Scott K. Vetter

Layered mafic sill complex beneath the eastern Snake River Plain: Evidence from cyclic geochemical variations in basalt

The eastern Snake River Plain in southern Idaho, western United States, is characterized by 1–2 km of Pleistocene to late Pliocene basalt overlying rhyolite caldera complexes. Cyclic variations in the chemical composition of basalts from 1136 m of scientific drill core show that the parent magmas of these lavas evolved by crystal fractionation at shallow to intermediate crustal depths, punctuated by episodic recharge with more primitive compositions and assimilation of adjacent wall rock. We have identified 10 upward fractionation cycles and four reversed cycles; assimilation of sialic crust was limited and mainly affects the oldest basalts, which directly overlie rhyolites. We infer that the crystal fractionation and/or recharge cycles took place in a series of sill-like intrusions at intermediate crustal depths that now form a layered mafic intrusion that underlies the eastern Snake River Plain at depth. This layered sill complex is represented by the ∼10-km-thick “basaltic sill” that has been imaged seismically at ∼12–22 km depth. The association of this mid-crustal sill complex with geochemical fractionation cycles in basalt supports the concept that exposed layered mafic intrusions may be linked to overlying basalt provinces that have since been removed by erosion.

Yellowstone plume-continental lithosphere interaction beneath the Snake River Plain

The Snake River Plain represents 17 m.y. of volcanic activity that took place as the North American continent migrated over a relatively fixed magma source, or hotspot. The identification of a clear seismic image of a plume beneath Yellowstone is compelling evidence that the Miocene to recent volcanism associated with the Columbia Plateau, Oregon High Lava Plains, Snake River Plain, Northern Nevada Rift and Yellowstone Plateau represents a single magmatic system related to a mantle plume. A remaining enigma is, why do radiogenic isotope signatures from basalts erupted over the Mesozoic–Paleozoic accreted terrains suggest a plume source while basalts erupted across the Proterozoic–Archean craton margin indicate an ancient subcontinental mantle lithosphere source? We show that ancient cratonic lithosphere like that of the Wyoming province superimposes its inherent isotopic composition on sublithospheric plume and/or asthenospheric melts. The results show that Yellowstone plume could have a radiogenic isotope composition similar to the mantle source of the early Columbia River Basalt Group and that the plume source composition has persisted to the present day.

Continental basalts of the Boise River Group near Smith Prairie, Idaho

The Boise River Group of late Cenozoic age consists of basaltic lavas and intercalated fluvial and lacustrine sediments deposited in drainages of the Boise River and Boise River South Fork from 1.8 to 0.2 m.y. Basalts of the Boise River Group can be divided into two groups based on major and trace element chemistry. Boise River Group 1 basalts (BRG 1), which comprise the three oldest flows in the Smith Prairie area, are silica-saturated olivine tholeiites characterized by low alkalis, Mg/Fe, and Ni, high concentrations of high field strength elements (HFSE), and enrichment in the heavy isotopes of Sr, Nd, and Pb. They are chemically similar to basalts of the Snake River Plain, and both units probably derive from a similar, enriched mantle lithosphere source. Some BRG 1 flows (Long Gulch, Rock Creek) record the affects of crustal assimilation and fractional crystallization. Boise River Group 2 basalts (BRG 2) are all younger than BRG 1 (< 0.7 m.y.) and are transitional between olivine tholeiites and alkali olivine basalts. BRG 2 basalts are characterized by high alkalis, Mg/Fe, and Ni, lower HFSE concentrations, and isotopic compositions of Sr and Nd near bulk earth. These lavas have distinct geochemical characteristics and are not cogenetic with the older, BRG 1 flows or with coeval lavas of the Snake River Plain. Chemical variations within individual flows are consistent with low-pressure crystal fractionation of the observed phenocryst phases (olivine + plagioclase). Chemical variations between different flows within the same group cannot result from low-pressure fractionation but can be modeled by combined high-pressure pyroxene fractionation and low-pressure olivine + plagioclase fractionation. The high Mg numbers and Ni of the younger BRG 2 basalts are not consistent with enrichment of the light field strength elements by crustal assimilation; this enrichment must reflect a mantle source region characteristic. The trace element and isotopic systematics of this source region are similar (but not identical) to the asthenospheric source inferred for ocean island basalts. In contrast, chemical and isotopic systematics of the BRG 1 basalts imply derivation from ancient subcontinental lithosphere which has been isolated from the asthenosphere for at least 1.5 eons. The subcontinental lithosphere was affected by an early enrichment in Rb/Sr which supported growth of high 87Sr/86Sr. A second enrichment event, resulting in Fe-Ti metasomatism of the lithosphere, occurred later in response to intrusion of partial melts from a mantle plume rooted in the underlying asthenosphere. This is second enrichment event is required by the high Nb/Rb ratios of the BRG 1 lavas, and of similar lavas of the Snake River Plain. The transition from saturated olivine tholeiites of Boise River Group 1 to transitional-alkalic lavas of BRG 2 in the Smith Prairie region implies a time-dependent change in mantle source region, with early magmas derived from a shallow lithospheric source giving way to younger magmas derived from a deeper asthenospheric source. The eruption of these younger lavas along the flanks of the Snake River Plain coeval with eruption of saturated tholeiites of the Snake River Group to the south further implies an axial zonation to magmatism in the Snake River Province. The nature of this axial zonation suggests that the deeper asthenospheric melts were preferentially tapped along the margins of the Snake River Plain but were blocked from reaching the surface in the axial zone. These time-space relationships are consistent with a model in which thinned lithosphere near the rift axis undergoes extensive partial melting which overwhelms any asthenospheric melts that may attempt passage. The lithosphere is too cool to melt along the rift margins (where pressure release by extensional thinning is minor); subsequent small volumes of asthenospheric melt are able to traverse this region more easily and erupt with little or no lithospheric contamination.

