Geochemical zoning and magnetic mineralogy at Fe,Ni-alloy–troilite interfaces of three iron meteorites from Morasko, Coahuila II, and Mundrabilla

Authors


Abstract

We combined high-resolution and space-resolved elemental distribution with investigations of magnetic minerals across Fe,Ni-alloy and troilite interfaces for two nonmagmatic (Morasko and Mundrabilla) IAB group iron meteorites and an octahedrite found in 1993 in Coahuila/Mexico (Coahuila II) preliminarily classified on Ir and Au content as IIAB group. The aim of this study was to elucidate the crystallization and thermal history using gradients of the siderophile elements Ni, Co, Ge, and Ga and the chalcophile elements Cr, Cu, and Se with a focus on magnetic minerals. The Morasko and Coahuila II meteorite show a several mm-thick carbon- and phosphorous-rich transition zone between Fe,Ni-alloy and troilite, which is characterized by magnetic cohenite and nonmagnetic or magnetic schreibersite. At Morasko, these phases have a characteristic trace element composition with Mo enriched in cohenite. In both Morasko and Coahuila II, Ni is enriched in schreibersite. The minerals have crystallized from immiscible melts, either by fractional crystallization and C- and P-enrichment in the melt, or by partial melting at temperatures slightly above the eutectic point. During crystallization of Mundrabilla, the field of immiscibility was not reached. Independent of meteorite group and cooling history, the magnetic mineralogy (daubreelite, cohenite and/or schreibersite, magnetite) is very similar to the troilite (and transition zone) for all three investigated iron meteorites. If these minerals can be separated from the metal, they might provide important information about the early solar system magnetic field. Magnetite is interpreted as a partial melting or a terrestrial weathering product of the Fe,Ni-alloy under oxidizing conditions.

Introduction

Extraterrestrial material provides a unique view into the early solar system and it records a large range of processes. Fe,Ni-alloys are expected to represent the cores of differentiated asteroids (Haack and McCoy 2003), which is known from their trace element compositional trends (e.g., Scott 1972). Originally, iron meteorites were classified in four groups (I–IV) using the siderophile elements Ga and Ge (Goldberg et al. 1951). Later classifications further subdivided these four groups into 13 classes by attaching letters to the group numbers based on the correlation among Ga, Ge, Ir, and Ni (e.g., Wasson 1967; Mittlefehldt et al. 1998). Within most iron meteorite groups, geochemical trends are largely consistent with fractional crystallization of the metallic melt and it is assumed that each group formed from a single molten metallic pool or core (Alexander and Calvin 2000). Different areas within the pools or cores can be distinguished by their trace element concentrations and especially S, P, and C affected partition coefficients and liquid–liquid immiscibility (e.g., Goldstein et al. 2009). After solidification, these bodies have probably been broken up by impacts long after the bodies had slowly cooled. Therefore, meteorites have later on suffered different stages and degrees of metamorphism, melting, particle irradiation, and hypervelocity impacts. This overprint affects the distribution of major and trace elements, as well as the presence of varying mineral phases. For iron meteorite materials, the partition coefficient between melt and metal phase depends on the concentration of Ni, S, P, and C in the melt (Chabot and Drake 2000; Chabot et al. 2006, 2007, 2008, 2011). Therefore, the knowledge of trace element distribution and gradients helps to elucidate the crystallization and thermal history of iron meteorites.

Ages of about 4.5 Ga for the IAB meteorites support the idea that partial melting, crystallization, and metal–silicate mixing occurred very early in the history of the solar system (e.g., Mittlefehldt et al. 1998 and references therein). Irons of the IAB and IIICD group belong to the nonmagmatic silicate-bearing irons, which are assumed to be the product of partial melting and impact events (e.g., Mittlefehldt et al. 1998; Wasson et al. 2007). The chemical behavior of the nonmagmatic irons is distinctly different from the magmatic groups, especially concerning the large variation in the siderophile elements Ni and Ge. These nonmagmatic irons show only small Ir fractionation. Together with the occurrence of trapped chondritic silicates in the metal, this chemical behavior is interpreted as an indication of rapid cooling (Wasson and Kallemeyn 2002; Wasson et al. 2007).

The IIAB irons are considered to belong to the magmatic iron meteorite groups, which show large concentration ranges of Ir and a strong negative correlation versus Au, As, and Ni. It is suggested that such a geochemical trend reflects efficient fractional crystallization (Wasson et al. 2007). Experimental studies concerning the nonmetals S, P, and C show that solid/liquid distribution coefficients depend on the concentration of these elements in the melt. During fractional crystallization, S, C, and P are enriched in the remaining liquid and, in a later stage, it is possible that the S-, C-, and P-concentrations in the melt reach the field of liquid–liquid immiscibility and the melts can separate (Ulff-Møller 1998; Chabot and Drake 2000; Wasson et al. 2007).

As iron is the major component of all metallic meteorites, a strong magnetization due to the presence of ferromagnetic minerals occurs (e.g., Rochette et al. 2009). Rochette et al. (2009, and references therein) proposed that many meteorite types can be identified purely on the basis of their magnetic susceptibility. However, compared with our knowledge of the magnetic mineralogy of terrestrial rocks, the current knowledge on the magnetic mineralogy is rather poor concerning extraterrestrial material.

The magnetic behavior of iron meteorites can be complex and is controlled by Fe,Ni-alloy, daubreelite, troilite, schreibersite, suessite, and magnetite (e.g., Coey et al. 2002; Hoffmann et al. 2010). Troilite [FeS] is hardly seen in magnetic measurements because of its very low magnetic susceptibility, and daubreelite [FeCr2S4] is ferrimagnetic below its Curie temperature at about −110 °C. The latter phase is assumed to control the magnetic properties of iron meteorites in cold environments as long as the meteorite is not warmed above a magnetic transition temperature at about −200 °C (e.g., Kohout et al. 2010). For cohenite ([Fe, Ni, Co]3C), a Curie point at 215 °C is reported, for schreibersite ([Fe,Ni]3P) at around 350–400 °C, and for suessite ([Fe,Ni]3Si) at 550–600 °C (Hoffmann et al. 2010; Weiss et al. 2010). Oshtrakh et al. (2011) reported a Curie temperature for rhabdite (a variety of schreibersite) at 345–355 K (72–82 °C), which suggests a significant, but not yet verified, variation in Curie temperatures with composition. The magnetic properties of these phases are not well known (Weiss et al. 2010). Hoffmann et al. (2010) assumed that the suessite Curie temperature (TC) has been often misinterpreted as due to magnetite. On the other hand, magnetite has been described in graphite nodules of the Canyon Diablo meteorite, where it was identified by Mössbauer spectroscopy (Coey et al. 2002). Coey et al. (2002) measured in these graphite nodules three Curie temperatures at 757, 587, and 297 °C and related them to kamacite, magnetite, and ferromagnetic graphite, respectively. Graphite normally is diamagnetic, but ferromagnetic ordering at local defect structures in the graphite lattice is described in Cervenska et al. (2009). Magnetic susceptibility in iron meteorites is mainly controlled by the Fe,Ni-alloy. An equilibrium phase diagram of the Fe-Ni system with Curie temperatures is presented in fig. 1 of Garrick-Bethell and Weiss (2010). Ni-poor alloy (<10% Ni) has a TC at 770 °C and pure Ni at 354 °C. The phase relations in the Fe-Ni system are complicated due to phase transitions at low temperatures and TC is not linearly related to composition.

Figure 1.

a) Macroscopic image of the Coahuila II meteorite, showing a troilite nodule surrounded by a mm-thick transition zone and Widmannstätten pattern in the Fe,Ni-alloy. In contrast to the Coahuila, which is a hexahedrite, the Coahuila II is an octahedrite. b) Macroscopic image of the Mundrabilla meteorite with interstices filled with troilite.

An earlier geochemical investigation of the iron meteorite of Morasko (Poznan, Poland) documented significant differences in trace element contents between the troilite phase and the bulk of Fe,Ni-alloy (Luecke et al. 2006; Szurgot et al. 2008). To reconstruct the crystallization and thermal history, we suspect that the trace element behavior in the troilite phase and at the contact between the Fe,Ni-alloy and troilite might be critical.

