OLIVINE COMPOSITIONS OF THE LOWER STEENS BASALT: PROBING INTO MAGMA HISTORIES OF EARLY FLOOD BASALT EVENTS

The composition of olivine phenocrysts helps us better understand how mafic magma reservoirs establish in the crust during a flood basalt event. The Steens Basalts occur mainly in southeastern Oregon. With an age of ~16.7 Ma and Mg# as great as 67, this stack of up to 200 flows is the oldest and most mafic expression of the Columbia River flood basalt event. Olivine and plagioclase are the most common mineral assemblage indicating magma residence at a low-pressure cotectic. Olivine makes up as much as 15% of the rock and is most commonly half or less of the mode. It occurs as euhedral phenocrysts from a few tenths to several mm in diameter. Olivine ranges from euhedral, clear to cracked or crushed looking, and is sometimes embayed. Iddingsite alteration at the margins and along cracks is common. Olivine also occurs as inclusions along concentric zones in plagioclase and as microphenocrysts in plagioclase glomerocrysts. Plagioclase occurs as phenocrysts to glomeroocrysts, or occur as radial snowflake textured clusters with trapped olivine and oxides. Plagioclase is not included in olivine.

Olivine phenocrysts from ten of the most mafic flows of the lower Steens Basalts range from (Fo₆₀) to Fo₈₈. Fo content roughly increases with MgO of the bulk rock (7 to 11 wt.%) and Mg# (58 to 67). A few samples have cumulus olivine based on correlation of Mg# with modal olivine. As the whole rock Mg# decreases the texture of the rock becomes coarser, plagioclase glomerocrysts become more abundant, and the forsterite content of the olivine phenocrysts slightly decreases. Core and mid-zone compositions of olivine tend to be the same and range from Fo₈₅ (at highest whole rock Mg#) to Fo₈₀ (at lowest whole rock Mg#) of the samples studied. Thin rims are typically different in composition than cores or interiors, thus the grains are normally or reversely zoned. Some samples have heterogeneous (normal and reverse) rims suggesting the magma was assembled from diverse parts of a reservoir with little equilibration. Most samples, however, have olivine with a thin less Fo-rich rim. The homogeneity of grain interiors suggests that olivine grew in a well-mixed equilibrated reservoir and was subsequently entrained en route to eruption.