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Northwest Africa (NWA) 6583 is a silicate-bearing iron meteorite with Ni = 18 wt%. The oxygen isotope composition of the silicates (∆′17O = −0.439 ‰) indicates a genetic link with the IAB-complex. Other chemical, mineralogical, and textural features of NWA 6583 are consistent with classification as a new member of the IAB-complex. However, some unique features, e.g., the low Au content (1.13 μg g−1) and the extremely reducing conditions of formation (approximately −3.5 ∆IW), distinguish NWA 6583 from the known IAB-complex irons and extend the properties of this group of meteorites. The chemical and textural features of NWA 6583 can be ascribed to a genesis by impact melting on a parent body of chondritic composition. This model is also consistent with one of the most recent models for the genesis of the IAB-complex. Northwest Africa 6583 provides a further example of the wide lithological and mineralogical variety that impact melting could produce on the surface of a single asteroid, especially if characterized by an important compositional heterogeneity in space and time like a regolith.
Silicate-bearing iron meteorites carry important information about both nebular and asteroidal processes. They are extremely diverse, particularly in terms of texture and chemical composition of the (Fe,Ni) metal, as well as in terms of texture and mineral composition of the silicate fraction. The possibility of measuring the oxygen isotope composition of the silicates in silicate-bearing irons has established a genetic relationship between several iron meteorite groups and known stony meteorite groups. A possible genetic link has been invoked between IIE irons and H chondrites and between IAB-complex irons and winonaites (Clayton and Mayeda 1996; Folco et al. 2004).
Models elaborated to explain the occurrence of silicate minerals (most commonly olivine, pyroxene, plagioclase, and silica polymorphs) within an (Fe,Ni) metal matrix are extremely complex and may involve impact melting on the surface of their parent asteroids, incomplete metal-silicate fractionation during core formation, oxidation/reduction processes, mixing of silicate and metallic melts, complete destruction and reassembling of asteroidal bodies, etc. (e.g., Mittlefehldt et al. 1998 and the references therein, for a review).
In this paper, we report on the structure, chemistry, mineralogy, and oxygen isotope composition of a unique, silicate-bearing iron meteorite named Northwest Africa 6583 (Meteoritical Bulletin no. 100, unpublished data). Based on the collected data, we establish its relationship with a known meteorite group, we define its petrogenesis (including its crystallization history and oxygen fugacity), and discuss the implications for the parent body of the IAB-complex.
Samples and Analytical Methods
Northwest Africa 6583 meteorite (hereafter NWA 6583) was purchased from a Moroccan dealer in October 2010 by Mr. Mirko Graul (Bernau, Germany). Before sampling, NWA 6583 was a single 1825 g iron meteorite with an irregular flattened shape, measuring 105 × 100 × 38 mm (Fig. 1). The upper portion of the meteorite is glossy-brown and presents approximately 40 rounded depressions from 0.5 to 1 cm in diameter (Fig. 1a). The portion originally embedded in soil (approximately 40% of the surface) is oxidized and partially covered by thin crusts of light-brown calcareous material (mostly soluble calcium salts) of secondary origin, i.e., caliche or calcrete (Figs. 1b and 1c). We did not observe any remnant of the fusion crust.
A 60.5 g etched end cut is on deposit at the Pisa University's Museo di Storia Naturale, while M. Graul holds the main mass. A further 30 g full slice was donated by the owner of the meteorite for further studies. A sample of 580 mg was extracted from the end cut for inductively coupled plasma-mass spectrometry (ICP-MS) analysis. Subsequently, the end cut and the slice were polished for metallographic, scanning electron microscope coupled with an energy-dispersive X-ray fluorescence analysis (SEM-EDX), and electron probe microanalysis (EPMA) studies. The total examined surface was approximately 48 cm2.
Backscattered and secondary electron (BSE, SE) images and semiquantitative chemical analyses were obtained at Pisa University's Dipartimento di Scienze della Terra with a Philips XL30 SEM-EDX, operating at 20 kV. Mineral compositions were determined with a CAMECA SX50 electron microprobe fitted with four wavelength-dispersive spectrometers at the Istituto di Geoscienze e Georisorse (IGG) of the Consiglio Nazionale delle Ricerche (CNR) in Padua. Running conditions were 15 kV accelerating voltage, 20 nA beam current, and 1 μm nominal beam spot. Counting times for the determined elements were 20 s and 10 s at peak and background, respectively. The manufacturer-supplied PAP procedure was employed for raw data reduction. Standards used for instrumental calibration were minerals (diopside, apatite, and sphalerite), synthetic compounds (MnTiO3, Cr2O3), and pure elements (Fe, Ni, Co, and Cu) for metal, sulfide, and phosphide analysis and minerals (diopside, orthoclase, albite,) and synthetic compounds (Al2O3, MnTiO3, Cr2O3, Fe2O3, NiO) for silicate analysis.