We therefore present in our study bulk and space-resolved major and trace element data across profiles from the troilite phase and its surrounding rim into the Fe,Ni-alloy and describe the magnetic minerals, which can be identified in the temperature range between −190 and 800 °C. We investigated the iron meteorites Morasko, Mundrabilla, and an octahedrite of 475 kg found in 1993 in Coahuila/Mexico (in the following named as Coahuila II), which vary in texture and therefore indicate different cooling histories. The aim of our study was to elucidate the crystallization and thermal history of magmatic and nonmagmatic iron meteorites with a focus on magnetic minerals at the metal–troilite interface using high-resolution major and trace element analyses and space-resolved elemental distribution in combination with Curie temperature determination of the magnetic phases.

Materials and Methods

The Morasko and Coahuila II meteorites contain isolated, rounded troilite nodules up to a size of about 3 cm with a macroscopically visible rim enclosed by the Fe,Ni-alloy matrix (Fig. 1a). Carbon in the marginal parts of the troilite nodules as well as in a several mm-thick transition zone between troilite and Fe,Ni-alloy can be macroscopically observed.

The Morasko meteorite was found in 1914. Our fragments from Poznan, Poland, are composed in principal of the FeNi minerals kamacite (α-Fe,Ni; <6% Ni); taenite (γ-Fe,Ni; >30% Ni), which makes about 98 vol%; and troilite (FeS) nodules (approximately 2 vol%). Trace minerals are schreibersite ([Fe,Ni]3P), cohenite ([Fe,Ni,Co]3C), sphalerite (ZnS), graphite (C), daubreelite (FeCr2S4), and silicate minerals (Na-pyroxene and plagioclase), which mostly occur in the margins of the troilite nodules (e.g., Idzikowski et al. 2010). This iron is classified as nonmagmatic IIICD (e.g., Geist et al. 2005), a subgroup of the IAB group.

The first Coahuila meteorite was found in 1837 (Rubinovich-Kogan et al. 1992). The piece we investigated is part of a big specimen (475 kg) found in 1993 by Rendón & Hochmann in the Sierra Arteaga of Coahuila (NE Mexico). This new meteorite is an octahedrite, whereas the well-known Coahuila meteorite is a hexaedrite. This new meteorite from Coahuila also exhibits characteristic rims around its troilite nodules, similar to those from Morasko, but is unclassified until now.

The first Australian iron meteorite from Mundrabilla was discovered in 1911. The fragment used for our investigation is part of a meteorite found there in 1966 and partly stored now in the Staatliches Museum für Naturkunde, Karlsruhe, Germany. This S-rich meteorite consists of about 75 vol% Fe,Ni-alloy and about 25 vol% troilite (+daubreelite). In this meteorite, the troilite does not occur as nodules, but as fillings of interstices in the Fe,Ni-alloy, partly of angular shape (cm-size). This meteorite also belongs to the IIICD group (Wasson and Kallemeyn 2002), but has a much higher troilite content than the two other meteorites and has not developed a transition zone between troilite and metal phase (Fig. 1b).

Optical microscopy in reflected light mode was used to detect the magnetic and nonmagnetic phases on highly polished surfaces of the iron meteorites at room temperature. We coated the polished surface of the samples with ferrofluid (EMG 807) from Ferrofluidics GmbH, which was diluted with distilled water in the ratio 1:10, dispersed on the polished section and covered by a glass slide. It was attracted by the ferrimagnetic, but not by the antiferromagnetic or paramagnetic phases. Scanning electron microscopy (SEM) in backscatter mode was performed using a LEO 1530 Gemini instrument (Laboratory for Electron Microscopy, KIT, Karlsruhe) in combination with energy dispersive X-ray (EDX) analyses (Noran System Thermo Fischer) to verify the mineral phases and characterize their textures. Chemical etching with nital (3% solution of HNO3 in ethanol for approximately 10 s at room temperature) was used to characterize the microstructures in the Fe,Ni-alloy. We concentrated our mineralogical study on the phases that occur near the boundary between Fe,Ni-alloy and troilite to relate them to the geochemical signatures.

For the bulk analysis of the Fe,Ni-alloy, the troilite phase, and the transition zone, a portion of the sample material was cut by a diamond saw and carefully broken between Teflon slabs to avoid contamination. Subsequent separation of the phases was done by pre-enrichment of the magnetic Fe,Ni-alloy using a hand magnet and separation of the weakly magnetic and nonmagnetic phases by hand picking of pure specimens under a binocular microscope.

Fifteen samples (six from Fe,Ni-alloy, five from troilite, and four from the transition zone) were digested in hot nitric acid of subboiled quality. For the analysis of the PGE and Au, an aliquot of the metal phase of each meteorite was digested in aqua regia. The solutions (including procedural blanks) were analyzed for their trace element contents using a double focusing high-resolution ICP-MS (Axiom, VG Elemental) or a quadrupole ICP-MS equipped with a collision cell (XSERIES 2, Thermo scientific). Sodium, Ca, Mg, and Fe were determined by AAS (Perkin Elmer AAnalyst200), S and C by CSA (Eltra CS2000). Details of the analytical procedure are described in Luecke et al. (2006). The isotope compositions δ34S and δ 13C of sulfur and carbon from the troilite phase and transition zone were measured with an Isoprime mass spectrometer (GV Instruments, UK) coupled online with an element analyzer (Euro EA). Raw data of S-isotope ratios were calibrated against Canyon Diablo troilite VCDT and C-isotope ratios versus V-PDB. Reported values are averages of three individual measurements each, with a standard deviation of <0.1‰.

The spatial distribution of major and trace elements across the troilite phase, the transition zone, and the Fe,Ni-alloy was determined by micro X-ray fluorescence (μXRF) using an Eagle III μXRF (Röntgenanalytik, Taunusstein, Germany). The μXRF is equipped with an air-cooled 50W X-ray tube with Rh-anode. The X-ray beam was focused by polycapillary to a diameter of 30 μm. This results in an elliptical focal spot of 30 × 40 μm. To optimize the tube spectra for the excitation of the elements Ni (Z = 28) to Mo (Z = 42), the tube was operated at 50 kV and the primary beam was filtered by a 50 μm Rh foil. For the determination of the elements from P (Z = 15) to Co (Z = 27), the tube was operated at 30 kV, using a 25 μm Al primary beam filter. From each iron meteorite, an area across the metal–troilite boundary was mapped (3 × 3 mm for Mundrabilla, 3.3 × 3.9 mm for Morasko, and 6.7 × 2.9 mm for Coahuila II) using a regular grid with a spacing of 50 μm equivalent to an area coverage of approximately 40% for Mundrabilla (3600 points) and Morasko (5600 points), and 100 μm equivalent to an area coverage of approximately 10% for Coahuila II (2800 points). Measuring times were 10 s per point for the elements P-Co and 90 s per point for Ni-Mo. In the middle of each grid, an additional high-resolution line scan with a point distance of 30 μm across the metal–troilite boundary was carried out, using extended measuring times (200 s per point for the Al-filter and 600 s per point for the Rh-filter) to improve detection limits and reproducibility. The spectra were evaluated by the fundamental parameter method implemented in the PyMca V4.4.0 spectra evaluation software (Solé et al. 2007).

As XRF is sensitive to matrix variations and PyMCA does not adjust the matrix variation iteratively, the measured points of each meteorite were at first grouped by k-means cluster analysis to obtain groups of homogeneous matrix composition and then the groups were recalibrated by adjusting the average of each group to the ICP-MS bulk data of the respective phases of each meteorite.

The mapping method yielded semiquantitative data of spatial distribution for 10 characteristic elements of the present meteorites given as elemental distribution maps across Fe,Ni-alloy, transition zone, and troilite in median concentration values with P10 and P90 (for data see the Appendix). For a measuring time of 100 s per point, detection limits are approximately 20 μg g−1 for Ga, Ge, Se, and Mo; 300 μg g−1 for Ni and S; 0.3% for P and Co (high D.L. for Co due to overlapping Fe line).

