First use of NanoSIMS ion probe measurements to understand volcanic cycles at Yellowstone

(Press-News.org) Boulder, Colo., USA - 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.

FEATURED ARTICLE
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; http://dx.doi.org/10.1130/G36862.1.

Other recently posted GEOLOGY articles (see below) cover such topics as Giant stromatolites of the Green River Formation, Colorado, USA; The "great hydration event" beneath the Colorado Plateau; and 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; http://dx.doi.org/10.1130/G36793.1.

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; http://dx.doi.org/10.1130/G36932.1.

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; http://dx.doi.org/10.1130/G36799.1.

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; http://dx.doi.org/10.1130/G36766.1.

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; http://dx.doi.org/10.1130/G36767.1.

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; http://dx.doi.org/10.1130/G36762.1.

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; http://dx.doi.org/10.1130/G36768.1.

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 ( END