Ore Geology Reviews 65 (2015) 400–412

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Magmatic controls on the genesis of Ni–Cu±(PGE) sulphidemineralisation on Mars

R.J. Baumgartner a,⁎, M.L. Fiorentini a, D. Baratoux b,e, S. Micklethwaite a, A.K. Sener c, J.P. Lorand d, T.C. McCuaig a

a Centre for Exploration Targeting, School of Earth and Environment, ARC Centre of Excellence for Core to Crust Fluid Systems, The University of Western Australia, 35 Stirling Highway,6009 Crawley, Perth, Western Australia, Australiab Observatoire Midi-Pyrénées, 14 Avenue Edouard Belin, 31400 Toulouse, Francec Matrix Exploration Pty Ltd., Shop 8 Pioneer Village, 7 Albany Highway, 6112 Armadale, Western Australia, Australiad Laboratoire de Planétologie et Géodynamique de Nantes, Université de Nantes and Centre National de la Recherche Scientifique (UMR 6112), 2 Rue La Houssiniére, BP 92208,44322 Nantes Cédex 3, Francee Institut de Recherche pour le Développement & Institut Fondamental d’Afrique Noire, Dakar, Senegal

⁎ Corresponding author.

http://dx.doi.org/10.1016/j.oregeorev.2014.10.0040169-1368/Crown Copyright © 2014 Published by Elsevie

a b s t r a c t

a r t i c l e i n f o

Article history:Received 18 August 2014Received in revised form 3 October 2014Accepted 9 October 2014Available online 30 October 2014

Keywords:MarsNi–Cu–PGE sulphidesMineral system analysisSulphide saturationSulphur assimilationLava channel

Widespread igneous activity, showing striking mineralogical, petrographical and chemical commonalities withterrestrial komatiites and ferropicrites, intensely affected, reshaped and buried the primary Martian crust. Thisstudy evaluates whether the igneous activity on Mars may have led to the formation of orthomagmatic Ni–Cu±(PGE) sulphide mineralisation similar to that associated with terrestrial komatiites and ferropicrites.Particular focus is laid on two different components of the Martian Ni–Cu±(PGE) sulphide mineral system:1) the potential metal and sulphur fertility of mantle sources and derived melts, and 2) the physicochemicalprocesses that enable sulphide supersaturation and batch segregation of metal-rich sulphide liquids.We show that potentiallymetal-richMartianmantlemelts likely reach sulphide saturationwithin 30wt.% crystalfractionation. This value is comparable to that calculated for the mineralised ferropicrites at Pechenga, Russia.However, the majority of known world-class Ni–Cu±(PGE) sulphide deposits associated with terrestrialkomatiites and ferropicrites originated due to the assimilation of crustal sulphur-rich substrate, thus promotingthe attainment of sulphide supersaturation and batch segregation of metal-rich sulphide liquids during earlystages ofmagma evolution. Given the high sulphur inventory ofMartian crustal reservoirs, ranging from sulphidebearingmagmatic rocks to sulphate-rich soils, regoliths and sedimentary deposits, it is likely thatmantle-derivedmelts assimilated significant amounts of crustal sulphur during ascent and emplacement. It is proposed thatchannelled lava flows, which potentially emplaced and incised into sulphur-rich crustal lithologies, may haveled to the formation of orthomagmatic Ni–Cu±(PGE) sulphide mineralisation on Mars.

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1. Introduction

Information on themorphology and physicochemical characteristicsof Martian igneous activity comes from optical systems on boardorbiting satellites (e.g. Christensen et al., 2001; Bibring et al., 2006;Boynton et al., 2007; Jaumann et al., 2007; Murchie et al., 2009;among others), in-situ observation on the Martian surface via MarsLanders and Rovers equipped with analytical instruments (e.g.Arvidson et al., 2006; Bish et al., 2013; Gellert et al., 2006; Ming et al.,2008; Squyres et al., 2006), and the investigation of the chemical andtextural characteristics of the Martian meteorites (e.g. Agee et al.,2013; Bridges and Warren, 2006; Humayun et al., 2013; Nyquist et al.,2001; Papike et al., 2009; Treiman, 2004), which represent fragmentsof the Martian crust ejected by asteroid/meteorite impacts. These datasets indicate that the Martian geological record comprises widespreadigneous activity, displaying distinctive physicochemical similarities

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with terrestrial mantle-derived mafic to ultramafic magmatism in theArchean and Proterozoic eons (e.g. Baird and Clark, 1984; Burns andFisher, 1990; Reyes and Christensen, 1994; Filiberto, 2008a; Lentzet al., 2011), such as komatiites (e.g. Mount Keith, Australia; Hill et al.,1995) and particularly ferropicrites (e.g. Pechenga, Russia; Hanski,1992).

This study evaluates whether the broad igneous activity, which in-tensely affected, reshaped and buried the primary Martian crust, mayhave led to the formation of orthomagmatic Ni–Cu±(PGE) sulphidemineralisation similar to that associated with terrestrial komatiitesand ferropicrites (e.g. Arndt et al., 2005; Maier, 2005; Maier et al.,1998; Naldrett, 1999, 2010; Song et al., 2011). The study by Burns andFisher (1990) first investigated the potential of Martian magmatic sys-tems to host metal-rich sulphide mineralisation. However, althoughsince then a significantly larger amount of data on the architectureand composition of the Martial mantle and crustal reservoirs has be-come available, only one study (West and Clarke, 2010) marginallyrevisited the question of whether and where such mineralisation

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potentially occur. We therefore assess for the first time the Ni–Cu±(PGE) sulphide potential of Martian volcanic systems by applyinga mineral systems analysis (McCuaig et al., 2010; Wyborn et al.,1994).

The mineral systems analysis approach allows the identification ofprospective regions across multiple scales. It is based on the hypothesisthat the genesis of sizeable mineralisation requires a combination ofscale-hierarchical temporally and spatially independent parametersand processes, operating from the scale of the mantle down to thescale of individual faults, intrusions and lithologies (McCuaig et al.,2010). The critical parameters and processes of the mineral system forNi–Cu±(PGE) sulphides in mafic to ultramafic magmatic systems in-clude: (1) melting of a suitable mantle source, which provides metalsand sulphur to the system, (2) an active lithosphere-scale pathway,which assures the delivery of high-flux melts from deep mantle reser-voirs to the uppermost crustal regions, (3) sulphide saturation, whichcan be attained through a wide range of mechanisms, 4) physical pro-cesses that concentrate sulphides, and (5) preservation (McCuaiget al., 2010). On Mars, a number of these factors remain unknown orare currently poorly constrained.

