Yellowstone Hotspot Track Research Articles

Convective Melting and Water Behavior around Magmatic-Hydrothermal Transition: Numerical Modeling with Application to Krafla Volcano, Iceland

Abstract Water is an essential component of rhyolitic magmas and nearly universally, silicic magmatism in the upper crust includes a transition from magma to water-saturated roof rocks. We have numerically simulated the effect of the addition of geothermal fluids to an intruded rhyolitic sill from the hydrothermal system contained within the porous felsitic roof rocks. Water uptake in the melt proceeds via its thermodynamically estimated saturation with a partial melt, corresponding to the fugacity of hydrothermal water in the melt-fluid zone at particular T–P-${X}_{H_2O}$ conditions. It is assumed that the exchange occurs until the melt fraction increases to the threshold melt fraction value εb ≈ 0.3–0.45. In this approximation, the amount of added water is the product of its solubility and the critical melt fraction εb. In two series of numerical experiments run at pressures of 200 and 50 MPa, the interaction of water-filled porous felsite with near liquidus rhyolite magma resulted in water absorption, induced partial melting creating a narrow several meters wide mushy zone, and sluggish convection below that distributed water across the intruded sill. At P = 200 MPa, the addition of about 1.5 wt% water results in stronger volume convection, causing the melting rate to increase to 20 m/year. However, the addition of <0.22 wt% water induced no melting on the magma/contact mush interface, and the intruded sill crystallizes without convection. We apply the results of these numerical experiments to hot and dry rhyolites of the Yellowstone hot spot track magmas and then to the 2009 AD rhyolite sampled by the IDDP-1 exploration well in Krafla (Iceland). An active contact between the hydrothermal system within felsite and hot 963°C rhyolite magma was accidentally crossed at the depth of 2100 m, with a very thin (<30 m) transition providing information for a partial verification of our theoretical model. With the parameters observed in 2009, including the water concentrations in the melt (1.8 wt%) and felsite (0.92 wt%) and the high temperature of the intruded magma (945°C–980°C), we obtained slow melting of the preheated felsite roof at a rate of about 1 m/year. This seems reasonable if the 2009 magma was intruded during the 1975–1984 Krafla Fires eruption. We additionally present new δD values (−118‰) and H2Otot (1.6–2.0 wt%) concentration and reinterpret published δD and H2Otot values for felsitic fragments from IDDP-1; we find these to be lower than the δD in the mantle-derived magmas or hydrothermal systems. We demonstrate that, in theory, the formation of a fluid with such a low δD can be provided by the addition of low-δD water from OH-bearing minerals in hydrothermally altered roof rock. This may happen during dehydration of epidote from the altered roof rocks, and, alternatively, may also proceed by the mechanism of thermal diffusion (the Soret effect) through the partially molten/hydrothermal transition zone controlled by fluid fugacity. The high-temperature gradient in the contact zone between magma and the geothermal system of about 15–17°/m with conditions at the cold end close to the critical point for the aqueous fluid further decreases the expected δD value at the hot end of the contact zone to less than −110‰.

Removal of the Northern Paleo-Teton Range along the Yellowstone Hotspot Track

Abstract Classically held mechanisms for removing mountain topography (e.g., erosion and gravitational collapse) require 10-100 Myr or more to completely remove tectonically generated relief. Here, we propose that mountain ranges can be completely and rapidly (<2 Myr) removed by a migrating hotspot. In western North America, multiple mountain ranges, including the Teton Range, terminate at the boundary with the relatively low relief track of the Yellowstone hotspot. This abrupt transition leads to a previously untested hypothesis that preexisting mountainous topography along the track has been erased. We integrate thermochronologic data collected from the footwall of the Teton fault with flexural-kinematic modeling and length-displacement scaling to show that the paleo-Teton fault and associated Teton Range was much longer (min. original length 190-210 km) than the present topographic expression of the range front (~65 km) and extended across the modern-day Yellowstone hotspot track. These analyses also indicate that the majority of fault displacement (min. 11.4-12.6 km) and the associated footwall mountain range growth had accumulated prior to Yellowstone encroachment at ~2 Ma, leading us to interpret that eastward migration of the Yellowstone hotspot relative to stable North America led to removal of the paleo-Teton mountain topography via posteruptive collapse of the range following multiple supercaldera (VEI 8) eruptions from 2.0 Ma to 600 ka and/or an isostatic collapse response, similar to ranges north of the Snake River plain. While this extremely rapid removal of mountain ranges and adjoining basins is probably relatively infrequent in the geologic record, it has important implications for continental physiography and topography over very short time spans.

