S.K. Frape

Abiogenic methanogenesis in crystalline rocks

Isotopically anomalous CH4-rich gas deposits are found in mining sites on both the Canadian and Fennoscandian shields. With δ 13CCH4 values from −22.4 to −48.5% and δDCH4 values from −133 to −372%, these methane deposits cannot be accounted for by conventional processes of bacterial or thermogenic methanogenesis. Compositionally the gases are similar to other CH4-rich gas occurrences found in Canadian and Fennoscandian shield rocks (Sherwood Lollar et al., 1993). However, the isotopically anomalous gases of this study are characterized by unexpectedly high concentrations of H2 gas, ranging from several volume percent up to 30 vol%. The H2 gases are consistently depleted in the heavy isotope, with δDH2 values of −619 to −659‰ 3H/4He ratios in the range of 0.4 × 10−8 to 4.4 sx 10−8 indicate that there is no resolvable component of mantle-derived He in these deposits. Based on these results a mantle-derived source for the C-bearing gases is unlikely. n nSeveral lines of evidence support an alternative abiogenic origin for the gases from Sudbury, Canada, and Juuka and Pori, Finland. The D-depleted H2 gas and calculated ΔD(CH4−H2) equilibration temperatures of 110–170°C at all three sites are in good agreement with results obtained for abiogenically produced CH4 and H2 in ophiolite sequences in Oman and the Philippines. Serpentinization and the hydration of ultramafic rocks are the proposed mechanisms for CH4 and H2 production in these ophiolite sequences. The widespread occurrence of serpentinized and altered ultramafic rocks at Juuka and at a number of other sites in both Canada and Finland implies that similar mechanisms may be involved in gas production at at least three sites on the shields. The origin of the gases at the remaining shield sites is discussed. Alternative hypothesis include (1) production of the rest of the shield gases by mixing between abiogenic endmembers and bacterially generated hydrocarbon gas such as identified elsewhere on the Canadian and Fennoscandian shields (Sherwood Lollar et al., 1993); and (2) production of a series of isotopically distinct abiogenic CH4 endmembers at each site due to variability in the isotopic composition of available carbon sources. We cannot conclusively distinguish between the alternative scenarios based on existing data. However, the evidence for an abiogenic CH4 endmember at at least three sites emphasizes the need for substantial revision of current theories of methanogenesis. Abiogenic processes of CH4 production may be considerably more widespread than previously anticipated.

Evidence for bacterially generated hydrocarbon gas in Canadian shield and fennoscandian shield rocks

Hydrocarbon-rich gases found in crystalline rocks on the Canadian and Fennoscandian shields are isotopically and compositionally similar, suggesting that such gases are a characteristic feature of Precambrian Shield rocks. Gases occure in association with saline groundwaters and brines in pressurized “pockets” formed by sealed fracture systems within the host rocks. When released by drilling activities, gas pressures as high as 5000 kPa have been recorded. Typical gas flow rates for individual boreholes range from 0.25 L/min to 4 L/min. The highest concentrations of CH4 are found in the deepest levels of the boreholes associated with Caue5f8Naue5f8Cl (and Naue5f8Caue5f8Cl) brines. N2 is the second major component of the gases and with CH4 accounts for up to 80 to >90 vol%. Higher hydrocarbon (C2+) concentrations range from <1 to 10 vol.%, with C1/(C2 = C3) ratios from 10−1000. Isotopically the gases show a wide range of values overall (σ13C = −57.5 to −41.1%; σD = −245 to −470‰) but a relatively tight cluster of values within each sampling locality. The Enonkoski Mine methanes are unique with σ13C values between −65.4 and −67.3‰ and σD values between −297 and −347‰. n nThe shield gases are not readily reconcilable with conventional theories of methanogenesis. The range of C1/(C2 + C3) ratios for the shield gases is too low to be consistent with an entirely bacterial origin. In addition, σDCH4 values are in general too depleted in the heavy isotope to be produced by thermogenic methanogenesis or by secondary alteration processes such as bacterial oxidation or migration. However, isotopic and compositional evidence indicates that bacterially derived gas can account for a significant component of the gas at all shield sites. Conventional bacterial gas accounts for 75–94 vol% of the occurrences at Enonkoski Mine in Finland. At each of the other shield sites, bacterial gas can account for up to 30–50 vol% of the total gas accumulation. This study and other recent evidence of active bacterial communities in deep hydrogeological environments emphasize the need for more comprehensive investigation of the role of microorganisms in the deep subsurface.

