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Thiel Mountains

Pallasite, PMG (main-group)
standby for theil mountains photo
Found December 7, 1961
85° 23.9′ S., 86° 35.4′ W. Geologists from the US Geological Survey found two fragments lying about 200 m apart on the ice at the NE base of Mt. Wrather, in the Thiel Mountains of Antarctica (now a dense collection area abbreviated TIL). This was only the third meteorite ever found in Antarctica. The larger fragment had a weight of 18 kg and the smaller one 10.6 kg. The fusion crust had been completely stripped and the surface smoothed and polished due to sandblasting by windblown rock and ice particles. Two smaller pallasites, TIL 07016 (3,490 g) and TIL 08004 (5,008 g), which were found in 2007 and 2008, respectively, are likely paired to the original pallasite.

 

Thiel Mountains is a typical member of the main-group pallasites. Trace element and O-isotopic studies suggest that pallasite metal crystallized from IIIAB liquids during fractional crystallization of the core and mantle; however, some recent studies rule out this scenario (Yang and Goldstein, 2006). Metallographic cooling rates of pallasites are not what would be expected given an origin at the core–mantle boundary. Instead, based on the size of the island phase in the cloudy zone of the pallasites, the cooling rates are 20× lower than those of IIIAB irons, implying that the irons were actually closer to the surface of the parent body than pallasites. In addition, the Re–Os chronometer suggests that pallasites formed 60 m.y. later than IIIAB irons, raising further doubt about a IIIAB core–mantle origin for main-group pallasites (E. Scott, 2007). Moreover, pallasites have a much younger range of CRE ages than the IIIAB irons (Huber et al., 2011).

 

The olivine grains in Thiel Mountains exhibit significant rounding, once considered to be due to thermodynamic processes that minimize the capillary forces along the olivine–metal interface over timescales on the order of 10 to 100 m.y. (Saiki et al., 2003). However, this rounding is now thought to occur primarily from resorption at high temperatures (above ~1250°C) in the presence of silicate melt (Boesenberg et al., 2012). The specimen of Thiel Mountains shown above is a 16.1 g partial slice.

Note that the name “Thiel Mountains” is sometimes misspelled as “Theil Mountains”.


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Seymchan

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’
(https://doi.org/10.1111/maps.13072)
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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Springwater

Pallasite, MG (main-group), anomalous silicate composition
high-Δ17O subgroup (see details on the Imilac page)
standby for springwater photo
Recognized Summer, 1931
52° 0′ N., 108° 18′ W. Three masses weighing 85, 41, and 23 pounds, having a combined weight of 149 pounds, were found in a field 100 miles west of Saskatoon in Saskatchewan, Canada. One of the masses was sent by the finder to the Nininger Meteorite Laboratory in Denver where it was verified as being meteoritic. The 117 pound (52 kg) main mass was found by Mike Farmer and team members in their 2009 expedition.

Springwater has anomalous compositions with respect to olivine fayalite, Sc, Cr, Mn, Zn, and phosphate REE contents. It has a phosphoran olivine composition distinct from most other main-group pallasites (present also in Brahin, Brenham, Rawlinna 001, and Zaisho), and many of its anomalous chemical compositions suggest that it crystallized from a late-stage, P-enriched, Si-depleted melt which cooled quickly through high temperatures (Boesenberg et al., 2004). In contrast, it has a Sc content that is the lowest among pallasites, and which is consistent with a minimally evolved melt (Mittlefehldt and Rumble III (2006). The mineral farringtonite was first found in this meteorite. Springwater silicates do not follow an igneous fractionation trend, but instead are more consistent with the loss of Fe through oxidation as a magmatic gas phase was introduced (Wasson and Choi, 2003). These oxidizing gases were concentrated in voids formed by core contraction and mantle collapse during cooling, and they introduced enrichments of the volatile siderophiles Ge and Ga into various pallasitic lithologies.

Springwater and the other PMG-as members might have formed in a more chemically heterogeneous mantle region than normal PMG members, though overall compositional and isotopic similarities still suggest a common parent body for all. It has been proposed that main-group pallasites could represent melt residues of the IIIAB iron parent body. However, due to the significantly slower cooling experienced by main-group pallasites compared to IIIAB irons, as well as the younger ages determined for main-group pallasites, a common parent body is not considered plausible by some (E. Scott, 2007). Moreover, pallasites have a much younger range of CRE ages than the IIIAB irons (Huber et al., 2011), although this may be explained by the higher space endurance of the irons.

