Petrology and Geochemistry of Oceanic Intraplate Sheet-Flow Basalts, Nauru Basin, Deep Sea Drilling Project Leg 89

Abstract

Reentry of Hole 462A during Leg 89 resulted in the penetration of a further 140 m of basalt sheet-flows similar to those found during Leg 61 at the same site. Twelve volcanic units (45 to 56) were recognized, comprising a series of rapidly extruded, interlayered aphyric and poorly clinopyroxene-plagioclase-olivine phyric, nonvesicular basalts. All exhibit variable, mild hydration and oxidation, relative to fresh oceanic basalts, produced under reducing, low-CO 2-activity conditions within the zeolite facies. Secondary assemblages are dominated by smectites, zeolites, and pyrite, produced by low-temperature reaction with poorly oxygenated seawater. No systematic mineralogical or chemical changes are observed with depth, although thin quenched units and more massive hypocrystalline units exhibit slightly different alteration parageneses. Chemically, the basalts are olivine-and quartz-normative tholeiites, characterized by low incompatible-element abundances, similar to mildly enriched MORB (approaching T-type), with moderate, chrondite-normalized, large-ionlithophile-element depletion patterns and generally lower or near-chrondritic ratios for many low-distribution-coefficient (K D) element pairs. In general, relative to cyclic MORB chemical variation, they are uniform throughout, although 3 chemical megagroups and 22 subgroups are recognized. It is considered that the megagroups represent separate low-pressure-fractionated systems (olivine + Plagioclase ± clinopyroxene), whereas minor variations within them (subgroups) indicate magma mixing and generation of near-steady-state conditions. Overall, relatively minor fractionation coupled with magma mixing produced a series of compositionally uniform lavas. Parental melts were produced by similar degrees of partial melting, although the source may have varied slightly in LIL-element content.

Geochemistry and Petrography of Cretaceous Sills and Lava Flows, Sites 800 and 802

“…Sills forming part of the oceanic crust of small basins, such as the Gulf of California (Saunders et al, 1982) and the Daito and Shikoku basins in the western Pacific (Marsh et al, 1980) are mainly tholeiites, although in a few cases relatively abundant alkalic basalt units may be present, such as in Hole 446A (Daito Basin). In the deeper water, open ocean environment, such as the Nauru Basin, the upper portion of the basaltic sequence constitutes a series of sills, although again they are tholeiitic in composition (Floyd, 1989). hi some cases submarine sills appear to be closely associated with seamounts or ocean islands.…”

“…Currently, we prefer the latter tectonic situation. There is little doubt that the mid-Cretaceous volcanic event was a major feature of the central-western Pacific (Larson, 1991) and is represented by different volcanic forms, such as reef-capped seamounts and ocean islands (with their attendent slumped volcaniclastic aprons), enormous oceanic plateaus, such as the Ontong-Java Plateau, and deep-sea sill-flow complexes, as in the Nauru Basin (for example, Winterer, 1973;Schlanger et al, 1981;Haggerty et al, 1982;Rea and Vallier, 1983;Schlanger and Moberly, 1986;Floyd, 1989). Leg 129 also demonstrated the existence of other deep-sea expressions of the mid-Cretaceous event in the form of sills intruding Early Cretaceous sediments (Site 800) and pre-Aptian lava flows (Site 802).…”

Site 803

“…Above 200 mbsf, the hole was significantly wider than the maximum opening of the density tool caliper. Hence, above this depth the log density measurements average about 1.4 g/cm 3 , compared with core measurements of about 1.6 g/cm 3 (see , 1980), Hole 504B on the Costa Rica Rift (Kempton et al, 1985), and the Nauru Basin (Floyd, 1986;Saunders, 1986). Averages for Manihiki Plateau lavas ("M"; Jackson et al, 1976) and Malaita basement basalts ("Ma"; J.…”

“…Mahoney, unpubl. data, 1989), which may represent an uplifted portion of the western edge of the Ontong Java Plateau (e.g., Kroenke, 1972), and to the Cretaceous sill and flow complex in the Nauru Basin to the east of the plateau (e.g., Floyd, 1986;Saunders, 1986). In conjunction with Nd and Sr isotopic studies, the above chemical characteristics have been interpreted to reflect mantle sources that are a combination of MORB and oceanic-island-type components (Mahoney, 1987;P.…”

Reactivity landscape of pyruvate under simulated hydrothermal vent conditions

“…Secondary minerals such as zeolites that are produced upon reaction of hot basalt melts with seawater are frequently observed together with sulfide minerals. For instance, clinoptilolite occurs at the Juan de Fuca Ridge (14) and stilbite was detected on the ocean floor at 1.1 km depth along with pyrite (15).…”