Mark E. Barley

Oceanic nickel depletion and a methanogen famine before the Great Oxidation Event

It has been suggested that a decrease in atmospheric methane levels triggered the progressive rise of atmospheric oxygen, the so-called Great Oxidation Event, about 2.4 Gyr ago. Oxidative weathering of terrestrial sulphides, increased oceanic sulphate, and the ecological success of sulphate-reducing microorganisms over methanogens has been proposed as a possible cause for the methane collapse, but this explanation is difficult to reconcile with the rock record. Banded iron formations preserve a history of Precambrian oceanic elemental abundance and can provide insights into our understanding of early microbial life and its influence on the evolution of the Earth system. Here we report a decline in the molar nickel to iron ratio recorded in banded iron formations about 2.7 Gyr ago, which we attribute to a reduced flux of nickel to the oceans, a consequence of cooling upper-mantle temperatures and decreased eruption of nickel-rich ultramafic rocks at the time. We measured nickel partition coefficients between simulated Precambrian sea water and diverse iron hydroxides, and subsequently determined that dissolved nickel concentrations may have reached ∼400 nM throughout much of the Archaean eon, but dropped below ∼200 nM by 2.5 Gyr ago and to modern day values (∼9 nM) by ∼550 Myr ago. Nickel is a key metal cofactor in several enzymes of methanogens and we propose that its decline would have stifled their activity in the ancient oceans and disrupted the supply of biogenic methane. A decline in biogenic methane production therefore could have occurred before increasing environmental oxygenation and not necessarily be related to it. The enzymatic reliance of methanogens on a diminishing supply of volcanic nickel links mantle evolution to the redox state of the atmosphere.

Increased subaerial volcanism and the rise of atmospheric oxygen 2.5 billion years ago

The hypothesis that the establishment of a permanently oxygenated atmosphere at the Archaean–Proterozoic transition (∼2.5 billion years ago) occurred when oxygen-producing cyanobacteria evolved is contradicted by biomarker evidence for their presence in rocks 200 million years older. To sustain vanishingly low oxygen levels despite near-modern rates of oxygen production from ∼2.7–2.5 billion years ago thus requires that oxygen sinks must have been much larger than they are now. Here we propose that the rise of atmospheric oxygen occurred because the predominant sink for oxygen in the Archaean era—enhanced submarine volcanism—was abruptly and permanently diminished during the Archaean–Proterozoic transition. Observations are consistent with the corollary that subaerial volcanism only became widespread after a major tectonic episode of continental stabilization at the beginning of the Proterozoic. Submarine volcanoes are more reducing than subaerial volcanoes, so a shift from predominantly submarine to a mix of subaerial and submarine volcanism more similar to that observed today would have reduced the overall sink for oxygen and led to the rise of atmospheric oxygen.

The Sholl Shear Zone, West Pilbara: evidence for a domain boundary structure from integrated tectonostratigraphic analyses, SHRIMP UPb dating and isotopic and geochemical data of granitoids

Abstract The Pilbara Block provides a record of Archaean continental growth involving the tectonic accretion of outboard island-arcs and collisions with other continental-scale fragments. This record of continental growth is balanced by breakup and strike-slip dismemberment of the continent. New SHRIMP UPb in zircon ages and SmNd data provide evidence in the West Pilbara which demonstrates that subduction-related and tectonic-accretion processes at the western margin of that ancestral continent between 3.15-2.78 Ga were coeval with, and genetically related to, crustal-scale tectonics and basin formation inboard of that margin. The tectonic division of the West Pilbara is defined by integrated tectonic analyses, geochronology, geochemistry and isotopic analyses. Geochronological studies clearly indicate that the western Pilbara comparises two domains with different recorded geohistories, whereas geochemistry and isotopic systematics reflect the changing tectonic regimes through time. In combination, these studies allow the development of a reconstruction of the relative positions of the domains through time on the western margin of the Pilbara Block. The supracrustal rocks of the northern Roebourne Lithotectonic Complex (Domain 6 in a Pilbarawide scheme) were formed in an island arc setting, facing an ocean to the north-west, prior to 3260 Ma, the time of emplacement of voluminous granitoids into the complex. In contrast, the supracrustal rocks of the southern Sholl Belt (Pilbara Domain 5) were formed in a back-arc setting behind a north-west-facing arc between 3125 and 3112 Ma, with more-or-less synchronous granite emplacement at about 3115 Ma. The two domains were tectonically juxtaposed, between 2991 and 2925 Ma, by the Sholl Shear Zone, a largely sinistral shear zone, with subsequent volcanism in both domains to about 2925 Ma. The Roebourne Lithotectonic Complex (Domain 6) is interpreted to be an allochthonous terrane, which formed north-east relative to its present position, but indigenous to the Pilbara Block rather than an exotic terrane. The East Pilbara is interpreted to have acted as a cratonic hinterland during the convergent margin tectonics that affected the two West Pilbara domains.

