Early Archaean Research Articles

Precambrian mafic magmatism in the Singhbhum craton, eastern India

The Singhbhum craton has a chequred history of mafic magmatism spanning from early Archaean to Proterozoic. However, lack of adequate isotopic age data put constraints on accurately establishing the history of spatial growth of the craton in which mafic magmatism played a very significant role. Mafic magmatism in the craton spreads from ca.3.3 Ga (oldest “enclaves” of orthoamphibolites) to about 0.1 Ga (‘Newer dolerite’ dyke swarms). Nearly contemporaneous amphibolite and intimately associated tonalitic orthogneiss may represent Archaean bimodal magmatism. The metabasic enclaves are appreciably enriched and do not fulfill the geochemical characteristics of worldwide known early Archaean (>3.0 Ga) mafic magmatism. The enclaves reveal compositional spectrum from siliceous high-magnesian basalt (SHMB) to andesite. However, the occurrence of minor depleted boninitic type within the assemblage has so far been overlooked. High magnesian basalt with boninitic character of Mesoarchaean age is also reported in association with supracrustals from southern fringe of the granitoid cratonic nucleus. The subcontinental lithospheric mantle (SCLM) below the craton is conjectured to have initiated during the early Archaean. Significantly, recurrence of depleted magma types in the craton is observed during the whole span of mafic igneous activity which has been vaguely related to “mantle heterogeneity”, although the alternative model of sequential mantle melting is also being explored.

Ancient orogens and modern analogues: an introduction

Abstract Plate-tectonics principles have been routinely applied to the study of Phanerozoic orogenic belts and, more controversially, to Precambrian orogens as far back as the Early Archaean. Recent advances in a variety of fields have vastly improved our understanding of ancient orogenic belts, so that realistic modern analogues can be entertained. This volume presents up-to-date syntheses of some classic modern and ancient orogenic belts as well as examples of some of the processes responsible for their evolution.

About this title - Ancient Orogens and Modern Analogues

Plate tectonics provide a unifying conceptual framework for the understanding of Phanerozoic orogens. More controversially, recent syntheses apply these principles as far back as the Early Archaean. Many ancient orogens are, however, poorly preserved and the processes responsible for them are not well understood. The effects of processes such as delamination, subduction of oceanic and aseismic ridges, overriding of plumes and subduction erosion are rarely identified in ancient orogens, although they have a profound effect on Cenozoic orogens. However, deeply eroded ancient orogens provide insights into the hidden roots of modern orogens. Recent advances in analytical techniques, as well as in fields such as geodynamics, have provided fresh insights into ancient orogenic belts, so that realistic modern analogies can now be applied. This Special Publication offers up-to-date reviews and models for some of the most important orogenic belts developed over the past 2.5 billion years of Earth history.

Was nitric oxide the first deep electron sink?

Evolutionary histories of enzymes involved in chemiosmotic energy conversion indicate that a strongly oxidizing substrate was available to the last universal common ancestor before the divergence of Bacteria and Archaea. According to palaeogeochemical evidence, O(2) was not present beyond trace amounts on the early Earth. Based on recent phylogenetic, enzymatic and geochemical results, we propose that, in the earliest Archaean, nitric oxide (NO) and its derivatives nitrate and nitrite served as strongly oxidizing substrates driving the evolution of a bioenergetic pathway related to modern dissimilatory denitrification. Aerobic respiration emerged later from within this ancestral pathway via adaptation of the enzyme NO reductase to its new substrate, dioxygen.

