Major element composition of phenocrysts from the volcanic basement of ODP Hole 125-786B, Izu forearc region, Philippine Sea

Early arc volcanism during Eocene to Oligocene in the Izu forearc region was investigated during ODP Legs 125 and 126 in 1989, and effusive and intrusive volcanics were recovered from Leg 125 Site 786. These rocks were all classified into boninites and associated rocks by Leg 125 Shipboard Scientific Party, and they concluded that boninitic volcanism had occurred before 40 Ma, and arc tholeiitic volcanism began after 40 Ma. In this study, lava flows and breccias that classified into boninite series are divided into two groups, tholeiite and boninite, based on petrographical and petrological properties. Both series are also distinguished by bulk rock composition. It is considered that the sources of both rock types have similar depleted compositions because of their similar, very low bulk HFSE concentrations. We suggest that boninitic and tholeiitic volcanism occurred closely in time and space, and reflected different temperature and water condition.

(Table 1) Compressional wave velocities for rocks from DSDP Hole 31-293 in the Philippine Sea

Compressional wave velocities measured in gabbroic rocks and metabasites recovered from Site 293 of Leg 31 in the Philippine Sea (on the Central Basin Fault) are correlative with seismic velocities determined for Layer 3. The lower crustal origin for these rocks suggested by this data is further supported by the similarity between these samples, dredge haul samples from fracture zones in the main ocean basins and rocks found in ophiolite complexes. These plutonic rocks were possibly introduced to the sea floor by movements along the Central Basin Fault, a major tectonic feature in the Philippine Sea, or formed as part of new ocean crust within a leaky transform fault.

Rare and trace element analyses from DSDP Holes 31-291 and 31-294 from the Philippine Sea

Metal-rich sediments were found in the West Philippine Basin at DSDP sites 291 (located about 500 km SW of the Philippine Ridge or Central Basin Fault) and 294/295 (located about 580 km NE of the Philippine Ridge). In both cases the metalliferous deposits constitute a layer, probably Eocene in age, resting directly above the basaltic basement at the bottom of the sediment column. The chemistry of the major (including Fe and Mn) and trace elements (including trace metals, rare earth elements, U and Th) suggest a strong similarity of these deposits to metalliferous deposits produced by hydrothermal activity at oceanic spreading centers. Well-crystallized hematite is a major component of the metal-rich deposits at site 294/295. We infer that the Philippine Sea deposits were formed at some spreading center by hydrothermal processes of metallogenesis, similar to processes occurring at oceanic spreading centers. A locus for their formation might have been the Philippine Ridge (Central Basin Fault), probably an extinct spreading center. We conclude that metallogenesis of the type occurring at oceanic spreading centers can take place also in marginal basins. This has implications for the origin of metal deposits found in some ophiolite complexes, such as those in Luzon (Philippines), which may represent fragments of former marginal basins rather than of oceanic lithosphere.

Rhenium and platinum-group element concentrations and Os isotope data for Philippine Sea Plate basalts from DSDP Legs 31, 58, and 59

Platinum-group elements (PGE), rhenium and osmium isotope data are reported for basalts from Deep Sea Drilling Project cores in the Philippine Sea Plate (PSP). Lithophile trace element and isotopic characteristics indicate a range of source components including DMM, EMII and subduction-enriched mantle. MORB-like basalts possess smooth, inclined chondrite-normalised PGE patterns with high palladium-PGE/iridium-PGE ratios, consistent with previously published data for MORB, and with the inferred compatibility of PGE. In contrast, while basalts with EMII-type lithophile element chemistry possess high Pt/Ir ratios, many have much lower Pd/Ir and unusually high Ru/Ir of >10. Similarly, back-arc samples from the Shikoku and Parece-Vela basins have very high Ru/Ir ratios (>30) and Pd/Ir as low as 1.1. Such extreme Pd/Ir and Ru/Ir ratios have not been previously reported in mafic volcanic suites and cannot be easily explained by variable degrees of melting, fractional crystallisation or by a shallow-level process such as alteration or degassing. The data appear most consistent with sampling of at least two mantle components with distinct PGE compositions. Peridotites with the required PGE characteristics (i.e. low Pd, but relatively high Ru and Re) have not been documented in oceanic mantle, but have been found in sub-continental mantle lithosphere and are the result of considerable melt depletion and selective metasomatic enrichment (mainly Re). The long-term presence of subduction zones surrounding the Philippine Sea Plate makes this a prime location for metasomatic enrichment of mantle, either through fluid enrichment or infiltration by small melt fractions. The Re-Os isotope data are difficult to interpret with confidence due to low Os concentrations in most samples and the uncertainty in sample age. Data for Site 444A (Shikoku Basin) give an age of 17.7+/-1.3 Ma (MSWD = 14), consistent with the proposed age of basement at the site and thus provides the first robust radiometric age for these samples. The initial 187Os/188Os of 0.1298+/-0.0069 is consistent with global MORB, and precludes significant metasomatic enrichment of Os by radiogenic slab fluids. Re-Os data for Sites 446A (two suites, Daito Basin) and 450 (Parece-Vela Basin) indicate ages of 73, 68 and 43 Ma, which are respectively, 30, 17 and >12 Ma older than previously proposed ages. The alkalic and tholeiitic suites from Site 446A define regression lines with different 187Os/188Osinitial (0.170+/-0.033 and 0.112+/-0.024, respectively) which could perhaps be explained by preferential sampling of interstitial, metasomatic sulphides (with higher time-integrated Re/Os ratios) by smaller percentage alkalic melts. One sample, with lithophile elements indistinguishable from MORB, is Os-rich (146 pg/g) and has an initial 187Os/188Os of 0.1594, which is at the upper limit of the accepted OIB range. Given the Os-rich nature of this sample and the lack of evidence for subduction or recycled crust inputs, this osmium isotope ratio likely reflects heterogeneity in the DMM. The dataset as a whole is a striking indication of the possible PGE and Os isotope variability within a region of mantle that has experienced a complex tectonic history.

Chemical composition of <0.001 mm grain size fraction from sedimentary rocks and bottom sediments of the Philippine Sea

Distriburtion and formation of clay minerals in different types of bottom sediments from the West Pacific are under consideration.