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M.
Guillong
*^{a},
A.
von Quadt
^{a},
S.
Sakata
^{b},
I.
Peytcheva
^{a} and
O.
Bachmann
^{a}
^{a}Department of Earth Sciences, ETH Zurich, 8092 Zurich, Switzerland. E-mail: guillong@erdw.ethz.ch
^{b}Department of Geology and Mineralogy, Kyoto University, Kyoto 606-8502, Japan

Received
8th January 2014
, Accepted 4th April 2014

First published on 4th April 2014

Zircon Pb–U dating has become a key technique for answering many important questions in geosciences. This paper describes a new LA-ICP-MS approach. We show, using previously dated samples of a large quaternary rhyolitic eruption in the Kos–Nisyros volcanic centre (the 161 ka Kos Plateau Tuff), that the precision of our LA-ICP-MS method is as good as via SHRIMP, while ID-TIMS measurements confirm the accuracy. Gradational age distribution over >140 ka of the Kos zircons and the near-absence of inherited cores indicate near-continuous crystallisation in a growing magma reservoir with little input from wall rocks. Previously undated silicic eruptions from Nisyros volcano (Lower Pumice, Nikia Flow, Upper Pumice), which are stratigraphically constrained to have happened after the Kos Plateau Tuff, are dated to be younger than respectively 124 ± 35 ka, 111 ± 42 ka and 70 ± 24 ka. Samples younger than 1 Ma were corrected for initial thorium disequilibrium using a new formula that also accounts for disequilibrium in ^{230}Th decay.

Due to the complexity^{17,18} of LA-ICP-MS for U–Pb zircon dating, the method requires care and results must be validated. However, it can now be shown that it is possible to generate accurate results with a good precision over extended periods of time using matrix matched standard reference materials and secondary standards.^{4,6,7,9,14,19–21}

As dating techniques improve, young samples are of increasing interest; they allow (1) improved stratigraphic constraints on active volcanoes, (2) investigation of magma chamber processes that occur over timescales less than a few 100 ka, and (3) inter-comparison of different dating methods at high resolution. The analysis of young zircons in the range of 50 ka to 10 Ma is challenging due to the low radiogenic Pb content, problems with common Pb contamination and the necessity to correct for the initial Th disequilibrium.^{22} However, new results presented in this paper on young zircons (in the range of 50 to 500 ka) from the Kos–Nisyros volcanic centre (Aegean arc) show that we can reproduce SHRIMP precision and accuracy, while analysing a lot more zircons with relative ease. We also show that the LA-ICP-MS technique can provide a powerful means of finding the youngest zircon within a population to estimate the eruption age of silicic units that would not be easily dateable otherwise. We stress, however, that this youngest zircon within a population can still be much older than the true eruption age.

Sample | Coordinates | Th/U magma ± 2SE (30%)^{a} |
Source^{b} |
Th/U zircons ± 2SE |
f
_{(Th/U)} ± 2SE |
---|---|---|---|---|---|

a Analytical uncertainty: for disequilibrium correction a minimum of 30% uncertainty is assumed for error propagation. b WR = whole rock analysis. | |||||

NS07 | N36°36.728 E 27°09.407 | 4.0 ± 0.1 (1.2) | WR analysis | 0.95 ± 0.11 | 0.236 ± 0.028 |

NS24 | N36°34.630 E 27°10.560 | 3.85 ± 0.43 (1.15) | WR analysis (= NISP2) | 0.89 ± 0.16 | 0.231 ± 0.041 |

NISP2(1) | N36°36.986 E 27°09.814 | 3.85 ± 0.43 (1.15) | WR analysis | 0.95 ± 0.22 | 0.246 ± 0.057 |

KPT04-36 | N36°46.097 E 27°06.990 | 3.3 ± 0.3 (1) | WR analysis | 0.83 ± 0.18 | 0.252 ± 0.056 |

KPT04-24 | N36°57.791 E 26°57.294 | 3.3 ± 0.3 (1) | WR analysis | 0.59 ± 0.05 | 0.181 ± 0.016 |

Laboratory & sample preparation | |

Laboratory name | Dept. of Earth Science, ETH Zurich |

Sample type/mineral | Zircons |

Sample preparation | Conventional mineral separation, 1 inch resin mount, 1 μm polish to finish |