Discovery of two new super-eruptions from the Yellowstone hotspot track (USA): Is the Yellowstone hotspot waning?

AbstractSuper-eruptions are amongst the most extreme events to affect Earth’s surface, but too few examples are known to assess their global role in crustal processes and environmental impact. We demonstrate a robust approach to recognize them at one of the best-preserved intraplate large igneous provinces, leading to the discovery of two new super-eruptions. Each generated huge and unusually hot pyroclastic density currents that sterilized extensive tracts of Idaho and Nevada in the United States. The ca. 8.99 Ma McMullen Creek eruption was magnitude 8.6, larger than the last two major eruptions at Yellowstone (Wyoming). Its volume exceeds 1700 km3, covering ≥12,000 km2. The ca. 8.72 Ma Grey’s Landing eruption was even larger, at magnitude of 8.8 and volume of ≥2800 km3. It covers ≥23,000 km2 and is the largest and hottest documented eruption from the Yellowstone hotspot. The discoveries show the effectiveness of distinguishing and tracing vast deposit sheets by combining trace-element chemistry and mineral compositions with field and paleomagnetic characterization. This approach should lead to more discoveries and size estimates, here and at other provinces. It has increased the number of known super-eruptions from the Yellowstone hotspot, shows that the temporal framework of the magmatic province needs revision, and suggests that the hotspot may be waning.

Experimental Melting of Hydrothermally Altered Rocks: Constraints for the Generation of Low-δ18O Rhyolites in the Central Snake River Plain

Abstract Quantifying the relative contributions of crustal versus mantle-derived melt is important for understanding how silicic magmas are generated, stored, and interact with country rock in trans-crustal magmatic systems. Low-δ18O rhyolitic ignimbrites and lavas erupted during Miocene volcanic activity in the central Snake River Plain (14–6 Ma) have been inferred to be the result of large-scale partial or bulk melting of pre-existing hydrothermally altered lithologies of the Idaho batholith and Challis volcanic field. In this study, we assess the melting behaviour of heterogeneously altered source materials via partial melting experiments over a range of run times at conditions of 750–1000°C and 1–2 kbar, and apply our observations to current models for the petrogenesis of low-δ18O rhyolites along the Yellowstone hotspot track. Partial melt produced in the experiments inherits the bulk oxygen isotope composition from hydrothermally altered peraluminous source materials independent of the melt fraction, excluding the possibility for preferential, disequilibrium melting of 18O-depleted mineral phases during incipient melting. We propose a new model to explain the generation of low-δ18O rhyolites in the central Snake River Plain, whereby mantle-derived magmas assimilate ∼30–40% of crustal material that was hydrothermally altered at high temperatures in two stages: (1) a preceding episode of hydrothermal alteration during intrusion of Eocene plutons (‘pre-existing source’); (2) syn-magmatic hydrothermal alteration within a nested caldera complex. During assimilation, dilution of peraluminous crustal lithologies with mantle-derived magma maintains the metaluminous character of rhyolites erupted along the Yellowstone hotspot track. These results link previous models favouring melting of either pre-existing or syn-magmatically altered lithologies for the generation of low-δ18O rhyolites along the Yellowstone hotspot track and provide direct experimental observation of the chemical processes occurring during assimilation processes in magmatic environments.

Elemental abundance patterns and Sr-, Nd- and Hf-isotope systematics for the Yellowstone hotspot and Columbia River flood basalts: Bearing on petrogenesis