Magnetic susceptibility as a function of temperature (−194 to 15 °C and from room temperature to 800 °C) was measured with a KLY-4S kappabridge (working at 300 A/m and 875 Hz) combined with a CS-L/CS-3 apparatus (AGICO company). Heating/cooling rates range between 3–4 and 11–14° min−1 for the low-temperature and high-temperature run, respectively. The high-temperature runs were done in an argon atmosphere to avoid mineral reactions with oxygen during heating (flow rate of 110 mL min−1). The raw data were corrected for the empty cryostat/furnace and normalized to room temperature values. In this study, the Curie temperature (TC) was determined graphically using the peak temperature method as described in Lattard et al. (2006).

Results

Optical and Electron Microscopy

We concentrated our mineralogical study on the phases that occur near the boundary between Fe,Ni-alloy and troilite (Figs. 1, 2a, and 2b) to relate them to the geochemical signatures (see below). Two different textural types have been distinguished. The Coahuila II (Fig. 1a) and Morasko (Fig. 2a) meteorites show several millimeter thick transition zones around the troilite nodules consisting of graphite intergrown with secondary terrestrial alteration products (e.g., Fe-oxides/-hydroxides), in addition to troilite, schreibersite, cohenite, and Fe,Ni-alloy. This type shows Neumann lines in the Fe,Ni-alloy (Fig. 2c) indicating shock pressures of <8 GPa (e.g., Heymann et al. 1966). For the complex microstructures of the Morasko iron, very slow cooling rates (0.1–100 °C Myr−1) at temperatures lower than 400 °C are suggested (Wojnarowska et al. 2008). The Mundrabilla meteorite shows no pronounced transition zones (Fig. 2b), and the polycrystalline mixture of roughly equant kamacite and taenite grains (Fig. 2d) indicates a static recrystallized texture. Such a texture and intergrowth is an indication for a fast cooling rate (500° yr−1; Scott 1982).

Figure 2.

Different textures of the Morasko (a and c) and Mundrabilla (b and d) irons. a) The porous transition zone in the Morasko iron is 2–3 mm wide and consists of a mixture of graphite (gr) and Fe-oxides, schreibersite, troilite, and Fe,Ni-alloy (FeNi). b) In the Mundrabilla iron, the transition is sharp but decorated by lath-like daubreelite (dau). (a) and (b) are backscattered electron images. (c) and (d) shows microstructures of the Fe,Ni-alloy under reflected light microscope after etching with nital. c) The Morasko Fe,Ni-alloy shows Neumann lines, which result from mechanical twinning at shock pressures >8 GPa (Heymann et al. 1966). d) The Mundrabilla Fe,Ni-alloy shows equigranular grains with triple junctions indicating recrystallization and small schreibersite (sch) inclusions mostly decorating grain boundaries of the Fe,Ni-alloy. The brown color is caused by ferrofluid and indicates the ferrimagnetic character of schreibersite.

Morasko Meteorite

In the Morasko meteorite, the Fe,Ni-alloy consists of complex microstructures with an intergrowth of magnetic and nonmagnetic phases. The dominating ferrimagnetic phase in the Fe,Ni-alloy is kamacite. Toward the troilite nodule, the abundance of cohenite and schreibersite, which are intergrown with the metal (Fig. 3a), increases. Cohenite has a very characteristic magnetic domain structure with a stripe-like pattern probably indicating twinning of the crystal (Fig. 3b). Iron and graphite occurs along cracks in cohenite indicating the break-down reaction Fe3C → 3Fe + C (e.g., Lipschutz and Anders 1961). The dark rim around the troilite nodule (Fig. 3a) consists of graphite intergrown with Fe-hydroxides and presumably very fine-grained magnetite. Ferrimagnetic pyrrhotite has not been observed in the troilite nodule, but daubreelite (FeCr2S4) occurs as tiny (<2 μm) rounded inclusions (Fig. 3c). Along grain boundaries or cracks, the Fe,Ni-alloy shows in places foamy structures with tiny crystals of magnetite (Fig. 3d).

Figure 3.

Microscopic images of the polished surface of the troilite–metal interface. Morasko: a) transition zone with troilite (tr), schreibersite (sch), cohenite (coh), and Fe,Ni-alloy (FeNi). b) Treatment with ferrofluid of the same detail as in (a) shows characteristic stripe-like and granulated Bitter pattern in cohenite (coh). Bitter pattern in Fe,Ni-alloy is very weakly developed. No Bitter pattern occurs in schreibersite and troilite. c) Rounded daubreelite (dau) inclusions in troilite. d) Euhedral magnetite (mt) crystals in porous Fe,Ni-alloy, which forms sharp boundaries with (unmolten) Fe,Ni-alloy. a, c, and d: backscattered electron images, b: reflected light image, section coated with ferrofluid.

Coahuila II Meteorite

The Fe,Ni-alloy of the Coahuila II meteorite also shows complex microstructures consisting of very delicate magnetic and nonmagnetic domains oriented in different directions (Figs. 4a and 4b). In metal with cracks between schreibersite and the homogeneous Fe,Ni-alloy, no Ni was detected by EDX (Fig. 4e). Schreibersite in the transition zone between Fe,Ni-alloy and troilite attract the ferrofluid indicating that this variety is a ferrimagnetic phase. The abundance of schreibersite is much lower than in the Morasko sample and no cohenite was observed in the studied transition zone. Tiny, submicron-sized euhedral crystals of schreibersite and magnetite occur in porous Fe,Ni-alloy rims between homogeneous Fe,Ni-alloy and the transition zone (Figs. 4c and 4d).

Figure 4.

Microscopic images of the polished surface of the troilite–metal interface. Coahuila II: a) Sequence of Fe,Ni-alloy (FeNi) with different Ni concentration (see 1 and 2 in EDX of e), schreibersite (sch), and troilite (tr). b) The same detail as in (a) shows delicate magnetic domain structures in Fe,Ni-alloy and larger magnetic domains in schreibersite of the transition zone. c) Locally, porous Fe,Ni-alloy occurs between transition zone and Fe,Ni-alloy. d) Euhedral schreibersite and magnetite (mt) crystals occur within the pores. a, c, and d: backscattered electron images, b: reflected light image, section coated with ferrofluid. e) Semiquantitative composition (EDX) of phases from (a). EDX of 1, 2, 3, and 4 refers to same numbers in (a).

Mundrabilla Meteorite

The Fe,Ni-alloy of the Mundrabilla meteorite shows an extremely fine intergrowth of magnetic and nonmagnetic domains. The etched microstructures show equigranular grains with triple junctions (Fig. 2d), which indicate recrystallization and therefore reheating of the sample. There is a sharp transition between the troilite and Fe,Ni-alloy (Figs. 1b and 2b). The sharp boundary is decorated by lathes of daubreelite, which contain even smaller lamellae of schreibersite (Fig. 5a). Foamy structures with magnetite crystals as we have observed in the Morasko and Coahuila II Fe,Ni-alloy occur only rarely in negligible amounts. EDX analyses of bright spots within the Fe,Ni-alloy revealed a higher Ni concentration (Figs. 5b and 5c). Interestingly, Fe,Ni-alloy from Mundrabilla contains some carbon (Fig. 5c), while the Coahuila II (Fig. 4e, EDX of 1 and 2) and Morasko (data not shown) do not show carbon in the Fe,Ni-alloy near the troilite nodule.

Figure 5.

Microscopic images of the polished surface of the troilite–metal interface. Mundrabilla: a) Lath-shaped schreibersite crystals (sch) in daubreelite (dau) at the Fe,Ni-alloy (FeNi)–troilite interface (see also Fig. 2b). b) Fe,Ni-alloy with higher Ni concentration (taenite?) occurs as rounded-to-elongated patches within Ni-poor Fe,Ni-alloy (kamacite). c) Semiquantitative composition (EDX) of phases from (b). Note that Fe,Ni-alloy from Mundrabilla contains carbon, while the one from Coahuila II (Fig. 4e, EDX of 1 and 2) does not.

