Early Archaean Research Articles

Earth’s oldest tsunami deposit? Early Archaean high‐energy sediments in the ca 3.48 Ga Dresser Formation (Pilbara, Western Australia)

AbstractDynamic sedimentary processes are a key parameter for establishing the habitability of planetary surface environments on Earth and beyond and thus critical for reconstructing the early evolution of life on our planet. This paper presents a sedimentary section from the ca 3.48 Ga Dresser Formation (Pilbara Craton, Western Australia) that contains high‐energy reworked sediments, possibly representing the oldest reported tsunami deposit on Earth to date. Field and petrographic evidence (e.g. up to 20 cm large imbricated clasts, hummocky bedding, Bouma‐type graded sequences) indicate that the high‐energy deposit represents a bi‐directional succession of two debrite–turbidite couplets. This succession can best be explained by deposition related to passage and rebound of tsunami waves. Sedimentary processes were possibly influenced by highly dense silica‐rich seawater. The tsunami was probably triggered by local fault‐induced seismic activity since the Dresser Formation was deposited in a volcanic caldera basin that experienced syndepositional extensional growth faulting. However, alternative triggers (meteorite impact, volcanic eruption) or a combination thereof cannot be excluded. The results of this work indicate a subaquatic habitat that was subject to tsunami‐induced high‐energy disturbance. Potentially, this was a common situation on the early Archaean Earth, which experienced frequent impacts of extraterrestrial bodies. This study thus adds to the scarce record of early Archaean high‐energy deposits and stresses the relevance of high‐energy depositional events for the early evolution of life on Earth.

Editorial: Biographical Memoirs, Volume 69

Editorial: Biographical Memoirs, Volume 69

Stephen Moorbath. 9 May 1929 — 16 October 2016

Stephen Moorbath was an intellectual with eclectic interests across the sciences and humanities. In 1939, as a ten-year-old, he fled from Germany to England with his father. Stephen spent almost the whole of the rest of his life—from schoolboy to university professor—in Oxford, where he became one of the world's leading isotope geochemists. His academic career began with setting up Europe's first radiometric rock and mineral dating laboratory. In this laboratory, together with numerous colleagues and students, he applied the lead–lead, rubidium–strontium, potassium–argon and, later, samarium–neodymium isotopic dating methods to the solution of many geological problems. He made major contributions in establishing the chronology for the geological and tectonic evolution of Precambrian crust in the Scottish Highlands and Islands, in West Greenland, Zimbabwe, South India, and Ghana. He developed isotopic criteria for assessing the relative contributions of mantle and crustal sources to Tertiary igneous rocks in Scotland, Andean volcanics and the late Archaean granitoids of West Greenland. He established dating methods for sedimentary rocks: Rb–Sr for shales and Pb/Pb for Archaean limestone. Stephen's abiding geological passion was the study of the early Archaean, especially the Godthåbsfjord and Isukasia areas of West Greenland.

High-temperature metamorphism and crustal melting at ca. 3.2 Ga in the eastern Kaapvaal craton, southern Africa

The question of whether high-grade metamorphism and crustal melting in the early Archaean were associated with modern-style plate tectonics is a major issue in unravelling early Earth crustal evolution, and the eastern Kaapvaal craton has featured prominently in this debate. We discuss a major ca. 3.2 Ga tectono-magmatic-metamorphic event in the Ancient Gneiss Complex (AGC) of Swaziland, a multiply deformed medium- to high-grade terrane in the eastern Kaapvaal craton consisting of 3.66–3.20 Ga granitoid gneisses and infolded greenstone remnants, metasedimentary assemblages and mafic dykes. We report on a 3.2 Ga granulite-facies assemblage in a metagabbro of the AGC of central Swaziland and relate this to a major thermo-magmatic event that not only affected the AGC but also the neighbouring Barberton granitoid-greenstone terrane.Some previous models have related the 3.2 Ga event in the eastern Kaapvaal craton to subduction processes, but we see no evidence for long, narrow belts and metamorphic facies changes reflecting lithospheric suture zones, and there is no unidirectional asymmetry in the thermal structure across the entire region from Swaziland to the southern Barberton granite-greenstone terrane as is typical of Phanerozoic and Proterozoic belts. Instead, we consider an underplating event at ca. 3.2 Ga, giving rise to melting in the lower crust and mixing with mantle-derived under- and intraplated mafic magma to generate the voluminous granitoid assemblages now observed in the AGC and the southern Barberton terrane. This is compatible with large-scale crustal reworking during a major thermo-magmatic event and the apparent lack of a mafic lower crust in the Kaapvaal craton as shown by seismic data.

