Atmospheric gas in modern and ancient halite fluid inclusions: A screening protocol

Accepted Manuscript Atmospheric gas in modern and ancient halite fluid inclusions: A screening protocol

Nigel Blamey, Uwe Brand PII: DOI: Reference:

S1342-937X(19)30018-8 https://doi.org/10.1016/j.gr.2018.12.004 GR 2074

To appear in:

Gondwana Research

Received date: Revised date: Accepted date:

4 July 2018 3 December 2018 12 December 2018

Please cite this article as: N. Blamey and U. Brand, Atmospheric gas in modern and ancient halite fluid inclusions: A screening protocol, Gondwana Research, https://doi.org/ 10.1016/j.gr.2018.12.004

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Atmospheric gas in modern and ancient halite fluid inclusions: a screening protocol

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Nigel Blamey1 Uwe Brand*2

1Department

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of Earth Sciences, University of Western Ontario, 1151 Richmond Street N., London, Ontario N6A 5B7 Canada ([email protected]) 2Department

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of Earth Sciences, Brock University, 1812 Sir Isaac Brock Way, St. Catharines, Ontario L2S 3A1 Canada ([email protected])

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Corresponding author

ACCEPTED MANUSCRIPT Abstract Halite possesses great potential for hosting and storing information vital to the reconstruction of Earth’s ancient climate, seawater chemistry and evolving atmosphere. Here, we propose a screening protocol that not only distinguishes between primary and secondary halite, but also

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identifies fluid inclusions that carry original gas trapped during the primary crystallization

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process. An integrated multi-analytical protocol is presented for sample preparation, petrographic evaluation, analytical measurements and distinguishing original gases and

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contents from contaminants. The screening protocol starts with the visual inspection of halite for primary chevron and hopper features and/or milky appearance. Then, petrography is used

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to distinguish between primary and secondary (diagenetic) crystal fabrics and inclusions. The

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trace element and isotope geochemistry speak directly to the composition of the depositional

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and/or diagenetic fluids and to the formation in marine, non-marine or diagenetic settings. Furthermore, rare earth elements and redox sensitive elements may address the redox

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conditions of the salt pan/flat brines, whereas microthermometry helps characterize the

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depositional/diagenetic environment’s temperature. Finally, gas extracted by quadrupole massspectrometer from gas bubbles in fluid inclusions is screened with concomitant CH4, CO2 and Ar

40Ar/36Ar

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contents to distinguish and quantify organic matter decomposition contributions and with to monitor modern atmospheric gas diffusion/leakage issues, and thus, contamination

of the primary gas contents. Our test case shows that halite collected from cores is well-suited in maintaining its original mineralogical texture, chemistry and gas content. Our integrated screening protocol suggests that the 815 Myr old halite from the Tonian Browne Formation of the Officer Basin, Australia

ACCEPTED MANUSCRIPT formed under atmospheric conditions of about 10.9 % pO2 (back calculation) or 9.6 % pO2 (mean calculation). The coeval Neoproterozoic gypsum collected in outcrop from Minto Inlet, Victoria Island, Canada underwent some mineralogical alteration. However, experiments suggest that depositional gypsum from outcrop/core may have potential of retaining vestiges of

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original gas in fluid inclusions.

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Highlights - Integrated screening protocol for halite Gas inclusions in modern and ancient halite/gypsum

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Trapped original atmospheric oxygen - Neoproterozoic

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Key Words: Halite; screening protocol; gas inclusions; atmospheric oxygen; Neoproterozoic

ACCEPTED MANUSCRIPT 1. Introduction The conceptual model of Earth’s ancient atmosphere is based on theoretical considerations, redox-sensitive element and isotope modelling, and/or by extrapolation using the appearances/disappearances of organisms in marine and terrestrial sediments (e.g.,

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Holland, 2006). Precambrian atmospheric reconstructions have been produced with many new

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proxies, but only a few studies have measured trapped whiffs of the ancient atmosphere (e.g., Blamey et al., 2016). Material such as amber initially thought to possess great potential in

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trapping atmospheric gas failed the scrutiny of a detailed screening process (Berner and Landis, 1988). Similarly, evaporites were mentioned in the 1950’s by Smits and Gentner (1950), and

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then again by Freyer and Wagener (1970, 1975) to possess the potential for trapped

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atmospheric nitrogen, oxygen, and argon in their inclusions. Freyer and Wagener (1970) in their

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pioneering work experimented with different cleaning protocols, sample preparation techniques, and the release of gas from inclusions using incremental heating of samples (cf.

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Zimmermann, 1972; Zimmermann and Moretto, 1996). They noticed a decrease in oxygen

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content depending on sample preparation method (i.e., dissolving, grinding and melting), but noted a major release of gas after heating of samples to temperatures greater than 350°C

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(Freyer and Wagener, 1975). They ascribed the large volume of gas released from the halite to either diagenesis, and/or degradation of organic matter that may impact the primary gas composition in the halite formed from air-saturated water. They concluded from their observations that gases extracted from their studied evaporites, including halite, may have an atmospheric origin. Freyer and Wagener (1975) further postulated that recrystallization would lower gas contents, and oxygen contents could be changed through the oxidation of organic

ACCEPTED MANUSCRIPT matter. But, it should still be possible with back calculations to attain original atmospheric gas values during the time of salt formation. Other analytical tools, such as FT-IR, Raman microspectrometry or Laser Raman Spectrometry are available for determining gas in halite fluid inclusion bubbles (e.g., Grishina

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1987; Wopenka et al., 1990; Siemann and Ellendorff, 2001), but they provide no easy means of

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measuring their actual concentrations (cf. Roedder, 1990). Thus, the quadrupole mass spectrometer is the instrument of choice for measuring the gas content in fluid inclusions in

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evaporites and other rocks and minerals (Norman and Sawkins, 1987; Blamey, 2012; Brand et al., 2015). However, thermal decrepitation to open inclusions has been replaced by the crush-

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fast-scan (CFS) method at room temperature (Blamey, 2012). This gives results with greater

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precision and sensitivity on smaller samples. The bulk crush method gives average gas contents,

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whereas the incremental crush method allows for the reduction of sample size while revealing potentially more heterogeneity in gas compositions with each sequential crush. Blamey et al.

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(2016) provided the first direct measurement of atmospheric oxygen for the Precambrian, and

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presented compelling arguments for the oxygen content in halite from the Browne Formation of Australia to represent the Tonian atmosphere. They provided a brief synopsis of the

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screening methodology used in the selection of the best material for gas analysis by mass spectrometer. Since it was brief, the main objective of this paper is to provide a detailed screening protocol that should be used for selecting the ‘best’ material to potentially extract primary and ‘original’ gas contents from evaporites, especially halite. Furthermore, using our integrated screening protocol we plan on re-examining the reliability and fidelity of

ACCEPTED MANUSCRIPT atmospheric oxygen on a sample suite of Neoproterozoic halite from Australia, and conduct an experiment testing the suitability of ancient gypsum.

2. Material & Methods

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We selected modern halite from several localities and settings including marine and non-

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marine environments for screening (Figure 1). We also used ancient evaporites from both

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outcrop and core for testing our integrated screening protocol (Figure 1). For the initial set-up of the procedure, ambient atmospheric laboratory-air trapped in 1 µL capillary tubes was used

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to confirm the mass spectrometer calibration and establish baseline parameters for gas

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contents trapped in bubbles of inclusions of modern evaporite minerals.

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2.1 Capillary Tubes

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Calibration of the mass spectrometer was accomplished by, a) bleeding minute amounts of ambient air into the system to determine the gas sensitivity factors (bleed-in-gas; BIG), and b)

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checking the calibration by crushing capillary tubes and releasing its trapped atmospheric gas

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into the main mass spectrometer chamber for measurement (1 µL of one tube equals one crush analysis; 1TCA). Millipore glass capillary tubes were cut into 2-3 cm lengths representing a

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volume of about 1 µL. One end of the tube was flame sealed, and subsequently they were placed overnight in a furnace at about 500°C to burn off contaminants. Cooled tubes were sealed with high-vacuum epoxy and allowed to cure for several days. Sealed capillary tubes were placed in the crushers, pumped overnight to high vacuum and then by ‘simulated incremental’ crush the released gas was introduced into the mass spectrometer for analysis. The 1TCA air slug was then compared to the BIG air component introduced under vacuum ( of

ACCEPTED MANUSCRIPT about 10-7 to 10-5 Torr) into the machine. Knowing the volume, barometric pressure, and composition of the ‘test’ air, the current generated by the machine can be used to calculate the contents of gas in mols/L (described in detail by Blamey et al., 2015).

