LA-ICPMS for In Situ U–Th Dating of Holocene Stalagmites

In the research article published recently in the ACS Analytical Chemistry journal, the researchers from the Nanjing Normal University, Nanjing, China, National Taiwan University, Taipei, Taiwan, and ETH Zurich, Zurich, Switzerland presented an in situ U–Th dating approach of carbonate speleothems using laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) with a detection efficiency of 1–2%.

A newly developed in situ U–Th dating technique utilizing laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) achieves a detection efficiency of 1–2%. This technique uses a 229Th–233U–236U isotope triple spike for real-time correction of instrumental mass discrimination and U/Th elemental fractionation. The method accurately dated individual layers in natural stalagmites ranging from 210 to 1,000 years, aligning with results from traditional multi collector-ICPMS methods. This approach is promising for applications in fields like paleoclimatology and archaeology, especially for dating recent Holocene stalagmites with high precision.

The original article

LA-ICPMS for In Situ U–Th Dating of Holocene Stalagmites

Chung-Che Wu, Chuan-Chou Shen, Detlef Günther, and Bodo Hattendorf

Analytical Chemistry 2024 96 (31), 12640-12648

DOI: 10.1021/acs.analchem.4c01114

licensed under CC-BY 4.0

Selected sections from the article follow. Formats and hyperlinks were adapted from the original.

Abstract

Here, we present an in situ U–Th dating approach of carbonate speleothems using laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) with a detection efficiency of 1–2%. By online addition of a ²²⁹Th–²³³U–²³⁶U isotope triple spike to the laser-generated aerosol, instrumental mass discrimination and U/Th elemental fractionation could be monitored and corrected. With this approach, the ²³⁴U/²³⁸U and ²³⁰Th/²³⁸U activity ratios of a flowstone sample in secular equilibrium could be accurately reproduced as unity with two-sigma uncertainties ±0.053 and ±0.050, respectively. The method was used for the determination of the formation ages of individual layers in natural stalagmites ranging between 210 and 1 thousand years ago (ka). The determined ages corresponded well with those obtained using conventional solution multi collector-ICPMS techniques after isotope separation. Particularly, Holocene stalagmites, as young as 1 ka, could be accurately dated with 2 standard error of ±76 years. This developed microdomain U–Th dating approach thus can be applicable for diverse research areas, such as paleoclimatology, oceanography, geomagnetism, and archeology.

Introduction

The U–Th absolute dating system (also called ²³⁰Th or ²³⁸U–²³⁴U–²³⁰Th–²³²Th) has been widely exploited to determine the timing and the process of Earth’s geological, environmental, and biotic evolution, based on materials as young as few years to over the last 600 thousand years (ka, hereafter). It has been applied to a variety of fields, such as paleoclimatology, (1) ocean circulation, (2) tectonic and seismic processes, (3) archeology, (4) and human evolution. (5)

Conventionally, U–Th dating approaches employ a solution-based protocol involving the isolation of U and Th from dissolved samples using solid phase extraction and measured using either thermal ionization mass spectrometry (TIMS) or multi collector-inductively coupled plasma mass spectrometry (MC-ICPMS). (6−13) These methods require sample sizes containing nanogram levels of U or Th and achieve standard errors of the age at the permil or even subpermil. However, these methods are restricted to a discrete millimeter-domain sampling strategy. Furthermore, time-consuming sample preparation is required, including dissolution, spiking, extraction, and isotope preconcentration. Laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS), can provide several advantages over those conventional methods as (i) spatially resolved analyses at a scale of 10–100 μm; (ii) high sample throughout; and (iii) the reduction of polyatomic interfaces from the solvent. (14)

A pioneering study of in situ U–Th dating using LA-ICPMS was presented by Stirling et al. (15) U–Th ages of natural samples were determined by using a quadrupled (λ = 266 nm) Nd:YAG laser coupled to a first-generation MC-ICPMS (VG P54). However, samples had to contain uranium concentrations of tens to hundreds of μg/g. Eggins et al. (16) subsequently demonstrated the feasibility of profiling U–Th isotopes in teeth and bone using an ArF (λ = 193 nm) excimer laser system coupled with an Agilent 7500s quadrupole ICPMS. In 2009, Hoffmann et al. (17) developed a micromill sampling technique for stalagmite samples with uranium concentration as low as 250 ng/g; but, the age uncertainty was as high as 10–20%. Due to the lower sensitivity and precision of LA-ICPMS, (18) when compared to solution-based U–Th dating approaches via TIMS and MC-ICPMS techniques, its application remained sparse. In addition, instrumental mass discrimination, (19) interelement fractionation, (20) and detector dead time, (21) are also fundamental restrictions affecting the accuracy and precision of isotope and element ratio determinations. Thus, over the past decades, only a handful of studies have accessed LA-ICPMS for U–Th dating. (17,22−25)

