Ian R. Fletcher

Reassessing the first appearance of eukaryotes and cyanobacteria.

The evolution of oxygenic photosynthesis had a profound impact on the Earth’s surface chemistry, leading to a sharp rise in atmospheric oxygen between 2.45 and 2.32u2009billion years (Gyr) ago and the onset of extreme ice ages. The oldest widely accepted evidence for oxygenic photosynthesis has come from hydrocarbons extracted from ∼2.7-Gyr-old shales in the Pilbara Craton, Australia, which contain traces of biomarkers (molecular fossils) indicative of eukaryotes and suggestive of oxygen-producing cyanobacteria. The soluble hydrocarbons were interpreted to be indigenous and syngenetic despite metamorphic alteration and extreme enrichment (10–20‰) of 13C relative to bulk sedimentary organic matter. Here we present micrometre-scale, in situ 13C/12C measurements of pyrobitumen (thermally altered petroleum) and kerogen from these metamorphosed shales, including samples that originally yielded biomarkers. Our results show that both kerogen and pyrobitumen are strongly depleted in 13C, indicating that indigenous petroleum is 10–20‰ lighter than the extracted hydrocarbons. These results are inconsistent with an indigenous origin for the biomarkers. Whatever their origin, the biomarkers must have entered the rock after peak metamorphism ∼2.2u2009Gyr ago and thus do not provide evidence for the existence of eukaryotes and cyanobacteria in the Archaean eon. The oldest fossil evidence for eukaryotes and cyanobacteria therefore reverts to 1.78–1.68u2009Gyr ago and ∼2.15u2009Gyr ago, respectively. Our results eliminate the evidence for oxygenic photosynthesis ∼2.7u2009Gyr ago and exclude previous biomarker evidence for a long delay (∼300 million years) between the appearance of oxygen-producing cyanobacteria and the rise in atmospheric oxygen 2.45–2.32u2009Gyr ago.

Dating low-grade metamorphic events by SHRIMP U-Pb analysis of monazite in shales

Although the U-Pb isotopic system is widely used to date magmatism and medium- to high-grade metamorphism, dating low-grade metamorphic events has been hampered by a scarcity of suitable minerals. We show that monazite forms in shales during low-grade metamorphism and can be dated by in situ U-Pb analysis, providing precise ages for low- temperature thermal events. In Proterozoic shales from the Pine Creek inlier of northern Australia, monazite and xenotime crystals that formed in a contact aureole yield ages synchronous with granite intrusion, ca. 1830 Ma. In the Pilbara craton of northwestern Australia, monazite growth 2192 ± 5 m.y. ago coincided with basaltic volcanism and rifting, an event apparently responsible for hydrothermal iron-oxide mineralization in a nearby giant iron-ore deposit. The mineralizing environment was highly oxic and bathed by meteoric fluids; thus, our results imply the existence of an oxygen-rich atmosphere by 2200 Ma. Given the abundance of shales in the stratigraphic record, and the likely widespread distribution of monazite in such rocks, in situ U-Pb dating of monazite may become an important chronometer for low-grade metamorphic events.

A comparative study of the geochemical and isotopic systematics of late archaean flood basalts from the pilbara and kaapvaal cratons

