Polymict, crystalline
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Fell October 1960
26° 27' S., 120° 22' E.

This fall, occurring around 1:00 P.M., was witnessed by two station workers who were opening a fence gate on the Millbillillie–Jundee track in Western Australia; however, no specimens were recovered until 1970. Searches by local Aboriginies have produced many well-preserved, oriented specimens with the shiny black fusion crust that is characteristic of these calcium-rich meteorites. Like most eucrites, these stones contain calcium-bearing minerals such as plagioclase and augite, which, when mixed with free metal that has been oxidized to magnetite, and then rapidly quenched from the molten state, produces a black, glossy fusion crust (O.R. Norton, 2002). Millbillillie is among the most thermally equilibrated of the eucrites, a type 6 in the metamorphic sequence of Takeda and Graham (1991). It contains a mixture of granulitic fragmental breccias and impact-melts, with a network of glassy veins and other shock features. It has been classified as a recrystallized polymict breccia containing lithologies of differing Mg# (Mg/Mg+Fe) (Yamaguchi et al., 1994).

In their paper, Not All Eucrites Are Monomict Breccias, Yamaguchiet al. (1997) concluded that Millbillillie, Sioux County, and several other eucrites previously believed to be monomict breccias are instead metamorphosed polymict eucrites. The variability in the clast textures suggests that many eucrites including Millbillillie are actually metamorphosed polymict breccias with a low abundance of exotic components. Additional evidence for this fact in Millbillillie is the presence of lithologies with significantly different Mg#, a likely result of brecciation events, probably at the floor of an impact crater, which was followed by homogenation and recrystallization of the clasts. Further evidence of a heterogeneous composition is the difference in 244Pu–Xe ages in the components—4.566 b.y. for the fine-grained component, and 4.507 b.y. for the coarse-grained component (Quitté et al., LPSC XXXI, #1441; Miura et al., 1998).

Mineralogical, geochemical, and petrological evidence compiled for Millbillillie suggests a formation chronology in which melting occurred very early, ~1.2 (± 1.2) m.y. after the closure of CAIs (Babechuk et al., 2010). According to Al–Mg systematics, core segregation occurred 2.5 (± 1.2) m.y. after CAIs (Schiller et al., 2010). Mantle fractionation occurred ~2 m.y. later and was followed by cratering processes that caused brecciation and mixing of fractionated impact melt with lithic fragments. This was then followed by a period of thermal annealing resulting in significant equilibration of the pyroxenes. A subsequent impact produced the network of glassy veins, which was then followed by another weak impact that produced fine microcracks. It has been established by Ar–Ar dating that a significant impact event occurred ~3.55 b.y. ago, resetting the chronometer to match one of several age clusters common for eucrites.

Crater retention ages that were calculated by Schmedemann et al. (2013) show an older cluster dated at 3.75 (+0.05/–0.21) b.y., which is linked to the formation of the Veneneia basin in the northern hemisphere. In addition, they recognized a younger cluster at 3.58 (+0.07/–0.12) b.y. which corresponds to the formation of the Rheasilvia basin, the major event thought to have spalled the Vestoids, and which also corresponds to the Ar–Ar age. These crater retention age clusters not only correspond to some of the measured HED age clusters, but also to the period of Late Heavy Bombardment that affected the Moon ~4.1–3.8 b.y. ago; however, these younger clusters attest to an extended period of bombardment on Vesta. Furthermore, the crater counting technique revealed a global age of ~4 b.y.

Millbillillie has a Kr–Kr-based cosmic-ray exposure (CRE) age of 23.57 (±1.87) m.y., including it within the largest of five common breakup clusters established at 5–7, 10–14, 17–25, 30–46 and 70–76 m.y. Meteorite examples of each of these CRE age clusters are (in m.y.) 6.93 (±0.33) Bouvante, 12 (±2) Juvinas, 23.57 (±1.87) Millbillillie, 36.37 (±2.08) Stannern, and 65–70 HaH 286. The ~22 m.y. old impact-ejection event was evidently a particularly large one since it comprises about one-third of all HED meteorites and includes representatives of all HED meteorite types (cumulate, brecciated, unbrecciated, and polymict eucrites, diogenites, and howardites) ejected at that time (Wakefield et al., LPSC XXXV, #1020; Bogard, 2009). In addition, the ~39 m.y. old impact-ejection event was statistically significant (Cartwright et al., 2012). Some anomalous eucrites have similar CRE age values, including 26.71 m.y. for both Pasamonte and Bunburra Rockhole, and 12.5 (±0.52) m.y. for Ibitira. In a similar CRE age study also based on Kr-systematics, Strashnov et al. (2013) have determined those ages for a group of eucrite falls (including Millbillillie). Although they also concluded that the majority of these eucrites correspond to only five common impact events, which occurred within the past ~50 m.y., the clusters differed slightly from those outlined above. Their five cluster events are dated at 10.6 (±0.4), 14.4 (±0.6), 21.7 (±0.4), 25.4 (±0.4), and 37.8 (±0.6) m.y. ago.

