Mark D. Schmitz

Calibrating the Cryogenian

Aging Snowball Earth Earths glacial cycles have varied dramatically over time; at one point glaciers may have covered nearly the entire planet. Correlating various paleoclimate proxies such as fossil and isotope records from that time hinges on the ability to acquire precise age estimates of rocks deposited around the time of this so-called “Snowball Earth.” Macdonald et al. (p. 1241) report new high-precision U-Pb dates of Neoproterozoic strata in the Yukon and Northwest Territories, Canada, to calibrate the timing of carbon isotope variation in rocks from other locations around the globe. Based on the estimated past positions of where these rocks were deposited, glaciers probably extended to equatorial latitudes. The overlap with the survival and, indeed, diversification of some eukaryotes in the fossil record suggests that life survived in localized ecological niches during this global glaciation. A volcanic tuff dated to 716.5 million years ago calibrates the timing of a global glaciation event and eukaryotic survival. The Neoproterozoic was an era of great environmental and biological change, but a paucity of direct and precise age constraints on strata from this time has prevented the complete integration of these records. We present four high-precision U-Pb ages for Neoproterozoic rocks in northwestern Canada that constrain large perturbations in the carbon cycle, a major diversification and depletion in the microfossil record, and the onset of the Sturtian glaciation. A volcanic tuff interbedded with Sturtian glacial deposits, dated at 716.5 million years ago, is synchronous with the age of the Franklin large igneous province and paleomagnetic poles that pin Laurentia to an equatorial position. Ice was therefore grounded below sea level at very low paleolatitudes, which implies that the Sturtian glaciation was global in extent.

Derivation of isotope ratios, errors, and error correlations for U‐Pb geochronology using 205Pb‐235U‐(233U)‐spiked isotope dilution thermal ionization mass spectrometric data

A comprehensive treatment of the derivation of U-Pb isotope ratios and their corresponding uncertainties from isotope dilution thermal ionization mass spectrometric measurements is presented. Standard parametric statistical methods of error propagation are utilized to convolve uncertainties associated with instrumental mass fractionation, tracer subtraction, blank Pb and U subtraction, and initial common Pb correction. Derivations include errors and error correlations for total sample U/Pb and Pb isotope ratios (including radiogenic and initial common Pb) for two- and three-dimensional isochron calculations, radiogenic U/Pb and Pb isotope ratios for concordia and radiogenic model age calculations, and the propagation of model age errors from radiogenic isotope ratios.

U-Pb zircon and titanite systematics of the Fish Canyon Tuff: an assessment of high-precision U-Pb geochronology and its application to young volcanic rocks

A large data set of single and multi-grain zircon and titanite analyses from a sample of the Oligocene Fish Canyon Tuff (FCT), a voluminous ash flow from the San Juan Mountains of Colorado and widely used 40Ar/39Ar geochronological standard, has been used to evaluate the influence of various sources of analytical and geological uncertainty on the calculated age of this tuff by the isotope dilution U-Pb zircon method. Twenty-three single zircon grains and seven small multi-grain fractions of the FCT yield an inverse-variance weighted mean 206Pb/238U date of 28.402 ± 0.023 Ma (2σ; MSWD 0.93) and a slightly older weighted mean 207Pb/235U date of 28.529 ± 0.030 Ma (MSWD 0.74), which are insensitive to common Pb corrections. Initial 230Th disequilibria calculated from a newly measured Th/U = 2.2 for FCT pumice shards indicate Th-deficiency for these zircons; its correction brings the weighted mean 206Pb/238U date to 28.478 ± 0.024 (MSWD 0.97), minimizing the discordance of the FCT zircon analyses. These calculations culminate in Concordia ages (Ludwig, 1998) for the crystallization of the FCT zircons of 28. 476 ± 0.029 Ma (MSWD 1.50) without propagating decay constant errors, or 28.498 ± 0.035 Ma (MSWD 1.03) with propagated decay constant errors. These results are in close agreement with a previous single-grain 206Pb/238U zircon date of 28.41 ± 0.05 Ma for the FCT (Oberli et al., 1990), however they do not corroborate the discordia lower intercept date of 27.52 ± 0.09 put forth by Lanphere and Baadsgard (2001). The 28.476 ± 0.064 Ma Concordia age for FCT zircons (including systematic decay constant errors and uncertainty in Pb/U tracer calibration) is substantially older than all published 40Ar/39Ar sanidine ages, which range from 27.5 to 28.05 Ma (Lanphere and Baadsgaard, 1997; Renne et al. 1998). A lengthy period (∼400 k.y.) of zircon residence in the magma chamber before eruption is commonly called upon to explain the age discrepancy. However, FCT titanite, which may remain open to Pb diffusive exchange until quenching upon eruption, yield a minimum 230Th-disequilibrium corrected weighted mean 206Pb/238U age of 28.395 ± 0.049 Ma (± 0.078 with tracer uncertainty), weakening residence time arguments, and supporting recent calls for re-evaluation of the 40K decay constant and the accepted ages for commonly used 40Ar/39Ar mineral standards. The FCT zircon Concordia age also provides an empirical constraint on the decay constants of the U isotopes, indicating the relative accuracy of the currently accepted decay constants to within the original counting statistic errors of Jaffey et al. (1971).

