Eugene I. Smith

A mantle melting profile across the Basin and Range, SW USA

This is the published version. Copyright 2002 American Geophysical Union. All Rights Reserved.

Polygenetic Quaternary volcanism at Crater Flat, Nevada

Abstract Alkali basalts erupted during the Quaternary at Crater Flat, Nevada, record a complex history of polycyclic and polygenetic volcanism. Magmas from the two main centers (Black Cone and Red Cone) are petrographically and geochemically similar, although field evidence suggests a number of separate eruptive events. High incompatible element concentrations, low Nb/La and high Zr/Y indicate that the magmas were derived by small degrees of partial melting from the lithospheric mantle. At Red Cone, a significant range of Sr, La, Ce, Ba and Th concentrations is observed with time (e.g., Sr range 1308–1848 ppm): the youngest samples having the more elevated values. However, there is only limited variation in the compatible trace elements (e.g., Sc and Ni). The array of compositions at Red Cone could not have been produced by changes in the degree of partial melting, or by fractional crystallization. Rather, a model of magma mixing is proposed between relatively enriched and depleted end-members. The cluster of Black Cone data falls consistently at the least-enriched end of the Red Cone sample arrays, suggesting that the Black Cone magma represents one of the mixing end-members. The modeling indicates that the magmatic plumbing systems of the two centers were linked, at least during the early stages of volcanism. Moreover, volcanic activity may have occurred at a number of sites along the length of the magmatic feeder zone during a single eruptive phase. This could have significant implications for volcanic hazard assessment in the region around Yucca Mountain, and the proposed nuclear waste repository.

The role of the mantle during crustal extension: Constraints from geochemistry of volcanic rocks in the Lake Mead area, Nevada and Arizona

One of the fundamental questions in areas of large-magnitude extension and magmatism is the role of the mantle in the extension process. The Lake Mead area is ideally suited for developing models that link crustal and mantle processes because it contains both mantle and crustal boundaries and it was the site of large-magnitude crustal extension and magmatism during Miocene time. In the Lake Mead area, the boundary between the amagmatic zone and the northern Colorado River extensional corridor parallels the Lake Mead fault system and is situated just to the north of Lake Mead. This boundary formed between 11 and 6 Ma during, and just following, the peak of extension and corresponds to a contact between two mantle domains. During thinning and replacement of the lithospheric mantle in the northern Colorado River extensional corridor, the lithospheric mantle in the amagmatic zone remained intact. Contrasting behavior to the north and south of this boundary may have produced the mantle domain boundary. The domain to the north of the boundary is characterized by mafic lavas with a lithospheric mantle isotopic and geochemical signature (ϵ Nd = -3 to -9; 87 Sr/ 86 Sr = 0.706-0.707). To the south of the boundary in the northern Colorado River extensional corridor, lavas have an ocean island basalt (OIB)-mantle signature and appear to have only a minor lithospheric mantle component in their source (ϵ Nd = 0 to +4; 87 Sr/ 86 Sr = 0.703-0.705). Mafic lavas of the northern Colorado River extensional corridor represent the melting of a complex and variable mixture of asthenospheric mantle, lithospheric mantle, and crust. Pliocene alkali basalt magmas of the Fortification Hill field represent the melting of a source composed of a mixture of asthenospheric mantle, high U/Pb (HIMU)-like mantle, and lithospheric mantle. Depth of melting of alkali basalt magmas remained relatively constant from 12 to 6 Ma during, and just after, the peak of extension but probably increased between 6 and 4.3 Ma following extension. Miocene and Pliocene low ϵ Nd and high 87 Sr/ 86 Sr magmas and tholeiites at Malpais Flattop were derived from a lithospheric mantle source and were contaminated as they passed through the crust. The shift in isotopic values due to crustal interaction is no more than 4 units in ϵ Nd and 0.002 in 87 Sr/ 86 Sr and does not mask the character of the mantle source. The change in source of basalts from lithospheric mantle to asthenospheric mantle with time, the OIB character of the mafic lavas, and the HIMU-like mantle component in the source are compatible with the presence of rising asthenosphere, as an upwelling convective cell, or plume beneath the northern Colorado River extensional corridor during extension. The Lake Mead fault system, a major crustal shear zone, parallels the mantle domain boundary. The Lake Mead fault system may locally represent the crustal manifestation of differential thinning of the lithospheric mantle.