Major and Trace Elements

Bulk concentrations of major and trace elements in the troilite, and in different parts of the transition zone (high and low P and C concentrations), and in the Fe,Ni-alloy (high Co concentrations) are given as averages of three single determinations in Table 1 for the three meteorites. The chalcophile elements like Cr, Cu, and V are enriched in the troilite compared with the Fe,Ni-alloy, whereas the siderophile elements like Ni, Co, Ge, and Ga are enriched in the metal (Jana and Walker 1997). Siderophile and chalcophile trace elements of the Fe,Ni-alloy are normalized to the average CI chondrites and Ni (Fig 6a). The trace element patterns of all three meteorites are similar to those of Canyon Diablo (D'Orazio and Folco 2003) and Odessa (Mittlefehldt et al. 1998). As Ni in troilite and the transition zone is distinctly lower compared with the Fe,Ni-alloy, the Ni normalization does not allow a comparison of trace element patterns of the three phases. Therefore, the trace elements of Fe,Ni-alloy, transition zone, and the troilite phase are additionally normalized to Fe (Figs. 6b–d).

Table 1. Bulk major and trace element data for troilite; transition zone (TZ); and Fe,Ni-alloy of Morasko, Coahuila II, and Mundrabilla.
 MoraskoCoahuila IIMundrabilla
TroiliteTZ centralTZ alloy facingFe,Ni-alloyTroiliteTZ centralTZ alloy facingFe,Ni-alloyTroiliteFe,Ni-alloy
  1. N.D. = not determined.

Fe (%)63.9534.276.293.361.640.954.290.261.3588.9
Ni (%)0.0790.0718.296.650.0290.0902.866.410.0127.43
S (%)34.8218.580.35N.D.31.1218.0117.53N.D.35.1N.D.
P (%)0.0250.0153.210.1260.0030.0271.030.110.0020.11
C (%)0.3641.952.75N.D.6.5537.822.7N.D.0.07N.D.
Na (μg g−1)210290130<0.01290180160<0.0121030
Mg (μg g−1)60505525503704453457
Ca (μg g−1)9002040<0.02164015<0.02265<0.02
Cr (μg g−1)730037001073.25700370029002.6111009.05
Mn (μg g−1)49018884<0.6381189280<0.6960<0.6
Zn (μg g−1)<2040<20<202088984342<2012.5<20
Cu (μg g−1)23216935115217157148106195144
V (μg g−1)37180.4<0.1271714<0.11290.1
Pb (μg g−1)5.33.65.2<1.26.69.19.9<1.21.45<1.2
Co (μg g−1)416214004750364484443501.354900
Ge (μg g−1)0.551.213.9418271492640.7127
Ga (μg g−1)0.120.152.81070.80.41.8920.2559
As (μg g−1)<0.6<0.6<0.68.8<0.6<0.6<0.68.55<0.611
Mo (μg g−1)1.90.635.56.11.21.18.45.652.655.45
W (μg g−1)<0.1<0.12.71.8<0.1<0.10.60.95<0.10.5
Pd (μg g−1)N.D.N.D.N.D.2.85N.D.N.D.N.D.2.23N.D.3.47
Pt (μg g−1)<0.04<0.040.37.8<0.040.20.16.12<0.043.02
Ru (μg g−1)<0.07<0.072.69.8<0.07<0.070.85.6<0.072.97
Rh (μg g−1)<0.01<0.010.51.68<0.01<0.010.21.37<0.010.84
Ir (µg g−1)<0.09<0.09<0.090.86<0.09<0.09<0.091.69<0.090.74
Au (µg g−1)N.D.N.D.N.D.1.3N.D.N.D.N.D.0.43N.D.1.46
δ34S (‰)−1.54−1.91−1.53N.D.−1.61−0.95−2.00N.D.−1.30N.D.
13C (‰)N.D.−5.76−21.08N.D.−6.51−5.85−6.40N.D.N.D.N.D.
Figure 6.

a) Bulk concentration of siderophile and chalcophile elements normalized to the CI chondrite and to Ni (D'Orazio et al. 2009). Elements are ordered from left to right by decreasing 50% condensation temperature. CI chondrite composition is from McDonough and Sun (1995) and Henderson and Henderson (2009). CI chondrite and Fe normalized concentrations of siderophile and chalcophile elements for (b) Fe,Ni-alloy; (c) transition zone; and (d) troilite of the Morasko, Coahuila II and Mundrabilla meteorites. Data for Canyon Diabolo and Odessa from D'Orazio and Folco (2003).

The Fe and chondrite normalized data show that the Fe,Ni-alloy of Morasko and Coahila II is slightly enriched in W, Ru, Pt, Rh, Co, Ga, and Ge, whereas Mundrabilla plots near one for W, Ga, and Ge and is depleted in Ru and Pt. The Fe,Ni-alloy of Morasko and Mundrabilla is depleted in Ir and enriched in Au, whereas Coahuila II plots for Ir near to 1 and shows a depletion in Au (Fig. 6b). All three Fe,Ni-alloys are strongly depleted in Cu.

The transition zones between troilite and metal of the Morasko and Coahuila II can be divided into two different regions: one alloy facing with a strong enrichment in P (up to approximately 3%) and 2.7% C (Morasko), respective 23% C (Coahuila II) and a second facing the troilite with low P concentrations and very high C contents (approximately 40%). In the Mundrabilla iron, no macroscopic transition zone occurs. Bulk Mo and W are clearly enriched in the P-rich transition zone of Morasko, slightly in the P-rich transition zone of Coahuila II, and depleted in the C-rich parts (Fig. 6c). The high C-content of the transition zone facing the troilite results from the occurrence of graphite together with secondary Fe-oxides/-hydroxides, whereas the carbon in the alloy-facing part of Morasko is mainly incorporated into cohenite. As Mo and W have a much higher affinity to C than to P, these elements are probably incorporated into the cohenite. At Coahuila II, no cohenite could be identified by SEM. In troilite of all three meteorites, the chalcophile element Cu is enriched and from all investigated meteorites, Mundrabilla shows the lowest Cu fractionation between troilite and metal phase (CuTr/CuFeNi approximately 1.4 for Mundrabilla and approximately 2 for Morasko and Coahuila II).

The sulfur isotope ratio (δ34S) of the troilite phase and the transition zone in all three meteorites is similar (−0.95 to −2‰) and slightly depleted (Table 1) compared to the zero point of the Canyon Diablo meteorite.

Carbon isotope ratios (δ13C) have been measured for different portions of the transition zone, which is enriched in carbon (Table 1). In the Coahuila II rim and troilite, the δ13C values range from −5.85 to −6.51‰, which is in agreement with the carbon isotopic composition of graphite for the Canyon Diablo and Magura iron meteorites (Deines and Wickman 1975). In the Morasko rim zone, two different portions were analyzed, one containing cohenite and the other graphite. While the graphite-rich portion is characterized by a similar carbon isotopic composition (−5.76‰) as the Coahuila II rim, the cohenite-rich portion shows a significantly lighter isotopic composition (−21.08‰).

Space-Resolved Elemental Distribution (μEDXRF)

Line scans (Fig. 7) and μXRF mappings (Fig. 8) show a clear spatial distribution of the investigated elements for troilite, Fe,Ni-alloy, and the minerals of the transition zone.

Figure 7.

High-resolution profiles across the troilite–Fe,Ni-alloy interface (a, b) Morasko, (c, d) Coahuila II, and (e, f) Mundrabilla with improved detection limits due to extended measuring times per point. The troilite (Tr), the transition zone (TZ), and Fe,Ni-alloy (FeNi) are marked. Note the clear enrichment of Mo at the contact of TZ and Fe,Ni-alloy at Morasko, the weak enrichment at Coahuila II, and the slight enrichment in the metal at Mundrabilla. sch: schreibersite, coh: cohenite, FeNi (2) as in Fig. 4a.

Figure 8.

Area distribution of S, Cr, Cu, Se, Ge, Ni, P in troilite, transition zone, and Fe,Ni-alloy for Morasko (3.3 × 3.9 mm, spacing 50 Vm), Coahuila II (6.7 × 2.9 mm, spacing 100 Vm), and Mundrabilla (3 × 3 mm, spacing 50 Vm). Note the point-shaped enrichment of P and Ni in the metal phase of Mundrabilla parallel to the troilite interface and the line-shaped Cr enrichments in the troilite.

