SHRIMP (Sensitive High Resolution Ion MicroProbe) is an instrument that for decades has used the radioactive decay of uranium into lead to measure geologic time. The accuracy and precision of this instrument has not been seriously reviewed in almost 20 years. This paper compares several dozen SHRIMP ages in our database with more accurate and precise methods to assess SHRIMP accuracy and precision. Analytical and geological complications are addressed to try to improve the method.
Advancements in technology have introduced new techniques to more quickly and cheaply date rocks with little sample preparation. A unique use of this method is to date shales and constrain when these rocks were first deposited. This approach can also time when such sequences were subsequently affected by heat or fluids after they were deposited. This is useful, as the formation of precious-metal-bearing systems or petroleum source rocks is commonly associated with such processes.
This work provides equations allowing the use of minerals with variable concentrations of parent and daughter isotopes as primary standards to correct for elemental fractionation during the analysis by laser ablation inductively coupled plasma mass spectrometry.
The article demonstrates a new technique that can be used to determine the age of calcite crystallisation using the decay of 176Lu to 176Hf. The technique is novel because (a) Lu–Hf radiometric dating is rarely applied to calcite and (b) this is the first instance where analysis has been conducted by ablating the sample with a laser beam rather than bulk dissolution. By using laser ablation the original context of the sample is preserved.
Calcite is frequently formed during alteration processes in the basaltic, uppermost layer of juvenile oceanic crust. Weathered oceanic basalts are hard to date with conventional radiometric methods. We show in a case study from the North Pamir, Central Asia, that calcite U–Pb age data, supported by geochemistry and petrological microscopy, have potential to date sufficiently old oceanic basalts, if the time span between basalt extrusion and latest calcite precipitation (~ 25 Myr) is considered.
Distinguishing between different dolomitization processes is challenging yet critical for resolving some of the issues and ambiguities related to the formation of dolomitic rocks. Accurate U–Pb absolute dating of dolomite by LA-ICP-MS could contribute to a better understanding of the dolomitization process by placing syngenetic, early diagenetic, and/or epigenetic events in the proper geological context.
To explore the U–Pb geochronometer in fluorite, the spatial distribution of uranium and other substituted elements in natural crystals is investigated using induced fission-track and synchrotron radiation X-ray fluorescence mapping. LA-ICP-MS U–Pb dating on four crystals, which preserve micrometer-scale variations in U concentrations, yields identical ages within analytical uncertainty. Our results show that fluorite U–Pb geochronology has potential for dating distinct crystal growth stages.
This work presents LA-ICP-MS U–Pb geochronology of epidote in hydrothermal veins. The challenges of epidote dating are addressed, and a protocol is proposed allowing us to obtain epidote U–Pb ages with a precision as good as 5 % in addition to the initial Pb isotopic composition of the epidote-forming fluid. Epidote demonstrates its potential to be used as a U–Pb geochronometer and as a fluid tracer, allowing us to reconstruct the timing of hydrothermal activity and the origin of the fluid(s).
We characterize three natural carbonate samples with elevated uranium/lead (U/Pb) ratios to demonstrate techniques improving the understanding of U incorporation in carbonates for U/Pb dating. With the rapidly accelerating application of laser ablation analyses, there is a great need for well-characterized reference materials that can serve multiple functions. Strontium (Sr) isotope analyses and U XANES demonstrate that these samples could be used as reference materials.
A new methodology for the micron-scale uranium–lead dating of carbonate minerals is proposed. It is based on the extraction of ages directly from pixel images (< 1 mm2) obtained by laser ablation coupled to a mass spectrometer. The ages are calculated with a robust linear regression through the pixel values. This methodology is compared to existing approaches.
Unetched and etched apatite grains from five samples were dated by U–Pb method using laser ablation inductively coupled plasma mass spectrometry. Our experiment indicates that etching needed for apatite fission track dating has insignificant effects on obtaining accurate U–Pb ages; thus, the laser ablation-based technique may be used for apatite fission track and U–Pb double dating.
This contribution presents a new reference material, ASH-15 flowstone with an age of 2.965 ± 0.011 Ma (95 % CI), to be used for in situ U–Pb dating of carbonate material. The new age analyses include the use of the EARTHTIME isotopic tracers and a large number of sub-samples (n = 37) with small aliquots (1–7 mg) each that are more representative of laser-ablation spot analysis. The new results could improve the propagated uncertainties on the final age with a minimal value of 0.4 %.
This paper serves as a guide to those interested in dating calcite by laser ablation. Within it are theoretical and practical limits of U and Pb concentrations (and U / Pb ratios), which would allow viable extraction of ages from calcite (and other minerals with moderate U / Pb ratios), and which type of instrumentation would be appropriate for any given sample. The method described uses a new detector array, allowing for lower detection limits and thereby expanding the range of viable samples.
