Vickie C. Bennett

Coupled 142Nd-143Nd Isotopic Evidence for Hadean Mantle Dynamics

The oldest rocks—3.85 billion years old—from southwest Greenland have coupled neodymium-142 excesses (from decay of now-extinct samarium-146; half-life, 103 million years) and neodymium-143 excesses (from decay of samarium-147; half-life, 106 billion years), relative to chondritic meteorites, that directly date the formation of chemically distinct silicate reservoirs in the first 30 million to 75 million years of Earth history. The differences in 142Nd signatures of coeval rocks from the two most extensive crustal relicts more than 3.6 billion years old, in Western Australia and southwest Greenland, reveal early-formed large-scale chemical heterogeneities in Earths mantle that persisted for at least the first billion years of Earth history. Temporal variations in 142Nd signatures track the subsequent incomplete remixing of very-early-formed mantle chemical domains.

∼ 3710 and ⪖ 3790 Ma volcanic sequences in the Isua (Greenland) supracrustal belt; structural and Nd isotope implications

Abstract The Isua supracrustal belt of southern West Greenland is the largest body of early Archaean supracrustal rocks, and as such has been the focus of numerous studies to characterise early Earth processes. Based on new geochronological constraints we re-interpret the Isua supracrustal belt stratigraphy and re-evaluate Nd isotopic constraints for the early Earth from Isua supracrustal belt rocks. Part of the belt (∼ 50%) consists of a package of mafic chloritic schists (the garbenschiefer unit), layered amphibolites and felsic rocks, mica (±kyanite) schists and banded iron formation (BIF), and is interpreted as a sequence of mafic to felsic volcanic/volcanosedimentary rocks, turbidites and pelites with minor gabbro. Graded felsic volcanic, in this package have yielded a UPb zircon age of ∼ 3710 Ma. Along the southern side of the belt, a package of amphibolites and ultramafic schists with BIF layers, interpreted as a mafic to ultramafic volcanic sequence, is cut by tonalite sheets with U-Pb zircon ages of 3790–3800 Ma. Thus the belt contains at least two unrelated supracrustal packages of different ages (⪖ 3790 Ma and ∼ 3710 Ma), that were probably juxtaposed in the early Archaean. Felsic volcanic rocks from a single ∼ 3710 Ma unit have consistent initial ϵ Nd values of +0.8 to + 1.8, suggesting minimal fractionation of Nd from Sm during later metamorphism. They may have formed from a depleted (but not ultra-depleted) mantle source only a short time before 3710 Ma, with or without a contribution from LREE-enriched crust of 3750–3870 Ma with ϵ Nd (3800) of + 2 to +4. There is debate as to whether the 3806 ± 2 Ma ‘A6’ felsic unit with ϵ Nd(3800) of + 1.0 to +2.9 is derived from volcanic rocks or from granitoid sheets. Thus in the published Isua supracrustal belt Nd database there are no samples which are definitely > 3710 Ma supracrustal or clearly mantle-derived rocks. Given the problems of locally intense alteration, the presence of supracrustal rocks of two or more ages and of older inherited crustal components in some samples, the Isua supracrustal belt is not the ideal locality to find definitive answers on early Archaean mantle evolution.

Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures

Biological activity is a major factor in Earth’s chemical cycles, including facilitating CO2 sequestration and providing climate feedbacks. Thus a key question in Earth’s evolution is when did life arise and impact hydrosphere–atmosphere–lithosphere chemical cycles? Until now, evidence for the oldest life on Earth focused on debated stable isotopic signatures of 3,800–3,700 million year (Myr)-old metamorphosed sedimentary rocks and minerals from the Isua supracrustal belt (ISB), southwest Greenland. Here we report evidence for ancient life from a newly exposed outcrop of 3,700-Myr-old metacarbonate rocks in the ISB that contain 1–4-cm-high stromatolites—macroscopically layered structures produced by microbial communities. The ISB stromatolites grew in a shallow marine environment, as indicated by seawater-like rare-earth element plus yttrium trace element signatures of the metacarbonates, and by interlayered detrital sedimentary rocks with cross-lamination and storm-wave generated breccias. The ISB stromatolites predate by 220 Myr the previous most convincing and generally accepted multidisciplinary evidence for oldest life remains in the 3,480-Myr-old Dresser Formation of the Pilbara Craton, Australia. The presence of the ISB stromatolites demonstrates the establishment of shallow marine carbonate production with biotic CO2 sequestration by 3,700 million years ago (Ma), near the start of Earth’s sedimentary record. A sophistication of life by 3,700 Ma is in accord with genetic molecular clock studies placing life’s origin in the Hadean eon (>4,000 Ma).