The Yellowstone hotspot track in the northwestern US has a 17-myr history that began with eruption of the voluminous Columbia River basalts (CRB), and produced an eastward track with the bimodal rhyolite and basaltic eruptive centers of the Snake River Plain (SRP) as the North American Plate drifted westwards. We have generated the first Hf isotopic data on basalts in this large igneous province along with bulk-rock trace-element abundance patterns and Sr-Nd isotopic ratios to assess the hypothesized kinship with co-located rhyolites throughout the hotspot track. The isotopic compositions for all three elements span much of the compositional range observed in mantle-derived basalts worldwide. Varying between a high of +11.3 in parts of the CRB and a low of −8.3 in some of the late SRP basalts, the new εHf values are geographically controlled, similar to the shift first documented with Sr and Nd isotopes across the “0.706 Sr line,” which marks the boundary between thin- accreted young ocean lithosphere and arc terranes in the west to thick, old cratonized crust in the east. This observation shows that there is significant lithospheric control on the isotopic compositions of both basalts and rhyolites associated with the Yellowstone hotspot track.Modeling of assimilation and fractional crystallization (AFC) processes in seven basaltic lava flows filling the 2-Ma Big Bend Ridge Caldera at Mesa Falls, Idaho, near the eastern end of the hotspot track suggests that the crustal contribution to the Nd and Hf concentration budgets increases systematically with time and was significant. A comparison of new and previously published bulk-rock major-oxide concentrations (e.g., the alkalis Na2O and K2O) with our previously published data for H2O, Cl, F and S in olivine-hosted melt inclusions suggests that the volatiles are sub-crustal, possibly from the lithospheric mantle, and degas extensively during magma ascent and crystallization.

Understanding the isotopic and chemical evolution of Yellowstone hot spot magmatism using magmatic-thermomechanical modeling

Large-scale melting and reworking of the crust and the upper mantle occurs in areas of mantle plume-crust interaction, generating voluminous and bimodal basaltic and silicic magmatism. Continental hot spot track magmas preserve geochemical and temporal records of these processes. Here, we apply and further develop the large scale (crust and the upper mantle) I2VIS magmatic-thermomechanical modeling code to investigate the potential origins of chemical and isotopic trends in the Yellowstone hot spot track, perhaps the best studied and constrained subcontinental mantle plume system on Earth. We model the propagation of melt via dikes in solid crust and via percolation in partially molten crust by using Lagrangian markers which also track the chemical and isotopic compositions of the melts that eventually erupt at the surface. Confirming the results of earlier geophysical and geochemical studies, we show that the eruptive activity at Yellowstone is best explained in terms of the formation of a ~15 km-thick mid-crustal mafic sill complex with its top at a depth of ~8 km that develops over a period of ~3 Myr. This sill complex releases rhyolitic fractionates and creates additional rhyolites by melting the surrounding crust, driving the voluminous rhyolitic volcanism which characterizes the Yellowstone system.We recognize three key trends in the evolution of a continental hot spot volcanic center such as Yellowstone: (1) frequent, small eruptions results in in larger erupted rhyolite volumes compared to rare and large eruptions, (2) erupted magmas tend to be initially comprised of a large degree of crustal melt, and the relative contribution of fractionates of basalts from the mantle increases with time, and (3), the initial depth of the crust which melts to produce eruptible rhyolites becomes shallower with time. The first of these trends is simply a function of the continuous supply of new basalt to the crust from the mantle. The latter trend, which is responsible for low-δ18O rhyolites, is produced by a combination of progressive melting of shallower crust as the system becomes hotter, repeated caldera collapses advecting shallow crust to a depth where it can melt, and the overplating and burial of shallow crust by repeated intrusions of basalt. The mid- and lower crust is not a significant source of erupted rhyolitic melts, as both the crustal melting and basalt fractionation takes place in the sill complex which forms near the bottom of the upper crust. The resulting modeled isotopic evolution of erupted magmas are a good match with the actual stable and radiogenic isotopic record of Yellowstone hot spot track volcanism preserved in zircon phenocrysts, and further replicates the results of recent geophysical imaging campaigns, giving us confidence that the assumptions and results of these thermomechanical models are broadly correct.

Caldera Life-Cycles of the Yellowstone Hotspot Track: Death and Rebirth of the Heise Caldera

Caldera Life-Cycles of the Yellowstone Hotspot Track: Death and Rebirth of the Heise Caldera

Anomalous mantle transition zone beneath the Yellowstone hotspot track

The origin of the Yellowstone and Snake River Plain volcanism has been strongly debated. The mantle plume model successfully explains the age-progressive volcanic track, but a deep plume structure has been absent in seismic imaging. Here I apply diffractional tomography to receiver functions recorded at USArray stations to map high-resolution topography of mantle transition-zone discontinuities. The images reveal a trail of anomalies that closely follow the surface hotspot track and correlate well with a seismic wave-speed gap in the subducting Farallon slab. This observation contradicts the plume model, which requires anomalies in the mid mantle to be confined in a narrow region directly beneath the present-day Yellowstone caldera. I propose an alternative interpretation of the Yellowstone volcanism. About 16 million years ago, a section of young slab that had broken off from a subducted spreading centre in the mantle first penetrated the 660 km discontinuity beneath Oregon and Idaho, and pulled down older stagnant slab. Slab tearing occurred along pre-existing fracture zones and propagated northeastward. This reversed-polarity subduction generated passive upwellings from the lower mantle, which ascended through a water-rich mantle transition zone to produce melting and age-progressive volcanism. The mantle transition zone in the western United States is perturbed along a path that mirrors the line of the Yellowstone hotspot track at the surface, according to analysis of tomographic data.

