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Smara

Eucrite
Polymict
standby for smara photo
Found April 2000
26° 41′ N., 11° 44′ W.

A single 12.87 kg stone was recovered from the Western Sahara. Smara is a fragmental breccia containing a variety of clast types including basalts, gabbros, microgabbros, and impact-melts, the latter measuring up to 1 cm in diameter. Continued research on this new find should reveal many more details about its formation history.

The probable source asteroid, 4 Vesta, has a diameter of ~525 km. It has an outer basaltic crust thin enough (~10–25 km) to have been completely excavated down to diogenitic material by the huge impact which left the 460-km-wide, ~13-km-deep crater near the south pole. Exposure age distributions of a statistical sampling of HED meteorites show that at least two major impact events occurred during its history—one at 22 m.y. ago and another at 39 m.y. ago. Many shallower impacts into eucritic strata also occurred. Some of the eucrite, diogenite, and howardite material was spalled into space by these impacts and entrained into the 3:1 and ν6 resonances. A recent search has identified twenty small Vesta-like asteroids, eight of which bridge the gap between Vesta and the 3:1 resonance, which are composed of both eucritic and diogenitic fragments. Consequently, HED fragments were perturbed from these dynamical escape hatches into Earth-crossing orbits on time scales of tens-to-hundreds of m.y., where they were eventually swept up by Earth’s gravity to land in our collections.

The general composition of eucrites consists of roughly equal amounts of anorthite, a plagioclase feldspar, and the clinopyroxene pigeonite. The parental magma was probably derived from the mafic mineral peridotite, a mixture of olivine, pigeonite, and plagioclase, which is the same mineral that forms the bulk of the Earth’s upper mantle. Eucrite differentiation and crystallization occurred very early in Solar System history, ~4.565 b.y. ago, while cooling and metamorphism within individual ejecta blankets lasted an additional ~600 m.y. The photo above shows a 1.9 g partial slice of Smara exhibiting its brecciated texture.


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Pasamonte

Achondrite, ungrouped
eucrite-like basaltic
(previously Eucrite-polymict)

standby for pasamonte photo
Fell March 24, 1933
36° 13′ N., 103° 24′ W. At 5:00 on a March morning, ~100 stones totaling 3–4 kg were heard and seen to fall in New Mexico, after putting on a display for observers in New Mexico, Colorado, Kansas, Oklahoma, and as far away as Texas and Wyoming. The fireball left a thick, twisting dust trail, perhaps a mile wide and hundreds of miles long, comprising perhaps thousands of tons of material. Grabbing his Kodak Brownie camera, a rare photo of the actual fireball in flight was taken by the quick-acting Charles M. Brown as it spiralled towards Earth (see below), and other images of the remnant twisted dust cloud were captured. Information was recorded in a note by Harvey Nininger describing the scene as photographed by Charles Brown: Great meteor of Mar. 24 1933. Photo by Chas. M. Brown and copyrighted by him. Nininger survey demonstrated that meteor was visible for 15 to 22 seconds. Cloud remained visible 3 hrs. or more. Diameter of luminous sphere was about 6 miles. Diameter of spiral train was about 1 mile. Meteorites from this fall were strewn along a path of 28 miles having a width of about 2 to 3 miles wide. The fall was in an E.N.E. to W.S.W. direction beginning about 25 mi. W.S.W. from Clayton New Mexico. Meteorites were preserved in America Meteorite Museum, U.S. 66, west of Winslow Arizona. The small stones were collected by ranchers along a distance of 28 miles near the Pasamonte Ranch, and these were subsequently identified as meteorites by Harvey Nininger, who had independently located the strewnfield after spending many months conducting eyewitness interviews.

