TEXTURAL AND COMPOSITIONAL EVIDENCE FOR MAGMA MIXING IN THE ALKALINE OFF-RIFT BúðAHRAUN FLOW, ICELAND
Crystal size distributions are being made in order to quantify the crystal phase populations texturally. This is being accomplished through the manual tracing of macrocrysts on a cut surface, and through X-ray mapping by electron microprobe (EPMA). The EPMA maps have revealed two compositionally distinct populations of ol macrocrysts in terms of Mg & Fe content. Work is currently being done to quantitatively characterize the compositions of both populations of macrocrysts in addition to microcrysts in the groundmass.
Analysis of cpx macrocrysts will be used to determine thermal and barometric conditions of crystallization. The presence of a particularly well sector zoned cpx macrocryst in thin section presents an opportunity to take point compositions through the electron microprobe in order to map out potential chemical differences across the crystal zones. The goal of mapping out these point compositions is to determine how significant of an effect sector zonation has on geobarometry. While this process is better understood for rift derived tholeiitic cpx, the off-rift alkaline cpx effects are not as well constrained.