Origins and evolution of rhyolitic magmas in the central Snake River Plain: insights from coupled high-precision geochronology, oxygen isotope, and hafnium isotope analyses of zircon

We present new high-precision CA-ID-TIMS and in situ U–Pb ages together with Hf and O isotopic analyses (analyses performed all on the same grains) from four tuffs from the 15−10 Ma Bruneau–Jarbidge center of the Snake River Plain and from three rhyolitic units from the Kimberly borehole in the neighboring 10−6 Ma Twin Falls volcanic center. We find significant intrasample diversity in zircon ages (ranges of up to 3 Myr) and in δ18O (ranges of up to 6‰) and eHf (ranges of up to 24 e units) values. Zircon rims are also more homogeneous than the associated cores, and we show that zircon rim growth occurs faster than the resolution of in situ dating techniques. CA-ID-TIMS dating of a subset of zircon grains from the Twin Falls samples reveals complex crystallization histories spanning 104–106 years prior to some eruptions, suggesting that magma genesis was characterized by the cyclic remelting of buried volcanic rocks and intrusions associated with previous magmatic episodes. Age-dependent trends in zircon isotopic compositions show that rhyolite production in the Yellowstone hotspot track is driven by the mixing of mantle-derived melts (normal δ18O and eHf) and a combination of Precambrian basement rock (normal δ18O and eHf down to − 60) and shallow Mesozoic and Cenozoic age rocks, some of which are hydrothermally altered (to low δ18O values) by earlier stages of Snake River Plain magmatism. These crustal melts hybridize with juvenile basalts and rhyolites to produce the erupted rhyolites. We also observe that the Precambrian basement rock is only an important component in the erupted magmas in the first eruption at each caldera center, suggesting that the accumulation of new intrusions quickly builds an upper crustal intrusive body which is isolated from the Precambrian basement and evolves towards more isotopically juvenile and lower-δ18O compositions over time.

A crustal-scale integrated geophysical and tectonic study of the Snake River Plain region, northwestern U.S.A.

ABSTRACTThe Pacific Northwest region of North America is a site of very complex tectonomagmatic activity. This activity is due to subduction of the Pacific plate, the associated Cascade chain of volcanoes, micro-plate interactions, and mantle plume activity to the east of the plate margin that produced the Yellowstone hotspot track along the Eastern Snake River Plain (ESRP). A number of recent geophysical and geological studies have produced new results that have drawn attention to the complex tectonic setting of the region east of the Cascade Range, and its tectonic evolution is the subject of considerable scientific interest and debate. Numerous seismic studies have specifically focused on the crustal and upper mantle structure of the ESRP and Yellowstone area. However, crustal-scale studies of the Western Snake River Plain (WSRP) are limited. We undertook an integrated analysis of new and existing geophysical data and geologic constraints to study the crustal structure of the WSRP and generated two-dimensional crustal models across it. We observed both differences and similarities in the structural and tectonic evolution of the eastern and western arms of the SRP based on our integrated analysis. From a broader perspective based on recent geological and geophysical studies in the surrounding region, the intersection of the two arms of the SRP emerges as a major element of a complex tectonic intersection that includes the High Lava Plains of eastern Oregon, the Northern Nevada rift, a southwestern extension of the ESRP into northern Nevada, as well as, faulting and volcanism extending north-westward to connect with the Columbia River basalt plateau region. Thus, the goal of this study is to advance our understanding of the tectonomagmatic evolution of the region and to encourage further studies in the region.

