Archaean Era Research Articles

Preservation of ancient and fertile lithospheric mantle beneath the southwestern United States.

Stable continental regions, free from tectonic activity, are generally found only within ancient cratons-the centres of continents which formed in the Archaean era, 4.0-2.5 Gyr ago. But in the Cordilleran mountain belt of western North America some younger (middle Proterozoic) regions have remained stable, whereas some older (late Archaean) regions have been tectonically disturbed, suggesting that age alone does not determine lithospheric strength and crustal stability. Here we report rhenium-osmium isotope and mineral compositions of peridotite xenoliths from two regions of the Cordilleran mountain belt. We found that the younger, undeformed Colorado plateau is underlain by lithospheric mantle that is 'depleted' (deficient in minerals extracted by partial melting of the rock), whereas the older (Archaean), yet deformed, southern Basin and Range province is underlain by 'fertile' lithospheric mantle (not depleted by melt extraction). We suggest that the apparent relationship between composition and lithospheric strength, inferred from different degrees of crustal deformation, occurs because depleted mantle is intrinsically less dense than fertile mantle (due to iron having been lost when melt was extracted from the rock). This allows the depleted mantle to form a thicker thermal boundary layer between the deep convecting mantle and the crust, thus reducing tectonic activity at the surface. The inference that not all Archaean crust developed a strong and thick thermal boundary layer leads to the possibility that such ancient crust may have been overlooked because of its intensive reworking or lost from the geological record owing to preferential recycling.

Isotopic evidence for microbial sulphate reduction in the early Archaean era.

Sulphate-reducing microbes affect the modern sulphur cycle, and may be quite ancient, though when they evolved is uncertain. These organisms produce sulphide while oxidizing organic matter or hydrogen with sulphate. At sulphate concentrations greater than 1 mM, the sulphides are isotopically fractionated (depleted in 34S) by 10-40/1000 compared to the sulphate, with fractionations decreasing to near 0/1000 at lower concentrations. The isotope record of sedimentary sulphides shows large fractionations relative to seawater sulphate by 2.7 Gyr ago, indicating microbial sulphate reduction. In older rocks, however, much smaller fractionations are of equivocal origin, possibly biogenic but also possibly volcanogenic. Here we report microscopic sulphides in approximately 3.47-Gyr-old barites from North Pole, Australia, with maximum fractionations of 21.1/1000, about a mean of 11.6/1000, clearly indicating microbial sulphate reduction. Our results extend the geological record of microbial sulphate reduction back more than 750 million years, and represent direct evidence of an early specific metabolic pathway--allowing time calibration of a deep node on the tree of life.

Chronological correlations between the Pilbara and Kaapvaal cratons

The early geological development of the Pilbara and Kaapvaal cratons has many features in common. Attempts have been made to correlate geologically similar features of the two cratons, and it has been postulated that they originated as contiguous components of a single continent, ‘Vaalbara’, during this time. The early geological histories of the Pilbara and Kaapvaal cratons are here compared in detail and the evidence that they were initially contiguous is assessed. These comparisons indicate significant differences in the chronologies of magmatic events within the granite–greenstone crusts of the Pilbara and Kaapvaal cratons. In addition, igneous correlatives emplaced during ca 2985 and 2782 Ma magmatic events on the Kaapvaal Craton have not been identified on the Pilbara Craton, and a well-defined 2760 Ma magmatic event, manifest as widespread emplacement of granitic rocks into the Pilbara granite–greenstone basement and eruption of flood basalts of the lower part of the Fortescue Group, is absent from the Kaapvaal Craton. Furthermore, similarities in first- and second-order transgression–regression cycles within the sedimentary supracrustal sequences may be attributable to global sea-level fluctuations, and thus may be irrevelant to the question of former contiguity. However, similarities in some aspects of the geological development of the Pilbara and Kaapvaal cratons imply that there were periods, extending for between 60 and 200 Ma, of the Archaean era during which the style of crust formation, intensity of volcanism and subaerial erosion, and magnitude of sea-level fluctuations may have varied on a global scale. Such similarities include the overall duration of formation of the granite–greenstone crusts from ca 3650 to 3100 Ma, the onset of craton-wide erosion in the interval ca 3125 to 3000 Ma, the major episodes of flood basaltic volcanism between 2760 and 2680 Ma, the predominance of chemical (carbonate and banded iron-formation) sedimentation between ca 2630 and 2440 Ma and the transition to widespread clastic sedimentation within the interval 2440 to 2200 Ma.

