Impactinduced Hydrothermal Systems

The formation of an impact crater is a short-lived catastrophic event that occurs when an asteroid or comet impacts on the surface of a planetary body (Shoemaker, 1963; Grieve, 1987; Melosh, 1989; Toon et al., 1997; Grieve and Therriault, 2004). The cratering process can be divided into three major stages (Gault et al., 1968): (1) contact and compression, (2) excavation, and (3) modification. The first two stages induce a shock deformation of the bedrock and cause intense fracturing on a micrometer to kilometer scale. These events are followed by the modification stage where the gravitational collapse of the transient crater causes a complex interaction of a series of concentric and radial faults, all of which provide excellent fluid pathways for the subsequently formed impact-induced hydrothermal system. Geochemical studies of impact structures have demonstrated that the temperature range of impact-induced hydrothermal systems can be favorable for thermophilic microbial communities (Osinski et al., 2001; Kirsimae et al., 2002; Naumov, 2002; Hode et al., 2003; Versh et al., 2005), and that these systems can remain active for thousands of years (Abramov and Kring, 2004; JSeleht et al., 2005).

Impact-induced hydrothermal systems (Newsom et al., 1986, 2001; McCarville and Crossey, 1996; Cockell and Lee, 2002), therefore, represent target areas of interest for sample return missions. Impact craters on Mars are easy to identify from orbit, and any associated hydrothermal mineral assemblages should be localized to a relatively narrow ring around the impact structures (Newsom et al., 2001; Hode et al., 2003).

Here, we report evidence for microbial biosignatures in an ancient impact-induced hydrothermal system in Siljan, Sweden. A variety of mineralized (some carbonaceous) features similar to partially mineralized hyperthermophilic biofilm remnants that occur in modern hot springs were discovered. These findings support the hypothesis that impact-induced hydrothermal systems may be favorable targets in the search for evidence of fossil life, a discovery that may have implications for the search for ancient life on Mars.

2. The Siljan Impact Structure

The Late Devonian (Bottomley et al., 1978; Reimold et al., 2005) Siljan Impact Structure, located in central Sweden (Fig. 1a), is the largest known impact structure in Western Europe with a present topographic expression of approximately 80 km. The target sequence consisted of a Proterozoic basement covered by Paleozoic sedimentary rocks of Ordovician to early Devonian age. Though the exact thickness of the sedimentary cover at the time of impact is unknown, Grieve (1988) estimated that the maximum amount of erosional unloading was 1 km. The Siljan Impact Structure still resembles a typical impact ring structure with a central uplifted peak; the central 30 km wide region of fractured, brecciated, and shocked Proterozoic igneous rock is surrounded by a peripheral trough comprised mainly of Paleozoic sedimentary rock.

Throughout the impact structure are traces of a post-impact hydrothermal system, as evidenced by the presence of hydrothermally precipitated mineral phases that include quartz, calcite, fluorite, pyrite, galena, and sphalerite (Hode et al., 2003; Komor et al., 1988a, b; Valley et al., 1988). Hydrothermal quartz occurs exclusively as vein and breccia filling in the crystalline basement inside the central uplifted region, whereas calcite, fluorite, and sulphide minerals are present in fractures and veins in the down-faulted sedimentary rocks that consist mainly

Pyrite Found Impact Breccia

Figure 1. (a) (Modified after Hode et al., 2003). Map showing the location of the Siljan Impact Structure located in Dalarna, south central Sweden. (b) Schematic overview of the Siljan Impact Structure with projected isotherms that outline three main fluid temperature regimes of the impact-induced hydrothermal system. The isotherms and the sedimentary rocks (marked as light grey in the figure) that lie parallel to the ring-shaped structure are preserved even after 1 km of erosional unloading due to the severe downfaulting (Fig. 2a) in the perimeter around the central uplift caused by the impact event. The star in Fig. 1b indicates the location of the Jutjarn Quarry from which the limestone sample described in this case study was collected.

Figure 1. (a) (Modified after Hode et al., 2003). Map showing the location of the Siljan Impact Structure located in Dalarna, south central Sweden. (b) Schematic overview of the Siljan Impact Structure with projected isotherms that outline three main fluid temperature regimes of the impact-induced hydrothermal system. The isotherms and the sedimentary rocks (marked as light grey in the figure) that lie parallel to the ring-shaped structure are preserved even after 1 km of erosional unloading due to the severe downfaulting (Fig. 2a) in the perimeter around the central uplift caused by the impact event. The star in Fig. 1b indicates the location of the Jutjarn Quarry from which the limestone sample described in this case study was collected.

of carbonates (Fig. 2a) in the eastern part of the structure (Hode et al., 2003). The formation temperature (Fig. 1b) of the hydrothermally precipitated minerals ranged from 342°C for the earliest formed quartz in the central part of the structure (Lindblom and Wickman, 1985) to below 75°C for minerals precipitated in the outer regions of the crater (Hode et al., 2003). A general calcite-fluorite-pyrite-galena-sphalerite-quartz paragenetic sequence for the low temperature hydrothermal minerals was proposed by Hode et al. (2003).