Instrumental mass discrimination, or so-called mass bias, is a common artifact in ICP-MS measurements and manifests itself in that the “measured” isotope ratios differ from “true” ratios. (11,26,27) Other isotope fractionation effects, which eventually occur during LA, (28) or transport of the laser-generated aerosol, (29) include U/Th elemental fractionation in particular. (15) These effects and the resulting bias need to be considered and corrected.

In this study, we used a “jet” interface setup (30) to achieve sufficient sensitivity for detection of the low abundant ²³⁰Th⁺ in particular in combination with a ²²⁹Th–²³³U–²³⁶U triple spike (TS) admixed to the LA generated aerosol for U–Th dating. The TS was continuously added by pneumatic nebulization with desolvation to assess the magnitudes of mass discrimination and U–Th elemental fractionation. The validation and accuracy of this developed method were evaluated by analyzing a series of speleothems, including a secular equilibrium flowstone and stalagmites covering an age span between 210 and 1 ka. In particular, we address the capability of this dating approach for Holocene stalagmite samples containing <20 μg/g U and as young as 1000 years with correspondingly low atomic ²³⁰Th/²³⁸U abundance ratios <0.5 ppm.

Experimental Section

The LA-ICPMS strategy for in situ U–Th dating used in this work is schematically shown in Figure 1. Solid sampling was carried out using a 193 nm ArF excimer LA system (GeoLas C, Lambda Physik, Göttingen, Germany) and the laser ablation aerosol was mixed online with the aerosol of a ²²⁹Th–²³³U–²³⁶U triple spike (TS). (11,12) The TS was added via solution nebulization using a desolvating nebulizer system (DSN 100, Nu Instruments Ltd., Wrexham, UK) with argon as the nebulizer gas. The mixed aerosols were introduced into a sector field-inductively coupled plasma mass spectrometer (SF-ICPMS, Element XR, Thermo Fisher Scientific, Bremen, Germany). The approach of standard additions for calibrating the LA-ICPMS was first proposed by the Houk Group. (31) Here, we use this setup with the “jet” interface configuration to enhance the sensitivity (30) to achieve sufficient intensities of ²³⁰Th in natural carbonates and young Holocene samples in particular.

Anal. Chem. 2024, 96, 31, 12640-12648: Figure 1. Schematic LA-ICPMS setup in this study. Solutions (i.e., TS 229Th–233U–236U or 1% HNO3) were introduced via the desolvating nebulizer system (DSN), mixed with the laser ablated sample aerosol, and transported to the ICP torch.

Operating Conditions of LA-ICPMS for In Situ U–Th Dating Analysis

Laser ablation sampling was performed in an airtight, cylindrical ablation chamber with a total volume of 63 mm³, allowing to place samples of up to 50 mm in diameter. (32) The laser aerosol was generated and transported in a He carrier gas. A typical measurement comprised data acquisition intervals of 40 s of instrumental background and 100–300 s of sample ablation, followed by 40 s to monitor the aerosol washout. To avoid artifacts caused by the higher ablation rate at the onset of the ablation, the first 5–10 s of each sample acquisition were excluded from the data evaluation.

To ensure stable signal intensities from the DSN, measurements were only carried out after a warm-up time of >6 h. The nebulizer Ar gas flow, hot gas, and membrane Ar gas flows were optimized on a daily basis, and typical values are listed in Table 1.

Anal. Chem. 2024, 96, 31, 12640-12648: Table 1. Operating Settings of the LA-ICPMS with Desolvated Solution Nebulization.

The ICPMS was stabilized with plasma on for at least 1 h, and the carrier gas flow rate was optimized by ablating NIST SRM 612 silicate glass using 16 μm spot size, 10 Hz laser repetition rate, and a scan speed of 2 μm/s. The ICPMS was operated at low-resolution mode (M/ΔM = 300). During the optimization, the ion optics were adjusted to ensure symmetric peak shapes before each analysis sequence or when necessary, and the mass offset was checked to ensure that the integration window was at the peak center.

