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During the Cryogenian (720 to 635 Ma ago) Snowball Earth glaciations, ice extended to sea level near the equator. The cause of this catastrophic failure of Earth's thermostat has been unclear, but previous geochronology has suggested a rough coincidence of glacial onset with one of the largest magmatic episodes in the geological record, the Franklin large igneous province. U-Pb geochronology on zircon and baddeleyite from sills associated with the paleo-equatorial Franklin large igneous province in Arctic Canada record rapid emplacement between 719.86 ± 0.21 and 718.61 ± 0.30 Ma ago, 0.9 to 1.6 Ma before the onset of widespread glaciation. Geologic observations and (U-Th)/He dates on Franklin sills are compatible with major post-Franklin exhumation, possibly due to development of mafic volcanic highlands on windward equatorial Laurentia and increased global weatherability. After a transient magmatic CO2 flux, long-term carbon sequestration associated with increased weatherability could have nudged Earth over the threshold for runaway ice-albedo feedback.

Despite being a prominent continental-scale feature, the late Mesoproterozoic North American Midcontinent Rift did not result in the break-up of Laurentia, and subsequently underwent structural inversion. The timing of inversion is critical for constraining far-field effects of orogenesis and processes associated with the rift’s failure. The Keweenaw fault in northern Michigan (USA) is a major thrust structure associated with rift inversion; it places ca. 1093 Ma rift volcanic rocks atop the post-rift Jacobsville Formation, which is folded in its footwall. Previous detrital zircon (DZ) U-Pb geochronology conducted by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) assigned a ca. 950 Ma maximum age to the Jacobsville Formation and led researchers to interpret its deposition and deformation as postdating the ca. 1090–980 Ma Grenvillian Orogeny. In this study, we reproduced similar DZ dates using LA-ICP-MS and then dated 19 of the youngest DZ grains using high-precision chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS). The youngest DZ dated by CA-ID-TIMS at 992.51 ± 0.64 Ma (2σ) redefines the maximum depositional age of the Jacobsville Formation and overlaps with a U-Pb LA-ICP-MS date of 985.5 ± 35.8 Ma (2σ) for late-kinematic calcite veins within the brecciated Keweenaw fault zone. Collectively, these data are interpreted to constrain deposition of the Jacobsville Formation and final rift inversion to have occurred during the 1010–980 Ma Rigolet Phase of the Grenvillian Orogeny, following an earlier phase of Ottawan inversion. Far-field deformation propagated >500 km into the continental interior during the Ottawan and Rigolet phases of the Grenvillian Orogeny.

New geochronologic and paleomagnetic data from the North American Midcontinent Rift (MCR) reveal the synchronous emplacement of the Beaver River diabase, the anorthosite xenoliths within it, and the Greenstone Flow—one of the largest lava flows on Earth. A U-Pb zircon date of 1091.83 (Formula presented.) 0.21 Ma (2 (Formula presented.)) from one of the anorthosite xenoliths is consistent with the anorthosite cumulate forming as part of the MCR and provides a maximum age constraint for the Beaver River diabase. Paired with the minimum age constraint of a cross-cutting Silver Bay intrusion (1091.61 (Formula presented.) 0.14 Ma; 2 (Formula presented.)), these data tightly bracket the age of the Beaver River diabase to be 1091.7 (Formula presented.) 0.2 Ma (95% CI), coeval with the eruption of the Greenstone Flow (1091.59 (Formula presented.) 0.27 Ma; 2 (Formula presented.))—which is further supported by indistinguishable tilt-corrected paleomagnetic pole positions. Geochronological, paleomagnetic, mineralogical and geochemical data are consistent with a hypothesis that the Beaver River diabase was the feeder system for the Greenstone Flow. The large areal extent of the intrusives and large estimated volume of the volcanics suggest that they represent a rapid and voluminous ca. 1,092 Ma magmatic pulse near the end of the main stage of MCR magmatism.