Oxygen isotope evolution of the Lake Owyhee volcanic field, Oregon, and implications for the low-δ18O magmatism of the Snake River Plain–Yellowstone hotspot and other low-δ18O large igneous provinces

The Snake River Plain–Yellowstone (SRP-Y) hotspot track represents the largest known low-δ18O igneous province; however, debate persists regarding the timing and distribution of meteoric hydrothermal alteration and subsequent melting/assimilation relative to hotspot magmatism. To further constrain alteration relations for SRP-Y low-δ18O magmatism, we present in situ δ18O and U–Pb analyses of zircon, and laser fluorination δ18O analyses of phenocrysts, from the Lake Owyhee volcanic field (LOVF) of east-central Oregon. U–Pb data place LOVF magmatism between 16.3 and 15.4 Ma, and contain no evidence for xenocrystic zircon. LOVF δ18O(Zrc) values demonstrate (1) both low-δ18O and high-δ18O caldera-forming and pre-/post-caldera magmas, (2) relative increases in δ18O between low-δ18O caldera-forming and post-caldera units, and (3) low-δ18O magmatism associated with extension of the Oregon-Idaho Graben. The new data, along with new compilations of (1) in situ zircon δ18O data for the SRP-Y, and (2) regional δ18O(WR) and δ18O(magma) patterns, further constrain the thermal and structural associations for hydrothermal alteration in the SRP-Y. Models for low-δ18O magmatism must be compatible with (1) δ18O(magma) trends within individual SRP-Y eruptive centers, (2) along axis trends in δ18O(magma), and (3) the high concentration of low-δ18O magmas relative to the surrounding regions. When considered with the structural and thermal evolution of the SRP-Y, these constraints support low-δ18O magma genesis originating from syn-hotspot meteoric hydrothermal alteration, driven by hotspot-derived thermal fluxes superimposed on extensional tectonics. This model is not restricted to continental hotspot settings and may apply to several other low-δ18O igneous provinces with similar thermal and structural associations.

Rheomorphic ignimbrites of the Rogerson Formation, central Snake River plain, USA: record of mid-Miocene rhyolitic explosive eruptions and associated crustal subsidence along the Yellowstone hotspot track

Rogerson Graben, USA, is critically placed at the intersection between the Yellowstone hotspot track and the southern projection of the west Snake River rift. Eleven rhyolitic members of the re-defined, ≥420-m-thick, Rogerson Formation record voluminous high-temperature explosive eruptions, emplacing extensive ashfall and rheomorphic ignimbrite sheets. Yet, each member has subtly distinct field, chemical and palaeomagnetic characteristics. New regional correlations reveal that the Brown’s View ignimbrite covers ≥3300 km2, and the Wooden Shoe ignimbrite covers ≥4400 km2 and extends into Nevada. Between 11.9 and ∼8 Ma, the average frequency of large explosive eruptions in this region was 1 per 354 ky, about twice that at Yellowstone. The chemistry and mineralogy of the early rhyolites show increasing maturity with time possibly by progressive fractional crystallisation. This was followed by a trend towards less-evolved rhyolites that may record melting and hybridisation of a mid-crustal source region. Contemporaneous magmatism-induced crustal subsidence of the central Snake River Basin is recorded by successive ignimbrites offlapping and thinning up the N-facing limb of a regional basin-margin monocline, which developed between 10.59 and 8 Ma. The syn-volcanic basin topography contrasted significantly with the present-day elevated Yellowstone hotspot plateau. Concurrent basin-and-range extension produced the N-trending Rogerson Graben: early uplift of the Shoshone Hills (≥10.34 Ma) was followed by initiation of the Shoshone Fault and an E-sloping half-graben (∼10.3–10.1 Ma). The graben asymmetry then reversed with initiation of the Brown’s Bench Fault (≥8 Ma), which remained intermittently active until the Pliocene.

Mid-Miocene record of large-scale Snake River–type explosive volcanism and associated subsidence on the Yellowstone hotspot track: The Cassia Formation of Idaho, USA

