Accuracy and precision of U–Pb zircon geochronology at high spatial resolution (7–20 μm spots) by laser ablation-ICP-single-collector-sector-field-mass spectrometry

Abstract

Use of small spots (≤20 μm) for laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U–Pb zircon geochronology is of increasing interest in the Earth sciences because the temporal record of geologic processes is often preserved on a fine scale within zircon grains. However, the systematic biases and external sources of uncertainty of U–Pb ages is poorly defined when measured on small spots by LA-ICP-single-collector-sector-field (SF)-MS instrumentation. This study addresses the accuracy and precision for small spots and specifically the extent to which short ablation times limit Pb/U Down-Hole Fractionation (DHF), which largely controls the accuracy of the U–Pb ages. Six zircon reference materials (91500, FC-1, R33, Temora 2, Plešovice, and Fish Canyon Tuff) were measured on spot sizes of 20, 15, 10, and 7 μm diameter. Laser fluence was increased from 3 to 6 J cm⁻² with decreasing spot size to compensate partially for decreasing U and Pb signals. 91500 zircon was the calibration reference material. Raw count rate data were processed using Iolite version 3.63 software with the U–Pb common approach data reduction scheme and smoothed cubic spline DHF correction model. Samples were ablated for 30 seconds and results processed for the first 28, 15, 10, and 7 seconds of ablation (masking the initial 2 seconds) in order to assess the accuracy and precision of U–Pb ages as a function of ablation time. Measured ²⁰⁶Pb/²³⁸U ratios for the six zircon reference materials increase steadily with ablation time, reflecting DHF, but exhibit somewhat different patterns of increase for different zircons, producing the major source of uncertainty for the U–Pb ages. A secondary source of uncertainty is differences between the ²⁰⁶Pb/²³⁸U (normalized to their accepted values) for different zircons near the start of ablation, which may reflect matrix-dependent instrumental mass bias in the ICP. Nonetheless, processing data from only the first 10 to 15 seconds of ablation (50 to 75 laser pulses) restricts the extent of DHF and time-resolved Pb/U variations between different zircons to a sufficient degree to give concordant U–Pb ages on 20 to 7 μm spots that are accurate and precise to better than 1.4% using LA-ICP-single-collector-SF-MS instrumentation.

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Introduction

U–Pb zircon geochronology is an important application of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) in the Earth sciences. Advances in the technologies commonly employed for the technique, particularly argon fluoride (ArF) excimer lasers, two-volume, rapid-response ablation cells, and more efficient ion extraction systems for mass spectrometry have permitted U–Pb zircon age measurements on an increasingly smaller spatial scale. Examples of recent LA-ICP studies using quadrupole MS,^1–4 single-collector sector-field (SF)-MS^5–10 and multi-collector SF-MS^10–13 commonly report U–Pb zircon ages on ablation spots of 35 to 25 μm width with depths of comparable to smaller length scales. With these ablation volumes, LA-ICP-MS U–Pb ages on zircon reference materials commonly achieve an accuracy of ca. 2% relative to U–Pb ages determined by isotope dilution – thermal ionization mass spectrometry (ID-TIMS) and an internal precision (measurement repeatability) on ²⁰⁶Pb/²³⁸U and/or ²⁰⁷Pb/²⁰⁶Pb ages of ca. 1% absolute.^12,14,15

There is much interest in increasing the spatial resolution of U–Pb zircon age analyses even further, using laser spot sizes of 20 μm or less.^{6,7,11,16–20} This is driven by the desire of geologists to analyze specific domains such as tiny inherited components within magmatic zircons and the thin rims of metamorphic zircons, or to analyze very fine-grained zircon grains formed, for example, as a result of rapid crystal growth in quenched magmas and as silt-sized detrital grains contained in mudstones. However, the accuracy and precision of LA-ICP-MS U–Pb ages as a function of laser fluence and ablation time for spot sizes of ≤20 μm are poorly defined, particularly for LA-ICP-quadrupole MS and single-collector SF-MS instruments.

LA-ICP-MS U–Pb zircon geochronology using spot sizes of ≤20 μm presents specific challenges. In particular, a limiting factor to the accuracy and precision of the U–Pb age measurements is progressive, volatility-controlled fractionation between radioactive ²³⁸U and ²³⁵U and radiogenic ²⁰⁶Pb and ²⁰⁷Pb as the ablation pit deepens.^21–24 This phenomenon is referred to as Down-Hole Fractionation (DHF) and is accelerated in analyses acquired for small-diameter laser spots, which have greater aspect (depth/width) ratios than large-diameter spots^24–26 for a given ablation time. DHF is also known to be matrix-dependent, even for different zircon specimens,^{3,4,9,27–30} so that some common zircon reference materials may not be appropriate calibration reference materials for unknown zircon using small-diameter laser spots. A related issue for LA-ICP-MS U–Pb zircon geochronology using small spots is the application of a laser fluence that removes sufficient quantities of U and Pb from the ablation pit per laser pulse to provide acceptable counting statistics and measurement precision, given that high fluence (5 J cm⁻²) has been reported to increase DHF of Pb/U in zircon and decrease accuracy of resulting U–Pb ages.⁴

This study examines the accuracy and precision for U–Pb ages measured on 20, 15, 10, and 7 μm static laser spots in zircon reference materials 91500, FC-1, R33, Temora 2, Plešovice, and Fish Canyon Tuff using an ESI NWR193 ArF excimer laser ablation system with a two-volume (TwoVol2) sample chamber coupled to a Nu AttoM ICP-SF-MS employing a single ion counter (IC) detector. The results demonstrate that the most accurate and precise ages are achieved by excluding the initial 2 seconds of ablation, which are unstable, and then integrating only the next ∼10–15 seconds of the analyte signals, which are characterized by the least Pb/U DHF.

Experimental

Zircon reference materials

The zircon reference materials analyzed in this study all have well-characterized U–Pb ID-TIMS ages, which are used to assess the accuracy of the LA-ICP-MS U–Pb age results. The zircon reference materials cover a wide range of U concentration, source rock lithology and age, providing a variety of U and Pb signal intensities and U/Pb DHF responses. Detailed information for each zircon is given in Table 1. Grain separates of each zircon were mounted in epoxy resin and polished with diamond paper to a flat surface in a single, 25 mm diameter epoxy round block.

