Showing papers by "Blair Schoene published in 2020"

Geologic evidence for an icehouse Earth before the Sturtian global glaciation

Abstract: Snowball Earth episodes, times when the planet was covered in ice, represent the most extreme climate events in Earth's history. Yet, the mechanisms that drive their initiation remain poorly constrained. Current climate models require a cool Earth to enter a Snowball state. However, existing geologic evidence suggests that Earth had a stable, warm, and ice-free climate before the Neoproterozoic Sturtian global glaciation [ca. 717 million years (Ma) ago]. Here, we present eruption ages for three felsic volcanic units interbedded with glaciolacustrine sedimentary rocks from southwest Virginia, USA, that demonstrate that glacially influenced sedimentation occurred at tropical latitudes ca. 751 Ma ago. Our findings are the first geologic evidence of a cool climate teetering on the edge of global glaciation several million years before the Sturtian Snowball Earth.

The lead-up to the Sturtian Snowball Earth: Neoproterozoic chemostratigraphy time-calibrated by the Tambien Group of Ethiopia

Abstract: Author(s): Park, Y; Swanson-Hysell, NL; MacLennan, SA; Maloof, AC; Gebreslassie, M; Tremblay, MM; Schoene, B; Alene, M; Anttila, ESC; Tesema, T; Haileab, B | Abstract: The Tonian-Cryogenian Tambien Group of northern Ethiopia is a mixed carbonate-siliciclastic sequence that culminates in glacial deposits associated with the first of the Cryogenian glaciations-the Sturtian "Snowball Earth. " Tambien Group deposition occurred atop arc volcanics and volcaniclastics of the Tsaliet Group. New U-Pb isotope dilution-thermal ionization mass spectrometry (ID-TIMS) dates demonstrate that the transition between the Tsaliet and Tambien Groups occurred at ca. 820 Ma in western exposures and ca. 795 Ma in eastern exposures, which is consistent with west to east arc migration and deposition in an evolving back-arc basin. The presence of intercalated tuffs suitable for high-precision geochronology within the Tambien Group enable temporal constraints on stratigraphic data sets of the interval preceding, and leading into, the Sturtian glaciation. Recently discovered exposures of Sturtian glacial deposits and underlying Tambien Group strata in the Samre Fold-Thrust Belt present the opportunity to further utilize this unique association of tuffs and carbonate lithofacies. U-Pb ID-TIMS ages from zircons indicate that Tambien Group carbonates were deposited from ca. 820 Ma until 0-2 m.y. before the onset of the Sturtian glaciation, making the group host to a relatively complete carbonate stratigraphy leading into this glaciation. New δ13C and 87Sr/86Sr data and U-Pb ID-TIMS ages from the Tambien Group are used in conjunction with previously published isotopic and geochronologic data to construct newly time-calibrated composite Tonian carbon and strontium isotope curves. Tambien Group δ13C data and U-Pb ID-TIMS ages reveal that a pre-Sturtian sharp negative δ13C excursion (referred to as the Islay anomaly in the literature) precedes the Sturtian glaciation by-18 m.y., is synchronous in at least two separate basins, and is followed by a prolonged interval of positive δ13C values. The composite Tonian 87Sr/86Sr curve shows that, following an extended interval of low and relatively invariant values, inferred seawater 87Sr/86Sr rose ca. 880-770 Ma, then subsequently decreased leading up to the ca. 717 Ma initiation of the Sturtian glaciation. These data, when combined with a simple global weathering model and analyses of the timing and paleolatitude of large igneous province eruptions and arc accretion events, suggest that the 87Sr/86Sr increase was influenced by increased subaerial weathering of radiogenic lithologies as Rodinia rifted apart at low latitudes. The following 87Sr/86Sr decrease is consistent with enhanced subaerial weathering of arc lithologies accreting in the tropics over tens of millions of years, lowering pCO2 and contributing to the initiation of the Sturtian glaciation.

