Project Hotspot: Mineral chemistry of high-MgO basalts from the Kimama core, Snake River Scientific Drilling Project, Idaho

Mineral compositions can be used to deduce magma crystallization temperatures and to infer key characteristics of magma source regions including delving into the plume or no-plume sources of intraplate basalts. To this end, mineral compositions in basalt acquired by the Snake River Scientific Drilling Project have been analyzed by electron microprobe. The samples are from the Kimama drill hole on the axis of the Central Snake River Plain, Idaho which was drilled through 1912 m of basalt and interbedded sediments. Five of the least evolved basalt flows (i.e., low Fe, Ti, and high Ni and Cr) were chosen based on semiquantitative analyses using a Bruker Tracer IV handheld X-ray fluorescence spectrometer. Phenocryst phases include olivine and plagioclase; many olivine phenocrysts also contain inclusions of Cr-Al-rich spinel. Groundmass phases are olivine, plagioclase, clinopyroxene, magnetite, and ilmenite. Olivine phenocrysts are normally zoned with cores of Fo 81-70; the rims of Fo 70-50 overlap with the compositions of olivine in the groundmass. Spinels included in olivines in the most MgO-rich lavas are Al-rich (up to 34 wt% Al2O3), similar to those in ocean island basalts (Barnes and Roeder, 2001) and some zone to higher Fe and Ti. Plagioclase phenocryst cores (An 76-65) overlap significantly with the compositions of groundmass plagioclase (An 72-40). Clinopyroxene is confined to the groundmass and creates an ophitic texture. Pyroxene compositions are typically: Wo 45-37, En 42-30, Fs 30-15 and more evolved pyroxenes trend towards Craters of the Moon pyroxenes which have lower Ca. Temperature and oxygen fugacity were calculated from magnetite-ilmenite pairs using QUILF (Anderson et al., 1993), which yielded temperatures of 750-1000°C and fO2 near or just below the QFM buffer. The magnetite-ilmenite pairs are all groundmass phases; thus, these are post-eruption temperatures and fO2 estimates. Olivine compositions were used to test if the source of the Snake River Plain basalts contains a subducted oceanic crustal component as suggested by Sobolev et al. (2005) and Herzberg (2011). The olivines in the Kimama core have Mn, Fe/Mn, and Ca concentrations that are similar to Hawaiian shield-building basalts, and are consistent with derivation of their parent magmas from pyroxenite sources, such as those hypothesized for some mantle plumes. However, Ni concentrations (500-1500 ppm) in olivines from Kimama are relatively low, and the olivines are too evolved (Fo <81) to be definitive with regard to the presence or absence of pyroxenite in the source.