Supplementary Text –
Unlocking the Zinc Isotope Systematics of Iron Meteorites
L.J. Bridgestock1, H. Williams2,3, M. Rehkämper1,4 *, F. Larner1,2, M.D. Giscard1,4, S.
Hammond5 , B. Coles1, R. Andreasen1, B.J. Wood2, K.J. Theis6, C. Smith4, G.K.
Benedix7, M. Schönbächler8
1
Department of Earth Science and Engineering, Imperial College London, SW7 2AZ, UK
2
Department of Earth Sciences, University of Oxford, OX1 3AN, UK
3
Department of Earth Sciences, Durham University, DH1 3LE, UK
4
5
Natural History Museum, London, SW7 5BD, UK
Centre for Earth, Planetary, Space & Astronomical Research, The Open University, Milton Keynes
MK7 6BJ, UK
6
Dept. School of Earth, Atmospheric and Environmental Sciences, University of Manchester, M13
9PL, UK
7
8
Department of Applied Geology, Curtin University, Bentley, Western Australia, 6845, Australia
Institut für Geochemie und Petrologie, ETH Zürich, Clausiusstrasse 25, 8092 Zürich, Switzerland
*Corresponding author: markrehk@imperial.ac.uk; +44 20 7594 6391
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Detailed information on analytical techniques
General materials and procedures
Following physical separation, the samples were handled in Class 10 laminar flow hoods
within Class 1000 clean rooms either at the MAGIC Laboratories, Imperial College London
(leaching, digestion of meteorite samples and preparation of all samples for isotopic analyses), or in
the geochemical laboratories of the Department of Earth Sciences, Durham University (digestion of
samples from experiments).
At the MAGIC Labs, the mineral acids 6 M HCl and 15.4 M HNO3 were purified from
AnalaR grade acids by sub-boiling distillation, whilst high-purity (distilled-quality) 8 M HBr and
28 M HF were purchased from VWR (UK). Dilutions were made using 18.2 MΩ cm grade H2O and
aqua regia was prepared as a 3 + 1 mixture of 6 M HCl + 15.4 M HNO3.
Leaching and digestion of samples
To remove surface contamination, all meteorite samples were sequentially leached at room
temperature with (i) 0.5 M HBr (1 hour, in ultrasonic bath), (ii) 6 M HCl (1 hour, in ultrasonic bath)
and (iii) aqua regia (30 min) with intermittent rinses in 18 MΩ cm H2O. The samples were then
washed in ethanol, dried and weighed. Approximately 20 to 30% of the original meteorite metal
was removed by leaching. Following leaching, the samples were digested in aqua regia at 80°C on a
hotplate.
The digestion of the sulfide nodules was incomplete and insoluble residues were removed
by centrifugation. Troilite, and schreibersite for TS3, were observed to dissolve in this first
digestion step. The insoluble residue of TS3, comprising silicates and chromite, was weighed after
drying and then dissolved in a second step to yield sample TS3-D2. This involved (i) treatment with
a 3 + 1 mixture of 28 M HF + 15.4 M HNO3 at 80 °C on a hotplate for 1 week, (ii) evaporation to
dryness and digestion in 6 M HCl at 80 °C on a hotplate for 2 days, and (iii) evaporation to dryness
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and digestion in aqua regia in a Parr bomb at 120°C for 2 days. Finally, all samples were dissolved
in 6 M HCl for aliquoting.
The separated silicate glass and metal fractions from the high-pressure experiments were
also cleaned prior to digestion, by treating with ethanol in an ultrasonic bath (20 min; to remove
residual organics) followed by 18 MΩ cm H2O (20 min). The samples were then digested using an
8 + 1 mixture of 28 M HF + 15.4 M HNO3 (silicates) or aqua regia (metal) at 130°C and finally
dissolved in 6 M HCl for aliquoting.
Aliquoting of sample solutions
Small (typically ~5%) aliquots of the 6 M HCl sample solutions were used for preliminary
Zn concentration measurements (to enable optimal application of the Zn double spike), whilst larger
aliquots were employed for subsequent double spike Zn isotope analyses. For the IIAB and IIIAB
irons, the latter measurements used up all available sample material, whilst solution remained for
IAB irons. Aliquots of the remaining IAB iron meteorite solution were employed to determine the
concentrations of Cr and other metals by ICP-MS at the Open University (see supplementary Table
S3.
