VARIATIONS IN WHOLE ROCK AND MINERAL COMPOSITION OF THE LOWER STEENS BASALT, SE OREGON

The Steens Basalt (~16.7 Ma) of SE Oregon is the oldest member of the Columbia River Flood Basalt (CRB) event. The Steens flows were fed by dikes near Steens Mountain, and originally covered ~50,000 km³. At Steens Mountain, 100-250 flows are stacked in over 900 m of near vertical exposure. Flows range from aphyric to extremely plagioclase phyric, and compound flows are abundant. The Steens Basalt is dominantly basalt (48-52 wt.% SiO₂), unlike the rest of the CRB, which are dominantly basaltic andesite in composition.

The Steens Basalt is subdivided into Upper and Lower units by previous workers, based on compositional distinctions. The lower flows are more mafic, tholeiitic and incompatible trace element poor, whereas the upper flows are less magnesian, mildly alkalic, and generally more enriched in incompatible trace elements. These changes up section are analogous to the cryptic layering typical of layered mafic intrusions.

New mineral chemistry on the Lower Steens Basalt reveals cryptic variations in olivine, clinopyroxene and plagioclase compositions, signaling temporal changes in magmatic conditions. A stratigraphically low and relatively less magnesian flow has homogeneous olivine (cores and rims Fo_76-78), suggesting a well-equilibrated magma reservoir. Olivine of <Fo₈₀ is unlikely to be in equilibrium with mantle; thus the liquid from which these olivines fractionated was differentiated to some degree. Above it, a more magnesian flow has diverse olivine signaling recharge, mixing and incomplete equilibration; cores are Fo_85-75, and rims are as low as Fo₆₃. Clinopyroxene Mg# and anorthite content are highest from this sample (Mg# 67-77, An_68-98), but samples above and below this height exhibit excursions to lower ranges (Mg# 60-73, An_66-88). The majority of olivine grains are normally zoned with respect to MgO, but many of the grains are remarkably homogeneous from core to rim. Reverse zoning is generally restricted to minor elements in both olivine (Ni) and clinopyroxene (Cr), but is particularly notable in the flows with the most homogeneous forsterite content. The chemostratigraphic record from the Lower Steens Basalt records development of a large crustal mafic magma system in which periods of recharge and mixing with resident magma alternate with periods of differentiation in well-mixed reservoirs.