Granitoid rock samples from the assumed center of the Keurusselkä impact site were subjected to a systematic study of fluid-inclusion compositions and densities in various microstructures of the shocked quartz. The results are consistent with the following impact-induced model of formation. After cessation of all major regional tectonic activity and advanced erosional uplift of the Fennoscandian shield, a meteorite impact (approximately 1.1 Ga) caused the formation of planar fractures (PFs) and planar deformation features (PDFs) and the migration of shock-liberated metamorphic fluid (CO2 ± H2O) to the glass in the PDFs. Postimpact annealing of the PDFs led to the formation of CO2 (±H2O) fluid-inclusion decorated PDFs. The scarce fluid-inclusion implosion textures (IPs) suggest a shock pressure of 7.6–10 GPa. The postimpact pressure release and associated heating initiated hydrothermal activity that caused re-opening of some PFs and their partial filling by moderate-salinity/high temperature (>200 °C) H2O (+ chlorite + quartz) and moderate-density CO2. The youngest postimpact endogenic sub- and nonplanar microfractures (MFs) are characterized by low-density CO2 and low-salinity/low-temperature (<200 °C) H2O.
Fennoscandia is one of the most densely crater-populated parts of the Earth (e.g., Henkel and Pesonen 1992; Abels et al. 2002; Dypvik et al. 2008). Altogether, 31 impact structures have been discovered and, in Finland, a total of 11 proven impact sites are recognized. Among these, the Keurusselkä is the most recent find. Unlike in all other Finnish impact sites, its geomorphology does not suggest impact history, but the structure was located based on shatter cones (Hietala and Moilanen 2004, 2007).
Lake Keurusselkä is located within the Paleoprotezoic (approximately 1.9 Ga) Central Finland Granitoid Complex (CFGC), characterized by Svecofennian syn- and postkinematic granitoid intrusions and intervening metamorphosed volcanic-sedimentary belts (Nironen 2003; Fig. 1). The area is crosscut by highly tectonized zones of shearing, fracturing, brecciation, and hydrothermal alteration.
The impact interpretation of the Keurusselkä structure was confirmed by the study of Ferrière et al. (2010). They measured the crystallographic orientations of planar fractures (PFs) and planar deformation features (PDFs) in quartz and feldspar grains from the shatter cone samples, which yielded an impact pressure estimate of 2–20 GPa for the structure. A late Mesoproterozoic Ar/Ar age of 1140 Ma for a local pseudotachylite vein (Schmieder et al. 2009) and a paleomagnetic age (1120 Ma) consistent with this age (Raiskila et al. 2011) are the current best age estimates for the structure. As such, it is one of the oldest and the most deeply eroded impact structures.
Fluid-inclusion studies of impact rocks are relatively scarce (e.g., Lindblom and Wickman 1985; Komor et al. 1988; Andersen and Burke 1996; Kirsimäe et al. 2002) perhaps due to postimpact origin of the fluid inclusions and thus by their indirect relation to the processes of crater formation.
The aim of this study was to characterize fluid compositions and densities of the inclusions in the previously documented decorated PDFs and PFs (see Ferrière et al. 2010), and other microstructures of the shocked quartz in the Keurusselkä samples. The evolution of the fluids, including interpretation of pressure-temperature conditions, is outlined.
Sample Description and Analytical Methods
Three conically shattered feldspar metaporphyry samples from the center of the Keurusselkä structure were selected for a detailed study (Fig. 1). They are fine- to medium-grained, relatively equigranular light reddish brown granitoids cut both along and perpendicular to prevailing lineation. Rock-forming minerals include K-feldspar, sodic plagioclase, quartz, and minor hornblende with hydrothermal alteration evidenced by patchy or striation-parallel chloritization and epidotization, as well as intense sericitization of plagioclase grain cores and albitization of their rims (Fig. 2).
Cathodoluminescence (CL) studies of quartz were performed at the Geological Survey of Finland (GTK, Espoo) using a scanning electron microscope (SEM) Jeol JSM 5900 LV equipped with the Oxford energy dispersive spectrometer (EDS) and Gatan MiniCL detector with a detection range of 185–850 nm to infer microtextural relationships between fluid inclusions and the host quartz. At the Department of Geosciences and Geography, University of Helsinki, an electron probe microanalyzer Jeol JXA 8600 equipped with the Oxford EDS Si (Li) thin window detector was used to search and image the planar microstructures, and to identify the host phases based on their chemical composition.
