P-rich patches and bands in P-poor olivine from the Baima layered intrusion (SW China) reveal disequilibrium textures and complicated magma chamber processes

In this paper we report for the first time complex P zoning patterns recorded by high-resolution X-ray mapping of P in olivine from the Baima mafic layered intrusion in SW China. It shows that olivine grains have distinctly irregular P-rich patches/bands that are crosscut and interlocked by P-poor olivine sections. The P-rich patches/bands contain 250 to 612 ppm P, up to four times higher than those for P poor olivine section (123 to 230 ppm). Under EBSD maps, P-rich patches/bands within a single olivine have the same crystallographic orientation, indicating that they were originally the same grain. Fe-rich melt inclusion and rounded magnetite are enclosed in the P-poor olivine sections. These textures thus clearly record a two-stage growth process of olivine. Followed by a secondary stage of dissolution and re-precipitation, P-rich olivine crystals formed in the first stage were dissolved by a disequilibrium Fe-rich ambient melt to form P-poor olivine section in the same grain, and were only preserved as P-rich patches/bands due to the slow diffusion of P.