Determination of halogen abundances in terrestrial and extraterrestrial samples by the analysis of noble gases produced by neutron irradiation

Thelackofareliabledatabaseforheavyhalogens(bromineandiodine)interrestrialandextraterrestrialsamples ismainlyduetotheanalyticalchallengesofdeterminingtheirverylowabundances( b 1ppm)inthematerialsof interest. The neutron irradiation noble gas mass spectrometric (NI-NGMS) technique initially developed in the 1960s is the only viable technique currently capable of determining concentrations below 1 ppb of iodine for small ( b 10 mg) sample sizes. We describe in detail the analytical protocols and provide a comprehensive and transparent overview of the data reduction procedures in order to fully explore the uncertainties of the tech-nique.We demonstrate how thecapabilities of modern massspectrometers used for Ar-Ar dating,canbe readily extendedtoincorporatehalogenmeasurements.Anewandcriticalassessmentoftheuseofstandardsispresent-edbasedonresultsfrommultipleirradiations,includingameteorite(Shallowateraubrite),scapolitemineralsin- troduced by Kendrick (2012) and a novel internal calibration method based on using barium.


Introduction
Halogens are present as minor and trace elements in most geological samples. Studies of halogens have been applied to crustal and oreforming fluids (e.g.; Böhlke and Irwin, 1992a,b;Turner and Bannon, 1992;Ballentine et al., 2002;Kendrick and Burnard, 2013) and more recently, the determination of heavy halogens (bromine and iodine) in mantle-derived samples has provided important constraints on the origin and the recycling of the major volatile elements in the Earth (Burgess et al., , 2009Sumino et al., 2010;Kendrick et al., 2011Kendrick et al., , 2012aKendrick et al., ,b, 2013a. However, low concentrations of halogens in the materials of interest, together with the lack of available techniques for their low level detection, means that reliable abundance data are relatively sparse compared to other volatile elements. Whilst the electron microprobe (EMP) can determine chlorine and fluorine abundances down to levels of 0.01 wt.%, bromine and iodine abundances are b 1 ppm in most minerals and rocks. Chlorine and bromine can be determined by instrumental neutron activation analysis (INAA) but require N 100 mg-size samples (e.g. Heinrich et al., 1993). Similarly, ion chromatography analysis requires crush-leaching of gram-sized samples to extract halogens (e.g. Bottrell and Yardley, 1988). Recently, Heinrich et al. (2003) assessed the potential of Br and Cl quantification by LA-ICP-MS. However the halogens tend to have low ionisation efficiencies leading to relatively low sensitivity. Using LA-ICP-MS for scapolite minerals, Hammerli et al. (2013) determined detection limits of about 8 ppm Br and N500 ppm for Cl. Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) has been used with detection limits of 10 ppm for F and Cl (Joachim et al., 2015) and SIMS has attained limits of 0.6 ppm for Br and 0.035 ppm for I (Kusebauch et al., 2015).
An alternative technique for halogen determination is based upon extension of the 40 Ar-39 Ar technique (Merrihue, 1965;Turner, 1965;Turner et al., 1971;Turner and Bannon, 1992;Böhlke and Irwin, 1992a, b;Irwin and Roedder, 1995). Although fairly routinely used for Cl, this technique has only recently become more widely adopted for Br and I following a revival in the 2000s by the Manchester Isotope Geochemistry and Cosmoschemistry group (Johnson et al., 2000;Kendrick et al., 2001;Burgess et al., 2002). Kendrick (2012) summarises in detail the basic concepts of the neutron-irradiation noble gas technique. The author suggested the use of the mineral scapolite as a standard for halogen determinations to replace the existing meteorite standards (Shallowater or Bjurböle). Subsequently, Kendrick et al. (2013a) have revised the halogen values of the scapolite standards, reducing them by 22% and 27% for Br and I, respectively. This correction has been Chemical Geology 437 (2016)

