Flotation mechanism of sulphide melt on vapour bubbles in partially molten magmatic systems

Abstract

The motion of dense Fe-rich immiscible sulphide liquids is generally supposed to be dominated by gravitational settling in crustal magma chambers, however they may become buoyant by attachment to low-density vapour bubbles to form compound drops, possibly contributing to the upward transfer of sulphur and transition metals in degassing magma bodies. Here, using numerical models, we consider constraints on the flotation of compound drops, and find a wide range of morphologies that would be expected to be capable of migrating through magmatic systems, including both melt- and mush-dominated domains, but also some situations where the compound drops will spontaneously separate to allow the vapour to rise while the sulphide phase is sequestered in the mush. The stability of compound drops is empirically related to a function f(Bo)=|σMV−σMS|σMS⋅1/BoS+|σVS−σMS|σMS⋅1/BoV, where M, V, and S denote silicate melt, vapour, and sulphide liquid, σ is surface tension (N⋅m−1), and Bo is the Bond number. Over a range of plausible conditions in magmas, f(Bo) must exceed values of 3 to 4 for capillary forces to overcome buoyancy forces tending to pull them apart. Using published thermodynamic models for vapour and sulphide solubility in silicate melts, we show that in many magmas the second boiling of vapour occurs synchronously with the saturation of sulphide liquid, increasing the potential for coupling of vapour and sulphide droplets. Diffusive coarsening produces mm-sized bubbles within 100-500 years, generating enough buoyancy to trigger the migration of compound drops. During compaction-driven expulsion of interstitial melt, compound drops can pass through constrictions between crystals in mush. The flotation of sulphide-vapour aggregates is likely to occur in crustal magma reservoirs and provides a feasible mechanism for the removal of sulphide liquid from crystal mushes, promoting its ability to participate in the generation of magmatic sulphide and porphyry copper deposits, emissions of the metals during volcanic eruptions, and even the remobilization of chalcophile metals sequestered in deep arc cumulates to generate porphyry Cu-(Au) deposits.