William S. Fyfe

The evolution of the earth's crust: Modern plate tectonics to ancient hot spot tectonics?☆

Abstract Present models of crust formation and destruction are reviewed. The importance of the continental erosion—hydrosphere—subduction linkage is stressed. Convective cooling of new ocean crust is a major process involved in the geochemical cycle. Whether or not continental accretion or removal is occurring depends critically on the mass of sediment subducted. Models of the evolution of crust are reviewed but it is suggested that the various influences of seawater—volcanic convective interaction were much greater in the past. It is proposed that massive mixing processes occur near the base of the continental crust when mantle magmas are ponded near the Moho interface and this process is responsible for the general geochemistry of granitic magmas. In general, ancient motions may suggest that “hot spot” phenomena dominated compared to modern ocean-floor spreading processes.

Bacteria as nucleation sites for authigenic minerals in a metal-contaminated lake sediment

Abstract Sediment samples contaminated with metals arising from mine tailings drainage were obtained from Lower Moose Lake in the Onaping region near Sudbury, Ontario, Canada. The samples were examined by electron microscopy, selected-area electron diffraction and energy-dispersive X-ray spectroscopy. Individual bacterial cells and their remains were prominent as nucleation sites for both metal sulfides and a complex polymorphic (Fe,Al)-silicate. The principal metal sulfide species associated with the bacteria were amorphous mackinawite (FeS 1 - x ) and microcrystalline millerite (NiS). Trace amounts of Cu and Zu were also detected in some of the sulfide precipitates. At least two structural forms of the (Fe,Al)-silicate were present, and energy-dispersive X-ray spectroscopy point analyses revealed corresponding differences in chemical composition. Poorly ordered limonitic clay-type phases had a granular morphology and contained less Fe than well-developed crystalline material which generated hexagonal diffraction patterns with reflections ( d = 4.60 and 2.55 Ȧ) characteristic of an interstratified chamositic clay.

Metallic ion binding by Bacillus subtilis: Implications for the fossilization of microorganisms

The silicification of bacterial cells has been followed in a laboratory simulation by electron microscopy and energy-dispersive X-ray spectroscopy. Structural degradation of the bacteria was not influenced by silica availability, but the remains of degraded cells did promote the precipitation of silica. In contrast, cells reacted with ferric iron before aging remained intact, and structures conforming to the original morphology of the bacteria were preserved by silicification. The binding of metallic ions by microbial cells, in particular the retention of iron, is therefore considered to be an important contributing factor to the fossilization of microorganisms.

Rate of serpentinization in seafloor environments

The permeability of partially serpentinized peridotites has been measured near room temperature and found to be in the range of 10−11 to 10−12 darcys or 10−7 to 10−8 cm2 sec−1, comparable to shaley materials. Serpentinites act as semipermeable membranes to salt ions. In ocean crust and deep groundwater environments serpentinization must be a continuous process, controlled only by water access to ultramafic rocks through their cover. Given this, it will generate high local strains and stresses with episodic cracking. The serpentinization process may also contribute to heat flow and ocean floor topography.

Spatial distribution of microbial methane production pathways in temperate zone wetland soils: Stable carbon and hydrogen isotope evidence

The identity and distribution of substrates that support CH4 production in wetlands is poorly known at present. Organic compounds are the primary methanogenic precursor at all depths within anoxic wetland soils; however, the distribution of microbial processes by which these compounds are ultimately converted to CH4 is uncertain. Based on stable isotope measurements of CH4 and ΣCO2 extracted from soil porewaters in two temperate zone wetlands, we present evidence that a systematic spatial distribution of microbial methanogenic pathways can exist in certain anoxic, organic-rich soils. CH4 production by the acetate fermentation pathway is favored in the shallow subsurface, while methanogenesis from the reduction of CO2 with H2 becomes more predominant in older, less reactive peat at depth. This distribution can account for many of the reported CH4 emission characteristics of wetlands, in particular, their sensitivity to changes in primary productivity, temperature, and hydrology. These factors play an important role in controlling the short-term supply of labile substrates to fermentive methanogens in the shallow subsurface where the most intense CH4 production occurs. Predominance of the CO2-reduction pathway at depth may help to explain reports of CH4 with a semifossil age in lower peat layers.

The gold—carbonate association: Source of CO2, and CO2 fixation reactions in Archaean lode deposits