Gravity-driven continental overflow and Archaean tectonics

Whether modern tectonic processes differ substantially from those in Archaean times (>2,500 Myr ago) remains controversial. One view1 is that Archaean tectonic processes were some combination of modern ones, occurring faster or more shallowly because of the larger heat output of the early Earth, but others2 have proposed that significantly different processes operated. Here I argue that gravitational spreading of Archaean continents would have caused them continuously and pervasively to ‘overflow’ onto adjacent ocean basins, and that this process would have naturally ceased at the end of the Archaean era. Because modern continental crust is believed to be ductile rather than brittle below a depth corresponding to a temperature of about 350–400 °C (ref. 3), it seems likely that such a ductile zone was universally present within the hotter Archaean continental crust. If the mean geothermal gradient of the continents had exceeded ∼25–30 °C km−1, then the resulting ductile zone would have caused continental overflow to occur, and such a process can account for many of the distinctive peculiarities observed in the Archaean geological record. The cessation of continental overflow corresponds naturally to the stabilizing ‘cratonization’ which marked the end of the Archaean era, with its timing dependent on the evolution of both the geothermal gradient in the continents and the depth of the ocean basins.

Vanadium partitioning and the oxidation state of Archaean komatiite magmas

The MgO content of komatiite lavas is an important measure of their formation temperature deep in the Archaean mantle, and forms the basis for models of the early Earth's thermal and chemical evolution1,2,3,4,5. Estimates of the primary MgO content of komatiites are sensitive to the oxidation state—characterized by the oxygen fugacity ( )—assumed for the magmas during their crystallization. Despite two decades of study, however, is still poorly constrained for these lavas. Here I present an estimate of the for komatiite flows, based on the systematics of vanadium partitioning between komatiitic liquid and olivine in six well-characterized komatiite flows of varying ages. This approach shows that the oxidation state of several of these Archaean lava flows was the same as, or possibly more oxidizing than, that of present-day oceanic basalts. These results may require a downward revision of the mantle melting temperature estimated for many komatiites by about 50 °C, and suggest that the mantle was unlikely to be much less oxidized during the Archaean era than at present.

Experimental melting of hydrous low-K tholeiite: evidence on. the origin of Archaean era tons

Experimental melting studies were performed on a natural high-Al basalt and a synthetic average Archaean tholeiite (AAT) composition (0.3 wt.% K20) with variable amounts of H20. Microprobe analyses of quenched melts (glass) from runs at 5-30 kbar and 750°-l100°C showed that typical Archaean tonalitic and trondhjemitic "grey gneiss" compositions were produced from the average Archaean tholei-ite over the entire experimental range, with 15% to less than 1 % H20. The high-Al basalt produced liquids too high in Al203 (18--23%) for realistic grey gneiss compositions. The persistent generation in our experiments of low-K calc-alkaline magmas directly by vapor-undersaturated partial melting of low-K Archaean tholeiite strongly suggests this mechanism for the origin of early continents. Temperatures of 850°-l000°C and pressures around 15 kbar are appropriate melting conditions. Ton­alitic magmas are favored by higher temperatures, lower pressures, and higher H20 contents in the source. Trondhjemitic magmas are favored by lower temperatures, higher pressures, and lower H20 contents. Heavy REE depletion of magmas would be possible for partial melting above 15 kbar because of the stability at higher pressures of residual garnet. Unfractionated REE patterns of magmas could result from melting at lower pressures, where garnet does not coexist with liquid. The low-K trends of melts are maintained by very refractory amphibole (up to 0.7 wt. % KzO) which coexists with liquid for bulk H20 contents of 2 wt. % or more. Shallow subduction-zone melting of amphibolite with magma extraction, and partial melting of amphibolite under deep-crustal metamorphic conditions are models for early crustal evolution which appear to satisfy the experimental constraints.

The petrography and geochemistry of the Clericy Pluton: an ultrapotassic pyroxenite-syenite suite of late Archaean age from the Abitibi region, Quebec

The Archaean Clericy Pluton is a small, bimodal suite of ultramafic and syenitic rocks located in the Abitibi region of Quebec. The syenite unit consists predominantly of alkali feldspar and (aegirine)-augite and contains numerous elongated mafic xenoliths. The ultramafic component is predominantly clinopyroxenite. Using the geochemical screen proposed by Foley et al. (1987) the mafic to intermediate members of the suite (53.8% < SiO 2 < 60.9%) are shown to possess an ultrapotassic character. Crystallization, mainly of clinopyroxene and alkali feldspar, explains the transition from ultramafic to syenitic compositions. Magma mixing between a minimum melt granitic liquid and syenite is required to explain the most evolved compositions. The Clericy rocks were probably emplaced in a small release zone which developed along one of a number of major transcurrent faults which formed as a consequence of oblique subduction at the end of the Archaean era.

