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High-Precision Measurement of 186Os/188Os and 187Os/188Os: Isobaric Oxide Corrections with In-Run Measured Oxygen Isotope Ratios Zhu-Yin Chu,*,† Chao-Feng Li,† Zhi Chen,†,‡ Jun-Jie Xu,§ Yan-Kun Di,∥ and Jing-Hui Guo† †

State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China ‡ College of Earth Sciences, University of Chinese Academy of Sciences, Beijing 100049, China § School of Ocean Sciences, China University of Geosciences, Beijing 100083, China ∥ Key Laboratory of Orogenic Belts and Crustal Evolution, MOE, School of Earth and Space Sciences, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: We present a novel method for high precision measurement of 186Os/188Os and 187Os/188Os ratios, applying isobaric oxide interference correction based on in-run measurements of oxygen isotopic ratios. For this purpose, we set up a static data collection routine to measure the main Os16O3− ion beams with Faraday cups connected to conventional 1011 amplifiers, and 192 Os16O217O− and 192Os16O218O− ion beams with Faraday cups connected to 1012 amplifiers. Because of the limited number of Faraday cups, we did not measure 184Os16O3− and 189Os16O3− simultaneously in-run, but the analytical setup had no significant influence on final 186Os/188Os and 187Os/188Os data. By analyzing UMd, DROsS, an in-house Os solution standard, and several rock reference materials, including WPR-1, WMS-1a, and Gpt-5, the in-run measured oxygen isotopic ratios were proven to present accurate Os isotopic data. However, 186Os/188Os and 187Os/188Os data obtained with in-run O isotopic compositions for the solution standards and rock reference materials show minimal improvement in internal and external precision, compared to the conventional oxygen correction method. We concluded that, the small variations of oxygen isotopes during OsO3− analytical sessions are probably not the main source of error for high precision Os isotopic analysis. Nevertheless, use of run-specific O isotopic compositions is still a better choice for Os isotopic data reduction and eliminates the requirement of extra measurements of the oxygen isotopic ratios. mong the osmium isotopes, 187Os is generated by β− decay of 187Re. The 187Re−187Os system has been widely used to constrain the ages of subcontinental lithospheric mantle, examine the recycling of oceanic crust into the mantle, and constrain the influx of extraterrestrial and continental components into the oceans.1 These applications require high precision measurements of 187Os/188Os ratios. In addition to the 187Re−187Os system, 186Os has received particular attention for the past 2 decades due to its potential ability to trace core− mantle interactions in planetary systems.2,3 186Os is produced by the radiogenic decay of 190Pt with a half-life of 468 Ga.4 Because of the very long half-life, combined with the very low abundance of 190Pt (0.014%), the natural variation in the 186 Os/188Os ratio is very small (∼80 ppm variation in the 186 Os/ 188 Os ratio for mantle evolution over 3 Ga). 3 Consequently, in contrast to the 187Re−187Os system, using the 190Pt−186Os system as a geochemical tracer requires extremely precise interference-free measurements.5,6 Negative thermal ionization mass spectrometry (N-TIMS) has high ionization efficiencies for Os (up to 1−5%),7,8 and is therefore widely used for Os isotopic analyses. However, with the N-TIMS method, Os ionizes as a negative trioxide ion, requiring oxygen isotope corrections (e.g., 190Os16O218O

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© XXXX American Chemical Society

interference with 192Os16O3), and more complicated oxygenbased interference corrections (e.g., 186W16O3 interference with 186 Os16O3). Conventionally, O isotope interferences have been corrected using a constant O isotopic composition throughout the analytical sessions. The O isotopic compositions used for the oxygen isotope corrections were either the Nier O isotopic composition or a composition determined for a given instrument using a Re standard and fixed O supply.3,9 Nevertheless, many previous studies such as Harper and Jacobsen,10 Liu et al.,11 Luguet et al.,5 Chu et al.,12 and Chatterjee et al.6 emphasized that the O isotopic composition could vary significantly during a single analysis or between analyses, probably due to the oxygen isotopic fractionation effect. Therefore, theoretically, it would be realistic if the isobaric oxide corrections can be done using oxygen isotopic ratios measured in-run.5,6,12 Recently, Luguet et al.5 reported a method to conduct isobaric oxide corrections with in-run measured oxygen isotopic ratios. However, because of the difficulty aligning the Faraday Received: May 4, 2015 Accepted: August 10, 2015

