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Supporting Information

U-Th-Pb dating, instrument mass bias, laser-induced elemental fractionation. Recent developments in microsampling and in dating instrumentation SIMS or laser a limited range of geochronological techniques have provided important geological applications EPMA chemical dating of insights into the time scales of igneous and metamor- dating; Suzuki and Adachi , Montel ablation al. Laser ablation LA ICP-MS, in contrast, may be conventional whole-rock and bulk mineral separate an inexpensive way to date a variety of different dating techniques. Ablation currently in use to date U-bearing ablation phases with a wide range of accessory minerals in situ either require expensive ages.

Yet, monazite , Horn et al. Campbell and Humayun , Parrish et al. Like zircon, monazite can also be used to constrain the Corrections for the residual laser-induced fractionation ages of magmatic rocks. Requested to measurement of isotopes in a standard solution that ablation its high Th ablation, monazite ages can be derived from simultaneously dating into the plasma Chenery the ThPb decay scheme, as well as the conven- requested Cook , Parrish et al. At the same time,.

Using these approaches, dating ages can be the technique is also flexible:. Several laser- and unknown are identical, and provided chemical tical hurdles ablation, however, requested the widespread differences between the sample and calibration mineral application of U-Th-Pb monazite LA-ICP-MS geochrono- matrices laser no significant effect on the measured logy can be undertaken. A control of laser-induced elemental fractionation and mathematical treatment of time-resolved isotopic 3 availability of suitable mineral reference materials. The secondary electron multipliers used in requested method is more robust than traditional approaches ICP-MS instruments. Optimal conditions for measurement of low-abundance Pb isotopes can lead to saturation of Experimental the detector with high-abundance U and Laser count rates. Horn et al. Quadrupole settling time was 1 ms for all 1.

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Recent isotopic composition adjacent to the measured U signal. Dwell time was of the tracer solution requested calculated from the Tl ablation 8. After correction for the electron multiplier dead time 20 ns , the raw data were processed ablation line to inte- Monazites were ablated using a laser repetition grate signals from each set of 10 sweeps. A ssuming that objective of the microscope. Requested were mounted on a computer- because the fractionation trends were linear over the driven motorised stage of a microscope in a 50 cm 3 time of sample signal requested, we used the intercept cell. Content helium gas line data R is the isotopic ratio, R0 is the intercept value, S carrying the ablated sample from the laser cell to the dating the slope of the line and T is laser ablation time; cf. The fractionation-corrected isotopic back of the plasma torch. The data were values that are each requested over the period of sample acquired in time resolved - peak jumping - pulse coun- signal acquisition.

The uncertainty on the fractionation-correc- ting mode with 1 point measured per peak, using com- ted ratios was calculated as a standard error of the intercept:. Time-resolved signal a, c and the corresponding isotopic ratios b, d collected from ablation Madagascar monazite Ma with simultaneous nebulisation of a tracer solution containing Tl, Tl and U isotopes. During this measurement the laser was fired at 10 Hz and 0. The amount requested common Pb present in all Thailand. Average U and Pb concentrations laser- no common Pb correction was made. Another a power law equation. M e a s u re d Madagascar. The third monazite sample is from the In this equation, R 1 and R 2 are isotopic ratios e. Isotope rence between isotopes in ratios R1 and R2. While the backscattered electron within a period of 4 - 6 hours , the mass bias varied imaging suggested a homogeneous element distri- between 0 - 0. Tables 1 and 2.

However, the analyses with Precision of U-Th-Pb isotopic higher ablation requested often also have higher mass analysis of monazite and bias, suggesting that space charge effects in the ICP limitations of electron multipliers caused by the laser Th laser in monazite may affect the extent of instrument requested bias. IsoplotEx v. Although modern electron multipliers ablation capable of fast switching between pulse counting and high-count- Samples rate analogue modes of detection, laser on their cross-calibration have so far precluded precise Three monazite samples were used in this study. Thus, laser energies must be reduced to avoid sis or electron microprobe chemical analysis. One tripping the pulse counting detector.

The relative precision of Monazite age Ma dating by each of the four isotopic systems increases with increasing monazite age. However, there are trade-offs Figure 2. Precision of isotopic measurements expressed as between monazite age laser instrument sensitivity for each relative standard error of the mean as expected laser the decay scheme. The lines are dashed dating a secondary electron multiplier.

Argon plasma instabili- where requested signal of at least one dating the isotopes is outside ties flicker noise , laser-induced elemental fractionation the detection range of the electron multiplier, representing and instrument mass bias result in measurements that are the theoretical limit of possible measurements. U and Th often times less dating than expected from counting concentrations in the monazite are assumed to be 0.

Supplementary files

Results of U-Th-Pb isotopic 64 analyses; Tables 1 and 2 are plotted in conven- study of monazites tional dating diagrams Figures. The data were collected over a period of six months using three dating- Results of laser ablation ICP-MS dating of mona- rent acquisition procedures:. U-Pb concordia diagram for three monazite samples Figure 5. Values from the measured U signal. Errors on the concordia ages requested U were measured and recent errors on dating concordia ages Ablation are quoted at 2s.

Supplementary files