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2.2 Modern Halite

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Modern halite was collected at seven localities, with four from non-marine environments and

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three from marine ones (Figure 1; cf. Blamey et al., 2016). The terrestrial halite came from natural and man-made ponds providing global coverage. The marine halite came from the

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Bahamas, Bonaire and South Africa. All material was cleaned and each reduced to 2-4mm3

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blocks with a mass of about 100 mg. The final cleaning step consisted of immersion in isopropanol alcohol for about 5 minutes. Afterwards, each sample was left to air dry before

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placing it, using forceps, into the crushing chamber. Depending on sample size and number of

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2.3 Ancient Evaporites

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inclusions, it may be possible to obtain gas from 5 to 12 crushes (cf. Blamey et al., 2016).

Ancient evaporites (gypsum and halite) were cleaned and prepared for gas analysis as described

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in section 2.2. The material for this part of the study came from six localities in Australia (#1) to

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(Asia #2-4), to Europe (#5) and to North America (#6; Figure 1; cf. Blamey et al., 2016). The samples range in age from Messinian (late Miocene) to Tonian (early Neoproterozoic). Table 1 gives an overview of chemical parameters for the various evaporites, with some clearly of a marine origin, whereas others have a non-marine origin. 3. Screening Methods A multitude of screening methods was used to identify evaporite samples with primary chemistry but more importantly with original gas contents trapped within fluid inclusions (cf.

ACCEPTED MANUSCRIPT Brand et al., 2015; Blamey et al., 2016). Petrography was the second step after visual inspection of material for chevrons, cornets, hoppers, milky appearance; all features rich in fluid inclusions (Figure 2; Schreiber and El Tabakh, 2000). The third step of microthermometry was used to bracket the temperature of formation, followed by the fourth step of trace chemistry consisting

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of Br, Mg, Sr and others to identify and differentiate between marine and non-marine, and

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depositional and diagenetic material. Sulphur and strontium isotopes are excellent tracers to differentiate not only between originally marine/non-marine and diagenetic sources, but as

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well as the amount of available oceanic sulphate (Hurtgen et al., 2002). Furthermore, the argon isotope composition is ideal in assessing any leakage of gas from fluid inclusions. Once these

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steps are complete, the methane, carbon dioxide and argon gas contents of halite may be

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to establish the fidelity of oxygen.

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evaluated relative to baseline parameters of modern counterparts and capillary tube contents

3.1 Petrography

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After removal of surficial contaminants, slices of halite are cleaved for mounting on glass slides.

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Cleaving of halite is the least disruptive process in obtaining ‘thick’ sections for petrographic analysis of fabrics and fluid inclusions. Slices about 1 mm thick were mounted on glass slides for

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petrographic and cathodoluminescence evaluation, but the latter was abandoned due to its inconclusive results. However, plain light petrography is an extremely valuable tool in identifying chevrons, cornets, hoppers, inclusion trails and other primary fabrics, but more importantly, inclusions with gas bubbles, clear halite devoid of inclusions, or halite of a secondary/diagenetic nature, or halite with overlapping primary and secondary inclusions (Figure 3; cf. Lowenstein and Hardie, 1985; Schreiber and El Tabak, 2000; Goldstein, 2001).

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3.2 Microthermometry For microthermometry, wafers of halite were cleaved to a thickness of approximately 1mm and placed into a freezer at about -5° to -8°C for up to five weeks to nucleate gas bubbles in the

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fluid inclusions. Our freezer temperature and duration deviates from that used by some authors

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ranging from -15° to -20°C for just a few days (Roberts and Spencer, 1995; Rigaudier et al.,

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2011). Cooled wafers were expeditiously loaded into the pre-cooled stage at -10°C of a LINKAM THMS-G600 heating-freezing stage system. This system was calibrated with synthetic and SYN-

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SLINK water standards, and condensation was minimized by placing desiccant into the stage.

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Once in place, samples were gradually heated starting at 0°C at a rate of 5°C/min until the system reached a pre-set maximum temperature of either 50° or 80°C. This procedure allows

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for garnering of homogenization temperatures (Th) from gas bubbles in inclusions.

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3.3 Trace element chemistry

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Trace chemistry is another valuable tool for differentiating primary halite from marine, continental and diagenetic forms, while rare earth element results are useful proxies of the

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redox state of the salt-precipitating brine or secondary fluid. The trace chemistry of halite and

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fluid inclusion may have a polygenetic origin, anything from depositional seawater and brines to meteoric surface water, groundwater and formation fluids (Roedder et al., 1987; Zimmermann, 2001). Thus, cleaning of samples after petrographic investigation is of utmost importance in obtaining chemical contents reflecting the solid or fluid source. Evaporite samples were carefully cleaned of detrital material, and then gently crushed by mortar and pestle under anhydrous ethyl alcohol to release chemical contributions from fluids within inclusions, and rinsed with copious amounts of the alcohol through a filtered funnel (cf.

ACCEPTED MANUSCRIPT Moretto, 1988). After air drying, crushed material was weighed to four decimal places and then dissolved in appropriately-sized volumes of distilled water. The acidified solution with the addition of ionization releasing agents was analysed by atomic absorption spectrophotometer and matched to NBS 633 for a certified Sr content of 2621 mg/kg (N=120) and error of 3.03%

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(cf. Brand and Veizer, 1980). A similar preparation procedure was used when analysing for REE

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elements in evaporites by ICP-MS. This procedure reduces, but not necessarily eliminates, the chemical contribution from fluid-inclusion brines, because some may be extremely small and

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beyond the reach of the crushing process. The procedure for analysing for Br contents is amended from Kovalevych et al. (2006) and consists of cleaning halite of all extraneous

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contaminants. Some halite was powdered under anhydrous alcohol to eliminate contribution of

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chemistry from brines trapped in fluid inclusions (Moretto, 1988). This was followed by pressing

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about 6 g of crushed halite with 1.5 g of wax into powder pellets for Br measurement by X-ray fluorescence.

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3.4 Stable and radiogenic isotopes

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Preparation of evaporites for sulphur isotopic analysis followed the method of Yanagisawa and Sakai (1983) modified by Hurtgen et al. (2002). Some isotope results of the Browne Formation

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halite are from Spear (2013). After cleaning and decontamination of the halite, BaSO4 was precipitated and SO2 gas was analysed on a mass spectrometer and results reported versus VCDT for 34S with an error of ±0.40 ‰ (see Spear, 2013 for details). Analysis of strontium isotopes followed standard analytical procedures, but involved more intense cleaning and removal of contaminants during sample preparation. Dissolution of halite in ultra-pure water and subsequent filtration removed most detrital and carbonate contaminants (cf. Tan et al.,

ACCEPTED MANUSCRIPT 2010). This was followed by standard separation sequencing and extraction of Sr for isotope analysis. All results were calibrated to NIST standard reference material NBS 987 adjusted to a value of 0.710247 (N=394) with a standard error (2) of ±0.000002 and standard deviation (2) of ±0.000032 (cf. Brand, 2004 for more details). Unirradiated argon isotope compositions were

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measured on cleaned modern and ancient halites. The isotopic composition was measured by

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vacuum stepwise heating using the extended Ar-Ar method on a Thermo Scientific ARGUS VI multi-collector mass spectrometer (Mark et al., 2010; Pujol et al., 2013; Stuart et al., 2016).

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3.5 Gas Chemistry

We concentrated on halite for gas analysis with the least amount of detrital, carbonate and

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other contaminants, while paying special attention to visible depositional fabrics of chevrons,

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hoppers and trails. Material with obvious diagenetic or secondary features was avoided and

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removed from further analysis. Of course, this will depend on the objectives of any particular study. Samples should be on average about 100 mg or more and about 2-4 mm3. Sample

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chunks were cleaned with isopropanol alcohol and allowed to air dry before placement into the

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crusher system. After loading of the crushers, units were pumped overnight to achieve a vacuum of about <10-8 Torr to remove ‘attached’ and fracture-associated gas in the halite.

