An igneous fragment from cluster IDP L2011#21: An analog for the source of pyrrhotite and taenite in comet 81P/Wild 2 captured in Stardust aerogel

Authors

  • Frans J. M. Rietmeijer

    Corresponding author
    • Department of Earth and Planetary Sciences, MSC03-2040, 1-University of New Mexico, Albuquerque, New Mexico, USA
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E-mail: fransjmr@unm.edu

Abstract

The silica glass extracted from the bulbous parts of Stardust tracks is riddled by electron-opaque nanograins with compositions that are mostly between pyrrhotite and metallic iron with many fewer nanograins having a Fe-Ni-S composition. Pure taenite nanograins are extremely rare, but exist among the terminal particles. Assuming that these Fe-Ni-S compositions are due to mixing of pyrrhotite and taenite melt droplets, it is remarkable that the taenite melt grains had discrete Fe/Ni ratios. This paper presents the data from an igneous pyrrhotite/taenite fragment of cluster IDP L2011#21, wherein the taenite compositions have the same discrete Fe/Ni clusters as those inferred for the Stardust nanograins. These Fe/Ni clusters are a subsolidus feature with compositions that are constrained by the Fe-Ni phase diagram. They formed during cooling of the parent body of this cluster IDP fragment. These specific Fe/Ni ratios, 12.5, 24, 40, and 53 atom% Ni, were preserved in asteroidal taenite that survived radially outward transport to the Kuiper Belt where it accreted into the (future) comet Wild 2 nucleus.

Introduction

The small area (30 cm2) collectors (SACs) that are flown mounted underneath the wings of high-flying aircraft at 17–19 km altitude in the lower stratosphere since May 22, 1981 as part of the NASA/Johnson Space Center Cosmic Dust Program occasionally contain dark mounts of hundreds of fragments ranging from “fines” (<5 μm) to interplanetary dust particle (IDP)-sized (approximately 10 μm) fragments. SACs are still flown today. Cluster IDPs would break apart upon inertial-impact collection (Brownlee et al. 1977). Since May 1985, the large area (300 cm2) collectors (LACs) saw a dramatic increase in collected cluster IDPs compared with their SAC incidences. Could it be true that the LACs were sampling specific events, e.g., meteor showers, or was there a dramatic increase in the incidence of cluster IDPs in the stratosphere after LACs were deployed? Using the NASA/JSC cosmic dust catalogs as a resource for statistical analyses should be done with the utmost caution, but they may be first-order information (Rietmeijer and Warren 1994; Rietmeijer and Jenniskens 2000; Mateshvili and Rietmeijer 2002). For example, when normalized to a background level, the data supported an almost constant annual supply of collectable cluster IDPs in the lower stratosphere on LACs and SACs (Rietmeijer 1997a). This incidence of cluster IDPs would be akin to a sporadic meteor background. The same resource showed that an average cluster IDP consists mostly of chondritic IDP-like particles with many fewer Mg,Fe-silicates and a very low amount of sulfides that are mostly 2–10 μm in size, plus a small fraction that is >19 μm (Rietmeijer 1997b). Messenger (2000) found that the most carbon-rich IDPs were fragments from cluster IDPs and that they had the highest D/H ratios compared with noncluster IDPs. Cluster IDPs were much sought after because their high (but generally unknown) porosity would reduce peak heating temperatures incurred during deceleration in the atmosphere, which would enhance the possibility of intact survival of their individual constituent grains.

Cluster IDP L2008#5, which was studied in some detail, included some unanticipated constituents such as particles with a plagioclase-like bulk composition and an approximately 10 μm nonchondritic particle with SiO2 = 80 wt% (fragment clu320) (Thomas et al. 1995). These particles were not out of the ordinary. Smaller, micron-sized pure silica and amorphous aluminosilica grains are present in some chondritic aggregate IDPs (for a review, see Rietmeijer 1998a, 2002). The diverse mineralogy of its fragments might be an indicator of cluster IDP origin, e.g., an asteroidal breccia for cluster IDP L2008#5 (Thomas et al. 1995), which is plausible, given that the regolith surfaces of the asteroid Eros (McCoy et al. 2001) and asteroid Itokawa surface (Fujiwara et al. 2006) show that mineral dust is present at asteroid surfaces with the possibility of clusters IDPs.