Aerobic bacterial pyrite oxidation and acid rock drainage during the Great Oxidation Event

The enrichment of redox-sensitive trace metals in ancient marine sedimentary rocks has been used to determine the timing of the oxidation of the Earth’s land surface. Chromium (Cr) is among the emerging proxies for tracking the effects of atmospheric oxygenation on continental weathering; this is because its supply to the oceans is dominated by terrestrial processes that can be recorded in the Cr isotope composition of Precambrian iron formations. However, the factors controlling past and present seawater Cr isotope composition are poorly understood. Here we provide an independent and complementary record of marine Cr supply, in the form of Cr concentrations and authigenic enrichment in iron-rich sedimentary rocks. Our data suggest that Cr was largely immobile on land until around 2.48 Gyr ago, but within the 160 Myr that followed—and synchronous with independent evidence for oxygenation associated with the Great Oxidation Event (see, for example, refs 4–6)—marked excursions in Cr content and Cr/Ti ratios indicate that Cr was solubilized at a scale unrivalled in history. As Cr isotope fractionations at that time were muted, Cr must have been mobilized predominantly in reduced, Cr(iii), form. We demonstrate that only the oxidation of an abundant and previously stable crustal pyrite reservoir by aerobic-respiring, chemolithoautotrophic bacteria could have generated the degree of acidity required to solubilize Cr(iii) from ultramafic source rocks and residual soils. This profound shift in weathering regimes beginning at 2.48 Gyr ago constitutes the earliest known geochemical evidence for acidophilic aerobes and the resulting acid rock drainage, and accounts for independent evidence of an increased supply of dissolved sulphate and sulphide-hosted trace elements to the oceans around that time. Our model adds to amassing evidence that the Archaean-Palaeoproterozoic boundary was marked by a substantial shift in terrestrial geochemistry and biology.

Supercontinent cycles and the distribution of metal deposits through time

Systematic temporal variations in the distribution of several important groups of metal deposits reflect the cyclic aggregation and breakup of large continents. In particular, metal deposits that form in continental basins or are associated with anorogenic magmatism were extraordinarily abundant in the Middle Proterozoic (2.0 to 1.4 Ga), corresponding to the assembly of the first large continents. It is important to note that peaks in the abundance of continental metal deposits also coincide with a postulated Late Proterozoic supercontinent (1.0 to 0.8 Ga) and the near maximum extent of Pangea. In contrast, metal deposits that form, or are preserved, in convergent-margin orogens were most abundant in the late Archean (2.9 to 2.6 Ga), corresponding to a period of high global heat flow and rapid stabilization of continental crust, and the past 200 m.y., which corresponds to the present tectonic cycle. Similar mineralization was also present, albeit less abundant, in Early Proterozoic orogens, as well as in Late Proterozoic and Phanerozoic orogens. Future metals exploration may benefit from the application of sequence stratigraphy, as used by the oil industry, to recognize such cycles, particularly in the Precambrian rock record.

Jurassic to Miocene magmatism and metamorphism in the Mogok metamorphic belt and the India‐Eurasia collision in Myanmar

