Ureilite
Monomict/Unbrecciated/Main Group
Olivine–orthopyroxene–augite, plagioclase-bearing
Fell October 7, 2008
20° 43.04' N., 32° 30.58' E.
In 2008, October 6 at 5:46 A.M., asteroid 2008 TC3 fell to Earth in northern Sudan. See the Almahata Sitta webpage for the complete story of the discovery of this meteorite, results of the consortium analyses, and new models for the petrogenetic history of the ureilite parent body. The 2008 TC3 meteorite was sent to NASA's Johnson Space Center in Houston (Zolensky) and Carnegie Institution of Washington (Steele) for analysis and classification, and Almahata Sitta was determined to be a polymict ureilite fragmental breccia composed of three main ureilite lithologies, along with a wide range of xenolithic clasts representing many different chondritic and achondritic lithologies in an assemblage similar to the polymict breccia Kaidun (Bischoff et al., 2010). The analyses indicate that all of the clasts came from the Almahata Sitta fall; e.g., detection of short-lived cosmogenic nuclides, very low weathering grade (W0–W0/1), multiple lithologies among fragments delimiting a strewn field, a high number of rare E-chondrite rock types found together, diffusion of PAHs among clasts (Sabbah et al., 2010), and the finding of new and unique meteorite fragments within a small area.
The heterogeneous composition of Almahata Sitta likely reflects an assemblage derived from a catastrophic collision(s) between ureilite and chondrite objects (Kohout et al., 2010). In an alternative scenario, these diverse clasts could have become gravitationally bound within a common debris disk composed of a disrupted ureilite asteroid, and this disk subsequently re-accreted into one or more smaller second-generation asteroids. This second-generation asteroid was then lightly sintered together through multiple low-energy impacts resulting in a bulk porosity of ~50%. This fine-grained, highly porous, weakly consolidated matrix material is possibly represented by the recovered specimen MS-168 and/or the C1+URE+OC+EH regolith breccia clasts AhS 91/91A and 671; this would be consistent with the reflectance spectra obtained for the asteroid (Goodrich et al., 2015, 2019).
Among the wide variety of xenolithic clasts recovered from the Almahata Sitta polymict ureilite fall is the 15.55 g clast MS-MU-012. This clast was initially analyzed at the Institut für Planetologie in Münster, Germany and classified as the first known plagioclase-bearing olivine–augite ureilite (Bischoff et al., 2015, #5092). In their analyses of MS-MU-012, Goodrich et al. (2015, 2022) determined that it was an olivine–orthopyroxene–augite ureilite, and documented a composition of ~52% olivine (Fo88.1 [±0.1]), 14% plagioclase-rich areas (An68.1 [±2]), 13% orthopyroxene, and 11% augite, with minor Si- and P-bearing kamacite and Cr-bearing troilite occurring interstitially.
Primary trapped melt inclusions are present in all three silicate phases, but most abundant in olivine which was the first to crystallize (Goodrich et al., 2022). The plagioclase in MS-MU-012 consists of both pristine crystals and those which have been shock-melted and recrystallized, with the unmelted pristine plagioclase having a composition of An68.4. This attests to its late-stage crystallization in which cooling of the residual melt to below 1180°C (plagioclase threshold) occurred prior to its complete extraction, in distinction to all other Hughes-type ureilites, leaving a significant amount of plagioclase within the mafic assemblage (Goodrich et al., 2016, 2022).
The MS-MU-012 clast is considered to be a paracumulate (a hybrid word derived from "partial melt residue" and "cumulate", described by Warren and Kallemeyn [1989] as a convective, mostly crystalline mush beneath a magma ocean), representing a mixture of residual olivine and cumulus pyroxene. A plausible formation scenario was presented in which an augite-saturated melt invaded a region composed of a residual olivine–pigeonite assemblage; following melt extraction, the lithology appears texturally similar to a cumulate (Berkley and Goodrich, 2001; D.W. Mittlefehldt, 2005). It is considered that the parental melts from which olivine–augite ureilites were formed originated at greater depths than melts parental to the ferroan olivine–pigeonite ureilites (Goodrich et al., 2004). On the other hand, crystallization of the olivine–augite ureilites occurred relatively late at ~3–4 m.y. after CAIs—after the parental melt had ascended to form a shallow intrusion and the degree of incremental fractional melting had reached ~15%. At this stage, most of the plagioclase had been removed and the magma had undergone reduction from IW–1.6 to IW–1.9, thus establishing higher olivine Fo compositions (Goodrich et al., 2009, 2022). In a study of Fe–Mg zoning profiles for reduction rims of olivine in MS-MU-012, Mikouchi et al. (2018) verified a typical fast cooling rate from 1200°C to 700°C of 0.2°C/hr at an oxygen fugacity of IW–1. Although the presence of plagioclase in MS-MU-012 is unique among main group ureilites, the meteorite is otherwise nearly indistinguishable from typical olivine–augite–[±orthopyroxene] ureilites with respect to mineralogy, O-isotopic composition, and petrographic characteristics (Goodrich et al., 2016). See the HaH 064 page for further information about the olivine–augite ureilite subgroup.