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Chemical Geology j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h e m g e o independently confirmed for Br by Hammerli et al. (2014) in scapolites, but no external standards or techniques have been used to verify I concentrations.
The introduction of scapolite standards has an impact on the way previous workers have reduced their data to estimate the halogen abundances in natural samples. We discuss the implications by describing different approaches to reduce the data. We compare results from five different irradiations using two different reactors. We introduce an alternative halogen standardisation method based on the barium content (when known independently) of irradiated samples, referred to from hereon as a barium calibration. Finally, the results obtained from the Shallowater meteorite and the barium correction are used to refine the I/Cl values of BB2/SP and BB1 scapolite standards of Kendrick et al. (2013a) and reduce their 2σ-errors from 23% (BB2/SP) and 15% (BB1) to 14% and 10%, respectively.
Integrated neutron fluences are typically in the range 10 18 -10 19 n·cm 2 with most halogen absorption reactions involving thermal and epithermal neutrons (Fig. 2) leading to conversion factors between 10 −6 and 10 −4 of the parent halogen atoms.
Using the approach of Chilian et al. (2006) for INAA, upon irradiating a sample containing an amount m (grams) of an element, then the quantity of a given nuclide i X (moles) produced by a (n, γ, β) reaction is given by: where M at is the atomic mass, θ is the isotopic abundance; Y is the fractional yield for branched isotope decay; σ th is the thermal neutron absorption cross section (2200 ms − 1 ); Q o = σ epi /σ th is the ratio of resonance integral to thermal neutron cross-section, f = φ th / φ epi with φ th and φ epi being the thermal and epithermal neutron fluences, respectively ( Table 2). The parameter f is usually not known a priori and will vary with reactor and irradiation position; however it can be determined experimentally from standards. The proportion of epithermal neutron-induced isotope production can be estimated as Q o / (Q o + f), or expressed by the F-factor as: As an example, our irradiation MN2014b carried-out in the GRICIT Facility at the TRIGA reactor Oregon gave f = 11.3 ± 2.3 (Table 2), with Br Q o = 11.9 ± 0.3 and I Q o = 24.9 ± 0.7 (Table 2), this indicates that 51% of the 80 Kr Br and 69% 128 Xe I is formed by epithermal neutron absorption.
In contrast, nucleogenic production of 38 Ar from 37 Cl has a relatively low Q o value (=0.7 ± 01) and therefore is used to monitor the thermal neutron fluence. Eq. (1) can be useful for irradiation planning if the epithermal/thermal fluence ratio of a reactor irradiation position has been previously characterised and remains relatively constant over time. However, Eq. (1) does not include the effects of neutron selfshielding on isotope production. Analytical formulae for correcting self-shielding effects in cylindrical samples (appropriate for samples irradiated in silica glass tubes) are given by Chilian et al. (2006Chilian et al. ( , 2008Chilian et al. ( , 2010. The effects of self-shielding are likely to be negligible for most samples used in 40 Ar-39 Ar irradiations, because of their relatively low mass (usually b0.01 g) and the low concentrations of target elements (for halogens typically at ppb-ppm levels). Self-shielding effects should be considered when irradiating samples with high concentrations of halogens (e.g. halogen salts), in which self-shielding by epithermal neutrons at resonance peaks could be severe (Chilian et al., 2006). For example, using the method of Chilian et al. (2006), it is estimated that epithermal self-shielding of a 1g cylindrical (radius = 1 cm; height = 2 cm) sample of iodide salt (NaI or KI) could lead to an erroneous underestimate of I abundance from 128 Xe I of up to 38%.
The halogen abundances in samples can be evaluated either from the derived neutron fluence, or from standards of known parent element composition. We compare these methods and discuss implications in Section 4.