The method was evaluated for two specific settings: plasma operating conditions, by maintaining a ThO⁺/Th⁺ intensity ratio below 1.5% and a ²³⁸U⁺/²³²Th⁺ sensitivity ratio near 1 when ablating NIST SRM 612 (further on referred to as “robust”), or optimization for the highest sensitivity of ²³²Th⁺, without restraints regarding ThO⁺ abundance and the ²³⁸U⁺/²³²Th⁺ intensity ratio. For the latter, the sensitivity for ²³²Th⁺ was 45% higher and the ²³⁸U⁺/²³²Th⁺ sensitivity ratio for NIST SRM 612 increased to ≈1.35. The purpose of this investigation is to evaluate whether U/Th sensitivity ratios from the triple spike could be used to monitor those from the laser aerosol.

To make use of the “jet” interface, an additional flow of 15–30 mL/min of N₂ gas was added to the aerosol to enhance sensitivity (Figure 1). Typical operating conditions are summarized in Table 1.

Despite the fact that the peak tailing of the instrument used can be improved by adjusting the retardation filter before the detector, (11,33) we did not consider this strategy. The reason was that we observed a substantial decrease in ion signal intensities with only moderate improvement in abundance sensitivity (Figure S1). Due to the fact that the count rate for ²³⁰Th dominated the uncertainty in the determined ages while ²³²Th was not abundant in the samples analyzed, we opted for higher signal intensities.

Standards and Carbonate Samples

Standard reference material, NIST SRM 612 with certified mass fractions of Th: 37.8 μg/g and U: 37.4 μg/g, (34) was used for optimization and to determine the extent of U–Th elemental fractionation for the laser-generated aerosol. The measured ²³⁸U⁺/²³²Th⁺ intensity ratio was normalized to the reference value of 0.957 for ²³⁸U⁺/²³²Th⁺ in NIST SRM 612 to yield a correction factor that was further applied to the carbonate samples. A second glass standard, NIST SRM 610, with 10-fold higher mass fractions of Th and U than NIST SRM 612, was used to estimate the magnitude of peak tailing.

A suite of natural geological carbonate materials, with ages spanning from secular equilibrium (>800 ka) to as young as 1 ka, were analyzed in this study (Table 2). A subsample of a 1.9-million-year-old secular equilibrium flowstone, WM-H2013, collected from the same stratigraphic layer of previously published sample WM1, (35) Wilder Mann Cave, Austria, was used to validate the approach. Stalagmite, YK23, with a deposition interval from 206 to 172 ka, was obtained from Yangkou Cave (29°020’N, 107°110′E), southwestern China. (36) Another stalagmite, CC\99-3-LONG-TR, was acquired from Crevice Cave (37°45′N, 89°50′W), southeastern Missouri, USA, and its formation age ranges from 83 to 65 ka. (37) Holocene stalagmites, TK16, TK07, and TK40, with formation ages from 10 to 1 ka, were collected from Klang Cave (8°200′N, 98°440′E), southern Thailand. (38,39) All speleothem ages were previously determined using the conventional solution MC-ICPMS U–Th dating approach in the HISPEC laboratory, National Taiwan University. (11)

Anal. Chem. 2024, 96, 31, 12640-12648: Table 2. Descriptions of Samples Used in This Study.

Reagents and Triple Spike Solutions

All solutions were prepared gravimetrically using subboiled nitric acid in cleaned PP vials (Sarstedt, Nümbrecht, Germany). All standards were prepared in 1% (w/v) nitric acid and high-purity water (UPW, Milli-Q Element water system, Millipore).

The ²²⁹Th–²³³U–²³⁶U triple spike solution was prepared from the ²³³U–²³⁶U isotope reference material, IRMM-3636 (Institute for Reference Materials and Measurements, Belgium), and a ²²⁹Th standard, NIST 4328C (Standard Materials Preparation Center, Ames Laboratory, USA). Following the methods given in Cheng et al., (12) the composition of the TS was determined as ²³⁶U/²³³U = 0.981345 ± 0.000025, ²³⁸U/²³³U = 0.00048184 ± 0.00000020, ²³⁵U/²³³U = 0.000052993 ± 0.000000055, and ²³⁴U/²³³U = 0.00036015 ± 0.00000040. The molar fractions of the TS solution used were 0.00813 pmol/g of ²²⁹Th, and 0.27216 pmol/g of ²³³U, yielding a total activity of <0.04 Bq/g.

