Three fragments totaling 740 g were found in the Libyan Desert. Two other stones that were recovered later, DaG 665 (363 g) and DaG 874 (64.6 g), are possibly paired. Dar al Gani 319 is one of only a handful of polymict members among a mostly monomict ureilite population. This particular meteorite has a brecciated structure and contains solar type rare gases, suggesting a surface regolith residence on the ureilite parent body. Diamonds and many types of inclusions have been found in this meteorite. The diverse clasts have been categorized into seven general groups (Ikeda et al., 2000):
coarse-grained mafic lithic clasts
fine-grained mafic lithic clasts
felsic lithic clasts and gabbroic clasts
dark clasts
sulfide- and metal-rich lithic clasts
chondrule and chondrite fragments
isolated mineral clasts, mostly single crystals
The main clast type present in polymict ureilites is the common Type I monomict ureilite, containing mainly olivine, pigeonite, and carbon. Type I ureilites crystallized from the residues that remained after the extraction of a partial melt component, and they probably represent mantle material. Type II ureilite clasts, which contain mainly olivine, augite, and orthopyroxene, along with minor sulfide and metal, are present in a much lower abundance. Magmatic inclusions, usually containing a feldspathic glass component, are present in olivine and orthopyroxene grains of the Type II clasts. Type II ureilites are magmatic cumulates formed by fractional crystallization of basaltic magmas derived from alkali-rich chondritic precursor material. They crystallized within shallow magma chambers at high cooling rates.
The abundant felsic clasts present in polymict ureilites are mainly porphyritic clasts composed of plagioclase and pyroxene phenocrysts in an alkali-rich groundmass. These felsic clasts have an O-isotopic composition consistent with the missing basaltic component of the ureilite parent body. They represent three distinct igneous lithologies corresponding to increasing formation depth (albitic, labradoritic, and ol–aug, respectively), and were produced by ~20% partial melting, primarily fractional melting along with some equilibrium batch melting (representing clasts with incompatible element depletion and enrichment, respectively) of the chondritic precursor material early in UPB history (Kita et al., 2004; Cohen et al., 2004). This was followed by rapid fractional crystallization of the felsic magma and loss through explosive volcanism, or alternatively, through loss of peripheral layers following an oblique collision with a smaller planetesimal (Downes et al., 2008). The crystallization age of one of the albitic feldspathic clasts, determined by both Mn–Cr and Al–Mg systematics (Kita et al., 2007), was found to be a very old 4.562 b.y. This concordance in chronometers may reflect the impact disruption thought to have occurred on the ureilite parent body, and the age attests to the formation of the ureilite parent body within ~10 m.y. of the collapse of the solar nebula.
A single unbrecciated ureilite (EET 96001) has been found to contain a consequential component (at least 32 vol%) of compositionally diverse K-rich feldspars, probably former crustal basalt material, located within FeS-rich veins (Warren et al., 2006). It is considered plausible that this assemblage of a shallow-formed feldspar and a more deeply occurring FeS-rich vein was then gently mixed with the ureilite groundmass at a time when the jumbling of diverse materials could have occurred; i.e., at the time of the catastrophic disruption and reassembly of the proto-ureilite parent body. During this period, rapid smelting reactions would have produced large volumes of CO gas having forces that could have mixed diverse materials, and also perhaps causing the explosive loss of most of the basaltic melt. Certain elemental ratios which establish distinct correlations at Fo ~82–85 constrain the timing of this disruption event to a period when parent body melting had started to produce relatively magnesian melts (Downes et al., 2007). These Mg-rich melts re-accreted to form inclusions within polymict ureilites like DaG 319.
The dark carbonaceous clasts found in DaG 319 have a CI-like texture, and may have formed as a late-accretion onto the ureilite parent body (Ikeda et al., 2003). Some of these dark clasts contain anhydrous minerals which have O-isotopes that are consistent with a ureilite origin. Some dark clasts are Fa-free, and show evidence within phyllosilicate-rich nodules, veins, and matrix of having experienced complete hydration by water highly depleted in 16O. Other dark clasts are Fa-bearing and similar to fayalites found in CV chondrites. These Fa-bearing clasts contain exotic rock fragments of basaltic and peridotitic textures with forsteritic olivines, and they experienced only mild hydration concurrent with the dark clast. These Fa-bearing clasts may have been indigenous to the regolith zone.
