Blair Schoene

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.

Correlating the end-Triassic mass extinction and flood basalt volcanism at the 100 ka level

New high-precision U/Pb geochronology from volcanic ashes shows that the Triassic-Juras- sic boundary and end-Triassic biological crisis from two independent marine stratigraphic sections correlate with the onset of terrestrial fl ood volcanism in the Central Atlantic Mag- matic Province to <150 ka. This narrows the correlation between volcanism and mass extinc- tion by an order of magnitude for any such catastrophe in Earth history. We also show that a concomitant drop and rise in sea level and negative δ 13 C spike in the very latest Triassic occurred locally in <290 ka. Such rapid sea-level fl uctuations on a global scale require that global cooling and glaciation were closely associated with the end-Triassic extinction and potentially driven by Central Atlantic Magmatic Province volcanism.

U-Pb dating of zircon in the Bishop Tuff at the millennial scale

Zircon from the Bishop Tuff (eastern California) was dated by the U-Pb isotope dilution thermal ionization mass spectrometry (ID-TIMS) method to evaluate time scales of magmatic evolution and applicability of the method to rocks younger than 1 Ma. The 206 Pb/ 238 U dates from 17 of 19 grains are equivalent with a weighted mean of 767.1 ± 0.9 ka that overlaps with the published 40 Ar/ 39 Ar sanidine age, requiring that most zircon crystallized immedi- ately before eruption. Dates from two grains that are 6 and 12 k.y. older indicate minimum times that a small portion of the grains resided in the magma. These fi ndings are consistent with chemical zoning patterns in zircon that suggest a relatively simple crystallization history, yet contrast with previous results of ion microprobe dating of zircon that show 50-80 k.y. of magma residence. The weighted mean 207 Pb/ 235 U date is ~52 k.y. older than the 206 Pb/ 238 U date, a difference that is attributed to excess 207 Pb from initial Pa/U disequilibrium. The results dem- onstrate that high-precision ID-TIMS geochronology can resolve magma chamber dynamics of <1 Ma silicic eruptions at the millennial scale.

U-Pb geochronology of the Deccan Traps and relation to the end-Cretaceous mass extinction

Dating the influence of Deccan Traps eruptions The Deccan Traps flood basalts in India represent over a million cubic kilometers of erupted lava. These massive eruptions occurred around the same time as the end-Cretaceous mass extinction some 65 million years ago, which famously wiped out all nonavian dinosaurs. Schoene et al. determined the precise timing and duration of the main phase of the eruptions, which lasted over 750,000 years and occurred just 250,000 years before the Cretaceous-Paleogene boundary. The relative contribution of these eruptions and of the Chicxulub impact in Mexico to the mass extinction remains unclear, but both provide potential kill mechanisms. Science, this issue p. 182 The main phase of the Deccan Traps eruption began 250,000 years before the end-Cretaceous extinction and lasted 750,000 years. The Chicxulub asteroid impact (Mexico) and the eruption of the massive Deccan volcanic province (India) are two proposed causes of the end-Cretaceous mass extinction, which includes the demise of nonavian dinosaurs. Despite widespread acceptance of the impact hypothesis, the lack of a high-resolution eruption timeline for the Deccan basalts has prevented full assessment of their relationship to the mass extinction. Here we apply uranium-lead (U-Pb) zircon geochronology to Deccan rocks and show that the main phase of eruptions initiated ~250,000 years before the Cretaceous-Paleogene boundary and that >1.1 million cubic kilometers of basalt erupted in ~750,000 years. Our results are consistent with the hypothesis that the Deccan Traps contributed to the latest Cretaceous environmental change and biologic turnover that culminated in the marine and terrestrial mass extinctions.