The bulk chemical analysis of the metal of NWA 6583 was performed at the Pisa University's Dipartimento di Scienze della Terra by conventional ICP-MS (Thermo PQII Plus), following the procedure described in D'Orazio and Folco (2003). To maximize analytical accuracy, the sample solutions were measured using the standard additions method instead of an external calibration. At the concentration levels of NWA 6583, the analytical uncertainties (RSD%) ranged from 2% to 5% for all elements but Re, for which RSD% ranges from approximately 10 to 20%.
Oxygen three-isotope analyses of NWA 6583 silicate inclusions were carried out at the Stable Isotopes Laboratory of CEREGE (Centre Européen de Recherche et d'Enseignement de Géosciences de l'Environnement) in Aix-en-Provence. Molecular oxygen was extracted using the IR-laser fluorination technique (Alexandre et al. 2006; Suavet et al. 2010). About 3 mg of pyroxene grains was extracted from the metal matrix to produce two powdered aliquots (labeled Sample A and Sample B), each weighing approximately 1.5 mg. The two aliquots were heated with a 30 W CO2 IR-laser in the presence of BrF5. The released gas was purified through cryogenic nitrogen traps and one KBr trap. Molecular oxygen was passed through a −114 °C slush and trapped in a nitrogen-cooled molecular sieve before being expanded at 100 °C directly to a dual-inlet mass spectrometer (ThermoQuest Finnigan Delta Plus). The isotope compositions are expressed in standard δ-notation, relative to Vienna standard mean ocean water (‰ versus V-SMOW): δ18O = ([18O/16O]sample/[18O/16O]standard − 1) × 1000 and δ17O = ([17O/16O]sample/[17O/16O]standard − 1) × 1000. ∆′17O was calculated as 1000 ln (1 + [δ17O/1000]) − λ1000 ln (1 + [δ18O/1000]) where λ = 0.5247 (Miller 2002). Measured δ18O and δ17O values of the samples were corrected on a daily basis using the quartz laboratory standard Boulangé itself calibrated against the international standard NBS28 (quartz sand) with δ18O = 9.6 ± 0.123 ‰, δ17O = 5.026 ± 0.075 ‰, and ∆′17O = 0 ± 0.026 ‰ (1σ, n = 23). Reproducibility of δ18OBoulangé, δ17OBoulangé, ∆′17OBoulangé analyses (±1σ) is 0.119 ‰, 0.061 ‰, and 0.024 ‰, respectively (n = 63).
Texture and Chemistry of the Main Minerals
Etched sections show that NWA 6583 is devoid of macroscopic Widmanstätten pattern. Northwest Africa 6583 shows, instead, a polycrystalline texture (Fig. 2) consisting of irregular, subequant crystals of (Fe,Ni) ataxitic metal with different orientations and sizes ranging from 0.5 to 22 mm (approximately 75% of crystals have sizes between 1 and 6 mm). Troilite, graphite, and schreibersite are the most abundant accessory minerals and they are easily identifiable with the naked eye (Fig. 2).
The bulk composition of the NWA 6583 (Fe,Ni) metal is characterized by high content of Ni (18.0 wt%) and moderately low Co content (0.43 wt%). The trace element distribution is close to the cosmic abundance. In greater detail, we observe relatively higher concentrations of the moderately volatile, chalcophile, and siderophile incompatible elements Cu, Ge, Ga, Sn, As, and Sb (Table 1), and lower concentrations of the refractory, highly siderophile, and compatible elements Ir, Re, and W, as evidence of a limited trace element fractionation.
Table 1. Inductively coupled plasma-mass spectrometry analysis of bulk metal of Northwest Africa 6583
Ni and Co are in wt%, the other elements are in μg g−1.
The (Fe,Ni) ataxitic crystals have homogeneous Ni-rich composition (Ni = 17.5 ± 0.4 wt%, Co = 0.40 ± 0.04 wt%, n = 100; Table 2). Their microstructure consists of ragged platelets (10–15 μm in size) arranged in octahedral orientation (Fig. 3a). In literature, such microstructure in Ni-rich (Fe,Ni) metal is known as martensitic structure (Buchwald 1975; Goldstein et al. 2009). The martensitic matrix is speckled with numerous small (from 3 to 6 μm wide and up to 200 μm long) spindles of kamacite (Ni = 6.9 ± 0.5 wt%, Co = 0.59 ± 0.05 wt%, n = 11; Table 2). Kamacite spindles are nucleated on tiny schreibersite crystals and are surrounded by a very thin (typically <0.5 μm) rim of taenite (Fig. 3b). In the outer portion (up to approximately 250 μm from the grain boundary) of the largest metal grains, the amount of kamacite spindles decreases. The (Fe,Ni) metal grains are separated from each other by small schreibersite crystals enveloped in kamacite.
