Seymchan Meteorite

displaying "facies" of both pallasite and iron meteorite
mingled within one or more fragmented metre-scale masses found in Siberia

Seymchan pallasite facies in iron, 204 kb]

Figure 1. A slice of the Seymchan meteorite, 143.92 grams, circa 11.2x6.0x0.45 cm in size. Note that the slice was sawn, polished, and then lightly etched to bring out the intergrowths in the metal phase. The slice is remarkable for displaying the contact between what appears (caveat: at the scale of the palm of your hand!) to be a lens or vein of pallasite "facies" material within an iron meteorite "facies". This is not unknown in Seymchan material and one has to consider whether pallasite is intruding iron, or vice versa, or if the two "facies" (metal-silicate and metal) are segregating one from the other, as coeval and immiscible melts. As with a smooth rock outcrop, a sawn slice shows just two dimensions of a 3-D reality, so interpret with caution! Immediate provenance: Eric Wichman / Michael Miller.


"Rock of the Month #127, posted for January 2012" ---

Seymchan is a meteorite find from the Magadan district of Far Eastern Russia, which is only now receiving wide attention, due to much larger recoveries in the past decade.

The sample above displays metal, largely a nickel-iron alloy (kamacite), surrounding the pallasite "lens" (olivine and kamacite), with coarse bronze-coloured axe-head crystals ("rhabdites") of the iron-nickel phosphide schreibersite at the boundary of pallasite and iron.

The first find in 1967 was followed by a second mass, reported total known weight 322.3 kg. The two pieces, 20 metres apart, were reportedly found in the bed of a meander channel, on the west side of the main branch of the Hekandue River, that flows north via the Kolyma River to enter the East Siberian Sea of the Arctic Ocean. The location is near 62° 54'N, 152° 26'E.

It is clear that the current recovered mass is far greater, more (perhaps much more) than 3 tonnes. The initial find was identified as a coarse octahedrite, a IIE iron meteorite with kamacite bandwidth 2.0 mm, and early analyses returning 9.47 and 9.15% nickel (Scott and Wasson, 1976; Grady, 2000, p.457). Seymchan was later reclassified as an "ungrouped iron" (Wasson and Wang, 1986). Most intriguing, the finds in the 2000s have revealed that perhaps 80% of the material is iron, but the remaining 20% is pallasite. The meteoritic literature is slowly catching up with this fascinating find. Photographs of this new (2004) material show great heterogeneity in olivine grain size and metal:olivine ratios with metal veins up to tens of cm in thickness. Glorieta Mountain and Brenham also contain large regions of metal lacking olivine. Neutron activation analysis of Seymchan pallasite metal and fragments of the iron "facies" shows that the two are identical, and thus parts of one fall (van Niekerk et al., 2007). The bulk metal contains roughly 0.53% Co and 9.3-9.5% Ni plus 18 ppm As, 662-694 ppb Ir and 2.1 ppm Au (3 analyses), and Seymchan is seen as a main group pallasite with affinities to the IIE irons (van Niekerk et al., 2007).

Seymchan pallasite slices, 232 kb]

Figure 2. Here are three polished, unetched thin slices, 28.29 (top), 22.50 and 6.66 grams, of the pallasitic facies of Seymchan. 1-centimetre cube for scale. The olivine grains are commonly angular, and range from <1 mm to 11x9 mm in section. Some faces are rounded, but many appear to be fractured, and very few approximate well-developed crystal habit.

Seymchan pallasite edge to iron mass, 127 kb]

Figure 3. One corner of a large rectangular slab of Seymchan, roughly 56x32x9 cm, mass circa 110 kg. The sample was sawn and polished, but is currently unetched, hence the metallographic fabric is not clearly visible on the sawn faces. The dimpled outer edge is visible, apparently of a very coarse pallasite facies. However, roughly 95 percent of the volume appears to be solid nickel-iron, possibly a straight-edged vein or dyke of metal intruding a pallasitic crystal mush (?). Note the olivines in the foreground, metal beyond, and coarse schreibersite rhabdites in the metal. Immediate provenance of the slab and the smaller slices above: Blaine Reed.

Mineral Chemistry

Some wavelength-dispersive electron microprobe analyses of mineral grains were conducted in November 2011, using the SX-50 instrument at the Department of Geology, University of Toronto (thanks to the capable assistance of Yanan Liu, lab manager). A summary of the results follows for the principal phases in the analysed sample: kamacite, schreibersite and Ni-poor magnesian olivine.

Summary microprobe data are available here, as a 185-kb pdf file, together with a photo of the analysed mount,
the raw material of which is shown in the following photograph.

