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The sequence of eruption, spatial pattern, and spatio-temporal relationships among the Neogene-Quaternary rhyolitic and basaltic lava along the Snake River Plain (SRP) in Idaho are analyzed applying the spatial methods of global and local Moran’s I, standard deviational ellipse, and Ripley’s K-function. The results of the analyses by the Moran’s I and K-function methods indicate a higher spatial autocorrelation, hence clustering, of rhyolitic lava compared to the more dispersed basaltic lava in each center of eruption along the SRP. The clustered nature of rhyolitic lava around each caldera either reflects the original spread and large thickness of the rhyolitic lava, or the absence of younger cover strata or lava like the distribution of rhyolite in the present caldera at the Yellowstone National Park. The standard deviational ellipses (SDEs) of the lavas indicate that younger basaltic lava that erupted from newer calderas overlapped older rhyolitic and basaltic lava as the position of the Yellowstone hotspot progressively migrated to the northeast along the SRP. The less eccentric SDEs of rhyolitic lava in each caldera probably reflect the original caldera-scale spread of viscous felsic lava, compared to the more eccentric and larger SDEs of basaltic lava which represent basalt’s wider and more directed spread due to its higher fluidity and ability to flow longer distances along the trend of the SRP. The alignment of the long axes of the lava SDEs with the trend of the Eastern SRP and the trend of systematic spatial overlap of older lava by successively younger basaltic lava corroborate the previously reported migration of the centers of eruption along the ESRP as the Yellowstone hotspot migrated to the northeast. [ABSTRACT FROM AUTHOR]

There are pressing concerns about the interplay between agricultural productivity, water demand, and water availability in semi-arid to arid regions of the world. Currently, irrigated agriculture is the dominant water user in these regions and is estimated to consume approximately 80% of the world's diverted freshwater resources. We develop an improved irrigated land-use mapping algorithm that uses the seasonal maximum value of a spectral index to distinguish between irrigated and non-irrigated parcels in Idaho's Snake River Plain. We compare this approach to two alternative algorithms that differentiate between irrigated and non-irrigated parcels using spectral index values at a single date or the area beneath spectral index trajectories for the duration of the agricultural growing season. Using six different pixel and county-scale error metrics, we evaluate the performance of these three algorithms across all possible combinations of two growing seasons (2002 and 2007), two datasets (MODIS and Landsat 5), and three spectral indices, the Normalized Difference Vegetation Index, Enhanced Vegetation Index and Normalized Difference Moisture Index (NDVI, EVI, and NDMI). We demonstrate that, on average, the seasonal-maximum algorithm yields an improvement in classification accuracy over the accepted single-date approach, and that the average improvement under this approach is a 60% reduction in county scale root mean square error (RMSE), and modest improvements of overall accuracy in the pixel scale validation. The greater accuracy of the seasonal-maximum algorithm is primarily due to its ability to correctly classify non-irrigated lands in riparian and developed areas of the study region. [ABSTRACT FROM AUTHOR]

Previous archaeological research in southern Idaho has suggested that climate change over the past 8000 years was not dramatic enough to alter long-term subsistence practices in the region. However, recent isotopic analyses of bison remains from cold storage caves on the Snake River Plain contest this hypothesis. These results, when examined against an archaeoclimate model, suggest that cold storage episodes coincided with drier, warmer phases that likely reduced forage and water, and thus limited the availability of bison on the open steppe. Within this context we build a risk model to illustrate how environment might have motivated cold storage behaviors. Caching bison in cold lava tubes would have mitigated both intra-annual and inter-annual food shortages under these conditions. Our analysis also suggests that skeletal fat, more than meat, may have influenced the selection, transport and storage of bison carcass parts. Deciphering when and how cold storage caves were utilized can ultimately provide a more comprehensive understanding of foraging behaviors in a broad range of hunting-gathering economies. [ABSTRACT FROM AUTHOR]

We present compelling isotopic evidence from ~15 Ma rhyolites that erupted coeval with the Columbia River Basalts in southwest Idaho's J-P Desert and the Jarbidge Mountains of northern Nevada at that suggests that the Yellowstone mantle plume caused hydrothermal alteration and remelting of diverse compositions of shallow crust in the area where they erupted. These rhyolites also constitute the earliest known Miocene volcanism in the vicinity of the Bruneau–Jarbidge and Twin Falls (BJTF) volcanic complexes, a major center of voluminous (10 3 –10 4 km 3 ) low-δ 18 O rhyolitic volcanism that was previously defined as being active from 13 to 6 Ma. The Jarbidge Rhyolite has above-mantle δ 18 O (δ 18 O of +7.9‰ SMOW) and extremely unradiogenic ε Hf (− 34.7) and ε Nd (− 24.0). By contrast, the J-P Desert units are lower in δ 18 O (+4.5 to 5.8‰), and have more moderately unradiogenic whole-rock ε Hf (− 20.3 to − 8.9) and ε Nd (− 13.4 to − 7.7). The J-P Desert rhyolites are geochemically and petrologically similar to the younger rhyolites of the BJTF center (the one exception being their high δ 18 O values), suggesting a common origin for J-P Desert and BJTF rhyolites. The presence of low-δ 18 O values and unradiogenic Nd and Hf isotopic compositions, both of which differ greatly from the composition of a mantle differentiate, indicate that some of these melts may be 50% or more melted crust by volume. Individual J-P Desert units have isotopically diverse zircons, with one lava containing zircons ranging from − 0.6‰ to + 6.5‰ in δ 18 O and from − 29.5 to − 2.8 in ε Hf . Despite this diversity, zircons all have Miocene U/Pb ages. The range of zircon compositions fingerprints the diversity of their source melts, which in turn allow us to determine the compositions of two crustal end-members which melted to form these rhyolites. These end-members are: 1) Archean basement with normal to high-δ 18 O and unradiogenic ε Hf and 2) hydrothermally altered, shallow, young crust with low-δ 18 O (0–1‰) and more radiogenic ε Hf . We suggest that the shallow crust's low-δ 18 O composition is the result of hydrothermal alteration which was driven by a combination of normal faulting and high heat fluxes from intruding Yellowstone plume-derived basalts shortly prior to the onset of silicic magmatism. Furthermore, zircon diversity in the J-P Desert units suggests rapid assembly of zircon-bearing melts of varying isotopic composition prior to eruption, creating well-mixed magmas with heterogeneous zircons. We suggest that this hydrothermal priming of the crust followed by remelting upon further heating may be a common feature of intraplate mantle plume volcanism worldwide. [ABSTRACT FROM AUTHOR]

