Petrography and Geochemistry of High Silica EPR Glasses: Implications for Melt Production and Transport

Abstract:

Highly evolved, silica-rich lavas have been recovered from several different locations along the global MORB system. Petrographic and geochemical data from basalt, andesite (52-55 % SiO₂), and dacite (56-59 % SiO₂) glasses collected on the East Pacific Rise from 8°-15°50^’N, give insight into the petrogenesis and eruption characteristics of these highly silicic melts. Although most geochemical variability in MOR basalts is commonly attributed to either low-pressure fractional crystallization of mantle derived melts or to a small degree of partial melting from a hydrated gabbro, the chemical anomalies in the trace element and isotopic abundances in the dacite and andesite samples at 8°37’N suggest that these lavas have had a more complicated petrogenesis. Andesite and dacite samples are characterized by significantly elevated Ce, Nd, Yb, Zr, Sr, and Ba concentrations. Coeval basaltic glasses from this region are typically “N-type” and “E-type”. The 8°37-8°39’N andesite glasses are predominantly phaneritc, with xenocrystic-clots of plagioclase commonly having discontinuous, oscillatory reverse zoning that vary from An₅₇ to An₇₇. Several of the samples contain gabbroic xenoliths composed of rounded olivines, euhedral clinopyroxene, and plagioclase xenocrysts with sieve textures. The composition of olivine xenocrysts from the andesitic glasses ranges from Fo₇₇ to Fo₈₁. Dacite glasses from 8°-15°50^’N are unanimously aphyric with sparse micro-xenocrysts. The composition of plagioclase grains varies from An₄₇ to An₆₆. Olivine xenocrysts from the 9°03’N locality average Fo₆₇ for their composition. However, the dacite glasses on the west portion of the 8°37’N 1-2 km offset are characterized by phaneritic samples with basaltic xenoliths that contain spherulitic-clots of plag/ol/cpx. Geochemical and petrologic modeling of the andesite and dacite lavas suggests that they are derived by the partial melting (3-5%) of a hydrated basaltic/gabbroic crust. Furthermore, the phase chemistry and textural relationships of the xenocrystic-clots in the high silica glasses suggests these melts were rapidly transported to the eruptions sites with little interaction with the existing basaltic plumbing system.