COMPARISON OF METAMORPHIC AND METEORITE IMPACT STRAIN RATES AND P-T-X PRODUCTS

High pressures and temperatures occur in both slow and fast geologic processes, but the products produced vary greatly as a function of strain rate and duration of the event. Peak pressures and temperatures in ultrahigh-pressure (UHP) metamorphic environments are typically 50 GPa and 800 C, but can be much larger than this in the shock waves produced by meteorite impacts (pressures sufficient to vaporize rocks at 100 GPa, melt granite at ~50 GPa, and produce unique lamellae in quartz at ~3.0-10 GPa. Typical metamorphic strain rates are of the order of 10^-1210^-15/sec (and in Gary Ernst’s beloved Franciscan rocks, may have been a meager 10^-16-10^-17/sec!) In contrast, strain rates at meteorite impact sites can be as large as 10⁸/sec. For intuition: a bat hitting a baseball generates a strain rate of ~10²/sec; tires hitting a bump in the road ~10³/sec. A high-speed projectile hitting a tank, ~10⁵/sec. Durations of metamorphic events are the order of ~10’s of million years whereas the duration of peak pressure and strain may be seconds or even less. Rocks have different effective “strengths” at different strain rates, generally reduced strength at high strain rates. Thus, in the field, structures such as faults and folds have very different characteristics at the extremes of strain rates. Particularly notable are the large zones of pseudotachylite and melt glass stringers along faults around impacts that do not occur in standard tectonic setting. Restricting discussion to pressures less than 50 GPa, the products of slow metamorphism and shock metamorphism at comparable pressures are very different because equilibrium conditions are not achieved in shock events. In metamorphic terrains, coesite, or quartz pseudomorphs after coesite, and diamonds, or graphite pseudomorphs after diamonds, are considered to be indicators of the highest pressures attained in the metamorphic P-T-X excursions. Coesite is interpreted to form at pressures between 2.5 and 3.0 GPa In contrast, minerals formed in shocked rocks may reflect an blurry P-T-X space traversed upon decompression. For example, the Coconino Sandstone shocked to 3.0 GPa has very little coesite, even though that is the equilibrium phase under shock conditions. However, Coconino Sandstone shocked to higher pressures (e.g., >>10 GPa) have as much as 30% coesite, formed as the rocks decompress through the coesite stability field.