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Depending on gas contents and number of inclusions up to 12 incremental crushes and gas releases may be realized from any specific halite sample. The liberated gas was analysed with a Pfeiffer Vacuum Prisma quadrupole mass-spectrometer in the crush fast-scan (CFS) peakhopping mode, and with the signal current sent to the computer for processing (cf. Norman and Sawkins, 1987). Background was measured for 10 cycles up to 1 second before and after each

ACCEPTED MANUSCRIPT liberated gas burst from each crush. Gases measured include H2, He, CH4, H2Og, N2, O2, Ar and CO2. Calibration of the mass spectrometer was achieved and maintained using the Scott Gas Minimix calibration standard supplemented by three natural fluid-inclusion gas standards. The

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gas contents of the capillary tubes were used to convert the mass spectrometer’s current signal

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to mols of gas, and confirm the N2, O2 and Ar ratios relative to modern values. Quantitative results were determined with the matrix inversion method of Isenhour and Jurs (1972), and

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detection limits were calculated and set to ~1x10-15 mols using the formula of Blamey et al. (2012, 2015). All gas results are presented in Appendices 1 (capillary tubes), 2 (modern halite),

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and 3 (ancient halite and gypsum).

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3.6 Statistics

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We used the PAST3 software (Hammer et al., 2001) to evaluate the results as well as for comparing the various gas contents and concentrations (argon and oxygen) of the

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Neoproterozoic halite (Browne Formation, Australia) and gypsum (Minto Inlet Formation,

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Canada). We also used it to evaluate the contribution of atmospheric gas or gas dissolved in

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seawater/brines in the halite fluid inclusions.

4. Screening Protocol We propose a screening protocol for selecting the best-preserved halite with suitable fluid inclusions and trapped ambient atmospheric gas in bubbles for analysis by mass spectrometry. Some steps may be more crucial to selecting material for primary gas, but as many screening steps should be applied to get the best possible result. The physical evidence garnered by petrography is deemed essential in selecting halite ranging from good to limited potential. The

ACCEPTED MANUSCRIPT presence of hoppers, cornets, chevrons, and of trails of inclusions are excellent signs of primary depositional characteristics retained by the halite (Figure 2; Schreiber and El Tabakh, 2000). Solid and fluid chemistry of the halite may serve as secondary proxies of identifying material with the greatest potential of having formed at or near the water/air interface and preserved

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its gas bubble content. With quantitative gas geochemistry of minerals in its infancy, the real

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power of elemental and isotopic results may be realized with further study.

4.1 Lithological selection

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In the meantime, evaporite sequences and material from cores from below the groundwater

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table should be considered prime targets for gas analysis. Also, material from areas of tectonic tranquility and overall geologic inactivity would be better targets than those from areas with

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high burial rates, temperatures, groundwater flow and generally high tectonic activity (Blamey

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et al., 2016). The presence of chevrons/cumulates and a milky appearance are all excellent

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indicators of primary fabrics and contents preserved by the halite (Schreiber and El Tarakh, 2000), whereas clear areas denote an absence of inclusions which is possible in both

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depositional and secondary halite.

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Selected material are almost exclusively from cores except for the gypsum from the Minto Inlet Formation. The Browne material from specific intervals considered depositional as well as secondary influences (Figure 4). Sample 309-3-6 (#1478) of the Empress E1A core was obtained from just below a horizon with obvious signs of sand deposition and formation of an efflorescence halite crust during surface exposure (Figure 4A; Spear, 2013). In contrast, sample 310-2-5 (#1482) came from bedded halite (Figure 4B), and sample C-2 (#1502.2) came from the bottom horizon of a massive halite sequence (Figure 4C). Thus, selecting halite from cores must

ACCEPTED MANUSCRIPT consider the sub- and suprajacent lithologies and their potential influence on the selected material in hosting and maintaining primary inclusions with original gas bubbles. The use of fossil evaporites from outcrop and of gypsum is indetermined as to their potential for retaining

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original gas in their fluid inclusions; only more study will answer this question.

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4.2 Petrographic evaluation

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Petrography is a strong test for identifying halite of depositional and diagenetic origins (cf. Roedder, 1984; Lowenstein and Hardie, 1985; Schreiber and El Tabak, 2000; Goldstein, 2001;

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Schoenherr et al., 2009; Benison, 2013; Spear et al., 2014). Fluid inclusions may or may not

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carry bubbles with gas, nor is there any assurance that the gas is primary, although some specific petrographic features may provide solid clues to their fidelity. Preserved fluid inclusion

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trails associated with chevrons are usually assumed to be a good indicator of an original and

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primary fabric (Figure 5A). Thus, excellent preservation is suggested for the Neoproterozoic

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halite from the Browne Formation (horizon #1502.2, Figure 5D). Another feature suggesting preservation is the ‘cubic’ nature of fluid inclusions (Figure 5A-5D), and in some instances, some

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or several inclusions may contain bubbles of air (gas; Figure 5A, 5B, 5C). Inclusions trails are well

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defined in the halite from the Salt Range, unfortunately, these small trails with inclusions are overprinted by large diagenetic inclusions (Figure 6A). In some instances, during burial and subsequent tectonic activity, halite may fracture and inclusion trails may form along fracture seams (Figure 6B). In addition, pre-existing and perhaps primary cubic fluid inclusions and trails may be distorted by burial and tectonic activity into odd and/or elongated shapes (left side of Figure 6B). These two types of inclusions may co-exist within a single, large halite crystal (cf. Figure 3). Halite that forms as cement during early burial tends to be clear and devoid of

ACCEPTED MANUSCRIPT inclusions (Schreiber and El Tabakh, 2000), which is easily distinguished from depositional halite with its cloudy, white and/or primary fabrics (chevrons, cornets, hoppers, cumulates, cf. Fig. 2; Roedder, 1984; Goldstein and Reynolds, 1994). 4.2 Microthermometry

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Microthermometry is a powerful tool for differentiating between depositional and burial

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(diagenetic) processes and thermal influences on halite and fluid inclusions (Roberts and Spencer, 1995). Fluid inclusions, if of a primary nature, will hold important information about

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the source seawater (if marine) as well as a temperature record of the halite-precipitating brine. Chevrons may be disrupted and fluid inclusions may be stretched under burial and

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sampling (drilling) conditions, which would also raise homogenization temperatures (Th) to

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levels far exceeding the depositional regime of 35-55°C plus the 3-6°C to account for bacterial

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presence (Schreiber and El Tabakh, 2000). Thus Th, in some instances, may be as high as 70°C (Lowenstein and Spencer, 1990). The associated problem of examining gas bubbles in fluid

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inclusions (e.g., Figure 5A) by microthermometry is overcome by selecting only single-phase

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fluid inclusions for homogenization temperature determinations (cf. Roberts and Spencer, 1995). Fluid inclusions tested for homogenization temperatures on Messinian, Hongchunping,

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and Browne Formation halites are all well within the depositional framework (Table 1). In contrast, homogenization temperatures of 55-100°C obtained from the Salt Range halite suggest a burial/heating influence, which is supported by their generally stretched, distorted and large secondary inclusions (Figure 6A).

4.3 Chemical analysis

ACCEPTED MANUSCRIPT The chemistry of the liquid trapped in fluid inclusions as well as that of the solid mineral may provide important information for differentiating between marine and non-marine halite and between depositional and diagenetic types (e.g., McCaffrey et al., 1987; Lazar and Holland, 1988; Ayora et al., 1994; Siemann and Schramm, 2000; Zimmermann, 2001; Horita et al., 2002).

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The chemistry derived from inclusion fluids is valuable in characterizing the composition of

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ancient seawater (e.g., Timofeeff et al., 2006). However, geochemical analyses are beset by several issues that complicate, a) their analyses and b) their interpretation. For example, the

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chemistry of fluids in inclusions may not be uncontaminated seawater, but may be sourced during syndepositional recycling from a second generation of water (sea or meteoric) that may

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have dissolved a pre-existing halite and reprecipitated as syndepositional halite (Timofeeff et

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al., 2001). The presence of undistorted chevrons should help alleviate this concern and

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differentiate depositional from syndepositional halite. Another issue, is that the brine trapped in fluid inclusions is not simply a product of evaporated and concentrated seawater. Instead,

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the fluid reflects complex changes with precipitation of the halite and other minerals on the

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evaporation pathway (cf. Table 1; Timofeeff et al., 2001). The chemistry of solid halite may be used to infer an environment of deposition, but this may be complicated by the mixing of

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contributions from the solid and fluid inclusion fractions. Moretto (1988) documented differences in trace chemistry based on ‘dry’ and ‘wet’ preparation methods, and this should be considered for the tracers such as Br, REE, S and Sr isotopes when they will be used as proxies of depositional conditions (McCaffrey et al., 1987).

