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Über dieses Buch

This book presents an integrated approach to the study of the evolution of the Archean lithosphere, biosphere and atmosphere, and as such it is a unique contribution to our understanding of the early Earth and life. The structural and geochemical make-up of both the oceanic and continental crust of the Archean Earth is documented in some case studies of various cratons, and the implications of the Phanerozoic plate and plume tectonic processes for the Archean geology are discussed in several chapters in the book. All chapters are process-oriented and data-rich, and reflect the most recent knowledge and information on the Archean Earth. The interdisciplinary approach of examining the evolution of the Archean crust, oceans, and life that we adopt in this book sets it apart from previous publications on Precambrian geology.

The book will be attractive to researchers in academia and in industry, and to senior undergraduate students, graduate students and faculty in earth and natural sciences.



1. Precambrian Greenstone Belts Host Different Ophiolite Types

We use in this study the new description and classification of ophiolites in order to identify potential ophiolite complexes in four Precambrian greenstone belts, ranging in age from 3.8 Ga to 2.0 Ga, and their tectonic origin. The mafic-ultramafic rock assemblages in the 3.8 Ga Isua (Greenland) and 3.5 Ga Barberton (South Africa) greenstone belts show geochemical signatures that are comparable to those of Phanerozoic suprasubductionzone ophiolites. The 2.7 Ga greenstone belts of the Wawa greenstone belts of the Superior Province (Canada) and the 1.95 Ga Jormua Complex (Finland) display subduction-unrelated geochemical patterns, and represent plume- and continental margin-type ophiolites, respectively. This geochemical and tectonic diversity of the Precambrian greenstone belts is reminiscent of the Phanerozoic ophiolites, and suggests that the modern plate tectonics operated as far back as the early Archean, albeit perhaps in a different mode.

Harald Furnes, Maarten de Wit, Yildirim Dilek

2. The Plume to Plate Transition: Hadean and Archean Crustal Evolution in the Northern Wyoming Province, U.S.A.

The 2.8–4.0 Ga record of crustal evolution preserved in the northern Wyoming Province of western North America provides insight into the role of plume- and plate-regimes in the generation of Hadean and Archean continental crust, and the associated elemental depletion of the primitive mantle. The most complete record is exposed in the Beartooth Mountains (Montana-Wyoming), which lie within the Beartooth-Bighorn magmatic zone (BBMZ) sub-province of the Wyoming Province. The BBMZ (> 100,000 km


) is characterized by a single, voluminous suite of Mesoarchean (~ 2.8–2.9 Ga) TTG (tonalite-trondhjemite-granodiorite) plutonic and metaplutonic rocks. In the Beartooth Mountains these Mesoarchean rocks are exposed along an ~ 100 km E-W cross-section, along which they intrude greenschist grade turbidites in the west (South Snowy block) and high grade, older gneisses in the east (Beartooth Plateau block). The most complete assemblage of pre-2.8 Ga crust is preserved as enclaves within the plutonic Mesoarchean rocks of the Beartooth Plateau block. These older gneisses consist of 3.1–3.5 Ga, tectonically interleaved meta-plutonic (principally TTG and associated migmatites) and metasupracrustal lithologies (e.g., quartzites, schists, banded iron formation, and a range of paragneisses).

The arc-like elemental abundances and enriched Pb and Nd isotopic systematics of the Mesoarchean magmatic suite and the 3.1–3.5 Ga older enclaves in conjunction with Lu-Hf data from 3.3 to 4.0 Ga detrital zircons suggest a model of crustal evolution that began with a Hadean, mafic proto-continent that likely developed over a zone of mantle upwelling. Lu-Hf systematics of the 3.6 to 4.0 Ga zircons suggest substantial recycling within the proto-continent in this interval, and that this recycling involved a low Lu/Hf (~ 0.1) system. A ubiquitous component of 3.2–3.4 Ga detrital zircons with more juvenile Hf isotopic compositions occurs throughout the northern Wyoming Province and suggests a major period of crustal growth and generation of TTG-suite rocks from more depleted sources. Following a period of relative quiescence (2.8–3.1 Ga) in the BBMZ, late Mesoarchean arc magmatism (TTG, adakites, etc.) largely reconstituted the older crust during a relatively brief period between 2.79 and 2.83 Ga; it has remained essentially undisturbed since that time.