Deep Time Paleobiology: Stromatolites–A Key to Decoding Primitive Ecosystems on Earth and Beyond

Finding the beginning of Earth's fossil record is a long-standing palaeontological challenge arising from the quest to understand the origin of life. Research in recent years has necessarily focused on determining the existence (or otherwise) of fossils in the Early Archaean rock record. Nonetheless, despite numerous reports of microfossils(?) and stromatolites, consensus on the existence of life in the Early Archaean has been elusive (e.g. Moorbath, 2005). However, new techniques and approaches are allowing more confident interpretation of the Archaean fossil record, and the nature of the earliest biosignatures can be used to inform our understanding of emergent ecosystems on Earth and perhaps on other terrestrial planets.Evidence is mounting that microbial ecosystems may have had a firm foothold as early as ~3.5 Ga (Tice and Lowe, 2004; Schopf, 2006; Hofmann et al., 1999; Allwood et al., 2006, 2007b; Westall et al., 2006; Westall and Southam, 2006). Significantly, there is now also evidence that the Early Archaean record may not be as meager and cryptic as previously thought. For example, the 3.43 Ga Strelley Pool Chert of the Pilbara Craton of Western Australia contains kilometer-scale tracts of a fossilized stromatolite (microbial?) reef (Allwood et al., 2006, 2007b) and provides a large suite of evidence that is consistent with life's existence. Moreover, the rapidity with which the Strelley Pool reef established itself on a newly-submerged landmass suggests that life was well established by that time, waiting in the wings in planktonic form until conditions favored sessile biofilm formation. The rich vault of information in such rocks as the Strelley Pool Chert may shed light not only upon life's antiquity, but also on the nature of early organisms and ecosystems, the environments that nurtured them, the processes that aided preservation of biosignatures and the palaeontological approaches needed to interpret them. This in turn will be a valuable guide in the search for—and interpretation of—ancient microbial biosignatures in the geologic record of other planets or moons.

3.35 Ga komatiite volcanism in the western Dharwar craton, southern India: Constraints from Nd isotopes and whole-rock geochemistry

We present field, petrographic, Sm–Nd whole-rock isochron and whole-rock geochemical data for komatiites from Sargur Group greenstone belts of the western Dharwar craton. Field evidence such as pillow structure indicates their eruption in a marine environment. Petrographic data reveal that the igneous mineralogy has been altered during post-magmatic hydrothermal alteration processes corresponding to greenschist- to lower amphibolite facies conditions with rarely preserved primary olivine and orthopyroxene. A 16-point Sm–Nd whole-rock isochron gives an age of 3352 ± 110 Ma for the timing of eruption of komatiite lavas. About 60% of the studied komatiite samples show Al-depletion whilst the remaining are Al-undepleted. The Al-depleted komatiites are characterised by high CaO/Al 2O 3 ratios (1.01–1.34) and low Al 2O 3/TiO 2 (5–16) whereas Al-undepleted komatiites show lower CaO/Al 2O 3 ratios (0.59–0.99) and higher Al 2O 3/TiO 2 (17–26). Trace element distribution patterns of komatiites suggest that most of the primary geochemical and Nd isotopic compositions are preserved with only minor influence of post-magmatic alteration processes and negligible crustal contamination. The chemical characteristics of Al-depleted komatiites, such as high (Gd/Yb) N together with lower HREE, Y, Zr and Hf, imply their derivation from deeper upper mantle with garnet (majorite?) involvement, whereas lower (Gd/Yb) N slightly higher HREE, Y, Zr and Hf suggest derivation from shallower upper mantle without garnet involvement. The observed chemical characteristics (CaO/Al 2O 3, Al 2O 3/TiO 2, MgO, Ni, Cr, Nb, Zr, Y, Hf, REE) indicate derivation of the komatiite magmas from different depths in a plume setting, whereas sub-contemporaneous felsic volcanism and TTG accretion can be attributed to an arc setting. In order to explain the spatial association of komatiite volcanism with contemporaneous mafic-felsic volcanism and TTG accretion we propose a combined plume-arc setting. Nd isotope data of the studied komatiites indicate depleted mantle reservoirs which may have evolved by early (>4.53 Ga) global differentiation of the silicate Earth as suggested by Boyet and Carlson [Boyet, M., Carlson, R.W., 2005. 142Nd evidence for early (>4.53 Ga) global differentiation of silicate Earth. Science 309, 577–581] or extraction of continental crust during the early Archaean.