4.3.1 Bromine

ACCEPTED MANUSCRIPT Of all the elements analysed from the fluid and solid matter of halite, Br is the best to define and characterize its depositional and diagenetic history (McCaffrey et al., 1987). McCaffrey et al. (1987) used the Br content to chart the evaporation pathway of seawater and the precipitation of halite from seawater once concentrated by a factor of 10.6 (Potter et al., 2004;

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Babel and Schreiber, 2014). They harvested salt and corresponding brine from crystallizing

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pans, with salt containing as much as 4.2 wt. % fluid inclusions (McCaffrey et al., 1987). Their experimental work suggests that halite should contain about 70 to 250 mg/kg Br with the lower

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value characteristic of the onset of precipitation on seawater’s continual evaporation pathway. Bromine values lower than 70 mg/kg may indicate some influx of ‘secondary’ seawater or

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meteoric water into the system during secondary-cycle precipitation of halite (Holser, 1979).

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The Br contents of our studied material are higher than 70 mg/kg, with two exceptions of the

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Messinian and the Salt Range material (Table 1). The Br value of 37 mg/kg for the Messinian is not for the solid fraction but of the fluid from inclusions (Lazar and Holland, 1988), and speaks

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directly to the issue of the complex relationship between fluid inclusion and solid chemistry of

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halite (cf. Moretto, 1988). In contrast, the low Br value for the Salt Range halite is for the solid component and suggests recycling or syndepositional alteration of the depositional halite by

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diagenetic fluids (Table 1; Holser, 1979; Brand and Veizer, 1980; Hovorka et al., 1993; Schreiber and El Tabakh, 2000; Timofeeff et al., 2001). Thus, the Br contents of halite from Bonaire, and from the Majiagou and Browne formations support original depositional marine conditions (Table 1). 4.3.2 Rare Earth Elements

ACCEPTED MANUSCRIPT Rare Earth Elements (REE) in carbonates and halite are measured to ascertain the redox conditions of the depositional or diagenetic fluid (Azmy et al., 2011). Measurements of REE on halites is rare, but low REE contents and a Ce* value of less than 1.0 speak to the relatively good preservation of the halite material and the precipitation from oxic fluids/seawater (Table

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1). In contrast, the uppermost sample from the Browne Formation suggests a different

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depositional or fluid history with anoxic water conditions (Ce* = 1.03, Table 1; Azmy et al., 2011). The application of REE chemistry to evaporites is in the preliminary stage and deserves

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more detailed studies as proxies of environmental conditions.

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4.3.3 Strontium isotopes

Strontium isotopes are widely used to differentiate between marine and non-marine

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carbonates and evaporites as well as those with a diagenetic history (e.g., Veizer, 1989;

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Hovorka et al., 1993; Brand, 2004; Tan et al., 2010). Also, the secular variation of Sr in seawater

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is well established (e.g., Veizer et al., 1999), and thus the Sr isotope composition of halite could serve as an invaluable proxy of original depositional and diagenetic conditions. The marine

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halite from the Bahamas and its Sr isotopic signature (0.709153) clearly reflects that of modern

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seawater (Elderfield, 1986; Brand et al., 2003), whereas that from the halite of Lake Polaris in Australia is a good indicator of its non-marine nature (0.732038, Table 1). Furthermore, the Sr isotope compositions of the halite from the Browne Formation (0.706696 – 0.706767) concur with values from carbonates considered primary for early-mid Neoproterozoic seawater (Halverson et al., 2007). Overall, the Sr isotope composition and interpretations support the conclusions reached based on Br contents and primary fabrics for the various halite material (Table 1, Figure 5).

ACCEPTED MANUSCRIPT 4.3.4 Other elements/isotopes The Sr and Mg elemental contents of halite may possess traits suitable for differentiating between depositional and diagenetic halite, but more work is need to establish solid pathways and trends (cf. Table 1). Sulphur isotopes are routinely analysed in carbonates and evaporites,

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and their compositions, if primary, can be matched to the secular variation curve of seawater

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sulphur and to assess variations in the sulphur cycle (e.g., Hovorka et al., 1993; Hurtgen et al., 2002). Furthermore, isotopes such as chlorine show great potential in tracing the evaporation

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pathway of evaporites from pre-concentrated seawater or a secondary influx of seawater and meteoric water (e.g., Eastor et al., 1999). Most of this research is in its infancy and requires

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more work to use these proxies as preservation indicators.

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4.4 Gas chemistry

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Extracting and examining the gas content of halite is not new, but the novelty is the concept

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that the gas, if primary, may reflect that of the ambient atmosphere (cf. Smits and Gentner, 1950; Fryer and Wagener, 1975). These early applications had problems in fully characterizing

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all gases trapped within bubbles but also with determining their concentrations, and most of

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these early results were limited to very large inclusions. With the CFS method and mass spectrometers, we can measure the gas content of tiny air bubbles trapped during the precipitation process in fluid inclusions of chevron trails in small samples (cf. Blamey et al., 2016). 4.4.1 Baseline (modern halite) The first step in characterizing the gas chemistry of bubbles trapped in fluid inclusions is the release of the gas at room temperature using the crush fast scan method and its immediate

ACCEPTED MANUSCRIPT analysis by mass spectrometer. Marine and non-marine halites from around the globe were collected to test for its fidelity in trapping atmospheric gas and documenting its global uniformity (Figure 1). To assess the impact of both internal and external sources of contamination, the pO2 content is plotted against carbon dioxide (and methane) an organic

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matter decomposition gas (Freyer, 1978). Organic matter remains have been noted in

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inclusions of ancient halite in keeping with an abundance of halophilic bacteria/algae in salt brines (Schreiber and El Tabakh, 2000; Vreeland et al., 2007). After incorporation, the organic

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matter may decompose and generate gas (Figure 7). In contrast, air trapped in capillary tubes, devoid of halophilic organisms, shows the ideal ambient air composition, whereas marine and

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non-marine halite show distinct and variable organic matter gas (OMG) contents. In contrast,

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halite formed in deeper seawater brine or trapping only fluid in inclusions may not record any

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gases (cf. Bonaire samples; Figure 7). The decomposition of organic matter tends to favour the production of CO2 over CH4 (cf. Freyer, 1978), a trend observed in modern halite and

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accompanied by concomitant changes in the nitrogen, oxygen and argon gas contents

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(Appendix 2). However, modern halite samples with low or relatively low CO2 and CH4, for a combined total of <10 mol %, exhibit a minimal impact on their major gas contents. Those

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samples contain oxygen levels similar to that of modern atmospheric air (Figure 7). 4.4.2 Linearity of gas contents

Another screening test for original and primary gas contents in evaporites is the ‘linearity’ test for multiple gases such as oxygen and OMG. Messinian halite exhibits extremely detailed chevron structures and trails of fluid inclusions (Blamey et al., 2016). The plot of twelve crushes for two runs of gas inclusion analysis of Messinian halite show stable trends of O2 and the CO2

ACCEPTED MANUSCRIPT and CH4 combination (OMG) for the two samples (Figure 8). Supported by petrographic and chemical features and microthermometry (Table 1), the results speak clearly to the stability of the analytical protocol as well as to the fidelity of the inclusion(s) selected for gas analysis. To emphasize, no noticeable increase or trend in OMG was observed with increasing number of

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crushes and analyses, while oxygen contents also remained relatively invariant (Figure 8).

