Petrography and Geochemistry of High-Silica East Pacific Rise Glasses: Implications for Melt Production and Transport from 8o37'N to 15o50'N
Analytics
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Abstract
Highly evolved, silica-rich lavas have been recovered from several different locations along the global MOR system. Over the course of two decades, basalt (50-51 wt.% SiO2), basaltic andesite (52-55 wt.% SiO2), andesite (56-61 wt.% SiO2), and dacite (62-67 wt.% SiO2) glasses were collected along the East Pacific Rise from 8o-15o50'N. The occurrence of highly differentiated MORB lavas is commonly attributed to either low-pressure fractional crystallization of mantle derived melts or variable degrees of partial melting, assimilation, and/or magma mixing. However, detailed petrographic and geochemical analyses have given insight into the production and eruption characteristics of these high-silica melts, specifically focusing on the chemical anomalies in the major and trace element abundances and their relationship to the diverse mineral assemblages present. Andesite and dacite samples are characterized by significantly elevated Ce, Nd, Yb, Zr, Sr, and Ba concentrations. Coeval basaltic glasses from the 8o37'N region are typically N-MORB and E-MORB. These variations in parental magma compositions suggest that the 8o37'N locality is fed by two separate parental mantle melts.Based on the petrographic evidence of rounded olivine, euhedral disequilibrium orthopyroxene, large crystal clots of plagioclase, and assimilated crystal clot material, the andesites on the east limb and the dacites on the west limb of 8o37'N were generated by similar processes. Although both lava types have different SiO2 content, both melts were produced by the partial melting and amalgamation of a deep, hydrated gabbroic source due to the upwelling of mantle melt (Type 1). Xenolith material within the west limb dacites is particularly similar to the east limb andesite xenoliths, suggesting that both these differentiated melts ascended through similar conduits and similar partially melted gabbroic compositions. The west limb dacites possibly experienced a slightly higher degree of fractional crystallization as multiple mineral assemblages (pyroxene and plagioclase) are observed to have dissolution cores, partially assimilated material incorporated into the groundmass glass, and the relative absence of olivine.The andesite lavas from the west limb and the dacites from the east limb at 8o37'N (Type 2) contain smaller sized crystal clots with no chill margins and are commonly composed of orthopyroxene, clinopyroxene, and plagioclase. The mineralogy of the crystal clots resembles the xenolith material observed in both the east limb andesites and west limb dacites (Type 1). In comparison to the Type 1 melts, the change in mineralogy with the deficiency of olivine and scarcity of large crystals, suggests that the melt ascended through a different composition of the overlying gabbro source. Such changes in composition and crystal dimension within the partially melted gabbro source advocates that the gabbro itself is partitioned into deep, coarse-grained cumulate layers (Type 1), and grades upward into shallow, medium-grained gabbro compositions (Type 2). Xenolith clots are smaller in scale and are either partially assimilated or rounded with chill margins. Further analysis of the plagioclase grains in the xenoliths suggests that they are not in equilibrium with the melt, opposed to the plagioclase groundmass phase. Thus the east limb dacites and the west limb andesites were generated as MORB mantle melt, that partially melted relatively shallow, hydrated gabbro crust with slightly more fractional crystallization occurring as the melts ascend. Geochemical and petrologic modeling of the andesite and dacite lavas suggests that they are derived by the partial melting (< 5%) of a hydrated gabbroic crust. Furthermore, the phase chemistry and textural relationships of the xenocrystic-clots in the high silica east limb andesite and west limb dacite glasses suggests these melts were rapidly transported to the eruption sites with only minor interaction with the existing basaltic plumbing system. Together, these characteristics observed at the 8o37'N deval suggest that the high-silica lavas have had a more complicated petrogenesis than previously proposed.