Interfacial tension between magmatic sulfide and silicate liquids: Constraints on kinetics of sulfide liquation and sulfide migration through silicate rocks

We have measured the interfacial tension γSF between sulfide and silicate melts over the temperature range from 1250 to 1325 °C by in situ measurement of the shapes of sessile drops of sulfide immersed in silicate melt, sitting upon level alumina substrates. The silicate melt was an alkali-free synthetic basaltic liquid. The sulfide melts were synthetic mixtures of Fe-Cu-Ni-S or Fe-S-O. The interfacial tension ranges from about 500 mJ m−2 for sulfide of FeS composition to approximately 600 mJ m−2 for sulfide containing 16 wt.% Cu or 6.5 wt.% Ni, and up to 650 mJ m−2 for Fe-S-O liquid containing 4 wt.% O. Contact angles vary from 150° to 180°, showing that sulfide liquid does not wet oxide minerals in silicate magmas. The density of sulfide melt does not depend strongly on Cu or Ni content, remaining between 4.0 and 4.5 g cm−3 over the range of Cu and Ni contents expected in sulfide magmas, but increases substantially to about 5.5 g cm−3 in Fe-S-O melts containing 4 wt.% O. Our results can be combined with conventional concepts of nucleation theory to indicate that when silicate magmas exist at small degrees of supersaturation, sulfide drops may nucleate rarely and at widely spaced intervals, leading to kinetic control of the compositions of the resulting droplets. In light of our results, we use simple scaling arguments to argue that sulfide magmas are not capable of migrating through partially molten silicate rocks by capillary forces alone, but may be forced through narrow grain boundaries or grain edges only by the flow of the enclosing silicate melt. During melt extraction by compaction of partially molten mantle peridotite, it is highly unlikely that silicate flow rates could be sufficiently rapid to permit the entrainment of droplets of sulfide liquid. At temperatures below the solidus of enclosing silicate rocks, sulfide melt will be free to travel along fractures or grain boundaries. The mobility of sulfide melts through completely solidified silicate rocks may account for the widespread observation that late-stage, highly fractionated sulfide liquids have escaped from cooling bodies of massive sulfide to form veins and disseminations of Cu-rich sulfide minerals in their silicate host rocks.