The occurrence of Fe-Ni metals in Fe sulfides is consistent with the large number of metallic Fe particles in planar defects that give the black color of NWA 2737 olivine (Beck et al. 2006). Metal phases in NWA 2737 are obvious products from the reduction reactions that took place during shock events; however, the mechanisms in silicates and sulfides were probably different.
Opaque trace phases in NWA 2737 markedly differ from what is currently reported in Martian meteorites, i.e., metal-deficient pyrrhotite (Fe1−xS; 0.05 < x < 0.13) associated with trace amounts of pentlandite, Cu-sulfides (chalcopyrite, cubanite) and iron disulfides (pyrite or marcassite; Lorand et al. 2005; Chevrier et al. 2011). Pyrite or marcasite is especially abundant in the Chassigny meteorite where single-phase pyrite blebs were reported (e.g., Floran et al. 1978; Rochette et al. 2001; Greenwood et al. 2000) in addition to metal-deficient pyrrhotites (Lorand, and Chevrier, in prep.). By contrast, no pyrite has been identified in NWA 2737.
Os-Ir alloys, Fe-Ni alloys, and troilitic pyrrhotites are essential minerals in carbonaceous chondrites (e.g., Brearley and Jones 1997; Kimura et al. 2011); however, these minerals occupy very distinct textural sites compared with NWA 2737, namely as complex refractory metal nuggets (RMN = Os-Ir-Ru-Rh-Pt-Re-Mo) enclosed in calcium-aluminum-rich inclusions for Os-Ir alloys, Fe-Ni metals of various origin (condensation, reduction of ferro-magnesian silicates, inside and outside chondrules or late-metamorphic minerals) and matrix component for the troilite, mostly replaced by poorly characterized sulfide phases during aqueous alterations on parent bodies. Ni metal particles in carbonaceous chondrites have various origins, including secondary product of heating in the parent body after aqueous alteration (Kimura et al. 2008) or pentlandite alteration product after impact-induced heating (e.g., Kimura and Ikeda 1992). In NWA 2737, both metal phases (i.e., taenite or tetrataenite and Fe-Os-Ir alloys) occur inside troilite and their origin is closely related to this sulfide. No replacive relationship between pentlandite and Fe-Ni metals has been observed.
Evidence of Sulfur Devolatilization Affecting Magmatic Sulfides in NWA 2737
Two lines of evidence support a magmatic origin for NWA 2737 sulfides: (1) their shape of immiscible sulfide melts—from nearly spherical droplets to polyhedral grains with convex-inward margins, (2) their pyrrhotite-rich assemblages. As documented in other Martian meteorites, sulfide liquid separated at a late-magmatic stage as suggested by (1) the overabundant intergranular sulfide blebs at boundaries between augite and olivine and sulfide pocket at triple junction of olivine crystals (now sealed by Ca carbonates), (2) the lack of solitary sulfide inclusions in cumulus silicates (olivine, chromite), (3) NWA 2737 troilite shows quite homogeneous but low Ni contents (1.0 ± 0.5) suggesting equilibration with evolved silicate melts.
It has long been shown that pyrrhotite Fe/S ratios correlate negatively with oxygen fugacity if sulfides occur as accessory minerals, acting as passive indicator of redox conditions (Barton 1970; Eggler and Lorand 1993; Lorand et al. 2005). Nonstoichiometry in pyrrhotites is caused by incorporation of trivalent iron that generates vacancies and decreases the M/S ratio (Vaughan and Craig 1978). Thus, the uniformly high (stoichiometric) M/S ratio of NWA 2737 pyrrhotites is at odds with the oxidizing crystallization conditions that are postulated for Chassigny (FMQ-1 log unit; Delaney and Dyar 2001; Treiman et al. 2007). Based on change in the vanadium valence with oxygen fugacity, Beck et al. (2006) even estimated a higher magmatic fO2 in NWA 2737 than in Chassigny meteorite (FMQ + 0.3 log unit). As shown by the log fS2-T diagram of Fig. 5, the FMQ-pyrrhotite buffer (FMQ-Po) imposes pyrrhotite compositions with M/S ≤ 0.9 (as observed for instance in nahklites, Chevrier et al. 2011) eventually crystallizing pyrite on cooling below 200 °C. By contrast, troilite can coexist stably with Fe-Ni alloys over a very narrow window close to the Fe-FeS curve, well below the conditions defined by FMQ-Po. By reducing oxygen fugacity via volatilization of oxygen, shock processes in NWA 2737 may have contributed to reduction of metal-deficient pyrrhotite compositions into troilite, while destroying the pyrite-like phases.