Geochemical and paleomagnetic variations in basalts from the Wendell Regional Aquifer Systems Analysis (RASA) drill core: Evidence for magma recharge and assimilation–fractional crystallization from the central Snake River Plain, Idaho

The temporal and magmatic evolution of central Snake River Plain (SRP; Idaho, USA) olivine tholeiites erupted within the past 4 m.y. is evaluated here. This investigation correlates and merges both geochemical and paleomagnetic measurements to constrain the volcanic history recovered from the 340 m Regional Aquifer Systems Analysis (RASA) test well located near Wendell, Idaho. Only a handful of studies have accomplished this task of shedding light on the chemical stratigraphy of the SRP and the petrogenesis of basalts with depth, and therefore through time. Paleomagnetic relationships suggest that time breaks between individual lava fl ows represent a few years to decades, time breaks between fl ow groups represent at least a couple of hundred years or possibly much longer, while significant hiatuses in volcanism, revealed by thick sediment packages or polarity reversals (both are evidenced in this well), are inferred to last thousands to tens of thousands of years. Major element geochemistry from 52 basaltic lava fl ows demonstrates near primitive compositions (i.e., ~10 wt% MgO) and tholeiitic iron enrichment trends, similar to lavas from the eastern SRP. Trace element concentrations are similar to those of ocean island basalts, with enriched Ba and Pb, and light rare earth element (REE)/heavy REE ratios similar to those of many Neogene volcanics of the western Cordillera. When combined, we identify a total of 11 fl ow groups, which we also classify as fractionation or recharge on the basis of decreasing or increasing MgO weight percent with depth. Taking into consideration these trends, we review the potential recharge, fractionation, and assimilation processes that characterize much of SRP olivine tholeiite, and conclude that assimilation, in combination with fractional crystallization, is the dominant petrogenesis for the basalts in the central SRP. Although fractionation of Wendell parent magmas was accompanied by assimilation of crustal material, this could not have been assimilation of ancient cratonic crust. The geochemical cycles observed in this well are inferred to represent fractionation and recharge of basaltic magma from a series of sill-like layered mafi c intrusions located in the middle crust, similar to what has been proposed for the processes that control the eruptive history of basalts in the eastern SRP.

Evidence for Cyclical Fractional Crystallization, Recharge, and Assimilation in Basalts of the Kimama Drill Core, Central Snake River Plain, Idaho: 5.5-Million-Years of Petrogenesis in a Mid-crustal Sill Complex

Basalts erupted in the Snake River Plain of central Idaho and sampled in the Kimama drill core link eruptive processes to the construction of mafic intrusions over 5.5 Ma. Cyclic variations in basalt composition reveal temporal chemical heterogeneity related to fractional crystallization and the assimilation of previously-intruded mafic sills. A range of compositional types are identified within 1912 m of continuous drill core: Snake River olivine tholeiite (SROT), low K SROT, high Fe-Ti, and evolved and high K-Fe lavas similar to those erupted at Craters of the Moon National Monument. Detailed lithologic and geophysical logs document 432 flow units comprising 183 distinct lava flows and 78 flow groups. Each lava flow represents a single eruptive episode, while flow groups document chemically and temporally related flows that formed over extended periods of time. Temporal chemical variation demonstrates the importance of source heterogeneity and magma processing in basalt petrogenesis. Low-K SROT and high Fe-Ti basalts are genetically related to SROT as, respectively, hydrothermally-altered and fractionated daughters. Cyclic variations in the chemical composition of Kimama flow groups are apparent as 21 upward fractionation cycles, six recharge cycles, eight recharge-fractionation cycles, and five fractionation-recharge cycles. We propose that most Kimama basalt flows represent typical fractionation and recharge patterns, consistent with the repeated influx of primitive SROT parental magmas and extensive fractional crystallization coupled with varying degrees of assimilation of gabbroic to ferrodioritic sills at shallow to intermediate depths over short durations. Trace element models show that parental SROT basalts were generated by 5-10% partial melting of enriched mantle at shallow depths above the garnet-spinel lherzolite transition. The distinctive evolved and high K-Fe lavas are rare. Found at four depths, 319 m, 1045 m, 1078 m, and 1189 m, evolved and high K-Fe flows are compositionally unrelated to SROT magmas and represent highly fractionated basalt, probably accompanied by crustal assimilation. These evolved lavas may be sourced from the Craters of the Moon/Great Rift system to the northeast. The Kimama drill core is the longest record of geochemical variation in the central Snake River Plain and reinforces the concept of magma processing in a layered complex.