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Atmospheric injection of sulfur from the Medusae Fossae forming events

Lujendra Ojha a,*, Suniti Karunatillake b, Kayla Iacovino c a Department of Earth and Planetary Sciences, Rutgers, The State University of New Jersey, Piscataway, NJ, USA b Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA, 708033, USA c Astromaterials Research and Exploration Science, Jacobs, NASA Johnson Space Center, Houston, TX, 77058, USA

ARTICLE INFO ABSTRACT

Keywords: A plethora of in-situ bulk chemical and mineralogical analyses, remote sensing data, and geochemical models Mars suggest that the Martian crust is sulfur(S)-rich, exceeding the S content of terrestrial basalts by almost an order of Atmosphere magnitude. The main source of crustal S on Mars is the volcanic exhalation of SO2 and H2S gases. Volcanic ash, Sulfur expelled along with gases, may be a sink for up to 30% of the exhaled S gas species. We analyze the elemental Pyroclastic composition of the Martian shallow sub-surface using the Mars Odyssey Gamma Ray Spectrometer to find areas Geochemistry Dust that are simultaneously enriched in constituents of major volcanic gases such as S, Cl, and H2O. A large sedimentary deposit of likely pyroclastic origin called the Medusae Fossae Formation (MFF) is located within this region of possibly extensive alteration by volcanic gases. Based on reasonable terrestrial analog estimates that the MFF scavenged, at maximum, 30% of the exhaled S gas species, we find that a significant amount of S (>1017 kg) would have been delivered to the atmosphere over the time it took for the MFF to be deposited on Mars. The mass of the S emitted from the MFF-forming event(s) is up to 8 orders of magnitude higher than the mass of S emitted from the largest Quaternary volcanic eruption on Earth. Thus, volcanic degassing from the MFF forming events could have significantly affected surface geological processes, climate evolution, and habitability of Mars.

1. Introduction S in the Martian soil and dust (e.g. Berger et al., 2016; Gellert et al., 2006; Yen et al., 2005). Remote sensing observations have found evidence for Sulfur (S) is the tenth most abundant element in the solar system and massive and widespread deposits of sulfates in association with outcrops þ its complex heterovalence states (S2- to S6 ) allow it to participate in and the bulk regolith throughout the planet (e.g. Gendrin et al., 2005; numerous geochemical and biochemical processes. Sulfur can play a Murchie et al., 2009; Wray et al., 2011; Carter et al., 2013; Ackiss and major role in the planetary differentiation process by allowing early Wray, 2014; Karunatillake et al., 2014, 2016), and Martian meteorites differentiation of Fe-Ni-S core and a largely silicate mantle and crust (e.g. are known to contain sulﬁdes and sulfates (e.g. Righter et al., 2009; Franz Stewart et al., 2007). Sulfur in the atmosphere can form sulfate aerosols et al., 2014; Ding et al., 2015). and play a powerful role in the global climate and the chemistry of the Sulfur on the Martian surface is ultimately derived from the mantle by atmosphere and surface environments (Bluth et al., 1993; McCormick various magmatic and hydrothermal processes (Settle, 1979; Bullock and et al., 1995; Mitchell and Johns, 1997; de Silva and Zielinski, 1998; Moore, 2007; Johnson et al., 2008; Craddock and Greeley, 2009; Gaillard Robock, 2000; Halevy et al., 2007; Robock et al., 2009). Sulfur may have and Scaillet, 2009; Righter et al., 2009; Franz et al., 2018). Magmatic also played a major role in the early oxygenation of the biosphere and the degassing occurs when volcanic gases are exsolved from melts during the coevolution of life on Earth (e.g. Lyons and Gill, 2010), and has been decompression of magma ascending to the near surface. The abundance recently found to be associated with organic compounds on Mars of sulfur soluble in an aluminosilicate melt is directly related to its FeO (Eigenbrode et al., 2018). As such, the knowledge of the bulk S content content. As Martian basalts contain twice as much FeO as their terrestrial and its spatial distribution on a planet is important to our understanding counterparts (Haggerty, 1978), the melts can dissolve up to 3–4 times of the planetary differentiation processes; sedimentary, geomorphic, and more S under similar conditions of pressure and temperature. Indeed, aqueous processes; climate evolution; and current and past habitability. thermodynamic modeling suggests that the S concentration of Martian Multiple lines of observations have shown that the Martian regolith is basalt melt is in the range of 4000–7000 ppm, which exceeds the S rich in sulfur. In-situ measurements have revealed high concentrations of content of terrestrial basalt melt by a factor of 4–7(Gaillard and Scaillet,