Late Quaternary aeolian dune systems in the Great Plains and western U.S. are valuable archives of past climate variability. This study investigates the St. Anthony dune field, located in the eastern portion of the Snake River Plain in southern Idaho. The current semi-arid climate of the region allows for a central core of active dunes that is approximately 450 km 2 in area and includes compound crescentic, blowout, falling, and climbing dunes that range in height from 15 to 121 m. The surrounding region is characterized by ∼20,000 km 2 of relic parabolic, linear, and complex linear dunes that are currently fixed by vegetation. This study examined nine samples collected from the fixed dunes that surround the active core of the St. Anthony dune field. Optically stimulated luminescence (OSL) dating results indicate that these fixed dunes were mobile and actively accreting ∼6.6 ka (middle Holocene) and 1.0–2.0 ka (late Holocene). Evidence for activation of a large region currently occupied by stable dunes suggests these periods were characterized by hydrologic deficit and/or warmer climate conditions than currently exists. This research provides new information regarding the depositional timing of the St. Anthony dunes and assists in providing evidence of palaeo-climatic conditions for the eastern Snake River Plain during the middle to late Holocene. [ABSTRACT FROM AUTHOR]

Researchers have established a Holocene pattern of Bison spp. diminution on the Great Plains of North America. This pattern, however, is less clear west of the Rocky Mountains. This lack of clarity stems from a relative paucity of paleontological and archaeological bison assemblages sufficiently large enough to understand local bison diminution. To begin filling this important gap in knowledge, we analyze a large bison assemblage from Baker Cave, a Late-Holocene archaeological site located on Idaho’s Snake River Plain. Measurements of humeri, radii, tibiae, metatarsals, and calcanei demonstrate that these animals were significantly smaller than Early-Holocene bison from both the Great Plains and Snake River Plain. Middle-Holocene bison from the Great Plains are generally larger than those from Baker Cave, but this size difference varies by skeletal element. The Baker Cave bison do fall within the range of Late-Holocene morphological variation present in both Snake River Plain and Great Plains bison populations. These results provide a necessary first step for understanding bison morphology in the region, but establishing a pattern of diminution west of the Rocky Mountains will require follow-up studies with other faunas. [ABSTRACT FROM PUBLISHER]

Abstract: The manifestation of brittle deformation within inactive slumps along the North Menan Butte, a basaltic tuff cone in the Eastern Snake River Plain, is investigated through field and laboratory studies. Microstructural observations indicate that brittle strain is localized along deformation bands, a class of structural discontinuity that is predominant within moderate to high-porosity, clastic sedimentary rocks. Various subtypes of deformation bands are recognized in the study area based on the sense of strain they accommodate. These include dilation bands (no shear displacement), dilational shear bands, compactional shear bands and simple shear bands (no volume change). Measurements of the host rock permeability between the deformation bands indicate that the amount of brittle strain distributed throughout this part of the rock is negligible, and thus deformation bands are the primary means by which brittle strain is manifest within this tuff. Structural discontinuities that are similar in appearance to deformation bands are observed in other basaltic tuffs. Therefore deformation bands may represent a common structural feature of basaltic tuffs that have been widely misclassified as fractures. Slumping and collapse along the flanks of active volcanoes strongly influence their eruptive behavior and structural evolution. Therefore characterizing the process of deformation band and fault growth within basaltic tuff is key to achieving a more complete understanding of the evolution of basaltic volcanoes and their associated hazards. [Copyright &y& Elsevier]

Abstract: 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. We present new Pb, Sr, and Nd data for a suite of 25 basalts collected from Western and Central Snake River Plain (SRP). The new isotope data, combined with previously published data from the SRP, provide a traverse of the Wyoming craton margin, from the 87Sr/86Sr = 0.706 line boundary of western SRP with Phanerozoic accreted terranes, east through the central and eastern SRP, to the Yellowstone Plateau. Low-K basalts from the western SRP, overlain by high-K basalts, provide a temporal record of regional source variation from ∼16.8 to 0.2 Ma. Principal Component Analysis (PCA) of the new and previously published SRP basalt Pb isotopes reveals that >97% of the total variability is accounted for by mixing between three end-members and is consistent with a sublithospheric Yellowstone hotspot mantle source with a radiogenic isotope composition similar to the mantle source of the early Columbia River Basalt Group (CRBG) and two continental lithosphere end-members, heterogeneous in age and composition. We use the SRP Pb, Sr, and Nd isotope data to model the Yellowstone Hotspot–continental lithosphere interaction by three component mixing between two continental lithospheric components, Archean lithosphere (CL1) that represents older lithosphere underlying the Yellowstone Plateau in the east, and Paleoproterozoic lithosphere (CL2) representing the younger lithosphere underlying the SRP in the west near the craton margin, and a sublithospheric end-member, representing the Yellowstone hotspot (PL). The results suggest a continuous flow of PL material westward as the NA continental lithosphere migrated over the upwelling hotspot along a shoaling gradient in the sub-continental mantle lithosphere. The model shows a decrease in Total Lithosphere end-members (CL1 + CL2) and the Lithosphere Ratio (CL1/CL2), from the craton interior at Yellowstone toward its western margin, consistent with geologic and geophysical evidence that the continental lithosphere beneath the SRP decreases in age and thickness from east to west. The Lithosphere Ratio shows step-like decreases from Yellowstone in the east to the 87Sr/86Sr = 0.706 line in the west, indicating that the SRP cuts across geochemically distinct parcels of lithospheric mantle, consistent with terrane accretion models for the craton margin. In the western SRP, young high-K basalts have a lower mass fraction of Total Lithospheric compared to the underlying low-K tholeiites, but the same Lithosphere Ratio, consistent with a recent (700–900 ka) decrease in lithosphere contribution between eruption of early low- and younger high-K basalts. [Copyright &y& Elsevier]