The 1.95-km-thick Cassia Formation, defined in the Cassia Hills at the southern margin of the Snake River Plain, Idaho, consists of 12 refined and newly described rhyolitic members, each with distinctive field, geochemical, mineralogical, geochronological, and paleomagnetic characteristics. It records voluminous high-temperature, Snake River−type explosive eruptions between ca. 11.3 Ma and ca. 8.1 Ma that emplaced intensely welded rheomorphic ignimbrites and associated ash-fall layers. One ignimbrite records the ca. 8.1 Ma Castleford Crossing eruption, which was of supereruption magnitude (∼1900 km 3 ). It correlates regionally and exceeds 1.35 km thickness within a subsided, proximal caldera-like depocenter. Major- and trace-element data define three successive temporal trends toward less-evolved rhyolitic compositions, separated by abrupt returns to more-evolved compositions. These cycles are thought to reflect increasing mantle-derived basaltic intraplating and hybridization of a midcrustal region, coupled with shallower fractionation in upper-crustal magma reservoirs. The onset of each new cycle is thought to record renewed intraplating at an adjacent region of crust, possibly as the North American plate migrated westward over the Yellowstone hotspot. A regional NE-trending monocline, here termed the Cassia monocline, was formed by synvolcanic deformation and subsidence of the intracontinental Snake River basin. Its structural and topographic evolution is reconstructed using thickness variations, offlap relations, and rheomorphic transport indicators in the successive dated ignimbrites. The subsidence is thought to have occurred in response to incremental loading and modification of the crust by the mantle-derived basaltic magmas. During this time, the area also underwent NW-trending faulting related to opening of the western Snake River rift and E-W Basin and Range extension. The large eruptions probably had different source locations, all within the subsiding basin. The proximal Miocene topography was thus in marked contrast to the more elevated present-day Yellowstone plateau.

Initiation of large-volume silicic centers in the Yellowstone hotspot track: insights from H2O- and F-rich quartz-hosted rhyolitic melt inclusions in the Arbon Valley Tuff of the Snake River Plain

During the onset of caldera cluster volcanism at a new location in the Snake River Plain (SRP), there is an increase in basalt fluxing into the crust and diverse silicic volcanic products are generated. The SRP contains abundant and compositionally diverse hot, dry, and often low-δ18O silicic volcanic rocks produced through time during the formation of individual caldera clusters, but more H2O-rich eruptive products are rare. We report analyses of quartz-hosted melt inclusions from pumice clasts from the upper and lower Arbon Valley Tuff (AVT) to gain insight into the initiation of caldera cluster volcanism. The AVT, a voluminous, caldera-forming rhyolite, represents the commencement of volcanism (10.44 Ma) at the Picabo volcanic field of the Yellowstone hotspot track. This is a normal δ18O rhyolite consisting of early and late erupted members (lower and upper AVT, respectively) with extremely radiogenic Sr isotopes and unradiogenic Nd isotopes, requiring that ~50 % of the mass of these elements is derived from melts of Archean upper crust. Our data reveal distinctive features of the early erupted lower AVT melt including: variable F concentrations up to 1.4 wt%, homogenous and low Cl concentrations (~0.08 wt%), H2O contents ranging from 2.3 to 6.4 wt%, CO2 contents ranging from 79 to 410 ppm, and enrichment of incompatible elements compared to the late erupted AVT, subsequent Picabo rhyolites, SRP rhyolites, and melt inclusions from other metaluminous rhyolites (e.g., Bishop Tuff, Mesa Falls Tuff). We couple melt inclusion data with Ti measurements and cathodoluminescence (CL) imaging of the host quartz phenocrysts to elucidate the petrogenetic evolution of the AVT rhyolitic magma. We observe complex and multistage CL zoning patterns, the most critical being multiple truncations indicative of several dissolution–reprecipitation episodes with bright CL cores (higher Ti) and occasional bright CL rims (higher Ti). We interpret the high H2O, F, F/Cl, and incompatible trace element concentrations in the context of a model involving melting of Archean crust and mixing of the crustal melt with basaltic differentiates, followed by multiple stages of fractional crystallization, remelting, and melt extraction. This multistage process, which we refer to as distillation, is further supported by the complex CL zoning patterns in quartz. We interpret new Δ18O(Qz-Mt) isotope measurements, demonstrating a 0.4 ‰ or ~180 °C temperature difference, and strong Sr isotopic and chemical differences between the upper and lower AVT to represent two separate eruptions. Similarities between the AVT and the first caldera-forming eruptions of other caldera clusters in the SRP (Yellowstone, Heise and Bruneau Jarbidge) suggest that the more evolved, lower-temperature, more H2O-rich rhyolites of the SRP are important in the initiation of a caldera cluster during the onset of plume impingement.

