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NanoSIMS Ion Probes Help Measure Very Sharp Concentration Gradients in Igneous Minerals

Super-eruptions are not the only type of eruption to be considered when evaluating hazards at volcanoes with protracted eruption histories, such as the Yellowstone (Wyoming), Long Valley (California), and Valles (New Mexico) calderas. There have been more than 23 effusive eruptions of rhyolite lava at Yellowstone since the last caldera-forming eruption ~640,000 years ago, all of similar or greater magnitude than the largest volcanic eruptions of the 20th century.

This study by Christy B. Till and colleagues is innovative because it is the first to use NanoSIMS ion probe measurements to document very sharp concentration gradients over very short distances in igneous minerals, which allow a calculation of the timescale between reheating and eruption for the magma body of interest.

Their results suggest that an eruption at the beginning of Yellowstone's most recent volcanic cycle was triggered within 10 months after reheating of a mostly crystallized magma reservoir following a 220,000-year period of volcanic quiescence. A similarly energetic reheating of Yellowstone's current subsurface magma bodies could end ~70,000 years of volcanic repose and lead to a future eruption over similar timescales. Fortunately, write the authors, any significant reheating event is likely to be identifiable by geophysical monitoring.

Months between rejuvenation and volcanic eruption at Yellowstone caldera, Wyoming
Christy B. Till et al., Arizona State University, Tempe, Arizona 85287, USA. Published online ahead of print on 1 July 2015;

Other recently posted GEOLOGY articles (see below) cover such topics as

  1. Giant stromatolites of the Green River Formation, Colorado, USA;
  2. The "great hydration event" beneath the Colorado Plateau; and
  3. The crustal structure of northwest Namibia.

Giant stromatolites of the Eocene Green River Formation (Colorado, USA)
Stanley M. Awramik and H. Paul Buchheim, University of California, Santa Barbara, California, USA. Published online ahead of print on 10 July 2015;

The ~50 million-year-old deposits of Green River Formation in Colorado contain the largest, columnar stromatolites (laminated structures produced by microbes) known from lake deposits. Some individual columns are over 5.5 meters tall and many are over seven meters wide. They are composed of carbonate layers (some silicified) that can be traced from the base to the top of the column and hence the stromatolites grew in water at least 5.5 m deep. The large size is due to several factors, including growth on tree stumps, the delivery of calcium-rich spring and river waters, and the growth of the stromatolites kept up with subsidence. The tree stumps provided elevated substrates as the lake flooded a woodland. The near-shore lake environment was supplied with calcium-rich waters that resulted in the precipitation of abundant calcium carbonate and enhanced stromatolite growth.

Timing of the Cenozoic "Great Hydration" event beneath the Colorado Plateau: Th-Pb dating of monazite in Navajo volcanic field metamorphic eclogite xenoliths
Daniel J. Schulze et al., University of Toronto, Mississauga, Ontario, Canada. Published online ahead of print on 10 July 2015;

The Colorado Plateau is a vast area in the southwestern United States of relatively undeformed crust surrounded by regions of intense deformation and great topographic relief. Although many studies have been undertaken on the plateau, there is no agreement on the mechanism(s) for its uplift or on the timing. Here we present evidence that the upper mantle beneath the plateau underwent a massive hydration event that caused expansion and a density decrease, resulting in the rise of the Colorado Plateau. We also dated the age of formation of the mineral monazite in samples of rock from beneath the plateau (brought to the surface in volcanic eruptions) at 28 million years before present, and present evidence that suggests that it was formed during this hydration event and thus marks the beginning of the rise of the Colorado Plateau.