Hydrofluoric (HF) acid vapor-etched (4 min) carbon-coated thick sections (approximately 200 μm) and secondary electron (SE) imaging were used to detect possible glass in the PDFs as described by Gratz et al. (1996).
Fluid-inclusion compositions were determined by conventional thermometric analysis (e.g., Shepherd et al. 1985; Samson et al. 2003). Doubly polished thick sections (approximately 200 μm) were used in the microthermometric measurements using Linkam THMSG600 heating–freezing stage. Accuracy levels of ±0.1 and ±2.0 °C were achieved for subzero and higher temperatures, respectively, by calibration using synthetic fluid inclusion standards (SynFlinc). Thiryt-two quartz grains were selected for fluid-inclusion analysis and the total number of analyzed inclusions is 256.
Isochore calculations for the CO2 and H2O inclusions were made using the equation of states for pure carbon dioxide and brine by Bottinga and Richet (1981) and Brown and Lamb (1989) with the aid of a Windows version of the computer program FLINCOR (Brown 1989).
Fluid Inclusion Petrography
The observed fluid inclusions in the shocked quartz can be grouped into four fluid-inclusion assemblages (FIA) based on their petrographic and textural relationships (Fig. 3): (1) isolated and solitary implosion halos (IPs), (2) decorated planar deformation features (PDFs), (3) planar fractures (PFs), and (4) sub- and nonplanar microfractures (MFs). Each FIA represents a group of cogenetic fluid inclusions.
Etching of quartz grains with HF revealed no glass in the PDFs. The PDFs are only visible as straight parallel trails of open fluid inclusions (Figs. 4a and 4b). For comparison, etching of plagioclase grains produced planar features, very similar to the PDFs in quartz grains, marked by aligned etch pits (Fig. 4c).
The grayscale SEM-CL image indicates that there is a slight luminescent difference between the host quartz and the quartz associated with the PDFs (Figs. 5a and 5b). The quartz in the PDFs is slightly more luminescent than the host quartz. Fluid inclusions occur as nonluminescent black dots arranged along the PDFs.
Compositionally, the fluid inclusions can be divided into CO2 and H2O dominant types. At room temperature, the CO2 inclusions occur as two-phase liquid–gas inclusions and one-phase liquid inclusions that nucleate a vapor bubble upon cooling. Generally, the size of the CO2 inclusions ranges from about 2 to 10 μm in the longest dimension. Due to the dark appearance and extremely small size of the CO2 inclusions, the microthermometric measurements were possible only from the inclusions ≥5 μm. H2O inclusions, easily recognizable by their clear appearance, are present either as two-phase liquid–gas inclusions or as one-phase liquid inclusions. Their size ranges from about 5 to 30 μm.
The IPs and PDFs contain solely CO2 inclusions, whereas the PFs and MFs contain both CO2 and H2O inclusions in separate trail sets, that is, some PFs and MFs have CO2 inclusions and others H2O inclusions. In all the FIAs, melting temperatures of the CO2 inclusions are close to −56.6 °C ± (0.1–0.3) °C, indicating the presence of practically pure CO2, and the homogenization temperatures (ThCO2) to liquid are from −19 to 25 °C. This indicates a large range in the density or molar volume (approximately 1.03–0.71 g mL−1 or approximately 42–62 mL mol−1) of the carbonic phase. The distribution of the ThCO2 indicates that each FIA has a single discrete population of CO2 inclusions (Fig. 6a).
In the PFs and MFs, the H2O inclusions show melting temperatures of ice from −7.5 to −2.5 °C and from −2.0 to −1.4 °C, indicating salinities from 11 to 4 and from 3.3 to 2.3 weight percent NaCl equivalent, respectively. As shown by the distribution of the homogenization temperatures to liquid (Fig. 6b), the PFs contain a single high-temperature (217–238 °C) and the MFs a single low-temperature (106–155 °C) population of aqueous inclusions. In the PFs, the high-temperature and moderate saline fluid is commonly associated with chlorite (Fig. 3f). In the PFs and MFs, the bulk densities of the aqueous inclusions are 0.88–0.92 and 0.94–0.97 g mL−1, respectively.
In the Keurusselkä impact structure, the sequence of events, fluid composition (CO2), and spatial occurrence of the inclusions are very similar to those of the Vredefort Dome, South Africa (Schreyer and Medenbach 1981; Schreyer 1983). In both instances, the target rocks are granitoids of high metamorphic grade terrain. In Vredefort, CO2 fluid is possibly derived from old inclusions of the lower crustal granulites.
