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.

pic This is Meteor Crater, Arizona, USA. Credit: Image courtesy NASA Earth Observatory/National Map Seamless Server.

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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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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source: Geological Society of America