Enhanced mantle-to-crust rhenium transfer in undegassed arc magmas.

Variations in the 187Os/188Os isotopic signature of mantle and mantle-derived rocks have been thought to provide a powerful chemical tracer of deep Earth structure. Many studies have inferred from such data that a long-lived, high-rhenium component exists in the deep mantle (187Re is the parent isotope decaying to 187Os, with a half-life of ∼42 billion years), and that this reservoir probably consists of subducted oceanic crust. The interpretation of these isotopic signatures is, however, dependent on accurate estimates of rhenium and osmium concentrations in all of the main geochemical reservoirs, and the crust has generally been considered to be a minor contributor to such global budgets. In contrast, we here present observations of high rhenium concentrations and low Yb/Re ratios in arc-type melt inclusions. These results indicate strong enrichment of rhenium in undegassed arc rocks, and consequently the continental crust, which results in a crustal estimate of ∼2 p.p.b. rhenium, as compared to previous estimates of 0.4–0.2 p.p.b. (refs 4, 5). Previous determinations of rhenium in arc materials, which were largely measured on subaerially erupted samples, are likely to be in error owing to rhenium loss during degassing. High mantle-to-crust rhenium fluxes, as observed here, require a revaluation of geochemical models based on the 187Re-187Os decay system.

The early Archaean Itsaq Gneiss Complex of southern West Greenland: the importance of field observations in interpreting age and isotopic constraints for early terrestrial evolution

Geochemical and isotopic studies of small volumes of variably preserved≥ 3600 Ma rocks in gneiss complexes are crucial for documenting early Earth history. In the Itsaq Gneiss Complex of the Nuuk region, West Greenland, there is dispute whether the granitic (sensu lato) orthogneisses dominating it are mainly products of a single ca. 3650 Ma crust formation “super event,” or whether they formed in several unrelated events between ca. 3850 and 3560 Ma. Which of these interpretations of the dates is correct has major implications regarding what the whole rock radiogenic isotopic record (Pb/Pb, Sm/Nd, Rb/Sr) reveals about continental crust formation and early terrestrial differentiation. There is also debate whether some West Greenland metasedimentary rocks with 12C/13C data interpreted as evidence for life are≥ 3850 Ma or only≥ 3650 Ma old. Establishing the correct age for these rocks is important for debates concerning early surficial environments and origin of life. Controversies have arisen because of different approaches taken by different workers, specifically with respect to how much emphasis is placed on field geology in interpreting dates and isotopic data. In this paper, field observations and sampling from low strain zones, where the origin and geological context of the rocks are best preserved and understood, are closely integrated with U-Pb zircon dates and cathodoluminescence (CL) imagery of the zircons. This approach shows that most single-phase, well-preserved, meta-granitoid samples have simple zircon populations dominated by oscillatory-zoned prismatic grains formed when their host magmas crystallized. On the other hand, migmatites and some strongly deformed-banded gneisses have much more complex zircon populations. The combined field evidence and zircon geochronology on the Itsaq Gneiss Complex demonstrate that 1) some areas contain exposed orthogneisses formed during multiple magmatic/thermal events between ca. 3850 and 3560 Ma and are not (as suggested by Kamber and Moorbath, 1998) dominated by ca. 3650 Ma granitoids containing abundant> 3650 Ma zircons inherited from cryptic, unexposed, older rocks; 2) abundant,≥ 3750 Ma granitoids are present, which are locally well-preserved; 3) some water-lain sediments reported as showing C isotope evidence for life were deposited as early as 3850 Ma; 4) the whole-rock Sm/Nd isochron approach fails to distinguish with any confidence 3650 Ma from 3800 Ma rocks, 5) however, it reinforces previous indications for markedly depleted (≥ + 2.5 ϵNd) domains in the pre-3750 Ma mantle.

U–Pb evidence of ∼1.7 Ga crustal tectonism during the Nimrod Orogeny in the Transantarctic Mountains, Antarctica: implications for Proterozoic plate reconstructions

