Flax Creek Meteorite

 Kentucky Geological Survey scientists used an assortment of KGS laboratory's analytical instruments to confirm the identity of two meteorites recently discovered in Kentucky. The meteorites are the first to be found in the Commonwealth since 1990. Quade Mott discovered the newly named "Handys Bend" meteorite on a farm near Wilmore, Kentucky in July of 2021. The "Flax Creek" meteorite was unearthed by Johnathan Baldwin nearly 8 months later near Crab Orchard, Kentucky. The confirmation of these meteorites brings the total number of Kentucky meteorites to 29. 

Flax Creek Genetic Classification: 

 IAB was the only group to plot coherently with Flax Creek across the entirety of chemical data, with the lone exception of the Au v. Cr plot (Figure 36). That graph shows the low-Au Flax Creek sample plotting between the low-Cr IAB subgroup and the high-Cr IIICD subgroup. This relationship may result from the significant chemical heterogeneity

among IAB irons and the significant range of IAB complex Cr concentrations which spans 3 orders of magnitude. However, Flax Creek remains an outlier in this plot. Flax Creek plotted concordantly or marginally with groups IIE or IIIAB on 17-of-28 plots each. However, Flax Creek plotted discordantly with IIE meteorites on four plots: Ni v. Co, Au v. Cr, Ni v. Au, Co v. Ir (Figure 35, Figure 36, Figure 40, Figure 41). Likewise, Flax Creek plotted discordantly with IIIAB group meteorites on four plots: Au v. Cr, Ni v.

Cr, Ni v. As, and Ni v. Au (Figure 36, Figure 38, Figure 40). These chemical

inconsistencies prohibit a genetic relationship between Flax Creek and IIE or IIIAB groups. The addition of Ga, Ge, and Re data would benefit genetic classification, as these elements

exhibit considerable intragroup partitioning. However, the consistent grouping of Flax Creek with IAB meteorites on 27-of-28 plots supports the interpretations that Flax Creek

is a member of the IAB complex.

The IAB complex represents the second largest iron meteorite group and the largest “non-magmatic” group (Scott, 2020). Group IAB was originally segmented into four distinct genetic groups, IA, IB, IIIC, and IIID. However, subsequent chemical analyses of

additional meteorites eventually revealed unbroken compositional trends connecting these once-segregated groups (Wasson and Kallemeyn, 2001). Today, many researchers recognize a single, albeit multifaceted, genetic classification now known as the IAB complex, or simply IAB. Yet, distinctions between IAB and IIICD are still widely employed, and a consensus designation has not been established (Goldstein et al., 2021).

Multiple IAB complex subgroups have been identified based upon compositional clustering within the complex. The subgroups can be distinguished using a number of trace

element relationships but may be best revealed using Ni and Au concentrations. These subclasses consist of a definitive main group (MG), two high-Au subgroups (sHH, sHL),

three low-Au subgroups (sLH, sLM, sLL), and an ungrouped population (un) that does not fit into any of the currently established subgroups (Wasson and Kallemeyn, 2001). Flax

Creek falls within the ungrouped category, at the margins of the main group (Figure 44). Chemical and isotopic compositions of silicate inclusions suggest a shared origin and

genetic relationship with primitive achondrite group winonaites (Benedix et al., 2000). IAB complex and winonaite meteorites now comprise the clan WIN-IAB-IIICD 

Isotopic data indicate that the IAB complex originated in the NC reservoir

(Goldstein et al., 2021). As a NC reservoir meteorite, Flax Creek originated in a larger body within the orbit of Jupiter, but its formation history remains somewhat enigmatic.

Narrow ranges and chemical trends of some elements such as Ir suggest that IAB meteorites did not form primarily from fractional crystallization of a magmatic body (Worsham et al., 2017). This assertion is supported by the presence of silicate inclusions

(Kirby et al., 2022; Slaby et al., 2017), like those exhibited in Flax Creek. The structure of taenite crystals and the presence of silicates and noble gasses in many IAB meteorites suggest rapid heating and cooling consistent with substantial impact events (Wasson and Kallemeyn, 2001). Several models propose the development of isolated melt pools that sank and embedded within the regolith of the impacted parent body (Benedix, 2000).

