NORTHWEST AFRICA 10503

A single 407 g meteorite was found in the Sahara Desert, possibly near Tan Tan, Morocco, and subsequently purchased by F. Kuntz in Zagora, Morocco on behalf of Planetary Studies Foundation in Galena, Illinois. An analysis was conducted at the University of Washington in Seattle (A. Irving and S. Kuehner), and NWA 10503 was determined to be a texturally-evolved meteorite exhibiting ~120° triple junctions with no relict chondrules visible. The elevated silicate FeO/MnO ratios are higher than those for ordinary chondrites and are consistent with a carbonaceous chondrite classification. In May 2016, a lot of smaller stones having a combined weight of 215 g was purchased by B. Hoefnagels. This group of stones was designated NWA 10859, and results of petrographic and isotopic analyses (A. Irving and S. Kuehner, UWS; K. Ziegler, UNM) led to the determination that they are paired with NWA 10503.

On an oxygen three-isotope diagram the values plot away from all other analyzed meteorites and along an extension of the trend line for the ungrouped pallasite Milton (K. Ziegler, UNM; see diagram below). NWA 10503 was initially classified as an ungrouped carbonaceous metachondrite that might be related to the Milton pallasite.

In an effort to better resolve potential genetic relationships that might exist among these meteorites, a Cr-isotopic analysis was conducted by Sanborn et al. (2018) for NWA 10503 as well as for olivine from the Milton pallasite. It is demonstrated on a coupled Δ17O vs. ε54Cr diagram (shown below) that both meteorites plot among the CV clan and plausibly share a genetic relationship.

In an effort to further resolve differences between the CV and CK chondrite groups, Yin and Sanborn (2019) analyzed Cr isotopes in a significant number and broad range of meteorites. Their study included samples from each of the three CV subgroups (oxA, oxB, Red), anomalous CV3 chondrites, a C3-ungrouped, several CK members, and other potential CV-related meteorites including NWA 10503 and Milton (see top diagram below). It is demonstrated that the CV and CK meteorites are clearly resolved into two distinct isotopic reservoirs. In addition, it is shown by the ε54Cr values that NWA 10503 plots among the CV-related meteorites. A coupled Δ17O vs. ε54Cr diagram plotting all of the meteorites in the study is shown in the bottom diagram below.

In a study of two newly recovered ungrouped carbonaceous meteorites, the unequilibrated chondrite NWA 11961 and the dunitic breccia NWA 12264, Irving et al. (2019) further populated the O-isotopic trend line previously defined by NWA 10503 and the Milton pallasite; they termed this the 'CX' trend. However, Cr isotope data obtained for all of these meteorites have resolved both NWA 11961 and NWA 12264 as potential new carbonaceous parent bodies distinct from that of NWA 10503 and Milton, the latter previously considered possible members of the CV-clan (see diagrams below).

Another ungrouped metachondrite with affinities to carbonaceous chondrites is NWA 2788. This meteorite has a metamorphic texture exhibiting ~120° triple junctions, elevated Fe/Mn and Ca/Na ratios, and an O-isotopic composition that plots very near to the TFL. Bunch et al. (2006) suggest that if a chondrule-bearing representative of this parent body is found and identified in the future it should be termed a 'CT chondrite' (see NWA 2788 photos, abstracts [1, 2], and isotopic plot).

Northwest Africa 10503 is a somewhat friable meteorite with features indicating a low degree of terrestrial weathering and a low shock stage. Two views of a 3.79 g fragment of NWA 10503 are shown above. In the top photo below are some of the larger stones representing the paired NWA 10859, while two stones of NWA 10503 with green-colored fusion crust are shown in the bottom photo.