Tungsten isotopes in bulk meteorites and their inclusions-Implications for processing of presolar components in the solar protoplanetary disk.

Europe PMC Funders Group Author Manuscript Meteorit Planet Sci. Author manuscript; available in PMC 2016 July 19. Published in final edited form as: Meteorit Planet Sci. 2015 September 3; 50(9): 1643–1660. doi:10.1111/maps.12488.

Europe PMC Funders Author Manuscripts

Tungsten isotopes in bulk meteorites and their inclusions— Implications for processing of presolar components in the solar protoplanetary disk J. C. Holst*, C. Paton, D. Wielandt, and M. Bizzarro Centre for Star and Planet Formation and Natural History Museum of Denmark, University of Copenhagen, DK-1350 Copenhagen, Denmark

Abstract


We present high precision, low- and high-resolution tungsten isotope measurements of iron meteorites Cape York (IIIAB), Rhine Villa (IIIE), Bendego (IC), and the IVB iron meteorites Tlacotepec, Skookum, and Weaver Mountains, as well as CI chondrite Ivuna, a CV3 chondrite refractory inclusion (CAI BE), and terrestrial standards. Our high precision tungsten isotope data show that the distribution of the rare p-process nuclide 180W is homogeneous among chondrites, iron meteorites, and the refractory inclusion. One exception to this pattern is the IVB iron meteorite group, which displays variable excesses relative to the terrestrial standard, possibly related to decay of rare 184Os. Such anomalies are not the result of analytical artifacts and cannot be caused by sampling of a protoplanetary disk characterized by p-process isotope heterogeneity. In contrast, we find that 183W is variable due to a nucleosynthetic s-process deficit/r-process excess among chondrites and iron meteorites. This variability supports the widespread nucleosynthetic s/r-process heterogeneity in the protoplanetary disk inferred from other isotope systems and we show that W and Ni isotope variability is correlated. Correlated isotope heterogeneity for elements of distinct nucleosynthetic origin (183W and 58Ni) is best explained by thermal processing in the protoplanetary disk during which thermally labile carrier phases are unmixed by vaporization thereby imparting isotope anomalies on the residual processed reservoir.

Introduction The process of planetesimal growth and planet formation in the nascent solar system was governed by accretion of dust into km-sized bodies and the subsequent differentiation of large bodies into a metallic core and silicate mantle (Kleine et al. 2002; Yin et al. 2002; Scherstén et al. 2006; Kruijer et al. 2013; Wittig et al. 2013). Constraints on the timescales of planetesimal melting and differentiation are provided by the short-lived 182Hf-182W chronometer (T1/2 = 8.90 ± 0.09 Myr; Vockenhuber et al. 2004). This decay scheme, comprising a lithophile parent and a siderophile daughter, effectively constrains metalsilicate differentiation events and has been applied to a range of meteoritic materials, with a focus on iron meteorites (Kleine et al. 2005; Markowski et al. 2006a; Scherstén et al. 2006; *

Corresponding author. [email protected]. Editorial Handling Dr. Edward Scott

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Qin et al. 2008a; Kruijer et al. 2013; Wittig et al. 2013). Its use relies on the critical assumption of isotopic homogeneity in the protoplanetary disk and among solids formed within the first few million years after initial collapse of the protosolar molecular cloud core.


Knowledge of the distribution of tungsten isotopes requires the assessment of their origin and introduction into the protosolar system. Tungsten isotopes are produced in the terminal phases of stellar evolution during the asymptotic giant branch phase of low and intermediate mass stars and during supernova nucleosynthesis. The two main processes responsible for W isotope synthesis are the slow and rapid neutron capture processes, the s-, and r-process, respectively (Burbidge et al. 1957; Cameron 1957; Arlandini et al. 1999; Ávila et al. 2011). Aside from 180W, which is a p-process isotope produced in highly energetic stellar environments by photodisintegration and/or proton capture, all W isotopes have contributions from both the s-, and r-process (Arlandini et al. 1999; Vockenhuber et al. 2007). Altogether, W isotopes provide a potential means for studying the distribution of p-, s-, and r-process carriers in the early solar system (Kleine et al. 2008; Qin et al. 2008a; Burkhardt et al. 2012b; Wittig et al. 2013). This is important, as variability in the abundances of key isotopes is ubiquitous in meteoritic material (e.g., Trinquier et al. 2009; Burkhardt et al. 2012a) suggesting a heterogeneous distribution of the carriers of nucleosynthetic isotope anomalies within the protoplanetary disk. The initial abundance of short-lived radionuclides 26Al, 53Mn, and 10Be has been shown to vary in the solid forming regions of the disk (Nyquist et al. 2009; Larsen et al. 2011; Wielandt et al. 2012; Schiller et al. 2015). For example, a recent high-resolution comparison of U-corrected Pb-Pb and 26Al-26Mg ages for three angrite meterorites supports a model of initial disk 26Al heterogeneity. Indeed, Schiller et al. (2015) showed that 26Al-26Mg ages for three rapidly cooled angrites are systematically younger by ~1.5 Myr relative to their assumption free absolute ages, requiring that the angrite parent body