The formation history of the howardite–eucrite–diogenite (HED) clan began with the early accretion of the parent asteroid, probably 4 Vesta, within ~1 m.y. of the first Solar System condensates. Based on its O-isotopic signature, the precursor material for this large asteroid is calculated to have had a chondritic H/CV-like (or possibly H/CR-like) composition (Rai et al., 2016). Within a short time the body began to melt from the heat of decay of short-lived radionuclides such as 26Al and 60Fe, eventually forming a magma ocean. Metal–silicate melting, differentiation, and fractionation then occurred 4,564.9 (±1.1) m.y. ago, or ~3 m.y. after CAI formation (based on Mn–Cr systematics; Trinquier et al., 2008; Al–Mg systematics; Schiller et al., 2010). Notably, mesosiderite clasts have similar ages within error. An alternative timeline based on the Hf–W system in a model reflecting a low mantle Hf/W ratio suggests that differentiation leading to core metal segregation occurred no later than 1.2 (±1.2) m.y. after the closure of CAIs (Babechuk et al., 2009). Late accretion occurred within 2 m.y. of CAIs, but after core formation had occurred as evidenced by the finding of highly siderophile elements still remaining in diogenite mantle material (Day et al., 2012).

The metallic core of Vesta eventually attained a radius of ~55–75 km. Active convective forces in the magma ocean promoted equilibrium crystallization conditions and initiated mantle fractionation, eventually leading to eucritic melts ~2.1 m.y. after CAIs. An olivine-rich dunite layer ~150 km thick initially crystallized around the metallic core. Recent studies of the unique 1.1 g olivine-rich meteorite QUE 93148 suggest that this might be a sample of the HED mantle layer (Goodrich and Righter, 2000; C. Floss, 2003), just as the dunitic meteorites NWA 2968 (Bunch et al., 2006) and MIL 03443 (Mittlefehldt, 2008) might be. However, due to its lower Co and Ni abundances than what would otherwise be expected for an olivine-rich mantle lithology or magma ocean cumulate, QUE 93148 could have actually originated on a distinct planetary body such as that of the main-group pallasites (Shearer et al., 2008; Shearer et al., 2010).

Basaltic volcanism occurred very soon after differentiation of the parent body—within a short interval commencing as early as ~7 m.y. after the formation of the Solar System, and spanning a period no longer than 17 m.y. (Misawa et al., 2005). Micron-sized zircons, associated with ilmenite, have been studied from various eucrites to obtain an accurate crystallization age. The 207Pb–206Pb ages for these zircons of ~4,554 (±20) m.y. represent the crystallization ages of extrusive eucritic lavas. However, it has been found that this crystallization event is best dated by zircons from the eucrite Igdi since those derived from Millbillillie reflect a slightly later volcanism or disturbance at 4,543 (±15) m.y. (Lee et al., 2009). Coincidentally, this eucrite thermal event occurred during the time period in which the Moon is considered to have formed—in the Giant Impact 30–110 m.y. after the beginning of the solar system (Hopkins and Mojzsis, 2012).

Next in the sequence to crystallize was a cumulate, orthopyroxene-rich, diogenite layer at least 13 km thick. Ultimately, residual liquids which were subjected to fractional crystallization (Holzheid and Palme, 2007) were extruded onto the surface. This period of volcanism produced basalt flows that solidified to form a thin crust ~10–15 km thick (Mayne et al., 2008; Wasson, 2012). This basaltic crustal rock was buried in turn by continual insulating flows of lava, resulting in its reheating and metamorphism and eventual formation of the Main Group–Nuevo Laredo trend eucrites. The late-stage ascent of a portion of this Main Group magma was contaminated with a crustal partial melt to become the incompatible-element-rich Stannern trend eucrites.