The incidence of cluster IDPs on the simultaneously flown LACs L2009 and L20011 stands out among all SACs and LACs flown prior to mid-1991 (Rietmeijer 1997a). Both collectors were exposed incrementally during several collection flights in June and July, 1991, and could have captured meteor stream particles that are associated with active comets (Rietmeijer 2000; Jenniskens 2006). This linkage is speculative, but not without some substance. Among the collected particles was cluster IDP L2011#21 that, in Cosmic Dust Catalog 13 (September 1992), is represented by fragment L2011F1. It is an 8 × 6 μm chondritic aggregate particle with a whisker-shaped grain, which is a reasonably reliable indicator for an extraterrestrial origin (Bradley et al. 1983), and by inference, the entire cluster IDP. Cosmic Dust Courier 12 (February 1997) listed four additional fragments, F4, F6, 11, and 12, of this cluster IDP, but no catalog-level information is available for these cluster fragments, i.e., optical properties, a scanning electron microscope image, and an energy disperse (chemical) spectrum. Fragment L2011F4 (assumed size: 20 μm) was step-heated to determine its noble gas content and 3He/4He ratio (4.46 × 10−3) and to estimate its atmospheric entry peak heating temperature (660 °C) (Nier and Schlutter 1993). Excess 3He in particles from cluster IDPs on the L2009 and L2011 LACs was consistent with those anticipated for heavily pre-irradiated regolith material on Kuiper Belt comets (Pepin et al. 2001). These particles, including cluster IDP 2011#21, once liberated from their sources, re-entered the inner solar system. Jupiter Family comet 81P/Wild 2 from the Kuiper Belt (Brownlee et al. 2004) contains similar minerals and constituents (chondrule fragments, CAIs) to those found in unequilibrated ordinary chondrites (UOCs) (Brownlee et al. 2006; Zolensky et al. 2008; and many others). The current model explaining this surprising finding foresees long-range dust transportation in the equatorial plane of dust that had formed in collisionally disrupted, evolving objects in the asteroid belt radially outward into the present-day region of the Kuiper Belt (Brownlee et al. 2006; Ciesla 2007). Thus, an interesting corollary for the minerals and organic compounds in cluster IDPs from L2009 and L2011 was held in cold storage for 4.56 Gyr with no apparent changes.

This paper seeks to understand the origin of taenite grains that were captured in the underdense, density-graded silica aerogel of the Stardust mission (Leroux et al. 2008a), using the analog of a low-Ni Fe-sulfide/taenite particle from cluster IDP L2011#21 that is linked to an igneous origin in the asteroid belt, which is consistent with the presence of protoenstatite (Schmitz and Brenker 2008), ortho- and clino-enstatite polymorphic intergrowths (Jacob et al. 2009) and igneous Ca-rich pyroxene (Leroux et al. 2008b) in comet 81P/Wild. Rietmeijer (2004a) initially proposed that low-Ni iron and taenite in fragment F6 of this cluster IDP were the result of pyrrhotite desulfurization delivered in comets (Rietmeijer 1988) due to impact-induced volatilization and/or photo- or ion-induced sputtering of sulfur from the surface of a large asteroid such as 433 Eros (Nittler et al. 2001), or the loss of an FeS-rich partial melt (Nittler et al. 2001). Alternatively, sulfidation of Fe,Ni alloys condensed in the solar nebula (Meibom et al. 2000) could have produced Fe,Ni-sulfides.

In light of the results from the Stardust mission that supported long-range, radially outward dust transportation from the inner solar nebula to beyond 40 AU (Brownlee et al. 2006), this paper identifies petrologic properties caused by atmospheric entry flash heating to define the original properties of fragment F6 from cluster IDP L2011#21 and to identify the nature of its parent body.

Experimental

Ultra-thin sections cut at UNM from fragment F6 were placed on a holey carbon thin-film that was supported on standard 200 mesh Cu grids. Individual grids were housed in a Gatan low-background, double-tilt specimen holder for analysis using a JEOL 2000FX analytical and transmission electron microscope (ATEM) that operated at an accelerating voltage of 200 keV. It was equipped with a Tracor-Northern TN-5500 energy dispersive spectrometer (EDS) for in situ analysis of elements >11 atomic number using a 15 nm probe size. Quantitative analyses were made using the Cliff and Lorimer (1975) thin-film procedure based on experimentally determined k-factors on natural standards. The error in major-element abundances is 5% relative. The probe size and beam current were monitored during analyses to ensure reproducible conditions. To test accuracy and precision of EDS analyses, ultra-thin sections of olivine and pyroxene standards were analyzed using the same experimental conditions as for the sample. Grain size was measured directly on TEM negatives with an error of <10% relative. Selected area electron diffraction (SAED) was used to determine the crystallographic properties with a relative error between 5 and 10% for the measured interplanar (d-spacing) values.

Observations

The available information for cluster IDP L2011#21 shows six individual particles, viz. a chondritic porous aggregate IDP (F1); an unknown type (F4); a nonchondritic Fe,Ni,S-particle (F6a); and three lenticular, amorphous aluminosilica particles (F6b, 11, 12) with a rich mineralogy including protophyllosilicates (Table 1). Particle F6a is the only known sulfide IDP with an associated Fe,Ni-metal grain (Fig. 1). This sulfide shares many of the petrographic features of previously described flash-heated sulfide IDPs from another cluster IDP (Rietmeijer 2004b). This sulfide is a single grain (6.2 × 5.8 μm) plus a smaller (1.7 × 0.9 μm) anhedral sulfide grain. Serial thin sectioning showed that the grain was approximately 6 μm thick. During ultramicrotome sectioning, the sulfide grain shattered into the typical laths of flash-heated sulfide IDPs (Rietmeijer 2004b). The largest sulfide grain contains a Fe,Ni-metal inclusion (1.7 × 1.5 μm) that is covered by a patch of silicate material. Smaller patches ranging from approximately 300 nm up to 1.5 × 0.5 μm in size occur randomly on the particle surface.