formed from material with an initial 26Al/27Al of . Furthermore, there is clear evidence that stable nonradiogenic isotopes, particularly the neutron-rich iron group nuclei, are not homogeneous at bulk planetesimal and individual grain scales (Andreasen and Sharma 2006; Trinquier et al. 2007, 2009; Larsen et al. 2011; Burkhardt et al. 2012a). Such heterogeneity could reflect incomplete mixing of freshly synthesized stellar ejecta that were injected (e.g., Ouellette et al. 2009, 2010) into the forming solar system with its associated protoplanetary disk. Alternatively, it could result from disk processing that generated isotopically distinct reservoirs from an initially well-mixed protoplanetary disk (Trinquier et al. 2009). Variability in the isotopes of tungsten has been shown for both calcium-aluminum-rich inclusions (CAIs; Burkhardt et al. 2008, 2012a; Kruijer et al. 2014) and several groups of iron meteorites (Qin et al. 2008a; Schulz et al. 2013; Wittig et al. 2013; Cook et al. 2014; Peters et al. 2014). The protoplanetary disk may thus have been characterized by W isotope heterogeneity for p-, s-, and r-process nuclides at the scale of parent bodies and down to individual refractory inclusions. We address this topic by the stable 180W and 183W isotopes in a twofold study employing novel analytical techniques, including both low- and high-resolution multiple-collector

Meteorit Planet Sci. Author manuscript; available in PMC 2016 July 19.

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inductively coupled plasma-mass spectrometry (MC-ICPMS). It has been shown that lowresolution measurements could be affected by molecular interferences, imparting spurious anomalies on W isotope data (Holst et al. 2011) and thereby possibly affecting ε180W and ε183W. The application of high-resolution mass spectrometry may thus be required to obtain accurate 180W and 183W results.


We report robust high-precision, high-resolution data for 180W/184W of a suite of wellstudied iron meteorites, chondrites, and a refractory inclusion to ascertain the degree of homogeneity of this rare heavy p-process nuclide in the young protoplanetary disk. Secondly, we report high-resolution measurements of 183W/184W, to assess the s- and rprocess variability in tungsten in the formation region of the investigated material. Importantly, if tungsten isotope ratios vary due to nucleosynthetic s- and r-process anomalies, internal normalization to 186W/184W or 186W/183W will impact chronology based on normalized 182W data. Thus, to construct a robust and accurate chronology of metal-silicate differentiation in the early solar system, it is important to assess the presence of nucleosynthetic variability in W isotopes.

Samples and Analytical Methods Iron Meteorites


Six samples were chosen to represent a variety of iron meteorite types, some of which have previously been characterized for their W isotope composition including 180W (Schulz et al. 2013). These included Cape York (IIIAB), Rhine Villa (IIIE), and Bendego (IC) as well as the IVB iron meteorites Tlacotepec, Skookum, and Weaver Mountains, all of which were chosen for direct comparison to the data set of Schulz et al. (2013). Furthermore, we provide 180W measurements of a bulk CI chondrite Ivuna and a CAI from CV3 chondrite NWA 8722. Sample preparation methods were modified from previously established procedures (Kraus et al. 1955; Faix et al. 1981; Salters and Hart 1991; Horan et al. 1998; Münker et al. 2001; Kleine et al. 2004, 2008). Iron meteorite chips of 0.63–2.9 g were cut using diamond coated saw-blades and polished with tungsten-free abrasive paper and diamond-coated dental drills. Prior to dissolution, sample chips were cleaned successively in ethanol and 0.05 M HNO3 in an ultrasonic bath. The W purification procedure for iron meteorites was modified from Kleine et al. (2004, 2008). All hydrofluoric acid used in the purification protocol was pre-cleaned on AG1 anion resin at a concentration of 4 M, which was found to reduce the W reagent blank by a factor of ~25. Initial sample digestion was achieved in 10–25 mL 6 M HCl + 0.06 M HF at 130 °C. Samples were then dissolved in 4:1 HNO3:H2O2 to remove organics and Os as a volatile oxide. Following this step, full dissolution was achieved in 6 M HCl + 0.06 M HF and each sample was taken up in 1 M HF + 0.1 M HNO3. This volume was then loaded on precleaned and conditioned 20 mL BioRad polypropylene columns containing AG1-X8, 200– 400 mesh. Sample matrix was subsequently eluted in 100 mL 1 M HF + 0.1 M HNO3, followed by elution of W and other high field strength elements (HFSE) in 100 mL 6 M HNO3 + 0.2 M HF. The HFSE fraction was dried and re-dissolved in 2 mL 1 M HF + 0.1 M HNO3. Here, it was critical to flux the sample at 100–130 °C to re-dissolve any sample tungsten adsorbed on the vial surface. Once fluxed, this cut was loaded onto a 1 mL anion Meteorit Planet Sci. Author manuscript; available in PMC 2016 July 19.

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column. Matrix elements were eluted with 5 mL 1 M HF + 0.1 M HNO3 and HFSE such as Zr and Hf, the latter a direct isobaric interference, were eluted in 2 mL 6 M HCl + 0.01 M HF. Finally, the purified W was recovered in 8 mL 6 M HCl + 1 M HF.