This article addresses two critical components of the Martian Ni–Cu±(PGE) sulphide mineral system: 1) the potential metal andsulphur fertility of mantle source regions and derived melts, and2) the physicochemical processes that enable sulphide supersatura-tion and metal concentration into segregating sulphide liquids. Wereview and summarise the findings of a large number of studies onthe physicochemical nature of the Martian mantle, including variationsin melting conditions and in the metal and sulphur budget of mantlereservoirs and partial mantle melts. Specifically, this study constrainsthe sulphide saturation history andmetal budget ofMartianmafic to ul-tramafic magmatic systems in terms of crystal fractionation, and drawscomparison to the evolution of terrestrial ferropicrites (especially themineralised ferropicrites at Pechenga, Russia). Finally, the study definesconstituent processes that potentially trigger sulphide supersaturationin metal-rich Martian silicate melts and shows that the mineralisationpotential in Martian mafic to ultramafic magmatic systems is compara-ble to its terrestrial equivalents.

2. Controls on the genesis of Ni–Cu±(PGE) sulphides

On Earth, the genesis and localisation of Ni–Cu±(PGE) sulphidemineralisation associated with mafic to ultramafic magmatic rocks,such as komatiites and ferropicrites, are first and foremost related tothe large-scale architecture of the lithosphere at the timeofmagmatism.The lateral dimension, thickness and geometry of lithospheric blocksexert a first order control on the formation and stabilisation of conti-nents, and control the location and geochemistry of mantle melts.Begg et al. (2010) and Mole et al. (2013, 2014) recently advocatedthat the lithosphere plays a key role in controlling magma chemistryand maximising magma flux to ultimately generate Ni–Cu±(PGE) sul-phide mineralisation.

At the scale of single volcanic systems, the attainment of sulphidesaturation and segregation of metal-rich sulphide liquid from silicatemagmas is the key factor in the genesis of Ni–Cu±(PGE) sulphidemineralisation. Due to the preference of noble metals to partitioninto sulphides, segregating sulphide liquids scavenge the metalsfrom the silicate magma (e.g. Arndt et al., 2005; Brügemann et al.,2000; Lesher and Campbell, 1993; Maier, 2005; Naldrett, 1999,2010; Patten et al., 2013; Peach et al., 1990, 1994), whereas physicalprocesses associated with the dynamic emplacement of magmasalong conduits and channels locally promote the mass concentrationof sulphides.

The amount of sulphide liquid segregating froma silicatemelt is, to afirst order, controlled by the difference between the sulphur concentra-tion (S) and the sulphur concentration at sulphide saturation (SCSS).The composition and metal tenor of the sulphide liquid, on the other

hand, are controlled by: 1) the metal budget of the silicate magma,2) the volume ratio between sulphide liquid and silicate magma(i.e. “R-factor”; Campbell and Naldrett, 1979), and 3) magma fluxand dynamics, as well the efficiency of the equilibration process be-tween the silicate melt and segregating sulphide liquid (Naldrett,2010).

The SCSS of a silicate melt is controlled by its chemical compositionand ambient factors, such as pressure, temperature and oxygen fugacity(e.g. Mavrogenes and O'Neill, 1999; O'Neill and Mavrogenes, 2002; Liand Ripley, 2005; Liu et al., 2007; Righter et al., 2009; Jugo et al., 2010;Ding et al., 2014). Fractional crystallisation in a crustal low-pressure en-vironment progressively reduces the SCSS of a melt (Mavrogenes andO'Neill, 1999) and promotes the attainment of sulphide saturation.However, this process usually accounts only for small portions ofsulphide liquid continuously segregating from crystallising silicatemagmas (fractional segregation). The majority of terrestrial world-class Ni–Cu±(PGE) sulphide mineralisation associated with mafic toultramafic magmas originated due to complex chemical mixing pro-cesses, which drive sulphide undersaturated silicate melts to sulphidesupersaturation and batch segregation of large amounts of sulphideliquid.

Mechanisms that trigger mineralising events for Ni–Cu±(PGE)sulphides include mingling between compositionally contrastingmelts in deep-seated magma chambers and (sub-) surface eruptions(e.g. Godel et al., 2011; Naldrett and vonGruenwaldt, 1989), or contam-ination of sulphide-undersaturated melts with crustal contaminants.Crustal contamination processes refer to assimilation of crustal silica-rich wall rocks (e.g. Irvine, 1975; Li and Naldrett, 1993, 2000; Li andRipley, 2005; Seat et al., 2009), addition of crustal volatiles, such asCO2 and H2O (e.g. Gorbachev and Kashirceva, 1986; Liu et al., 2007;Song et al., 2003) and, most importantly, assimilation of crustal sulphurby devolatilisation, partial melting or bulk assimilation of sulphide/sulphate bearing country rocks (e.g. Bekker et al., 2009; Fiorentiniet al., 2012; Konnunaho et al., 2013; Lesher and Campbell, 1993;Ripley and Al-Jassar, 1987; Ripley and Li, 2003, 2013; Ripley et al., 1999)

3. Martian magmatism

3.1. Morphology, distribution and age of Martian igneous activity

The surface expression of Martian volcanic activity may besummarised and subdivided into twomain categories: 1) central volca-noes that originate fromprolonged concentrated effusive eruptions, and2) volcanic plains, mainly situated around the Tharsis and Elysium(Fig. 1) volcanic provinces (Grott et al., 2013; Werner, 2009). Theearliest visible morphological features of Martian volcanism rangefrom remnants of shield volcanos and lava plains (Grott et al., 2013;and references therein), to recently discovered calderas (Michalskiand Bleacher, 2013) that are potentially common in Noachian crustalterrains (N3.7 Ga; Fig. 1), providing evidence for widespread andlarge-scale volcanism in early eras. However, igneous activity graduallydeclined over time, thus forming scattered large volcanic provinces/plains (e.g. Syrtis Major, Hesperia Planum, volcanic plains of theTharsis and Elysium region; Fig. 1) during the Hesperian (3.7–3.0 Ga) era (e.g. Hiesinger and Head, 2004; Werner, 2009; Rampeyand Harvey, 2012; and references therein). Most recent volcanism(Amazonian; 3.0 Ga to present) predominately formed individuallarge shield volcanoes restricted to the Tharsis and Elysium igneousprovinces (Fig. 1).

The Tharsis and Elysium igneous provinces (Fig. 1) are by far the vol-umetrically largest (i.e. 3x108 and 3.5x106 km3 respectively; Phillipset al., 2001; Platz et al., 2010; Werner, 2009; Grott et al., 2013) igneousfeatures on the Martian surface. The enormous volumes of individualigneous provinces on Mars are related to partial mantle melting associ-ated with long-lived mantle plumes producing magmas below an im-mobile lithosphere (e.g. Zuber, 2001). However, given the lack of

Fig. 1. Chronostratigraphicmap ofMars (redrawn after Tanaka et al., 2014) showing surfaces of common age (i.e. Noachian, Hesperian, Amazonian). Volcanic terrains are subdivided sep-arately in greyscale levels. Landing sites are highlightedwith stars (for interpretation of the references to color in this figure legend, the reader is referred to theweb version of the article).

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knowledge and understanding of the global and regional architectureof the Martian lithosphere, potential lithospheric controls on thespatial distribution of plume-related Martian igneous activity re-main unclear.