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DOI: 10.1021/acs.analchem.5b01689 Anal. Chem. XXXX, XXX, XXX−XXX

Article

Analytical Chemistry Table 1. Cup Configuration for Os Isotopic Analysis amplifier (Ω) mass main oxide

L4

L3

L2

L1

central

H1

H2

H3

H4

1011 230 198 16 Pt O2

1011 233 185 Re16O3

1011 234 186 Os16O3

1011 235 187 Os16O3

1011 236 188 Os16O3

1011 238 190 Os16O3

1011 240 192 Os16O3

1012 241 192 Os16O217O

1012 242 192 Os16O218O

cups to collect all the Os16O3− isotopes, 192Os16O217O−, and 192 Os16O218O− in one static collection, they utilized a peakhopping routine; they measured the main OsO3− peaks for 8 s in sequence 1 and O isotopic compositions at masses 236−243 for 4 s in sequence 2, with 4 s of settling time after each peak hop. The method leads to a ∼ 50% loss of measurement time for the main OsO3− peaks. Alternatively, Chatterjee et al.6 recommended measuring O isotopic compositions before and after an analysis via a separate routine and using the mean value as an approximation of the inrun O isotopic composition. However, as sometimes the oxygen isotopic ratios can vary significantly within a measurement run,5,6,11,12 this approach only provides a quasi in-run method. Meantime, the total measurement time for their Oisotope measurements (1 block of 10 cycles with 2 s on the 241−242 peaks and 5 s idle time per cycle) was just approximately 2 min (combining pre- and post-run determinations), such that the measurement precision on oxygen isotopic ratios could probably not be ascertained. Here we present a new static data collection method to determine the in-run oxygen isotopic composition. We measure the main Os 16 O 3 − masses, excluding 184 Os 16 O 3 − and 189 Os16O3−, on Faraday cups connected to conventional 1011 ohm amplifiers and determine the in-run 17O/16O and 18O/16O by simultaneously measuring 241 (192Os16O217O−) and 242 (192Os16O218O−) on Faraday cups connected to 1012 ohm amplifiers. Osmium isotope analyses of reference materials based on this method are presented below.

