GZ7 and GZ8 – Two Zircon Reference Materials for SIMS U‐Pb Geochronology

Here, we document a detailed characterisation of two zircon gemstones, GZ7 and GZ8. Both stones had the same mass at 19.2 carats (3.84 g) each; both came from placer deposits in the Ratnapura district, Sri Lanka. The U‐Pb data are in both cases concordant within the uncertainties of decay constants and yield weighted mean 206Pb/238U ages (95% confidence uncertainty) of 530.26 Ma ± 0.05 Ma (GZ7) and 543.92 Ma ± 0.06 Ma (GZ8). Neither GZ7 nor GZ8 have been subjected to any gem enhancement by heating. Structure‐related parameters correspond well with the calculated alpha doses of 1.48 × 1018 g−1 (GZ7) and 2.53 × 1018 g−1 (GZ8), respectively, and the (U‐Th)/He ages of 438 Ma ± 3 Ma (2s) for GZ7 and 426 Ma ± 9 Ma (2s) for GZ8 are typical of unheated zircon from Sri Lanka. The mean U mass fractions are 680 μg g−1 (GZ7) and 1305 μg g−1 (GZ8). The two zircon samples are proposed as reference materials for SIMS (secondary ion mass spectrometry) U‐Pb geochronology. In addition, GZ7 (Ti mass fractions 25.08 μg g−1 ± 0.18 μg g−1; 95% confidence uncertainty) may prove useful as reference material for Ti‐in‐zircon temperature estimates.


Supplement 1. Details for analytical procedures in the five ID-TIMS laboratories ID-TIMS analyses at NIGL
Isotope ratio measurements were made using a Thermo Scientific Triton thermal ionization mass-spectrometer. Lead was either measured in peak hopping (dynamic) mode on a single MasCom secondary electron multiplier (SEM) for lower beam intensities, or more typically, by a dynamic combined Faraday-SEM peak hopping mode whereby mass 205 was cycled with masses 201, 203 and 204 in the axial SEM, and measured simultaneously with either mass 202 and 205, or masses 205-208 in Faraday cups with 10 11 Ω resistors. Linearity and dead time corrections on the SEM were monitored using repeated analyses of the NBS 982, NBS 981 (Catanzaro et al. 1968) and CRM U500 (Condon et al. 2010) standards. Uranium was run as an oxide (UO 2 ) and measured in static mode on Faraday detectors equipped with 10 12 Ω resistors. Raw U and Pb data were filtered using the Tripoli software program . The Pb and U mass fractionation was calculated in real-time based on the isotopic composition of the double spiked ET2535 (v. 3.0) tracer. Uranium oxide measurements were corrected for isobaric interferences using an 18 O/ 16 O value of 0.00205. The Pb measurements were corrected for isobaric interferences of BaPO 3 and Tl using the monitoring masses of 201 and 203. Data reduction, date calculation/presentation and uncertainty propagation were done in ET_Redux . Uranium blanks were assumed to be 0.1 pg ± 0.01 pg (1s). Common Pb was attributed entirely to laboratory blank using a calculated Pb isotopic composition for the laboratory blank of 206 Pb/ 204 Pb = 18.10 ± 0.05; 207 Pb/ 204 Pb = 15.55 ± 0.02 and 208 Pb/ 204 Pb = 37.82 ± 0.16 (1s uncertainties). Note that at NIGL, several additional analyses were subjected to chemical abrasion (CA-ID-TIMS) for comparison, and that procedure used an abridged approach following Mattinson (2005).

ID-TIMS analyses at University of Oslo
Zircon fragments were first cleaned with HNO 3 , H 2 O and acetone, and then weighed on a microbalance. Fragments were transferred to Krogh-type Teflon bombs and dissolved in HF at 195 °C after adding a 202 Pb-205 Pb-235 U spike. Analysis of the Pb and U isotope compositions was done by means of a Finnigan MAT262 mass spectrometer. Further analytical details are documented elsewhere (Corfu 2004). All isotope ratios and ages were corrected for fractionation, spike, blank ( 206 Pb/ 204 Pb = 18.3; 207 Pb/ 204 Pb = 15.555) and initial common Pb (based on Stacey and Kramers 1975). Errors were calculated by quadratic propagation of the main sources of uncertainty using an in-house program. The U-Pb ratio of the spike used is adapted to 206 Pb/ 238 U = 0.015660 for the EARTHTIME ET100 solution, as obtained with the EARTHTIME ET2535 spike  at the NERC Isotope Geosciences Laboratory.