Insights for the melt migration, the volcanic activity and the ultrafast lithosphere delamination related to the Yellowstone plume (Western USA)

The Yellowstone–East Snake River Plain hotspot track has been intensely studied since several decades and is widely considered to result from the interaction of a mantle plume with the North American plate. An integrated conclusive geodynamic interpretation of this extensive data set is however presently still lacking, and our knowledge of the dynamical processes beneath Yellowstone is patchy. It has been argued that the Yellowstone plume has delaminated the lower part of the thick Wyoming cratonic lithosphere. We derive an original dynamic model to quantify delamination processes related to mantle plume–lithosphere interactions. We show that fast (∼300 ka) lithospheric delamination is consistent with the observed timing of formation of successive volcanic centres along the Yellowstone hotspot track and requires (i) a tensile stress regime within the whole lithosphere exceeding its failure threshold, (ii) a purely plastic rheology in the lithosphere when stresses reach this yield limit, (iii) a dense lower part of the 200 km thick Wyoming lithosphere and (iv) a decoupling melt horizon inside the median part of the lithosphere. We demonstrate that all these conditions are verified and that ∼150 km large and ∼100 km thick lithospheric blocks delaminate within 300 ka when the Yellowstone plume ponded below the 200 km thick Wyoming cratonic lithosphere. Furthermore, we take advantage of the available extensive regional geophysical and geological observation data sets to design a numerical 3-D upper-mantle convective model. We propose a map of the ascending convective sheets contouring the Yellowstone plume. The model further evidences the development of a counter-flow within the lower part of the lithosphere centred just above the Yellowstone mantle plume axis. This counter-flow controls the local lithospheric stress field, and as a result the trajectories of feeder dykes linking the partial melting source within the core of the mantle plume with the crust by crosscutting the lithospheric mantle. This counter-flow further explains the 50 km NE shift observed between the mantle plume axis and the present-day Yellowstone Caldera as well as the peculiar shaped crustal magma chambers.

Crustal magmatism and lithospheric geothermal state of western North America and their implications for a magnetic mantle

The western North American lithosphere experienced extensive magmatism and large-scale crustal deformation due to the interactions between the Farallon and North American plates. To further understand such subduction-related dynamic processes, we characterize crustal structure, magmatism and lithospheric thermal state of western North America based on various data processing and interpretation of gravimetric, magnetic and surface heat flow data. A fractal exponent of 2.5 for the 3D magnetization model is used in the Curie-point depth inversion. Curie depths are mostly small to the north of the Yellowstone–Snake River Plain hotspot track, including the Steens Mountain and McDermitt caldera that are the incipient eruption locations of the Columbia River Basalts and Yellowstone hotspot track. To the south of the Yellowstone hotspot track, larger Curie depths are found in the Great Basin. The distinct Curie depths across the Yellowstone–Snake River Plain hotspot track can be attributed to subduction-related magmatism induced by edge flow around fractured slabs. Curie depths confirm that the Great Valley ophiolite is underlain by the Sierra Nevada batholith, which can extend further west to the California Coast Range. The Curie depths, thermal lithospheric thickness and surface heat flow together define the western edge of the North American craton near the Roberts Mountains Thrust (RMT). To the east of the RMT, large Curie depths, large thermal lithospheric thickness, and low thermal gradient are found. From the differences between Curie-point and Moho depth, we argue that the uppermost mantle in the oceanic region is serpentinized. The low temperature gradients beneath the eastern Great Basin, Montana and Wyoming permit magnetic uppermost mantle, either by serpentinization/metasomatism or in-situ magnetization, which can contribute to long-wavelength and low-amplitude magnetic anomalies and thereby large Curie-point depths.

Crustal-scale recycling in caldera complexes and rift zones along the Yellowstone hotspot track: O and Hf isotopic evidence in diverse zircons from voluminous rhyolites of the Picabo volcanic field, Idaho

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 10.44±0.27 Ma (U–Pb) Tuff of Arbon Valley (TAV), a chemically zoned and normal-δ18O (δ18O magma=7.9‰) unit with high, zoned 87Sr/86Sri (0.71488–0.72520), and low-εNd(0) (−18) and εHf(0) (−28). The TAV and an associated post caldera lava flow possess the lowest εNd(0) (−23), indicating ∼40–60% derivation from the Archean upper crust. Normal-δ18O rhyolites were followed by a series of lower-δ18O eruptions with more typical (lower crustal) Sr–Nd–Hf isotope ratios and whole rock chemistry. The voluminous 8.25±0.26 Ma West Pocatello rhyolite has the lowest δ18O value (δ18Omelt=3.3‰), 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-δ18O zircon values). The significant (4–5‰) decrease in magmatic-δ18O values in Picabo rhyolites is accompanied by an increase in zircon δ18O heterogeneity from ∼1‰ variation in the TAV to >5‰ variation in the late-stage low-δ18O 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-δ18O 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-δ18O alteration may be related to: (1) deeply buried and unexposed older deposits of Picabo-age or Twin Falls-age low-δ18O 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-δ18O 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.