Laser ablation system | |

Make, model & type | Resonetics resolution 155 |

Ablation cell & volume | Laurin Technics 155, constant geometry, aerosol dispersion volume <1 cm^{3} |

Laser wavelength | 193 nm |

Pulse width | 25 ns |

Energy density/fluence | ∼2.0 J cm^{−2} |

Repetition rate | 5 Hz |

Spot size | 30 μm |

Ablation rate | ∼75 nm per pulse |

Sampling mode/pattern | Single hole drilling, 5 cleaning pulses |

Carrier gas and flow | 100% He, 0.7 l min^{−1} |

Ablation duration | 40 seconds |

ICP-MS Instrument | |

Make, model & type | Thermo Element XR SF-ICP-MS |

Sample introduction | Ablation aerosol only, squid aerosol homogenisation device |

RF power | 1500 W |

Make-up gas flow | ∼0.95 l min^{−1} Ar (mixed with He inside ablation cell funnel) |

Detection system | Single detector triple mode SEM, analog, Faraday |

Masses measured | 202, 204, 206, 207, 208, 232, 235, 238 amu |

Integration time per peak | 12 ms (masses 202, 204), 20 ms (masses 208, 232, 235, 238), 40 ms (masses 206, 207) |

Total integration time per reading | 0.202 seconds |

Dead time (ns) | 8 ns |

Typical oxide rate (ThO/Th) | 0.18% |

Typical doubly charged rate (Ba^{++}/Ba^{+}) |
3.5% |

Data processing | |

Gas blank | 10 second prior to each ablation spot |

Calibration strategy | GJ-1 used as a primary reference material, Plesovice, 91500 & Temora used as secondary's for quality control |

Reference material information | GJ-1 ^{206}Pb/^{238}U 0.09761 ± 0.0002 (Wtd mean of TIMS analysis)^{35} |

Data processing package used | Iolite 2.5 with VizualAge |

Mass discrimination | Mass bias correction for all ratios normalized to the primary reference material |

Common-Pb correction | No common-Pb correction applied |

Uncertainty level & propagation | Ages are quoted at 2 SE absolute, propagation is by quadratic addition. Reproducibility of reference material uncertainty is propagated^{10} |

Quality control/validation | Plesovice: Wtd ave. ^{206}Pb/^{238}U age = 338.3 ± 1.7 (2 SE, MSWD = 3.1, n = 16 in 3 sessions), Temora: Wtd ave. ^{206}Pb/^{238}U age = 421.6 ± 2.0 (2 SE, MSWD = 3.4, n = 18 in 3 sessions), 91500: Wtd ave. ^{206}Pb/^{238}U age = 1064.8 ± 3.3 (2 SE, MSWD = 0.44, n = 8 in 1 session) |

Th disequilibrium correction and error propagation |
^{206}Pb/^{238}U ages of the samples were corrected using eqn (3). Errors from (206/238) measured, f_{(Th/U)}, λ_{238} (1.55125 × 10^{−10}, 0.11% 2δ),^{36} and λ_{230} (9.17055 × 10^{−06}, 0.15% 2δ)^{37} are propagated |

Zircon ages can be corrected for common Pb contamination using different methods. The most commonly used approach is via measuring ^{204}Pb and using common Pb isotope ratios to subtract the common Pb component from the radiogenic Pb isotopes. It is also widely used to calculate the intercept of the measured ratios with the concordia curve assuming a common Pb composition^{38} or applying the routine proposed by Andersen.^{39} These common Pb corrections were considered, but due to the very low 204 counts and the isobaric interference from ^{204}Hg, it was not possible to measure the ^{204}Pb counts with sufficient precision. The ^{207}Pb signal was also very low and therefore not precise enough to reliably correct for common Pb. All age results presented in this work are therefore not common Pb corrected. With the help of VizualAge and a live Concordia diagram common Pb contaminated signals are easily recognised as a shift of the data point ellipse away from the Concordia towards common Pb composition as shown in the ESI, Fig. 1–7.† Integration intervals are, where possible, set to exclude any common Pb. As few as 1 analysis (KPT samples) to up to 50% of analyses (NS07) with remaining common Pb were discarded based on live Concordia diagrams and the offset to the Concordia as is shown for all samples in the ESI, Fig. 1–7.† These Concordia plots show results prior to the Th disequilibrium correction and are therefore expected to be discordant.^{40} Although all masses were measured, only the ^{206}Pb/^{238}U age is considered precise and accurate enough. The other ratios (^{207}Pb/^{235}U, ^{208}Pb/^{232}Th and ^{207}Pb/^{206}Pb) could not be measured with sufficient precision and accuracy, partly due to the influence of common Pb on these ratios and the low abundance in such young zircons.