procedural Os blank was 0.2−0.3 pg with a 187Os/188Os of ∼0.16. Sample Loading. Osmium was loaded onto platinum filaments (99.995% purity Pt wire supplied by H. Cross Company, Moonachie, NJ; 0.03 mm thick, 0.72 mm wide) with Ba(OH)2 as an ion emitter. Prior to loading, the filaments were degassed in air by heating to bright red for 10 min. The amount of Os loaded on the Pt filament for the solution standards was 50 ng. Osmium analyte extracted from international rock reference materials was taken up with 2 μL of HBr and loaded onto the filaments. After being dried down completely using a filament current of ∼1.5 A, 1 μL of Ba(OH)2 activator was loaded to cover the Os load and dried down, also using a filament current of ∼1.5 A; subsequently, the filament current was tuned to ∼2 A lasting for 2 s and then turned off. Thermal-Ionization Mass Spectrometry. Osmium isotope ratios were measured on a Thermo Fisher TRITON Plus thermal-ionization mass spectrometer (TIMS) operated in the negative ion mode7,8 at the Institute of Geology and Geophysics, Chinese Academy of Sciences. During the analytical session, the nine Faraday cups of the instrument were connected to seven 1011 Ω amplifiers and two 1012 Ω amplifiers. A gain calibration of the amplifiers was performed once a day. The cup configuration for Os isotope analysis is shown in Table 1. With the static multi-ion-collection routine, 186Os16O3−, 187 Os16O3−, 188Os16O3−, 190Os16O3−, 192Os16O3−, 192 Os16O217O−, and 192Os16O218O− ion beams were measured, but 184 Os 16 O 3 − and 189 Os 16 O 3 − ion signals and WO3 − interferences were not monitored simultaneously in-run, due to the limited number of Faraday cups. 185Re16O3− was measured to monitor 187 Re 16 O 3 − interference with 187 Os16O3−. 198Pt16O2− was measured to monitor PtO2− interferences. In addition, in order to make a perfect peak alignment for masses 241 ( 192 Os 16 O 2 17 O − ) and 242 (192Os16O218O−) with other Os16O3− peaks, the zoom and focus parameter were optimized once a day. Usually, the dispersion and focus voltages were set to 10 V and −1.5 V, respectively. Isotope data were collected in 20 blocks comprising 20 cycles, each with 8 s integration time. Idle time between blocks was set to 8 s. Peak center and lens focuses were performed every fifth block using the 188Os16O3− (mass 236) ion beam in the center cup. The baseline was measured for 30 s (idle time 10 s) every fifth block after deflecting the ion beam. The ion current during sample analysis was kept at 80% to 120% of the value at the beginning of the measurement using 188Os16O3− as pilot signal. Signals on mass 234 (186Os16O3−) was usually tuned to 200−300 mV for the solution standards and 100−200 mV for the analytes separated from rock standards, and corresponding voltages up to 6−8 V and 2.5−5.4 V for the 192 Os16O3−, respectively. Oxygen pressure in the source was maintained at 2−3 × 10−7 mbar by bleeding oxygen gas into the source chamber. A sample analysis took ∼70 min. The ionization temperature for the OsO3− measurement was usually 800−850 °C.

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EXPERIMENTAL SECTION Reference Materials. The Johnson Matthey Maryland University solution (UMd),5,6 the Durham Romil Osmium Standard (DROsS),5,6 and the Johnson Matthey solution from the Institute of Geology and Geophysics, Chinese Academy of Sciences (hereafter named as IGGCAS) were used for the test measurements. Three whole-rock reference materials, including WPR-1 (Peridotite)13−15 and WMS-1a (Massive sulfide)16 from the Canadian Certified Reference Materials Project (CCRMP), and Gpt-5 (Chromitite)16−20 from the Institute of Geophysical and Geochemical Exploration (IGGE), China, were analyzed for 186Os/188Os and 187Os/188Os ratios to further verify the proposed method. Sample Digestion for Rock Reference Materials and Os Extraction. Several aliquots of Gpt-5 (0.5 g), WMS-1a (0.5 or 1 g), and WPR-1 (2 g) were weighed into precleaned Pyrex borosilicate glass Carius tubes and digested with reverse aqua regia following the procedures described in Shirey and Walker.21 In brief, Gpt-5 and WPR-1 were digested with 3 mL of concentrated HCl and 6 mL of concentrated HNO3 at 240 °C for 72 h. Differently, because of its higher sulfur content (∼28%), WMS-1a was digested with 3 mL of concentrated HCl and 9 mL of concentrated HNO3 at 220 °C for 72 h. Next, Os was extracted from the aqua regia solution into CCl4 and subsequently back-extracted into 4 mL of HBr.22 Finally, the Os samples were further purified via microdistillation.23 The total B

DOI: 10.1021/acs.analchem.5b01689 Anal. Chem. XXXX, XXX, XXX−XXX

Article

Analytical Chemistry

ratios obtained from step 2. The calculations of steps 1−6 could be performed iteratively but it was found to be not necessary. Finally, instrumental mass fractionation was corrected using the 192 Os/188Os = 3.0835,6 together with an exponential law.26 All of the above calculations were performed cycle-by-cycle.

