OXYGEN ISOTOPE COMPOSITION OF PLAGIOCLASE FROM THE STEENS BASALT, COLUMBIA RIVER BASALT PROVINCE, SE OREGON

Oxygen isotopic compositions can provide important insights into mantle source of magmas, differentiation processes that affect magmas during ascent through the crust, and weathering or hydrothermal interactions of rocks post-eruption. The Steens Basalts (~16.7 Ma) represent the earliest and most mafic part of the Columbia River Flood Basalt event. New whole rock, mineral chemical and radiogenic isotope data suggest that generation of the lower Steens lavas is dominated by recharge and fractional crystallization punctuated by periods during which the magma reservoirs had sufficient time to thoroughly mix and equilibrate. Upper Steens lavas are in general more evolved and appear to have a stronger assimilation component. δ¹⁸O in plagioclase was analyzed in 15 samples from lower and upper Steens lavas to help further characterize the differentiation processes responsible for producing these magmas.

δ¹⁸O in plagioclase from all samples is restricted, with values ranging from 5.9 to 6.6‰; two outliers reach values of 7.4 (in lower Steens) and 8.5‰ (in upper Steens). These values are elevated relative to a mantle source and are likely due to assimilation of crustal material with a higher δ¹⁸O. Values for upper Steens are barely higher on average (mode at 6.2) relative to lower Steens lavas (mode at 6.1). A diffuse positive correlation exists between δ¹⁸O and SiO₂. Two trends are discernable, one with slightly increasing δ¹⁸O (5.9 to 6.3‰) over 48-53 wt.% SiO₂ (and corresponding decrease in Mg# from 65 to 40). The second trend shows markedly increasing δ¹⁸O (5.9 to 8.5‰) over the same SiO₂ range. The former trend is defined by most of the data, and we suggest it is caused by crystal fractionation outcompeting recharge as the magma system evolves. The excursions to high δ¹⁸O document that crustal assimilation did occur but that it was mostly a minor part of the O mass balance compared to the combination of crystal fractionation (expected to elevate δ¹⁸O by a scant permil over this compositional range) and recharge (hypothesized to buffer δ¹⁸O to a parental composition at 6‰).