Earth's oldest stable crust in the Pilbara Craton formed by cyclic gravitational overturns

During the early Archaean, the Earth was too hot to sustain rigid lithospheric plates subject to Wilson Cycle-style plate tectonics. Yet by that time, up to 50% of the present-day continental crust was generated. Preserved continental fragments from the early Archaean have distinct granite-dome/greenstone-keel crust that is interpreted to be the result of a gravitationally unstable stratification of felsic proto-crust overlain by denser mafic volcanic rocks, subject to reorganization by Rayleigh–Taylor flow. Here we provide age constraints on the duration of gravitational overturn in the East Pilbara Terrane. Our U–Pb ages indicate the emplacement of ~3,600–3,460-million-year-old granitoid rocks, and their uplift during an overturn event ceasing about 3,413 million years ago. Exhumation and erosion of this felsic proto-crust accompanied crustal reorganization. Petrology and thermodynamic modelling suggest that the early felsic magmas were derived from the base of thick (~43 km) basaltic proto-crust. Combining our data with regional geochronological studies unveils characteristic growth cycles on the order of 100 million years. We propose that maturation of the early crust over three of these cycles was required before a stable, differentiated continent emerged with sufficient rigidity for plate-like behaviour.

Most ancient evidence for life in the Barberton greenstone belt: Microbial mats and biofabrics of the ∼3.47 Ga Middle Marker horizon

The Middle Marker – horizon H1 of the Hooggenoeg Formation – is the oldest sedimentary horizon in the Barberton greenstone belt and one of the oldest sedimentary horizons on Earth. Herein, we describe a range of carbonaceous microstructures in this unit which bear resemblance to phototrophic microbial biofilms, biosedimentary structures, and interpreted microfossils in contemporaneous greenstone belts from the Early Archaean. Post-depositional iron-rich fluid cycling through these sediments has resulted in the precipitation of pseudo-laminated structures, which also bear resemblance, at the micron-scale, to certain microbial mat-like structures, although are certainly abiogenic. Poor preservation of multiple putative microbial horizons due to coarse volcaniclastic sedimentation and synsedimentary fragmentation by hydrothermal fluid also makes a conclusive assessment of biogenicity challenging. Nonetheless, several laminated morphologies within volcaniclastic sandstones and siltstones and coarse-grained volcaniclastic sandstones are recognisable as syngenetic photosynthetic microbial biofilms and microbially induced sedimentary structures; therefore, the Middle Marker preserves the oldest evidence for life in the Barberton greenstone belt. Among these biosignatures are fine, crinkly, micro-tufted, laminated microbial mats, pseudo-tufted laminations and wisp-like carbonaceous fragments interpreted as either partially formed biofilms or their erosional products. In the same sediments, lenticular objects, which have previously been interpreted as bona fide microfossils, are rare but recurrent finds whose biogenicity we question. The Middle Marker preserves an ancient record of epibenthic microbial communities flourishing, struggling and perishing in parallel with a waning volcanic cycle, an environment upon which they depended and through which they endured. Direct comparisons can be made between environment-level characters of the Middle Marker and other Early Archaean cherts, suggesting that shallow-water, platformal, volcanogenic-hydrothermal biocoenoses were major microbial habitats throughout the Archaean.