Pasamonte was determined to be an unequilibrated (Type 2 in the metamorphic sequence of Takeda and Graham, 1991) basaltic meteorite that has retained some primary Fe–Mg zoning in pigeonite grains. While it was previously classified as an unequilibrated monomict eucrite representing the type specimen for ‘Pasamonte-type’ lithologies in polymict eucrites, detailed examination has resulted in its reclassification as a polymict breccia. Pasamonte contains a variety of basaltic lithologies, granulites, granulitic breccias, and impact-melt breccias. In addition, it contains pyroxene of both equilibrated and unequilibrated types with differing zoning types. Pasamonte exhibits evidence of mild thermal annealing by the variation it exhibits in pyroxene lamellar wavelengths, a factor related to cooling rate. This feature, along with the Fe-enriched zones adjacent to pyroxenes fractures and reversed zoning in pyroxenes, provides evidence supporting a low degree post-magmatic metasomatic equilibration process associated with an Fe-rich dry vapor lasting ~60 years (Schwartz and McCallum, 2003; McCallum et al., 2004; Barrat et al., 2011). This duration can be contrasted to the 25,000 years of annealing experienced by the highly metamorphosed eucrite Haraiya (Type 7). By contrast, the eucrite NWA 049 represents a sample that experienced a high degree of post-magmatic metasomatic equilibration. Pasamonte has a very old crystallization age of 4.58 b.y., and a young cosmic ray exposure age of only 7.7 m.y.

In 1981, the Basaltic Volcanism Study Project (BVSP) assigned Pasamonte, along with Nuevo Laredo and Lakangaon, to the Nuevo Laredo Trend eucrites, which were formed from fractional crystallization of Main Group melts; however, this assignment of Pasamonte might have been based on incomplete data. The three subgroups of the noncumulate group of eucrites have been separated based on the molar Mg/(Mg+Fe) (here abbreviated Mg#) versus an incompatible element such as Ti, as follows:

  1. Main Group (primary basalt): Mg# ~ 0.38–0.41; Ti ~ 3–4 mg/g
  2. Stannern trend (primary partial melt): Mg# similar to main series; Ti up to 5.7 mg/g
  3. Nuevo Laredo trend (fractional crystallization): Mg# extends from Main Group to 0.32; Ti = 5.7 mg/g

A plot of the three subgroups shows a convergence at the center of the Main Group, implying that a genetic relationship (i.e., same parent body) exists among them, and a possible derivation of the two trends from the primary melts of the Main Group. Currently, the Main Group is combined with the Nuevo Laredo Trend to form a single series, while the Stannern Trend represents Main Group magma that has been contaminated by a crustal partial melt.