As already described in the preceding chapter, the siderophile elements Ni, Co, Ge, and Ga are enriched in the Fe,Ni-alloy, whereas the more chalcophile elements Cr, Cu, and Se are concentrated in troilite. These elements are nearly homogeneously distributed within the troilite and Fe,Ni-alloy of the Morasko und Coahuila II meteorite. Within the transition zone, a strong zonation and strong concentration gradients are observed. In the Morasko and Coahuila II meteorites, point- and line-like enrichments of P (up to 23%), Ni (up to 30%), and Co (up to 1%) are observed within the transition zones between Fe,Ni-alloy and troilite (Figs. 7a–d, 8a, and 8b) at the side facing the troilite. This observation is in agreement with microscopic observations according to which schreibersite is concentrated in the transition zone between troilite and Fe,Ni-alloy (Figs. 3a and 4a). At the side facing the Fe,Ni-alloy, the Ni content is relatively constant, but significantly lower than in the alloy (approximately 4% instead of 6–8% in the alloy). In this part, cohenite and graphite are observed by SEM in Morasko, whereas in Coahuila II, the metal is depleted in Ni (Figs. 3a, 4a, 7b, and 7d). The Ge- and Ga-concentrations increase steadily (from approximately 30 to 130 μg g−1 Ge and 10 to 40 μg g−1 Ga) with decreasing distance to the alloy (Figs. 7a and 7c). In this part, Morasko shows a strong enrichment of Mo (up to 70 μg g−1), whereas in Coahuila II, Mo only displays a very weak (up to 30 μg g−1) enrichment slightly above the detection limit of the μXRF for Mo (Figs. 7b and 7d). In the Mundrabilla meteorite, Cr is strongly enriched (up to approximately 15%) directly at the boundary between troilite and Fe,Ni-alloy and also along parallel lines within the troilite phase. This is consistent with the presence of lath-shaped daubreelite exsolutions at the metal–troilite boundary (Fig. 2b). In the Mundrabilla meteorite, where the transition zone is absent, schreibersite is enriched along lines in the metal phase, parallel to the contact with the troilite (Fig. 8c). Between the schreibersite or taenite enrichment and the troilite, a concentration gradient of Ni from approximately 4% at the troilite contact to 7% at 1 mm distance from the contact occurs. In this area, cohenite is observed microscopically (Fig. 3a). For Coahuila II and Mundrabilla, Mo is slightly enriched (to approximately 10 μg g−1) in the metal phase (Fig. 7). This is consistent with experimental results for the Fe-Ni-S system of Chabot et al. (2007), where Mo is enriched in the first crystallizing metal phase.

Temperature-Dependence of Magnetic Susceptibility

Mass-specific magnetic susceptibility differs clearly between Fe,Ni-alloy (30–50 × 10−3 m3 kg−1) and the troilite and transition zone (0.4–2.7 × 10−3 m3 kg−1). Although there is no significant difference between troilite and the transition zone (Table 2), there exist few variations, which reflect some heterogeneity in the distribution and amount of ferro- and paramagnetic phases.

Table 2. Mass susceptibility in 10−3 m3 kg−1 of Fe,Ni-alloy, transition zone, and troilite nodule of the three meteorites.
 Fe,Ni-alloyTransition zoneTroilite
Morasko38.650.490.41
Coahuila II52.652.681.76
Mundrabilla30.602.33

Temperature-dependent magnetic susceptibility curves (k-T curves) were done to better characterize the magnetic minerals at the Fe,Ni-alloy–troilite interface. k-T curves of the troilite and transition zone are complex and we observed no significant difference between these two regions (Fig. 9). At low temperatures, all samples show a magnetic transition (Tm) at about −150 °C, characterized by an increase in susceptibility, and a Curie temperature (TC) between −113 and −118 °C. Two low-temperature magnetic transition temperatures are described for daubreelite, a magnetic transition (Tm) at about −195 °C and a TC between −103 and −125 °C (Kohout et al. 2010). While our TC fits excellently to the reported TC of daubreelite, Tm is outside our measurement range. The strong increase in susceptibility resembles that one known for the Verwey transition (TV) in magnetite at −152 °C.

Figure 9.

Magnetic susceptibility as function of temperature. Left side: troilite or transition zone, right side: Fe,Ni-alloy. During heating in argon atmosphere, mineral reactions or phase transformations occur. In (e), traces of Fe,Ni-alloy occur as magnetic susceptibility does not reach the temperature axis >600 °C. For further explanations, see text. a, b: Morasko; c, d: Coahuila II; e, f: Mundrabilla.

Troilite is antiferromagnetic with a Néel temperature of 315 °C. In our k-T curves, no transition temperature around 300 °C occurs. Instead, there is a faint TC at about 140 and 150 °C in the Coahuila II and Mundrabilla samples, respectively. This temperature is most likely related to schreibersite, which occurs in the transition zone of the Coahuila II (Fig. 4b) and Mundrabilla meteorite (Fig. 5a). The Morasko troilite does not show a TC at about 150 °C, and microscopically no magnetic schreibersite was observed. Instead, there is a peak at 176 °C. We interpret this peak as the TC of cohenite, which has been observed microscopically as ferrimagnetic phase (Fig. 3b). All troilite and transition zone samples show an increase in magnetic susceptibility above about 250–300 °C (Morasko and Coahuila II) or 400 °C (Mundrabilla), indicating new formation of magnetite (TC at 590 °C) during thermal treatment. The strong Hopkinson peak (cuspid peak before reaching TC) indicates small grain sizes for magnetite. A transformation of pyrrhotite into magnetite during thermal treatment is a frequent phenomenon seen in k-T curves, even if measurements are done in an argon atmosphere (e.g., Kontny et al. 2000) and we assume that this transformation also occurs in troilite. However, magnetite must also originally be in the sample because (a) there is a distinct increase in magnetic susceptibility at −150 °C, the TV of magnetite; (b) magnetic susceptibility at room temperature is significantly higher than in the paramagnetic region above the TC of magnetite (Figs. 9a and 9c). Even the Mundrabilla troilite shows traces of magnetite, which are not related to the heating of the sample, indicated by the TV at −147 and a slightly lower magnetic susceptibility above TC at 588 °C (Fig. 9e). Heating and cooling curves are not reversible, but the TC at about 590 °C is. This behavior indicates that cohenite and schreibersite are no longer stable phases when heated up to 700 °C.

A general feature in the Fe,Ni-alloy of the three meteorites is a TC near 770 °C (763–777 °C; Figs. 9b, 9d, and 9f) indicating kamacite. The Coahuila II Fe,Ni-alloy shows a prominent increase in susceptibility above about 300 °C probably indicating phase transitions in the Fe-Ni system (see magnetic phase relations described in Garrick-Bethell and Weiss 2010). Little variation in susceptibility in the low-temperature curve of the Morasko and Coahuila II Fe,Ni-alloy may indicate the presence of further magnetic phases, which are, however, masked by the strong magnetic susceptibility of the Fe,Ni-alloy.

Heating up to 800 °C in an argon atmosphere causes phase transitions in the Fe,Ni-alloy of all three meteorites. The cooling curve is not reversible, but shows two TCs: (a) a minor one at 770 °C; and (b) a major one at about 630–650 °C.

For Fe and Fe,Ni-alloy with Ni <10%, a TC of 770 °C is reported (Xiong et al. 2011). The slightly higher TC in two of our samples is in accordance with a small Co concentration in the irons, which increases TC (e.g., Kalinin et al. 1970). Cobalt concentrations of about 0.5% are analyzed in the Fe,Ni-alloy of all three meteorites (see Table 1).

Discussion

Geochemical Classification and Characterization of Magnetic Minerals in the Meteorites

We have investigated the geochemistry and mineralogy of different magnetic phases from the Fe,Ni-alloy, troilite, and transition zone from the Morasko, Coahuila II, and Mundrabilla irons. According to the classification given in Choi et al. (1995), the Morasko and Mundrabilla meteorites belong to the nonmagmatic IIICD group (Fig. 10). Although the Mundrabilla meteorite is consistent in many properties with the IIICD group, a low Ir concentration in the metal and the high amount of troilite defines this meteorite as an anomalous member of the IIICD group (Choi et al. 1995).

Figure 10.

Discrimination diagrams (a: Ge versus Ni, b: Ir versus Ni, c: Ge versus Au, d: Ir versus Au) with the fields of the IAB/IIICD and IIAB iron meteorite groups (from Mittlefehldt et al. 1998).

