Iron, IC, octahedrite
Low Re, Os, and Ir subgroup
click on photo for a magnified view
5° 55' N., 73° 0' W.
On April 21, 1810 near the Columbian town of Tocavita Hill, Cecilia Corredor was chasing a broody hen searching for her eggs when she discovered a strange-looking iron mass that was cold to the touch (retazosdelavida.blogspot.com, 2020). The 612.5 kg iron meteorite was hauled to the nearby town of Santa Rosa de Viterbo, where in 1818, the blacksmith put it to use as an anvil. In 1823, the Museum of Bogotá purchased the meteorite from Cecilia for 20 piastres (100 francs), and in about 1874, the large iron mass was put on exhibit atop an elaborate stone column in the town plaza (see photo below). Additional smaller masses associated with a common shower were recovered over the following years, including a 100.5 kg mass found in 1942, all having a combined weight of at least 825 kg (V.F. Buchwald, 1975). Although initial analyses were conducted using artificially reheated material, it was ultimately determined that Santa Rosa is a member of the IC chemical group (J. Wasson, 1970).
Santa Rosa Iron Atop Stone Column, Alongside Henry A. Ward
Photo credit: Henry A. Ward
In an effort to gain more factual information about the different iron masses from Columbia, variously named Santa Rosa, Tocavita, and Rasgalla, and to obtain the main mass for a comprehensive scientific study, Prof. Henry A. Ward (1834-1906) traveled to Columbia in the spring of 1906 (Henry A. Ward, 1907; H. Plotkin, 2004 #5038). When he looked out of his hotel window, he was surprised to see the 612.5 kg iron meteorite on reverent display in the town's central plaza. Ward tried to acquire the iron meteorite by negotiating an agreement with the Governor, whereby Ward would erect a statue of the President of Columbia, General Rafael Reyes who was born in Santa Rosa, in exchange for the meteorite. Late one night after hosting a dinner party for the town residents, Ward inlisted the help of 50 soldiers to quietly lower the meteorite from its pedestal and remove it by oxcart. However, before the iron mass could be successfully transferred out of the country, the Chief of the Colombian police seized it, and the Minister of Public Instruction directed that its removal be prohibited. Nevertheless, Ward was able to obtain a 147.5 kg end section, which he subsequently distributed to museums around the world (see Ward's sketch of the sectioned main mass at this link). Tragically, a few months later Henry Ward was struck by a car and killed while crossing Delaware Avenue in Buffalo, New York, becoming the city's first automobile-related fatality (retazosdelavida.blogspot.com, 2020).
Early descriptions of Santa Rosa published by Cohen, Brezina, and Ward state that it is a polycrystalline octahedrite composed of individual kamacite assemblages measuring 23 cm across that exhibit different orientations; this is easily seen in the etched specimen above. Very fine fissures filled with schreibersite separate the individual kamacite areas. Cigar-shaped troilite cylinders with similar length (46 cm), width (410 mm), and orientation are present in a relatively even distribution of about one per 20 cm² (V.F. Buchwald, 1975). Schreibersite surrounds the troilite along with a thin rim of cohenite. A more in-depth description of the IC irons can be found on the Arispe page.
Both highly siderophile element (HSE) data and W and Mo isotopic data were acquired by Tornabene et al. (2020 #2391; 2021 #1531; 2023) for each of the IC irons. It was demonstrated that the members of this group derive from a common parental melt and are related through varying degrees of fractional crystallization and solidliquid mixing processes: Arispe was the first to crystallize following ~7% fractional crystallization, whereas the crystallization of Santa Rosa involved a more complex solidliquid mixing scenario.
Tornabene et al. (2020, 2021, 2023) determined that the IC irons can be divided into two subgroups on the basis of whether the meteorites have high or low abundances of Re, Os, and Ir (see diagrams below). They posit that the two subgroups represent mixing between solids and liquids in various stages of evolution following a severe collision. The best fit model of Tornabene et al. (2023) that accounts for the IC group HSE concentrations has an initial core liquid composed of ~ 18 wt% S, 2 wt% P, and 0.03 wt% C. The high Re abundance subgroup represents mixtures of contemporaneously evolved solids and liquids, while the low Re abundance subgroup represents variable mixtures of early-formed solids present after <1% fractional crystallization in combination with evolved trapped metal persisting after 9% fractional crystallization, with the assumption that these latter meteorites are composed of >99% evolved liquid component. They derived a pre-exposure 182W model age for their IC group (excluding the anomalous Winburg) of 1.0 (±0.3) m.y. after CAIs. With respect to the disparate HSE data (µ182W isotope value) obtained for the anomalous IC iron Winburg, Tornabene et al. (2023) concluded that it either experienced late-stage alteration due to an impact event, or it represents a later metalsilicate segregation process that would date to 12.7 (±0.9) m.y. after CAIs. In addition, they used mass balance calculations to ascertain the mass of the IC parent body core, finding it to be ~25% of the mass of the IC parent body.
Highly Siderophile Element Data for IC Iron Subgroups
High abundance subgroup: Arispe, Nocoleche, Murnpeowie, Union County, Mount Dooling, Winburg
Low abundance subgroup: Bendego, NWA 2743, Etosha, Chihuahua City, St. Francois County, Santa Rosa
Diagrams credit: Tornabene et al., 51st LPSC, #2391 (2020)
Using nucleosynthetic Pt isotope anomalies to correct for CRE-induced W isotope variations in iron meteorites, Spitzer et al. (2021) calculated new HfW differentiation ages for the iron chemical groups. It was determined that the IC irons have the oldest differentiation age among the established magmatic iron groups at 1.0 (±0.7) m.y. after CAIs (see diagram below).
HfW Ages Corrected for Nucleosynthetic Pt Isotope Anomalies
Diagram credit: Spitzer et al.EPSL vol. 576, art. 117211 (2021, open accesslink)
'Nucleosynthetic Pt isotope anomalies and the Hf-W chronology of core formation in inner and outer solar system planetesimals'
To learn more about the relationship between this and other iron chemical groups, click here. The specimen of Santa Rosa shown above is a 19.8 g etched partial slice. Two different views of the current 411 kg Santa Rosa main mass curated at the National Museum of Columbia are shown below.
Santa Rosa Main Mass, National Museum of Colombia
Photo credit: Omar Gaona The Astrolabe, vol. 9, #76-905 (2010)