Evaluating the construction and evolution of upper crustal magma reservoirs with coupled U/Pb zircon geochronology and thermal modeling:A case study from the Mt. Capanne pluton (Elba, Italy)

Evaluating mechanisms and rates for magma transport and emplacement in the upper crust is important in order to predict the thermal and rheological state of the crust, and understand the relationship between plutonism and volcanism. U–Pb geochronology on zircon is commonly used to constrain magma emplacement and storage time in the crust, but interpreting complex zircon age populations in terms of in-situ crystallization versus crystallization at a deeper level is not trivial. This study focuses on the Mt. Capanne pluton in Elba (Italy), a well-documented example of arc-related laccolith emplaced in the upper continental crust. Previous studies proposed that the Mt. Capanne intrusion was accreted in less than 10 000 yr by distinct and mappable magma pulses. Here, we couple high-precision ID-TIMS U–Pb zircon geochronology with numerical thermal simulations to evaluate emplacement rates, test different emplacement models, inform zircon age interpretations and evaluate the potential for melt storage during construction of the Mt. Capanne pluton. Our results require that the Mt. Capanne intrusion was built in at least 250 000 yr by multiple magma injections. A variety of emplacement scenarios show that melt was preserved for <60 000 yr after each pulse and that the maximum eruptible volumes were approximately equal to the volume of each pulse. Our results also require that the majority of zircon crystallization occurred in zircon saturated reservoirs at deeper crustal levels prior to final magma emplacement and cooling, which has implications for using zircon U–Pb geochronology to infer upper crustal magma residence times

A thermotectonic framework for the growth and stabilization of the eastern Kaapvaal craton, southern Africa

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2006.Includes bibliographical references.Continents are an amalgamation of crust and mantle lithosphere assembled over -4 Gyr and are therefore our best record of the early Earth. Exposures of rocks -3.0-3.7 Ga cover -20,000 km2 of eastern S. Africa and Swaziland, and provide a record of the continental assembly and subsequent stabilization of the eastern Kaapvaal craton. This thesis uses structural, geochronological, thermochronological and isotopic constraints to examine the tectonothermal processes responsible for the growth and stabilization of this portion of Mesoarchean lithosphere. Field mapping was focused on terrane-bounding shear zones and syntectonic plutons, and in combination with ID-TIMS U-Pb zircon geochronology and Sm-Nd analysis, places sub-Myr constraints on the timing, distribution, and kinematics of magmatism and deformation during growth and modification of continental lithosphere. Detailed U-Pb apatite and titanite thermochronological datasets are used in combination with finite difference numerical modeling to determine non-linear temperature-time paths for rocks between -650-300 °C from 3.45-3.08 Ga - providing a sensitive indicator of tectonic and magmatic processes in the middle to lower crust.(cont.) From 3.2 to 3.3 Ga, multiple microcontinental fragments with distinct age and Nd isotopic characteristics were assembled along an oblique subduction zone boundary, with the Barberton Greenstone belt representing a lithospheric suture zone. During continental assembly and orogeny, strain was partitioned into 3236-3220 Ma syntectonic plutons and terrane-bounding transcurrent shear zones bordering the margins of the previously stabilized ca. 3.66-3.45 Ga Ancient Gneiss Complex. Subsequent 3.2-3.1 Ga reactivation of crustal anisotropies within the lithospheric suture zone - represented broadly by the Barberton Greenstone Belt - controlled the thermal and compositional reorganization of the crust through transtensional tectonics, exhumation of deep-crustal gneiss terranes, and upward migration of granitic batholiths. This final period of crustal modification was responsible for juxtaposing low-grade greenstone supracrustal rocks against middle- to lower-crustal gneiss terranes, and ultimately led to crustal stabilization in the greenstone belt and vicinity. These results support a model in which the stabilization of the Kaapvaal craton was a piece-wise transition resulting from lithospheric thickening and crustal thinning over, hundreds of Myr.by Robert Blair Schoene.Ph.D