* Corresponding author. Department of Earth and Planetary Sciences, Rutgers, The State University of New Jersey, Piscataway, NJ, USA. E-mail address: [email protected] (L. Ojha). https://doi.org/10.1016/j.pss.2019.104734 Received 30 October 2018; Received in revised form 25 June 2019; Accepted 2 September 2019 Available online xxxx 0032-0633/© 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: Ojha, L. et al., Atmospheric injection of sulfur from the Medusae Fossae forming events, Planetary and Space Science, https://doi.org/10.1016/j.pss.2019.104734 L. Ojha et al. Planetary and Space Science xxx (xxxx) xxx

2009). The volume and the species of the degassed sulfur-bearing gases and ~90–99 wt% of ash (Sparks et al., 1997). Volcanic ash is comprised strongly depends on the atmospheric pressure at venting conditions, the of a mixture of amorphous aluminosilicate glass and crystalline materials magma oxygen fugacity (fO2), and the concentrations of other volatile (Heiken and Wohletz, 1992) with its specific mineralogy dictated by the elements present (e.g., H, C, Cl) (Wallace and Carmichael, 1994; Gaillard source magma. A significant amount of exhaled volcanic gases can be and Scaillet, 2009; Iacovino, 2015). The lower gravity and atmospheric scavenged by fine ash particles during heterogenous reactions within density of Mars compared to Earth facilitates rapid degassing of volatiles erupting plumes (Oskarsson, 1980; Varekamp et al., 1984; De Hoog et al., during magma ascent (Wilson and Head, 1983), and the relatively low 2001; Delmelle et al., 2007, 2018; Self et al., 2008). This may occur as a viscosity of basaltic magmas compared to terrestrial rhyolitic composi- combination of: 1) chemisorption reactions at temperatures between 190 tions further amplifies the degassing process. The amount of S degassed is and 800 C(Rose, 1977; Witham et al., 2005); 2) direct precipitation of affected in equal magnitude by the fO2 and magmatic water content, and elemental sulfur at moderate temperatures (400–500 C) (DiFrancesco maximum sulfur degassing is obtained for hydrated-oxidized melts et al., 2015, 2016; King et al., 2018; Nekvasil et al., 2019); and 3) (Gaillard et al., 2012). Other factors such as temperature, melt compo- chemisorption reactions to form sulfate and precipitation to form sition, and fS2 play a secondary role in controlling the volume of degassed halogen salt coatings on ash grains at high temperatures (600–1200 C) sulfur (Galliard and Scaillet, 2014). (Ayris et al., 2013; Henley et al., 2015; King et al., 2018; Palm et al., þ The transition from S2- to S6 in silicate melts occurs over a narrow 2018; Renggli and King, 2018). fO2 interval between quartz-fayalite-magnetite oxygen fugacity buffer Water- and acid-soluble species scavenged by ash may mobilized if (QFM) and QFM þ2(Jugo et al., 2010). H2S is the dominant form of deposits are infiltrated with groundwater (requiring low pH in the case of gaseous S in melts with lower fO2, lower H2O, and higher pressure, acid-soluble species). Pristine ash samples preserve their scavenged whereas SO2 is dominant in melts with higher fO2, higher H2O, and lower volatiles, and water and acid may be used to leach them in the laboratory, pressure. In some cases, S2 may also make up a significant portion of thus allowing for direct measurement of the bulk composition and con- exhaled S gas. The fO2 of Martian basalts encompasses a wide range of centration of volatile-bearing precipitates. Comparison of leachate vol- values, as evidenced from Martian meteorites, ranging from QFM-5 to atile concentrations with the total mass of erupted material gives an QFMþ1.5 (Bunch and Reid, 1975; Herd et al., 2001, 2002; Wadhwa, estimate of volatile mass returned to the surface as fallout. Such in- 2001; Sautter et al., 2002; Goodrich et al., 2003; Herd, 2003; Dyar et al., vestigations for eruptions where total eruptive volatile flux is indepen- 2005; Shearer et al., 2006; Righter et al., 2008). H2S may have dominated dently known have found that ash can scavenge as much as 30% of the during the early stages of Martian history when degassing occurred from sulfur and 10–30% of the total chlorine emitted (Rose, 1977; Varekamp a reduced magma ocean at higher eruptive pressure (Gaillard et al., et al., 1984; De Hoog et al., 2001; de Moor et al., 2005; Witham et al., 2012). The assimilation of crustal materials in the melt would have led to 2005; Delmelle et al., 2007, 2018; Self et al., 2008) thus limiting the the increase in the oxidation state of magmas over time, thus, the amount of SO2 and halogens injected into the atmosphere and making dominant sulfur species during the past ~4 Ga on Mars was likely SO2 surface deposits a potentially important sink for erupted volatiles. (Gaillard et al., 2012). Thermodynamic modeling within the above The 1974 Fuego Volcano