Abstract

Shock-metamorphosed zircons were recovered from an upper Eocene impact ejecta layer, probably from the Chesapeake Bay impact structure, found at three Deep Sea Drilling Project–Ocean Drilling Program Sites (612, 903, and 904) on the upper continental slope off New Jersey. The shocked zircons exhibit granular textures, and some have planar features. X-ray diffraction patterns show that some of the shocked zircons contain a high- pressure ZrSiO4 polymorph with a scheelite-type structure that was first produced in laboratory experiments in 1969. The presence of the high-pressure ZrSiO4 polymorph in some of the zircons indicates that they have been shock metamorphosed at pressures between 20 and 90 GPa. The discovery of this phase in naturally occurring shocked zircons confirms previous predictions that this phase would be found in shock-metamorphosed rocks from impact structures, and it provides a new tool for calibrating peak shock pressures in impact structures and ejecta. In addition, because the high-pressure ZrSiO4 polymorph has now been found in naturally occurring samples, it can be given a mineral name.

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