Figure 10 shows the discrimination diagrams, according to which the iron meteorites can be classified. It becomes clear that for Ir concentrations of about 0.5–1.7 ppm and low Ni concentrations of about 6–7 wt%, which have been analyzed for our samples, the discrimination between IAB and IIICD is difficult. Also, the concentration ranges for the nonmagmatic IAB and IIICD, and the magmatic group IIAB overlap. IIAB are described as having the highest S-contents and the largest known range in Ir concentrations of any magmatic group of iron meteorites, with an extremely steep negative correlation versus Au (Wasson et al. 2007).

The partition coefficients for Au are much lower and Au shows a considerably bigger concentration range. Therefore, elemental correlations using Au instead of Ni are proposed to be more discriminative (Wasson 1999; Wasson and Kallemeyn 2002). This correlation allows a clear distinction between the groups IAB and IIICD. The latter is now regarded as a subgroup of IAB, and also a clear distinction compared with the IIAB is possible. Based on this classification, Morasko is classified as IAB and Mundrabilla as intermediate between IAB sLL and sLM (low Ni, low Au; low Ni, medium Au; Wasson and Kallemeyn 2002), whereas Coahuila II is currently not assigned to one of the groups. Compared with the well-known IIAB Coahuila meteorite, we observe considerably lower Ir concentrations in the Coahuila II meteorite (1.7 μg g−1 Ir in Coahuila II instead of 14.63 μg g−1 in Coahuila, Petaev and Jacobsen 2004; 14.4 μg g−1, Wasson et al. 2007). Plotting Co, Ge, W, and Ir against Au, as proposed by Wasson and Huber (2006) and Wasson et al. (2007), Coahuila II separates from Morasko and Mundrabilla. Coahuila II plots in or near the field of IIAB meteorites, but Ir concentrations are too low for the low Au content. Morasko and Mundrabilla are plotting within the IAB field (Fig. 10). Therefore, Coahuila II can be probably regarded to be an octahedrite of the IIAB group.

The magnetic mineralogy is very similar in all three investigated iron meteorites, independent of group and cooling history. The only difference is the higher amount of schreibersite and cohenite in the well-developed graphite-rich transition zone of the Coahuila II and Morasko meteorites. Although the transition zone in the Morasko and Coahuila II irons is characterized by ferrimagnetic schreibersite and/or cohenite, their magnetic susceptibility cannot be distinguished from those of the troilite. The main reason for this behavior is the occurrence of minor magnetite in both areas, which is indicated by the k-T curves. Magnetite shows a much higher susceptibility than cohenite and schreibersite (Fig. 9). As magnetite was not observed microscopically in the troilite and transition zone, the magnetite grains are probably very small, and they are either an evidence for terrestrial oxidation or a product of heating during the terrestrial impact. Microscopically, magnetite has only been identified as μm-sized tiny euhedral crystals in porous Fe,Ni-alloy, which occurs as irregular patches at the boundary of Fe,Ni-alloy toward the transition zone (Figs. 3d and 4d). This texture indicates some kind of reheating under oxidizing conditions, probably due to the impact on Earth. Minor amounts of magnetite, kamacite, and other nonmagnetic phases are also described for graphite nodules of the Canyon Diablo meteorite (Coey et al. 2002). These authors measured three Curie temperatures at 757, 587, and 297 °C in the graphite nodules and related them to kamacite, magnetite, and ferromagnetic graphite, respectively. In the graphite-enriched transition zone of iron meteorites in our study, we also observed as magnetically dominant phase magnetite. However, neither a Curie temperature for kamacite (TC: 770 °C) nor ferromagnetic graphite (TC: 300 °C) has been measured, but we identified additionally to magnetite (TC: 590 °C), daubreelite (TC: −115 °C), and cohenite (TC: 176 °C) or ferrimagnetic schreibersite (TC: 140–150 °C) as magnetic phases. These latter phases might contribute significantly to the magnetization of iron meteorites, although their amount compared with the Fe,Ni-alloy is minor.

Crystallization of Metal and Other Phases

The different textures of Morasko and Coahuila II compared with Mundrabilla imply significant differences in the cooling history. The distribution of minerals in the meteorites of Morasko and Coahuila II suggests that metal and S-, P-, and C-rich phases were separated by liquid immiscibility of melt liquids and then solidified.

Scott (1972) proposed that the siderophile element concentrations in 10 of the 13 iron meteorite groups are caused by crystal-liquid (magmatic) fractionation (Liu and Fleet 2001). However, from a melt containing Fe, Ni, and S in a bulk composition of the investigated meteorites, the metal phase will initially crystallize (Hsieh et al. 1982; Chabot et al. 2007). This will lead to enrichment of S in the residual melt until the eutectic point is reached and eutectic mixture of metal phase and sulfide phase would be solidified. This would result in a solid metal phase and an intermixture of troilite and metal.

Structures, similar to this scenario, have been observed experimentally (Chabot et al. 2007) and in the sulfide-rich Sahara 03505 iron meteorite (D'Orazio et al. 2009). Over recent years, the influence of the light elements S, C, and P on the distribution coefficient between solid metal phase and melt has been studied experimentally (Chabot et al. 2007, 2009, 2011; Hayden et al. 2011). According to these studies, the distribution coefficients of siderophile elements increase with increasing S-content in the melt phase. Nickel has only a minor influence on the distribution of trace elements between solid and molten metal phase (Chabot et al. 2007). This behavior results in a strong enrichment of Co, Ni, Ge, Ga, and the PGE in the metal phase, as well as a weak enrichment of Mo. This is also in agreement with the results of our study in which we observe approximately 5000 μg g−1 Co, 100 μg g−1 Ga, and 100–400 μg g−1 Ge in the metal phase and only low concentrations in troilite. Cr, Cu, and other chalcophile elements enrich significantly until the system crystallizes at the eutectic point (Chabot et al. 2009). According to the μEDXRF-results, we observe 6000–10000 μg g−1 Cr, approximately 200 μg g−1 Cu and 30–300 μg g−1 Se in troilite. In such Fe-Ni-S–dominated system, a finely crystallized eutectic mixture of metal and sulfide phase should be present, similar to that observed in the Sahara 03505 meteorite (D'Orazio et al. 2009). Instead, we can observe massive troilite nodules in the metal phase. In the eutectic system, slow cooling rates may produce coarser crystallites but not a separation of melts, and thus no droplets would be observed. This may be the case for Mundrabilla with its high S-content and low C- and P-contents. Nevertheless, the big cm-sized pure troilite fillings of interstices between metal crystals (Fig. 1b) do not appear as a typical eutectic crystallization as observed in the Sahara 03505 (D'Orazio et al. 2009).

In the presence of P and/or C and low S-contents in the melt, the system becomes more complicated. Both the systems Fe-S-P and Fe-S-C are characterized by relatively large regions with a liquid–liquid immiscibility (Raghavan 1988; Ulff-Møller 1998; Chabot and Drake 2000; Goldstein et al. 2009). In this case, the crystallization of the metal phase leads to an accumulation of S, P, and C in the melt until the area of liquid–liquid immiscibility is reached and separation into S-, P-, and possibly also C-rich melt occurs. This can lead to a strong enrichment of schreibersite and cohenite at the interface between metal and troilite. In the Coahuila II and Morasko meteorites, a boundary layer with an enrichment of Ni and P (schreibersite) around the troilite nodules can be observed, followed on the metal facing side by a zone depleted in Ni. SEM-images show that this area can also be enriched in metal carbides (cohenite) as observed in the Morasko meteorite (Figs. 4a and 7). In the Morasko meteorite, Mo is significantly enriched at the troilite–metal interface on the side facing the metal. To a lesser extent, this is also the case for the Coahuila II meteorite. Cohenite and Mo-rich carbides have been observed in graphite spherules of presolar dust (e.g., Jadhav 2009). Consequently, Mo is incorparted into the cohenite or bound as carbide and enriched in the contact zone, where a small amount of C-rich melt separated between solid metal, P-rich melt, and S-rich melt (Fig. 7). Our interpretation is that the Ni-depleted metal between schreibersite and Fe,Ni-alloy marks the point where the crystallization of schreibersite started after the field of liquid immiscibility was reached and two melts separated. At Morasko in the region between schreibersite and Fe,Ni-alloy, Ni-poor cohenite was observed by SEM (Figs. 3a and 7b).

