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

Chondrite meteorites contain an ancient record of the formation of the solar system. Two important
components of chondrites, chondrules and refractory inclusions, were formed in the protoplanetary
disk environment in the earliest stages of solar system history. Chondrules and some refractory
inclusions underwent brief high-temperature heating episodes that resulted in melting followed by
rapid cooling. Thermal histories are interpreted from chemical and isotopic studies of natural
samples, as well as laboratory studies of analog compositions: typical peak temperatures are around
1500 C and typical cooling rates are in the range of tens to hundreds of degrees per hour. Even
though it is clear that these high-temperature events affected large volumes of disk material, there
is much debate about the heating mechanism. Several different models exist, including heating by
direct radiation from the protosun, heating in shock fronts produced in a variety of settings within
the disk, and impacts between planetesimals. In order to refine and assess the different models, we
need to determine robust constraints on the heating process from chondrules and refractory
inclusions themselves. New astronomical observations of extrasolar planets and protoplanetary
disks such as HLTauri (see reference below) are giving us exciting new insights into the formation of
planetary systems, and studies of chondrites provide direct samples of these environments.
This PhD project will combine observations of chondrules and refractory inclusions in chondrites
with experimental studies designed to reproduce textures, mineral assemblages, mineral
compositions and isotopic effects. The objective is to determine constraints on the chondruleforming
mechanism that can be used as input parameters for chondrule formation models. Previous
experiments have placed broad constraints on the heating process, but there are still many
unanswered questions. From observations at the sub-millimetre scale, we can make inferences
about processes that operated on a scale of thousands of kilometres (see figure). Experiments will
be carried out in a one-atmosphere furnace with programmable temperature control. Studies of natural samples and experimental run products will use microbeam analytical techniques including
scanning electron microscopy, electron microprobe analysis, Raman spectroscopy and secondary ion
mass spectrometry.