Compositions of Basaltic Glasses from the East Pacific Rise and Siqueiros Fracture Zone, Near 9°N

Abstract

The compositions of 45 natural basalt glasses from nine dredge stations and six Deep Sea Drilling Project Leg 54 sites near 9°N on the East Pacific Rise have been determined by electron microprobe. These comprise 19 distinct chemical groups. Seventeen of these fall in the range of the eastern Pacific tholeiite suite, which is characterized by marked enrichment in FeO*, TiO 2 , K 2 O, and P 2 O 5 as CaO, MgO, and A1 2 O 3 all decrease. Based on trace elements, an estimated 50-75 per cent fractionation of plagioclase, clinopyroxene, and olivine is required to produce ferrobasalts from parental olivine tholeiites. Additional chemical variations occur which require source heterogeneities, differences in the degree of melting, different courses of shallow fractionation, or magma mixing to explain. Glass compositions from within the Siqueiros fracture zone are mostly less fractionated than those from the flanks of the Rise, and show chemical differences which require variations in the depth of melting or highpressure fractionation to explain. Some of them could not be parental to East Pacific Rise flank ferrobasalts. Two remaining glass groups, from dredge hauls atop a ridge and a seamount, respectively, have distinctly higher K 2 O, P 2 O 5 , and TiO 2 as well as lower CaO/Al 2 O 3 and SiO 2 at corresponding values of MgO than the tholeiite suite. These abundances, and whole-rock Y/Zr, Ce/Y, Nb/Zr, and isotopic abundances indicate that these basalts had a deeper, less depleted mantle source than the Rise tholeiite suite. Trace element abundances preclude the "ridge" basalt type from being a hybrid between the "seamount" basalt type and any East Pacific Rise tholeiite so far analyzed. The East Pacific Rise glasses from 9°N compare very closely to glasses dredged and drilled elsewhere on the East Pacific Rise. However, glass compositions from Site 424 on the Galapagos Rift drilled during Leg 54, as well as glasses and basalts dredged from the Galapagos and Costa Rica rifts, indicate that a greater degree of melting prevailed along much of the Galapagos Spreading Center than anywhere along the East Pacific Rise. PROCEDURES Samples were selected aboard the Glomar Challenger and from dredge hauls archived at the Scripps Institution of Oceanography. The sample selection for the DSDP materials was guided by the availability of glasses, and by identification of distinct petrographic types. The dredge sample selection was guided by published analyses and descriptions (Batiza et al., 1977; Johnson, 1979; Batiza and Johnson, this volume; Schrader et al., this volume). Of particular interest were olivine-spinel phyric oceanites (dredge SD-7, Siqueiros Expedition, Siqueiros fracture zone, described by Schrader et al., this volume), alkalic olivine basalts (dredge SD-8, Siqueiros Expedition, Siqueiros fracture zone, described by Batiza et al., 1977), and various MgO-rich and MgO-poor basalts that have been used elsewhere in fractionation and mixing calculations (Batiza et al., 1977; Batiza and Johnson, this volume). Samples were an...

Petrological Systematics of Mid-Ocean Ridge Basalts: Constraints on Melt Generation Beneath Ocean Ridges

“…For example, in the late 1970s and early 1980s attention focussed on chemical variations as a function of proximity to tectonic offsets along axis such as ridge offsets and propagating rifts [Hekinian and Thompson, 1976;Schilling and Sigurdsson, 1979;Natland and Melson, 1980;Christie and Sinton, 1981;Sinton et al, 1983;Bender et al, 1984;Langmuir and Bender, 1984]. In these and subsequent studies, evidence was found for local variations in the extent of melting and degree of fractionation.…”

The isotope geochemistry of abyssal peridotites and related rocks

“…The transform edge effect has become part of the litany of possible effects which are commonly cited to account for variations in crustal thickness (Fox, et al, 1980) and in basalt composition seen near to fracture zones (Natland and Melson, 1980;Perfit, et al, 1983;Bender, et al, 1983;Langmuir and Bender, 1984). A case can be made that there has not yet been a definitive demonstration of a transform edge effect.…”

“…It has long been postulated that transform faults have an effect on the composition and structure of the nearby oceanic crust (Fox, et al, 1980;Natland and Melson, 1980;Stroup and Fox, 1981;Langmuir and Bender, 1.984). This effect is thought to be due to the conduction of heat across the transform away from the active ridge segment.…”

“…They called upon the thermal effect of the transform edge to explain these observations. Natland and Melson (1980) proposed that the presence of relatively old lithosphere on the plate opposite the ridge-transform intersection of a fracture zone could result in cooling of the upwelling asthenosphere near the end of a rift. This cooling would cause melting in the mantle to become both shallower in depth and lower in total extent near the transform.…”

“…First is the effect of differences in source composition (Natland and Melson, 1980). Different source compositions can mimic the effects of differing mantle temperatures with respect to basalt compositions.…”