Geochemistry of ash and pumice of ODP Hole 132-810C

Air-fall volcanic ash and pumice were recovered from 22 intervals in upper Miocene-Pleistocene nannofossil oozes cored in Hole 810C on Shatsky Rise, northwest Pacific. Shatsky Rise is near the eastern limit of ash falls produced by explosive volcanism in arc systems in northern Japan and the Kuriles, more than 1600 km away. Electron probe analyses establish that the ash beds and pumice pebbles are andesitic to rhyolitic in composition, and belong to both tholeiitic and high-alumina lineages similar to tephra from Japanese volcanoes. High-speed winds in the polar-front and subtropical jets are evidently what propelled the ash for such a distance. The pumice arrived by flotation, driven from the same directions by winds, waves, and currents. It is not ice-rafted debris from the north. One thick pumice bed probably was deposited when a large pumice mat passed over Shatsky Rise. Far more abundant ash occurs in sediments cored at DSDP Sites 578 through 580, about 500 km west of Shatsky Rise. Most of the ash and pumice at Shatsky Rise can be correlated with specific ash beds at 1, 2, or all 3 of these sites by interpolating to precisely determined magnetic reversal sequences in the cores. Most of the correlations are to thick ash layers (5.7 +/- 3.0 cm) at one or more sites. These must represent extremely large eruptions that spread ash over very wide areas. Whereas several of the thicker correlative ashes fell from elongate east-trending plumes directed from central Japan, the majority of them - dating from about 2 Ma - came from the North Honshu and Kurile arc systems to the northwest. This direction probably was in response to both long-term and seasonal fluctuations in the location and velocity of the polar-front jet, and to more vigorous winter storm fronts originating over glaciated Siberia.

Chemical composition of glass inclusions from ODP Holes 157-953C and 157-956B

We report S concentrations and relative proportions of (SO4)2- and S2- in OL- and CPX-hosted glass inclusions and in host glassy lapilli from Miocene basaltic hyaloclastites drilled north and south of Gran Canaria during ODP Leg 157. Compositions of glass inclusions and lapilli resemble those of subaerial Miocene shield basalts on Gran Canaria and comprise mafic to more evolved tholeiitic to alkali basalt and basanite (10.3-3.7 wt.% MgO, 44.5-56.9 wt.% SiO2). Glass inclusions fall into three groups based on their S concentrations: a high-sulfur group (1050 to 5810 ppm S), an intermediate-sulfur group (510 to 1740 ppm S), and a low-sulfur group (<500 ppm S). The most S-rich inclusions have the highest and nearly constant proportion of sulfur dissolved as sulfate determined by electron microprobe measurements of SKa peak shift. Their average S6+/S_total value is 0.75+/-0.09, unusually high for ocean island basalt magmas. The low-sulfur group inclusions have low S6+/S_total ratios (0.08+/-0.05), whereas intermediate sulfur group inclusions show a wide range of S6+/S_total (0.05-0.83). Glassy lapilli and their crystal-hosted glass inclusions with S concentrations of 50 to 1140 ppm S have very similar S6+/S_total ratios of 0.36+/-0.06 implying that sulfur degassing does not affect the proportion of (SO4)2- and S2- in the magma. The oxygen fugacities estimated from S6+/S_total ratios and from Fe3+/Fe2+ ratios in spinel inclusions range from NNO-1.1 to NNO+1.8. The origin of S-rich magmas is unclear. We discuss (1) partial melting of a mantle source at relatively oxidized fO2 conditions, and (2) magma contamination by seawater either directly or through magma interaction with seawater-altered Jurassic oceanic crust. The intermediate sulfur group inclusions represent undegassed or slightly degassed magmas similar to submarine OIB glasses, whereas the low-sulfur group inclusions are likely to have formed from magmas significantly degassed in near-surface reservoirs. Mixing of these degassed magmas with stored volatile-rich ones or volatile-rich magma replenishing the chamber filled by partially degassed magmas may produce hybrid melts with strongly varying S concentrations and S6+/S_total ratios.