The ureilites of the olivine–augite type define a broad range of Δ17O values, consistent with their formation from melts originating at variable depths and crystallizing after ascent to shallower depths. Members of this type are represented by a relatively small number of samples resolved into the following subdivisions (after Goodrich et al., 2022):
1. Hughes cluster
a. Archetypal Hughes-type samples
Hughes 009
MS-MU-012 (+plagioclase)
FRO 90054/93008
FRO 90228
NWA 3222 (37–41 vol% augite)
NWA 11754
AhS clast #15 (75.5 g)
b. Other Hughes-type samples (low-augite, graphite-bearing)
EET 96293/96314/96331
NWA 2895
MIL 091004
Calama 001
EET 87720 clast Gr23
2. Other augite-bearing ureilites
HaH 064
ALH 82106/82130/84136
EET 87511/87523/87717
EET 87517
META78008
LEW 88774
LEW 85440/88012/88201/88281
Y-74130
Ramlat as Sahmah 530
NWA 11900
DaG 999 (single aug-bearing clast)
Y-74123 (aug within melt veins)
Y-790981 (aug within melt veins)
It was stated by Goodrich et al. (2022) that a number of the Hughes-type members, including MS-MU-012, are considered archetypal representatives having accordant O-isotopic compositions (Δ17O ~ –1‰) and olivine Fo values, and which together define a mass-dependent fractionation trend with a slope of 0.46 (±0.11). Goodrich et al. (2022) conducted a trace element analysis of augite in MS-MU-012 which showed it to have essentially identical REE abundances to that in Hughes 009 and FRO 90054. In addition, they recognized that carbon phases are lacking in these Hughes-type samples and proposed some possible reasons for this: (i) exhaustion of graphite in the reduction process; (ii) loss of graphite from the cumulus pile as a flotation fraction; (iii) failure of graphite to ascend due to the small-scale of the melt conduits; reason (ii) may be the most plausible. The above characteristics, which are shared in common by the Hughes-type samples, attest to an origin of this group of ureilites from a common parental magma unit on the UPB (see diagrams below).
Oxygen 3-Isotope Diagram for Hughes-Type Ureilites
click on image for a magnified view
Fo in Olivine vs. Δ17O for Hughes-Type Ureilites
Symbols the same as (a) above
click on image for a magnified view
Diagrams credit: Goodrich et al., MAPS, vol. 57, #8, p. 1606 (2022)
'The first main group ureilite with primary plagioclase: A missing link in the differentiation of the ureilite parent body'
(https://doi.org/10.1111/maps.13889)
The oxygen isotope composition of AhS 3005 indicates a fractional crystallization origin from the same parental melt as the ureilitic trachyandesites MS-MU-011 and MS-MU-035, as well as the plagioclase-bearing olivine–orthopyroxene–augite ureilite MS-MU-012 (Goodrich et al., 2022 #1065). This fractionation relationship is also attested by the similarity of the plagioclase An compositions between the two AhS 3005 sectors and MS-MU-011 and MS-MU-035. Goodrich et al. (2022) proposed a fractionation sequence for these three ureilites as follows: AhS 3005 labradorite–opx sector → MS-MU-011 → AhS 3005 oligoclase–augite sector → MS-MU-035. They also demonstrated that these three samples along with MS-MU-012 originated from a more evolved parental source melt associated with the magnesian labradoritic rock type rather than the earlier crystallized albitic rock type that is more prevalent in polymict ureilites.