Abstract: Rhyolites of the Picabo volcanic field (10.4–6.6 Ma) in eastern Idaho are preserved as thick ignimbrites and lavas along the margins of the Snake River Plain (SRP), and within a deep (>3 km) borehole near the central axis of the Yellowstone hotspot track. In this study we present new O and Hf isotope data and U–Pb geochronology for individual zircons, O isotope data for major phenocrysts (quartz, plagioclase, and pyroxene), whole rock Sr and Nd isotope ratios, and whole rock geochemistry for a suite of Picabo rhyolites. We synthesize our new datasets with published Ar–Ar geochronology to establish the eruptive framework of the Picabo volcanic field, and interpret its petrogenetic history in the context of other well-studied caldera complexes in the SRP. Caldera complex evolution at Picabo began with eruption of the (U–Pb) Tuff of Arbon Valley (TAV), a chemically zoned and normal-δ 18O (δ 18O ) unit with high, zoned 87Sr/86Sri (0.71488–0.72520), and low- (−18) and (−28). The TAV and an associated post caldera lava flow possess the lowest ε Nd(0) (−23), indicating ∼40–60% derivation from the Archean upper crust. Normal-δ 18O rhyolites were followed by a series of lower-δ 18O eruptions with more typical (lower crustal) Sr–Nd–Hf isotope ratios and whole rock chemistry. The voluminous West Pocatello rhyolite has the lowest δ 18O value (δ 18Omelt ), and we correlate it to a 1,000 m thick intracaldera tuff present in the INEL-1 borehole (with published zircon ages 8.04–8.35 Ma, and similarly low-δ 18O zircon values). The significant (4–5‰) decrease in magmatic-δ 18O values in Picabo rhyolites is accompanied by an increase in zircon δ 18O heterogeneity from variation in the TAV to variation in the late-stage low-δ 18O rhyolites, a trend similar to what is characteristic of Heise and Yellowstone, and which indicates remelting of variably hydrothermally altered tuffs followed by rapid batch assembly prior to eruption. However, due to the greater abundance of low-δ 18O rhyolites at Picabo, the eruptive framework may reflect an intertwined history of caldera collapse and coeval Basin and Range rifting and hydrothermal alteration. We speculate that the source rocks with pre-existing low-δ 18O alteration may be related to: (1) deeply buried and unexposed older deposits of Picabo-age or Twin Falls-age low-δ 18O volcanics; and/or (2) regionally-abundant late Eocene Challis volcanics, which were hydrothermally altered near the surface prior to or during peak Picabo magmatism. Basin and Range extension, specifically the formation of metamorphic core complexes exposed in the region, could have facilitated the generation of low-δ 18O magmas by exhuming heated rocks and creating the large water-rock ratios necessary for shallow hydrothermal alteration of tectonically (rift zones) and volcanically (calderas) buried volcanic rocks. These interpretations highlight the major processes by which supereruptive volumes of magma are generated in the SRP, mechanisms applicable to producing rhyolites worldwide that are facilitated by plume driven volcanism and extensional tectonics. [Copyright &y& Elsevier]

The central Snake River Plain (CSRP) of southern Idaho and northern Nevada, USA, forms part of the Columbia River-Yellowstone large igneous province. Volcanic rocks of the province are compositionally bimodal (basalt-rhyolite), and the rhyolites produce a broadly time-transgressive record of a hotspot which is currently located under Yellowstone. Snake River Plain rhyolites represent hot (>850 °C), dry magmas and have field characteristics consistent with high emplacement temperatures. Individual ignimbrite sheets reach 1,000 km and exhibit little to no compositional zonation on a large scale but reveal considerable complexity on a crystal scale, particularly with regard to pyroxene compositions. Multiple pyroxene compositions may exist in a single ignimbrite which, along with multiple glass compositions in widely dispersed fallout tephra, suggests complex storage of rhyolite prior to eruption. Unlike most igneous rocks, the mineral cargo of the CSRP rhyolites exhibits little isotopic variability, with unimodal Sr/Sr values returned from plagioclase grains inferred to represent the combination of strong crystal-melt coupling and rapid diffusional re-equilibriation. All the rhyolites within the CSRP have a characteristic low- δO signature; with >20,000 km of rhyolite exhibiting this depletion, the CSRP represents the largest low- δO province on Earth. The low-O nature of the rhyolites requires assimilation of hydrothermally altered materials which may be from altered Eocene batholithic rocks or from down-dropped intra-caldera tuffs. The wide range of crustal assimilants, with highly variable radiogenic isotope characteristics, available in the CSRP is permissive of a variety of petrogenetic models based on radiogenic isotopic data. [ABSTRACT FROM AUTHOR]

The phase relations have been investigated experimentally at 200 and 500 MPa as a function of water activity for one of the least evolved (Indian Batt Rhyolite) and of a more evolved rhyolite composition (Cougar Point Tuff XV) from the 12·8–8·1 Ma Bruneau–Jarbidge eruptive center of the Yellowstone hotspot. Particular priority was given to accurate determination of the water content of the quenched glasses using infrared spectroscopic techniques. Comparison of the composition of natural and experimentally synthesized phases confirms that high temperatures (>900°C) and extremely low melt water contents (<1·5 wt % H2O) are required to reproduce the natural mineral assemblages. In melts containing ∼0·5–1·5 wt % H2O, the liquidus phase is clinopyroxene (excluding Fe–Ti oxides, which are strongly dependent on fO2), and the liquidus temperature of the more evolved Cougar Point Tuff sample (BJR; ∼940–1000°C) is at least 30°C lower than that of the Indian Batt Rhyolite lava sample (IBR2; 970–1030°C). For the composition BJR, the comparison of the compositions of the natural and experimental glasses indicates a pre-eruptive temperature of at least 900°C. The composition of clinopyroxene and pigeonite pairs can be reproduced only for water contents below 1·5 wt % H2O at 900°C, or lower water contents if the temperature is higher. For the composition IBR2, a minimum temperature of 920°C is necessary to reproduce the main phases at 200 and 500 MPa. At 200 MPa, the pre-eruptive water content of the melt is constrained in the range 0·7–1·3 wt % at 950°C and 0·3–1·0 wt % at 1000°C. At 500 MPa, the pre-eruptive temperatures are slightly higher (by ∼30–50°C) for the same ranges of water concentration. The experimental results are used to explore possible proxies to constrain the depth of magma storage. The crystallization sequence of tectosilicates is strongly dependent on pressure between 200 and 500 MPa. In addition, the normative Qtz–Ab–Or contents of glasses quenched from melts coexisting with quartz, sanidine and plagioclase depend on pressure and melt water content, assuming that the normative Qtz and Ab/Or content of such melts is mainly dependent on pressure and water activity, respectively. The combination of results from the phase equilibria and from the composition of glasses indicates that the depth of magma storage for the IBR2 and BJR compositions may be in the range 300–400 MPa (∼&leq;13 km) and 200–300 MPa (∼&leq;10 km), respectively. [ABSTRACT FROM AUTHOR]