To attain a sufficiently low interference level of 180Hf on 180W, a third column step was necessary for all samples. Prior to this step, samples were converted to NO3− form, and then dissolved in 1 M HF + 0.1 M HNO3. Again, hot plate fluxing at 130 °C was necessary before loading onto 1 mL anion columns. The elution protocol was identical to the second column step. For some samples, the third column step was repeated to reduce the 180Hf/180W to ≤10−3. Silicate Samples For chondrite and CAI samples of 1–3 g, as well as NIST 3163 W and NIST SRM 361 standards and a BCR-2 rock standard, a new W purification protocol designed for silicate matrices was applied. This was modified from Fritz et al. (1961) and Strelow et al. (1972) and utilized a cation matrix separation step followed by an anion purification step. The advantage of this approach is that W is eluted directly from the cation column, thereby avoiding the potential loss of W due to column saturation. This enables larger samples to be processed on fewer parallel columns, as the cation column efficiently retains most major elements. We used AG50W-X8, 200–400# and samples were loaded in 4–6 column volumes (c.v.) 0.25 M HNO3 + 0.1 M HF + 0.1% H2O2 depending on sample size. Prior to loading, they were fluxed at 100 °C for 1–2 days to ensure proper oxidation of Cr to Cr3+. At this oxidation state, Cr is strongly adsorbed on the cation resin and hence effectively separated from W. Residual adsorbed tungsten and other HFSE were eluted in 2.5 c.v. 0.1 M HF. After collection of W, adsorbed Fe was eluted in 2.5 c.v. 1 M HF followed by removal of matrix elements with 10 c.v. 6 M HCl.


The second column consisted of pre-cleaned and conditioned AG1-X4, 200–400 mesh. The converted and fluxed samples were loaded in 1–4 mL 1 M HF. Aluminum and other matrix elements were eluted in 1 M HF, followed by elution of Ti, Hf, and Zr in 2 M HCl + 0.1% H2O2. Residual Hf was removed with 2 c.v. 6 M HCl + 0.01 M HF followed by W elution in 4 c.v. 6 M HCl + 1 M HF. As with the above protocol, the second column step was repeated 1–3 times to ensure that 180Hf/180W ≤10−3. The final two column steps used 0.2 mL columns to minimize resin-derived organics and minimize procedural blanks. Following W recovery, all samples were re-dissolved six times with 4:1 concentrated HNO3: 30% H2O2 to remove residual Os and organic molecules. We note that residual organics can cause (1) a significant decrease in the ionization efficiency of W in the plasma source mass spectrometer resulting in variable instrumental mass fractionation and poor standard-sample intensity matching and (2) differences in uptake rate between sample and standard, also resulting in poor sample-standard signal intensity matching. Moreover, an organic molecular interference has been known to cause significant effects on 183W (Kleine et al. 2002, 2004), necessitating reduction in resin-derived and sample-related organics. Total procedural blanks were ~4.5 ng for the most elaborate six-column chemistry, and result from the use of large columns and large quantities of reagents as well as the use of 350 mL Teflon beakers during sample handling. In addition, a substantial part of the procedural blank is caused by the use


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of steel-jacketed Parr bombs during silicate sample digestion. However, owing to sample sizes containing 450–3000 ng W, coupled with the relatively small magnitude of the potential isotopic anomalies, this blank level has a negligible impact on the results, given the uncertainties of our measurements.


The Elemental Hf/W Ratio of CAI BE The Hf/W ratio of the CV chondrite refractory inclusion dubbed CAI BE was determined by doping with a mixed 180Hf-186W tracer as described in Holst et al. (2013). A doped 0.3% aliquot of the bulk sample was passed over a cation column (1 mL AG50W-X8) to remove most matrix elements. It was then converted to NO3− form and fluxed on a hotplate before analysis. The measurements were performed on a ThermoFisher X-series II quadrupole ICPMS. In the same run, we conducted a tracer calibration against Alfa Aesar solution standards of known concentration and isotopic composition to obtain the elemental Hf/W ratio in the mixed tracer. The tracer calibration is necessary as the tracer composition changes with time due to the variable behavior of Hf and W when stored in solution. Mass Spectrometry Samples were fluxed on a hotplate at 130 °C in 0.5 M HNO3 + 0.1 M HF and run on the ThermoFisher Neptune Plus at the Centre for Star and Planet Formation in Copenhagen, using a Cetac Aridus II desolvating nebulizer sample introduction system and combining a sampler Jet Cone with the skimmer X-cone. The typical sample aspiration rate for this introduction system was ~0.05 mL min−1. The large sample sizes permitted the measurement of each sample in both low- and high-resolution mode.


Tungsten isotopes were measured in static mode using seven Faraday collectors with the following configuration: 183W in the axial collector; 178Hf, 180W, and 182W in the L3, L2, and L1 collectors on the low-mass side of the axial Faraday; and 184W, 186W, and 188Os in the H1, H2, and H3 collectors on the high mass side of the axial Faraday. All masses were measured using Faraday detectors connected to amplifiers with 1011 Ω feedback resistors, with the exception of 178Hf and 180W, which were measured using 1012 Ω feedback resistors. 178Hf was monitored to correct for 180Hf interference on 180W. Sample-standard bracketing using the NIST 3163 W solution standard was applied to correct for instrumental mass fractionation using the exponential law, and data were acquired in both low- and highresolution modes for each sample. Total sensitivity of the instrument for W in low- and highresolution mode was 1200 and 120 V/ppm, respectively. Low-resolution measurements were acquired based on techniques described in Holst et al. (2013). Measurements conducted in high-resolution mode were performed on the low-mass side of the peak, at a position allowing for an effective mass resolving power (M/ΔM) of ~4500. Samples and standards were analyzed with signal intensities matched to better than 5%. The typical intensity on mass 180 (180W+180Hf) was 25–60 mV. Isobaric interferences from 184,186Os were monitored on 188Os, and the 184Os/184W ratio was ~1 × 10−6 for the high-Os IVB iron meteorite samples. The Os interference was substantially less for silicate samples and was thus negligible in all cases. Therefore, only the interference from 180Hf on 180W was significant and required correction. Although our ion exchange protocol reduced the 180Hf/180W to ≤10−3, we are experienced minute levels of a few picograms of Hf-blank