Placing this history in a global context suggests that Hadean-Eoarchean crust formed in diachronous and spatially diverse environments that were both plume-like (e.g., Pilbara, northern Wyoming Province) and subduction-like (e.g., West Greenland). The relative importance of plume-type crustal growth declined and subduction-type growth increased through time as a consequence of a progressive decline in terrestrial heat production and mantle potential temperature, with a concomitant increase in hydrous mantle melting in subduction zones.

Paul A. Mueller, David W. Mogk, Darrell J. Henry, Joseph L. Wooden, David A. Foster

3. The Archaean Karelia and Belomorian Provinces, Fennoscandian Shield

The Archaean bedrock of the Karelia and Belomorian Provinces is mostly composed of granitoids and volcanic rocks of greenstone belts whose ages vary from c. 3.50 to 2.66 Ga. Neoarchaean rocks are dominant, since Paleoarchaean and Mesoarchaean granitoids (> 2.9 Ga) are only locally present. The granitoid rocks can be classified, based on their major and trace element compositions and age, into four main groups: TTG (tonalite-trondhjemite-granodiorite), sanukitoid, QQ (quartz diorite-quartz monzodiorite) and GGM (granodiorite-granite-monzogranite) groups. Most ages obtained from TTGs are between 2.83–2.72 Ga, and they seem to define two age groups separated by a c. 20 m.y. time gap. TTGs are 2.83–2.78 Ga in the older group and 2.76–2.72 Ga in the younger group. Sanukitoids have been dated at 2.74–2.72 Ga, QQs at c. 2.70 Ga and GGMs at 2.73–2.66 Ga. Based on REE, the TTGs fall into two major groups: low-HREE (heavy rare earth elements) and high-HREE TTGs, which originated at various crustal depths. Sanukitoids likely formed from partial melting of subcontinental metasomatized mantle, whereas the GGM group from partial melting of pre-existing TTG crust.

The Karelia and Belomorian Provinces include a large number of generally NNW-trending greenstone belts, whose tectonic settings of origin may include an oceanic plateau, island arc and/or continental rift. The ages of volcanic rocks in these greenstone belts vary from 3.05 to 2.70 Ga.

Migmatized amphibolites occur as layers and inclusions in TTGs and fall into two main groups on the basis of their trace element contents. Rocks of the first group have flat or LREE-depleted trace element patterns, resembling the modern mid-ocean ridge basalts. Rocks of the second group are enriched in LILE and LREE may in part represent metamorphosed dykes with assimilated and/or diffused crustal signatures from their TTG country rocks.

Metamorphism of the TTG complexes occurred under upper amphibolite and granulite facies conditions at c. 2.70–2.60 Ga. The pressures of the regional metamorphism were mostly c. 6.5–7.5 kbar as constrained by geobarometry, and the corresponding temperatures were c. 650–740 °C. The granulites near the western boundary of the Karelia Province were equilibrated at c. 9–11 kbar and 800–850 °C. Subduction-related eclogites in the Belomorian Province were metamorphosed at pressures up to 20 kbars in two stages around 2.88-2.81 Ga and c. 2.72 Ga. In other greenstone belts the observed metamorphic conditions show significant variations. In the central parts of the Ilomantsi greenstone belt the observed metamorphic P and T values are c. 3–4 kbars and 550–590


C, and in the Kuhmo greenstone belt 16–17 kbar and 650–690 °C, respectively.

Neoarchaean accretion of exotic terranes at c. 2.83–2.75 Ga and the subsequent collisional stacking at ~ 2.73–2.68 Ga were instrumental in the construction of the current crustal architecture of the Karelia Province. The Svecofennian orogeny strongly modified, however, this Neoarchaean crustal structure during the early Proterozoic

Pentti Hölttä, Esa Heilimo, Hannu Huhma, Asko Kontinen, Satu Mertanen, Perttu Mikkola, Jorma Paavola, Petri Peltonen, Julia Semprich, Alexander Slabunov, Peter Sorjonen-Ward

4. Archaean Elements of the Basement Outliers West of the Scandinavian Caledonides in Northern Norway: Architecture, Evolution and Possible Correlation with Fennoscandia

Meso- and Neoarchaean basement rocks occur in two coastal outliers west of the Scandinavian Caledonides in North Norway, i.e. in western Troms (West Troms Basement Complex) and the Lofoten-Vesterålen area. When restored, these two outliers appear to have been assembled together in a cratonic-marginal position at the northern edge of the Fennoscandian shield in the Neoarchaean, and they share a similar tectono-magmatic history prior to Palaeoproterozoic events. This is confirmed by radiometric dating and similarity of sedimentary/volcanic units, intrusive/magmatic events, structural architecture and metamorphic events, and the mechanisms of amalgamation.

