SANTA CLARA

A single 63 kg mass was found in Durango, Mexico. Because it contains 17.9% nickel, it forms a macroscopically featureless surface structure of taenite. However, very fine, non-intersecting laths of kamacite in a fine plessitic matrix can be seen on a microscopic level.

The dozen irons comprising group IVB are enriched in refractory siderophile elements (e.g., Os, Ir, W, Re, Ru, Mo, and Pt) and depleted in volatile siderophiles (e.g., Au, Cr, Cu, As, Ga, Ge) (Campbell and Humayun, 2004). Condensation calculations indicate that these siderophile abundances may have resulted from a multi-stage condensation process, in which fractionations in both the nebula and the molten core occurred. Using the concentration ratio of Re and O measured for both solid and liquid metal, a fractional crystallization model was determined (Walker et al., 2008). They found that formation of different IVB irons occurred after varying degrees of fractional crystallization of the evolving liquid, having a range of ~15% to ~70% (representing Cape of Good Hope and Tinnie, respectively). Both S and P are depleted in IVB irons, and a simple fractional crystallization model for the this group gives an estimate for the initial S content of the molten core of 1 (±1) wt%. This indicates that 28% of the core material that formed from the later-crystallized, S-rich, residual liquid is not yet represented in our collections (N. Chabot, 2004). Other elemental ratios indicate that oxidizing conditions existed on the parent body during core differentiation, resulting in the loss of Fe to the silicate phase (McCoy et al., 2008).

It was stated by Campbell and Humayun (2005) that the depletion of moderately volatile elements in IVB irons is similar to that observed in the angrite meteorites, and that the angrites may best represent the hypothesized silicate portion of the IVB parent body. It was also suggested that the Fe/Mn ratio of the IVB silicate shell would probably have been high (~200) compared to the Earth (~60), although probably higher still than that of the angrite silicate shell (~120). To learn more about the relationship between the IVB and other iron chemical groups, click here. The above specimen is a 10.0 g partial slice.

---