GEOCHEMICAL DIVERSITY OF QUATERNARY OLIVINE THOLEIITES ON THE EASTERN SNAKE RIVER PLAIN

Quaternary-Holocene eastern Snake River Plain (ESRP) olivine tholeiite shield volcanoes overlie the Miocene-Quaternary age-progressive track of the Yellowstone hotspot system in Idaho. We present the results of a comprehensive geochemical study of tholeiite corehole samples taken from a representative part of the ESRP where there is good stratigraphic control. Incompatible trace elements range by factors of ~7 to 10, and major elements, especially MgO, and K₂O/TiO₂ and CaO/Al₂O₃ ratios range by factors of ~2 to 3. Initial ⁸⁷Sr/⁸⁶Sr ratios range from ~0.706 to 0.708, and ¹⁴³Nd/¹⁴⁴Nd ratios range from ~0.5122 to 0.5125. Distinct compositional clusters on co-variant plots suggest that each group of lava flows represents a single shield-building monogenetic eruption derived from a separate magma batch. The most primitive compositions for each series range from high MgO and low La to low MgO and high La, and some of these compositions recur in time and space along several eruptive axes. Wide ranges of incompatible trace element abundances are decoupled from relatively minor variations in ¹⁴³Nd/¹⁴⁴Nd vs. ⁸⁷Sr/⁸⁶Sr ratios, which are related to source heterogeneity and are not related to assimilation of old Archean crust. Trace element and major element (MELTS) fractionation models of two representative starting compositions (MgO=11 and 7.7 wt. %) at 1, 4, 7 and 15 kbar indicate that observed compositional ranges are greater than those enabled by simple fractional crystallization at various crustal levels. They more likely reflect complexities related to low degrees of partial melting, heterogeneity in the source regions, and polybaric crystallization. The large number of mafic vents and compositionally evolved volcanic systems on the ESRP implies a substantial volume of mafic magma injected at depth. Compositional diversity attributed to source heterogeneity may also be due to mixing of primary mafic melts with previously injected magmas in crustal- or sub-crustal realms.