SOUTH BYRON


Iron, ungrouped
Ataxite
(eponymous member of the South Byron Trio)

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Found 1915
43° 2' N., 78° 4' W.

A single terrestrially-weathered (surface loss of a few mm) iron meteorite weighing ~6 kg was found west of South Byron, Genesee County, New York. The meteorite was analyzed at the Chicago Field Museum (Nichols and Farrington) and classified as a nickel-rich (17.8%) ataxite.

As described in the Handbook of Iron Meteorites Volume 3 (Vagn F. Buchwald, 1975), shock-melted mm-scale troilite–daubreelite inclusions, sometimes associated with primary chromite grains, are scattered throughout an otherwise homogeneous FeNi-metal matrix. Under high magnification, µm-scale kamacite platelets and associated schreibersite crystals can be resolved forming a micro-Thomson (Widmanstätten) structure. The presence of daubréelite in South Byron and the similar Babb's Mill (Troost's Iron) is attributed by Corrigan et al. (2018) to a late-stage reduction process, possibly related to the exhaustion of water utilized in the oxidation of metallic iron to form FeO.

South Byron is grouped with two other irons, Babb's Mill (Troost's) and Inland Forts (ILD) 83500, and the grouplet of three is termed the South Byron Trio (SBT). The existence of a possible genetic link between the SBT and the Milton pallasite has been investigated by numerous investigators (e.g., Jones et al., 2003 [#1683]; Reynolds et al., 2006 [#5233]; McCoy et al., 2017 [#2241], 2019 [GCA]; Corrigan et al., 2017 [#2556]; Corrigan et al., 2018 [#2527]). All of these meteorites have significant characteristics in common, including similar metal compositions (siderophile element abundance patterns reflecting oxidizing conditions), similar metal structures (kamacite spindles and associated schreibersite), O-isotope values that plot along a consistent trend line, similar Δ17O-isotopic compositions of chromites, and overlapping Mo-isotopic values. The differences observed in siderophile element and platinum group element abundances could be attributed to fractional crystallization processes, or as Hilton et al. (2019) have inferred, to formation on separate parent bodies that accreted from similar precursor materials in a similar isotopic region (CC) of the protoplanetary disk (see respective diagrams below).

Siderophile Element Concentrations for SBT and Milton
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click on image for a magnified view (V, Cr, and Mn not included)

Diagram credit: McCoy et al., 48th LPSC, #2241 (2017)

Oxygen Isotopes for SBT and Milton
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Diagram credit: McCoy et al., 48th LPSC, #2241 (2017)

Oxygen Isotopic Composition of Chromites for SBT and Milton
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Diagram credit: McCoy et al., GCA, vol. 259 (2019)
'The Milton pallasite and South Byron Trio Irons: Evidence for oxidation and core crystallization'
(https://doi.org/10.1016/j.gca.2019.06.005)

Molybdenum Isotopes for SBT and Milton
(µ notation denotes deviation from terrestrial standards in parts per million)
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Diagram credit: Hilton et al., 49th LPSC, #1186 (2018)

HSE Abundances for SBT and Milton
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Diagram credit: Hilton et al., GCA, vol. 251, pp. 217–228 (2019)
'Genetics, crystallization sequence, and age of the South Byron Trio iron meteorites:
New insights to carbonaceous chondrite (CC) type parent bodies'
(https://doi.org/10.1016/j.gca.2019.02.035)

Investigators have explored the possibility of a genetic relationship between IVB irons and other meteorite groups, including the South Byron Trio. Based on O-isotopic analyses utilizing chromite grains from the IVB irons Warburton Range and Hoba, Corrigan et al. (2017) found that IVB irons plot close to the Milton–South Byron trio grouping (MSB in diagram below). The O-isotopic compositions of the IVB irons and the MSB grouping also fall within the range of the oxidized CV–CK chondrites. Moreover, Corrigan and McCoy (2018) found that both IVB irons and the MSB grouping show evidence for early oxidation (e.g., both have a similar high Ni content of ~15.5–18 wt% and ~15–18 wt%, respectively), as well as evidence for late reduction (e.g., both contain reduced mineral phases such as troilite, daubréelite, and schreibersite).

standby for o-isotopic relationship between groups diagram
Diagram credit: Corrigan et al., 48h LPSC, #2556 (2017)
Hoba: Δ17O = –3.4 [±0.2] ‰
Warburton Range: Δ17O = –3.4 [±0.4] ‰
Milton–South Byron trio: Δ17O = ~ –3.6 [±0.6] ‰

Further comparative studies of Milton, the South Byron Trio, and IVB irons were presented by McCoy et al., 2019). Despite the fact that these meteorites have similarities in their Δ17O values, metallography, and HSE enrichments, the significantly higher depletion of volatile siderophiles among the IVB irons was attributed to parent body devolatilization subsequent to removal of the silicate shell. For such a degassing process to occur unimpeded, core crystallization on the IVB parent body would have proceeded from the center outwards. This is consistent with observations in which the earliest crystallizing, low-Ni irons have the slowest cooling rates due to core–outward crystallization. In contrast, it was determined by McCoy et al. (2019), and supported by Zhang et al. (2021), that the SBT parent body was stripped of its silicate shell prior to crystallization. Rapid cooling and solidification started at the core–mantle boundary and proceeded inward in the following sequence: Milton ⇒ South Byron (at ~1% solidification) ⇒ Babb's Mill (at ~2–8% solidification) ⇒ ILD 83500 (at ~35–42% solidification). This crystallization sequence would have prevented a similar volatile degassing process as hypothesized for the IVB parent body (McCoy et al., 2019). Zhang et al. (2022) used fractional crystallization modeling and determined that the initial bulk composition of the SBT metallic core contained 8 (±2) wt% S and 1.5 (±0.3) wt% P, and that the group members represent ≤37% crystallization of the parental core liquid.

