Noah McLean

238U/235U Systematics in Terrestrial Uranium-Bearing Minerals

A Better Date Uranium-lead (U-Pb) dating, which is one of the most commonly used methods of radiometric dating for old terrestrial materials, operates by comparing the ratio of trace levels of U with its nuclear decay product Pb. This dating method, and the similar Pb-Pb method, assumes that the ratio between the two most common U isotopes (238U and 235U) is constant. By accurately measuring the 238U/235U ratio in a suite of minerals representing a range of tectonic environments, Hiess et al. (p. 1610; see the Perspective by Stirling) demonstrate that this ratio is more variable than was previously thought. The variability does not reflect any systematic bias with time, location, or temperature—suggesting that ideally 238U/235U should be determined for every sample to calculate ages. In the absence of such data, a revised 238U/235U ratio for zircon minerals could significantly modify previous age estimates using U-Pb and Pb-Pb dating techniques. Highly variable uranium isotope ratios highlight the need for a revised approach to radiometric dating. The present-day 238U/235U ratio has fundamental implications for uranium-lead geochronology and cosmochronology. A value of 137.88 has previously been considered invariant and has been used without uncertainty to calculate terrestrial mineral ages. We report high-precision 238U/235U measurements for a suite of uranium-bearing minerals from 58 samples representing a diverse range of lithologies. This data set exhibits a range in 238U/235U values of >5 per mil, with no clear relation to any petrogenetic, secular, or regional trends. Variation between comagmatic minerals suggests that 238U/235U fractionation processes operate at magmatic temperatures. A mean 238U/235U value of 137.818 ± 0.045 (2σ) in zircon samples reflects the average uranium isotopic composition and variability of terrestrial zircon. This distribution is broadly representative of the average crustal and “bulk Earth” 238U/235U composition.

Zircon U-Pb Geochronology Links the End-Triassic Extinction with the Central Atlantic Magmatic Province

Life Versus the Volcanoes Correlating a specific triggering event, such as an asteroid impact or massive volcanism, to mass extinction events is clouded by the difficulty in precisely timing their occurrence in the geologic record. Based on rock samples collected in North America and Morocco, Blackburn et al. (p. 941, published online 21 March) acquired accurate ages for events surrounding the mass extinction that occurred ∼201 million years ago, between the Triassic and Jurassic Periods. The timing of the disappearance of marine and land fossils and geochemical evidence of the sequential eruption of the Central Atlantic Magmatic Province imply a strong causal relationship. Climate change triggered by massive volcanism set the stage for the era of dinosaurs. The end-Triassic extinction is characterized by major losses in both terrestrial and marine diversity, setting the stage for dinosaurs to dominate Earth for the next 136 million years. Despite the approximate coincidence between this extinction and flood basalt volcanism, existing geochronologic dates have insufficient resolution to confirm eruptive rates required to induce major climate perturbations. Here, we present new zircon uranium-lead (U-Pb) geochronologic constraints on the age and duration of flood basalt volcanism within the Central Atlantic Magmatic Province. This chronology demonstrates synchroneity between the earliest volcanism and extinction, tests and corroborates the existing astrochronologic time scale, and shows that the release of magma and associated atmospheric flux occurred in four pulses over about 600,000 years, indicating expansive volcanism even as the biologic recovery was under way.

Time Scales of Metamorphism, Deformation, and Crustal Melting in a Continental Arc, North Cascades USA

The record of metamorphism, deformation, and melting in the North Cascades continental arc provides insights into the timing and mechanisms of extensional unroofing that followed crustal thickening. The Skagit Gneiss (North Cascades) is composed of variably deformed 90–45 Ma tonalitic to granitic intrusive rocks. These lithologies and metasedimentary rocks are migmatitic. Zircon and monazite from different textural varieties of migmatite from three outcrops along an east-west transect were analyzed using the isotope dilution–thermal ionization mass spectrometry (ID-TIMS) technique. The data reveal two main migmatization pulses: (1) 68–63 Ma and (2) 53–47 Ma. In the westernmost locality, leucosome zircon yields group 1 dates, and mesosome (sillimanite-garnet-biotite gneiss) zircon is as young as 61 Ma. Leucosome zircon in the easternmost outcrop yields only group 2 dates, and biotite gneiss contains 47 Ma zircon. The third outcrop has leucosome dates from both groups: 65 Ma and 53 Ma. Monazite from leucosomes and mesosomes likely records the timing of prograde metamorphism (ca. 69 Ma) and later synkinematic ± fluid-mediated growth (49–46 Ma). These results indicate that partial melting occurred during 71–61 Ma metamorphism and lasted through 46 Ma deformation associated with exhumation. The results and field observations suggest the presence of ductilely deforming crust from 68 to 46 Ma. The similarity between the youngest U-Pb dates in the migmatites and cooling and basin-filling ages suggests a link between ductile and brittle processes over a range of structural levels in the arc, from a zone of crustal melting, flow, and migmatite crystallization to Earth9s surface.

