The electron microprobe was used to determine the bulk composition of immiscible sulfide globules trapped in the glass phase of 25 fresh submarine basalt samples from the Mid-Atlantic Ridge. Twenty-three samples represent a spectrum of primitive through differentiated tholeiites from the FAMOUS dive area; two are differentiated basalts from the Reykjanes Ridge. The analyzed globules range in diameter from 11 to 233 µm. On the average, they constitute only 0.0022 volume percent of the rocks and contain less than 1.5 percent of the sulfur. Compositions of the globules change with differentiation as measured by Fe/(Fe+Mg) or TiO2 content of the host glass. Globules in glass containing 0.66 to 1.0 wt percent TiO2 typically contain 20 to 26 wt percent Ni + Cu and have an average atomic Ni/Cu of 1.6. With differentiation toward 1.6 wt percent TiO2, Ni + Cu content of the globules falls to less than 10 wt percent and atomic Ni/Cu falls to 0.4.

Sulfur content of the host glasses shows a strong correlation with FeO content, increasing from 840 ppm to 1,370 ppm as FeO content increases from 8.0 to 12.6 wt percent. Reference to experimental studies shows that this relationship is consistent with sulfur saturation of the host glass at liquidus temperatures. Crystal fractionation is considered to be the dominant factor in keeping the differentiating melt at sulfur saturation.

The sulfide globules may have persisted in the basaltic melt from its place of formation by partial melting in the mantle, or they may have exsolved from the melt as it became sulfur-saturated in a high-level magma chamber. Globule abundance and composition indicate adjustment to the composition of the melt in which they were trapped. Material balance calculations suggest that one-third of the Cu and commensurate amounts of S, Ni, and Fe have settled from the magma as immiscible globules.

The sulfide globules contain less than 4 wt percent magnetite, compatible with low fo2 in the magma. Three sulfide phases coexisted in the globules at about 600 °C: monosulfide solid solution, intermediate solid solution, and pentlandite. At lower temperatures, the intermediate solid solution has broken down, and the monosulfide solid solution has exsolved a second generation of pentlandite.

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