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4.4.3 Organic Matter decomposition

Modern salt brines teem with life, bacterial and algal, especially of the halophilic varieties and

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thus the potential is high that some may get incorporated into the halite and its fluid inclusions

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during the precipitation process (e.g., Schreiber and El Tabakh, 2000; Vreeland et al., 2007). Although, the micro-organisms may survive in the fluid inclusions for a significant amount of

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time, it is most likely they start to decompose soon after being trapped in the halite, and during

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this process produce carbon dioxide and methane;, organic matter decomposition gases (OMG)

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while consuming some of the trapped oxygen. This process and the accompanying trend of increasing OMG with concomitantly decreasing O2 is clearly visible in the modern marine halites

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(Figure 7). In samples with high OMG production, the argon gas channel may be influenced by

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the organic matter decomposition process. But, we have determined that the impact on the original gas contents of nitrogen and oxygen is minimal if the combined OMG contribution is less than 10 mol% (Figure 7). This decomposition pathway discernable in modern halite will serve as a guide in identifying gas contents and compositions that are relatively unimpeded by the gas-altering impact of organic matter decomposition.

4.4.4 Geographic/age comparison

ACCEPTED MANUSCRIPT The direct comparison of gas results of halite from the same age should provide another line of evidence for fidelity of gas results. For example, the better preserved and less impacted inclusions by the production of organic matter decomposition gases from both marine and nonmarine halite define the field marking the primary signal of modern atmosphere (Figure 7). This

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clearly speaks to the fact that both marine and non-marine halite, if preserved, may record the

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same global atmospheric gas conditions. Samples 310-2-5 (Empress 1A core) and 1466-25 (Lancer 1 core) have mean oxygen values of 8.98 and 8.71 mol %, respectively (Appendix 3),

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which speaks clearly to their compatibility despite their geographic separation of several

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hundred kilometers (Blamey et al., 2016).

4.4.5 Gas content calculations

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We can determine the window of original gas compositions by calculating the mean value of all

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primary results, and these may be supplemented by subprime ones. Subprime results of gas

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inclusions pass most screening tests and thus satisfy most categories of preservation. Of a total of 38 crushes and gas analyses, only five gas contents of the Browne halite are deemed the best

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based on the full set of screening parameters, with an additional five results satisfying most

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screening parameters (Appendix 3). The mean oxygen value for the best-preserved Browne Formation halite is 10.0 %, while the mean is 9.6 % for the subprime material. Another way of calculating the oxygen content is by determining the linear regression for the covariance between the oxygen contents and that of the concomitant organic matter decomposition gases. The intercept set at a plausible level of carbon dioxide (and potentially methane) is another possible method to determine the ambient atmospheric oxygen and potentially carbon dioxide contents. Back calculation suggests that the oxygen content for the best Browne halite gas

ACCEPTED MANUSCRIPT results falls, depending on ambient carbon dioxide content, between 10.9 % (<1 % CO2) and 9.8 % (at ~11% CO2). All these results suggest that atmospheric oxygen content during the Tonian was, on average, about 10.1 %, a value similar to that advocated by Blamey et al. (2016).

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5. Discussion

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The Blamey research group (Blamey et al., 2016) advocated that halite with primary lithology,

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fabric, trace and gas chemistry may hold the clue to air trapped during crystallization in the depositional environment. It may be possible that during burial and secondary diagenetic

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alteration of halite some external gases may exchange or get incorporated into secondary fluid

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inclusions. At this stage of the research the release of fluid inclusion gases by crushing is random, which makes it difficult to exclude some secondary inclusions from being crushed and

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gas released for analysis (cf. Figure 3). These examples clearly show the intimate nature of

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primary and secondary inclusions, and thus the problem with releasing gas from just primary or

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secondary or both types of inclusions during a crush step. We should be able to identify largescale crush anomalous gas emissions by dramatic changes in the crush-result pathway (cf.

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Figure 8). Furthermore, contamination of the primary gas content and compositions may arise

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with diffusion of gases during the burial history of evaporites. However, wholesale alteration is easy to identify in the gas contents of ancient evaporites if their values and ratios are similar to those encountered in modern halite. It is the more subtle changes of gas contents, especially of older material from the Precambrian period with their ‘low’ oxygen contents where diffusion/leakage may be a real concern, as well as in samples with both primary and secondary inclusions (cf. Figure 3). Primary oxygen contents and their relationship to atmospheric oxygen evolution is of fundamental importance to microbial life in the oceans, and the subsequent rise

ACCEPTED MANUSCRIPT of animals (e.g., Lyons et al., 2014; Blamey et al., 2016). We already demonstrated that internal ‘activities’ or the mixing of primary and secondary inclusion gases during the crushing process may lead to mixed results (sec. 4.5.3, Figures 3, 6). With their long geologic history, Precambrian halite may experience significant impacts during their burial history from tectonic

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to halokinetic upheaval compounded by contamination by other ancient or modern

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atmospheric gases during exhumation. However, the impermeable integrity of halite is a wellknown fact from their sealing properties of hydrocarbon deposits (e.g., Warren, 1986;

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Schoenherr et al., 2009), as well as being a host rock of choice for storing nuclear waste material (e.g., Roedder et al., 1987).

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5.1 Diffusion

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Diffusion of gas both in and out-of halite may present a considerable problem to obtaining with

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high probability original gas contents, especially for material with complex depositional-burial histories (Babel and Schreiber, 2014). Although, salt and halite have a general reputation of an

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impermeable substance, tests are required to substantiate this assertion. Zimmermann and

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Moretto (1996, p. 414) conducted extensive experiments on primary and diagenetic salt to “…determine the departure temperatures of gases…”, namely the release or diffusion of gas

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with incremental heating mimicking burial or exposure to radioactive processes (Figure 9). With sequential heating between 80-320°C, the first component to depart was adsorbed or intergranular water, with water from fluid inclusions released above this thermal limit (Zimmermann and Moretto, 1996). The log diffusion coefficient per radius square of a hypothetical sphere is miniscule at temperatures usually experienced by ancient halite documented by their thermal history during burial and possibly subsequent exhumation (Figure

ACCEPTED MANUSCRIPT 9, Table 1; Zimmermann and Moretto, 1996). Thus, under common basinal burial temperatures (~ 100-150°C), gas trapped in the tiny bubbles within fluid inclusions should be safe from extraction, release or cross contamination (cf. Moore et al., 2001). 5.2 Leakage

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The potential issue of leakage remains the greatest problem in obtaining measurements of

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original ancient air trapped in amber and halite and other minerals (e.g., Berner and Landis, 1988; Blamey et al., 2016; Yeung, 2017). Argon-36 is considered primordial whereas argon-40, a

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stable isotope of Ar, has been accumulating in the atmosphere from the decay of radioactive potassium-40 (Pujol et al., 2013). Noble gases are rare and their contents are extremely

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sensitive to input from decay processes, facilitating the tracing of post-depositional water-rock-

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mineral interactions. If bubbles hosted by fluid inclusions leaked during their burial history,

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eventually, their 40Ar/36Ar ratio should approach that of modern air at 298.56 (Lee et al., 2006). The 40Ar/36Ar measured in modern (297.21) and Messinian (294.31) halites are similar to that

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measured in modern air (Figure 10A). The slightly lower 40Ar/36Ar ratio measured in the

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Messinian halite speaks directly to its excellent preservation and hosting primary gas contents. The 40Ar/36Ar of the Tonian Brown Formation halite ranges from 285.85 to 435.95 with a mean

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of 320.81 (Figure 10B). If the gas ratio measured was subject to leakage from the fluid inclusion and bubbles, then the value should be closer to that measured in modern air. If the gas ratio was not subject to the addition of radioactive decay 40Ar, then the values should be somewhat lower concentrating about a modelled ratio of 273 to 280 or 290 (cf. Pujol et al., 2013; Stuart et al., 2016). It appears neither leakage and replacement by modern atmospheric air component or unaltered by radioactive decay is the explanation for the measured ratios (Figure 10B).

ACCEPTED MANUSCRIPT Instead, the measured argon in the Tonian halite is the product of input from the radioactive decay of potassium and its retention within the aqueous phase of the fluid inclusion. It further confirms the reason for slightly elevated Ar contents measured in some of the samples (crushes) from the Browne halite (Fig. 11). Most importantly, the 40Ar/36Ar ratios speak clearly

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to the retention of primary gas content and composition within the fluid inclusion of the 815

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Myr old halite from the Brown Formation, and thus it confirms the original and primary nature of the measured oxygen of Blamey et al. (2016) and in this study.

