MILTON

A 2,038 g rock was found by G. Wennihan while clearing the rocks from his soybean field in Fairfax, Missouri. Because of its unusually heavy weight and its rusty appearance, the rock was tossed into the back of his pickup truck to save. The mass was eventually cut in half and the unique appearance of the interior raised speculations of a space origin. However, it was not until friend and geology student, B. Rogers, took the strange rock to the geology department of Northwest Missouri State University that 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; he immediately recognized that it was a pallasite. Samples of the pallasite were sent to the University of New Mexico, Institute of Meteoritics, and to UCLA for a thorough analysis.

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-isotope data indicate that Milton is not genetically related to other pallasites. It lacks taenite cloudy zones and has not experienced shock reheating, which indicates it has the fastest cooling rate among pallasites of >5000°K/m.y. (Yang et al., 2010). Milton has olivine that 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 pallasites. Milton also has O-isotopic ratios which are distinct from all other pallasite groups, but as with the Eagle Station grouplet, it reflects a relationship with the carbonaceous chondrite anhydrous mineral mixing line (slope = 0.94 ±0.01).

Some ungrouped irons have O-isotopic ratios that are similar to Milton, but are significantly different in their 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 (South Byron, ILD 83500, and Babb's Mill) have similar structures to the metal in Milton, including kamacite spindles and associated schreibersite, and that they may constitute a grouplet that originated on a common parent body. All of these irons and the Milton metal experienced a similar oxidation history during core formation, as evidenced by the presence of FeO-rich olivine, chromite, and phosphate, as well as depletions of other easily oxidized elements (McCoy et al., 2008). Siderophile abundances are also similar among them all. In addition, the common presence of volatile siderophile elements in these irons demonstrate that they were not derived from high-temperature condensation processes as was the case for other high-Ni iron groups. It is also indicated that the Milton pallasite is a product of an earlier stage of fractional crystallization compared to that of the main-group pallasites.

In addition to the irons above, several other ungrouped ataxites may be members of this high-Ni iron group, including El Qoseir, Illinois Gulch, Morradal, Nordheim, and Tucson. However, significant differences that exist between their refractory element contents compared to those in the South Byron trio requires further work (Kissin, 2010).

Silicate phases in Milton are enriched in siderophile and highly siderophile elements, which mostly partition into metal phases (Hillebrand et al., 2004). Because of the unusual homogeneity of the metal and silicates in Milton, it proved to be a good tool for J. Hillebrand (2004) in determining the in situ metal/silicate partition coefficients of a pallasite. Milton is a unique representative of its parent asteroid, and it demonstrates that the petrogenesis of pallasites must have occurred in a similar way on different parent bodies. The specimen of Milton shown above is a 40.1 g partial slice sectioned from an 85 g slice, and acquired from the owner of the main mass. The top photo below shows the cut face of a 500 g end section, and the bottom, the natural suface of a 677 g end section.

Photos courtesy of Jay Piatek