Geochemical and Smue5f8Nd isotopic data are reported for igneous rocks of the 2.76 to 2.69 Ga Fortescue Group of the Pilbara Craton, Western Australia, and the ∼ 2.70 Ga Ventersdorp Supergroup of the Kaapvaal Craton, southern Africa. The metamorphic history of the Fortescue Group has also been investigated using whole-rock Rbue5f8Sr and Pbue5f8Pb isochron techniques. Igneous of both sequences have mixed tholeiitic and calc-alkaline affinities and show immobile element correlations indicating derivation from chemically heterogeneous, LREE-enriched sources. Both sequences have similar negative ϵNd values (between −1.5 and −4.4 for the Fortescue Group and 0 and −3.4 for the Ventersdorp Supergroup) which are not correlated with Mg/(Mg + Fe2+). The negative ϵNd values of the Fortescue Group rocks probably do not result from crustal assimilation within crustal-level magma chambers during or following differentiation as the Fortescue Group felsic differentiates have ϵNd values within the ranges of values found in associated mafic rocks and zircon xenocrysts are absent in the felsic differentiates of the Fortescue Group. Instead, these results are consistent with either extensive crustal contamination of primitive (komatiitic or picritic) Fortescue Group parent magmas prior to their differentiation or derivation from negative ϵNd (or enriched) mantle sources. The presence of zircon xenocrysts in some Ventersdorp samples provides compelling evidence of some crustal contamination, although correlations between immobile element ratios (such as La/Yb, Ti/Zr, Ti/Sc and V/Zr) and ϵNd indicate derivation from chemically heterogeneous, enriched mantle sources. A whole-rock Smue5f8Nd isochron date of 3308 ± 138 Ma, ∼ 600 Ma older than the time of eruption indicated by Uue5f8Pb zircon data and within, ∼ 300 Ma of the formation of the Kaapvaal granite-greenstone terrane, may date an Smue5f8Nd fractionation event within the subcontinental lithospheric mantle sources of the Ventersdorp rocks. Rbue5f8Sr and Pbue5f8Pb isochrons obtained for several flows from the Fortescue Group register hydrothermal events occurring between ∼ 2.45 and 2.0 Ga and probably associated with burial metamorphism. Similarities in the field geology, geochemistry and isotopic characteristics of the Fortescue and Ventersdorp sequences and Phanerozoic examples of continental flood basalt volcanism suggest a common mode of origin, possibly involving the interaction of asthenospheric mantle plumes with subcontinental lithospheric mantle sources which have been modified by subduction processes.

Metamorphic replacement of mineral inclusions in detrital zircon from Jack Hills, Australia: Implications for the Hadean Earth

The Hadean (before 4.0 Ga) crust has long been considered to comprise mainly primitive mafic and ultramafic rocks. However, mineral inclusions in detrital zircons as old as 4.4 Ga from Jack Hills, Australia, have been interpreted to be magmatic and to provide evidence for extensive granitic crust. In situ U-Pb dating of monazite and xenotime inclusions in 4.25–3.35 Ga detrital zircons from Jack Hills shows that these inclusions are not magmatic, but formed during metamorphism at either 2.68 Ga or 0.8 Ga. Monazite-xenotime thermometry of intergrowths in the inclusions and the quartz-muscovite rock matrix constrain temperatures to between 420–475 °C, corresponding with conditions during peak regional metamorphism. Petrography and U-Pb geochronology of zircon inclusions from other localities show that the replacement of primary inclusions may commence in the igneous host rock and continue through weathering, sedimentation, and diagenesis. With increasing metamorphic grade, the inclusion assemblage increasingly reflects the composition of the rock matrix. In Jack Hills, most of the inclusions have the same composition and abundances as the metamorphic matrix, consistent with their formation during metamorphism. The titanium content of quartz inclusions indicates formation temperatures of 350–490 °C, supporting a metamorphic origin. Several lines of evidence indicate that at least some of the muscovite inclusions are also secondary. The lack of apatite inclusions in zircons from Jack Hills, relative to zircon in common granitic rocks, suggests that secondary minerals may have replaced primary apatite. Thus, detrital zircon may not be impermeable to post-depositional fluids, raising doubts about the use of the mineral inclusions they contain to infer initial magma chemistry. These results call for a reassessment of the source melts of the Hadean zircons and the composition of the earliest crust.

Sm‐Nd model ages across the margins of the Archaean Yilgarn Block, Western Australia — III. The western margin

Earlier studies of the nature and extent of Proterozoic activity around the Archaean Yilgarn Block, based on Sm‐Nd model ages, have now been extended by an examination of the western margin. New data from the western Yilgarn Block indicate that rocks of the Chittering Metamorphic Belt (2.8–2.9 Ga) are significantly younger than rocks in the other three metamorphic belts of the Western Gneiss Terrain (literature values ≥3.0 Ga). This suggests possible continental growth by marginal accretion during the Archaean. Data from Yilgarn Block granitoids indicate a complex and extended period of generation, ranging from 2.7 to 3.1 Ga. There is a major time break at the western margin of the Yilgarn Block, marked by the Darling Fault. All rocks sampled W of the fault have Proterozoic model ages. This confirms the existence of a Proterozoic mobile belt along the western edge of the Yilgarn Block and provides further evidence that the present Darling Fault follows a zone of Precambrian crustal discontinuity. The rema...