Three voluminous rhyolitic ignimbrites have been identified along the southern margin of the central Snake River Plain. As a result of wide-scale correlations, new volume estimates can be made for these deposits: ~350 km for the Steer Basin Tuff and Cougar Point Tuff XI, and ~1,000 km for Cougar Point Tuff XIII. These volumes exclude any associated regional ashfalls and correlation across to the north side of the plain, which has yet to be attempted. Each correlation was achieved using a combination of methods including field logging, whole rock and mineral chemistry, magnetic polarity, oxygen isotope signature and high-precision Ar/Ar geochronology. The Steer Basin Tuff, Cougar Point Tuff XI and Cougar Point Tuff XIII have deposit characteristics typical of 'Snake River (SR)-type' volcanism: they are very dense, intensely welded and rheomorphic, unusually well sorted with scarce pumice and lithic lapilli. These features differ significantly from those of deposits from the better-known younger eruptions of Yellowstone. The ignimbrites also exhibit marked depletion in δO, which is known to characterise the SR-type rhyolites of the central Snake River Plain, and cumulatively represent ~1,700 km of low δO rhyolitic magma (feldspar values 2.3-2.9‰) erupted within 800,000 years. Our work reduces the total number of ignimbrites recognised in the central Snake River Plain by 6, improves the link with the ashfall record of Yellowstone hotspot volcanism and suggests that more large-volume ignimbrites await discovery through detailed correlation work amidst the vast ignimbrite record of volcanism in this bimodal large igneous province. [ABSTRACT FROM AUTHOR]

Major oxide, trace element and volatile (HO, CO, S, F, and Cl) compositions have been analyzed for olivine-hosted melt inclusions in eight basalt samples from Yellowstone National Park and the Snake River Plain (SRP) to identify the least differentiated melt compositions and assess the volatile budget of the Yellowstone hotspot. Melt-inclusion chemistry was evaluated to understand potential overprinting effects in the shallow mantle and crust of magmas derived from deeper levels. Maximum water concentrations of 3.3 wt% and CO up to 1,677 ppm have been observed in olivine-hosted melt inclusions from the Gerritt Basalts at Mesa Falls, Idaho (SRP region), which is significantly higher than the maximum concentrations measured in lavas from other hotspots such as Hawaii (~0.8-0.9 wt%). Maximum water concentrations were generally observed in the least differentiated melt inclusions in terms of incompatible major oxide concentrations, indicating that high water concentrations are characteristic of the mantle or perhaps lower crust rather than resulting from differentiation enhancement within the shallow crust, even taking into account the fact that water behaves as an incompatible element during crystal fractionation. Enrichment in Ba coupled with depletion in Th in many of the melt inclusions and their host rocks is a characteristic of many arc lavas and may indicate that volatiles in Yellowstone-Snake River Plain basalts could have a subduction zone origin. [ABSTRACT FROM AUTHOR]

Abstract: Pollen analysis of soils overlain by welded tuffs of the Yellowstone hotspot provides a radiometrically-dated chronology of the vegetation 12–7Ma for central Snake River Plain. The welded tuffs, produced as the Yellowstone hotspot migrated eastward, overlie soils containing perfectly-preserved pollen, occasionally containing cytoplasm. The pollen assemblage is dominated by Artemisia (sagebrush; ave. 32%, range 11–61%) and Poaceae (grass; ave. 21.3%, range 4–55%). This assemblage is very similar to that found in the area today, except the welded-tuff samples contain low percentages (<5%) of arboreal taxa not found in the current flora: Podocarpus, Cedrus, Taxus, Aesculus, and Ulmus. The values of Artemisia and Poaceae are much higher than those found in samples of the same age (12–7Ma) from the Pacific Northwest (which are dominated by tree pollen) and the Great Salt Lake basin (which are dominated by pine [Pinus], and greasewood [Sarcobatus]). Furthermore, the welded-tuff samples from the Snake River Plain contain a diverse herbaceous taxa characteristic of modern sagebrush steppe, and a diverse fungal spore assemblage including the root symbiont Glomus and the dung fungus Sporormiella. These results show that sagebrush steppe was widespread on the western Snake River Plain 12Ma, at least 5Ma earlier than previously recognized. [Copyright &y& Elsevier]

Abstract: SHRIMP analysis of U/Pb ages of detrital zircons in twelve late Miocene to Pleistocene sand samples from six drill cores on the Snake River Plain (SRP), Idaho, suggests that an ancestral Lost River system was drained westward along the northern side of the SRP. Neoproterozoic (650 to 740 Ma, Cryogenian) detrital zircon grains from the Wildhorse Creek drainage of the Pioneer Mountains core complex, with a source in 695 Ma orthogneiss, and which are characteristic of the Big Lost River system, are found in Pliocene sand from cores drilled in the central SRP (near Wendell) and western SRP (at Mountain Home). In addition to these Neoproterozoic grains, fluvial sands sourced from the northern margin of the SRP contain detrital zircons with the following ages: 42 to 52 Ma from the Challis magmatic belt, 80 to 100 Ma from the Atlanta lobe of the Idaho batholith, and mixed Paleozoic and Proterozoic ages (1400 to 2000 Ma). In contrast, sands in the Mountain Home Air Base well (MHAB) that contain 155-Ma Jurassic detrital grains with a source in northern Nevada are interpreted to represent an integrated Snake River, with provenance on the southern, eastern and northern sides of the SRP. We propose that late Pliocene and early Pleistocene construction of basaltic volcanoes and rhyolitic domes of the Axial Volcanic Zone of the eastern SRP and the northwest-trending Arco Volcanic Rift Zone (including the Craters of the Moon volcanic center), disrupted the paleo-Lost River drainage, confining it to the Big Lost Trough, a volcanically dammed basin of internal drainage on the Idaho National Laboratory (INL). After the Axial Volcanic Zone and Arco Volcanic Rift Zone were constructed to form a volcanic eruptive and intrusive highland to the southwest, sediment from the Big Lost River was trapped in the Big Lost Trough instead of being delivered by surface streams to the western SRP. Today, water from drainages north of the SRP enters the Snake River Plain regional aquifer through sinks in the Big Lost Trough, and the water resurfaces at Thousand Springs, Idaho, about 195 km to the southwest. Holocene to latest Pliocene samples from drill core in the Big Lost Trough reveal interplay between the glacio-fluvial outwash of the voluminous Big Lost River system and the relatively minor Little Lost River system. A mixed provenance signature is recognized in fine-grained sands deposited in a highstand of a Pleistocene pluvial-lake system. [Copyright &y& Elsevier]