Distinctive tonalite-trondhjemite-granodiorite (TTG) gneisses (2.92-2.67 Ga) and intervening meta-volcanic and sedimentary units, e.g. the Ringvassøya greenstone belt (2.83–2.6 Ga), make up a significant portion of the West Troms Basement Complex. The TTG gneisses, likely magmatic in origin, were deformed, metamorphosed up to granulite facies and locally migmatized during periods of accretion and collisional/convergent tectonism at c. 2.9–2.8 Ga, 2.75–2.7 and 2.7–2.67 Ga. The final Neoarchaean stage (c. 2.67 Ga) caused high-grade metamorphism, resetting and comprehensive migmatization along presumed Neoarchaean terrane boundary shear zones prior to intrusion of an extensive Palaeoproterozoic mafic dyke swarm (2.4–2.2 Ga). The subsequent Palaeoproterozoic evolution involved rifting and basin formation (2.4–1.9 Ga), intrusion of an extensive magmatic suite of anorthosite-mangerite-charnockite-granite (1.8–1.7 Ga) and Svecofennian accretion and collisional orogenesis at c. 1.8–1.67 Ga.

In terms of correlation of the Archaean components with Fennoscandia and/or Laurentia, a closer connection to Fennoscandia is inferred from gravity-magnetic trends beneath the Caledonides, age constraints and tectono-magmatic evolution, and extrapolation of intervening tectonic basement windows present in the Caledonides. Provinces such as the Kola-Norwegian, Belomorian and Karelian provinces of the northwestern Fennoscandian shield of Russia, Finland and north Sweden, display obvious similarities with respect to supracrustal units of similar age and geological setting and their tectono-magmatic evolution.

Steffen G. Bergh, K. Kullerud, P.I. Myhre, F. Corfu, P.E.B. Armitage, K.B. Zwaan, E.J.K. Ravna

5. A Review of the Geodynamic Significance of Hornblende-Bearing Ultramafic Rocks in the Mesoarchean Fiskenæsset Complex, SW Greenland

The Fiskenæsset Complex, SW Greenland, is characterized by layered anorthosite, leucogabbro, gabbro, and ultramafic rock association. Ultramafic rocks consist mainly of hornblendite, hornblende peridotite, hornblende pyroxenite, and dunite. Despite upper amphibolite to granulite facies metamorphism, poly-phase deformation and multiple granitoid intrusions, primary igneous layers and mineral assemblages have been well preserved. Petrographic studies, including SEM-BSE imaging, reveal the presence of igneous hornblende occurring as an interstitial mineral to olivine, clinopyroxene, orthopyroxene, plagioclase, and chromite, as well as inclusions in these minerals, consistent with a hydrous mantle source. Large negative Nb-anomalies in whole-rock samples and hornblende grains suggest that the magmas of the Fiskenæsset Complex originated from a hydrous sub-arc mantle peridotite. Water was recycled to the source of the Fiskenæsset rocks through subduction of hydrated oceanic crust. Phanerozoic hornblende-bearing mafic and ultramafic rocks are typically associated with supra-subduction zone ophiolites and magmatic arcs. Recycling of water to the upper mantle via subduction of oceanic crust not only resulted in the generation of hornblende-rich rocks, but also played an important role in the formation of TTG-dominated Archean continental crust.

Ali Polat

6. The Precambrian Geology of the North China Craton: A Review and Update of the Key Issues

This paper reviews current thinking with respect to the main issues concerning the nature and evolution of the North China Craton (NCC). Because literature on the NCC is so voluminous, it of necessity focuses on specific aspects that have wider applicability to the nature of Precambrian cratons in general. The assembly of the craton is examined and opposing views evaluated. The overall distribution of Precambrian rocks is placed within the favoured model for subdividing the craton, and recent advances in determining the ages of key lithological associations are presented. The more controversial topic of where the NCC was placed in the Nuna/Columbia and Rodinia supercontinents is canvassed, although no definitive conclusions can be drawn. Finally, the removal of continental lithosphere from beneath the eastern part of the craton in the Phanerozoic is briefly examined, since this phenomenon has wider implications for the global preservation of Precambrian crust.