Table 1. Fragments of cluster IDP L2011#21(modified after Rietmeijer 1998b)
 Size (μm)Petrology
  1. Cluster particle L2011#21-F6 consisted of two subparticles. Note: the fragments of this cluster IDP were discussed generally, viz. (1) a possible asteroidal origin of cluster IDP L2011#21 (Rietmeijer 1998b) and (2) the possible flash heating origin of the silicate component in fragment F6 (Rietmeijer 2004a).

F18 × 6No data
F420 (assumed)3He/4He = 4.46 × 10−3 (Nier and Schlutter 1993)
F6

(a) 9 × 5.8

(b) 14.2 × 1.5

(a) FeS/FeNi particle and associated Mg,Fe-silicates (this paper), and

(b) amorphous aluminosilica matrix with opal-A, protophyllo-silicates and amorphous Fe-rich inclusions; turbostratic illite (d001 = 1.1 nm) grains (190 × 50 nm to 1750 × 330 nm), poorly ordered K-bearing dufrenite (760 × 570 nm), polycrystalline titanite (135 × 45 nm), disordered Ca,Al,Fe grains (390 × 180 nm)

1141 × 9Vesicular amorphous aluminosilica matrix with ordered circular features; silica, mullite, wollastonite, kaolinite-pyrophyllite grains, rare TiO2, and Bi-chloride
1225 ×  (8.5–0.3)Amorphous aluminosilica matrix with protophyllosilicates; microphenocrysts of tridymite with zircon and Cr,Pb inclusions, mullite, alkali-feldspar, low K2O di,tri-octahedral chlorite-smectite, Fe-oxides, TiO2, and ilmenite
Figure 1.

Transmission electron microscope images of two ultramicrotome thin sections cut from fragment F6a from cluster IDP L2011#2. Sulfides in flash-heated particles tend to shatter during ultramicrotome sectioning, which causes poorly transparent sections and shattering into narrow “laths” in approximately 70 nm thin sections. Fig. 1A shows the main sulfide with the smaller sulfide grains (arrowhead) and the covering fine-grain Fe-oxide rim as well as the chondritic material (lower right). This section shows excessive sample loss including the taenite (FeNi) grain. Fig. 1B is a larger section better showing the smaller sulfide grain (next to the scale bar) and fine-grained Fe-oxide rim (arrowhead) as well as the location of the FeNi grain.

The sulfide is covered by a continuous Fe-oxide rim that on one side is 175–350 nm thick, but only approximately 75 nm on the opposite side of the particle. The polycrystalline Fe-oxide rim consists of randomly distributed grains of equant hexagonal grains (approximately 80–135 nm) and elongated hexagonal grains (up to 75 × 105 nm). Adjoined grains show equilibrium triple-junctions. These Fe-oxide rims are a common feature of IDPs that developed in response to atmospheric entry flash heating (Zolensky et al. 1994; Rietmeijer 1998a).

Pyrrhotite in the solar nebula is thought to have formed indirectly by sulfidation of an intermediate troilite phase (Zolensky and Thomas 1995). This modal was an elegant solution to an earlier observation that sulfides cannot be direct solar nebula condensates (Kerridge et al. 1979). Instead, Kerridge et al. (1979) suggested that low-Ni iron metal reacting with H2S at approximately 400 °C might yield subsulfur, Fe,Ni-sulfide plus taenite. However, the formation of pyrrhotite plus taenite will require a Fe-Ni-S melt at temperature of 650 °C and higher (Kullerud et al. 1969). The Fe-Ni phase diagram shows that equilibrium formation of Fe,Ni grains requires FeNi-melt temperatures between 1400 and 1500 °C. This temperature brackets the formation temperature of the particle presented. The petrographic texture of this fragment of cluster IDP L2011#21 is inconsistent with aggregation of pyrrhotite and taenite as might have occurred in the solar nebula. The observed texture of the taenite grain nicely welded in the larger pyrrhotite grain and the thermal range of formation are highly suggestive of an igneous origin for this particle.

