Christopher M. Fedo

Detrital Zircon Analysis of the Sedimentary Record

The composition of “heavy,” or accessory, detrital minerals in sediments and sedimentary rocks has been a topic of quantitative study for at least the last seventy years, beginning with the first issue of the Journal of Sedimentary Petrology in May 1931 (Tyler 1931, Pentland 1931). Zircon has since played a prominent and complex role in interpreting the composition and history of modern and ancient sediments. Because zircon is highly refractory at Earth’s surface, it occurs in virtually all sedimentary deposits and so provides a critical link in understanding the source history of a deposit. Twenhofel (1941), in a pioneering paper on the frontiers of sedimentary mineralogy and petrology, noted that the simple presence of detrital zircon would be of little value in determining its source: “Zircons from a half dozen sources with as many different properties may be present in a sediment and merely be identified as zircon. Parent rocks cannot be positively identified on such data . The variety or varieties must be identified and their optical properties determined.” From very early on, then, it was recognized that detrital zircon would be a powerful tool in understanding provenance, and thus, sedimentary dispersal systems. Interpretive goals matured considerably in the subsequent decades, especially with major advances in microscopy, mineral chemistry, isotope tracer geochemistry, and geochronology, each addressing different aspects of provenance, sedimentation, and Earth history. The hundreds of published studies utilizing detrital zircon in the last 20 years indicate the increasing success in assessing provenance, paleogeography, and tectonic reconstructions. Selected studies are highlighted in this review to illustrate ways in which detrital zircon can be used for interpreting the stratigraphic record, and thus, the past surface conditions of Earth. In it we will outline the quantitative techniques involved in the sampling protocol and interpretation of data and then …

Quartz and Feldspar Stability, Steady and Non‐Steady‐State Weathering, and Petrogenesis of Siliciclastic Sands and Muds

The kinetic and thermodynamic properties of quartz, plagioclase, and K‐feldspars, which constitute 70 to 80% of the upper crust, provide a framework for prediction of mineralogical and chemical changes involved in the production of siliciclastic sediments. Chemical weathering of bedrock may produce weathering profiles with distinct mineralogical zones, compositionally much different from the parent rock. Mass wasting of such profiles produces sediments that reflect the mineralogy of the zones exposed to mechanical erosion, rather than the composition of fresh bedrock. The relative rates of chemical weathering and mechanical erosion determine which mineralogical zones are exposed to mass wasting, and therefore control the compositions of siliciclastic sediments. Stable rates of chemical weathering and erosion result in steady‐state weathering, so that thickness and the mineralogical composition of eroded soil zones, and therefore the mineralogy of derived sediments, remain unchanged while steady‐state weathering prevails. Non‐steady‐state weathering occurs where climate and tectonism vary. Changing conditions alter rates of chemical weathering and physical erosion, resulting in exposure of different, and sometimes all, weathering zones of profiles, or exposure of bedrock. Such conditions result in production of sediments with diverse mineralogy, reflecting incipiently‐to‐highly weathered zones of profiles. Steady‐ and non‐steady‐state weathering of granitic rocks therefore can be assessed by study of variations in quartz and feldspar contents of sands and variations in bulk compositions of muds, as shown for sediments derived from the Sierra Nevada and Bega batholiths, and the Appalachian Piedmont. Compositional variation of sediments, or its absence, are controlled by the relative rates of chemical weathering and erosion and provides insight into climatic and tectonic conditions in source lands, as well as information about provenance composition.

Geochemistry of shales from the Archean (~3.0 Ga) Buhwa Greenstone Belt, Zimbabwe: Implications for provenance and source-area weathering

