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NWA 1912

standby for northwest africa 1912 photoMesosiderite, subgroup 2C (subgroup 2B in Metbull 88)
Found 2002, no coordinates recorded

A very small fusion-crusted stone weighing only 13.52 g was purchased in Erfoud, Morocco by M. Farmer in March 2003. Analysis and classification was completed at Northern Arizona University. Although the MetBull #88 lists NWA 1912 as belonging to subgroup 2B, it exhibits only minor recrystallization and has a matrix composed predominantly of large orthopyroxene grains along with some plagioclase, features that are consistent with assignment to subgroup 2C. Furthermore, it is considered likely that NWA 1912 is a member of the NWA 1827 pairing group, assigned to subgroup 2C (Bunch et al., 2004).

Northwest Africa 1912 is an unshocked meteorite (S1) that shows only minor signs of weathering. The specimen in the photo above is a 0.64 g partial slice, which exhibits green orthopyroxene fragments in a metallic matrix, along with lesser amounts of anorthitic plagioclase, chromite, troilite, and silica.


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Chinguetti

Mesosiderite, group 1B
standby for chinguetti photo
Found 1916
20° 15′ N., 12° 41′ W. In 1916, a French Legion captain named Gaston Ripert, along with his Arab guide, led his soldiers through the Western Sahara Desert in the Adrar region of Mauritania. His guide brought him to a giant metallic meteorite mass, said to have been the source of iron for Arab blacksmiths. Smaller masses were scattered about the area, one of which, weighing 4.5 kg, was collected from on top of the giant mass. Capt. Ripert had no map, compass, or measuring stick, and was only able to make very cursory observations. According to his later recollections, the find location was about 10 hours by camel to the southeast of Chinguetti, among the dunes of Ouarane (in earlier transcribed notes, the location was said to be about 45 km to the southwest of Chinguetti and to the west of Aouinet N’Cher). The large metallic mass was described as measuring 100 m in width and 40 m in height, with one side polished by the wind into a mirrored finish. The base was deeply carved by the wind, and metallic, needle-like projections covered the summit of the mass; these projections could not be removed by their best efforts.

Many subsequent expeditions to the area, particularly those by Théodore Monod, Directeur de l’Institut Francaise d’Afrique Noire in Paris, failed to locate any sign of this giant meteorite among the dunes. It was therefore assumed that Capt. Ripert had misidentified a blackened, quartzite–sandstone rock outcropping as the main mass from which the smaller fragments were cleaved. However, Capt. Ripert remained steadfast in his story throughout his life.

Modern radiometric dating techniques have been applied to this mystery to determine the CRE age, terrestrial age, and the pre-atmospheric size of the 4.5 kg Chinguetti mass (Welten et al., 2001). Methods employed have established a CRE age of 66 (±7) m.y, similar to that of the Estherville and Crab Orchard mesosiderites. The terrestrial age was calculated to be less than 18 (±1) t.y, a relatively short interval which is inconsistent with the description given by Capt. Ripert—that of a mass having a deeply wind-carved base. Perhaps most importantly, the pre-atmospheric diameter of Chinguetti was determined to be only ~1.2 m given a shielding depth of ~15 cm, which calls into serious doubt the existence of the giant meteoritic mass.

Based on the metamorphic textures of matrix silicates, the mesosiderites were assigned to specific subgroups (Powell, 1971; Floran, 1978, Hewins, 1984), with Chinguetti being assigned to the least metamorphosed subgroup-1. In his scheme, Hewins proposed a further division of the least metamorphosed category based on plagioclase abundance: a higher abundance for group 1A (24%) and a lower abundance for group 1B (21%). Visit the Bondoc page for a more thorough description of this grouping scheme. The photo above shows a 0.58 g micromount of this very rare mesosiderite. A more representative photo of Chinguetti exhibited at the Muséum National d’Histoire de Paris can be seen at their website.


See also the online article by Richard Greenwood (2014), ‘The meteorite that vanished’.


<!– For additional information on the Chinguetti meteorite, watch the XiveTV documentary ‘The Meteorite That Vanished’ on YouTube. This is the story of three adventurers ’ daring attempt to crack the Sahara ’s greatest mystery and to establish once and for all whether the world ’s largest meteorite lies beneath the shifting dunes of Mauritania. –>

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Vaca Muerta

Mesosiderite, group 1A
standby for vaca muerta photo standby for vaca muerta photo
Found during or before 1861
25° 45′ S., 70° 30′ W. approx. The first of many large masses of this polymict, metal–silicate breccia was found by a prospector in Atacama, Chile, ~100 miles SE of Taltal. The discovery was first reported by the Chilean geologist Ignacy Domeyko in 1862, in a landscape described as vegetation-free, sand and gravel covered open plains, crossed by dry river beds. Possibly many tons of Vaca Muerta were found and subsequently broken apart by 19th century miners to retrieve the metal-rich portion for smelting.

