Laboratory measurements of porosity parameterization and wavespeed dependence under confining pressure up to 200 MPa in shocked Chicxulub peak-ring granitoids.

Abstract:

In a rocky body with no tectonic activity, bolide impacts may be the only significant form of crustal recirculation. Although the physical properties of shocked rocks at ambient pressures and temperatures are beginning to be known, in-situ properties at depth are still poorly understood. In the case of Chicxulub, the shock affected depth could be up to 100 km. It is therefore important to better understand crack-parameterization of shocked rocks at high-pressure in order to better constrain near surface elastic properties.

With the aim of quantifying damage in exotic rocks, we carried out high pressure ultrasonic pulse transmission measurements on numerous granitoid and melt core samples retrieved during drilling of IODP/ICDP Expedition 364 into the peak ring of the Chicxulub structure. Analysis of thin sections and He and Hg porosimetry on representative portions additionally assists in characterizing the shock-produced microcracks. Hg porosimetry confirms high porosities in the damaged granitoids and indicates wide, continuous ranges of microcrack apertures from 10 nm to 100 µm. These pore structures have been additionally observed in SEM with varied inter and intra granular porosity. Conversely, in the melts, Hg porosimetry suggests a narrower Gaussian distribution of pore dimensions ranging from 10 nm to 1 µm sharply centered at ~100 nm. As expected, ultrasonic P and S phase velocities increase nonlinearly with confining pressure up to 200 MPa as much as 25% in the granitoids. Even at the highest pressures these rocks retain a significant deficit in wave speed relative to that expected on the basis of their mineralogical composition; up to a 40% velocity deficit exists at 200 MPa in the investigated samples. This pressure dependence is largely due to the progressive closure of microcracks and the wavespeed/pressure curve specifics are likely sensitive to the specific crack parameters of each individual sample.