Abstract Phyllites from the Archean (~3.0 Ga) Buhwa Greenstone Belt, Zimbabwe, were deposited on a stable cratonic platform. Analyses of the phyllites generally define a single geochemical group based on major-and trace-element abundances. The phyllites are strongly depleted in CaO, Na2O, and Sr with respect to average Archean upper crust. By contrast, K2O, Ba, and Rb are enriched several times relative to average Archean upper crust, reflecting basin-scale K metasomatism. Transition metals are somewhat depleted, whereas high field strength elements are typically enriched relative to average Archean upper crust. Samples generally have fractionated LREE patterns (average CeN/SmN = 2.4) with small negative Eu anomalies (average Eu/Eu* = 0.78) and generally flat HREEs (average GdN/ YbN = 1.3). Such geochemical characteristics suggest that the source dominantly consisted of tonalite with less common occurrences mafic volcanic rocks and granite. Mixing calculations, which massbalance the REEs and Th/Sc, suggest 70% tonalite, 15% mafic volcanic rocks, and 15% granite as possible proportions for the source of the phyllites. A nearby early Archean (~3.5 Ga) continental nucleus preserves similar lithologies and is the probable source. Intense chemical weathering of the source terrane is indicated by premetasomatized chemical index of alteration values of 95–100, nearly complete depletion of CaO and Na2O, and high Al2O3/Na2O ratios. Fine-grained sediments of comparable age and presumed tectonic setting elsewhere in southern Africa show similar geochemical characteristics, implying that source-area compositions and weathering intensities were similar. These mature platformal deposits suggest a tectonically stable environment where intense chemical weathering took place at ~3.0 Ga across southern Africa.

Potassic and sodic metasomatism in the Southern Province of the Canadian Shield: Evidence from the Paleoproterozoic Serpent Formation, Huronian Supergroup, Canada

Abstract Major element geochemistry, petrographic observations, and microprobe analyses of the Paleoproterozoic Serpent Formation, Huronian Supergroup, Canada, suggest that these strata experienced both potassium and sodium metasomatism. Major element data, plotted in Al2O3CaO∗ + Na2OK2O compositional space, form an array nearly identical to data from sub-Huronian paleosols reported elsewhere, suggesting that Serpent fluvial systems captured materials from entire weathering profiles. Potassium metasomatism was concentrated in shales, and represents the illitization of kaolinite, which is a residual product of plagioclase weathering. Sodium metasomatism principally affected silt- and sand-sized particles. Microprobe analyses of plagioclase grains indicate that they are nearly pure albite (>Ab96). Corresponding whole-rock major element data plot in bulk albite space on an (Al2O3K2O)CaO∗Na2O ternary diagram, whereas basement rocks plot in bulk andesine and oligoclase space. End member compositions and the absence of luminescence when bombarded with an electron beam indicate that the albites are of secondary origin. Extremely pure albite cements (>Ab99) commonly fringe detrital grains. There has been little replacement of detrital K-feldspars, despite the complete albitization of plagioclase grains. The timing of metasomatism, at least the last stage, is constrained to the period between ∼ 1700 and 1750 Ma (based on geochronologic data reported elsewhere), when the Southern Province of the Canadian Shield experienced considerable felsic plutonism, possibly related to arc collision. This plutonism may have resulted in the heating of deep basinal brines. Stresses associated with orogenic build-up, or gravity driven groundwater circulation working in concert with the plutonism, expelled these brines through the Serpent Formation (and the entire Huronian Supergroup), thoroughly metasomatizing the region. Such processes associated with collisional tectonics are akin to the development of Mississippi Valley-type PbZn mineralization in the mid-continent of the United States.

Paleoclimatic control on the composition of the Paleoproterozoic Serpent Formation, Huronian Supergroup, Canada: a greenhouse to icehouse transition

