MILTON


Pallasite, ungrouped
(possibly CV-clan-related)
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Found October 2000
40° 17' 15" N., 95° 22' 36" W.

While clearing rocks from his soybean field in Fairfax, Missouri, G. Wennihan found an unusually heavy one that had a rusty appearance. He tossed this 2,038 g rock into the back of his pickup truck to save. The large mass was eventually cut in half, and the unique appearance of the interior raised speculations that it originated in space. Eventually a friend of his who was a geology student, B. Rogers, took the strange rock to the geology department of Northwest Missouri State University where it was cleaned and examined. Although assistant geology professor Richard Felton and several faculty members examined the rock, it was Dr. Renee Rohs who recognized its resemblance to an Imilac specimen that she had seen years earlier while attending a class taught by Dr. Van Schmus (Horejsi and Cilz, 2002). Reasonably, the rock was taken to Dr. Van Schmus at the University of Kansas for his qualified opinion, and he immediately recognized that it was a pallasite. Samples of the pallasite were sent to the Institute of Meteoritics at University of New Mexico and to UCLA for thorough analyses.

Milton has a high abundance of small, angular olivines (73 vol%, Fo84.1) within an FeNi-metal matrix (Jones et al., 2003). The metal composition is relatively homogenous with respect to siderophile and highly siderophile elements. Chemical, mineral, and O-isotopic data indicate that Milton is not genetically related to other pallasites. The metal in Milton lacks taenite cloudy zones and shows no evidence of shock reheating, which attests to the fastest cooling rate among pallasites at >5000K/m.y. (Yang et al., 2010). The olivine in Milton is zoned in Ca and Cr, and has a higher molar Fe/Mn ratio than that of other pallasites. Likewise, the composition of the FeNi-metal is different from that of the main-group and Eagle Station pallasite groups. In addition, Milton has O-isotopic ratios that are distinct from all other pallasite groups, and as with the Eagle Station group, Milton demonstrates a relationship with the carbonaceous chondrite anhydrous mineral mixing line (slope = 0.94 ±0.01). Notably, the O-isotopic ratios for both Milton and the Eagle Station group pallasites plot proximate to an extension of the trend line for the CV–CK chondrites.

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Diagram credit: Gregory et al., 47th LPSC, #2393 (2016)

Some ungrouped irons have similar O-isotopic ratios to Milton but have significantly different iron chemistry, which excludes a genetic relationship. However, it has been argued by Reynolds et al. (2006) that the high-Ni irons which comprise the South Byron trio (Babb's Mill [Troost's], South Byron, and Inland Forts [ILD] 83500) have similar structures to the metal in Milton, including kamacite spindles and associated schreibersite, and therefore they might constitute a grouplet that originated on a common parent body. All of these irons and the metal in Milton experienced a similar oxidation history during core formation, as evidenced by the presence of FeO-rich olivine, chromite, and phosphate, as well as the depletions in other easily oxidized elements (McCoy et al., 2008). Siderophile element abundances for these four meteorites were shown by McCoy et al. (2017) to have very similar values. Moreover, isotopic compositions (Mo, Ru, and W) and HSE abundances of the IVB irons and the Milton–South Byron trio grouping fall within the range of the oxidized CV–CK chondrites (Hilton et al., 2018).

Carbonaceous vs. Non-carbonaceous Irons
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Diagram credit: Hilton et al., 49th LPSC, #1186 (2018)

McCoy et al. (2017) also recognized that the presence of volatile siderophile elements in these meteorites indicates they were not derived from a high-temperature condensation process contrary to other high-Ni iron groups such as IVA, but instead oxidation (nebular or parent body) and fractional crystallization were the dominant formation processes. The Milton pallasite is a product of an early stage of fractional crystallization compared to the main-group pallasites, as well as with regards to its fractional crystallization sequence among the South Byron trio irons. Based on HSE abundance patterns, Hilton et al. (2018) concluded that Babb's Mill (Troost's) was first in the sequence (representing the first 1% of crystallized melt) followed soon thereafter by South Byron (2%), with ILD 83500 having the highest content of incompatible P being last in the sequence (42%). If Milton is part of a core–mantle boundary then crystallization apparently proceeded inwards similar to the crystallization process some envision for the IVA and IIIAB irons following mantle removal on their respective parent bodies. In their analyses of O-isotopic composions in chromite for these four meteorites, McCoy et al. (2017) demonstrated that they plot along a similar trend line on an oxygen three-isotope diagram (see below). Together with previous petrographic and geochemical data, this new O-isotopic data provides strong evidence supporting a common source parent body.

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

In addition to the irons mentioned above, several other ungrouped ataxites could be members of this high-Ni iron group, including El Qoseir, Illinois Gulch, Morradal, Nordheim, and Tucson. However, because of the significant differences that exist in their content of refractory elements compared to that in the South Byron trio, further work is needed in order to establish a definitive connection (Kissin, 2010). Investigators have also 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) found that IVB irons share close similarities to the Milton–South Byron trio grouping (see diagram below). The O-isotopic compositions of the IVB irons and the South Byron trio–Milton grouping also fall within the range of the oxidized CV–CK chondrites. Moreover, Corrigan and McCoy (2018) found that both IVB irons and the Milton–South Byron trio 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).

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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] ‰

Silicate phases in Milton are enriched in the siderophile and highly siderophile elements which typically partition into metal phases (Hillebrand et al., 2004). Because of the unusual homogeneity of its metal and silicates, Milton has served as a good tool for J. Hillebrand (2004) to determine the in situ metal/silicate partition coefficients of a pallasite. Milton is a unique representative of its parent asteroid, and demonstrates that the petrogenesis of pallasites must have occurred in a similar way on multiple parent bodies. Interestingly, the ungrouped metachondrite NWA 10503 has been conjectured to have a possible affinity to the Milton pallasite (Irving et al., 2016). Not only does this unique meteorite share with Milton an association with carbonaceous chondrites as attested by their elevated silicate FeO/MnO ratios, but it also falls along an extension of the trend line established by Milton on an oxygen three-isotope diagram (see below).

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Diagram credit: Irving et al., 79th MetSoc, #6461 (2016)

In an effort to better resolve potential genetic relationship that might exist between Milton and the CV chondrites, a Cr-isotopic analysis of olivine from the Milton pallasite was conducted by Sanborn et al. (2018). It is demonstrated on a coupled Δ17O vs. ε54Cr diagram (shown below) that Milton plots among the CV clan and plausibly shares a genetic relationship, but also that Eagle Station plots closer to the CK (or CO) chondrite group. It could be inferred that both the CV and CK planetesimals experienced a similar petrogenetic history in a similar isotopic reservoir of the nascent solar system.

Chromium vs. Oxygen Isotope Plot
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click on diagram for a magnified view

Diagram credit: Sanborn et al., 49th LPSC, #1780 (2018)

The specimen of Milton shown above is a 40.1 g partial slice sectioned from an 85 g slice that was acquired from the owner of the main mass, J. Piatek. The top two photos below show the cut face of a 500 g end section and the natural suface of a 677 g end section of Milton. The bottom photo shows a 2 cm-wide magnified image of an interior slice of Milton, courtesy of Dr. Laurence Garvey.

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Photos courtesy of Dr. Jay Piatek

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Photo courtesy of Dr. Laurence Garvey—Arizona State University