| 10 Jun 2026
Causes and mitigation of U-Pb fractionation during LA-ICPMS analyses of zircon using nanosecond excimer laser systems
Abstract. This work investigates the causes of measurement biases in U-Pb dating of zircon by laser ablation inductively coupled mass spectrometry (LA-ICPMS) and possible methods for correction. Plasma-induced biases include oxidative loss of U, which is normally minimized by restricting nebulizer Ar flow at the cost of lower sensitivity, mass bias and ionization efficiency, which together normally produce a negative bias of several tens of percent for 206Pb/238U. There is also an ablation bias due to the relative volatility of Pb, which depends to some extent on the structural state of the zircon. We suggest that oxidation of elements in the plasma is caused by turbulent entrainment of oxygen from the surrounding air and point out a flaw in the design of some movable two-stage ablation chambers that can result in variable degrees of oxidation as position is changed. Measurements of NIST glass and zircon include significant ablation bias even when scanned. The main cause of increasing 206Pb/238U ablation bias seen in zircon and other minerals appears to be sequestration of Pb-depleted melt in fallback and along the sides of the pit. Integrated signal profiles measured using laser pulses at 0.2 Hz combined with modelling of U/Pb fractionation suggest that the earliest Pb/U measurements (first 10 pulses) are affected by decreasing fractionation from a melt pool as it becomes increasingly depleted in Pb. This trend is opposed by deposition of depleted material as fallback, which dominates signal loss for the first 10 pulses but rapidly decreases. The fractionation sequence from the first 10 or so pulses is therefore chaotic. Ratios from the following 50 or so pulses show an approximately linear increase in fractionation. Normalized data from these pulses give trends with higher intercepts and lower slopes for standards with higher radiation damage during the same session. Fractionation and signal decay subsequently rise more slowly but remain linear, probably because fractionation is dominated by deposition in the deepening pit. An ablation fractionation model is proposed based on the drop in measured signal but this cannot be used to estimate accurate bias-free 206Pb/238U ratios because of the number of unconstrained parameters. The best approach for calibrating against an unknown is a direct comparison of all or part of the standard ratio profile with the sample profile after multiplication by a calibration factor. The factor that results in the best fit should represent a ratio of unbiased 206Pb/238U between standard and sample. Software is included to process and calibrate data. Data from Precambrian zircon with well-established 207Pb/206Pb ages suggest that radiation damage below the metamict state results in little bias to discordance. Reverse discordance from metamict zircon appears to be approximately proportional to U concentration. Increased accuracy of 206Pb/238U ages using nanosecond laser systems can most likely be achieved through design improvements rather than data processing. Sensitivity should be increased and ablation bias decreased by reducing the volume in the sampling cup to minimize fallback. Plasma cooling in a flow of N2 gas from a liquid nitrogen dewar should enable increased sensitivity without oxidation of U. The use of H2, instead of He as a carrier gas should reduce fallback and cost but would require design changes to prevent exposure to the plasma before complete venting of O2 from the ablation chamber.
The manuscript entitled, “Causes and mitigation of U-Pb fractionation during LA-ICPMS analyses of zircon using nanosecond excimer laser systems” investigates the fractionation behavior of Pb from U during successive pulses of a laser on (primarily) both crystalline and metamict zircon, and offers possible explanations for these observations that are ubiquitous in laser-ablation geochronologic analyses. Further to explain possible causes of fractionation, the authors present code to correct for this behavior, but in the end, recognize that it has many of the same shortcomings of other data reduction schemes when it comes to zircon that has undergone radiation damage. The data and ideas presented in the manuscript are useful to the geochronologic community with a few caveats. Though the paper was organized fairly well, it lacked a thorough explanation of instrument conditions for each experiment that was performed in the study, was commonly worded ambiguously, and there was insufficient detailed explanation of the ideas and presentation of the data, including figures, to live up to its potential. Many concepts are discussed but not shown (e.g., pit morphology during ablation is discussed early on, but only shown in a figure later on), and others are shown but not discussed (the images of the melted zircon in the laser pits is a complete unknown), but nevertheless, the data and reasoning (which is often hard to follow) are sound, and, if modified, the presentation of this study will be quite useful to those who perform or collect laser-ablation data, whether it be geochonologic data or otherwise.