High-Precision U-Pb Calibration of Carboniferous Glaciation and Climate History, Paganzo Group, NW Argentina

The duration and geographic extent of Carboniferous glacial events in southern Gondwana remain poorly constrained despite recent evidence for a more dynamic glacial history than previously considered. We report 10 high-precision (2! ± <0.1%) U-Pb ages for the Permian-Carboniferous Paganzo Group, NW Argentina, that redefi ne the chronostratigraphy of the late Paleozoic Paganzo and Rio Blanco Basins, and signifi cantly refi ne the timing of glacial events and climate shifts in the western region of southern Gondwana. Radiometric calibration of the Paganzo Group indicates three pulses of Carboniferous glaciation in the mid-Visean, the late Serpukhovian to earliest Bashkirian, and between the latest Bashkirian to early Moscovian. An abrupt shift in depositional style from high-sinuosity single-storied fl uvial deposits and clay-rich paleosols to low-sinuosity multi storied feldspathic fluvial deposits inter calated with eolianites and calcic paleosols is constrained to the latest Moscovian and earliest Kasimovian. These constraints indicate a relatively abrupt climate shift from humid-subhumid to nonseasonal semiarid regional climate conditions that occurred signifi cantly earlier than previously inferred (Early Permian). This period of high-latitude aridity was contemporaneous with a shift to dryland depositional environments and a major vegetation regime shift documented throughout the Pangean paleotropics in the Pennsylvanian.

Mesozoic thermal evolution of the southern African mantle lithosphere

The thermal structure of Archean and Proterozoic lithospheric terranes in southern Africa during the Mesozoic was evaluated by thermobarometry of mantle peridotite xenoliths erupted in alkaline magmas between 180 and 60 Ma. For cratonic xenoliths, the presence of a 150–200 °C isobaric temperature range at 5–6 GPa confirms original interpretations of a conductive geotherm, which is perturbed at depth, and therefore does not record steady state lithospheric mantle structure. Xenoliths from both Archean and Proterozoic terranes record conductive limb temperatures characteristic of a “cratonic” geotherm (∼40 mW m−2), indicating cooling of Proterozoic mantle following the last major tectonothermal event in the region at ∼1 Ga and the probability of thick off-craton lithosphere capable of hosting diamond. This inference is supported by U–Pb thermochronology of lower crustal xenoliths [Schmitz and Bowring, 2003. Contrib. Mineral. Petrol. 144, 592–618]. The entire region then suffered a protracted regional heating event in the Mesozoic, affecting both mantle and lower crust. In the mantle, the event is recorded at ∼150 Ma to the southeast of the craton, propagating to the west by 108–74 Ma, the craton interior by 85–90 Ma and the far southwest and northwest by 65–70 Ma. The heating penetrated to shallower levels in the off-craton areas than on the craton, and is more apparent on the southern margin of the craton than in its western interior. The focus and spatial progression mimic inferred patterns of plume activity and supercontinent breakup 30–100 Ma earlier and are probably connected. Contrasting thermal profiles from Archean and Proterozoic mantle result from penetration to shallower levels of the Proterozoic lithosphere by heat transporting magmas. Extent of penetration is related not to original lithospheric thickness, but to its more fertile character and the presence of structurally weak zones of old tectonism. The present day distribution of surface heat flow in southern Africa is related to this dynamic event and is not a direct reflection of the pre-existing lithospheric architecture.