Samples and neutron irradiation
Irradiation standards used in this study include the hornblende age standard Hb3gr (Turner et al., 1971), three scapolite minerals (BB1, SP2 and BB2) previously described by Kendrick (2012) and the Shallowater aubrite meteorite, a commonly used I-Xe standard (Gilmour et al., 2006).
Prior to irradiation, each sample and monitor were wrapped in aluminum foil. They were placed in a quartz tube, evacuated and sealed to a maximum length of 6.5 cm using a flame. Hb3gr hornblendes were positioned in the bottom, middle and top of each quartz tube. Shallowater was positioned in the middle of the tube. Scapolite standards were placed at the bottom and top of each tube. This procedure has been used in five irradiations carried-out between 2012 and 2014 so that results can be directly compared. Sample batches MN2012b, MN2012f (both in 2012) and MN2013 (in 2013) were irradiated in the RODEO facility (High Flux Reactor, Petten, Netherlands) with a constant rotation during irradiation. In 2014, irradiations were carried-out in ICIT (MN2014a) and GRICIT (MN2014b) facilities of the TRIGA Reactor, Oregon State University (OSU). As the halogen-derived noble gas isotopes are produced by low energy thermal neutrons and epithermal neutrons, the irradiation cans were not Cd-shielded. The Petten irradiations were each irradiated continuously for 24 h duration. At OSU samples were irradiated for a few hour intervals each day over several weeks to give a total irradiation time of 278 and 205 h for MN2014a and MN2014b respectively. All the irradiation details are provided in Table 2.

Noble gas extraction and mass spectrometry
Noble gas isotopic measurements were made using either MS1, a custom built single focusing magnetic sector mass spectrometer , epithermal (also referred to as resonance neutrons) (φ epi ) and the fast neutron (φ fast ) components. Nucleogenic isotope production is dominated by thermal neutron absorption, however strong resonances at higher energy mean that it is essential to use standards to estimate the irradiation parameters (modified from http://atom.kaeri.re.kr). (15 cm, 90°geometry) that has been in operation for nearly 50 years, 1 or a Thermofisher Scientific ARGUS VI mass spectrometer, one of the latest generation of multi-collector instruments (Table 3).

MS1
The MS1 mass spectrometer sensitivity is optimised for the measurement of low abundance Xe isotopes. It has a Baur-Signer source and a detector slit configured for high transmission (sensitivity of 3 × 10 −4 A/Torr 40 Ar). The ion beams are detected using either a Faraday cup capable of measuring 10 −6 -10 −15 A using a 10 11 Ohm amplifier or a continuous dynode channeltron measuring 10 −13 -10 −19 A.
Noble gases are extracted from samples using a tantalum resistance furnace operating over a temperature range of 400-1800°C. Samples are loaded via a conventional Pyrex glass "Christmas Tree" holding up to 12 samples. Prior to sample analyses, the furnace is outgassed several times at 1750°C for 30 min and blanks are determined by heating the empty furnace to each target temperature steps (600°C, 1400°C, 1600°C and 1700°C). Samples are dropped into the furnace with an iron slug operated by an external hand magnet. Each heating step is for 30 min during which noble gases are purified of active gases using a getter (SEAS NP10) at 400°C. Once extracted, the noble gases are transferred to the inlet manifold of the mass spectrometer where the noble gases (Ar, Kr, Xe) are gettered for a further 5 min (SEAS ST172) at 250°C. Argon, Kr and Xe are transferred onto a charcoal cold-finger cooled with liquid nitrogen for 5 min in order to concentrate them into the mass spectrometer inlet manifold. The charcoal finger is then warmed for 5 min to 50°C and the noble gases expanded into the mass spectrometer. Isotopic analyses are made statically with Ar, Kr and Xe isotopes measured sequentially seven times (50 min total duration). Argon isotopes are measured on the Faraday collector whilst the less abundant Kr and Xe isotopes are measured using the channeltron. Each heating, purification and isotopic analysis step takes a total duration of approximately 1.5 h. Aliquots of air are analysed daily using a similar procedure to the samples in order to check the sensitivity and mass discrimination of the MS1 mass spectrometer.