Sequential closure of K–Ca and Rb–Sr isotopic systems in Archaean micas

Comparative K–Ca and Rb–Sr data for extremely radiogenic muscovites (eCa=2300 to 18,700; 87Sr/86Sr=52 to 991) from two unmetamorphosed Archaean pegmatitic rocks from the Jack Hills in the northwestern part of the Yilgarn Craton yield model ages ranging from 2580 Ma to 2100 Ma, with the K–Ca dates being younger than Rb–Sr. The muscovites occur as two textural types: radial clusters of bladed crystals (`rosettes) in a massive `pegmatitic granite, and large euhedral `books in cross-cutting pegmatites. There are marked differences between the dates from the two mineral forms as well as between the two isotopic systems, and no individual date can be used to provide a point for [P–]T–t plots. The data can be interpreted collectively in terms of delayed closure of the isotopic systems in a slowly cooled terrane. The closure temperature modelling demonstrates that the diffusion coefficient of Ca in muscovite is approximately an order of magnitude higher than that of Sr. A curved (assumed exponential) cooling trajectory, with extremely slow cooling (∼0.1°C Ma−1) at ∼2100 Ma, is required to reproduce the youngest K–Ca dates. n nBiotites (ϵCa ∼150) from unmetamorphosed Archaean rocks from the southwest of the craton show differences between K–Ca and Rb–Sr model ages (Rb–Sr data from Libby and de Laeter, 1979) that resemble the differences in the Jack Hills muscovite dates. For biotites which have been affected by regional reheating at (or uplift until) ∼500 Ma, the responses of the two isotopic systems are similar, but the analysed samples are not sufficiently radiogenic (ϵCa<40) to permit identification of any closure sequence. n nMuscovites with a common cooling history but differing closure temperatures define empirical cooling curves in 87Sr*/87Rb vs 40Ca*/40K plots. These can be used in conjunction with the K–Ca/Rb–Sr concordia to provide an improved minimum age estimate for the samples. When only single samples (or very similar samples) are available, the difference between the K–Ca and Rb–Sr dates gives a `quality control indication of the discrepancy between the Rb–Sr date and crystallisation age.

Sm-Nd, Pb-Pb and Rb-Sr geochronology of the Manfred Complex, Mount Narryer, Western Australia

The Sm-Nd, Pb-Pb and Rb-Sr systems in Manfred Complex samples ranging from metaleucogabbro to metapyroxenite indicate formation of the complex ∼ 3700 Ma ago and multiple metamorphic disturbances over the succeeding ∼ 2000 Ma. The isotopic responses to metamorphism were largely determined by the accessory mineral contents of the rocks and by country rock influences. n nWhole-rock data give a five-point Sm-Nd isochron date of 3680±70 Ma, and the six samples with least radiogenic Pb give a Pb-Pb isochron date of 3689±146 Ma. Whole-rock Rb-Sr data reflect the widespread Yilgarn ‘event’ at ∼ 2700 Ma, but some samples show additional Rb-Sr disturbance < 2000 Ma ago. n nData for various mineral fractions show substantial internal re-equilibration of Sm-Nd during granulite facies metamorphism at ∼ 3320 Ma, and further disturbance at least as recently as ∼ 3100 Ma. About half of the samples examined show evidence of partial Pb isotopic re-equilibration or U gain at ∼ 2900 Ma, possibly a time of retrograde amphibolite facies metamorphism. n nA high apparent source μ = 10.2 ± 0.4 from the Pb-Pb whole-rock isochron could indicate assimilation of significant quantities of older (? > 4000 Ma) crustal material. The measured initial ϵNd = −0.2 ± 0.7 is consistent with this, but is not definitive. n nThe various dates correspond well to those determined by Kinny et al. (Kinny, P.D., Williams, I.S., Froude, D.O., Ireland, T.R. and Compston, W., 1988. Precambrian Res., 38: 325–341) and Wijbrans (Wijbrans, J.R., 1985, Ph.D. thesis. Australian National University), and complement the geological framework presented by Myers (Myers, J.S., 1988. Precambrian Res., 38: 309–323).

