MATRIX IN CARBONATE IMPACT BRECCIAS: COMMINUTION AND PRECIPITATION FROM SUPERCRITICAL WATER (SCW)?

It is widely held that large meteorite impacts generate breccia matrix through the process of comminution. Fine-grain particles in matrix form from fragmentation of the target rock through blasting, fracturing, crushing, cleaving, grinding, and vibration. The component particles of impact breccia matrix also can include melt that retain geochemical traces related to the impactor. Particles ultimately are cemented together, and diagenesis follows. An additional, potentially significant, yet previously unrecognized, component in generating matrix particles may be mineral precipitates from supercritical water (SCW), especially in carbonate target rock successions and marine impacts.

SCW occurs under high pressure and temperature, beyond a thermodynamic critical point at 22 MPa and 374 °C. SCW has many of the characteristics of a gas but behaves as a relatively low-density fluid with distinctive properties. SCW exists dominantly as monomeric molecules that lack surface tension, viscosity, and the attendant solubility characteristics of subcritical water. With this lack of solubility, salts are precipitated. For example, supercritical water oxidation (SCWO) reactors have seen application in treatment of water contaminated with miscible hydrocarbons, sludge, and other pollutants.

Meteorite impacts generate both extreme pressure and temperature that greatly exceed conditions necessary for the formation of SCW. Large impacts are recognized from features such as high-pressure polymorphs of silicate minerals (> 2 GPa), shatter cones (2 – 30 GPa), and planar deformational features (PDFs) in quartz (5 – 35 GPa). Substantial melts occur in impacts at pressures ≥ 40 GPa. In the contact and compression phase of impact cratering, near ground zero, temperatures are thought to exceed 2000 °C. Even the residual heat of impact would keep temperatures elevated for an extended time; hydrothermal geochemical signatures are common in impact breccias. Given that roughly one-third of large, ancient impact structures (~50) are found in carbonate target successions, and several of these are regarded as marine impacts, the potential for forming SCW-precipitated particles in breccia matrix may be appreciable. Thermobarometry potentially provides a key test for this hypothesis.