Statistical geochemistry reveals disruption in secular lithospheric evolution about 2.5 Gyr ago

The Earth has cooled over the past 4.5 billion years (Gyr) as a result of surface heat loss and declining radiogenic heat production. Igneous geochemistry has been used to understand how changing heat flux influenced Archaean geodynamics, but records of systematic geochemical evolution are complicated by heterogeneity of the rock record and uncertainties regarding selection and preservation bias. Here we apply statistical sampling techniques to a geochemical database of about 70,000 samples from the continental igneous rock record to produce a comprehensive record of secular geochemical evolution throughout Earth history. Consistent with secular mantle cooling, compatible and incompatible elements in basalts record gradually decreasing mantle melt fraction through time. Superimposed on this gradual evolution is a pervasive geochemical discontinuity occurring about 2.5 Gyr ago, involving substantial decreases in mantle melt fraction in basalts, and in indicators of deep crustal melting and fractionation, such as Na/K, Eu/Eu* (europium anomaly) and La/Yb ratios in felsic rocks. Along with an increase in preserved crustal thickness across the Archaean/Proterozoic boundary, these data are consistent with a model in which high-degree Archaean mantle melting produced a thick, mafic lower crust and consequent deep crustal delamination and melting—leading to abundant tonalite–trondhjemite–granodiorite magmatism and a thin preserved Archaean crust. The coincidence of the observed changes in geochemistry and crustal thickness with stepwise atmospheric oxidation at the end of the Archaean eon provides a significant temporal link between deep Earth geochemical processes and the rise of atmospheric oxygen on the Earth.

Towards accurate numerical calibration of the Late Triassic: High- precision U-Pb geochronology constraints on the duration of the Rhaetian

Numerical calibration of the Late Triassic stages is arguably the most controversial issue in Mesozoic stratigraphy, despite its importance for assessing mechanisms of environmental perturbations and associated biologic consequences preceding the end-Triassic mass extinction. Here we report new chemical abrasion–isotope dilution– thermal ionization mass spectrometry zircon U-Pb dates for volcanic ash beds within the Aramachay Formation of the Pucara Group in northern Peru that place precise constraints on the age of the Norian- Rhaetian boundary (NRB) and the duration of the Rhaetian. The sampled ash bed–bearing interval is located just above the last occurrence of the bivalve Monotis subcircularis, placing this stratigraphic sequence in the uppermost Norian, perhaps ranging into the earliest Rhaetian. Zircon U-Pb dates of ash beds constrain the deposition age of this interval to be between 205.70 ± 0.15 Ma and 205.30 ± 0.14 Ma, providing precise constraints on the age of the NRB. Combined with previously published zircon U-Pb dates for ash beds bracketing the Triassic-Jurassic boundary, we estimate a duration of 4.14 ± 0.39 m.y. for the Rhaetian. This ends a prolonged controversy about the duration of this stage and has fundamental implications for the rates of paleoenvironmental deterioration that culminated in the end-Triassic mass extinction.

(U-Th)/He thermochronometry constraints on unroofing of the eastern Kaapvaal craton and significance for uplift of the southern African Plateau

The timing and causes of the >1.0 km elevation gain of the southern African Plateau since Paleozoic time are widely debated. We report the first apatite and titanite (U-Th)/He thermochronometry data for southern Africa to resolve the unroofing history across a classic portion of the major escarpment that encircles the plateau. The study area encompasses ∼1500 m of relief within Archean basement of the Barberton Greenstone Belt region of the eastern Kaapvaal craton. Titanite dates are Neoproterozoic. Apatite dates are Cretaceous, with most results clustering at ca. 100 Ma. Thermal history simulations confirm Mesozoic heating followed by accelerated cooling in mid- to Late Cretaceous time. The lower temperature sensitivity of the apatite (U-Th)/He method relative to previous thermochronometry in southern Africa allows tighter constraints on the Cenozoic thermal history than past work. The data limit Cenozoic temperatures east of the escarpment to ≤35 °C, and appear best explained by temperatures within a few degrees of the modern surface temperature. These results restrict Cenozoic unroofing to less than ∼850 m, and permit negligible erosion since the Cretaceous. If substantial uplift of the southern African Plateau occurred in the Cenozoic as advocated by some workers, then it was not responsible for the majority of post-Paleozoic unroofing across the eastern escarpment. Significant Mesozoic unroofing is coincident with large igneous province activity, kimberlite magmatism, and continental rifting within and along the margins of southern Africa, compatible with a phase of plateau elevation gain due to mantle buoyancy sources associated with these events.

