Methane inclusions in shocked quartz from the Gardnos impact breccia, South Norway

A meteorite impact structure of Late-Proterozoic age has recently been recognized at Gardnos in the Hallingdal area, South Norway. The structure is situated in pre-1100 Ma gneisses and metasediments, and consists of a zone of autochtonous breccia, a suevite (impact melt) breccia and a series of unbrecciated crater fill sediments. In the late Silurian, the impact structure was overrun by Caledonian nappes, leading to low-grade metamorphic recrystallization (T_max ≤ 400°C, P_max ≥ 2.5 kbar). Shocked quartzite from the autochtonous breccia is impregnant by fine-grained carbonaceous material, giving it a nearly black colour in hand specimens. Micro-Raman spectroscopy shows this material to be poorly crystalline. Planar fractures, typical of shocked quartz, are outlined by graphite inclusions and by trails of secondary fluid inclusions. The fluid inclusions comprise methane inclusions, with minor carbon dioxide and no visible water (X_CH4 ≥ 96 mol%) and water inclusions with moderate salinity. The methane inclusions show H1, H2 and S2 types of microthermometric behaviour; H1 inclusions show a peak of homogenization temperatures to the liquid at -84 to -112°C. At T ≤ 400°C, this corresponds to isochore pressures of 1 to 2.5 kbar, which is compatible with a Caledonian cooling and uplift path, but not with the extreme pressure at the moment of the impact, nor the low pressures (P ≤ 0.2 kbar) encountered in the shallow parts of the impact structure during the final stages of post-impact cooling. Methane was formed in-situ by reaction between solid carbonaceous material and aqueous metamorphic fluid, and was trapped as the partly open planar fractures healed. The methane-rich fluid inclusions in shocked quartz from the Gardnos impact structure are thus only indirectly related to the meteorite impact and not at all to hydrocarbons of a deep (mantle) origin.