Table 2. Electron microprobe average compositions of martensite, kamacite, troilite, and schreibersite of NWA 6583 (units in wt%). Standard deviations (1σ) are reported in parentheses
N. of analyses
The (Fe,Ni) metal of NWA 6583 shows an anomalously high content of Si (martensite, average = 0.13 ± 0.02 wt%, n = 100; kamacite, average = 0.12 ± 0.03 wt%, n = 11; Table 2). These contents are much higher than those commonly observed in iron meteorites, where Si is generally below 35 μg g−1 (0.0035 wt%; Wai and Wasson 1969). Two notable exceptions are Horse Creek and Tucson, which contain 2.5 wt% and 0.8 wt% Si, respectively (Wai and Wasson 1969). High concentrations of Si (from approximately 0.3 to 4 wt%) are typical of the (Fe,Ni) metal of enstatite chondrites and aubrites (Brearley and Jones 1998).
Troilite is the most abundant accessory mineral (approximately 4.5 vol%); it occurs as rounded blebs up to 5 mm in maximum length. The troilite crystals close to the external surface of the meteorite (<2 mm) show lamellar twinnings, melting textures, and are polycrystalline (Fig. 4a). The composition of troilite is homogeneous and characterized by moderately high contents of Cr, Mn, and Ti (0.83 ± 0.07 wt%, 0.15 ± 0.04 wt%, and 0.13 ± 0.02 wt%, respectively, n = 37; Table 2). Troilite crystals often include, preferentially at their margins, small crystals of different species of Mn-Fe-Mg-Zn-Cu-Ni sulfides, Mg-silicates, and rare Fe-Ni phosphide (see next section). Native copper has been occasionally found at the troilite-(Fe,Ni) metal interface.
Graphite is the second most abundant accessory mineral (approximately 1.5 vol%). It usually occurs as subspherical rosettes 50–500 μm in diameter, occasionally reaching 10 mm (Figs. 2, 4b, and 5a,b), and as isolated lamellar crystals up to 0.7 mm in length (Fig. 4c).
Schreibersite (approximately 1 vol%) occurs as three different types: (1) large skeletal crystals up to 8 mm in maximum length, sometimes enveloping troilite crystals; (2) tiny (1–30 μm) crystals at the core of the kamacite spindles (Fig. 3b); (3) slightly larger euhedral crystals at the (Fe,Ni) metal grain interface. Two analyses of the large skeletal crystals show a Ni/Fe atomic ratio of 0.27 (Table 2). Some large schreibersite crystals located close (<2 mm) to the external surface of the meteorite show evidence of melting. This is documented by the occurrence of rims with very fine-grained eutectic textures, which are thicker on the crystal side closer to the external surface of the meteorite.
Terrestrial weathering products are represented by iron hydroxides that are concentrated along the (Fe,Ni) grain boundaries (Fig. 2a), around troilite crystals (Fig. 4d), and on the external surface (Fig. 1).
Accessory Sulfide Inclusions in Troilite
About 40% of troilite crystals contain small inclusions of various sulfide minerals. The most common sulfides belong to the alabandite–niningerite series (Figs. 4d,e, 5, and 6a; Table 3). These crystals are wedge-shaped and are located very close to the troilite crystal boundaries. The variable compositions may be distinguished on the basis of the different reflectivity in SEM-BSE images (the higher the Mg/Mn ratio, the lower the BSE reflectivity). Some crystals are compositionally zoned, showing an irregular decrease of the Mg/Mn ratio from core to rim (Fig. 5c).
Table 3. Representative electron microprobe analyses of Fe-Mn-Mg-Zn sulfides of NWA 6583 (units in wt%)
The concentrations of P, Ti, and Co are below the detection limit of 0.08, 0.03, and 0.06, respectively.
Cation formula (a.p.f.u.) based on 2 atoms
In the FeS-MnS-MgS compositional triangle (Fig. 6a; Keil 2007), the data points form a continuous trend from Mg-free alabandite (Mg = 0.07 wt% and Mn = 52 wt%) to Mn-bearing niningerite (down to Mg = 15.5 wt% and Mn = 19.7 wt%) with less variable Fe content (from Fe = 9.40 wt% to Fe = 19.8 wt%). These sulfides contain very low or negligible Ca (<0.32 wt%) and Cr (<0.37 wt%; Table 3). Niningerite usually has higher Fe, Ca, and Cr contents than alabandite (Table 3).
In association with sulfides of the alabandite–niningerite series, we also found a Fe-Zn-Mn sulfide mineral. This mineral forms 50–200 μm anhedral crystals characterized by (Mn+Fe)/Zn atomic ratio close to 2 (Figs. 5b and 6b). Normalizing the cations to a total of one, the formula can be written as: (Fe0.42Zn0.34Mn0.23)S. Recently, Ma et al. (2012) described a new hexagonal mineral species, buseckite (Fe0.46,Zn0.32,Mn0.16,Mg0.04)S1.01, found within the Zakłodzie ungrouped enstatite-rich achondrite (Fig. 6b). The Fe-Zn-Mn sulfide we found in NWA 6583 could be, in fact, buseckite; however, X-ray diffraction data are needed to confirm this hypothesis.
