Nick M W RobertsGeochronology and Tracers Facility

British Geological Survey

Brief HistoryTraditional bulk dissolution methodca. 1987, Oxford Isotope Lab, Stephen Moorbath

- Pb-Pb in Archean limestone - Moorbath et al.,1987- Pb-Pb young marble – Jahn, 1988- U-Pb in coral limestone – Smith & Farquhar, 1989- Pb-Pb – metamorphism – Taylor & Kalsbeek, 1990- U-Pb - diagenesis – Smith et al., 1991

1998 – First Quaternary Dates- U-Pb in speleothem – Richards et al., 1998

2014 – First LA-ICP-MS publication- U-Pb dating of diagenetic cement – Li et al., 2014- U-Pb dating of fracture-fill veins – Roberts & Walker, 2016- U-Pb Reference Material for LA-ICP-MS – Roberts et al., 2017

See Reviews :- Jahn & Cuvellier, 1994- Rasbury & Cole, 2009- Roberts et al., 2020

The LA-ICP-MS MethodLaser ablation for sampling of 5-150 μm diameter spotsSingle age determined from >5 to 10s of spots per sampleNormalisation to a carbonate reference material- WC1 254 ± 7 Ma (2s)- Sample data are limited by this ~2.5% uncertaintyMeasurement can use Q-, SC- or MC- ICP-MS instrumentationSample requirements:- Thin sections (ideally >100μm thick)- Polished blocks- Chips in epoxy resin

Limitations (1)Low UraniumUranium is typically very low in most carbonates (< 1 ppm), but can be high (>1 ppm) in a variety of settings. The LA-ICP-MS method can be successfulat 10s ppb.

High initial PbIncorporation of Pb during crystallisation/formationof carbonates is the biggest hurdle, initial Pb is typically much higher than the Pb derived from decayof U.

Predicting high U/Pb in carbonates is not easy, and assuch, a study will often involve screening of manysamples to find those most suitable.

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Limitations (2)Initial disequilibriumA limitation for accurate dating of young (i.e. <10 Ma) carbonates, isinitial disequilibrium in the U-Pb decay chain, such as excess 234U.

We are still learning and accumulating data on the scale of thisproblem for different settings.

Regarding geochronology, speleothem research is the only fieldwhere this has been discussed in any detail.

234U is typically in excess in groundwater (rather than in secularequilibrium); a correction is used to calculate accurate 238U/206Pb ages.

If the excess 234U is not known or measurable (as is the case forsamples beyond the range of U-Th), then it has to be estimated, limitingthe confidence of U-Pb ages.

The grey histogram is a literature-based compilation of 234U/238U activity ratios. The coloured curves show the effect on 238U/206Pb ages if the initial ratio is different to 1.0. For example, a sample with a measured age of 4 Ma, will have a true age of 3 Ma if the 234U/238Uinitial was as high as 3.

Compilation of U and Pbconcentrations in different carbonates, based on measurements in the BGS lab.

Applications (1)

Since 2014, the LA-ICP-MS method has been applied to a range of different carbonate types and settings.

Most applications have been limited to calcite, but dolomite and other forms of carbonate have been successfully dated too.

As the method progresses, more reference materials comprising different matrices will be required.

• Slickenfibres• Fracture-filling veins

• syn-tectonic and post-tectonic• hydrothermal

• Breccia cement• Paleosol/caliche• Diagenetic cement• Speleothems• Authigenic limestone

see LA-CARB.COM

Published dates range from the Quaternary to the Precambrian boundary, with variable precision:

Applications (2)

Faulting, fracture-fill and fluid-flow processes

Hydrocarbon charge events

Dates from the main breccia cement within the damage zone of the ‘Frontal Faults’exposed at Flamborough Head, UK. From Roberts et al., in review, Solid Earth

Frontal Faults at Selwicks Bay, UK

2.5 cm

Vuggy calcite from deep mines in Sweden. Individual crystals grew over a protracted period from ~55 to 12 Ma.(Ellipses on the BSE images show location of spots and are coloured according to their age). From Drake et al., in prep.