A goal of undergraduate geoscience programs is to prepare students for the geoscience workforce. In order to understand the extent to which programs succeed at this goal, we identified desired workforce skills, compiled undergraduate geoscience program requirements, and mapped workforce skills onto faculty responses to the 2016 National Geoscience Faculty Survey categorized by respondent-provided course names for majors-level courses (n = 1037). We used a Bayesian statistical model to determine the posterior probability of students practicing desired workforce skills in undergraduate geoscience programs, using program profiles as the model prior, conditioned by the likelihood of a student practicing workforce skills in each course category based upon survey responses. Probabilistic Monte Carlo simulations of students in each example program were used to calculate the likely number of courses in which a student practiced each skill. Activities to help students develop skills related to geologic reasoning, working as part of a team, quantitative skills (algebra), applying skills in new scenarios, evaluation of scientific literature, temporal thinking, spatial thinking, written communication, and managing uncertainty are frequently encountered by students; on average, students encounter these activities at least seven times during an undergraduate program. In contrast, students encounter activities to help them develop an understanding of societal relevance and systems thinking, on average, fewer than three times during an undergraduate program. Our results provide a snapshot, via instructors' reporting, of the state of workforce skill development across undergraduate degree programs, and both the approach and data set can be built upon for future studies.

The Mascall fauna is a well-known middle Miocene (Barstovian) mammalian assemblage in the Pacific Northwest. It has been collected for over 100 years and collecting intensity has increased since the establishment in 1975 of a national monument enclosing the type area of the formation. Despite its importance to biostratigraphy, biogeography of Barstovian taxa, and paleoecological studies, the fauna at the type locality has not been taxonomically examined in more than 50 years. Evaluation and classification of the stratigraphy of the Mascall Formation (Bestland et al. 2008) has prompted a faunal revision in order to place taxa within the new stratigraphic framework. Here we report on the fauna from the type area of the Mascall Formation in central Oregon, and conclude that 20 taxa are new to the fauna, and several taxa previously assigned to distinct species are synonymized. We also place specimens and taxa within a robust stratigraphic framework, calibrated with new U-Pb radioisotopic ages for the Mascall Tuff (15.122±0.017 Ma), the most fossiliferous layer in the formation, and the Kangaroo Tuff (13.564±0.018 Ma), the upper most tuff in the formation.

The Pilbara craton of northwestern Australia is known for what were, when reported, the oldest known microfossils and paleosols on Earth. Both interpretations are mired in controversy, and neither remain the oldest known. Both the microfossils and the paleosols have been considered hydrothermal artefacts: carbon films of vents and a large hydrothermal cupola, respectively. This study resampled and analyzed putative paleosols within and below the Strelley Pool Formation (3.3 Ga), at four classic locations: Strelley Pool, Steer Ridge, Trendall Ridge, and Streckfuss, and also at newly discovered outcrops near Marble Bar. The same sequence of sedimentary facies and paleosols was newly recognized unconformably above the locality for microfossils in chert of the Apex Basalt (3.5 Ga) near Marble Bar. The fossiliferous Apex chert was not a hydrothermal vein but a thick (15 m) sedimentary interbed within a sequence of pillow basalts, which form an angular unconformity capped by the same pre-Strelley paleosol and Strelley Pool Formation facies found elsewhere in the Pilbara region. Baritic alluvial paleosols within the Strelley Pool Formation include common microfossil spindles (cf. Eopoikilofusa) distinct from marine microfossil communities with septate filaments (Primaevifilum) of cherts in the Apex and Mt Ada Basalts. Phosphorus and iron depletion in paleosols within and below the Strelley Pool Formation are evidence of soil communities of stable landscapes living under an atmosphere of high CO2 (2473 ± 134 ppmv or 8.8 ± 0.5 times preindustrial atmospheric level of 280 ppm) and low O2 (2181 ± 3018 ppmv or 0.01 ± 0.014 times modern). [ABSTRACT FROM AUTHOR]