Two more accessory sulfides have been sporadically found in NWA 6583 in association with troilite: an Fe-Cu sulfide with a Fe/Cu atomic ratio close to 1 (most likely chalcopyrite), and an Fe-Ni sulfide (both analyzed by SEM-EDX only). The first has been found in the external portions of troilite, while the second has been found in the melted rim of a large troilite where it forms the very thin (from 1 to 5 μm) rim of small troilite crystals (Fig. 5d).
Northwest Africa 6583 contains crystals of Mg and Mg-Ca silicates that may be found as isolated euhedral crystal embedded in the (Fe,Ni) metal matrix or included within troilite (Figs. 7a–c), or as clusters of tens of individuals with euhedral to subhedral textures even when they abut the metal matrix. In both the studied slices, the clusters of silicates occur close to the external surface of the meteorite, in correspondence with the bottom of the small depressions (Figs. 1, 2, 4f, and 7d). The size of the silicate crystals ranges from a few tens of micrometers up to 1 mm. Their composition is homogeneous. The most common silicate is enstatite, very close to the Mg2Si2O6 endmember composition (Fs0.44 ± 0.22, Wo1.57 ± 0.15; n = 17; Table 4). Some crystals, especially those forming the clusters, show complex intergrowths with diopside and troilite (Figs. 7b–f and 4f). Diopside never forms isolated crystals. Its composition (Fs1.52 ± 0.75, Wo44.34 ± 5.98; n = 9; Table 4) is close to the CaMgSi2O6 endmember composition, even though it is slightly more iron-rich than the coexisting enstatite. Olivine is much less abundant. The few euhedral crystals found within troilite, or in the metal matrix, are very close to the Mg2SiO4 endmember composition, Fa1.3 (SEM-EDX analyses only).
Table 4. Electron microprobe average compositions of pyroxenes of NWA 6583 (units in wt%). Standard deviations (1σ) are reported in parentheses
No. of analyses
Two aliquots of pyroxene extracted from NWA 6583 were analyzed to obtain the oxygen isotope composition, yielding an average δ17O = 2.059 ‰, δ18O = 4.767 ‰, ∆′17O = −0.439 ‰ (Table 5).
Table 5. Oxygen isotope composition of two subsamples of pyroxene crystals separated from NWA 6583
1 σ ‰
1 σ ‰
1 σ ‰
The precision (1σ) was estimated from repeated measurements of an in-house quartz standard (see text).
A New Member of the IAB-Complex
Northwest Africa 6583 was previously classified as an ungrouped iron meteorite (Meteoritical Bulletin no. 100, unpublished data). Our subsequent data on the oxygen isotope composition of the silicate inclusions of NWA 6583 plot below the terrestrial mass fractionation line within the field of the silicate inclusions of the IAB-complex iron meteorites and winonaites (δ17O = 2.059‰, δ18O = 4.767 ‰, and ∆′17O = −0.44 ‰; Fig. 8 and Table 5). Several lines of evidence (e.g., oxygen isotope composition, chemistry, mineralogy) suggest that IAB-complex irons and winonaites are genetically related (Clayton and Mayeda 1996; Benedix et al. 2000), i.e., that they formed in parent bodies within the same oxygen reservoir of the early solar system and, possibly, in the same parent body. The oxygen isotope composition of NWA 6583 thus establishes a strong petrogenetic relationship with the related IAB-complex iron meteorites and winonaites.
Only the 6.1% of the entire iron meteorite population (66 of the 1074 classified irons; Meteoritical Bulletin Database, accessed March 2013) has a Ni content in the 13.5–22.5 wt% range, i.e., a range defined by the NWA 6583 Ni content (18 wt%) ±25%. Most of these meteorites belong to the group IVB (14 irons) and to the IAB-complex (27 irons), while 23 are ungrouped irons and only one belongs to the group IIF (Repeev Khutor; Kracher et al. 1980). Northwest Africa 6583 is very different from IVB irons for its two-to-three orders of magnitude higher Ga, Ge, and Cu contents and for its one-to-two orders of magnitude lower Re, Ir, W, and Mo (Walker et al. 2008; Fig. 9). In turn, NWA 6583 is similar to members of the IAB-complex as well as to some ungrouped irons like Gebel Kamil and Yamato-791076 (Wasson et al. 1989; Wasson and Kallemeyn 2002; D'Orazio et al. 2011). The high Ge/Ga ratio and the low Ni/Ir ratio of Repeev Khutor, IIF iron group (16.6 and 4.8, respectively), exclude any relationship between NWA 6583 (Ge/Ga = 2.4 and Ni/Ir = 94.7) and the IIF irons group.
