Iron, IVB, ataxite
standby for santa clara photo
Found 1976
24° 28' N., 103° 21' W.

A single 63 kg iron mass was found in Durango, Mexico and subsequently purchased by Arizona State University—Center for Meteorite Studies. 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 scale. 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 14 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 might have resulted from a multi-stage condensation process, in which fractionations occurred in both the nebula and the molten core. Using the concentration ratio of Re and O measured for both solid and liquid metal, a fractional crystallization model was developed by Walker et al. (2008). They found that different IVB irons were formed under varying degrees of fractional crystallization of an evolving liquid, over a crystallization interval of ~15–70% (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 which 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 high-Ni content that is observed (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 its members, is consistent with 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 not only attest to one of the fastest cooling rates among iron groups, but also record 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) calculated that this impact event occurred while 25% of the outermost portion of the core was still molten, and that IVB irons were derived from the previously solidified portion within. Solidification of the innermost portion of the core proceeded after mantle removal (and possibly removal of the remaining liquid core), evidenced 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 was 220–340 km in total diameter. The low-Ni IVB subgroup with the slowest cooling rate (200°C/m.y.) was located near the center of the core, while the high-Ni subgroup that crystallized late and had the fastest cooling rate (4700°C/m.y.) was located ~62 km from the center.

It was reported 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 calculated length of time that the magnetic field persisted on the angrite parent body (8 m.y.) is concordant with 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). Rare silica inclusions no larger than 25 µm in size have been reported in Santa Clara (and IVB Warburton Range), associated with sulfide nodules (Teshima and Larimer, 1983). The presence of these inclusions in a nearly Si-depleted FeNi-metal host attests to formation under high-temperature reducing conditions (but more oxidizing than for E chondrites). Further research is needed to better understand the nature of IVB parent body.

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.