ID-TIMS analyses at University of Geneva
Isotopic analyses were performed on a Thermo Scientific Triton mass spectrometer equipped with a MasCom discrete dynode electron multiplier. The linearity of the multiplier was calibrated using CRM U500 (Condon et al. 2010), NBS 982 and NBS 983 (Catanzaro et al. 1968) solutions. The deadtime for the SEM was determined to be constant at 22.5 ns for up to a count rate of 1.3 × 10 6 s -1 and at a Faraday/SEM yield between 93 % and 94 %. During the time of the measurements isobaric interferences from BaPO 2 + or Tl + were monitored by measuring masses 201 and 203 in spiked, and masses 202 and 205 in unspiked samples. Since no statistically significant signal was observed on the controlled masses, no correction was applied. Lead-isotope fractionation was corrected based on average Pb fractionation factors determined by EARTHTIME 202 Pb-205 Pb-233 U-235 U tracer and measurements of NBS 981 (Catanzaro et al. 1968) standard (0.13 % amu -1 ± 0.02 % amu -1 ). The U mass fractionation for the same analyses was calculated using the 233 U/ 235 U ratio of the double spike solution (0.99506 ± 0.01 %, 1s). The average U fractionation factor was 0.08 % amu -1 ± 0.02 % amu -1 (1s All common Pb in the zircon analyses was attributed to the procedural blank with the following Pb isotopic composition: 206 Pb/ 204 Pb = 18.18 ± 0.12; 207 Pb/ 204 Pb = 15.50 ± 0.05; 208 Pb/ 204 Pb = 37.58 ± 0.16 (1s). Uranium blanks were <0.1 pg and did not influence the degree of discordance at the age range of the studied samples, therefore a value of 0.0005 pg (±50 %) was used in all data reduction. The initial statistics, data reduction and age calculation were done using the TRIPOLI and Redux software . The accuracy of the measured data was assessed by repeated analysis of the EARTHTIME ET100 synthetic solution (Condon et al. 2008) yielding an internal reproducibility in 206 Pb/ 238 U dates of better than 0.05 %. The ET100 synthetic solution measured with EARTHTIME 202 Pb-205 Pb-235 U-238 U tracer yielded a mean 206 Pb/ 238 U ratio of 100.249 Ma ± 0.014 Ma (MSWD = 1.6; n = 10).

ID-TIMS analyses at Boise State University
Isotopic determinations were performed using an IsotopX PhoeniX-62 TIMS. A correction for mass-dependent Pb fractionation was applied based on repeated measurements of NBS 982 (Catanzaro et al. 1968) Pb [on both the Daly ion counter [0.16 ± 0.03 %) amu -1 ; 1s] and the Faraday cups [0.10 × (1 ± 0.02 %) amu -1 ; 1s]. Uranium was run as an oxide (UO 2 ) and measured in static mode on Faraday detectors equipped with 10 12 Ω resistors. The U mass fractionation for the same analyses was calculated using the 233 U/ 235 U ratio of the double spike solution (0.99506 ± 0.01 %, 1s). Raw U and Pb data were filtered using the Tripoli software program ) and the U-Pb dates and uncertainties for each analysis were calculated using the algorithms of Schmitz and Schoene (2007). Uranium oxide measurements were corrected for isobaric interferences using an 18 O/ 16 O value of 0.00206. More analytical details are reported elsewhere (Davydov et al. 2010, Schmitz andDavydov 2012).
Uncertainties are based upon non-systematic analytical errors, including counting statistics, instrumental fractionation, tracer subtraction, and blank subtraction. All nonradiogenic Pb was attributed to laboratory blank with a mean isotopic composition determined by total procedural blank measurements. These error estimates should be considered when comparing the 206 Pb/ 238 U dates with those from other laboratories that used tracer solutions calibrated against the EARTHTIME gravimetric standards.

ID-TIMS analyses at Princeton University
Mass spectrometry was performed using an IsotopX PhoeniX-62 TIMS. Lead analyses were performed using a two-sequence method in peak-hopping mode, switching the axial mass from 204 to 205. The axial mass was measured in a Daly photomultiplier ion counter and higher Pb masses were measured in Faraday cups. This allows for two static analyses in which mass 205 was used to monitor the Faraday-Daly gain in real time. A correction for mass-dependent Pb fractionation was applied based on repeated measurements of NBS 982 (Catanzaro et al. 1968) Pb [on both the Daly ion counter [0.18 × (1 ± 0.02 %) amu -1 ; 1s] and the Faraday cups [0.09 × (1 ± 0.02 %) amu -1 ; 1s] to account for differences in detector specific mass fractionation in the Daly setup. Deadtime on the Daly detector was also monitored by measuring NBS 982 over a large dynamic range up to a count rate of 2.5 × 10 6 s -1 . Although the calculated value drifts by about a nanosecond on a yearly time-scale, the correct value for the measurement period was applied to these data.
Uranium was measured as the oxide UO 2 by a two sequence method similar to Pb, but in this case alternating mass 272 and 267 in the Daly ion counter. The latter is used to monitor Daly-Faraday gain drift, and the former is used to monitor the 18 O/ 16 O composition used to correct for isobaric interferences on UO 2 that arise from, e.g., mass 18 O 16 O 233 U interfering with mass 16 O 16 O 235 U. The Daly gain correction was applied to mass 272 ( 18 O 16 O 238 U). Uranium mass fractionation was monitored cycle-by-cycle from the deviation of measured 233 U/ 235 U from the known tracer 233 U/ 235 U using the EARTHTIME ET535 tracer composition reported in Condon et al. (2015).
Data reduction was performed using the programs Tripoli and U-Pb Redux . All non-radiogenic Pb was attributed to laboratory blank with a mean isotopic composition determined by total procedural blank measurements. This uncertainty in this composition was negligible given the size of zircon fragments chosen for analysis.