High-precision U-Pb zircon ages on SE Newfoundland tuffs now bracket the Avalonian Lower–Middle Cambrian boundary. Upper Lower Cambrian Brigus Formation tuffs yield depositional ages of 507.91 ± 0.07 Ma (Callavia broeggeri Zone) and 507.67 ± 0.08 Ma and 507.21 ± 0.13 Ma (Morocconus-Condylopyge eli Assemblage interval). Lower Middle Cambrian Chamberlain's Brook Formation tuffs have depositional ages of 506.34 ± 0.21 Ma (Kiskinella cristata Zone) and 506.25 ± 0.07 Ma (Eccaparadoxides bennetti Zone). The composite unconformity separating the Brigus and Chamberlain's Brook formations is constrained between these ages. An Avalonian Lower–Middle Cambrian boundary between 507.2 ± 0.1 and 506.3 ± 0.2 Ma is consistent with maximum depositional age constraints from southwest Laurentia, which indicate an age for the base of the Miaolingian Series, as locally interpreted, of ≤ 506.6 ± 0.3 Ma. The Miaolingian Series' base is interpreted as correlative within ≤ 0.3 ± 0.3 Ma between Cambrian palaeocontinents, although its exact synchrony is questionable due to taxonomic problems with a possible Oryctocephalus indicus -plexus, invariable dysoxic lithofacies control of O. indicus and diachronous occurrence of O. indicus in temporally distinct δ 13C chemozones in South China and SW Laurentia. The lowest occurrence of O. indicus assemblages is linked to onlap (epeirogenic or eustatic) of dysoxic facies. A united Avalonia is shown by late Early Cambrian volcanics in SW New Brunswick; Cape Breton Island; SE Newfoundland; and the Wrekin area, England. The new U-Pb ages revise Avalonian geological evolution as they show rapid epeirogenic changes through depositional sequences 4a–6. [ABSTRACT FROM AUTHOR]

Sedimentary deposits north of the western Snake River Plain host Idaho's first and only producing oil and gas field. They consist of the lower to middle Miocene Payette Formation, the middle to upper Miocene Poison Creek and Chalk Hills Formations, and the Pliocene to lower Pleistocene Glenns Ferry Formation. Using new geochronology, palynomorph biostratigraphy, and geologic mapping, we connect updip surface features to subsurface petroleum play elements. The Payette Formation is a likely main source of the hydrocarbons, and acts as one of the reservoirs in the unnamed basin. Here, we redefine the Payette Formation as 0 to ~3,500 ft (0 to ~1,000 m) of mudstone, with lesser amounts of sandstone overlying and interbedded with the Columbia River Basalt Group and Weiser volcanic field. Index palynomorphs, including Liquidambar and Pterocarya, present in Idaho during and immediately after the middle Miocene climatic optimum, and new U--Pb ages of 16.39 and 15.88 Ma, help establish the thickness and extent of the formation. For the first time, these biostratigraphic markers have been defined for the oil and gas wells. The Poison Creek Formation is sandstone interbedded with mudstone that is ~800-1,800 ft (250-550 m) thick. The Chalk Hills Formation is a tuffaceous siltstone, claystone, and sandstone that is as much as ~4,200 ft (1,280 m) thick. New U--Pb ages are 10.1, 9.04, and 9.00 for the Poison Creek Formation, along with maximum depositional ages of 10.7 to 9.9 Ma for four samples from the Poison Creek Formation. A single U--Pb age of 7.78 Ma was determined from pumice low in the Chalk Hills Formation. Like the Payette Formation, the Poison Creek Formation can be a reservoir, whereas the Chalk Hills Formation acts as a sealing mudstone facies. The overlying sandstone, siltstone, and conglomerate of the Glenns Ferry Formation act as the overburden to the petroleum system in the subsurface, and were important for burial and hydrocarbon maturation. The Glenns Ferry Formation is up to 500 ft (150 m) thick in the study area, as much has been eroded. Whereas the Payette and Poison Creek Formations were deposited during the mid-Miocene climatic optimum amongst and above volcanic flows, the Chalk Hills and Glenns Ferry Formations were deposited within ancient Lake Idaho during an overall increase in aridity and cooling after the mid-Miocene climatic optimum. [ABSTRACT FROM AUTHOR]