An–Fe/Mg Comparisons to Magnesian Labradoritic Lithology
click on image for a magnified view
Diagram credit: Goodrich et al., 53rd LPSC, #1065 (2022)
'Almahata Sitta 3005: A New Sample Of Ureilitic Crust And
New Insights Into Differentiation Of The Ureilite Parent Asteroid'
The broad diversity of lithologic types present in 2008 TC3 constituted <30% of all material recovered. However, given that the vast bulk of 2008 TC3 is thought to have been lost as fine dust (≥99.9% of the estimated 42–83 ton pre-atmospheric mass), the asteroid was likely composed predominantly of very fine-grained, highly-porous, weakly-consolidated matrix material, possibly represented by the recovered specimen MS-168 and/or the C1+URE+OC+EH regolith breccia clasts AhS 91/91A and 671; this would be consistent with the reflectance spectra and other data obtained for the asteroid (Goodrich et al., 2015, 2019; Bischoff et al., 2022). Examples of some of the diverse samples that have been recovered are listed below (Bischoff et al., 2010, 2015, 2016, 2018, 2019; Horstmann and Bischoff, 2010, 2014; Hoffmann et al., 2016; Fioretti et al., 2017; Goodrich et al., 2018, 2019):
unique chondrite: MS-CH, type 3.8 [± 0.1], has petrographic and isotopic affinities to R-chondrites, but is mineralogically anomalous
Bencubbin-like carbonaceous chondrite: MS-181, a 58.6 g chondrule-like clast containing metal globules and silicates in a 60:40 ratio, having an O-isotopic composition consistent with bencubbinites
C2 carbonaceous chondrite: AhS 202, 20.057 g, w/ tremolite amphibole (Fioretti et al., 2017 #1846 [photo]; Goodrich et al., 2019 #1551, 2020 #1223 [O plot]; Hamilton et al., 2020 #1122; Hamilton et al., 2021; Miller et al., 2021 #2360 [O–Cr plot]; Dodds et al., 2022 #2158)
C1+URE+OC+EH regolith breccia: AhS 91/91A and 671 (Goodrich et al., 2018 #1321 [photo], 2019 #1551, #1312, article, 2021 #1331, article); MS-266 (polymict breccia with C1 clast; Bischoff et al., 2022)
niningerite-bearing, fine-grained ureilitic fragment (linking E chondrites): MS-20
sulfide-metal assemblage in a fine-grained ureilitic fragment: MS-158, -166
ungrouped enstatite- and metal-rich achondrite fragments: MS-MU-019 (complete mass; cut section photo credit: Bischoff et al.2022; characteristics similar to NWA 8173/10271); MS-MU-036 (similar to MS-MU-019, Itqiy, and NWA 2526 [Bischoff et al., 2016; Zhu et al., 2021]); AhS 38 (similar to MS-MU-019 and Itqiy but contains olivine [Goodrich et al., 2018]); AhS 60 (possible E IMR analogous to Portales Valley [Goodrich et al., 2018])
the first known plagioclase-bearing olivine–orthopyroxene–augite ureilite lithology: MS-MU-012 (Goodrich et al., 2022)
trachyandesitic clasts: (1) MS-MU-011 (view 1), MS-MU-011 (view 2), MS-MU-011 (aka ALM-A); plagioclase-enriched (~70 vol%) with pockets of gemmy olivine (photo courtesy of Stephan Decker) likely sampling the UPB crust, or possibly an alkali- and water-rich localized melt pocket; calculated Ar–Ar age of ~4.556 b.y. and Pb–Pb age of ~4.562 b.y. (Bischoff et al., 2013, 2014; Delaney et al., 2015; Turrin et al., 2015; Amelin et al., 2015); (2) MS-MU-035; anorthoclase and/or plagioclase-enriched (~65 vol%) (Bischoff et al., 2016); (3) MS-277, 11.03 g; (4) MS-MU-065, 54.7 g
andesitic clast: AhS 3005, 16.84 g, composed of two different sectors: 1) "labradorite–opx" (plag cores = An50–53); 2) "oligoclase–augite" (plag cores = An30–35) (Goodrich et al., 2022 #1065)
Special thanks to Siegfried Haberer and Stephan Decker for providing specimens of this special meteorite and many of its xenolithic clasts to the scientific and collector communities. The photo of MS-MU-012 shown above is a 0.14 g partial slice, while the photo below shows the main mass.
