Posted on Leave a comment

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. –>

Posted on Leave a comment

NWA 1878

Mesosiderite, group 0B
Home Page
mouseover for alternate view angle

Purchased June 2003
no coordinates recorded A complete, fusion-crusted, stony-iron meteorite weighing 1,374 g was purchased in Morocco on behalf of A. and G. Hupé. A sample was sent to the University of Washington in Seattle (A. Irving and S. Kuehner) for analysis and classification, and NWA 1878 was determined to be a type-B mesosiderite. It was further ascertained that the mass was paired with the 728 g mesosiderite NWA 1817 (A. Irving and S. Kuehner, UWS), purchased in Morocco in January of 2003 for N. Oakes, as well as being paired with several other independently classified masses (e.g., NWA 1979 and NWA 2042) for a total combined weight of at least 6.4 kg. Continued research was conducted by Bunch et al. (2014) on these specimens and a large number of mesosiderite samples previously considered to represent a separate fall. It was eventually determined that all of these mesosiderites represent a single strewn field (totaling at least 80 kg) comprising mesosiderites of differing subgroups (see also NWA 1827).

Northwest Africa 1817 (=1878) was described as a coarse-grained, unbrecciated, plutonic igneous-textured assemblage of spheroidal FeNi-metal–silicate clusters together with a larger component of silicate material (predominantly orthopyroxene with lesser amounts of plagioclase), along with minor silica, troilite, chromite, and merrillite, plus rare olivine grains and clasts of eucritic and diogenitic composition (Bunch et al., 2004). Oxygen isotope values for NWA 1817 were obtained at Carnegie Institution, Washington D.C. (D. Rumble, III), and the meteorite plots in the field of the mesosiderites on an oxygen three-isotope diagram (see below).
Diagram courtesy of the Meteoritical Bulletin: Oxygen Isotope Plots Direct Link Ample petrographic evidence exists to support the hypothesis for a two-stage irradiation history for mesosiderites (Hidaka and Yoneda, 2011). In the first stage, occurring >4.4 b.y. ago, the silicate component of a large (~200–400 km diameter) parent body was irradiated near the surface, prior to mesosiderite formation. Subsequent to differentiation of this planetesimal, a low velocity collision occurred with a large (~50–150 km diameter) iron projectile ~4.4 b.y. ago, melting and mixing the cool silicate layer of the planetesimal with the molten FeNi-metal of the projectile forming complex breccias. The partial or total collisional disruption and gravitational reassembly of the target body is considered a strong likelihood by some investigators (Haack et al., 1996), while others favor a scenario in which a severe impact caused molten metal from the differentiated, molten core of the planetesimal itself to be mixed with the cooler silicates from the mantle (Scott et al., 2001).

After a brief period of rapid cooling resulting from the mixing of cold and hot material, NWA 1878 experienced very slow cooling at ~0.01 °C/year consistent with deep burial of the mesosiderite precursor material under an extensive debris blanket and/or within lava flows (Sugiura and Kimura, 2015). Other mesosiderites including NWA 1242, Crab Orchard, ALH 77219, and A-882023 also cooled much more slowly from peak temperatures down to intermediate temperatures, while others including Estherville, Vaca Muerta, NWA 2924, and Dong Ujimqin Qi experienced rapid cooling over the same temperature range indicative of a residence nearer the surface.

Over time, reduction processes were initiated, while episodic impact events on this large, slowly cooling body caused remelting, metal–silicate mixing and brecciation, formation of quench textures, mixing of deep silicates and near-surface silicates of eucritic and diogenitic compositions, regolith gardening, and degassing, ultimately resetting the Ar–Ar chronometer to reflect an age of ~3.6–3.9 b.y. Thereafter, impact excavation and ejection from the mesosiderite meteoroid occurred, with calculated CRE ages of various mesosiderites reflecting multiple excavations over the past 10–150 m.y.

Mesosiderites have been historically classified from type 1 to 4 in order of increasing degrees of thermal metamorphism due to impact-generated reheating. An in-depth analysis of the mesosiderite ALHA77219 was conducted by Agosto et al. in 1980, and they determined that the matrix is fine-grained and that the pigeonite grains are anhedral rather than coarsely poikiloblastic, features consistent with minimal recrystallization and a classification as type 1B. Recently, Sugiura (2013) developed criteria for establishing the most primitive mesosiderite, the study of which could elucidate the earliest history of the mesosiderite parent body. He determined that NWA 1878 was the most primitive mesosiderite known based primarily on the following indicators:

  1. a wider range of silicate compositional heterogeneity (especially for plagioclase)
  2. a smaller pyroxene lamellae width
  3. a smaller spheroidal metal grain size
  4. lack of corona formation in olivine (or coronal onset lacking chromite)

Additional discrimination criteria for primitiveness were identified and investigated by Sugiura et al. (2013), such as the Al/(Al+Cr) ratio and the Ti concentration in Cr-spinel. It was determined that NWA 1878 was more primitive than either the 1B ALHA77219 or the 1A Vaca Muerta mesosiderites, and accordingly, NWA 1878 was designated the first mesosiderite of metamorphic type 0B. The photo of NWA 1878 shown above is a 4.0 g partial slice acquired from M. Farmer. The top photo below shows a slice of NWA 1817 exhibiting the spherical metal grains (courtesy of M. Graul), while that below shows a portion of the main mass (courtesy of N. Oakes).
Photo source: Encyclopedia of Meteorites


Posted on Leave a comment

Bondoc

standby for bondoc photoMesosiderite, group 4B
Found 1956, 12° 20′ N., 122° 52′ E. approx.

