Ophiolites are believed to preserve pieces of ocean crust exhumed and emplaced on land; however, these are invariably dismembered and tectonically modified. Extrusive basalts and sheeted dike sequences of the ocean crust are fortuitously exposed in the walls of rift valleys at both fast- and slow-spreading oceanic ridges, but intact exposures of gabbro, interpreted from seismic modeling to comprise in excess of two-thirds of the oceanic crust, are exceedingly more rare. These circumstances conspire to make the origin of the lower oceanic crust one of the most enigmatic puzzles under investigation by earth scientists.
In 1997, Ocean Drilling Program (ODP) Leg 176 successfully deepened Hole 735B, at the Atlantis II Fracture Zone on the Southwest Indian Ridge (Fig. F1), from the 500-m depth cored 10 yr ago during Leg 118 to a total depth of 1508 m. The combined results of drilling during Legs 118 and 176 (Robinson, Von Herzen, et al., 1989; Dick, Natland, Miller, et al., 1999) provide us with the most complete, intact section of lower ocean crust ever sampled. The gabbroic ocean crust at this site consists of several intervals of primitive olivine gabbro and troctolite. Each of these intervals is composed of many smaller intrusions, but shipboard geochemical analysis of major element oxides indicate a series of cycles relating to major pulses of magma with coherent fractionation sequences (Dick, Natland, Miller, et al., 1999). As a complement to other mineral chemistry and crustal evolution studies undertaken by the shipboard science party and various shore-based researchers, this paper reports the results of an investigation into sulfide mineralization and the presence, or absence for the most part, of platinum-group elements (PGEs) in the gabbros from this location.
PGEs are chalcophile and siderophile and are known to commonly occur with sulfide minerals in natural rock systems (Naldrett, 1989). PGE-bearing phases have been reported from ophiolite complexes (Prichard et al., 1986; Ohnenstetter et al., 1991; Pederson et al., 1993) generally associated with sulfide minerals or disseminated chromite layers. Although reports of PGE concentrations in ophiolitic gabbroic rocks are rare (e.g., Lachize et al., 1991; Prichard and Lord, 1990), gabbroic rocks from the plutonic sequence near the fast-spreading ridge at Hess Deep contain as much as 7.4 ppb Pt, 3.5 ppb Pd, and 3.1 ppb Au (Prichard et al., 1996). Troctolites associated with the ultramafic section at Hess Deep contain significantly higher concentrations of noble metals (36 ppb Pt and 54 ppb Pd) (Prichard et al., 1996), but dislocation of the section between the sites where gabbro and ultramafic rocks were recovered makes establishing any cogent relationship tenuous. Little is known about differences in PGE concentrations between ocean crust formed at fast- and slow-spreading ridges because of the lack of sampled exposures.
Igneous sulfides reported from the Leg 118 cores include pyrrhotite and chalcopyrite, with very rare pentlandite. Troilite is also present as a low-temperature exsolution product (Alt and Anderson, 1991; Natland et al., 1991). These sulfides are interpreted to have formed predominantly by accumulation of immiscible sulfide droplets. Although PGEs are efficiently scavenged from magmas by accumulation of immiscible sulfides, no systematic survey of PGE contents in the gabbros cored during Leg 118 has been reported.
This study was aimed at documenting the sulfide mineralogy and composition of gabbros recovered during Leg 176 in an effort to determine host phases of PGE mineralization in cores from Hole 735B, particularly to determine if PGE mineralization is hosted in primary magmatic phases or in secondary phases. Because of a strong alteration overprint, the only documented study of PGE mineralization in seafloor gabbros (Prichard et al., 1996) proved equivocal in determining whether PGE mineralization had been hosted in primary sulfides and reprecipitated as independent phases during alteration or if they were precipitated directly as independent phases. Given the overall extremely low levels of alteration in the gabbros from Hole 735B, we hoped to be able to see through any hydrothermal overprint and determine if platinum-group minerals (PGMs) are preserved as primary phases. Although PGE concentrations in oceanic gabbros may be low, the vast amount of ocean crust requires research into the distribution of highly refractory chalcophile elements to evaluate the global PGE budget and determine how PGEs are extracted from the mantle and concentrated in the ocean crust.