Juvenile crust formation in the Zimbabwe Craton deduced from the O-Hf isotopic record of 3.8–3.1 Ga detrital zircons

Hafnium and oxygen isotopic compositions measured in-situ on U-Pb dated zircon from Archaean sedimentary successions belonging to the 2.9–2.8Ga Belingwean/Bulawayan groups and previously undated Sebakwian Group are used to characterize the crustal evolution of the Zimbabwe Craton prior to 3.0Ga. Microstructural and compositional criteria were used to minimize effects arising from Pb loss due to metamorphic overprinting and interaction with low-temperature fluids. 207Pb/206Pb age spectra (concordance >90%) reveal prominent peaks at 3.8, 3.6, 3.5, and 3.35Ga, corresponding to documented geological events, both globally and within the Zimbabwe Craton. Zircon δ18O values from +4 to +10‰ point to both derivation from magmas in equilibrium with mantle oxygen and the incorporation of material that had previously interacted with water in near-surface environments. In εHf-time space, 3.8–3.6Ga grains define an array consistent with reworking of a mafic reservoir (176Lu/177Hf ∼0.015) that separated from chondritic mantle at ∼3.9Ga. Crustal domains formed after 3.6Ga depict a more complex evolution, involving contribution from chondritic mantle sources and, to a lesser extent, reworking of pre-existing crust. Protracted remelting was not accompanied by significant mantle depletion prior to 3.35Ga. This implies that early crust production in the Zimbabwe Craton did not cause complementary enriched and depleted reservoirs that were tapped by later magmas, possibly because the volume of crust extracted and stabilised was too small to influence (asthenospheric) mantle isotopic evolution. Growth of continental crust through pulsed emplacement of juvenile (chondritic mantle-derived) melts, into and onto the existing cratonic nucleus, however, involved formation of complementary depleted subcontinental lithospheric mantle since the early Archaean, indicative of strongly coupled evolutionary histories of both reservoirs, with limited evidence for recycling and lateral accretion of arc-related crustal blocks until 3.35Ga.

Millions of Boreal Shield Lakes can be used to Probe Archaean Ocean Biogeochemistry

Life originated in Archaean oceans, almost 4 billion years ago, in the absence of oxygen and the presence of high dissolved iron concentrations. Early Earth oxidation is marked globally by extensive banded iron formations but the contributing processes and timing remain controversial. Very few aquatic habitats have been discovered that match key physico-chemical parameters of the early Archaean Ocean. All previous whole ecosystem Archaean analogue studies have been confined to rare, low sulfur, and permanently stratified lakes. Here we provide first evidence that millions of Boreal Shield lakes with natural anoxia offer the opportunity to constrain biogeochemical and microbiological aspects of early Archaean life. Specifically, we combined novel isotopic signatures and nucleic acid sequence data to examine processes in the anoxic zone of stratified boreal lakes that are naturally low in sulfur and rich in ferrous iron, hallmark characteristics predicted for the Archaean Ocean. Anoxygenic photosynthesis was prominent in total water column biogeochemistry, marked by distinctive patterns in natural abundance isotopes of carbon, nitrogen, and iron. These processes are robust, returning reproducibly after water column re-oxygenation following lake turnover. Evidence of coupled iron oxidation, iron reduction, and methane oxidation affect current paradigms of both early Earth and modern aquatic ecosystems.

Stagnant lids and mantle overturns: Implications for Archaean tectonics, magmagenesis, crustal growth, mantle evolution, and the start of plate tectonics

