Phosphate and feldspar mineralogy of equilibrated L chondrites: The record of metasomatism during metamorphism in ordinary chondrite parent bodies

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AbstractIn ordinary chondrites (OCs), phosphates and feldspar are secondary minerals known to be the products of parent‐body metamorphism. Both minerals provide evidence that metasomatic fluids played a role during metamorphism. We studied the petrology and chemistry of phosphates and feldspar in petrologic type 4–6 L chondrites, to examine the role of metasomatic fluids, and to compare metamorphic conditions across all threeOCgroups. Apatite in L chondrites is Cl‐rich, similar to H chondrites, whereas apatite inLLchondrites has lower Cl/F ratios. Merrillite has similar compositions among the three chondrite groups. Feldspar in L chondrites shows a similar equilibration trend toLLchondrites, from a wide range of plagioclase compositions in petrologic type 4 to a homogeneous albitic composition in type 6. This contrasts with H chondrites which have homogeneous albitic plagioclase in petrologic types 4–6. Alkali‐ and halogen‐rich and likely hydrous metasomatic fluids acted during prograde metamorphism onOCparent bodies, resulting in albitization reactions and development of phosphate minerals. Fluid compositions transitioned to a more anhydrous, Cl‐rich composition after the asteroid began to cool. Differences in secondary minerals between H and L,LLchondrites can be explained by differences in fluid abundance, duration, or timing of fluid release. Phosphate minerals in the regolith breccia, Kendleton, show lithology‐dependent apatite compositions. Bulk Cl/F ratios forOCs inferred from apatite compositions are higher than measured bulk chondrite values, suggesting that bulk F abundances are overestimated and that bulk Cl/F ratios inOCs are similar toCI.