High-resolution temporal and stratigraphic record of Siletzia’s accretion and triple junction migration from nonmarine sedimentary basins in central and western Washington

The presence of early Eocene near-trench magmatism in western Washington and southern British Columbia has led to speculation that this area experienced ridge-trench interaction during that time. However, the effects of this process as they are preserved in other parts of the geologic record are poorly known. We present high-precision U-Pb zircon geochronology from Paleogene nonmarine sedimentary and volcanic sequences in central and western Washington that preserve a record of tectonic events between ca. 60 and 45 Ma. The data reveal that the Swauk, Chuckanut, and Manastash Formations formed a nonmarine sedimentary basin along the North American margin between ≤59.9 and 51.3 Ma. This basin experienced significant disruption that culminated in basinwide deformation, uplift, and partial erosion during accretion of the Siletzia terrane between 51.3 and 49.9 Ma. Immediately following accretion, dextral strike-slip faulting began, or accelerated, on the Darrington–Devil’s Mountain, Entiat, Leavenworth, Eagle Creek, and Straight Creek–Fraser fault zones between 50 and 46 Ma. During this time, the Chumstick Formation was deposited in a strike-slip basin coeval with near-trench magmatism. Faulting continued on the Entiat, Eagle Creek, and Leavenworth faults until a regional sedimentary basin was reestablished ≤45.9 Ma, and may have continued on the Straight Creek–Fraser fault until 35–30 Ma. This record of basin disruption, volcanism, and strike-slip faulting is consistent with ridge-trench interaction and supports the presence of an oceanic spreading ridge at this latitude along the North American margin during the early Eocene.

Algorithms and software for U-Pb geochronology by LA-ICPMS

The past 15 years have produced numerous innovations in geochronology, including experimental methods, instrumentation, and software that are revolutionizing the acquisition and application of geochronological data. For example, exciting advances are being driven by Laser-Ablation ICP Mass Spectrometry (LA-ICPMS), which allows for rapid determination of U-Th-Pb ages with 10s of micrometer-scale spatial resolution. This method has become the most commonly applied tool for dating zircons, constraining a host of geological problems. The LA-ICPMS community is now faced with archiving these data with associated analytical results and, more importantly, ensuring that data meet the highest standards for precision and accuracy and that interlaboratory biases are minimized. However, there is little consensus with regard to analytical strategies and data reduction protocols for LA-ICPMS geochronology. The result is systematic interlaboratory bias and both underestimation and overestimation of uncertainties on calculated dates that, in turn, decrease the value of data in repositories such as EarthChem, which archives data and analytical results from participating laboratories. We present free open-source software that implements new algorithms for evaluating and resolving many of these discrepancies. This solution is the result of a collaborative effort to extend the U-Pb_Redux software for the ID-TIMS community to the LA-ICPMS community. Now named ET_Redux, our new software automates the analytical and scientific workflows of data acquisition, statistical filtering, data analysis and interpretation, publication, community-based archiving, and the compilation and comparison of data from different laboratories to support collaborative science.

