Evidence of phosphorous cycling in the slab-mantle interface from apatite in retrogressed eclogites

We studied apatite from retrogressed eclogites in a serpentine matrix from the North Motagua Mélange (Guatemala) to understand P behavior in the subduction interface. The retrogressed eclogites display eclogite-, blueschist-, and ep-amphibolite-facies assemblages at different modal abundances and have been linked to two rehydration events in the interface. Textural analyses, mineral assemblages, and inclusions indicate that apatite increases in size and crystallizes or overgrows from eclogite- to blueschist-epidote-amphibolite transition facies, while chemical dissolution at epidote-amphibolite-facies suggested by the record of highly altered appearance of apatite. In our samples, all apatite crystals are halogen-poor and relatively enriched in Sr, Y, and Pb. When normalized to chondrites, the eclogite and blueschist apatite change from depleted to enriched light rare earth elements (REE) patterns and also contain B, As, and Ba. In contrast, epidote-amphibolite apatite display depleted light and middle REE patterns and are enriched Cr, Mn, and U. Our data suggest fluid-rock interaction with two different fluids, an Al-, K-, and P-rich fluid that crystallized apatite, and the second was Na-, Si-, and Ca-rich that dissolved apatite, indicating that P in the subduction interface moves readily between fluid and solid phases, a behavior that it known to its cycle on Earth’s surface.