Primitive achondrites are a remarkable family of meteorites, marking the transition between unmolten chondrites and fully differentiated achondrites (Fig. 1). As such, these meteorites sampled partially melted planetesimals, formed only a few million years (My) after the formation of the first solids in the Solar System. Primitive achondrites have coarse-grained igneous achondritic textures, resulting from extensive thermal metamorphism, while retaining some chondritic features such as relict chondrules and chondritic bulk rock compositions (Krot et al. 2014). The peculiar nature of primitive achondrites constrains their parent body petrogenesis, which is yet to be fully understood. Primitive achondrites are subdivided into different groups on the basis of their petrologies and chemical and isotopic compositions, including the acapulcoites–lodranites, winonaites, brachinites, and ureilites (Krot et al. 2014). While brachinites and ureilites have very distinctive petro-logic characteristics, the boundary between winonaites and acapulcoites is not so obvious. In fact, the similarities between winonaites and acapulcoites may have been hiding an entirely new class of primitive achondrites (Stephant et al. 2023).
ACAPULCOITES OR WINONAITES: THE DIFFICULTY OF CLASSIFICATION
Acapulcoites and winonaites are both relatively fine-grained rocks with achondritic textures that mainly consist of olivine, low-Ca and Ca-rich pyroxene, plagioclase, metallic Fe–Ni, troilite, chromite, and phosphates (Kimura et al. 1992; Keil and McCoy 2018). The main geochemical criteria commonly used to discriminate winonaites and acapulcoites are bulk elemental compositions, relative abundances of mineral components, chemical compositions of pyroxene and chromite, and the iron contents of olivine (also known as fayalite content or Fa) and low-Ca pyroxene (also known as ferrosilite content or Fs) (cf., Fig. 2; Stephant et al. 2023). However, some of these parameters were set about 30 years ago, when the first specimens of both acapulcoites and winonaites were discovered. With the increase of meteorite finds further populating these two groups, previously strong distinguishing criteria have become blurry, with significant overlap between winonaite and acapulcoite compositions (cf., Fig. 2; Keil and McCoy 2018). As a result, numerous meteorites were initially classified as acapulcoites or ungrouped achondrites before ultimately being reclassified as winonaites on the basis of their oxygen isotopic compositions, often argued to be the only trustworthy criteria—or their cosmic exposure ages, which do not match the acapulcoite restricted cluster of 4–7 My. Such somewhat regular misclassifications highlight the compositional ambiguity existing between winonaites and acapulcoites. One such ambiguous meteorite is Northwest Africa (NWA) 090, a meteorite found in Morocco in 2000 whose recent study yielded surprising results.
NORTHWEST AFRICA (NWA) 090: THE KEY TO UNVEILING A METEORITE GROUP HIDING IN PLAIN SIGHT
NWA 090 (Fig. 1), a meteorite initially classified as an acapulcoite, presented similar petrological, mineralogical, and geochemical characteristics as seven other primitive achondrites (i.e., Dhofar 1222, NWA 725, NWA 1052, NWA 1054, NWA 1058, NWA 1463, and NWA 8614). The intriguing aspect of these samples is that five of their classifications have been changed from acapulcoites or ungrouped achondrites to winonaites, highlighting the peculiar and ambiguous nature of these primitive achondrite samples. Moreover, these eight meteorites are primitive in nature, as evidenced by the presence of relict chondrules, low metamorphic temperatures, and high abundances of opaque phases. These features led Irving and Rumble (2006) to suggest some of these samples could represent the chondritic precursor of winonaites. Interestingly, the modal abundances and major mineral compositions of NWA 090 and the seven other samples transition between acapulcoites and winonaites. While the modal abundances of olivine and opaque phases are most similar to winonaites, their pyroxene, olivine, and chromite compositions seem closer to acapulcoites. This ambiguity advocates for a strong need to revisit the petrological, mineralogical, and geochemical criteria that distinguish acapulcoites from winonaites. Stephant et al. (2023) propose new petrological and chemical criteria (i.e., combined olivine iron content, FeO/MnO ratio, and potassium content of plagioclase) to distinguish acapulcoites and winonaites (cf., Fig. 3). Remarkably, not only does this new classification scheme allow us to separate acapulcoites and winonaites, but it also provides evidence that this group of eight transitioning primitive achondrites are actually their own, previously unrecognized group of primitive achondrites.
TISSEMOUMINITES: A NEW PRIMITIVE ACHONDRITE GROUP SOURCING A DISTINCT PARENT BODY?
The oxygen isotopic compositional differences between different meteorite groups are a good proxy to assess the distinct provenance of planetary materials (e.g., Greenwood et al. 2017). Remarkably, the oxygen isotope compositions of these eight samples are resolvable from those of both acapulcoites and winonaites (cf., Fig. 4). Although closely related to winonaites, this new group of eight specimens likely originate from a parent body distinct from those of both winonaites and acapulcoites. This discovery is further supported by distinct molybdenum isotopic compositions between winonaites and one meteorite in this peculiar primitive achondrite group, NWA 725 (Worsham et al. 2017). As a result, Stephant et al. (2023) propose to name this new group of eight primitive achondrite “tissemouminites,” in reference to the village of Tissemoumine (Morocco) where the first sample of this group to be officially classified, NWA 725, was found.
In addition to the discovery of a new group of primitive achondrites, our work shows that the redefinition of petrological, mineralogical, and geochemical classification criteria for primitive achondrites can be used by the community for meteorite classification without the absolute necessity for oxygen isotope measurements. Ultimately, the differing petrogenetic features recorded by winonaites and tissemouminites will enable us to learn more about the evolution of bodies in the early Solar System. Thermal modeling of tissemouminite and winonaite parent bodies, which potentially formed from a common reservoir of material in the nebula and then evolved on different paths, will enhance our understanding of the diversity of planetesimals present in the early Solar System and how they partially melted and evolved.