3.2. Mantle sources, partial melting and derived melts

Significant information on the geochemical and mineralogicalcharacteristics of Martian volcanic rocks, their parental melts and po-tential mantle sources, is derived from the Martian meteorites. Theyare subdivided into three major groups (i.e. Shergottites–Nakhlites–Chassignites: SNC meteorites) and several ungrouped specimens, suchas the polymict regolith breccias NWA 7034, NWA 7533 and pairedsamples (Agee et al., 2013; Humayun et al., 2013), and orthopyroxeniticALH 84001 (Mittlefehldt, 1994). The Shergottites range from Fe-richand subalkaline pyroxene-phyric basalts (i.e. basaltic Shergottites)and porphyritic olivine basalts (i.e. olivine-phyric Shergottites), tostrongly porphyritic/poikilitic lherzolites (i.e. lherzolitic Shergottites),whereas Nakhlites and Chassignites represent clinopyroxenites anddunites showing mostly cumulate textures (e.g. Treiman, 2004;Bridges and Warren, 2006; Basu Sarbadhakari et al., 2009, 2011;Papike et al., 2009; Gross et al., 2011, 2013; Aoudhejane et al.,2012; among others).

Data sets derived from hyperspectral remote imagery and gamma-ray spectroscopy, which have been translated to global element andmineral abundance maps (e.g. Bandfield, 2002; Boynton et al., 2007;Koeppen and Hamilton, 2008; Poulet et al., 2007), confirm the wide-spread occurrence of predominately Fe-rich (either tholeitic or alkaline)magmatic rocks. Furthermore, theMars Exploration Rovers Spirit (MER-A) and Opportunity (MER-B), as well as the Mars Science Laboratory(MSL) Curiosity (Fig. 1), investigated alkaline and Fe-rich magmatic

rocks at the Gusev Crater, Meridiani Planum and Gale Crater landingsite, respectively (McSween et al., 2006, 2008, 2009; Schmidt et al.,2014; Stolper et al., 2013). Elemental correlations amongMartian igne-ous rocks, particularly the Martian meteorites (Dreibus and Wänke,1985; Wänke et al., 1994), in combination with the premise that bulkMars has a chondritic composition, imply that the primitive Martianmantle is rich in iron (~18 wt.% FeO) and volatiles, such as N, F, P andS (Dreibus andWänke, 1985), relative to theprimitive terrestrialmantle(McDonough and Sun, 1995).

Constraints on the physicochemical conditions (e.g. pressure, tem-perature and oxygen fugacity) of mantle melting are based on silicateinclusion studies in early magmatic phases (e.g. Harvey and McSween,1992) and mass-balance calculations among bulk-rock and cumulusminerals in the Martian meteorites (e.g. Basu Sarbadhakari et al.,2009; Gross et al., 2011). Additional information comes from partialmelting and crystallisation experiments on the inferred compositionof the Martian mantle and primitive surface rocks (e.g. Bertka andHolloway, 1994a, 1994b; Filiberto, 2008b; Filiberto and Dasgupta,2011; Filiberto et al., 2008; Monders et al., 2007; Musselwhite et al.,2006), as well as thermodynamic modelling results compared withspectral remote sensing data sets (e.g. Baratoux et al., 2011, 2013).

These studies highlight distinctive temporal and spatial variationsin melt generation in the Martian mantle, such as b5 to 30% mantlemelting at inferred P-T conditions in the range of ~1–5 GPa and1300–1550 °C respectively, as well as under relatively reduced (−3.5to −1 below FMQ) conditions. These physicochemical variations maybe explained by modification of the conditions of partial melting inrelation to planetary cooling (Baratoux et al., 2011), chemical heteroge-neities in the mantle, such as variations of the water content and/oroxygen fugacity (e.g. Balta and McSween, 2013; Herd et al., 2002;Treiman, 2004; Tuff et al., 2013), interaction andmixing between rather

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reduced primitive magmas and variably oxidised crustal components(e.g. Borg et al., 2002; Nyquist et al., 2001), or most likely by a co-variation of these parameters.

4. Sulphur and metal budget of the Martian mantle and derivedmelts

4.1. Siderophile and chalcophile metal budget

The metal inventory of the Martian meteorites, as well as magmaticrocks discovered and analysed at several landing sites, provides impor-tant constrains on the potential metal budget of mantle reservoirs andderived partial melts. The rather unevolved and primitive members ofthe SNC meteorites (i.e. Y 980459, Musselwhite et al., 2006; NWA6234, Gross et al., 2013; NWA 5789, Gross et al., 2011; LAR 06319,Basu Sarbadhakari et al., 2009) have Ni concentrations ranging between180 and 240 ppm (see references in Meyer, 2012; Table 1 and Fig. 2a),being comparable with the Ni concentrations (160–180 ppm) in theAdirondack-class basalts at the Gusev Crater landing site (Gellertet al., 2006; Ming et al., 2008; Table 1 and Fig. 2a). The SNC meteoritesalso contain significant PGE concentrations (NN1 ppb PGEtot; Joneset al., 2003; Brandon et al., 2012; Fig. 2b), with total abundances thatare at the same order of magnitude if compared with mantle meltsand derived magmatic rocks on Earth.

Elemental correlations among chondrites and the SNC meteoritesargue for significantly lower Ni and Cu concentrations in the Martianprimitive mantle (i.e. 400 ppm Ni and 6 ppm Cu, Table 1; Dreibus andWänke, 1985; Wänke et al., 1994), if compared with the primitive ter-restrial mantle (i.e. 1960 ppm Ni and 30 ppm Cu, Table 1; McDonoughand Sun, 1995). By considering that Ni and Cu become progressivelymore siderophile and chalcophile with decreasing pressure (Li and

Table 1Inferred Ni, Cu and S concentrations in theMartian core, mantle, partial mantle melts andseveral crustal reservoirs. Values for nickel, copper and sulphur in the terrestrial core andmantle are given in brackets. See text for discussions.

Ni Cu S

Core and mantleMetallic core 7.6a (5.2b) wt.% – 14.2a (1.9b) wt.%Primitive mantle 400a (1960c)

ppm6a (30c)ppm

700–2200d (250c)ppm

Magmatic rocksOlivine-phyricShergottitese

180–240 – 1600–1700

Primitive Gusev basaltsf,g 160–180 – –

Soils and regolithsViking 1 and 2h – – 2.4–3.8 wt.%Mars pathfinderi – – 1.6–2.6MER-A spiritj 154–717 – 0.4–4.3MER-B opportunityk 590–1090 – 1.8–2.9MSL curiosityl 446–455 – 2.0–2.2

Sulphate depositsLayered sulphatedepositsd

– – 8–12

a Wänke et al. (1994).b McDonough (2003).c McDonough and Sun (1995).d Gaillard et al. (2013), and references therein.e References in Meyer (2012): Martian meteorites Y 980459, NWA 5789, NWA 6234,

LAR06319.f Gellert et al. (2006).g McSween et al. (2008).h Clark (1993).i Bell et al. (2000).j Ming et al. (2008).k Rieder et al. (2004).l Schmidt et al. (2014), soil targeted at Rocknest (“Portage soil”), Gale Crater.

