DAR AL GANI 013

A single fragment weighing 205 g was found in the Libyan Sahara Desert. As with most other R chondrites, Dar al Gani 013 is a chondritic genomict breccia enriched in solar-wind-implanted rare gases. This meteorite contains both equilibrated and highly unequilibrated lithic and mineral clasts together with impact-melt fragments, all incorporated within a fine-grained, olivine-rich, recrystallized matrix. Dispersed sulfide grains in the matrix are responsible for the darkening typically observed in R chondrites (Kallemeyn et al., 1996). The high noble metal abundance (Pt, Os, Ir, Au, Ru) present in the pre-oxidized FeNi-metal component of the R chondrites was exsolved during the oxidation phase; these were subsequently combined with sulfides, with the Pt residing in the unequilibrated phase tetraferroplatinum in some members.

Dar al Gani 013 contains clasts and CAIs that are more primitive than those found in most other R chondrites. Some primitive clasts have a petrologic subtype as low as 3.2 and are composed of magnesian chondrules (~0.4 mm in diameter, similar to those in H chondrites) and chondrule fragments embedded within a high proportion of fine-grained, porous, Fe-rich, olivine-pyroxene matrix. These clasts represent unaltered, highly oxidized accreted material.

Fe-bearing oxides, including nearly pure magnetite, hercynite-rich chromite, nearly pure chromite, magnetite–chromite solid solution, and ilmenite, exist together with NiO in R chondrites with variable degrees of alteration (Isa and Rubin, 2010; Isa et al., 2013). The composition of these oxide minerals is unrelated to the meteorite petrologic type, but instead likely reflects water+metal reactions. The primitive CAIs that are present are represented by unaltered phases, which include objects with Mg-rich spinel cores rimmed by diopside, along with abundant fassaite and perovskite (Rout and Bischoff, 2008).

At a later period, thermal metamorphism at depth produced the R4–R6 lithologies (R4–5 = 550–690°C; R6 = 880°C), which was followed by impact gardening. Eventually, low-shock lithification resulted in the unequilibrated breccias we find as meteorites. A modified version of the Van Schmus–Wood classification scheme has been proposed by Berlin and Stöffler (2004) to accommodate the R chondrite metamorphic variation present in the pyroxene, feldspar, and sulfides, especially the lack of low-Ca pyroxene in types 5 and 6:

Measurements of the coarser-grained feldspars present in higher petrologic types provide the most accurate ages; an Ar–Ar age of 4.53 (±0.01) b.y. has been determined for an equilibrated lithology of Rumuruti, which was age corrected for K decay bias to give an age of 4.56 b.y. (Trieloff et al., 2007; Buikin et al., 2006). A minimum range of ~170 m.y. in 39Ar1–40Ar ages was determined by Dixon et al. (2003) for various R chondrites. They contend that the younger ages of some clasts may be the result of an early impact and subsequent deep burial with slow cooling in a thick regolith or rubble-pile. Alternatively, a late impact could have reset the 39Ar–40Ar chronometer of those lithologies exhibiting younger ages, with shallow burial and rapid cooling occurring afterwards. This latter scenario is consistent with the very early age of breccia formation, the measured CRE ages, and the abundant solar rare gases characteristic of this group. It is equally plausible that the extremely brief metamorphic history (crystallization 4.56 b.y. ago), the pervasive brecciation, and the high abundance of solar-wind-implanted rare gases could reflect a collisional disruption and gravitational reassembly of the Rumuruti parent body soon after its formation (Buikin et al., 2007).

The DaG 013 meteorite is almost unshocked (S1) and was initially given a weathering grade of W4 consistent with a high degree of terrestrial weathering based on the weathering scale of Wlotzka, (1993). More recently, a more useful weathering index (wi) was developed by Rubin and Huber (2005) for those oxidized meteorite groups lacking significant FeNi-metal phases, such as the CK and R chondrite groups. This index is based on the modal abundance of brown-stained silicates as visually determined on a thin section in transmitted light at ~100× magnification. It is thought that the brown staining in R chondrites (and CK chondrites) is caused by the terrestrial decomposition and mobilization of sulfides (mainly pyrrhotite and pentlandite) which are typically prevalent in these meteorites; e.g., Rumuruti, designated wi-0, contains 8.0 wt% sulfides. It was determined that DaG 013 has a weathering index of wi-5, signifying severe weathering.

Dar al Gani 013 has a CRE age of 9.7 m.y. based on cosmogenic 21Ne, with an average age of 8.0 ±1.2 m.y. based on all cosmogenic nuclides studied (Schultz et al., 2005). Noble gas analyses of the known Northwest Africa R chondrites were conducted by Vogel et al. (2014). They have tentatively placed these numerous R chondrites into ~16 groupings representing possible common source craters and/or fall events. One of these groups comprises meteorites with a CRE age of ~10 m.y., and along with DaG 013 includes R chondrites with the NWA series designations 755, 845, 851, 978, 1471, 2198, and 5069.