Abstract: Basaltic volcanism in the western Snake River Plain underwent an abrupt change circa ~700 ka to 900 ka, from low-K tholeiitic basalt and ferrobasalt to high-K transitional alkali basalt. The low-K tholeiitic basalts share major element, trace element, and isotopic characteristics with olivine tholeiites of the eastern Snake River Plain, and must have been derived by similar processes from similar sources. In contrast, the younger high-K alkali basalts share major element, trace element, and isotopic characteristics with plume-derived alkali basalts of ocean islands suites like Hawaii. We conclude that this abrupt transition reflects either or both the erosion of pre-existing mantle lithosphere in the wake of the Yellowstone–Snake River plume, or the depletion of this lithosphere in fusible components so that it no longer contributed to the overall mass flux of magma. The abruptness of the transition implies that it may have a catastrophic origin, such as lithospheric delamination caused by a Rayleigh–Taylor instability beneath the Idaho batholith. [Copyright &y& Elsevier]

Abstract: We report new He, Nd and Sr isotope results for basalts from the northwestern United States. The new 3He/4He results for olivine phenocrysts in basalts from the eastern Snake River Plain (SRP), the Owyhee Plateau (OP) and the Oregon High Lava Plains (HLP), together with published He isotope data for Yellowstone and the Cascades volcanic arc, delineate distinct mantle sources for each of these sub-provinces. All basalts from the eastern SRP (8 Quaternary localities plus 1 Miocene locality) have 3He/4He ratios higher than observed in normal mid-ocean ridge basalts, but overlapping with ranges observed in hotspot-related oceanic islands. For a lateral distance of some 400 km along the SRP, 3He/4He ranges from ~11 R A in the west to >19 R A adjacent to Yellowstone. Such high ratios have not been observed elsewhere in the western U.S., and are consistent with the presence of a mantle plume. The lateral gradient in 3He/4He suggests that the proportion of plume-derived He decreases westward, but this interpretation is complicated by possible addition of crustal helium during open-system crystal fractionation in some SRP basaltic magmas. Although crustal contamination may modulate 3He/4He in basalts along the SRP, the effect is not strong and it does not obscure the elevated 3He/4He mantle source signature. In contrast, young basalts from the HLP and the OP have 3He/4He values of 8.8–9.3 R A, within the range for mid-ocean ridge basalts; these data reflect a shallow asthenospheric source with no discernible influence from the Yellowstone hotspot. Basalts from Newberry volcano have slightly lower 3He/4He (7.6–8.3 R A), within the range for other Cascades arc lavas (7.0–8.4 R A). Three alternative explanations are possible for the origin of the high 3He/4He signature along the SRP: (1) multi-component mixing of (a) magmas and/or CO2-rich fluids derived from plume mantle having high 3He/4He, (b) continental lithosphere having low 3He/4He, and (c) shallow asthenospheric mantle (MORB source); (2) a mantle plume beneath Yellowstone that has an unusual combination of He, Nd and Sr isotope characteristics; or (3) a continental lithospheric mantle that experienced ancient enrichment of 3He relative to (U+Th). The isotope relations between He–Nd and He–Sr, along with other considerations, generally favor the first explanation, but the other possibilities cannot be ruled out at the present time. [Copyright &y& Elsevier]

Abstract: This paper evaluates the Neogene kinematic evolution of the Eastern Snake River Plain (ESRP) to estimate the volume of mafic magma transferred from mantle to crust during passage over the Yellowstone hotspot. Four major kinematic processes affected the ESRP including densification due to emplacement of gabbro into the middle crust, subsidence and consequent filling of a 6–km–deep basin with volcanic and plutonic rocks, 20% extension via dike injection or faulting/ductile attenuation, and lower crustal flow from ESRP to the adjacent Basin and Range. Two kinematic models involving these processes are proposed. Overall, the two models are viable only if the initial crustal thickness were 43–47–km–thick; otherwise, the lost mass of the ESRP is not balanced by the excess mass in the Basin and Range. In the first model (densification, eruption, and extension by dike injection), the ESRP crust receives mantle-derived gabbro equivalent to a 16–km–thick layer, but would need to lose 10 km of crust via lower crustal flow. In the second model (densification, eruption, and extension by faulting and ductile attenuation), the ESRP crust receives a 9.4 km equivalent thickness of mantle-derived gabbro, and would need to lose 6 km of crust via lower crustal flow. The cumulative amount of mantle-derived gabbro in the ESRP crust derived by this kinematic analysis (equivalent to a 9.4–16–km–thick layer) is nearly identical to the amount estimated independently using a mass balance petrologic model, suggesting it is a robust result. [Copyright &y& Elsevier]