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5.3 Test Case – Neoproterozoic Browne halite

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Blamey et al. (2016) presented an evaluation of the gas contents in halite from the Tonian Browne Formation of the Officer Basin of Australia. They evaluated the N2, O2 and Ar ratios and

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concluded that Tonian atmosphere possessed an oxygen level of about 10.9 ±1.4% (Blamey et

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al., 2016). Using the screening parameters advanced here and bracketed by the results of the

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modern halite, we propose, based on the strictest interpretation, that the Tonian atmosphere contained about 10.0 % oxygen (N=5 results, Appendix 3). With a more liberal approach, a

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further five results considered subprime suggest a combined level of 9.6 % for this time. Back

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calculations using OMG levels of less than 1% and 11% suggest oxygen levels of 10.9 % and 9.8 %, respectively (Figure 11). Considering the sum of these results gives 10.1 % for the Tonian atmosphere encapsulated in bubbles within fluid inclusions formed during deposition of the Browne Formation halite.

5.4 Experimental – Neoproterozoic Minto Inlet gypsum

ACCEPTED MANUSCRIPT Of all the evaporite minerals, halite is the most studied for its fluid chemistry, solid chemistry, isotopic compositions – both liquid and solid, for their microthermometry and gas chemistry (cf. Babel and Schreiber, 2014; Blamey et al., 2016). Other evaporite minerals, among them gypsum and anhydrite, are usually just mentioned peripherally in sedimentary and stratigraphic studies

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(cf. Thomson et al., 2015), with little attention paid by researchers to their diagenetic evolution

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(Murray, 1964). We have a golden opportunity to compare the gas results of the Browne to a coeval evaporite (gypsum) of intercontinental geographic separation (Australia to Canada), and

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evaluate the depositional and diagenetic history of the gypsum from the Minto Inlet Formation. The Tonian Minto Inlet Formation of the Shaler Supergroup on Victoria Island, Canada consists

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of alternating restricted evaporites and open ocean limestones (Rainbird et al., 1996), which are

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only slightly older than the Browne Formation of Australia (Thomson et al., 2015). Thus, a

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comparison of gas results is warranted to address the question of fidelity of atmospheric gas content determinations. Unfortunately, in comparison to halite and available information on

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determining primary features and depositional conditions, little is known about features in

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ancient gypsum. Murray (1964) provides some insight into features to distinguish between depositional and diagenetic or replacement gypsum/anhydrite. More importantly, he describes

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a formational-replacement-diagenetic cycle of sedimentary gypsum, where depositional gypsum is replaced during burial by anhydrite, and, when this anhydrite is exhumed it in turn is replaced by gypsum (Moore, 1964). Thus, it is of utmost importance to differentiate between sedimentary (depositional) gypsum and the diagenetic variety. We hope to face this challenge by extracting gases from the Minto Inlet evaporite – gypsum- material obtained from core and surface exposures of the southwest study area on

ACCEPTED MANUSCRIPT Victoria Island (Thomsen et al., 2015, fig. 1). Figure 12 is a plot of gas, oxygen and argon garnered from three gypsum samples of the Minto Inlet Formation, with combined OMG gases (methane and carbon dioxide) well below the threshold of 10 % (Appendix 3). The gas results of the Minto gypsum are compared to the primary gas (oxygen and argon) compositions of the

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Browne halite of Australia. Oxygen gas contents of Minto sample 1(g) are similar and overlap to

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some degree with the best gas results from the Browne Formation, whereas in contrast Minto samples 2 and 3 are significantly enriched in their oxygen and argon contents (Figure 12, Table

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2). Table 2 summarizes the statistical comparison between the different groups of results, and there appears to be no significant difference between Minto sample 2 and 3 for both oxygen

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and argon. At first glance there is no significant difference in Ar between combined results of

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samples 2&3 and those of sample 1 and 1g (Table 2); with a difference of about 0.1 units.

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Despite the similarity in argon, there is a significant difference in the oxygen contents of combined results of samples 2&3 and those of sample 1 and 1g (Table 2). Most importantly,

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there are no significant differences in oxygen and argon contents between the best of the

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Browne Formation halite and of the Minto Inlet gypsum. This is encouraging and suggests that a study of integrated petrography, chemistry (solid and liquid) and gas analyses is warranted to

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fully explore the potential of gypsum as an archive of primary atmospheric gas contents and compositions (Table 3). 5.5 Challenge In 2017, Yeung (2017, p.67) “… reanalyzed the data recently collected from Neoproterozoic (815±15 Ma) halites in the Officer Basin, Australia.” Before he did his ‘analysis’, Yeung, (2017, p. 67) argued “… that these halites (Browne Formation) are consistent with an atmospheric O 2

ACCEPTED MANUSCRIPT concentration of <10 % PAL…” and “…not ~50 % PAL as previously reported (Blamey et al., 2016).” He supports his claim by assuming (Yeung, 2017, p. 67) “…that atmospheric N 2 has not changed significantly since the Neoproterozoic…” drawing evidence from the Phanerozoic. Blamey et al. (2016), and this study make no such assumptions, instead we just measured the

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gas content of bubbles trapped in the fluid inclusions of halite. Yeung (2017, p.69) further

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suggests the possibility of “…contamination by post-depositional constituents such as modern air…” and cites high post-depositional temperature of >100°C to facilitate the process, through

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“…diffusive gas loss…”. His speculations are in direct contrast with experimental outcomes and direct measurements. The extensive experimental work of Zimmermann and Moretto (1996)

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discussed in section 5.1 stand in stark contrast. Furthermore, the measured 40Ar/36Ar ratios in

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the Browne halite argue for, 1) original levels of gas within the gas bubbles, and 2) the

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accumulation of radiogenic argon from 40K decay in the gas inclusions against any leakage and diffusion (Figure 10). The homogenization temperatures (Table 1) and primary fabric of

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chevrons (Figure 5D) with the Ar isotope compositions (Figure 10) argue against any type of

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type leakage induced by elevated burial temperatures and tectonic activity and confirm their fluid and gas integrity. We, thus, reject his claim of a Tonian atmosphere with <10% PAL O2 and

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<60% PAL Ar with no plausible explanation for either, in clear contrast to the results of this study and that of Blamey et al. (2016) based on actual measurements of oxygen and argon gas contents and compositions.

6. Summary

ACCEPTED MANUSCRIPT In summary, halite with its solid, fluid and gas phases with an integrated screening protocol (Table 3) may serve as archives to resolve gas contents that reflect its ambient environment. The screening protocol must include as many facets as outlined in Table 3 to assure success in obtaining primary gas signatures. Also, the sampling protocol must look for ways to continually

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improve on the selection of the most suitable material for gas analysis, and for that we offer

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vigilance in seeking out inclusions of a cubic nature in conjunction with trails clearly indicative of a depositional origin. Also, the mimicking of other gases (such as organics) for inorganics

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during mass spectrometric analysis may be an issue that needs attention. The parameters offered up in Table 3 are suggestions that should ensure success in obtaining primary gas

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compositions. Following these and other parameters are essential, because of the miniscule

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nature of fluid inclusions that may carry gas bubbles, to assure gas fidelity. Only, more studies

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consisting of integrated lithology, petrography, microthermometry, chemical (elemental and isotopic compositions) and gas measurements on primary fluid inclusions of evaporites, halite

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and depositional gypsum. This will provide more insight into the evolution of the atmosphere

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Conclusions

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and the hydrosphere, and ultimately the biosphere and life on planet Earth.

The integrated evaluation of halite, modern and ancient, marine and non-marine, using everything from visible observations to gas chemistry suggests that evaporites (including gypsum) hold great potential as gas and chemistry archives. 1) Visual and petrographic examinations can identify material with high potential of having primary growth structures and pristine gas stored by the halite archive,

ACCEPTED MANUSCRIPT 2) The petrography of macro- and microscopic features is highly valuable in guiding the researchers to the best material of two-phase inclusions in halite, 3) Microthermometry in conjunction with solid and fluid geochemistry is an essential aspect of the screening protocol, among the chemistry, Br and Sr isotopes have proven

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potential as reliable proxies,

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4) The Argon isotope ratio is an excellent indicator of the ‘sealed’ nature of the fluid inclusions and thus of the primary/secondary compositions of their gas contents,

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5) The organic matter gas (OMG) content is a good tracer of gas contributions from organic matter decomposition; material abundant and commonly trapped in fluid inclusions,

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6) The combined OMG and Ar gas evaluation allows for the characterization of oxygen and

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carbon dioxide contents that may reflect ambient atmospheric conditions at time of

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halite crystallization,

7) The comprehensive re-evaluation of measured gas contents in the halite from the

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Tonian Browne Formation suggests that the average atmospheric oxygen content

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ranged from 9.6 to 10.9 % depending on coeval carbon dioxide content, and 8) Gypsum, an underutilized evaporite material, holds potential in being a supplementary

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archive for hosting and trapping original gas contents, but detailed studies are required to realize its full potential.

