Deep regolith fragmental breccia

standby for dar al gani 319 photo
Found 1997
27° 01.68' N., 16° 21.52' E.

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):

  1. coarse-grained mafic lithic clasts
  2. fine-grained mafic lithic clasts
  3. felsic lithic clasts and gabbroic clasts
  4. dark clasts
  5. sulfide- and metal-rich lithic clasts
  6. chondrule and chondrite fragments
  7. 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, and are associated with three distinct igneous lithologies—in order of increasing formation depth: albitic (An~032), labradoritic (An~3370), and olivine–augite feldspathic clasts (Cohen et al., 2004; Goodrich et al., 2016, 2017). In addition, minor abundances of magnesian anorthite-rich (An~8990, Fo~93) and ferroan anorthite-rich (An>95, Fo~5070) clasts were identified by Goodrich and Wilson (2014). These feldspathic clasts were formed 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 ureilite parent body (UPB) history (Kita et al., 2004; Cohen et al., 2004; Goodrich et al., 2017). This melting 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 might reflect the impact-disruption thought to have occurred on the UPB, and the age attests to the formation of the UPB within ~10 m.y. of the collapse of the solar nebula. Other chronometric systems such as the short-lived Hf–W chronometer indicate accretion of the UPB occurred within ~1 m.y. of solar system history (Budde et al., 2014).

Two plagioclase feldspar-enriched (~70 vol%) trachyandesitic clasts, designated MS-MU-011 and MS-MU-035, were found as part of the Almahata Sitta polymict ureilite fall. It is considered that these clasts likely sample the UPB crust, or possibly an alkali- and water-rich localized melt pocket, and they are similar to several albitic clasts identified in polymict ureilites by Goodrich et al. (2017). These two trachyandesitic clasts have an Ar–Ar isochron age for glass-bearing pyroxene of 4.556 (±0.015) b.y., which is ~1 m.y. younger than other felsic clasts found in DaG 319 or any other polymict ureilites (Bischoff et al., 2014). This age constrains the timing of the UPB disruption event to ≥6.5 m.y. after CAIs. Also part of the Almahata Sitta fall is MS-MU-012, which is the first known plagioclase-bearing olivine–augite ureilite lithology, with an O-isotopic composition consistent with a primary origin on the UPB (Bischoff et al., 2015; Goodrich et al., 2016). Similarly, the 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 responsible for mixing diverse materials and causing explosive volcanism accompanied by loss of most of the basaltic melt. Certain elemental ratios which establish distinct correlations at Fo~8285 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 such as DaG 319.

The dark carbonaceous clasts found in DaG 319 have a CI-like texture and may reflect late-stage accretion onto the UPB (Ikeda et al., 2003). Some of these dark clasts contain anhydrous minerals that have O-isotopes which 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 are 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 host 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 in 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 chondrule fragments identified in DaG 319 show similarities to those of both known and unknown ordinary chondrites, likely representing impactors onto the ureilite parent body. In addition, both incompletely equilibrated and equilibrated chondrite fragments are present which are likely R chondrite material, possibly unique from R chondrites in our collections (Goodrich et al., 2016). The olivines in these 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. Isolated PO chondrules of type 3.1–3.2 and type 4 have been identified in DaG 319 having OC or RC compositions (Goodrich et al., 2016). A complex L/LL chondrite breccia found in DaG 319 was characterized by Goodrich and Gross (2015).

Other exotic clasts identified in DaG 319 and other polymict ureilites include those representing material from E chondrites or aubrites, and clasts derived from objects not yet represented in our collections (Downes et al., 2008). Notably, certain anorthite–fassaite-bearing clasts (An >95) have chemical and O-isotopic compositions that are similar to angrites, and these may have originated on the angrite parent body (Goodrich and Wilson, 2014). The first representative sampling of a quenched angrite clast found in DaG 319 was described by Goodrich et al. (2015). Components in this small (~1 × 1.5 mm) angrite clast show some similarities to those in the basaltic/quenched angrites A-881371 and LEW 87051. In a contrasting view, Cohen et al. (2004) suggest that most of the anorthite-rich plagioclase clasts likely derive from ureilitic precursor material. Anorthite-rich clasts (>An90) would be produced from material having Ca/Al ratios 2×CI and which experienced a high degree of fractional batch melting (~18%). Si-bearing metals identified by Ross et al. (2009) in polymict ureilites are considered to be remnants of the disrupted UPB core that were subsequently accreted to the regolith of the reconstituted UPB.

Notably, a mm-sized, gabbroic troctolitic clast (γ-8) with compositions (e.g., trace elements and An of plagioclase; Fe/Mg, Fe/Mn, Mg#, and CaO and Cr2O3 contents of olivine) similar to the ungrouped achondrite NWA 7325 was found in DaG 319 (Kita et al., 2004; Goodrich et al., 2014). Furthermore, Boyle et al. (2017) identified a Ca-plagioclase clast in the polymict ureilite NWA 10657 with identical An and FeO values to plagioclase in NWA 7325. In addition, a number of other mafic silicates and plagioclase clasts have been identified in polymict ureilites which have textures and certain mineral compositions similar to NWA 7325 (see diagram below). The occurrence of similar clasts in both NWA 7325 and polymict ureilites (the clasts are considered to be exogenous in the ureilites) should not be surprising given the fact that the two parent bodies also share similar O-isotopic compositions and likely formed within the same nebular region. Further comparisons utilizing a possible second sampling of the NWA 7325 parent body, NWA 11119 (photo courtesy of Carl Agee), should be elucidating.

standby for ureilite vs nwa 7325 diagram
Diagram credit: Boyle et al., 48th LPSC, #1219 (2017)

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 subgroups 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 subgroups. 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 as 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; it probably had a diameter of ~200 km (Goodrich et al., 2007).

An excellent petrographic thin section micrograph of DaG 319 can be seen on John 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.

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