An atlas of apatite and merrillite in martian meteorites: REE Geochemistry and a new tool for shergottite classification

The phosphates, apatite and merrillite, are accessory phases in all martian meteorites. Although apatite is commonly used to assess volatile content and speciation in martian meteorites, merrillite is at least twice as abundant in most samples, but poorly understood. Given that shergottites are divided into enriched, intermediate, and depleted subgroups based on bulk differences in light rare earth element (LREE) abundance and isotopic compositions, an understanding of phosphate mineral behavior is essential to deciphering the petrogenetic differences between these groups because they are the main REE-bearing phases. This study examines 10 enriched shergottites, six intermediate shergottites, and four depleted shergottites to investigate systematic variations in phosphate mineralogy and geochemistry. Two nakhlites, a chassignite, ALH 84001, and two pairs of NWA 7034 were also examined to cover all martian meteorite types known to date. Fourteen of the shergottites were previously classified into enriched, intermediate, and depleted subgroups based on bulk rock REE trends and La/Yb ratios. The remaining six shergottites had not been subgrouped during classification. All samples were elementally mapped using the XFM beamline at the Australian Synchrotron, which provided the relative abundance of merrillite, apatite, K-feldspar, and maskelynite within each sample (the same can be achieved with electron microprobe or SEM). We show that it is possible to classify shergottites from a single representative thin section using apatite to merrillite ratios (A¹⁰/M, where A¹⁰ is apatite abundance × 10) and K-feldspar to phosphate ratios (K¹⁰/P, where K¹⁰ is K-feldspar abundance × 10). Enriched shergottites typically have A¹⁰/M of 1.08 to 8.72 and K¹⁰/P of 1.85 to 13.34; intermediate shergottites have A¹⁰/M ranging from 0.5 to 0.96 and K¹⁰/P of 0.36 to 0.94; and depleted shergottites have A¹⁰/M ranging from 0.26 to 0.42 and K¹⁰/P of 0.09 to 0.39. Calculating these ratios thus provides a quick and straightforward method of chemically classifying shergottites that avoids the need to destroy samples for bulk rock REE analysis.