Chemical analysis of ODP Hole 104-642E

During Ocean Drilling Program Leg 104 a 900-m-thick sequence of volcanic rocks was drilled at Hole 642E on the Vøring Plateau, Norwegian Sea. This sequence erupted in two series (upper and lower series) upon continental basement. The upper series corresponds to the seaward-dipping seismic reflectors and comprises a succession of about 122 flows of transitional oceanic tholeiite composition. They have been subdivided into several formations consisting of flows related to each other by crystal fractionation processes, magma mixing, or both. Major- and trace-element chemistry indicates affinities to Tertiary plateau lavas of northeast Greenland and to Holocene lavas from shallow transitional segments of the Mid-Atlantic Ridge, such as Reykjanes Ridge. The tholeiitic magmas have been derived from a slightly LREE-depleted mantle source. Two tholeiitic dikes that intruded the lower series derive from an extremely depleted mantle source. Interlayered volcaniclastic sediments are dominantly ferrobasaltic and more differentiated. They appear to come from a LREE-enriched mantle source, and may have been erupted in close vicinity of the Vøring Plateau during hydroclastic eruptions. The two tholeiitic dikes that intruded the lower series as well as some flows at the base of the upper series show evidence of assimilation of continental upper crustal material.

Petrographic and chemical characteristics of abyssal tholeiites at DSDP Leg 63 Holes

Tholeiitic basalts were obtained from basaltic basement ranging in age from 6 to 17 m.y. on IPOD/DSDP Leg 63. The main rock types encountered at all sites but 473 are basaltic pillow lavas. Although many of these pillow basalts are highly or moderately altered, fresh glass is usually present. At Site 473, we recovered coarse-grained, massive basalts; no clearly defined pillowed forms were observed. Phenocrysts or microphenocrysts present in the Leg 63 basalts are Plagioclase and clinopyroxene at Site 469; olivine, Plagioclase, and spinel at Site 470; and olivine, Plagioclase, and clinopyroxene at Sites 472 and 473. Olivines of the basalts from Holes 470A and 472 (Fo85-88) are generally more magnesian than those of the Hole 473 basalts (Fo77-81). Also, plagioclases of Holes 470A and 472 basalts (An70-85) are generally more calcic than those of Holes 469 and 473 basalts (An66-72). Geochemical study of the Leg 63 basalts indicates that in all cases they are large-ion-lithophile (LIL) element depleted tholeiites like typical abyssal tholeiites. In particular, they are very similar in composition to those described from the eastern Pacific, although the degree of iron enrichment found in the Leg 63 basalts is not as extensive as in basalts from the Galapagos spreading center. Hence, the geochemical evidence of the Leg 63 basalts is compatible with their formation at a spreading center. Compositional variations in Leg 63 basalts from any single drill hole is small. Major and trace element data indicate that the samples from Holes 469 and 473 are more fractionated in chemical composition than are the samples from Holes 470A and 472; this compositional variation may be largely ascribed to differences in the extent of shallow-level fractional crystallization of similar parental magma. The Hole 472 samples, however, show a LIL element character distinct from the other Leg 63 samples.

Chemical composition of rocks and minerals from the ophiolite complex in the Hunter Fracture Zone

A collection of dredge samples from the Hunter Fracture Zone includes holocrystalline massive and cumulose basic and ultrabasic rocks and volcanites of the ophiolite complex: from basalts to rhyolites. The ultrabasic rocks are largely serpentinized harzburgites and lherzolites; their relict mineralogy is typical of peridotite considered to be the refractory residue of partial melting of the mantle. Cumulate textured ultramafic rocks probably are related to the cumulate gabbro and granodiorite rather than to the residual mantle material. The gabbroic rocks are dominantly cumulate textured Pl-Opx-Cpx±Ol gabbronorite and Pl-Cpx±Ol gabbros; the mineral features of these rocks are the result of their crystallization at moderate pressure (in a moderate level magma chamber). The massive Pl-Cpx±Ol gabbros are less common. Green and brown-green Ca-amphibole has partially or totally replaced the clinopyroxene in many samples. There is an overlap in mineral chemistry between the cumulate rocks and the Opx-Cpx-Pl volcanic rocks and boninites. It is interpreted as an indication that the cumulate rocks were co-genetic with Opx-Cpx-Pl volcanic rocks and that they both constitute remnants of an island arc volcanic-plutonic series. The petrologic evidence indicates that ophiolite gabbroic rocks were derived from an island-arc rather than from a mid-ocean ridge.