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addition during sample preparation immediately prior to mass spectrometry. To account for this, in addition to matching the concentration of W in the bracketing standard with the sample, the former was also doped with Hf standard solution to a level matching that of the sample, so that it best reflected the measurement conditions of unknowns.


Each analysis comprised a total of 1678 s of baseline measurements (obtained on peak) and 839 s of data acquisition (100 scans integrated over 8.39 s) interspaced by 120 s washouts using the Cetac QuickWash accessory module. All data reduction was conducted off-line using the freely distributed Iolite data reduction package, which runs within Igor Pro (Paton et al. 2011). Baseline intensities were interpolated using a smoothed cubic spline, as was instrumental drift with time. Typical baseline levels were 0.5–2 mV of total tungsten. Iolite’s smooth spline auto choice was used in all cases, which determines a theoretically optimal degree of smoothing based on variability in the reference standard throughout an analytical session, which corresponds typically to 12–24 h of continuous measurement without adjusting the instrument’s tuning parameters. For each analysis, the mean and standard error of the measured ratios were calculated, using a 2 SD threshold outlier rejection. Individual sample analyses were combined to produce an average, weighted by the propagated uncertainties of individual analyses. The reported analytical uncertainties include the propagated 2 SD error on the bracketing standard for each analytical session. W isotope data are reported in the ε notation as deviations from the NIST 3163 W standard in parts per 104:

(1)


(2)

and were internally normalized using the exponential law. As shown in equations (1) and (2) this was performed separately using both 186W/184W = 0.92767 and 186W/183W = 1.98594 (Völkening et al. 1991) to test for isotopic anomalies on the normalizing isotopes. The two normalization schemes are denoted (6/4) and (6/3), respectively. For (6/3) normalized data, refer to Table 1. We prefer (6/4) normalization due to the potential presence of analytical artifacts (see Discussion section) affecting the measured 183W, causing shifts in (6/3) normalized data.

Results Tungsten isotope data for iron meteorites, chondrites, and CAI BE are summarized below and presented in Table 1 and Fig. 1. Low-Resolution Data For ε180W, there is no resolved effect of using (6/3) normalization instead of (6/4) and all measurements overlap for the two different normalizations. For analyses performed in low-


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resolution mode, there is an apparent spread of ε180W with a negative anomaly for Rhine Villa and positive anomalies for Cape York and all three measured IVB iron meteorites. The positive ε180W for Cape York is intermediate between the terrestrial standard value and the IVB iron group that span a range in ε180W of ~2–6. The CI chondrite Ivuna and the CV chondrite refractory inclusion CAI BE both have ε180W overlapping with the terrestrial value. Also, all three processed standards, NIST 3163, BCR-2, and SRM 361, overlap within uncertainty with unprocessed NIST 3163. As with ε180W, the ε182W of all processed standards are in good agreement with the unprocessed terrestrial standard. The ε182W of the iron meteorites range from −4.1 to −3.1 with Bendego and Tlacotepec having the most negative anomalies. Ivuna and CAI BE have ε182W between −1.5 and −1.8 for (6/4) normalization and between −2.1 and −2.8 for (6/3) normalization. We observe this clear discrepancy only for the two silicate samples and the cause is explained in the Discussion section. The ε183W of processed standards also overlap with the terrestrial value. In contrast, there is a clear negative ε183W (6/4) for Cape York, Rhine Villa, and Bendego, corresponding to a positive ε184W (6/3). We explore this minor deficit relative to the standard in The Distribution of 183W section. Note that for Skookum ε183W = 0.25 ± 0.06, which is most likely caused by an interference on mass 183 (see Organic Interferences section and cf. Kleine et al. 2002, 2004; Holst et al. 2011). Ivuna has ε183W of 0.36 ± 0.12, whereas CAI BE ε183W = 1.00 ± 0.11. We include W isotope data measured in low-resolution mode for Allende from Holst et al. (2013), as these data were obtained under identical analytical conditions and substantiate our conclusions regarding 183W variability (Table 1).