Half a million years of magmatic history recorded in a K-feldspar megacryst of the Tuolumne Intrusive Complex, California, USA

Abstract: K-feldspars reach megacrystic size (>3 cm) relative to their groundmass in many granitoid plutons and some volcanic rocks. However, the nature of the growth environment and the time scales for megacrystic growth remain poorly constrained. Chemical abrasion–isotope dilution–thermal ionization mass spectrometry with trace-element analysis (CA-ID-TIMS-TEA) U-Pb geochronology was carried out on zircon inclusions from the core and rim of one K-feldspar megacryst sampled from the interior of the Tuolumne Intrusive Complex (TIC), California, USA. Combined with new zircon ages from the groundmass, these data can test if K-feldspar megacrysts are igneous and capable of recycling and transport in the magmatic system or whether they formed by textural coarsening in low-melt-fraction or subsolidus conditions. The zircon ages reveal that the megacryst core is 0.5 m.y. older than the rim, which itself is older than the groundmass. Core ages match zircon dates from the TIC’s porphyritic Half Dome unit, and rim and groundmass ages overlap with the younger Cathedral Peak unit. Trace elements of the zircons from the megacryst core and rim are similar and less evolved than the groundmass zircons. The core-to-rim age progression of zircon inclusions is inconsistent with subsolidus K-feldspar coarsening, but instead indicates that megacrysts in the TIC grew in an igneous environment over at least 0.5 m.y., and that growth likely occurred spanning two or more intrusive episodes. This supports models of an increasingly maturing magmatic system, where crystal recycling from older into younger magma batches is common.

U–Pb ID-TIMS geochronology using ATONA amplifiers

Abstract: We document the performance of new ATONA (‘aA to nA’) amplifiers installed on an Isotopx Phoenix thermal ionisation mass spectrometer (TIMS) at Princeton University and evaluate their suitability for high-precision analyses of Pb and U isotopes in pg- to ng-size samples characteristic for U–Pb geochronology. The new amplifiers are characterised by low and stable noise levels comparable to 1012 to 1013 ohm resistors, response time 10−14 A (1 mV relative to a 1011 ohm amplifier or ca. 60 kcps). As such currents are routinely achievable for major Pb peaks of interest (205–208Pb) in natural samples containing more than ca. 10 pg Pb* (radiogenic Pb), we expect ATONA-Faraday detection to find broad applications in U–Pb geochronology. Its practical use for low-blank, radiogenic samples continues to require ion counting for 204Pb, either with a fixed Faraday–ion counter gain or using a dynamic two-step (e.g. FaraDaly) method. Routine adoption of ATONA-Faraday collection in place of ion counting for most major Pb and U isotopes has the potential to increase sample throughput and precision, both improving the accessibility of isotope dilution (ID)-TIMS geochronology and pushing this technique towards better reproducibility.

Polyphase Zircon Growth during Slow Cooling from Ultrahigh Temperature: an Example from the Archean Pikwitonei Granulite Domain

Abstract: Quantifying the timescales of Archean ultrahigh temperature (UHT) metamorphism is essential for constraining the style of plate tectonics on the early Earth. However, such timescales can be difficult to quantify, due to the antiquity of Archean rocks and the extreme thermal conditions of UHT metamorphism. We constrain the timescales of Archean UHT metamorphic processes recorded by a single rock sample from the Pikwitonei granulite domain (northwestern Superior Province), through the integration of two U–Pb zircon petrochronologic techniques. In this study we combine: (1) high-spatial resolution laser ablation split-stream inductively coupled mass spectrometry (LASS) on in situ zircon (in thin section) and hand-picked zircon; and (2) high-precision isotope dilution thermal ionization mass spectrometry (ID-TIMS) analyses on microsampled fragments from the same hand-picked zircon analysed by LASS. Phase equilibria modelling and Zr-in-rutile thermometry suggest the rock followed a P–T path characterized by decompression at > 960 °C, followed by near-isobaric cooling at ∼0·8 GPa. In situ LASS zircon analyses could be interpreted to record zircon growth at broadly ∼2665 Ma, though the large uncertainties on isotopic dates make potentially distinct growth episodes difficult to distinguish. ID-TIMS U–Pb dates of zircon fragments reveal a polyphase zircon growth history over a 24 Ma duration, from 2673 to 2649 Ma. Zircon trace element compositions, textures, and microstructural relationships, as well as evaluation of zircon-garnet equilibrium, suggest zircon grew during melt crystallization, after UHT decompression and garnet resorption. Variable Ti concentrations within zircon domains indicate: (1) zircon crystallized through the temperature interval of ∼875 °C to ∼730 °C, potentially in isolated rock domains with variable zircon saturation temperature; and/or (2) zircon crystallized over a narrower temperature interval in isolated rock domains with variable aTiO2 and/or aSiO2. Collectively, the data suggest the west-central Pikwitonei granulite domain reached peak UHT conditions prior to 2673 Ma, after which suprasolidus conditions in the lower crust persisted for at least 24 Ma. Such an interpretation would be impossible if based on either the LASS or ID-TIMS zircon data alone, which highlights the utility of applying both techniques in tandem to constrain metamorphic timescales in ancient UHT terranes.