Unlike other feldspathic arenites in the ∼ 12 km thick Huronian Supergroup, strata of the 250–350 m thick Serpent Formation contain abundant plagioclase. Deposited in a distal alluvial setting, the Serpent Formation consists of arenites with minor shale and siltstone. The plagioclase abundance could reflect unroofing of a unique source relative to other Huronian units, or could be due to less intense chemical weathering. Major-, trace- and rare earth-element (REE) geochemistry of the Serpent Formation are used to evaluate these two possibilities. Serpent Formation source rocks form part of the Archean Superior Province, which mainly consists of tonalitic, granitic and supracrustal assemblages and their metamorphosed equivalents. In Al2O3CaO* + Na2OK2O (ACNK) space, shales define a linear array which indicates that the unweathered source contained plagioclase and K-feldspar in the ratio of 5:1. This particular array indicates that Serpent fluvial systems tapped a variably weathered carapace developed on the Superior Province, rather than purely fresh bedrock. A modelled mixture of 80% tonalite- and 20% granite-group rocks reproduces REE and trace element characteristics of the Serpent Formation sandstones and shales: average Eu/Eu*= 0.87, LaN/SmN= 4.4, GdNYbN= 1.8 and Th/Sc= 1.65. Mass balance considerations indicate that less than 5% supracrustal-group rocks can be added at the expense of the granite-group. A unique source for Serpent detritus is rejected because underlying and overlying units have approximately the same provenance composition. Plagioclase:K-feldspar ratios in Serpent sandstones range from 2:1 to 1:2, which achieves mass balance among source, mudstone and sandstone compositions when plotted in ACNK space. Achievement of mass balance implies that sandstones and mudstones were derived from the same weathering profiles. These findings indicate that plagioclase preservation in the Serpent Formation is related to less intense (compared with other Huronian arenites) paleoweathering conditions, probably a harbinger of the widespread continental glaciation recorded in the overlying Gowganda Formation.

Questioning the evidence for Earth's earliest life—Akilia revisited

It has been argued that apatite crystals containing inclusions of isotopically light graphite in a quartz-pyroxene rock from the island of Akilia, southwest Greenland, represent the earliest (older than 3.85 Ga) traces of life on Earth. Although the age and protolith of this rock have been subjects of vigorous discussions, the occurrence of isotopically light graphite inclusions in Akilia apatite has so far not been debated in the literature. We present here the results of petrographic analysis of 17 different Akilia samples, including the actual sample (G91-26) used in the original study. Our finding that none of the apatite crystals in these samples contain graphite inclusions indicates that the Akilia apatite has no bearing on claims pertaining to a past record of life on Earth.

EARLY CAMBRIAN EDIACARAN-TYPE FOSSILS FROM CALIFORNIA

Abstract Ediacara-type fossils are rare in the southwestern United States, and Cambrian occurrences of soft-bodied Ediacaran-type fossils are extremely rare. We report both discoidal and frondlike fossils comparable to Ediacaran taxa from the western edge of the Great Basin. We describe one specimen of a discoidal fossil, referred to the form species ?Tirasiana disciformis, from the upper member of the Lower Cambrian Wood Canyon Formation from the Salt Spring Hills, California. Two fragmentary specimens of frond-like soft-bodied fossils are described from the middle member of the Lower Cambrian Poleta Formation in the White Mountains, California, and the upper member of the Wood Canyon Formation in the southern Kelso Mountains, California. On the basis of similarities with fossils from the lower member of the Wood Canyon Formation and from the Spitzkopf Member of the Urusis Formation of Namibia, these specimens are interpreted as cf. Swartpuntia. All fossils were collected from strata containing diagnostic Early Cambrian body and trace fossils, and thus add to previous reports of complex Ediacaran forms in Cambrian marine environments. In this region, Swartpuntia persists through several hundred meters of section, spanning at least two trilobite zones.

Microscale heterogeneity of Fe isotopes in >3.71 Ga banded iron formation from the Isua Greenstone Belt, southwest Greenland

We present Fe isotope compositions for magnetite crystals from >3.7 Ga banded iron formation (BIF) from the Isua Greenstone Belt and younger, spatially related pyrite obtained in situ by secondary ion mass spectrometry (SIMS). Across a distance of several millimeters, individual magnetite crystals of BIF show δ 56 Fe values up to 2‰, and within single magnetite-rich layers, there is a >2‰ range in δ 56 Fe. The highest positive δ 56 Fe values are consistent with those predicted by the equilibrium fractionation factor for oxidation of ferrous to ferric Fe in aqueous solution at a likely Early Archean ocean temperature of ∼70 °C, and further suggest that subsequent precipitation of ferric oxides was sufficiently slow that kinetic fractionation effects were minimal. These high values also imply that the supply of oxidant was limited. Heterogeneity in δ 56 Fe values is attributed to diagenetic reactions occurring in limited-volume pore waters, isolated from the bulk ocean (δ 56 Fe = 0‰), with development of isotope reservoir effects. Secondary pyrite in BIF and a conglomerate also show a distinct enrichment in heavy Fe isotopes, interpreted as redistribution and/or fractionation during postformational events.