Table 1 Host rock information for zircon reference materials with preferred ID-TIMS ages and U concentration ranges^a

Zircon	Host rock	ID-TIMS Ages (Ma, ±2 s)		U range (mg g^−1)	References
		^206Pb/^238U	^207Pb/^206Pb
a Ages (Ma = million years) determined by isotope dilution thermal ionization mass spectrometry (ID-TIMS) with 2-sigma uncertainties (95% confidence limits, univariate normal). CA-TIMS are ID-TIMS ages determined after chemical abrasion to remove discordant domains of zircon. Listed U concentrations are for zircon grains untreated by chemical abrasion.
91500	Porphyroblastic syenite gneiss with pegmatite, Kuehl Lake, Ontario, Canada	1062.4 ± 0.8	1065.4 ± 0.6	70–90	Wiedenbeck et al.^31
FC-1	Olivine gabbroic anorthosite, northern Duluth Complex, Forest Center, Minnesota. USA	1099.9 ± 1.1	1099.0 ± 0.6	218–1510	Paces and Miller^32
R33	Biotite-hornblende monzodiorite in 60 m thick dioritic dyke, Braintree Complex, Vermont, USA	419.26 ± 0.39 (ref. 33)	422.37 ± 0.36 (ref. 34)	103–398 (ref. 33)	Black et al.,^33 Mattinson^34 (CA-TIMS)
Temora 2	Middledale gabbroic diorite, Lachlan Orogen, New South Wales, Australia	416.78 ± 0.33 (ref. 33)	420.13 ± 0.30 (ref. 34)	82–320 (ref. 33)	Black et al.,^33 Mattinson^34 (CA-TIMS)
Plešovice	Potassic syenitic to granitic granulite, Bohemian Massif, Czech Republic	337.16 ± 0.11 (ref. 15)	337.96 ± 0.61 (ref. 15)	465–3084 (ref. 35)	Sláma et al.^35 (CA-TIMS), ages recalculated by Horstwood et al.^15 with updated EARTHTIME tracer spike calibration
Fish Canyon Tuff	Dacite ignimbrite, San Juan Mountains, Colorado, USA	28.478 ± 0.024	Precise data not available	214–845	Schmitz and Bowring^36

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Instrument and experiment set-up

The zircon grains were analyzed in the Mineral Isotope Laser Laboratory (MILL) at Texas Tech University. Locations of spots for LA-ICP-MS analysis were chosen based on high-contrast backscattered electron (BSE) imaging to avoid fractures, inclusions, and growth domain boundaries. Table 2 gives the instrumentation and experimental conditions used in this study.

Table 2 Instrument conditions and experimental set-up parameters

ICP-MS instrument
Make, model & type	Nu Instruments AttoM ICP-MS
Sample introduction	Laser ablation
RF power (W)	1300 W
Cool gas (Ar; l min^−1)	13.0 l min^−1
Aux gas (Ar; l min^−1)	0.85 l min^−1
Ar make-up gas flow (l min^−1)	0.7 l min^−1
Detection system	MasCom Electron Multiplier
Laser ablation system
Make, model & type	ESI NWR193 193 nm ArF excimer laser
Ablation cell and volume	TwoVol 2 (two volume ablation chamber)
Laser	Coherent ExciStar XS excimer laser
Pulse width (ns)	5 ns
Fluence (J cm^−2)	3–6 J cm^−2
Repetition rate (Hz)	5 Hz
Laser spot size (μm)	7, 10, 15, 20 μm diameter
Sampling mode/pattern	Static ablation/circular
Carrier gas	He
Cell carrier gas flow (l min^−1)	0.830 l min^−1
Data collection/method parameters
Gas blank (s)	10 s
Ablation duration (s)	30 s
Calibration strategy	91500 zircon used as calibration reference material
Isotopes measured (m/z) + dwell time per peak (μs or ms)	^202Hg (200 μs); ^204(Hg + Pb) (200 μs); ^206Pb (400 μs); ^207Pb (1 ms); ^208Pb (200 μs); ^232Th (200 μs); ^235U (2 ms); ^238U (200 μs)
Number sweeps per cycle	40
Total time per cycle (s)	0.1984 s
Analysis mode	Deflector jump
IC dead time (ns)	9.2 ns
Detection mode	Pulse counting
Data processing details
Processing software	Iolite (v3.63)
Data reduction scheme	UPb_CommonApproach
Down-hole correction model	Smoothed cubic spline
^238U measured or calculated	Calculated from ^235U using ^238U/^235U = 137.818
Statistics for baselines	Mean with no outlier rejection
Statistics for normal selections	Mean with 2SD outlier rejection

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Experiment details
Experiment	Exp-I	Exp-II	Exp-III	Exp-IV
Spot size	20 μm	15 μm	10 μm	7 μm
Fluence	3.0 J cm^−2	3.5 J cm^−2	4.5 J cm^−2	6.0 J cm^−2
Laser pulse repetition rate	5 Hz	5 Hz	5 Hz	5 Hz

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The ICP-MS was first tuned for sensitivity and stability in solution mode using a synthetic solution containing approximately 1 ng g⁻¹ U, Th and Pb. Further tuning was then performed in LA mode using line scans (3 J cm⁻², 10 Hz, 25 μm spot, 2 μm s⁻¹ scan rate) on the NIST SRM 612 soda-lime glass to achieve a mean intensity in counts per second (cps) of ∼1 million on mass 238 and ThO/Th < 0.5% by adjusting the He carrier gas flow rates, torch X–Y–Z position, and auxiliary Ar and Ar make-up gas flow rates. All measurements for isotopes of U, Th, Pb, and Hg were collected in deflector peak-jumping mode. ²⁰⁴Pb was measured to monitor for the presence of common Pb in the zircon grains. ²⁰²Hg was measured in order to correct for the isobaric interference of ²⁰⁴Hg on ²⁰⁴Pb. Because large signals for ²³⁸U in U-rich zircons were affected by detector dead-time intensity losses on the ion counter^37,38 of the AttoM, and uncertainties associated with drift and gain corrections of attenuated ion counter signals,⁸ the ²³⁸U cps in each analysis was calculated from measured ²³⁵U cps using a fixed ²³⁸U/²³⁵U of 137.818.³⁹

Four sets of LA-ICP-MS experiments were performed using the following spot size and fluence combinations: 20 μm–3.0 J cm⁻², 15 μm–3.5 J cm⁻², 10 μm–4.5 J cm⁻² and 7 μm–6.0 J cm⁻². Laser fluence was increased progressively from 3 to 6 J cm⁻² with decreasing spot size from 20 to 7 μm in order to compensate partially for decreasing signal intensities associated with decreasing sample mass. Nonetheless, the mean intensities for the measured U and Pb isotopes decreased with decreasing spot size. For instance, the mean intensity of ²⁰⁶Pb for 15 replicate analyses of 91500 zircon (10 second ablations) decreased from 68 235 (±2226), 36 799 (±2007), 19 111 (±731), to 13 845 (±349) cps, for 20, 15, 10, to 7 μm spots, respectively.

Each experiment was carried out by firing the laser (at 5 Hz) for 30 seconds, after 10 seconds of gas background measurement, and followed by 20 seconds of monitored washout time. A standard-sample-standard bracketing approach was adopted, using 91500 zircon as the calibration reference material. The other five zircons analyzed in this study (FC-1, R33, Temora 2, Plešovice, and Fish Canyon Tuff) were treated as unknowns. Ten analyses of each of the five zircons were carried out in each of the four experiments, bracketed by two or five analyses of 91500, to derive weighted averages and uncertainties of the age measurements. The measured signal intensities for each experiment were collected in a single time-resolved-analysis (TRA) file. Individual analyses were separated and identified in the TRA file by reference to the time-stamped laser-log file.