Hugo K. H. Olierook, Kai Rankenburg, Stanislav Ulrich, Christopher L. Kirkland, Noreen J. Evans, Stephen Brown, Brent I. A. McInnes, Alexander Prent, Jack Gillespie, Bradley McDonald, and Miles Darragh
Using a relatively new dating technique, in situ Rb–Sr geochronology, we constrain the ages of two generations of mineral assemblages from the Tropicana Zone, Western Australia. The first, dated at ca. 2535 Ma, is associated with exhumation of an Archean craton margin and gold mineralization. The second, dated at ca. 1210 Ma, has not been previously documented in the Tropicana Zone. It is probably associated with Stage II of the Albany–Fraser Orogeny and additional gold mineralization.
U–Pb ages of zircon crystals are used to determine the provenance and depositional age of strata of the Triassic Chinle and Moenkopi formations and the Permian Coconino Sandstone of northern Arizona. Primary source regions include the Ouachita orogen, local Precambrian basement rocks, and Permian–Triassic magmatic arcs to the south and west. Ages from fine-grained strata provide reliable depositional ages, whereas ages from sandstones are compromised by zircon grains recycled from older strata.
The dating of carbonates by laser ablation inductively coupled plasma mass spectrometry is improved by an additional, newly characterised reference material and adapted data evaluation protocols: the shape (diameter to depth) of the ablation crater has to be as similar as possible in the reference material used and the unknown samples to avoid an offset. Different carbonates have different ablation rates per laser pulse. With robust uncertainty propagation, precision can be as good as 2–3 %.
Nick M. W. Roberts, Kerstin Drost, Matthew S. A. Horstwood, Daniel J. Condon, David Chew, Henrik Drake, Antoni E. Milodowski, Noah M. McLean, Andrew J. Smye, Richard J. Walker, Richard Haslam, Keith Hodson, Jonathan Imber, Nicolas Beaudoin, and Jack K. Lee
Here we review current progress in LA-ICP-MS U–Pb carbonate geochronology and present strategies for acquisition and interpretation of carbonate U–Pb dates. We cover topics from imaging techniques and U and Pb incorporation into calcite to potential limitations of the method – disequilibrium and isotope mobility. We demonstrate the incorporation of imaging and compositional data to help refine and interpret U–Pb dates. We expect this paper to become a go-to reference paper for years to come.
Bau, M., Romer, R. L., Lüders, V., and Beukes, N. J.:
Pb, O, and C isotopes in silicified Mooidraai dolomite (Transvaal Supergroup, South Africa): implications for the composition of Paleoproterozoic seawater and `dating' the increase of oxygen in the Precambrian atmosphere, Earth Planet. Sc. Lett., 174, 43–57, 1999.
Corrigan, C. M. and Harvey, R. P.:
Multi-generational carbonate assemblages in martian meteorite Allan Hills 84001: implications for nucleation, growth, and alteration, Meteorit. Planet. Sci., 39, 17–30, 2004.
Drake, H., Roberts, N. M., and Whitehouse, M. J.:
Geochronology and Stable Isotope Analysis of Fracture-fill and Karst Mineralization Reveal Sub-Surface Paleo-Fluid Flow and Microbial Activity of the COSC-1 Borehole, Scandinavian Caledonides, Geosciences, 10, 56, https://doi.org/10.3390/geosciences10020056, 2020.
Hill, C. A., Polyak, V. J., Asmerom, Y., and Provencio, P.:
Constraints on a Late Cretaceous uplift, denudation, and incision of the Grand Canyon region, southwestern Colorado Plateau, USA, from U-Pb dating of lacustrine limestone: U-Pb age of lacustrine limestone, Tectonics, 35, 896–906, 2016.
Lee, M. R., Lindgren, P., and Sofe, M. R.:
Aragonite, breunnerite, calcite and dolomite in the CM carbonaceous chondrites: High fidelity recorders of progressive parent body aqueous alteration, Geochim. Cosmochim. Ac., 144, 126–156, 2014.
Ray, J. S., Veizer, J., and Davis, W. J.:
C, O, Sr and Pb isotope systematics of carbonate sequences of the Vindhyan Supergroup, India: age, diagenesis, correlations and implications for global events, Precambrian Res., 121, 103–140, 2003.
Ring, U. and Gerdes, A.:
Kinematics of the Alpenrhein-Bodensee graben system in the Central Alps: Oligocene/Miocene transtension due to formation of the Western Alps arc, Tectonics, 35, 1367–1391, 2016.
Roberts, N. M. W. and Walker, R. J.:
U-Pb geochronology of calcite-mineralized faults: Absolute timing of rift-related fault events on the northeast Atlantic margin, Geology, 44, 531–534, 2016.
Absolute chronological constraints are crucial in Earth and planetary sciences. In recent years, U–Pb dating of carbonates has provided information on the timing of, for example, diagenesis, faulting, or hydrothermalism. These studies have targeted relatively young terrestrial carbonates up to 300 million years old. By dating 3.9 billion-year-old martian carbonates in situ using the U–Pb chronometer, we show that this system is robust in ancient samples that have had a relatively simple history.
Absolute chronological constraints are crucial in Earth and planetary sciences. In recent years,...