Linking deep and shallow crustal processes during regional transtension in an exhumed continental arc, North Cascades, northwestern Cordillera (USA)

The North Cascades orogen (northwestern USA) provides an exceptional natural laboratory with which to evaluate potential temporal and kinematic links between processes operating at a wide range of crustal levels during collapse of a continental arc, and particularly the compatibility of strain between the upper and lower crust. This magmatic arc reached a crustal thickness of ≥55 km in the mid-Cretaceous. Eocene collapse of the arc during regional transtension was marked by magmatism, migmatization, ductile flow, and exhumation of deep crustal (8–12 kbar) rocks in the Cascades crystalline core coeval with subsidence and rapid deposition in nonmarine basins adjacent to the core, and intrusion of dike complexes. The Skagit Gneiss Complex is the larger of two regions of exhumed deep crust with Eocene cooling ages in the Cascades core, and it consists primarily of tonalitic orthogneiss emplaced mainly in two episodes of ca. 73–59 Ma and 50–45 Ma. Metamorphism, melt crystallization, and ductile deformation of migmatitic metapelite overlap the orthogneiss emplacement, occurring (possibly intermittently) from ca. 71 to 53 Ma; the youngest orthogneisses overlap 40 Ar/ 39 Ar biotite dates, compatible with rapid cooling. Gently to moderately dipping foliation, subhorizontal orogen-parallel (northwest-southeast) mineral lineation, sizable constrictional domains, and strong stretching parallel to lineation of hinges of mesoscopic folds in the Skagit Gneiss Complex are compatible with transtension linked to dextral-normal displacement of the Ross Lake fault zone, the northeastern boundary of the Cascades core. The other deeply exhumed domain, the 9–12 kbar Swakane Biotite Gneiss, has a broadly north-trending, gently plunging lineation and gently to moderately dipping foliation, which are associated with top-to-the-north noncoaxial shear. This gneiss is separated from overlying metamorphic rocks by a folded detachment fault. The Eocene Swauk and Chumstick basins flank the southern end of the Cascades core. In the Swauk basin, sediments were deposited in part at ca. 51 Ma, folded shortly afterward, and then covered by ca. 49 Ma Teanaway basalts and intruded by associated mafic dikes. Directly after dike intrusion, the fault-bounded Chumstick basin subsided rapidly. Extension directions from these dikes and from Eocene dikes that intruded the Cascades core are dominantly oblique to the overall trend of the orogen (275°–310° versus ∼320°, respectively) and to the northwest-southeast to north-south ductile flow direction in the Skagit and Swakane rocks. This discordance implies that coeval extensional strain was decoupled between the brittle and ductile crust. Strain orientations at all depths in the Cascades core contrast with the approximately east-west extension driven by orogenic collapse in coeval metamorphic core complexes ∼200 km to the east. Arc-oblique to arc-parallel flow in the Cascades core probably resulted in part from dextral shear along the plate margin and from along-strike gradients in crustal thickness and temperature.

Geochronology and Thermochronology

This book is a welcome introduction and reference for users and innovators in geochronology. It provides modern perspectives on the current state-of-the art in most of the principal areas of geochronology and thermochronology, while recognizing that they are changing at a fast pace. It emphasizes fundamentals and systematics, historical perspective, analytical methods, data interpretation, and some applications chosen from the literature. This book complements existing coverage by expanding on those parts of isotope geochemistry that are concerned with dates and rates and insights into Earth and planetary science that come from temporal perspectives.

Synthetic U-Pb ‘standard’ solutions for ID-TIMS geochronology [abstract only]

Despite being the most abundant mineral in the Earth’s crust and one commonly used for Ar/Ar geochronology, plagioclase has been little studied with respect to Ar diffusion kinetics. We present results of Ar and Ar diffusion experiments and Ar/Ar geochronometry on intercumulus plagioclase from a gabbro in the Rustenburg Layered Series of the Bushveld Complex (BC). The plagioclase is relatively uniform compositionally, typically ranging from An60-An70. Age spectra are variably discordant, having profiles consistent with degassing of the 2.06 Ga BC by the ~1.3 Ga Pilanesberg Complex. Single ~400μm neutron-irradiated plagioclase crystals were wrapped in platinum packets and incrementally heated with a diode laser between 400 and 1600 °C. Temperature was controlled and monitored with an optical pyrometer, calibrated to ± 10°C (1σ). We calculated diffusion coefficients (D/a) from fractional release data assuming a spherical geometry. The calculated values yield ~linear arrays in Arrhenius plots. From least-squares regression of the low temperature values (<1000°C; the first ~10-20% of the total Ar and Ar released), we quantified the diffusion parameters Ea and Do/a . Between 400 and 800 °C, ArCa and ArK behave somewhat differently: ArCa has an apparently higher Ea (36-42 kcal/mol) than ArK (27-40 kcal/mol). However, at higher T, the calculated coefficients D/a for each isotope are equivalent, and both share the same apparent kinetic function. Four experiments (one is shown below) with different heating schedules yield closure temperatures, TC (calculated for dT/dt = 1000 C/Ma), of 305-339°C and 214-319°C for ArCa and ArK, respectively. The ArCa/ ArK spectra and microprobe data suggests that the lower TC determined for ArK may be an artifact of K-rich inclusions or exsolution lamellae, or grain-scale compositional zoning. We conclude that the ArCa results are more indicative of natural Ar diffusion in this plagioclase. Primitive andesite magmas as restitemelt metatexitic systems from ascending cold diapirs A. CASTRO*, T. GERYA, I. MORENO-VENTAS C. FERNÁNDEZ AND A. GARCÍA CASCO Dpto. de Geología, U. de Huelva, 21071, Huelva, Spain (*correspondence: dorado@uhu.es) ETH Zurich, Switherland Dpto. de Geodinámica y Paleont., U. de Huelva, Spain Dpto. de Mineralogía y Petrología, U. de Granada, Spain