In 1985, a mining engineering student named Edmundo Martinez explored the Atacama Desert region from which Vaca Muerta was originally found by miners a century earlier (Pedersen et al., 1992). Field work by Martinez and his associates led to the characterization of many sites in the strewn field and the recovery of both undisturbed and disturbed masses of this mesosiderite, in sizes of 13 g–312 kg and 2 g–850 kg, respectively. In addition, they found a large pile of Vaca Muerta igneous clasts discarded by 19th century miners who were seeking the more metal-rich stones for smelting. Almost 4,000 kg of this meteorite has been accounted for, while the pre-atmospheric mass is estimated to have been over 6,000 kg. Calculations indicate that the Vaca Muerta meteoroid had a pre-atmospheric diameter of ~100–140 cm (Reedy et al., 1994). The extensive strewnfield has dimensions of 11.5 × 2.1 km.

A two-stage irradiation history has been proposed for mesosiderites by Hidaka and Yoneda (2011). In the first stage, occurring <4.4 b.y. ago, the silicates were irradiated near the surface, prior to mesosiderite formation. Subsequent to differentiation of the mesosiderite parent body, a low velocity collision of a large (~50–150 km diameter) iron projectile and a large (~200–400 km diameter), still molten iron planetesimal occurred ~4.4 b.y. ago, melting and mixing the cool silicate layer of the planetesimal with the molten Fe-metal of the projectile into complex breccias. The partial or total collisional disruption and gravitational reassembly of the target body is considered a strong likelihood by some (Haack et al., 1996). Others favor a scenario in which the molten metal that became mixed with cooler silicates was derived from the differentiated core of the mesosiderite planetesimal itself through a severe impact, without a catastrophic disruption and reassembly (Scott et al., 2001).

Based on in-depth mineralogical and textural studies of numerous samples of a massive >80 kg mesosiderite, portions of which having been described and classified by several independent labs, Bunch et al. (2014) have proposed an alternate petrogenetic history for the mesosiderites. Because they did not find any eucritic inclusions in any of the fragments from this very large mesosiderite, and the O- and Cr-isotopic compositions of the orthopyroxene and plagioclase in this mesosiderite are virtually identical to those of diogenites, they developed a scenario in which a small, previously cooled iron-rich asteroid collided with a still warm (~800°C), previously differentiated diogenitic asteroid at a low angle and relatively low velocity (<6 km/s). The frictional shear forces created in this oblique impact produced small rotating molten metal spherules which mechanically incorporated diogenite fragments, resulting in a minimally-shocked, fragmental stony-iron layer of debris.

Each of these scenarios envision a brief period of rapid cooling due to the mixing of warm and cold material, followed by very slow cooling and annealing consistent with deep burial of the mesosiderite precursor material under an extensive debris blanket and/or subsequent lava flows. Other evolutionary stages considered to have occurred include reduction processes and episodic impact events, the latter resulting in re-melting, metal–silicate mixing and brecciation, formation of quench textures, mixing of deep silicates and near-surface silicates, regolith gardening, thermal metamorphism, and degassing which reset the Ar–Ar age chronometer to reflect an age of ~3.6–3.9 b.y. Thereafter, the impact excavation and ejection from depth of the mesosiderite meteoroid (~138 m.y. ago for Vaca Muerta) initiated a second stage of irradiation (Bajo and Nagao, 2011). Other calculated CRE ages of mesosiderites reflect different excavations occurring over the past 10–150 m.y. A terrestrial age of <2000 years was calculated for Vaca Muerta (Jull et al., 2009).

Mesosiderites of subgroup 1 have experienced the least thermal metamorphism and are more commonly richer in eucritic enclaves (Ikeda et al., 1990), some of which have escaped significant recrystallization. Vaca Muerta eucritic enclaves or clasts, also known as ‘eucritic pebbles’, are mostly characterized as monogenic basalts with granular to gabbroic textures, composed primarily of silicate (93–98 wt% as plagioclase and pigeonite) with minor amounts of tridymite (derived from both the primary eucritic material and through reduction processes sustained by phosphorus in FeNi-metal), phosphates, oxides, and various opaques. The major and trace element compositions of the Vaca Muerta clasts are most commonly similar to cumulate eucrites having a more magnesian composition (ilmenite-free; Mg# ≥0.47), whereas other mesosiderites might contain more ferroan clasts (ilmenite-bearing; Mg# ≤0.46) and show similarities to non-cumulate eucrite, diogenite, and dunite material. The eucritic clasts were formed during the very early stages of magmatism on the mesosiderite parent body ~4.563 (±0.015) b.y. ago, representing both magmatic rocks (ilmenite-bearing) and cumulate and/or residual rocks (ilmenite-free). The significant petrographic differences that exist between these mesosiderite eucritic clasts and the HED meteorites may indicate they originated on separate parent bodies.

