Shock reequilibration and destruction of fluid inclusions: Comparing results of single crystal shock experiments with crystalline basement rocks from The Ries Crater, Germany and porous sedimentary rocks from Meteor Crater, Arizona

Fluid inclusions, nanoliter volumes of fluid trapped within minerals as they precipitate, often record hydrothermal processes. While fluid inclusions are nearly ubiquitous in terrestrial rocks which form in the presence of a fluid phase, fluid inclusions have been reported in only a few meteorites despite compelling evidence of aqueous alteration in many other planetary samples. Single crystal shock experiments as well as field studies examining fluid inclusions trapped in quartz in crystalline basement rocks from the Ries Crater, Germany and porous sedimentary rocks from Meteor Crater, Arizona demonstrate that fluid inclusions trapped prior to impact reequilibrate and may be destroyed as a result of shock metamorphism. In all three studies, most two-phase fluid inclusions decrepitate or collapse as a result of brittle deformation at low shock pressures forming single phase inclusions. No two-phase fluid inclusions were observed in single crystal samples exceeding 6 GPa. However, field samples of crystalline basement samples contain some two-phase inclusions in shock stage 1 rocks (10-35 GPa). Rare two-phase inclusions were observed in sedimentary rocks which contained planar fractures, however no two phase inclusions were observed in sedimentary samples containing shock lamellae and/or diaplectic glass. At higher shock pressures the number of inclusion vesicles decreases significantly in all three sample sets suggesting that fluid inclusions are destroyed by plastic deformation and/or phase changes in the host mineral. No inclusion vesicles were observed in single crystal samples shocked to pressures above 12 GPa. Fluid inclusions were also extremely rare in crystalline basement and sedimentary samples containing shock lamellae and/or diaplectic glass. However, infrequent observations of single phase inclusions in the natural samples demonstrate the heterogeneous nature of shock deformation in polycrystalline samples. Comparison of field studies with single crystal shock experiments confirms that impact processing may result in reequilibration of fluid inclusions at low shock pressures and the destruction of fluid inclusions in moderate to highly shocked materials, erasing key evidence of hydrothermal processes in some planetary materials. However, fluid inclusions in naturally shock metamorphosed polycrystalline rocks may survive higher shock pressures than those observed in single crystal experiments. In addition, fluid inclusions in crystalline basement rocks may be able to withstand slightly higher shock pressures than those trapped in porous sedimentary rocks. However, it is difficult to directly compare these two sample sets due to higher temperature conditions at comparable shock pressures created in sedimentary rocks as a result of the collapse of pore spaces. In addition, the results of these field studies indicate that the pre-impact fluid inclusion record is destroyed by moderate to high impact pressures. As a result, fluid inclusion assemblages observed in breccias and melt rocks shocked to pressures greater than 10-35 GPA within large impact craters likely record post-impact hydrothermal systems rather than pre-impact fluid processes.