A pressure-temperature phase diagram for zircon at extreme conditions

Abstract

Hypervelocity impact processes are uniquely capable of generating shock metamorphism, which causes mineralogical transformations and deformation that register pressure (P) and temperature (T) conditions far beyond even the most extreme conditions created by terrestrial tectonics. The mineral zircon (ZrSiO 4) responds to 26 shock deformation is various ways, including crystal-plasticity, twinning, 27 polymorphism (e.g., transformation to the isochemical mineral reidite), formation of 28 granular texture, and dissociation to ZrO 2 + SiO 2 , which provide robust 29 thermobarometers that record different extreme conditions. The importance of 30 understanding these material processes is twofold. First, these processes can mobilize 31 and redistribute trace elements, and thus be accompanied by variable degrees of 32 resetting of the U-Pb system, which is significant for the use of zircon as a 33 geochronometer. Second, some features described herein form exclusively during 34 shock events and are diagnostic criteria that can be used to confirm the hypervelocity 35 origin of suspected impact structures. We present new P-T diagrams showing the 36 phase relations of ZrSiO 4 polymorphs and associated dissociation products under extreme conditions using available empirical and theoretical constraints. We present case studies to illustrate zircon microstructures formed in extreme environments, and present electron backscatter diffraction data for grains from three impact structures (Mistastin Lake of Canada, Ries of Germany, and Acraman of Australia) that preserve different minerals and microstructures associated with different shock conditions. For each locality, we demonstrate how systematic crystallographic orientation relationships within and between minerals can be used in conjunction with the new phase diagrams to constrain the P-T history. We outline a conceptual framework for a zircon-based approach to 'extreme thermobarometry' that incorporates both direct observation of high-P and high-T phases, as well as inferences for the former existence of phases from orientation relationships in recrystallised products, a concept we refer to here as 'phase heritage'. This new approach can be used to unravel the pressure-temperature history of zircon-bearing samples that have experienced extreme conditions, such as rocks that originated in the Earth's mantle, and those shocked during impact events on Earth and other planetary bodies.

U‐Pb dating of zircons from an impact melt of the Nördlinger Ries crater

“…; Timms et al. ; McGregor et al. ), anatase (TiO 2 ), and Fe‐rich inclusions on a nm‐scale are discernible (see Figs.…”

Shock‐induced phase transformation of anorthitic plagioclase in the eucrite meteorite Northwest Africa 2650

“…; Timms et al. ). In NWA 2650, the lamellae‐shaped texture of reidite indicates that it might also have formed through solid‐state transformation.…”

In situ U–Pb analysis of shocked zircon from the Charlevoix impact structure, Québec, Canada

“…2015b; Timms et al. ), may open an entire new avenue in the field of impact‐related geo‐ and thermochronology.…”

“…; Timms et al. ; Kenny et al. ) that exhibit relic planar microstructures (e.g., zircon grain #10 in SI‐3; Fig.…”