Sm‐Nd model ages across the margins of the Archaean Yilgarn Block, Western Australia—II; southwest transect into the Proterozoic Albany‐Fraser Province

Abstract This paper continues a survey of Sm‐Nd age relationships at the margins of the Yilgarn Block, Australias largest Archaean craton. Model ages have been determined along an irregular transect extending approximately 120 km from the extreme SW of the Western Gneiss Terrain of the Yilgarn Block through the western extremity of the Albany‐Fraser Province, a major Proterozoic mobile belt. The ages demonstrate the significance of the Manjimup and Pemberton Lineaments as major crustal discontinuities. The sequence of ages across the lineaments strongly supports accretionary models of Precambrian crustal evolution, although some aspects of the age sequence can also be interpreted as mixing trends. Within the Western Gneiss Terrain, the Manjimup Lineament marks a change from older (c. 3.1 Ga) to younger (c. 2.7 Ga) Archaean gneisses. Further south the Pemberton Lineament, marking the northern tectonic boundary of the Albany‐Fraser Province, defines a change from Archaean (c. 2.7 Ga) to Proterozoic (c. 2.1...

Evidence for microbial life in synsedimentary cavities from 2.75 Ga terrestrial environments

Fluviolacustrine sediments of the 2.75 Ga Hardey Formation (Fortescue Group, Australia) preserve pendant columnar structures with stromatolitic lamination within synsedimentary cavities. The millimeter-sized, finger-like columns strongly resemble microbialites from modern basaltic caves and indicate the likely presence of microbial biofilms. The ancient microbial columns are preserved by chert and locally occur as reworked clasts, indicating a near-depositional age for the structures. Sulfur isotopic analysis of pyrite in the columns and adjacent carbonaceous matrix yields δ34SCDT (CDT—Canyon Diablo troilite) values between −8.5‰ and +19‰, showing significant fractionation characteristic of biological cycling of sulfur. Organic matter in cavity ceilings and shale matrix has δ13CPDB (PDB—Peedee belemnite) values between −55‰ and −43‰, suggesting the presence of methanotrophs. Our results suggest that 2.75 Ga terrestrial environments supported a microbial ecosystem, including microbes that inhabited synsedimentary hollows, extending the fossil record of coelobionts by ~1.5 b.y. Subsurface cavities represent a new habitable microenvironment for early life on Earth, and an analogue for ancient life on Mars.

Dating sedimentary rocks using in situ U-Pb geochronology of syneruptive zircon in ash-fall tuffs <1 mm thick

Absolute ages for sedimentary rocks are required to construct a temporal framework in which to decipher Earth9s history. The most reliable method for dating Precambrian sedimentary rocks is U-Pb geochronology of zircon in intercalated volcanic rocks. However, extracting sufficient zircons involves destructive mineral separation procedures, often requiring several kilograms of sample. We have dated felsic tuffs 207 Pb/ 206 Pb dates between ca. 2600 Ma and ca. 2680 Ma, all consistent with their stratigraphic position and available age constraints. The dates represent the timing of syndepositional volcanism and provide reliable estimates for the age of deposition. Zircons in a 0.5 mm tuff band 15 mm above the Jeerinah impact layer provide an age of 2632 ± 7 Ma, which represents a close approximation for the timing of a major asteroid impact, the strewn field of which is distributed across two Archean cratons. In situ geochronology of zircon in felsic tuffs requires only small volumes of sample and is especially useful for drill core samples where material is limited. It avoids the destruction of samples associated with mineral separation, retaining textural information and avoiding potential contamination. This approach adds significantly to the number of sedimentary rocks that can be dated accurately.