An important consideration when evaluating a purported biosignature is to determine whether the paleoenvironment in which it was found could have supported life. Thermo-philic and hyperthermophilic microorganisms thrive in hydrothermal systems where fluid temperatures remain below the boiling point (Stetter, 2006). Fluid inclusion microthermometry, which in this study included a pressure correction for 1 km erosional unloading, indicates that the formation temperature of the hydrothermal calcite ranged between 76°C and 110°C (Hode et al., 2003), a temperature interval that overlaps with the temperature range for hyperthermophiles (<80°C to 100°C at 1 atm, Stetter, 1996, 2006). The mineral assemblage within the veins and oil-bearing fluid inclusions indicates that the

Figure 2. Sample site in the Jutjarn Quarry in Dalarna, Sweden. (a) Downfaulted Ordovician limestone in the Jutjarn Quarry on the eastern side of the Siljan Impact Structure. (b) (Modified after Hode et al., 2003.) Close-up of the hydrothermal calcite vein that was investigated in this study.

circulating hydrothermal solutions were anoxic and rich in hydrocarbons (Vlierboom et al., 1986). The occurrence of metal sulphides in the veins of the Siljan Impact Structure is consistent with a scenario in which reduced metal ions could have acted as electron donors, possibly together with molecular hydrogen. The geochemical characteristics of the hydrothermal fluid are similar to those reported for fluid associated with the 1,640 Ma McArthur River (also known as HYC) lead-zinc ore deposits and host sediments in Northern Territory, Australia, which are hypothesized to have supported subsurface anaerobic chemolitho-trophic communities (Logan et al., 2001).

3. Materials and Methods

The findings of this case study are based on the results of an extensive submicro-scopic study of one hand sample (#S39-01b-01, Figs. 2b and 3), which is part of a larger sample set of rocks associated with the Siljan Impact Structure that was collected by T. Hode (Hode et al., 2003; Hode, 2005). Sample #S39-01b-01, a piece of limestone collected from exposed outcrop (Fig. 2a) at the active Jutjarn Quarry, contains calcite-filled veins (Fig. 3) that precipitated at 90-110°C (determined by micro-thermometry with a pressure correction for 1 km erosional unloading).

The Jutjarn Quarry is located a few kilometers outside the central uplift of the im-pact structure in an area that was most likely located below the impact melt sheet when the hydrothermal system was active. The current surface features, hydrothermal veins, and mineral assemblages were exposed after ~1 km of erosional unloading and are, thus, remnants of a deep reaching impact-induced hydrothermal system. The calcite vein from which the case study sample was collected cross-cuts the tilted layers of downfaulted sediments, indicating that the vein post-dates the impact event. Since no regional thermal event has occurred after the impact and no traces of hydrothermal mineralization induced by the Caledonian orogeny (~400 Ma) have been reported in the area, the hydrothermal mineral assemblages could only have been formed by the residual heat from the actual impact event.

The sample was sawn into serial sections that were prepared in three different ways: one serial section was chelator-etched with 2% (m/v) EDTA for 30 minutes, another was acid-etched with 30% (v/v) HCl for 30 seconds, and a third "control split" was not etched. Splits of the etched sections, fractured to fit on specimen holders for SEM analysis, were individually washed in distilled deionized water prior to and after etching. Immediately after the water-rinsed specimens dried, they were gold coated. Splits of the non-etched control sample were washed in distilled deionized water twice prior to gold coating.

The specimens were investigated with an Hitachi S-4300 field emission scanning electron microscope (FE-SEM) at the Swedish Museum of Natural History, and a FEI Sirion FESEM at Portland State University, both of which were equipped with an Oxford Instruments EDS detector.

Figure 3. Hand specimens and thin sections of the samples investigated. (a) Sawn hand specimen with calcite vein cross-cutting the limestone matrix. (b) (Modified after Hode et al., 2003.) Microphotograph shows the boundary between the calcite vein and the carbonate matrix. The layer of calcite crystals adjacent to the wall of the vein contain smaller crystals than those formed toward the center of the vein, an observation consistent with mineral precipitation from cooling hydrothermal fluids. (c) Photograph of a doubly polished thin section used for fluid inclusion microthermometry. Pyrite crystals (shown with arrows) indicate a reducing fluid environment. (d) Photomicrograph showing oil-rich fluid inclusions. Microthermometry indicated that the minimum formation temperature of the calcite crystals ranged between 90°C and 110°C after pressure correction, and a salinity of 0-2.4 eq wt% NaCl. The formation temperature confirms that the mineral assemblage originated from hydrothermal fluids and not from later-stage groundwater fluids (see also Fig. 2b). The oil in the Siljan Impact Structure is present either in fluid inclusions or in cracks and pore spaces in the rock.