ARGUS VI
The Thermo Fisher Scientific ARGUS VI is a static vacuum mass spectrometer with 13 cm, 90°extended geometry ion optics principally designed for the multi-collection of Ar isotopes in neutron-irradiated samples (m/z 36, 37, 38, 39 and 40). The collector array consists of five Faradays (in positions H2, H1, Axial, L1 and L2) and a low mass compact discrete dynode (CDD) ion counting multiplier. This design enables simultaneous collection of 40 Ar on H2 or H1; 39 Ar on H1 or Ax; 38 Ar on Axial or L1; 37 Ar on L1 or L2 and 36 Ar on L2 or CDD. Coupled to its low volume (680 cm 3 ), the instrument utilizes a X-and Z-focused Nier type bright source giving sensitivities in excess of 1 × 10 − 3 A/Torr 40 Ar at 200 μA trap current and 4.5 kV acceleration voltage.
In order to measure the xenon isotopes, the accelerating voltage is reduced to 2.5 kV and the filament trap current increased to 230 μA. The ion source is optimised for sequential analysis of Ar, Kr and Xe isotopes. The sensitivities are: 1.24 × 10 − 12 cm 3 STP Ar/A, 7.73 × 10 −13 cm 3 STP Kr/A and 9.45 × 10 −13 cm 3 STP Xe/A. The gases are extracted using a 10.6 μm wavelength CO 2 laser (CETAC Fusion CO 2 ) with a 3 mm beam diameter. The 55 W output CO 2 laser allows for the controlled stepped heating of samples. Samples are loaded into 3 mm diameter holes drilled into an Al sample holder. The view-port window and cover slip are made of ZnS. For Hb3Gr hornblende, noble gases are extracted in three laser heating steps of 25 s each: 1) 0.5% to 3% of the laser power; 2) 3% to 6%; and 3) 7% to 9%. The scapolite standards are heated in a single step by slowly increasing the laser power from 6% to 10% over 180 s. For the Shallowater meteorite, at least eight individual heating steps are used from 0.5% (30 s) to 9% (30 s). Purification time on a hot getter (NP10) is 4 min, followed by five minute transfer onto a liquid nitrogen cooled charcoal finger. Following transfer, the charcoal finger is warmed for 5 min to 60°C to release noble gases. The purified unseparated noble gases are expanded into the mass spectrometer. Introducing Ar, Kr and Xe at the same time reduces any elemental fractionation between Kr and Ar associated with their sequential release by heating the charcoal finger. Xenon isotopes (128-136) are detected first by magnetic switching of the peaks on the CDD. Argon data are collected using multicollection with the five Faraday cups. Finally, krypton isotopes ( 86 Kr, 84 Kr, 82 Kr and 80 Kr) are collected in a single step on H2 ( 86 Kr), H1 ( 84 Kr), AX ( 82 Kr) and L1 ( 80 Kr). The multicollection of krypton isotopes is achieved by applying a small voltage to the entrance of each Faraday cup (Thermo Fisher Scientific, Deflection Plate Technology). The isotopic analyses of nine cycles takes approximately 20 min, at the end of which isotope abundances are determined by regression to inlet time.

Determining abundances of K, Ca and Cl
All measurements made on samples and irradiation standards are subject to the corrections outlined below. Argon data are corrected in order of instrumental background, mass discrimination, radioactive decay of 37 Ar Ca and 39 Ar K , and neutron interference reactions. These are conventional corrections applied for 40 Ar-39 Ar age dating (e.g. McDougall and Harrison, 1999) and a summary is provided for completeness in Supplementary section A. The irradiation parameters (J, α and β) are calculated using Hb3Gr monitor. The hornblende Hb3gr was chosen as a monitor by Turner et al. (1971) due to is homogeneity in terms of composition and age. For this study we used the value determined by Schwarz and Trieloff (2007) of 1074.9 ± 3.5 Ma.
K, Ca and Cl (moles) are determined using Eqs.
(3) to (5). Further details are provided in the Supplementary section A.