One concern I have for the study is that it is lacking in repeatability. As someone who collects considerable laser-ablation data, instrument conditions can commonly dictate day-to-day behavior of ablated material analyzed by an ICPMS. Data presented herein appears to be done on multiple occasions, though it is commonly unspecified, and the authors (correctly) point out that one run cannot be compared to another that was run on a different day. Most of the experiments shown indicate trends rather than absolute value corrections and thus do not necessarily need to be repeated, but differences in down-hole fractionation between one day and the next begs a question about how much we can rely on conclusions based on a data from a single session. That said, most of the conclusions do not require that the experiments be repeated, however, I see a good reason to compare datasets that were collected in a single session rather than multiple sessions, and given that the time to repeat some of these experiments is not extreme, I encourage the authors to conduct at least the experiment shown in Figure 11 on the same day, and possibly comment on why they might be different from day to day. In the case of understanding the processes, this would be an important focus area.
A second main comment I would make refers to this and several LA fractionation studies: pulses analyzed in this study seem to go well beyond the number of pulses used in a typical ablation. I don’t know how deep the pits were, but they were only performed on ‘standards,’ which are much bigger than typical grains. I suspect that many unknown grains would not withstand 250 pulses at 3.5 j/cm2. Though it is interesting to see the behavior beyond a typical depth, it should be mentioned what a typical depth is and what part of the profile one should pay most attention to. I am also not sure that the authors ever make an attempt to explain why the slope of the fractionation curve changes at a specific depth or pulse count. I would be interested to know if they have any ideas.
At times, I feel that the reference list is rather small. Many researchers have looked at laser induced elemental fractionation (LIEF, which is not mentioned here), and many others have discussed the physical coupling between the incident radiation and crystal structure which created the ablation plume. Though including this background material is not strictly necessary, it would add a lot of insight into the processes investigated herein.
Though I have many comments below that I think could add to the clarity and significance of the manuscript, I commend the authors on tackling a difficult problem, the solution to which could have a rather large impact on our ability to solve geologic problems.
Line item comments:
73. Need more instrument information. What kind of cell?
74. I belive this is meant to be j/cm2
77. Where is the table of instrument conditions? See Horstwood et al., 2016 for suggestions.
120. If the idea is increased turbulence entraining oxygen, then moving the torch closer to the cones should not be the same as increasing the Ar flow, no? Please explain.
146. Another possibilty is that it is an interference. Please explain why you think O is increased during the ablation of calcite over zircon. I imagine the spots are larger, for one.
147. Monazite is a phosphate, but you just said there was no detectable change for monazite.
180. A picture is worth 1000 words. Not everyone has this system, so use of "normally" isn't really appropriate. The newest designs tend to have a fixed cup and a stage enclosed in the cell. Somewhat surpisingly, these also have differences in apparent oxide production from one location to another.
190. But earlier it was noted that oxidation decreases through time?
212. Is this your data or someone else's conclusion?
244. What study are the 206/238 ratios of the NIST glasses from?
266. I believe you mean TRA?
270. Ambiguous. Does this fs laser have a shorter wavelength too? Please expand.
298. And elswhere: micron is usually abreviated μm.
300. How many pulses?
302. What were the conditions of this pass?
304. An order of magnitude reduction from an uneven surface? How deep was the pit? Was the laser out of focus at the bottom of the pit? Why does the pit look so strange?
306. It is unclear how this experiment was performed. Also, why would you expect lower ratios at the bottom of the pit? Don't we expect the analyses from the bottom of the pit to have higher ratios? Please elaborate on these points.
310. This is not clear, and not explained in the text. What was done to create these laser pits, and why does it represent a typical ablation?
324. Same question here about the pit - is there an intention to create a different style of ablation and pit? It isn't clear in the text.
398. Why not just use 90Zr? Why use an oxide?
407. Pulses, not passes.
427. Are there any other possible reasons? Possibly a change in the coupling of the laser with the material, or other? Is there anyway to test this?
431. This is where an explanation such as that given in figure 12 is necessary.
448. This is confusing. Are you saying that the radius of the base of the pit reduces is reduced by 20% at the bottom than the top? What do you mean that the emission (emission of what?) is reduced to half with a 20% reduction in area? If the mass of the ablation plume is proportional to area, then a 30% reduction leads to half the area (0.7^2 = 0.49).