Figure 3. Hand specimens and thin sections of the samples investigated. (a) Sawn hand specimen with calcite vein cross-cutting the limestone matrix. (b) (Modified after Hode et al., 2003.) Microphotograph shows the boundary between the calcite vein and the carbonate matrix. The layer of calcite crystals adjacent to the wall of the vein contain smaller crystals than those formed toward the center of the vein, an observation consistent with mineral precipitation from cooling hydrothermal fluids. (c) Photograph of a doubly polished thin section used for fluid inclusion microthermometry. Pyrite crystals (shown with arrows) indicate a reducing fluid environment. (d) Photomicrograph showing oil-rich fluid inclusions. Microthermometry indicated that the minimum formation temperature of the calcite crystals ranged between 90°C and 110°C after pressure correction, and a salinity of 0-2.4 eq wt% NaCl. The formation temperature confirms that the mineral assemblage originated from hydrothermal fluids and not from later-stage groundwater fluids (see also Fig. 2b). The oil in the Siljan Impact Structure is present either in fluid inclusions or in cracks and pore spaces in the rock.

Rutherford backscattering (RBS) element analysis was performed on the specimens with the use of a nuclear microprobe located at the Nuclear Microprobe Facility at the Lund Institute of Technology, Sweden (Malmqvist et al., 1993). All of the RBS data were analyzed with the SIMNRA software developed at the Max Planck Institute of Plasma Physics. An RBS spectral plot represents the energy loss of the accelerated alpha particles after they scatter from nuclei in the sample. The alpha-particle beam size was set to approximately 5 |im for the experimental analyses made during this study.

Incoming alpha particles that undergo RBS loose a portion of their energy due to the transfer of momentum that occurs during their collision with a target nuclei and atomic electrons encountered prior to and after the collision. The amount of energy lost by a backscattered alpha particle during an elastic scattering event depends on the mass of the sample nucleus encountered and the depth at which the scattering event occurs in the sample. The measured yield (i.e. the relative number of counts) from a specific element in the sample depends on the probability that the projectile will collide with it (i.e., a function of the effective size or scattering cross section of the nucleus) and on the concentration of the element in the sample.

An RBS spectrum thus represents the total number of backscattered alpha particles (y-axis) plotted as a function of their respective energies (x-axis). As discussed in the next section, the SIMNRA analysis of the data not only revealed which elements are present in the sample, but also illustrated the relative proportion of these elements at the point analysis site.

4. Results

A comparison of FE-SEM photomicrographs of etched vs. non-etched specimens revealed that biogenic-like features were partly encased in the hydrothermal minerals that precipitated within the impact-induced fractures (Figs. 4, 5, 6a, 8). Biogenic-like features associated with calcite crystals and calcite/pyrite grain boundaries included bundles of thread-like objects (Figs. 4 and 5) and stretched, curved films (Figs. 4, 6a, and 8).

The montage of SEM images of the vein shown in Fig. 4 provide a spatial context for the various biogenic-like features observed in the specimen. Though several hydrothermal assemblages of various temperature regimes and mineral facies were investigated for similar structures, only the low-temperature calcite-filled veins contained evidence of biofilm-like remnants.

Evidence that the film-like feature shown in Fig. 6a is calcified is illustrated by a comparison of the Rutherford backscattering (RBS) spectra shown in Figs. 6b, c: the position of the slopes that represent Ca in each spectrum start at 1,700 KeV. In other words, Ca was encountered at the immediate surface for both points where analyses were performed. If the film was not calcified, then the incoming alpha particles would have lost a portion of their energy as they penetrated through the film before they encountered the underlying carbonate, and the positions of the slopes that represent Ca in both spectra would have been located at different energies.

The depth sensitivity of the RBS element analysis revealed that the C content of the curved film-like feature shown in Fig. 6a is approximately three times as high as it is in the underlying carbonate and in the adjacent calcite grain. Spectral modeling with the SIMNRA program, which can provide a rough estimate of the individual components of the total spectrum, revealed that the higher C concentration in the curved film-like remnant (Fig. 6c) does not extend into the calcite grain situated beneath it (i.e., if it did, then the summation spectrum shown in Fig. 6c would be flat, as it is for

