NORTHWEST AFRICA 7317


CR6
(possibly NWA 011-related)

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Purchased June 2012
no coordinates recorded

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 was classified as a recrystallized, texturally evolved CR6 chondrite paired to NWA 2994 [4,756 g], NWA 3250 [916 g], NWA 6901 [1,197 g], and NWA 6921 [1,749 g] (T. Bunch and J. Wittke, NAU; A. Irving, UWS). At least one other stone presumed to belong with this pairing group remains to be studied. This pairing group constitutes one of only a few rare CR6 chondrites to be classified. See a photo of two whole stones exhibited in the 'Encyclopedia of Meteorites' by Marcin Cimala. As with NWA 3100 and NWA 2994, NWA 7317 contains evidence of relict barred chondrules in thin section, logically disqualifying it as a "primitive" achondrite, and not meeting the requisite advanced 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), while that of NWA 3250 (see CR chondrite comparison plot) was previously determined at the Open University, UK (I. Franchi and R. Greenwood; Δ17O = –1.72). Including the plot from NWA 3100 (University of Western Ontario; T. Larson and F. Longstaffe), they all fall within the field for CR chondrites. The anomalous achondrites Tafassasset and LEW 88763 are also geochemically and isotopically related to the CR parent body, but they have experienced a higher degree of thermal metamorphism and recrystallization. Since relict chondrules have now been reported in the texurally-evolved Tafassasset (as well as in LEW 88763), it would perhaps be more appropriately termed CR6 as well.

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., 2005—Northern 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, have elevated δ15N, and share 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 diagram below includes NWA 6901, which is paired with NWA 7317, and it is apparent that it plots very near the CR chondrite field.

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Diagram credit: Sanborn et al., 45th LPSC, #2032 (2014)

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.

The specimen of NWA 7317 shown above and in the top photo below is a 3.07 g partial slice. The excellent photos at the bottom show the main mass and a petrographic thin section micrograph of NWA 7317.

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Photo courtesy of Tomasz Jakubowski

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Photo courtesy of Peter Marmet