Spherules and basement analyses from DSDP Hole 5-32

Lower Miocene basaltic glass spherules from DSDP Site 32 pelagic sediments in the eastern Pacific are compositionally diverse, and new analyses and interpretations have been added to those of earlier workers. The spherules are of titanian ferrobasalt which is compositionally similar to highly evolved abyssal basalts and to some oceanic island eruptives, and they were most likely shaped during intense lava fountaining during a number of separate eruptions. These eruptions tapped distinct but related magma batches in terms, for example, of distinctively high TiO2 and FeO* contents. Their age overlaps that of some of the eruptions of the Columbia River Plateau Basalts, but they are compositionally distinct from most of the latter basalts. Although about 15 m.y. old, they show little alteration. The low chlorine and sulfur contents compared to those of abyssal ferrobasalts are consistent with degassing prior to quenching during subaerial eruptions, and rule out production of the spherules by submarine fountaining. Lava fountaining alone is insufficient to account for the distance of about 100 km from even the closest possible seamount source. Instead, large phreatomagmatic eruption columns reaching at least 15 km and including lava fountaining immediately after the initial explosion are required. Alternatively, and deemed less likely, is their deposition by turbidites derived from Pioneer Seamount.

Experimental petrology of basement basaltic rocks from ODP Leg 127/128 samples

The relatively fresh basement basaltic rocks cored at Sites 794 and 797 during ODP Legs 127 and 128 show compositional variations suggesting the following: (1) the aphyric rocks might be differentiated from compositional equivalents of the aphyric sample with the lowest FeO*/MgO (Sample 127-797C-12R-4, 35-37 cm); and (2) the plagioclase-phyric rocks (i.e., another constituent of the basement basaltic rocks from the sites) may be derivatives from the same parents; in this case, however, crystallized plagioclase was not effectively removed. Melting experiments were conducted for Sample 127-797C-12R-4, 35-37 cm, and the differentiation processes for the basement basaltic rocks were assessed. The high-pressure melting-phase relation can not account for the compositional variation of the aphyric rocks, suggesting that the variation was developed at relatively low pressure where olivine and plagioclase fractionation was followed by Ca-rich clinopyroxene fractionation. The density of Sample 127-797C-12R-4,35-37 cm, is comparable to that of plagioclase at some depth, but at still relatively low pressure, making it possible that the liquidus plagioclase was retained in the successive liquids to produce the plagioclase-phyric rocks. According to backtrack calculation assuming the olivine maximum fractionation, Sample 127-797C-12R-4, 35-37 cm, was differentiated from primary picritic high-Al basalt magma. The estimated primary magma composition was experimentally proved to coexist with harzburgite mantle at about 14 kbar, suggesting relatively shallow production (approximately 40-50 km below surface) of the rifting-related primary magma.