Some researchers propose IAB formation in a rapidly cooled impact-induced melt within a porous, and dry planetesimal. (Wasson and Kallemeyn, 2001). Benedix et al(2000) proposed a model that combines partial melting and incomplete differentiation of a chondritic parent body to create multiple isolated magmatic iron pools of various depths. A subsequent massive impact event caused fragmentation, mixing, and reassembly of the parent body. Based on trends among select isotopic compositions and HSE abundancies

across the complex, Worsham et al (2017) suggests that the IAB subgroups formed on a minimum of three separate parent bodies in fairly close proximity. Chemical and structural

differences across subgroups also imply that subgroups formed via different means, from fractional crystallization in magmatic cores to impact-generated melts. Consequently, Worsham (2017) argues that the IAB complex should no longer be considered a single

group as its subgroups are not genetically related. Historical estimates of cooling rates for the IAB complex are highly variable, ranging from 1-10º C/My up to over 1,000º C/My (Goldstein et al., 2021). These discrepancies likely reflect a large range of cooling conditions between IAB subgroups

(Worsham et al., 2017), and may contribute to the irregular nature of the Flax Creek Widmanstätten structure. Additionally, temperatures did not maintain conditions necessary to melt plagioclase or facilitate the escape of noble gases. In some cases, cooling

occurred too quickly to allow growth of large kamacite crystals (Wasson and Kallemeyn, 2001). Flax Creek cooled slowly enough to develop medium-to-coarse grained kamacite

platelets, but the meteorite also contains silicate inclusions with low-melting points. These

apparent contradictions are common among IAB complex irons and contribute to the ambiguity regarding their formation in the early solar system. IAB Ni concentrations exhibit a sizable range of concentrations, from less than 6%

(Benedix, 2000) to over 30% (Choi, Ouyang, and Wasson, 1994), and Flax Creek falls on the low-Ni end of this group (Table 3). This large range of composition reflects the

compound origin of these meteorites. The range of Widmanstätten patterns for IAB complex meteorites records similar variability. Structural classifications across this group

span the entirety of taxonomies, from hexahedrites through ataxites (Benedix, 2000). Main72

group IAB members exhibit a unique pattern like that of Flax Creek (Figure 22), with knobby kamacite lamellae and curved kamacite borders (Goldstein et al,. 2021) structurally

controlled by schreibersite and cohenite precipitates.

Phosphides and carbides found in IAB complex members are consistent with those observed in Flax Creek. Schreibersite is practically universal among IAB members, often

in the form of macroprecipitates (Figure 27, Figure 28, Figure 29). Rhabdites are also widely distributed (Figure 23, Figure 25) (Scott and Wasson, 1975). Cohenite and haxonite

are common to ubiquitous, with the former often developing macroprecipitates (Figure 26),

like those exhibited in Flax Creek (Scott and Wasson, 1975). Large cohenite and schreibersite grains often act as barriers that impede kamacite plate growth (Goldstein et al., 2021). The majority of IAB meteorites contain elliptical troilite nodules with varying

graphite and silicate contributions. These nodules are often encased in cohenite and schreibersite macroprecipitates (Wasson and Kallemeyn, 2002). While troilite nodules weren’t encountered in the Flax Creek specimen, the mineral which primarily encased the silicates is unknown due to replacement by secondary weathering minerals. Silicate inclusions are rare in most iron meteorite groups but nearly ubiquitous

among IAB complex irons (Wasson and Kallemeyn, 2001). Olivine, orthopyroxene and

plagioclase feldspar are the most common silicate minerals found in IAB meteorites (Scott

and Wasson, 1975). Most silicates inclusions are of non-fractionated, chondritic origin, but

a subset of IAB irons contain igneous inclusions likely produced by silicate partial melting and fractionation (Ruzicka, 2014). Among this subset, some IAB meteorites exhibit felsic

silicates commonly encountered in terrestrial rhyolitic-to-andesitic rocks, such as silica and

alkali feldspathic minerals (Kirby et al., 2022; Ruzicka, 2014; Slaby et al., 2017).