Sulfide and Fe,Ni-metal

The sulfide has a mottled interior texture that toward the grain boundary becomes a distinct zone characterized by vesicles that increase in abundance, size, and shape from spherical to euhedral hexagonal grains, where it abuts the Fe-oxide rim. This zoned texture of sulfide IDPs is due to precipitation of platy Fe-oxide grains and sulfur loss (Rietmeijer 2004b). Oxidation leads to a defect structure for heated sulfide, 2+Fe1-3δ3+FeδS, wherein □ represents an iron vacancy and δ is the iron deficit (Condit et al. 1974) (Table 2). The defect structure due to Fe-oxidation and the structurally different sulfides as a function of S-content caused a disordered structure. The sulfide and intermediate Fe-Ni-S compositions are shown in Fig. 2, wherein all pure-Fe data are rim compositions. The low-Ni (1–3 atom%) sulfide compositions range from 52 to 61 S atom% (Table 2) in a continuous trend from Fe7S8 (pyrrhotite) to Fe3S4 (greigite) to Fe2S3 (a spinel type sulfide after greigite sulfidation) (Fig. 2). This compositional range from FeS to FeS2 and the defect structure of sulfide in this sample developed in response to atmospheric entry flash heating (Rietmeijer 2004b). The Ni content of sulfides, the intermediate Fe-Ni-S, and taenite grains are shown in Fig. 3.

Table 2. Sulfide composition (atom%) listed according to increasing S-content
 FeNiSStructural formulaδDefect structure
Minimum45.52.352.5Fe0.87Ni0.04S0.122+Fe0.643+Fe0.240.12S
Average42.01.956.1Fe0.75Ni0.03S0.252+Fe0.253+Fe0.500.25S
Maximum38.01.460.6Fe0.63Ni0.02S0.333+Fe0.670.33S
Figure 2.

Fe versus S (atom%) contents for low-Ni sulfides, the Fe-oxide rim, and the taenite inclusion in fragment F6a from cluster IDP L2011#2. The intermediate Fe-Ni-S compositions suggest postformation heating.

Figure 3.

Fe versus Ni (atom%) for sulfides (solid squares) and taenite (open squares) in fragment F6a from cluster IDP L2011#2.

The single-crystal and polycrystalline SAED patterns for the Fe,Ni-inclusion indicate randomly oriented taenite subgrains with variable unit cell dimensions (Table 3). While the fit between observed and ideal cubic unit cell values is not perfect, the data are consistent with disordered γ-taenite of variable Ni-contents. In fact, this taenite grain consists of domains with distinct Ni-contents, viz. 12.5, 24.2 ± 2.3, 39.7 ± 3.9, and 52.9 ± 3.1 atom% Ni. The calculated Ni content of the original presumably homogenous taenite crystal, 34 ± 14.5 atom% Ni, is preserved in a compositionally homogeneous central band that on both sides is flanked by domains with 24.2 and 52.9 atom% Ni. This compositional zoning is interpreted as a subsolidus feature (see below).

Table 3. Observed diffraction maxima (nm) taenite data J.C.P.D.S. file 23-297
hkl Range (observed)γ-taenite; 24.8 atom% Niγ-taenite; 100 atom% Ni
1000.36–0.350.3570.352
1110.21–0.200.2060.203
2200.12–0.130.1260.124

Silicates and Amorphous Materials

The silicates (Fig. 4) are (1a) Al-bearing (5.8 wt% Al2O3) endiopside (En66.5Wo33.5; Ca0.6Mg1.1Al0.2Si2O6), (1b) low-Al (0.5–4.5 wt% Al2O3) enstatite, and (2a) low-Fe olivine (Fo85–88Fa15–12) and (2b) high-Fe olivine (Fo58Fa42) (Table 4). In addition, there is a significant fraction of amorphous material with deep metastable eutectic serpentine dehydroxylate (O = 7) or smectite-dehydroxylate (O = 22) compositions (Fig. 5). These oxygen numbers refer to the assumed oxygen values that yield an internally consistent structural formula (Rietmeijer et al. 2002; Rietmeijer 2009a) (Table 4). The amorphous materials typically contain Al and rare Mn. These amorphous serpentine- and smectite-dehydroxylate compounds may contain ferric iron (Table 4). Their presence in this particle probably suggests that silicate melting and iron oxidation had occurred at some time, perhaps during atmospheric entry flash heating, although an earlier (unknown) heating event cannot be excluded. The Si- and CI-normalized bulk composition of the silicates and amorphous compounds are depleted in Ca, Cr, Mn, Fe, and Ni (Fig. 6). More severe depletions occurred in amorphous Mg,Al-silica compounds (Fig. 4: the data points Si >22 atom% and Mg <13 atom%).

