High-temperature events in the protoplanetary disk: Formation conditions of chondrules and refractory inclusions

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Chondrite meteorites contain an ancient record of the formation of the solar system. Two importantcomponents of chondrites, chondrules and refractory inclusions, were formed in the protoplanetarydisk environment in the earliest stages of solar system history. Chondrules and some refractoryinclusions underwent brief high-temperature heating episodes that resulted in melting followed byrapid cooling. Thermal histories are interpreted from chemical and isotopic studies of naturalsamples, as well as laboratory studies of analog compositions: typical peak temperatures are around1500 C and typical cooling rates are in the range of tens to hundreds of degrees per hour. Eventhough it is clear that these high-temperature events affected large volumes of disk material, thereis much debate about the heating mechanism. Several different models exist, including heating bydirect radiation from the protosun, heating in shock fronts produced in a variety of settings withinthe disk, and impacts between planetesimals. In order to refine and assess the different models, weneed to determine robust constraints on the heating process from chondrules and refractoryinclusions themselves. New astronomical observations of extrasolar planets and protoplanetarydisks such as HLTauri (see reference below) are giving us exciting new insights into the formation ofplanetary systems, and studies of chondrites provide direct samples of these environments.This PhD project will combine observations of chondrules and refractory inclusions in chondriteswith experimental studies designed to reproduce textures, mineral assemblages, mineralcompositions and isotopic effects. The objective is to determine constraints on the chondruleformingmechanism that can be used as input parameters for chondrule formation models. Previousexperiments have placed broad constraints on the heating process, but there are still manyunanswered questions. From observations at the sub-millimetre scale, we can make inferencesabout processes that operated on a scale of thousands of kilometres (see figure). Experiments willbe carried out in a one-atmosphere furnace with programmable temperature control. Studies of natural samples and experimental run products will use microbeam analytical techniques includingscanning electron microscopy, electron microprobe analysis, Raman spectroscopy and secondary ionmass spectrometry.