Upper mantle seismic structure beneath the Pacific Northwest: A plume-triggered delamination origin for the Columbia River flood basalt eruptions

We invert teleseismic P and S body waves constrained by an ambient-noise surface wave model and Moho depth inferred from receiver function analysis (Gao et al., 2011) to image mantle structures continuously from the surface to the base of the upper mantle. The major structures coincide with prominent geological features. We focus on a NE-Oregon structure, termed here the Wallowa anomaly, which coincides with the source area for the ∼16Ma Columbia River flood basalt eruptions and a circular area of topographic relief created during and after these eruptions. Resolution tests indicate that the curtain-like structure previously interpreted as Farallon lithosphere connects with the Wallowa anomaly above 150km along the northeast margin of the Wallowa anomaly. This connection, along with the pre-flood basalt magmatic and tectonic history of the Pacific Northwest, lead us to conclude that arrival of the Yellowstone plume to south-central Oregon initiated delamination of remnant Farallon lithosphere from the base of NE Oregon, exposing ocean crust to Yellowstone asthenosphere. This hypothesis accounts for the propagation of flood basalt volcanism far north of the Yellowstone hotspot track, and for the high-silica composition of most of the flood basalt magmas.

Crust and upper mantle electrical conductivity beneath the Yellowstone Hotspot Track

Combining long-period magnetotelluric data from the spatially uniform EarthScope USArray and higher-resolution profiles, we obtain a regional three-dimensional electrical resistivity model in the Snake River Plain and Yellowstone areas (Idaho and Wyoming, United States), and provide new constraints on the large-scale distribution of melt and fluids beneath the Yellowstone hotspot track. Contrary to what would be expected from standard mantle plume models, the electromagnetic data suggest that there is little or no melt in the lower crust and upper mantle directly beneath Yellowstone caldera. Instead, low mantle resistivities (10 Ωm and below), which we infer to result from 1%–3% partial melt, are found 40–80 km beneath the eastern Snake River Plain, extending at least 200 km southwest of the caldera, beneath the area of modern basaltic magmatism. The reduced resistivities extend upward into the mid-crust primarily around the edges of the Snake River Plain, suggesting upward migration of melt and/or fluid is concentrated in these areas. The anomaly also shallows toward Yellowstone, where higher temperatures enhance permeability and allow melts to ascend into the crust. The top of the conductive layer is at its shallowest, in the upper crust, directly beneath the modern Yellowstone supervolcano.

Slab fragmentation, edge flow and the origin of the Yellowstone hotspot track

The Snake River Plain/Yellowstone (SRP/Y) volcanic province is widely considered a classic example of a plume generated continental hotspot. Here we present new S-wave and P-wave tomographic images that suggest an alternative subduction-related process by which volcanism along the SRP/Y hotspot track results from slab fragmentation, trench retreat, and mantle upwelling at the tip and around the truncated edges of the descending plate. Our seismic images of the upper mantle in the depth range 300–600 km show that the subducted oceanic plate extends locally eastward well into the mantle beneath stable North America. The break-up and along-strike fragmentation of the descending plate is related in both time and space to the onset of flood volcanism and the formation of the SRP/Y hotspot. A sub-horizontal branch of the subducting oceanic plate, orphaned from the descending plate by the northward migration of the Mendocino triple junction, resides in the mantle transition zone (400–600 km) directly beneath the SRP/Y track. Its truncated northern edge is parallel to the northwestern margin of the hotspot track and marks the southern edge of a slab gap. A number of recent numerical and physical tank model studies suggest that a rapidly retreating and severely fragmented downgoing plate drives mantle flow around both the tip and the edges of the descending slab. Our seismic results show that the morphology of the subducting slab is appropriate for generating large-scale poloidal flow (producing flood volcanism) in the upper mantle during the re-initiation phase of slab descent ca 20 Ma and for generating smaller-scale toroidal and poloidal upwellings (producing hotspot volcanism) around both the leading tip and northern edge of the slab as it descends into the deeper upper mantle. Plate reconstructions are consistent with the timing and position of both flood and hotspot volcanism.