REIDITE AND SHOCK-TWINNED ZIRCON IN POLYMICT BRECCIA FROM THE ORDOVICIAN ROCK ELM IMPACT STRUCTURE, USA

Reidite is a high-pressure polymorph of ZrSiO₄ that forms >30 GPa [1] and is an important accessory mineral in studies of shock metamorphism as its formation conditions have been experimentally constrained. However, naturally occurring reidite is rare; it has only been reported from three impact structures, where it occurs in ejecta and impact melt-bearing breccias [2,3,4,5] that record shock pressures from 35 to 60 GPa (shock stages II-III, [6]). Here we report a new occurrence of reidite in polymict breccia from Rock Elm, a deeply eroded, 6.5 km diameter, Middle Ordovician impact structure in Wisconsin, USA [7]. The breccia contains lithic clasts and quartz grains with planar fractures, formed during shock deformation of the underlying Cambrian Mt. Simon sandstone. Reidite was documented in zircons using electron backscatter diffraction (EBSD) [8], and has two habits: (1) crystallographically oriented sets of micrometer wide lamellae, and (2) randomly orientated sub-micrometer granules along margins and within intragrain voids. Both habits are inferred to have formed simultaneously, and record the variable response of zircon at the grain scale to high-pressure shock deformation in a porous sedimentary target rock. A shocked zircon with impact microtwins in {112} [9] was also documented in the same sample by EBSD. The presence of reidite at Rock Elm provides a record of significantly higher pressures for exposed rocks (>30 GPa) than previous reports based on shocked quartz (<10 GPa) [7], and along with shock-twinned zircon, further confirms a high pressure impact origin for the breccia. Rock Elm is the fourth impact structure where reidite has been recorded, and the first occurrence of reidite in a porous target rock, the Mt. Simon sandstone. With an inferred minimum age of ~450 Ma, Rock Elm contains the oldest preserved reidite in the geological record thus far described.

This work was supported by the National Science Foundation (EAR-1145118).

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