Relating stratigraphic position to numerical time using age-depth models plays an important role in determining the rate and timing of geologic and environmental change throughout Earth history. Astrochronology uses the geologic record of astronomically derived oscillations in the rock record to measure the passage of time and has proven a valuable technique for developing age-depth models with high spatial-temporal resolution. However, in the absence of anchoring dates, many astrochronologies float in numerical time. Anchoring these chronologies relies on radioisotope geochronology (e.g., U-Pb, 40Ar/39Ar), which produce high-precision, randomly distributed point estimates of age. In this study, we present a new R package, astroBayes for a Bayesian inversion of astrochronology and radioisotopic geochronology into age-depth models. Integrating both data types allows reduction in uncertainties related to interpolation between dated horizons, and the resolution of subtle changes in sedimentation rate, especially when compared to existing Bayesian models that use a stochastic random walk to approximate sedimentation variability. The astroBayes inversion also incorporates prior information about sedimentation rate, superposition, and the presence/ absence of major hiatuses. The resulting age-depth models preserve both the spatial resolution of floating astrochronologies, and the accuracy and precision of modern radioisotopic geochronology. We test the astroBayes method using two synthetic data sets designed to mimic real-world stratigraphic sections. Model uncertainties are relatively constant with depth, and predominantly controlled by the precision of the radioisotopic dates and are significantly reduced relative to "dates-only" random walk models. Since the resulting age-depth models leverage both astrochronology and radioisotopic geochronology in a single statistical framework they can resolve ambiguities between the two chronometers. Finally, we present a case study of the Bridge Creek Limestone Member of the Greenhorn Formation where we refine the age of the Cenomanian-Turonian Boundary. [ABSTRACT FROM AUTHOR]

The geologically rapid appearance of fossils of modern animal phyla within Cambrian strata is a defining characteristic of the history of life on Earth. However, temporal calibration of the base of the Cambrian Period remains uncertain within millions of years, which has resulted in mounting challenges to the concept of a discrete Cambrian explosion. We present precise zircon U–Pb dates for the lower Wood Canyon Formation, Nevada. These data demonstrate the base of the Cambrian Period, as defined by both ichnofossil biostratigraphy and carbon isotope chemostratigraphy, was younger than 533 Mya, at least 6 My later than currently recognized. This new geochronology condenses previous age models for the Nemakit–Daldynian (early Cambrian) and, integrated with global records, demonstrates an explosive tempo to the early radiation of modern animal phyla. [ABSTRACT FROM AUTHOR]

Abstract: Alkaline rocks and carbonatites, including nepheline syenites, are well established as mantle-derived magmatic products, but the nature and location of their mantle sources is debated. Some workers have used [...]