Some of the residual liquid, or more likely a separate REE-enriched liquid, was trapped at depths of up to ~10 km and underwent late fractionation and re-equilibration processes to produce the cumulate eucrites, dated at ~60–100 m.y. after CAI formation based on Hf–W, Sm–Nd, and Lu–Hf systematics (Touboul et al., 2008). Thereafter, surface eucritic material was impact brecciated to form a regolith, which was combined with chondritic and diogenitic clasts and lithified to form the polymict howardite members of the HED clan (additional classification information for the HED clan can be found on the Kapoeta page).

Vesta has an average diameter of ~506 km, with an outer basaltic crust thin enough (~15–25 km) to have been completely excavated down to diogenitic material by an impactor ~37–44 km in diameter traveling 5.5 km/sec (Ivanov and Kamyshenkov, 2013). The resulting impact basin near the south pole, named Rheasilvia (after the mother of Romulus and Remus, the mythical mother of the Vestals), is ~460 km wide and ~20 km deep. Some investigators believe this excavation event occurred ~4.48 b.y. ago, while others provide evidence for a significantly more recent event consistent with the fact that the Vestoids remain in close proximity.

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3-D image of Vesta (Anaglyph Red/Blue); credit: NASA's Dawn Spacecraft
See the Dawn Low-Altitude Mapping Orbit-derived Global Geologic Map of Vesta (pdf format)—Williams et al., #1126 (2015)

Many shallower impacts into eucrite layers also occurred between 4.1 and 3.5 b.y. ago which reset many radiometric chronometers (Bogard and Garrison, 2003). Some of the eucrite, diogenite, and howardite material was spalled into space by these impacts and was entrained deep into the 3:1 and v6 resonances. Searches have identified over 1,000 small (< 10 km in diameter) Vesta-like asteroids (Vestoids) composed of both eucritic and diogenitic fragments, which are thought to have been created by a late impact event 3.5 b.y. ago (Scott et al., 2009). Some of these Vestoids bridge the gap between Vesta and the 3:1 resonance gap. From the various dynamical escape hatches, Vestoids like the near-Earth asteroids 1983 RD, 1980 PA, and 1985 DO2, were perturbed into Earth-crossing orbits on time scales of tens to hundreds of m.y. Exposure age distributions of a statistical sampling of HED meteorites show that at least two major impact events occurred around 22 m.y. and 39 m.y. ago on one or more of these Vestoids.

The basaltic eucrite Bunburra Rockhole, which was tracked by the Desert Fireball Network as it fell in Western Australia in 2007, was recovered during an organized field search in 2008 (Bland et al., 2009). Its precisely calculated orbit is consistent with an Aten-type asteroid with a semi-major axis <1 AU. Bunburra Rockhole has an O-isotopic composition distinct from that of Vesta-derived eucrites (Towner et al., 2010) and is thought to represent a unique basaltic parent asteroid. Interestingly, all other petrographic characteristics studied so far are similar to the known basaltic eucrites (Spivak-Birndorf et al., 2010). Bunburra Rockhole was likely delivered from the inner main belt through the v6 resonance, demonstrating how material ejected from Vesta and the related Vestoids can be delivered to Earth in a like manner through an evolving orbit. The ungrouped eucrite-like achondrite A-881394 has an O-isotopic composition very similar to that of Bunburra Rockhole, suggesting that the two meteorites may be related.

The general composition of eucrites consists of roughly equal amounts of anorthite, a plagioclase feldspar, and the clinopyroxene called pigeonite. The source magma was probably derived from the mafic mineral peridotite, a mixture of olivine, pigeonite, and plagioclase, and is the mineral forming the bulk of the Earth's upper mantle. The age of eucrite crystallization is ~4.5 b.y., no later than ~16.2 m.y. after the formation of Allende CAIs. Cooling and metamorphism within an ejecta blanket lasted ~600 m.y. longer. The photo of Millbillillie above shows an 8.4 g oriented individual with radial flow lines, along with a 4.0 g partial slice showing the interior, coarse-grained texture consisting of anorthite and pigeonite.

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