Revisiting problem of chilled margins associated with marginal reversals in mafic–ultramafic intrusive bodies

Numerous observations on mafic–ultramafic layered intrusions, sills and dykes show that chilled margins always develop as an integral part of their marginal reversals and possess the following features: (a) they are commonly much more evolved or primitive than bulk intrusion compositions, (b) evolved chilled margins are composed of the low temperature cotectic assemblages of relevant magmatic systems and (c) tend to be compositionally similar in intrusions formed from different parental magmas, (d) fine-grained chilled margins are notably absent in many intrusions, with contact rocks being represented by medium- to coarse-grained cumulates. The anomalous features of chilled margins can be partly attributed to contamination, intratelluric inhomogeneity of magma, changes in composition of intruding magma, loss of magma from the chamber, supercooling, etc. A major process still remains, however, illusive, but appears to be universally operating along the cooling margins of magmatic bodies in a liquid state, being gravity-independent and temperature gradient-driven. We recognize this not yet specified process as Soret fractionation and explain the above observations in the following way. Primary chilled margins do not commonly survive because of intensive remelting by heat flux from the interior of the chamber. The subsequently formed “secondary chilled margins” represent cumulates that crystallized from liquids produced by temperature gradient-driven Soret fractionation. At high temperature gradients the process tends to produce similar cotectic liquids crystallizing gabbronorite (or gabbro) from all parental magmas of a given magmatic system, resulting in compositionally similar “secondary chilled margins” that are more evolved than bulk compositions. At low temperature gradients the process produces liquids that are only slightly more fractionated than the parental magma and form “secondary chilled margins” that are more primitive than bulk compositions. This interpretation suggests that, apart from the rare cases of chilled margins that survived remelting, they should not be used as monitors for parental magma compositions of intrusive bodies, even if all conventional complicating factors were not operative.