Solid Earth Sciences (Mar 2017)
In-situ high spatial resolution LA-MC-ICPMS 230Th/U dating enables detection of small-scale age inversions in speleothems
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 230Th (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 230Th/238U measurement performed on a stalagmite from Hϋttenblӓserschachthöhle, western Germany, having U concentrations of 2–7 μg g−1 and with 230Th 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 (F1 and F2) for 230Th/238U and 234U/238U, 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.
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