Figure 4. Montage of SEM micrographs that illustrate the relative distribution of various microbial-like features within the calcite vein. (a) Overview image of the investigated EDTA-etched calcite vein. Pyrite assemblages comprise the topographical highs since etching removed the top few tenths of micrometers of the surrounding calcite. Arrows indicate where the various features showed in Figs. 4b-g are located. All of the biogenic-like features were exposed by etching, an indication that they are located within the mineral matrix and are not later-stage contaminations. The crack along the right side of the image is the center of the vein. The overview image is a montage of three low-magnification SEM images. (b) A bundle of threads exposed by etching (close-up and further description of the feature is shown in Fig. 5c). (c) Microbial-like feature with a threaded and torn polymer-like structure. It is partly embedded in the calcite matrix, which indicates that it was trapped when the crystal was formed, and lies within the same calcite crystal as the feature shown in Figs. 4b and 5c. (d) Thread-like feature attached and stretched between pyrite (topographical high) and the calcite matrix (close-up shown in Fig. 5a). (e) Sheet-like feature (indicated by arrow) exposed after etching is located at the border between a pyrite crystal and the calcite matrix. See Fig. 6 for further discussion. (f) Sheet-like feature (indicated by arrow) wrapped around the edge of a pyrite aggregate. The feature is fully pyri-tized and no evidence for carbonaceous matter could be found. Note how the etching has removed the surrounding calcite leaving the pyrite exposed as topographical highs. See Fig. 8 for further discussion. (g) A pyrite framboid located inside the calcite matrix. Pyrite framboids are often found in reducing hydrothermal systems rich in carbonaceous matter (e.g., McKinley et al., 2000).

the summation spectrum that represents the adjacent calcite grain (Fig. 6b). Figure 7 shows the simulated SIMNRA spectral curve superimposed on the spectrum of the point analysis of the film-like feature (Fig. 6c). The simulated total spectrum was generated by summing the simulated spectral contributions of Ca, C, and O in the sample (Fig. 7). The dashed line in Fig. 7 indicates the shape of the simulated carbon curve of the spectrum from the point analysis of the adjacent calcite grain (Fig. 6b). The difference in

Figure 5. SEM photomicrograph images that illustrate the thread and polymer-like objects associated with the fossilized biofilm remnants. The features could not be remnants of fibrous illite as the EDS analysis indicated the absence of K and Al. (a) Threads partly embedded in a calcite matrix. The white rectangle marks the enlarged area on the right-hand side of the image, showing detail of the partly embedded threads. (b) A twisted film with threads attached to a calcite crystal (upper left corner) and clay fragment (lower right corner). (c) A bundle of threads situated on top of calcite and clay. The threads are broken on the lower right portion of the bundle, which indicates that they are brittle

Figure 5. SEM photomicrograph images that illustrate the thread and polymer-like objects associated with the fossilized biofilm remnants. The features could not be remnants of fibrous illite as the EDS analysis indicated the absence of K and Al. (a) Threads partly embedded in a calcite matrix. The white rectangle marks the enlarged area on the right-hand side of the image, showing detail of the partly embedded threads. (b) A twisted film with threads attached to a calcite crystal (upper left corner) and clay fragment (lower right corner). (c) A bundle of threads situated on top of calcite and clay. The threads are broken on the lower right portion of the bundle, which indicates that they are brittle and partially mineralized.

Figure 6. Photomicrograph and Rutherford backscattering (RBS) spectra of a purported microbial feature (also shown in Fig. 4e) and surrounding calcite grains. (a) SEM image that shows a smooth though curved film-like feature stretched between calcite (Ca) and pyrite (Py) crystals. Filled white circles indicate points of analysis. (b) RBS spectrum of the calcite grain (Ca) around and beneath the upper portion of the curved film-like feature (filled white circle to the upper right in Fig. 6a indicates point of analysis). The x-axis shows the energy of the backscattered alpha particles, which is determined by the cross-section (size) of the target nuclei and depth in the sample at which the scattering event took place. The y-axis shows the number of counts at a given energy. Carbon, oxygen, and calcium peak slopes are labeled in the spectrum (see also Fig. 7). (c) RBS spectrum of the curved film-like feature indicates an elevated (3x) carbon content (filled white circle to the lower left in Fig. 6a indicates point of analysis).

Figure 6. Photomicrograph and Rutherford backscattering (RBS) spectra of a purported microbial feature (also shown in Fig. 4e) and surrounding calcite grains. (a) SEM image that shows a smooth though curved film-like feature stretched between calcite (Ca) and pyrite (Py) crystals. Filled white circles indicate points of analysis. (b) RBS spectrum of the calcite grain (Ca) around and beneath the upper portion of the curved film-like feature (filled white circle to the upper right in Fig. 6a indicates point of analysis). The x-axis shows the energy of the backscattered alpha particles, which is determined by the cross-section (size) of the target nuclei and depth in the sample at which the scattering event took place. The y-axis shows the number of counts at a given energy. Carbon, oxygen, and calcium peak slopes are labeled in the spectrum (see also Fig. 7). (c) RBS spectrum of the curved film-like feature indicates an elevated (3x) carbon content (filled white circle to the lower left in Fig. 6a indicates point of analysis).

Was this article helpful?

0 0

Post a comment