High-Resolution Data As for low-resolution data, all ε180W in high-resolution mode is consistent when using (6/3) and (6/4) normalization. In contrast to the low-resolution data, the high-resolution ε180W of Cape York and Rhine Villa is not resolved from the terrestrial standard. The ε180W values for Cape York and Rhine Villa are 0.40 ± 1.0 and 0.03 ± 1.4, which is in excellent agreement with terrestrial standard. The weighted mean ε180W of Bendego is also in closer agreement with the terrestrial value when measuring in high-resolution mode. For CAI BE and Ivuna, there is no indication of anomalous ε180W. Again, we note that all processed standards agree with the terrestrial ε180W. Interestingly, the general lack of ε180W anomalies is contrasted by the IVB iron group that in high resolution still displays clearly resolved ε180W excesses ranging from 2 to 6 ε with Tlacotepec having the highest measured ε180W of 5.80 ± 2.0. The measured ε182W is identical to the pure W standard for all processed standards. Highresolution data for iron meteorites show the same range of ε182W from −4 to −3 ε and each sample measurement is identical in both low- and high-resolution mode. Ivuna and CAI BE both overlap within uncertainty with the average value for chondritic meteorites of −1.9 ± 0.1 (Kleine et al. 2004). However, due to effects on ε183W (or ε184W), the ε182W for Cape York, Rhine Villa, Bendego, Ivuna, and CAI BE vary according to the applied normalization scheme. This will be discussed in the Discussion section.


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As for ε182W, the ε183W of processed NIST 3163, SRM 361, and BCR-2 overlap with the unprocessed bracketing standard. Moreover, the IVB irons have terrestrial ε183W with no indication of variability. The deficits in ε183W for Cape York, Rhine Villa, and Bendego in low resolution are confirmed in high-resolution mode and range between −0.15 and −0.11, whereas Ivuna and CAI BE are positive with ε183W of 0.36 ± 0.12 and 0.51 ± 0.17, respectively. Note that CAI BE is less positive in high resolution compared to low resolution (see the Organic Interferences section). The validity and implications of the observed spread in ε183W among different types of bulk meteorites are explored in The Distribution of 183W section. Lastly, we determined the Hf/W ratio for CAI BE following the analytical protocol for elemental ratio determination as described by Holst et al. (2013) and found a Hf/W of 0.27 ± 0.016. This corresponds to ~0.3 ε of ingrowth on 182W if a canonical initial 182Hf/180Hf of 9.85 × 10−5 is assumed (Burkhardt et al. 2012b).

Discussion Reproducibility of W Isotope Data The reproducibility of our analytical protocols is different for metal and silicate samples. This difference is due to the application of different chromatographic procedures, matrix effects, and lower W concentrations in silicate samples.


The reproducibility of the isotopic measurements for iron meteorites was evaluated based on the 2 SD uncertainty of low- and high-resolution measurements of the NIST SRM 361 Fe-Ni steel standard with tungsten concentrations comparable to that of iron meteorite samples. For Fe-Ni-rich samples in low-resolution mode, the 2 SD is ε180W (6/4) = 1.3, ε182W (6/4) = 0.16, and ε183W (6/4) = 0.06. In high-resolution mode it is ε180W (6/4) = 1.3, ε182W (6/4) = 0.14, and ε183W (6/4) = 0.08. For silicate matrices, the long-term 2 SD was assessed from measurements of column processed NIST 3163 and USGS rock standard BCR-2. In low-resolution mode the external reproducibility for ε180W (6/4) = 1.2, ε182W (6/4) = 0.10, and ε183W (6/4) = 0.05. In highresolution mode ε180W (6/4) = 2.0, ε182W (6/4) = 0.14, and ε183W (6/4) = 0.11. We note that our 2 SD on ε180W is comparable to those of recent studies (Schulz et al. 2013; Cook et al. 2014), despite our study including measurement of 180W in high-resolution of small silicate samples with low W concentrations such as CAI BE (~300 ng W). In addition, the ε182W of all iron meteorites and chondritic samples yield values that are consistent with literature data (e.g., Kleine et al. 2005; Scherstén et al. 2006; Qin et al. 2008a; Schulz et al. 2013). Tungsten isotopic anomalies outside of the estimated reproducibility can have several causes including (1) cosmic ray induced production/burnout during a prolonged meteoroid phase, (2) nucleosynthetic anomalies, or (3) alpha-decay of rare, long-lived 184Os. As small anomalies in the abundances of nonradiogenic W isotopes help constrain the extent and nature of nucleosynthetic variability in the solar protoplanetary disk it is important to assess their cause.


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However, before drawing conclusions from the data set, there are a number of potential analytical artifacts that must be identified and accounted for. Repeat measurements of several standards, including processed synthetic and rock standards as well as a NIST SRM 361 steel standard yield W isotope data that are indistinguishable from the terrestrial value as determined from an unprocessed NIST 3163 solution standard. Nevertheless, deficits and excesses in ε183W are observed in bulk meteorites and also, conditions may differ for meteoritic samples relative to standards due to various effects including isobaric interferences, nonkinetic mass fractionation, nuclear charge related odd-even effects (Shirai and Humayun 2011; Willbold et al. 2011; Kruijer et al. 2012), and organic interferences. These potential effects are evaluated before the data are interpreted in terms of early solar system processes. 180Hf

Interference Correction Apart for IVB irons, all samples show ε180W values identical to the terrestrial standard, indicating that interference correction for 180Hf on 180W does not result in inaccuracies beyond the 2σ reproducibility of our method. The accuracy of the Hf interference correction was empirically tested by doping pure W solutions with variable amounts of Hf. These experiments show that the 180Hf interference correction is accurate for interference levels of up to ~8000 ppm (Fig. 2), which is substantially beyond the levels encountered in meteorite samples (5 × CI and implies that the reservoir from which the Earth formed had lost ~80% of its material prior to or during accretion. Such extensive material loss is highly unlikely indicating that the W isotope variability is not controlled by differential incorporation of SiC (i.e., the s-process signature) among bulk solar system objects.