Evolution of a mafic volcanic field in the central Great Basin, south central Nevada

Evolution of a mafic volcanic field is investigated through a study of Pliocene age rocks in the Reveille Range in south central Nevada. Pliocene activity began with the eruption of relatively abundant hawaiite (episode 1, 5-6 Ma), which was followed by trachytic volcanism (4.3 Ma) and by a second episode of lower-volume hawaiite and basanite (episode 2, 3.0-4.7 Ma). Incompatible elements indicate an asthenospheric source. Isotopically, episode 2 basalts cluster around 87Sr/86Sr=0.7035 and œNd=+4.2, but episode 1 samples vary to high 87Sr/86Sr (up to 0.7060) over a narrow range of end (+0.8 to +4.5). Trachytic rocks (MgO--0.5%) are isotopically akin to the episode 1 basalts. Geochemical variation requires the addition of a crustal component (high 87Sr/86Sr, Sr/Nd, Pb/La, low œNd) to the episode 1 hawaiites and trachytic samples, probably by assimilation of carbonate-rich sedimentary wall rock. The volcanic field developed in at least two eruptive cycles of approximately equal duration. Basanites (deeper and lower percentage melts) appear only in the younger episode. Eruptive episodes were apparently linked to separate melting events in the mantle. Through time, basalts were produced in diminishing volumes by lower percentage melting, magma generation and storage was at greater depths, and magma ascent was at higher velocities. Spatially, the melting anomalies were large in the Pliocene but progressively diminished in size so that by Pleistocene time, volcanism was restricted to a small area near the northern end of the initial outbreak.

Eruptive probability calculation for the Yucca Mountain site, USA: statistical estimation of recurrence rates

Investigations are currently underway to evaluate the impact of potentially adverse conditions (e.g. volcanism, faulting, seismicity) on the waste-isolation capability of the proposed nuclear waste repository at Yucca Mountain, Nevada, USA. This paper is the first in a series that will examine the probability of disruption of the Yucca Mountain site by volcanic eruption. In it, we discuss three estimating techniques for determining the recurrence rate of volcanic eruption (λ), an important parameter in the Poisson probability model. The first method is based on the number of events occurring over a certain observation period, the second is based on repose times, and the final is based on magma volume. All three require knowledge of the total number of eruptions in the Yucca Mountain area during the observation period (E). Following this discussion we then propose an estimate of E which takes into account the possibility of polygenetic and polycyclic volcanism at all the volcanic centers near the Yucca Mountain site.

Structural and geochemical constraints on the reassembly of disrupted mid-Miocene volcanoes in the Lake Mead-Eldorado Valley area of southern Nevada

In the Lake Mead-Eldorado Valley (LMEV) area of southern Nevada, mid-Tertiary volcanic and plutonic rocks in the River, McCullough, and Eldorado mountains lie in the upper plate of a regional detachment structure. The detachment structure and strike-slip faults of the Lake Mead fault zone are temporally and kinematically related. Strike-slip systems and normal faults (Eldorado Valley fault) serve as boundaries between regions of variable extension in the upper plate of this detachment. Geochemical correlation and geometric reconstructions suggest that prior to extension, the LMEV area was characterized by three stratovolcano complexes, each above or adjacent to a chemically correlative pluton. Geochemical correlation techniques are useful tools that may have general application in reconstructing structurally disrupted volcanic-plutonic terranes.

Non-hotspot volcano chains produced by migration of shear-driven upwelling toward the East Pacific Rise

While most oceanic volcanism is associated with the passive rise of hot mantle beneath the spreading axes of mid-ocean ridges (MOR), volcanism occurring off-axis reflects intraplate upper-mantle dynamics and composition, yet is poorly understood. Off the south East Pacific Rise (SEPR), volcanism along the Pukapuka, Hotu-Matua, and Sojourn ridges has been attributed to various mechanisms, but none can reconcile its spatial, temporal, and geochemical characteristics. Our three-dimensional numerical models show that asthenospheric shear can excite upwelling and decompression melting at the tip of low-viscosity fingers that are propelled eastward by vigorous sublithospheric flow. This shear-driven upwelling is able to sustain intraplate volcanism that progresses toward the MOR, spreads laterally close to the axis, and weakly continues on the opposite plate. These predictions can explain the anomalously fast eastward progression of volcanism, and its spatial distribution near the SEPR. Moreover, for a heterogeneous mantle source involving a fertile component, the predicted systematics of volcanism can explain the geochemical trend along Pukapuka and the enriched anomaly of SEPR mid-oceanic ridge basalt at 16°–20.5°S. Our study highlights the role of horizontal asthenospheric flow and mantle heterogeneity in producing linear chains of intraplate volcanism independent of a (deep-rooted) buoyancy source.

