Differential migration of immiscible liquids in gabbroic crystal mush: the Skaergaard Intrusion, East Greenland

Abstract

Ferro-basaltic liquids intersect a binode during fractionation, splitting into two immiscible silicate liquids, one Si-rich and the other Fe-rich. The two liquids have very different physical properties: the Si-rich liquid is more buoyant and viscous, preferentially wetting plagioclase, whereas the Fe-rich liquid preferentially wets grains of mafic minerals and Fe-Ti oxides. The density difference means that when a crystal-poor magma body intersects the binode, it is likely to undergo compositional stratification, with implications for the compositions of any eruptions. When the interstitial liquid in a crystal mush unmixes, the density difference is compounded by the differences in wetting properties, leading to complex behaviour if capillary forces act in a different direction to that of gravity. The extent and scale of differential migration of unmixed liquids may thus play an important role in the genesis of the Daly Gap, while the preferential partitioning of P, PGE and REE into the Fe-rich conjugate has significant implications for formation of economically important deposits. The Skaergaard Intrusion of East Greenland records abundant evidence for differential migration of immiscible silicate liquid conjugates. At the intrusion scale, the roof rocks are more silica-rich than the cumulates at the floor, consistent with intersection of the binode by the bulk magma leading to the accumulation of the buoyant Si-rich conjugate in the roof mush. This is supported by the localised presence of reverse modal layering on the floor, interpreted as bodies of dense Fe-rich liquid ponded at the top of thin (<1m) mush. Large-scale lateral migration of Fe-rich liquid along fractures developing in the solidifying wall mush may have been the underlying cause of metasomatism to form localised 100m-scale patches of replacive pyroxenite. Evidence of metre-scale differential migration within the crystal mush on the intrusion floor is provided by paired silicic and mafic late-stage segregations, recording downwards penetration and disruption of the floor mush by dense Fe-rich liquid, coupled with limited upwards flow of viscous Si-rich liquid. Further evidence of metre-scale differential migration is provided by compositional rims developed on the tops and bases of (almost) fully-solidified blocks in the floor cumulates of material solidified at the roof, fragmented and released during contemporaneous seismic activity. The mafic rims at their tops formed by the ponding of downwards-moving Fe-rich liquid while the felsic rims at their bases formed by the ponding of upwards-moving Si-rich liquid against the impermeable autoliths, consistent with the extent of rim development being a function of autolith shape, rather than composition. Differential migration on the cm-scale, driven by capillarity and the differences in wetting properties of the two immiscible conjugates, is suggested as the mechanism by which poorly-defined cm-scale micro-rhythmic layering is superimposed on graded modal layering. The Skaergaard examples of differential migration provide the opportunity to constrain the length- and time-scales of differential migration of unmixed immiscible silicate conjugates, to quantify the effects of capillarity and emulsion coarsening, and to assess the more general importance of liquid immiscibility on petrogenetic evolution.