Chemical and mineral compositions of basalts and volcanic glasses from DSDP Sites 65-483 and 65-485

Major and rare earth element (REE) data for basalts from Holes 483, 483B, and 485A of DSDP Leg 65, East Pacific Rise, mouth of the Gulf of California, support a simple fractional crystallization model for the genesis of rocks from this suite. The petrography and mineral chemistry (presented in detail elsewhere) provide no evidence for magma mixing, but rather a simple multistage cooling process. Based on its lowest TiO2 content (0.88%), FeO*/MgO ratio (0.95 with total Fe as FeO), and Mg# (100 Mg/Mg + Fe" = 70), sample 483-17-2-(78-83) has been selected as the most primitive primary magma of the samples analyzed. This is supported by the REE data which show this sample has the lowest total REE content, a La/Sm_cn (chondrite-normalized) = 0.36, and Eu/Sm_cn = 1.05. Because other samples analyzed have higher SiO2, lower Mg#, and a negative Eu anomaly (Eu/Sm_cn as low as 0.89), they are most likely derivative magmas. Wright-Doherty and trace element modelling support fractional crystallization of 14.1% plagioclase (An88), 6.7% olivine (Fo86), and 4.7% clinopyroxene (Wo41En49Fs10) from 483-17-2-(78-83) to form the least differentiated sample with Mg# = 63. The La/Sm_cn of this derivative magma is almost identical to the parent magma (0.35 to 0.36), but the other samples have higher La/Sm_cn (0.45 to 0.51), more total REE, and lower Mg# (60 to 56). Both Wright-Doherty and trace element modelling indicate that the primary magma chosen cannot produce these more evolved samples. For the major elements, the TiO2 and P2O5 are too low in the calculated versus the observed (1.38 to 1.90; 0.11 to 0.17, respectively, for example). Rayleigh fractionation calculates a lower La/Sm_cn and requires about 60% crystal removal versus 40% for the Wright-Doherty. These more evolved samples must be derived from a parent magma different from the one selected here and, unfortunately, not sampled in this study. A magma formed by a smaller degree of partial melting with slightly more residual clinopyroxene left in the mantle than for sample 483-17-2-(78-83) is required.

Geochemistry of basaltic rocks from ODP Hole 183-1137A on the Kerguelen Plateau

Cretaceous basalts recovered during Ocean Drilling Program Leg 183 at Site 1137 on the Kerguelen Plateau show remarkable geochemical similarities to Cretaceous continental tholeiites located on the continental margins of eastern India (Rajmahal Traps) and southwestern Australia (Bunbury basalt). Major and trace element and Sr-Nd-Pb isotopic compositions of the Site 1137 basalts are consistent with assimilation of Gondwanan continental crust (from 5 to 7%) by Kerguelen plume-derived magmas. In light of the requirement for crustal contamination of the Kerguelen Plateau basalts, we re-examine the early tectonic environment of the initial Kerguelen plume head. Although a causal role of the Kerguelen plume in the breakup of Eastern Gondwana cannot be ascertained, we demonstrate the need for the presence of the Kerguelen plume early during continental rifting. Activity resulting from interactions by the newly formed Indian and Australian continental margins and the Kerguelen plume may have resulted in stranded fragments of continental crust, isolated at shallow levels in the Indian Ocean lithosphere.

Chemical and isotope compositions of rocks dredged from the Shatsky Rise

Magmatic rocks of the Shatsky Rise form two groups replacing one another in time. The earlier ferrotholeiites enriched in potassium compose large massifs. Trachybasalts form seamounts and neotectonic ridges. Composition of volcanites indicates that two sources of magmatism took part in their formation: a depleted source characteristic of basalts of mid-ocean ridges and a ''plume'' source participating in formation of oceanic plateaus.

Geochemistry at DSDP Holes 80-550 amd 80-548

At Sites 548 and 550 of DSDP Leg 80 several condensed sedimentary sections contain various types of polymetallic crusts. The relationships between mineralogic and geochemical data in the sections have been studied in the context of the biostratigraphic and sedimentologic results. The diagenetic evolution during periods of low accumulation rate varies according to depth and sedimentary environment. At Site 548 on the continental margin, the phosphatic and manganiferous crusts are similar to those related to upwelling influences before Late Cretaceous deposition. At Site 550 the upper Paleocene cherts, deposited directly on oceanic crust, are overlain by pelagic brown clays containing diagenetic manganiferous concretions characterized by very high Sr and Ba contents. The origin of these small nodules is probably related to the authigenesis of fecal pellets. The upper Eocene indurated section is made up of authigenic zeolites, clays, and Fe-Mn phases and is similar to the volcanic-sedimentary deposits described in deep basins and seamounts of the Pacific. These crusts and a polynucleated nodule within the overlying sediments have geochemical characteristics (high Ni, Co, and Cu contents) similar to those formed in the deep ocean under volcanic influences during periods of low sedimentation rates or sedimentary hiatuses. Volcaniclastic material is ubiquitous and peculiarly abundant in Eocene sections and can be related to the volcanic formation of Iceland in the North Atlantic.