The sets of isochores for the different fluid inclusion types in the different FIAs corresponding to the fluid densities derived from their homogenization temperatures are presented in Fig. 7. At Keurusselkä, the isochores for the CO2 inclusions in the PDFs cross the P–T field of the Svecofennian metamorphism, suggesting the peak metamorphic origin for the fluids. We suggest that the impact-driven momentary overpressure caused the collapse (implosion) of the pre-existing peak metamorphic CO2 inclusions and the migration of the liberated fluid to the glass in the PDFs. The marked density difference of the CO2 inclusions between the different FIAs suggests their trapping over a wide range of P–T conditions.
In grayscale CL-images, the Keurusselkä PDFs showed weak luminescence compared with the host quartz, which would be more characteristic of tectonic deformation lamellae than PDFs. However, Hamers and Drury (2011) showed that annealed PDFs in old impact sites like Vredefort show red luminescence, and it is only the young amorphous PDFs that are nonluminescent. Due to the nonluminescent background quartz in our samples, it is possible that the red luminescence would be weakly visible in grayscale CL images. The annealed character of the Keurusselkä PDFs is also supported by the fact that no amorphous component was observed by the HF etching of the samples.
The IPs represent remnants of dense CO2-bearing Svecofennian peak metamorphic fluid inclusions that have collapsed during momentary overpressure relative to their original internal pressure. These kinds of conditions could have been achieved by tectonic isobaric cooling (e.g., Vityk and Bodnar 1995) or impact driven (Elwood Madden et al. 2004). Due to the proven impact history of the Keurusselkä site (Ferrière et al. 2010), the original peak metamorphic inclusions should no longer exist in the rocks of the central uplift area, and the IPs must therefore be impact-derived features. If this is the case, the experimental work by Elwood Madden et al. (2004) indicates their formation at pressures from 7.6 to 10 GPa. This is consistent with the pressure range of 2–20 GPa suggested by Ferrière et al. (2010).
The homogenization temperatures of aqueous inclusions define two populations (PFs and MFs) trapped at different P–T conditions (Fig. 6b). In the PFs, the temperature range of the H2O inclusions (217 and 238 °C) is markedly higher than in the MFs (106–155 °C), and thus, the brine in the PFs may represent a fluid responsible for the initial stage of the postimpact hydrothermal alteration of the crater rocks. However, these homogenization temperatures represent absolute minimum trapping temperatures. During the approximately 1.1 Ga of continental erosion, several kilometers of bedrock have been lost from the surface, including all impact lithology. The crustal depth at the time of the impact is difficult to estimate, and thus the true trapping temperatures of the aqueous inclusions in the PFs remain open. In the nonplanar MFs, the low-salinity/low-temperature H2O fluids represent the youngest observed fluid in the shocked quartz. This fluid possibly represents a very late hydrothermal system recharged by meteoric waters.
Based on petrographic/textural relationships of the FIAs and fluid densities of the different fluid-inclusion types in the shocked quartz, a tentative postimpact cooling history of the granitoid rocks is indicated in Fig. 7. The suggested P–T path is further defined by the two sets of intersecting isochores for the compositionally contrasting fluid-inclusion types (i.e., CO2 and H2O) in the PFs and MFs located in the two-phase subsolvus region of the CO2–H2O–NaCl system.
Several kilometers underneath the Keurusselkä impact site, the shock pressure of 7.6–10 GPa resulted in the implosion of the pre-existing Svecofennian peak-metamorphic CO2-rich fluid inclusions, as indicated by the presence of IPs in the shocked quartz. The postimpact integration of the CO2 from glass to the annealed quartz caused the formation of PDFs now decorated by the dense CO2 inclusions. Under the central uplift area of the impact site, the formation of the impact-induced hydrothermal system caused re-opening of some PFs and subsequent trapping of moderate-density CO2 and saline H2O inclusions. The fluid-inclusion data suggest high initial temperatures for the hydrothermal fluids (>200 °C). A possible late re-activation of the shear zones and associate circulation of meteoric waters is indicated by the low-salinity H2O inclusions in the MFs (<200 °C).
The authors thank K. Kinnunen and B. Johanson from the Geological Survey of Finland (GTK) for providing the Keurusselkä samples and for the SEM-CL imaging, respectively. The constructive criticisms and comments by associate editor C. Koeberl, R. Bodnar, A. van den Kerkhof, T. Öhman, K. Kirsimäe, M. Hamers, and an anonymous reviewer led to considerable improvement of the manuscript, for which the authors are very grateful.