The lower plate is the dominant agent in modern convergent margins characterized by active subduction, as negatively buoyant oceanic lithosphere sinks into the asthenosphere under its own weight. This is a strong plate-driving force because the slab-pull force is transmitted through the stiff sub-oceanic lithospheric mantle. As geological and geochemical data seem inconsistent with the existence of modern-style ridges and arcs in the Archaean, a periodically-destabilized stagnant-lid crust system is proposed instead. Stagnant-lid intervals may correspond to periods of layered mantle convection where efficient cooling was restricted to the upper mantle, perturbing Earth's heat generation/loss balance, eventually triggering mantle overturns. Archaean basalts were derived from fertile mantle in overturn upwelling zones (OUZOs), which were larger and longer-lived than post-Archaean plumes. Early cratons/continents probably formed above OUZOs as large volumes of basalt and komatiite were delivered for protracted periods, allowing basal crustal cannibalism, garnetiferous crustal restite delamination, and coupled development of continental crust and sub-continental lithospheric mantle. Periodic mixing and rehomogenization during overturns retarded development of isotopically depleted MORB (mid-ocean ridge basalt) mantle. Only after the start of true subduction did sequestration of subducted slabs at the core-mantle boundary lead to the development of the depleted MORB mantle source. During Archaean mantle overturns, pre-existing continents located above OUZOs would be strongly reworked; whereas OUZO-distal continents would drift in response to mantle currents. The leading edge of drifting Archaean continents would be convergent margins characterized by terrane accretion, imbrication, subcretion and anatexis of unsubductable oceanic lithosphere. As Earth cooled and the background oceanic lithosphere became denser and stiffer, there would be an increasing probability that oceanic crustal segments could founder in an organized way, producing a gradual evolution of pre-subduction convergent margins into modern-style active subduction systems around 2.5 Ga. Plate tectonics today is constituted of: (1) a continental drift system that started in the Early Archaean, driven by deep mantle currents pressing against the Archaean-age sub-continental lithospheric mantle keels that underlie Archaean cratons; (2) a subduction-driven system that started near the end of the Archaean.

Earliest microbial trace fossils in Archaean pillow lavas under scrutiny: new micro-X-ray absorption near-edge spectroscopy, metamorphic and morphological constraints

Abstract Filament-shaped titanite microtextures in early Archaean ( c. 3.47 Ga) metabasalts from the Barberton Greenstone Belt of South Africa have been argued to represent Earth's oldest microbial trace fossils, but petrological data on the environmental conditions that led to the formation of these microtextures are sparse. We present here new metamorphic constraints on the equilibrium mineral assemblage containing the titanite microtextures, together with Fe speciation redox variations in the chlorite matrix surrounding the microtextures and morphospace analysis of the microtextures to test their biogenicity. Thermodynamic phase diagram modelling using a calculated rock microdomain composition indicated that the titanite mineral assemblage is stable at temperature conditions of T =240–360°C. The high-resolution quantitative mapping results combined with micro-X-ray absorption near-edge spectroscopy at the Fe K -edge in chlorite revealed that the titanite microtextures are located in low-temperature, high XFe 3+ chlorite bands, veins and microdomains, supporting an origin as abiotic, retrograde mineral cooling textures. These new data, combined with a late c. 2.9 Ga U–Pb date for the titanite, are incompatible with the existing biological model, which invokes microbial micro-tunnel formation followed by titanite ‘infilling’ and preservation in the early Archaean sub-seafloor. The continuum of titanite morphologies reported herein indicates that titanite morphology cannot be used as reliable evidence in support of a biogenic origin in these metavolcanic rocks. It is proposed that the titanite microtextures of purported biogenic origin from other greenstone belts, such as the Pilbara Craton of Western Australia, also deserve to be scrutinized by high-resolution petrological investigations. Supplementary material: Representative electron microprobe analyses of retrograde minerals and µXANES spectra and corresponding fitted pre-edge peaks for all spot analyses are available at https://doi.org/10.6084/m9.figshare.c.3498465

Lithostratigraphy and structure of the early Archaean Doolena Gap greenstone belt, East Pilbara Terrane, Western Australia