Agee, 1996, 2001), the low metal concentrations in the Martian mantlemay be the consequence of the relatively lower equilibration pressureduring core formation (e.g. Tang and Dauphas, 2014; Wood et al.,2006), due toMars relatively lowmass (~1/10 that of Earth). The signif-icant PGE signatures in the Shergottite meteorites (NN1 ppb PGEtot;Jones et al., 2003; Brandon et al., 2012; Fig. 2b) indicate that themantleof Mars, similar to Earth, exhibits elevated abundances and near chon-dritic ratios. The accretion of a late-veneer appears to be the most rea-sonable explanation for the inferred PGE inventory of the Martianmantle (see discussions in Jones et al., 2003; Brandon et al., 2012).

4.2. Sulphur budget

Theoretical accretion models, which are based on the assumptionthat bulk Mars has a chondritic composition, suggest that Mars is avolatile-rich planet with an estimated bulk sulphur content of ~5 wt.%(Table 1; Wänke et al., 1994). Similar models imply significantly lesssulphur in bulk Earth (b 0.6 wt.%, Dreibus and Palme, 1996). By assum-ing that sulphur is entirely sequestered in the Martian interior duringplanetary differentiation, the Martian core has been estimated tocontain 14–18 wt.% S (Table 1; Dreibus and Wänke, 1985; Wänkeet al., 1994). Calculations on the metal-silicate partitioning at core–mantle equilibration conditions suggest that the primitive Martianmantle potentially exhibits between 700 and 2200 ppm S (Table 1;Gaillard et al., 2013). The sulphur concentration in the primitive terres-trial mantle is estimated to be one order of magnitude lower (i.e.250 ppm S, Table 1; McDonough and Sun, 1995). Given the low massof Mars, the relatively high sulphur abundance in the Martian mantleis consistentwith the increasingly siderophile nature of sulphurwith in-creasing pressure (Li and Agee, 2001).

Knowledge on the sulphur capacity of Martianmantlemelts provideadditional constraints on the potential sulphur inventory of Martianmantle reservoirs. Sulphides in the terrestrial primitivemantle, contain-ing ~250 ppm bulk S (Table 1), are estimated to be exhausted by 15–25% partial melting (Hamlyn et al., 1985; Keays, 1995; Lorand et al.,1999). FeO-rich and reduced Martian mantle melts that form at 30%partial melting and pressures of 1.5 GPa, potentially have a SCSS of upto ~4300 ppm S (Righter et al., 2009; Ding et al., 2014; Table 1). Byassuming that most primitive Martian basalts formed from melts de-rived by b25–30% partial mantle melting at pressures of ~1.5 GPa, andthat sulphides entirely enter silicate melts upon sulphide saturation,mass balance calculations among the Martian mantle and the sulphurcapacity of derived melts suggest that the bulk Martian mantle likelycontains ≥700 ppm S (Ding et al., 2014).

The relatively young (b500 Ma) Shergottite group meteorites,which contain 1600–2700 ppm sulphur (Table 1; see references inMeyer, 2012), provide important information on the fate of sulphurduring ascent and emplacement of Martian mantle melts. Given thefact that the Shergottite meteorite Y 980459, which potentially repre-sents a rather primitive melt composition (Musselwhite et al., 2006),contains 1600–1700 ppm sulphur (Table 1; Shirai and Ebihara, 2004),it is assumed that its sulphur concentration is representative of allShergottites. Differences between the potential (i.e. up to 4300 ppm)and detected (i.e. as low as 1600 ppm; Table 1) sulphur content in theShergottite meteorites potentially reflect different degrees of partialmelting, mantle sulphur heterogeneities, sulphide-undersaturatedpartial mantle melting, sulphide fractionation and/or sulphurdegassing duringmelt ascent/emplacement, and impact induced sulphurvolatilisation (Ding et al., 2014; Gaillard and Scaillet, 2009; Gaillard et al.,2013; Lorand et al., 2012; Righter et al., 2009). However, the magmaticsulphur contents of Shergottite meteorites may have been also alteredby assimilation of crustal sulphur-material during melt ascend and/oremplacement (e.g. Franz et al., 2014), postmagmatic (hydrothermal)alteration (Bridges et al., 2001), impact melt generation during or priorto Meteorite ejection (e.g. Walton et al., 2014), and terrestrial alteration(Schwenzer et al., 2013).

Fig. 2.Whole rockMgOversusNi (a) and Pd/Ir (b) forMartianmeteorites and crustal rocks analysed at theGusevCrater landing site (Jones et al., 2003;Gellert et al., 2006;Ming et al., 2008;Brandon et al., 2012; references inMeyer, 2012). Trendlines for terrestrial komatiites (Barnes and Fiorentini, 2012) and ferropicrites (Hanski, 1992; Hanski and Smolkin, 1995; Stone et al.,1995; Brügemann et al., 2000; Gibson et al., 2000; Crocket et al., 2005; Ichiyama et al., 2006; Fiorentini et al., 2008; Goldstein and Francis, 2008; Kitayama and Francis, 2012) are shown forcomparative purposes.

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5. Sulphur and metals in the evolution of Martian magmas

5.1. Sulphide saturation in terms of crystal fractionation

As the attainment of sulphide saturation is the key in the formationof sulphide mineralisation, this section explores and tracks the sulphurcapacity of Martian mantle melts during cooling and crystal fraction-ation in a low-pressure environment (see Ripley and Li, 2013, for com-parison), utilising the MELTS thermodynamic calculator (Ghiorso andSack, 1995) in conjunction with the SCSS equation of Righter et al.(2009). The SCSS trends are calculated for reduced (i.e. FMQ-2) and an-hydrous Martian mantle melts (Table 2), such as the ones derived frompartial mantle melting simulations with pMELTS (Baratoux et al., 2011)and melting experiments (Bertka and Holloway, 1994b) at 1.5 GPa and30% melting. Furthermore, this study investigates the SCSS of the meltsparental to the Shergottitemeteorites Y 980459 and LAR 06319, and theAdirondack basalt analysed at the Gusev Crater landing site, as thesespecimens potentially represent rather unfractionated mantle melts(Basu Sarbadhakari et al., 2009; Filiberto et al., 2008; Gross et al.,2011). The potential SCSS trend for the ferropicrite melt parental tothe world-class Ni–Cu±(PGE) sulphide mineral deposit at Pechenga,

Table 2Normalised compositions (wt.%) of Martian and terrestrial melts used to calculate SCSS.