Data reduction

Raw data were dead time corrected online within the Nu AttoLab software. Exported data were reduced offline using Iolite (version 3.63) data reduction software²² and the “U–Pb Common Approach” data reduction scheme (DRS). Several DHF correction models are available in Iolite (linear, exponential, running median) but the smoothed cubic spline⁴⁰ fit the TRA data most closely over the entire 30 seconds of each ablation, and was used in all cases. Corrections were not made for the rare occurrence of common Pb in the zircons, as detected by ²⁰⁴Pb cps significantly above gas background. Final age calculations, histograms, weighted means and Concordia diagrams were made using the Iolite VizualAge DRS⁴¹ and Isoplot/Ex v. 4.15.⁴²

Initially, the entire 30 seconds of the time-resolved signal for each zircon analysis was processed, excluding each of the initial 2 and final 2 seconds of ablation, where the signal intensities tended to be unstable and thus produced larger residuals for the DHF correction model using Iolite. This gave results for 28 seconds after the start of laser ablation, or 26 seconds (130 laser pulses) of total integration time. To evaluate the accuracy and precision of shorter ablation times, the same data sets were reprocessed by masking the final 15, 20 and 23 seconds of ablation, respectively, to produce results for 15, 10 and 7 seconds after the start of ablation. This corresponds to 13 seconds (65 pulses), 8 seconds (40 pulses) and 5 seconds (25 pulses) of total integration times, respectively.

Results

DHF patterns of the zircons by spot size

Fig. 1 plots the average trace of the smoothed cubic spline curve fit by the Iolite “U–Pb Common Approach” DRS to time-resolved, ID-TIMS-normalized ²⁰⁶Pb/²³⁸U ratios for the six zircon reference materials analyzed in this study. The data are averages of 15 replicate measurements for zircon 91500 and 10 replicate measurements for the other zircons. The raw ²⁰⁶Pb/²³⁸U data are normalized to the accepted ID-TIMS ²⁰⁶Pb/²³⁸U in order to compare the ratios for the different zircons on the same scale.

Fig. 1 Time series (s = seconds) of smoothed cubic spline curves calculated in Iolite through averages of the raw ²⁰⁶Pb/²³⁸U of replicate measurements of zircon 91500 (n = 15) and the other zircon reference materials analyzed in this study (n = 10). Each curve is normalized to the accepted ID-TIMS ²⁰⁶Pb/²³⁸U ratio (91500 = 0.17917,³¹ FC-1 = 0.18587,³² R33 = 0.06721,³³ Temora 2 = 0.06679,³³ Plešovice = 0.053694,¹⁵ Fish Canyon Tuff = 0.004415.³⁶ Note the larger ²⁰⁶Pb/²³⁸U scale for the 7 μm curves. The normalized ²⁰⁶Pb/²³⁸U increase with ablation time as a result of DHF for all zircon reference materials, but more steeply and with more variable patterns for the smaller spots (10 and 7 μm) compared to the larger spots (20 and 15 μm).

The ratios for normalized raw ²⁰⁶Pb/²³⁸U shown in Fig. 1 all increase with ablation time, reflecting DHF of more volatile Pb from more refractory U as the ablation pit deepens, but exhibit somewhat different patterns of increase for different zircons. For instance, for the 20 μm spots, all except zircon FC-1 display rather linear trends of increasing ²⁰⁶Pb/²³⁸U but zircon R33 has a distinctly steeper slope. ²⁰⁶Pb/²³⁸U of zircon FC-1 on a 20 μm spot initially increase rapidly but after 15 seconds of ablation increase along a shallower slope. This is consistent with the results of other studies that have found different DHF patterns for different zircons.^{3,4,9,27–30}

The relative increase in ²⁰⁶Pb/²³⁸U after 28 seconds of ablation is much greater for the 7- and 10 μm spots than for the 15- and 20 μm spots, with the increase for zircon FC-1 consistently the largest. For instance, after 28 seconds, the ²⁰⁶Pb/²³⁸U increases by 88% for zircon FC-1 and 51% for zircon 91500 on a 7 μm spot, compared to 26% for zircon FC-1 and 18% for zircon 91500 on a 20 μm spot.

The relative increases in ²⁰⁶Pb/²³⁸U are not entirely consistent from spot size to spot size. For instance, on a 20 μm spot, zircon R33 shows almost as large of an increase in ²⁰⁶Pb/²³⁸U (24%) after 28 seconds of ablation as zircon FC-1 (26%) but more than the others (17–19%). In contrast, on a 15 μm spot, zircon Temora 2 shows almost as large of an increase in ²⁰⁶Pb/²³⁸U (31%) after 28 seconds of ablation as zircon FC-1 (32%) but more than the others (26–28%).

For the 10- and 7 μm spot sizes, the greatest divergence in ²⁰⁶Pb/²³⁸U patterns between the different zircons occurs after about 15 seconds of ablation. For instance, on a 7 μm spot, after 15 and 28 seconds, respectively, the ²⁰⁶Pb/²³⁸U ratio increases by 33% and 88% (factor of 2.6 increase) for zircon FC-1, 35% and 81% (factor of 2.3 increase) for zircon Temora 2, and 25% and 51% (factor of 2.0 increase) for zircon 91500. In comparison, for a 15 μm spot, there is a more uniform increase in ²⁰⁶Pb/²³⁸U by a factor of 2.0 for each zircon: after 15 and 28 seconds, respectively, the ²⁰⁶Pb/²³⁸U increases by 17% and 32% for zircon FC-1, 15% and 31% for zircon Temora 2, and 13% and 26% for zircon 91500.

A more-subtle difference between the zircons is a variation in their ID-TIMS-normalized ²⁰⁶Pb/²³⁸U just after the initial 2 seconds of ablation: these ratios vary from 0.95–0.98 for the 20 μm spots, 0.94–0.97 for the 15 μm spots, 0.93–0.95 for the 10 μm spots and 0.91–0.98 for the 7 μm spots. Because these variations occurred before ablation pits deepened significantly, they may reflect zircon matrix-dependent instrumental mass bias, possibly due to Pb/U fractionation in the ICP,^43,44 rather than DHF at the ablation site, as is described in more detail in the Discussion section below.

U–Pb ages of the zircons by spot size

Table 3 presents a summary of the weighted average LA-ICP-MS ²⁰⁶Pb/²³⁸U ages and 2-sigma standard error (2SE, absolute and relative%) for 10 replicate analyses of the zircon reference materials at the various spot sizes (20, 15, 10, 7 μm) and ablation times (28, 15, 10, 7 seconds). Also listed are the % differences (age offsets) between the weighted average LA-ICP-MS ²⁰⁶Pb/²³⁸U ages and the accepted ID-TIMS ²⁰⁶Pb/²³⁸U ages. These age offsets are plotted in Fig. 2 where the accepted ID-TIMS ²⁰⁶Pb/²³⁸U ages and a relative 2% uncertainty are illustrated by the blue-shaded horizontal band. Details of the individual U–Pb analyses with internal precision (measurement repeatability) at 2 sigma (2σ) and propagated systematic errors following recommendations of Horstwood et al.¹⁵ are given in Tables A1–A4 in the ESI.†

Table 3 Weighted average ²⁰⁶Pb/²³⁸U ages at various spot sizes and ablation times for zircon reference materials^a

