Abstract

The Siljan Ring in central Sweden is a Devonian impact crater of the complex type. The ∼52-km-wide crater occurs in Precambrian Dala series granites (1.65 Ga). Deep drilling in the crater provides samples from depths down to 6000 m; these are supplemented by surface samples from the crater and surrounding regions. Secondary fluid inclusions occur in matrix quartz of nearly all granites examined. Most inclusions contain low-salinity water (0-2 wt% NaCleq) and water vapor; no CH4 or H2 has been detected. In surface samples, peak trapping temperatures are 327-342 °C on the crater's inner plateau, and minimum trapping temperatures are 135-225 °C near the annular moat. In the subsurface, trapping temperatures distinguish two sample populations separated by a flat-lying 60-m-thick dolerite at 1200 m below sea level (-1200 m). In the deep sample group below -1200 m, estimated trapping temperatures increase with depth along gradients between 17 and 32 °C/km, from ∼250-300 °C just below the dolerite to ∼320-440 °C at -5711 m. Trapping temperatures in samples above -1200 m are hotter than would be predicted by extrapolation of the deep trends and are believed to reflect hydrothermal activity generated by impact heating. This interpretation is supported by the presence of hotter temperatures of fluid-inclusion trapping in samples from inside the crater compared to outside, and by crosscutting relations which show that some fluid-inclusion trails postdate impact-related features such as planar elements in quartz. Impact-heated fluids circulated through a circular domain of rock approximately 52 km wide and at least 1.2 km thick. The large scale of this circulation system indicates that impact-generated hydrothermal activity can be a significant mechanism for heat dissipation following bolide impacts on water-bearing planets.

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