Tectonic evolution of the North China Block: from orogen to craton to orogen

Abstract The North China Craton contains one of the longest, most complex records of magmatism, sedimentation, and deformation on Earth, with deformation spanning the interval from the Early Archaean (3.8 Ga) to the present. The Early to Middle Archaean record preserves remnants of generally gneissic meta-igneous and metasedimentary rock terranes bounded by anastomosing shear zones. The Late Archaean record is marked by a collision between a passive margin sequence developed on an amalgamated Eastern Block, and an oceanic arc–ophiolitic assemblage preserved in the 1600 km long Central Orogenic Belt, an Archaean–Palaeoproterozoic orogen that preserves remnants of oceanic basin(s) that closed between the Eastern and Western Blocks. Foreland basin sediments related to this collision are overlain by 2.4 Ga flood basalts and shallow marine–continental sediments, all strongly deformed and metamorphosed in a 1.85 Ga Himalayan-style collision along the northern margin of the craton. The North China Craton saw relative quiescence until 700 Ma when subduction under the present southern margin formed the Qingling–Dabie Shan–Sulu orogen (700–250 Ma), the northern margin experienced orogenesis during closure of the Solonker Ocean (500–250 Ma), and subduction beneath the palaeo-Pacific margin affected easternmost China (200–100 Ma). Vast amounts of subduction beneath the North China Craton may have hydrated and weakened the subcontinental lithospheric mantle, which detached in the Mesozoic, probably triggered by collisions in the Dabie Shan and along the Solonker suture. This loss of the lithospheric mantle brought young asthenosphere close to the surface beneath the eastern half of the craton, which has been experiencing deformation and magmatism since, and is no longer a craton in the original sense of the word. Six of the 10 deadliest earthquakes in recorded history have occurred in the Eastern Block of the North China Craton, highlighting the importance of understanding decratonization and the orogen–craton–orogen cycle in Earth history.

Early Archaean hydrothermal fluids; a study of inclusions from the ∼ 3.4 Ga Buck Ridge Chert, Barberton Greenstone Belt, South Africa

Fluid inclusions have been studied from cavities and breccias in one of the oldest well-preserved, relatively low-grade metamorphosed sedimentary sequences on Earth. The fluid inclusion-bearing samples are from the uppermost part of the early Archaean (∼ 3.4 Ga) Buck Ridge volcano-sedimentary complex in the Barberton Greenstone Belt (South Africa), which was affected by hydrothermal alteration during and immediately after its deposition. In banded, silicified sandy sediments this alteration is characterised, among other features, by quartz-cemented hydrothermal breccias, layer-bound stockworks of cm-size quartz veins and quartz-filled cavities below stacked pebbles. Quartz grains in these hydrothermal deposits contain numerous fluid inclusions filled with either two-phase aqueous or mixed H 2O–CO 2 fluid. The aqueous inclusions are divided into three different groups, based on their microthermometric characteristics: type H A (relatively high salinity), type M A (intermediate salinity) and type L A (relatively low salinity). Optical and microthermometric characteristics of the fourth type of inclusions, the mixed H 2O–CO 2 fluid (type C) indicate that it was homogeneous at the time of trapping, with only some minor changes occurring after trapping. Type H A and type C fluids appear as immiscible fluids in their host crystal, most probably resulting from unmixing of the hydrothermal fluid. Phase relations in the CO 2–H 2O system show that the unmixing must have occurred at a relatively low pressure of ∼ 100 bars. This conclusion is in line with the geological context of the samples, which indicates a shallow environment of deposition for the sediments, (almost) contemporaneous hydrothermal activity, and a very low metamorphic grade. However, it is contrary to the first impression given by the homogeneous character of type C inclusions, which suggests much higher P– T conditions at the time of trapping. The aqueous component of type C fluids may have progressively mixed with high-salinity aqueous fluids (type H A), resulting in a dilution trend from H A to M A to L A fluids. The hydrothermal fluids from the Buck Ridge Chert in the Barberton Greenstone Belt are strikingly similar to those encountered in present-day systems. They show minor differences with older (ca. 3.8 Ga) fluids preserved in hydrothermally altered rocks from the Isua Greenstone Belt in Greenland.