eruptions in Guatemala was one of the first constraints illustrates that Martian volcanism may have expelled gases volcanoes recognized as releasing excess sulfur and produced more than 3 with up to ~60 mol% SO2 (Gaillard and Scaillet, 2009). The H2S/SO2 0.2 km of basaltic ash (Rose, 1977; Rose et al., 2008). During this ex- ratio in exhaled volcanic gas may be altered during and after eruption via plosion, as much as 33% of the degassed S fell back to Earth as acid the removal of SO2 by dissolution in surface water or underground aerosol particles absorbed on the ash (Rose, 1977). Given the estimated aquifer, sulfate aerosol formation in the atmosphere, and chemical re- mass of the ash deposit, Rose (1977) estimated that 2.2 1011 g of S must actions between gas and ash in the plume (see review by King et al., 2018 have been released to the atmosphere. The 1982 eruptions of the El and references therein). Halogens such as chlorine (Cl) and fluorine (F) Chichon volcano in Chiapas, Mexico, produced approximately 0.38 km3 are also released to the atmosphere in the form of HCl, NaCl, KCl; and HF, of andesitic ash deposit (Varekamp et al., 1984). The fine ash particles SiF by volcanic degassing (Giggenbach, 1996 Oppenheimer, 2003), scavenged around 30% of the sulfur emissions from the eruption and although in a minor amount. carried it to the ground (Varekamp et al., 1984), which equates to Degassing is the key mechanism responsible for the transfer of sulfur ~4 1011 g of scavenged S. The total mass of S injected into the atmo- from the mantle to the atmosphere on Mars (Franz et al., 2018). The sphere by this event equates to ~1 1013 g(Krueger, 1983). The amount of volcanically injected atmospheric S on Mars has been previ- 1982–1983 eruptions of Galunggung in West Java, Indonesia produced ously estimated from theoretical considerations. Based on geochemical ~0.37 km3 of andesitic to high-Mg basalt ash (Gourgaud et al., 1989; modeling, it has been suggested that degassing from the Tharsis volcanic Gerbe et al., 1992). The analysis of ash leachate from this event shows region alone could have made 20–60 m thick global-equivalent-layer that 35% of the exhaled S was scavenged by volcanic ash from the (GEL) of sulfate minerals (Gaillard and Scaillet, 2009). Similarly, John- eruptions. Self et al. (2008) estimate that 75% of the exhaled volatiles son et al. (2008) estimated SO2 emission in the range of may reach the tropospheric-stratospheric boundary via eruptions, while 1.19–4.76 1014 kg from the Tharsis igneous province, which could 25% may form a ground-level fog with the lava flow. This is in reasonable have supported greenhouse warming up to 25 K greater than what could agreement with de Hoog et al.’s [2001] estimate for S deposited onto be caused by CO2 alone. Righter et al. (2009) proposed that degassing of clasts versus that free to circulate, where Galunggung observations sug- 2400 ppm S from Fe-rich Martian melts over Mars’s geological history gest 35% of exhaled S phases adsorb on pyroclastic debris depositing might have produced all the observed S on the surface. Craddock and quickly, while the rest circulates in the atmosphere. Although the exact Greeley (2009) estimated that as much as 1 bar of sulfur may have been mechanics of sulfur uptake and their relative efficiency are not well released via juvenile degassing throughout Martian history. It has been understood (e.g. Schmauss and Keppler, 2014; Delmelle et al., 2018), ash estimated that ~11% of the terrestrial primitive mantle sulfur may have leachate studies on Earth have clearly demonstrated that volcanic ash can been outgassed over geological time on Earth (Canfield, 2004). scavenge a significant fraction of exhaled sulfur. McLennan and Grotzinger (2009) assumed a similar rate of degassing on While several works in the past have estimated the amount of sulfur Mars and estimated the total mass of surficial sulfur to exceed 2.3 that could have been injected into the Martian atmosphere from theo- 1022 g, which equates to a global equivalent layer of 2-km thick sedi- retical considerations (Johnson et al., 2008; Craddock and Greeley, 2009; mentary layer with S content equal to average soil (~6% SO3)(McLennan Gaillard and Scaillet, 2009; Righter et al., 2009), constraints from the and Grotzinger, 2009). Collectively, these theoretical estimates illustrate compositional analyses of available Martian geochemical data has so far that a large amount of volcanic S has been injected into the Martian at- been lacking. In this work, we rely on the elemental geochemistry of the mosphere by volcanism (also see Nekvasil et al., 2019). near-surface regolith of Mars provided by The Mars Odyssey Gamma Ray At the terminus, eruption plumes are composed of ~1–10 wt% of gas Spectrometer Suite (GRS) to provide an estimate of the volume of S gases