The AGeS2 (Awards for Geochronology Student research 2) Program: Supporting Community Geochronology Needs and Interdisciplinary Science

Geochronology is essential in the geosciences. It is used to resolve the durations and rates of earth processes, as well as test causative relationships among events. Such data are increasingly required to conduct cutting-edge, transformative, earth-science research. The growing need for geochronology is accompanied by strong demand to enhance the ability of labs to meet this pressure and to increase community awareness of how these data are produced and interpreted. For example, a 2015 National Science Foundation (NSF) report on opportunities and challenges for U.S. geochronology research noted: While there has never been a time when users have had greater access to geo-chronologic data, they remain, by and large, dissatisfied with the available style/ quantity/cost/efficiency (Harrison et al., 2015, p. 1). And the 2012 National Research Council NROES (New Research Opportunities in the Earth Sciences) report (Lay et al., 2012, p. 82) recommended: [NSF] EAR should explore new mechanisms for geochronology laboratories that will service the geochronology requirements of the broad suite of research opportunities while sustaining technical advances in methodologies. The AGeS (Awards for Geochronology Student research) program is one way that these calls are being answered

Early formation of the Moon 4.51 billion years ago

Establishing the age of the Moon is critical to understanding solar system evolution and the formation of rocky planets, including Earth. However, despite its importance, the age of the Moon has never been accurately determined. We present uranium-lead dating of Apollo 14 zircon fragments that yield highly precise, concordant ages, demonstrating that they are robust against postcrystallization isotopic disturbances. Hafnium isotopic analyses of the same fragments show extremely low initial 176Hf/177Hf ratios corrected for cosmic ray exposure that are near the solar system initial value. Our data indicate differentiation of the lunar crust by 4.51 billion years, indicating the formation of the Moon within the first ~60 million years after the birth of the solar system

Evaluating uncertainties in the calibration of isotopic reference materials and multi-element isotopic tracers (EARTHTIME Tracer Calibration Part II)

A statistical approach to evaluating uncertainties in the calibration of multi-element isotopic tracers has been developed and applied to determining the isotopic composition of mixed U-Pb (202Pb-205Pb-233U-235U) tracers used for accurate isotope dilution U-Pb geochronology. Our experiment, part of the EARTHTIME initiative, directly links the tracer calibration to first-principles measurements of mass and purity that are all traceable to SI units, thereby quantifying the accuracy and precision of U-Pb dates in absolute time. The calibration incorporates new more accurate and precise purity measurements for a number of commonly used Pb and U reference materials, and requires inter-relating their isotopic compositions and uncertainties. Similar methods can be used for other isotope systems that utilize multiple isotopic standards for calibration purposes. We also detail the inter-calibration of three publicly available U-Pb gravimetric solutions, which can be used to bring the same first-principles traceability to in-house U-Pb tracers from other laboratories. Accounting for uncertainty correlations in the tracer isotope ratios yields a tracer calibration contribution to the relative uncertainty of a 206Pb/238U date that is only half of the relative uncertainty in the 235U/205Pb ratio of the tracer, which was historically used to approximate the tracer related uncertainty contribution to 206Pb/238U dates. The tracer uncertainty contribution to 206Pb/238U dates has in this way been reduced to <300 ppm when using the EARTHTIME and similarly calibrated tracers

U-Pb geochronology at 100ppm age uncertainty

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An integrative biostratigraphic, chemostratigraphic, and sequence stratigraphic perspective of the Ordovician–Silurian boundary on Anticosti Island (Canada)