Isotope Ratio Measurement Protocol

The SF-ICPMS used in this work exhibited a pronounced tailing near the base of the mass peaks (Figure S2), which can lead to detectable contributions from intense ion beams to the ion signals at positions as far as 4 m/z units away. This can lead to substantial overestimation of not only the ion signal of ²³⁰Th⁺ in particular but also the TS isotopes and ²³⁴U⁺. It was compensated by using intensity ratios determined at 6 m/z position relative to ²³²Th⁺ or ²³⁸U⁺ while ablating NIST SRM 610. Peak tailing from ²³⁸U⁺ and ²³²Th⁺ was observed to be appreciably symmetric and highly similar (Figure S2). Tail corrections were thus carried out by assuming an identical contribution from the low and high mass sides of the ²³²Th⁺ and ²³⁸U⁺ peaks. Intensity ratios for m/z 239 and 240 relative to ²³⁸U⁺ were used to correct for peak tailing at positions of +1 and +2 m/z distance while correction factors for −1, −2, −3, and −4 m/z differences were determined via intensity ratios measured for m/z 231, 230, 229, and 228 relative to ²³²Th⁺. This assumes that ²²⁹Th⁺ and ²³³U⁺ are not present in NIST SRM 610, while there is no indication in the spectra for a substantial abundance of ²³⁰Th⁺ from the decay of ²³⁸U. All steps involved in the data reduction are outlined in the Supporting Information.

For analyses of the carbonates, the integration times were adjusted to account for isotopic abundance, in particular, for ²³⁰Th⁺ to improve counting statistics (see Table 1).

Bias Corrections

Previously published LA U–Th dating protocols usually determined mass discrimination factors through (a) the LA-aerosol of ²³⁵U/²³⁸U from NIST 61x series standards (x = 0, 1, 2); (15,22,23) (b) a secular equilibrium standard; (17) and (c) canonical ²³⁸U/²³⁵U ratios of 137.88 for natural samples. (22)

Here, a new protocol using ²²⁹Th–²³³U–²³⁶U TS admixed to the laser aerosol was used to assess the mass discrimination via the measured ²³⁶U⁺/²³³U⁺ intensity ratio and their known molar concentration ratio with the reference value of 0.981345. (11) For mass discrimination correction, we used the exponential law (40) and all measured intensity ratios were adjusted using the same factor. (10,41,42)

The U⁺/Th⁺ intensity ratios after correction for mass discrimination were further normalized by the relative change in the TS ²³³U⁺/²²⁹Th⁺ intensity ratios and the offset of the NIST SRM 612 ²³⁸U⁺/²³²Th⁺ ratio that was measured multiple times in the same sequence.

Data Acquisition and Processing

Ion signals at m/z 229, 230, 231, 233, 234, 235, and 236 were acquired using pulse counting mode and m/z 238 in analog mode, while m/z 232 was monitored either by pulse counting for the carbonate samples or analog mode for the NIST SRMs. To avoid artifacts from detector cross-calibration for natural samples, all U–Th ages were calculated based on the intensities measured for ²³⁵U by pulse counting multiplied by 137.818 (±0.65‰, 2σ) (43) to determine ²³⁸U abundances. The dead time of the electron multiplier detector was monitored, and the value did not change significantly over a period of six months. Data processing included subtraction of instrumental blanks followed by correction for peak tailing, instrumental mass discrimination, as well as U/Th elemental fractionation (details as in Supporting Information). Uncertainties from all processing steps were propagated to the U–Th age calculation and are given as two standard errors (2 S.E.) unless otherwise noted. The decay constants used for the age calculations were 9.1705 × 10^–6 yr^–1 for ²³⁰Th, 2.8221 × 10^–6 yr^–1 for ²³⁴U, (12) and 1.55125 × 10^–10 yr^–1 for ²³⁸U. (44)

Results and Discussion

Detection Efficiency

The detection efficiency (DE) is the fraction of ions detected per atom introduced into the ICP. The DE for laser ablation sampling was estimated by using confocal microscopy (PLuneox, Sensofar, Terrassa, Spain) to measure the volumes of ablation materials on NIST SRM 612 (crater size 16 μm, repetition rate 20 Hz, and scan speed 2 μm/s) and a carbonate flowstone (crater size 90 μm, repetition rate 20 Hz, and scan speed 5 μm/s) and assuming quantitative material transport from the ablation site to the ICP. The value obtained with this approach thus rather reflects the lower limit of the actual DE.