Abstract: The Bruneau–Jarbidge eruptive center of the central Snake River Plain in southern Idaho, USA produced multiple rhyolite lava flows with volumes of <10km3 to 200km3 each from ~11.2 to 8.1Ma, most of which follow its climactic phase of large-volume explosive volcanism, represented by the Cougar Point Tuff, from 12.7 to 10.5Ma. These lavas represent the waning stages of silicic volcanism at a major eruptive center of the Yellowstone hotspot track. Here we provide pyroxene compositions and thermometry results from several lavas that demonstrate that the demise of the silicic volcanic system was characterized by sustained, high pre-eruptive magma temperatures (mostly ≥950°C) prior to the onset of exclusively basaltic volcanism at the eruptive center. Pyroxenes display a variety of textures in single samples, including solitary euhedral crystals as well as glomerocrysts, crystal clots and annealed microgranular inclusions of pyroxene±magnetite±plagioclase. Pigeonite and augite crystals are unzoned, and there are no detectable differences in major and minor element compositions according to textural variety — mineral compositions in the microgranular inclusions and crystal clots are identical to those of phenocrysts in the host lavas. In contrast to members of the preceding Cougar Point Tuff that host polymodal glass and mineral populations, pyroxene compositions in each of the lavas are characterized by single rather than multiple discrete compositional modes. Collectively, the lavas reproduce and extend the range of Fe–Mg pyroxene compositional modes observed in the Cougar Point Tuff to more Mg-rich varieties. The compositionally homogeneous populations of pyroxene in each of the lavas, as well as the lack of core-to-rim zonation in individual crystals suggest that individual eruptions each were fed by compositionally homogeneous magma reservoirs, and similarities with the Cougar Point Tuff suggest consanguinity of such reservoirs to those that supplied the polymodal Cougar Point Tuff. Pyroxene thermometry results obtained using QUILF equilibria yield pre-eruptive magma temperatures of 905 to 980°C, and individual modes consistently record higher Ca content and higher temperatures than pyroxenes with equivalent Fe–Mg ratios in the preceding Cougar Point Tuff. As is the case with the Cougar Point Tuff, evidence for up-temperature zonation within single crystals that would be consistent with recycling of sub- or near-solidus material from antecedent magma reservoirs by rapid reheating is extremely rare. Also, the absence of intra-crystal zonation, particularly at crystal rims, is not easily reconciled with cannibalization of caldera fill that subsided into pre-eruptive reservoirs. The textural, compositional and thermometric results rather are consistent with minor re-equilibration to higher temperatures of the unerupted crystalline residue from the explosive phase of volcanism, or perhaps with newly generated magmas from source materials very similar to those for the Cougar Point Tuff. Collectively, the data suggest that most of the pyroxene compositional diversity that is represented by the tuffs and lavas was produced early in the history of the eruptive center and that compositions across this range were preserved or duplicated through much of its lifetime. Mineral compositions and thermometry of the multiple lavas suggest that unerupted magmas residual to the explosive phase of volcanism may have been stored at sustained, high temperatures subsequent to the explosive phase of volcanism. If so, such persistent high temperatures and large eruptive magma volumes likewise require an abundant and persistent supply of basalt magmas to the lower and/or mid-crust, consistent with the tectonic setting of a continental hotspot. [Copyright &y& Elsevier]

Abstract: This paper estimates mantle mass transfer to the crust along the Yellowstone–Snake River Plain (YSRP) hotspot track by applying a simple mass-balance analysis to rhyolite petrogenesis. Mass-balances are based primarily upon distinctive Nd- and Sr-isotopic characteristics of rhyolite, basalt, and pre-existing crust. We present new Nd- and Sr-isotopic data for basalts that are coeval with hotspot volcanism. Many of these basalts exhibit moderate to strong crustal interaction having e Nd as low as –18 and 87Sr/86Sri of 0.715, while others overlap petrographically, compositionally and isotopically with primitive post-hotspot olivine tholeiites (ε Nd =–4.4±1.3; 87Sr/86Sri =0.7067±0.001). We also present new Nd- and Sr-isotopic, petrologic, and U–Pb zircon geochronologic data for YSRP rhyolites associated with Picabo and Heise volcanic fields, some of which occur in 3.2 and 1.6-km-deep boreholes. Rhyolites of the central and eastern YSRP have clustered and primitive ranges of ε Nd (–8±2) and 87Sr/86Sri (0.712±0.003) composition, in contrast to heterogeneous Archean country rocks (ε Nd =–23 to –52 and 87Sr/86Srm =0.702 to 0.893). Using these values for country rock and basalt, we calculate a conservative Neodymium Crustal Index of ∼0.4 for YSRP rhyolite. We present a simple mass-balance model of rhyolite petrogenesis which budgets sequentially for mantle-derived mass within erupted rhyolites, and cogenetic pluton and restite masses. Our mass-balance estimates indicate that the amount of mantle-derived mass added to the crust between 11 and 4 Ma is equivalent to a layer of gabbro ∼14-km-thick or ∼340,000 km3. The corresponding average mass flux of 0.05 km3/yr is comparable to many other global hotspot systems. [Copyright &y& Elsevier]

Abstract: The geochemistry of NW USA rhyolites correlates strongly with geography and the nature of the underlying basement terranes. Rhyolites from the Snake River Plain-Yellowstone (SRPY) province have higher 87Sr/86Sr, 207Pb/206Pb, and lower 143Nd/144Nd than those from the Oregon High Lava Plains (HLP) province, reflecting a dominant influence of Precambrian cratonic crust east of the western Idaho suture zone versus accreted oceanic terranes of Phanerozoic age to the west. Rhyolites from the cratonic domain show significant enrichments of Th, U, and LREE/HREE, whereas B concentration and especially B/Nb and B/Rb are systematically higher west of the tectonic boundary. This decoupling of B from the other incompatible elements is best explained in terms of distinctive magmatic sources east and west of the suture zone. B isotopic composition [δ 11B] was measured for natural and synthetic glasses via multiple multiplier laser ablation-ICP-MS. δ 11B values are systematically lighter in SRPY rhyolites (−5.6 to −8.9 ‰) compared to those from the HLP (−0.8 to −3.1 ‰). These data are consistent with strongly fluid-depleted and/or metamorphosed sources for SRPY rhyolites, whereas HLP sources resemble those of typical oceanic basalts, and could reflect melting of juvenile basalt-derived protoliths in the crust. B isotope ratios of low-δ 18O rhyolites are indistinct from those with normal δ 8O, suggesting that δ 11B values are not strongly affected by hydrothermal processes that alter source materials with meteoric water. Considering all data, it is likely that B compositions of the rhyolites are inherited from their sources in the crust. Although low δ 11B (<0 ‰) is also observed in many mantle-derived basalts (i.e., OIB, MORB), in SRPY rhyolites it is associated with enrichments of elements (e.g., U, Th, Rb, LREE) typically concentrated in continental crust, and thus old high-grade metamorphic continental crust is inferred to be11B-depelted. If rhyolite protoliths were originally metasediments, it is likely that bulk B and δ 11B were selectively removed by metamorphic dehydration reactions and transported to the surface via 11B-enriched fluids. [Copyright &y& Elsevier]