Growth-fault structure and stratigraphic architecture of the Buck Ridge volcano-sedimentary complex, upper Hooggenoeg Formation, Barberton Greenstone Belt, South Africa

The ∼3.45–3.42Ga Buck Ridge volcano-sedimentary complex (BR-vsc) forms the uppermost part of the Hooggenoeg Formation, in the early Archaean Onverwacht Group of the Barberton Greenstone Belt, South Africa. The complex consists mainly of massive and pillowed basalts and felsic, quartz-plagioclase porphyritic rocks, which are capped by pervasively silicified sedimentary rocks. Deposition of the felsic extrusive and (volcani-)clastic sedimentary rocks occurred in shallow water to emersion, during an extensional tectonic regime. The extensional regime is expressed by a number of listric normal faults that transect the entire ∼2km thick complex. The syndepositional (i.e. growth fault) character of the faults is indicated by variations in thickness and facies of rock units across the faults, and by systematic rotation of the fault blocks. Broadly contemporaneous with the faulting, felsic rocks intruded the complex into the highest levels. The geometry of these intrusive bodies was controlled by the major growth faults. Extension also created space for the intrusion of approximately bedding-perpendicular felsic and mafic dykes, and black chert veins. The new field observations and U–Pb zircon SHRIMP data have pinned the timing of events during and after deposition of the BR-vsc. The growth-fault model accounts well for previously unexplained structural observations that include both extensional and compressional features. Recognition of an early extensional phase of deformation demonstrates further similarities between the tectonic histories of the Barberton Greenstone Belt and the greenstone belts of the east Pilbara in Australia.

Geological constraints on detecting the earliest life on Earth: a perspective from the Early Archaean (older than 3.7 Gyr) of southwest Greenland

At greater than 3.7 Gyr, Earth's oldest known supracrustal rocks, comprised dominantly of mafic igneous with less common sedimentary units including banded iron formation (BIF), are exposed in southwest Greenland. Regionally, they were intruded by younger tonalites, and then both were intensely dynamothermally metamorphosed to granulite facies (the highest pressures and temperatures generally encountered in the Earth's crust during metamorphism) in the Archaean and subsequently at lower grades until about 1500 Myr ago. Claims for the first preserved life on Earth have been based on the occurrence of greater than 3.8 Gyr isotopically light C occurring as graphite inclusions within apatite crystals from a 5 m thick purported BIF on the island of Akilia. Detailed geologic mapping and observations there indicate that the banding, first claimed to be depositional, is clearly deformational in origin. Furthermore, the mineralogy of the supposed BIF, being dominated by pyroxene, amphibole and quartz, is unlike well-known BIF from the Isua Greenstone Belt (IGB), but resembles enclosing mafic and ultramafic igneous rocks modified by metasomatism and repeated metamorphic recrystallization. This scenario parsimoniously links the geology, whole-rock geochemistry, 2.7 Gyr single crystal zircon ages in the unit, an approximately 1500 Myr age for apatites that lack any graphite, non-MIF sulphur isotopes in the unit and an inconclusive Fe isotope signature. Although both putative body fossils and carbon-12 enriched isotopes in graphite described at Isua are better explained by abiotic processes, more fruitful targets for examining the earliest stages in the emergence of life remain within greater than 3.7 Gyr IGB, which preserves BIF and other rocks that unambiguously formed at Earth's surface.