Table 4. Olivine and pyroxene grain compositions and the amorphous magnesiosilica and ferromagnesiosilica materials with serpentine- and smectite-dehydroxylate stoichiometry associated with pyrrhotite/taenite in cluster IDP L2011#21
 OlivineOlivineEnstatiteEnstatiteEndiopside
SiO239.238.360.760.058.1
Al2O30.20.44.55.9
MgO45.926.438.732.621.2
CaO1.014.8
Cr2O30.20.4
MnO0.40.3
FeO14.9534.30.20.15
Sum100.0599.8100.0100.05100.0
      
Si0.981.052.022.032.00
Al0.010.010.180.24
Mg1.721.081.931.611.08
Ca0.040.55
Cr0.010.01
Mn0.010.01
Fe2+0.310.790.010.01
      
Fo/En84.757.899.597.066.5
Fa/Fs15.342.20.52.4
Wo0.633.5
 Serpentine dehydroxylateSmectite dehydroxylate
SiO246.242.465.358.2
Al2O31.13.11.7
MgO46.9543.331.527.3
MnO0.4
FeO5.4
Fe2O314.312.7
Sum100.0100.099.999.9
     
Si1.911.797.857.33
Al0.050.150.26
Fe3+0.210.41
Al0.29
Mg2.892.735.645.13
Mn0.01
Fe2+0.18
Fe3+0.250.80
     
Mg/(Mg+Fe+Ca)94.185.6100.080.9
Fe/(Mg+Fe+Ca)5.914.419.1
Figure 4.

Mg versus Si (atom%) for Mg,Fe-silicates (open squares) and Mg-silicates (dots) in fragment F6a from cluster IDP L2011#2.

Figure 5.

Mg versus Si (atom%) showing the distribution of stoichiometric crystalline silicates, viz. olivine (O = 4; open circles) and pyroxenes (O = 6; open squares), amorphous silicates O = 7 (dots) and O = 22 (solid squares); and nonstoichiometric silica-rich amorphous materials (solid triangles) in fragment F6a from cluster IDP L2011#21.

Figure 6.

Si- and CI-normalized bulk compositions for all silicates (solid squares) and the amorphous Mg,Al-silica compounds (open squares) in fragment F6a from cluster IDP L2011#2.

Ferric iron in these compounds is most likely due to oxidation during atmospheric entry flash heating that affected the low-Ni Fe-sulfides and caused the Fe-oxide rim. The systematic differences in rim thickness could suggest a leading edge that developed during deceleration in the atmosphere. Chondritic aggregate IDP-like material is seen attached to the surface of Fe-sulfide and silicate fragments of cluster IDPs, on Fe-sulfide IDPs and silicate IDPs (Brownlee et al. 1977; Rietmeijer 2004b). Flash melting and ultra-rapid quenching from the 660 °C peak heating temperature (Nier and Schlutter 1993) of this attached material might have produced amorphous Mg,Al-silica compounds and those with deep metastable eutectic serpentine- and/or smectite-dehydroxylate compositions (Rietmeijer 2009a). The amorphous compounds with a serpentine dehydroxylate composition attached to the surface of particle F6 have a strong resemblance to GEMS (glass with embedded metal and sulfides; Bradley 1994a) (Fig. 7). Are these objects surviving GEMS or GEMS-like objects that had formed during atmospheric entry heating?

Figure 7.

GEMS-look-alike objects attached to sulfide in fragment F6a from cluster IDP L2011#2. Rounded Fe-oxides grains (bright spots) abound. The scale bar is 250 nm.

Parent Body Processing

Alternatively, the pyroxenes and olivine are original silicate minerals from the parent body of this fragment of cluster IDP L2011#21. In this scenario, coexisting Al-bearing enstatite and endiopside would indicate temperatures of approximately 1400 °C (Huebner and Turnock 1980; Lindsley and Andersen 1983). The presence of alumina could indicate lithostatic pressure or high cooling rates. The olivine compositions suggest that temperatures were >1100 °C and probably closer to approximately 1500 °C (Davidson and Mukhopadhyay 1984). The continuous chemical series from Fe7S8 to Fe2S3 is an atmospheric entry feature (Rietmeijer 2004b). Pyrrhotite was the original sulfide. Pyrrhotite is a common phase in meteorites and IDPs as are metallic iron and Fe,Ni-metal, although the relative proportions of sulfides and metals vary among and within meteorite groups. For a review, I refer the reader to Papike (1998). The presence of pyrrhotite and Fe,Ni-metal indicates melting temperatures between 1100 and 1350 °C (Hsieh et al. 1987) forming Fe,Ni,S, S-rich Fe-S, and low-Ni Fe-Ni-S melts in the presence of Fe,Ni crystals (Waldner and Pelton 2004). The observed sulfide and Fe,Ni metal compositions (Fig. 1) are consistent with equilibration at approximately 600 °C at 1 atm (Waldner and Pelton 2004), followed by subsolidus Fe,Ni zoning. In summary, the combination of low-Ni sulfides, taenite, Mg-rich pyroxenes, and olivine point to an asteroidal source that supported Fe,Ni,S and Mg,Ca,Al-silicate melts. That is, a fractionated igneous parent body wherein core-mantle formation had occurred and that could support volcanic activity such as lava fountaining (Wilson and Keil 1997). The corollary being that (1) the amorphous serpentine-dehydroxylate and smectite-dehydroxylate (Table 4) and low-Mg,Al silica compounds (Fig. 6) formed during atmospheric entry flash heating, and (2) the original subsolidus Fe,Ni zoning was not disturbed during peak flash heating at 660 °C (cf. Nier and Schlutter 1993).

