Evidence for fractionation of Quaternary basalts on St. Paul Island, Alaska, with implications for the development of shallow magma chambers beneath Bering Sea volcanoes

St. Paul Island is the youngest volcanic center in the Bearing Sea basalt p rovince. We have undertaken a field, petrographic, and geochemical study of select St. Paul volcanic rocks in order to better understand their differe ntiation; specifically, to test the hypothesis that magmas erupted from ind ividual Bering Sea basaltic volcanoes are not related by shallow-level proc esses such as crystal fractionation. Petrographically, all of the St. Paul volcanic rocks are olivine-, plagioclase-, and clinopyroxene-phyric. Textur al features and modal contents of olivine phenocrysts, however, vary widely throughout the spectrum of basalt compositions. Although differing in size and abundance, olivine phenocrysts in all rock compositions are euhedral a nd commonly skeletal, suggesting rapid growth during ascent or eruptive que nching. None, however, display reaction textures with surrounding groundmas s liquid. Compositionally, the St. Paul volcanic rocks are basalts and teph ritic basalts and all have high contents of normative nepheline (8% to 16%) . Concentrations of many major and incompatible trace elements display no c lear correlations with bulk-rock SiO2 and MgO contents or modal abundances of phenocrysts, suggesting that much of the compositional diversity of thes e magmas reflects variable mantle sources and degrees of partial melting. S imilarly, chondrite-normalized REE patterns show variable degrees of light REE enrichment (La-n = 70-90) that do not correlate with bulk-rock mg-numbe rs. in contrast, concentrations of compatible trace elements (Ni, Cr, and C o) are positively correlated with MgO contents and modal percentages of oli vine phenocrysts. Maximum forsterite contents of olivine phenocryst cores i n most St. Paul rocks decrease with decreasing bulk-rock mg-number and are similar to the calculated equilibrium range. This is evidence that the high mg-numbers are magmatic and do not result from olivine accumulation. Inste ad, major and compatible trace element mass balance calculations support de rivation of the low mg-number lavas from the high mg-number lavas mainly by olivine fractionation, which, in turn, implies that St. Paul magmas may ha ve temporarily resided in crustal magma chambers prior to eruption. (C) 199 9 Elsevier Science B.V. All rights reserved.