Stratigraphic continuity and early deformation of the central part of the Coppin Gap Greenstone Belt, Pilbara, Western Australia

This article focuses on the discrepancy between the often advocated stratigraphic continuity of greenstone belts in the Pilbara Craton (Australia) on the one hand, and the observations of syndepositional deformation and other tectonic disturbance on the other hand. The Coppin Gap Greenstone Belt of the East Pilbara consists predominantly of mafic, intermediate and felsic rocks, with minor amounts of silicified sedimentary rocks of the early Archaean Warrawoona Group. Several well-defined zones subparallel to the bedding show intense brittle to ductile deformation, testifying to differential movements of the tectono-stratigraphic slices they delimit. These deformation zones were at least partly related to west-block-down normal faults and accommodated upper block to the west transport. However, new U–Pb zircon dates, geochemical analyses and detailed field observations show that the overall stratigraphic succession in the Coppin Gap Greenstone Belt has not been fundamentally disturbed. The normal faults occur in several irregularly superposed arrays of km-spaced faults, part of which were active during deposition. Normal faults also occur throughout the entire belt. Combined, the early deformation structures indicate east–west extension over a minimum period of 20 Ma, from ∼3469 Ma onwards. Likely, the extension took place over an even longer period of time, during much of the time of deposition of the Warrawoona Group, from ∼3490 Ma onwards. Syndepositional extension of the Warrawoona Group was geometrically unrelated to the doming of granitoid batholiths. Surficial (i.e. uppermost crustal) gravitational collapse during or shortly after deposition may have caused the apparent contradictory co-existence of deformation zones and stratigraphic continuity. Although the deformation has not caused major disruptions of the stratigraphic succession, the precision of the U–Pb SHRIMP ages is insufficient to rule out excisions or duplications of stratigraphy on the order of a few million years.

The rise of continents—An essay on the geologic consequences of photosynthesis

Earth accreted 4567 Myr ago from largely homogeneous material. From this initial capital of matter, differentiation formed the chemical and physical compartments of core, mantle, continents, ocean and atmosphere, that characterize Earth today. Differentiation was, and still is, driven by energy from various sources including radioactive heat and relic heat from accretion. With evolution of photosynthesis, living organisms acquired the ability to harvest Solar energy and channel it into geochemical cycles. On our present Earth, the primary production from life contributes 3 times more energy to these cycles than Earth's internal heat engine. We hypothesize that the emergence of this energy resource modified Earth's geochemical cycles and ultimately stimulated the production of granite during the earliest Archaean, which led to the first stabilization of continents on Earth. Such biological forcing may explain the unique presence of granite on Earth, and why stable continents did not form during the first half billion years of Earth's history.

Integrated Pb- and S-isotope investigation of sulphide minerals from the early Archaean of southwest Greenland

We present high-spatial resolution secondary ion mass spectrometry (SIMS) measurements of Pb and S isotopes in sulphides from early Archaean samples at two localities in southwest Greenland. Secondary pyrite from a 3.71 Ga sample of magnetite–quartz banded iron formation in the Isua Greenstone Belt, which has previously yielded unradiogenic Pb consistent with its ancient origin, contains sulphur with a mass independently fractionated (MIF) isotope signature (Δ 33S = + 3.3‰). This reflects the secondary mineralization of remobilized sedimentary S carrying a component modified by photochemical reactions in the early Archaean atmosphere. It further represents one of the most extreme positive excursions so far known from the early Archaean rock record. Sulphides from a quartz–pyroxene rock and an ultramafic boudin from the island of Akilia, in the Godthåbsfjord, have heterogeneous and generally radiogenic Pb isotopic compositions that we interpret to represent partial re-equilibration of Pb between the sulphides and whole rocks during tectonothermal events at 3.6, 2.7 and 1.6 Ga. Both these samples have Δ 33S = 0 (within analytical error) and therefore show no evidence for MIF sulphur. These data are consistent with previous interpretations that the rock cannot be proven to have a sedimentary origin. Our study illustrates that SIMS S-isotope measurements in ancient rocks can be used to elucidate early atmospheric parameters because of the ability to obtain combined S and Pb-isotope data, but caution must be applied when using such data to infer protolith. When information from geological context, petrography and chronology (i.e. by Pb isotopes) is combined and fully evaluated, Δ 33S signatures from sulphides and their geological significance can be interpreted with a higher degree of confidence.