According to Wasson and Kallemeyn (2002), irons belonging to the large IAB-complex should have Au > 1.3 μg g−1, As > 10 μg g−1, Co > 0.39 wt%, Sb > 0.18 μg g−1, and 0.4 ≤ Ge/Ga ≤ 7. Northwest Africa 6583 matches this classification scheme for all elements but Au (1.13 μg g−1; Table 1). Despite its Au contents, the iron meteorite NWA 5804 (1.04 μg g−1; Meteoritical Bulletin no. 100, unpublished data) has been classified as IAB-ungrouped. Wasson and Kallemeyn (2002) in their work about the IAB-complex reported that, despite their Au contents, Zacatecas 1792, NWA 176, and Bocaiuva could be considered as IAB-related (?). Northwest Africa 6583, NWA 5804, NWA 176, Bocaiuva, and Zacatecas 1792 define a positive trend in the diagram Ni versus Au and it is parallel to the overall trend defined by the IAB-complex irons (Fig. 10). However, the creation of a new low Au IAB-subgroup is premature at this stage, also because the oxygen isotope composition of Bocaiuva and NWA 176 (∆′17O = −4.39 ‰ and ∆′17O = −5.21 ‰, respectively, Malvin et al. 1985; Liu et al. 2001) is very different from the majority of IAB-complex members (NWA 6583 inclusive).
Silicate inclusions in iron meteorites are reported in the IAB, IIE, and IVA iron groups (e.g., Haack and McCoy 2003). The occurrence of silicate inclusions in NWA 6583 is consistent with IAB-complex iron classification. Their overall mineral composition is most similar to the reduced assemblages found in silicate inclusions in IAB-complex irons, in winonaites, enstatite chondrites, and aubrites. However, the whole-rock oxygen isotope compositions of enstatite chondrites and aubrites plot on the terrestrial fractionation line (Δ′17O from 0.15‰ to −0.37‰, Clayton et al. 1984), ruling out a possible link between NWA 6583 with enstatite chondrites (EH and EL) and aubrites (Fig. 8). The mineral compositions of olivine and pyroxene in NWA 6583 (olivine Fa1.3, enstatite Fs0.44 ± 0.22, Wo1.57 ± 0.15, diopside Fs1.52 ± 0.75, Wo44.3 ± 6.0; Table 4) match the compositions of the silicate inclusions of the most reduced IAB-complex irons like Burkhala, Pine River, Kendall County, Elephant Moraine-84300, and of the winonaites Pontlyfni, Queen Elizabeth Range-945335, Yamato-8005, -75300, -74025, -75303, -75261 (Table 6).
Table 6. Main features of NWA 6583 and IAB-complex irons and winonaites containing low Fe pyroxenes and olivine, and Fe-Mn-Mg-Zn sulfides
Northwest Africa 6583 contains Fe-Mn-Mg-Zn sulfides, as frequently observed in the most reduced inclusions of the IAB-complex. However, while in IAB-complex irons Fe-Mn-Mg sulfides occur as almost Mg-free alabandite, in NWA 6583 they give rise to an almost complete alabandite–niningerite solid solution (Fig. 6a). Note that this is the first report of such an extended solid solution in the alabandite–niningerite series. The Mn-rich composition of the (Fe,Zn,Mn)S in NWA 6583 (Mn approximately 25 wt%; Table 3), although atypical of the IAB-complex, is similar to that observed in the Waterville IAB-complex iron (Mn ranges from 2.31 to 23.4 wt%; Weinke et al. 1979) (Fig. 6b). Thus, the overall mineralogical composition of the inclusions is also consistent with the classification of NWA 6583 as a new member of the IAB-complex.
Petrogenesis of NWA 6583
Textural and Chemical Constraints
The silicate inclusions in the NWA 6583 metal consist of euhedral to subhedral individual crystals and polycrystalline aggregates with subhedral textures (Fig. 7a). The contacts between silicate inclusions and the surrounding metal matrix are igneous and defined by the euhedral to subhedral crystal boundaries of the silicates (Figs. 7b and 7c). Complex textural relationships are observed between enstatite and troilite (Fig. 7f). Their contacts are decorated by fine intergrowths of diopside plus sulfide that require further studies. No angular silicate inclusions (evidence of clastic origin) commonly reported in silicate inclusions in iron meteorites (e.g., Benedix et al. 2000) were observed in NWA 6583. Textural relationship thus indicates that NWA 6583 formed through the crystallization of a melt composed of two immiscible liquids of silicate and metallic compositions. The occurrence of silicate inclusions consisting exclusively of individual crystals of enstatite suggests very high melt temperatures close to, or above, the enstatite melting point approximately 1570 °C.
The metal composition of NWA 6583 is close to the cosmic abundances (from 0.4 × CI for Ir to 18.8 × CI for Sb) and shows a modest enrichment of incompatible elements (Fig. 9). The compositions of the silicates are broadly chondritic. These lines of evidence indicate the formation of the parent melt from an unfractionated precursor and limited parent-body processing.