Zircon	n	^206Pb/^238U age (Ma)	±2SE	% 2SE	% age offset	Zircon	n	^206Pb/^238U age (Ma)	±2SE	% 2SE	% age offset
a SE = Standard Error (= (standard deviation)/√n). n = number of spots analysed. Age offsets (%) are calculated relative to the ID-TIMS age (see text for further information). b 1–3 of the 10 spot analyses of FC-1 at 15 μm were omitted because they showed unusually heterogeneous Pb/U ratios, possibly associated with grain fractures, particularly in the later parts of the 30 second (s) ablations.
20 μm						15 μm
20 μm 28 s						15 μm 28 s
91500	15	1062.7	3.2	0.30	0.03	91500	15	1061.9	7.2	0.68	−0.05
FC-1	10	1142.3	5.3	0.46	3.89	FC-1	7^b	1138.0	10.0	0.88	3.50
Temora 2	10	417.3	1.7	0.41	0.12	Temora 2	10	420.3	6.8	1.62	0.84
R33	10	427.8	1.1	0.26	2.03	R33	10	420.7	3.1	0.74	0.33
Fish Canyon	10	29.3	0.3	0.89	2.71	Fish Canyon	10	28.3	0.4	1.49	−0.73
Plešovice	10	343.2	1.3	0.38	1.80	Plešovice	10	335.9	2.5	0.74	−0.36
20 μm 15 s						15 μm 15 s
91500	15	1062.9	5.5	0.52	0.05	91500	15	1062.9	7.3	0.69	0.05
FC-1	10	1127.0	6.8	0.60	2.50	FC-1	8^b	1125.0	10.0	0.89	2.32
Temora 2	10	421.1	2.4	0.57	1.04	Temora 2	10	418.9	5.0	1.19	0.51
R33	10	423.3	2.4	0.57	0.95	R33	10	420.8	2.9	0.69	0.36
Fish Canyon	10	28.7	0.3	1.01	0.74	Fish Canyon	10	28.2	0.4	1.53	−1.12
Plešovice	10	339.3	1.2	0.35	0.64	Plešovice	10	335.7	2.1	0.63	−0.42
20 μm 10 s						15 μm 10 s
91500	15	1061.8	5.4	0.51	−0.06	91500	15	1064.0	11.0	1.03	0.15
FC-1	10	1121.4	7.1	0.63	1.99	FC-1	8^b	1126.0	12.0	1.07	2.41
Temora 2	10	421.6	1.9	0.45	1.16	Temora 2	10	418.4	5.0	1.20	0.39
R33	10	419.3	3.6	0.86	0.00	R33	10	420.4	4.9	1.17	0.26
Fish Canyon	10	28.3	0.4	1.31	−0.70	Fish Canyon	10	28.1	0.4	1.46	−1.33
Plešovice	10	338.8	1.8	0.53	0.50	Plešovice	10	334.4	3.6	1.08	−0.81
20 μm 7 s						15 μm 7 s
91500	15	1062.8	8.5	0.80	0.04	91500	15	1062.7	9.4	0.88	0.03
FC-1	10	1120.4	7.9	0.71	1.90	FC-1	9^b	1127.6	7.3	0.65	2.56
Temora 2	10	424.8	3.6	0.85	1.92	Temora 2	10	417.4	4.7	1.13	0.15
R33	10	420.6	4.8	1.14	0.31	R33	10	418.9	2.9	0.69	−0.10
Fish Canyon	10	28.5	0.5	1.72	−0.03	Fish Canyon	10	28.9	0.4	1.28	1.59
Plešovice	10	340.9	2.6	0.76	1.12	Plešovice	10	334.2	1.8	0.54	−0.87
10 μm						7 μm
10 μm 28 s						7 μm 28 s
91500	15	1062.2	7.0	0.66	−0.02	91500	15	1062.0	7.4	0.70	−0.04
FC-1	10	1136.5	8.9	0.78	3.37	FC-1	10	1172.7	9.7	0.83	6.66
Temora 2	10	416.5	4.3	1.03	−0.07	Temora 2	10	418.1	5.3	1.27	0.32
R33	10	422.9	5.7	1.35	0.86	R33	10	420.2	8.7	2.07	0.21
Fish Canyon	10	28.2	0.3	0.99	−0.84	Fish Canyon	10	27.4	0.5	1.76	−3.96
Plešovice	10	342.5	2.8	0.82	1.59	Plešovice	10	334.2	4.2	1.26	−0.87
10 μm 15 s						7 μm 15 s
91500	15	1062.7	9.7	0.91	0.03	91500	15	1061.9	9.3	0.88	−0.05
FC-1	10	1098.6	8.7	0.79	−0.08	FC-1	10	1157.8	8.1	0.70	5.30
Temora 2	10	418.1	6.0	1.44	0.32	Temora 2	10	410.8	5.9	1.44	−1.43
R33	10	421.5	5.6	1.33	0.52	R33	10	412.1	6.8	1.65	−1.72
Fish Canyon	10	28.8	0.6	2.08	1.13	Fish Canyon	10	27.9	0.6	2.19	−2.10
Plešovice	10	335.2	2.6	0.78	−0.57	Plešovice	10	338.5	3.7	1.09	0.41
10 μm 10 s						7 μm 10 s
91500	15	1061.0	14.0	1.32	−0.13	91500	15	1062.0	13.0	1.22	−0.04
FC-1	10	1104.0	15.0	1.36	0.41	FC-1	10	1134.0	11.0	0.97	3.14
Temora 2	10	412.0	6.1	1.48	−1.15	Temora 2	10	408.9	6.8	1.66	−1.89
R33	10	419.5	6.3	1.50	0.05	R33	10	419.2	7.5	1.79	−0.02
Fish Canyon	10	28.3	0.6	2.09	−0.80	Fish Canyon	10	28.4	0.5	1.65	−0.13
Plešovice	10	335.7	4.6	1.37	−0.42	Plešovice	10	340.8	3.2	0.94	1.09
10 μm 7 s						7 μm 7 s
91500	15	1062.0	15.0	1.41	−0.04	91500	15	1061.0	14.0	1.32	−0.13
FC-1	10	1105.0	14.0	1.27	0.50	FC-1	10	1092.0	16.0	1.47	−0.68
Temora 2	10	412.7	7.7	1.87	−0.98	Temora 2	10	410.9	8.3	2.02	−1.41
R33	10	420.2	6.1	1.45	0.21	R33	10	424.5	9.5	2.24	1.24
Fish Canyon	10	28.6	0.6	2.21	0.25	Fish Canyon	10	28.9	0.5	1.77	1.34
Plešovice	10	337.0	4.1	1.22	−0.04	Plešovice	10	335.9	2.2	0.65	−0.36

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Fig. 2 ²⁰⁶Pb/²³⁸U age offsets (%) with respect to the accepted ID-TIMS ²⁰⁶Pb/²³⁸U ages for the weighted means of the replicate LA-ICP-MS measurements of each zircon sample. Vertical bar length represents the 2-standard error (2SE%) uncertainty of the LA-ICP-MS measurements. For each zircon reference material, the 28 second (s) ablation result (red color) is reprocessed for masked ablation times of 15 (violet), 10 (green), and 7 (blue) seconds (from left to right, respectively). Blue-shaded horizontal field in each plot represents a ±2% age offset with respect to the accepted ID-TIMS age, represented by the brown horizontal line.

Wetherill U/Pb (²⁰⁷Pb/²³⁵U vs.²⁰⁶Pb/²³⁸U) Concordia ages for the zircons determined by LA-ICP-MS, with their absolute 2SE and mean square weighted deviation (MSWD) and % age offsets relative to the ID-TIMS ages, are listed in Table 4 and plotted in Fig. 3.