2 L. Ojha et al. Planetary and Space Science xxx (xxxx) xxx that would have been delivered to the Martian atmosphere during large more than underlying primary igneous rocks at key volcanic provinces pyroclastic eruptions. In section 2, we describe the GRS instrument suite mantled by dust (Viviano et al., 2019). Thus, the near global coverage, and our methodology. In section 3, we present the S, Cl, and H2O mass sensitivity to decimeter depth, and association with fine sediments makes fractions (as percentage, wt%) from a region on Mars that is significantly GRS chemical maps an ideal dataset for a study of this nature. enriched in these elements. We describe a large sedimentary unit of likely Previously, GRS maps have been used to study chlorine enrichment in pyroclastic origin on Mars called the Medusae Fossae Formation (MFF) sedimentary deposits (e.g. Keller et al., 2006; Diez et al., 2009), global and provide reasonable lower bounds on the amount of sulfur that would bulk soil hydration by sulfates (e.g. Karunatillake et al., 2014, 2016), have been delivered to the Martian atmosphere when this unit was bulk crustal compositions (e.g. Boynton et al., 2007; King and McLennan, deposited on Mars. We then discuss the significance of our results. 2010), and igneous processes (e.g. Baratoux et al., 2014; Hood et al., 2016; Susko et al., 2017). Several chemical maps were unavailable in fi 2. Methods nal form to prior workers due in part to overlapping spectral peaks at contrasting intensity corresponding to different elements, particularly fi The shallow subsurface composition of the Martian crust to decimeter affecting S (cf., Evans et al., 2007). Continued re nements to GRS depths has been derived by GRS. GRS measures the spectrum of gamma spectral analyses and forward modeling from spectra to concentration photons emitted from the Martian surface; characteristic spectral peaks (Boynton et al., 2007; Evans et al., 2007; Karunatillake et al., 2007) from specific nuclear reactions allow the quantification of most major allowed Al, Ca, and S abundance trends to be analyzed in a series of rock-forming elements, along with select minor and trace elements (Al, subsequent works (Baratoux et al., 2014; Karunatillake et al., 2014, 2016; Ca, Cl, Fe, H, K, S, Si, Th) (Boynton et al., 2007). Peak area above the Hood et al., 2016; Susko et al., 2017; Ojha et al., 2018) as also used in this continuum can be used to infer the wt% of each element over an area of work. the planet’s surface, leading to chemical abundance maps excluding We further resolve the discrepancy between the mapping resolution approximately the 50 latitudes where H increases rapidly (Fig. 1). and the analytical resolution in prior works (e.g., Karunatillake et al., Recent synthesis of Mars Reconnaissance Orbiter (MRO) Compact 2014) by rendering chemical maps at the same coarse resolution as used Reconnaissance Imaging Spectrometer for Mars (CRISM) derived for analyses. As in our prior work (Ojha et al., 2018), that helps prevent mineralogy and GRS derived chemistry also suggests that GRS-chemistry apparent variations in the maps that lack spatial precision to support fi may represent fine sediments – such as of eruptive or eolian provenance – interpretations. Speci cally, spatial smoothing needed for numerical precision causes spatial autocorrelation (Karunatillake et al., 2011, 2012) and limits the spatial resolution of the S wt% map. Therefore, we use 5 5 bins (referred to as “pixels” in the typical parlance of other remote sensing datasets) for the Cl and H2O wt% data set, and re-binned S wt% from the original 10 10 resolution to 5 5. Fig. 1 shows the resulting wt% maps for Cl, S, and H2O. We sought to find regions on Mars that are statistically enriched in elements such as S, Cl, and H2O which are major constituents of volcanic gases (Oppenheimer, 2003). Fluorine, despite its significance to magmatic conditions and surficial weathering as discussed in Gale crater works (e.g., Cousin et al., 2017; Forni et al., 2015) is unavailable in GRS-derived chemical maps. To that end, we used an enhanced Student’s t-test parameter, ti, that measures the error-weighted deviation for each element from its bulk-average on Mars at each 5 5 GRS grid (Kar- unatillake et al., 2009):