(1) |

As zircon grows from a melt, Th gets fractionated from U, imparting a disequilibrium in ^{230}Th (an intermediate product in the ^{238}U decay series) that has to be corrected to get an accurate age. The initial Th disequilibrium correction has been described in detail in several papers.^{22,40–43} This fractionation is described by f_{(Th/U)} (eqn (2)) that needs to be determined.

(2) |

There are several approaches to quantify the fractionation factor f_{(Th/U)}. With LA-ICP-MS and SHRIMP, Th/U in the mineral is directly measured. Th/U in the magma is typically considered as a constant and can be estimated by whole rock or volcanic glass analysis. However, the assumption of a homogeneous magma with constant Th/U is questionable. Instead of assuming a constant Th/U in the magma, it is possible to assume a constant fractionation and variations in Th/U in the magma. An investigation of these two approaches^{43} found larger scatter in the data using the constant fractionation compared to the constant magma value. A constant magma value implies varying fractionation, while a constant fractionation would imply variations in the Th/U ratio within the magma. Reality might be in-between and the assumption of a constant Th/U in the magma was weighted more^{43} and this approach was also used in this work. The uncertainty on Th/U in the magma was found to be 14% estimated from isotope dilution Th/U measurements.^{43} Another approach to overcome the problem of unknown Th/U in the magma uses the difference between the measured ^{208}Pb/^{232}Th age which is not affected by disequilibrium and the ^{206}Pb/^{238}U age to get f_{(Th/U)}.^{44} Our data did not allow this approach as the ^{208}Pb/^{232}Th age has too high uncertainty and variability possibly due to common Pb contamination.

The influence on the corrected age of changing the Th/U values in the magma from 2 to 4 has been found to be 1.4% for 1 Ma old zircons, decreasing for older zircons.^{40} This is a problem for high precision TIMS dating with uncertainties of 0.1% but not for LA-ICP-MS where the uncertainty for such young samples is ∼1–10%. The typical Th disequilibrium correction for zircons is in the range of 50 to 110 ka.^{41} The correction usually is accurate to ±15 ka due to the above mentioned uncertainties in the Th/U ratio determination in the magma.^{41}

The growth of the ^{206}Pb/^{238}U ratio can be calculated correctly with this eqn (1) only after ^{230}Th reaches radioactive equilibrium. This equation does not accurately correct when ^{230}Th is still in disequilibrium which is the case up to about 300 ka. In fact, the isotopic growth curve defined from eqn (1) does not pass the origin point when t = 0 and ^{206}Pb = 0. Therefore, for the chronology of the zircon crystals with ages below 1 Ma, a more accurate correction must be applied as shown in eqn (3). The initial assumptions and the derivation of eqn (3) can be found in the ESI† and in the study of Sakata et al., 2013.^{44}

(3) |

As the ages measured in this work are between 70 ka and 400 ka, eqn (3) was used to get accurate results. A comparison of ages determined using both equations is discussed below. Th/U whole rock data available on the Kos Plateau Tuff are limited and the precision based on few samples and bulk chemistry analysis is likely overestimating the uncertainty on Th/U in the magma itself. Therefore we assume an uncertainty on Th/U in the magma of 30% and propagate this uncertainty to the corrected age.

The approach of using an old SRM to quantify these very young zircons might be affected by small differences in the ablation behaviour between the SRM and the samples, especially the downhole fractionation. Parameters that can have and influence include the amount of radiation damage, trace element composition, transparency, crystal orientation and carrier gas composition and are currently under investigation and discussion.^{47,48} The individual contributions to the uncertainty on the final age were calculated. The most important one (∼65% of the total error) is the uncertainty from the measured ^{206}Pb/^{238}U ratio. The remaining 35% comes in approximately even parts from the uncertainty on the decay constants λ_{232} and λ_{238} and the uncertainty of the fractionation factor f_{(Th/U)}.