Fe,Ni Compositions in IDPs and Comet Wild 2

Fragment F6 of Cluster IDP 2011#21

The bulk composition of the Fe,Ni-metal inclusion, 34 atom% Ni, is preserved in a compositionally homogeneous central band that is flanked on both sides by adjoining domains with average compositions of 24 and 53 atom% Ni (Table 5). In addition, narrow granular zones of taenite nanocrystals (approximately 12 atom% Ni) are present directly in contact with pyrrhotite. This taenite zone is covered by a layer of granular Fe-oxide nanocrystals with Ni = 0–4 (atom%) and traces of Cr forming the rim on this particle. Kamacite nanocrystals (<15 nm) embedded in equilibrated aggregates and kamacite with a magnetite rim were reported in chondritic porous aggregate IDPs (Bradley 1994b). This kamacite-iron oxide association might point to sulfide oxidation during atmospheric entry flash heating of the IDPs, but there is no firm evidence to support this mode of formation. Kamacite is present in EOC meteorites (Table 5). Kamacite occurs as discrete single-crystals, polycrystalline grains, and zoned Fe,Ni grains (Brearley and Jones 1998).

Table 5. Ni (atom%) contents in Fe,Ni sulfides and Fe,Ni metal from selected sources: IDP L2005E40 (Rietmeijer 2004b), aggregate IDPs (Dai and Bradley 2001), unequilibrated and equilibrated ordinary chondrites (Brearley and Jones 1998), and taenite (this paper)
Fe,Ni-Sulfides
8183344–58~10 μmIDP L2005E40
4–1023  0.5–5 μmAggregate IDPs
4–10  40100–500 nmAggregate IDPs
Taenite
 25–3033.553UOCs
525–40 (core) 50–55 (rim)EOCs
1221–2834–4450–57This paper

The Fe-Ni phase diagram has a shallow eutectic of 67 atom% Ni at 1436 °C. It appears that Fe,Ni metal compositions in natural extraterrestrial materials favor the Fe-saturated part of this phase diagram for Fe,Ni-sulfides and Fe,Ni alloys independent of their grain size. The discrete Fe,Ni alloy compositions (Table 5) show a remarkable match with the complex topology of Fe/Ni the subsolidus region in the Fe-Ni phase diagram (wt% Ni) below the range of 450 and 400 °C (Zhang et al. 1994). The 5–12 atom% Ni compositions include both kamacite and taenite. Assuming time to equilibrate in the subsolidus stability field, the composition at Ni = 12 atom% (Table 5) is coincident with a metastable phase boundary in this Ni versus temperature phase diagram (Zhang et al. 1994). The 21–28 and 34–44 atom% Ni groups (Table 5) match the spinodal decomposition region within the metastable two-phase field in this phase diagram. The latter group is also consistent with the Fe3Ni (awaruite) eutectoid composition diagram (Zhang et al. 1994). The 50–57 atom% Ni composition matches the range of ordered and disordered FeNi in the Ni versus temperature phase diagram (Zhang et al. 1994). This remarkable match between the Fe,Ni compositions in natural materials and the low-temperature phase diagram is presented without consideration of grain size and cooling rates in the asteroids that were parent bodies of UOC and equilibrated ordinary chondrites (EOC) meteorites, IDP L2005E40, and cluster IDP 2011#21.

Fe,Ni in Wild 2 Aerogel

Fragment F6 of cluster IDP 2011#21 (Fig. 1) that is dominated by: (1) large pyrrhotite grains with an associated Fe,Ni-grain, (2) fine-grained material with a close to chondritic composition (Fig. 6), and (3) high-Mg olivine and pyroxenes closely resembles Stardust particle Febo. This terminal particle in track 57 (C2009,2,57,1[TP], Febo) consists of a pyrrhotite grain (approximately 3 × 2 μm) with an attached submicron kamacite grain and a large fine-grained region that partially encloses a forsterite (Fo97) grain with an attached enstatite (En98) grain (Joswiak et al. 2012; fig. 19). The composition of fine-grained material is within approximately 3 times chondritic (Joswiak et al. 2012). The attached silica glass is probably melted Stardust aerogel material. The remarkable similarity between this Wild 2 particle and the cluster IDP fragment underscores the radial transport from the region of the asteroid belt to the Kuiper Belt. The major difference is the presence of taenite in the cluster IDP fragment (and in UOC and EOC meteorites) and an apparent dearth of taenite among captured Wild 2 particles (Table 6), including the MSG (metal and sulfide in glass) material (Joswiak et al. 2012), which is an unnecessary and undesirable acronym for material already known as Stardust glass. The most abundant Fe,Ni phase appears to be kamacite that is mostly present as metal inclusions in olivine and pyroxene including igneous porphyritic, chondrule-like Wild 2 particles (Nakamura et al. 2008b). Taenite was found in two terminal particles (Table 6), which is prima facie evidence that taenite was among the dust that accreted into comet Wild 2 nucleus.

