Pallasite, PMG-am (main-group, anomalous metal composition)
low-Δ17O subgroup
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.

Subsequent high-precision triple oxygen isotope analyses of a broad sampling of main-group pallasites (Sterley, Imilac, Huckitta, Springwater, Brenham, Giroux, Fukang, Esquel, Brahin, and Seymchan) and selected members of the HED group (Tatahouine, Stannern, and Juvinas) were conducted by Ali et al. (2013, 2014). 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 bimodality does exist for these pallasites based on several factors: Δ17O values, MgO content in olivines, bulk olivine abundance, concentration density of olivine grains, and paleointensity. They were able to resolve systematic variations among the main-group pallasites which indicates the existence of two distinct subgroups as follows:

1. low-Δ17O (ave. –0.172 ±0.007), more olivine-rich (olivine/metal = 2.9); e.g., Esquel, Seymchan, Brahin, Fukang, Giroux
2. high-Δ17O (ave. –0.213 ±0.011), less olivine-rich (olivine/metal = 2.0); e.g., Imilac, Brenham, Springwater, Huckitta, Sterley

This O-isotopic bimodality has been attributed to several possible scenarios, including the existance of multiple parent bodies, the sampling of different locations on a common parent body, and/or, varibility in the degree of impactor contamination. Utilizing the paleointensity data of Tarduno et al. (2012) for the low-Δ17O Esquel and the high-Δ17O Imilac, Ali et al. (2014) ascertained that they each formed at different depths (40 km and 10 km, respectively) on one or more parent bodies.

standby for pallasite bimodality photo
Image courtesy of Ali et al., 45th LPSC, #2390 (2014)

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 at the bottom can be seen a section of an exquisitely solidified crystal mush.

standby for seymchan photo

standby for seymchan photo
Photos courtesy of Ivan Koutyrev—Finmet