Epithermal neutron production and Br, I and Ba abundances
Earlier studies showed that that absorption of epithermal neutrons at resonance peaks in the cross-sections of 79 Br and 127 I account for about 30% of 80 Kr Br and 40-50% of 128 Xe I produced during irradiation (Böhlke and Irwin, 1992b;Johnson et al., 2000;Kendrick, 2012). Therefore the noble gas/parent halogen production ratios require calibration by analysing a standard with known concentrations of Br and I. Initially, Table 3 Mass spectrometer ion source and noble gas characteristics of the MS1 and ARGUS VI mass spectrometers. Sensitivities and isotope ratios are calculated over a month long-period (1σ).

MS1
ARGUS VI  Turner (1965) used potassium iodide and potassium bromide to measure the integrated neutron fluence. Both Kr and Xe from these salts were measured by isotope dilution in that work. This approach was used until the end of the 1980s. However, self-shielding processes will almost certainly have impacted on the correction factors, as discussed earlier.
To circumvent this effect a meteorite standard was introduced to calibrate the I-Xe system. In the following section, we consider three different approaches to monitor noble gas isotope production from epithermal neutrons: the Shallowater aubrite (Johnson et al., 2000), scapolite minerals (Kendrick, 2012) and a novel technique based on barium in samples. In order to test the different corrections we use a set of six Mid-Ocean Ridge Basalt (MORB) samples irradiated in MN2012b that were analysed on both the ARGUS VI and MS1 mass spectrometers. Details about these samples can be found in the Supplementary data section B.
Noble gases are corrected for neutron-induced fission of 235 U (Kr and Xe; Supplementary data section A.8) and atmospheric contamination using natural isotopes least affected by the irradiation procedure (i.e. 36 Ar, 84 Kr and 130 Xe).
First the thermal neutron production ratio is calculated as: where φ th is determined from Hb3gr (Eq. A.21), using Y 128I = 0.94, σ th = 6.16 ± 0.04 b (Supplementary data - By dividing Eq. (6) by Eq. (7), we can estimate the iodine factor (F I ). This factor is used to determine the amount of 128 Xe produced from 127 I by epithermal neutrons.
Conventionally the 128 Xe/ 129 Xe ratio is obtained from the meteorite standard via the slope of concordant heating steps using an I-Xe isotope correlation diagram. The data from the meteorite typically shows discordant low temperature steps and concordant data for high temperature steps (e.g. Hohenberg, 1967 - Fig. 1).
Stepped heating of Shallowater was carried-out on the ARGUS VI using the CO 2 fusion laser (Fig. 3). As the methodology for xenon measurements is new on the ARGUS VI mass spectrometer, we compared our analyses of Shallowater samples from the same irradiation using the RELAX mass spectrometer (S. Crowther -pers. comm.). RELAX (Refrigerator Enhanced Laser Analyser for Xenon) is an ultrasensitive resonance ionisation time-of flight mass spectrometer with micro-channel plate detectors designed for the analyses of very small amounts of xenon (Gilmour et al., 1991(Gilmour et al., , 1994Crowther et al., 2008). The lower sensitivity of the ARGUS VI compared to RELAX for Xe results in fewer temperature steps despite larger sample sizes (2.5 mg versus 0.5 mg). However, for the same irradiation MN2014a (OSU), the 129 Xe/ 128 Xe ratios are in excellent agreement (0.646 ± 0.042 -ARGUS VI and 0.654 ± 0.014 (RELAX). For the Petten reactor, we can only compare Shallowater data for MN2013 (ARGUS VI) to a previous irradiation MN2012f (RELAX) however the 129 Xe/ 128 Xe value should be relatively constant over time for the same irradiation position within a reactor. Using the ARGUS VI we obtained ( 129 Xe/ 128 Xe) MN2013 of 1.83 ± 0.15 (1σ) for Shallowater and using RELAX we obtained a ( 129 Xe/ 128 Xe) MN2012f ratio of 1.869 ± 0.004 (1σ). To calculate the production of 80 Kr Br by epithermal neutrons, the epithermal fluence is calculated using the 128 Xe/I value obtained from Shallowater: The amount of 80 Kr produced by 79 Br is given by, with σ th and σ ep given in Table 1 and Supplementary data - Tables C.1 and C.3. The correction factors (F 79Br , F 81Br , F 127I and F 131Ba ) calculated using this method are shown in the Table 4 for all irradiations.   (Ludwig, 2012). The lower error on the slope for the OSU irradiated sample is explained by the much larger sample size used.