Table 6. Kamacite and taenite grains in particles from 81P/comet Wild 2 captured in Stardust tracks
Track & particleCompositionDimensionsCommentsSource
  1. Except when present as inclusions in olivine and pyroxene, these Fe,Ni-alloy occurrences also include pyrrhotite. These Fe,Ni grain occurrences do not include the electron-opaque inclusions in vesicular low-Mg silica Stardust glass (Leroux et al. 2008a; Rietmeijer et al. 2008; Tomeoka et al. 2008; Joswiak et al. 2012).

  2. Sources: (1) Joswiak et al. (2012), (2) Bridges et al. (2010), (3) Nakamura et al. (2008a), and (4) Nakamura et al. (2008b).

Kamacite
C2,7,10,3No data  1
C2009,2,57,1(TP)No data  1
C2009,2,57,120Fe/(Fe+Ni) = 0.95 at0.5 μm (remnant) 1
C2009,20,77,2Fe/(Fe+Ni) = 0.94 el. wt% Micron-size rounded core in Fe-rich olivine1
C2044,0,41,0,0No dataSize of measured grains 5–10 μmEDX spectrum2
C2054,0,35,6,0(TP)Fe0.8Ni0.2-Fe0.9Ni0.1Round nanometer grainInclusions in porphyritic olivine/pyroxene particle3,4
C2081,1,108,1(TP)No dataNanometer – ~5 μm grainsInclusions in porphyritic olivine/pyroxene particle4
Taenite
C2044,0,38,1(TP)Fe/(Fe+Ni) = 0.9 el. wt%~3 μm; rounded (abraded?) 1
C2009,6,61,1(TP)No data  1

Fe-Ni-S Nanograins in Stardust Glass

The surviving Wild 2 minerals defied our expectations of comet dust. The closest direct observation of dust ejected from an active comet nucleus was comet P/Halley. The fly-by at a distance of 8030–8890 km (Vega missions) and later 600 km (Giotto mission) from its nucleus detected nanometer ferromagnesiosilica dust grains and few FeS grains (Jessberger et al. 1988; Fomenkova et al. 1992). These grains were probably the smallest fragments of larger aggregates that were released from the nucleus. The Stardust mission flew past the comet at a distance of 236 km and captured numerous ultrafine-grained dust particles and fewer particles ranging from approximately 1 to 10 μm that had left the comet nucleus (near) surface as weakly bonded aggregates (Zolensky et al. 2006). On the basis of size alone, the F6 sulfide/taenite fragment would probably have survived hypervelocity impact intact as a terminal particle. The attached amorphous materials, pyroxenes and olivine, would have melted and/or evaporated, and commingled with molten Stardust aerogel leaving the “ghosts grains” with “Mg,Al” compositions that were found in Stardust glass (Leroux et al. 2008a; Tomeoka et al. 2008; Rietmeijer 2009a).

The vesicular low-Mg Stardust glass is riddled with electron-opaque Fe,Ni,S-nanograins that include compositions ranging from: (1) FeS (pyrrhotite) to Fe (Fe-metal), (2) intermediate composition between Fe- and Ni metal, and (3) mixed Fe,Ni,S grains. Pure Ni grains were not yet identified among the electron-opaque nanograins in Stardust glass and in MSG (metal and sulfide in glass) material (Joswiak et al. 2012). In a subset of samples analyzed so far, the electron-opaque nanograin compositions in Stardust glass when plotted in a Fe-Ni-S (atom%) ternary diagram fan out from Ni-free and low-Ni (approximately 2 atom% Ni) pyrrhotite to iron metal and between pyrrhotite and kamacite, Ni <10% at (Zolensky et al. 2006; Leroux et al. 2008a, fig. 12; Tomeoka et al. 2008, fig. 6b; Velbel and Harvey 2009, fig. 2). These trends can be either continuous or discontinuous, but it is possible that discontinuity is a low sampling bias. In some tracks, the electron-opaque nanograins are mostly low Ni,S-iron grains (Leroux et al. 2008a, fig. 12; Tomeoka et al. 2008, fig. 6b). In other tracks, the electron-opaque nanograins have higher Ni contents for subsulfur FeS grains (Fig. 8a) or show a more dramatic scattering of low-S Fe-Ni-S nanograins with variable high-Ni contents, e.g., C2054,0,35,24,1 (Fig. 8b). In addition to this allocation, similar distributions of Fe,Ni,S compositions were observed in allocation C2054,0,35,24,7 (Leroux et al. 2008a, fig. 12), C2054,0,35,16,8 (Velbel and Harvey 2009), and C2054,0,35,16,6 (Tomeoka et al. 2008, fig. 6b). The question is whether these nanograin compositions are random or the result of pyrrhotite–taenite interactions in this type B Stardust track. To explore the latter, tie lines were drawn between pyrrhotite and the Fe,Ni-metal compositions in fragment F6 of cluster IDP 2011#21, viz. (1) Ni = 12.5 atom%, (2) Ni = 24.2 ± 2.3 atom%, (3) Ni = 39.7 ± 3.9 atom% and Ni = 52.9 ± 3.1 atom% Ni (cf. Table 5). The results (Figs. 8a and 8b; dashed lines) show that the apparent randomness of Fe-Ni-S grain compositions disappeared. A tie line drawn from pyrrhotite through the nanograin composition in C2054,0,35,16,8 with the highest Ni content observed among these nanograins so far (Velbel and Harvey 2009) intersects the Fe-Ni join of the ternary Fe-Ni-S diagram at 60 atom% Ni that matches the eutectic composition in the Fe-Ni system, but it also matches a eutectoid in the Fe-Ni system below 500 °C (cf. Zhang et al. 1994).

