Petrological, petrofabric, and oxygen isotopic study of five ungrouped meteorites related to brachinites

Abstract

Northwest Africa (NWA) 6112, Miller Range (MIL) 090206 (plus its pairs: MIL 090340 and MIL 090405), and Divnoe are olivine‐rich ungrouped achondrites. We investigated and compared their petrography, mineralogy, and olivine fabrics. We additionally measured the oxygen isotopic compositions of NWA 6112. They show similar petrography, mineralogy, and oxygen isotopic compositions and we concluded that these five meteorites are brachinite clan meteorites. We found that NWA 6112 and Divnoe had a c axis concentration pattern of olivine fabrics using electron backscattered diffraction (EBSD). NWA 6112 and Divnoe are suggested to have been exposed to magmatic melt flows during their crystallization on their parent body. On the other hand, the three MIL meteorites have b axis concentration patterns of olivine fabrics. This indicates that the three MIL meteorites may be cumulates where compaction of olivine grains was dominant. Alternatively, they formed as residues and were exposed to olivine compaction. The presence of two different olivine fabric patterns implies that the parent body(s) of brachinite clan meteorites experienced diverse igneous processes.

Al Huwaysah 010: The most reduced brachinite, so far

“…The mineralogical assemblage occurring in Al Huwaysah 010 is a widespread fine-grained assemblage where opaque phases are associated with Ca-poor orthopyroxene. This texture is similar to that observed in Divnoe, NWA 6112, MIL 090206, MIL 090304, MIL 090405, and other brachinite/brachinite-like meteorites, where opaque phases occurring in the fine-grained assemblage along with orthopyroxene are mainly represented by Fe-sulfide and Fe-Ni metal (Goodrich et al, 2011(Goodrich et al, , 2017Hasegawa et al, 2019;Irving et al, 2013;Rumble III et al, 2008). Moreover, in Al Huwaysah 010, almost all olivine-olivine, olivine-augite, and olivine-chromite grain boundaries are lined with such fine-grained assemblages and they also occur as patches within olivine (Figure 2) and even in olivine chadacrysts enclosed in Ca-rich clinopyroxene (Figure 1b).…”

“…Olivine in Al Huwaysah 010 (Table 1) has a higher amount of Mg (Fo 83.3 ) compared to all the known brachinites, where the composition usually ranges from Fo 64 to Fo 70 . These differences, however, become less pronounced when specific brachinites (or brachinite‐like achondrites) are considered where olivine ranges from Fo 72 to Fo 80 (Crossley et al., 2020; Goodrich et al., 2011; Greenwood et al., 2017; Hasegawa et al., 2019; Irving et al., 2013). A clear difference may also be observed between the value of molar Fe/Mn (29.4) in olivine of Al Huwaysah 010 and those of other brachinites (52–77).…”

“…As for the detailed processes of the petrogenesis of brachinites, there is currently no overall consensus among the scientific community. Even the petrofabric seems to show differences if distinct meteorites are considered (Hasegawa et al., 2019) implying that the parent body(s) of the brachinite group of meteorites experienced diverse igneous processes. Nevertheless, the group of brachinites is still poorly constrained.…”

Heterogeneous accretion of Earth inferred from Mo-Ru isotope systematics

“…Three of the samples are classified as ungrouped achondrites (NWA 6112, NWA 5363, NWA 1058), but their chemical and petrologic properties suggest they may be linked to brachinites. Accordingly, these meteorites are sometimes also classified as 'brachinite-like' meteorites (Day et al, 2012;Hasegawa et al, 2019).…”

“…for samples with N>3. c NWA 6112 is petrologically similar to brachinites, however, based on the Fa composition slightly outside the range of brachinite and hence classified as ungrouped achondrite. Recently,Hasegawa et al (2019) re-investigated oxygen isotopes, petrography, mineralogy, and olivine fabric of NWA 6112 and concluded that the sample belongs to the brachinite clan meteorites.…”