Rather than an s-process deficit, the observed ε183W anomalies in chondrites and CAI BE are likely related to a preferential incorporation of r-process-enriched material relative to Earth, and iron meteorites. This is consistent with previous conclusions based on Mo isotopes in Allende type B CAIs (Burkhardt et al. 2011). Due to uncertainties in the isotopic composition and the exact nature of the r-process carrier(s) in the early solar system, it is not possible to quantify the degree of r-process enrichment as a function of the observed anomalies. However, we suggest that the magnitude of the 183W anomalies could be related to the selective incorporation of one or more anomalous r-process carriers rather than by bulk depletion/enrichment of SiC in planetesimals and planetary objects. Indeed, this is perfectly consistent with a thermal processing scenario in which thermally labile carrier grains, possibly carrying the r-process signature, are enriched in the gaseous CAI precursor reservoir and, in turn, would imply that chondrites, Earth, and iron meteorite parent bodies formed from the residual processed disk material. Furthermore, such a model predicts higher degrees of thermal processing in the inner versus outer protoplanetary disk (cf. Trinquier et al. 2009), consistent with the observation that carbonaceous chondrites are distinct in their ε183W from that of Earth and iron meteorites, the former having formed further from the Sun. The r-process excess observed in carbonaceous chondrites results from either a lower degree of thermal processing (e.g., Ivuna) and/or a higher abundance of incorporated CAIs (Allende), carrying the r-process signature from thermally labile precursor grains (Trinquier et al. 2009; Burkhardt et al. 2012a). In summary, the stable W isotope variability among bulk solar system objects is consistent with a thermal processing scenario, as has previously been proposed to explain bulk heterogeneity in Ti, Cr, Sr, Mo, and Ni isotopes (Trinquier et al. 2009; Burkhardt et al. 2012a; Steele et al. 2012; Paton et al. 2013). Due to the nucleosynthetic origin of W isotopes, these signatures are not readily explained by varying abundances of presolar SiC in bulk planetary objects. Instead a viable explanation is an excess of r-process material in CAIs and carbonaceous chondrite parent bodies such as those of CI and CV chondrites. Ultimately, this is explained by thermal processing in the solar nebula where thermally labile r-process carrier phases were vaporized in the inner protoplanetary disk to form part of the gaseous precursor reservoir to CAIs. These, as well as pristine outer disk material enriched in r-process nuclei, were then later incorporated into carbonaceous chondrites, making them distinct from Earth and iron meteorite parent bodies.


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Conclusions Our results for tungsten isotopes give constraints on the nucleosynthetic make-up of the protoplanetary disk and the processing which governed isotope variability. The conclusions are summarized as follows:

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1.

We observe no 183W deficit in any processed standards. Such deficits have been reported previously as caused by a nuclear-charge–related fractionation during sample preparation. As our analytical method does not result in resolved deficits, there is no requirement for correction of the (6/3) normalized data or ε183W (6/4) for meteorite samples.

2.

In low-resolution mode, we observe spurious effects of organic interferences on mass 183 (0.25−0.5ε). However, high-resolution data for chondrites and CAI BE do not show this effect and are consistent with an s-process deficit/r-process excess. Thus, for high-precision measurements of 183W/184W, it is important to apply high-resolution mass spectrometry. In addition, 180W data may be affected by molecular interferences as indicated by the positive ε180W of Cape York. This signature is resolved in high-resolution mode, further supporting the use of high mass resolution mass spectrometry for high-precision W isotope measurements.

3.

There are no resolved effects of meteoroid phase cosmic ray exposure on either ε180W or ε183W. Substantial effects are observed for ε182W in iron meteorites with long exposure ages, in agreement with previous reports, yet the ε180W and ε183W data appear to be unaffected by cosmic rays.

4.

The constant ε180W for the studied irons (except the IVB group), chondritic materials, and a refractory inclusion provides evidence for pprocess homogeneity in the young protoplanetary disk and is in agreement with recent results from Cook et al. (2014). In contrast to a recent study by Schulz et al. (2013), there is no indication of widespread heterogeneity of 180W and progressive p-process nuclide homogenization in the disk. Rather, p-process nuclei of several elements (e.g., Sr, Mo, Ba, and W) appear to have been homogeneous when the solar system formed. The elevated ε180W for IVB iron meteorites is likely due to their high Os/W ratio suggesting that it is a result of long-term accumulation from the αdecay of 184Os, as was recently suggested by Peters et al. (2014). We did not determine an 184Os/180W isochron for the samples studied here but point out that the variable ε180W within the group IVB irons is qualitatively compatible with 184Os decay in samples with high and variable Os/W ratios.

5.

Variability in ε183W in bulk meteoritic objects and inclusions (−0.15 ± 0.07 to 0.51 ± 0.17) reflects variable incorporation of nucleosynthetically anomalous presolar components. As such, Ivuna, Allende, and CAI BE are s-process depleted/r-process enriched compared to Earth and iron meteorites. This is likely due to the preservation of a


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thermally labile r-process carrier(s) in Ivuna (which accreted in the outer protoplanetary disk) and the enrichment of r-process carrier material in Allende and the CAI (which condensed from a gas following thermal processing). 6.