1 2 3 4 5 6

SiO2 47.45 46.19 46.28 50.13 48.62 46.65TiO2 0.40 0.45 0.50 0.49 0.77 2.31Al2O3 8.85 7.43 10.64 6.09 6.80 10.18FeOtot 19.23 20.93 19.37 16.03 20.57 15.50MnO 0.17 0.58 0.43 0.44 0.49 0.20MgO 15.06 15.90 12.15 18.37 13.31 14.87CaO 6.90 6.58 7.92 7.31 7.30 8.65Na2O 1.63 1.38 2.13 0.81 1.29 0.40K2O 0.31 0.03 0.02 0.16 1.03P2O5 0.56 0.55 0.31 0.69 0.21

1) Melt composition from pMELTS partial melting simulations on the primitive Martianmantle at 1.5 GPa and 30 wt.% partialmelting (Baratoux et al., 2011); 2)melt compositionderived from partial melting experiments on the primitive Martian mantle at 1.5 GPa and30 wt.% partial melting (Bertka and Holloway, 1994b); 3) composition of the Adirondackbasalt analysed at the Gusev Crater landing site; 4–5) whole rock composition of the oliv-ine-phyric Shergottites Y 980459 (Greshake et al., 2004) and LAR 06319 (after 10 wt.% ol-ivine cumulate substraction; Basu Srabadhakari et al., 2011); 6) Pechenga ferropicrite(Hanski and Smolkin, 1995).

Russia (Table 2; Hanski and Smolkin, 1995), is calculated and shownfor comparative purpose.

Modelled fractional crystallisation sequences in the Martian meltsare broadly consistent with the mineralogy of the olivine-phyricShergottite meteorites, and follow the order spinel → olivine → ±orthopyroxene → clinopyroxene → feldspar (Fig. 3). Fractionated spi-nels usually evolve from chromite through ulvöspinel/titanomagnetiteto magnetite, whereas olivine and pyroxene become progressively Fe-rich. In particular, two pyroxene fractionation trends may be broadlydistinguished: 1) Mg-rich orthopyroxene via pigeonite to Fe-rich augite,and 2) Ca-rich augite becoming more Fe-rich with differentiation. Thecalculated initial SCSS values for the Martian melts are in the range of~3400–5000 ppm, being at the maximum for the melt from the partialmantle melting experiment at 1.5 GPa (Fig. 4). By way of comparison,the melt parental to the ferropicrites at Pechenga has a calculated SCSSof ~3400 ppm on the liquidus. The melt SCSS values gradually drop dueto melt cooling and crystal fractionation, whereas slopes vary with thecrystallising mineral species. The sulphur capacities strongly decreaseduring initial olivine crystallisation, whereas the attainment of ol ±opx ± cpx crystallisation is characterised by more gentle SCSS slopes. Fi-nally, SCSS values may progressively increase during late stage plagio-clase fractionation due to increasing FeO contents in the magma (Fig. 4).

The variation of sulphur contents in the melts as a function of theirdifferentiation (i.e. sulphur trajectories) is calculated for initial concen-trations of 1000, 1600 and 3000 ppm S (Fig. 4). The lower bound isrepresentative for a melt produced by 25% melting of the primitiveterrestrial mantle, whereas 1600 ppm S represents the bulk sulphur in-ventory of the Shergottite meteorite Y 980459 (Shirai and Ebihara,2004). The upper bound (i.e. 3000 ppm S) is utilised to investigate theinfluence of potentially high initial sulphur contents on the sulphide sat-uration state of progressively fractioning Martian melts. The Martianmantle melts used in this study may reach sulphide saturationwithin 3–15 and 17–30 wt.% melt differentiation, assuming 3000and 1600 ppm initial sulphur, respectively. Conversely, assuming1000 ppm initial sulphur, sulphide saturation would not be achieveduntil crystal fractionation is well advanced (N32 wt.%). By way of com-parison, the melt parental to the mineralised ferropicrites at Pechenga,Russia, potentially having up to 1000 ppm S on the liquidus, may be sat-urated in sulphide within ~25 wt.% melt differentiation. Given the factthat sulphur abundances in most Martian mantle melts are expectedto be relatively higher than in the terrestrial ones (i.e. N1000 ppm S;

Fig. 3. Fractional crystallisation sequences calculated for primitiveMartianmantlemelts and ferropicrites (see text and Table 2 for details). The upper limits of spinel stability are uncertain(see discussion in Asimow et al., 1995).

Fig. 4. Sulphur content at sulphide saturation (SCSS) versus melt crystallisation in primitiveMartian melts and the melts parental to the ferropicrites at Pechenga, Russia (see text andTable 2 for details). All calculations are performed at FMQ-2 and using a pressure of 1 bar.The trajectories for sulphur in the liquids are calculated for 1000, 1600 and 3000 ppm initialsulphur. The lower bound is representative for a melt produced by 25%melting of the prim-itive terrestrial mantle, 1600 ppm S represents the bulk sulphur inventory of the Shergottitemeteorite Y 980459 (Shirai and Ebihara, 2004), and 3000 ppm S is utilised to investigate theinfluence of potentially high initial sulphur contents on the sulphide saturation state of Mar-tian melts. Sulphur is assumed to be perfectly incompatible during silicate/oxide fraction-ation. Sulphide saturation is attained at the intersections of the curves representing thesulphur content and the SCSS of the fractionated melts. The grey fields indicate the degreeof fractionation at which sulphide saturation is attained for the range of considered Martianmelts at respective initial sulphur concentrations. The vertical bold dashed line correspondsto the attainment of sulphide saturation in the melt parental to the Pechenga ferropicrites,assuming an initial sulphur concentration of 1000 ppm S.

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see section “Sulphur and metal budget of the Martian mantle andderived melts”), these simulations indicate that most Martian mantlemelts and terrestrial ferropicrites have a comparable potential toreach sulphide saturation and liquid segregation.

5.2. Metal fractionation in terms of crystal fractionation and sulphidesaturation

Sulphide-undersaturated silicate fractionation, both during meltascent and/or emplacement, strongly influences the metal budget ofany magma. The concentrations of several (highly) siderophile andchalcophile elements, such as Cu and the palladium-group platinum-group elements (PPGEs: Pt, Pd and Rh), substantially increase duringfractional crystallisation. This process is considered to be the conse-quence of the moderate to strong incompatibility of these elements inmost silicate and oxide minerals (e.g. Capobianco and Drake, 1990;Peach et al., 1994; Righter et al., 2004). Conversely, Ni and iridium-group platinum-group element (IPGEs: Os, Ir and Ru) concentrationsprogressively decrease duringmelt differentiation. These elements par-tition into crystallising spinel, olivine and pyroxene (e.g. Hill et al., 2000;Lee and Tredoux, 1986; Locmelis et al., 2011, 2013; Righter et al., 2004),or form small IPGE-rich phases (i.e. sulphides or alloys in the Os–Ir–Rusystem) predominately enclosed within Cr-rich spinel (e.g. Barnes andFiorentini, 2008; Fiorentini et al., 2004).