The homogeneous composition of the silicate crystals suggests equilibrium crystallization at very high temperatures, whereas the subliquidus history recorded by the surrounding metal is compatible with a relatively fast cooling. Northwest Africa 6583 is made of 0.5–20 mm precursor γ-iron crystals (Fig. 2). They are much smaller than the γ-iron crystals of most iron meteorite groups; as an example, IIIAB irons have their largest γ-iron crystals >2 m (Buchwald 1975). The small size of the NWA 6583 ataxitic (Fe,Ni) metal crystals suggests a rapid cooling of the metal liquid through the γ-iron + liquid and the γ-iron stability fields. Polycrystalline textures with small size (from 2 to 40 cm) of γ-iron crystals are a common feature of IAB-complex irons (Wasson and Kallemeyn 2002). Polycrystalline textures of similar grain size (namely, similar precursor γ-iron crystals) are observed in Mundrabilla (grain size 2–5 cm, Buchwald 1975) and interpreted by Scott (1982) as the result of cooling rates of the order of approximately 6.3 °C yr−1 at solidification temperatures. The lack of Widmanstätten pattern in the (Fe,Ni) metal grains suggests cooling rates faster than approximately 0.01 °C yr−1 at temperatures between 500 and 700 °C (Goldstein et al. 2009). Their martensitic structure documents very fast cooling rates at even lower temperatures between 650 and 290 °C for a metal composition Fe82Ni18. Below the temperature of the onset of the reaction γ → γ + α of 650 °C relatively high cooling rates are indeed required to preserve γ-iron in a metastable state down to the α2 (martensite) inversion temperature of approximately 290 °C (Goldstein et al. 2009).
Redox Formation Conditions
The very low Fe content of silicate inclusions; the occurrence of graphite and Mg-Mn sulfides; the occurrence of Cr, Mn, and Ti in troilite; and the high Si content of (Fe,Ni) metal indicate that NWA 6583 equilibrated under very low oxygen fugacity conditions. In this section, we will examine our data to constrain the redox conditions under which NWA 6583 formed.
In most meteorites, Fe is partitioned among metal, sulfide, and silicates. In highly reducing environments, Fe is virtually excluded from silicates (Reid and Cohen 1967), and its siderophile and chalcophile behavior increases. The oxygen fugacity under which an assemblage containing Fe-Ni-(Si) metal and pyroxenes equilibrated may be estimated on the basis of the FeO content of enstatite following the equation:
where fO2 is the oxygen fugacity, is the Gibbs free energy difference of the reaction
and , , and are the activities of FeSiO3 in enstatite, Fe in the metal, and Si in the metal, respectively. Evidence for equilibrium conditions of the NWA 6583 phases involved in the reaction includes the prevailing euhedral shapes of the enstatite crystals and their homogeneous composition (Tables 2 and 4).
An equilibrium temperature of approximately 900 °C has been estimated using the two-pyroxene geothermometer of Wells (1977). The activity of FeSiO3 in pyroxene has been calculated using the ideal site mixing model of Ghiorso and Carmichael (1980), while and have been assumed to be equal to their mole fractions (e.g., Kilburn and Wood 1997), respectively. The value of (P,T) was calculated at 1 bar and 900 °C using the thermodynamic values from Berman (1988) for ferrosilite, and from Pankratz and Mrazek (1982) for iron, silicon, and oxygen. The value of oxygen fugacity calculated in this way is −3.8 log units below the iron-wüstite (IW) buffer (∆IW = −3.8). A similar approach was used by Benedix et al. (2005) for IAB-complex irons and Righter and Drake (1996) for IAB-complex and IIE irons, winonaites, acapulcoites, SNC, and ordinary chondrites.
Experimental studies indicate that monosulfides of the lithophile elements Mg and Mn are stable only at very low fO2, i.e., ∆IW < −3; (Siebert et al. 2004; Berthet et al. 2009).
The stability of graphite is controlled by oxygen fugacity, pressure, and temperature. At a given oxygen fugacity, graphite stability increases with pressure and decreases with temperature (Rubin 1997). However, as the parent bodies of meteorites generally have small diameters, graphite may be stable only at low values of oxygen fugacity. As an example, within an asteroidal body with a mean density of 3340 kg m−3 and a diameter of 100 km, graphite could be stabilized only at ∆IW < 1 at a depth of 10 km and a temperature of 900 °C.
Kilburn and Wood (1997) showed how the siderophile behavior of Si increases as oxygen fugacity decreases. In their experiments, the (Fe,Ni) metal phase contained 0.37 wt% Si at ∆IW = −4.0. Extrapolating the data of Kilburn and Wood (1997) to the Si content of the metal of NWA 6583 (0.13 wt%), we estimate a ∆IW value of −3.5.
Chromium, like other lithophile elements (e.g., Mn and Ti), shows an increase in its chalcophile behavior as the oxygen fugacity decreases. Thus, the relatively high Cr content of troilite of NWA 6583 (approximately 0.8 wt%; Table 2) is again suggestive of highly reducing conditions.
