Pallasite, MG (main-group), anomalous metal composition
low-Δ17O subgroup (see details on the Imilac page)
standby for seymchan photo
Found June 1967
62° 54′ N., 152° 26′ E. As reported by Mednikov (1967), Zvetkov (1967), and Tsvetkov (1969), a large mass was found in a stream bed of the Yasachnaya River (flowing into the river of Hekandue, a left tributary of the river of Jasachnaja) by a geologist, F. A. Mednikov, about 150 km from Seymchan, in the Magadan Region of the USSR (V. Buchwald, 1975). The thumbprinted, triangular-shaped mass weighed 272.3 kg. In October of that year, a further search of the area by I. H. Markov, utilizing a mine detector, resulted in the recovery of an additional mass weighing 51 kg. The large mass was provided to the Academy of Sciences, USSR.

A small section of the iron was analyzed by J. Wasson (1974) and it was determined to be a member of chemical group IIE. Subsequently, a more precise elemental analysis of the IIE iron group members by J. Wasson and J. Wang (1986) found that Seymchan (and another IIE member, Lonaconing) had many elemental trends that deviated strongly from typical IIE group members, and therefore, Seymchan (and Lonaconing) was reclassified as an ungrouped iron.

During a 2004 expedition to the original Seymchan discovery site, D. Kachalin found additional masses having a combined weight of ~50 kg. Remarkably, many of the new masses (~20%) were found to contain silicates with a pallasitic texture, something not discovered previously during studies of only small sections of the original mass. This heterogeneous mixture, including portions composed of only FeNi-metal and portions containing silicates in a pallasitic texture, is similar to the iron–pallasite mixtures found previously in both the Brenham and the Glorieta Mountain pallasites. With knowledge of the existence of a pallasitic structure in Seymchan, and of its identical chemical composition to the original Seymchan iron masses, an O-isotopic analysis was conducted; the values plot within the field of the main-group pallasites. Notwithstanding its similar chemical composition to that of the main-group members, Seymchan is an anomalous pallasite due to its high Ir content (van Niekerk et al., 2007).

Previous O-isotopic analyses for main-group pallasites and the HED meteorites indicated that these two groups have values that are very similar. In a high precision comparative analysis of the oxygen three-isotope composition between olivines from five main-group pallasites and representative HED samples, including eucrite and diogenite material, Jabeen et al. (2013) determined that a clear distinction exists, thus demonstrating that these meteorite groups originated on separate parent bodies. In another study investigating the close O-isotopic relationship between main-group pallasites, mesosiderites, and the HED clan, Ziegler and Young (2007) discovered that non-homogenized samples of main-group pallasite olivines exhibit a bimodality in 17O values, which also distinguishes their origin from that of the mesosiderites and the HED clan. In their follow-up of this report, a more refined O-isotopic analysis was conducted by Greenwood et al. (2008), but their results did not support a bimodality in 17O values; however, they definitively established that the parental source of main-group pallasites was different from that of mesosiderites and the HED clan.

More recently, Ali et al. (2018) employed improved laser fluorination techniques to increase the precision of triple oxygen isotope data for 25 MG pallasites and selected members of the HED group. Their results, together with geochemical and other data, not only demonstrate that the HEDs are not genetically related to the main-group pallasites, but also that a significant bimodality exists for these pallasites based on several factors: Δ17O values, MgO content in olivines, bulk olivine abundance, concentration density of olivine grains, and paleointensity. They clearly demonstrated that two statistically distinct subgroups are resolved, and that these subgroups likely represent at least two asteroidal parent bodies with each having homogeneous olivine compositions.

  1. high-Δ17O-bearing (ave. –0.166 [±0.014] ‰) subgroup; e.g., Acomita, Ahumada, Brenham, Finmarken, Huckitta, Imilac, Jay Bird Springs, La’gad 002, Marjahlati, Otinapa, Pallasovka, Somervell County, South Bend, Springwater, Sterley, Thumrayt 001
  2. low-Δ17O-bearing (ave. –0.220 [±0.009] ‰) subgroup; e.g., Brahin, Esquel, Fukang, Giroux, Hambleton, Krasnojarsk, Mount Dyrring, Newport, Seymchan

standby for pallasite bimodality diagram
Diagram credit: Ali et al., MAPS, vol. 53, #6, p. 1228 (2018)
‘The oxygen isotope compositions of olivine in main group (MG) pallasites: New measurements by adopting an improved laser fluorination approach’
The specimen shown above is a 91 g slice of the Seymchan pallasite exhibiting abundant silicates in an FeNi-metal matrix. The top photo below shows a 35 kg Seymchan individual, while the bottom photo shows a section of an exquisitely solidified crystal mush. standby for seymchan photo

standby for seymchan photo
Photos courtesy of Ivan Koutyrev—Finmet

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