The numerous biotic, climatic, and tectonic events of the Devonian cannot be correlated and investigated without a wellcalibrated time scale. Here, we updated the calibration of the Devonian time scale using a Bayesian age-depth model that incorporates radioisotopic ages and astrochronology durations. We used existing radioisotopic ages collected and harmonized in the last two geologic time scale compilations, as well as new U-Pb zircon ages from Emsian {Hercules I K-bentonite, Wetteldorf, Germany: 394.290 ± 0.097(0.21)[0.47] Ma} and Eifelian K-bentonites {Tioga B and Tioga F K-bentonites, Fayette, New York, USA: 390.82 ± 0.18(0.26)[0.48] Ma and 390.14 ± 0.14(0.23)[0.47] Ma, respectively}. We anchored floating astrochronology stage durations on radioisotopic ages and chained astrochronologic constraints and uncertainty together to extrapolate conditioning age likelihoods up or down the geologic time scale, which is a new method for integrating astrochronology into age-depth modeling. The modeling results in similar ages and durations for Devonian stages regardless of starting biostratigraphic scaling assumptions. We produced a set of rescaled biostratigraphic zonations, and a new numerical calibration of Devonian stage boundary ages with robust uncertainty estimates, which allow us to evaluate future targets for Devonian time scale research. These methods are broadly applicable for time scale work and provide a template for an integrated stratigraphic approach to time scale modeling. [ABSTRACT FROM AUTHOR]

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 (1). The absence of [...]

Tandem in situ and isotope dilution U-Pb analysis of zircons from pyroclastic volcanic rocks and both glacial and non-glacial sedimentary strata of the Pocatello Formation (Idaho, northwestern USA) provides new age constraints on Cryogenian glaciation in the North American Cordillera. Two dacitic tuffs sampled within glacigenic strata of the lower diamictite interval of the Scout Mountain Member yield high-precision chemical abrasion isotope dilution U-Pb zircon eruption and depositional ages of 696.43 ± 0.21 and 695.17 ± 0.20 Ma. When supplemented by a new high-precision detrital zircon maximum depositional age of ≤670 Ma for shoreface and offshore sandstones unconformably overlying the lower diamictite, these data are consistent with correlation of the lower diamictite to the early Cryogenian (ca. 717-660 Ma) Sturtian glaciation. These 670-675 Ma zircons persist in beds above the upper diamictite and cap dolostone units, up to and including a purported "reworked fallout tuff," which we instead conclude provides only a maximum depositional age of ≤673 Ma from epiclastic volcanic detritus. Rare detrital zircons as young as 658 Ma provide a maximum depositional age for the upper diamictite and overlying cap dolostone units. This new geochronological framework supports litho- and chemostratigraphic correlations of the lower and upper diamictite intervals of the Scout Mountain Member of the Pocatello Formation with the Sturtian (716-660 Ma) and Marinoan (≤650-635 Ma) low- latitude glaciations, respectively. The Pocatello Formation thus contains a more complete record of Cryogenian glaciations than previously postulated. [ABSTRACT FROM AUTHOR]

The Lu-Hf and Sm-Nd isotope systems in kimberlite-borne, Archean to Proterozoic lower-crustal granulite xenoliths from southern Africa display significant parent-daughter fractionations and corresponding isotopic anomalies resulting in the decoupling of these generally well correlated systems. Systematic compositional and mineralogical controls on decoupling have also been identified. Opposite senses of divergent evolution from the terrestrial Hf-Nd isotope array are demonstrated in mafic granulite samples versus felsic and metasedimentary granulite samples. The former have superchondritic Lu/Hf and [sup.176]Hf/[sup.177]Hf ratios at subchondritic Sm/Nd and [sup.143]Nd/[sup.144]Nd ratios, whereas the latter have subchondritic Lu/Hf and [sup.176]Hf/[sup.177]Hf ratios at a wide range of subchondritic to superchondritic Sm/Nd and [sup.143]Nd/ [sup.144]Nd ratios. This decoupling is shown to be related to the mineralogical sensitivity of crustal protolith Lu/Hf ratios to fractionation upon anatexis in the presence not only of residual garnet and accessory zircon, but also the residual oxide minerals (rutile versus iron-titanium oxides). The isotope diversity observed in these deep-crustal melt residues supports suggestions that the Hf-Nd systematics of complementary crustal melts may be useful fingerprints for deciphering the mineral assemblage and geochemical character of granitoid magma sources, and that together these systematics can yield insights into the origin and differentiation of continental crust. Keywords: Lu-Hf, Sm-Nd, U-Pb, lower crust, xenoliths, granulite, zircon, garnet, rutile.