All these data indicate that the mineral assemblage of NWA 6583 formed under very low oxygen fugacity conditions approximately −3.5 ∆IW; Fig. 11). These conditions are intermediate between those characterizing enstatite chondrites and aubrites (from −6 to −4 ∆IW; Righter and Neff 2007), and IAB-complex irons and winonaites (from −2.8 to −1.5 ∆IW; Righter and Drake 1996; Righter and Neff 2007).
Formation Process and Geological Setting
The fact that NWA 6583 is the product of the crystallization of a very high-temperature melt consisting of a mixture of metal and subordinate silicate liquids with virtually undifferentiated composition suggests formation through total melting of a chondritic precursor. Total melting is characteristically attained through impact melting. Consistently, the textural and mineralogical records of a relatively rapid cooling history at subliquidus temperatures suggest that NWA 6583 crystallized in a surface or subsurface environment of the parent body. Furthermore, silicate inclusions are frequently observed in impact melt irons (e.g., Buchwald 1975; Schrader et al. 2010).
Impact melt rocks on asteroids may occur in a variety of geological settings, including dikes and veins in the basements of craters, melt lumps and melt sheets in the breccia layer at the crater floor, melt splashes at the top or within the ejecta blanket (e.g., Stöffler et al. 1991; Taylor et al. 1993; Keil et al. 1997), or effectively differentiated metal and silicate melt sheets at the crater floor (e.g., Vickery and Melosh 1983; Gaffey and Gilbert 1998; Folco et al. 2004; Ruzicka et al. 2005). Molten debris liberated during catastrophic breakup of the parent asteroid, and eventually reassembled in an offspring rubble-pile, is an additional possibility (e.g., Benedix et al. 2000; Asphaug et al. 2011).
The polycrystalline texture and the modest enrichment in incompatible elements in the metal favor crystallization of NWA 6583 in a subsurface environment, e.g., veins and dikes in the basements of craters, or melt lumps and melt sheets within the breccia layer at the crater floor, where slow enough cooling rates could allow some chemical fractionation of the metal and the formation of its polycrystalline texture. The lack of metal-silicate fractionation in NWA 6583 rules out crystallization in large differentiated melt sheets at crater floor (e.g., Folco et al. 2004).
Lastly, the martensitic structure within metal grains indicative of very fast cooling rates in the 650 °C–290 °C temperature range (see the 'Textural and Chemical Constraints' section) suggests, however, final cooling in a surface environment. This could be explained through excavation during a later impact, most likely by spallation to account for the lack of deformation in NWA 6583.
Some troilite crystals are polycrystalline and show melting textures and lamellar twinning. Some schreibersite crystals also show melting texture. These effects are localized within 2 mm from the external surface of the meteorite and are due to reheating during ablative flight through the Earth's atmosphere of NWA 6583. The melting of schreibersite indicates that the temperature was higher than 1000 °C (Axon 1963).
Implication for the IAB-Complex Parent Body
The IAB-complex is the second largest iron meteorite group. It is characterized by a large variability in the composition of the metal (Wasson and Kallemeyn 2002), type and composition of the silicate inclusions (Benedix et al. 2000), and oxygen fugacity conditions of formation (Righter and Drake 1996; Righter and Neff 2007). Because of this large variability, the origin and evolution of the IAB-complex iron meteorites are a matter of debate (e.g., Haack and McCoy 2003).
Wasson and Kallemeyn (2002) argued that the main group was not formed by fractional crystallization due to its unfractionated (or nearly so) geochemistry. Rather, its formation involved modest crystal segregation with solid and melt essentially in equilibrium in isolated impact melt pools on the surface of a porous chondritic body, and multiple impact-induced melting events created the different subgroups. Other formation models include crystallization of a sulfur- and carbon-rich core in a partially differentiated object (Kracher 1985; McCoy et al. 1993), or catastrophic breakup and reassembly of a partially differentiated object at its peak temperature (namely, 1200–1400 °C) 4.4–4.54 Ga ago (Benedix et al. 2000). These clustered ages would support the catastrophic breakup model for IAB-complex iron parent body. Alternatively, they would document a period of intense bombardment in the history of the IAB-complex iron parent body.
The impact melting scenario envisaged for NWA 6583 in the previous section fits the model by Wasson and Kallemeyn (2002). Furthermore, it is difficult to explain the intimate coexistence in NWA 6583 of metal and silicate liquids (which are characterized by contrasting densities) within the context of planetary differentiation.
Remarkably, NWA 6583 shows a number of peculiarities relative to other members of the IAB-complex group of meteorites. Northwest Africa 6583 is extremely reduced: the record of its unusual mineral composition widens the range of redox formation conditions of the IAB-complex irons down to ∆IW approximately −3.5 (Fig. 11). Furthermore, NWA 6583 has a chemical composition characterized by high Ni and very low Au (i.e., Ni = 18 wt%, Au = 1.13 μg g−1), which deviates somewhat from the classification compositional scheme proposed by Wasson and Kallemeyn (2002) for the IAB-complex irons. The unusual geochemical composition and redox formation condition of NWA 6583 thus significantly extend the properties of the IAB-complex. This, once again, poses the question whether the members of the IAB-complex irons formed in a single parent asteroid or in several similar parent asteroids.