Thermal processing is further supported by the observed correlation of 183W and 58Ni, as these anomalous isotopes of the two elements are produced in different stellar environments. Moreover, the generation of isotopic anomalies through thermal processing of dust is backed by other isotope data (e.g., Cr and Ti) that suggest widespread thermal processing in the protoplanetary disk during the earliest stages of planetesimal formation.

Acknowledgments The Centre for Star and Planet Formation is funded by the Danish National Research Foundation. We thank I. Leya, M. Humayun, and G. Herzog as well as E. Scott for constructive reviews that improved the quality of the manuscript.

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Europe PMC Funders Author Manuscripts Meteorit Planet Sci. Author manuscript; available in PMC 2016 July 19.

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Fig. 1.

Tungsten isotope data for iron meteorites, Ivuna, Allende (from Holst et al. 2013), and CAI BE as well as rock, steel, and processed solution standards. Top and bottom panels show low- and high-resolution data, respectively. Errors are 2 SE for samples with n > 1 and propagated with the 2 SD uncertainty on the bracketing standard for each analysis.


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Fig. 2.

Hf doping experiment of terrestrial NIST 3163 W standard showing the impact of the direct isobaric 180Hf interference on the final, interference corrected ε180W ratio. The interference correction is effective to less than 1ε for 180Hf levels beyond 8000 ppm. Given that no samples have 180Hf interference higher than 3000 ppm, the interference correction is considered valid for all meteoritic samples.

Europe PMC Funders Author Manuscripts Meteorit Planet Sci. Author manuscript; available in PMC 2016 July 19.

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Europe PMC Funders Author Manuscripts Fig. 3.


Plot showing ε183W/184W versus ε182W/184W for all investigated iron meteorites, chondrites Ivuna (CI) and Allende (CV), and refractory inclusion CAI BE. Also shown are data for USGS rock standard BCR-2 and the NIST 3163 W solution standard. Filled symbols represent magmatic iron meteorites and all symbols are as in Fig. 1. All plotted data are normalized using the exponential mass fractionation law and 186W/184W, see text. The arrow indicates the direction of s-process deficit/r-process excess forming a mixing line with solar system material at an initial ε182W/184W of −3.51 (Burkhardt et al. 2012b). Note that the ordinate intercept for bulk irons may be slightly higher than solar system initial due to decay of 182Hf on the parent body prior to core formation. Gray data symbols represent measured data, not corrected for ingrowth on 182W from the decay of 182Hf. For Ivuna, the decay correction is based on the most recent Hf/W ratio for this meteorite (Barrat et al. 2012) whereas for CAI BE, we have directly determined the Hf/W ratio on a sample aliquot. For Allende, the decay correction is calculated using a chondritic Hf/W ratio of 1.2. The large uncertainty associated with decay correction of Ivuna means that it is not possible to distinguish an analytical artifact from a nucleosynthetic excess on 183W (see text). However, when also considering that Allende and CAI BE plot on the expected mixing line between solar system material and a component characterized by an s-process deficit/r-process excess (in agreement with earlier reports; Burkhardt et al. 2012b), we conclude that Ivuna’s composition is most likely a nucleosynthetic anomaly.


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Fig. 4.

Measured ε180W for iron meteorites Ivuna and CAI BE as well as standards in highresolution mode. All samples except the IVB group irons overlap with the terrestrial ε180W suggesting no widespread early solar system heterogeneity in the rare p-process isotope 180W as previously proposed (Schulz et al. 2013). Errors are 2 SE propagated with the 2 SD of the bracketing standard in each analytical session.


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Europe PMC Funders Author Manuscripts Fig. 5.


ε62Ni versus ε183W (6/4). 62Ni data are from Steele et al. (2011, 2012). Symbols are as in Fig. 1 and filled symbols represent magmatic iron meteorites. The Ni isotopic composition of Ivuna is assumed to be identical to Orgueil as measured by Steele et al. (2012) and Cape York is assumed to have the same Ni isotope composition as Lenarto (IIIAB) as also measured by Steele et al. (2012). We make no corrections for an odd-even isotope effect, resulting in an apparent ε183W deficit (Shirai and Humayun, 2011; Kruijer et al. 2012). Allende W isotope data are taken from Holst et al. (2013). Errors on W isotope data are 2 SE propagated with the 2 SD of bracketing standards for each session (except for Allende, which is 2 SE, n = 5). Errors on nickel isotope data are 2 SE (Steele et al. 2012). The correlation between Ni and W isotopes is most likely related to a coupled incorporation of nucleosynthetic carrier(s). The Ni isotope variability is ascribed to variations in the abundance of neutron-poor 58Ni (Steele et al. 2012) and the correlation of 58Ni and 183W could result from a nebular sorting process such as thermal processing (see text).


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Table 1

Tungsten isotope data for iron meteorites, chondrites, inclusion CAI BE, and terrestrial standard materials


ε180W(6/4)

Sample Cape York (IIIAB)

Rhine Villa (IIIE)

Bendego (IC)

Tlacotepec (IVB)

Weaver Mts (IVB)

Skookum (IVB)

Ivuna (CI)

CAI BE (CV incl.)