ci m ti ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi; (1) 2 2 sm;i þ s

where ci is the wt% of an element, m is the global arithmetic mean wt%, sm,i is the numerical uncertainty of ci, and s is the standard deviation of the data. Here we define areas of significant enrichment as those with ti values greater than 1.5 (i.e. statistical confidence in directional deviation > 94%). The correlation coefficient ( \Þ between two elements (a, b) was computed as follows: 1 XN a μ b μ \ ¼ i a i b ; ða;bÞ N 1 σ σ (2) i¼1 a b

μ σ where N is the number of observations, a and a are the mean and μ σ standard deviation respectively of a, and b and b are the mean and standard deviation of b. To assess the significance of the apparent correlation, we resampled the original data set a thousand times and calculated the slope, y-intercept, and the correlation coefficient for each new subsample. This process of ‘bootstrap’ provides better information on the characteristics of the dataset than statistical parameters (mean, standard deviation, correlation coefficients) computed from the full data Fig. 1. GRS derived abundance maps for S, Cl, and H2O. GRS derived elemental set (Efron, 1979). abundance maps for S (a), Cl (b), and H2O (c) in wt%, overlain on a Mars Orbiter Laser Altimeter (MOLA) shaded relief (Smith et al., 1999). The outline of the We estimate the mass of S release to the atmosphere by a simple mass Medusae Fossae Formation (MFF) is shown in black. balance relation. Given the mass of a volcanic deposit (Mdeposit) and the