Anticosti Island, Canada, has long been recognized as an exceptional OrdovicianâSilurian boundary succession with the potential to serve as one of the best records of climatic, oceanographic, and biological events associated with the Late Ordovician mass extinction. However, differing interpretations as to the position of the Hirnantian Stage within the stratigraphic succession due to the paucity of diagnostic graptolites, the apparent absence of a typical Hirnantia fauna within the Upper Ordovician Ellis Bay Formation, and lateral facies variability among outcrops has hindered the study of the OrdovicianâSilurian boundary on the island, particularly in the eastern half of the outcrop belt. Definitively identifying the stratigraphic position of the Hirnantian Stage within the succession is therefore critical for understanding this classic OrdovicianâSilurian boundary section, as well as for the integration of data from Anticosti into our global understanding of the Late Ordovician mass extinction. Here, we take an integrative approach to studying the Ellis Bay and lowermost Becscie formations, combining new paleobiological, geochemical, radiometric, and sequence stratigraphic constraints from ongoing fieldwork with existing biostratigraphic, geochemical, and palynological studies in the context of newly measured stratigraphic sections. These formations record six depositional sequences bounded by regionally traceable but subtle unconformities, often mantled by thin siliciclastic veneers reworked into transgressive lag facies. Many of these unconformities have gone unrecognized despite more than a century of work at certain localities. Furthermore, despite previous controversy, multiple lines of evidence favor a Hirnantian age for the entire Ellis Bay and lowermost Becscie formations, including newly recognized occurrences of Hirnantia and Hindella in the lower Ellis Bay Formation, a two-phased positive carbon isotope excursion, with the second phase reaching ~6â° in the Laframboise Member of the Ellis Bay Formation, and a U-Pb TIMS age of 443.61 ± 0.52 Ma from zircons in a bentonite from the mid-Ellis Bay Formation. While graptolite and conodont biostratigraphy support this age model, determination based on chitinozoan biozonation is more equivocal but may be controlled by facies preferences. Conodont, brachiopod, and chemostratigraphic data additionally suggest that the Hirnantian Stage may extend slightly into the lower Becscie Formation on the western end of Anticosti and well into the lower Becscie Formation in the eastern part of Anticosti. Our reappraisal of a classic OrdovicianâSilurian boundary section has important implications for understanding the sequence of climatic, environmental, and biological events throughout the Late Ordovician mass extinction. Given that the Ellis Bay and lowermost Becscie formations are indeed Hirnantian in age (encompassing ~2 My), these formations record six fourth-order depositional sequences of approximately ~333 ky. Furthermore, comparison of the Hirnantian of Anticosti to coeval exposures suggests that other regions may be incomplete at the level of the fourth-order cycles that occur in the Ellis Bay Formation. Resulting uncertainties in correlations based on unconformities and interpretations of stratigraphic architecture may therefore greatly complicate global correlation of Hirnantian records. Further study of this issue is critical, as stratigraphic architecture is expected to be an overarching control on the expression of oceanographic, climatic, and biotic events at a regional scale, complicating the interpretation of the pattern and drivers of the Late Ordovician mass extinction

Science Priorities for the Extraction of the Solid MSR Samples from their Sample Tubes NASA-ESA Mars Rock Team

editorial reviewedPreservation of the chemical and structural integrity of samples that will be brought back from Mars is paramount to achieving the scientific objectives of MSR. Given our knowledge of the nature of the samples retrieved at Jezero by Perseverance, at least two options need to be tested for opening the sample tubes: (1) One or two radial cuts at the end of the tube to slide the sample out. (2) Two radial cuts at the ends of the tube and two longitudinal cuts to lift the upper half of the tube and access the sample. Strategy 1 will likely minimize contamination but incurs the risk of affecting the physical integrity of weakly consolidated samples. Strategy 2 will be optimal for preserving the physical integrity of the samples but increases the risk of contamination and mishandling of the sample as more manipulations and additional equipment will be needed. A flexible approach to opening the sample tubes is therefore required, and several options need to be available, depending on the nature of the rock samples returned. Both opening strategies 1 and 2 may need to be available when the samples are returned to handle different sample types (e.g., loosely bound sediments vs. indurated magmatic rocks). This question should be revisited after engineering tests are performed on analogue samples. The MSR sample tubes will have to be opened under stringent BSL4 conditions and this aspect needs to be integrated into the planning