Both the Vaca Muerta eucritic clasts (see example 1, example 2, example 3, and right photo above) and the metallic-melt host phase have CRE ages based on Cl–Ar and Sm–Gd of 138 (±11) m.y., which reflects only irradiation received during transit to Earth (Albrecht et al., 2000; Hidaka and Yoneda, 2011). The eucritic pebbles have an additional Kr–Kr-based cosmogenic age of >60 m.y., which reflects the additional exposure received by the precursor material during residence at shallow depth on the parent asteroid prior to impact ejection. Studies of cosmogenic 131Xe that was produced while the meteoroid was in transit to Earth indicates that the progenitor of Vaca Muerta had been located deep within the source object (Bajo et al., 2011). In addition, rare examples (0–6 vol%; Prinz et al., 1980) of large, coarse-grained, magnesian olivine (dunite) clasts have been found in Vaca Muerta and some other mesosiderites, but it is still unresolved whether they represent higher-level cumulates or mantle material (Greenwood et al., 2015).

Mandler and Elkins-Tanton (2013) proposed a formation scenario for such dunites that involves a two-stage crystallization process: first, an equilibrium crystallization process from the late-stage liquid after 60–70% solidification of the global magma ocean; second, a fractional crystallization process within an ascended, high-level (crustal) pluton composed of the former extracted residual melt, ultimately resulting in the formation of a thin lower-crustal dunite layer along with more shallow olivine diogenite, diogenite, and cumulate eucrite lithologies. On the other hand, if the dunitic clasts are actually derived from mantle material, a scenario is required to explain how such material was incorporated into the regolith. However, it was argued by Barrat and Yamaguchi (2014) that magma chamber processes are unable to explain the chemical diversity of the diogenites (e.g., the range of heavy-REE ratios in diogenitic orthopyroxenes), and that neither assimilation of wallrock nor incorporation of a trapped melt component can account for this diversity. They contend that the diversity is more likely the result of variability in the respective initial parental melt compositions.

The diverse suite of silicate clasts (e.g., basalts, gabbros, orthopyroxenites, dunites) present in Vaca Muerta and other mesosiderites have O-isotopes consistent with an origin on the mesosiderite parent body rather than a xenolithic origin (Greenwood et al., 2009, 2013, 2015). Because mesosiderites have isotopic signatures (e.g., 17O, 54Cr, and 50Ti [Rüfenacht et al., 2018]) that are virtually identical to the HED clan meteorites, many investigators consider that mesosiderites originated on the HED parent body, or at least within similar nebular reservoirs (see diagrams below). Further information regarding the origin of the dunitic clasts in our collections can be found on the NWA 2968 page. standby for o-isotopic diagram
Diagram credit: Greenwood et al., 2017
For an explanation of the diagram components see the open access article in Chemie der Erde, vol. 77, p. 25 (2017)
‘Melting and differentiation of early-formed asteroids: The perspective from high precision oxygen isotope studies’
(http://dx.doi.org/10.1016/j.chemer.2016.09.005)

standby for 54Cr vs 17O diagram photo
Diagram credit: Wasson and Göpel, 77th MetSoc, #5446 (2014) However, studies of the Eu/Sm systematics suggest that the HED clan meteorites and the mesosiderites did not originate on a common parent body. The silicates in mesosiderites have a Eu/Sm ratio higher than CI and so exhibit a positive Eu anomaly, while the howardites of the HED clan have a Eu/Sm ratio less than CI and show a negative Eu anomaly. Further evidence supporting separate parent bodies rather than distinct regions on a common parent body includes the fact that the CRE ages are not correlated; most howardites fall into two clusters of ~21 and ~38 m.y., while the mesosiderites have widely ranging ages from <10 to 340 m.y. Moreover, in contrast to the mesosiderite parent body, the HED parent body shows no evidence of a crustal melting episode, or a metal–silicate mixing event (Rubin and Mittlefehldt, 1993). In accord with this viewpoint, it was shown by D. Mittlefehldt (2014) that mesosiderites contain basaltic clasts that have a wide range of anomalous characteristics, while howardites do not. He cited these anomalies as evidence for the fact that clast formation succeeded metal–silicate mixing on the parent body, and he also noted that petrologic data are indicative of clast formation preceding impact gardening. Therefore, the apparent lack of such anomalies in components of howardites is inconsistent with a common parent body for both HED and mesosiderite meteorites. In another study of the highly siderophile element (HSE) contents of mesosiderites, Xu et al. (2011) found that the HSE patterns are very similar to those in FeNi-metal from H-group chondrites, and they suggest a genetic link might exist there.