It has been demonstrated that the HED parent body was relatively homogenous in its O-isotopic composition. In a study of a number of eucrites having anomalous O-isotopic ratios and/or anomalous chemical compositions, textures, or ages, evidence was presented indicating that Pasamonte must have originated on a parent body distinct from that of the other HED meteorites (Scott et al., 2008, 2009). For example, its significant displacement from the Eucrite Fractionation Line (EFL)—plotting ~4.7 standard deviations from the eucrite/diogenite mean Δ17O value—cannot be reasonably explained by the admixture of foreign impactor contaminates, by terrestrial weathering processes, or by an isotopically heterogeneous parent body. Pasamonte has a pyroxene Fe/Mn ratio of 29, which is at the lower range (28–40) of typical eucrites. Moreover, its chromites have compositions which are much more Al-rich and Ti-poor than in other eucrites. It is reasonable to assume that Pasamonte was derived from one of many Vesta-sized asteroids that likely existed early in Solar System history, prior to the Late Heavy Bombardment period ~3.5–4.1 b.y. ago. Notably, the paired brecciated, vesiculated basalts PCA 82502 and PCA 91007 have O-isotopic compositions which are virtually identical to Pasamonte (see diagram below), and they have similar anomalously high abundances of certain siderophile elements (Ni, Ir, Os) as well; it could be inferred that they formed in a common nebular region (Scott et al., 2009). standby for pasamonte-pca o diagram
Diagram credit: Mittlefehldt et al., 47th LPSC, #1240 (2016) As presented by Sanborn and Yin (2014) [#2018], a Δ17O vs. ε54Cr diagram is one of the best available diagnostic tools for determining genetic (parent body) relationships among meteorites, constrained by the degree to which isotopic homogenization occurred on their respective parent bodies. Moreover, Sanborn et al. (2015) demonstrated that ε54Cr values are not affected by aqueous alteration. Currently, a number of anomalous eucritic meteorites are known, including Ibitira, Pasamonte, NWA 1240, PCA 82502/91007, Bunburra Rockhole, A-881394, EET 92023, and Emmaville, each of which are resolved from typical eucrites and the HED parent body both isotopically and compositionally; notably, the latter four anomalous eucritic meteorites share close similarities in their O-isotopes and might be genetically related (Barrett et al., 2017; see O-isotopic diagram). However, a high-resolution Δ17O diagram presented by Mittlefehldt et al. (2018) appears to distinguish many of these meteorites from each other, with two couples—Pasamonte with PCA 91007 and Bunburra Rockhole with EET 92023—showing overlapping values (see diagram below). Asuka 881394 is exceptional in having the oldest measured U–Pb age of any achondrite. Employing the precise 238U/235U value of 137.768, 2015), Wimpenny et al. (2019) determined the most precise and accurate Pb–Pb isochron age for A-881394 of 4.56495 (± 0.00053) b.y. By comparison, the ungrouped NWA 11119 (likely related to NWA 7325) has an Al–Mg age relative to D’Orbigny of 4.5648 (± 0.0003) b.y. (Srinivasan et al., 2018). standby for pasamonte ox diagram
Diagram credit: Mittlefehldt et al., 49th LPSC, #2700 (2018) Another useful tool to help resolve potential genetic relationships among meteorites is the pyroxene Fe/Mn ratio. While Fe and Mn do experience nebular fractionations, they are not readily fractionated during parent body igneous processing, and therefore different Fe/Mn values are inherent in different parent objects. Mittlefehldt et al. (2017) utilized a number of eucrites and anomalous eucrite meteorites, including A-881394, EET 92023, Ibitira, and Emmaville, to compare the Fe/Mn and Fe/Mg ratios in low-Ca pyroxenes. Consistent with the O-isotopic results, these four meteorites plot in separate locations on an Fe/Mn vs. Fe/Mg coupled diagram, which suggests that they derive from separate parent bodies (see top two diagram below). Moreover, despite the fact that Pasamonte and PCA 82502/91007 are similar with respect to both Δ17O and ε54Cr values (see Sanborn et al., 2016, #2256), these two meteorites are resolved on an Fe/Mn vs. Fe/Mg coupled diagram, which suggests that they derive from separate parent bodies as well (see bottom diagram below). standby for fe/mn vs. fe/mg diagram
Diagram adapted from Mittlefehldt et al., 47th LPSC, #1240 (2016)

standby for fe/mn vs. fe/mg diagram
Diagram credit: Greenwood et al., 48th LPSC, #1194 (2017) It is known that ureilites, generally considered to originate from a common parent body, have a relatively wide degree of variability in Δ17O, but a relatively narrow degree of variability in ε54Cr. By comparison, Sanborn et al. (2014) inferred that the similar degrees of variability that exist among these anomalous eucritic meteorites could likewise reflect a common origin from a single Vesta-like parent body distinct from typical eucrites (see diagram below). Several exceptions to this hypothesis have since been identified including the following: NWA 1240 plots away from the common HED field; PCA 82502/91007 is resolved from the other anomalous eucrites by both O-isotopes and pyroxene Fe/Mn ratio; A-881394 has significantly different oxygen isotopes, Ti/Al and Fe/Mn values, and bulk composition compared to HEDs (Mittlefehldt et al. (2015); and EET 92023 exhibits significant differences in O-isotopes, Cr-isotopes (Sanborn et al., 2016, #2256), and pyroxene composition compared to HEDs and other anomalous achondrites. EET 92023 shares similar O- and Cr-isotopes to A-881394 and Bunburra Rockhole indicating that they each formed within a common isotopic reservoir. Under the hypothesis that Δ17O values serve equally well as a discriminator compared to ε54Cr values, all of these anomalous meteorites could derive from numerous unique parent bodies distinct from Vesta (see diagram below). Furthermore, although Bunburra Rockhole and A-881394 have the same oxygen and chromium isotope compositions, new in-depth analyses of Bunburra Rockhole conducted by Benedix et al. (2017, and references therein) have revealed that these two meteorites have very different textures and mineral chemistries; e.g., Bunburra Rockhole has plagioclase with An8790, while A-881394 has plagioclase with An98. Based on their results, they consider it likely that these two meteorites also derive from separate parent bodies.

