Mesosiderite, group B4
standby for bondoc photo
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. Wang and Hsu (2019) used Pb–Pb chronometry to date 53 merrillite crystals associated with FeNi-metal in the Youxi mesosiderite. Based on the low REE abundances in the Youxi merrillite compared to that in eucrites, they contend that it was formed by oxidation of P in metal during the metal–silicate mixing event rather than during magmatic activity. They derived an age of 3.950 (±0.080) b.y. which they consider represents the timing of merrillite development during the mesosiderite-forming event. An equally plausible timing for the metal–silicate mixing event was ascertained by Haba et al. (2019) using high-precision U–Pb dating of zircons in several mesosiderites. Based on these results they contend that the metal–silicate mixing event occurred 4.52539 (±0.00085) b.y. ago. They propose a scenario in which a hit-and-run collision disrupted the northern hemisphere of Vesta leading to ejecta debris reaccreting to the opposite, southern hemisphere (see schematic diagram below). The deeply buried mesosiderite meteorites were ejected into Earth-crossing orbits by later impacts.

Schematic Illustration of Mesosiderite Formation
Crust (yellow); Mantle (blue); Core (red); Collisional debris (green)
standby for mesosiderite formation scenario diagram
Diagram credit: Haba et al., Nature Geoscience, vol. 12, #2, p. 512, (2019)
'Mesosiderite formation on asteroid 4 Vesta by a hit-and-run collision'

It is notable that the O-isotopic values of the mesosiderites are almost identical to those of the HED suite of meteorites, implying that a genetic link exists between these disparate groups (Greenwood et al., 2006). Conversely, multiple line of evidence presented by D.W. Mittlefehldt (2021), including petrological (e.g., modal, textural, and redox data), compositional (e.g., incompatible lithophile trace elements), and observational (Dawn at Vesta), indicate that separate parent bodies were probably involved.

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). Additional discrimination criteria for primitiveness were investigated by Sugiura et al. (2013). They determined that NWA 1878 was more primitive than other mesosiderites in category 1, so it was designated the first metamorphic type 0.

Hewins proposed a further division for the least metamorphosed, at that time category 1, based on plagioclase abundance: a higher abundance for group A1 (24%) compared to a lower abundance for group B1 (21%). A further division of the more highly metamorphosed categories 2 and 3 was based on whether plagioclase or orthopyroxene matrix predominates (groups A2/A3 and B2/B3, 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# [= molar MgO/(MgO + FeO)] 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 may have assimilated a higher proportion of solidified, weakly fractionated (higher Ir, lower Ni and Au) metal compared to group A. Furthermore, the concentrations of Ga and Ge are lower in the metal of category 1 mesosiderites than in metal 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 B2 and B3 as having some melt-rock textures and assigned them to a new igneous group B4, reassigning the previous members of group 4 to A4. However, this reinterpretation has left groups B2 and B3 unrepresented. More recently, Hewins established a group C2 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 B4. For a more in-depth treatment, see R. Hewins, Meteoritics, vol. 23 (1988).

Bondoc was classified as a member of group B3 under the Floran scheme, and was reclassified as B4 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. Bondoc has a cosmic-ray exposure age of 166 (±40) m.y. The specimens shown above are an 8.3 g partial slice (left) and a 32.6 g slice from a golfball-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
click on photo for a magnified view