Chemistry of gabbroic rocks from ODP Hole 118-735B

Gabbroic rocks and their late differentiates recovered at Site 735 represent 500 m of oceanic layer 3. The original cooling of a mid-ocean ridge magma chamber, its penetration by ductile shear zones and late intrusives, and the subsequent penetration of seawater through a network of cracks and into highly permeable magmatic hydrofracture horizons are recorded in the metamorphic stratigraphy of the core. Ductile shear zones are characterized by extensive dynamic recrystallization of primary phases, beginning in the granulite facies and continuing into the lower amphibolite facies. Increasing availability of seawater during dynamic recrystallization is reflected in depletions in 18O, increasing abundance of amphibole of variable composition and metamorphic plagioclase of intermediate composition, and more complete coronitic or pseudomorphous static replacement of magmatic minerals. Downcore correlation of synkinematic assemblages, bulk-rock oxygen isotopic compositions, and vein abundance suggest that seawater is introduced into the crust by way of small cracks and veins that mark the end of the ductile phase of deformation. This "deformation-enhanced" metamorphism dominates the upper 180 and the lower 100 m of the core. In the lower 300 m of the core, mineral assemblages of greenschist and zeolite facies are abundant within or adjacent to brecciated zones. Leucocratic veins found in these zones and adjacent host rock contain diopside, sodic plagioclase, epidote, chlorite, analcime, thomsonite, natrolite, albite, quartz, actinolite, sphene, brookite, and sulfides. The presence of zircon, Cl-apatite, sodic plagioclase, sulfides, and diopside in leucocratic veins having local magmatic textures suggests that some of the veins originated from late magmas or from hydrothermal fluids exsolved from such magmas that were subsequently replaced by (seawater-derived) hydrothermal assemblages. The frequent association of these late magmatic intrusive rocks within the brecciated zones suggests that they are both artifacts of magmatic hydrofracture. Such catastrophic fracture and hydrothermal circulation could produce episodic venting of hydrothermal fluids as well as the incorporation of a magmatically derived hydrothermal component. The enhanced permeability of the brecciated zones produced lower temperature assemblages because of larger volumes of seawater that penetrated the crust. The last fractures were sealed either by these hydrothermal minerals or by late carbonate-smectite veins, resulting in the observed low permeability of the core.

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.

Chemical composition of phosphorites, Fe-Mn nodules and crusts, and basalts from the West Pacific

The monograph gives the first systematic description of ore-bearing guyots from the West Pacific. It is mostly based on data obtained in numerous expeditions of Russian vessels during 1984-1992. Ore deposits located on upper parts of all slopes and tops of the guyots include phosphorites associated with cobalt- and platinum-rich ferromanganese crusts. Location, origin and prospecting of mineral deposits are discussed on the base of new data on metallogenic factors (geodynamics, tectonics, magmatism, sedimentation and morphostructures).

Chemical analysis of ash layers in ODP Leg 104 holes

Numerous fresh ash layers comprise about 0.3% by volume of Neogene to Holocene sediments drilled at Leg 104 Sites 642 and 643 (Vøring Plateau, North Atlantic). Median grain sizes of the ashes are about 100 /µm and maximum grain sizes range up to 1200 µm. Rhyolitic pumice shards dominate, with minor bubble wall shards. Basaltic shards are poorly vesicular and blocky or round. Phenocrystic plagioclase, zircon, and clinopyroxene occur in the rhyolitic, plagioclase, and clinopyroxene phenocrysts and basaltic lithics in the basaltic tephra. Quartz, amphibole, clinozoisite, and rutile are interpreted as xenocrysts. All ash layers are well-sorted and represent distal fallout from major explosive eruptions. Most ashes are rhyolitic (high-K and low-K) in composition, some are bimodal (tholeiitic and rhyolitic). Early Miocene tephra is dominantly basaltic. Iceland is inferred to be the likely source region for most ashes. Late Miocene high-K rhyolites may have originated from the K-rich Jan Mayen magmatic province. One Quaternary layer with biotite and alkali feldspar phenocrysts may have been derived from Jan Mayen Island. Four individual Pliocene to Holocene ash layers from Sites 642 and 643 can be correlated fairly well. Upper Miocene layers are tentatively correlated as a sequence between Sites 642 and 643. Average calculated layer frequencies are about three layers/m.y. through the Pliocene and Pleistocene and five to eight layers per m.y. through the middle and late Miocene, suggesting rather continuous volcanic activity in the North Atlantic. Episodic magmatic activity during Neogene epochs in this part of the North Atlantic, as postulated in the literature, cannot be confirmed.

Lithologic and geochemical composition of samples from the Circum Superior Belt, North America

The ca. 1880 Ma Circum-Superior Large Igneous Province (LIP) consists of a number of discontinuous segments known to cover a significant portion of the margin of the Superior Province craton in North America. New geochemical and isotopic data from western segments of this LIP support a common origin for the these segments and suggest that magmatism in the Lake Superior region may have been fed through the ~ 600 km long Pickle Crow dyke from a source north of the Fox River Belt in northeastern Manitoba. The Fox River Belt, Pickle Crow dyke and sections of the Hemlock Formation in the Lake Superior region possess trace element signatures which are similar to those of more recent oceanic plateaux. The Hemlock Formation displays a heterogeneous geochemical signature. This chemical heterogeneity can in part be explained by lithospheric contamination and possibly by source heterogeneity. The tectonomagmatic setting in which these igneous rocks were formed could have involved a mantle plume. Evidence supporting a plume origin includes high MgO volcanic rocks, high calculated degrees of partial melting and geochemical signatures similar to those of oceanic plateaux.

