Mesosiderite, group 4B
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.

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 golfball-sized, silicated iron nodule (right).