IAB complex silicate minerals, including alkali-feldspars and silica polymorphs,

vary among individual irons and range from clusters of several small, sub-100 µm

inclusions encased in the metallic matrix to large, cm-sized inclusions (Ruzicka, 2014;

Wasserburg, 1968; Wasson and Kallemeyn, 2001). Alkali-feldspars inclusions of some

IAB meteorites are limited to inclusions in troilite and graphite nodules (Slaby et al., 2017;

Wasson and Kallemeyn, 2001). The Flax Creek silicate inclusions are encased in section

of surface corrosion that borders two coarse-grained mineral assemblages (Figure 30).

These silicate grains may represent the remnants of a larger aggregate inclusion that served

as nucleus for the assemblage. If other minerals phases were present in this assemblage,

weathering and oxidation has severely altered the metallic and less-stable phases. However,

secondary minerals resulting from terrestrial weathering are an implausible source of silica

inclusions (Rubin, 1997).

Silica inclusions account for the large majority of Flax Creek silicate minerals

(Figure 30). Tridymite and cristobalite are the only silica polymorphs commonly found

within iron meteorites, and neither have been encountered outside of IAB, IIE, IVA, and

ungrouped irons (Rubin, 1997; Scott et al., 1996; Ruzicka, 2014). A subset of IIE

meteorites contain silica-rich glass and fractionated, alkali-silica inclusions (Kirby et al.,

2022; Ruzicka, 2014). Group IIE meteorites likely formed from impact induced metal and

silicate immiscible in a chondritic parent body (Kirby et al., 2022; Wasson, 2061). Alkali-

feldspar grains in the IIE Miles meteorite have been dated to less than 3.5 Ga, further

complicating the complex formation histories of these meteorites (Kirby et al., 2022).

As a result of complex and multifaceted formation processes, IIE group irons are

structurally and chemically heterogeneous and exhibit broad trends across the group

(Wasson, 2016). Flax Creek shares general structural, chemical, and mineralogical trends

with a few individual IIE irons (Wasson, 2016), but Flax Creek’s chemical trends are incompatible with IIE chemical group trends as a whole. IVA meteorites likewise formed

via complex processes that involved fractional crystallization of a molten core and trapped impact-induced melts (Scott et al., 1996; Wasson et al., 2006). However, Flax Creek is

structurally, chemically, and mineralogically distinct from IVA group meteorites (Ruben

et al., 2022). The presence of silica, plagioclase, and alkali-feldspar inclusions within Flax Creek

supports the IAB complex genetic interpretation for the meteorite. Silicate melt inclusions

are limited to few genetic groups, and the IAB complex is the only group that contains these mineral phases and shares structural and chemical characteristics with Flax Creek.

In addition to silicate inclusions and shared chemical trends resolvable on binary plots, Flax Creek shares common mineralogical similarities with IAB complex meteorites.

Schreibersite, cohenite, and rhabdites are nearly ubiquitous among the IAB group, with the former two minerals often developing macroprecipitates. These precipitates often act as

barriers impeding kamacite growth, as is observed in the Flax Creek specimen. Main group IAB members exhibit a course pattern with knobby kamacite lamellae and curved kamacite

borders. These structural characteristics are indicative of the unique origin of IAB main

group meteorites and are prominently exhibited in Flax Creek, which falls on the margins of the IAB main group. The IAB complex includes a diverse, but genetically resolvable, group of meteorites, and the chemical, mineralogical, and structural attributes of Flax Creek are consistent with IAB complex meteorites. The bulk of observations and data confirm that Flax Creek is most consistent with the IAB complex.