The interdependence between sulphide-undersaturated meltfractionation and metal enrichment/depletion is usually reflected in amoderate to strong correlation between whole-rock MgO and Ni aswell as Pd/Ir, and has been recognised to be a key characteristic in ter-restrial mafic to ultramafic volcanic rocks, such as komatiites andferropicrites (Fiorentini et al., 2010, 2011). Similarly, the significantMgO/metal correlations in the Martian meteorites, ranging from lowNi (usually b120 ppm) and high Pd/Ir (up to ~100) in the relativelyMgO-poor basaltic Shergottites, to high Ni (usually N250 ppm) andlow Pd/Ir (up to ~2) in the lherzolitic Shergottites (Fig. 2a and b),

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indicate that crystallising oxides and silicates progressively scavengepart of the metals (predominately Ni and the IPGEs) of their parentalmagmas. A similar but rather weak MgO–Ni correlation at higherabsolute Ni concentrations is also observed in the magmatic rocksanalysed at the Gusev Crater landing site (Fig. 2a).

The segregation of metal-rich sulphide liquids strongly decreasesthe metal budget of a silicate melt. The most primitive olivine-phyricShergottites usually have significant PGE (NN1 ppb PGEtot; Jones et al.,2003; Brandon et al., 2012) and Ni (180–240 ppm) concentrations,which correlate with whole-rock MgO contents (Fig. 2a and b). Con-versely, the relatively evolved olivine-phyric Shergottite Dhofar 019,which shows rather low Ni (i.e. 74 ppm; Fig. 2a), does not follow theMartian meteorite trend and, most importantly, shows a rather pro-nounced PGE depletion (i.e. bb1 ppb PGEtot; Brandon et al., 2012).Considering the sulphide-undersaturated nature of the parental meltof this group of Martian meteorites, the Ni and PGE depletion in thisspecimen may be explained by the attainment of sulphide (super-)saturation and sulphide liquid segregation during melt ascent and/orcrustal emplacement.

6. Discussion on the genesis of Ni–Cu±(PGE) sulphidemineralisation on Mars

6.1. Sulphide supersaturation in Martian magmas: The need of anexternal trigger

The attainment of sulphide supersaturation and batch segregationof sulphide liquid is a crucial factor in the genesis of Ni–Cu±(PGE)sulphide mineralisation. The SCSS analysis in Fig. 4 indicates thatMartian mantle melts emplacing at shallow crustal levels (i.e. low-pressures) may reach sulphide saturation within 30 wt.% silicate frac-tionation, predominately during orthopyroxene/clinopyroxene ratherthan olivine crystallisation (Fig. 3). Careful examinations of the sulphidemineralogy in the Martian meteorites (e.g. Burgess et al., 1989; Gattacaet al., 2013; Lorand et al., 2005, 2012) offer an independent insight intothe potential characteristics of magmatic sulphide mineral systems onMars. Disseminated sulphides in the olivine-phyric Shergottites usuallyoccur in conjunctionwith primarymagmatic pyroxenes. This co-geneticrelationship is consistentwith the S-SCSS analysis on fractionatingMar-tian mantle melts carried out in this study (Figs. 3 and 4).

The metal tenor of a segregating sulphide liquid is strongly depen-dent on the metal concentration and the fractionation history of itsparental silicate melt. For instance, by assuming ~200 ppm Ni to berepresentative for Martian mantle melts (Table 1), and for an olivine/melt Ni distribution coefficient (DNi) of 5 (Filiberto et al., 2009), themetal concentration may drop to ~50 ppm within 30 wt.% of olivinefractionation. This is consistent with the pyrrhotite dominated sul-phide mineralogy observed in the Martian meteorites, whereaspentlandite is virtually absent (Lorand et al., 2005, 2012). Similarly toEarth, mineralising processes in Martian mafic to ultramafic magmasmost probably require the attainment of sulphide supersaturation andbatch segregation of sulphides during the very early stages of magmafractionation (preferentially within 10 wt.% silicate crystallisation), toprevent the substantial sequestration of part of the metals (preferen-tially Ni and the IPGEs) by fractionating silicates and oxides. We exam-ine below the various processes that may facilitate the attainment ofsulphide saturation in Martian mantle melts.

6.2. Potential trigger for sulphide supersaturation in Martian magmas

6.2.1. Addition of volatilesThe assimilation of crustal volatiles is a potential mechanism by

which terrestrial mafic to ultramafic magmas may reach sulphidesupersaturation and batch segregation of metal-rich sulphides to forma mineralisation. The addition of H2O has a small negative effect onthe sulphur solubility in mafic melts. Laboratory experiments carried

out by Liu et al. (2007) imply that the addition of high amounts ofwater (up to ~10 wt.% H2O) may be necessary to lower the SCSS of amafic magma by a factor of 1/2. Martian regolith in several low-thermal inertia and high-albedo equatorial highland regions may con-tain up to 10wt.% pore ice (Rapp, 2006; and references therein). Highlyvesicular and volatile-rich magmatic rocks, being indicative of signifi-cant interaction between magma and water/ice, have been discoveredat the Gusev Crater landing site (Schmidt et al., 2008). Notwithstandingthis, Martianmagmaswould need to assimilate and dissolve exceeding-ly high amounts of ice-rich regolith to substantially lower their SCSS.This process is inhibited by the low solubility of water in basalticmelts at low pressures (e.g. Berndt et al., 2002).

6.2.2. Addition of silica-rich crustal rocksThe assimilation of crustal silica significantly lowers the SCSS of

silicatemagmas and is recognised to be a trigger for the formation of sev-eral orthomagmatic Ni–Cu±(PGE) sulphidemineralisation on Earth (e.g.Irvine, 1975; Li and Naldrett, 1993; Li et al., 2002; Seat et al., 2009). TheMartian crust has long been viewed as dominated by mafic rocks.Recent discoveries, however, altered this view. Crustal materialsthat spectrally resemble dacites to quartzofeldspathic rocks havebeen identified by TES and THEMIS global spectral surveys (e.g.Bandfield et al., 2004). The occurrence of anorthositic rocks (e.g.Carter and Poulet, 2013; Wray et al., 2013), as well as silica-rich (sed-imentary) deposits (e.g. Skok et al., 2010; Squyres et al., 2008), has beendetected by CRISM spectral analysis. Fragments of feldspar-rich litholo-gies (potentially anorthosites) have been documented using laserinduced breakdown spectroscopy (LIBS) onboard MSL Curiosity at theGale Crater landing site (Sautter et al., 2014). The widespread occur-rence of silica-rich rocks may be also inferred from monzonitic clastsin the regolith breccias NWA 7034, NWA 7533 and paired samples(Humayun et al., 2013). Finally, comparison of the density of Martianbasalts with geophysical constrains on the density of the Martiancrust suggests the existence of a buried felsic crustal component(Baratoux et al., 2014). Given these large uncertainties, it is uncon-strained but possible that Martian mantle melts significantlyinteracted with crustal silica-rich reservoirs to reach sulphidesupersaturation.

