Evidence for Fractionation and Recharge in Shallow Basaltic Magma Chambers: Kimama Butte, Snake River Plain, Idaho

Small, monogenetic shield volcanoes are the most prominent feature of the Snake River Plain, Idaho. Even though monogenetic shields are very short lived, it is not uncommon for compositional heterogeneity to be present within a single volcano. Kimama Butte shows distinct changes in major and trace element compositions and in olivine and plagioclase from the earliest, most distal flows to the late-stage summit eruptions. The rocks from Kimama Butte display a strong Fe-enrichment trend typical of fractionating tholeiitic basalt. Major and trace element compositions range from: 6.9- 5.5% MgO, 15.6-18% Fe2O3, 15-13% Al2O3, 3.2-4.3% TiO2, 9.5-10.1% CaO, 0.6-0.8% K2O, and P2O5 0.6 -1.1%. P, the most incompatible element, increases by a factor of 1.8 and Ce increases by 1.7 times. Trace elements (Ni<80 and Cr<200 ppm) , olivine compositions (about Fo65), and low Mg# (<65) show that none of the lavas are primary melts from the mantle. High iron concentrations may be due to differentiation not great depth of origin. Element variations are best explained by the fractionation of plagioclase and olivine. For example, Al2O3 declines with increasing differentiation and Ca varies only slightly across the entire range of lavas. Moreover, normalized trace element patterns have a prominent negative Sr anomaly as a result of plagioclase fractionation. MELTS simulations were performed with a constant parent but varying pressure, water content, and fO2 to determine the conditions which best fit the observed lava compositions. The phase assemblage (no pyroxenes or oxides) and major element variations are best explained by approximately 25-30% crystallization of olivine and plagioclase at low-pressure (<3 kb to as low as 100 b). Electron microprobe analyses of plagioclase (An72) and olivine (Fo75) phenocrysts agree with the predicted mineral compositions from MELTS. However, low-pressure fractionation alone cannot explain the observed the strong enrichments of incompatible elements. Instead, they can be modeled as the result of magma recharge into the evolving chamber. Calculations suggest that recharge rate was about 0.6 the crystallization rate. Assimilation of ferrogabbro intrusions or of the Phosphoria Formation at great depth may not be necessary to explain the incompatible element enrichments.