ST. GENEVIEVE COUNTY


Iron, IIIF, octahedrite
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Found during Fall of 1888
37° 58' N., 90° 19' W.

A single spheroidal mass of 244 kg was found by Mr. Z. Murphy, a surveyor, ~1 mile west of the hamlet of Punjaub, which is near present-day Lawrenceton, Missouri. The St. Genevieve mass was bought by Ward several years later. Because of the long terrestrial age, several mm has been removed from the surface and terrestrial oxides are evident.

Group characteristics of the IIIF magmatic irons include very low-Co, high-Cr, and low-P contents. Other meteorites constituting group IIIF include Moonby, Clark County, Nelson County, and Oakley. In their paper ‘Chemical classification of iron meteorites–VIII’, Wasson and Scott (1975) determined that Oakley was the fifth member of a common grouping, which was thereby established as group IIIF. Since then the number of irons in this group has increased to nine. However, it should be noted that fractional crystallization modeling conducted by Zhang et al. (2021) indicated that four of the IIIF members—Binya, Cerro del Inca, Moonbi, and St. Genevieve County—do not fit their model's Ga–As and Ge–As trends. Therefore, they have tentatively suggested that these four irons represent a separate parent body from that of the other group members.

Zhang et al. (2022) demonstrated that many of the interelement trends for the IIIF irons are inconsistent with fractional crystallization of a common parental source magma, and the trends differ significantly from those of any other magmatic iron group. With this insight, as well as the observation of large variations in both the structural class (including fine, medium, coarse, and very coarse) and kamacite bandwidths among the IIIF group members, they concluded that either the IIIF irons experienced a more complex petrogenesis than other groups or they represent more than one parent body.

Based on O, Cr, Ti, and Ni stable-isotopic data, Warren (2011) recognized the existence of two distinct taxonomic super-groups: those which accreted inside the orbit of Jupiter where thermal processing occurred under reducing conditions, termed 'non-carbonaceous' (NC), and those which accreted outside of its orbit where thermal processing occurred under oxidizing conditions, termed 'carbonaceous' (CC); the difference in redox conditions is attributed to differences in the ice, dust, and gas abundances. In a study of W, Mo, and Ru isotopes in iron meteorites, Kruijer et al. (2017) recognized that both of these reservoirs were coeval and remained spatially separated within the protoplanetary disk for a prolonged period (~ 3.6–4.8 m.y., inferred from timing of CR and CB parent body accretion, respectively) due to the rapid growth of proto-Jupiter (~30 M core at 2.9–3.2 AU within the first 0.6 m.y.; Desch et al., 2018).

Notably, the pyroxene pallasites NWA 1911 and Zinder were found to contain metal with a composition that is chemically identical to that of group IIIF irons, specifically the Moonbi grouplet composed of Moonbi, St. Genevieve County, and Cerro del Inca (Boesenberg et al., 2017; Humayun et al., 2018; Zhang et al., 2022 [see Fig 5]). However, nucleosynthetic Mo and Ru isotope data obtained by Kruijer et al. (2017) and Worsham et al. (2019) for three IIIF irons indicate they formed in the carbonaceous region of the Solar System beyond Jupiter, whereas the negative ε54Cr and δ26Mg values of Zinder indicate that it formed in the non-carbonaceous, inner region of the Solar System (Wimpenny et al., 2019). See the Protoplanetary Disk page for more information about these two regions of the solar nebula.

Interestingly, Rubin (2018, Fig. 1) has shown that group IIIF irons are poorly resolved from the NC-iron groups with respect to Ni and Ir contents. Acquisition of stable isotope data (Mo, Ru, and W) for these pallasites and other IIIF irons is in progress to ascertain their correct protoplanetary reservoir and to further evaluate any genetic connections that may exist. Thus far, Pape et al. (2022 #6478) has reported results for ε182W values of the IIIF members obtained after correcting for CRE-induced W isotope variations. The results establish a Hf–W model age for the IIIF parent body of ~2.2 m.y. after CAIs, which is older than the previous age of 3.0 (±1.2) m.y. after CAIs calculated from less inclusive data. This revised IIIF differentiation age is now the oldest among CC irons (excluding the South Byron Trio) and about the same as the IVA group among NC irons calculated at 2.2 (±0.2) m.y. after CAIs (Spitzer et al., 2021; see diagram below and data chart). Molybdenum isotope data will be available soon.

Hf–W Ages Corrected for Nucleosynthetic Pt Isotope Anomalies
Note: the IIIF age indicated below is prior to the revised age of ~2.2 m.y. after CAIs
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Diagram credit: Spitzer et al. EPSL vol. 576, art. 117211 (2021, open access link)
'Nucleosynthetic Pt isotope anomalies and the Hf-W chronology
of core formation in inner and outer solar system planetesimals'
(https://doi.org/10.1016/j.epsl.2021.117211)

The specimen of St. Genevieve shown above is a 55.4 g etched partial slice exhibiting a fine to medium Thomson (Widmanstätten) structure. The photo below shows the entire mass as photographed by Ward.

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