  We give a de Finetti type representation for exchangeable random coalescent
trees (formally described as semi-ultrametrics) in terms of sampling iid
sequences from marked metric measure spaces. We apply this representation to
define versions of tree-valued Fleming-Viot processes from a $\Xi$-lookdown
model. As state spaces for these processes, we use, besides the space of
isomorphy classes of metric measure spaces, also the space of isomorphy classes
of marked metric measure spaces and a space of distance matrix distributions.
This allows to include the case with dust in which the genealogical trees have
isolated leaves.