To calculate the DE for U, a homogeneous distribution in the materials was assumed, which is reasonable for the NIST SRM 612. For the natural flowstone, we used a U content of 3.8 μg/g, which is the mean value previously determined for the same specimen using solution-based analyses (in the laboratory of HISPEC, unpublished data). The DE for the DSN was determined by aspirating a U standard (Merck AG, Darmstadt, Germany) using a pneumatic nebulizer (MicroMist) with an aspiration rate of 1.3 μL/s and assuming quantitative transfer of the dissolved U to the ICP.

After the optimization for LA, DSN, and ICPMS, DEs for LA aerosol were 1.2 and 1.7% for NIST SRM 612 and flowstone, respectively, and 1.3% for the DSN with a 0.01 ng/g U standard. These results indicate that the different sample aerosols are detected with similarly high absolute sensitivity.

Instrumental Background

Background intensities were recorded for 40 s with the laser off prior to each run, under the same operating parameters as during laser ablation. Background intensities measured were typically <1 cps at m/z 230, <300 cps at m/z 232, <10 cps at m/z 234, <100 cps at m/z 235, and ≈1000 cps at m/z 238. Based on these values, the contribution of the instrumental background to the age of a 200-ka stalagmite sample, with Th contents of 1–10 ng/g and U contents of 1.9–3.3 μg/g, (36) as an example, would count for less than 3.5% to the ²³⁰Th⁺ ion signal, <0.02% to ²³²Th⁺, <1% to ²³⁴U⁺, <0.2% to ²³⁵U⁺, and 0.02% to ²³⁸U⁺, in our experiments (90 μm diameter circular laser spot, 20 Hz repetition rate, and 5 μm/s scan speed).

Mass Spectral Peak Tailing

The correction factors for peak tailing were obtained by analyzing NIST SRM 610 while introducing a blank sample (1% HNO₃) instead of the TS. Ion signals for ²³²Th⁺ and ²³⁸U⁺ were recorded in analog detection mode and all other signals using pulse counting. The resulting intensity ratios of m/z 228, 229, 230, and 231 relative to ²³²Th⁺ were 1.74 × 10^–6, 3.20 × 10^–6, 5.57 × 10^–6, and 2.26 × 10^–5 respectively. Intensity ratios of m/z 239 and 240 relative to ²³⁸U⁺ were 2.26 × 10^–5 and 6.01 × 10^–6.

Instrumental Mass Discrimination

The instrumental mass discrimination observed for the TS solution exhibited an unusual behavior for a sector field ICPMS. While typically the heavier isotope has a higher transmission in the MS, the opposite was observed here (Figure 2A, B). Even without laser ablation sampling, the measured mean intensity ratio ²³⁶U⁺/²³³U⁺ was always lower (0.973261) than the reference value (0.981345) (Figure S3). This unusual behavior is most likely a result of the low overall mass discrimination seen with the “jet” interface (45) and the E-scan acquisition mode. This was verified by comparing the data from the normally used electrostatic mass scan (E-scan) with results obtained by a magnetic mass scan (B-scan) for the TS ²³⁶U⁺/²³³U⁺ intensity ratios. The B-scan value was 0.983169 (see Figure S3), which was more in line with the expected mass discrimination of a sector field ICPMS and barely distinguishable from the reference value. An even stronger deviation of the TS mass discrimination was observed when also laser aerosol was introduced, which could be caused by the respective operating conditions and was not studied in more detail. To allow for a faster scan rate during the laser ablation analyses, we opted for the E-scan, assuming that mass bias correction by the exponential law is also applicable to inverse mass discrimination. When introducing the TS together with the laser-generated aerosol from NIST SRM 612, no substantial variation of the measured ²³⁶U⁺/²³³U⁺ for the TS was observed when varying the aerosol mass load or for changing the ICP operating conditions from robust settings (i.e., ²³⁸U⁺/²³²Th⁺ sensitivity ratio near 1, Figure 2A) to settings that yielded the highest sensitivity for Th (²³⁸U⁺/²³²Th⁺ ≈ 1.35, Figure 2B).

