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Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95447/1/eost7407.pd

Chemical composition of igneous rocks and origin of the sill and pillow-basalt complex at DSDP Site 61-462

The sill and pillow complex cored on Deep Sea Drilling Project Leg 61 (Site 462) is divided into two groups, A and B types, on the basis of chemical composition and volcanostratigraphy. The A-type basalt is characterized by a higher FeO*/MgO ratio and abundant TiO2, whereas the B-type basalt is characterized by a lower FeO*/MgO ratio and scarcity of TiO2. The A type is composed of sills interbedded with hyaloclastic sediments, and the B type consists of basalt sills and pillow basalt with minor amounts of sediment. However, the structure of pillow basalts in the B type is atypical; they might be eruptive. From paleontological study of the interbedded sediments and radiometric age determination of the basalt, the volcanic event of A type is assumed to be Cenomanian to Aptian, and that of B type somewhat older. The oceanic crust in the Nauru Basin was assumed to be Oxfordian, based on the Mesozoic magnetic anomaly. Consequently, two events of intraplate volcanism are recognized. It is thus assumed that the sill-pillow complex did not come from a normal oceanic ridge, and that normal oceanic basement could therefore underlie the complex. The Site 462 basalts are quartz-normative, and strongly hypersthene-normative, and have a higher FeO*/MgO ratio and lower TiO2 content. Olivine from the Nauru Basin basalts has a lower Mg/(Mg + Fe**2+) ratio (0.83-0.84) and coexists with spinel of lower Mg/(Mg + Fe**2+) ratio when compared to olivine-spinel pairs from mid-ocean ridge (MAR) basalt. The glass of spinel-bearing basalts has a higher FeO*/(FeO* + MgO) ratio (0.58-0.60) than that of MAR (<0.575). Therefore, the Nauru Basin basalts are chemically and mineralogically distinct from ocean-ridge tholeiite. That the Nauru Basin basalts are quartz-normative and strongly hypersthene-normative and have a lower TiO2 content suggests that the basaltic liquids of Site 462 were generated at shallower depths (<5 kbar) than ocean-ridge tholeiite: Site 462 basalts are similar to basalts from the Manihiki Plateau and the Ontong-Java Plateau, but different from Hawaiian tholeiite of hot-spot type, with lower K2O and TiO2 content. We propose a new type of basalt, ocean-plateau tholeiite, a product of intraplate volcanism

Trace element evidence from seamounts for recycled oceanic crust in the Eastern Pacific mantle

We present trace element data for 80 samples from about 50 seamounts in the east equatorial Pacific near the East Pacific Rise. These data indicate that the heterogeneous mantle source that supplies the seamounts consists of two components: (1) an extremely depleted component, much more depleted than estimates of the source of depleted MORB; and (2) an enriched component even more enriched than average OIB. The depleted component shows large variations in Zr/Hf, Nb/Ta, Rb/Cs, Ce/Pb, and Th/U that are correlated with each other and with La/Sm, indicating that these paired elements do fractionate from each other in some oceanic basalts. The order of incompatibility of trace elements we find differs slightly from that found elsewhere. For example, for seamounts, we find that D ≈ D < D ≈ D. In comparison with Th and U, the enriched component shows anomalous enrichments of Ta and Nb. Since such fractionations are characteristic of subduction zones, we suggest that the most likely ultimate source of the enriched component is recycled ocean crust