Chemiscal composition of basalts from DSDP Legs 69 and 70 Holes

Chemical compositions and 1-atm. phase relations were determined for basalts drilled from Holes 501, 504A, 504B, 505, and 505B on Legs 68, 69, and 70 of the Deep Sea Drilling Project. Chemical, experimental, and petrographic data indicate that these basalts are moderately evolved (Mg' values from 0.60 to 0.70), with olivine plus Plagioclase and often clinopyroxene on the liquidus. Chemical stratigraphy was used to infer that sequential influxes of magma into a differentiating magma chamber or separate flows from different magma chambers or both had occurred. Two major types of basalt were found to be inter layered: Group M, a rarely occurring type with major element chemistry and magmaphile element abundances within the range of the majority of ocean-floor basalts (TiO2 = 1.3%, Na2O 2.5%, Zr = 103 ppm, Nb = 2.5 ppm, and Y = 31 ppm); and Group D, a highly unusual series of basalt compositions that exhibit much lower magmaphile element abundances (TiO2 = 0.75-1.2%, Na2O = 1.7-2.3%, Zr = 34-60 ppm, Nb = 0.5-1.2 ppm, and Y = 16-27 ppm). The liquidus temperatures of the Group D basalts are high (1230- 1260°C) compared with those of other ocean-floor basalts of similar Mg' values. They have high CaO/Na2O ratios (5-8) and are calculated to be in equilibrium with unusually calcic Plagioclase (An78-84). The two basalt groups cannot be related by fractionation processes. However, constant Zr/Nb ratios (>40) for the two groups suggest a single mantle source, with differences in magmaphile element abundances and other element ratios (e.g., Zr/Ti, Zr/Y, Ce/Yb) arising through sequential melting of the same source. Magmas similar to Group D, if mixed with more typical mid-ocean-ridge basalt (MORB) magmas in shallow magma chambers, could provide a source for the highly calcic Plagioclase phenocrysts that appear in more common (i.e., less depleted) phyric ocean-floor basalts.

Geochemistry of sediments beneath the Tonga-Kermadec arc

The fate of subducted sediment and the extent to which it is dehydrated and/or melted before incorporation into arc lavas has profound implications for the thermo-mechanical nature of the mantle wedge and models for crustal evolution. In order to address these issues, we have undertaken the first measurements of 10Be and light elements in lavas from the Tonga-Kermadec arc and the sediment profile at DSDP site 204 outboard of the trench. The 10Be/9Be ratios in the Tonga lavas are lower than predicted from flux models but can be explained if (a) previously estimated sediment contributions are too high by a factor of 2-10, (b) the top 1-22 m of the incoming sediment is accreted, (c) large amounts of sediment erosion are proposed, or (d) the sediment component takes several Myr longer than the subducting plate to reach the magma source region beneath Tonga. The lavas form negative Th/Be-Li/Be arrays that extend from a depleted mantle source composition to lower Th/Be and Li/Be ratios than that of the bulk sediment. Thus, these arrays are not easily explained by bulk sediment addition and, using partition coefficients derived from experiments on the in-coming sediment, we show that they are also unlikely to result from fluid released during dehydration of the sediment (or altered oceanic crust). However, partial melts of the dehydrated sediment residue formed at ~800 °C during the breakdown of amphibole +/- plagioclase and in the absence of cordierite have significantly lowered Th/Be ratios. The lava arrays can be successfully modelled as 10-15% partial melts of depleted mantle after it has been enriched by the addition of 0.2-2% of these partial melts. Phase relations suggest that this requires that the top of the subducting crust reaches temperatures of ~800 °C by the time it attains ~ 80 km depth which is in excellent agreement with the results of recent numerical models incorporating a temperature-dependent mantle viscosity. Under these conditions the wet basalt solidus is also crossed yet there is no recognisable eclogitic signal in the lavas suggesting that on-going dehydration or strong thermal gradients in the upper part of the subducting plate inhibit partialmelting of the altered oceanic crust.