Simon A. Wilde

7. How to Make a Continent: Thirty-five Years of TTG Research

After more than 35 years of TTG (tonalite-trondhjemite-granodiorite) research, we still face many questions about the origin and tectonic significance of these peculiar rocks. What we do know is that TTGs are similar in composition regardless of age, they have high La/Yb, Sr/Y, Sr and Eu/Eu*, they decrease in abundance relative to calc-alkaline granitoids at the end of the Archean, and they are not made in oceanic arcs, shallow levels of oceanic plateaus or at ocean ridges. Furthermore, oxygen isotopes in TTG zircons require interaction of TTG sources with the hydrosphere, and the existence of Hadean continental crust inferred from detrital zircon suites remains problematic. Although we now realize that TTGs require amphibole and garnet fractionation and sources that are at least 50 km deep, what we do not know are the relative roles of (1) melting versus fractional crystallization and (2) melting of slabs versus melting of thickened mafic crust. The mechanisms and rates of slab dehydration control the stability of garnet and amphibole in subduction zones. From what we know about early Archean greenstones, they are more altered than later ones, and thus they would appear to bring more water and fluid-mobile elements into subduction zones, at least by the late Archean when plate tectonics became widespread. Hotter slabs in the Archean should contribute to higher volatile release rates. This may explain the trace element changes we see in TTGs at the end of the Archean.

To make continental crust today we need to start at a continental subduction zone where we produce both calc-alkaline (CA) and TTG magmas, and combine the felsic components in a ratio of about 3 parts CA to 2 parts TTG. In contrast, to make an Archean continent, we need nearly 100 % of the TTG component, and may begin, at least before about 3 Ga, by melting the roots of oceanic plateaus.

Kent C. Condie

8. Recycling of Lead at Neoarchean Continental Margins

A time-fixed Pb-Pb model of 2.7 ± 0.1 Ga mantle-derived granitoids from different Archean cratons suggests that the Pb isotope heterogeneity of Neoarchean granitoids can be explained by sediment recycling and subduction at oceanic and continental margins consisting of different-aged crustal segments. Recycling of crustal Pb to the mantle wedge gave rise to increasingly radiogenic mantle sources for the granitoids as the accretion of oceanic island arcs (OIA) and Mesoarchean microcontinents proceeded, leading to the formation of young (< 3.2 Ga) continental margins (YCM). Materials recycling at old (> 3.2 Ga) continental margins (OCM) encompassing fragments of Paleo- and Eoarchean protocrust provided the high- or low-µ Pb isotope signatures, depending on the age and U/Pb ratio of the crustal lead sources.

Jaana Halla

9. Crustal Evolution and Deformation in a Non-Plate-Tectonic Archaean Earth: Comparisons with Venus

Evidence for modern plate tectonics in the Archaean is equivocal to absent, and alternative environments for formation and deformation of greenstone sequences are summarized. We focus on proposals for an unstable stagnant lid basaltic plateau crust, with cratonization occurring initially above major mantle plumes. Archaean continental drift initiated as a result of mantle traction forces acting on newly-formed subcontinental mantle keels, with further cratonic growth occurring as a result of terrane accretion to the leading edges of the migrating cratonic nuclei.

Venus is presented as an analogue for a non-plate-tectonic Archaean Earth. Despite the absence of evidence for characteristic plate tectonic environments on Venus (i.e. subduction = trenches and volcanic arcs; seafloor-spreading = volcanic ridges and transforms), the form, scale, and geometry of folds, brittle-ductile shear zones, and faults interpreted on the surface of Venus from radar imagery are comparable to mid-upper crustal structures on Earth. Anastomosing rifts link coronae interpreted to form above upwelling mantle plumes. The Lakshmi Planum highland plateau in the western Ishtar Terra region of Venus lacks extensive, regional-scale internal deformation structures, but a fold-thrust belt produced mountains on its northern margin, folds and sinistral strike-slip faults occur on its NW margin, and both regional dextral and sinistral strike slip belts occur in a zone of lateral escape to its NE. Rift zones are present along the southern margin to Lakshmi Planum. The scale and kinematics of structures in western Ishtar Terra closely resemble those of the Indian-Asia collision zone, and we propose that lateral displacement of some coronae and ‘craton-like’ highlands or plana result from mantle tractions at their base in a stagnant lid convection regime, i.e. a similar regime as interpreted to have preceded development of plate tectonics on Earth.

