A single moderately weathered stone weighing 587 g was found in Algeria and sold in Tucson in January 2009 to M. Farmer and J. Strope. After the stone was cut into thin slices on a wire-saw (A. Karl), leaving ~450 g after accounting for the cutting loss, samples were sent to several investigators (T. Bunch and J. Wittke, NAU; M. Weisberg, KCCU) for study and classification. A preliminary classification was given for NWA 5492 as an anomalous member of the CB chondrite group.
Although textural similarities exist with CH and CB chondrites, several differences distinguish NWA 5492 from them. As is typical for the CB group, NWA 5492 is a breccia composed of type-I chondrules (e.g., PP, PO, BO, CC, RP, POP, Al-rich), lithic clasts (e.g., chondrule-rich, impact-melt, granular fine-grained), FeNi-metal nodules (isolated and in clusters), and abundant sulfides (troilite, daubreelite). As with CH chondrites, NWA 5492 lacks matrix material, has depletions in volatiles, and contains abundant FeNi-metal (22.6 vol%, comparable to CH chondrites). However, NWA 5492 does not contain the refractory element enrichments seen in CH chondrites (Friend et al., 2011; Weisberg et al., 2011, 2012). Importantly, the chondrule textural types present in NWA 5492 and the ungrouped metal-rich chondrite GRO 95551 are significantly different from those of ordinary chondrites (Weisberg et al., 2015). The lack of matrix material in NWA 5492 and GRO 95551 is not well understood, and could be from a different cause than in CH and CB chondrites, but it might be attributed to rapid accretion of chondrules in a very high temperature environment possibly associated with low amounts of ambient dust.
Silicates in NWA 5492 are much more highly reduced than those in E, CH, and CB chondrites, and they are primarily composed of virtually pure enstatite. Some chondrule silicates in NWA 5492 and GRO 95551 contain sub-µm- to µm-sized metal blebs, which also attest to highly reducing conditions during formation (Weiberg et al., 2015). The olivine abundance in NWA 5492 is similar to that of E chondrites (5.9 vol%), but the olivine in NWA 5492 is more highly magnesian (Fa0.10.7) than in E chondrites or in any other metal-rich chondrite group; olivine in GRO 9551 is similarly highly magnesian (ave. Fa1.3).
In comparison to E chondrites, and to all other chondrite groups, significant differences in elemental ratios and elemental abundances are observed. Moreover, the metal in NWA 5492 and GRO 9551 is unlike that in E, CH, and CB chondrites in that it contains very low Si; in that respect it is more similar to H-chondrite metal (Humayun and Weisberg, 2012; Weiberg et al., 2015). Notably, this unique breccia contains no CAIs or AOAs (although rare Al-rich chondrules have been reported by Weisberg et al., 2015), and it contains both a higher abundance of sulfides and a higher Na content in the plagioclase-rich mesostasis than is found in CH or CB chondrites. By comparison, the high abundance and variability of sulfides present in E chondrites is not observed in NWA 5492.
An O-isotopic composition was determined for NWA 5492 (D. Rumble, CIW), and its position on a three-isotope plot is unique among chondritic meteorites; sample clasts plot within two distinct reservoirs. The main plot (a), based on a sample of smaller-sized reduced chondrules and fragments that likely represent the parent body, lies in an unoccupied region above the TFL, just below the OC field and just above the EC field (Δ17O = 0.43; NWA 5492a/CR,CH plot). The other less prevalent O-isotopic plot (b), which is the result of analyzing certain barred chondrule clasts, lies along the CR trend line (Δ17O = -2.28; NWA 5492b/CR,CH plot). This plot (b) might represent impacts of diverse objects, possibly related to the CR Clan, on the NWA 5492 parent body. Notably, plot (a) lies near, or overlaps that determined for chondrules in GRO 95551, which supports the formation of both meteorites in a similar oxygen reservoir. Further O-isotopic analyses of NWA 5492 and GRO 95111 conducted by Weisberg et al. (2015) demonstrates that some overlap exists with O and E chondrites.
Diagram credit: Weisberg et al., 42nd LPSC, #1198 (2011)
Weisberg et al. (2015) observe that the overlap that exists in O-isotope values between the chondrules from NWA 5492 and GRO 95551, and those from O, E, and R chondrites, is indicative of a close relationship among them. They suggest that the chondrules (or their precursor material) which constitute these disparate meteorites likely formed in a similar nebular environment and/or experienced similar petrogenetic processes. Warren (2011) determined that the isotope signatures of Δ17O, ε54Cr, ε50Ti, and ε62Ni can be utilized to resolve carbonaceous from noncarbonaceous meteorites; the carbonaceous meteorites have positive values for all of these elements, while the noncarbonaceous meteorites have negative values. An example coupled Δ17O vs. 54Cr diagram is shown below to demonstrate the separation between carbonaceous and noncarbonaceous meteorites. It can be seen that the 'G chondrite' GRO 5551, and by inference NWA 5492, plot in the noncarbonaceous field.
Diagram credit: P. Warren, GCA, vol. 75, p. 6916 (2011)
'Stable isotopes and the noncarbonaceous derivation of ureilites, in common with nearly all differentiated planetary materials'
The HfW age for NWA 5492 is very old, falling within 0.7 m.y. of CAI formation, which possibly dates the stage of nebular metalsilicate condensation (Friend et al., 2011). It is proposed that NWA 5492 represents a new chondrite type having close affinities (e.g., chemical, textural, isotopic, metal composition) to GRO 95551. Both meteorites share elemental abundance ratios that may have some relationship to E and H chondrites, but which plot away from known chondrite groups. Evidence favors a nebular condensate origin (vs. impact origin) for the majority of the components in NWA 5492 (Weisberg et al., 2011); it is composed of the type of material that is believed to have accreted to form the primitive Earth.
Compared to the highly reduced, metal-rich, low-FeO group of chondrites designated 'HH chondrites', silicates in NWA 5492 and GRO 95551 are much more reduced and have a lower Ni content in metal. Such extensive reduction features preclude them from being the precursor material of the HH chondrites or silicates in IIE irons (Humayun and Weisberg, 2012; Weisberg et al., 2015). The CRE age of NWA 5492 is ~8 m.y. (Friend et al., 2011).
It has been demonstrated that a plot of two meteorites on a coupled Δ17O vs. ε54Cr diagram is one of the best diagnostic tools for determining possible genetic relationships between them (Sanborn et al., 2014). High-precision Cr isotopic analyses (ε53Cr and ε54Cr) of both NWA 5492 and GRO 95551 were conducted by Sanborn et al. (2015). The results show that NWA 5492 and GRO 95551 are indistinguishable in their Cr-isotopic compositions, consistent with their similarity in O-isotopic compositions previously determined. Furthermore, although the ε54Cr values show that these two meteorites are not associated with any of the carbonaceous chondrite groups, only through the combined Δ17O vs. ε54Cr plot is it evident that NWA 5492 and GRO 95551 are also not genetically related to the E chondrite groups.
Data obtained thus far indicate that these two meteorites derive from a similar unique isotopic reservoir, but further research might show that they actually share a common parent body. It was recommended by Weisberg et al. (2015) that these two meteorites be termed 'G chondrites' after the first described type specimen GRO 95551. The specimen of NWA 5492 shown above is a 2.15 g thin partial slice, ~1 mm in thickness. The small vesicles that are visible in the magnified image are attributed to degassing of hydrated clasts during impacts onto the parent body (Perron et al., 2008). The photos below show the high free metal component in this meteorite specimen and in a full slice curated by the Chicago Field Museum, as well as a close-up image from Michael Farmer.