A discovery of nanoscale sulfide droplets in MORB glasses: Implications for the immiscibility of sulfide melt and silicate melt

Abstract. Sulfur forms an immiscible liquid upon saturation in magma, and sulfide droplets were commonly found in fresh mid-ocean ridge basalt (MORB) magmas. In this paper, scanning electron microscopy (SEM) determined that MORB samples were primarily fine-grained and weakly phyric, with hypocrystalline to vitreous textures. A focused ion beam cut from the MORB glasses examined by transmission electron microscopy (TEM) revealed a range of nanoscale sulfide droplets (10–15 nm), featuring rounded shapes and smooth edges. Texturally, the droplets were crystalline and homogeneous in composition. Elemental S, Na, Fe, Cu, and Ni were evenly distributed within the droplets, while the content of element Si, Al and O are less in the droplets. Previous reports have elucidated the immiscibility between sulfide and silicate melts, and the structure of the silicate melt also affects the size distribution of sulfide droplets. This is the first report on nanoscale sulfide droplets within MORB glasses, and those results indicated that nanoscale sulfide droplets were the initial phase of sulfide saturation; such insight may prove useful in understanding how siderophile and chalcophile elements behaved during sulfide crystallization. In addition, this study determined the immiscibility of sulfides and silicate melts occurred in the early nanometer stage, the immiscibility of sulfides in magmatic Ni-Cu sulfide deposits was the key to the formation of magmatic Ni-Cu sulfide deposits. Therefore, all immiscibility phenomena may occur in the nanometer stage during magma evolution.