In the Wawa-Abitibi Subprovince of the Superior Craton in Canada, the formation of granite greenstone sequences in a plume-related volcanic plateau and subsequent deformation can be generated through geodynamic processes similar to those on Venus without having to invoke modern-style plate tectonics. 3D S-wave seismic tomographic images of the Superior Province reveal a symmetrical rift in the sub-continental lithospheric mantle (SCLM) beneath the Wawa-Abitibi Subprovince, with no evidence for ‘fossil’ subduction zones. Major gold deposits and kimberlites are located above rift-bounding faults in the SCLM. Early rift structures localized subsequent deformation and hydrothermal fluid flow during N-S shortening and lateral escape ahead of a southwardly moving indenter (the Northern Superior Craton—Hudson’s Bay terrane) in the ca. 2696 Ma Shebandowanian orogeny. The geometry of reverse and strike-slip shear zones in the Abitibi Subprovince of the SE Superior Province is similar to that of shear zones developed in western Ishtar Terra, Venus, which also formed ahead of a rigid indenter whose displacement is attributed to mantle tractions. Similarly, shortening and rift inversion in the Abitibi is ascribed to cratonic mobilism where displacement of the N Superior Province ‘proto-craton’ resulted from mantle flow acting upon its deep lithospheric keel. Deformation in other Archaean cratons previously interpreted in terms of plate tectonics may also be the result of similar, mantle-driven processes.

Lyal B Harris, Jean H Bédard

10. Accreted Turbidite Fans and Remnant Ocean Basins in Phanerozoic Orogens: A Template for a Significant Precambrian Crustal Growth and Recycling Process

Convergent margin settings involving accretion of large turbidite fans with slivers of oceanic basement reflect important cites of continental crustal growth and recycling. Accreted crust consists of an upper layer of recycled arc and/or crustal detritus (turbidites) underlain by a layer of tectonically imbricated upper oceanic crust, and/or thinned continental crust, along with underplated magmatic material. When oceanic crust is converted to lower crust, it represents a juvenile addition to the continent. This two-tiered accreted crust is commonly of average continental crustal thickness and isostatically balanced near sea level. The Paleozoic part of the Tasman Orogen (Lachlan-type) of eastern Australia is the archetypical example of a turbidite-dominated accretionary orogen. The Neoproterozoic Damaran Orogen of SW Africa is similar to the Lachlan-type except that it was incorporated into Gondwana via a continent-continent collision, whereas the Mesozoic Rangitatan Orogen of New Zealand illustrates the transition of convergent margin from a Lachlan-type to more typical accretionary wedge type orogen. The spatial and temporal variations in deformation, metamorphism, and magmatism across these orogens illustrate how large volumes of turbidite and their relict oceanic basement eventually become stable continental crust. The timing of deformation and metamorphism recorded in these rocks reflects the crustal thickening phase, whereas post-tectonic granitoids and volcanic deposits constrain the timing of chemical maturation and cratonization. Cratonization and chemical maturation of continental crust is fostered in these orogenic settings because turbidites represent fertile sources for magma genesis, particularly for the S-type granites that are common in these orogens. The structural style and lithotectonic assemblages of the three Phanerozoic examples is remarkably similar to the Archean Jardine turbidites, which were accreted to the Wyoming craton by 2.8 Ga. Recognition of similar orogens in the Archean is important for the evaluation of crustal growth models, particularly for those based on detrital zircon age patterns, because crustal growth by accretion of the upper ocean crust or by underplating of mafic magmas does not readily result in the formation of zircon-bearing magmas at the time of accretion. This crust only produces significant zircon when and if it partially melts, which may be long after the actual time of accretion. Consequently, the significance of this process over earth history is distorted compared to more zircon-rich orogenic processes in probability density-based analyses of crustal growth, but is recorded in Lu-Hf model ages of zircons from post-accretion magmas.