The early Archaean East Pilbara Terrane of the Pilbara Craton represents the archetypical granite dome – greenstone keel terrain and preserves much of the primary geological features of dome-and-keel formation. Here we present the first detailed lithostratigraphic and structural transect from marginal gneisses of the Muccan Granitic Complex through 6km of steeply dipping strata of the western Doolena Gap greenstone belt.Based on overprinting relationships, we identified five deformational events (D1–D5) that can be correlated between four structural domains: (i) marginal orthogneisses of the polyphase Muccan Granitic Complex, (ii) a dominantly mylonitic shear zone (South Muccan Shear Zone) representing a granite dome – greenstone keel detachment zone, (iii) a Central Fold Belt of dominantly mafic greenschists, and (iv) a lower greenschist- to sub-greenschist facies southern Low-Strain Belt.The D1 event in the Low-Strain Belt is recorded in syn-depositional normal faulting (f1) during eruption of deep water low vesicular pillow basalts of the ca. 3.47Ga Mount Ada Basalt, followed by a conformable transition towards shallow water stromatolites in the Duffer Formation. We interpret the D1 event as the result of upper-crustal extension and subsequent uplift above the Muccan Granitic Complex.The D2 event shows the most intense deformation with syn-anatectic tight folds within the Muccan Granitic Complex and tight to isoclinal transposed folds (F2) and fluid-induced normal faults (f2) within the Central Fold Belt. The D2 event resulted in a regional EW composite S1/2 foliation, and in the development of the mylonitic South Muccan Shear Zone along the dome-keel interface and within the Central Fold Belt, mineral lineations (L2) in conjunction with increasing southward D2l-tectonite. This D2 event is not observed in a ∼1km-thick sandstone package (ascribed to the <3.43Ga Strelley Pool Formation) in the Low-Strain Belt that was deposited over an angular unconformity indicating 45° SSW tilting of the underlying Mount Ada Basalt and Duffer Formation. This constrains the D2 event to between 3.47 and 3.43Ga.We propose that the D1 and D2 events record locally significant initiation of Muccan Granitic Complex doming and associated sagduction of greenstone keel rocks, suggesting that partial convective overturn initiated early in the evolution of the East Pilbara Terrane.

Early evolution of the Eukaryota

AbstractThe evolution of eukaryotes represents one of the most fundamental transitions in the history of life on Earth; however, there is little consensus as to when or over what timescale it occurred. Review of recent hypotheses and data in a phylogenetic context yields a broadly coherent account. Critical re‐assessment of the palaeontological record provides convincing evidence for the presence of crown‐group eukaryotes in the late Palaeoproterozic, and stem‐group eukaryotes extending back to the early Archaean. Despite their relatively early establishment, crown‐eukaryotes appear not to have become ecologically significant until the middle Neoproterozoic. I argue that this billion‐year delay was due to the singular, contingent evolution of crown‐group animals and their unique capacity to drive co‐evolutionary change.

Riftogenesis attractor structures in the lithosphere of the Baikal Rift System: Nature and formation mechanism