3 L. Ojha et al. Planetary and Space Science xxx (xxxx) xxx concentration of S in the deposit (Cs), the mass of the S present in that TIU (Fig. 2), suggesting that the source of the high concentration of S, Cl, deposit (Smass) is given by: and H2O in the S-Cl-H enriched region can be associated with fine particles, at least in the top few centimeters of the regolith. S ¼ M C : mass deposit s (3) The S-Cl-H enriched region includes a vast sedimentary deposit called Assuming that the volcanic deposit scavenged X% of the exhaled S, the Medusae Fossae Formation (MFF) (Tanaka, 2000; Bradley et al., 2002; Hynek, 2003; Kerber et al., 2011; Ojha et al., 2018; Ojha and the S released to the atmosphere (SatmÞ is given by the following relation: Lewis, 2018), that was likely emplaced during the Hesperian era (Kerber Smass and Head, 2010) [3.7–3.0 Ga] (Fig. 1; Fig. 4). The lack of boulders along Satm ¼ : (4) ðX=100Þ erosional contacts (Malin and Edgett, 2000; Hynek, 2003), different crater depth-diameter ratios than areas known to be caprock (Barlow, 3. Results and discussion 1993), erosional morphology (Schultz and Lutz, 1988; Malin and Edgett, 2000; Hynek, 2003), low density (Ojha and Lewis, 2018), and low The only area of Mars that is simultaneously enriched in S, Cl, and thermal inertia and radar signals (Hynek, 2003) are highly suggestive fi H2O is the equatorial region among the large volcanic provinces of that the MFF is composed of ne-grained materials. At present, the areal Tharsis, Elysium, and Apollinaris Patera (hereafter referred to as S-Cl-H extent of the MFF exceeds 2 106 km2 (20% of the continental United enriched region) (Fig. 2), likely associated with the siliciclastic fines that States), but the original extent of the MFF could have exceeded mantle these areas (cf., Viviano et al., 2019). The correlation coefficient 5 106 km2 (50% of the continental United States) (Bradley et al., 2002; between S and Cl within the Cl-enriched region (i.e. area with ti >¼ 1.5 Dunning et al., 2018; Dunning, 2019). The current volume of the MFF for Cl in Fig. 2) is 0.76, with S modeling ~60% of the Cl variance (Fig. 3). exceeds 1.4 106 km3, equivalent to ~10-m global-equivalent-layer In comparison, the correlation coefficient between S and Cl for the (GEL), making it one of the largest sedimentary deposits on the Martian mid-latitude is 0.62, with S modeling only ~38% of Cl variance Martian surface (Bradley et al., 2002). (Ojha et al., 2018). The results from the bootstrap method (correlation A variety of formation mechanisms have been proposed to explain the 0.76 0.04) clearly highlight that the high correlation between S and Cl depositional history of the MFF including glacial activity emplacing is not due to statistical outliers (Fig. 3). Given our prior observations of H paleo-polar deposits which incorporated eolian sediments (Schultz and vs S trends at regional scales, the positive S intercept to regression Lutz, 1988), direct atmospheric deposition of suspended eolian sediment (Fig. 3a) would be consistent with the availability of S to associate with (Tanaka, 2000), coarse-grained ignimbrite deposits from pyroclastic non-Cl phases. That also converges with the positive S intercept observed density currents (Scott and Tanaka, 1982; Mandt et al., 2008), in H-S bivariate space, likewise attributed to excess S in a reduced state, fine-grained distal ashfall from volcanic eruptions (Tanaka, 2000; Brad- such as elemental deposits (Karunatillake et al., 2014). ley et al., 2002; Hynek, 2003; Kerber et al., 2011; Ojha and Lewis, 2018; The S-Cl-H enriched region of Mars has a relatively low thermal Ojha et al., 2018), and water-ice (Watters et al., 2007; Wilson et al., inertia (Fig. 2 (d)). Thermal inertia is a key property of a regolith that 2018). Several aspects of these proposed hypotheses have been critically depends on particle size, the degree of induration, rock abundance, and assessed to arrive at the conclusion that the formation is most likely exposure of bedrock within the top few centimeters of the subsurface. composed of volcanic ash, ignimbrites, or loess (see Mandt et al., 2008 Fine particles such as volcanic ash and dust have low thermal inertia; and reference therein for a review). bright unconsolidated fines have thermal inertia between 28 and The MFF has a mean S and Cl wt% of 2.62 0.24 and 0.65 0.04 135 J m 2 K 1 s 1/2 (‘thermal inertia unit’ or TIU) (e.g. Putzig et al., respectively, which compared to the mid-latitudinal average is higher by 2005). The mean thermal inertia of the S-Cl-H enriched region is < 100 a factor of 1.2x and 1.4x respectively. While S, Cl, and H2O show higher

Fig. 2. Contour map showing regions on Mars with significant enrichment of S (a), Cl (b), H2O (c), and the overlap of the contours for all the elements (d). The contour labels correspond to a modified ‘t’ parameter (Equation (1)) that show regions with significant enrichment of S, Cl, and H2O compared to the bulk-average of

Mars. The background is the shaded relief map of Mars from Mars Orbiter Laser Altimeter (MOLA). (d) The overlap of the 1.5-ti contour for S, Cl, and H2O overlaid on top of the daytime thermal inertia map from Thermal Emission Spectrometer. The yellow ‘X’ marks the location of Apollinaris Patera.