Goldschmidt Conference Abstracts 2005

The c. 3.49 Ga Dresser Formation, a lower member of the Warrawoona Group in the North Pole Dome region, Western Australia, is a barite/black chert dominated, mafic volcanic hosted, shallow water (l100m), seafloor exhalative hydrothermal deposit. Carbonaceous matter (CM) aggregates, varying in size from 1mm to g20mm, have been isolated from black cherts in drill core from the Dresser Mine area. With total organic carbon (TOC) contents from 0.19% to 0.31%, organic petrology and SEM-EDS studies of whole rock samples reveals the CM as aggregated forms around siliceous nuclei in the majority of samples. Transmission electron microscopy (TEM) of carbonaceous concentrates show this aggregate material to be composed of sub-micron sized granular-textured particulates. The observed morphology is suggestive of the remains of microbial clusters or colonies. TEM investigation of one sample, however, showed it to be dominated by soot particles such as have been reported from the Sudbury impact and K-T boundary. The presence of soot particles supports previous evidence of meteoritic impact in the Pilbara during the early Archaean. Reflectance measurements (%Ro) of the CM have yielded three dominant groupings: 1) 0.5-1.5; 2) 2.4-2.8; and 3) 3.2-3.8. Different Ro populations in a single sample record several hydrothermal events, as well as relative proximity to a venting source. These results, combined with the bulk C-isotope values of the CM (d13C -32.1 to -38.2p PDB, n=11), are suggestive of a biogenic source, and as such, may represent remnants of some of the earliest primitive life forms on Earth.

U-Pb zircon (ID-TIMS and SHRIMP) evidence for the early ordovician intrusion of metagranites in the late Proterozoic Canaveilles Group of the Pyrenees and the Montagne Noire (France)

Abstract Depending on the quality of the zircon grains available for analysis, two methods may be used to date igneous rock emplacement, namely U-Pb TIMS with isotope dilution or in situ U-Pb SIMS (SHRIMP). Both methods have been used to determine, in a precise and accurate manner, the emplacement age of the granitic protolith of the various orthogneisses in the Pyrenean Axial Zone. More specifically, four representative samples of G1, G2 and a “transition gneiss” yielded reliable datings with an average age of 473 ± 4 Ma for each sample. The surrounding sediments of the Canaveilles Group were constrained by zircon grains from interlayered metarhyodacite and dated at 581 ± 10 Ma using the SHRIMP method, clearly giving this group a late Proterozoic (Vendian) age. Finally, the Somail orthogneiss of the Montagne Noire, equivalent to that of the Canigou, yielded an age of 471 ± 4 Ma with the in situ U-Pb method, which is identical to the dating of the Pyrenean samples. In addition, most of the studied orthogneisses recorded a wide range of significant concordant inherited ages spanning from early Archaean (3.5 Ga) to Pan-African/Cadomian (600–800 Ma). Bearing in mind the calc-alkaline affinity of the studied rocks, this work demonstrates the huge contrast between the active Gondwana margin in the north (“South European terrane”) and the remarkably homogeneous continental plate that existed from Arabia to Morocco during the Ordovician.