Aerobic respiration in the Archaean?

The Earth's atmosphere during the Archaean era (3,800-2,500 Myr ago) is generally thought to have been anoxic, with the partial pressure of atmospheric oxygen about 10(-12) times the present value. In the absence of aerobic consumption of oxygen produced by photosynthesis in the ocean, the major sink for this oxygen would have been oxidation of dissolved Fe(II). Atmospheric oxygen would also be removed by the oxidation of biogenic methane. But even very low estimates of global primary productivity, obtained from the amounts of organic carbon preserved in Archaean rocks, seem to require the sedimentation of an unrealistically large amount of iron and the oxidation of too much methane if global anoxia was to be maintained. I therefore suggest that aerobic respiration must have developed early in the Archaean to prevent a build-up of atmospheric oxygen before the Proterozoic. An atmosphere that contained a low (0.2-0.4%) but stable proportion of oxygen is required.

Disequilibrium silicate mineral textures: fractal and non-fractal features

IGNEOUS rocks formed from lava flows of the Archaean era (>2,700 million years ago) are often found to contain disequilibrium-textured crystals characterized by spherulitic, branching or dendritic morphologies that occur in layers near the flow surface. Well-known examples are the plagioclase spherulites of basalts and the platy and branching spinnifex-textured olivines and pyroxenes of komatiites1,2. Here we present evidence that, over a finite range of length scales, some disequilibrium textures are scale invariant. This observation implies that over this range of length scales their random patterns can be quantitatively characterized by a unique number, the fractal dimension3. We also demonstrate that some textures have a crossover from fractal to non-fractal behaviour. It is known that most disequilibrium crystals arise in part from rapid cooling and represent the case where the growth rates of the crystals are large compare to the diffusion rates in the silicate melt1,2,4. We therefore formulate a quantitative model for the growth that is based on a variant of diffusion-limited aggregation (DLA) 5.

Crustal structure along the northern ‘Fennolora’ profile

Many data, notably concerning the great change in Earth's bulk heat flow which has occurred since the Archaean era, suggest that late Archaean and early Proterozoic tectonic processes were most probably different from Phanerozoic plate tectonics. Preserved structures in the Earth's crust revealed by the seismic refraction/wide-angle reflection method can help in understanding the different crustal tectonic processes that have influenced the evolution of the area. P-wave analysis for the northern part of the ‘FENNOLORA’ deep seismic refraction line clearly shows the difference between the central Swedish Svecofennian crust and the Archaean crust in the north. Along the Archaean part of the FENNOLORA line, high apparent velocities have been recorded in the upper crust. The central Svecofennian crust, on the other hand, shows normal upper crustal velocities. Data from the Archaean part of the FENNOLORA profile give support to the concept of an ensialic development in this region during the late Archaean and early Proterozoic.

Rare-earth concentrations in 3.5-billion-year-old Onverwacht cherts: An indicator for early Precambrian crustal environments

3.5-billion-year-old cherts from Onverwacht Group, Swaziland System in the Barberton area, South Africa were analyzed for rare earth elements (REE), Sc, Hf, Fe, Cr, Co and Na by means of instrumental neutron activation analysis (INAA). The Onverwacht cherts showed less fractionated REE abundances compared with those in younger cherts and other sedimentary rocks. The chondrite-normalized La/Yb ratios observed for the samples from the older Theespruit Formation and younger Hooggenoeg Formation of the Onverwacht Group are 1.2∼2.5, and 4.6, respectively. These values are smaller than the La/Yb values observed for the Fig Tree Group (5.5) and Moodies Group (7.8), which overlie the Onverwacht Group in the Barberton area, indicating an increasing trend for La/Yb ratio in the sedimentary rocks with time in the Swaziland System. The results appear to indicate that the relative fractionation between light and heavy REE in the crustal region which had supplied REE to the sedimentary rocks increased with time in the Archaean era from nearly chondritic La/Yb ratio to about the present-day value in a very short time span of a few hundred million years. Absence of Ce anomaly, together with variable Eu anomalies in the REE patterns, possibly shows reducing conditions in the sedimentary environments. It is also suggested that the negative Eu anomalies commonly observed for the post Archaean sedimentary rocks were derived by accumulation of plagioclase in the lower continental crust by sinking of plagioclase in magmas in the upper crust.