The comparison between age data acquired using LA-ICP-MS in this study with the published data using SHRIMP^{27} shows remarkable agreement. Zircon ages cover the same range between the eruption age of 161 ka (^{40}Ar/^{39}Ar sanidine dating^{23}) and approximately 350 ka (Fig. 1), with very similar uncertainties (although errors on Th-corrections were not propagated in the SHRIMP results^{27}). Ages are evenly distributed in this range, with no major gaps in crystallization that can be seen within the precision of the method (ESI, Fig. 8 and 9†). It reinforces previous interpretations that zircons have been crystallizing over an extended period of time (>140 ka), recording the growth and maturation of the silicic magma reservoir in the upper crust.^{27} Based on the precision we obtain, it is not possible to distinguish between several discrete crystallisation events and a real continuous crystallisation in the given time frame.

Fig. 1
^{206}Pb/^{238}U age results for SHRIMP^{27} (narrow error bars), ID-TIMS (medium error bars) and LA-ICP-MS (wide error bars) for Kos Plateau Tuff samples including 2 standard errors. Only LA-ICP-MS and ID-TIMS results have the errors from the Th disequilibrium correction propagated. 2 old cores found with LA-ICP-MS and integrations with common Pb are not plotted. |

ID-TIMS measurements on 7 selected grains (ESI Table 2†) show again a remarkable overlap with SHRIMP and LA-ICP-MS ages. The age of 6 grains falls well within the range of all SHRIMP and LA-ICP-MS data. One zircon with an age of 410 ± 27 ka can be treated as an outlier. The distribution of ID-TIMS ages from 187 ± 15 ka to 262 ± 16 ka supports both the accuracy of the LA-ICP-MS measurements and the interpretation of zircon crystallization over an extended time. All details on the ID-TIMS method and measurements are given in the study of von Quadt et al., 2014.^{34}

Although no old grains were found using SHRIMP, two inherited cores stand out in the LA-ICP-MS analyses. Precise ages on those cores cannot be given, as they are a mixture of young overgrowth and an old core, but they are very old, with ages in the range of 1 to 2 Ga (see ESI, Table 1†). In general, LA-ICP-MS is much more sensitive in finding inherited cores as LA removes about 10 times more material per analysis than SHRIMP (crater depth for LA is 10–15 microns while SHRIMP craters are only 1–2 microns deep).

As our LA-ICP-MS method has shown its high degree of accuracy and precision especially in comparison to ID-TIMS data, we dated 3 younger, hitherto undated samples from the nearby Nisyros volcano. Using the youngest zircons as the best estimate of the eruption age, results yield (1) 124 ± 35 ka for the Lower Pumice (NISP2(1)), (2) 111 ± 42 ka for the Nikia Flow (NS24), and 70 ± 24 ka for the Upper Pumice (NS07) as shown in Fig. 2. All errors are given as 2 SE. Only a limited number of zircons could be separated from these samples, especially NS24, as they may have been zircon-undersaturated prior to eruption.^{25,26,49} By analysing more zircon grains from these samples it is likely that the spread increases, therefore the youngest zircon found can only be interpreted as maximum ages.

Fig. 2 U–Pb age results for Nisyros samples including 2SE. Eruption ages for these units are assumed to be at the youngest zircon age or younger. |

The different Th disequilibrium corrections (eqn (1) and (3)) also have a significant impact on the final age for young samples. Using sample NS07, we show that the higher the measured Th/U, the less is the correction (Fig. 4). The correction is as previously reported in the range of 70–110 ka (ref. 41) towards older age compared with the uncorrected age. For ages above 300 ka, the correction model of either eqn (1) or eqn (3) has limited influence on the final age. For ages younger than 300 ka, the correction model becomes important. A zircon with an age as young as 70 ka calculated using eqn (3) is more than 50% older (110 ka) when using eqn (1), which is outside the error bar in that case.