6.2.3. Magma mixingMineralising events in the terrestrial magmatic Ni–Cu–PGE–S

system also include the mixing of compositionally contrasting magmasin large-scale magmatic intrusions (e.g. mineralisation associated withthe Bushveld Igneous Complex; Naldrett and von Gruenwaldt, 1989),where progressively fractionating magma bodies are repeatedlyinjected and contaminated with pulses of fresh (i.e. more primitive)magma. This process, however, primarily relies on the slow coolingand fractionatinghistory of thepre-emplacedmagmabody. This scenar-io is unlikely to be achieved in surficial Martian lava flows, which mayflow far from the source vent due to their low viscosity (e.g. Baratouxet al., 2009; Chevrel et al., 2014; Jaeger et al., 2010), butmay be a poten-tial mechanism in deep-seated Martian magmatic intrusions andplutons such as those discovered in the northern plains (Michautet al., 2013).

6.2.4. Addition of sulphurThe most efficient way to reach sulphide saturation and sulphide

liquid segregation in a sulphide-undersaturated silicatemelt is the addi-tion of sulphur in amounts that exceed the capacity of the magma todissolve it (e.g. Lightfoot et al., 1994; Mainwaring and Naldrett, 1977;Naldrett, 1999, 2010; Ripley and Li, 2013). The Martian crust isrecognised to be extremely rich in sulphur (Table 1), with abundancesestimated to be typically 2 orders of magnitude higher than those of ig-neous and sedimentary rocks on Earth (e.g. Gaillard and Scaillet, 2009;Gaillard et al., 2013). Hence, the assimilation of crustal sulphur appearsto be the most promising and feasible mechanism to promote sulphide

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supersaturation in Martian magmas. The sulphur inventory of variouscrustal reservoirs, as well as processes potentially leading to the con-tamination of Martian magmas by crustal sulphur, is discussed in detailin the further chapters.

6.3. Crustal sulphur-rich reservoirs

Sulphur-bearing lithologies on the Martian surface occur as eithersulphide-bearing volcanic rocks or sulphate-dominated sediments,such as individual layered sulphate deposits, regolith and dust. Layeredsulphate deposits may be subdivided into layered sedimentary plains(e.g. Poulet et al., 2008) and crater (e.g. Thomson et al., 2011) as wellas canyon (e.g. Chapman and Tanaka, 2001) fillings with thicknessesof up to several hundred metres. These layered sulphate deposits,which have been mainly detected by orbital Thermal Emission Spec-troscopy onboard Mars Global Surveyor (TES), Mars Odyssey Orbiter(THEMIS) and Mars Express (OMEGA), contain up to ~8–12 wt.% sul-phur (Table 1), which corresponds to ~10-20 wt.% sulphate. The Mar-tian regolith predominately occurs as a mixture of sulphur-rich dust(i.e. soil) and lithic fragments (i.e. ejecta), which formed by impact-related fragmentation and brecciation (impact gardening) of the pre-dominately magmatic Martian crust (Hartmann and Neukum, 2001).Sulphur concentrations in Martian regoliths have been determined atlanding sites (i.e. Viking 1 and 2, Clark, 1993; Mars Pathfinder, Bellet al., 2000; Mars Exploration Rovers Spirit and Opportunity, Riederet al., 2004, Gellert et al., 2006; Ming et al., 2008; Mars Science Labora-tory Curiosity, Schmidt et al., 2014; Fig. 1). These data sets imply thatthe Martian regoliths are uniformly enriched in sulphur, with concen-trations that range between 0.4 and 4.3 wt.% S (Table 1).

The sulphate-rich nature of theMartian global regolith as well as thewidespread occurrence of sulphate-rich deposits is recognised to be theconsequence of a) sedimentary sequestration of volcanic-exhalativesulphur (Bibring et al., 2006) and b) weathering of sulphide-bearingmafic to ultramafic rocks in an oxidised atmosphere (e.g. Dehoucket al., 2012; King and McSween, 2005). Layered sulphate deposits arepredominately associated with Late Noachian to Hesperian crustalterrains, providing evidence for widespread igneous activity and sul-phur exhalation during these eons (Bibring et al., 2006). Regolith thick-ness primarily depends on the relative impact crater density(cumulative impact gardening) and, in turn, the surface age of certainMartian crustal terrains (Hartmann and Neukum, 2001; Warner et al.,2014). Stratigraphic units exposed since the early Noachian are estimat-ed to be mantled by thick (tens to hundreds of metres) regolith layers,whereas Hesperian to Early Amazonian regoliths presumably have athickness of less than ~20 metres (e.g. Hartmann and Neukum, 2001;Warner et al., 2014). Finally, the youngest units on the Martian surface(i.e. Amazonian) are assumed to be essentially free of any regolith cover(Hartmann and Neukum, 2001).

6.4. Erosion and assimilation of sulphur-rich lithologies by Martian lavas

Regional volcanic flow features on the Martian surface includesimple sheet flows, forming smooth lava plains (Fig. 5a) sharing typicalcharacteristics with mare-type wrinkled ridges, and multiple complexlobed sheet flows (Fig. 5b) composed of periodically emplaced singlelava flows (Greeley and Spudis, 1981; Grott et al., 2013). Numerous in-dividual effusive flows appear to be structurally controlled formingchannels and partially inflated lava tubes. These structurally controlledoutflows mainly occur on the flanks and peripheries of large shield vol-canoes (Williams et al., 2005; Wilson and Mouginis-Mark, 2003), andare associated with large fissure-fed flood lava provinces, such asthose in the Cerberus Fossae plains (Fig. 5c), which partially inciseinto older Martian crust (e.g. Hurowitz et al., 2010).

Despite the presence of the rather short-lived outflow channels inthe vicinity of Martian volcanic provinces, most Martian crustal terrainsare dominated by sinuous/anastamosing outflow channels (e.g. Kasei

Vallis; Fig. 5d), which extend for distances of up to thousands ofkilometres. Although some exceptions exist, these channel systemsusually donot appear in close spatial conjunctionwith effusive volcanism.They have been mostly interpreted as the product of erosion byflowing water due to catastrophic aqueous outbursts from the sub-surface (e.g. Baker et al., 1991; Clifford and Parker, 2001; Milton,1973). However, several studies (e.g. Leverington and Maxwell,2004; Leverington, 2007, 2009, 2011; Hopper and Leverington,2013; Dundas and Keszthelyi, 2014) questioned this hypothesisbased onmorphological, geochemical andmineralogical considerations,and concluded that erosion due to the turbulent emplacement of low-viscosityMartian lava appears to be an intriguing alternative hypothesisfor the formation of these channel systems.

The formation of deeply incised lava channels as a product of ther-mal/mechanical erosion by lava flowing in the turbulent regime is acommon feature within terrestrial volcanic provinces (e.g. Griffiths,2000; Huppert and Sparks, 1985; Williams et al., 1998, 2001), andmay also be the best explanation for the widespread occurrence ofsinuous rilles on planetary bodies other than Earth and Mars, such asthe Moon (e.g. Hulme, 1973; Williams et al., 2000). The capability oflava to thermally/mechanically erode crustal substrate primarily relieson: 1) its viscosity, 2) the dynamic state of the flow (i.e. laminar versusturbulent), which is dependent on lava viscosity and the flow geometry(i.e. flow depth and ground slope), and 3) the hardness and erosionresistance (i.e. erodibility) of the substrate (e.g. Huppert and Sparks,1985; Greeley et al., 1998; Kerr, 2009; Williams et al., 1998, 2001). Forinstance, analytical/numerical modelling of the low-viscosity (b10 Pa·s)as well as high-temperature (up to ~1600 °C) komatiite magmasat Kambalda, Australia, implies that they potentially eroded up to~1 m/day of basalt substrate and up ~23 m/day of weakly consolidatedsedimentary substrate (e.g. sulphide-bearing sedimentary rocks;Williams et al., 1998).