Ca isotope fingerprints of early crust-mantle evolution

The utility of 40Ca/ 44Ca as a tracer of pre-existing crustal contributions in early Archaean cratons has been explored to identify traces of Hadean crust and to assess the style of continental growth. The relatively short half-life of 40K (∼1.3 Gy) means that its decay to 40Ca occurs dominantly during early Earth History. If Archaean crust had a significant component derived from a more ancient protolith, as anticipated by “steady state” crustal evolution models, this should be clearly reflected in radiogenic 40Ca/ 44Ca ratios (or positive initial ε Ca) in different Archaean cratons. A high precision thermal ionisation technique has been used to analyse the 40Ca/ 44Ca ratios of plagioclase separates and associated whole rocks in ∼3.6 Ga (early Archaean) samples from Zimbabwe and West Greenland. Three out of four tonalite, trondhjemite, granodiorite (TTG) suite samples from Zimbabwe display initial 40Ca/ 44Ca ratios indistinguishable from our measured modern MORB value (i.e., ε Ca(3.6) ∼ 0). Greenland samples, however, are very diverse ranging from ε Ca(3.7) = 0.1 in mafic pillow lavas and felsic sheets from the Isua supracrustal belt, up to very radiogenic signatures (ε Ca(3.7) = 2.9) in both mafic rocks of the Akilia association and felsic TTG from the coastal Amîtsoq gneisses. At face value, these results imply the Zimbabwe crust is juvenile whereas most Greenland samples include an earlier crustal component. Yet the west Greenland craton, as with many Archaean localities, has experienced a complex geological history and the interpretation of age-corrected initial isotope values requires great care. Both felsic and mafic samples from Greenland display ε Ca(3.7) so radiogenic that they are not readily explained by crustal growth scenarios. The presence of such radiogenic 40Ca/ 44Ca found in low K/Ca plagioclases requires Ca isotope exchange between plagioclase and whole rock during later metamorphic event(s). In addition the unexpectedly radiogenic Ca isotope ratios in some mafic samples reflect anomalous K/Ca ratios as a result of intense K-metasomatism ∼3.6 Ga. Thus Ca isotope measurements are not a robust tracer of crustal growth in the presence of intense tectono-metamorphic processes. Coupled with other isotope data, however, the degree of overprint can be estimated and the 40Ca/ 44Ca ratio of a little disturbed sample hints at a small contribution of Hadean protocrust in the coastal part of the Godthåbsfjord area (Southwest Greenland). In the majority of Zimbabwe TTG samples, unradiogenic initial Ca isotope ratios point to very little prior crustal history and minor subsequent disturbance. We thus infer that the modest initial ε Nd ∼0.8 of the Zimbabwean samples is representative of the depleted mantle at ∼3.6 Ga. Furthermore, Ca isotope systematics provide little support for a “steady state” model of crustal growth.

An overview of adakite, tonalite–trondhjemite–granodiorite (TTG), and sanukitoid: relationships and some implications for crustal evolution

Examination of an extensive adakite geochemical database identifies two distinct compositional groups. One consists of high-SiO 2 adakites (HSA) which is considered to represent subducted basaltic slab-melts that have reacted with peridotite during ascent through mantle wedge. The second group consists of low-SiO 2 adakites (LSA) which we interpret to have formed by melting of a peridotitic mantle wedge whose composition has been modified by reaction with felsic slab-melts. The chemical composition of less differentiated (primitive) Archaean tonalite–trondhjemite–granodiorite (TTG) magmas evolved from 4.0 to 2.5 Ga. Mg# (molecular Mg/(Mg+Fe 2+), Ni, and Cr contents increased over this period of time and we interpret these changes in terms of changes in the degree to which the TTG magmas interacted with mantle peridotite. Over the same period, concentrations of (CaO+Na 2O) and Sr also increased, as the amount of plagioclase, residual from basalt melting, decreased in response to increased pressures at the site of slab-melting. In the Early Archaean, it appears that these interactions were very rare or absent thus leading to the conclusion that subduction was typically flat and lacked the development of a mantle wedge. In contrast, the relatively lower heat production by ∼2.5 Ga meant that slab-melting occurred at greater depth, where plagioclase was no longer stable, and where the development of a thick mantle wedge ensured interaction between the slab-melts and mantle peridotite. Close compositional similarities between HSA and Late Archaean TTG ( T<∼3.0 Ga) strongly suggest a petrogenetic analogy. However, an analogy between the older Archaean TTG and HSA is not complete because evidence for mantle wedge interaction is missing in most Early Archaean TTGs. Late Archaean sanukitoids and the compositionally similar Closepet-type granites have compositions significantly different from TTG of all ages. However, they show some affinity with LSA which could be considered as their possible analogue. These magmas are all thought to result from melting of a mantle peridotite whose composition has been modified by reaction with slab-melts. We propose that all these magmas are directly linked to slab-melting. Archaean TTG and HSA represent slab-melts that have interacted with peridotite to varying extent, whereas sanukitoids, Closepet-type granites, and LSA correspond to melts of peridotite previously metasomatised by slab-melt. The changes observed from Early Archaean TTG to Late Archaean TTG and to sanukitoids reflect change in both the nature and efficiency of interaction between slab-melt and mantle wedge peridotite. Comparisons between all of these rocks suggest that ancient styles of subduction that have operated since at least ∼3.3 Ga persist in a limited way today. The secular changes in the degree and style of these interactions is a direct consequence of the cooling of Earth that modified the thermal and dynamic parameters at the subducted slab–mantle wedge interface.