Abstract: Butte 5206, a small monogenetic olivine tholeiite basalt (OTB) shield in the Eastern Snake River Plain (ESRP), erupted during late-Pleistocene time along the Arco–Big Southern Butte Volcanic Rift Zone. Geochemical data from Butte 5206 and other basalts that erupted in the vicinity, including correlated samples from coreholes, are evaluated for possible mechanisms of magma evolution in the ESRP. Thermodynamic and mass balance models demonstrate that the evolution of Butte 5206 magmas reflect a two-stage process: (1) fractional crystallization of relatively primitive magma, possibly at multiple crustal levels, and (2) mixing of primitive and evolved liquids in a shallow reservoir prior to eruption. Application of this model to other basalts and local eruptive centers in the region suggest that similar processes are active during the petrogenesis of many ESRP basalts, and that the process of intrusion and mixing is cyclic throughout Pleistocene to Holocene time. [Copyright &y& Elsevier]

Abstract: The structure and composition of the Yellowstone–Snake River Plain (YSRP) volcanic system were analyzed using gravity data taken at over 30,000 stations in the YSRP and surrounding region. Additional constraints were provided by tomographic seismic velocity models, new heat flow and temperature information, GPS-derived strain rates, earthquake locations, and chemical analyses of volcanic rocks. P-wave velocity models and velocity–density data based on petrologic information were used to constrain three-dimensional density models. Rheology and strength properties were calculated at selected locations and compared to earthquake focal depths. Results of this study suggest that the lower crust of the Snake River Plain (SRP) has been thickened by the addition of an underplated layer composed primarily of clinopyroxene, having a density of 3200 kg/m3. A mid-crustal high-density sill is composed of a series of gabbroic lenses inter-fingering with the granitic upper crust. This geometry yields a bulk composition comparable to diorite and a density of 2900 kg/m3. The mid-crustal sill varies from 4 to 11 km in thickness, resulting in a series of SE-NW-trending gravity highs observed along the axis of the SRP. The mid-crustal sill extends up to 20 km southeast of the SRP volcanic field and causes asymmetry of the gravity field. The Yellowstone Plateau volcanic field density model reveals low-density partial melt 10 km beneath the caldera that shallows under the northeastern caldera, and continues laterally 20 km north of the caldera boundary and notably increases the previously estimated size of the magma reservoir by ~20%. The caldera melt body has a density of 2520 kg/m3 and a significantly lower value of 2470 kg/m3 for the northeastern caldera melt body. Southwest of Yellowstone, the crustal section occupied by the mid-crustal sill in the SRP and the partial melt in Yellowstone constitute a transition area between the active Yellowstone magma system and the now volcanically quiescent SRP, with a density of 2820 kg/m3. Strength models show that the crust of the YSRP becomes progressively stronger and cooler with increasing distance from Yellowstone, and tectonic earthquakes within the Yellowstone caldera are unlikely to nucleate below 4–6 km depth, thus limiting the maximum magnitude of earthquakes to M ≤6.5. [Copyright &y& Elsevier]

Abstract: Basaltic magmatism associated with the Yellowstone hotspot has been widely attributed to upwelling of a mantle plume, yet the temporal and spatial distribution of these magmas and their compositional characteristics are distinctive from oceanic hotspot magmatism. Fundamental questions concern the influence of continental cratonic lithosphere in producing the differences, and the extent to which upper plate processes contribute to magma production. To better understand scenarios of melt generation, P–T conditions are estimated for segregation of primitive Snake River Plain (SRP) basalts from the mantle. Combined with analysis of trace element and seismic constraints, we conclude from this that (1) melt production was concentrated at depths between roughly 70–100 km, (2) mantle temperature was only slightly higher than ambient conditions with a maximum potential temperature of 1450 °C, and (3) the mantle source was relatively fertile (Mg#<90). These results suggest that the seismically imaged plume below Yellowstone is significantly cooler than upwellings beneath Hawaii, Iceland and many other oceanic “hotspots”. Our findings, in combination with other geochemical and geodynamic considerations, are permissive of magma generation within the ancient lithospheric mantle keel associated with the Wyoming craton. Plume contributions, while not excluded, involve physical and geochemical implications that suggest they are subordinate. [Copyright &y& Elsevier]

