Petrographic insights into phosphate formation in L and LL ordinary chondrites

The origin and formation mechanisms of phosphate minerals in L and LL ordinary chondrites remain an open question. Here we report detailed quantitative data on the abundances, grain sizes and compositions of apatite and merrillite in L, LL and L/LL chondrites, covering a total analysed area of ∼2730 mm2. Our results show that the abundances of phosphate minerals are slightly higher in L than in LL chondrites and reveal distinct petrographic differences between merrillite and apatite. Merrillite abundance is anti-correlated with Fe, Ni metal content and increases with petrologic type, as does grain size – indicating grain coarsening processes – supporting the formation of this mineral during thermal metamorphism. In contrast, apatite shows no clear trend in abundance or grain size with petrologic type. Although chlorine contents are similar between apatites from L and LL chondrites, the small dispersion of Cl concentration and its decrease with petrologic type in LL chondrites indicate that apatites in these chondrites undergo equilibration with increasing thermal metamorphism, a pattern not observed in L chondrites. Although the Cl contents in apatites are fairly homogeneous across the samples, scarce F-rich apatites have been observed in some meteorites, leading to significant variability in Cl#. However, due to low concentrations and analytical challenges, we argue that comparisons between different OC groups should primarily be based on Cl concentrations. Furthermore, apatite and merrillite have different grain size distributions in L chondrites, but similar ones in LL chondrites, where they also occur as close assemblages, suggesting a petrogenetic relationship between apatite and merrillite in LL chondrites, but not in L chondrites. Overall, the present study suggests that phosphates did not have exactly the same history in L and LL chondrites.