Photosynthetic microbial mats in the 3,416-Myr-old ocean.

Recent re-evaluations of the geological record of the earliest life on Earth have led to the suggestion that some of the oldest putative microfossils and carbonaceous matter were formed through abiotic hydrothermal processes. Similarly, many early Archaean (more than 3,400-Myr-old) cherts have been reinterpreted as hydrothermal deposits rather than products of normal marine sedimentary processes. Here we present the results of a field, petrographic and geochemical study testing these hypotheses for the 3,416-Myr-old Buck Reef Chert, South Africa. From sedimentary structures and distributions of sand and mud, we infer that deposition occurred in normal open shallow to deep marine environments. The siderite enrichment that we observe in deep-water sediments is consistent with a stratified early ocean. We show that most carbonaceous matter was formed by photosynthetic mats within the euphotic zone and distributed as detrital matter by waves and currents to surrounding environments. We find no evidence that hydrothermal processes had any direct role in the deposition of either the carbonaceous matter or the enclosing sediments. Instead, we conclude that photosynthetic organisms had evolved and were living in a stratified ocean supersaturated in dissolved silica 3,416 Myr ago.

Characterisation of early Archaean chemical sediments by trace element signatures

Rare earth element (REE) plus yttrium (Y) patterns of modern seawater have characteristic features that can be used as chemical fingerprints. Reliable proxies for marine REE+Y chemistry have been demonstrated from a large geological time span, including Archaean banded iron formation (BIF), stromatolitic limestone, Phanerozoic reef carbonate and Holocene microbialite. Here we present new REE+Y data for two distinct suites of early Archaean (ca. 3.7–3.8 Ga) metamorphosed rocks from southern West Greenland, whose interrelationships, if any, have been much debated in recent literature. The first suite comprises magnetite-quartz BIF, magnetite-carbonate BIF and banded magnetite-rich quartz rock, mostly from the Isua Greenstone Belt (IGB). The REE+Y patterns, particularly diagnostic anomalies (Ce/Ce*, Pr/Pr*), are closely related to those of published seawater proxies. The second suite includes banded quartz-pyroxene-amphibole±garnet rocks with minor magnetite from the so-called Akilia Association enclaves (in early Archaean granitoid gneisses) of the coastal region, some 150 km southwest of the IGB. Rocks of this type from one much publicised and highly debated locality (the island of Akilia) have been identified by some workers [Nature 384 (1996) 55; Geochim. Cosmochim. Acta 61 (1997) 2475] as BIF-facies, and their 13C-depleted signature in trace graphite interpreted as a proxy for earliest life on Earth. However, REE+Y patterns of the Akilia Association suite (except for one probably genuine magnetite-rich BIF from Ugpik) are inconsistent with a seawater origin. We agree with published geological and geochemical (including REE) work [Science 296 (2002) 1448] that most of the analysed Akilia rocks are not chemical sediments, and that C-isotopes in such rocks therefore cannot be used as biological proxies. Application of the REE+Y discriminant for the above two rock suites has been facilitated in this study by the use of MC-ICP technique which yields a more complete and precise REE+Y spectrum than was available in many previous studies.

Comment on “Petrogenesis of an early archaean (3.4 Ga) norite dyke, Isua, West Greenland: evidence for early Archaean crustal recycling?”

Comment on “Petrogenesis of an early archaean (3.4 Ga) norite dyke, Isua, West Greenland: evidence for early Archaean crustal recycling?”