R chondrites are a unique group of highly oxidized meteorites with a higher volume of olivine (FeO-rich), a lower volume of pyroxene, and essentially no FeNi-metal as compared to all other chondrite groups. It has been suggested that the oxidation of the R chondrites occurred as a result of persistent equilibration with the nebular gas (Kallemeyn et al., 1996). The oxidant was probably water, and the oxidation of the various R chondrite components likely occurred near the snow line of the protoplanetary disk where water ice was abundant (Rout et al., 2009). A comparison of refractory lithophile abundances among R chondrites, E chondrites, O chondrites, and C chondrites indicated that the R-chondrite parent body formed at a heliocentric distance greater than O chondrites and less than C chondrites (Khan et al., 2013). Pre-terrestrial hydrous phases identified by Jamsja et al. (2011) in R chondrite genomict breccias include laihunite, goethite, anhydrite, and jarosite. Features related to oxidation conditions in R chondrites are not related to the metamorphic temperatures, i.e., petrologic type (Isa et al., 2011, 2013).

Notably, rumurutiites have the highest Δ17O values of any meteorite group, reflecting the isotopic composition of the accreted water. During metasomatic oxidation, FeNi-metal and pyroxene reacted with water to form olivine, while the remaining metal became enriched in Ni to form awaruite (Isa et al., 2010). Rubin and Li (2019) studied the transformational history of magnetite in a number of meteorite groups. For R chondrites, they concluded that magnetite was formed through metasomatic oxidation of primary FeNi-metal, but that it was mostly transformed to fayalite and secondary kamacite during subsequent thermal metamorphism under reducing conditions. They found that a small percent (~5%) of unequilibrated R chondrites have preserved a nearly pure magnetite component (e.g., Clast A in PCA 91241, MET 01149), while a relative few equilibrated R chondrites contain Cr-stabilized magnetite (e.g., Dhofar 1514, LAP 04840, Sierra Gorda 010). In addition, a small number of equilibrated R chondrites have been reported to contain magnetite grains of an unresolved composition (e.g., JaH 955, NWA 11304, NWA 12333, NWA 12470, Y-984084).

The compositional makeup of known R chondrites reflects that of a regolith breccia to a greater degree (~48%) than any other asteroidal meteorite group. The R chondrites share certain similarities with H and L ordinary chondrites, such as refractory element depletions and siderophile element abundances, and they have close similarities in chondrule size, textural type, and O-isotopic compositions. It is considered that these chondrite groups likely formed in the same O-isotopic reservoir during a similar timeframe within the nebular midplane (Greenwood et al., 2000). On the other hand, the R chondrites differ from the ordinary chondrites in having higher matrix/chondrule ratios and higher abundances of volatile and refractory lithophile elements, and they exhibit differences in other petrographic trends as well. Of particular interest is the higher bulk-rock Δ17O of R chondrites compared to ordinary chondrites, with a resulting higher FeO content and lower FeNi-metal content. The triple increase in Zn relative to ordinary chondrites is consistent with formation in a lower temperature, more oxidizing environment—between that of ordinary chondrites and carbonaceous chondrites. Rumuruti chondrites are enriched in high-temperature early nebular condensates still rich in refractory HREE, contrary to the later formed CV-group chondrites that contain a larger component of LREE (Khan et al., 2014).

The difference in the O-isotopic abundances is greater between the R chondrites and ordinary chondrites than it is among the H, L, and LL ordinary chondrite groups, further resolving this group from the ordinary chondrite groups (Weber et al., 1997). Similar to carbonaceous chondrites, R chondrites have a high olivine content within a high proportion of matrix, reflecting their highly oxidized nature, indicating that they formed at a large heliocentric distance—perhaps even close to that of the least oxidized carbonaceous chondrites. Rumuruti chondrites have a higher Δ17O value than that of any other Solar System material, and therefore do not fall on the same O-isotope trend line as ordinary chondrites (Weisberg, et al., 1991). Oxygen-isotopic variations among equilibrated clasts suggest that some R chondrite material had higher Δ17O values than those determined from Rumuruti bulk rock (Δ17O = +2.9‰) (Greenwood et al., 2000). A study of unequilibrated R chondrite components was conducted by Isa et al. (2012). They revealed that olivines in R chondrite chondrules comprise two broad O-isotopic compositional groups: the first with low Δ17O values of ~–4‰ plotting on the CCAM line, and the second with high Δ17O values of up to ~3‰ plotting along the TFL. When compared to the highest values found in OC chondrules (up to ~1.6‰), the even higher values obtained for R chondrites suggest that the R chondrite precursor material had higher Δ17O values, attesting to differences in their respective formation environments. It has been argued by some that R chondrites and ordinary chondrites accreted from similar nebular components, but that R chondrites accumulated a higher abundance of the high-Δ17O oxidant water residing in the fine-grained matrix olivine component. The proportion of oxygen retained from this water was calculated to be ~23%.

