Geochemistry of Basalts from the Nauru Basin, Deep Sea Drilling Project Legs 61 and 89: Implications for the Origin of Oceanic Flood Basalts

Abstract

At Site 462 in the Nauru Basin, western Pacific Ocean, 56 lithologic units have been recovered from an extensive flood basalt province. Fossil evidence suggests that the lavas were emplaced during the interval 100-115 Ma, some 30 m.y. after formation of the underlying Jurassic ocean crust. The lithologic units can be broadly divided into three chemical units, the lowermost two of which are chemically monotonous, suggesting rapid eruption of basalt from a compositionally homogeneous magma chamber. All the basalts are hypersthene-(hy-) rich tholeiites, with approximately chondritic La/Sm, La/Yb, Zr/Nb, La/Ta, and Th/Hf ratios. Chemically they resemble, in part, "transitional" mid-ocean ridge basalts (MORB) from areas such as the Reykjanes Ridge, although Rb, Ba, and K contents are very low and similar to those of "normal" MORB. Their 87 Sr/ 86 Sr ratios are higher than in N-type MORB (Fujii et al., 1981). The chemistry of the Nauru basalts differs from that of continental flood basalts, which tend to be strongly enriched in large-ion lithophile (LIL) elements, although the extent to which the differences result from sialic contamination or source variability is not clear.

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

“…Although the low abundances of these elements and their distinctive ratios in the tholeiitic basalts are akin to some types of mid-ocean ridge basalt (MORB) (Bougault et al, 1980;Saunders, 1984), their flat REE patterns distinguishes them from typical N-type MORB. These distinctive chemical features find their counterpart in clearly different isotopic ratios, which implies the two groups were derived from different mantle sources (Castillo et al, this volume).…”

“…(Winterer, 1973;Schlanger et al, 1981). These submarine provinces are generally considered to represent episodes of crustal thickening soon after or as a continuation of axial activity (Saunders, 1986;Mahoney, 1987) generated above the inflated heads of large mantle plumes during the early Cretaceous (Richards et al, 1989). The tholeiitic basalts of the oceanic plateaus have a broad MORB-like geochemistry, although they exhibit a number of distinctive features that set them apart (Batiza et al, 1980;Floyd, 1989).…”

“…, and the Nauru Basin (Saunders, 1984;Floyd, 1986). Mendoza failed rift data from Pearce et al (1986).…”

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.…”