Geochemistry of boninite and harzburgite of ODP Leg 125 basement samples

Holes drilled into the volcanic and ultrabasic basement of the Izu-Ogasawara and Mariana forearc terranes during Leg 125 provide data on some of the earliest lithosphere created after the start of Eocene subduction in the Western Pacific. The volcanic basement contains three boninite series and one tholeiite series. (1) Eocene low-Ca boninite and low-Ca bronzite andesite pillow lavas and dikes dominate the lowermost part of the deep crustal section through the outer-arc high at Site 786. (2) Eocene intermediate-Ca boninite and its fractionation products (bronzite andesite, andesite, dacite, and rhyolite) make up the main part of the boninitic edifice at Site 786. (3) Early Oligocene intermediate-Ca to high-Ca boninite sills or dikes intrude the edifice and perhaps feed an uppermost breccia unit at Site 786. (4) Eocene or Early Oligocene tholeiitic andesite, dacite, and rhyolite form the uppermost part of the outer-arc high at Site 782. All four groups can be explained by remelting above a subduction zone of oceanic mantle lithosphere that has been depleted by its previous episode of partial melting at an ocean ridge. We estimate that the average boninite source had lost 10-15 wt% of melt at the ridge before undergoing further melting (5-10%) shortly after subduction started. The composition of the harzburgite (<2% clinopyroxene, Fo content of about 92%) indicates that it underwent a total of about 25% melting with respect to a fertile MORB mantle. The low concentration of Nb in the boninite indicates that the oceanic lithosphere prior to subduction was not enriched by any asthenospheric (OIB) component. The subduction component is characterized by (1) high Zr and Hf contents relative to Sm, Ti, Y, and middle-heavy REE, (2) light REE-enrichment, (3) low contents of Nb and Ta relative to Th, Rb, or La, (4) high contents of Na and Al, and (5) Pb isotopes on the Northern Hemisphere Reference Line. This component is unlike any subduction component from active arc volcanoes in the Izu-Mariana region or elsewhere. Modeling suggests that these characteristics fit a trondhjemitic melt from slab fusion in amphibolite facies. The resulting metasomatized mantle may have contained about 0.15 wt% water. The overall melting regime is constrained by experimental data to shallow depths and high temperatures (1250? C and 1.5 kb for an average boninite) of boninite segregation. We thus envisage that boninites were generated by decompression melting of a diapir of metasomatized residual MORB mantle leaving the harzburgites as the uppermost, most depleted residue from this second stage of melting. Thermal constraints require that both subducted lithosphere and overlying oceanic lithosphere of the mantle wedge be very young at the time of boninite genesis. This conclusion is consistent with models in which an active transform fault offsetting two ridge axes is placed under compression or transpression following the Eocene plate reorganization in the Pacific. Comparison between Leg 125 boninites and boninites and related rocks elsewhere in the Western Pacific highlights large regional differences in petrogenesis in terms of mantle mineralogy, degree of partial melting, composition of subduction components, and the nature of pre-subduction lithosphere. It is likely that, on a regional scale, the initiation of subduction involved subducted crust and lithospheric mantle wedge of a range of ages and compositions, as might be expected in this type of tectonic setting.

Prominent deformation features and interpretation of sediment core CRP-2/2A (Table 1)

Sediment deformation features in CRP-2/2A were described during normal logging procedures and from core-scan images. In this paper the origin of soft-sediment folding, contorted bedding, microfaulting, clastic dykes, shear zones and intraformational breccias is discussed. The features have a stratigraphic distribution related to major unconformities and sequence boundaries. Hypotheses for the origins of sediment deformation include hydrofracturing, subglacial shearing, slumping, and gas hydrate formation. Shear zones, microfaults, clastic dykes and contorted bedding within rapidly deposited sediments, suggest that slumping in an ice-distal environment occurred in the early Oligocene. A till wedge beneath a diamictite at 364 mbsf the mid-Oligocene section represents the oldest evidence of grounded ice in CRP-2/2A. Shear zones with a subglacial origin in the early late Oligocene and early Miocene sections of the core are evidence of further grounding events. The interpretation of sediment deformation in CRP-2/2A is compared to other Antarctic stratigraphic records and global eustatic change between the late Eocenel/early Oligocene and the middle Miocene.

Basalt density, basement age, and intrusive/extrusive relations from DSDP Legs 2 through 7, 9, and 14

Densities of layer 2 basalt recovered during the Deep Sea Drilling Project have been found to decrease steadily with age, a finding ascribed to progressive submarine weathering in the context of sea-floor spreading. The least-squares solution for 52 density measurements gives a rate of decrease in density of (Delta p)/(Delta t) = -0.0046 g per ccm m.y. = -16 percent per 100 m.y., which is in excellent agreement with earlier estimates based on observed chemical depletion rates of dredged oceanic basalt. Weathering of sea-floor basalt, should it penetrate to any considerable depth in layer 2, will decrease layer 2 seismic refraction velocities, act as a source of geothermal heat, and substantially influence the chemistry of sea water and the overlying column of sediment.