The ~226-km-diameter M-type asteroid, 16 Psyche, has been considered by some to be the mesosiderite parent body. The visible and near-infrared reflectance spectra of 16 Psyche indicate a surface composed of metal and pyroxene consistent with mesosiderites. A study of 16 Psyche conducted by Viikinkoski et al. (2018) has determined that its density (3.99 [±0.26] g/cm–3) is a good match to values calculated by Britt and Consolmagno (2003) for both mesosiderites (~4.25 g/cm–3) and the stony-iron Steinbach (~4.1 g/cm–3), but is inconsistent with that of iron meteorites (~7.8 g/cm–3).

A recent detailed spectroscopic survey of main-belt and near-Earth asteroids has identified certain differentiated asteroids having high concentrations of high-Ca pyroxene, abundant plagioclase, minor olivine, and a significant metal component. These asteroids, including the S-type asteroids 17 Thetis and members of the Merxia and Agnia families, may be similar to the metal–pyroxene-rich mesosiderites (Sunshine et al., 2004). In a similar way, utilizing near-IR spectrography of asteroids located near the border of the 3:1 orbital resonance located at 2.50 AU (a Kirkwood Gap associated with Jupiter), Fieber-Beyer et al. (2010, 2011) found that close similarities exist in the absorption spectra of eleven members (out of twelve members studied) of the Maria asteroid family (total of ~80 members) and the absorption spectra obtained for mesosiderites. The Maria family, which is likely the result of collisional disruption, contains HED-type pyroxenes and exhibits spectral reddening likely caused by FeNi-metal. Fieber-Beyer et al. (2011) ascertained that the near-Earth asteroid 1036 Ganymed is mineralogically similar to diogenites, with pyroxenes identical to those in Johnstown, and they also determined that the spectra of the asteroid is consistent with asteroids from the Maria family. Dynamical models of the Maria family predict that it is a probable source of some or all mesosiderites delivered to Earth (see diagram below). standby for maria asteroid family diagram
Diagram credit: Fieber-Beyer et al., Icarus, vol. 213, #2, p. 534, (2011)
‘The Maria asteroid family: Genetic relationships and a plausible source of mesosiderites near the 3:1 Kirkwood Gap’
(https://doi.org/10.1016/j.icarus.2011.03.009)
Based on silicate matrix textures, Vaca Muerta has been placed into group 1A. (see the Bondoc page for further information about the grouping scheme). It is theorized by one investigative team that Vesta and the Vestoids, along with the isotopically similar mesosiderites and IIAB irons, were themselves the products of a catastrophic breakup of an even larger parent object which they have named ‘Opis’ (Irving et al., 2009). In Greek mythology, Opis was the wife of Saturn, whose children were Jupiter, Neptune, Pluto, Juno, Ceres, and Vesta. The left photo above shows a 9.1 g end section of a metallic-melt host phase of Vaca Muerta, while the right shows a 51.6 g end section of a naturally faceted, eucritic inclusion likely formed from a localized impact melt of an existing cumulate eucrite lithology (photos not to the same scale).


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Vermillion

Pyroxene Pallasite
Vermillion grouplet
standby for vermillion photo
Found May, 1991, recognized 1995
39° 44.18′ N., 96° 21.68′ W. A 34.36 kg mass was found by M. and G. Farrell while planting in a grain field in Marshall County, Kansas, in the vicinity of the Black Vermillion River. This was purchased by a dealer with the belief that it was a Brenham mass. Upon cutting, it was discovered to be unique from Brenham. Vermillion consists of ~86 vol% FeNi-metal and ~14 vol% silicates with grains much smaller than normal (Boesenberg et al., 1995). The silicates consist of olivine (~93 vol%), orthopyroxene (~5 vol%), chromite (~1.5 vol%), and merrillite (~0.5 vol%). Vermillion was compared to the pallasite Yamato 8451 (54.9 g) which has a very similar silicate composition consisting of olivine (~94 vol%), orthopyroxene (~4.8 vol%), clinopyroxene (~1.1 vol%), and merrillite (~0.1 vol%) reported by Yanai and Kojima (1995). Pyroxene accounts for ~0.7 and ~1.6 vol% of the silicate fraction of Vermillion and Y-8451 respectively, the remainder being mostly olivine. By comparison, the main-group pallasites contain all olivine with only trace amounts of pyroxene. The coexistence of both olivine and pyroxene in these two pallasites might indicate a lower crystallization temperature. In light of the compositional and isotopic similarities between Vermillion and Y-8451, Boesenberg et al. (1995) proposed they be recognized as a grouplet.

