samples will help us understand the boundary conditions that describe the range of physical conditions under which PUB lavas form.