Transformations to granular zircon revealed: Twinning, reidite, and ZrO₂in shocked zircon from Meteor Crater (Arizona, USA)

Abstract

Granular zircon in impact environments has long been recognized but remains poorly understood due to lack of experimental data to identify mechanisms involved in its genesis. Meteor Crater in Arizona (USA) contains abundant evidence of shock metamorphism, including shocked quartz, the high-pressure polymorphs coesite and stishovite, diaplectic SiO 2 glass, and lechatelierite (fused SiO 2). Here we report the presence of granular zircon, a new shocked-mineral discovery at Meteor Crater, that preserve critical orientation evidence of specific transformations that occurred during formation at extreme impact conditions. The zircon grains occur as aggregates of sub-micrometer neoblasts in highly shocked Coconino Sandstone (CS) comprised of lechatelierite. Electron backscatter diffraction shows that each grain consists of multiple domains, some with boundaries disoriented by 65° around <110>, a known {112} shock-twin orientation. Other domains have {001} in alignment with {110} of neighboring domains, consistent with the former presence of the high-pressure ZrSiO 4 polymorph reidite. Additionally, nearly all zircon preserve ZrO 2 + SiO 2 , providing evidence of partial dissociation. The genesis of CS granular zircon started with detrital zircon that experienced shock twinning and reidite formation at pressures from 20 to 30 GPa, ultimately yielding a phase that retained crystallographic memory; this phase subsequently recrystallized to systematically oriented zircon neoblasts, and in some areas partially dissociated to ZrO 2. The lechatelierite matrix, experimentally constrained to form at >2000 °C, provided the ultrahigh-temperature environment for zircon dissociation (~1670 °C) and neoblast formation. The capacity of granular zircon to preserve a cumulative pressure-temperature record has not been recognized previously, and provides a new method for investigating histories of impact-related mineral transformations in the crust at conditions far beyond those at which most rocks melt.

Linking shock textures revealed by BSE, CL, and EBSD with U‐Pb data (LA‐ICP‐MS and SIMS) from zircon from the Araguainha impact structure, Brazil

“…), and its back‐transformation to neoblastic zircon (Cavosie et al. b; Timms et al. , ; Cavosie et al.…”

Pantasma: Evidence for a Pleistocene circa 14 km diameter impact crater in Nicaragua

“…These orientations only form in zircon that first transformed to reidite, and then back‐transformed to zircon, leaving high‐angle misorientations that can be ascribed to either reidite formation (90°/<110>), or {112} twin formation (65°/<110>) (Cavosie et al. , , ; Erickson et al. ; Timms et al.…”

“…Detection of the former presence of reidite in granular zircon is a recently developed method that has been used to identify evidence of former reidite in glass from Meteor Crater (Cavosie et al. ), impact melt rock from the Acraman impact structure in Australia (Timms et al. ), and in Australasian tektites (Cavosie et al.…”

Documentation of shock features in impactites from the Dhala impact structure, India

“…; Cavosie et al. ). This texture is thought to develop syn‐ or post shock deformation at high temperature in grain parts that contained defects, where the effective energy barrier required for nucleation is lower than in other parts of the grains (Timms et al.…”