Continental crustals regimes as factors in the formation of sedimentary ore deposits

Although the formation of metalliferous mineral deposits by processes active in sedimentary basins depends ultimately on physical and chemical conditions at the site of deposition, the style, scale and timing of mineralization are influenced by tectonic and magmatic events, which relate to the evolution of the crustal environment as a whole. During the Archaean era, when geothermal gradients were high and igneous activity widespread, sedimentary processes played little part in mineralization. The development of cratonic crustal massifs at the beginning of the Proterozoic era was followed by an increase in the variety and size of deposits formed in sedimentary basins. From this time onward the cratonic cover formations have provided important sites for the deposition of metals. These metals entered the basins from a variety of sources, some being derived from weathering and erosion of the basement and others being transferred to surface levels by magmas and volatiles originating in the mantle or lower crust. It is suggested that the timing of mineralization is related to that of the major magmatic and tectonic happenings determined by the global tectonic regime.

Definition of ‘Archaean’ — comment and a proposal on the recommendations of the International Subcommission on Precambrian Stratigraphy

The proposed definition of the Archaean-Proterozoic boundary at 2500 Ma (Sims, 1980) is unsatisfactory as it is difficult to use, contravenes the international stratigraphic guide and does not allow for revisions of isotopic decay constants. Instead it is proposed that the boundary be set at the cooling age of the Great Dyke of Zimbabwe (2.5 Ga), which provides an equivalent definition in the rock . The division of the Archaean at 2.9 Ga and 3.5 Ga (Sims, 1980) is also unsatisfactory and instead reference successions are proposed in the Rhodesian craton, the Pilbara block in Australia and in Greenland, to serve as possible future stratotypes for Archaean eras.

The first 800 million years of Earth's history

The Earth grew initially from hot planetesimals of reduced material, and began to differentiate early into a metal-rich core, a silicate-rich mantle and a volcanic surface. Accretion ended with volatile-rich oxidized material, which did not interact with the core because of a mantle barrier. Quenching of magmas at the surface and foundering of cool material was effective in cooling the Earth by 1600 J per gram of magma. Magmas transported radioactive elements and volatiles to the outer 50 km. The lower mantle became stranded below the critical adiabat for melting. The core may have been fully molten. Radiogenic and non-radiogenic heat sources produced a surface heat flow that decreased from ca . 1.3 x 10 14 W at 4.45 Ga B.P . to the present value of 4 x 10 13 W. In spite of uncertainties in the production and transport of heat, it seems safe to conclude that intense volcanism occurred throughout the pre-Archaean era before survival of crust. From phase-equilibrium data for peridotites and calc-alkaline rocks, it is deduced that a range of ultrabasic to basic magmas was produced from a peridotitic upper mantle, and that calc-alkaline magmas were produced during foundering of crust. An anorthositic crust was not formed. From observations of lunar basins, it is concluded that a minimum of ca . 500-1000 impact basins were formed on Earth in ca . 100—200 Ma before 3.95 Ga B.P. Plausible calculations suggest that ca . 20 times as many basins were formed in the preceding ca . 300 Ma. Impact debris would be cooled by water in wet crustal rocks and oceans. It is assumed that after 4.0 Ga B.P. impacts would have only temporary local effects on the crust, and emphasis is placed on upper-mantle convection as the main driving force for geochemical transport. Sedimentation and metamorphism were important factors during foundering of crust. ‘Oceanic' and ‘continental' regions developed above up-welling and down-welling segments of convection cells, and these regions resembled greenstone belts and high-grade regions of the Archaean era. As heat flow declined, polygonal tectonics was replaced by symmetrical linear tectonics during the Archaean era, and then by asymmetrical linear tectonics during the Proterozoic era.

