TEXTURAL AND COMPOSITIONAL EVIDENCE FOR MAGMA MIXING IN THE ALKALINE OFF-RIFT BúðAHRAUN FLOW, ICELAND

The Búðahraun flow is an alkaline basalt flow located ~100km away from the main rift zone in western Iceland. The rock has a primitive composition (8-12% MgO), and it is derived from a cinder cone and not from a fissure eruption. It is porphyritic with macrocryst phases of olivine (ol), clinopyroxene (cpx) and plagioclase feldspar (plag) making up ~30% of a thin section. It is also vesicular, and has a microcrystalline groundmass consisting of ol, cpx, plag and a Fe-Ti oxide. The pre-eruptive histories of alkaline flank zone magmas are not well understood as compared to the voluminous tholeiitic lavas of the main rift zone. Textural analysis, mineral chemistry and geothermobarometry allow for the details of the magmatic plumbing system to be revealed.

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.