Situated south of the eastern Himalayan syntaxis at the western margin of the Shan-Thai terrane the highgrade Mogok metamorphic belt (MMB) in Myanmar occupies a key position in the tectonic evolution of Southeast Asia. The first sensitive high-resolution ion microprobe U-Pb in zircon geochronology for the MMB shows that strongly deformed granitic orthogneisses near Mandalay contain Jurassic (~170 Ma) zircons that have partly recrystallized during ~43 Ma high-grade metamorphism. A hornblende syenite from Mandalay Hill also contains Jurassic zircons with evidence of Eocene metamorphic recrystallization rimmed by thin zones of 30.9 plus or minus 0.7 Ma magmatic zircon. The relative abundance of Jurassic zircons in these rocks is consistent with suggestions that southern Eurasia had an Andean-type margin at that time. Mid- Cretaceous to earliest Eocene (120 to 50 Ma) I-type granitoids in the MMB, Myeik Archipelago, and Western Myanmar confirm that prior to the collision of India, an up to 200 km wide magmatic belt extended along the Eurasian margin from Pakistan to Sumatra. Metamorphic overgrowths to zircons in the orthogneiss near Mandalay date a period of Eocene (~43 Ma) high-grade metamorphism possibly during crustal thickening related to the initial collision between India and Eurasia (at 65 to 55 Ma). This was followed by emplacement of syntectonic hornblende syenites and leucogranites between 35 and 23 Ma. Similar syntectonic syenites and leucogranites intruded the Ailao Shan-Red River shear belt in southern China and Vietnam during the Eocene-Oligocene to Miocene, and the Wang Chao and Three Pagodas faults in northern Thailand (that most likely link with the MMB) were also active at this time. The complex history of Eocene to early Miocene metamorphism, deformation, and magmatism in the MMB provides evidence that it may have played a key role in the network of deformation zones that accommodated strain during the northwards movement of India and resulting extrusion or rotation of Indochina.

Late Archean convergent margin tectonics and gold mineralization: A new look at the Norseman-Wiluna Belt, Western Australia

On a world scale, the most important hydrothermal gold deposits occur either in Late Archean (ca. 2700 Ma) greenstone belts or at late Paleozoic to Quaternary convergent plate boundaries. In the latter environments, epithermal and porphyry-hosted deposits form during subduction-related magmatism in volcanic arcs, and deep-level meso- thermal deposits form during deformation in continental margin orogenic belts. The mesothermal deposits are similar to Archean gold deposits in greenstone belts such as the Norseman-Wiluna Belt in Western Australia. We here suggest that this similarity exists because the Norseman-Wiluna Belt is an orogenic belt with a tectonic history similar to that of younger mineralized convergent margins, such as the North American Cordillera. It is most likely that the Norseman-Wiluna Belt resulted from the interaction of lithospheric plates during the major period of continental growth and stabilization that occurred in the Late Archean, the distribution of gold mineralization being controlled by convergent margin tectonics.

The Late Archaean bonanza: metallogenic and environmental consequences of the interaction between mantle plumes, lithospheric tectonics and global cyclicity

Abstract The Late Archaean records periods of intense magmatism and the development of prodigious metallogenic provinces of Ni, Fe, Cu Zn and Au deposits. In particular, the period 2.74-2.66 Ga represents one of the most widespread episodes of ultrabasic and basic volcanism preserved in the geological record, as well as anomalously widespread granitoid magmatism. Extensive assemblages of this age, which comprise komatiites and komatiitic basalts derived from mantle plumes, together with tholeiites and calc-alkalic volcanic rocks, are preserved on most Late Archaean cratons. Intense submarine volcanism in plume-like environments resulted in rich komatiite-hosted Ni mineralization in continental-margin basins, and Cu Zn sulphide mineralization in extensional volcanic arcs. Peak submarine magmatism was accompanied by marine transgression and thereby flooding of previously exposed continental crust. Elevated hydrothermal activity and widespread su☐ic conditions in submarine basins are reflected by sulphide-rich carbonaceous sedimentary rocks, that contain the organic remains of bacterial communities, and banded iron formations (BIF). A major episode of mesothermal gold mineralization accompanied accretionary tectonics, as metal-and carbon-rich submarine volcanic and sedimentary successions were subducted or incorporated into nascent continental crust by 2.59 Ga. Comparison with Neoproterozoic and Phanerozoic tectonic and metallogenic patterns indicates that the period 2.78-2.59 Ga represents the first half of an ∼ 360 m.y. global tectonic cycle. This period records the breakup of a supercontinent and the opening and closing of marginal basins along long-lived convergent margins of the external ocean to that supercontinent. Enhanced magmatic events between 2.74 and 2.66 Ga were most likely the result of intrabasinal mantle plumes and a subsequent global plume-breakout event. Together, the plume events were responsible for the extreme environmental conditions during the Late Archaean relative to both the preceding and succeeding periods of Earth history. Interactions between mantle plumes and long-lived convergent margins of a Pacific-type ocean were responsible for the prodigious metal inventory of Late Archaean granitoid-greenstone terranes. Extensive convergent-margin and plume magmatism during that period, coupled with episodic periods of low-angle subduction underplating by oceanic lithosphere, may also have been the cause of development of the buoyant, continental mantle lithosphere that is responsible for the preservation of these highly mineralized cratons. It is also likely that the bonanza metallogenic provinces in the Witwatersrand basin and Paleoproterozoic orogens of West Africa and Laurentia-Baltica reflect interactions of mantle plumes with long-lived convergent margins of the external ocean.

