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.
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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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.
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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.
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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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