standby for pasamonte o-cr diagram
Diagram adapted from Sanborn and Yin, 45th LPSC, #2018 (2014) One more anomalous eucrite-like achondrite, classified as NWA 12338, has joined this disparate group. This unbrecciated basaltic meteorite is geochemically similar to eucrites but has some differences in its texture and mineralogy, and it has O-isotopic values that plot in a distinct space above the HED trend (see diagram below). standby for nwa 12338 ox diagram
Diagram credit: Guo et al., 50th LPSC, #1583 (2019) Because there are now a number of eucrite-like meteorites that are not grouped with normal eucrites for various reasons, it was proposed that the term ‘eucrite’ be used as a description of a rock type rather than to imply an origin on the presumed HED parent body Vesta. The photo above shows a partially fusion-crusted 18.93 g specimen of Pasamonte acquired from the Robert Haag Collection. standby for sonic boom photo standby for pasamonte photo
The left photo above captures the shock-generated condensation cloud at the moment when a jet breaks the sound barrier. Compare this to the Pasamonte fireball photo on the right. The corkscrew appearance of the dust train attests to the spinning motion of the incoming object over an extended period of time.

The photo below shows the persistent dust cloud of the Pasamonte meteorite showing the effects of adiabatic processes.
standby for pasamonte photo


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NWA 10674

Eucrite, polymict, anomalous
Home Page
mouseover for alternate view angle

Purchased October 2015
no coordinates recorded Four pieces of a polymict eucrite having a combined weight of 1,193 g were purchased by M. Jost from a Moroccan dealer. A type sample was sent to the University of Washington in Seattle (A. Irving and S. Kuehner) for analysis and classification, and NWA 10674 was determined to be an anomalous polymict eucrite breccia.

Northwest Africa 10674 is a complex breccia composed predominantly of eucritic clasts (orthopyroxene with exsolved pigeonite and calcic plagioclase) having variable textures, mixed with disaggregated eucrite-related clastic material. Minor phases in the eucrite breccia include silica polymorph, chromite, ilmenite, troilite and fayalite, as well as sparse diogentic clasts (~2 vol%). The meteorite is unusual in that it contains abundant metal-bearing vitric breccia clasts (20 vol%) composed of magnesian silicates (orthopyroxene and olivine) interspersed with grains of FeNi-metal (kamacite) within glass.

Interestingly, Warren et al. (2017) identified several cm-scale, metal-rich (12–17 vol%), ovoid silicate nodules in the polymict eucrite Camel Donga. These nodules are enriched in Ni and trace siderophile elements and lack other evidence for in situ reduction. They concluded that the metal nodules reflect the addition of an impactor component to the material that was precursory to Camel Donga. They speculate that this was a complex two-stage process involving metasomatic alteration by a reducing fluid that originated as a metal- and volatile-rich carbonaceous-chondritic (e.g., CM-type) contaminant. See the Camel Donga page for a photo and further details about this alteration process.

The specimen of NWA 10674 shown above is a 7.08 g partial slice acquired from the Space Jewels Switzerland Collection of Marc Jost. Click on the top photos below to see magnified images. Another representative slice of NWA 10647 is also shown below, courtesy of Marc Jost. standby for lodran photo standby for lodran photo
click on photos for a magnified view

standby for nwa 10674 slice photo It is noteworthy that the anomalous howardite NWA 3197 exhibits a striking similarity to NWA 10674 in both its exterior and interior appearance; compare the photo of NWA 10674 individuals and slices below to photos of NWA 3197 on the webpage of J. Utas here. Among the differences that do exist between these two meteorites is the fact that no recrystallized H chondrite clasts have been observed in the NWA 10674 samples analyzed. standby for nwa 10674 main mass photo
Photo courtesy of Marc Jost