Atmospheric Sulfur in Archean Komatiite-Hosted Nickel Deposits

Early Ore Formation Ore deposits contain most of the worlds metal resources, from commonly used metals such as iron, to precious and expensive metals such as platinum. Understanding how these ancient deposits form may lead to more efficient metal extraction and give clues about early Earth. Bekker et al. (p. 1086) studied sulfur and iron isotopes in 2.7-billion-year old Fe-Ni sulfide deposits from Canada and Australia and found that most of the metal-scavenging sulfur was originally atmospheric in origin. Photochemical reactions in the ancient oxygen-free atmosphere produced sulfide that eventually circulated to the sea floor and mixed with newly erupted komatite magmas. Thus, global surface processes in the oceans, atmosphere, and on continents are geochemically linked to ore-forming processes within Earth. The source of sulfur in economic iron-nickel sulfide deposits is primarily derived from the atmosphere. Some of Earth’s largest iron-nickel (Fe-Ni) sulfide ore deposits formed during the Archean and early Proterozoic. Establishing the origin of the metals and sulfur in these deposits is critical for understanding their genesis. Here, we present multiple sulfur isotope data implying that the sulfur in Archean komatiite-hosted Fe-Ni sulfide deposits was previously processed through the atmosphere and then accumulated on the ocean floor. High-temperature, mantle-derived komatiite magmas were then able to incorporate the sulfur from seafloor hydrothermal sulfide accumulations and sulfidic shales to form Neoarchean komatiite-hosted Fe-Ni sulfide deposits at a time when the oceans were sulfur-poor.

Tectonic evolution of the Late Archaean to Early Proterozoic Mount Bruce Megasequence Set, western Australia

The Mount Bruce Megasequence Set (formerly the Mount Bruce Supergroup) was deposited on the Pilbara Craton during the late Archaean and early Proterozoic. It comprises two megasequences, the Chichester Range Megasequence and the overlying Hamersley Range Megasequence. Each megasequence comprises three supersequences or supersequence packages whose tectonic history can be explained in terms of Phanerozoic-style tectonics. The basal Nullagine Supersequence, which comprises mostly subaerial basalt and terrigenous sedimentary rock, was deposited during an episode of WNW-ESE directed crustal extension that was initiated at about 2770 Ma. The overlying Mount Jope Supersequence, which comprises mostly basalt and tuff, was deposited during the subsequent development of a WNW-ESE trending rift before about 2690 Ma. Mudrock, carbonate sedimentary rock and iron formation in the overlying circa 2690 to circa 2600 Ma Marra Mamba Supersequence Package were deposited on the ensuing divergent craton margin. An unconformity or condensed succession covering a time span of as much as 130 m.y. separates the top of the Chichester Range Megasequence from the Hamersley Range Megasequence during which time an oceanic island arc is interpreted to have collided with the southern Pilbara Craton margin. Subsequently, the subducting oceanic plate flipped from southwest dipping to northeast dipping, and convergence resulted in the formation of a marginal subduction-related orogen with associated arc and backarc volcanism. Iron formation and distal arc-derived tuffs of the Brockman Supersequence Package were deposited on a siliciclastic-starved platform in a neutral continental backarc setting at around 2470 Ma. The overlying circa 2440 Ma Woongarra Supersequence was also deposited in a backarc continental setting and comprises iron formation, felsic tuffs, and probably coeval mafic rocks. The latter two rocks types were derived from a backarc magmatic belt. Shortening of the southern Pilbara Craton margin resulted in the development of a backarc compressive cratonic basin in which iron formation and siliciclastic and carbonate sedimentary rock of the Turee Creek Supersequence were deposited, and ultimately resulted in a continent-continent collision. The Mount Bruce Megasequence Set and its contained iron formations are preserved in the relatively distal hinterland of this collisional orogen.