Systematics of xenocrystic contamination: preservation of discrete feldspar populations at McCullough Pass Caldera revealed by 40Ar/39Ar dating

Single crystal 40Ar/39Ar dating of K-feldspars from silicic volcanic rocks containing xenocrysts often yields a spectrum of ages slightly older than those of juvenile sanidine phenocrysts. In contrast, feldspars from thin, low-volume units of the Tertiary (14 Ma) McCullough Pass Tuff define discrete age populations at ∼14 Ma, ∼15 Ma, and ∼1.3 Ga, reflecting the time of eruption, xenocrysts from an older ignimbrite exposed in the caldera wall, and Proterozoic basement K-feldspars, respectively. Conductive cooling and diffusion modelling suggests preservation of such discrete populations is likely only when xenocrystic material is incorporated into the magma very near or at the surface, or is engulfed in thin, rapidly cooled pyroclastic flows during emplacement. Incorporation of xenocrysts into the subvolcanic magma chamber, into thick rhyolite domes or lava flows, or into large, welded ignimbrite sheets will result in partial or total resetting of the K/Ar isotopic system. Similarly, petrographic evidence such as exsolution lamellae may be homogenized under these conditions but not in thin ignimbrites. Extremely low diffusion rates for disordering of the Al–Si tetrahedral siting of basement feldspars suggests that they will retain their ordered structural state given rhyolitic magma temperatures. Thus, even when petrographic and K/Ar isotopic evidence for xenocrystic contamination is obscured, it may be preserved in the form of Al–Si ordering.

Volcanic Hazard Assessment Incorporating Expert Knowledge: Application to the Yucca Mountain Region, Nevada, USA

Multiple-expert hazard/risk assessments have considerable precedent, particularly in the Yucca Mountain site characterization studies. A certain amount of expert knowledge is needed to interpret the geological data used in a probabilistic data analysis. As may be the situation in science, experts disagree on crucial points. Consequently, lack of consensus in some studies is a sure outcome. In this paper, we present a Bayesian approach to statistical modeling in volcanic hazard assessment for the Yucca Mountain site. Specifically, we show that the expert opinion on the site disruption parameterp is incorporated into the prior distribution, π(p), based on geological information that is available. Moreover, π(p) can combine all available geological information motivated by conflicting but realistic arguments (e.g., simulation, cluster analysis, structural control, ..., etc.). The incorporated uncertainties about the probability of repository disruptionpeventually will be averaged out by taking the expectation over π(p). We use the following priors in the analysis: (1) priors selected for mathematical convenience: Beta (r,s) for (r,s) = (2, 2), (3, 3), (5, 5), (2, 1),(2, 8), (8, 2), and (1, 1);and (2) three priors motivated by expert knowledge. Sensitivity analysis is performed for each prior distribution. Our study concludes that estimated values of hazard based on the priors selected for mathematical simplicity are uniformly higher than those obtained based on the priors motivated by expert knowledge. And, the model using the prior, Beta (8, 2), yields the highest hazard (=2.97 × 10-2. The minimum hazard is produced by the “three-expert prior” (i.e., values ofpare equally likely forp = 10-3, 10-2,and 10-1. The estimate of the hazard is 1.39 × 10-3, which is only about one order of magnitude smaller than the maximum value. The term, “hazard, ” is defined as the probability of at least one disruption of a repository at the Yucca Mountain site by basaltic volcanism for the next 10,000 years.

Intraplate volcanism at the edges of the Colorado Plateau sustained by a combination of triggered edge-driven convection and shear-driven upwelling

Although volcanism in the southwestern United States has been studied extensively, its origin remains controversial. Various mechanisms such as mantle plumes, upwelling in response to slab sinking, and small-scale convective processes have been proposed, but have not been evaluated within the context of rapidly shearing asthenosphere that is thought to underlie this region. Using geodynamic models that include this shear, we here explore spatiotemporal patterns of mantle melting and volcanism near the Colorado Plateau. We show that the presence of viscosity heterogeneity within an environment of asthenospheric shearing can give rise to decompression melting along the margins of the Colorado Plateau. Our models indicate that eastward shear flow can advect pockets of anomalously low viscosity toward the edges of thickened lithosphere beneath the plateau, where they can induce decompression melting in two ways. First, the arrival of the pockets critically changes the effective viscosity near the plateau to trigger small-scale edge-driven convection. Second, they can excite shear-driven upwelling (SDU), in which horizontal shear flow becomes redirected upward as it is focused within the low-viscosity pocket. We find that a combination of “triggered” edge-driven convection and SDU can explain volcanism along the margins of the Colorado Plateau, its encroachment toward the plateaus southwestern edge, and the association of volcanism with slow seismic anomalies in the asthenosphere. Geographic patterns of intraplate volcanism in regions of vigorous asthenospheric shearing may thus directly mirror viscosity heterogeneity of the sublithospheric mantle.