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Fig. 3. Correlation between GRS derived S and Cl for the Cl-enriched region. (a) The black dots show the real data from the GRS-derived data and the black line shows the mean regression line for S and Cl in the Cl-enriched region. The slope of the line is a ratio that relates the wt% of sulfur to the wt% of chlorine within the Cl- enriched region. (b)–(d) Results from the bootstrap method showing the histogram for the slope, Y-intercept, and the correlation coefﬁcient from 1000 different resampled dataset.

2000; Bradley et al., 2002; Hynek, 2003; Mandt et al., 2008; Kerber et al., 2011; Ojha and Lewis, 2018; Ojha et al., 2018) however suggest the source of enrichment is most likely due to interaction with volcanic gases. We disfavor an aeolian origin for the bulk of the MFF because, despite its huge volume, the MFF is pervasively indurated, welded, or jointed on a regional scale. Ancient loess deposits on Earth seldom display these properties regionally (Mandt et al., 2008). Water-ice is unstable at any depth near the equator of Mars (Schorghofer and Ahar- onson, 2005; Schorghofer, 2007), so while minor amount of ice may be trapped within the MFF (Watters et al., 2007; Wilson et al., 2018), the bulk of the MFF volume cannot be explained by the deposition of water-ice. Fig. 4. The overlap of the 1.5-ti contour for S (green), Cl (red), and H2O (blue) The volcanic origin of the S and Cl enrichment at the MFF is further overlaid on top of the MOLA shaded relief. The black lines show the geological supported by the observed S/Cl molar ratio at the MFF. The experi- boundary of the MFF. Symbols are the same as Fig. 2(d). mentally determined pressure dependent solubility of S and Cl in melts (e.g., Carroll and Webster, 1994; Webster et al., 1999) indicate that both enrichment separately (Fig. 1), the spatially colocated enrichment is species should be present in low abundances in vapors produced during distinct for the MFF (Fig. 4). If the MFF was only enriched in S, then a early degassing of ascending mafic magma (Aiuppa et al., 2004). How- variety of processes could have been responsible for such enrichment ever, both may play a major role in shallow-depth degassing of basaltic including oxidative weathering of S-bearing minerals, interaction with melts (Gerlach and Graeber, 1985; Metrich et al., 1993), and significant fi fl magmatic or hydrothermal S-bearing gas, immiscible sul de uids, and solubility contrast between the species makes S/Cl molar ratios a useful fi sul de minerals (King and McSween, 2005). Similarly, if the MFF was tracer in understanding the degassing processes and eruptive phenom- only enriched in Cl, then processes including chemical alteration through ena. For example, S is inferred to degas at low pressure from the hydrothermal activity and leaching of Cl by liquid water could have been sulfide-saturated basaltic magma of the mid-oceanic ridge basalt responsible for the enrichment. The collective enrichment of major vol- (MORB), whereas in oxidized, water-rich magmas sulfur exsolves into the canic gases (S, Cl, and H2O) in the MFF and plethora of previous works vapor phase at high pressure (Spilliaert et al., 2006; Burton et al., 2007). fi that have identi ed the MFF as a likely pyroclastic deposit (Tanaka, Chlorine remains dissolved in the melt up to very shallow depths; hence,