Table 4 U/Pb Wetherill concordia ages at various spot sizes and ablation times for zircon reference materials^a

Zircon	n/rejects*	Age (Ma)	±2SE	MSWD	% age offset	Zircon	n/rejects*	Age (Ma)	±2SE	MSWD	% age offset
a SE = Standard Error (= (standard deviation)/√n). n = number of spots analysed. MSWD = mean square weighted deviation. s = seconds. Age offsets (%) are calculated relative to the ID-TIMS age. Concordia ages obtained using VizualAge functions in Iolite.^41 Some reference zircon analysis sets include 1–4 individual analyses that were excluded from the concordia age calculation, and are shown in the table as a number of ‘rejects’ (*). The reference zircon analysis set is labelled in the table as “Discordant” where all or most of the analyses in a reference zircon analysis set are discordant and a concordia age cannot be calculated. However, all discordant analyses are included and plotted in Fig. 3 for illustrative purposes.
20 μm						15 μm
20 μm 28 s						15 μm 28 s
91500	15/0	1063.0	2.6	0.93	0.1	91500	15/0	1063.0	3.7	1.59	0.1
FC-1	Discordant					FC-1	Discordant
Temora 2	10/0	418.0	1.0	2.76	0.3	Temora 2	10/1	423.3	1.7	5.02	1.6
R33	10/0	428.6	1.0	2.53	2.2	R33	10/1	422.6	1.4	4.07	0.8
Fish Canyon	10/0	29.28	0.11	2.43	2.8	Fish Canyon	10/0	28.34	0.17	3.48	−0.5
Plešovice	10/2	342.4	0.7	1.76	1.6	Plešovice	Discordant
20 μm 15 s						15 μm 15 s
91500	15/1	1064.2	3.6	0.90	0.2	91500	15/0	1064.0	4.8	1.35	0.2
FC-1	Discordant					FC-1	Discordant
Temora 2	10/0	419.5	1.3	1.72	0.6	Temora 2	10/0	419.6	2.0	2.82	0.7
R33	10/1	423.7	1.5	2.07	1.1	R33	10/0	421.7	1.7	2.52	0.6
Fish Canyon	10/0	28.75	0.15	2.78	0.9	Fish Canyon	10/0	28.22	0.22	1.78	−0.9
Plešovice	10/2	340.7	0.9	2.63	1.1	Plešovice	10/0	335.3	1.0	1.14	−0.5
20 μm 10 s						15 μm 10 s
91500	15/0	1063.2	4.3	0.56	0.1	91500	15/1	1063.7	6.3	1.69	0.1
FC-1	10/2	1111.3	2.5	2.06	1.1	FC-1	10/3	1113.9	4.3	2.57	1.4
Temora 2	10/0	421.8	1.7	1.15	1.2	Temora 2	10/0	417.5	2.3	1.82	0.2
R33	10/0	421.1	1.8	3.47	0.4	R33	10/0	419.2	2.2	1.57	0.0
Fish Canyon	10/0	28.38	0.18	2.96	−0.3	Fish Canyon	10/0	28.14	0.27	1.53	−1.2
Plešovice	10/1	338.8	1.0	1.88	0.5	Plešovice	10/0	334.7	1.2	1.16	−0.7
20 μm 7 s						15 μm 7 s
91500	15/0	1062.7	5.6	1.39	0.0	91500	15/1	1064.4	8.1	1.11	0.2
FC-1	10/2	1103.0	3.0	3.89	0.4	FC-1	10/4	1115.5	5.8	1.39	1.5
Temora 2	10/0	425.0	1.9	1.39	2.0	Temora 2	10/2	415.6	3.1	1.01	−0.3
R33	10/1	422.3	2.3	2.04	0.7	R33	10/0	417.4	2.6	1.20	−0.5
Fish Canyon	10/0	28.53	0.23	3.09	0.2	Fish Canyon	10/0	27.89	0.36	0.51	−2.1
Plešovice	10/0	340.0	1.3	1.33	0.9	Plešovice	10/1	333.9	1.6	1.05	−0.9
10 μm						7 μm
10 μm 28 s						7 μm 28 s
91500	15/0	1063.4	5.3	0.37	0.1	91500	15/1	1064.7	6.1	0.60	0.2
FC-1	Discordant					FC-1	10/3	1155.6	4.3	5.25	5.2
Temora 2	10/0	415.1	1.9	1.91	−0.4	Temora 2	10/0	417.7	2.7	2.09	0.2
R33	10/1	421.3	2.1	2.76	0.5	R33	10/1	420.7	3.0	1.85	0.3
Fish Canyon	10/1	28.19	0.26	0.88	−1.0	Fish Canyon	10/3	27.68	0.32	0.93	−2.8
Plešovice	10/3	340.9	1.0	4.00	1.1	Plešovice	10/2	336.6	1.2	2.62	−0.1
10 μm 15 s						7 μm 15 s
91500	15/0	1062.8	6.8	0.66	0.0	91500	15/0	1062.2	7.8	0.80	0.0
FC-1	10/0	1102.8	4.1	2.34	0.3	FC-1	10/3	1144.2	4.8	4.24	4.1
Temora 2	10/0	416.1	2.5	1.40	−0.2	Temora 2	10/0	412.4	3.1	2.93	−1.1
R33	10/1	420.2	2.7	1.49	0.2	R33	10/0	414.0	3.1	3.46	−1.3
Fish Canyon	10/0	28.85	0.34	2.76	1.3	Fish Canyon	10/1	27.61	0.35	1.64	−3.1
Plešovice	10/2	335.9	1.2	1.81	−0.4	Plešovice	10/2	340.6	1.5	3.82	1.0
10 μm 10 s						7 μm 10 s
91500	15/0	1063.4	8.1	1.51	0.1	91500	15/0	1062.7	9.5	0.66	0.0
FC-1	Discordant					FC-1	10/2	1131.0	5.6	1.81	2.9
Temora 2	10/0	414.6	3.0	2.67	−0.5	Temora 2	10/0	410.9	3.7	2.52	−1.4
R33	10/0	416.6	2.9	1.94	−0.7	R33	10/0	420.4	3.7	2.62	0.3
Fish Canyon	10/0	28.19	0.44	1.30	−1.0	Fish Canyon	10/0	28.16	0.41	0.92	−1.1
Plešovice	10/1	335.8	1.4	3.67	−0.4	Plešovice	10/2	341.6	1.7	1.70	1.3
10 μm 7 s						7 μm 7 s
91500	15/0	1063.6	9.8	1.15	0.1	91500	15/0	1062.6	11.1	1.02	0.0
FC-1	Discordant					FC-1	Discordant
Temora 2	10/2	411.7	4.1	3.68	−1.2	Temora 2	10/0	412.9	4.6	3.80	−0.9
R33	10/0	414.7	3.6	3.72	−1.1	R33	10/0	426.6	4.4	3.36	1.7
Fish Canyon	10/0	28.58	0.52	0.92	0.4	Fish Canyon	10/0	28.91	0.48	1.44	1.5
Plešovice	10/2	334.2	1.9	4.69	−0.9	Plešovice	10/2	337.3	2.0	1.66	0.1

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Fig. 3 Wetherill U–Pb Concordia age plots for analyses of the six zircon reference materials (labeled at right) made at variable spot sizes of 20, 15, 10, and 7 μm plotted from left to right respectively. Ages in Ma are given along the Concordia curve. Ablation integration durations are color-coded as 28 (blue), 15 (green), 10 (black) and 7 (red) seconds (s). Concordia ages (Ma) (for concordant analyses) with ±2 sigma uncertainty are given within each panel with same color codes, except where the analyses are discordant and a Concordia age cannot be calculated. Data for individual analyses are listed in Tables A1–A4 of the ESI.†