Scapolite standards
Kendrick (2012) introduced scapolite (SP, BB2 and BB1) as a potential new standard for the analysis of Cl, Br and I. Subsequently, Kendrick et al. (2013a) irradiated the scapolite standards with a high thermal fluence making the correction for epithermal production negligible (b2%). Following the recommendation of Kendrick et al. (2013a) the revised halogen abundances of the scapolite standard values are given in Table 5.
In order to calculate halogen abundances in samples the halogen ratios measured in the scapolite minerals are equated to the halogen-derived noble gases formed during irradiation. The notations π and ω were proposed by Kendrick (2012) The results for π and ω are presented in Table 6. In order to obtain a comparable set of corrections using different approaches, we apply a common notation of correction F-factors-as follows. The production of noble gas proxy isotopes from thermal neutrons is given by: In the same way, it is possible to determine F 81Br and F I by using Eqs. (15) and (16) Using halogen ratios is advantageous because it is independent of the sample mass. The correction factor is also independent of the value of the thermal fluence as this term is common to both the sample and standard and cancels out in Eq. (12).
For MN2012b and MN2013a, the scapolite standards were run on the MS1 and ARGUS VI. A small discrepancy exists between Br/Cl and I/Cl values for scapolites from the same irradiation, analysed on different mass spectrometers. Chlorine measurements of scapolite are similar (4.0 ± 0.4 wt.% -MS1 and 4.0 ± 0.3 wt.% -ARGUS VI) on both instruments, thus the discrepancies result from the determination of 80 Kr Br and 128 Xe I . We suggest that the difference is probably related to the measurement procedure. On the ARGUS VI, Xe and Kr isotopes are analysed within 2 min and 4 min respectively following inlet, whereas on the MS1 Xe isotopic analysis does not begin until 12 min after that the gas is admitted into the source of the mass spectrometer, and after 9 min for Kr. Xenon and krypton isotopes are relatively rapidly ionised and depleted in the ion source thus a relatively long delay in measurement could impair the accuracy of extrapolation of data to inlet time into the spectrometer. To establish any effects that the calibration discrepancy may have on sample Br and I determinations, aliquots of six glass MORB samples from the East Pacific Rise (Castillo et al., 2002) were analysed on the MS1 and ARGUS VI mass spectrometers and the noble gas data were used to determine halogen abundances from the scapolite standard data obtained from each spectrometer (  Fig. 4 where the slopes of the linear regressions deviate b 5% from the 1:1 line for Cl and I and b 15% for I. Thus it is concluded that samples analysed on different mass spectrometers give consistent data as well as highlighting the importance of analysing standards and samples on the same instrument.

