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RESEARCH PRODUCT

In-situ high spatial resolution LA-MC-ICPMS 230Th/U dating enables detection of small-scale age inversions in speleothems

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Abstract We present an in-situ method for Th and U isotope measurements by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) to determine possible age inversions of stalagmites, using a 213 nm Nd:YAG laser connected to an MC-ICPMS. Due to the low ion beam intensity of 230 Th (20–120 counts per second, cps), we carefully optimized the operating parameters to get highest possible ion beam intensities, i.e., laser fluence (25 J cm −2 ), spot size (110 μm), pulse repetition rate (20 Hz), scan speed (4 μm s −1 ), integration time (1000 s), and He and Ar gas flow (∼0.9 L min −1 and ∼0.6 L min −1 respectively). A precision (2 relative standard error, 2RSE) of better than 1.8% was obtained for a single 230 Th/ 238 U measurement performed on a stalagmite from Hϋttenblӓserschachthohle, western Germany, having U concentrations of 2–7 μg g −1 and with 230 Th beam intensity of less than 100 cps. Compared to previous studies (Hoffmann et al., 2009), this is the about same precision, however at lower U concentrations. The data are corrected and calibrated by two factors ( F 1 and F 2 ) for 230 Th/ 238 U and 234 U/ 238 U, respectively, using a carbonate material (flowstone in secular equilibrium). We obtained an age uncertainty (2 SE, 2σ) of ca. 9 ka at ca. 215 ka. Most data agree with solution MC-ICPMS results obtained on the same sample within their uncertainties. The reproducibility of the LA-MC-ICPMS age data is within 4.5% (2RSE) as determined from 3 to 4 repeated analyses. With a spot size of 110 μm and spatial resolution of about 400 μm or higher, it is possible to see much more details in thin growing layers than conventional solution analysis, where mixed layer sampling cannot be avoided. Potential age inversions in small regions are revealed, which cannot be detected by solution analysis due to the insufficient spatial resolution.