Multispectral analyses techniques on X-Ray data in meteorite research

The investigations for meteorite characterization and classification often involve the use of SEM-X-Ray Maps to obtain the modal composition. To extract the major amount of information from the different elemental maps related to the whole thin section area, usually one approach is the multispectral X-Ray map data analyses. In this approach, the X-Ray maps of 18 elements are stacked in a single file resulting in a multispectral cube in which each pixel is associated to a “spectrum” of the elements occurring in the X-ray maps. Then, there are several algorithms to classify and calculate the modal mineralogy: these can be grouped in supervised and unsupervised. If properly used, keeping into account the edge pixel effects at grain boundaries and the dependence on the counting scale used for each elemental map, during X-ray map acquisition, multispectral data methods can be very promising to gather information on meteorites. For this research, we are using multispectral analyses to explore the chance to better localize and identify minerals or textures not yet described during the routine analyses for the meteorite classification, in other words, further accessory minerals or unexpected minerals or mineral-chemistry. We are currently investigating a thin section of Northwest Africa 14897, ordinary chondrite recently classified in 2022 as LL7 (Moggi Cecchi et al., 2022). This interesting meteorite has a markedly equilibrated texture and belongs to a group of chondrites that is considered by some authors one of the joining links between chondrites and primitive achondrites (Li et al., 2020; Friedrich et al., 2014; Grady et al., 2014). It has a cata-clastic breccia texture consisting of equilibrated olivine and orthopyroxene clasts in a fine matrix mainly consisting of the same minerals, together with diffuse recrystallized plagioclase, and no chondrules. Due to extensive weathering (W4) iron oxides are the most common opaques with no tracks of relict kamacite or taenite. Accessory phases include troilite, Ti-chromite, chlorapatite and merrillite, plus minor tetrataenite and secondary calcite. A modal estimate obtained from the SEM X-ray spectral maps provided the following results: 48% olivine, 25% low-ca pyroxene, 9% ca-pyroxene, 11% sodic plagioclase, 3% Fe-oxides, 0.6% chromite, 0.4 Cl-apatite, 3% calcite. For this investigation we used ENVI+IDL. We customized a routine in IDL for stacking the BSE image and elemental maps of the whole thin section in a unique multispectral file. By this method, besides comparing the modal composition obtained from SEM data with those resulting from the multispectral analyses on the whole section, we will search for further eventually occurring mineralogical phases in this meteorite.