Acknowledgements We thank Dr. G. Giuliani (Université de Lorraine) and the anonymous reviewer for their incisive comments on the manuscript. This work was supported by University of Western Ontario research grant #2017-1 to N. Blamey and by Natural Sciences and Engineering Research Council

ACCEPTED MANUSCRIPT of Canada (NSERC) Discovery grant #7961-2015 to U. Brand. A special thank you to C. Lécuyer (Univ. of Lyon) and A. Davis (Brock University) for some pictures of fluid inclusions in Messinian, Silurian and Ordovician halites. We thank M. Lozon (Brock University) for drafting the figures, R. Linnen (University of Western Ontario) for making the microthermometry system available to

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us, K. Azmy (Memorial University) for REE analyses, and D. Buhl (Ruhr University, Bochum,

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Germany) for strontium isotope analyses. We thank the authors of Blamey et al. (2016), R. Rainbird (Geological Survey of Canada), and D. Schoombee and E. van Vurren (Marina Salt Co.,

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South Africa) for sample material.

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Declaration of Interest The authors have no financial, personal or other related conflicts.

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Moore, J.N., Norman, D.I., Kennedy, B.M., 2001. Fluid inclusion gas compositions from an active magmatic-hydrothermal system: a case study of the Geysers geothermal field, USA. Chemical Geology, 173, 3-30.

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Moretto, R., 1988. Observations on the incorporation of trace elements in halite of Oligocene salt beds, Bourg-en-Bresse Basin, France. Geochimica et Cosmochimica Acta, 52, 2809-2814. Murray, R.C., 1964. Origin and diagenesis of gypsum and anhydrite. Journal of Sedimentary Petrology, 34, 512-523. Norman, D. I. & F. J. Sawkins, 1987. Analysis of volatiles in fluid inclusions by mass spectrometry. Chemical Geology, 61, 1-10. Potter, J., Siemann, M.G., Tsypukov, M., 2004. Large-scale carbon isotope fractionation in evaporites and the generation of extremely 13C-enriched methane. Geology, 32, 533-536.

ACCEPTED MANUSCRIPT Pujol, M., Marty, B., Burgess, R., Turner, G., Phillopot, P., 2013. Argon isotopic composition of Archaean atmosphere probes early Earth geodynamics. Nature 498, 87- 91. Rainbird, R.H., Jefferson, C.W., Young, G.M., 1996. The early Neoproterozoic sedimentary succession B of northwestern Laurentia: correlations and paleogeographic significance. Geological Society of America Bulletin, 108, 454-470.

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Rigaudier, T., Lécuyer, C., Gardien, V., Sue, J.P., Martineau, F., 2011. The record of temperature, wind velocity and air humidity in the dD and d18O of water inclusions in synthetic and Messinian halites. Geochimica et Cosmochimica Acta, 75, 4637-4652.

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Roberts, S.M. & Spencer, R.J., 1995. Paleotemperatures preserved in fluid inclusions in halite. Geochimica et Cosmochimica Acta, 59, 3929-3942.

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Roedder, E., 1984. The fluids in salt. American Mineralogist, 69, 413-439.

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Roedder, E., 1990. Fluid inclusion analysis – prologue and epilogue. Geochimica et Cosmochimica Acta, 54, 495-507.

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Roedder, E., d’Angelo, W.M., Dorrzapf, A.F., Aruscavage, P.J., 1987. Composition of fluid inclusions in Permian salt beds, Palo Duro Basin, Texas, U.S.A. Chemical Geology, 61, 79-90.

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Schoenherr, J., Reuning, L., Kukla, P.A., Littke, R., Urai, J.L., Siemann, M., Rawahi, Z., 2009. Halite cementation and carbonate diagenesis of intra-salt reservoirs from the late Neoproterozoic to early Cambrian Ara Group (South Oman Salt Basin). Sedimentology, 56, 567-589.

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ACCEPTED MANUSCRIPT Spear, N., Holland, H.D., Garcia-Veígas, J., Lowenstein, T.K., Giegengack, R., Peters, H., 2014. Analyses of fluid inclusions in Neoproterozoic marine halite provide oldest measurement of seawater chemistry. Geology, 42, 103-106. Stuart, F.M., Mark, D.F., Gandanger, P., McConville, P., 2016. Earth-atmosphere evolution based on a new determination of Devonian atmosphere Ar isotopic composition. Earth and Planetary Science Letters, 446, 21-26.

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Tan, H., Ma, H., Li, B., Zhang, X., Xiao, Y., 2010. Strontium and boron isotopic constraint on the marine origin of the Khammuane potash deposits in southeastern Laos. Chinese Science Bulletin, 55, 3181-3188.

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ACCEPTED MANUSCRIPT Yanagisawa, F. & Sakai, H., 1983. Thermal decomposition of barium sulfate-vanadium pentaoxide silica glass mixture for preparation of sulfur dioxide in sulfur isotope ratio measurements. Analytical Chemistry, 55, 985-987. Yeung, L.Y., 2017. Low oxygen and argon in the Neoproterozoic atmosphere at 815 Ma. Earth and Planetary Science Letters, 480, 66-74.

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Zimmermann, J.L., 1972. L’eau et les gaz dans les principals familles de silicates. Memoirs Science de la Terre, 22, 188 p.

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Zimmermann, H., 2001. On the origin of fluids included in Phanerozoic marine halite – basic interpretation strategies. Geochimica et Cosmochimica Acta, 65, 35-45.

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Zimmermann, J.L. & Moretto, R., 1996. Release of water and gases from halite crystals. European Journal of Mineralogy, 8, 413-422.

ACCEPTED MANUSCRIPT Figure Captions Figure 1. Global distribution of modern and ancient evaporite localities. Modern halite material is from 1-Australia, 2-Israel, 3-Peru, 4-Bonaire, 5-Bahamas, 6-New Mexico, U.S.A., and 7-South Africa. Ancient evaporite material (halite and gypsum) is from 1-Australia (Browne Formation;

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Blamey et al., 2016), 2-China (Hongchunping Formation; Meng et al., 2011), 3-Pakistan (Salt

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Range Formation; Kovalevych et al., 2006), 4-China (Majiagou Formation; Meng et al., 2014), 5Italy (Messinian; Rigaudier et al., 2011), and 6-Canada (Minto Inlet Formation; Thomson et al.,

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2015).

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Figure 2. Hand specimens of halite with formational features deemed primary in nature. A)

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chevrons in Permian Lower Andres halite in core from the Palo Duro Basin, southwestern

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United States. B) milky (cloudy) halite from the Messinian of Sicily (Realmonte mine; cf.

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Rigaudier et al., 2011).

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Figure 3. Primary and secondary fluid inclusions in halite from the Ordovician Red Head Rapids Formation and Silurian Salina Group B. A) Multiple fluid inclusions some with gas bubbles close

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to smaller secondary inclusions (lower left corner, Red Head Rapids Formation). B) Fluid inclusion-rich halite superimposed by larger and distorted fluid inclusions (lower center-right, Salina Group B).

Figure 4. Sections of the Empress and Lancer drill cores from the Officer Basin, Australia. Numbers designate core interval and core obtained by the Geological Survey of Western

ACCEPTED MANUSCRIPT Australia (GSWA-Empress 1A: E1A 309-3-6, 1478 m; E1A 310-2-5, 1482 m; E1A C-2, 1502.2 m; Blamey et al., 2016).