Abstract: Integration of geophysical and geological data show that the Yellowstone hotspot resulted from a mantle plume interacting with the overriding North America plate, a process that has highly modified continental lithosphere by magmatic and tectonic processes and produced the 16-17Ma, 700-km-long Yellowstone-Snake River Plain (YSRP) silicic volcanic system. Accessibility of the YSRP allowed large-scale geophysical projects to seismically image the hotspot and evaluate its kinematic properties using geodetic measurements. Seismic tomography reveals a crustal magma reservoir of 8% to 15% melt, 6km to 16km deep, beneath the Yellowstone caldera. An upper-mantle low-P-wave-velocity body extends vertically from 80km to 250km beneath Yellowstone, but the anomalous body tilts 60°WNW and extends to 660km depth into the mantle transition zone. We interpret this conduit-shaped low-velocity body as a plume of up to -3.5% Vp and -5.5% Vs perturbation that corresponds to a 1-2% partial melt. Models of whole mantle convection reveal eastward upper-mantle flow beneath Yellowstone at relatively high rates of 5cm/yr that deflects the ascending plume into its west-tilted geometry. A geodynamic model of the Yellowstone plume constrained by Vp and Vs velocities and attenuation parameters suggests low excess temperatures of up to 120K, corresponding to a maximum 2.5% melt, and a small buoyancy flux of 0.25Mg/s, i.e., properties of a cool, weak plume. The buoyancy flux is many times smaller than for oceanic plumes, nonetheless, plume buoyancy has produced a ~400-km-wide, ~500-m-high topographic swell centered on the Yellowstone Plateau. Contemporary deformation derived from GPS measurements reveals SW extension of 2-3mm/yr across the Yellowstone Plateau, one-fourth of the total Basin-Range opening rate, which we consider to be part of Basin-Range intraplate extension. Locally, decadal episodes of subsidence and uplift, averaging ~2cm/yr, characterize the 80-year Yellowstone caldera monitored history and are modeled as hydrothermal-magmatic sources. Moreover a recent episode, 2004-2009, of accelerated uplift of the Yellowstone caldera at rates up to 7cm/yr has been modeled as resulting from magmatic recharge of a 10-km-deep sill at the top of the crustal magma reservoir. Regionally, gravitational potential energy of the Yellowstone swell drives the lithosphere southwest and “downhill” from the Yellowstone Plateau 400km where it coalesces with Basin-Range province-wide westward extension. Based on the geometry and its assumed 660km depth, we extrapolate the plume source southwest to its original location at 17Ma and 600km southwest and 200km north of the YSRP. Importantly, this location is beneath the southern part of the Columbia Plateau flood basalt field of the same age and implies that the Yellowstone mantle plume may be the common source for both of these large volcanic fields. Our time-progression model suggests that the original plume head rose vertically behind the Juan de Fuca plate, but at ~12Ma it lost the protection of the subducting plate from eastward mantle flow and encountered cooler, thicker continental lithosphere, becoming entrained in eastward upper-mantle flow. These results reveal that Yellowstone plume-plate processes have had a profound effect on Late Cenozoic geologic evolution and topography of a large part of the western U.S. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

The Snake River Plain (SRP), a 90-km-wide topographic depression in southern Idaho, is a topographically anomalous feature in the western U.S. Previous seismic studies focused on the northeastern SRP to study its relationship with the Yellowstone hotspot. We present new teleseismic shear-wave splitting data from six broadband seismic stations deployed along the axis of the SRP from June 2000 to September 2001. We also analyze splitting at HLID, a permanent station of the National Seismic Network located ∼100 km north of the plain. Splitting of individual teleseismic phases is consistent at all stations within 2σ errors, and we favor the interpretation of anisotropy with a single horizontal fast axis, although a dipping-axis interpretation is statistically permitted at two of the stations. Our station fast directions, as well as shear-wave splitting data from numerous other stations throughout the Basin and Range, are best explained by a lattice preferred orientation of olivine due to horizontal shear along the base of the plate associated with the gravitational spreading of buoyant plume-like upwelling material beneath eastern Nevada into a southwestward flowing asthenosphere (with respect to a fixed hotspot reference frame). This parabolic asthenospheric flow (PAF) model for the Great Basin is attractive because it explains the observed high elevations, high mantle buoyancy, low-velocity anomaly beneath eastern Nevada, high heat flow, and depleted geochemistry of some erupted basalts. The lack of Pliocene–Recent major volcanism in eastern Nevada suggests that a significant amount of the buoyancy flux is due to compositional buoyancy. Our splitting station delay times vary in a way not predicted by the PAF model, and can be explained by: a zone of aligned magma-filled lenses and/or partially molten dikes beneath the SRP lithosphere, a depleted olivine-rich residuum underneath the sides of the eastern SRP, and/or the effect of lateral lower crustal flow from beneath the eSRP toward its adjacent flanks. [Copyright &y& Elsevier]

We report new U-Pb detrital zircon sensitive high-resolution ion microprobe (SHRIMP) age data (702 grains) from 13 samples collected from Miocene to Holocene sedimentary deposits in the western Snake River Plain region. These samples effectively show that modern stream sediments of the Snake River system reliably and repeatedly record the detrital zircon age populations that are present as sources in their drainage basins across the Cordilleran thrust belt and Basin and Range Province. We use this framework and the provenance of Neogene sedimentary rocks in the region to test the effect of the migrating Yellowstone hotspot on regional drainage patterns in southern Idaho since the middle Miocene. Our results indicate that Neogene paleodrainages were first directed radially away from the tumescent Yellow-stone highland, then subsequently reversed their flow toward the subsiding Snake River Plain basin. This occurred in east-progressing time-constrained intervals starting at 16 Ma. In northern Nevada, the drainage divide is represented by a northeast-trending, southeast-migrating crest of high topography. Specifically, middle to late Miocene (16-10 Ma) sedimentary deposits of the western Snake River Plain and Oregon-Idaho graben contain early to middle Eocene (52-42 Ma) detrital zircon populations sourced in Challis magmatic rocks north of the Snake River Plain. Middle Jurassic (160 Ma) and middle to late Eocene (42-35 Ma) detrital zircons, sourced from rocks in northern Nevada, are not present. Late Eocene detrital zircons from Nevada are present in two younger than 7 Ma sedimentary units of the Idaho Group along the Oregon-Idaho border. This indicates that by the late Miocene, southeastward headward erosion of the paleo-Owyhee River into the Owyhee Plateau had captured drainage from north-central Nevada and directed it northwestward toward the subsiding western Snake River Plain. The modern Owyhee Plateau is still a topographic high, in contrast to the modern Snake River Plain, suggesting that lowering of the regional Snake River Plain base level, rather than crustal subsidence, drove stream capture. By the late Pliocene (3 Ma), Middle Jurassic detrital zircons are recorded in the Glenns Ferry Formation and Tuana Gravel of the central Snake River Plain, suggesting that surface subsidence reversed the flow direction of paleo-Salmon Falls Creek from southward into Nevada to northward toward Idaho. Miocene strata of the western Snake River Plain lack recycled Proterozoic detrital zircons that are ubiquitous in sedimentary rocks of the central and southeast Idaho thrust belts. Such detrital zircons appear on the central and western Snake River Plain in early Pliocene to Holocene (4-0 Ma) deposits. This records capture of drainage from the eastern Snake River Plain. The Yellowstone hotspot controlled the east-migrating continental divide, in the wake of which formed the western-draining, and progressively eastward-collecting, Snake River system. [ABSTRACT FROM AUTHOR]