Quantitative radiometric and biostratigraphic calibration of the Pennsylvanian–Early Permian (Cisuralian) time scale and pan-Euramerican chronostratigraphic correlation

A quantitative biostratigraphic and radiometric calibration is presented for the Pennsylvanian through Early Permian global time scale, based upon high-precision, isotope dilution–thermal ionization mass spectrometer (ID-TIMS) U-Pb zircon ages for interstratified ash beds in the parastratotype sections of the southern Urals of Russia. Twenty-four ash-bed ages in three outer ramp and basinal sections of the Pre-Uralian foredeep bracket the biotic definitions of global stages and regional substages from the base of the Upper Pennsylvanian Kasimovian Stage to the base of the Lower Permian Artinskian Stage; four additional ash-bed ages in two sections of the eastern slope of the Urals constrain the global Bashkirian and Serpukhovian Stages. Quantitative stratigraphic methods (CONOP9) are applied to a compilation of over 2000 bioevents in 22 stratigraphic sections supplemented by our dated volcanic horizons to refine the Pennsylvanian–Early Permian global time scale. Significant shifts in the duration of several stages are demonstrated, ranging from one to six million years, compared with prior estimates. The unprecedented density of radiometric calibration points for the Pennsylvanian–Permian transition provides a high-resolution (∼0.1-Ma) global chronostratigraphic standard for testing and improving biostratigraphic correlations across Euramerica. We integrate radiometric ages, biostratigraphic correlation, and cyclostratigraphic tuning of major cyclothems to the long-period (404-ka) eccentricity cycle to elucidate the tempo, magnitude, and forcing of eustatic changes and cyclothemic deposition associated with the waxing and waning of Gondwanan ice sheets, and establish a pan-Euramerican chronostratigraphic framework for most of Pennsylvanian and Early Permian time.

Evaluation of Duluth Complex anorthositic series (AS3) zircon as a U-Pb geochronological standard: New high-precision isotope dilution thermal ionization mass spectrometry results

U-Pb zircon geochronology is increasingly called upon to achieve the resolution of absolute time at the 0.1% to 1% level for rocks of Phanerozoic to Hadean age. Doing so requires accurate calibration of the several methods (conventional isotope dilution thermal ionization mass spectrometry [ID-TIMS], Pb evaporation, high-resolution ion microprobe [e.g. SHRIMP], and laser ablation inductively coupled plasma mass spectrometry [LA-ICPMS]) currently in use, in numerous laboratories, for the analysis of U and Pb isotopes in accessory minerals. Toward this end, the geochronological community would benefit from the establishment, distribution and widespread analysis of one or more standard reference materials. Among the candidates is natural zircon from the Duluth Complex anorthositic series of the Midcontinent Rift system of North America. These zircons, first dated by conventional ID-TIMS at 1099.1 ± 0.5 Ma, have been subsequently adopted as a geochronological standard by a number of high resolution ion microprobe facilities. A new and independent analysis of the systematics of a large set of single zircons (n = 27) from the same mineral separate yields indistinguishable 207Pb/206Pb, upper intercept, and U-Pb concordia dates for the AS3 zircons. The concordia date, based on a subset of 12 concordant and equivalent zircons, of 1099.1 ± 0.2 Ma (±1.2 Ma considering systematic uncertainties in Pb/U tracer calibration and U decay constants) is indistinguishable from previously published results. We further document the absence of inherited Pb in the AS3 zircons, and discuss strategies for avoiding certain domains within the AS3 zircons exhibiting small amounts of radiation-induced, surface and fracture-correlated, recent Pb loss. Although the AS3 zircons do not represent the ideal (and elusive) homogeneous closed U-Pb system, we conclude that these and similar zircons from the Duluth Complex anorthositic series can provide a suitable geochronological reference standard for numerous U-Pb zircon analytical methods, given appropriate preparation guided by the results of this study. Our high-precision data set also serves as a useful confirmatory test of the currently accepted U decay constants.

