Three fragments weighing together 722 g were purchased in Erfoud, Morocco. Classification was performed at UCLA, and Northwest Africa 978 was determined to be an R3.8 chondrite. Although it was initially assigned a relatively low weathering grade of W2, a more useful weathering index (wi) has since been 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 this meteorite group; e.g., Rumuruti wi-0 contains 8.0 wt% sulfides. Northwest Africa 978 was determined to have a weathering index of wi-5, or severely weathered. This meteorite has been weakly shocked (S3).
In their noble gas studies, Schultz et al. (2005) suggested pairing several R chondrite finds of similar petrologic type with NWA 978 (21Ne-based CRE age of 10.8 (±0.4) m.y.), including NWA 755, NWA 845, NWA 851, and NWA 1471. Subsequent noble gas analyses of the known Northwest Africa R chondrites were conducted by Vogel et al. (2011, 2014). Their ~16 groupings vary slightly from the previous pairings suggested by other investigators, and they propose an ~10 m.y. CRE age group representing possible common source craters and/or fall events which includes R chondrites with the NWA series designations 755, 845, 851, 978, 1471, 2198, and 5069, and possibly DaG 013.
This is a unique group of chondrites having a higher volume of olivine (FeO-rich), a lower volume of pyroxene, and essentially no FeNi-metal as compared to all other chondrite groups. This is thought to be the result of metasomatic oxidation in which FeNi-metal and pyroxene reacted with water to form olivine (Isa et al., 2010). While R chondrites share certain similarities with ordinary chondrites including refractory element depletions and siderophile element abundances, they differ in volatile element abundances and petrologic trends. The triple increase in Zn relative to ordinary chondrites and the sparsity of metal, or lack thereof, observed in some R chondrites, indicates that this group experienced highly oxidizing conditions both in the nebula and on the parent asteroid.
Parent body metamorphism in an oxidizing, water-rich environment is attested by the hydroxyl-rich minerals amphibole, phlogopite, and apatite present in the R6 chondrites LAP 04840 and MIL 11207. It is considered that these chondrites experienced metamorphism of insoluble organic matter at high temperatures (~720°C) and at significant depth (tens of km) within a lithologic unit in which water with a high D/H ratio was pervasive. This deep burial is considered to be the result of reassembly following impact disruption on the R chondrite parent body (McCanta et al., 2006, 2008). In a study of these two hydroxyl-bearing R chondrites, Gross et al. (2017) reasoned that such high abundances of hydrous phases could only be established under conditions of high water vapor pressure (20700 bar), and such pressures could only be maintained on this relatively small body through the emplacement of a solid ice shield.
The difference in O-isotopic abundances between the R chondrites and the ordinary chondrites is greater than it is among the H, L, and LL ordinary chondrite groups, further resolving the R 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. Because R chondrites have the highest 17O value of any other Solar System material, they plot on a different O-isotope trend line than the other chondrites. This plot is farther from the carbonaceous and enstatite chondrites than it is from the ordinary chondrites (Rubin and Kallemeyn, 1989). Rumuruti chondrites of type 3 contain noble metals such as platinum, osmium, and germanium that originated as nebular condensates. In higher metamorphic types, the volatile germanium phases are decomposed, while other noble metal phases are transformed into mostly arsenides and tellurides.
In addition to NWA 978 (R3.8), several other unbrecciated R chondrites have been recovered so far, including the following: Sah 99527 and NWA 1668 (both R5); HaH 119, Ouzina, NWA 053, NWA 800, and Sah 98248 (all R4); NWA 753 (R3.9); Carlisle Lakes (R3.8); NWA 755 (R3.7); and Acfer 217 (R3.85). The photo above shows a 3.5 g slice of NWA 978, while the top photo below shows the fresh fusion crust on the 361.6 g main mass. The bottom image is an excellent petrographic thin section micrograph of NWA 978, shown courtesy of Peter Marmet.