Transformations to Granular Zircon Revealed: Meteor Crater, Arizona

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.

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Subduction zone interaction: Controls on arcuate belts
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The climate archive dune-sedimentary record of annual wind intensity
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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.

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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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27 July 2016
GSA Release No. 16-44

Kea Giles, Managing Editor,
GSA Communications
+1-303-357-1057
kgiles@geosociety.org

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Meteor Crater, Arizona
Meteor Crater, Arizona, USA. Image courtesy NASA Earth Observatory/National Map Seamless Server.