Individual calcite crystals with SIMS C-O isotope measurements (yellow ellipses) and U-Pb spots (dashed ellipses). The method of SIMS C-O isotope analysis has shown remarkable variation within single crystals, here related to microbially induced anaeorobic oxidation of methane (AOM), and methanogenesis (see Drake et al., 2017, 2019). Diagram shows model for AOM in deep granite-hosted fractures, whereby extreme depletion in δ13C of calcite occurs during oxidation of biogenic methane (Drake et al., 2015). From Ivarsson et al., in prep

Applications (3)Dating of calcite within basement fractures of the Rona Ridge. The calcite mineralisation propped open the fractures, which were capped by sediments, allowing hydrocarbon migration possibly enhanced by seismic activity (graded bedding shows sediment infill within deep fractures (1-2 km). From Holdsworth et al., 2019.

Dating of hydrocarbon charge in the Namibe Basin, Angola, via bituminous calcite and dolomite.The Santonian age of the calcite overlaps that of nearby volcanism, indicating a link between magmatism, hydrocarbon migration and possibly maturation.From Rochelle-Bates et al., in prep.

References

- Drake, H., et al., 2017. Isotopic evidence for microbial production and consumption of methane in the upper continental crust throughout the Phanerozoic eon. Earth and Planetary Science Letters, 470, 108-118.- Drake, H., et al., 2015. Extreme 13 C depletion of carbonates formed during oxidation of biogenic methane in fractured granite. Nature communications, 6, 1-9.- Drake, H., et al., 2019. Timing and origin of natural gas accumulation in the Siljan impact structure, Sweden. Nature communications, 10, 1-14.- Holdsworth, R.E., et al., 2019. Natural fracture propping and earthquake-induced oil migration in fractured basement reservoirs. Geology, 47, 700-704.- Jahn, B.M. and Cuvellier, H., 1994. Pb-Pb and U-Pb geochronology of carbonate rocks: an assessment. Chemical Geology, 115, 125-151.- Li, Q., et al., U–Pb dating of cements in Mesozoic ammonites. Chemical Geology, 376, 76-83.- Jahn, B.M., 1988. Pb–Pb dating of young marbles from Taiwan. Nature, 332, 429-432.- Moorbath et al., 1987. First direct radiometric dating of Archaean stromatolitic limestone. Nature, 326, 865-867.- Nicholson, S.L., et al., 2020. Pluvial periods in Southern Arabia over the last 1.1 million-years. Quaternary Science Reviews, 229, 106112.- Rasbury, E.T. and Cole, J.M., 2009. Directly dating geologic events: U‐Pb dating of carbonates. Reviews of Geophysics, 47.- Richards, D.A., et al., 1998. U-Pb dating of a speleothem of Quaternary age. Geochimica et Cosmochimica Acta, 62, 3683-3688.- Roberts, N.M.W., et al., 2020. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb carbonate geochronology: strategies, progress, and limitations. Geochronology, 2, 33-61.- Roberts, N.M.W., et al., 2017. A calcite reference material for LA‐ICP‐MS U‐Pb geochronology. Geochemistry, Geophysics, Geosystems, 18, 2807-2814.- Roberts, N.M.W., and Walker, R.J., 2016. U-Pb geochronology of calcite-mineralized faults: Absolute timing of rift-related fault events on the northeast Atlantic margin. Geology, 44, 531-534.- Smith, P.E. et al., 1991. Direct radiometric age determination of carbonate diagenesis using U-Pb in secondary calcite. Earth and Planetary Science Letters, 105, 474-491.- Taylor, P.N. and Kalsbeek, F., 1990. Dating the metamorphism of Precambrian marbles: Examples from Proterozoic mobile belts in Greenland. Chemical Geology: Isotope Geoscience Section, 86, 21-28.

Sedimentary carbonatesFrom the very young, to the very old…

Speleothem from cave in Yemen. Base is dated at 1.04 MaFrom Nicholson et al., 2020, QSR

Spherulite (deposition) and cone-in-cone (fluid overpressure) phases of calcereous concretion dated from upper Ediacaran sediments of Finnmark.From Meinhold et al., in review, Geological Magazine

Micritic limestone, Huaibei Gp., North China Craton.

From Lan et al., in review.