EVIDENCE FOR MELT SEGREGATION TOWARDS FRACTURES IN THE HOROMAN MANTLE PERIDOTITE COMPLEX

MELT segregation from a solid matrix in the upper mantle is the first step in the formation of magmas. The mechanisms of melt segregation proposed so far have been based on two different physical models: the percolation of interstitial melt driven by a density difference between melt and matrix, associated with compaction and deformation of the matrix1; or the suction of interstitial melt into fractures from a surrounding porous matrix2-4. Although the latter process was originally invoked to explain the occurrence of dunite and pyroxenite-gabbro dykes with zones depleted in melt component on both sides2,5-7, most of these zones are discordant, small in scale and clearly formed by reaction between peridotite and basaltic melt8. I have suggested9 that thick, concordant dunite zones in the most depleted peridotite layers of the Horoman peridotite complex formed by suction of partial melt towards fractures later filled by dunite. Here I report a detailed mineralogical variation across the dunite which, when set in its geological and petrographic context, provides clearer evidence for melt segregation by dynamic forcing3, rather than passive percolation. This mechanism of melt segregation may play an important role in the formation of primary magmas with low production rate and large geochemical variability.