Anal. Chem. 2024, 96, 31, 12640-12648: Figure 2. Instrumental mass discrimination for the 229Th–233U–236U TS and NIST SRM 612 obtained by varying the mass load of the laser-generated aerosol. (A) 238U+/232Th+ sensitivity ratio ≈1 and (B) 238U+/232Th+ sensitivity ratio ≈1.35 and maximum sensitivity for 232Th+. Top panels shows intensity ratios for the TS uranium isotopes 236U+/233U+ and the bottom panels for NIST SRM 612 238U+/235U+ together with the sensitivity obtained for 238U+ (gray bars) under these conditions. Reference values for TS and NIST SRM 612 are indicated by the dotted lines.

The measured intensity ratio ²³⁸U⁺/²³⁵U⁺ obtained for NIST SRM 612 in these experiments (Figure 2A, B) was always found to be higher than the reference value, which is assumed to be predominantly related to the mixed detection mode. While ²³⁵U⁺ was recorded using pulse counting, ²³⁸U⁺ had to be measured in analog mode.

These results indicate that mass discrimination of the uranium isotopes is barely affected by changes in the mass load of laser sampling. However, the detection mode of ICPMS appeared to have a much stronger impact. Correction for instrumental mass discrimination is preferably carried out using isotopes measured in the same detector mode which is, in this case, possible via the TS ²³⁶U⁺ and ²³³U⁺ ion signals.

U–Th Elemental Fractionation

U–Th elemental fractionation for the LA and TS aerosols was found to be changing with the mass load introduced to the ICPMS. The effect was less pronounced when using robust ICP operating conditions (Figure 3A); in contrast, conditions for maximum sensitivity led to (a) a substantial increase in the U/Th sensitivity ratio, and (b) a gradual decrease of U⁺/Th⁺ intensity ratios when the mass load was increased (Figure 3B). It is not clear at this stage why there is a loss of U relative to Th when increasing the mass load to the ICP but the effect is seen for both the TS and the laser-generated aerosol. It is also in line with observations by Kroslakova and Günther, (46) who observed a relative sensitivity loss of U⁺ vs Th⁺ in LA-ICPMS experiments when the mass load in the ICP was increased.

Anal. Chem. 2024, 96, 31, 12640-12648: Figure 3. U–Th elemental fractionation for the 229Th–233U–236U TS and NIST SRM 612 obtained for varying the mass load of the laser generated aerosol. (A) 238U+/232Th+ sensitivity ratio ≈1, and (B) 238U+/232Th+ sensitivity ≈1.35 and maximum sensitivity for 232Th+. Top panels show intensity ratios for the TS 233U+/229Th+ and the bottom panels for NIST SRM 612 238U+/232Th+ together with the sensitivity obtained for 238U+ (gray bars) under these conditions. Reference values for TS and NIST SRM 612 are indicated by the dotted lines.

Irrespective of the optimization, however, the fractionation for the laser-generated aerosol appeared to be more pronounced than for the triple spike. Conditions yielding a U/Th sensitivity ratio near 1 (Figure 3A) showed less than 5% offset in the ²³³U⁺/²²⁹Th⁺ intensity ratios, while the LA sensitivity ratios ²³⁸U⁺/²³²Th⁺ were higher than the reference by up to 12%. For conditions providing the highest sensitivity, the ratio observed for TS was offset by 16–24% while ratios from LA were found to be up to 50% higher than the reference value. The correlation between the normalized sensitivity ratios is shown in Figure 4 for both optimization conditions and spot sizes >32 μm. For conditions yielding a sensitivity ratio of ²³⁸U⁺/²³²Th⁺ near 1, a slope of 0.93 was obtained, which increased to 1.7 when maximizing the sensitivity for Th. For dating purposes, we thus opted for ICPMS operating conditions with a ²³⁸U⁺/²³²Th⁺ sensitivity ratio near 1 because the elemental fractionation observed with LA closely resembled that for the TS.