Tectonic controls on fault zone flow pathways in the Rio Grande rift, New Mexico, USA
Randolph T. Williams et al., University of Wisconsin, Madison, Wisconsin 53706, USA. Published online ahead of print on 1 July 2015;

Geologists have long recognized the potential of faults in the upper crust to act as conduits for subsurface fluid flow, and the importance of such flow to petroleum geologists and hydrologists has spurred considerable research. However, basin scale controls on fault zone architecture and permeability structure remain poorly understood. Williams and colleagues utilized calcite cements in faults as a geochemical record of fluid source to evaluate tectonic controls on fluid migration through fault zones during the development of the Rio Grande rift. Their work demonstrates that extension and syntectonic sedimentation result in a predictable spatial and temporal distribution of fault zone permeability structures, resulting in flow pathways which transmit fluids from different stratigraphic levels depending on slip magnitude and basin position. As the general pattern of sedimentation and faulting observed in the Rio Grande rift is similar to most other rift basins around the world, these results provide a fundamental first step toward accurate prediction of where fault zones will serve as conduits for fault-parallel flow and where they will be barriers, constraining fluid transport pathways in extensional tectonic environments.

Gas-driven filter pressing in magmas: Insights into in-situ melt segregation from crystal mushes
Mattia Pistone et al., National Museum of Natural History, Smithsonian Institution, Washington, D.C. Published online ahead of print on 2 July 2015;

Gas-driven filter pressing is a process that allows us to explore the roots of volcanoes. It consists of buildup and subsequent release of gas pressure promoting silicic melt expulsion from gas-rich, crystal-rich magmas stalled at depth in Earth's crust. The chemical and physical conditions at which gas-driven filter pressing operates remain poorly constrained. We present novel experimental data that illustrate how the crystal content of the magma dictates the ability of gas-driven filter pressing to segregate melt. Two laboratory-synthesized, crystal-bearing (34-80% crystals) magmas of different composition (haplogranite and dacite) and water content (2.1 and 4.2 wt% in the melt) were investigated using in situ, high temperature (500-800 degrees C) synchrotron X-ray tomographic microscopy with high spatial (3-micron/pixel) and temporal resolution (~8 seconds per three-dimensional dataset). Our results show that gas-driven filter pressing is promoted in situations where magmas inflate slowly relative to buildup of pressure and expulsion of melt. Gas-driven filter pressing operates efficiently below the maximum packing of bubbles and crystals (~74%), whereas, above this threshold, magmas tend to fracture and gas escapes through fractures. These observations offer a likely explanation for the generation of crystal-poor melts that may be eruptible at active volcanoes.

Tracking the Tristan-Gough mantle plume using discrete chains of intraplate volcanic centers buried in the Walvis Ridge
John M. O'Connor and Wilfried Jokat, Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany. Published online ahead of print on 1 July 2015;

Resolving the age-distance relation of volcanism along the Walvis has important implications for understanding and modeling plate motion and the role and character of mantle flow. But tracking the location of the Tristan-Gough plume might not be practicable if most of the complex morphology of the massive Walvis Ridge is related to the proximity of the South Atlantic mid-ocean ridge. O'Connor and Jokat use the discovery of discrete chains of intraplate volcanic centers buried in the Walvis Ridge, in combination with new information about the age distance relation of volcanism, morphology and crustal structure, to distinguish between buried plume and mid-ocean ridge segments. The continuity of the age-distance relation between widely separated plume segments implies a connection to a deep or constantly moving source in the mantle. Discovering buried or disrupted plume tracks in other primary hotspot trails could improve our understanding of the relationship between plates and the deep mantle.

From symmetric necking to localized asymmetric shearing: The role of mechanical layering
Thibault Duretz and Stefan M. Schmalholz, University of Lausanne Géopolis, Lausanne, Switzerland. Published online ahead of print on 1 July 2015;

The development of localized zones of deformation in rocks is a commonly observed feature. It is also well documented that rocks are often layered. We study the deformation of layered rocks under extension using numerical models. We report two modes of deformations: (1) distributed thinning accommodated by continuous boudinage of the different layers, and (2) strain localization into shear zones cutting across the layers. In the first mode, the overall style of deformation remains symmetric whereas, in the second mode, strain localization induces layer offset and an overall asymmetric style of deformation. Our results indicate that the rheology of the rock matrix is a key parameter. While a Newtonian rheology favors distributed symmetric boudinage, a non-Newtonian rheology (power-law creep) promotes the development of asymmetric shear zones. This mode of shear localization does not require complex rheological coupling mechanisms or material softening and thus represents one the simplest mechanisms for the formation of ductile shear zones.

