PRESSURE-TEMPERATURE-TIME CONSTRAINTS FOR SHOCK VEIN DEVELOPMENT: A CASE STUDY AT VREDEFORT

Numeric computing software (via MathWorks MATLAB) has been developed to calculate pressure-temperature-time (P-T-t) conditions experienced by shock veins in metaquartzites of the Witwatersrand Supergroup in the Vredefort impact structure. This approach accommodates the pressure due to the passage of the shock front, subsequent rarefaction unloading, and associated heating and cooling rates. Our model is not based on assuming the conditions required for stishovite and coesite formation, but rather explores the P-T-t environment derived from reconstruction of the Vredefort impact crater based on the works of especially Turtle and Pierazzo (1998), Turtle et al., (2003) and Ivanov (2005), as well as the shock vein study of Spray and Boonsue (2018).

The shock veins pervasively crosscut the host rock, varying in thickness up to a maximum of ≈1 mm in width (average ≈150 µm width) and can be traced for at least 1 m on well-exposed outcrops. They form most commonly between quartz grains but also crosscut larger grains and incorporate grains and fragments of grains as suspended clasts into the matrix.

Coesite is present within the shock vein matrix as new crystals that nucleated from the shock melt. Coesite is also developed within matrix-suspended clasts, and along vein wall/shock melt interfaces formed via solid-state transformation. Within clasts, stishovite forms acicular crystals that appear to grow into the clast from the melt/clast interface. Coesite is more abundant than stishovite, with the former forming a jig-saw texture within larger clasts while retaining cores of α-quartz. In smaller clasts coesite forms along the rim of the clast with cores of α-quartz.

The maximum temperature reached within the melt veins is ~4080 °C before they begin to cool via conduction. Within 270 ms after shock front passage the conditions begin to enter the stability field of stishovite, followed by the coesite stability field at ~1.22 s, the β-quartz stability field at ~1.92 s, and finally the ∝-quartz stability field, causing the β-quartz to revert to α-quartz much later at ~11.6 s; well after the rarefaction wave has passed. The P-T-t path indicates that the polymorphs formed under their normal stability field conditions during and following rarefaction wave decompression.