Anal. Chem. 2024, 96, 31, 12640-12648: Figure 4. Correlation of U/Th elemental fractionation for laser aerosol 238U+/232Th+ and TS 233U+/229Th+ under different ICP operating conditions. Measured sensitivity ratios are normalized to the respective reference values. Representative spot sizes of 32, 44, 60, and 90 μm were performed under robust plasma operating conditions and maximizing sensitivity 232Th+, respectively.

Secular Equilibrium Sample

The accuracy of the U⁺/Th⁺ intensity ratios obtained for carbonate samples was examined by using a natural sample that is sufficiently old to have reached secular equilibrium so that ²³⁴U/²³⁸U and ²³⁰Th/²³⁸U activity ratios are 1 across the sample. It is, however, difficult to verify by visual or chemical criteria whether the ablated material originated from a closed system of the carbonate. A consistent set of data from individual measurements, however, should allow us to infer true secular equilibrium.

Results of replicate analyses of the flowstone WM-H2013, are presented in Figure 5 and Table S1. The mean of the activity ratios [²³⁴U/²³⁸U]_activity and [²³⁰Th/²³⁸U]_activity were 1.035 ± 0.053 (n = 25, 2 S.D.) and 1.003 ± 0.050 (n = 25, 2 S.D.), respectively. Without U/Th elemental correction using the TS, the [²³⁰Th/²³⁸U]_activity was only 0.928 ± 0.031, indicating that elemental fractionation had occurred to some extent during the analysis of the carbonate sample but could be successfully corrected for. The corrected data had a within-run precision ranging from ±1.9 to ±5.9% for [²³⁴U/²³⁸U]_activity and between ±2.0 and ±7.0% (2 S.E.) for [²³⁰Th/²³⁸U]_activity. Experiments were carried out with a crater size of 90 μm, a repetition rate of 20 Hz, and a scan speed of 5 μm/s.

Anal. Chem. 2024, 96, 31, 12640-12648: Figure 5. Repeat measurements of (A) [234U/238U]activity and (B) [230Th/238U]activity together with the respective atomic abundance ratios for the secular equilibrium flowstone WM-H2013. In panel (B), green and light green dots, respectively, denote 230Th/238U ratios with and without U/Th elemental correction using the TS. Error bars indicate within-run precision (2 S.E.). Solid and dashed lines denote the mean and the repeatability (2 S.D.).

Natural Holocene Stalagmite Samples

Five stalagmite samples (Table 2), which had previously been dated using solution-based MC-ICPMS analyses (36,38,39) were used to further validate the proposed approach. The samples were analyzed using line scans that were placed as close as possible and in parallel to deposition layers whose ages had been determined before. Ablation was carried out using the previously optimized robust operating conditions (Table 1) and ablated under 90 μm spot size, 20 Hz laser repetition rate, and a scan rate of 5 μm/s. Measurements typically lasted for 120 s and consumed less than 40 μg of the carbonate samples.

For YK23, the U–Th age determined by LA-ICPMS was 198 ± 38 ka, which agrees with the published solution MC-ICPMS date of 193.46 ± 0.99 ka (Figure 6A and Table S2). (36) For CC\99-3-LONG-TR, LA U–Th ages were determined at depths of 40 and 55 mm from the top, resulting in ages of 65.3 ± 2.1 and 71.7 ± 4.2 ka, which differed by less than 3% from solution ages of 67.33 ± 0.19 and 73.23 ± 0.22 ka by MC-ICPMS (Figure 6A and Table S2). The reason for the higher uncertainty in the YK23 data would be due to a larger error of the propagation process.

Anal. Chem. 2024, 96, 31, 12640-12648: Figure 6. Comparison of U–Th dates for natural stalagmite samples obtained by the LA-ICPMS setup (this study) and conventional solution MC-ICPMS. The dark red line indicates a 1:1 relationship. The gray dots and dashed line indicate the dates and their regression line without U/Th elemental correction using the TS. (A) All samples with ages from modern up to 210 ka. (B) Only Holocene samples (<10 ka). Error bars indicate the propagated within-run precision (2 S.E.).