Abstract The Pacific margin of East Antarctica records a long tectonic history of crustal growth and breakup, culminating in the early Paleozoic Ross Orogeny associated with Gondwanaland amalgamation. Periods of older tectonism have been proposed (e.g. Precambrian Nimrod and Beardmore Orogenies), but the veracity of these events is difficult to document because of poor petrologic preservation, geochronologic uncertainty due to isotopic resetting, and debated geological field relationships. Of these, the Nimrod Orogeny was originally proposed as a period of Neoproterozoic metamorphism and deformation within crystalline basement rocks of the Nimrod Group, based on ∼1000 Ma K–Ar mineral ages. Later structural and thermochronologic study attributed major deformation features in the Nimrod Group to Ross-age basement reactivation. Yet, new SHRIMP ion microprobe U–Pb zircon age data for gneissic and metaigneous rocks of the Nimrod Group indicate a period of deep-crustal metamorphism and magmatism between ∼1730–1720 Ma. Igneous zircons from gneissic Archean protoliths show metamorphic overgrowths of ∼1730–1720 Ma, and an eclogitic block preserved within the gneisses contains zircons yielding an average metamorphic crystallization age of ∼1720 Ma. Deformed granodiorite that intrudes the gneisses and associated metasedimentary rocks yields a concordant zircon crystallization age of ∼1730 Ma. Despite scant petrologic evidence for these metamorphic and igneous events, the zircon ages from these diverse rock types indicate major crustal thickening, possibly due to collision, in the late Paleoproterozoic. We therefore recommend revival of the term Nimrod Orogeny to describe Paleoproterozoic tectonic events in rocks of the East Antarctic shield. Similarities in the ages of igneous and metamorphic events in the Nimrod Group and geologic units elsewhere in present-day East Antarctica, southern Australia and southwestern North America suggest they may have played a role in early supercontinent assembly. In particular, similarity with the Laurentian Mojave province is consistent with Proterozoic plate reconstructions joining ancestral East Antarctica with western Laurentia.

Evidence for rhenium enrichment in the mantle wedge from submarine arc–like volcanic glasses (Papua New Guinea)

The low Re abundance in arc-type volcanic rocks characterized by high 1 8 7 Os/ 1 8 8 Os ratios is an unsolved puzzle of the 1 8 7 Re- 1 8 7 Os isotope system, leaving a significant gap in our understanding of the evolution of the upper mantle-continental crust system. Here we report new observations of high Re concentrations in fresh, submarine-erupted-i.e., relatively undegassed-island arc-like volcanic glasses dredged from the eastern Manus Basin, offshore Papua New Guinea. These observations, together with previously published reports of high Re concentrations in arc-type melt inclusions, indicate that undegassed arc-type volcanic rocks and the mantle wedge are enriched in Re. Consequently, the Re concentration in the continental crust is likely to be as high as ∼2 ppb, much higher than previously estimated. The low Re concentrations in subaerial arc-type volcanic rocks are probably due to Re loss during magma degassing.

Targeting the impactors: siderophile element signatures of lunar impact melts from Serenitatis

Abstract Highly siderophile element compositions of lunar impact melt breccias provide a unique record of the asteroid population responsible for large cratering events in the inner Solar System. Melt breccias associated with the 3.89 Ga Serenitatis impact basin resolve at least two separate impact events. KREEP-rich melt breccias representing the Apollo 17 poikilitic suite are enriched in highly siderophile elements (3.6–15.8 ppb Ir) with CI-normalized patterns that are elevated in Re, Ru and Pd relative to Ir and Pt. The restricted range of lithophile element compositions combined with the coherent siderophile element signatures indicate formation of these breccias in a single impact event involving an EH chondrite asteroid, probably as melt sheet deposits from the Serenitatis Basin. One exceptional sample, a split from melt breccia 77035, has a distinctive lithophile element composition and a siderophile element signature more like that of ordinary chondrites, indicating a discrete impact event. The recognition of multiple impact events, and the clear signatures of specific types of meteoritic impactors in the Apollo 17 melt breccias, shows that the lunar crust was not comprehensively reworked by prior impacts from 3.9 to 4.5 Ga, an observation more consistent with a late cataclysm than a smoothly declining accretionary flux. Late accretion of enstatite chondrites during a 3.8–4.0 Ga cataclysm may have contributed to siderophile element heterogeneity on the Earth, but would not have made a significant contribution to the volatile budget of the Earth or oxidation of the terrestrial mantle. Siderophile element patterns of Apollo 17 poikilitic breccias become more fractionated with decreasing concentrations, trending away from known meteorite compositions to higher Re/Ir and Pd/Pt ratios. The compositions of these breccias may be explained by a two-stage impact melting process involving: (1) deep penetration of the Serenitatis impactor into meteorite-free lower crust, followed by (2) incorporation of upper crustal lithologies moderately contaminated by prior meteoritic infall into the melt sheet. Trends to higher Re/Ir with decreasing siderophile element concentrations may indicate an endogenous lunar crustal component, or a non-chondritic late accretionary veneer in the pre-Serenitatis upper crust.