Given a scenario in which the SBT and Milton derive from a common parent body, it was recognized by McCoy et al. (2017, 2019) and others that the inferred sequence of crystallization would be from the core-mantle boundary inwards. This process would be consistent with the formation of an early core dynamo, evidence for which has been demonstrated through paleomagnetic studies of CV meteorites (e.g., Elkins-Tanton and Weiss, 2009; Weiss et al., 2010; Elkins-Tanton et al., 2011; Carporzen et al., 2010, 2011; Gattacceca et al., 2013, 2016). However, see the Allende page for an alternate paleomagnetism hypothesis proposed by Fu et al. (2021).

The cosmic ray exposure (CRE) age for South Byron based on K systematics was reported to be 210 (±70) m.y. (Voshage, 1967). Because the two other SBT irons, ILD 83500 and Babb's Mill (Troost's Iron), have small recovered masses, Spitzer et al. (2021) reasoned they should also have low CRE-induced neutron affects. Analyses of these two irons revealed that they actually have small negative ε192Pt, ε194Pt, and ε196Pt values consistent with a lack of CRE-induced neutron-capture reactions. Upon further investigation, these Pt isotope anomalies were resolved as nucleosynthetic in origin, the first such identification in a meteorite. The presence of these nucleosynthetic Pt isotope anomalies in iron meteorites means that previous CRE-corrected 182W isotope data gave metal–silicate segregation ages that were ~0.8 m.y. too old. New calculations were made based on the revised pre-exposure ε196Pt value and the revised pre-exposure 182W values. Based on the new Hf–W ages, they determined that the carbonaceous (CC) irons differentiated ~1–2 m.y. later than non-carbonaceous (NC) irons (~3–4 m.y. and ~1–2 m.y. after CAIs, respectively). However, given the present level of precision in the model data, the accretion times for both CC and NC irons are the same at ~1 m.y. after CAIs. With respect to this same issue, Kaminski et al. (2020) concluded that the older ages of NC magmatic irons compared to CC magmatic irons is not related to any difference in accretion timing, but instead reflects a delay in CC irons reaching their FeS melting point due to inherently lower sulfur abundances. Notably, the SBT irons have a core formation age that is ~1 m.y. older than the other CC irons and similar to that of NC irons, which Spitzer et al. (2021) ascribe to an earlier parent body accretion and/or a lower abundance of accreted water ice which prolonged the heating stage leading to differentiation (see diagram below).

Hf–W Ages Corrected for Nucleosynthetic Pt Isotope Anomalies
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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)

It is proposed by Zhang et al. (2022) that the compositional trends of the parent bodies of the early-formed CC irons, and those of the comparatively later-formed CC chondrites, were in large part established by an aerodynamic size-sorting process associated with a pressure bump (or gravitational instability such as the streaming instability; Simon et al., 2021 and references therein) sustained by a rapidly growing Jupiter. The coarser particles, including CAIs containing refractory metal nuggets with high HSE abundances, were concentrated in the high-pressure region nearest to Jupiter, while the finer particles, including matrix material containing volatiles such as S, occupied the radial space progressively outward from Jupiter (see diagram below). For the CC irons there is an anticorrelation between the HSE abundances and the S concentrations. The composition of the CC planetesimals at any given heliocentric distance corresponds to the S/HSE ratio of the precursor size-sorted particles, and informs of their accretion location. As shown by Zhang et al. (2022), a similar systematic ordering by component size is observed for the CC chondrites, which are thought to have orbits located outward from Jupiter in the sequence CO ⇒ CV ⇒ CM ⇒ CI; similarly, they show a corresponding increase in their S/HSE ratios with distance. Based on the S/HSE ratios of the earliest CC planetesimals, now represented only by their iron cores (or fragments of local metal ponds; Kaminski et al., 2020), the IVB and IID parent bodies would have contained ~20 vol% CAIs.

Composition and Spatial Arrangement of CC-iron Precursors
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Diagram credit: Zhang et al., Science Advances, vol. 8, #37, (2022, open access link)
'Compositions of carbonaceous-type asteroidal cores in the early solar system'
(https://doi.org/10.1126/sciadv.abo5781)

It is notable that the ungrouped iron Willow Grove shares Mo and 183W isotopic compositions with the South Byron Trio and the Milton pallasite (Corrigan et al., 2022). Further details about the South Byron Trio and its possible relationship to the Milton pallasite can be found on the Milton page. The specimen of South Byron shown above is a 4.5 g polished part slice originally acquired by a fellow collector from the Impactika Meteorite Collection of Anne Black.