CHEMICALLY DISTINCT, BUT TEMPORALLY EQUIVALENT, LAVAS IN THE WESTERN SNAKE RIVER PLAIN, IDAHO

The Western Snake River Plain (WSRP), Idaho, comprises one arm of the arc-shaped lowland long recognized as the Snake River Plain physiographic province, and contains aerially extensive and compositionally diverse Pleistocene basalts. Monogenetic volcanoes in the WSRP produced a wide variety of landforms including maars, tuff cones, scoria cones and shields, and the deposits they produced include palagonite tuffs, lahars, scoria beds, pillowed lava flows, and subaerial pahoehoe. This work presents new ⁴⁰Ar/³⁹Ar dates, geochemical data, and Sr, Nd, and Pb isotopic compositions for WSRP basalts to examine compositional variations within time and space. In general, magmas experienced a shift to lower ⁸⁷Sr/⁸⁶Sr and greater ¹⁴³Nd/¹⁴⁴Nd, ²⁰⁶Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb beginning about 1 Ma. Similarities in the isotopic signatures of mildly alkaline basalts and tholeiitic ferrobasalts, which overlap temporally and spatially, suggest the parent magmas originated in a common source but experienced different types and degrees of crystal fractionation and crustal assimilation. MELTS computer simulations of both equilibrium crystallization and fractional crystallization show that a mildly alkaline parental melt could produce residual liquids having many of the major oxide characteristics of tholeiitic ferrobasalts. However, models could not produce the observed K₂O/P₂O₅ ratios, thus necessitating assimilation of a high-P contaminant. Further, we use the isotopic signatures of these melts to speculate on the nature of the asthenospheric mantle from which the magmas were derived and how this source region compares to that of neighboring basaltic provinces.