Microstructural dynamics of central uplifts: Reidite offset by zircon twins at the Woodleigh impact structure, Australia: COMMENT

Abstract

The recent work of Cox et al. (2018) shows reidite lenses crosscut and offset by shock microtwin lamellae in zircon, clearly showing the order of events from a very short timescale. They explain that reidite formed at the peak shock pressure and that shock microtwins formed after shock pressure was released. However, the details of the interpretations and implications are imprecise. The paper over-interprets poorly constrained processes, leading to unwarranted assertions about processes in the central uplift of impacts. Firstly, the choice to represent the interpretations on a pressuretemperature (P-T) diagram (Cox et al.’s figure 3) is unfortunate, as temperature is not constrained, and the results are entirely pressure dependent (Fig. 1). The authors do not explain where the temperature data in their P-T graph originated. The known constraints on zircon-toreidite transition and microtwin formation are not strongly affected by temperature, and the temperature of transformation cannot be constrained. The uppermost post-impact temperature is constrained by the preservation of reidite, which decomposes at ~1200oC (Kusaba et al., 1985). A more appropriate temperature path might show that the temperature increasing after the passage of the shock wave, such that reidite formed closer to the initial ambient temperature of 200oC. The temperature of shock microtwin formation has no known relevant constraints.