Evidence from phosphorus X‐ray mapping for a multistep process in the formation of olivine phenocrysts in FeO‐rich porphyritic chondrules

Abstract Phosphorus X‐ray maps of olivine phenocrysts in many type II (FeO‐rich) porphyritic chondrules LL3.00 Semarkona and CO3.05 Y 81020 reveal multiple sets thin dark/bright (P‐poor/P‐rich) layers that resemble oscillatory zoning. Such discrete are generally not evident BSE images or Fe, Cr, Ca, Al, Mg, Mn because rapid diffusion these cations at high temperatures smoothed out their initial distributions, thereby mimicking normal igneous In contrast, the relatively slow P preserves original dendritic hopper morphologies crystals; skeletal structures formed during quenching after chondrule melting. The crystals were filled with low‐P cooling one more subsequent heating events, mainly involving melting mesostasis. Crystallization mafic silicates depleted mesostasis FeO MgO enriched it silico‐feldspathic components. Sectioning grains particular orientations can produce apparent zoning P. Strong evidence a secondary event is H5k. Phenocryst H5k‐2 this has relict core (with rhythmic layers) was fractured severed; overlain by set differently oriented subparallel P‐poor layers. Chondrule C6f from contains large multi‐lobed phenocryst still cavities, partially outlined P‐poor/P‐rich P‐rich rims surrounding could reflect short period grain growth late‐stage reheating event.