Palaeontological evidence for an Oligocene divergence between Old World monkeys and apes

Apes and Old World monkeys are prominent components of modern African and Asian ecosystems, yet the earliest phases of their evolutionary history have remained largely undocumented. The absence of crown catarrhine fossils older than ∼20 million years (Myr) has stood in stark contrast to molecular divergence estimates of ∼25–30 Myr for the split between Cercopithecoidea (Old World monkeys) and Hominoidea (apes), implying long ghost lineages for both clades. Here we describe the oldest known fossil ‘ape’, represented by a partial mandible preserving dental features that place it with ‘nyanzapithecine’ stem hominoids. Additionally, we report the oldest stem member of the Old World monkey clade, represented by a lower third molar. Both specimens were recovered from a precisely dated 25.2-Myr-old stratum in the Rukwa Rift, a segment of the western branch of the East African Rift in Tanzania. These finds extend the fossil record of apes and Old World monkeys well into the Oligocene epoch of Africa, suggesting a possible link between diversification of crown catarrhines and changes in the African landscape brought about by previously unrecognized tectonic activity in the East African rift system.

Ages and origins of rocks of the Killingworth Dome, south-central Connecticut: implications for the tectonic evolution of southern New England

The Killingworth dome of south-central Connecticut occurs at the southern end of the Bronson Hill belt. It is composed of tonalitic and trondhjemitic orthogneisses (Killingworth complex) and bimodal metavolcanic rocks (Middletown complex) that display calc-alkaline affinities. Orthogneisses of the Killingworth complex (Boulder Lake gneiss, 456 ± 6 Ma; Pond Meadow gneiss, ∼460 Ma) were emplaced at about the same time as eruption and deposition of volcanic-sedimentary rocks of the Middletown complex (Middletown Formation, 449 ± 4 Ma; Higganum gneiss, 459 ± 4 Ma). Hidden Lake gneiss (339 ± 3 Ma) occurs as a pluton in the core of the Killingworth dome, and, on the basis of geochemical and isotopic data, is included in the Killingworth complex. Pb and Nd isotopic data suggest that the Pond Meadow, Boulder Lake, and Hidden Lake gneisses (Killingworth complex) resulted from mixing of Neoproterozoic Gander terrane sources (high 207Pb/204Pb and intermediate εNd) and less radiogenic (low 207Pb/204Pb and low εNd) components, whereas Middletown Formation and Higganum gneiss (Middletown complex) were derived from mixtures of Gander basement and primitive (low 207Pb/204Pb and high εNd) sources. The less radiogenic component for the Killingworth complex is similar in isotopic composition to material from Laurentian (Grenville) crust. However, because published paleomagnetic and paleontologic data indicate that the Gander terrane is peri-Gondwanan in origin, the isotopic signature of Killingworth complex rocks probably was derived from Gander basement that contained detritus from non-Laurentian sources such as Amazonia, Baltica, or Oaxaquia. We suggest that the Killingworth complex formed above an east-dipping subduction zone on the west margin of the Gander terrane, whereas the Middletown complex formed to the east in a back-arc rift environment. Subsequent shortening, associated with the assembly of Pangea in the Carboniferous, resulted in Gander cover terranes over the Avalon terrane in the west; and in the Middletown complex over the Killingworth complex in the east. Despite similarities of emplacement age, structural setting, and geographic continuity of the Killingworth dome with Oliverian domes in central and northern New England, new and published isotopic data suggest that the Killingworth and Middletown complexes were derived from Gander crust, and are not part of the Bronson Hill arc that was derived from Laurentian crust. The trace of the Ordovician Iapetan suture (the Red Indian line) between rocks of Laurentian and Ganderian origin probably extends from Southwestern New Hampshire west of the Pelham dome of northcentral Massachusetts and is coverd by Mesozoic rocks of the Hartford basin.