Sulphur chemistry of the ODP Site 206-1256 volcanic section

Sulfide mineralogy and the contents and isotope compositions of sulfur were analyzed in a complete oceanic volcanic section from IODP Hole 1256D in the eastern Pacific, in order to investigate the role of microbes and their effect on the sulfur budget in altered upper oceanic crust. Basalts in the 800 m thick volcanic section are affected by a pervasive low-temperature background alteration and have mean sulfur contents of 530 ppm, reflecting loss of sulfur relative to fresh glass through degassing during eruption and alteration by seawater. Alteration halos along fractures average 155 ppm sulfur and are more oxidized, have high SO4/Sum S ratios (0.43), and lost sulfur through oxidation by seawater compared to host rocks. Although sulfur was lost locally, sulfur was subsequently gained through fixation of seawater-derived sulfur in secondary pyrite and marcasite in veins and in concentrations at the boundary between alteration halos and host rocks. Negative d34S[sulfide-S] values (down to -30 per mil) and low temperatures of alteration (down to ~40 °C) point to microbial reduction of seawater sulfate as the process resulting in local additions of sulfide-S. Mass balance calculations indicate that 15-20% of the sulfur in the volcanic section is microbially derived, with the bulk altered volcanic section containing 940 ppm S, and with d34S shifted to -6.0 per mil from the mantle value (0 per mil). The bulk volcanic section may have gained or lost sulfur overall. The annual flux of microbial sulfur into oceanic basement based on Hole 1256D is 3-4 * 10**10 mol S/yr, within an order of magnitude of the riverine sulfate source and the sedimentary pyrite sink. Results indicate a flux of bacterially derived sulfur that is fixed in upper ocean basement of 7-8 * 10**-8 mol/cm**-2/yr1 over 15 m.y. This is comparable to that in open ocean sediment sites, but is one to two orders of magnitude less than for ocean margin sediments. The global annual subduction of sulfur in altered oceanic basalt lavas based on Hole 1256D is 1.5-2.0 * 10**11 mol/yr, comparable to the subduction of sulfide in sediments, and could contribute to sediment-like sulfur isotope heterogeneities in the mantle.

Geochemistry of igneous rocks recovered from a transect across the Mariana Trough, arc, fore-arc, and trench, at DSDP Leg 60 Holes and DM17

Major and trace element analyses are presented for 110 samples from the DSDP Leg 60 basement cores drilled along a transect across the Mariana Trough, arc, fore-arc, and Trench at about 18°N. The igneous rocks forming breccias at Site 453 in the west Mariana Trough include plutonic cumulates and basalts with calc-alkaline affinities. Basalts recovered from Sites 454 and 456 in the Mariana Trough include types with compositions similar to normal MORB and types with calc-alkaline affinities within a single hole. At Site 454 the basalts show a complete compositional transition between normal MORB and calc-alkaline basalts. These basalts may be the result of mixing of the two magma types in small sub-crustal magma reservoirs or assimilation of calc-alkaline, arc-derived vitric tuffs by normal MORB magmas during eruption or intrusion. A basaltic andesite clast in the breccia recovered from Site 457 on the active Mariana arc and samples dredged from a seamount in the Mariana arc are calc-alkaline and similar in composition to the basalts recovered from the Mariana Trough and West Mariana Ridge. Primitive island arc tholeiites were recovered from all four sites (Sites 458-461) drilled on the fore-arc and arc-side wall of the trench. These basalts form a coherent compositional group distinct from the Mariana arc, West Mariana arc, and Mariana Trough calc-alkaline lavas, indicating temporal (and perhaps spatial?) chemical variations in the arc magmas erupted along the transect. Much of the 209 meters of basement cored at Site 458 consists of endiopside- and bronzite-bearing, Mg-rich andesites with compositions related to boninites. These andesites have the very low Ti, Zr, Ti/Zr, P, and rare-earthelement contents characteristic of boninites, although they are slightly light-rare-earth-depleted and have lower MgO, Cr, Ni, and higher CaO and Al2O3 contents than those reported for typical boninites. The large variations in chemistry observed in the lavas recovered from this transect suggest that diverse mantle source compositions and complex petrogenetic process are involved in forming crustal rocks at this intra-oceanic active plate margin.