Crustal structure of northwest Namibia: Evidence for plume-rift-continent interaction
Trond Ryberg, Helmholtz Centre Potsdam-GFZ German Research Centre for Geosciences, Potsdam, Germany. Published online ahead of print on 1 July 2015;

The causes for the formation of Large Igneous Provinces and hotspot trails are still a matter of considerable dispute. Seismic tomography and other studies suggest that hot mantle material rising from the core-mantle boundary might play a significant role for breaking-up of continents and formation of hotspot trails. We present the first deep-seismic sounding images of the crust from the landfall area of the Walvis Ridge at the Namibian coast to constrain processes of plume-lithosphere interaction, the formation of continental flood basalts and associated intrusive rocks. Our study identified a narrow region (<100 km) of high seismic velocity anomalies in the middle and lower crust, interpreted as a massive mafic intrusion into the lower crust. Our findings question the active role of a plume during Gondwana break-up. The superposition of the northward propagating South Atlantic rift and a locally impinging plume head were equally responsible for the increased magmatic activity in the crust of NW Namibia. This led to the formation of a locally confined lower crustal magmatic body and associated feeding systems for the Etendeka flood basalts.

Fluvio-deltaic avulsions during relative sea-level fall
Edmonds et al., Indiana University, Bloomington, Indiana, USA. Published online ahead of print on 10 July 2015;

As humans continue to alter the earth, it is critical to understand how different parts of the environment will respond. For example, sea-level is changing almost everywhere and influencing coastal rivers systems. Understanding how rivers respond to changes in sea-level is important since rivers are sites of commercial, agriculture, and ecological importance. Presently, sea-level is rising on most coasts making it difficult to study examples of modern rivers that experience relative sea level fall. We studied the Goose River in Labrador because it is one of the few rivers in the world experiencing rapid relative sea level fall due to glacial rebound. We found, using a combination of field data and modeling, that as relative sea-level falls rivers move to new locations on their floodplains. This is surprising because previously such movement was thought to be associated with sea-level rise. Moreover, we find that faster rates of sea-level fall cause more river movement. Our results suggest that both relative sea-level rise and fall will induce river movement, which can be catastrophic and significantly alter the coast, cause flooding, and displace humans.

The evolution of volcanic plume morphology in short-lived eruptions
K.N. Chojnicki et al., Scripps Institution of Oceanography, La Jolla, California, USA. Published online ahead of print on 10 July 2015;

Volcanic plumes and analogue laboratory jets change shape over time. In the analogue jets, the change in shape corresponds to a change in the internal flow dynamics resulting from changes in source conditions. Based on our laboratory findings, we propose a method of using changes in volcanic plume shape to infer changes in eruption conditions for plumes from short-lived eruptions. This method offers near real-time assessment of changes in eruption conditions that may be useful for evaluating eruption hazards as an eruption evolves.

Dramatic volcanic instability revealed by InSAR
L.N. Schaefer et al., Michigan Technological University, Houghton, Michigan, USA. Published online ahead of print on 10 July 2015;

Interferometric synthetic aperture radar (InSAR) was used to measure ground deformation during explosive eruptions on 27 and 28 May 2010 at Pacaya volcano, Guatemala. Satellite imagery produced using space-borne and airborne radar data reveal ~3 m of along-slope movement of the southwest sector of the edifice during these eruptions. This is the largest measured slope instability witnessed in a single event at a volcano that did not result in a catastrophic landslide. This rapid and extreme movement is particularly concerning given the history of sector collapse and persistent activity at this volcano. These findings emphasize the utility of high-resolution InSAR measurements for monitoring deformation and potential catastrophic slope instability at volcanoes worldwide.