Barium calibration
Barium is routinely determined in geological samples, including Mid-Ocean Ridge Basalts (MORB) using a range of analytical techniques including EMP, ICPMS, XRF and ICPES. Since barium shows strong resonances in neutron absorption at epithermal neutron energies, the 131 Xe determinations provide an independent monitor of epithermal fluence in addition to the meteorite and scapolite standards. Barium has the added advantage of being determined in the same sample for which halogens are being measured, which helps to remove any uncertainties associated with neutron flux variations that may exist when using standards located up to a few cm's from a sample's position in an irradiation tube. Turner (1965) analysed neutron-produced noble gas isotopes from trace elements including Ba. He noted that in meteorites 131 Xe is produced by (n, γ, β) reactions from both 130 Te and 130 Ba. In principle, it is not possible to distinguish between 131 Xe produced from Te or Ba unambiguously (Turner, 1965). Meteorites contain relatively high levels of Te and the concentrations Te and Ba are often comparable. However in MORB samples Te shows low abundances of about 3 ± 1 ppb (Yi et al., 2000). In contrast Ba in MORB is consistently above N 1 ppm (http:// www.earthchem.org/petdb/ With ( 131 Xe/ 136 Xe) nf = 0.453 ± 0.013 and ( 134 Xe/ 136 Xe) nf = 1.246 ± 0.036 (Ozima and Podosek, 2002). This correction accounts for between 1 and 3% of the measured 131 Xe in MORB samples.
The Ba abundances are calculated following a similar approach as outlined above for Br (Eq. (9)) by substituting appropriate terms: There is uncertainty in the cross section values for Ba in the literature, and epithermal fluence can be up to 23% higher using the values reported by Mughabghab (2006). As a consequence correction factors for Br and I are higher by up to 9% and 13%, respectively, and the sample concentrations of these elements will be therefore underestimated by the same amount. Here the different published cross-sections are reviewed in order to identify the most reasonable value for future studies.
A summary of all the cross section values for 130 Ba (n.γ) (published and in web-based compilations) are given in Supplementary data - Table C.4. The National Nuclear Data Center's NUDAT compilation gives a thermal cross-section value of 11.3 ± 1.0 b (http://www.nndc. bnl.gov), based on combining the measured values for the production of the ground state (8.8 ± 0.9 b) and the isomeric state (2.5 ± 0.3 b) (Fig. 1). These two measurements were made in 1959 and 1967 respectively using equipment of very low resolution that was perhaps not able to correct for competing decays from the irradiated samples. A compilation by Mughabghab (2006) replaces these values with 7.7 ± 0.9 b and 0.98 ± 0.05 b for ground and isomeric states respectively, giving a combined total cross section of 8.7 ± 0.9 b (http://www.nndc.bnl.gov/atlas/ atlasvalues.html). The origin of the ground state value is not given by Mughabghab (2006). However the isomeric cross section is from Heft's (1968) measurement using high-resolution Ge detectors. In comparison, Heft's ground state cross section is 6.52 ± 0.40 b. Table 6 Average values of noble gas-halogen correction factors calculated using scapolite standards (BB2/SP and BB1) for the four irradiations discussed in this study. FnX values are calculated using Eq. (14), π is calculated using Eq. (10), π* is the factor for 81 Br and ω is calculated using Eq. (11). Uncertainties are given at 1σ and represent the internal precision of each irradiation. Dauenhauer and Krane (2012) determined the isomeric cross section of 0.596 ± 0.037 b, which differs from Heft's (1968) value due to changes in the accepted value of the gamma-ray branching from the isomeric level. Heft used a then-current value of 40% in his analysis whilst Dauenhauer and Krane (2012) used the now-current value of 55%. If we correct Heft's ground state cross section for the 15% difference of the gamma-ray branching, the isomeric cross section of Dauenhauer and Krane (2012) and (Heft, 1968) would be identical. The ground state cross section of Dauenhauer and Krane (2012) (7.15 ± 0.34 b) is slightly larger than that of Heft (1968) because of their differing values of isomeric cross section. The effects of these corrections somewhat cancel out, so that the total cross section of 7.75 ± 0.34 b (Dauenhauer and Krane, 2012) and 7.48 ± 0.40 b (Heft et al., 1978) overlaps. Both are in substantial disagreement with the value of Mughabghab (2006) based on the early low-resolution measurements. We therefore choose to use the most recent value for the cross-section of 130 Ba of 7.75 ± 0.34 b. For the resonant neutron cross section, the value proposed by Dauenhauer and Krane (2012) is 197.3 ± 10.0 b obtained by combining the ground state and the metastable state. This is 11% higher than the most recent values found in the database of Pritychenko and Mughabghab (2012). This discrepancy remains unexplained although it is possible that the value for the metastable state has not been included in current databases. To maintain consistency, we use the resonant integral data of Dauenhauer and Krane (2012).
We use this new set of factors to correct the six MORB data (Castillo et al., 2002) and compare the final results to the ones obtained using the scapolite standards ( Fig. 6 and   ppb Bromine 30 Fig. 6. Comparison of the calibration of bromine and iodine concentrations using neutron fluences derived from barium versus direct calibration using scapolite standards. We show the 1:1 line with an envelope of 10% error for bromine and 5% error for iodine. Error bar is 1σ. abundances by 7.4%. In contrast the difference between barium and scapolite calibrations for I is b5%. We attribute this variation to the contrast in thermal/epithermal neutron cross sections (Q o ) for target elements involved. Using the data in Table 1, Q o values are very similar for 127 I and 130 Ba at 24.9 ± 0.7 and 25.5 ± 1.7 respectively, whereas 80 Br has a much lower Q o of only 11.9 ± 0.3. Therefore, 128 Xe/I and 131 Xe/Ba production rates should be similar and largely independent of the neutron energy distribution of the irradiation.