Subsequent studies have determined that Vermillion shares a similar pyroxene composition, mineralogy, O-isotope composition, and REE pattern not only with Yamato 8451, but also with the more recently discovered pyroxene pallasite Choteau (8,474 g), and it was proposed by Gregory et al. (2016) that these three pyroxene pallasites be recognized as members of a new pyroxene-pallasite grouplet termed ‘Vermillion pallasites’. standby for o-isotopic diagram
Diagram credit: Gregory et al., 47th LPSC, #2393 (2016) However, significant differences that exist between these three Vermillion pallasites are not yet resolved, including differences in texture (Y-8451 contains 4× the vol% of silicates as Vermillion) and siderophile trace element composition, as well as the presence of the carbide cohenite in Vermillion. In addition, although Vermillion, Y-8451, and Choteau all contain both low- and high-Ca pyroxenes of similar compositions, some differences are evident. Vermillion and Y-8451 contain both pyroxene types in the form of large grains, inclusions in olivine, and grains bordering olivine. However, in Choteau high-Ca pyroxene has only been identified as inclusions in olivine, while low-Ca pyroxene is present as both individual grains and along boundaries of high-Ca pyroxene grains (Gregory et al., 2016). Furthermore, plagioclase has not been found in either Vermillion or Y-8451, but is present in Choteau as inclusions in both low- and high-Ca pyroxene and as small veins in low-Ca pyroxene. Notably, plagioclase is also present in the ungrouped pyroxene pallasite NWA 10019, but it is compositionally distinct to that in Choteau.

The olivine fayalite composition of these pyroxene-bearing pallasites plots at the magnesian end of the main-group pallasite range. As a comparison, the Eagle Station pallasite group has the most ferroan composition, as well as a high Ge/Ga ratio in the metal and a unique O-isotope composition. The high Ir content in the Eagle Station pallasites suggests crystallization from the inner core region of its parent body, below the core–mantle interface in which the main-group and pyroxene-bearing meteorites probably formed on their respective parent bodies.

Siderophile trace element and oxygen isotopic compositions clearly resolve the pyroxene-bearing meteorites from the main-group and Eagle Station pallasites, and therefore they represent additional parent bodies on which pallasite-like textures were formed. In addition to the Vermillion trio, several other pyroxene-bearing meteorites have been found and studied. A 46 g pyroxene-rich pallasite named Zinder was found in Niger in 1999. It contains 28 vol% pyroxene and 27 vol% olivine in a network of FeNi-metal. In 2003, a 53 g pyroxene-rich pallasite designated NWA 1911 was purchased in northwest Africa. It has a modal composition of about 24% FeNi-metal and 75% silicates, with the silicates consisting of 34.5% orthopyroxene and 40.2% olivine. Separate fragments of the ungrouped pyroxene-bearing pallasite NWA 10019, weighing together 606 g, were found in 2015.

On a Ni vs. Au coupled diagram, Vermillion plots just outside of the low-Au end of the IAB main group irons, and also plots along an extention of the low-Au, medium-Ni (sLM) subgroup into lower Au compositions (Wasson and Kallemeyn, 2002). Furthermore, the O-isotopic composition of Vermillion is within the range of IAB irons, and therefore, Vermillion could be considered genetically related to members of the low-Au division of the IAB iron-meteorite complex.

Based on all of the data gathered so far, it could be concluded that the pallasites in our collections represent at least seven separate parent bodies: 1) main-group; 2) Eagle Station group; 3) Milton; 4) Choteau + Vermillion + Y-8451; 5) Zinder + NWA 1911; 6) NWA 10019; 7) LoV 263. In addition, several pallasites with anomalous silicates (e.g., Springwater) and anomalous metal (e.g., Glorieta Mountain) could possibly increase the number of unique parent bodies. A beautiful etched slice of Choteau is shown in ‘Meteorite Picture of the Day’ for June 20, 2013, courtesy of Ruben Garcia. The specimen of Vermillion pictured above is a 67.9 g partial slice. The photo below shows a close-up of the etched reverse side. standby for vermillion photo


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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”.