Abstract Abundant carbonate is a ubiquitous feature of Archaean lode gold deposits — both in domains of alteration enveloping veins, and in rocks enclosing auriferous sediments. Detailed studies of chemical mass balance in metabasalts, progressively altered towards gold-bearing quartz carbonate veins disposed within shear zones at Yellowknife, reveal massive additions of CO2, K, Si and Fe accompanying mineralisation, with concomitant depletions of Na. Coherent behaviour of Al, Sc, Zr (also V and Nb) provides a reference for constraining the volume relations during hydrothermal alteration. In the peripheral regions of alteration, depletions of Ca and Mg result in overall volume reduction, but these elements are added to veins and their immediate alteration envelopes where there is a large positive volume change. Precious metals, together with Cr, Ni, Cu, Zn, Pb, Rb, Cd and Ba, have been added to the veins and altered wall rocks. Whereas quartz, noble metals and other trace elements have been precipitated from hydrothermal fluids, solution of Ca, Mg (and some Fe), indigenous to peripheral alteration regions, combining with CO2 from the mineralising reservoir, appears to be the process for forming the abundant ferri-dolomite gangue. Observations of many Archaean Au deposits reveals that carbonate chemistry reflects the nature of wall rocks, with wall rocks donating the bivalent metal cations, and hydrothermal fluids the CO2. Alteration reactions at Yellowknife involved hydrolysis of albite accompanied by fixation of Kaqueous to produce muscovite, with Na loss; and hydrolysis of chlorite + epidote with CO2 fixation to form ferri-dolomite. Studies of oxygen isotopes, Fe 2+ ( Fe 2+ + Fe 3+ ) , and structure reveal that Au, quartz and carbonate were precipitated in the presence of fluids of probable metamorphic origin ( δ 18 O + 8–9‰ ), at low redox potential, and at ambient temperatures of 400–450°C, during episodes of hydrofracturing. The abundant CO2 and K, required for extensive carbonate—muscovite replacement alteration, could be supplied by fluids released during prograde metamorphism under greenschist or greenschist—amphibolite facies conditions, where the relative proportion of CO 2 H 2 O is in the order of 0.2 to 0.5, and K Na ⋍ 1 . Given high CO2CO in the hydrothermal reservoir, these molecules may act as complexing agents for transport of Au and other rare elements (e.g., W, Pd, Ni, Cr).

Chloritization of the hydrothermally altered bedrock at the Igarap Bahia gold deposit, Carajs, Brazil

The Igarapé Bahia gold deposit has developed from weathering of a near-vertical hydrothermal Cu (Au) mineralization zone. The unweathered bedrock composed of chlorite schists is mainly metamorphosed basalts, pyroclastic and clastic sedimentary rocks and iron formation. Contents and Fe/(Fe + Mg) ratios of chlorites increase from distal country rock towards the mineralization zone, which can be attributed to different water/rock ratios and locations in a hydrothermal system. In the hydrothermal system high salinity fluids convected through basin-floor rocks, stripping metals from the recharge zones with precipitation in discharge zones. The chlorite with lower Fe/(Fe + Mg) ratios indicates alteration by relatively unreacted Mg-rich fluids, occurring within recharge zones. By contrast, the chlorite with higher Fe/(Fe + Mg) ratios in the mineralization zone formed from solutions rich in Fe, Mn, Au, Cu, H2S and SiO2 within a discharge zone. The iron formation could also be formed within the discharge zone or on the basin floor from the Fe-rich fluids. The distal country rock with less chlorite content is a hydrothermal product at low water/rock ratios whereas the proximal country rock and the host rock with more chlorite content formed at high water/rock ratio conditions. The Al(IV) contents of chlorites indicate that the formation temperatures of these rocks range from 204 to 266 °C, with temperatures slightly increasing from distal country rock towards the mineralization zone.

Evolution of stable carbon isotope compositions for methane and carbon dioxide in freshwater wetlands and other anaerobic environments

Two types of distribution for αC values are observed in anaerobic environments when δ13C–ΣCO2 and δ13C–CH4 values are measured across gradients of depth or age of organic debris. The type-I distribution involves a systematic increase in αC values with depth as a result of decreasing δ13C–CH4 and increasing δ13C–ΣCO2 values. This behavior corresponds to a progressive increase in the prevalence of methanogenesis by the CO2 reduction pathway relative to acetate fermentation. Utilization of autotrophically formed acetate by methanogens would also cause an increase in αC values. The type-II distribution occurs when both δ13C–CH4 and δ13C–ΣCO2 values decrease with depth, resulting in approximately constant αC values. This condition corresponds with a strong dependence of methanogens on porewater ΣCO2 as a carbon source by way of either the CO2 reduction pathway or utilization of autotrophically formed acetate. Freshwater wetlands possess both types of αC value distribution. Wetlands with type-I distributions exhibit curves with slopes that vary probably as a function of deposition and preservation of labile organic carbon. An abundance of labile substrates in anaerobic soils yields steeper curves because aceticlastic methanogenesis predominates and δ13C–CH4 and δ13C–CO2 values are high. Diminished transfer of labile carbon to the methanogenic zone results in an increased prevalence of the CO2 reduction pathway, yielding low δ13C–CH4 values and shallowly sloping curves. Aerobic oxidation of organic matter or decay involving sulfate reduction produces CO2 with low δ13C values, which also will contribute to shallowly sloping curves. The size of the dissolved CO2 pool can influence the sensitivity of δ13C–CO2 values to change during methanogenesis. Regression curves of δ13C–CH4 and δ13C–ΣCO2 values from four wetlands with type-I distributions intersect at δ13C–CH4 = −40.7 ± 6.1‰ (1σ) and δ13C–ΣCO2 = −23.9 ± 4.8‰ (1σ). These values are similar to δ13C values for methyl and carboxyl moieties within acetate produced by anaerobic degradation of fresh C3 plant matter. A low abundance of acetate during aceticlastic methanogenesis will result in minimal expression of metabolic kinetic isotope effects (KIEs) and production of CH4 and CO2 with δ13C values similar to the intramolecular distribution of sedimentary acetate. The type-II distribution is prevalent in marine environments, probably because of substrate depletion in the sulfate reduction zone. The type-I distribution does occur in marine settings where deposition rates of organic matter are high. Landfills possess only the type-I distribution of αC values and exhibit unusually steep curves, possibly because methanogenesis occurs predominantly from acetate produced by fermentation at mesophilic temperatures. The high abundance of acetate in landfill leachate may permit varying degrees of expression of the KIE associated with aceticlastic methanogenesis. Outgassing of 12CO2 may contribute further to the steepening of αC curves in landfills and other anaerobic environments possessing a type-I distribution. Defining the type of αC distributions in different wetlands could reduce uncertainty in estimating the δ13C value of CH4 emissions. Hence, the prevalence of type-I vs. type-II αC distributions in wetlands may have practical importance for the refinement of global CH4 budgets that rely on 13C/12C ratios for mass balance.