7.15 – Deep Fluids in the Continents

The debate on the origin and evolution of the fluids in crystalline rocks is very much an ongoing research topic. A number of experimental and research sites, such as the ones discussed in this article, will continue to produce hydrogeochemical information well into the middle of this century as researchers around the globe attempt to understand the hydrogeology and geochemistry of crystalline rock environments.

Sulfate in brines in the crystalline rocks of the Canadian shield

Abstract Deep groundwaters in crystalline rocks typically are very saline and are characterized by a rather unique Ca-Na-Cl-dominated chemistry. Sulfate is present in variable amounts and may be linked to both the geochemical evolution of these fluids as well as to recent processes initiated through mining activities. It is possible to distinguish on the basis of isotopic compositions between brine sulfate and secondary sulfate formed by oxidation of Sulfides: The latter is characterized by δ 34 S values which reflect the local mineral sulfide precursor and δ 18 O close to or below 0%. SMOW. The isotopic composition of the brine sulfate is characterized by δ 18 O and δ 34 S values which resemble marine isotopic compositions at some localities, at others they could be explained as being of magmatic/hydrothermal origin. It is likely that the sulfate participated in the geochemical evolution of these brines. Thus, its isotopic composition reflects geochemical processes rather than a primary origin. No evidence for the influence of bacterial reduction was found.

Challenges for Coring Deep Permafrost on Earth and Mars

A scientific drilling expedition to the High Lake region of Nunavut, Canada, was recently completed with the goals of collecting samples and delineating gradients in salinity, gas composition, pH, pe, and microbial abundance in a 400 m thick permafrost zone and accessing the underlying pristine subpermafrost brine. With a triple-barrel wireline tool and the use of stringent quality assurance and quality control (QA/QC) protocols, 200 m of frozen, Archean, mafic volcanic rock was collected from the lower boundary that separates the permafrost layer and subpermafrost saline water. Hot water was used to remove cuttings and prevent the drill rods from freezing in place. No cryopegs were detected during penetration through the permafrost. Coring stopped at the 535 m depth, and the drill water was bailed from the hole while saline water replaced it. Within 24 hours, the borehole iced closed at 125 m depth due to vapor condensation from atmospheric moisture and, initially, warm water leaking through the casing, which blocked further access. Preliminary data suggest that the recovered cores contain viable anaerobic microorganisms that are not contaminants even though isotopic analyses of the saline borehole water suggests that it is a residue of the drilling brine used to remove the ice from the upper, older portion of the borehole. Any proposed coring mission to Mars that seeks to access subpermafrost brine will not only require borehole stability but also a means by which to generate substantial heating along the borehole string to prevent closure of the borehole from condensation of water vapor generated by drilling.

New Sampling Devices for Environmental Characterization of Groundwater and Dissolved Gas Chemistry (CH4, N2, He).

The objective of this paper is to highlight an underutilized resource in groundwater chemistry investigations and to introduce an innovative set of sampling devices designed to take advantage of this resource. While boreholes drilled for oil and gas exploration have long been used to derive hydrogeologic information (1, 21, boreholes drilled for mineral exploration have rarely been taken advantage of. Throughout crystalline rock terrains on the Canadian and Fennoscandian Shields in particular, large numbers of uncased exploration boreholes extend from the surface and from underground mine workings. These boreholes intersect hydrogeologic regimes ranging from near-surface meteoric groundwaters to deep saline formation waters and brines (3, 4 ) . This paper will demonstrate how sampling devices developed at the University of Waterloo can be used to take advantage of such borehole networks and to provide a wealth of geochemical and isotopic data on both dissolved gas and groundwater chemistries (5,6). While the use of uncased boreholes does not permit the detailed stratigraphic interval sampling control of conventional cased and packered sampling instrumentation, it nonetheless presents certain distinct advantages. By taking advantage of existing boreholes, this sampling approach eliminates the high cost of mounting an independent drilling program and provides an economical reconnaissance tool. The technique is ideally suited to provide rapid, low-cost evaluation of groundwater properties in unconventional hydrogeologic settings such as underground excavations and to provide preliminary data on which to base the selection of boreholes for more extensive casing installation and instrumentation. A variety of downhole sampling devices have been developed in recent years (4, 7-10). Each however has limitations with respect to flexibility of sample type and sample size, maximum operating pressures and depth, portability, and adaptability to nonideal field conditions. In order to overcome these deficiencies, the Waterloo samplers were designed to be narrow-diameter (3.18 cm), self-contained units (with internal power units and triggering devices), capable of withstanding external pressures of up to 10 000 KPa and of operating in both freshwater