Source and Crystallization Characteristics of Basalts in the Kimama core: Project Hotspot Snake River Scientific Drilling Project, Idaho

Mineral chemistry and petrography of basalts from the Kimama drill core recovered by Hotspot: Snake River Scientific Drilling Project, Idaho establish crystallization conditions of these lavas. Twenty-three basalt samples, from 20 individual lava flows were sampled from the upper 1000 m (of the 1912 m drilled) core drilled on the axis of the Snake River Plain, and represent approximately 3 m.y. of volcanism (rocks at the bottom of the hole are ~6 Ma). Rock from the upper 1000 m are typically fresh, while those lower in the core are more altered and are less likely to preserve fresh phenocrysts to analyze. Intratelluric phenocrysts (pre-eruption) are: olivine, plagioclase and Cr-spinel inclusions in olivine and plagioclase; groundmass phases (post-eruption) are: olivine, plagioclase, clinopyroxene, magnetite and ilmenite. Olivine core compositions range from Fo84-68, plagioclase cores range from An80-62, clinopyroxene ranges in composition from Wo47-34, En47-28, Fs30-15, spinel inclusions are Cr (up to 20 wt % Cr2O3) and Al-rich (up to 35 wt % Al2O3) and evolve to lower concentrations of Cr and Al and higher Fe and Ti, chromian titanomagnetite to magnetite, and ilmenite are groundmass oxide phases. Thermobarometry of Kimama core basalts indicates that the phenocryst phases crystallized at temperatures of 1155 to 1255°C at depths of 7 to 17 km, which is within or near the seismically imaged mid-crustal sill. Plagioclase hygrometry suggests that these lavas are relatively anhydrous with less than 0.4 wt % H2O. Groundmass phases crystallized at lower temperatures (<1140°C) after eruption. Oxygen fugacity inferred from Fe-Ti oxide equilibria is at or just below the QFM buffer. The origin of the basaltic rocks of the Snake River Plain has been attributed to a mantle plume or to other, shallow mantle processes. Mineral and whole rock major and trace element geochemistry of the olivine tholeiites from the Kimama core are used to distinguish between these two sources (deep or shallow mantle). Whole rock compositions were corrected for plagioclase and olivine fractionation to calculate primary liquids to estimate mantle potential temperatures. Olivine phenocrysts have the pyroxenite source characteristics of low Mn and Ca, but a peridotite source characteristic of low Ni. Thus, trace element models were used to test whether there is pyroxenite in the source of the Snake River Plain basalts, as hypothesized for Hawaii and other plume-related hotspots (e.g., Sobolev et al., 2005; Herzberg, 2011). Olivine chemistry and trace element models establish that the basalt source is a spinel peridotite, not a pyroxenite. The average mantle potential temperature obtained for these samples is 1577°C, 177°C hotter than ambient mantle, suggesting that the basaltic liquids were derived from a thermal plume. Silica activity barometry shows that melt segregation occurs between 80 and 110 km depth, which is within or very near the spinel stability field, and suggests that the lithosphere has been eroded by the plume to a maximum depth of 80 km, and recent mantle tomography suggests that it may be even thinner.