Seymchan-Brahin-Brenham, 235 kb]

Figure 4. Two thin slices of coarse pallasite facies of Seymchan, unpolished, perfect material for preparation of polished mounts and thin sections. The left-hand half of the top-left slice furnished the mount for microprobe work. Note thin metal veinlets between the olivine crystals. These and other samples of Seymchan can be compared with polished slices of Brenham (lower left) and Brahin (lower right). Immediate provenance of the two rough-hewn slices of Seymchan: Blaine Reed; Brenham and Brahin: KD Meteorites.

Olivine appears to be essentially unzoned, average composition Fa11.6, Fe/Mn~41, with Ni always <110 ppm, the detection limit. This very low nickel content is well-known in pallasites. In particular, an ion microprobe (SIMS) study of olivine in seven pallasites (Reed et al., 1979) estimated the Ni contents in the olivine of seven pallasites at 22-41 ppm, significantly lower than then-published results by the less-sensitive electron microprobe. Their samples included olivine from Brenham, Eagle Station and Springwater, as well as terrestrial olivines from St. John's Island (Zabargad), Rhum in Scotland and the Bushveld complex in South Africa. The low Ni content of the pallasite olivines can be taken as an indication that the olivine has equilibrated with the iron phase, which served as a collector for most of the nickel. The metal phase appears to be a better "scavenger" of nickel than iron sulphides, which in most terrestrial mafic intrusions are analogous to meteoritic metal as "collectors" of less-common metals.

Kamacite The kamacite averages 6.59 wt.% Ni and 0.50 wt.% Co, plus traces of Si, S and P. Note that the nickel value is about 70% of published analyses for bulk metal. This reflects a) appreciable Ni in the Ni-rich alloy taenite in the iron meteorite facies of Seymchan (see Harju et al., 2011; and the SEM discussion, below), and b) additional Ni in the Ni-rich phosphide. A traverse of a mm-scale mass, and analyses in thin veinlets between olivine, suggest that the metal is quite homogeneous. A small (<0.2 mm) metal bleb hosted by olivine was relatively Co-rich (0.59%).

Schreibersite, the Fe-Ni phosphide, averages 39.9% Ni, 14.8% P and 0.13% Co, plus up to 446 ppm Si and 500 ppm S. It appears quite homogeneous, the estimated formula (Fe,Ni,Co)3.06P.

A brief scanning electron microscope examination of the microprobe-analysed, carbon-coated polished mount, courtesy of Dr Giovanni di Prisco, provided some new information, including: 1) the occurrence of taenite lamellae in kamacite (Fig. 5); 2) Ni-bearing goethite at the kamacite-olivine interface, confirming minor terrestrial oxidation of metal; and 3) minor remobilization of troilite (with minimal Ni and Cr contents) into thin veinlets.

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Figure 5. Three BSE images of select features. Left: This image shows a taenite lamella in a matrix of kamacite. On the right, a typical occurrence of the phosphide schreibersite, with a cracked appearance, decorates the contact of kamacite with olivine. Note the zonation in the taenite, clearly visible in backscattered electron imagery, enhanced photographically for clarity. The bright margins of the taenite reflect a higher content of the heavier atom (Ni, element 27, in Fe, element 26). There is an M-shaped diffusion profile, with perhaps 3 times as much Ni at the margins of the Ni-rich lamellae as in the host kamacite. The small round object at the lower left is an artifact, probably a bubble induced in the carbon thin film by the electron beam. Field of view 2.7 mm. Centre: a study in partial terrestrial alteration of the metal (kamacite, white) and chromite (medium grey, lower centre and left of image). Against the kamacite is a thin film of pitted Ni-bearing goethite. The rectangular pitted area against chromite is also goethite, with both Ni and Cr contents. Field of view 0.27 mm. Right: a rounded kamacite bleb with a rind of troilite, in olivine. Field of view 0.20 mm.

Textures in Seymchan

Here are some initial thoughts on the diversity of textural features in the illustrated material. Photographs of polished slabs of pallasites are quite instructive (see Norton and Chitwood, 2008, amongst many publications and web sites).

A. Some, such as Albin, Huckitta and Eagle Station, display angular olivine, evidently fragments in breccia cemented by a metal matrix. The brecciation may reflect an impact event, consistent with the non-magmatic, impact-related origin that has been suggested for IIE irons. At the least, a dynamic magmatic environment is indicated, as in a rapidly-convecting magma chamber or conduit (so-called taxite fabric, in the terminology used in nickel exploration). Most pallasites are relatively sulphur-poor, and thus troilite and other sulphides are minor phases. Some, such as Newport, contain native copper, further evidence of the low S content (Buseck, 1968). A specimen photographed at the Tucson show in February 2014 (Fujihara, 2014) shows a further variant: an etched face of metal enclosing rounded, heiroglyph-like chromite inclusions.