Hayden et al. (2011) investigated the trace element partitioning in the Fe-S-C system by partial melting a mixture of Fe–metal containing approximately 1.5% C and FeS. By heating above 1150 °C (above the eutectic point of the Fe-S-C system), two immiscible liquids, a S-rich melt and a C-rich melt, were observed. In systems running below this point, solid metal and S-rich melt dominated the systems. Above the eutectic point, the distribution coefficients of several trace elements changed drastically: elements like Mo and W with a strong affinity to C are enriched in the C-rich melt. According to this observation, if the field of liquid immiscibility is reached above the Fe-C eutectic (1153°), Mo and W will be strongly enriched in the C-rich liquid (Hayden et al. 2011). This is in agreement with our observations on the transition zones between metal and troilite of the Morasko samples, where in high-resolution profiles across the transition zone, the Mo enrichment is clearly separated from the schreibersite with its extremely high P and Ni contents (Figs. 7a and 7b). In this scenario, two (or three) separate melts form, S-rich and C-rich, and possibly an additional P-rich melt. Due to its affinity to C, Mo is strongly enriched in the C-rich melt. For the macroscopically and microscopically similar Coahuila II iron, only a very weak enrichment of Mo is observed in the transition zone.

The Mundrabilla meteorite shows a completely different behavior. Microscopically, neither Widmanstätten pattern nor Neumann lines can be observed, although the former are macroscopically described (Buchwald 1977). The lack of microscopically visible Widmanstätten pattern and the equigranular grains with triple junctions indicates reheating after the initial crystallization (e.g., Yang et al. 2011). The daubreelite lamellae at the troilite–metal interface and along parallel lines in the troilite (Figs. 7c and 8c) probably also indicate a temperature gradient perpendicular to the metal–sulfide contact during this reheating process.

The missing transition zone between metal phase and troilite in Mundrabilla, not showing a schreibersite- and graphite-rich rim, may be an indication that the P- and C-content of the initial melt was lower than in the Morasko and Coahuila II meteorites and the S-content was much higher. With up to 35% troilite, this iron is extraordinary S-rich and the field of liquid immiscibility cannot be reached above the eutectic point of the Fe-S system. In this case, first, the Fe,Ni-phase will crystallize and S, C, and P concentrations will increase and μm-sized, often isolated crystals of schreibersite and cohenite will occur close to the metal–troilite boundary as observed in our Mundrabilla sample, where a line of tiny schreibersite crystals is observed in the metal phase parallel to the metal–troilite boundary.

Another possibility is that due to faster cooling, μm-sized droplets of P-rich melt were enclosed in the crystallizing metallic melt (see Figs. 7c and 8c). The gradual decrease in Ni from schreibersite (indicated by the Ni peak in Figs. 7f and 8c) to the troilite contact from approximately 6–7% to approximately 4% Ni accompanied by an increase in Fe may be an indication for diffusive transport of Ni in the metal close to the troilite contact.

In the literature (e.g., Goldstein et al. 2009), there is agreement that the nonmagmatic IAB and IIICD iron meteorites experienced a crystallization history and environment substantially different from the fractional crystallization of an undisturbed metallic melt (magmatic groups). This is in agreement with the element distributions of this study comparing the IAB meteorite Morasko with Coahuila II. Both show similar element distribution maps along profiles from the Fe,Ni-alloy toward the troilite phase (Fig. 8) and similar mineralogical phases, suggesting a similar crystallization history with slow cooling rates. Both meteorites show concentration gradients of siderophile elements (Ga, Ge) in the cohenite and graphite-rich transition zone with increasing concentration near the metal contact. This can be explained by diffusive transport of these elements during the cooling period after solidification of the metal and the C-rich melt.

Cohenite Formation and Carbon Distribution

Both the Morasko and Coahuila II meteorites contain significant concentrations of carbon in the transition zone between Fe,Ni-alloy and troilite. Microscopically, graphite and cohenite (only in Morasko) have been identified as carbon-bearing phases. According to Buchwald (1977), cohenite is a typical phase of group I and is present, but in smaller amounts, in group IIAB meteorites. In our Coahuila II sample, graphite is abundant, but no cohenite occurs (Table 1). Although graphite is also described in Mundrabilla (Buchwald 1977), it does not occur in the sample of our study.

A systematic difference in the carbon isotopic composition from graphite and cohenite is described in several studies (Satish-Kumar et al. 2011 and references therein). It is interpreted to reflect isotopic equilibrium due to isotopic fractionation properties of the phases involved. According to the experimental determination of carbon isotope fractionation between iron carbide melt and carbon (Satish-Kumar et al. 2011), there exists a temperature-dependence of carbon isotope fractionation between cohenite and graphite. This equilibrium fractionation can be used to estimate the formation temperatures of planetary material. Under the precondition that the isotope fractionation reflects isotope equilibrium between the two phases in the Morasko transition zone, the strong isotope fractionation of about 15‰ between cohenite and graphite suggests equilibrium temperatures of about 500 °C. Slightly higher temperatures of 650 and 600 °C were obtained for the Canyon Diablo and Magura iron meteorites, respectively (see fig. 6 in Satish-Kumar et al. 2011). Goldstein et al. (1976) suggested that cohenite and metal equilibrated at temperatures below 600 °C, which is in agreement with the temperatures obtained from carbon isotope fractionation. Both Morasko and Canyon Diablo belong to the nonmagmatic IAB group iron meteorites with slow cooling rates. The relatively low equilibrium temperatures, therefore, might rather be interpreted as closure temperatures for the isotopic systems than formation temperatures of the meteorites. This interpretation is in agreement with the study of Sugiura (1998), according to which the carbon isotopic composition systematically changes with cooling rates and a large fractionation is observed between the phases for the most slowly cooled iron meteorites.

Cohenite is described almost exclusively in meteorites containing 6–8 wt% Ni and the temperature at which cohenite forms is between 650 and 610 °C (Brett 1966). Cohenite in the Morasko iron meteorite is interpreted as a stable thermodynamic phase crystallized from C-rich melt. It also should be mentioned that at high pressures, cohenite coexists with liquid metal (Chabot et al. 2008). This observation is in agreement with the occurrence of a large single crystal (magnetic domain pattern showing the same orientation in all grains), which formed at the troilite–metal interface. The grains show strong fracturing, along which Fe and graphite have formed.

Shock and Reheating of the Iron Meteorites

While the reheating of the Mundrabilla meteorite is interpreted as cosmical, in the Morasko and Coahuila II meteorites, probably local high-explosive shock-related reheating during terrestrial impact is recognized. For the latter interpretation, we consider the following observations: (1) Along impact-induced cracks in cohenite, the assemblage graphite and iron (Fe3C → 3Fe + C) occurs (Figs. 3a and 3b), indicating a local reheating above 650–700°C (e.g., Lipschutz and Anders 1961). (2) The porous structure in the Fe,Ni-alloy with euhedral μm-sized magnetite (Morasko) or schreibersite and magnetite (Coahuila II) crystals in pores suggests partial melting at the metal–troilite interface. Similar textures, albeit with different mineralogy from those shown in Fig. 3d with a sharp boundary between molten and nonmolten material, and tiny crystals formed within the molten part have been observed in experimentally shocked pyrrhotite ore at 30 GPa (see fig. 3c and 3d in Mang et al. 2013). As schreibersite and magnetite form under clearly different oxygen fugacity, they do not represent equilibrium conditions. Such heterogeneous conditions require addition of oxygen to the strongly reduced meteoritic system and we suggest that the tiny euhedral magnetite crystal formed locally from melt during the terrestrial impact. Fe-oxides are also observed along cracks in the Fe,Ni-alloy (Figs. 4a and 4b). We interpret the common occurrence of magnetite seen in the k-T curves of the troilite and transition zone (the signal of magnetite cannot be seen in the Fe,Ni-alloy because of low amount and magnetic susceptibility compared with the iron) and microscopically in the Fe,Ni-alloy as a product of local reheating during the impact on Earth.

Conclusions

Fe,Ni-alloy–troilite interfaces of two nonmagmatic group IAB (Morasko and Mundrabilla) and one up to now ungrouped iron meteorite (Coahuila II: preliminarily assigned to IIAB group based on Ir and Au content in this study) were microscopically and geochemically studied and the magnetic mineralogy of the different zones was investigated.

