Chemical Trends in the Ice Springs Basalt, Black Rock Desert, Utah

The Holocene Ice Springs volcanic field of west-central Utah consists of 0.53 km3 of tholeitic basalts erupted as a sequence of nested cinder cones and associated lava flows. Whole rock x-ray fluorescence and atomic absorption analysis of ninety-six samples of known relative age document statistically significant inter- and intra- eruption chemical variations. Elemental trends include increases in Ti, Fe, Ca, P, and Sr and decreases in Si, K, Rb, Ni, Cr, and Zr with decreasing age. Microprobe analyses of microphenocrysts of olivine, plagioclase, and Fe-Ti oxides and of groundmass olivine, plagioclase, and clinopyroxene indicate limited chemical variation between mineral assemblages of the eruptive events. Petrographic analyses have identified the presence of minor amounts of silicic xenoliths, orthopyroxene megacrysts, and plagioclase xenocrysts. Potassium-argon determinations establish the existence of excess argon in the basaltic cinder (30.05 x 10 -12 moles/gm) and in distal lava flows (8.29 x 10 -12 moles/gm) which suggest apparent ages of 16 and 4.3 million years respectively. Strontium isotopic data (Puskar and Condie, 1973) show systematic variations from oldest eruptions (87Sr/86Sr=0.7052) to youngest eruptions (87Sr/86Sr=0.7059). Theoretical evaluation of observed major element, trace element, isotopic, and thermophysical properties of the lavas and cinders limits the importance of proposed magmatic differentation processes. The data are compatible with model involving crystal fractionation, crustal assimilation, and magma mixing. Initial modification of mantle derived melts resulted from olivine fractionation at depth. Subsequent combination of 6 to 8% fractionation of plagioclase, minor olivine, and magnetite at shallow depths, less than one percent assimilation of silicic crustal basement rocks, and interaction of compositionally similar magma pulses explains the overall inter-eruption chemical trends. The intra-eruption variations follow the overall trends and a similar combination of processes of lesser magnitude accounts for the intra-eruption variations