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Supplementary Text
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Analytical Details
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General
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After drilling or scooping, ~50 to ~150 mg of sample is dropped into a quartz sample cup
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which is then transported into an oven, evacuated, and heated monotonically over about
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one hour to a temperature of ~900oC. The heating schedule varied somewhat among the
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individual analyses, as did the ultimate temperature achieved; these differences have no
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observable effect on Cl isotopic composition. During heating a He carrier gas sweeps
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evolved gases into the SAM analytical suite, including a small split of the gas flow into
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the QMS. The QMS has an electron impact ion source and a pulse-counting secondary
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electron multiplier and can scan the entire mass range from 2 to 527 Da, although
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typically a smaller mass range (e.g., 2-150 Da) is scanned during EGA sequences. The
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QMS has a mass resolution of unity, so species with the same nominal mass-to-charge
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ratio are not distinguished. As discussed below, this necessitates correction for isobaric
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interferences. Correction is also required for SEM deadtime, which increases with count
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rate. At about a million cps the correction is ~1%; for all Cl isotope measurements
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reported here the deadtime correction is < 1.5%.
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Isobaric Species
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We compute Cl isotope ratios from the HCl peaks after making any necessary corrections
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for isobaric species (see equation 1 and associated definition of variables in main text).
S1
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Potential isobars we have identified include Ar, hydrocarbons, oxygen, H2S, CS2 and
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acetonitrile. Here we quantify these isobars and develop correction schemes as needed.
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Compared to HCl, the amount of Ar present in the samples is negligible. The
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geochronology experiment undertaken on the Cumberland sample by SAM (Farley et al.,
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2014) determined an 36Ar concentration of 55 pmol/g, compared with an EGA-based HCl
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concentration in the same sample of ~7 mmol/g (Ming et al., 2014), yielding a 36Ar/HCl
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ratio of less than 10 ppm. The same logic precludes a substantial contribution from 38Ar
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on mass 38. Inspection of the abundances of parent ions indicates negligible contributions
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to M38 from CS2 and acetonitrile. Similarly, the contribution to mass 36 from 18O18O is
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extremely small – the natural abundance of 18O is just ~0.2%, so this isotopologue will be
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just 4 ppm of the 16O16O signal at mass 32. The mass 32 signal in our samples includes
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both 16O16O and 32S, so it provides an upper limit on the amount of O2 evolved from the
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samples. 4 ppm of the mass 32 signal in our samples is always very small compared to
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the total signal at mass 36.
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Given these simplifications, the important isobars for Cl isotope ratio determination are
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H234S, C3H2 and C3, so:
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RCl= (M38 – kHC38M39)/ (M36 – ksM34 – kHC36M39)
(S1)
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We correct for H2S using the primary H2S signal at mass 34 (corrected for very minor
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contributions from 18O16O) and the isotopic composition of sulfur (ks = 34S/32S ~ 0.0448).
S2
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Following previous work (Farley et al., 2014) we correct for hydrocarbons using the
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C3H3 fragment at mass 39 (kHC38 = C3H2/C3H3 ~ 0.19, kHC36 = C3/C3H3 ~ 0.02). Below we
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assign exact values and uncertainties to these correction factors.
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Over the course of the EGA run the relative intensity of the isobars compared to the HCl
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signal varies, causing highly time-variable uncertainties in the corrected HCl abundances
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and the resulting isotopic ratios. Denoting the numerator of equation S1 as N and the
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denominator as D, the uncertainty in Cl isotope ratio determined on a given sweep of the
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mass spectrum is:
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(σRCl/RCl) = [(σ(σ(σDD
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
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where



S2)
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σ (σkHC38M39)2((σkHCkHC(σM39M39
(S3)
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σD(σkSM34)2((σksks(σM34M34
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kHC36M39)2((σkHC36kHC36(σM39M39

(S4)
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SAM blank and background measurements indicate kHC36 varies between about 0.17 and
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0.21, presumably reflecting small variations in the relative abundance of various organic
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contaminants in the system. This limited variability in organic composition is further
S3
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supported by the near constancy of the purely hydrocarbon ratios M40/M39 and
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M41/M39 within and between EGA runs. We adopt a best estimate of kHC38 = 0.19 ±
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0.048 (1σ). Owing to the overwhelming abundance of HCl on mass 36 even in blank
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runs, the value of kHC36 is difficult to determine directly. Based on runs with very high
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organic concentrations (e.g, OD2; see below) and on the fragmentation pattern of likely
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organic molecules in SAM (e.g., Freissinet et al., 2015; Glavin et al., 2013) we adopt a
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value of kHC36 = 0.02 ±0.02. Modeling shows that selection of even a far higher value
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(e.g., 0.1) would make a quantitative but not a qualitative difference to our results.
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The isotopic composition of sulfur dictates kS; to accommodate possible isotopic
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variability and mass bias we adopt a best estimate and uncertainty of kS=0.0448 ± 0.002.
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In all of our samples the hydrocarbon correction greatly dominates the Cl isotope ratio
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uncertainty.
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Inspection of equations S2-S4 indicates three sources of uncertainty, arising from: a) the
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instrumental bias; b) the absolute signal counts; and c) the isobar corrections. We believe
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instrumental bias is a systematic uncertainty, and provide justification for this conclusion
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below. Because it is systematic to all of our measurements, we do not include
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instrumental bias explicitly in the following computations. The uncertainty associated
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with absolute signal intensity is random, so declines with the total number of counts
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measured. Thus this error decreases with increasing signal intensity and when multiple
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sweeps are pooled. Uncertainty in isobaric corrections is systematic and depends on the
S4
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systematic uncertainty in the correction factors and on the time-varying relative intensity
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of the isobaric signal to the total signal on the Cl isotope measurements.
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Mass Bias
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Both the value and the uncertainty in mass bias (β, equations 1 and S1) are difficult to
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estimate. Note that β is a catch-all term that includes both mass-dependent discrimination
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in the oven and mass spectrometer, and also imperfections in peak shape. Determination
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of Cl isotopes was not a designed application of SAM, so neither an in-situ Cl isotope
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standard nor a pre-launch calibration are available. SAM determined the 36Ar/38Ar ratio
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of both terrestrial and Mars atmospheric Ar, and this ratio involves very similar mass
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spectrometer bias effects (e.g., peak shape effects) to those relevant to the Cl isotope
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ratio. On a pre-launch measurement with the SAM flight model a 38Ar/36Ar ratio of 0.189
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± 0.001 was obtained on terrestrial air. This is within error (<5 ‰) of the accepted value
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of 0.1883 ± 0.0002 (Renne et al., 2001). In addition, SAM measured (Atreya et al., 2013)
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a Mars atmospheric 38Ar/36Ar ratio that is within stated error of the 38Ar/36Ar ratio
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inferred from the trapped atmosphere component in martian meteorites (Wiens et al.,
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1986). Both of these observations suggest that mass bias associated with the mass
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spectrometric measurement is close to unity.
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