A diverse suite of ultrahigh-temperature (UHT) granulite xenoliths from kimberlites of the central Kaapvaal craton (South Africa) record the deep crustal manifestation of Neoarchean Ventersdorp extensional tectonics and magmatism. U-Pb geochronological data for texturally characterized metamorphic zircon and monazite from a number of metasedimentary (garnet + quartz [+ or -] sapphirine [+ or -] sillimanite [+ or -] antiperthite) UHT granulites are combined with published geothermobarometry to elucidate the pressure-temperature-time (P-T-t) path of metamorphism. Prograde metamorphic growth of zircon and monazite indicates ramping of lower crustal temperatures [greater than or equal to] 25 Ma prior to peak UHT metamorphic conditions at ca. 2720-2715 Ma. The subsequent cooling path is established by dating of 2690 Ma zircon in retrograde kyanite-rich leucosomes, indicating initial cooling rates of [greater than or equal to] 10[degrees]C/Ma. Preservation of peak and prograde monazite dates through the UHT conditions support a very high volume diffusion closure temperature for this mineral, similar to that of zircon (>1000[degrees]C). The 5-10 Ma duration of UHT metamorphism in the lower crust is directly correlative with the rapid eruption of the Ventersdorp flood basalts through the central Kaapvaal craton at 2714 [+ or -] 8 Ma, and associated crustal melting, plutonism, and widespread extensional tectonics. UHT metamorphism is postulated to have occurred in response to nonuniform intracratonic lithospheric thinning and superimposed magmatic heat advection. The links between upper and lower crustal manifestations of 2.7 Ga magmatism and extension support the hypothesis that the present-day flat seismic Moho of the central and western Kaapvaal craton is a lower crustal rheological response to Ventersdorp tectonomagmatism. The lithospheric thermal gradient necessitated by the UHT lower crustal granulites demands a transient removal of the thermal lithosphere localized along the north-northeast-trending axis of the Ventersdorp rift at 2.72 Ga. However, contemporaneous maintenance of a coherent mantle keel with 'cratonic' thermal gradients 200 km distal to the rift is required in order to preserve ancient (3.2-2.9 Ga) diamondiferous lithosphere. The contrast between the craton-wide geography of crustal extension and the more restricted localization of thermal effects in the lithospheric mantle embodies a profound crust-mantle decoupling in the Kaapvaal craton during intracratonic rifting, and bears upon our understanding of the generation and preservation of cratonic lithospheric mantle keels. Keywords: U-Pb geochronology, zircon, monazite, UHT metamorphism, sapphirine, granulite, lower crust, Ventersdorp Supergroup, Kaapvaal craton, rifting.

Garnet U‐Pb dating by laser ablation‐inductively coupled plasma‐mass spectrometry requires the development of matrix‐matched reference materials of variable chemistry and U mass fraction for accurate analysis. Additional calibration of existing primary reference materials is also justified based on the relatively poor calibration of some of the widely available primary reference materials that are currently utilised by the geoscience community. We present a micro sampling workflow combined with a refined ID‐TIMS methodology for the generation of high precision (~ 0.1%) U‐Pb dates from domains within garnet single crystals. Using this workflow, we calibrated two new natural andradite reference materials, the Jumbo andradite (And99; 110.34 ± 0.03 (0.04) [0.13] Ma, n = 7, MSWD = 1.21) and the Tiptop andradite (And87; 209.57 ± 0.11 (0.13) [0.26] Ma, n = 6, MSWD = 1.39). We also present additional calibration of the widely utilised Willsboro‐Lewis andradite primary reference material (And90; 1024.7 ± 9.5 (9.6) [9.6] Ma (2s; overdispersed), n = 6). Wafers of the Jumbo and Tiptop andradite reference materials are available from the authors upon request. [ABSTRACT FROM AUTHOR]