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Millbillillie

Eucrite
Polymict, crystalline
standby for millbillillie photo
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. As with 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., 31st LPSC (2000) #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., 35th LPSC, #1020 [2004]; 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, in addition to impact-generated heating (John T. Wasson, 2016), eventually forming a magma ocean. Metal–silicate melting, differentiation, and fractionation then occurred 4.5649 (±0.0011) b.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 1.09 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 (D. Mittlefehldt, 2008; Greenwood et al., 2015) are thought to 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 (±0.020) b.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 (±0.015) b.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 ν6 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. Interestingly, all petrographic characteristics studied so far are similar to the known basaltic eucrites (Spivak-Birndorf et al., 2010). However, Bunburra Rockhole has O- and Cr-isotopic compositions distinct from that of Vesta-derived eucrites, and therefore it is thought to represent a unique basaltic parent asteroid (e.g., Towner et al., 2010, Benedix et al., 2017). Bunburra Rockhole was likely delivered from the inner main belt through the ν6 secular resonance, demonstrating how material ejected from Vesta and the related Vestoids can be delivered to Earth in a like manner through an evolving orbit. Moreover, although Bunburra Rockhole and the ungrouped eucrite-like achondrite A-881394 have the same oxygen and chromium isotope compositions, new in-depth analyses of Bunburra Rockhole conducted by Benedix et al. (2017, and references therein) have revealed that these two meteorites have very different textures and mineral chemistries; e.g., Bunburra Rockhole has plagioclase with An8790, while A-881394 has plagioclase with An98. Based on their results, they consider it likely that these two meteorites derive from separate parent bodies. Further details about the anomalous eucrites can be found on the Pasamonte page.

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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Dho 007

Eucrite
Polymict breccia, cumulate
(or mesosiderite inclusion)

standby for dhofar 007 photo
Found 1999
18° 20.1′ N., 54° 10.9′ E.

This coarse-grained, brecciated eucrite, with a total known weight of ~27 kg, was found broken into 37 fragments on the desert floor of Oman. The find site is a flat plain, covered with carbonate stones, interspersed with a quartz–carbonate sand. The area is overgrown by low shrubs. Based on structure, petrography, bulk chemistry, and REE patterns, this eucrite was classified as a cumulate by Dr. Marina Ivanova at the Vernadsky Institute of Geochemistry of the Russian Academy of Sciences, Moscow.

In contrast to noncumulate eucrites, cumulate eucrites have higher Mg contents (Mg# ~44–65), REE abundances much lower than chondritic, and are LREE-enriched with positive Eu anomalies (18–23 × CI). The source magma was probably derived from the mineral peridotite, a mixture of olivine, pigeonite, and plagioclase, and is the mineral forming the bulk of the Earth’s upper mantle. Cumulate eucrites formed by fractional crystallization in a magma chamber about 4.5 b.y. ago. On the basis of augite lamellae widths and Ca zoning profiles, a burial depth for cumulate eucrites was calculated to be 7–8 km, with a cooling rate of 0.16–0.2°C/t.y.

The composition of Dhofar 007 consists of roughly equal proportions of anorthite, a Ca-rich plagioclase feldspar, and the clinopyroxenes pigeonite and augite, along with minor amounts of Fe-metal, troilite, chromite, and phosphates. Glassy melt-veins pervade these components, reflecting an impact-shock history. During studies of Dhofar 007 by Yamaguchi et al (2003), they identified various xenolithic inclusions, including a polymineralic impact-melt clast, a Mg-rich orthopyroxene fragment, and a recrystallized plagioclase grain, demonstrating a polymict nature for this meteorite.

Another clast that was studied which represents a large portion of this meteorite is coarse-grained with a granular texture, composed primarily of equal amounts of pyroxene (pigeonite and augite) and plagioclase, along with minor silica and metallic phases. The derivation of the metallic phases is suggested by Yamaguchi et al. (2006) to have occurred through the injection of a metallic component during a high temperature impact-shock event on the mesosiderite parent body. They describe a scenario by which the eucrites were formed at a distance from the actual impact location of a large metallic projectile, while the mesosiderites were located in close proximity to the impact. Dhofar 007 is unusually enriched in siderophile elements, including Ni, Ir, Os, Au, Pd, and Co, similar to the abundances found in metal of mesosiderites. The platinum group elements also have ratios that are similar to those in metallic portions of mesosiderites.