Experiment-I: 20 μm and 3 J cm⁻²

For 20 μm spots, offsets between weighted average LA-ICP-MS ²⁰⁶Pb/²³⁸U ages and the accepted ID-TIMS ²⁰⁶Pb/²³⁸U ages are 2.0% or less for all six zircon reference materials when ablation intervals are shortened to 10 and 7 seconds (Table 3). However, the age offset for zircon FC-1 increases to 2.5% and 3.9% for 15 and 28 second ablation intervals, respectively. The age offset for zircon from the Fish Canyon Tuff is 2.7% for the 28 second ablation interval. Internal precision on the LA-ICP-MS ²⁰⁶Pb/²³⁸U ages worsens progressively from 0.26–0.89% 2SE for the 28 second ablation intervals to 0.35–1.0% 2SE for the 15 second ablation intervals to 0.45–1.3% 2SE for the 10 second ablation intervals to 0.71–1.7% 2SE for the 7 second ablation intervals (Table 3).

Offsets between Wetherill U/Pb Concordia ages determined by LA-ICP-MS and the accepted ID-TIMS ages are 2.0% or less for all six zircon reference materials when ablation intervals are shortened to 10 and 7 seconds (Table 4). However, zircon FC-1 is discordant when processed for 15 and 28 second ablation intervals. The Concordia age offsets for the Fish Canyon Tuff and R33 zircon are 2.8% and 2.2% for the 28 second ablation interval, respectively (Table 4). MSWD ranges from 0.9 (91500 zircon) – 2.8 (Fish Canyon, Temora 2 zircons) for the 15 and 28 second ablation intervals and 0.6 (91500 zircon) – 3.9 (FC-1 zircon) for the 7 and 10 second ablation intervals.

Experiment-II: 15 μm and 3.5 J cm⁻²

In the 15 μm spot experiment, offsets between weighted average LA-ICP-MS ²⁰⁶Pb/²³⁸U ages and the accepted ID-TIMS ²⁰⁶Pb/²³⁸U ages are 1.6% or less for all zircon reference materials (except FC-1) for all 4 ablation time intervals (Table 3). The age offset for zircon FC-1 varies from 3.5% to 2.3% to 2.4% to 2.6% for 28- to 15- to 10- to 7 second ablation intervals, respectively. Internal precision on the LA-ICP-MS ²⁰⁶Pb/²³⁸U ages varies from 0.68–1.6% 2SE for the 28 second ablation intervals to 0.63–1.5% 2SE for the 15 second ablation intervals to 1.1–1.5% 2SE for the 10 second ablation intervals to 0.5–1.3% 2SE for the 7 second ablation intervals (Table 3).

Offsets between Wetherill U/Pb Concordia ages determined by LA-ICP-MS and the accepted ID-TIMS ages are 2.1% or less for all zircon reference materials (except FC-1 and Plešovice) for all four ablation time intervals (Table 4). Zircon FC-1 is discordant when processed for 15- and 28 second ablation intervals and Plešovice is discordant when processed for the 28 second ablation interval. MSWD ranges from 1.6 (91500 zircon) – 5.0 (Temora 2 zircon), 1.1 (Plešovice zircon) – 2.8 (Temora 2 zircon), 1.2 (Plešovice zircon) – 2.6 (FC-1 zircon) and 0.5 (Fish Canyon Tuff zircon) – 1.4 (FC-1 zircon) for the 28, 15, 10, and 7 second ablation intervals, respectively.

Experiment-III: 10 μm and 4.5 J cm⁻²

For 10 μm spots, offsets between weighted average LA-ICP-MS ²⁰⁶Pb/²³⁸U ages and the accepted ID-TIMS ²⁰⁶Pb/²³⁸U ages are 1.1% or less for all zircon reference materials for the 15, 10 and 7 second ablation time intervals (Table 3). For the 28 second ablation interval, the age offset for zircon FC-1 is 3.4%. Internal precision on the LA-ICP-MS ²⁰⁶Pb/²³⁸U ages varies from 0.66–1.3, 0.78–2.1, 1.3–2.1 and 1.2–2.2% SE for the 28, 15, 10, and 7 second ablation intervals, respectively (Table 3).

Offsets between Wetherill U/Pb Concordia ages determined by LA-ICP-MS and the accepted ID-TIMS ages are 1.3% or less for all zircon reference materials (except FC-1) for all four ablation time intervals (Table 4). Zircon FC-1 is discordant when processed for the 28, 15 and 7 second ablation intervals. MSWD ranges from 0.4 (91500 zircon) – 4.0 (Plešovice zircon), 0.7 (91500 zircon) – 2.8 (Fish Canyon Tuff zircon), 1.3 (Fish Canyon Tuff zircon) – 3.7 (Plešovice zircon) and 0.9 (Fish Canyon Tuff zircon) – 4.7 (Plešovice zircon) for the 28, 15, 10, and 7 second ablation intervals, respectively.

Experiment-IV: 7 μm and 6.0 J cm⁻²

In the 7 μm spot experiment, offsets between weighted average LA-ICP-MS ²⁰⁶Pb/²³⁸U ages and the accepted ID-TIMS ²⁰⁶Pb/²³⁸U ages are 1.4% or less for all zircon reference materials for the 7 second ablation time interval (Table 3). For the 10 and 15 second ablation intervals, the age offsets for zircon FC-1 are 3.1% and 5.3%, respectively, whereas offsets for the other zircon are 2.1% or better. For the 28 second ablation intervals, the age offsets for zircon FC-1 and Fish Canyon Tuff are 6.7% and 4.0%, respectively, whereas offsets for the other zircon are 0.9% or better. Internal precision on the LA-ICP-MS ²⁰⁶Pb/²³⁸U ages varies from 0.70–2.1, 0.70–2.2, 0.94–1.8, and 0.65–2.2% SE for the 28, 15, 10, and 7 second ablation intervals, respectively (Table 3).

Excluding zircon FC-1 and Fish Canyon Tuff, offsets between Wetherill U/Pb Concordia ages determined by LA-ICP-MS and the accepted ID-TIMS ages are 1.7% or less for the other zircon reference materials for all four ablation time intervals (Table 4). Zircon FC-1 is discordant when processed for the 7 second ablation interval, and has age offsets of 2.9%, 4.1% and 5.2%, respectively, for the 10, 15 and 28 second ablation intervals. Fish Canyon Tuff zircon has age offsets of 3.1% and 2.8%, respectively, for the 15 and 28 second ablation intervals. Because Fish Canyon Tuff zircon has a comparatively young age (∼28 Ma), it has low contents of ²⁰⁶Pb and ²⁰⁷Pb that may have compromised the precision of the Pb/U ratio measurements. MSWD ranges from 0.6 (91500 zircon) – 5.2 (FC-1 zircon), 0.8 (91500 zircon) – 4.2 (FC-1 zircon), 0.7 (91500 zircon) – 2.6 (R33 zircon) and 1.0 (91500 zircon) – 3.8 (Temora 2 zircon) for the 28, 15, 10, and 7 second ablation intervals, respectively.