Decreasing oxygen fugacity at the whole-rock scale may account for the very homogeneous troilite compositions analyzed within prophyritic inclusions that are expected to have been closed systems during the shock event(s). However, this decrease was coeval with shock-induced sulfur vaporization. Compared with the regularly faceted enclosed sulfides, sulfide blebs that occur interstitially with the main silicates display highly denticulated shapes showing signs of volume loss (now filled up with Ca-carbonates). Their spongy textures may result from bubbling of S vapor. Fe-Ni alloys formed in situ, by volume diffusion of Ni-Fe, where S preferentially escaped. This is the reason why NWA 2737 Ni-Fe alloys are tiny grains, sometimes enclosed, sometimes marginal with respect to troilite, never forming rims around troilite as commonly reported for Ni3Fe awaruite-replacing BMS phases in serpentinized terrestrial peridotites that experienced low-temperature (<200 °C) desulfurization reactions (e.g., Lorand 1985). The highly heterogeneous distribution of metals between troilite blebs is a straightforward consequence of devolatilization which is by definition a very heterogeneous process depending on the diffusion rate of S through the pyrrhotite lattice, pressure twins, defects, temperature, pressure, etc.
There are several lines of evidence suggesting that sulfur may have started to escape at high temperature, consistent with the temperature range postulated for the first shock event (400–500 °C). Copper seems to have been vaporized from BMS grains, resulting in very low Cu content (no Cu sulfide and scarcely detected Cu concentrations in troilite). The total removal of pyrite from NWA2737 can be related to its incongruent melting to pyrrhotite + sulfur that occurs above 740 °C at P = 1 bar (Kullerud et al. 1969). Shock-related heating to this temperature should have also resulted in the breakdown of any pre-existing pentlandite which decomposes above 610 °C (Kullerud et al. 1969). Kimura et al. (2011) recently suggested that tiny Ni-rich (51–69 wt% Ni) Fe-Ni-Co metal phases associated with troilite in some CM chondrites are pentlandite decomposition product formed by impact-induced heating. Ordered tetrataenite (gamma ordered fcc, equiatomic FeNi) is stable below 320 °C covering a very narrow compositional range (Ni/Fe = 0.5 ± 0.02); however, it undergoes phase transformation above 320 °C (Yang et al. 1997). In ordinary chondritic meteorites, tetrataenite formed from homogeneous low-Ni paramagnetic fcc taenite phase by metamorphism and very slow cooling (1–100 °C Myr−1, Clarke and Scott 1980), i.e., 5–7 orders of magnitude slower than that experienced by NWA2737 postshock assemblages on the parent body, giving rise to complex exsolution patterns in the metal phase (e.g., Yang et al. 1997; Uehara et al. 2011). Our SEM study did not revealed exsolution microtexture inside NWA 2737 alloy grains; the latter exsolved from the troilite, perhaps from former pentlandite.
Evaluation of crystallization temperatures for NWA Fe-Ni alloys can be only semiquantitative, as no crystallographic parameters could be obtained from such tiny grains: synthetic taenite coexisting with troilite above 400 °C display less than approximately 45% Ni at 725 °C (Karup-Møller and Makovicky 1995), and no more than 30% at 900 °C, as shown by quenched immiscible sulfide blebs in highly reduced terrestrial basalts from the Disko island (Greenland) (Pedersen 1979; see also Kullerud et al. 1969). Compared with the aforementioned compositions, NWA 2737 FeNi alloys are more Ni-rich, a feature that reflects either small scale equilibrium or re-equilibration of the Fe-Ni partitioning between alloy and troilite to lower temperature. It is notable that Ni, a minor element in the troilite diffused into the alloy creating a Ni-poor dark halo around some of the coarsest grains. Then, the Fe-Ni alloys may have served as nuclei for pentlandite growth. Newly formed pentlandite flames are shaped in a form expected from homogeneous nucleation process in the typology of pentlandite exsolution (Etschmann et al. 2004); according to that typology, NWA 2737 pentlandite typically exsolved over a temperature range of 200–300 °C. It is worth noting that the tie-lines linking NWA troilite composition with taenite in the Fe-Ni-S system are not consistent with those calculated at 25 °C (Misra and Fleet 1973), nor with mineral pairs occurring in terrestrial ultramafic rocks that display troilite + awaruite compositions (Ni2Fe-Ni3Fe; cf. Lorand 1985, 1988). The diffusion of Ni between troilite and taenite was blocked at higher temperature, thus leaving residual Ni in NWA troilite slightly higher than expected at 25 °C. Nickel and Fe diffuse in pyrrhotite structures at about 10−9 cm2 s−1 at 480 °C for Fe in Po and 720 °C in NiS (Ewers 1972; MacQueen 1979). Metal ions move between hexagonal close packed S2- ions due to the defective metal sublattice. In nondefective metal sublattice, diffusion coefficients are seriously decreased; likewise, shock pressure has a blocking effect on Fe-Ni diffusion by decreasing the S-S interatomic distance. Both parameters may have had blocking effects on the Ni-Fe partitioning between troilite and pentlandite and between troilite and alloys.