Formation scenarios and classification schemes for the main-group pallasites can be found on the Imilac page. Shown above is a 3.5 g specimen of Springwater purchased from M. Killgore in 1995. The excellent photo below is a large etched slice of Springwater exhibiting the typical distribution of silicate and metal, shown courtesy of Sergey Vasiliev. standby for springwater photo
Photo courtesy of Sergey Vasiliev—SV-meteorites.com


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Mount Vernon

Pallasite, PMG (main-group)
standby for mount vernon photo
Found ~1868, recognized 1902
36° 56′ N., 87° 24′ W. A mass of 351 pounds was found on the farm of Capt. S. Fruit, about 7 miles northeast of Hopkinsville, Kentucky, but it was not recognized as a meteorite until 1902. During the intervening years, the owner had used the meteorite as a boot scraper, and had finally sent a sample to E. Ulrich, of the U.S. Geological Survey, but only when the increasing price of zinc and lead made him curious about its true nature. Mr. Ulrich subsequently acquired the specimen for the U.S. National Museum. The Catalogue of Meteorites fourth edition lists Mount Vernon as a fall during the year 1868, but this claim is in question. Only Marjalahti, Omolon, and Zaisho are undisputed pallasite falls.

An extended terrestrial exposure has led to significant oxidation of the exterior of the mass, which measured 33 × 36 × 55 cm when acquired by the museum. Mount Vernon is composed of unusually large and compact blebs of olivine ranging from 5 mm to 25 mm in diameter with only a minimal network of FeNi-metal containing thin veinlets of phosphide. It was estimated by E. R. D. Scott (1977) that the size of the largest olivine grains in Mount Vernon reached a size of at least 30 cm.

This pallasite is much less stable to terrestrial oxidation than many others. The specimen pictured above is a 24.5 g partial slice, which unfortunately has experienced further deterioration during its residence in humid Florida. The photo below shows a 1,507 g partial slice, which was acquired by the owner from the Robert Haag Collection, previously obtained from the United States National Museum, Smithsonian Institution. Synopses of several different proposed formation histories of the main-group pallasites can be found on the Imilac page.

standby for mount vernon photo
Photo courtesy of the Dr. J. Piatek Collection


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La’Gad 002

Pallasite, PMG-am (anomalous metal composition)
high-Δ17O subgroup (see details on the Imilac page)
standby for al mahbas photo standby for al mahbas photo
Found August 2004
27° 14′ 25.7′ N., 8° 50′ 49.5′ W. A meteorite having the appearance of a pallasite and weighing ~2–5 kg was found by a nomad about 30 km southeast of Al Mahbas in Saguia el Hamra, Western Sahara. Along with this mass, a few other complete individuals weighing ~16–68 g were recovered which still exhibit black fusion crust and fresh metal. Numerous smaller fragments found around the ~10 m find area contain only oxidized metal. It was reported that the nomad who found the large mass had transported it a few miles away before he hid it under a small tree in a wadi, only to later forget the exact location.

In November of 2004, meteorite dealer M. Farmer traveled to the original find location with a metal detector and recovered a number of oxidized fragments and <100 g of fresh material, the latter being utilized for classification at ASU (L. Garvie), UCLA (J. Wasson), and UNM (K. Ziegler). Because this find is located within a mined, prohibited military zone only ~2 km from the Algerian border, a thorough search could not be completed. A preliminary classification of this pallasite under the provisional designation NWA 2683 was completed at Northern Arizona University (T. Bunch and J. Wittke). The meteorite was determined to be shocked to stage S2 and severely weathered to grade W5 (the FeNi-metal matrix has been transformed into hematite). The metal composition of La’gad 002 is high in Ir and very low in Ni (J. Wasson), most similar to Marjalahti. It is designated an anomalous pallasite based on a very high proportion of plessite to kamacite of 50% (up to 98% plessite in some pairings).

La’gad 002 is likely paired with the pallasites NWA 10015, NWA 10023, NWA 10252 and NWA 11720 based on their similar isotopic and geochemical compositions as well as their high content of plessitic metal. The photos above show two lighting angles of an oxidized slice weighing 1.26 g. The photo below shows a 65.3 g fusion crusted individual in the Dr. J. Piatek Collection. standby for al mahbas photo
Photo courtesy of Dr. J. Piatek