Colin W. Taylor

Fluid flow through rough fractures in rocks I: high resolution aperture determinations

Abstract Full characterisation of rough fracture surfaces and the resulting variable apertures is an important step in the drive towards an improved understanding of the factors which control fluid flow through rocks. A number of surface profiling techniques exist, such as needle or laser profiling. However, these techniques are difficult and time consuming to apply to the total area of a fracture because they involve measuring a large number of parallel profiles, which are then difficult to align accurately. Hence, although a single profile may have high resolution in the z-direction and the direction of the profile, the fracture surface has a much lower resolution due to alignment errors. We have developed and improved considerably a previously existing optical concept for obtaining the topography of a fracture surface where the information about surface heights is measured simultaneously. The method relies on the construction of high fidelity transparent models of the fracture surface, which are imaged while covered with dyed and undyed water. The resulting images are converted to topography by a simple calibration procedure that makes use of the Lambert–Beer law. This method utilises in-house OptiProf™ software, which provides considerably more control over the imaging process than previous attempts at optical profiling. We have applied the technique to a range of rock fractures and test pieces, and have found the technique to work reliably, with lateral resolutions of 15 μm, and a vertical resolution of 15 μm, although all these could be improved by increasing the pixel count and the bit-depth of the camera, respectively.

Sulfur isotope signatures for rapid colonization of an impact crater by thermophilic microbes

In the 23-km-diameter Haughton impact structure, Canadian High Arctic, in sulfate-rich bedrock, widespread hydrothermal sulfide mineralization occurred in breccias formed during the impact. The sulfides exhibit extreme sulfur isotopic fractionation relative to the original sulfate, requiring microbial sulfate reduction by thermophiles throughout the crater. This evidence of widespread microbial activity demonstrates that colonization could occur within the lifetime of a moderately sized, impact-induced hydrothermal system. The pyrite was subsequently oxidized to jarosite, which may also have been microbially mediated. The successful detection of evidence for microbial life suggests that it would be a valuable technique to deploy in sulfate-rich impact terrain on Mars.

Preservation of biological markers in clasts within impact melt Breccias from the Haughton impact structure, Devon Island

The 39 +/- 2 Ma Haughton impact structure on Devon Island comprises a thick target succession of sedimentary rocks, mainly carbonates. The carbonates contain pre-impact organic matter, including fossil biological markers. Haughton is located in an area where no major thermal event has affected the sedimentary succession after heating caused by impact. This makes Haughton uniquely suitable for studies concerning the preservation of fossil biological markers following an impact event. Melt breccia is the most common impactite at Haughton. It is composed of clasts of the target, mainly carbonates, embedded in a fine groundmass. The groundmass is composed of material that was melted during impact. In this study, fossil biological marker maturity parameters (tricyclic terpane-hopane ratio and pregnane-sterane ratio) and an aromatic maturity parameter [methylphenanthrene ratio (MPR)] were used to compare the degree of thermal alteration in different size fractions of carbonate clasts (<0.5-4 cm in diameter) and between edges and centers of large carbonate clasts (15-20 cm in diameter). The data show that fossil biological markers can be preserved and detected in isolated large and small fractions of carbonate clasts that are embedded in an impact melt. The results also indicate that there is a thermal gradient from the center of a clast to the edge of a clast, which suggests that biological markers are more likely to be found preserved in the center of a clast. The thermal maturity values point to a higher degree of thermal alteration in the melt breccia carbonate clasts than in the coherent carbonate bedrock.

Characterization of rough-walled fractures in crystalline rocks

Abstract The full characterization of the rough surfaces of fractures and their resulting apertures is an important step in the drive toward an improved understanding of the factors which control fluid flow through rocks. This is crucial in igneous and metamorphic rocks, since fractures in these rocks may form the only significant pathways for fluid migration. Here we describe a three-pronged approach for the full characterization of rough fracture surfaces in a selection of crystalline rocks using a suite of software developed in house. Firstly, profiling is carried out using an optical method, which converts images of epoxy fracture surfaces covered with dyed water into topographies using the Lambert-Beer Law. Many hardware and software (OptiProf™) developments give this method the upper hand over previous attempts at spectrophotometric analysis. It is not possible to profile every fracture surface, therefore numerical modelling of fluid flow is carried out using synthetic fractures with rough fracture surfaces that are representative of the natural rock fractures. ParaFrac™ allows the analysis and parametrization of fracture surfaces and apertures. SynFrac™ enables the numerical synthesis of fracture surfaces and apertures with prescribed basic parameters. Both procedures take full account of the complex matching properties of the fracture surfaces as a function of wavelength, as well as anisotropy within the properties defining the fracture surfaces and their resulting aperture. They have been rigorously tested on a large suite of synthetic fractures as well as real rock fractures. These tests have allowed relationships between the standard deviation of surface asperity heights, the fractal dimension and the matching parameters to be related to the resulting aperture of the fractures.

A microbial role in the construction of Mono Lake carbonate chimneys

Lacustrine carbonate chimneys are striking, metre-scale constructions. If these were bioinfluenced constructions, they could be priority targets in the search for early and extraterrestrial microbial life. However, there are questions over whether such chimneys are built on a geobiological framework or are solely abiotic geomorphological features produced by mixing of lake and spring waters. Here, we use correlative microscopy to show that microbes were living around Pleistocene Mono Lake carbonate chimneys during their growth. A plausible interpretation, in line with some recent works by others on other lacustrine carbonates, is that benthic cyanobacteria and their associated extracellular organic material (EOM) formed tubular biofilms around rising sublacustrine spring vent waters, binding calcium ions and trapping and binding detrital silicate sediment. Decay of these biofilms would locally have increased calcium and carbonate ion activity, inducing calcite precipitation on and around the biofilms. Early manganese carbonate mineralisation was directly associated with cell walls, potentially related to microbial activity though the precise mechanism remains to be elucidated. Much of the calcite crystal growth was likely abiotic, and no strong evidence for either authigenic silicate growth or a clay mineral precursor framework was observed. Nevertheless, it seems likely that the biofilms provided initial sites for calcite nucleation and encouraged the primary organised crystal growth. We suggest that the nano-, micro- and macroscale fabrics of these Pleistocene Mono Lake chimneys were affected by the presence of centimetre-thick tubular and vertically stacked calcifying microbial mats. Such carbonate chimneys represent a promising macroscale target in the exploration for ancient or extraterrestrial life.