Braided Fluvial to Marine Transition: The Basal Lower Cambrian Wood Canyon Formation, Southern Marble Mountains, Mojave Desert, California

ABSTRACT The Lower Cambrian Wood Canyon Formation, recognized over a 39,000 km2 belt through the southern Great Basin and Mojave Desert, was examined in the southern Marble Mountains, eastern Mojave Desert, southeastern California. There it crops out as a well-exposed 90-130 m thick, feldspathic sandstone nonconformably overlying Proterozoic basement. Previous investigators have suggested a tide-dominated, nearshore, shallow marine origin for the entire unit in the eastern Mojave, based on broad-scale stratigraphic trends and localized fossil occurrences in the upper member of the formation. In contrast to a regional approach, the present study uses a detailed lithofacies analysis in a smaller study area to suggest that a fluvial to marine transition occurs within these strata. The middle and upper members of the Wood Canyon Formation are recognized in the southern Marble Mountains and are divisible into six distinct lithofacies that possess gradational contacts where exposed: cross-stratified sandstone (Facies A); mediumpebble conglomerate (Facies B); planar-stratified sandstone (Facies C); sandstone and thin mudstone (Facies D); mudstone with intercalated sandstone (Facies E); and horizontally laminated siltstone (Facies F). Facies A-D, contained within the middle member, comprise the bulk of Wood Canyon sediments and record the depositional characteristics of a transition zone from a distal alluvial braid plain to a tidally influenced, fluvial-dominated braid-delta complex. Cross stratification in the braid-plain sediments displays a strongly unimodal pal ocurrent pattern, suggestive of downstream-migrating sinuous-crested megaripples. Braid-delta sediments, however, show a more dispersed paleocurrent pattern and contain a sparse trace fossil assemblage. A change from fluvial-dominated to marine-dominated processes is recorded in upper member rocks (Facies E and F). Facies E contains many of the traits characteristic of a low- to mid-tidal flat sequence, such as coarsely interlayered bedding, flaser and lenticular bedding, microripple marks, and locally abundant Rusophycus and Planolites trace fossils. Facies F, which dominantly consists of horizontally laminated siltstone, is interpreted as the product of an estuarine lagoon that developed behind and within a discontinuous barrier of stranded braid bars.

Depositional setting and paleogeographic implications of earth's oldest supracrustal rocks, the >3.7 Ga Isua Greenstone belt, West Greenland

Abstract New structural and stratigraphic mapping in the Isua greenstone belt, West Greenland has revealed that the exposed lithologic succession is quite different to that depicted in earlier studies. The key to a better understanding of the stratigraphy has been the recognition of intense strain and metasomatic effects combined with ductile fault structures that segment the belt into a number of tectonic slices. In some of these slices, deformation and metasomatism are somewhat lower than in surrounding slices (though still significant), which permit the recognition of primary depositional features that may be used to compare with lithologies from elsewhere in the belt. The belt is dominated by amphibolite that in a number of places show well-defined pillows. Pillow breccias and basaltic debris flows also occur within this package. Strongly recrystallized ultramafic bodies that occur in the belt are interpreted as intrusions or komatiitic flows. The most common sedimentary rock type is chert/banded iron-formation. These lithologies have been strongly affected by brittle and ductile deformation in combination with coarse recrystallization. Siliciclastic detrital rocks such as conglomerate and sandstone are much less common in the belt, and where present, have intrabasinally derived sources. Highly deformed quartzo-feldspathic schist crops out in a number of places in the belt and most likely has multiple origins. Previously these schists have been considered to be felsic volcaniclastic rocks, though recent mapping, and geochemical, and isotopic studies support the hypothesis that at least some of these schists are highy deformed and carbonated tonalitic gneiss sheets or replaced pillow-lava successions. Carbonate rocks in the belt are now considered to be mostly, or entirely, replacement in origin, where metasomatizing fluids have particularly utilized amphibolite–chert contacts; additionally, some carbonate may represent the products of early sea-floor alteration. We envision a temporarily emergent, below wave base, sea-floor origin for the succession based on the assemblage of lithologies.