Figure 5. Petrographic images of primary inclusions and inclusion trails in ancient halite. A) well-

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preserved inclusions in chevron trails in halite from the Messinian, inclusion #1 is a typically

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one-phase, whereas #2 shows evidence of a trapped gas bubble in an inclusion. B) Square inclusions with most containing only fluid, with a few with gas (#3) in the Ediacaran

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Hongchunping Formation (Meng et al., 2011). C) Close-up of fluid inclusions and gas-carrying bubbles (#4) in the Ordovician Majiagou Formation (Meng et al., 2014). D) Detailed depiction of

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chevrons with small to large inclusions along the inclusion trails of the Browne Formation

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(Spear et al., 2014; Blamey et al., 2016).

Figure 6. Diagenetic and secondary fluid inclusions in ancient halite. A) small primary inclusions

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along inclusion trails combined with much larger and irregularly-shaped secondary inclusions

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from the Ediacaran Salt Range Formation (modified from Kovalevych et al., 2006). B) Diagenetic inclusions along fracture planes and crystal boundaries from the Tonian Browne Formation

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(Blamey et al., 2016).

Figure 7. Compilation of gas contents in modern non-marine and marine halite material and gas trapped in capillary tubes (Fig. 1). Gas results O2 and OMG (organic matter gases: CH4 and CO2) are from non-marine halite: Dead Sea, Israel; Australia, Peru and USA, and from marine halite:

ACCEPTED MANUSCRIPT Bahamas and Bonaire. The field defines range of probable concentrations of pO2 and OMG gases with primary contents.

Figure 8. Typical gas results extracted with the low-temperature incremental crush-fast scan

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(CFS) method from inclusions of the Messinian halite (OMG – CH4 and CO2; O2 – oxygen). The

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CFS method randomly crushes and releases gas into the mass spectrometer, of note is the maintenance of the linearity of gas contents with number of sequential crushes of the same

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sample.

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Figure 9. Kinetic study results of carbon dioxide and hydrogen gas liberation using diffusion

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models. Diffusion is expressed as the logarithmic diffusion coefficient (D) per grain dimension

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(a, halite), with temperatures ranging from 60 to 560°C (Zimmermann and Moretto, 1996).

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Note: mH – milky halite, cH – clear halite.

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Figure 10. Isotopic argon (40Ar/36Ar) ratio of modern, Messinian, and Tonian Browne Formation halites. A) The 40Ar/36Ar of modern halite varies from 294.2±0.5 (error) to 303.2±5.8, the

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Messinian halite (7.3 to 5.2 Ma) varies from 290.6±2.0 to 300.2±8.6, and the Browne Formation (815±15 Ma) varies from 285.9±5.0 to 436.0±4.2 (Blamey et al., 2018). The baseline 40Ar/36Ar value of 298.56 for the modern atmosphere is from Lee et al. (2006). B) Close-up of 40Ar/36Ar values in halite from the Tonian Browne Formation. The stippled area corresponds to Tonian atmospheric 40Ar/36Ar based on computations and modelling by Pujol et al. (2013).

ACCEPTED MANUSCRIPT Figure 11. Oxygen and OMG gas contents in Tonian halite from the Browne Formation, Officer Basin, Australia. Reference gas contents and trends of trapped gas (capillary tubes) and modern halite (non-marine and marine). Fields for ‘primary’ signals includes natural variation and preparation and measurement errors (machine), with cut-off limits of primary screening

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parameters for Tonian evaporites set at: <1.3 % for CH4, ~ 0.86 % for Ar (cf. Pujol et al., 2013),

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and <10.4 % for CO2.

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Figure 12. Incremental-crush gas results of oxygen and argon from a suite of Neoproterozoic halite (Browne Formation, Officer Basin, Australia) and gypsum (Minto Inlet Formation, Victoria

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Island, Canada). Tonian Browne Formation halite results = solid symbol (Blamey et al., 2016),

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and the Tonian Minto Inlet Formation gypsum gas results = open symbol (Appendix 3; locality in

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Thomson et al., 2015).

ACCEPTED MANUSCRIPT

Th

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87Sr/86Sr

K+

Ca2+

Mg2+

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mMol/kg

0.732038

-

-

-

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H2O

Sr Br 34S REE Ce* 40Ar/36Ar mg/kg ‰ ppm °C

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Loc/Fm/Sample #

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Table 1. Geochemistry of mineral (Sr, Br, 34S, 87Sr/86Sr, REE, Ce*; 40Ar/36Ar), extracted fluids (K+, Ca2+, Mg2+), and homogenization temperatures (Th) of halite and gypsum from the Tonian Browne and Minto Inlet formations, Ediacaran Hongchunping & Salt Range formations and Ordovician Majiagou formation, and Messinian and modern counterparts (Lake Polaris, Bahamas, Bonaire). ______________________________________________________________________________ _____________________________________________________________________________

-

-

-

-

-

0.709153

-

-

-

-

640

-

-

-

-

294.3

21-42

10 163 1640 1699

+27.1 -

-

-

-

12 -

-

18-35

-

-

-

53 -

+32.1 -

55-100

388

-

1271

4 -

-

+15.9 -

0.70561

-

-

-

-

Lake Polaris, Australia LP 1-1 LP 1-2 -

297.2 -

-

5 -

70 -

307

<20 <7

Bonaire

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535

CE

Majiagou2 ZJY-47-21

Hongchunping3 CH-2E Salt Range4 SR-7(mean)

Minto Inlet5

37 1900

PT

Messinian1

ED

Bahamas

-

M

9 6 -

-

-

ACCEPTED MANUSCRIPT

115 E 1482 553 E 1492 149 E 1497

46 93 3498

45 +15.4 41

-

388

3550

-

0.75 -

393

46.5 3440

-

55

0.23

-

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0.706696 40.7

19 1.03 17 4283 4 0.99 35 423

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Browne6 E 1478

17 323 3741 31 0.95 L 1466.5 8 107 +15.1 0.706767 88 0.82 320.8 ______________________________________________________________________________ ____________________________________________________________________________ Note: Loc – location; Fm – formation; 1- Lazar and Holland, 1988; Rigaudier et al. 2011; 2-Meng et al., 2014; 3- Meng et al., 2011; 4- Kovalevych et al., 2006; 5-Walter et al., 2000; 6-Blamey et al., 2016, 2018 & this study.

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E 1502.2

ACCEPTED MANUSCRIPT Table 2. Statistical analysis (t-test)* of argon and oxygen gas contents in Neoproterozoic gypsum (Minto Inlet Formation [MIF], Canada) and halite (Browne Formation, Officer Basin, Australia)

Sample

N

Mean

St Dev

St Err

p

Argon

MIF-2 MIF-3

8 8

0.954 0.898

0.103 0.084

0.036 0.030

0.249

Oxygen

MIF-2 MIF-3

8 8

20.23 20.19

1.29 2.44

Argon

MIF-2&3 MIF-1 MIF-1(g)

16 8 4

0.926 1.013 0.878

MIF-2&3 MIF-1 MIF-1(g)

16 8 4

20.21 11.81 11.27

MIF-1(g) Browne

4 10

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0.095 0.174 0.048

0.024 0.061 0.024

0.124

1.89 0.80 0.62

0.47 0.28 0.31

0.0001

0.048 0.114

0.024 0.036

0.522

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0.963

0.346

0.0001

MIF-1(g) 4 11.27 0.62 0.31 0.139 Browne 10 9.59 2.03 0.64 ______________________________________________________________________________ Note: MIF-1(g) – gas results deemed primary; *PAST3 (Hammer et al., 2001).

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Oxygen

0.878 0.838

0.46 0.86

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Argon

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Oxygen

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Parameter

ACCEPTED MANUSCRIPT Table 3. Summary of tools used in the protocol screening for primary fluid inclusions and gas bubbles. ______________________________________________________________________________ _____________________________________________________________________________

Surface

Contam. phase Efflores.

chevrons <1 cornets

cubic >65 age

15age

<1.0

<250

app

single-

app* hoppers

double

cloudy

trails

34S

REE

age

<1

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Gas Chemistry 87Sr Br

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Free of 55°C

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Macro OMG

MicroGeochemistry Micro thermometry Ar H2S H2

natural

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Petrography

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Visual

app

levels

(0.1-9.0)

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CE

PT

ED

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______________________________________________________________________________ _____________________________________________________________________________ Note: Contam. = contamination; Efflores. = efflorescence; age app = age appropriate levels, *Pujol et al. 2013; OMG – (0.1 to 9.0) – levels in modern halite

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