Cross-calibration of I/Cl ratio in scapolites (BB2/SP and BB1)
The scapolite Br and I abundances of Kendrick (2012) were subsequently revised by 22% and 27% respectively by Kendrick et al. (2013a). The authors attributed the revision to irradiation of samples in a strongly thermalized neutron flux, therefore negating the requirement to estimate the epithermal neutron contribution via inclusion of a meteorite standard. Bromine concentrations were subsequently confirmed using LA-ICPMS (Hammerli et al., 2014), whereas the I abundances in scapolites have not been cross-calibrated using an independent iodine standard.
The Shallowater aubrite step heating data, as well as the Ba correction, are used here to re-evalute, the I/Cl ratio for SP/BB2 (n = 21) and for BB1 (n = 10) standards irradiated between 2012 and 2014 (Tables 7A and 7B and Fig. 7).
This brings BB1 and SP/BB2 scapolite standards to the same level of certainty as SY scapolites, not measured in that study. Despite this improvement, it seems clear than the correction based on Shallowater aubrite or the barium correction produce smaller errors as shown in the Supplementary data (Table B1) for the ALARCON set of samples.

Conclusion
The main findings of this study are summarised as follows. 1) We provide details of the analytical procedures for obtaining Ar, Kr and Xe isotope determinations using the ARGUS VI multicollector noble gas mass spectrometer. We compare the data obtained with our existing MS1 mass spectrometer to demonstrate the improvement in performance and data quality of the multicollector instrument.
2) The general principles and data reduction procedures are described in detail for the conversion of halogen-derived noble gas isotopes to halogen abundances. We show that the use of standards of known concentrations of each element is preferred, over direct calculation based on neutron flux determinations.

Table 7B
Re-determination of I/Cl ratio in BB1 standards. The calculated value is the same as the value calculated by Kendrick et al. (2013a,b) and the error bar is reduced from 15% to 10% (2σ). 3) Step-heating of Shallowater meteorite standard is recommended to generate a correlation line on a plot of 129 Xe/ 132 Xe vs. 128 Xe/ 132 Xe and to obtain the 128 Xe/ 127 I ratio for an irradiation. 4) A barium calibration, which is completely independent of other standards, shows excellent agreement with both Shallowater and the newly revised I/Cl ratio for scapolite standards for the determination of iodine. This correction can be used as an internal standard within each irradiation if Ba contents have been independently determined. 5) Scapolite standards are a very good standard for bromine and can also be used as an iodine standard, however we recommend to use Shallowater aubrite or the barium calibration for iodine. The value of I/Cl ratio for BB2/SP scapolite has been revised downward by 8.5%. The new recommended value for BB2/SP is (1.19 ± 0.09) ×10 − 6 (1σ). For BB1, we recommend a I/Cl ratio of (25.9 ± 1.3) ×10 −6 (1σ).  Fig. 7. Scapolite standards used to monitor the production of 38 Ar Cl , 80 Kr Br and 128 Xe I in four irradiations have good reproducibility. The absolute I/Cl ratios recommended for the standards have been revised using a combination of techniques described in this study.