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

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