The chemistry of some Brazilian soils: Element mobility during intense weathering

Abstract Chemical data for 56 elements in soils from three regions of Brazil (Bahia, Amazon, Goias) indicate that intense chemical weathering leads to a product soil essentially in the system, SiO2Al2O3Fe2O3H2O. The major minerals of the leached soils are quartz, kaolinite, gibbsite, goethite and hematite. Trace elements show a wide range of behaviour from those which are enriched as for some bio-important elements (B, Cl, Mo, Se, Sn, I) and refractory metals (Sc, Zr, Nb, Gd, Tm, Th) to the majority which are severely leached. Trace-element behaviour appears to be largely controlled by the dominant clay or Al2O3SiO2 minerals, or the degree of weathering. Consideration of the chemistry of lateritic soils and groundwaters indicates that the development of deep lateritic profiles requires times in the order of tens of millions of years in a very stable continental setting.

Fluids and thrusting

Abstract Recent studies of the structure of the continental crust by COCORP and BIRPS have shown that major thrust structures in the lower crust are ubiquitous. Plate-tectonic processes of continental crust destruction or construction at sites where subduction or collision occurs (the Himalayas or the Andes) involve thrusting of continental materials or ocean crust beneath continents, and local overthrusting may accompany transcurrent motion. In all such cases massive fluid flow must occur. As first stressed by M.K. Hubbert and W.W. Rubey, thrusting mechanisms require that fluid pressures are close to, or greater than, lithostatic pressures. When thrusting occurs, compaction of the underplate must occur rapidly, expelling pore water. Crustal thickening leads to gradual heating of the underplate with prograde metamorphism near the top and even melting at the base. If the overthrust plate is thick and hot at the base, retrograde metamorphism will occur at the base of the overthrust plate and rising fluids will encounter inverted thermal gradients. In such a region veins need not occur but leaching phenomena may dominate. If an underthrust plate is hydrated, and/or contains sedimentary aquifers, large fluid volumes are expelled through shear zones into the overthrust plate. The scale of fluid-release processes can be large. Thus, in thin-skinned tectonics where the overthrust plate is 10–15 km thick, induced fluid release can easily reach 4 · 109 g m−2 of thrust surface. The flow of such fluids will be controlled by the thrust surface and lithology, both of which will influence hydraulic fracture mechanisms and spacing of fractures. The chemistry of thrust-derived fluids may be highly variable depending on lithology and the time constants of thrusting. Vertical thermal and redox environments will be similarly dependent. In the subduction process, impressive quantities of fluids must pass back up the thrust surface, and such fluids have recently been directly observed during drilling. Dewatering of spilites, serpentinites and sediment layers of the underthrust oceanic lithosphere must produce massive fluid flow back to the surface but some of these fluids may form hydrated minerals in the overlying mantle and, at great depth, flux mantle melting. Extreme metasomatism in blueschist belts must result from such fluids. A case of particular interest involves the cessation of subduction when a high-level slab reaches thermal equilibrium. In general, flow regimes in the thrust and fault zones associated with collisions follow a sequence from conditions of high T-P with locally derived fluids at low water/rock ratios during initiation of the structures, to high fluxes of reduced metamorphic fluids along conduits as the structures propagate and intersect hydrothermal reservoirs. Later in the tectonic evolution, and at shallower crustal levels, there may be incursion of oxidizing near-surface fluid reservoirs into the faults. These fluids may have extremely low δ 18O-values, where mountain ranges form on rebound faults, and high-altitude depleted fluids penetrate down the rebound structure. Extensive mineralization may be associated with such thrust-derived fluids. Examples which will be discussed include the Mother Lode Au deposits of California, U.S.A., and the large U deposits of Lagoa Real, Brazil.