Two different texture types were distinguished: Morasko and Coahuila II contain troilite nodules with a several mm-thick carbon- and phosphorous-rich transition zone and the Fe,Ni-alloy shows Neumann lines. The transition zone is characterized by schreibersite, cohenite, and graphite, with schreibersite enriched close to the troilite, whereas cohenite and graphite are enriched close to the metal. The minerals from the transition zone, troilite, and the Fe,Ni-alloy are interpreted as having been crystallized from immiscible melts, either by fractional crystallization and C- and P-enrichment in the melt until the field of immiscibility is reached, or by partial melting at temperatures slightly above the eutectic point. The zonation indicates that, at first, the metal solidified, then the C-rich melt, followed by the P-rich and at last the S-rich melt. Large single crystals of cohenite and schreibersite and diffusive concentration gradients in the transition zone indicate slow cooling rates, which is in agreement with observations from experimental studies (e.g., Goldstein et al. 2009). Mundrabilla differs in that it shows no geochemically visible transition zone and the Fe,Ni-alloy is re-equilibrated, indicating a penetrating reheating event after crystallization of the phases. The magnetic mineralogy of the three iron meteorites is similar, with schreibersite (and cohenite: Morasko), daubreelite, and magnetite as magnetic phases in the troilite and transition zone (only in Morasko and Coahuila II). The Curie temperatures of cohenite and schreibersite (nonmagnetic and magnetic varieties) from this study (180 °C and 140–150 °C, respectively) differ from those described in the literature (215 °C and 350–400 °C, respectively; Weiss et al. 2010). Further systematic rock magnetic and magnetic mineralogy studies including mineral chemical investigations are needed to understand the magnetic properties of these ferrimagnetic phases in dependence of their chemical composition. The here-presented study has shown that temperature-dependent magnetic susceptibility measurements are a rapid tool for detecting these phases.

The chondrite and Fe normalized trace element distribution of the three meteorites in the Fe,Ni-alloy near the interface to the troilite nodules is similar, but elements like W, Ru, Pt, Rh, Ga, and Ge are slightly enriched in Morasko and depleted in Mundrabilla, albeit both meteorites are classified as IAB group. Both are depleted in Ir. The Coahuila II is characterized by a depletion of Au and Ir plots near 1. All other elements are very similar to those of the Morasko meteorite. On the basis of the Ir/Au ratio, we assume that Coahuila II can be regarded to be an octahedric IIAB. The transition zones of Morasko and Coahuila II are characterized by a significant C- and P-enrichment, and Mo is enhanced in the C-rich part indicating that Mo is incorporated into cohenite, which indicates that in the late cooling stage, the liquid departed to three immiscible melts, S-rich forming troilite, P-rich forming schreibersite, and C-rich one forming cohenite. The point where the crystallization of schreibersite started, after the immiscible melts departed, is probably marked by the occurrence of metal depleted in Ni. Especially, Ge shows a clear gradient within the cohenite from schreibersite toward the Fe,Ni-alloy indicating diffusive transport after solidification.

Both sulfur and carbon isotope ratios are typical for primarily crystallized phases in iron meteorites. The strong carbon isotope fractionation in the transition zone of Morasko of about 15‰ suggests equilibrium temperatures of about 500 °C for cohenite and graphite, which are interpreted as closure temperatures for the isotopic system. The growth of mm-sized cohenite crystals is in agreement with slow cooling rates.

The challenge of meteorite magnetic studies is the discrimination of cosmogenic and Earth-related features. Magnetic mineralogy studies like the one we present here are essential to understand the origin of the magnetic record. Magnetic phases like cohenite and schreibersite, which are magnetically not well understood, are predestined to carry primary magnetic records from the formation of the iron meteorites. Cohenite as a main carrier of natural remanent magnetization is reported for the Abee meteorite by Sugiura and Strangway (1981) and for lunar basalts by Gattacceca et al. (2010). Magnetite formation is assumed to be a product of oxidation and local shock-related reheating when the meteorites struck Earth. Magnetic signals of magnetite therefore have to be eliminated before obtaining a characteristic magnetic record from the meteorite.

Acknowledgments

The authors thank A. Muszynski (Institute of Geology, University of Poznań, Poland) for providing the sample of the Morasko meteorite, J. Hochmann (Saltillo, NE Mexico) and W. Stinnesbeck (University of Heidelberg) for a sample of the Coahuila II meteorite, and U. Gebhardt (Staatliches Museum für Naturkunde, Karlsruhe, Germany) for the meteorite sample of Mundrabilla (Australia). One of the authors (W.L.) deeply appreciates A. Muszyński for his intensive help becoming familiar with Poznań people, its landscape, and geology during a long-lasting stay at his university in Poland. W.L. thanks W. Stinnesbeck (University of Heidelberg, Germany) and E. Frey (Staatliches Museum für Naturkunde, Karlsruhe, Germany) for introduction in their research area in northern Mexico and finally U. Gebhardt for her permission to inspect the meteorite collection of the Staatliches Museum für Naturkunde (Karlsruhe, Germany). This study would not have been possible without the help of C. Haug, C. Mößner, K. Nikoloski, B. Oetzel, G. Preuss (IMG), P. Lied (IAG), and V. Zibat (LEM). The fruitful discussions with Z. Berner (KIT, Karlsruhe) are thankfully acknowledged. We are grateful to Ian Ray for improving the English language and his information about the discovery of the irons. The manuscript benefited from very careful and constructive comments of M. Petaev, an anonymous reviewer, and the associate editor C. Koeberl.

Editorial Handling

Dr. Christian Koeberl

Appendix

Table A1. Median (Med.), P10, and P90 variation range of the µEDXRF mapped areas of the distinct mineral phases troilite, Fe,Ni-alloy (FeNi), and the transition zone (TZ) within the investigated meteorites.

  MoraskoCoahuila IIMundrabilla
Troilite P-rich TZ TZ Fe,NiTroilite P-rich TZ TZ Fe,NiTroilite TZ Fe,Ni
  1. n.d. = not detectable.

  n 315 70 111 154499 13 103 85119 97 250
S (%)Med.34>4.5>2.7>0.739>4>3>238>33>6
P1028 2 0.7 0.427 2 1 131 25 2
P9037 6 8.1 2.343 4 16 241 37  
Se (µg g−1)Med.280>40>30>10250>170>50>10n.d. n.d. n.d.
P10230 20 10 0220 10 10 0  
P90330 70 60 30310 190 160 20  
Cu (µg g−1)Med.264>205>92 118213>97>33 46214>200>77
P10209 164 61 90165 67 7 20137 102 22
P90468 305 175 154361 105 180 69281 308 138
Cr (%)Med.0.7>0.1 0.1>00.6>0.1 0.1>01.4 1.5>0.4
P100.6 0 0.1 00.5 0 0 01.1 1.1 0.1
P900.8 0.1 0.2 0.10.6 0.1 0.3 0.12.5 12 0.9
Fe (%)Med.63<61<90<9159<75<92 9059<63<85
P1060 58 81 8855 54 79 8955 57 83
P9069 68 94 9269 76 95 9264 69 88
Ni (%)Med.0 17 4<60 19 3<61<3<7
P100 14 3 60 16 1 50 2 6
P902 18 6 71 20 4 63 4 9
Co (%)Med.n.d.<0.3<0.35<0.71n.d.<0.24 0.2<0.51n.d.<0.2<0.49
P10 0.25 0.25 0.65 0.16 0.2 0.46 0.1 0.44
P90 0.37 0.46 0.75 0.25 0.3 0.58 0.3 0.59
Ge (µg g−1)Med.15<41<81<44723<62 59<34618<32<121
P104 21 36 3757 23 31 2924 4 84
P9039 76 157 52045 69 87 38847 86 155
Ga (µg g−1)Med.9 6 9<7412 13 11<6016 11<19
P102 1 2 432 1 2 324 1 4
P9022 25 24 10235 14 33 9532 24 42
P (%)Med.n.d. 17 n.d. n.d.n.d. 17 n.d. n.d.n.d. n.d. n.d.
P10 10   4    
P90 19   24    

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