A single meteorite weighing 1,096 g was purchased in Morocco by T. Jakubowski and M. Cimala of PolandMet. The stone was analyzed at the University of Washington in Seattle (A. Irving) and NWA 7317 was classified as a recrystallized, texturally evolved CR6 chondrite likely paired with NWA 2994 (4,756 g), NWA 3250 (916 g), NWA 6901 (1,197 g), NWA 6921 (1,749 g), and NWA 8548 (244.8 g) (T. Bunch and J. Wittke, NAU; A. Irving, UWS; C. Agee, UNM). See also a photo of two whole stones exhibited in the Encyclopedia of Meteorites by Marcin Cimala. More recently, the ungrouped achondrite NWA 11112 (528 g, photo courtesy of Kai Ke) and the primitive achondrite NWA 12869 (4,300 g) have been analyzed and are presumed to be related to this pairing group. As with NWA 3100 (CR6), NWA 7317 and pairings contain evidence of relict barred chondrules in thin section, logically disqualifying it as a primitive achondrite and not meeting the requisite advanced degree of thermal metamorphism to be termed a type 7 or metachondrite.
This meteorite has been very weakly shocked (S2) and has experienced very minor terrestrial weathering (W0/1). The oxygen isotopic composition for NWA 7317 was determined at the Carnegie Insitution in Washington DC (D. Rumble, III; CR chondrite comparison plot), while that of NWA 3250 CR chondrite comparison plot) was previously determined at the Open University, UK (I. Franchi and R. Greenwood; Δ17O = 1.72). Including the plot for NWA 3100 (University of Western Ontario; T. Larson and F. Longstaffe), all of these meteorites fall within the field of the CR chondrites. In addition, the meteorites Tafassasset and LEW 88763 are also geochemically and isotopically consistent with a CR-like parent body and have experienced a high degree of thermal metamorphism and recrystallization. Since relict chondrules have also been reported in samples from each of these texurally-evolved meteorites, and with NomCom presently lacking a type 8 category, they would perhaps be more appropriately designated CR6. However, if the metamorphic continuum were to include type 8 as a completely recrystallized end point as proposed by Irving et al. (2019 abstract), then a type 7 designation for all of these meteorites would be appropriate.
A cooperative study was undertaken of a number of previously ungrouped achondrites, primitive achondrites, and silicated irons which have O-isotopic compositions that plot along the CR oxygen isotope trend line (Bunch et al., 2005Northern Arizona University, University of Washington, and University of Western Ontario). From the meteorites that were studied, including NWA 3100, NWA 801, Tafassasset, NWA 011 pairing group, LEW 88763, Sombrerete, and NWA 468, it was determined that some or all of them may have originated in the core, mantle, crust, and chondritic regolith of a large, at least partially differentiated CR-type parent body that was subsequently collisionally disaggregated. Compared to all other meteorite groups, both CR chondrites and IIC irons have significant δ183W excesses and elevated δ15N as well as similar Mo isotope systematics, and therefore a genetic link is inferred (Kruijer et al. (2017; Budde et al., 2018).
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Diagram credit: Bunch et al., 36th LPSC, #2308 (2005)
Continued research on this front has been ongoing (e.g., Bunch et al., 2005; Floss et al., 2005, [MAPS vol. 40, #3]; Irving et al., 2014 [#2465]; Sanborn et al., 2014 [#2032]). As provided in the Sanborn et al. (2014) abstract, a coupled Δ17O vs. ε54Cr diagram is one of the best diagnostic tools for determining genetic relationships among meteorites. Moreover, Sanborn et al. (2015) demonstrated that ε54Cr values are not affected by aqueous alteration. The diagrams below include the NWA 7317 pairings NWA 6901, 6921, and NWA 2994, and it is apparent that they plot within the CR chondrite field.
Diagram credit: Sanborn et al., 45th LPSC, #2032 (2014)
17O vs. ε54Cr and ε50Ti for CR Carbonaceous Achondrites
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Diagrams credit: Sanborn et al., GCA, vol. 245, pp. 577596 (2019)
'Carbonaceous Achondrites Northwest Africa 6704/6693: Milestones for Early Solar System Chronology and Genealogy' (https://doi.org/10.1016/j.gca.2018.10.004)
However, results of a study of the paired meteorite NWA 6901 conducted by J. Zipfel (2014, #5346) led to a different conclusion. He determined that despite having similar oxygen and chromium isotopic values, this meteorite has a major element composition that is inconsistent with a derivation from a CR-like source. He suggests that the infiltration of a trace element-rich melt phase similar to that of the phosphates present in the ungrouped achondrite NWA 011 (possibly CR-related) could explain the trace element abundance pattern of NWA 6901. These two meteorites also plot near each other on a coupled Δ17O vs. ε54Cr diagram (see above), and therefore he proposes that these meteorites might be related.
A noble gas study was conducted by Miura and Okazaki (2019) for the paired NWA 3250. They determined an average CRE age of 34 m.y. based on 21Ne, with the assumption that heavy shielding conditions existed in the meteoroid. In addition, utilizing PuXe chronometry for NWA 3250, they calculated an absolute crystallization age relative to Angra dos Reis (4.5578 b.y.) that is 107 (±24) m.y. older.
The specimen of NWA 7317 shown above and in the top photo below is a 3.07 g partial slice. The photos at the bottom show the main mass and two petrographic thin section micrographs of NWA 7317 shown courtesy of Peter Marmet. The thin section on the right contains a rare relict chondrule near the top of the image.
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Photo courtesy of Tomasz Jakubowski
click on photos for a magnified view
Photos courtesy of Peter Marmet