David A. Foster, Paul A. Mueller, Ben D. Goscombe, David R. Gray

11. Biogenicity of Earth’s Earliest Fossils

The abundant and diverse assemblage of filamentous microbial fossils and associated organic matter permineralized in the ~ 3465 Ma Apex chert of northwestern Australia—widely regarded as among the oldest records of life—have been investigated intensively. First reported in 1987 and formally described in 1992 and 1993, the biogenicity of the Apex fossils was questioned in 2002 and in three subsequent reports. However, as is shown here by use of analytical techniques unavailable twenty years ago, the Apex filaments are now established to be

bona fide

fossil microbes composed of three-dimensionally cylindrical organic-walled (kerogenous) cells. Backed by a large body of supporting evidence of similar age—other microfossils, stromatolites, and carbon isotopic data—it seems clear that microbial life was present and flourishing on the early Earth ~ 3500 Ma ago.

J. William Schopf, Anatoliy B. Kudryavtsev

12. In situ Morphologic, Elemental and Isotopic Analysis of Archean Life

A number of key questions in Archean palaeobiology require study at the micrometre (µm) to nanometre (nm) scale. These include: identifying the transition from a prebiotic world to one containing life; distinguishing true signs of life from abiotic artifacts; identifying the first appearance of important groups of microbes (e.g. cyanobacteria) and metabolic pathways (e.g. sulfur processing, iron processing, anoxygenic and oxygenic photosynthesis); and, determining the transition from a purely prokaryotic world to one including eukaryotes. Here I outline four complementary

in situ

microanalysis techniques that are now providing new evidence in our quest to solve these important scientific questions. The integrated use of these techniques is illustrated by way of a case study from the 3430 Ma Strelley Pool Formation of Western Australia.

David Wacey

13. Archaean Soils, Lakes and Springs: Looking for Signs of Life

Microbial life in Archaean non-marine settings like soils, lakes and springs would have faced several challenges. These would have included exposure to UV light; aridity, salinity and temperature changes; and nutrient availability. Current understanding is that none of these challenges would have been insurmountable. Microbial organisms of Archaean marine environments are likely to have been similar in their lifestyles and habits to those of the Archaean terrestrial world. Non-marine stromatolites, microbial filaments, microbial borings and microbially-induced sedimentary structures might therefore have been preserved. But Archaean subaerial surfaces would have been very prone to erosion by wind and rain, so the oldest fossil ‘soils’ of subaerially weathered surfaces (up to 3.47 Ga) are mostly identified using geochemistry. However, some ancient duricrusts like calcretes have been reported. Archaean lacustrine microbial life may have included stromatolites of the Tumbiana Formation of Western Australia. The case that these were lacustrine rather than marine is critically assessed, with the conclusion that the stratigraphy provides the strongest supporting evidence here. Archaean terrestrial hot springs, though often mentioned in origin of life studies, are not yet known from the rock record. In the Palaeoproterozoic to present these silica and carbonate-precipitating environments are commonly found in proximity to volcanic sediments and faults, where the deposits form terraced mounds, fissure ridges and hydrothermal lakes. It remains plausible that life could have existed and even evolved in these hypothesised Archaean hot-spring settings, and there is cause for optimism that the evidence for this might one day be found.

Alexander T. Brasier

14. Rare Earth Elements in Stromatolites—1. Evidence that Modern Terrestrial Stromatolites Fractionate Rare Earth Elements During Incorporation from Ambient Waters

Ancient chemical sediments may provide critical information about early microbial life and ancient environmental conditions. For example, the rare earth element (REE) content and fractionation patterns of Archean and Proterozoic banded iron formations (BIF) and other chemical sediments are thought to preserve the REE patterns of ancient seawater, and as such have been employed to investigate secular trends in seawater chemistry through geologic time. Recently it was suggested that REEs could provide evidence for distinguishing between biotic and abiotically precipitated chemical sediments. However, it is important to underscore that very little is actually known about how stromatolites and other microbialites obtain their REE concentrations and fractionation patterns, including what biological processes, if any, the REEs may record. Here, we present REE concentration and fractionation patterns for modern, lacustrine stromatolites and the ambient waters within which they form. We show that the REE patterns of the stromatolites are highly fractionated compared to the ambient waters. Specifically, the stromatolites exhibit heavy REEs (HREE) enrichments relative to upper crustal proxies (i.e., shale composites), whereas the ambient waters are substantially depleted in the HREEs. We propose that surface complexation and subsequent preferential incorporation of HREEs by organic ligands associated with bacterial cell walls, microbialite biofilms, and/or exopolymeric substances may explain the HREE enrichments of the stromatolites.

Karen H. Johannesson, Katherine Telfeyan, Darren A. Chevis, Brad E. Rosenheim, Matthew I. Leybourne


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