The results of fluid regime and seismic source studysuggest that the formation and arrangement of the riftogenesis attractor structures (RAS) in the lithosphereof the Baikal Rift System (BRS) are caused by thedynamics and energy release of the fluid systems. Themechanism of RAS formation and functioning arebased on the processes of selforganization in fluidizedmelts and rocks at different decompression regimes.The absolute majority of fluid systems of the lithosphere are open dissipative ones with selforganizationprocesses typical for them; the degree of manifestationof these selforganization processes depends on theenergy state of the system and the position relative tothe equilibrium state [1]. The lithosphere was selforganized on a global level in the Early Archaean,when the energy potential of dissipation was maximaland degassing was of an areal character. As the energypotential of fluid transfer decreased, the style of allendogenous processes changed from areal to belt andlinear at the boundary between the Archaean and Proterozoic. Later on, processes of lithosphere fluidization were located along linear zones of energy dissipation (deep faults). The deeper mantle levels werereached by a fault, the higher the solvent ability andenergy capacity of a fluid were, the stronger the selforganization of matter in the fault was. As a result,hightemperature massdemanding deep fluid systemsformed and then melting sources, zones of regionalmetasomatosis, and autonomous orebearing fluidsystems were organized on their basis in the upperlithosphere.The role played by deep fluids in the formation ofregional tectonic structures is exceptionally great, soany tectonic zone of higher fluid conductivitybecomes a selforganizing system in the future. Notethat the role played by tectonic factor is obviouslydeterminative at the first stage, but fluid systemsbecome so at the following stages. It is fluid systemsthat are forces breaking and restructuring the lithosphere after the influence of the tectonic factor finished. For the structures of this kind, three evolutionstages are distinguished: (1) disturbance of stationarity, (2) growth of the system’s energy potential withenergy absorption and the system’s selforganization,and (3) reduction of the energy potential and fade outof the system as an active fluidconductor. Analysis ofthe global geochronological data on the “lifetime” offluidized zones in the lithosphere indicates that theselonglived systems evolve under an oscillating regime.In the hierarchy of natural systems, intracontinental rift systems, consisting of deep fault zones andzones functioning in a relatively narrow thermodynamical regime, are classified as mesosystems [2]. It isknown that processes of different kinds run in openstationary systems, but the average parameters of theseprocesses do not change in any chosen time interval,while they vary in different parts of the system [3].These systems that change in space and time are identified as dynamic ones [4], and it follows from the theory that dissipative dynamic systems in geological–geophysical media must have spatiotemporal attractors, which are classified as riftogenesis attractors interms of the lithosphere of the rift zone; riftogenesisattractors are the zones in the medium and time periods, where and when earthquakes with normal faultingmechanisms dominate. Based on the data on earthquake source parameters in the BRS lithosphere, threeRASs are distinguished (Fig. 1) [5], and the timeattractors reflect the “assembling” structure in termsof an evolutionary scenario with bifurcation of tripleequilibrium [6]. In accordance with the actualismprinciple, RASs are considered as attributes of theBRS Cenozoic dynamics [7], and the three stages ofvolcanic activation are explained by consecutive origination and development of the three RASs: firstly inthe South Baikal Basin (Late Cretaceous–Paleocene),then in the Khubsugul Basin (Eocene–Oligocene),and finally in the Muya Basin (postOligocene). Two“rifting” stages [8], growing rates of rift processes, andpropagation of riftogenesis in the southwest andnortheast directions from the South Baikal Basin arerelated to the appearance and twoway evolution of the

The faint young Sun problem revisited with a 3-D climate–carbon model – Part 1

Abstract. During the Archaean, the Sun's luminosity was 18 to 25% lower than the present day. One-dimensional radiative convective models (RCM) generally infer that high concentrations of greenhouse gases (CO2, CH4) are required to prevent the early Earth's surface temperature from dropping below the freezing point of liquid water and satisfying the faint young Sun paradox (FYSP, an Earth temperature at least as warm as today). Using a one-dimensional (1-D) model, it was proposed in 2010 that the association of a reduced albedo and less reflective clouds may have been responsible for the maintenance of a warm climate during the Archaean without requiring high concentrations of atmospheric CO2 (pCO2). More recently, 3-D climate simulations have been performed using atmospheric general circulation models (AGCM) and Earth system models of intermediate complexity (EMIC). These studies were able to solve the FYSP through a large range of carbon dioxide concentrations, from 0.6 bar with an EMIC to several millibars with AGCMs. To better understand this wide range in pCO2, we investigated the early Earth climate using an atmospheric GCM coupled to a slab ocean. Our simulations include the ice-albedo feedback and specific Archaean climatic factors such as a faster Earth rotation rate, high atmospheric concentrations of CO2 and/or CH4, a reduced continental surface, a saltier ocean, and different cloudiness. We estimated full glaciation thresholds for the early Archaean and quantified positive radiative forcing required to solve the FYSP. We also demonstrated why RCM and EMIC tend to overestimate greenhouse gas concentrations required to avoid full glaciations or solve the FYSP. Carbon cycle–climate interplays and conditions for sustaining pCO2 will be discussed in a companion paper.