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Fig. 5. S/Cl molar ratio for the GRS mapped regions of Mars. gases emitted by explosive volcanic activity have higher S/Cl molar ratios than those emitted by persistent gas emission (e.g. Allard et al., 2005; Oppenheimer et al., 2006; Burton et al., 2007). Halogens only prevail over S (e.g. S/Cl and S/F gas ratios of <1) in the late stages of (residual) degassing, such as at lateral (secondary) vents and in the final phases of effusive eruptions (Aiuppa, 2002, 2009; Oppenheimer et al., 2011; Mather et al., 2012). Combining the range of S and Cl estimate of the MFF, we calculate an average molar S/Cl ratio of 4.1 0.4 for the MFF (Fig. 5) which is consistent with the > 1 S/Cl molar ratios expected of explosive volcanic exhalation. It should be noted however that no region on Mars has a molar S/Cl ratio of less than 3.5 suggesting that volcanic exhalation may have interacted with much of the Martian regolith (Fig. 5). While higher S/Cl ratios occur elsewhere, MFF remains significant for the underlying enrichment of both S and Cl. The high S/Cl elsewhere may also reflect leaching of the more aqueously mobile Cl phases or volatilization compared to S (Zhao et al., 2018). Our MFF observations compare favorably with studies targeted at examining the variability of the S/Cl ratio across the in-situ soil profile. For example, at Gusev Crater and Meridiani Planum, S/Cl mass ratio can range from 1.8 to 3.6 (i.e., 2–4 M ratio) as often noted for surface observations to as high as 4–5(4–6 M ratio) even at ~15 cm shallow depths (Karunatillake et al., 2013). 18 We estimate the current mass of the MFF to be 2.5 10 kg based on Fig. 6. Minimum and maximum estimates of sulfur that would have been the gravity-derived mean density of 1765 kg m 3 (Ojha and Lewis, degassed into the Martian atmosphere as a function of how much S was retained 2018), and an estimated volume of 1.4 106 km3 (Bradley et al., 2002). by the fine particles of the MFF. If the MFF retained 100% of the total exhaled Given the mean S content of 2.62 wt % for the MFF, the mass of S within gas then the mass of sulfur degassed into the atmosphere would equate to zero. the current deposit of the MFF is 6.5 1016 kg, assuming compositional homogeneity at depth. If the S content of the MFF is approximately 30% of the total mass of the exhaled S over the time of the deposition of the of the total mass of the exhaled S over the time of the deposition of the MFF, the mean amount of S injected into the atmosphere exceeds MFF, then the mean amount of S injected into the atmosphere exceeds 2 1015 kg. Similarly, given the mean Cl content of 0.65 wt % for the 17 17 2.2 10 kg, equivalent to 4.4 10 kg of SO2. For reference, the mean MFF and assuming that approximately 10% of the total mass of the total mass of the present-day Martian atmosphere is ~2.5 1016 kg. The exhaled Cl was scavanged by the MFF the mean amount of Cl injected percentage of S gas scavenged by ash may vary widely depending upon into the atmosphere exceeds 1.63 1017 kg. ash composition, mineralogy, and average particle surface area, but 30% Sulfur yields for ancient or unmonitored eruptions on Earth are is among the highest values reported from leachate studies on terrestrial commonly estimated to a first order via a petrological approach. eruptions and so represents the most conservative estimate for total S Assuming all erupted S gas was dissolved in basaltic melt prior to the yield from the MFF. If the fine particles of the MFF scavenged less than eruption, one can use measured pre-eruptive S concentrations to calcu- 30% of the S, then this estimate could be higher by up to a factor of 5 late the total S yield, so long as the volume, density, and the crystal (Fig. 6). Further, the MFF has undergone significant erosion since its content of the erupted material are known. We also computed the mass of emplacement (e.g. Dunning, 2019); similar calculation for the unknown the exhaled S from the MFF following a similar approach. Given the original volume of the MFF would yield an even higher amount of SO2 published estimate for the range of S concentrations expected in pre- injected into the atmosphere. Alternatively, if the enrichment of S is erupted Martian basalts (4000–7000 ppm; Gaillard and Scaillet, 2009), limited to the first few meters of the MFF (~1% of the volume of the and the known volume of the MFF, we estimate that 2.8 1013 to MFF), then the mean mass of S within the current deposit of the MFF 1.8 1016 kg of S would have been delivered to the Martian atmosphere equates to ~7 1014 kg. Assuming that this mass is approximately 30% over the depositional timescale. This is an extreme minimum value for

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