Shock-produced high-pressure (La, Ce, Th)PO4 polymorph revealed by microstructural phase heritage of monazite

The responses of uranium-bearing accessory minerals to shock metamorphism have received growing interest, because under extreme pressure and temperature conditions, these phases can form unique microstructures and/or polymorphs and their radiometric ages can be partially to wholly reset. This study...

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Bibliographic Details
Published in:Geology (Boulder) 2019-06, Vol.47 (6), p.504-508
Main Authors: Erickson, Timmons M, Timms, Nicholas E, Pearce, Mark A, Cayron, Cyril, Deutsch, Alex, Keller, Lindsay P, Kring, David A
Format: Article
Language:English
Subjects:
Backscatter
Bavaria Germany
Canada
Central Europe
Cerium
Crystallography
Deformation
Devon Island
Electron backscatter diffraction
Europe
Geology
Germany
Haughton impact structure
High pressure
high temperature
igneous and metamorphic rocks
Lamellae
Lanthanum
Metamorphism
Metamorphism (geology)
Microstructure
Mineralogy
Minerals
Monazite
nonsilicates
Nunavut
Orientation
Orientation relationships
P-T conditions
Petrology
Phosphates
planar deformation features
Polymorphism
Pressure
Queen Elizabeth Islands
Reversion
Ries Crater
Shock metamorphism
temperature
Thorium
Uranium
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Summary:The responses of uranium-bearing accessory minerals to shock metamorphism have received growing interest, because under extreme pressure and temperature conditions, these phases can form unique microstructures and/or polymorphs and their radiometric ages can be partially to wholly reset. This study presents new, high-resolution electron backscatter diffraction microstructural analyses of shock-deformed monazite, (La, Ce, Th)PO4, from the Haughton Dome, Nunavut, Canada, and the Nordlinger-Ries Crater, southern Germany. At each locality, shocked monazite grains contain distinctive lamellae comprising interlocking laths in four systematic crystallographic orientations. Modelling of orientation relationships among these lath-textured microstructures reveals that they were produced by reversion from lamellae of a previously undescribed high-pressure polymorph with a tetragonal symmetry. This is the first report of polymorphism found in natural (La, Ce, Th)PO4. The orientation relationships, lamellar forms, and ready reversion to monazite at low pressures are consistent with a deviatoric transformation. The former presence of this new (La, Ce, Th)PO4 polymorph is detected in moderately to highly shocked rocks, suggesting that it may be common at other impact craters.
ISSN:0091-7613
1943-2682
DOI:10.1130/G46008.1