These observations support models of global Mars soil formation that describe soils as having varying amounts of three main components: (1) S‐, Cl‐, and Zn‐enriched global dust, (2) regional fines, and (3) local gravel, sand, and finer material [e.g., McSween and Keil , 2000 ; Bishop et al ., 2002 ; McGlynn et al ., 2011 ; Berger et al ., 2016a ]. Active sand sheets are depleted in the global dust component and dominated by components 2 and 3, as indicated by low S, Cl, and Zn. Assuming that the global dust unit represents a well‐mixed sampling of the Martian crust, the S, Cl, and Zn enrichments indicate that fluid‐mobile element enrichments are a widespread feature of the modern Martian surface. These elements have been added to the crustal materials that have been physically broken down into dust particles. The enrichment was likely due to volcanic emissions and/or fluids that mobilized these elements to the surface where they were distributed with the igneous dust particles.

Fluid‐mobile element enrichments (S, Cl, and Zn) in Martian soils have been linked, in part, to enrichments in Martian dust [ Yen et al ., 2005 ; Ming et al ., 2008 ; Berger et al ., 2016a ]. Lower Zn, S, and Cl in the active Bagnold sand dune compared to Gale soil (Tables 2 and S1 ) agrees with MER APXS analyses of the El Dorado sand sheet in Gusev Crater, which is also lower in Zn (~130 ppm) than local basaltic soils (270 ± 80) [ Ming et al ., 2008 ]. MER APXS also found elevated Zn in undisturbed, dusty soil surfaces relative to disturbed soils, which led to the interpretation that the Martian dust is enriched in Zn [ Yen et al ., 2005 ; Ming et al ., 2008 ]. An MSL APXS study of airfall dust on the science observation tray could not quantify Zn or Ge because the sample volume was too small [ Berger et al ., 2016a ]. Nevertheless, the correlation of Zn with S and Cl in the sand and soils is consistent with and enrichment of Zn in the dust, as observed by the MER rovers.

Although the bulk chemical compositions of Martian soils reflect the largely basaltic crust, Zn is enriched in the soils relative to crustal values predicted from Martian meteorites (Table 1 ). Gale soils are similar in composition to the basaltic soils analyzed by APXS in Gusev Crater and Meridiani Planum (e.g., Laguna Class soils) [ Yen et al ., 2005 , 2013 ; Gellert et al ., 2006 ; Ming et al ., 2008 ; O'Connell‐Cooper et al ., 2017 ]. Mean Zn in Gale soil (320 ± 40 ppm) is in range with basaltic soils at Meridiani Planum (320 ± 80) and Gusev Crater (270 ± 80). Because Ge is near or below the 30 ppm detection limit in soils, we cannot conclude with certainty if they are enriched relative to the bulk crust and Martian meteorites.

4.2 Primary Enrichment Processes

The widespread occurrence of Zn and Ge in the diverse sedimentary rocks of Gale at concentrations significantly greater than those of the Martian meteorites and modeled crust (Table 1) points toward primary enrichment processes involving fluids. We define these as primary enrichment processes to distinguish them from secondary diagenetic processes that also affected Zn and Ge. We propose below that the magnitude of the Zn and Ge enrichment was most likely due to hydrothermal activity, but we first examine the amount of meteoritic input and the possible presence of magmatic sulfides in the parent rocks.

Inferring the Zn and Ge enrichment process(es) from Curiosity's data set includes uncertainty for four main reasons. (1) Curiosity has been investigating rocks in a sedimentary system, which can induce varying degrees of mixing, averaging, and sorting of the source rocks. (2) Zn is readily mobilized by fluids in sediment at low temperatures [e.g., McBride, 1994; Degryse et al., 2009; Mertens and Smolders, 2013]. These first two processes likely diluted and obscured concentrated primary Zn and Ge deposits. (3) The APXS detection limit for Ge (30 ppm) is 10 times higher than the levels in Martian meteorites and crustal models (<3 ppm); thus, the relative magnitude of Ge enrichment and Ge mobility below 30 ppm is uncertain. (4) We cannot unambiguously determine the Zn‐ and Ge‐bearing phases, or whether they are associated with crystalline minerals or the substantial X‐ray amorphous fraction observed by CheMin [Vaniman et al., 2014; Morris et al., 2015; Treiman et al., 2016; Rampe et al., 2017; Yen et al., 2017]. Despite these sources of uncertainty, we discuss plausible enrichment pathways for Zn and Ge in Gale.

