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Transformations to granular zircon revealed: Meteor Crater, Arizona

July 27, 2016

Boulder, Colo., USA - Having been reported in lunar samples returned by Apollo astronauts, meteorites, impact glass, and at a number of meteorite craters on Earth, granular zircon is the most unusual and enigmatic type of zircon known. The mechanisms and transformations that form this distinctive granular zircon have, until now, remained speculative because it has not been produced in shock experiments.

A new study of granular zircon from Meteor Crater in Arizona, USA, by Aaron J. Cavosie and colleagues, uses electron backscatter diffraction to unravel specific mineral transformations and pressure-temperature conditions involved in its genesis.

Mapping the orientation of recrystallized zircon domains (neoblasts) shows that making granular zircon first involves forming twins, followed by transformation to the high-pressure mineral reidite, all at extreme pressure and temperature, far beyond those found in Earth's crust. While at high temperature, the grains recrystallize to form the distinctive small neoblasts that define granular zircon, and then partially react to zirconia if high temperature persists.

These results, which include the first new shocked mineral discovery at Meteor Crater in more than 50 years, provide new insights into extreme impact conditions at inaccessible sites where granular zircon occurs, such as the surface of the Moon and collisions among asteroids.

FEATURED ARTICLE

Transformations to granular zircon revealed: Twinning, reidite, and ZrO2 in shocked zircon from Meteor Crater (Arizona, USA)


Aaron J. Cavosie et al., TIGeR (The Institute for Geoscience Research), Department of Applied Geology, Curtin University, Perth, WA 6102, Australia. This article is OPEN ACCESS online at http://geology.gsapubs.org/content/early/2016/07/19/G38043.1.abstract.

GEOLOGY articles are online http://geology.gsapubs.org/. Representatives of the media may obtain complimentary articles by contacting Kea Giles at the e-mail address above. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published. Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.

Other recently posted GEOLOGY articles are highlighted below:Persistent slip rate discrepancies in the eastern California (USA) shear zone

Eileen L. Evans et al., U.S. Geological Survey, 345 Middlefield Road, MS 977, Menlo Park, California 94025, USA. This article is online at http://geology.gsapubs.org/content/early/2016/07/19/G37967.1.abstract.

Taking advantage of a new approach for estimating fault slip rates from GPS observations, Eileen Evans and colleagues address a long-standing puzzle in the eastern California shear zone, where slip rates estimated from GPS measurements often disagree with those estimated from geologic observations. To model slip rates, they consider many potentially active faults, and find that eastern California shear zone deformation is well described by 10 microplates. Discrepancies with geologic slip rates appear to be persistent, especially on the Calico and Garlock faults. This result may be inconsistent with the presence of distributed off-fault deformation in the eastern California shear zone. Understanding localized discrepancies on the Calico and Garlock faults may hold important clues for understanding how fault systems deform and evolve in time, and for earthquake hazard in eastern California.



Subduction zone interaction: Controls on arcuate belts


Ágnes Király et al., Laboratory of Experimental Tectonics, Department of Sciences, Università degli Studi Roma Tre, Largo San Leonardo Murialdo 1, 00146 Rome, Italy. This article is online at http://geology.gsapubs.org/content/early/2016/07/19/G37912.1.abstract.

Subduction is a unique process on Earth, where a heavy plate sinks into the mantle along the boundary of two plates. Subduction zones can occur close to each other creating unique and very complex geological settings. This study first addresses how can two neighboring subduction zone interact with each other. We used 3D numerical modeling with the aid of a high-performance supercomputer to understand the interaction between the two oppositely subducting plates. The subduction zones strongly affect each other if plate edges are at distances <~600 km. In this case the mantle flow around the subducting plates combines into a single, large convective cell, trench migration slows down, and stress increases progressively with decreasing slab distance. This study can help to improve our understanding of tectonically and geologically complex areas such as the Mediterranean area between Africa and Europe or the complex boundary between Eurasia and the Pacific plate.



The climate archive dune-sedimentary record of annual wind intensity


Sebastian Lindhorst, University of Hamburg, Center for Earth System Research; and Christian Betzler Sustainability (CEN), Institute for Geology, Bundesstrasse 55, 20146 Hamburg, Germany. This article is online at http://geology.gsapubs.org/content/early/2016/07/19/G38093.1.abstract.

The understanding of the long-term wind-field variability is most relevant for calibrating climate models and for predicting the socio-economic consequences of regional climate shifts. Continuous, instrumental-based weather observations reach back only less than two centuries; the geological record, however, contains an archive of past wind activity that is basically unread. For the first time, we show that eolian dunes bear a high-resolution record of past wind strength. Grain size variations of dune sands through time are compared with historical wind observations and are exemplarily used to reconstruct 20th century wind-intensity in the southern North Sea area, beyond the time covered by historical wind observations. The approach can be used in both recent and fossil dune systems to gain long-term data series of wind intensity in areas and for time periods lacking an instrumental record. Potential applications include the validation of climate models, the reconstruction of supra-regional wind systems and the monitoring of future shifts in the climate system.



Quartz-in-garnet inclusion barometry under fire: Reducing uncertainty from model estimates


Kyle T. Ashley et al., Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, 2275 Speedway Stop C9000, Austin, Texas 78712, USA. This article is online at http://geology.gsapubs.org/content/early/2016/07/19/G38211.1.abstract.

Kyle Ashley and colleagues conducted heating experiments on quartz inclusions under pressure within garnet from a variety of tectonic environments. Ashley et al. found that the pressure inside the inclusions increased during heating, as a result of the different responses to changing temperature and pressure of the inclusion versus host. The magnitude of the pressure increase was less than that calculated using a one-dimensional elastic model. The authors therefore derived a correction to the elastic model, which improves the precision of this barometer and allows better pressure determination for mineral growth.



Avulsion flow-path selection on rivers in foreland basins


Douglas A. Edmonds et al., Indiana University Department of Geological Sciences and Center for Geospatial Data Analysis, Bloomington, Indiana 47405, USA. This article is online at http://geology.gsapubs.org/content/early/2016/07/19/G38082.1.abstract.

Rivers are dynamic features that move, twist, and wiggle as they carry water and sediment from mountains to the coast. Possibly the most dramatic river movement is the avulsion, which is the wholesale abandonment of the present river course in favor of a new location on the adjacent floodplain. Understanding this process is critical because it causes widespread flooding and deposits huge volumes of sediment that are good reservoirs for oil and natural gas. The process of river avulsions is a bit mysterious because they usually occur every 500 to 1000 years making them hard to observe. To overcome this barrier we took advantage of the new cloud-computing platform developed by Google called Earth Engine. Google Earth Engine provides unmatched access to satellite imagery from which we searched and found 55 instances of river avulsions through S. America and India/Nepal. On each avulsion we measured key attributes of how the river moves and found predictable relationships. For instance, avulsions have a characteristic shape where for every 1 km of lateral movement 5 km of river in a downstream direction is abandoned. These results place important constraints on river avulsion size that could aid in hazard prediction and recovering oil and natural gas.

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