SAM BOWRING - PIONEERING COLLABORATION BETWEEN EARTH SCIENCE AND COMPUTER SCIENCE

Sam Bowring anticipated and pioneered the development of a first of-its-kind collaboration between geochronologists and computer scientists to develop what he called “cradle to tomb” cyber infrastructure to support the flow of data for geochronology from the field to the laboratory to publicly accessible online databases. He first approached Jim Bowring in 1996 to discuss the problems and to explore solutions, and the work continued to advance from that point. Sam was adamant that earth scientists should not need to engage in computer science to develop tools to support their work, and that there was a great opportunity instead for collaboration with computer scientists to advance science, working together to create software that drives improvements in precision, accuracy, reproducibility, availability, and education. The word “Cyberinfrastructure” would not be coined until 1998, yet Sam Bowring foresaw the need for its key elements and processes. Over the last twenty years, Sam led the efforts to reify these ideas in concert with the authors and others. He worked with colleagues to establish EARTHTIME in 2001 as a community driven initiative with the goal of calibrating Earth history and developing the geochronological techniques necessary to produce high precision dates. These techniques include the collaborative development of robust open source software tools to support the scientific workflows of geochronologists including data reduction, analysis, and archiving. The resulting tools Tripoli and ET_Redux, developed with the authors and others, now play a key role in U-Pb geochronology. One of the most important aspects of his vision and these efforts was his anticipation of open data and the integration of these tools with EarthChem data repositories, especially Geochron.org, developed with author Doug Walker. Sam and Doug understood the power of open access to data and Sam insisted early on that ET_Redux be able to compile archived analyses from data repositories to potentially support novel advances. The overarching theme of Sam Bowring’s vision for collaborative development of cyber infrastructure is the potential to improve education at all levels by insisting on transparency and reproducibility.

EARTHTIME: Teaching Geochronology to High School Students in the USA

As part of the EARTHTIME outreach initiative, we have developed an educational module that teaches students about the basics of geochronology and how geologic time is measured. The exercises focus on the uranium-lead (U-Pb) dating method using the mineral zircon with applications to solving geological problems. During several Lab Day tutorials, students from local high schools attended a day of workshops, participating in hands-on exercises and a discussion of geochronology in earth science research. Student performance and learning impact were assessed using pre-tests (1 week before the event) and two post-tests (1 week after the event and 4 months after the event). These revealed that students greatly appreciated the hands-on exercises and that the exercises resulted in a significant increase in knowledge. We also developed and tested a “Lab Day on the road,” where scientists traveled to a local high school to introduce hands-on exercises and lead discussions related to geochronology. To further develop the module, a teacher workshop was conducted to identify educators’ needs and perspectives. The teaching methods, developed iteratively during 2 years of Lab Days, were incorporated in a Geochronology Lesson Plan and Material Kit. The finalized lesson plan is a 90- to 120-min educational module, downloadable at http://www.earth-time.org/Lesson_Plan.pdf, along with supporting spreadsheets and a video demonstration of the material. An EARTHTIME geochronology kit, linked to the lesson plan activities, is available by request to K-12 teachers in the USA.