Double spiking for Zn isotope analyses
A preliminary determination of the Zn concentrations was carried out using an abbreviated
version of the protocol that was applied for the isotope analyses. Appropriate aliquots of the sample
solutions were then mixed with a
64
Zn –
67
Zn double spike, characterized by
64
Zn/67Zn ≈ 2.5 (see
Arnold et al., 2010), and refluxed for at least 12 hours on a hotplate to ensure spike-spike sample
equilibration.
For most samples, the spike addition was calculated to provide a molar ratio of spike Zn to
natural Zn (ZnSp/ZnNat) of about 1, to optimize the precision and accuracy of the analyses (Arnold et
al., 2010). Test measurements that were carried out for spiked Zn standard solutions with ZnSp/ZnNat
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ratios of about 0.4 to 1.7 returned δ66Zn values that were indistinguishable, within uncertainty
(±0.05 to ±0.10‰; 2sd), to data obtained for solutions with ZnSp/ZnNat = 1. Based on this, all
samples were analyzed relative to ‘matching standards’ that featured similar ZnSp/ZnNat ratios and
similar Zn concentrations. These matching standard solutions were prepared as mixtures of the
MAGIC ‘London Zn’ in-house standard solution (Arnold et al., 2010) with the Zn double spike.
Standard applications of the double spike protocol for stable isotope analyses (e.g., Siebert
et al., 2001) assume that that isotopic variations other mass dependent isotope effects are absent. In
principle, this requirement can be problematic for meteorites because numerous elements feature
isotopic anomalies of radiogenic, nucleosynthetic or cosmogenic origin in such samples
(Rehkämper et al., 2012). However, previous isotopic studies, which did not apply a double spike
for mass bias correction, found no evidence for non-mass dependent Zn isotope variations between
terrestrial materials, lunar rocks and soils, and various meteorites, including chondrites, achondrites,
pallasites as well as IAB complex and IIIAB irons (Luck et al., 2005; Herzog et al., 2009; Moynier
et al., 2007; Moynier et al., 2011; Ghidan and Loss, 2011; Barrat et al., 2012; Paniello et al., 2012).
The only exception is the unconfirmed finding of Ghidan and Loss (2011), who reported a nonmass dependent Zn isotope effect of about 10‰ for the
68
Zn/64Zn ratio of the ungrouped iron
Redfields.
Separation of Zn for isotope analyses
Following addition of the double spike, the 6 M HCl sample solutions were evaporated to
dryness, and redissolved in 1 M HCl for separation of Zn from the sample matrix by anion
exchange chromatography. The separation was carried out following a procedure modified from
Arnold et al. (2010), using small Teflon columns filled with 200 µl of AG MP1 100-200 mesh
resin. After loading of the 1 M HCl sample solutions, matrix elements were rinsed from the resin
with 1 M HCl (8 ml) and Zn was subsequently eluted with 0.01 M HCl (6 ml). The Zn fractions
were evaporated to dryness, re-dissolved in 1 M HCl (1 ml) and processed through the same anion
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exchange chemistry for a second time. The dried residues of the Zn fractions were then twice taken
up in 15.4 M HNO3 (100 µl) and evaporated to dryness, to convert to nitrate form and oxidize
residual organics.
Double spike Zn isotope analyses by MC-ICP-MS
The isotopic analyses were carried out with a Nu Plasma HR MC-ICP-MS instrument at the
MAGIC Laboratories using protocols modified from Arnold et al. (2010). Sample introduction
utilized either an Aridus (CETAC Technologies) or a DSN-100 (Nu Instruments) desolvation
system fitted with nominal 100 µl/min glass nebulizers.
The analyses involved simultaneous collection of the ion beams of 64Zn+, 66Zn+, 67Zn+, 68Zn+
and
62
Ni+,
137
Ba2+ (for interference correction) using Faraday cups equipped with 1011 Ω resistors.
Data collection was performed in 3 blocks of 20 x 5 s integrations, with a 15 s electronic
background measurement preceding each block. All samples and standards were prepared for
analysis by dissolution in an appropriate volume of 0.1 M HNO3, and nitric acid wash solutions
were of the same molarity. The runs typically employed solutions with Zn concentrations of 40 to
60 ng/ml in 0.1 M HNO3, and instrumental sensitivities of better than 110 V (µg/ml)-1 were
generally attained for Zn.