Figure 8.

Fe-Ni-S (atom%) ternary diagrams from two different Stardust glass allocations showing the individual Fe-Ni-S nanograin compositions scattered throughout the low-Mg silica Stardust glass (modified from Leroux et al. 2008a). The dashed tie lines are drawn between pyrrhotite and the Fe,Ni metal compositions in fragment F6 of cluster IDP 2011#21 (Table 5). (Reproduced with the kind permission of Dr. H. Leroux and the Meteoritical Society)

Discussion

Track C2054,0,35 was made by a Wild 2 aggregate that contained taenite and pyrrhotite, as either individual grains or intergrowths like fragment F6 of cluster IDP 2011#21. The size of the taenite or sulfide/taenite grain that caused the Fe-Ni-S nanograin compositions in this track is unknown, but micrometer-sized pyrrhotite grains when shot into aerogel at 6.1 km s−1 faithfully reproduced the Fe,S nanoparticles in stardust glass (Ishii et al. 2008). It seems plausible then that the Fe-Ni-S nanograins were caused by hypervelocity impact of an igneous particle that was a few micrometers in size, wherein the original parent body subsolidus Fe/Ni distributions were preserved when resident in the Kuiper Belt. During hypervelocity capture in Stardust aerogel, the pyrrhotite and taenite ensemble was flash-melted and numerous nanometer droplets with variable Fe-Ni-S compositions spalled-off that were quenched into the electron-opaque nanograin inclusions that are scattered throughout the vesicular low-Mg and Mg-free silica Stardust glass.

Comet P/Wild 2 presented an unanticipated glimpse of the evolving asteroid belt wherein collisions among its forming bodies indiscriminately produced mineral debris from a wide range of differentiated and undifferentiated asteroids. This process continues today and makes it possible to collect meteorites, micrometeorites, cluster IDPs, chondritic porous aggregate IDPs, and nonchondritic IDPs. The captured comet Wild 2 dust included large solid particles as big as 50 μm that could have their analogs among the fragments of cluster IDPs (Brownlee et al. 2011). The pyrrhotite/taenite fragment discussed here is an example of a nonchondritic IDP that was ejected from a differentiated asteroid that as part of a cluster IDP was returned after 4.56 Gyr in cold storage in the Kuiper Belt. This fragment, which is the first pyrrhotite with an enclosed Fe,Ni-metal grain among IDPs, was part of a cluster that also contained amorphous aluminosilica fragments with a rich variety of protophyllosilicates (Table 1). These particles co-accreted with fragment F6, but they are not necessarily from the same parent body. Still, it seems possible that similar particles rich in protophyllosilicates were transported to the Kuiper Belt where they could accrete into the growing comet nuclei. The only question being their survival during hypervelocity (6.1 km s−1) capture in underdense, density-graded silica aerogel, but based on size alone, these amorphous aluminosilica grains might survive as terminal particles with preservation of the asteroidal protophyllosilicates. Indigenous comet Wild 2 layer silicates have yet to be identified among Stardust samples (Joswiak et al. 2012). It is unlikely that aqueous alteration had occurred in this comet's nucleus (Brownlee et al. 2012), albeit not impossible via hydrocryogenic alteration of amorphous materials (Rietmeijer 2011), or during (unspecified) secondary aqueous alteration that led to in situ cubanite, cubanite/pyrrhotite, and pyrrhotite/pentlandite assemblages below 210 °C (Berger et al. 2011).

During 2014, the European Space Agency Rosetta mission will sample dust ejected from comet 67P/Churyumov-Gerasimenko. It will be much closer to its nucleus than the Stardust mission had approached its target. Comet 67P/Churyumov-Gerasimenko is a short-period Jupiter Family comet that, based on the Stardust mission results, should prepare to encounter mineral dusts from collisions in the evolving asteroid belt in the protoplanetary disk. At its much closer distance to the nucleus, it might find much bigger particles than those encountered by the Stardust mission.

Acknowledgments

I thank George Flynn and Mike Zolensky for their reviews. The author was supported by NASA LARS (Laboratory Analysis of Returned Samples) grant NNX11AC36.

Editorial Handling

Dr. Donald Brownlee

Ancillary