450. This sentence makes a claim that forcing an analytical change would create a physical change which is impossible, and is one of the reasons that this section is confusing.
452. It is strange that rather than examine pit shape, depth change per pulse, etc., with the instrumentation used within this study that the authors refer to a study 30 years prior, when instrumentation would have been significantly different. Why not make and measure pits in this study?
458. It seems magical that the first pulses have only plasma-biased effects. Are you implying that there is no laser-induced fractionation during the first pulse?
484. What was the spot size? Did you perform any experiments with a different spot size? It would be interesting to know if these trends are related to the aspect ratio or just the first 10 pulses.
486. Why? Doesn't the previous statement imply that the first several pulses are the most important? Fig 11 shows early pulses in 11A but seems to start at 5 in 11B. There needs to be more explanation.
487. Why did you do this? If the Y-axis can't be compared, how do we know the slopes can be compared? Is it possible that the tuning conditions are partly responsible for the change in signal during ablation? Can you repeat the experiment and show that the patterns look the same between sessions?
492. Do you mean pulses 5-15?
493.This wording is ambiguous. It looks to me that the trend in the first several pulses can not be distinguished in the latter pulses. This is different than was stated earlier, which was that the first several pulses were unchanging.
508. The wording here is confusing. The authors speak of the base and the wall of the pit, but the deposition shown in the figure is both on the base (only at the edge) and the wall of the pit. There is a disconnect between the language used and the conception of the process which needs to be changed/improved.
513. “unrealistic”: Is is just that the angle of the deposit on the figure is exaggerated?
515. Interesting that a moat at the base of the pit is ubiquitous. One might assume that laser energy is uneven across the beam (highest at edges), but the process described here would put that assumption into question, or at least imply that the deposition and reablation at the wall is different than its aspect ratio might suggest.
518. I don't believe it is explicitly stated here, but I think the authors are implying that the amount of deposition on the walls of the laser pit increases with depth. Possibly because the height of the wall increases with depth, yielding more surface area for deposition and less space for the ablation plume to escape the pit. Would be nice to state this more clearly. An image similar to that of figure 12, but with a more thorough explanation, would be rather helpful in doing so.
526. Show this figure earlier in the manuscript and either expand on it here or where it is earlier.
533. Should be labeled B1?
546. This can't be seen in any of the diagrams. Why include it?
547. What does 1 refer to?
552. Are you implying that we shouldn't expect any recycling? Did you model it with some recycling? What are the results from that?
558. Why not point to all these phenomena in the figure? It is difficult to follow with text only.
560. Better to say U signal than U concentration.
561. This discussion goes back and forth between discussing one sample and both samples. It needs to be rewritten for clarity.
583. Fix subscript.
593. I recall this recommendation, however, it is hard for me to believe that Gehrels was ablating his sample for 250 pulses. Please check this work to make sure it is correct.
598. Normalized to another reference material? To expected plasma bias? Is the the 68 ratio? This is also confusing.
601; What do you mean by this? The explation has to be here in the text, not in a supplement.
606. This isn't much different from just the average of of the data points (0.2 vs 0.15%). Are you assuming that the intercept is correct or some other part of the curve?
624. Please state here whether this approach is applied to large datasets or just single data points.
635. What are the analytical conditions here? Same as all the other experiments? Is 250 pulses typical for an experiment run on a quadrupole? How deep are these holes? Describing the analytical conditions and showing and describing laser pits, etc. in each of these experiments would go a long way.
645. Above it states that the ZrO decays more rapidly in metamict zircon, but here it says the the average is the same, so it must mean that ZrO in metamict zrn starts out at a higher value, but the decay rate is faster, leading to a similar average. This must say something about the difference in interaction of the laser with metamict vs crystalline zrn.
647. Were the pits deeper? Showing profiles of the data (ZrO, 68, U) of the different metamict zircon vs pristine zircon would be helpful to make predictions here.
654. What is meant here by discordance - the different between the measured 68 and 76 or between 68 and the accepted value? The former would have a larger uncertainty.
715. This is highly confusing. It would appear that there are 3 different fractionation terms: cloud, deposit, signal. The "0" explanation is for deposit; this is understandable. The second explanation is for a cloud?? The third would be for a signal, but how can one discuss the signal of a pulse relative to the following ablation cloud?
DR Tables. These are fairly raw and could be organized with the reader in mind.