Discussion

Accuracy and precision of U–Pb ages for 20 and 15 μm spots

For 20 and 15 μm spots, more accurate U–Pb ages are achieved when 10 and 7 second ablation intervals are processed rather than 28 and 15 second ablation intervals. For the 10 and 7 second ablation intervals, offsets between Wetherill U/Pb Concordia ages determined by LA-ICP-MS and the accepted ID-TIMS ages are 1.4% and 2.1% or less, respectively, for each of the zircon reference materials. In contrast, FC-1 is discordant for the 20 and 15 μm spots and 28 and 15 second integrations; Plešovice is discordant for a 20 μm spot and 15 second ablation integration; and the offset for the Fish Canyon Tuff zircon is 2.8% relative to the ID-TIMS age for the 20 μm spot and 28 second ablation integration (Table 4).

There is a progressive loss in precision using the shorter ablation integration times for each of the zircon reference materials. For instance, for the 20 μm spot, the 2SE on the LA-ICP-MS ²⁰⁶Pb/²³⁸U ages increases from 0.26–0.89% (average 0.45%) for the 28 second ablation intervals to 0.45–1.3% (average 0.72%) for the 10 second ablation intervals, and 0.71–1.7% (average 1.0%) for the 7 second ablation intervals for all six zircon reference materials (Table 3). Thus, 10 second ablation intervals, applying 50 laser pulses to the sample, provide the combination of most accurate and precise U–Pb ages for 20 and 15 μm spots.

Accuracy and precision of U–Pb ages for 10 and 7 μm spots

For 10 μm spots, LA-ICP-MS U/Pb Wetherill Concordia ages are within 1.3% of their ID-TIMS ages for all zircon reference materials (except FC-1) irrespective of whether the 28, 15, 10 and 7 second ablation intervals are processed (Table 4). Zircon FC-1 gives discordant results for 28, 10 and 7 second ablation intervals but a Concordia age within 0.3% of the ID-TIMS age for the 15 second ablation interval. Precision improves with length of ablation time: 2SE on the LA-ICP-MS ²⁰⁶Pb/²³⁸U ages averages 1.6%, 1.5%, 1.2%, and 0.94% for the 7, 10, 15, and 28 second ablations, respectively (Table 3). Thus, 15 second ablations (75 pulses) provide the best compromise between accuracy and precision for 10 μm spots.

For 7 μm spots, zircons FC-1 and Fish Canyon Tuff give LA-ICP-MS U/Pb Wetherill Concordia ages either ∼3–5% offset from their ID-TIMS ages or discordant (Table 4). The most accurate LA-ICP-MS U/Pb Wetherill Concordia ages are achieved for the 10 second ablation intervals where offsets from ID-TIMS ages are 2.9% for FC-1, 1.1% for Fish Canyon Tuff and 0–1.4% for the other zircon reference materials. Precision represented by 2SE on the LA-ICP-MS ²⁰⁶Pb/²³⁸U ages varies from 0.97–1.8% (average 1.4%) for the 10 second ablation intervals (Table 3).

Zircon matrix effects on U–Pb ages for 7–20 μm spots

Differences in DHF between different zircons, specifically the zircon used as the calibration reference material versus zircons treated as unknowns, are the limiting control on the accuracy of LA-ICP-MS U–Pb zircon geochronology.²¹ As shown in Fig. 1, ²⁰⁶Pb/²³⁸U ratios of different zircons increase at different rates and with changing patterns as ablation and DHF proceeds. Accumulated radiation damage and trace element substitution into the zircon structure are thought to be the primary controls on the DHF patterns of zircon with crystallographic orientation and crystal color having only secondary effects.^9,27–30 In this regard, zircon FC-1, which exhibits more extreme DHF compared to other zircons in this study (Fig. 1), has been shown to possess large amounts of radiation damage based on its Raman spectrum.²⁸ This is also the case for actinide-rich domains in Plešovice zircon.³⁵

Several methods have been used to minimize differences in DHF between different zircons: low-laser-fluence (soft) ablation⁴⁵ or ablation rastering²¹ to limit pit depth; thermal annealing of grains prior to analysis to provide more uniform ablation characteristics;^27,28 and post-analysis correction of U–Pb dates as a function of radiation dose relative to the calibration reference material.⁴ In the study here, using LA-ICP-single-collector-SF-MS instrumentation, the most accurate and precise U–Pb ages for a variety of zircon matrices are achieved by limiting the ablation time to the first 10 seconds or 50 laser pulses for most of the spot sizes tested at 20 μm or less. This relatively short ablation time restricts the extent of DHF and thus the time-resolved Pb/U variations between different zircons by limiting pit depth. However, in contrast to low fluence ablation, high U and Pb sensitivities can be achieved for most zircons, even for spots ≤20 μm, using moderate to high fluence (3–6 J cm⁻²), short-time ablations. Spots ≤20 μm are difficult to form by rastering as the laser beam diameter and raster step size need to be much smaller than the total area ablated. Pre- and post-analysis methods such as thermal annealing and radiation dose corrections are possible to apply to small spot analyses but at the expense of lengthening the time for sample throughput.

As noted above in the Results section, ID-TIMS-normalized ²⁰⁶Pb/²³⁸U ratios just after the initial 2 seconds of ablation are all below unity, which probably represents instrumental mass bias. The calibration reference material is used to correct for instrumental mass bias but the 2 second ID-TIMS-normalized ²⁰⁶Pb/²³⁸U ratios vary between different zircons by 2–3% for the 20, 15 and 10 μm spots, and 7% for the 7 μm spots, and do not converge to a common starting value (Fig. 1). Thus, the different zircons require somewhat different mass bias correction factors, which represents another source of matrix-dependent uncertainty, in addition to DHF, contributing to the commonly reported overall 2% offset of LA-ICP-MS ²⁰⁶Pb/²³⁸U ages from their ID-TIMS ages.^14,15

A possible cause of the variable zircon-matrix-dependent instrumental mass bias is that the size distribution of ablated particles differs between the zircons, particularly at the start of ablation when the largest particles are likely produced (>0.8 μm in silicate glasses⁴³). Large particles are vaporized and ionized incompletely in the ICP, biasing ratios of volatile/refractory elements such as Pb/U.^43,44 Removal of the large particles using a size separation device placed between the ablation chamber and ICP torch has been shown to be effective in reducing volatile/refractory element fractionation in silicate glass,⁴⁴ and may be one way to reduce variability in initial Pb/U mass bias between different zircons.

Comparison to previous studies on ≤20 μm spots

Most previous studies on the accuracy and precision of LA-ICP-MS U–Pb zircon ages for ≤20 μm spots have been carried out using LA-ICP-multi-collector SF-MS instrumentation.^{7,11,16,18–20} These studies typically limited depth of ablation to 3–10 μm by applying either short ablation times, as was found to give more accurate results in this study, or low laser fluence (soft) ablation.