4.2.1 Meteoritic Input Much of the surface of Mars preserves impacts dating back to the Noachian period [e.g., Carr and Head, 2010], and Zn‐ and Ge‐bearing impactor material is likely a component of the modern surface [e.g., Yen et al., 2006]. The meteoritic components of lunar and Martian soils have been linked compositionally to CI chondrites [Taylor, 1982; Yen et al., 2006], which contain average Zn (310 ± 12 ppm) and Ge (33.2 ± 0.3 ppm) concentrations [Lodders, 2003] that are enriched ~3 times and ~10 times, respectively, over Martian meteorites. Thus, chondritic input of Zn and Ge may constitute part of the Gale enrichment. Nickel is a useful tracer for meteoritic material on Mars with APXS [Yen et al., 2006] because CI chondrites contain 10640 ± 210 ppm Ni [Lodders, 2003] and the element is quantifiable by the MER and MSL APXS instruments at >50 ppm [Gellert et al., 2006]. Yen et al. [2006] estimated a 1% to 3% chondritic contribution to soil based on elevated Ni in MER APXS analyses. Based on MSL APXS Ni values, a 1% to 4% meteoritic input was estimated for Yellowknife Bay mudstones in Gale [McLennan et al., 2013]. We note that CM and CR chondrite impactor clasts are common in howardite meteorites and may represent the chondritic component of Martian soils more accurately than CI chondrites [Zolensky et al., 1996]. However, compared to CI chondrites, CM and CR chondrites contain more Ni (12,300–13,100 ppm), less Zn (100–180 ppm), and less Ge (18–26 ppm) [Lodders and Fegley, 1998]. Therefore, the chondritic Zn and Ge component may be overestimated by CI chondrites. We also emphasize that the apparent mobilization of Ni in Gale sediments (e.g., in Ni‐rich Mg sulfates) [VanBommel et al., 2016] adds uncertainty to estimates of meteoritic input. We find that the magnitude of the Zn and Ge enrichments cannot be accounted for by meteoritic material alone. The molar ratio Ge/Ni is 0.0025 in CI chondrites and 0.17 in Gale (excluding the Ge‐rich Garden City vein and Ni‐rich Mg sulfates). Molar Zn/Ni in CI chondrites is 0.26 and 1.4 in Gale. Therefore, the mean Ni in Gale (500 ppm) does not represent sufficient meteoritic material to account for the elevated Zn and Ge. The 1–4% addition of chondritic material predicted by Ni content amounts to an addition of 0.3–1.3 ppm Ge and 3–12 ppm Zn, which is relatively insignificant.

4.2.2 Magmatic Sulfides Both Zn and Ge are chalcophile when sulfur activity is elevated, and they can be accommodated in magmatic sulfide minerals. Estimated sulfide concentrations at sulfide saturation (SCSS) values for Gale Crater melts range from ~6000 ppm for Jake_M class samples to ~13,500 ppm for some Bathurst Inlet class samples [Izawa et al., 2016]. Melts with compositions similar to Bathurst Inlet class rocks may have been particularly efficient carriers of sulfides and chalcophile elements, due to their high FeO* (up to 23 wt %), which correlates with both high SCSS and low viscosity [Izawa et al., 2016]. Melts with compositions similar to Jake_M and Bathurst Inlet may also have contributed to sulfide mineralization through assimilation of crustal S during emplacement in a manner analogous to some terrestrial magmatic sulfide deposits [Burns and Fisher, 1990; Baumgartner et al., 2015]. A further source of magmatic enrichment in Zn, Ge, and other chalcophile elements may be the Martian mantle source for these magmas. Elevated alkalis in Bathurst Inlet and Jake_M rocks indicate metasomatic events in the source mantle [Stolper et al., 2013; Schmidt et al., 2014b], and Zn and Ge may have been concentrated by associated processes. Although magmatic sulfide mineralization may have provided an important source of initial enrichment in chalcophile elements, including Zn and Ge, further enrichment by another secondary process would have been necessary to reach the Zn and Ge concentrations observed by the APXS. Evidence for sulfide minerals on the Martian surface comes from pyrolysis results from the Sample Analysis at Mars (SAM) instrument [McAdam et al., 2014], but sulfide minerals have not been confirmed above 1 wt % by CheMin. Pyrrhotite is present in Martian meteorites, including the regolith breccia samples of the NWA 7034 group [Agee et al., 2013], but concentrations of both Zn and Ge in specific mineral phases are poorly constrained for Martian meteorites, with some exceptions [Humayun et al., 2013, 2016], and are almost always below the detection limit of electron probe microanalysis (100 ppm).