In addition to the main Faraday measurements, possible PtO2−, WO3− and ReO3− interferences were quantified by measuring masses 227 (195Pt16O2−), 231 (183W16O3−), 233 (185Re16O3−) on the secondary electron multiplier (SEM) IC1 B, with virtual masses 244.35, 248.64, 250.75 in the center cup sequentially during peak jumps, before and after each Faraday measurement. Baseline was measured at the start for 30 s (idle time 10 s) after deflecting the ion beam. We measured the 195 16 Pt O2−, 183W16O3−, and 185Re16O3− ion beams with SEM IC1 B rather than IC1 C in order to avoid strong ion beams (188Os16O3−, 189Os16O3−, and 190Os16O3−) entering the H3 or H4 cups connected to the 1012 Ω amplifiers. Each SEM measurement included 1 block of 10 cycles, with 2 s integration time per sequence and idle time of 1 s between sequences, for a total analysis time of ∼2 min. Data Processing. Following measurements, data were exported as Excel files and reprocessed off-line. The off-line data reduction included the following steps: (1) elimination of the 190Os16O218O− contribution to the signal of mass 240 based on 190Os16O3− ion beam intensity, to get the net 192Os16O3− signal (termed as 240In). In this calculation we roughly used the Nier value24 of the oxygen isotope composition; (2) determination of the 17O/16O and 18O/16O ratios with the following equations:5−8 O 1 ⎛ 241I ⎞ ⎟ = ×⎜ 16 3 ⎝ 240 I n ⎠ O

(1)

O 1 ⎛ 242 I ⎞ ⎟ = ×⎜ 16 3 ⎝ 240 I n ⎠ O

(2)

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RESULTS AND DISCUSSION PtO2−, PtCl−, ReO3−, WO3−, and PtO3− Interferences. PtO2− Interferences. Polyatomic PtO2− interferences are listed in Table S-1. Because of the low abundance of 17O and 18O over 16O, the main PtO2− interferences just include the 198 Pt 16 O 18 O − interference with 184 Os 16 O 3 − and the 198 16 17 − Pt O O interference with 183W16O3−.5 As we have not measured 184Os16O3−, the 198Pt16O18O− interference with 184 Os16O3− does not require to be considered. Nevertheless, the 198Pt16O17O− interferes with 183W16O3− and can further affect the WO3− interference corrections on OsO3−. In this study, the 195Pt16O2− signal was in the range of 10 000−445 000 cps, corresponding to a 198Pt16O17O− signal of ∼10−40 cps. It can be inferred that the signal on mass 231 was partly contributed from 198Pt16O17O−. PtCl− Interferences. Among PtCl− ions, 195Pt37Cl− and 198 37 − Pt Cl ions would affect masses 232 (184Os16O3−) and 235 187 ( Os16O3−) (Table S-1), respectively. In addition, 194Pt37Cl− and 196Pt35Cl− interfere with 231W16O3− and 198Pt35Cl− and 196 37 − Pt Cl interfere with 233Re16O3− and would further affect WO3− and ReO3− interference corrections. However, if PtCl− ions were present, signals on mass 231 would be greater than that at mass 233 as both 194Pt37Cl− and 196Pt35Cl− are expected to be larger peaks compared to 198Pt35Cl− and 196Pt37Cl−.5 Fortunately, the absence of such observations during detailed scans on the IC ruled out the presence of PtCl−. Anyway, as we have not concentrated on 184Os measurement in this study, 195Pt37Cl− interference with 184OsO3− did not need to be considered. Further, no strong ion beam signal was observed on mass 231, such that the 198Pt37Cl− interference with 187OsO3− could be neglected due to the low natural abundance of 198Pt and 37Cl compared to 194Pt, 196Pt, and 35Cl. WO3− Interferences. In this study, the ion beam intensities on mass 231, after removing the 198Pt16O17O− contribution, generally ranged from