At 4:20 P.M. in Weld County, Colorado, four explosions were heard followed by the fall of 28 stones ranging in weight from a few grams to 23.5 kg. A crowd of mourners attending the burial service for John Moore Sr. at the Elwell Cemetery, two miles west of Johnstown, witnessed the fall. A few of the attendees walked across the road to a field and proceeded to dig up a 6.8 kg stone, now on display in the Denver Museum of Natural History. The fragment size distribution of this fall is reversed from the usual, with the larger fragments being found at the beginning of the strewn field ellipse rather than at the end. This can be explained by the breakup of a large fragment late in its flight. The total collected weight of Johnstown is about 40.3 kg.
Johnstown is a rare calcium-poor diogenite consisting of medium green orthopyroxene (hypersthene) crystals broken up in a light green brecciated matrix. It is a monomict breccia derived from a plutonic rock best classified as a cumulate orthopyroxenite with minor eucritic components. Some of these components are transitional to eucrites through the Y-75032-type diogenites and the Binda-type cumulate eucrites. These two types occupy a compositional gap between pigeonite crystallization in eucrites and orthopyroxene crystallization in diogenites. Accessory FeNi-metal and sulfides (troilite and pentlandite) have been identified by Gounelle and Alard (2009). They argue that the high content of highly siderophile elements (HSE) in diogenites suggests that the production of FeNi-metal did not result from silicate reduction or through desulfurization of sulfides, nor was the troilite a result of sulfurization of FeNi-metal. Instead, they believe an HSE-rich magma underwent sequential crystallization to produce an FeNi-metal and, from an immiscible sulfide melt component, troilite.
Johnstown shows evidence for the presence of a low percentage of trapped interstitial melt in the form of plagioclase and olivine crystals, which occupies the space between orthopyroxene crystals (Barrat, 2004). A wide range of siderophile element abundances might indicate a trace metal component within orthopyroxene, while a varied LREE content between orthopyroxene and the bulk rock provides evidence for an LREE-enriched component in Johnstown. Through trace element studies it was determined that the wide range of incompatible elements found among diogenites attests to their crystallization from a diversity of parental magma sources over a relatively short period of time, with the youngest sources experiencing up to ~90% fractional crystallization (Schiller et al., 2010). These magma sources were shown to be distinct from those which crystallized basaltic eucrites. The onset of diogenite formation was dated at 0.71.3 m.y. after CAIs, which predates the crystallization of basaltic eucrites.
The HREE-enriched parental magmas that crystallized some diogenites such as Tatahouine are consistent with derivation from a melt composed of a harzburgitic cumulate at low pressure located below the eucritic crust. A scenario for the formation of Johnstown begins with crystallization of an orthopyroxene-rich cumulate pile within a magma pluton or shallow layered intrusion within the eucritic crust, or alternatively, between a crustal layer and a convecting magma ocean, on its parent body. Remelting within this cumulate layer followed, causing a buoyant condition and subsequent intrusion into the overlying crust as diapirs (Barrat et al., 2008). In a similar case, formation of some HREE-enriched diogenites may have involved remelting of the residue from a source that sustained a high degree of melting (Stolper, 1977). Consequently, the previous theory surmising a sequential crystallization of diogenites and eucrites from a common, evolving parental melt is no longer favored. Based on elemental ratios, it is now believed that eucrites crystallized from parental melts which were not genetically related to those of diogenites. In any case, subsequent brecciation and thermal metamorphism of surface lavas occurred, resulting in the formation of ordinary eucrites and howardites. Variable thermal histories also affected diogenites, some experiencing multiple reheating events, with the result that both equilibrated (elemental homogeneity) and unequilibrated (igneous-zoned) diogenites were formed (Yamaguchi et al., 2010). The Johnstown diogenite was ultimately excavated at depth at high temperatures and ejected from the asteroid.
It is still under debate whether the source for diogenites was the asteroid 4 Vesta, or if the petrographic evidence is more indicative of a different parent body (Irving et al., 2014). Geological mapping of Vesta by the Hubble space telescope does show the sort of layered impact cratering that could serve as the source of these meteorites. Visible, near-infrared, and mid-infrared spectral studies of Vesta and HED meteorite samples have been conducted, which demonstrate that a compositional fit could exist between Vesta and the spectrum obtained for the mineral abundances in laboratory spectra for Johnstown, as well as for howardite and eucrite samples (Donaldson Hanna and Sprague, 2008, 2009). While it may be true that some investigators find the laboratory models consistent with the surface of Vesta, as a heterogeneous mixture composed primarily of howardite material at specific longitudes (155° to 0°) and eucrite material at others (90° to 130°), others do not yet find convincing evidence for Vesta being the source for diogenites.
Based on REE patterns, the diogenites Johnstown, Bilanga, Roda, A-881548, and Dhofar 700 exhibit similar very low Eu/Eu* values and significant light REE depletions, and they may constitute a subgroup (Barrat et al., 2010). The investigators argue that these diogenites with very low Eu/Eu* ratios were contaminated by <10% of a low degree (<5%) eucritic crustal melt phase associated with a very large Eu anomaly. This scenario would be consistent with diogenites forming concurrent with or later than the formation of eucrites.
To see an alternative classification system for the diogenites based on mineralogical and petrographical features, proposed by Beck and McSween (2010) and modified by Wittke et al. (2011), click here. The photo above shows the cut face of a 7.0 g specimen. Below is the reverse side with minor fusion crust visible on the top edge, and the bottom image is an excellent petrographic thin section micrograph of Johnstown, shown courtesy of Peter Marmet.
click on image for a magnified view
Photo courtesy of Peter Marmet