Volcanic–plutonic parity and the differentiation of the continental crust

The continental crust is central to the biological and geological history of Earth. However, crustal heterogeneity has prevented a thorough geochemical comparison of its primary igneous building blocks—volcanic and plutonic rocks—and the processes by which they differentiate to felsic compositions. Our analysis of a comprehensive global data set of volcanic and plutonic whole-rock geochemistry shows that differentiation trends from primitive basaltic to felsic compositions for volcanic versus plutonic samples are generally indistinguishable in subduction-zone settings, but are divergent in continental rifts. Offsets in major- and trace-element differentiation patterns in rift settings suggest higher water content in plutonic magmas and reduced eruptibility of hydrous silicate magmas relative to dry rift volcanics. In both tectonic settings, our results indicate that fractional crystallization, rather than crustal melting, is predominantly responsible for the production of intermediate and felsic magmas, emphasizing the role of mafic cumulates as a residue of crustal differentiation.

4.10 – U–Th–Pb Geochronology

U–Th–Pb geochronology was the first and is still the fastest-growing method of measuring the rates of geologic processes through deep time and calibrating the absolute dates of events in Earth history. A variety of high U–Th minerals are found in nearly all crustal rocks, making this system the most widely applicable to understanding continental evolution. The decay of three parent isotopes to three distinct radiogenic daughter isotopes of Pb also provides a means of assessing open system behavior. This chapter describes (1) the basics of the U–Th–Pb decay chains and dating equations; (2) the different methods of visualizing U–Th–Pb data and identifying open-system behavior; (3) the various geologic processes that cause open system behavior; (4) the benefits and drawbacks of the three most popular methods of analyzing Pb/U and Pb/Th isotope ratios, namely, isotope dilution thermal ionization mass spectrometry (ID-TIMS), secondary ion mass spectrometry (SIMS), and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS); (5) the limits to precision and accuracy in U–Th–Pb geochronology; and (6) some examples of exciting current and future research directions. Though not comprehensive, this chapter is meant to be informative to both the beginners and experienced users of U–Th–Pb geochronology and provide the tools and background necessary to delve deeper into one of earth sciences most exciting and rapidly evolving fields.

Rates and mechanisms of Mesoarchean magmatic arc construction, eastern Kaapvaal craton, Swaziland

The mechanisms and time scales of magmatism and deformation during orogenesis are important for developing models of lithospheric growth. We summarize the first detailed study of a well-preserved Mesoarchean crustal section from the oldest portion of the Kaapvaal craton, southern Africa, which records the complex interactions between deformation and magmatism during craton assembly ca. 3.3–3.2 Ga. We use chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb zircon geochronology and apatite thermochronology in conjunction with geological mapping to show that the tonalitic to granitic Usutu magmatic suite intruded into the ca. 3.66–3.45 Ga Ancient gneiss complex over a period of ∼16 Ma ca. 3236–3220 Ma as discrete pulses of magma with variable intrusive styles. Usutu rocks retain magmatic fabrics that preserve a history of NW-SE regional shortening, consistent with synchronous deformation recorded in the adjacent Barberton greenstone belt. U-Pb zircon dates of ca. 3.28–3.23 Ga and apatite cooling dates from the Nhlangano gneiss SE of the Ancient gneiss complex reveal that they represent an older, largely undocumented period of crustal growth and modification. Synchronicity in magmatism and similarity in kinematics of deformation north and south of a previously suggested continental suture in the Barberton greenstone belt lead us to propose doubly vergent subduction zones that were active from at least ca. 3.28 to 3.22 Ga. Geochemistry of the Usutu suite reveals differences in magma sources from north to south, which, when combined with Nd isotopic signatures, are consistent with their production in a subduction zone. The combination of <1 Ma precision on crystallization dates with field observations and geochemistry allows us to track the evolution of this magmatic system with temporal resolution unprecedented for Archean rocks.