SANTA CLARA

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%. Zhang et al. (2022) also used fractional crystallization modeling and determined that the initial bulk composition of the IVB metallic core contained 0.5 (±0.5) wt% S and 0.45 (±0.02) wt% P, and that the group members represent ≤78% crystallization of the parental core liquid.

Other elemental ratios indicate that oxidizing conditions existed on the parent body during core differentiation, which promoted the loss of ~72% of the Fe to the silicate phase and resulted in 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.

Numerical models were employed by Neumann et al. (2018) to better understand the relatively late timing of metal–silicate separation for IVB irons at ~2.9 (±0.6) m.y. after CAIs as inferred by Kruijer et al. (2014) from Hf–W chronometry. They derived a preferred multi-stage differentiation scenario as follows:

Following accretion of an ~220 km-diameter planetesimal ~0.1 (to ~0.5) m.y. after CAIs, and partial melting ~0.2 m.y. later, the extraction and ascension of an early silicate partial melt removes ~90% of the radiogenic 26Al component from the interior. This results in the formation of a shallow magma ocean (SMO) having a melt content of >50%. This silicate melt also sequesters a solid FeNi-metal component that is suspended within the SMO by convective forces. Over time, a separate, lower mantle magma ocean (MMO) is generated, metal particles gravitationally settle, and a proto-core evolves. The metal in this proto-core has a low 182W composition due to its early isolation from silicates, ~1.5 m.y. after CAIs, and exclusion of parental 182Hf. Therefore, the proto-core should reflect a Hf–W-based metal–silicate separation age significantly older than the observed age of ~2.9 (±0.6) m.y. after CAIs.

This discrepancy in the timing of metal–silicate separation was resolved in their preferred model which invokes a second stage of differentiation. After an extended period of gradual cooling (~5.4 [3–11] m.y.), both the solid and the remaining liquid metal within the SMO, which had now become enriched in 182W through 182Hf decay, precipitate downward through the MMO and into an eventually fully molten core. This late-segregated metal then mixes with the low 182W, early-segregated metal thereby establishing a "combined" Hf–W age for metal–silicate separation of ~2.9 (±0.6) m.y. after CAIs. Core crystallization is complete by ~30 m.y. after CAIs.

Schematic of the Evolution of the IVB Parent Body

click on photo for a magnified view

Diagram credit: Neumann et al., 81st MetSoc, #6209 (2018)
Full article in Journal of Geophysical Research: Planets (American Geophysical Union), vol. 123, #2, pp. 421-444, (2018)
'Multistage Core Formation in Planetesimals Revealed by Numerical Modeling and Hf‐W Chronometry of Iron Meteorites'
(https://doi.org/10.1002/2017JE005411)

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. 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 (see diagram below). 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 speculated that the angrites might 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).

With the advent of better investigative techniques, scientists have explored the possibility of a genetic relationship between IVB irons and other meteorite groups based on O-isotopic analyses. Utilizing chromite grains from IVB irons Warburton Range and Hoba, Corrigan et al. (2017) concluded that IVB irons are not genetically related to angrites, but that their respective parent bodies experienced similar petrogenetic histories. They also found that IVB irons share close similarities to the South Byron trio irons (Babb's Mill [Troost's], South Byron, and Inland Forts [ILD] 83500) and the Milton pallasite (see top diagram below). Moreover, isotopic compositions (e.g., O, Mo, Ru, and W) and HSE abundances of the IVB irons and the South Byron trio–Milton grouping fall within the range of the oxidized CV–CK chondrites (Hilton et al., 2018). In addition, Cr-isotopic analyses conducted by Sanborn et al. (2018) has provided further evidence for a genetic link between many of these disparate meteorites, possibly on a common, now disrupted CV-like parent body (see bottom diagram below).

Rare silica inclusions no larger than 25 µm in size have been reported in Santa Clara (and in IVB Warburton Range) associated with sulfide nodules (Teshima and Larimer, 1983). Despite the fact that this group of irons is considered to have formed from a volatile-depleted precursor under oxidation conditions (Campbell and Humayun, 2005), it was inferred that the presence of these inclusions in a nearly Si-depleted FeNi-metal host attests to formation under high-temperature, possibly late-stage 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.