SHRIMP UPb zircon geochronology of granites in the Arunta Inlier, central Australia: implications for Proterozoic crustal evolution

Abstract New SHRIMP UPb zircon isotopic ages are reported for ten granitic suites in the eastern Arunta Inlier. These data, combined with previous UPb zircon studies in the region, constrain the timing of major Palaeoproterozoic igneous events within the Arunta Inlier, and establish a geochronological framework for understanding lithostratigraphic relationships, tectonic events, and geochemical and isotopic evolution of the granites in the region. Most granites in the Arunta Inlier, including both syn-tectonic and post-tectonic types, yield zircon ages in the range of 1770-1710 Ma, which are significantly younger than the 1880-1850 Ma “orogenic” Barramundi Igneous Association recognised throughout many other Proterozoic terrains in northern Australia. No rocks of the Barramundi Association have been identified in this study. The previously proposed Barramundi-type suites, including the Jervois and Dneiper granites, yield ages of 1771 ± 6 and 1771 ± 15 Ma, respectively. Many of the granite suites contain inherited zircons with ages in the range of 1890-1830 Ma, similar to the age of the Barramundi Association. This, and the age of the Atnarpa Igneous Complex (Zhao and Cooper, 1992), which is coeval with, but geochemically distinct from, the Barramundi Association, suggest that the 1880-1850 Ma orogenic event affected the Arunta Inlier, but has been obscured by the younger tectonic events. The youth of tectonic events in the Arunta Inlier as compared with other Proterozoic terrains of northern Australia may have resulted from the inlier being located on the southern margin of the Northern Australian Orogenic Province during the Palaeoproterozoic, with the generation of granites in the Arunta Inlier possibly being related to subduction. The combined data indicate that major granitic activity occurred at: 1880-1850, 1820, 1770-1750, 1730-1710, 1660-1650 and 1615-1590 Ma. This result is broadly consistent with earlier summaries of the other Proterozoic terrains in northern Australia, although the intensity of each episode varies in different regions.

≥ 3850 Ma BIF and mafic inclusions in the early Archaean Itsaq Gneiss Complex around Akilia, southern West Greenland? The difficulties of precise dating of zircon-free protoliths in migmatites

Abstract The southern part of the early Archaean Itsaq Gneiss Complex (southern West Greenland) on Akilia and adjacent islands consists of polyphase dioritic–tonalitic–granitic injection components with inclusions of metavolcanic amphibolites, chemical sediments such as banded iron formation (BIF), gabbros and ultramafic rocks. Incipient in situ partial melting and strong deformation during several Archaean tectonothermal events strongly modified these injection components, so that they are now mostly banded, schlieric migmatites with neosome produced during several events. The margins of many inclusions have been loci for either segregation of neosome and/or higher strain—obliterating the relationship between the inclusions and the older components of the migmatites. An added complication is that none of the inclusion lithologies in the southern part of the complex contain protolith zircon, which would provide precise, direct dates. Instead, minimum ages of inclusions are obtained by dating invasive components of the granitic (sensu lato) migmatites. Previous age determinations of the inclusions were centred on a ∼200 m long body of amphibolites, BIF and ultramafic rocks on the southwestern corner of Akilia. Geochronology of this locality has been controversial, with our proposed age of ⩾3850 Ma making the inclusion the worlds oldest-dated sedimentary and mafic rocks. We continue the debate on the age of the inclusion on southwest of Akilia, and we broaden it by presenting mapping and zircon dating studies of inclusions on islands with 20 km of Akilia. This (1) addresses the contentious ages of these rocks, (2) examines the broader issue of how the complex field relationships in migmatites can lead to geochronological controversies, and (3) addresses the problem of precise dating of rocks in gneiss complexes which do not carry zircon from their protoliths. On Qilanngaarsuit (island), a gneiss sheet cutting a metaperidotite inclusion has an age of ∼3850 Ma. On an islet ∼2 km west of Akilia, tonalite and quartz-diorite which intrude and envelop two mafic amphibolite and hornblendite inclusions have dates of ∼3850 Ma also. With our proposed ages of ⩾3850 Ma, these inclusions are older than similar rocks in the 3700–3800 Ma Isua supracrustal belt. At other investigated localities ∼3850 Ma zircon dates have not been obtained from the orthogneisses enveloping or intruding inclusions. At two localities invasive leucogranite neosome and a discordant diorite sheet cutting inclusions have ages of 3600–3660 Ma—a period of known tectonothermal events including crustal melting and intrusion of gabbros and diorites. At three localities, banded-schlieric orthogneisses adjacent to supracrustal, gabbroic and ultramafic rocks contain 3800–3700 Ma igneous components. In these cases, the inclusions could still be ⩾3850 Ma and be equivalent in age to similar rocks on Akilia, but this has been masked by the local tectonothermal history and greater amount of neosome development. An alternative explanation is that some younger (