The instrumental mass bias encountered during the Zn isotope analyses was corrected using
the double spike technique with methods adapted from Arnold et al. (2010). For this, the “raw”
measured isotope ratios of
64
Zn/67Zn,
66
Zn/67Zn and
68
Zn/67Zn were processed offline with a
spreadsheet-based implementation of a data reduction technique originally developed by Siebert et
al., (2001). Corrections for spectral interferences from
136
64
Ni+ and Ba2+ ions (132Ba2+,
134
Ba2+ and
Ba2+) were also subjected to mass bias correction. The required corrections, however, were very
small in all cases and typically at levels of 64Ni+/64Zn+ and 134Ba2+/67Zn+ of about 1 x 10-5 or less.
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The Zn isotope compositions of the samples were determined relative to previously prepared
matching standard solutions (see above) and differences in the 66Zn/64Zn ratio (R) between samples
and standards are reported as δ66Zn:
δ66Zn = [(RSample/RStandard) – 1] x 1,000
In the manuscript, all δ66Zn values are reported relative to the widely adopted (but no longer
available) JMC Lyon Zn reference solution (Marèchal et al., 1999). To this end, the results were
corrected for an isotopic offset of δ66Zn = +0.12 ± 0.04‰ for London Zn relative to Lyon Zn
(Larner and Rehkämper, 2012; Arnold et al., 2010). The reported Zn concentrations of the samples
were determined by isotope dilution using the 67Zn/68Zn ratios measured in the double spike runs.
The Zn concentration and isotope data reported in Tables 1, 2 (main manuscript) and the
associated uncertainties take into account the laboratory blank and the precision of the mass
spectrometric data. Blank corrections and the calculation of uncertainties are further detailed below
and in supplementary Table S1.
Blank corrections for Zn
Four analyses yielded a total procedural Zn blank of 5 ± 2 ng (total range). The Zn
concentrations and δ66Zn values of all meteorite samples were corrected for this blank contribution.
The latter correction assumed a ‘normal’ terrestrial isotope composition of δ66Zn = 0.25 ± 0.25‰
for the contamination (Cloquet et al, 2008; Larner and Rehkämper, 2012).
The silicate and metal samples from the partitioning experiments featured sufficient Zn
(>150 µg) such that blank corrections were negligible and hence not applied to the Zn isotope and
concentration data.
Uncertainties of the Zn isotope and concentration data
Uncertainties for data of meteorite samples
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For the Zn concentrations, the total uncertainty u-Zn was calculated by determining the
effect on the calculated Zn abundances that is introduced by (i) the uncertainty of the mass
spectrometric runs (u-ms) and (ii) the uncertainty of the blank mass (u-blm), such that:
u-Zn2 = u-ms2 + u-blm2
For the Zn isotope data, the total uncertainty (u-δ66Zn) was obtained by determining the effect on
the δ66Zn results, which is introduced by (i) the precision of the mass spectrometric analyses (u-ms),
(ii) the uncertainty of the blank mass (u-blm), and (iii) the uncertainty in the isotope composition of
the Zn blank (u-bli), whereby:
u-δ66Zn2 = u-ms2 + u-blm2 + u-bli2
The detailed uncertainty calculations are shown in supplementary Table S1. The calculations
utilize a total procedural blank mass and isotope composition of 5 ± 2 ng and δ66Zn = 0.25 ±
0.25‰, respectively.
The typical within-run (internal) precision of a single δ66Zn measurement, which utilized
~25 ng of natural Zn, was about ±0.05‰ (2se, standard error of the mean). In comparison, the
typical reproducibility of δ66Zn values determined from replicate analyses of a London Zn – Zn
double spike mixture (each consuming ~25 ng natural Zn) was ±0.05‰ to ±0.20‰ and better than
±0.15‰ in most cases (2sd; Table S1). The larger within-session reproducibility was generally
taken as the mass spectrometric uncertainty for the δ66Zn values. Exceptions are samples TS1 and
TS2, which were inadvertently overspiked to ZnSp/ZnNat ≈ 7 to 11 and analyzed at low 66Zn+, 68Zn+
ion bean intensities. The Zn isotope analyses of these samples were carried out relative to London
Zn – Zn double spike mixtures with similar ZnSp/ZnNat ratios and the within-run precision of the
measurements was applied to characterize the mass spectrometric uncertainty of the δ 66Zn data, as
this was larger than the within-session reproducibility. For the Zn concentration data, the mass
spectrometric uncertainties were calculated by propagating the within-run precision (±2sd) of the
67
Zn/68Zn ratios determined for the spike-sample mixtures through the isotope dilution equations.