Teleconnection between the Intertropical Convergence Zone and southern westerly winds throughout the last deglaciation
Vincent Montade et al., Institut des Sciences de l'Evolution de Montpellier, Montpellier, France. Published online ahead of print on 10 July 2015;

The linkages of climatic changes between widely separated regions rely on atmospheric teleconnections coupled with oceanic circulation changes. However, specific impacts of such mechanisms that may interact at a different time scales are partly undetermined and their studies are crucial in the context of present-day rapid climate changes. One solution is to investigate past atmospheric and oceanic changes in order to understand how connections are setting up during a period in which rapid climatic changes were recorded such as the last deglaciation. Using paleoclimatic records and climate simulations, we demonstrate that atmospheric teleconnection represent one of the main driver to quickly transfer abrupt climatic changes. In particular, we show the key role of tropical Atlantic region in promoting north-south atmospheric teleconnections throughout the abrupt climatic events of the last deglaciation.

Supercritical-flow structures on a Late Carboniferous delta front: Sedimentologic and paleoclimatic significance
Dario Ventra et al., Faculty of Geosciences, Utrecht University, Utrecht, Netherlands. Published online ahead of print on 10 July 2015;

Dario Ventra (Utrecht University, the Netherlands) and coworkers describe the first outcrop example of sedimentary structures related to particularly energetic, catastrophic waterflows, preserved in Late Carboniferous sedimentary rocks of northwest England, approximately 320 million years old. Such structures, called "cyclic steps," were recently recognized only from laboratory experiments and deep-sea-bottom images, and add now to the known range of sediment-bed configurations (such as ripples, dunes and antidunes) characterizing most beaches, rivers and deserts worldwide. When preserved in ancient rocks, such as sandstone and conglomerates, such structures enable scientists to recognize past environmental conditions, the orientation of ancient winds and waterflows, and to better predict the lithological configuration of the subsurface for resource extraction. The structures discussed in this article assumed giant proportions and developed on the subaqueous front of a paleoequatorial delta at times of early assemblage of the megacontinent Pangea. The paleogeography of Pangea is known to have induced particularly intense monsoonal conditions and severe fluvial floods. Just such a catastrophic flood is recognized as the most likely cause for the anomalously large size and full preservation of these spectacular cyclic steps, which add to the geological toolkit required to reconstruct past environments and events in Earth's history.

Late Glacial and Holocene glacier fluctuations at Nevado Huaguruncho in the Eastern Cordillera of the Peruvian Andes
Nathan D. Stansell et al., Northern Illinois University, DeKalb, Illinois, USA. Published online ahead of print on 10 July 2015;

Paleoclimate records from the tropical Peruvian Andes provide insight into how the atmosphere responded to past shifts in Pacific and Atlantic Ocean conditions, and expand our knowledge of past climatic changes beyond the instrumental era. This paper presents new records of glacial variability from Nevado Huaguruncho in the Eastern Cordillera of Peru spanning the last ~15,000 years based on a combination of radiocarbon-dated lake sediments and cosmogenic isotope exposure ages on moraine boulders. These combined records from the same watershed produce an unusually high resolution and well-integrated glacial history. Glaciers advanced at the start of the Antarctic Cold Reversal at ca. 14.1 ka, and retreated until ca. 12 ka. Glaciers then re-advanced multiple times during the early Holocene from ca. 11.6 to 10.3 ka while conditions in the Eastern Cordillera were relatively dry, indicating that glaciers expanded in response to colder conditions. This pattern of glacier fluctuations during the Late Glacial and early Holocene was associated with changing tropical Atlantic sea-surface temperatures that were out-of-phase with temperature changes in the Northern Hemisphere. Glaciers at Huaguruncho underwent modest oscillations during the middle and late Holocene, and a final advance occurred under colder and wetter conditions during the Little Ice Age from 0.4 to 0.2 ka when it was also cold in the Northern Hemisphere.

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