Furthermore, evidence of high temperature diffusion in NWA 2737 troilite is the occurrence of detectable amounts of chromium in the troilite grains in contact with chrome spinel. For the oxygen fugacity of the FMQ-Po buffer, chromium mainly occurs as trivalent chromium (Murck and Campbell 1986; Mallmann and O’Neill 2009) which preferentially partitions into octahedral sites of Cr-spinel. Its chalcophile behavior, recorded by NWA 2737 troilite, suggests that Cr was able to entering the octahedral sites of troilite; however, trivalent Cr produces a charge excess which must be balanced by an increasing number of vacancies in the pyrrhotite sublattice as follows:
Reaction 1 may also accommodate lattice deformation resulting from the smallest size of trivalent chromium compared with Fe and Ni. However, no correlation between Cr contents and metal deficiency has been noted in NWA 2737, which makes the hypothesis of divalent chromium replacing divalent Fe on a one for one basis more likely. Divalent chromium is also of interest for any discussion of redox reactions involved in NWA 2737 petrogenesis. Divalent chromium becomes abundant in reduced melts for oxygen fugacity close to the buffer iron-wustite (IW; Roeder and Reynolds 1991). Enstatite chondrites, the most reduced natural mineral assemblages known in the solar system (equilibrated at IW-5 to IW-7 log units), contain Cr-bearing troilite showing up to 3.0 wt% Cr (Ikeda et al. 1997). Thus, NWA 2737 troilite Cr-bearing compositions are consistent with the dramatic decrease in oxygen fugacity conditions resulting from impact-related devolatilization, coupled with high-temperature heating allowing Cr to diffuse from Cr-spinel to adjacent sulfides. Unfortunately, no diffusion coefficients are available for chromium in pyrrhotite structures.
Origin of Os-Ir Alloys
Like Fe-Ni alloys, the two Fe-Os-Ir alloys detected with the SEM obviously formed from the Fe-Ni sulfide assemblage; however, the fact that these alloys are systematically offset from the troilite mass raise some question about their origin: whether they formed as troilite exsolution products or from former Os-Ir-richer minerals is presently unclear, yet several lines of evidence argue for the former interpretation. Os-Ir alloys are among the most refractory metals known in the solar system, their crystallization temperature ranging between 2500 and 3060 °C (Bird and Bassett 1980). Ru-poor Os-Ir alloys have been reported along with highly reduced minerals (SiC, Fe silicide, carbides, Fe) in podiform chromitites (Bai et al. 2000); thus, their occurrence in NWA 2737 is consistent with the overall reducing process this meteorite has experienced. Both Os and Ir are strongly chalcophile and can be incorporated into the octahedral sites of pyrrhotites at wt percent concentration levels at magmatic (950 °C) temperatures (Bullanova et al. 1996; Alard et al. 2000; Lorand and Alard 2001). Trivalent Os and Ir enter the pyrrhotite if octahedral metal vacancies are abundant, thus deforming the pyrrhotite sublattice (Li et al. 1996; Barnes et al. 2001). At a given temperature, the solubility of Ir and Os in pyrrhotite falls exponentially with decreasing fS2, reaching part-per-million concentration levels for troilite compositions (Lorand and Alard 2001; Peregoedova et al. 2004); Peregoedova et al. (2004) found Ir-rich mineral exsolution in pyrrhotite-like phase containing only 10 ppm Ir. Accordingly, by reducing the fugacity of sulfur and eliminating octahedral vacancies of pre-existing monoclinic-type pyrrhotite, the shock-induced S devolatilization may have been capable of triggering exsolution of NWA 2737 Fe-Os-Ir alloys.
Platinum-group elements are ultratrace elements in Martian meteorites, present at part-per-billion concentrations (Jones et al.  and reference therein). For NWA 2737, a mass balance calculation of the Os and Ir content of BMS can be attempted by assuming similar bulk-rock Os and Ir contents in NWA 2737 and Chassigny (1.8 ppb Os and 1.2 ppb Ir; Jones et al. 2003) and both elements exclusively partitioned into the Fe-Ni sulfide phases. NWA 2737 troilite would not host more than approximately 10–20 ± 10 ppm of both Os and Ir. Taken at face values, these calculated concentrations fall within the low-T range of terrestrial metal-rich pyrrhotite measured by laser-ablation ICPMS (Lorand and Alard 2001), and consistent with experimentally determined Ir-saturation levels for troilite. Unfortunately, these solubility data cannot be refined by in situ analyses, as NWA 2737 troilite grains are too small to be analyzed with LA-ICPMS techniques. One may also surmise that Os-Ir alloys nucleated from previous pentlandite flames or because the troilite was locally Os-Ir oversaturated, due to slower diffusion rate of Os and Ir compared with the volatilization rate of S. Brenan et al. (2000) reported a self-diffusion coefficient of Os in pyrrhotite of similar order of magnitude as those of Fe and Ni that also formed alloys inside troilite. The formation of Fe-Os-Ir alloys was coeval with vaporization of host troilite that displaced the alloy grains away from the sulfide. The fact that only two Fe-Os-Ir grains were detected, both at a submicrometric scale, is consistent with a local (not widespread) nucleation mechanism. The Ru-poor compositions of the Fe-Os-Ir alloys is consistent with the higher preference of Ru for Fe-Ni sulfides (Fleet and Stone 1991; see also Li et al. 1996). As shown in Fig. 5, Os-Ir alloys can coexist with Ru-bearing sulfides over a broad range of sulfur fugacity because the Ru sulfidation curve is located several log units below that of Os-Ir.