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The quoted mass spectrometric uncertainties are justified by results obtained for replicate
analyses of (i) Toluca 1, conducted within the same measurement session and (ii) Canyon Diablo 1
and 2, carried out on different days with different sample introduction systems. In all cases, the
analyses yielded δ66Zn values (and Zn concentrations) that are identical within the reproducibility
(Table 1, main manuscript).
The total uncertainties of the δ66Zn data are dominated by the mass spectrometric
reproducibility for the majority of the samples (Table S1). The blank corrections were small or
insignificant in this case because the samples featured more than 200 ng of natural Zn, equivalent to
>96% natural sample Zn. Uncertainties stemming from the δ66Zn blank corrections are only
important for the Zn-poor Toluca nodules TS1, TS2 and the Zn-depleted IIAB irons (Table S1),
which featured mass fractions of indigenous natural Zn that were as low as ~60% (Table 1, main
manuscript). The uncertainties of the δ66Zn blank corrections are, furthermore, dominated by the
blank mass component (u-blm) for all meteorites but the IIIAB irons (Table S1). As a consequence,
the total uncertainties calculated for δ66Zn would not change significantly, even if the isotope
composition of the blank is assigned a larger uncertainty of ±0.50‰ (Table S1).
The uncertainties of the Zn concentrations are dominated by the blank correction rather then
the mass spectrometric uncertainty for all samples but three IAB irons (Table S1). However, the
[Zn] uncertainties are generally excellent for the great majority of the samples at better than ±1.1%
(Table S1). The exceptions are (again) the Zn-depleted samples – the IIAB irons and TS1, TS2 –
which have uncertainties of between about ±3% and ±14%.
Uncertainties for data of samples from experiments
It was not necessary to apply blank corrections to the Zn isotope and concentration data for
samples from the partitioning experiments. The uncertainties given for these results (Table 2, main
manuscript) are, therefore, based solely on the precision of the mass spectrometric analyses. As
ample Zn was available for these samples, the quoted uncertainties denote the 2sd reproducibility
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calculated from 2 or 3 individual runs of the samples. For sample A170 metal, which was only
analyzed once, the uncertainty is based on the reproducibility (±2sd) obtained for bracketing runs of
a matching standard solution. For the Zn concentration results shown in Table 2, the uncertainties
are better than ±1% (2sd) throughout.
Additional measures taken for quality control of Zn isotope data
Six spiked solutions of London Zn were processed though the ion exchange chemistry and
analyzed relative to unprocessed solutions with similar ZnSp/ZnNat ratios. In all cases, the processed
and unprocessed standard solutions had identical Zn isotope compositions, within analytical
uncertainty. This demonstrates that the double spike technique corrects for any isotopic
fractionations that may be induced during column chemistry, for example as a result of incomplete
yields.
No directly suitable geological standard reference materials (SRMs) are available to
ascertain and document the quality of our Zn isotope analyses for iron meteorites. In the absence of
such materials, measurements were carried out for alternative SRMs for which published δ66Zn data
are available – a metal-rich ferromanganese (Fe-Mn) nodule and two basalts. In all cases, these
analyses provided results that are in excellent agreement with published reference values (Table 1,
main manuscript).
Concentration measurements for Cr and other elements at the Open University
Additional concentration analyses for Cr and other elements were carried out for the metal
samples of the IAB irons and Toluca inclusion TS3-D2 (Table 1, supplementary Table S2).
These measurements were performed at the Open University, using an Agilent 7500a
quadrupole ICP-MS, fitted with a quartz spray chamber and Babington nebulizer. Count rates were
in the order of 2 – 3 x 107 cps/(µg/ml), oxide interferences were kept low at 0.3% (determined from
CeO+/Ce+), and doubly charged species at <0.9%, as monitored by Ce2+/Ce+ (Rogers et al., 2006).
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The analyses were standardized against a suite of five synthetic multi-element standards, made from
Specpure plasma grade single element solutions, which covered the range in element concentrations
of the samples themselves. Within-run drift was monitored and corrected for using an online multielement internal standard (Be, Rh, In, Tm, Re and Bi). The detection limits were well below the
measured concentrations and precision (assessed as repeatability of an unknown) was better than
±2% RSD for elements heavier than Ge, and ±4% RSD for those elements lighter than Ge.
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