Wasson and Kallemeyn (2002) suggested that the IAB-complex formed in different impact melt pools on the surface of a single parent asteroid. This model finds support on their similar metal composition and the overlapping oxygen isotope composition of the IAB-complex main group and subgroups sLH, sLM, and sLL. Sombrerete, member of the sHL subgroup, is, however, an exception, having an oxygen isotope composition (∆17O = −1.39‰; Clayton and Mayeda 1996), which is much more enriched in 16O than the typical IAB composition. Therefore, sHL could have formed in a distinct parent asteroid (Wasson 2011).
The oxygen isotope composition of NWA 6583 is typical of the IAB-complex irons (Fig. 8). Thus, the unusual geochemical composition and redox formation conditions of NWA 6583 provide a further example of the lithological and mineralogical variety that impact melting could produce on the surface of a single asteroid. Such variety requires that the surface of the parent body of the IAB-complex irons was characterized by an important compositional heterogeneity in space and time (e.g., a regolith) and by an important heterogeneous distribution of reducing agents (e.g., C- or S-rich materials?). For example, carbonaceous chondrite-like xenoliths are reported in HED meteorites and H chondrites by Gounelle et al. (2003) and Briani et al. (2012), respectively, and interpreted as fossil micrometeorites. Similar to what is suggested for the parent bodies of the HED meteorites, Vesta, and H chondrites, the parent body of the IAB-complex irons might have formed in a region of the solar system subject to a flux of exotic reducing materials (e.g., materials with carbonaceous chondritic composition).
Summary and Conclusions
Northwest Africa 6583 is a silicate-bearing iron meteorite. In terms of texture and chemical composition, NWA 6583 can be classified as a Ni-rich (18 wt%) polycrystalline (grain size up to 22 mm) ataxite. The oxygen isotope composition of its silicates indicates a genetic link with the IAB-complex irons. The bulk metal composition of NWA 6583 matches the classification scheme for the IAB-complex with the exception of lower Au content (1.13 μg g−1). The overall mineralogical composition of the inclusions is consistent with several inclusions of the IAB-complex irons. Thus, NWA 6583 has been classified as a new member of the IAB-complex.
The textural features of the silicate inclusions of NWA 6583 indicate that the meteorite is the product of the crystallization of a very high temperature melt (≥1570 °C) consisting of a mixture of metal and subordinate silicate liquid. The chemical composition of the metal is close to the cosmic abundance, suggesting that the metallic liquid formed through total melting of a chondritic precursor. Total melting is considered to result from impact melting. An impact melting origin is also consistent with the occurrence of silicate inclusions and with the rapid cooling history at subliquidus temperatures recorded by the meteorite texture. According to Scott (1982), polycrystalline textures with similar grain size in Mundrabilla record cooling rate of approximately 6.3 °C yr−1. Furthermore, the lack of the Widmanstätten pattern in NWA 6583 indicates cooling rates faster than approximately 0.01 °C yr−1 at temperatures comprised between 500 and 700 °C; Goldstein et al. 2009). It is probable that the crystallization of NWA 6583 occurred in a subsurface environment to allow the nucleation of numerous γ-iron crystals and a modest enrichment in incompatible elements. The martensitic structure of the metal indicates a very fast final cooling in a surface environment.
The proposed model for NWA 6583 fits the model by Wasson and Kallemeyn (2002) for the petrogenesis of the IAB-complex irons. Nevertheless, some unique elements distinguish NWA 6583 from known IAB-complex irons: the low Au content of the metal, the extremely reducing condition of formation (approximately −3.5 ΔIW), and the occurrence of an almost complete alabandite–niningerite solid solution of the Fe-Mn-Mg sulfides. All these features extend the properties of the IAB-complex. Probably NWA 6583 and the other IAB-complex irons with similar oxygen isotope composition could form in the same parent body. Indeed, impact melting can generate a wide variety of rocks, especially if the parent body is characterized by an important compositional heterogeneity in space and time (e.g., a regolith). Impactors with different contents of reducing agents could also play an important role in determining the large range of the redox conditions of the IAB-complex.
This work was supported by the Italian Ministry of Education, University and Research (MIUR-PRIN 2008 project, code 008222KBS_005). Luigi Folco is supported by the Italian Programma Nazionale delle Ricerche in Antartide (PNRA) and the Italian Ministero degli Affari Esteri Progetti di Grande Rilevanza. The authors are grateful to Raul Carampin for assistance during electron microprobe analyses at CNR (Consiglio Nazionale delle Ricerche) IGG in Padua, and to Mirko Graul for generously providing us the study samples of NWA 6583. S. Kissin and D. Schrader are thanked for constructive reviews, and Nancy Chabot for editorial handling.