Modeling techniques and laboratory melting experiments (e.g., Gardner-Vandy and Lauretta, 2011; Gardner-Vandy et al., 2014; Sosa et al., 2017; Lunning et al., 2017) have demonstrated that an FeO-rich (oxidized) R chondrite-like precursor asteroid can undergo low-degree partial melting (14–22%) at 1120–1140°C at oxygen fugacity IW–IW+1 to produce a brachinite-like residue and a complementary evolved melt. Melt clasts with a basaltic andesite composition have been identified in DaG 013 and in the R chondrites PCA 91241 and LAP 031275 by Lunning et al. (2018, 2019); the melt clast in LAP 031275 was determined to be an L chondrite impact melt. These melt clasts are alkali-depleted analogs of lithologies attributed to SiO2-rich crustal components of the ureilite (ALM-A, MS-MU-011/035) and brachinite (GRA 06128/9) parent bodies which formed by disequilibrium fractional partial melting. However, results of further investigation by Lunning et al. (2020) have allowed to distinguish between disequilibrium fractional partial melting through radiogenic heating as in the SiO2-rich achondrites, and disequilibrium fractional crystallization after impact melting as in the R chondrite melt clasts.

A component of primitive, highly refractory forsterites which are Ca-rich and FeO-poor have been found in DaG 013. These forsterites have similar O-isotope ratios (16O-enriched) and chemical compositions to those found in some ordinary and carbonaceous chondrites. It was proposed by Pack et al. (2004) that these were formed as open-system condensates in a nebula setting, and that this entire suite of forsterites might have originated in a single reservoir where the dust/gas ratio was ~4–5 × the solar ratio. It is thought that they formed early in Solar System history during a period intermediate between the formation of CAIs and the onset of chondrule formation. In DaG 013, a small number of rare FeNi-metal grains were identified in forsterites of type-3 clasts. Notably, the R4-anom chondrite Sierra Gorda 010 (with similarities to LAP 031156) contains an unusually high abundance of magnetite (~4 wt%) that is enriched in the refractory oxides of Cr, Ti, and Al (M. Ivanova, MetBull 108).

A study of twenty R chondrites, including DaG 013, was undertaken to identify remnant CAIs and Al- and spinel-rich objects; 101 CAIs were eventually identified (Rout and Bischoff, 2007, 2008). The CAIs that were identified contain highly altered phases, some composed of secondary alteration products such as nepheline and sodalite thought to have formed in a nebular setting. High abundances of oxides are present. Some refractory inclusions are composed of concentric spinel with Al-rich diopside rims, while other concentric spinel-rich inclusions contain abundant fassaite, hibonite, or alteration products rich in Na, K, Cl, and Al (probably nepheline and sodalite). The most abundant CAI type found in R chondrites is an irregular-shaped, complex spinel-rich inclusion, possibly associated with hibonite, plagioclase, and fassaite, along with secondary alteration products such as oligoclase that typically has diopside rims and a high Al content. Some of the least altered, type 3 lithologies contain concentric fassaite-rich CAIs that include rare perovskite along with its secondary transformation product ilmenite.

Similarities were observed between these CAIs from R chondrites and those found in CO and CM chondrites, as well as to those in the ordinary and E chondrite groups. Based on Δ17O values, the CAIs in R chondrites were divided into 16O-rich (~ –23‰ to –26‰), 16O-depleted (~ –2‰), and heterogeneous (–25‰ to +5‰) (Rout et al., 2009). In a like manner to CAIs of other chondrite groups, R chondrite CAIs were likely formed in an 16O-rich nebular region, with some sustaining subsequent isotopic exchange with an 16O-depleted nebular gas or through metasomatism on the parent asteroid. In contrast to CAIs from other chondrite groups, no melilite or grossite was observed.

Comparative analyses of the known asteroid types and a suite of ungrouped and rare meteorites in multiple forms (bulk, powder, polished section), including the R chondrites Awsserd (R4) and NWA 12472 (R3), were conducted by Krämer Ruggiu et al. (2021) utilizing petrographic, spectroscopic, and albedo data. They concluded that these meteorites are best matched to the K-type asteroids (see diagram below).

This unique chondrite group was originally named for the Carlisle Lakes, Australia (49.5 g) specimen but was subsequently renamed for the only fall of the group—a 67 g stone from Rumuruti, Kenya (see photo below). There have recently been numerous new R chondrite finds in the hot and cold deserts of Africa, Australia, and Antarctica. The specimen of DaG 013 shown above is a 0.232 g cut fragment. Compare the effects of terrestrial weathering on DaG 013 (wi-5) to the fresh fall of the type specimen Rumuruti (wi-0).