Many studies have reported the presence of sulfide globules in olivine crystals and matrix glasses, which indicated S-saturated fractionation during magmatic evolution (Patten et al., 2012; Yang et al., 2014). On the one hand, well-developed sulfide globules in MORBs are thought to take shape prior to the host magma quenching, like those recorded by Yeats and Mathez (1976) and Patten et al. (2012), which are 5–50 μm in size and round in shape (Yeats and Mathez, 1976; Holzheid, 2010; Patten et al., 2012). The solubility of sulfur in basalt is related to changes in melt composition, temperature, and oxygen and sulfur fugacity (Mavrogenes and O’Neill, 1999). On the other hand, some embayed sulfides similar to those recorded by Yang et al. (2013) are thought to form due to partial sulfide dissolution during decompression. Therefore, the saturation of sulfide droplets may be related to a decrease in pressure or temperature. In this study, the discovery of nanoscale sulfide droplets in natural MORB glasses demonstrates a new method of S-saturated fractionation. The recorded droplets (small in size and rounded in shape) are characterized by smooth edges and crystalline features. Consequently, sulfide saturation may have initially commenced in the form of nanobead droplets.

Although sulfide droplets are common in fresh MORB glasses such as phenocrysts and glass, microcrystalline aggregates of plagioclase and olivine, and vesicle walls (Yeats and Mathez, 1976; Czamanske and Moore, 1977; Patten et al., 2012; 2013; Yang et al., 2014), the micro-sized droplets have different textures ranging from a fine-grained intergrowth of monosulfide solid solution (MSS) and intermediate solid solution (ISS) to coarse-grained intergrowth of MSS and ISS accompanied by oxide and pentlandite (Yeats and Mathez, 1976; Czamanske and Moore, 1977). The textural differences of these droplets might be controlled by component nucleation and diffusion rates (Czamanske and Moore, 1977). For example, Patten et al. (2013) reported that fine-grained and coarse-grained droplets refer to the primitive sulfide liquid, and the zoned sulfide droplets correspond to the stage where MSS crystallizes and co-exists with a residual sulfide liquid (the quenched ISS) (Patten et al., 2013). Compared with those micro-sized sulfide droplets with various textures (Patten et al., 2013; Yang et al., 2013), the nanoscale sulfide droplets recorded in this paper differed significantly in terms of internal textures showing homogeneous compositions without internal fractionation of MSS and ISS phases. Furthermore, the sulfide droplets in this study are crystalline, suggesting the formation of their lattices during this stage (Figures 4D–F).

The sulfide droplet sizes varied at different stages of sulfur saturation, and the droplet sizes also controlled the textural differences of the droplets. Holzheid (2010) demonstrated the polymerization degree of the silicate liquid controlled the sulfide droplet size distribution. Patten et al. (2012) speculated a positive correlation between the droplet diameter increase and the degree of phase segregation. Furthermore, Peach et al. (1990) reported that the size of a globule should reflect its residence time in the melt and the frequency with which it contacts and consumes other globules. Ostwald’s ripening plays an important role in globule size change (Yang et al., 2014). The experimental results indicate that when the composition of silicate melt reaches sulfide saturation immiscible, the phase separation of sulfide nanoscale droplets and silicate nanoscale liquids/melts promptly emerges. Small droplets then disappear, enlarging the larger droplets, and this process is generally accompanied by a reduction in interface free energy (Mazurin and Porai-Koshits, 1984). In this study, the nanoscale sulfide droplets are small (10–15 nm) and have rounded shapes and smooth edges. In summary, sulfides appear to form very shortly before eruption, followed by immediate supercooling.

Sulfide plays an important role in the distribution of chalcophile elements during mantle melting and basalt melt differentiation (Mathez, 1976; Peach et al., 1990; Bézos et al., 2005). As sulfide melt droplets segregate, the droplets scavenge siderophile and chalcophile elements such as Ni, Cu, Pt, and Pd from the magma (Holzheid, 2010). Most micro-sized sulfide grains in MORBs contained MSS, the first solid to crystallize from a sulfide liquid, and ISS, which crystallizes from the remaining liquid (Patten et al., 2013). Therefore, the different chalcophile elements in MSS and ISS typically exhibit distinct behaviors. For instance, platinum-group elements (PGE) are scavenged by immiscible sulfides during the evolution of mafic magmas (Peach et al., 1990; Keays, 1995; Rehkämper et al., 1999; Song et al., 2006), but the iridium-subgroup of PGE (Os, Ir, Ru, and Rh) normally behaves as compatible elements and partition into MSS (Patten et al., 2013; Yang et al., 2014). Co and Re have a slight preference for MSS, whereas Cu, Zn, Au, Ag, Sn, Te, Cd, Bi, and Pb partition into ISS (Patten et al., 2013). The sulfide droplets reported by Yang et al. (2014) contained Ni-Fe-rich MSS and Cu-Fe-rich ISS phases.

However, existing literature does not precisely specify the timing of the chalcophile elements’ incorporation into sulfide droplets. Unlike micro-sized droplets, the nanoscale sulfide droplets in this study contain Fe, Cu, Ni, and S (Figure 6). These elements might have been scavenged during initial sulfide saturation and behaved similarly in the droplets. With the formation of sulfide beads, the elements Si, O, and Al are depleted at the same time. Based on the nanoscale sulfide droplets reported in this paper, it can be inferred that chalcophile elements enter sulfide droplets in the early stage and are uniformly distributed in the range of sulfide droplets. As sulfide droplets in MORB provide unique records of near-liquidus phase relations in basaltic systems, such an insight can help understand how chalcophile elements behave during sulfide crystallization. With longer residence times, sulfide droplets gradually evolve and develop different textures, including MSS and ISS, with each chalcophile element showing distinct preferences.

Immiscibility is an important factor in the process of magmatic evolution and mineralization. During magma immiscibility, a single component melt can decompose into two kinds of melts with completely different compositions. After magma immiscibility, the large-scale separation of conjugated two-phase melts affects the overall evolution of magma. Early immiscibility begins in the magma evolution; the greater the impact on magma evolution, the greater its geochemical and petrological significance (Veksler et al., 2008). The immiscibility between sulfide droplets and silicate melts is similarly pivotal in magma evolution and has been well-documented in previous studies. According to the elemental maps, the sulfide droplets reported here are relatively rich in Fe, Cu, Ni, and S (Figure 6) but lack Si, O, and Al (Figure 7), indicating that the immiscibility between sulfide and silicate melts occurred during the early stage. According to these results, the immiscibility of sulfide and silicate melt occurs at the nanoscale stage, so it can be inferred that the immiscibility in the process of magma evolution may occur at the nanoscale stage. The basic principle of magmatic sulfide deposit origin stems from the sulfur saturation of magma (Mungall and Naldrett, 2008). Sulfide droplets form when sulfur saturation occurs. Magmatic sulfide deposits form when sulfide droplets become sufficiently concentrated in a certain area of the magmatic body (Mungall and Naldrett, 2008). Studies have shown that the formation of magma-type Ni-Cu sulfide deposits is related to S-saturation in mantle-derived basic and ultramafic magmas and the separation and enrichment of immiscible sulfide droplets (Arndt et al., 2005). Moreover, the physicochemical action of mineralization in a magmatic sulfide system is primarily controlled by the dynamics and kinetics of transported sulfide droplets (Robertson et al., 2015). Therefore, clarifying the relationship between sulfide droplets and silicate melt is crucial for comprehending the formation process of magmatic sulfide deposits.