Rare sulfide-rich and metal-rich clasts contain a silicate-rich matrix similar to that found in the Allende-like CV3 chondrites (Ikeda et al., 2003). The silicate-rich matrix of the sulfide-rich clasts has an oxygen isotopic composition that plots along the CCAM mixing line near to the other ureilitic clasts, and it may represent the precursor material of the unbrecciated ureilites at a moderate depth. Consistent with this scenario, the reduction of FeO in the silicate-rich matrix material, coupled with the subsequent removal of residual Fe within the melt, would have produced a final residue similar to the low Fe content of unbrecciated ureilites. In a like manner, the somewhat lighter oxygen composition of the silicate-rich matrix of the reduced metal-rich clasts suggests that they may represent the precursor material of unbrecciated ureilites at a deeper location on the ureilite parent body.
The O-isotopic compositions of the chondrule fragments and equilibrated chondrite fragments are similar to those of ordinary chondrites (though perhaps different than those known) and R chondrites, respectively, and likely represent impactors onto the ureilite parent body. Furthermore, olivines in the equilibrated chondrite fragments have many characteristics in common with R chondrites, including a low Mg#, low Ca and Cr contents, low metal content, and high Ni content, as well as the presence of chromite and pyrrhotite phases (Downes and Mittlefehldt, 2006). It has also been reported that other carbonaceous chondrite group members contain similar R chondrite-like clasts.
Other exotic clasts identified in DaG 319 and the other polymict ureilites include those from an enstatite chondrite or achondrite parent body, a low-Mn ordinary chondrite, and clasts derived from objects not yet represented in our collections (Downes et al., 2008). Notably, certain anorthite–fassaite-bearing clasts have chemical and O-isotopic compositions which are similar to angrites, and these may have originated on the angrite parent body. Still, Cohen et al (2004) suggest that anorthite-rich plagioclase clasts may be derived from ureilitic precursor material. Anorthite-rich clasts (>An90) would be produced from material having Ca/Al ratios 2×CI and which experienced fractional batch melting of a high degree (~18%). Si-bearing metals identified by Ross et al. (2009) in polymict ureilites are argued to be remnants of the disrupted UPB core which later accreted to the regolith of the reconstituted UPB.
Although the Type I and Type II mafic ureilitic clasts show a range of O-isotopic compositions, their presence within a single polymict ureilite demonstrates that they all were formed on a common parent body. In a similar manner, while O-isotopic deviations among the various ureilite groups preclude them from being related by igneous processes, the heterogeneity of the polymict ureilites suggests that there was a common parent body for all ureilite groups. Furthermore, the olivine compositions within a single thin section of polymict ureilite EET 87720 was found to span the entire range of olivine compositions recorded for unbrecciated ureilites, and the Mg# distribution is nearly identical to that of unbrecciated ureilites—two more factors which demonstrate a common origin for all ureilites (Downes and Mittlefehldt, 2006). Any measurable differences that do exist among individual olivines can be attributed to the fact that widespread impact gardening occurred subsequent to the collisional disruption and reassembly of the proto-ureilite parent asteroid, thus producing the compositional diversity observed.
The ureilite parent body was just large enough to attain temperatures high enough to produce partial melting (up to ~30%) promoting low degrees of basaltic melt migration, but less than that necessary to produce extensive melting and formation of a magma ocean. Due to this low degree of melting, perhaps caused by the sudden onset of cooling following impact disruption and reassembly, coupled with rapid melt extraction due to abundant smelting-produced CO+CO2, ureilites have retained the chemical and isotopic heterogeneity of the original carbonaceous chondrite-like asteroid, represented by the unmelted clast population. From chemical and isotopic compositions, it can be inferred that the composition of the UPB was similar to alkali-rich CV-like chondrite material, and that it was intermediate in size between undifferentiated chondritic asteroids and those asteroids large enough to have experienced melting, differentiation, and core formation—probably having a diameter of ~200 km (Goodrich et al., 2007).
A petrographic thin section micrograph of DaG 319 can be seen on J. Kashuba's page. This ureilite has a weathering grade of W2 and shock features consistent with low shock. The DaG 319 specimen shown above weighs 5.7 g.