Major elements oxides and mineralogy of primary minerals in ODP Hole 195-1201D basalts

The basement cored at Site 1201 (west Philippine Basin) during Ocean Drilling Program Leg 195 consists of a 91-m-thick sequence of basalts, mostly pillow lavas and perhaps one sheet lava flow, with a few intercalations of hyaloclastite and interpillow sedimentary material. Hydrothermal alteration pervasively affected the basalt sequence, giving rise to a variety of secondary minerals such as K-Fe-Mg-clay minerals, oxyhydroxides and clay minerals mixtures, natrolite group zeolites, analcite, alkali feldspar, and carbonate. The primary minerals of pillow and sheet basalts that survived the intense hydrothermal alteration were investigated by electron microprobe with the aim of characterizing their chemical composition and variability. The primary minerals are mostly plagioclase, ranging in composition from bytownite through labradorite to andesine, chromian-magnesian-diopside, and spinels, both Ti magnetite (partially maghemitized) and chromian spinel. Overall, the chemical features of the primary minerals of Site 1201 basalts correspond to the primitive character of the bulk rocks, suggesting that the parent magma of these basalts was a mafic tholeiitic magma that most likely only suffered limited fractional crystallization and crystallized at high temperatures (slightly below 1200°C) and under increasing fO2 conditions. The major element composition of clinopyroxene suggests a backarc affinity of the mantle source of Site 1201 basement.

Chemical composition of basalts from coarse debris of the Kara Sea bottom sediments

Study of basaltic debris from the Kara Sea bottom has shown its similarity to traps of the Eastern Siberia in mineralogy, structures and chemical composition. In comparison with oceanic tholeiites, the source of traps and Kara Sea basin basaltic melts was enriched in REE and some other incompatible elements. K-Ar dating of two samples of supposed autochtonous location from the eastern part of the Kara Sea basin has shown 209 and 218 Ma - younger than traps (247-248 Ma). Origin of Siberian traps used to connect with action of the mantle plume (Iceland plume, according to geodinamic reconstruction). Our new age data may be interpreted as an evidence of the Siberian plate moving over the head of plume.

Elemental analyses (XRF), total organic carbon (TOC), grain size analyses, lamination index of sediment core SO130-289KL

The Dansgaard-Oeschger oscillations and Heinrich events described in North Atlantic sediments and Greenland ice are expressed in the climate of the tropics, for example, as documented in Arabian Sea sediments. Given the strength of this teleconnection, we seek to reconstruct its range of environmental impacts. We present geochemical and sedimentological data from core SO130-289KL from the Indus submarine slope spanning the last ~ 80 kyr. Elemental and grain size analyses consistently indicate that interstadials are characterized by an increased contribution of fluvial suspension from the Indus River. In contrast, stadials are characterized by an increased contribution of aeolian dust from the Arabian Peninsula. Decadal-scale shifts at climate transitions, such as onsets of interstadials, were coeval with changes in productivity-related proxies. Heinrich events stand out as especially dry and dusty events, indicating a dramatically weakened Indian summer monsoon, potentially increased winter monsoon circulation, and increased aridity on the Arabian Peninsula. This finding is consistent with other paleoclimate evidence for continental aridity in the northern tropics during these events. Our results strengthen the evidence that circum-North Atlantic temperature variations translate to hydrological shifts in the tropics, with major impacts on regional environmental conditions such as rainfall, river discharge, aeolian dust transport, and ocean margin anoxia.

Geochemistry of minerals at DSDP Holes 68-501 and 69-504B

The interaction of seawater with basalts in DSDP Hole 501 and the upper part of Hole 504B (Costa Rica Rift) produced oxidative alteration and a zonation of clay minerals along cracks. From rock edges to interiors in many cracks the following succession occurs, based on microscopic observations and microprobe analysis: iron hydroxides (red), "protoceladonite" (green), iddingsite (orange), and saponite (yellow). Clay minerals replace olivines and fill vesicles and cracks. Other secondary minerals are phillipsite, aragonite, and unidentified carbonates. Some glass is transformed to Mg-rich palagonite. Bulk rock chemistry is related to the composition of the secondary minerals. The zonation can be interpreted as a succession of postburial nonoxidative and oxidative diagenesis similar to that described in the Leg 34 basalts.