Mineral resources of Southern Africa in time-stratigraphic perspective

The tectono-stratigraphic setting of mineral deposits in southern Africa is reviewed with reference to 5 outline maps portraying stages in the evolution of the subcontinent. The Early and Middle Archaean Eras are characterized by deposits of Cr, Au and Fe; the Late Archaean by Au, U and Ni. In the Early Proterozoic the intrusion of vast mafic complexes supplied Cr, Ni, Pt, Cu and V-Fe ore, while sedimentary basins received Fe, Mn, Cu, Pb and Zn in marginal seas. The Middle Proterozoic was a singularly poor metallogenic Era, producing only pegmatite minerals and a variety of Cu deposits. The Late Proterozoic is typified by widespread occurrence of Cu, Pb, Zn and local U in geosynclinal belts; pegmatite minerals formed on a large scale in rejuvenated basement. Carbon is the most important element of Phanerozoic deposits, accounting for the fossil fuels besides appearing as diamond in kimberlite. It also contributed to the precipitation of sandstone-type U. Post-Gondwana morphology and climate effected further surface concentration and dispersal of minerals.

Evidence for land emergence in the early to middle Precambrian

The geological record provides abundant evidence of major periods of post-orogenic uplift followed by denudation of subaerial continental terrains within the Archaean era and at the Archaean-Proterozoic boundary 2600 ± 200 Ma ago. Archaean greenstone belts, with subaerial volcanoes at least 2 km high, formed in narrow basins bordered by lands with relief of perhaps 4 km. The erosion of the borderlands gave rise to substantial terrigenous debris in the late greywackes and molasse of the greenstone belts. The high rate of radiogenic heat production in the Archaean led to more plate production, more subduction and the creation of thick-root orogenic belts, and thus eventually to substantial uplift, erosion and production of voluminous clastic sediments in the early Proterozoic. Many of these detrital beds underwent later sedimentary recycling; remnant successions are up to 13 km thick.

Precambrian thermal régimes

Regional studies of Archaean and Proterozoic complexes provide the basis for one approach to the problem of reconstructing the thermal regimes of Precambrian eras. Such studies have a bearing both on the PT conditions under which metamorphism and deformation of continental crustal rocks took place and on the conditions of magma-generation at depth. Although several lines of evidence, including the wide distribution of low-pressure metamorphic facies series, are consistent with the inference that Archaean geothermal gradients tended to be steep, the characters of many complexes formed before 2700 Ma suggest PT ranges not far from those indicated by rocks which suffered deformation and metamorphism in Phanerozoic zones of high heat flow. Tectonic patterns developed on a global scale in early Proterozoic times suggest that lateral variations of heat flow in continental crust did not conform to the present pattern, zones of high heat flow being developed within as well as at the borders of continental rafts. There are, nevertheless, indications that the lithosphere had locally attained thicknesses comparable with those found today in many cratonic regions soon after the end of the Archaean era.

The Delineation of Deep Crustal Magnetic Bodies from Total Field Aeromagnetic Anomalies

Studies were made of accurate, total field aeromagnetic data of excellent uniformity over an area in N. W. Ontario extending from 48°45′N to 53°N latitude and from 86°W to 94°W longitude. The rocks in this area are of early Precambrian age (Archaean era). The regional effects from the data were removed. The residual values were then analytically filtered to obtain the total field stripped of its near-surface, high amplitude components. Eighty-five individual anomalies were selected and analyzed to determine the horizontal dimensions and the depths to the tops and bottoms of the causative bodies and the magnitude and direction of total polarization vectors associated with the bodies. The bodies were assumed to be vertical rectangular prisms of arbitrary polarization. A brief summary of the method used is presented.The horizontal dimensions of the bodies vary from 0.8km (.5mile) to 5.1kms (3.2miles). The depths to the top of the bodies in the filtered aeromagnetic map are found to be in the range 4.8kms (3miles) to 8kms (5miles) from the surface of the ground. The depths to the bottom of the bodies are between 17.7kms (11miles) and 24kms (15miles) for most of the anomalies, with a mean depth of 20kms (12.4 miles), which is interpreted as the mean depth to the Curie point geotherm. The horizontal vectors of the polarizations are subparallel to the magnetic lineation of the filtered aeromagnetic map. An interesting and surprising result of this analysis is the detection of the presence of alternatingly normal and reverse polarization roughly in the East-West direction. The mean inclination and declination for normal polarizations are 67.6° and 258.7° respectively whereas those for reverse polarizations are -67.9° and 81.3° respectively. The consistency in the directions of magnetization in the two clusters for normal and reverse polarization indicates strongly the absence of any dominant component of magnetization parallel to the present field. On the basis that the magnetizations are permanent and the reversals are due to field or selfreversal, the calculated pole position for the rocks of Archaean era is found to be at a latitude of 31.5°N and a longitude of 136.9°W. The intensity of magnetization of the bodies lies within a wide range from .0006 to .349cgs emu with the average of .055cgs emu.