. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb
geochronology of carbonate minerals, calcite in particular, is rapidly
gaining popularity as an absolute dating method. The high spatial resolution
of LA-ICP-MS U–Pb carbonate geochronology has benefits over traditional
isotope dilution methods, particularly for diagenetic and hydrothermal
calcite, because uranium and lead are heterogeneously distributed on the
sub-millimetre scale. At the same time, this can provide limitations to the method,
as locating zones of radiogenic lead can be time-consuming and “hit or
miss”. Here, we present strategies for dating carbonates with in situ
techniques, through imaging and petrographic techniques to data
interpretation; our examples are drawn from the dating of fracture-filling
calcite, but our discussion is relevant to all carbonate applications. We
review several limitations to the method, including open-system behaviour,
variable initial-lead compositions, and U–daughter disequilibrium. We also
discuss two approaches to data collection: traditional spot analyses guided
by petrographic and elemental imaging and image-based dating that utilises
LA-ICP-MS elemental and isotopic map data.

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb carbonate geochronology: strategies, progress, and limitations

An improved method of U–Pb dating of vein calcite formed during deformation is used to determine the age and cause of folding along the south coast of England. Fractures arising from folding of Late Cretaceous Chalk of southern England occurred 34.7 ± 1.7 myr ago. Underlying Jurassic strata have veins within fractures with ages of 55, 48–42, 39–37, 34–31 and 25 Ma, with 34–31 Ma being the tectonic culmination. Folding was slightly younger than the age of the youngest strata in the overlying Solent Group, suggesting that folding terminated basin sedimentation. This age of north–south shortening is inconsistent with attribution to intraplate forces from the mainly younger Alps, but is plausibly a result of the Pyrenean Orogen, which evolved from 50 to 28 myr ago with a late Eocene culmination. A modified method for carbonate laser ablation inductively coupled plasma mass spectrometry U–Pb dating is presented that uses measured 232 Th and 208 Pb as a better monitor of common Pb, and it has distinct advantages over existing methods. Initial common lead determined on samples conforms closely to model compositions calculated by earlier workers, with free regressions giving the most robust dates. The modified method is applicable to structural geology, carbonate diagenesis and to dating of carbonate relevant to palaeo-environmental and archaeological studies. Supplementary material : Details of sample locations and characteristics, photographs of sample localities, microscopic images of veins, details of the experimental procedure and U–Pb data for all samples and reference materials are available at https://doi.org/10.6084/m9.figshare.c.3997086

/pdf/vein-calcite-dating-reveals-pyrenean-orogen-as-cause-of-33l7le4jw8.pdf

Vein calcite dating reveals Pyrenean orogen as cause of Paleogene deformation in southern England

We report U-Pb absolute ages of calcite cements from a diffuse vein network documented in the Bighorn Basin (Wyoming, USA), where distinct systematic vein sets developed at the front of the thin-skinned Sevier orogen, during Laramide layer-parallel shortening, and during thick-skinned Laramide thrusting and folding. The U-Pb age distribution illustrates: (1) an outward (eastward) transmission of Sevier orogenic stress (from 89.7 ± 2.9 Ma [2σ] in the west, and from 75.3 ± 2.8 Ma in the east); and (2) an inward (westward) development of Laramide-related fracturing and folding (72 ± 3.0 Ma and 45.4 ± 1.8 Ma, respectively, in the east; 60.5 ± 4.6 Ma and 27.9 ± 1.1 Ma, respectively, in the west), which is in accordance with the known sequence of exhumation of the major Laramide basement arches. Our results also show that the stress related to Laramide compression first overprinted the stress related to Sevier compression in the sedimentary cover around major basement uplifts. This study highlights the utility of U-Pb calcite geochronology as a powerful tool for constraining complex sequences of deformation in orogenic forelands.

/pdf/u-pb-dating-of-calcite-veins-reveals-complex-stress-4bvq72ydow.pdf

U-Pb dating of calcite veins reveals complex stress evolution and thrust sequence in the Bighorn Basin, Wyoming, USA

Improving paleohydrological and diagenetic reconstructions in calcite veins and breccia of a sedimentary basin by combining Δ47 temperature, δ18Owater and U-Pb age

We present a new approach to laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) U‐Pb dating of carbonates based on selection and pooling of pixels from 2‐D elemental and isotopic ratio maps. This image mapping technique is particularly useful for targeting subdomains in samples with complex geological histories. Key major and trace elements that are sensitive to detrital components, postformational fluid ingress, mineralogical changes, or diagenetic overprinting are measured along with the Pb and U isotopic data. Laser sampling is undertaken along successive linear rasters that are compiled into maps using the Monocle add‐on for Iolite, with one pixel in the map corresponding to one time slice of the time‐resolved signal. These element, element ratio, and isotope ratio maps can be overlain over photomicrographs or scanning electron microscopy images to spatially link compositional data to textural and structural features. The pixels corresponding to likely homogeneous age domains can be isolated by applying appropriate selection criteria (e.g., Th < 0.3 ppm, Mg/Ca < 0.004) and pooled into pseudo‐analyses using a proxy for the parent/daughter ratio (e.g., 207Pb/235U, 238U/208Pb) to retrieve the largest possible spread of the data points on isochron diagrams. The approach is best suited for analytical setups capable of rapidly or simultaneously scanning over a large mass range and can yield a precision of ±1% or better on quadrupole instruments depending on U concentration, 238U/204Pb, and age of the sample. The sample‐specific filtering criteria for selection and rejection of data and their rationale can be reported, resulting in more transparency with regard to data processing.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2018GC007850

An image mapping approach to U-Pb LA-ICP-MS carbonate dating and applications to direct dating of carbonate sedimentation.

U-Pb dating of calcite is an emerging but rapidly growing field of application in geochronology with great potential to inform problems in landscape, basin and mountain belt evolution, through age determination of diagenetic cements, vein mineralisation and geological formations difficult to date otherwise. In this brief, we present isotope dilution U-Pb isotope measurements on a sample of calcite (WC-1) that has been and will continue to be used as a reference material for in-situ U-Pb Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) dating, and which is suitable to be distributed to the geochronological community. We present in-situ measurements using LA-ICP-MS to demonstrate the suitability of WC-1 for use as a U-Pb dating reference material, in spite of it not being isotopically homogeneous. The WC-1 calcite sample is 254.4 ± 6.4 Ma old, and comprised of 85 to 98% radiogenic lead. It presents a suitable reference material that can facilitate dating of calcite ranging in age from Precambrian to late Neogene age.

/pdf/a-calcite-reference-material-for-la-icp-ms-u-pb-50zxw5nca7.pdf

Christopher J. Smith

Papers

A calcite reference material for LA-ICP-MS U-Pb geochronology