After learning of the possible existence of meteoritic iron found on the Bondoc Peninsula on the island of Luzon in the Philippines, Harvey Nininger enlisted the help of a friend who lived in Manilla, John Lednicky, to assist in the recovery of the main mass from its remote jungle location. After three and a half years of extraordinary effort, the single 888.6 kg mass of Bondoc and several smaller fragments were finally delivered to the American Meteorite Museum in Sedona. The extraordinary story of the Bondoc meteorite recovery is shared on the website of Jason Utas, which also includes historical and rare photos of the meteorite (see also below for another account of the recovery, courtesy of the Beyer Archive, University of Philippines).

Contained within the stony-iron mass comprising ~11 area% are baseball-sized spheres of iron which contain silicate inclusions on a still smaller scale. A large, metal-free, lenticular, nodule was found, consisting mostly of green pyroxene with minor amounts of plagioclase and fine opaques (Garvie et al., 2010). The major-element compositions of the components in this nodule are mostly comparable to those of known diogenites, and the nodule has experienced high degrees of metamorphism attested by the coarse grain sizes, 120° triple junctions, and compositional equilibration. The investigating team proposed that this pyroxenite nodule is a fragment from the deep, primitive crust of a differentiated parent body.

Consistent with other mesosiderites, Bondoc has an Ar–Ar gas retention age of ~3.9 b.y., probably identifying a very slow cooling rate under a thick debris blanket following the collisional disruption and gravitational reassembly of the parent body. Bondoc has a cosmic-ray exposure age of 166 (±40) m.y.

Based on the metamorphic textures of the matrix silicates, a scheme was developed (Powell, 1971; Floran, 1978) which assigned the mesosiderite group members into one of four textural categories; 1) minimally recrystallized, 2) moderately recrystallized, 3) highly recrystallized, or 4) intergranular melt rock. However, clear differences in bulk composition among these four categories prompted a reinterpretation of this scheme (Hewins, 1984).

Hewins proposed a further division of the least metamorphosed category 1 based on plagioclase abundance: a higher abundance for group 1A (24%) compared to a lower abundance for group 1B (21%). A further division of the more highly metamorphosed categories 2 and 3 was based on whether plagioclase or orthopyroxene matrix predominates (groups 2A/3A and 2B/3B, respectively). The more basaltic, plagioclase-rich members of class A are enriched in an anorthitic, cumulate eucrite-like component, while the more ultramafic, orthopyroxene-rich members of class B are enriched in a diogenite-like component. The more plagioclase-rich compositional class A contains a larger diopside component and has a lower Mg# than the orthopyroxene-rich compositional class B.

Through other studies, it was determined that the Ir/Ni ratios (or better still, a plot of Ir/Ni vs. Au/Ni) for matrix metal of mesosiderites is diagnostic for membership in group A or B, reflecting values of 0.000036 or 0.000051, respectively (Wasson and Rubin, 1985). According to Kong et al. (2008), group B might have assimilated a higher proportion of solidified, weakly fractionated (higher Ir, lower Ni and Au) metal than group A did. Furthermore, the concentrations of Ga and Ge are lower in the metal of category 1 mesosiderites than in that of more highly metamorphosed mesosiderites (Wasson et al., 1974). This is believed to have occurred as a result of reduction from silicates to metal during metamorphism.

Hewins reinterpreted the metamorphic orthopyroxene-rich groups 2B and 3B as having some melt-rock textures and assigned them to a new igneous group 4B, reassigning the previous members of group 4 to 4A. However, this reinterpretation has left groups 2B and 3B unrepresented. More recently, Hewins established a group 2C to accommodate the granular texture and very low plagioclase content (0–5%) of certain paired Antarctic orthopyroxinitic mesosiderites. However, the subsequent identification of igneous clasts in these mesosiderites led to their reassignment to group 4B (for a more in-depth treatment, see R. Hewins, Meteoritics, vol. 23, 1988).

Bondoc was classified as a member of group 3B under the Floran scheme, and was reclassified as 4B under the Hewins scheme, due to the presence of silicate melt matrices, poikilitic textures, resorbed olivine grains, and in light of its crystallization sequence. This melt rock formed as an impact melt into which cold clasts were mixed. The specimens shown above are an 8.3 g partial slice (left) and a 32.6 g slice from a golf ball-sized, silicated iron nodule (right).

History of the Discovery of the Mulanay Meteorite (Bondoc Peninsula): Courtesy of the University of Philippines, Beyer Archive
standby for lodran photo