ε183W(6/4)

ε180W(6/3)

ε182W(6/3)

ε184W(6/3)

n

LRa

1.27 ± 0.4

−3.37 ± 0.08

−0.08 ± 0.05

1.54 ± 0.4

−3.25 ± 0.05

0.05 ± 0.03

4

HRb

0.40 ± 1.0

−3.40 ± 0.12

−0.12 ± 0.08

0.43 ± 1.3

−3.20 ± 0.13

0.08 ± 0.05

5

LR

1.08 ± 0.7

−3.67 ± 0.10

−0.09 ± 0.07

−0.72 ± 0.5

−3.53 ± 0.09

0.06 ± 0.04

5

HR

−0.03 ± 1.4

−3.63 ± 0.10

−0.11 ± 0.07

0.29 ± 1.2

−3.50 ± 0.09

0.07 ± 0.04

6

LR

−0.81 ± 1.3

−4.16 ± 0.09

−0.16 ± 0.11

−0.20 ± 1.2

−3.91 ± 0.11

0.11 ± 0.06

10

HR

0.14 ± 1.6

−4.11 ± 0.06

−0.15 ± 0.07

0.48 ± 1.1

−3.90 ± 0.11

0.10 ± 0.05

8

LR

3.83 ± 0.8

−3.85 ± 0.10

0.02 ± 0.06

3.82 ± 0.8

−3.87 ± 0.07

−0.01 ± 0.04

5

HR

5.80 ± 2.0

−3.88 ± 0.11

0.03 ± 0.09

5.50 ± 1.8

−3.91 ± 0.05

−0.02 ± 0.05

3

LR

1.90 ± 1.3

−3.13 ± 0.11

0.05 ± 0.09

1.90 ± 1.2

−3.19 ± 0.06

−0.05 ± 0.08

9

HR

2.20 ± 1.2

−3.27 ± 0.19

0.00 ± 0.08

2.40 ± 1.2

−3.19 ± 0.15

0.00 ± 0.05

6

LR

2.66 ± 0.8

−3.29 ± 0.08

0.25 ± 0.06

2.31 ± 1.2

−3.64 ± 0.07

−0.16 ± 0.04

10

HR

2.94 ± 1.1

−3.27 ± 0.10

0.04 ± 0.08

2.92 ± 0.8

−3.32 ± 0.08

−0.03 ± 0.06

6

LR

−0.12 ± 2.3

−1.77 ± 0.13

0.39 ± 0.12

−0.69 ± 2.2

−2.16 ± 0.15

−0.26 ± 0.08

1

HR

0.38 ± 2.2

−1.98 ± 0.23

0.36 ± 0.12

−0.37 ± 2.3

−2.50 ± 0.31

−0.24 ± 0.08

1

LR

0.35 ± 1.7

−1.50 ± 0.13

1.00 ± 0.11

−1.31 ± 1.8

−2.79 ± 0.10

−0.67 ± 0.07

1


HR

1.34 ± 1.4

−2.21 ± 0.30

0.51 ± 0.17

0.30 ± 1.5

−2.92 ± 0.25

−0.34 ± 0.11

1

Allende (CV3)c

LR

–

−1.84 ± 0.34

0.20 ± 0.12

–

−2.06 ± 0.20

−0.13 ± 0.18

5

NIST 3163

LR

−0.08 ± 1.0

−0.04 ± 0.05

−0.02 ± 0.03

−0.16 ± 0.9

0.01 ± 0.05

0.02 ± 0.02

9

HR

0.18 ± 1.3

0.05 ± 0.08

0.03 ± 0.07

0.08 ± 1.3

0.01 ± 0.09

−0.03 ± 0.04

10

LR

−0.03 ± 0.8

0.00 ± 0.09

−0.09 ± 0.04

0.23 ± 0.7

0.13 ± 0.10

0.06 ± 0.05

10

HR

0.22 ± 1.5

0.06 ± 0.11

−0.06 ± 0.08

0.42 ± 1.8

0.10 ± 0.08

0.04 ± 0.06

10

LR

−0.80 ± 1.3

0.01 ± 0.16

−0.04 ± 0.06

−0.54 ± 1.2

0.09 ± 0.08

0.03 ± 0.04

17

HR

−0.50 ± 1.3

0.05 ± 0.14

−0.05 ± 0.08

−0.30 ± 1.2

0.08 ± 0.08

0.04 ± 0.05

10

BCR-2

NIST SRM 361

a

ε182W(6/4)

Low-resolution mode.

b

High-resolution mode MC-ICPMS data. The ε notation is the deviation from the terrestrial standard in parts per 10,000. (6/4) designates data that

are normalized to 186W/184W, whereas (6/3) are for normalization to 186W/183W.

c

Data from Holst et al. (2013). Quoted values are the weighted means of multiple sample analyses during one analytical session and errors for samples with n > 1 are 2 SE including the propagated 2 SD of the bracketing standard for each session. Errors on samples with n = 1 are 2 SE.


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Nanoimprinted comb structures in a low bandgap polymer: thermal processing and their application in hybrid solar cells.

Tungsten doped titanium dioxide nanowires for high efficiency dye-sensitized solar cells.

The source crater of martian shergottite meteorites.

Influence of polymer compatibility on the open-circuit voltage in ternary blend bulk heterojunction solar cells.

The oriented growth of tungsten oxide in ordered mesoporous carbon and their electrochemical performance.

Semiconducting carbon nanotube aerogel bulk heterojunction solar cells.

pbs quantum dots capped with amorphous ZnS for bulk heterojunction solar cells: the solvent effect.