Seismic anisotropy, mantle fabric, and the magmatic evolution of Precambrian southern Africa

The observed seismic anisotropy of the southern African mantle from both shear-wave splitting and surface wave observations provides important constraints on modes of mantle deformation beneath this ancient continent. We find that the mantle anisotropy beneath southern Africa is dominated by deformational events in Archean times occurring within the lithosphere, rather than present-day processes in the sublithospheric mantle. Consequently, the distribution and magnitude of anisotropy provide valuable data to constrain the mantle’s role in the tectonic evolution of this region. The pattern of mantle anisotropy reveals several noteworthy characteristics. First, mantle anisotropy is closely associated with the Great Dyke of the Zimbabwe Craton, with values of the splitting fast polarization direction, ϕ, parallel to the Dyke. This correspondence with the Great Dyke is likely not due to the present-day Dyke structure but instead is most probably due to the emplacement of the Dyke parallel to pre-existing mantle fabric within the Zimbabwe craton. This deformation thus predates dike emplacement and is no younger than Neo-Archean in age. Second, there is a spatially continuous arc of mantle anisotropy extending from the western Kaapvaal Craton to the northeastern Kaapvaal and Limpopo Belt. All along the arc, ϕ is subparallel to the trend of the arc. Given the crust/mantle chronology associated with these regions, the anisotropy likely represents deformation that occurred at ~2.9 to ~2.6 Ga during collisional accretion of both the western Kimberley and northern Pietersburg blocks onto the seismically isotropic eastern shield of the Kaapvaal, with accretion on the northern ramparts of the Kaapvaal ultimately culminating in the Neo-Archean Limpopo orogen. The anisotropy-inferred arc of deformation reveals diverse zones of both strong and weak coupling between the crust and mantle, as measured by the coherence between mantle deformation and geologically-inferred surface deformation. In particular, there is high coherence between surface and mantle deformation at the southwestern and northeastern ends of the arc, which implies strong crust-mantle coupling in these regions. Conversely, apparent decoupling exists in the northwestern portion of the arc, where northeast to southwest trending anisotropy cuts across north to south trending structures, such as the surface outcrop and aeromagnetic expressions of the Kraaipan Greenstone Belts. Independent seismic evidence from seismic reflection profiling supports the conclusions that these north-south-trending crustal features are superficial and confined to the upper crust. We present evidence that the mantle fabric producing seismic anisotropy constitutes fossil structure in the mantle that is subsequently reactivated, much like the more commonly acknowledged reactivation of crustal structures. In particular, we argue that Neo-Archean collisional orogenesis imparted a mechanical anisotropy to the mantle that controlled the subsequent magmatic history of cratonic southern Africa. We furthermore suggest that four major Precambrian magmatic events: the Great Dyke, the Ventersdorp, Bushveld, and the Soutpansberg, all represent extensional failure along planes oriented parallel to the local splitting fast polarization direction. Each of these events is interpreted to be a collisional rift, similar to the Baikal rift of northern Eurasia, where the stress field associated with collision produces extension and rifting for orientations at a small angle to the direction of the collision. Precise crustal geochronology associates both Ventersdorp and Great Dyke magmatism with the earliest and latest phases of the Limpopo collision, respectively. Similarly, the Bushveld magmatic event is temporally linked to the ~ 2.0 Ga reactivation of Neo-Archean structures in the Limpopo and surrounding areas by the Magondi Orogen, and the Soutpansberg is related to the ~1.9 Ga Kheis Orogen. Since the timing of these basaltic intrusions is controlled by temporal variations in lithospheric stress associated with orogenesis, it implies either that the melting process is genetically related to the evolution of the far-field collision, or that there was a semi-permanent reservoir of basaltic magma residing in the sublithospheric mantle during the ~1 billion-year time period spanned by these magmatic events. The existence of an extensive magma reservoir would argue for elevated temperatures just beneath the lithosphere during this time. Splitting delay times, δ t , a measure of the magnitude of anisotropy, reveal geologically controlled variations in the strength of anisotropy. In particular, the Meso-Archean Kaapvaal shield, the area that was not exposed to ~2.9 Ga and later deformational events, is effectively isotropic. We observe two areas where the anisotropic/isotropic transition is relatively sharp. The north-south boundary appears to coincide with the east-west trending Thabazimbi-Murchison Lineament. In the west, the boundary has been observed in the vicinity of Kimberley, South Africa, near the Colesberg Magnetic Lineament. The Eastern Shield has been relatively devoid of the kind of rifting and magmatic events seen elsewhere in cratonic southern Africa since the Meso-Archean, suggesting that the Eastern Shield lithosphere is mechanically stronger than surrounding areas. This relative strength difference may in part be due to the absence of the mechanical anisotropy inferred for the surrounding areas.