As examples of the short ablation time approach, Gehrels et al.¹¹ reported U–Pb ages on ∼15 μm wide × 6 μm deep pits (12 seconds of ablation or ∼48 laser pulses) for 11 zircon reference materials by LA-ICP-multi-collector-SF-MS with Pb isotopes measured on Channeltron ICs and U isotopes on Faraday cups, with a laser fluence of 4 J cm⁻². Weighted mean ²⁰⁶Pb/²³⁸U ages of 10 measurements for each zircon reference material were within 2% of the ID-TIMS ages. The average 2SE of each of the means for all age measurements was 1.3%. Johnston et al.¹⁶ used the same instrumentation as Gehrels et al.¹¹ for U–Pb age measurements on 12–14 μm wide × 4–5 μm deep pits (8–10 seconds of ablation or ∼32–40 laser pulses) for nine zircon reference materials. The results of Johnston et al.¹⁶ provided an accuracy and precision similar to that of Gehrels et al.¹¹ However, Fish Canyon Tuff zircon yielded ages ∼4–5% too young, which Johnston et al.¹⁶ suggested was due to non-linearity of the Channeltron ICs at low intensities of Pb isotopes. Xie et al.²⁰ determined U–Pb ages on 6–7 μm wide × 3 μm deep pits (∼6 seconds of ablation or 25 laser pulses) for six zircon reference materials with Pb and U isotopes both measured on a LA-ICP-MC-SF-MS instrument with discrete-dynode IC detectors and a laser fluence of 4 J cm⁻². They obtained a precision (2σ) for the weighted mean ²⁰⁶Pb/²³⁸U age of <1% and an accuracy of <1% offset to the nominal reference age.

As an example of the low fluence (soft) ablation approach, Lana et al.¹⁸ measured U–Pb ages on 20 μm wide × 5–10 μm deep pits (40 seconds of ablation or 240 laser pulses) for seven zircon reference materials using a LA-ICP-MC-SF-MS instrument with Pb and U isotopes analyzed on discrete-dynode ICs and Faraday cups, respectively, with a laser fluence of 1–2 J cm⁻². Accuracy and precision of ²⁰⁶Pb/²³⁸U ages were reported as between 0.5 and 1.0% (RSD). The soft ablation approach may be of limited value, however, for spots smaller than 20 μm, where the ablated sample volume becomes small, particularly for LA-ICP-single-collector-SF-MS instrumentation, which is less sensitive than multi-collector instruments.

The accuracy and precision of LA-ICP-MS U–Pb zircon ages for ≤20 μm spots have only rarely been evaluated using LA-ICP-quadrupole and single-collector SF-MS instrumentation, and not until now for a wide variety of zircon matrices as was done here. For instance, Sack et al.¹⁷ reported U–Pb ages on 10 μm wide × ≤15 μm deep pits (∼30 seconds of ablation or ∼150 laser pulses) for an ID-TIMS dated zircon reference material with U and Pb isotopes measured by pulse counting using a LA-ICP-quadrupole ICP-MS instrument with a laser fluence of 1.8–3 J cm⁻². Thirty analyses of the zircon by LA-ICP-MS gave a weighted mean ²⁰⁶Pb/²³⁸U age that is accurate to within 1.9% of the ID-TIMS age. However, the precision was poor, up to 6% (2SD) uncertainty on individual spots. Kooijman et al.⁶ reported U–Pb ages on 12 μm wide × 20 μm deep pits (36 seconds of ablation or ∼360 laser pulses) for 91500 and Plešovice zircon with U and Pb isotopes measured by LA-ICP-single-collector-SF-MS and a laser fluence of ∼5 J cm⁻². Weighted mean ²⁰⁶Pb/²³⁸U ages for the two zircon-reference materials were within 1% of the ID-TIMS ages. 2SE on the mean ²⁰⁶Pb/²³⁸U ages for the zircon was 1.1% and 2SD for individual analyses was 3–6%.

The 50 laser pulses preferred for the LA-ICP-single-collector SF-MS measurements in this study are at the high end of the range (25–48 pulses) needed to produce analyses of similar accuracy and precision by LA-ICP-multi-collector SF-MS, as reported by Gehrels et al.,¹¹ Johnston et al.¹⁶ and Xie et al.²⁰ The ability of LA-ICP-multi-collector SF-MS instruments to determine U–Pb ages of similar accuracy and precision with even fewer (<50) laser pulses reflects their greater analyte sensitivity and simultaneous measurement capability, which in principle provides more precise isotopic ratio data over a given ablation time. Thus, while LA-ICP-single-collector SF-MS instruments can produce U–Pb zircon ages of accuracy and precision comparable to LA-ICP-multi-collector SF-MS instruments for spot sizes as small as about 7–10 μm, LA-ICP-multi-collector-SF-MS measurements would be preferred for even smaller (<7 μm) spots. On the other hand, LA-ICP-single-collector SF-MS instruments do not require cross-calibration and drift correction of multiple detectors, making them fit-for-purpose for routine U–Pb zircon age measurements on spot sizes ≥7–10 μm. This is particularly true when sufficient laser fluence is applied for precise measurement of ²³⁵U, allowing the calculation of ²³⁸U from a fixed ²³⁸U/²³⁵U ratio, and avoiding corrections for detector non-linearity^37,38 of large, measured signals of ²³⁸U.

Conclusion

²⁰⁶Pb/²³⁸U ratios for the six zircon reference materials increase steadily with ablation time, reflecting DHF, but exhibit somewhat different patterns of increase for different zircons, producing the major source of uncertainty for the U–Pb ages. In particular, zircon FC-1 exhibits more extreme DHF compared to the other zircons consistent with large amounts of radiation damage based on its Raman spectrum.²⁸ This leads to discordant U–Pb ages for FC-1 when corrected for DHF using zircon 91500 and ablation times of 28 seconds.

A secondary source of uncertainty is differences of typically 2–3% between ID-TIMS-normalized ²⁰⁶Pb/²³⁸U ratios for different zircons near the start of ablation. This may reflect zircon matrix-dependent instrumental mass bias, possibly due to Pb/U fractionation in the ICP,^43,44 rather than DHF at the ablation site.

For 20, 15 and 7 μm spots, the most accurate ages are given by the 10 second (50 laser pulse) ablation intervals. For 10 μm spots, the 15 second (75 laser pulse) ablation intervals give the most accurate data for all zircon reference materials. These conditions provide the smallest offsets between Wetherill LA-ICP-MS U/Pb Concordia ages and the accepted ID-TIMS ages, which are 1.4% or less for each of the zircon reference materials (except FC-1 for the 7 μm spot). Precision determined as 2SE of the weighted mean LA-ICP-MS ²⁰⁶Pb/²³⁸U ages for the zircon reference materials averages 0.72% for the 20 and 15 μm spots using the 10 second ablation intervals, 1.2% for the 10 μm spots using the 15 second ablation intervals, and 1.4% for the 7 μm spots using the 10 second ablation intervals.

Thus, this study suggests that short (10–15 second) ablation times (50–75 pulses) restrict the extent of DHF and time-resolved Pb/U variations between different zircons to a sufficient degree to give U–Pb ages on 7–20 μm spots that are both accurate and precise to better than 1.4% using LA-ICP-single-collector ICP-SF-MS instrumentation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The Mineral Isotope Laser Laboratory (MILL) at Texas Tech University (TTU) is supported in part by the Pevehouse Endowment Fund and start-up funding from TTU. Chris Reudelhuber is thanked for assistance with the LA-ICP-MS analyses in the MILL. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

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Footnote

† Electronic supplementary information (ESI) available: Individual U–Pb analyses are given in Tables A1–A4. See DOI: 10.1039/c8ja00321a

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