The Th/U ratio in the magma measured by whole rock data or assumed influences in a limited way the final age as was investigated and is shown in Fig. 4. The grey lines represent the corrected ages using two different Th/U ratios for the magma. The measured Th/U ratio was found to be 4 for this sample (whole rock analysis, Table 1). Changing Th/U in the magma to either 8 or 2, instead of 4, changes the corrected age by about +10 or −20 ka respectively, which is always within the uncertainty. As the variation of Th/U in the zircons is lower than a factor of 4, all corrected ages assuming a constant fractionation factor f_{(Th/U)} and varying Th/U ratios in the magma would fall between the two grey lines. Based on these findings, the method including the Th disequilibrium correction gives accurate results for the LA-ICPMS technique. However, more precise and accurate Th/U ratios in the magma are needed for high precision ID-TIMS dating. A possibility might be analysing melt inclusions in zircons that represent the local magma composition and fractionation factors can be determined for individual zircons.

To further improve the method, a reliable common Pb correction is necessary. Possibilities include better detection of ^{207}Pb and the ^{207}Pb/^{206}Pb ratio by increasing the repetition rate and/or crater size and increasing the dwell time on mass 207, keeping ^{238}U in the pulse counting mode to avoid detector cross-calibration problems. Due to the low count rate on Pb isotopes, a bias in calculated ratios is possible as the mean-of-ratios can be different from the ratio-of-means. A simple comparison between the two calculations showed an average bias of the ^{206}Pb/^{238}U ratio smaller than 3% for all 11 good analyses of sample NS07 well below the precision of these measurements and without down-hole fractionation correction.

Th disequilibrium correction is essential for zircons younger than 10 Ma and the improved equation (3) should be used for zircons younger 0.5 Ma. The influence from the uncertainty of f_{(Th/U)} is relatively small in comparison to the overall uncertainty for analysis with LA-ICP-MS. Nevertheless errors from the correction should be propagated.

The evenly spread age distribution of the Kos Plateau Tuff samples (from 320 ka to 180 ka) is interpreted as recording near-continuous crystallisation of zircons in a persistent magma chamber. A probability density plot (ESI Fig. 8†) does not show a normal distribution of the KPT ages that would be expected from a sharp single crystallisation age. Therefore the calculation of a weighted mean of all the 132 zircons from all 3 techniques (SHRIMP, ID-TIMS and LA-ICP-MS, ESI Fig. 9†) is meaningless in this case, as it results in an weighted mean age of 238.3 ± 7.1 ka with a MSWD of 7.8 (see also previous discussion^{50}).

The fact that only 2 inherited cores were found, despite the very large number of analyses, suggests that the silicic magma reservoir was mostly isolated from its upper crustal wall rocks. Such a near closed-system fractionation is confirmed by the unradiogenic isotopic ratios of the Kos Plateau Tuff^{25,27,51} and is consistent with thermal models of upper crustal reservoirs that interacted in a limited way with their wall rocks.^{27,50,52,53} The fact that only very few zircons were found with ages in the range of 20 ka before eruption is also consistent with the interpretation that the magma was rejuvenated prior to eruption by fresh recharge,^{24} reducing the growth and even causing resorption of zircons.

The ages of the youngest zircons found in the Nisyros samples agree with previously determined stratigraphy^{25,54} and provide the first direct geochronological estimates of the eruption ages for the following three deposits: (1) Lower Pumice, (2) Nikia Flow, and (3) Upper Pumice. We stress that these are maximum ages, as they may all be zircon-undersaturated prior to eruption.^{26} It is demonstrated that the Kos–Nisyros system took at least several tens of thousands of years (from 160 ka, eruption of KPT to ∼120–130 ka, maximum eruption age of the Lower Pumices on Nisyros) to generate another explosive silicic eruption after the caldera-forming Kos Plateau Tuff. All three units also display an extended range of zircon ages, suggesting similar crystal recycling to that observed for the Kos Plateau Tuff^{27} and many other eruptions (e.g., see reviews by Costa, 2008 and Simon et al., 2008).^{41,55} A comparison between zircons with and without chemical abrasion using LA-ICP-MS is an ongoing project with the Kos Plateau Tuff and related units.^{34}

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## Footnote |

† Electronic supplementary information (ESI) available: Table with all results of LA-ICP-MS and ID-TIMS analyses, ESI Fig. 1–9, derivation of the Th disequilibrium correction. See DOI: 10.1039/c4ja00009a |

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