FRIGN zircon—The only terrestrial mineral diagnostic of high-pressure and high-temperature shock deformation

Abstract

Minerals that record high-pressure deformation from meteorite impact are relatively common on Earth; however, minerals that record both high-pressure and high-temperature effects of impact are rare, and they occur mostly in meteorites. Here, we report new occurrences of a type of granular zircon that is the only terrestrial mineral known that uniquely records both high-pressure and high-temperature conditions diagnostic of impact. Granular zircon grains in impact melt rock from the Luizi impact structure (Democratic Republic of the Congo) and in glass from the recently described Pantasma structure (Nicaragua) consist of ~1-µm-sized neoblasts in multiple domains that are systematically oriented such that all (001) poles are approximately orthogonal and coincide with a <110> direction from adjacent domains. The neoblast orientations are produced uniquely by transformation to the high-pressure polymorph reidite and its subsequent reversion to neoblastic zircon at high-temperature conditions, as evidenced by the occurrence of ZrO2 . We here refer to such granular grains that preserve orientation evidence for reversion from reidite as “former reidite in granular neoblastic” zircon, or FRIGN zircon, in order to distinguish this type of granular zircon from those that do not record high-pressure conditions. In addition to the localities described here, examples of what can now be classified as FRIGN zircon have been documented at Meteor Crater (Barringer crater, USA), at the Acraman impact structure (Australia), and in Australasian tektites. Occurrences thus span five continents, range in age from Precambrian (ca. 590 Ma) to recent (ca. 49 ka), and serve as a diagnostic indicator of impact metamorphism that may be present throughout much of the geological record. FRIGN zircon further has potential to be found in extraterrestrial samples, where granular zircon has previously been reported.