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Abstract. The loss of radiogenic Pb from zircon is known to be a major factor that can cause inaccuracy in the U–Pb geochronological system; hence, there is a need to better characterize the distribution of Pb loss in natural samples. Treatment of zircon by chemical abrasion (CA) has become standard practice in isotope dilution–thermal ionization mass spectrometry (ID-TIMS), but CA is much less commonly employed prior to in situ analysis via laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) or secondary ionization mass spectrometry (SIMS). Differentiating the effects of low levels of Pb loss in Phanerozoic zircon with relatively low-precision in situ U–Pb dates, where the degree of Pb loss is insufficient to cause discernible discordance, is challenging. We show that U–Pb isotopic ratios that have been perturbed by Pb loss may be modeled by convolving a Gaussian distribution that represents random variations from the true isotopic value stemming from analytical uncertainty with a distribution that characterizes Pb loss. We apply this mathematical framework to model the distribution of apparent Pb loss in 10 igneous samples that have both non-CA LA-ICP-MS or SIMS U–Pb dates and an estimate of the crystallization age, either through CA U–Pb or 40Ar/39Ar geochronology. All but one sample showed negative age offsets that were unlikely to have been drawn from an unperturbed U–Pb date distribution. Modeling apparent Pb loss using the logit–normal distribution produced good fits with all 10 samples and showed two contrasting patterns in apparent Pb loss; samples where most zircon U–Pb dates undergo a bulk shift and samples where most zircon U–Pb dates exhibited a low age offset but fewer dates had more significant offset. Our modeling framework allows comparison of relative degrees of apparent Pb loss between samples of different age, with the first and second Wasserstein distances providing useful estimates of the total magnitude of apparent Pb loss. Given that the large majority of in situ U–Pb dates are acquired without the CA treatment, this study highlights a pressing need for improved characterization of apparent Pb-loss distributions in natural samples to aid in interpreting non-CA in situ U–Pb data and to guide future data collection strategies.
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U-Pb memory behavior in Chicxulub's peak ring — Applying U-Pb depth profiling to shocked zircon
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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 10 12 to 10 13 ohm resistors, response time <0.5 s, exceptional gain stability <1 ppm and a vast dynamic range theoretically allowing to quantify signals from aA (10 −18 A) to nA (10 −9 A) level. We measured a set of Pb standards, synthetic U–Pb solutions and natural zircons at currents of 2 × 10 −16 to 2 × 10 −12 A (corresponding to intensities of 20 μV to 200 mV relative to a 10 11 ohm amplifier) to assess the utility of ATONA in replacing ion counting for the smallest samples. The results show a clear precision benefit of using ATONA-Faraday detection over Daly ion counting for ion currents of >10 −14 A (1 mV relative to a 10 11 ohm amplifier or ca. 60 kcps). As such currents are routinely achievable for major Pb peaks of interest ( 205–208 Pb) 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 204 Pb, 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.more » « less
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{"Abstract":["This dataset consists of detrital zircon U-Pb data from samples from the Amundsen Sea Embayment. The data are a product of the publication: "Reconstructing Eocene Antarctic river drainage from provenance analysis of Amundsen Sea Embayment sediments".\n\nIncluded are data from three Holocene sediment samples strategically located around the embayment to help characterise the detritus currently being eroded and deposited in the Amundsen Sea. Samples comprise of locations proximal to Pine Island Glacier and Thwaites Glacier, with a further sample to the north of Thwaites Glacier. A fourth sample consists of Cretaceous mudstone from a drill core from site PS104_20-2 in the Amundsen Sea Embayment.\n\nA second file contains a compilation of selected detrital zircon U-Pb dates from potential source areas around West Antarctica. These include the sedimentary rocks of the Swanson Formation and Ellsworth-Whitmore Mountains, as well as all published West Antarctic subglacial till data. Data from moraines in the Transantarctic Mountains are also included."]}more » « less
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The Snowball Earth hypothesis predicts global ice cover; however, previous descriptions of Cryogenian (720-635 Ma) glacial deposits are limited to continental margins and shallow marine basins. The Tavakaiv (Tava) sandstone injectites and ridges in Colorado, USA, preserve a rare terrestrial record of Cryogenian low-latitude glaciation. Injectites, ridges, and chemically weathered crystalline rock display features characteristic of fluidization and pervasive deformation in a subglacial environment due to glacial loading, fluid overpressure, and repeated sand injection during meltwater events. In situ hematite U-Pb geochronology on hematite-quartz veins, which crosscut and are cut by Tava dikes, constrain sand injection at ~690-660 Ma. We attribute early Tava sand injection episodes to basal melting associated with rifting and geothermal heating, and later injections to meltwater generation during ~661 Ma Sturtian deglaciation. A modern analog is provided by the Ross Embayment of Antarctica, where rift-related faults border sediment-filled basins, overpressurized fluids circulate in confined aquifers below ice, and extensive preglacial topography is preserved. Field evidence and geochronology in Colorado further highlight that deep chemical weathering of Proterozoic bedrock and denudation associated with the Great Unconformity predate Cryogenian injection of fluidized sand, consistent with limited glacial erosion.more » « less
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1016/j.chemgeo.2019.07.029