Butcher Ridge igneous complex: A glassy layered silicic magma distribution center in the Ferrar large igneous province, Antarctica

Abstract: The Butcher Ridge igneous complex, Antarctica, is an ∼6000 km3 hypabyssal silicic intrusion containing rhythmically layered glassy rocks. Baddeleyite U-Pb geochronologic analysis on a sample of the Butcher Ridge igneous complex yielded an age of ca. 182.4 Ma, which confirms that it was emplaced synchronously with the Ferrar large igneous province. Rocks of the Butcher Ridge igneous complex vary from basaltic andesite to rhyolite, and so the inferred volume of the Butcher Ridge igneous complex makes it the most voluminous silicic component of the Ferrar large igneous province. Major-element, trace-element, and isotopic data combined with binary mixing, assimilation-fractional crystallization (AFC), and energy-constrained AFC models are consistent with formation of Butcher Ridge igneous complex silicic rocks by contamination of mafic Ferrar parental magma(s) with local Paleozoic plutonic basement rocks. Field and petrographic observations and evidence for alkali ion exchange suggest that the kilometer-long, meter-thick enigmatic rhythmic layering formed as a result of secondary hydration and devitrification of volcanic glass along parallel fracture networks. The regularity and scale of fracturing/layering imply a thermally driven process that occurred during shallow emplacement and supercooling of the intrusion in the upper crust. We suggest that layering observed in the Butcher Ridge igneous complex is analogous to that reported from terrestrial and Martian cryptodomes, and therefore it is an ideal locality at which to study layering processes in igneous bodies.

Astronomically forced hydrology of the Late Cretaceous sub-tropical Potosí Basin, Bolivia

Abstract: Periodic variations in Earth’s orbital parameters force climate on local and global scales, with global responses particularly sensitive to the presence of ice sheets and their associated feedbacks. Therefore, determining whether orbital forcings influenced sedimentary records of the past, and if so, which had such an effect, can shed light on Earth’s climate sensitivity and global ice volume. To this end, we present a fieldand drone-based cyclostratigraphy of the predominantly lacustrine El Molino Formation of the Late Cretaceous–Early Paleogene Potosí Basin in present day Bolivia, which contains carbonate mud parasequences that record fluctuating hydrological conditions, including ephemeral marine connections, from 73 Ma to 64 Ma. We introduce a novel methodology for incorporating drone imagery into a quantitative, three-dimensional stratigraphic model that generates an upward-younging quantity comparable to stratigraphic height, and we find that our model outperforms our own field measurements of stratigraphic height. We project drone imagery at two sites into the stratigraphic model to construct time series of outcrop color, which vary systematically with facies and track basin water depth. Spectral analysis of these time series reveals sedimentary periodicities corresponding to short eccentricity, precession, and semi-precession, which are corroborated with measurements of magnetic susceptibility from mudstones. We generate independent age models at both study areas from four new U-Pb chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA– ID–TIMS) ages, which are consistent with an orbital interpretation for observed sedimentary periodicities. Importantly, we observe the presence of obliquity-scale periodicity in sedimentation during a period of marine connection, suggesting that sea level oscillations were driven by obliquity. This observation is consistent with previous claims about the presence of a small, orbitally forced Antarctic ice sheet during the latest Cretaceous.