Matthew E. Brueseke

Mid-Miocene rhyolite volcanism in northeastern Nevada: The Jarbidge Rhyolite and its relationship to the Cenozoic evolution of the northern Great Basin (USA)

We present new physical, geochemical, geochronologic, and oxygen isotope constraints on the mid-Miocene Jarbidge Rhyolite in northeastern Nevada (USA), providing new constraints on the tectonomagmatic evolution of the Cenozoic northern Great Basin. Widespread extension due to rapid collapse of the Nevadaplano began at ca. 17–16 Ma across the northern Great Basin. Coeval with this event was compositionally bimodal basalt-rhyolite volcanism that is often attributed to the inception of the Yellowstone hotspot. The most widespread mid-Miocene volcanic units in northeastern Nevada are lavas and domes of the Jarbidge Rhyolite. The thickest and most areally extensive exposures of these lavas include, and are found just west of, the Jarbidge Mountains, Nevada. This study focuses on Jarbidge Rhyolite directly south of the central Snake River Plain, adjacent to the thickest exposures in the vicinity of Jarbidge, Nevada. Textures on a range of scales indicate that the Jarbidge Rhyolite consists primarily of phenocryst-rich lavas. Laser 40 Ar/ 39 Ar ages for sanidine are consistent with effusive eruption of metaluminous to slightly peraluminous ferroan calc-alkalic rhyolite from 16.1 to 15.0 Ma; prior K-Ar ages suggest that some activity occurred over a slightly longer duration. Major and trace element data, coupled with new stable and prior radiogenic isotope measurements, suggest that Jarbidge Rhyolite magmas formed primarily via melting of quartzofeldspathic crust. The Jarbidge Rhyolite lavas are geochemically dissimilar from younger Snake River Plain rhyolites (e.g., lower MgO, lower Nb, higher Rb/Nb) and are more similar to coeval rhyolites erupted to the west on or adjacent to the Oregon Plateau. The distribution of the Jarbidge Rhyolite lavas in northeastern Nevada reflects an intimate association with temporally and spatially coincident extension rather than the Yellowstone hotspot.

New isotopic evidence bearing on bonanza (Au-Ag) epithermal ore-forming processes

New Cu, S, and Pb isotope data provide evidence for a magmatic source of metal(loid)s and sulfur in epithermal Au-Ag deposits even though their ore-forming solutions are composed primarily of heated meteoric (ground) waters. The apparent isotopic discrepancy between ore metals and ore-forming solutions, and even between the ore and associated gangue minerals, indicates two different sources of epithermal ore-forming constituents: (1) a shallow geothermal system that not only provides the bulk of water for the ore-forming solutions but also major chemical constituents leached from host rocks (silica, aluminum, potassium, sodium, calcium) to make gangue minerals and (2) metals and metalloids (As, Te, Sb, etc.) and sulfur (±Se) derived from deeper magma bodies. Isotopic data are consistent with either vapor-phase transport of metal(loids) and sulfur and their subsequent absorption by shallow geothermal waters or formation of metallic (Au, Ag, Cu phases) nanoparticles at depth from magmatic fluids prior to encountering the geothermal system. The latter is most consistent with ore textures that indicate physical transport and aggregation of nanoparticles were significant ore-forming processes. The recognition that epithermal Au-Ag ores form in tectonic settings that produce magmas capable of releasing metal-rich fluids necessary to form these deposits can refine exploration strategies that previously often have focused on locating fossil geothermal systems.

The leading wisps of Yellowstone: Post–ca. 5 Ma extension-related magmatism in the upper Wind River Basin, Wyoming (USA), associated with the Yellowstone hotspot tectonic parabola

The upper Wind River Basin in northwest Wyoming (USA) is located ~80– 100 km southeast of the Yellowstone Plateau volcanic field. While the upper Wind River Basin is a manifestation of primarily Cretaceous to Eocene Laramide tectonics, younger events have played a role in its formation, including Eocene Absaroka volcanism, Cenozoic lithospheric extension, and the migration of the North American plate over the Yellowstone hotspot tail. New 40Ar/39Ar ages coupled with existing K-Ar results from intrusives and lavas in the upper Wind River Basin show that igneous activity younger than ca. 5 Ma occurred locally. Field and geochemical data show that these <ca. 5 Ma upper Wind River Basin magmas were either erupted or emplaced along normal fault zones at different locations and range in composition from tholeiitic basalt (Spring Mountain) to calc-alkaline basaltic andesite through dacite (Lava Mountain, Crescent Mountain, and Wildcat Hill), and include a lamprophyre intrusion (Pilot Knob). Together, these igneous rocks define the Upper Wind River Basin volcanic field (UWRB). All UWRB rocks have large ion lithophile element enrichments, high field strength element depletions, and other geochemical characteristics associated with subduction and that are identical to those of the Miocene Jackson Hole volcanics, even though the former erupted in an intraplate setting. Our results suggest that UWRB magmatism, as well as the Jackson Hole volcanics and other small-volume, similarly aged intermediate to felsic magmatism in eastern Idaho, are the result of the interaction between the North American plate and the progression of the tectonic parabola associated with the Yellowstone hotspot tail.

Environmental and Track Factors That Contribute to Abrasion Damage

Sites with known occurrences of mud pumping or other track concerns were investigated to determine the prevalence of concrete bottom tie abrasion and environmental and track conditions that could contribute to its occurrence. Field investigations showed that it occurs in diverse geographic locations around the U.S. and is a source of continued maintenance concern for railroads. Water appeared to be a significant factor involved in concrete bottom tie abrasion. Ballast fouling, center-binding cracking, rail surface profile variations, and large track movement during loading was seen in locations with concrete bottom tie abrasion. Bumps or track stiffness changes were often found at locations of abrasion damage. Specifically, some locations with known stiff track conditions exhibited significant abrasion damage. INTRODUCTION Concrete railroad ties are being more frequently used in the railroad industry. Railroad ties are used to transmit loads from the train and rail to the subgrade and also to hold rail gage. Concrete ties are used in heavy-haul rail lines and high-speed rail lines because of their ability to carry large, repeated loads for a very long time. Concrete ties can last 50 years or longer when fabricated properly and the track is properly designed, built, and maintained. In order to achieve this life span, prestressed concrete tie thickness and prestressing forces are fabricated to resist design positive and negative bending moments. These design criteria are meant to prevent excessive deflections and gage widening during train loading and prevent ties from failing through breakage. If the tie section properties change during use, there is a potential for a loss in moment capacity, gage widening, tie breakage, and ultimately derailment [1]. Abrasion loss on the concrete tie sides and bottom could provide such a moment capacity reduction. On July 18, 2013, 10 cars on a northbound train on the Metro-North Hudson Line in the Bronx, NY containing municipal solid waste derailed, causing

Diverse mid-Miocene silicic volcanism associated with the Yellowstone-Newberry thermal anomaly

827,700 in damage [2] [3] [4]. At the location of derailment, the ballast was severely fouled with gray mud. The gray mud was mostly from ground up concrete fines from concrete ties that had lost section on the bottom. The ties also had center-binding cracking from high negative moments in the tie center that occurred or were