Compared to cumulate eucrites, the cooling history of Dhofar 007 is more complex. Following the shock-heating/melting event in which FeNi-metal was injected, recrystallization occurred. Thereafter, excavation from depth caused very rapid cooling at high temperatures (850–1200°C) in pyroxenes, resulting in the formation of very thin augite exsolution lamellae. This cooling rate is on the order of 10,000 times higher than that of cumulate eucrites cooled at depth. Subsequent burial by an extensive ejecta blanket led to very slow cooling at lower temperatures (~700°C down to at least 300°C) as evidenced by the metallic phases, in a similar manner to that of mesosiderites. Later, moderate impact-shock events produced brecciation and melt veins. Based on these anomalies in siderophile content and cooling history, Yamaguchi et al. (2003, 2006) proposed that Dhofar 007 might possibly be a silicate fraction from a mesosiderite.

Despite these anomalies, the petrology, REE content, Mg#, FeO/MnO ratios, mineralogy, and textures are all consistent with a cumulate eucrite classification. However, utilizing an oxygen three-isotope diagram, Greenwood et al. (2017) demonstrate that Dhofar 007 plots far away from the eucrite fractionation line (see diagram below). Whether this reflects impactor contamination on the eucrite parent body (4 Vesta) or a separate parent body is still an open question. standby for o-isotopic diagram
Diagram credit: Greenwood et al., Chemie der Erde, vol. 77, p. 25 (2017)
‘Melting and differentiation of early-formed asteroids: The perspective from high precision oxygen isotope studies’
(open access: http://dx.doi.org/10.1016/j.chemer.2016.09.005)
The unbrecciated eucrite-like achondrite EET 92023 exhibits important petrological and O-isotopic similarities to the coarse-grained clasts in Dhofar 007, and it has been considered that the two might be genetically related. In their study of EET 92023, Yamaguchi et al. (2017) determined that its source rock experienced a multistage thermal history including a very early impact by a IAB or IVA iron projectile, but evidence indicates that its anomalous O-isotopic signature is indigenous rather than the result of impactor contamination. It has not yet been determined if the similar O-isotopic signature of Dhofar 007 is also indigenous or is instead due to impactor contamination.

In addition to Dhofar 007 and EET 92023, a number of anomalous eucritic meteorites are known including Ibitira, Pasamonte, NWA 1240, PCA 82502/91007, Bunburra Rockhole, A-881394, and Emmaville, each of which are resolved from typical eucrites and the HED parent body both isotopically and compositionally. It is notable that EET 92023, Bunburra Rockhole, and A-881394 have close O-isotope values, which suggests the possibility of a genetic relationship (Barrett et al., 2017, O-isotopic diagram 1; Mittlefehldt et al., 2018, O-isotopic diagram 2).

Another useful tool to help resolve potential genetic relationships among meteorites is the Fe/Mn ratio. While Fe and Mn do experience nebular fractionations, they are not readily fractionated during parent body igneous processing, and therefore different Fe/Mn values are inherent in different parent objects. Mittlefehldt et al. (2017) utilized a number of eucrites and anomalous eucrite meteorites, including A-881394, EET 92023, Ibitira, and Emmaville, to compare the Fe/Mn and Fe/Mg ratios for low-Ca pyroxenes. Contrary to the results from O-isotopic analyses, these four meteorites plot in separate locations on an Fe/Mn vs. Fe/Mg coupled diagram, which suggests that they derive from separate parent bodies (see diagram below). Moreover, although Bunburra Rockhole and A-881394 have overlapping oxygen and chromium isotope compositions, new in-depth analyses of Bunburra Rockhole conducted by Benedix et al. (2017, and references therein) have revealed that these two meteorites have very different textures and mineral chemistries; e.g., Bunburra Rockhole has plagioclase with An8790, while A-881394 has plagioclase with An98. Based on their results, they consider it likely that these two meteorites derive from separate parent bodies. Further details about the anomalous eucrites can be found on the Pasamonte page. standby for anom eucrite diagram
Diagram adapted from Mittlefehldt et al., 47th LPSC, #1240 (2016) Based on the calculated CRE age and Kr–Kr age of ~12–15 m.y., a terrestrial age for Dhofar 007 has been estimated to be 70 t.y. (Takeda et al., 2007). A previous estimate given by Miura and Nagao (2003) was 20 t.y. Further information about the cumulate eucrites can be found on the NWA 1836 page. The photo above shows a 1.45 g partial end section of Dhofar 007 showing both fine- and coarse-grained clasts intruded by black shock veins. The photo below shows one of the fusion-crusted fragments as it was found lying on the desert plain.

standby for dhofar 007 photo