Implications of Geochemistry in support of Palaeo-Proterozoic Tectonothermal Evolution of Bhopalpatnam Granulite Belt, Bastar Craton, Central India

Abstract: Bhopalpatnam Granulite Belt which occur along SW margin of Bastar Craton and NE shoulder of Pranhita-Godavari Rift comprise of charnockite (enderbitic variety), garnet-sillimanite-biotite gneiss, quartzo-feldspathic gneiss and corundum bearing aluminous gneiss. High La/Yb ratio, low Eu anomaly (Eu/Eu*=1.0), high LREE/HREE ratio with uniform REE pattern, high La/Sc ratio (0.53-6.43), high Th/Sc ratio (0.03-2.56), low Ni (5.52-20.95), low Cr (31.05-117.05) and uniform Zr/Hf distribution pattern indicate a Proterozoic character. Distribution pattern of K2O, Na2O and CaO in ternary diagram show quartz–monzonite–granodiorite trend for the bulk rocks indicating that the bulk rock composition is close to TTG of early Archaean, which might have supplied the sediments for the rocks of Bhopalpatnam Granulite Belt. Geochemical and mineralogical evidence indicate an argillaceous protolith for garnet – sillimanite - biotite gneiss and corundum bearing aluminous gneiss, whereas an arkosic protolith for quartzo-feldspathic gneiss. The geochemical signatures also suggest an active continental margin setting for the rocks of Bhopalpatnam Granulite Belt with prominent Nb and Ta anomaly favouring a subduction environment between Bastar Craton and East Dharwar Craton. This is in conformity with the finding of the earlier workers suggesting a clockwise P-T path based on the combined fluid inclusion and mineral phase equilibria. The LILE geochemistry of charnockite suggests a bi-phase evolution. High LREE/HREE ratio portrays a highly evolved nature of the charnockitic melt generated through partial melting of the continental crust at the final stage of the granulite facies metamorphism during collision between Bastar and East Dharwar Cratons.

Zircon SHRIMP dating confirms a Palaeoarchaean supracrustal terrain in the southeastern Kaapvaal Craton, southern Africa

We report SHRIMP zircon ages for felsic volcanic rocks of the early Archaean Nondweni greenstone belt (NGB) located in northern KwaZulu-Natal Province, South Africa. The NGB is part of a chain of greenstone belt remnants that occurs south of the well-known Barberton greenstone belt (BGB). Two samples of felsic volcanic rocks of the basal Toggekry Formation yielded zircon ages of ~3.53Ga, whereas significantly younger ages of ~3.41Ga were obtained for two samples of felsic rocks from near the top of the Witkop Formation. The latter date now constrains the age of stromatolite-like structures in cherts associated with the felsic rocks, which probably represent some of the oldest preserved evidence of life on Earth. The zircon ages also indicate that the NGB correlates well chronologically with the BGB and is one of the oldest volcano-sedimentary successions in the southeastern Kaapvaal Craton. Geochemical data for the felsic NGB rocks suggest formation by partial melting or crystal fractionation of a mafic source that was contaminated by older crust, possibly in a back-arc environment. Initial εNd values for NGB rocks range from −1.2 to +0.3 and suggest involvement of pre-greenstone continental crust that was possibly extensive in the southeastern part of the Kaapvaal Craton during Palaeoarchaean times.

Volcaniclastic habitats for early life on Earth and Mars: A case study from ∼3.5 Ga-old rocks from the Pilbara, Australia

