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. Santa Clara metal contains little to no cloudy zone microstructure and lower Ni concentrations in taenite, which is evidence for a modest shock reheating to above 400°C for a period of years or 1000°C for a duration lasting only seconds (Goldstein et al., 2009).

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 by 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 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 ~72% of the Fe to the silicate phase and the observed high-Ni content (McCoy et al., 2008). Bland and Ciesla (2010) attributed the depletion of volatile elements to incomplete condensation from a hot disk 0.3 m.y. after CAIs, at a location of 0.5–1.5 AU.

Yang et al. (2009) determined that the varied CRE history of the IVB group, as well as the wide range of cooling rates measured for members of the group, suggest that a multiple breakup of the parent body and/or removal of the insulating mantle occurred. Goldstein et al. (2010) found that Ni concentration profiles measured along the kamacite–taenite interface attest to one of the fastest cooling rates among iron groups, but also encompass a wide range of cooling rates in a similar manner to IVA and IIIAB irons. However, in contrast to IVA and IIIAB irons, which crystallized inwards following mantle removal, low-Ni IVB irons cooled slower than high-Ni IVB irons consistent with concentric crystallization from the center outwards while temperatures were buffered by an insulating silicate mantle. That being said, to establish the wide variation in cooling rates that exists among IVB irons, the mantle would have to have been stripped prior to cooling below 600°C. Goldstein et al. (2010) and Yang et al. (2010) proposed that this impact event occurred while 25% of the outer core was still molten, and assert that IVB irons were derived from the 75% of the inner core that was solid, measuring 35–160 km. The inner core completed its final solidification after mantle removal (and possibly removal of the remaining liquid core), attested to by the wide variation in cooling rates among IVB irons. The core in which IVB iron crystallization occurred was calculated to have been 110–170 km in diameter on a pre-disrupted asteroid that had measured 220–340 km in diameter. The low-Ni IVB subgroup with the slowest cooling rate (475°K/m.y.) was located near the center of the core, while the high-Ni subgroup that crystallized late and had the fastest cooling rate (5000°K/m.y.) was located 62 km from the center.

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. In addition, the length of time that the measured magnetic field persisted on the angrite parent body (8 m.y.) was concomitant to the crystallization period of the IVB irons. They speculate that the angrites may serve as a good representation of the hypothesized silicate portion of the IVB parent body. However, 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), and probably higher still than that of the angrite silicate shell (~120). More research is needed.

To learn more about the relationship between the IVB irons and other iron chemical groups, click here. The specimen of Santa Clara shown above is a 10.0 g partial slice.

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