B. Others, such as Springwater, are notable for their smoothly rounded olivines, also in metal, but with a texture more reminiscent of liquid immiscibility (e.g., cooking oil with a little water, vinegar or milk). Some of these olivines are fairly coarse, 5 mm or more, as in Brenham, Esquel and Fukang. Some of these olivines appear "floating" in metal and thus are not constrained by adjacent olivines. The large olivines are often subhedral to subrounded in crystal habit. These seem to be formed in a more sheltered setting that the breccias (?).

Large sections of pallasites often display large relict angular olivine masses, 5-30 cm in size, which may reflect intrusion of olivine layers by molten metal (Scott, 1977). The rounded olivine grains in meteorites such as Brenham, Springwater and Krasnojarsk are suggestive of sulphide-silicate liquid immiscibility generating fabrics analogous to net- textured sulphides, ore textures found in magmatic nickel-copper deposits on Earth (Scott and Taylor, 1990).

Seymchan seems to display both angular and rounded olivines in different volumes of the pallasite facies. The "breccia" form predominates in the material I have seen, but some smaller volumes contain more rounded olivines (e.g., see first photo, above), if less perfectly spheroidal than may be seen in Springwater.

Tentative Conclusions

In summary, some pallasites, including Seymchan, display more than one, and as many as three textural "facies". Each individual meteorite, even the largest, represent at most a tiny volume of its asteroidal parent body. All samples of each meteorite were originally within centimetres to metres to perhaps (Canyon Diablo?) tens of metres of each other, a very small compass compared with the multi-kilometre scale of many terrestrial intrusions. The only exceptions may be regolith breccias, which can include fragments hurled by impact across the original surface of the parent body.

I have over 250 articles on pallasites to hand and, given time, may find a perfect explanation of the textural variation shown on those pages. Right now, the state of the art may be expressed by Scott et al. (2010), an elegant blend of science and fine illustrations. Evidence such as discrepancies in cooling rates between pallasite metal and iron meteorites argues against the classic explanation of pallasites as accreting at the boundary of a metallic asteroidal core and a silicate-dominated mantle. Instead, many pallasites and some classes of irons may have formed as melt pools at and near the asteroidal surface. Some of these melts may have penetrated deep into the crust of the parent body, as fracture-hosted breccias. Helpful analogies to these processes are probably at hand on Earth. Obvious examples include impact structures, particularly the sulphide-rich Sudbury igneous complex and its fractured basement, in Canada, and the giant rift-related intrusions of Noril'sk and Talnakh in Siberia. Back on the small scale, it seems appropriate to analyse samples of both "breccia" and massive-iron facies of Seymchan, to compare with the coarse-facies material analysed here. There's a lot to explore in a meteorite find of this scale!

References

Buseck,PR (1968) Mackinawite, pentlandite, and native copper from the Newport pallasite. Mineral.Mag. 36, 717-725.

Fujihara,G (2014) The 2014 Tucson show. Meteorite 20 no.1, 8-9, spring issue.

Grady,MM (2000) Catalogue of Meteorites. Natural History Museum, London, 690pp.

Harju,ER, Rubin,AE and Wasson,JT (2011) A new evaluation of pallasite cooling rates. Lunar and Planetary Science 42, abstract 2595, 2pp.

Norton,OR and Chitwood,LA (2008) Field Guide to Meteors and Meteorites. Springer-Verlag London Limited, 287pp.

Reed,SJB, Scott,ERD and Long,JVP (1979) Ion microprobe analysis of olivine in pallasite meteorites for nickel. Earth Planet.Sci.Letts. 43, 5-12.

Scott,ERD (1977) Formation of olivine-metal textures in pallasite meteorites. Geochim.Cosmochim.Acta 41, 693-710.

Scott,ERD and Taylor,GJ (1990) Origins of pallasites at the core-mantle boundaries of asteroids. Lunar and Planetary Science 21, 1119-1120.

Scott,ERD and Wasson,JT (1976) Chemical classification of iron meteorites - VIII. Groups IC, IIE, IIIF and 97 other irons. Geochim.Cosmochim.Acta 40, 103-115.

Scott,ERD, Goldstein,J and Yang,J (2010) Formation of stony-iron meteorites in early giant impacts. Planetary Science Research Discoveries, http://www.psrd.hawaii.edu/June10/pallasites-origin.html.

van Niekerk,D, Greenwood,RC, Franchi,IA, Scott,ERD and Keil,K (2007) Seymchan: a main group pallasite - not an iron meteorite. Meteoritics & Planetary Science 42, A154.

Wasson,JT and Wang,J (1986) A nonmagmatic origin of group-IIE iron meteorites. Geochim.Cosmochim.Acta 50, 725-732.

Graham Wilson, 19 December 2011, last revised 18 October 2012, 15 April 2013, 15 and 21 April 2014

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