By applying similar lava erosion modelling techniques to Mars, ithas been shown that channelised low viscosity lava flows on theMartian surface may be able to erode up to ~2 m/day of basalt sub-strate (Williams et al., 2005). Potential erosion rates in case of lavaoverflowing weakly consolidated material, such as Martian regolithand/or the sedimentary sulphate deposits of the Late Noachian andHesperian, are expected to be comparable with values calculatedfor the lavas at Kambalda, Australia. Given the potentially high erod-ibility of Martian sulphur/sulphate rich regoliths and sedimentarydeposits, their assimilation by flowing lava appears to be the mostpromising mechanism to drive sulphide supersaturation and batchsegregation of sulphide liquids to form mineralised environmentson Mars (Fig. 6).

7. Summary and conclusions

This study evaluates whether the igneous activity, which affected,reshaped and buried the primaryMartian crust,may have led to the for-mation of orthomagmatic Ni–Cu±(PGE) sulphide mineralisation, simi-lar to that associated with terrestrial komatiites and ferropicrites. Indoing so, focuswas laid on the criticalmagmatic processes of theminer-al system for Ni–Cu±(PGE) sulphides in mafic to ultramafic systems,namely potential metal and sulphur fertility of the Martian mantleand partial mantle melts, and the physical/chemical processes that en-able sulphide supersaturation and segregation of potentially metal-rich sulphide liquids.

The main outcomes are:

• Sulphur concentrations in the primitive Martian mantle (i.e. between700 and 2200 ppm S; Table 1) are estimated to be substantially higherthan in the terrestrial one (i.e. 250 ppm S; Table 1). The mantles ofboth planets contain sub-chondritic but significant PGE abundances,whereas Ni and Cu concentrations may be substantially lower in theMartian mantle (i.e. 400 ppm Ni and 6 ppm Cu; Table 1) than in the

Fig. 5. a) Simple lava flow in an area north west of Alba Mons (CTX images: P18_007998_2268_XN_46N120W; D05_029030_2292_XN_49N121W; P20_008789_2266_XN_46N121W).b) Lava flow field composed of multiple lobed flows south of Arsia Mons (CTX images: P11_005177_1606_XN_19S127W; G18_025325_1607_XN_19S127W; P07_003621_1585_XN_21S126W). c) Lava channel system in the Elysium Plains which potentially formed by low viscosity lava (HiRISE image: ESP_016361_1870). d) Mola shaded reliefimage of Kasei Valles and its surroundings.

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terrestrial mantle (i.e. 1960 ppm Ni and 30 ppm Cu; Table 1).• Martian mantle melts transport significant amounts of Ni(160–240 ppm Ni; Table 1, Fig. 2) and PGE (NN1 ppb PGEtot) intothe crust. However, the inferred Ni concentration in Martian mantlemelts are relatively low if compared with those parental to terrestrialkomatiites and ferropicrites (Fig. 2). Given the compatibility of Ni andpart of the PGEs in crystallising silicates and oxides in sulphide-undersaturated systems, mineralising processes in Martian mantlemelts, similar to their terrestrial equivalents, most likely requirethe early (b10 wt.% crystal fractionation) attainment of sulphidesaturation and sulphide liquid segregation.

• Terrestrial ferropicrites and Martian mantle melts that emplace in acrustal low-pressure environment potentially have a high gap be-tween their inferred initial sulphur concentration and SCSS (Fig. 4).Simulations on the S and SCSS during crystal fractionation implythat the Martian melts potentially reach sulphide saturation within

30 wt.% crystal fractionation. This value is similar to that calculatedfor the melt parental to the mineralised ferropicrites at Pechenga,Russia (i.e. ~25 wt.% silicate fractionation).

• Polymetallic sulphide mineralisation associated with terrestrialkomatiites and ferropicrites predominately formed due to batch seg-regation of metal-rich sulphide liquids triggered by thermal and/ormechanical erosion and assimilation of sulphur/sulphate-rich countryrocks. Quantitative models on the erosivity of both terrestrialkomatiites and channelled Martian lava flows imply erosion poten-tials of the same order of magnitude when emplaced over weaklyconsolidated substrate. Given the widespread occurrence of sulphur/sulphate-rich soil, regolith and individual sulphate deposits on Mars,it is concluded that Martian lava flows that incised into thesesulphur-rich lithologies (Fig. 6) potentially assimilated substantialamounts of crustal sulphur to reach sulphide supersaturation.

• Due to the dependency of the Martian regolith thickness on the

Fig. 6. Schematic cross section through a Martian lava channel that incises into regolith covered crustal terrain (figure not drawn to scale). See Table 1 and text for discussion regardingregolith composition and thickness.

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relative impact crater density and, in turn, the age of certain Martiancrustal terrains, it is argued that relatively young (i.e. Late Hesperianto Amazonian) channelled Martian lava flows that incised into rela-tively older (i.e. Noachian to Hesperian) sulphur-rich Martian crust,may be the most promising candidates to have formed Ni–Cu±(PGE) sulphide mineralisation.

It is concluded thatMartianprimitivemantlemelts,which potential-ly emplace and incise into crustal sulphur/sulphate-rich material aschannelled lavaflows, have a capacity to reach sulphide supersaturationand batch segregation of metal-rich immiscible sulphide liquids that iscomparable to that for terrestrial komatiites and ferropicrites in the Ar-chean to Proterozoic eons. However, whereas PGE concentrations inMartian and terrestrial primitive mantle melts may be at comparablelevels, the relatively low Ni and Cu abundances in the Martian mantleand derivedmelts imply that sulphide liquids segregating fromMartianmelts may have relatively lower Ni and Cu tenors.

Acknowledgements

R. Baumgartner acknowledges the receipt of an UWA InternationalPostgraduate Research Scholarship (IPRS). The research was financiallysupported by ARC Centre of Excellence for Core to Crust Fluid Systems(CCFS), Matrix Exploration Pty Ltd., and the French “ProgrammeNational de Planétologie” (INSU). M.L. Fiorentini also acknowledgessupport from the Australian Research Council through Linkage ProjectLP120100668 and the Future Fellowship Scheme (FT110100241). Ananonymous reviewer is acknowledged for constructive comments,which improved the quality and readability of the article. Special thanksare due to F. Pirajno for his review and editorial handling. This is contri-bution 508 from theARCCentre of Excellence for Core to Crust Fluid Sys-tems (CCFS).

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