Within the context of present and future in situ missions to Mars to investigate its habitability and to search for traces of life, we studied the habitability and traces of past life in ∼3.5 Ga-old volcanic sands deposited in littoral environments an analogue to Noachian environments on Mars. The environmental conditions on Noachian Mars (4.1–3.7 Ga) and the Early Archaean (4.0–3.3 Ga) Earth were, in many respects, similar: presence of liquid water, dense CO 2 atmosphere, availability of carbon and bio-essential elements, and availability of energy. For this reason, information contained in Early Archaean terrestrial rocks concerning habitable conditions (on a microbial scale) and traces of past life are of relevance in defining strategies to be used to identify past habitats and past life on Mars. One such example is the 3.446 Ga-old Kitty’s Gap Chert in the Pilbara Craton, NW. Australia. This formation consists of volcanic sediments deposited in a coastal mudflat environment and is thus a relevant analogue for sediments deposited in shallow water environments on Noachian Mars. Two main types of habitat are represented, a volcanic (lithic) habitat and planar stabilized sediment surfaces in sunlit shallow waters. The sediments hosted small (<1 μm in size) microorganisms that formed colonies on volcanic particle surfaces and in pore waters within the volcanic sediments, as well as biofilms on stabilised sediment surfaces. The microorganisms included coccoids, filaments and rare rod-shaped organisms associated with microbial polymer (EPS). The preserved microbial community was apparently dominated by chemotrophic organisms but some locally transported filaments and filamentous mat fragments indicate that possibly photosynthetic mats formed nearby. Both microorganisms and sediments were silicified during very early diagenesis. There are no macroscopic traces of fossilised life in these volcanic sediments and sophisticated instrumentation and specialized sample preparation techniques are required to establish the biogenicity and syngenicity of the traces of past life. The fact that the traces of life are cryptic, and the necessity of using sophisticated instrumentation, reinforces the challenges and difficulties of in situ robotic missions to identify past life on Mars. We therefore recommend the return of samples from Mars to Earth for a definitive search for traces of life.

The role of geochronology in understanding continental evolution

Abstract Geochronology has become one of the most essential tools in reconstructing processes of continental growth and evolution, and in situ dating of minerals has become common practice through the development of high-resolution ion microprobes and laser ablation inductively coupled plasma mass spectrometry techniques. Zircon has established itself as the most robust and reliable mineral to record magmatic and metamorphic processes. The combination of mineral ages with Sm–Nd, Lu–Hf and O isotopic systematics constrains magma sources and their evolution, and a picture is emerging that supports the beginning of modern-style plate tectonics in the early Archaean. Major fields for future research in geochronology include the search for very old crustal remnants, the establishment of Precambrian supercontinents, reconstruction of magmatic and tectonic processes in accretionary orogens, verification of ancient high-pressure rocks, and the reconstruction of detailed metamorphic histories by dating minerals in their original textural settings.

Progressive mixing of meteoritic veneer into the early Earth’s deep mantle

Komatiites are ancient volcanic rocks, mostly over 2.7 billion years old (from the Archaean era), that formed through high degrees of partial melting of the mantle and therefore provide reliable information on bulk mantle compositions1. In particular, the platinum group element (PGE) contents of komatiites provide a unique source of information on core formation, mantle differentiation and possibly core–mantle interaction2, 3, 4, 5, 6, 7, 8. Most of the available PGE data on komatiites are from late Archaean (~2.7–2.9 Gyr old) or early Proterozoic (2.0–2.5 Gyr old) samples. Here we show that most early Archaean (3.5–3.2 Gyr old) komatiites from the Barberton greenstone belt of South Africa and the Pilbara craton of Western Australia are depleted in PGE relative to late Archaean and younger komatiites. Early Archaean komatiites record a signal of PGE depletion in the lower mantle, resulting from core formation. This signal diminishes with time owing to progressive mixing-in to the deep mantle of PGE-enriched cosmic material that the Earth accreted as the ‘late veneer’ during the Early Archaean (4.5–3.8 Gyr ago) meteorite bombardment.

Evidence for an early Archaean granite from Bastar craton, India

Abstract: Granitoids in the early Archaean are believed to be potassium-poor tonalite–trondhjemite–granodiorite rocks. Only after continental crust attained sufficient thickness did true (relatively potassium-rich) granites form. No record of true granite prior to 3.4 Ga is available. We report a 3.6 Ga true granite from the Archaean Bastar craton in India. In contrast to the typical early Archaean granitoids, which are commonly deformed into gneisses, this granite is relatively undeformed. The age and composition of the granite implies that continental crust of the Bastar craton attained sufficient thickness to permit intracrustal melting at 3.6 Ga. Supplementary material: Representative major element, trace element and REE composition of the Dalli-Rajhara granite samples and a summary of SHRIMP U-Pb zircon data for the granite sample D-9 are available at http://www.geolsoc.org.uk/SUP18337 .