Due to the high sensitivity using the “jet” interface, particular focus was laid on Holocene stalagmite samples (<10 ka) with correspondingly low ²³⁰Th abundance. The young parts of samples TK16, TK07, and TK40 were analyzed at distances corresponding to previously determined U–Th ages of 1–10 ka. (38,39) The results obtained (Figure 6B and Table S2) were in good agreement with the reported values. For example, for one depth at 24.5 mm from top of stalagmite TK16, three duplicate LA-ICPMS U–Th ages were 1.060 ± 0.076, 1.130 ± 0.086, and 1.130 ± 0.086 ka, deviating by less than 5% from the reported age of 1.1142 ± 0.0024 ka. (38) Another 1-ka stalagmite, TK07, analyzed two duplicate LA U–Th dates at 4 mm from the top, yielded LA-ICPMS U–Th ages of 1.83 ± 0.10 and 1.847 ± 0.088 ka, in line with a solution MC-ICPMS age of 1.8031 ± 0.0060 ka. (39) At depths of 18 and 38 mm, TK07 yielded LA-ICPMS ages of 2.53 ± 0.15 and 5.77 ± 0.28 ka, comparable to the solution MC-ICPMS age of 2.758 ± 0.020 and 5.642 ± 0.022 ka, respectively. (39) Without the U/Th elemental correction using the TS on the other hand, all dates were underestimated by 2.5% for the youngest 1 ka samples and up to 35% for the 210 ka stalagmite samples (Figure 6).

Conclusions

An LA-ICPMS method for direct, spatially resolved dating of speleothems was developed and characterized in this work.

A high sensitivity “jet” interface and online addition of a dissolved isotope TS containing defined amounts of ²²⁹Th, ²³³U, and ²³⁶U were evaluated for direct determination of the abundance ratios of ²³⁰Th, ²³²Th, ²³⁴U, and ²³⁵U in carbonate samples. The online addition of the TS allowed for a direct assessment of and correction for changes in instrumental mass discrimination and U/Th elemental fractionation. Previous applications of LA-ICPMS for U–Th dating had to rely on, for example, correcting mass discrimination via naturally occurring uranium isotopes with highly differing abundances and correction for U/Th elemental fractionation using silicate glass standards. The TS on the other hand allows to correct instrumental artifacts because its ion signals are always measured parallel with the target isotopes of thorium and uranium. The effectiveness of this approach became evident in the substantially smaller deviations when applying online U/Th correction. The unaccounted loss of Th in that case caused the resulting ages to be underestimated by as much as 35% for the 210 ka sample. For Holocene samples, the deviation was between 2.5 and 10%. Using the TS for the U/Th elemental correction, on the other hand, all determined ages were in agreement with those obtained by established solution MC-ICPMS analyses after matrix separation. The precision of the LA-ICPMS method, however, is substantially poorer than the established MC-ICPMS analyses, which is attributed to matrix effects as well as the sequential isotope detection of the instrument used.

An additional challenge arose from the fact that more isotopes needed to be monitored, which reduced the signal correlation and counting time for ²³⁰Th in particular. Using a multicollector instrument with sufficient sensitivity would certainly alleviate this issue.

The method allowed us to determine the ages of different stalagmite samples covering a time span between 210 and 1 ka and U-contents between 1 and 20 μg/g. Ages could be reproduced with accuracies of <10% in all cases and <4% in most of them. Additionally, the ²³⁴U/²³⁸U and ²³⁰Th/²³⁸U activity ratios of a flowstone sample in a secular equilibrium and could be reproduced accurately using this LA-ICPMS approach. The age uncertainties are certainly larger than attainable with the conventional time and labor-intensive solution-based approach, but the accuracy obtained here should allow for a more rapid and less destructive age classification of samples than the conventional techniques.

It allows for an accurate reconstruction of age profiles in Holocene carbonates in particular and thus enables to identify the formation period of specific regions of interest, which contain specific markers as identified by complementary techniques. For example, combining isotope proxies of climate transitions or trace elements indicating volcanic activity may then assign an age more precisely with higher spatial resolution. The onset or termination of Holocene global-extensive climatic anomalies, such as the 9.3, 8.2, 5.5, 4.2, and 2.8 ka events, (47,48) could thus be more accurately extracted and constrained. Further applications include microdomain sampling studies, i.e., dating of thin-layers and slow-growing rate samples, where the spatial resolution is crucial, or small, archeological samples, where sample consumption needs to be minimized. We expect that this approach can make substantial contributions to various fields such as paleoclimatology, oceanography, geomagnetism, and archeology.