Sulfide globules in mid-ocean ridge basalts (MORB), and the effect of oxygen abundance in FeSO liquids on the ability of those liquids to partition metals from MORB and komatiite magmas

Abstract

The coexistence of sulfide liquids (or more accurately, FeSO liquids) with silicate liquids in a variety of magma systems is well known, as is the ability of immiscible FeSO liquids to segregate Ni, Cu and platinum-group metals into orebodies. Evidence for association of FeSO liquids with magmas that form mid-ocean ridge basalts (MORB) comes from sulfide globules in basalt glass from Siqueiros Fracture Zone, East Pacific Rise; three fracture zones in the Indian Ocean; Lau Basin; and ∼ 23 previously published oceanic localities. Approximately 40% of the sulfur originally present in magma at Siqueiros Fracture Zone exsolves as a sulfide liquid during fractional crystallization. Virtually all of the Cu, and 20% of the Ni, originally in the magma is extracted by the sulfide liquid. Significant variations in the ability of FeSO liquids to partition Ni Cu exist among MORB and other mafic magmas, such as komatiites and layered mafic intrusions. Nernst partition coefficients (sulfide to silicate melt; D S L ) are 2–3 times higher for Ni in MORB than in komatiite, and ∼ 5 times higher for Cu. Reasons for this variation are not entirely clear, but a possible contributing factor is that FeSO liquids in MORB appear to have a higher oxygen content than those in komatiites. That FeSO liquids associated with MORB magma are oxygen-rich is suggested by low observed values at Siqueiros Fracture Zone for the NiFe exchange coefficient K D3 [1.21−4.56; K D3 = ( Fe Ni ) olivine ( Fe Ni ) sulfide ]. It has been previously shown that natural magmatic FeSO liquids are rich in O 2− [ O ( S + O) is apparently always greater than ∼ 0.49], and that K D3 is inversely correlated with O ( S + O) . K D3-values below ∼ 12 would result from FeSO liquids with O ( S + O) > 0.49 . K D3-value of> 20 would be impossible (values from 12 to 20 would be unlikely) because they would require an FeSO liquid poor in O 2−. This argument for low K D3 in magmas is strengthened by the low K D3-values of MORB, whose globules are inarguably of magmatic origin. K D3 in komatiites is higher than in MORB (indicating a lower oxygen fugacity in komatiite magma) and varies from ∼ 5 to > 20, with values of > 20 coming from highly metamorphosed komatiites. Fleet and MacRae argued on the basis of experimental results that K D3-values of < 20 could not result from equilibration at magmatic temperatures and that orebodies associated with komatiites would have to be of hydrothermal origin. But those experiments were carried out at O ( S + O) = 0 , an unrealistic value for magmatic FeSO liquids. K D3-values in komatiites of > 20 could not have resulted from magmatic processes, but might have come from metamorphism under oxygen-deficient conditions.