Composition of bottom sediments from the Guinea Basin

Geomorphology of the Guinea Basin is described along with sediments from cores collected on the abyssal plain, within the abyssal hill zone, and in the eastern part of the Chain Fracture Zone. Stratigraphic differentiation of deep-sea sediments was based on diatom analysis, geochemical and lithological data. Holocene and Pleistocene were identified by these criteria. The lower boundary of Holocene is was found from a marked decrease in CaCO3 concentration and total diatom count. Mineral and chemical compositions are given for coarse silt fraction of various Late Pleistocene sediments. It is shown that this facial complex is determined by tectonic position of the Guinea Basin.

Abundance and chemical and mineral compositions of nodules from the Pacific Ocean

The monograph highlights extensive materials collected during expeditions of P.P. Shirshov Institute of Oceanology. We consider facial conditions of nodule formation, regularities of their distribution, stratigraphic position, petrography, mineral composition, textures, geochemistry of nodules and hosting sediments. Origin of iron-manganese nodules in the Pacific Ocean is considered as well.

Petrology of basalts from DSDP Hole 66-487

Basaltic rocks recovered at the Middle America Trench area off Mexico are typical plagioclase-olivine phyric abyssal tholeiites containing less than 0.2 wt.% K2O. Phenocrysts of plagioclase and olivine usually make up the aggregate. Plagioclase phenocrysts are Ca-rich and up to An90. Olivine phenocrysts, which are always attached to plagioclase phenocrysts, are magnesian, Fo88 to Fo89, and contain 0.2 to 0.3 wt. % of NiO. Plagioclase phenocrysts contain numerous glass inclusions with the Mg/Mg+Fe atomic ratio of 0.70 to 0.73, which is distinctly higher than the same ratio of the bulk rock (0.62-0.63). Olivine of Fo88 to Fo89 is equilibrated with the liquid with an Mg/Mg+Fe atomic ratio of about 0.7, assuming the KDMg-Fe between liquid and olivine of 0.3. Small droplets of glass within glass inclusions in plagioclase are more enriched in K2O and volatiles than the host glass. This enrichment may have been caused by the extraction of Al2O3 as plagioclase from the trapped liquid and implies its immiscibility. Aggregates of plagioclase with small amounts of olivine may have been floated from more primitive magma with an Mg/Mg+Fe atomic ratio of about 0.7, judging from the chemical characteristics mentioned above. Flotation must have occurred at relatively high pressure. Large crystals of plagioclase and smaller crystals of olivine are xenocryst rather than phenocryst. Parental magma of Leg 66 basalt was high-MgO olivine tholeiite.

Geochemistry of glass shards in the volcanic ash layers of the Izu-Bonin Arc

To examine the processes and histories of arc volcanism and of volcanism associated with backarc rifting. 130 samples containing igneous glass shards were taken from the Plioccne-Quatemai^ succession on the rift Hank (Site 788) and the Quaternary fill in the basin fill of the Sumisu Rift (Sites 790 and 791). These samples were subsequently analyzed at the University of Illinois at Chicago and Shizuoka University. The oxides determined by electron probe do not account for the total weight of the material; differences between summed oxides and 100% arise from the water contents, probably augmented by minor losses thai result from alkali vaporization during analysis. Weight losses in colorless glasses are up to 9%; those in brown glasses (dacitcs to basalts) arc no more than 4.5%; shards from the rift-flank (possibly caused by prolonged proximity to ihc scafloor) generally have higher values than those from the rift-basin fill How much of the lost water is magmatic, and how much is hydrated is uncertain; however, although the shards absorb potassium, calcium, and magnesium during hydration in the deep sea, they do so only to a minor extent that does not significantly alter their major element compositions. Therefore, the electron-probe results are useful in evaluating the magmatism recorded by the shards. Pre- and syn-rift Izu-Bonin volcanism were overwhelmingly dominated by rhyolile explosions, demonstrating that island arcs may experience significant silicic volcanism in addition to the extensive basaltic and basaltic andestic activity, documented in many arcs since the 1970s, that occurs in conjunction with the andesitic volcanism formerly thought to be dominant. Andesitic eruptions also occurred before rifting, but the andesitic component in our samples is minor. All the pre- and syn-rift rhyolites and andesites belong to the low-alkali island-arc tholeiitic suite, and contrast markedly with the alkali products of Holocene volcanism on the northernmost Mariana Arc that have been attributed to nascent rifting. The Quaternary dacites and andesites atop the rift flank and in the rift-basin fill are more potassic than those of Pliocene age, as a result of assimilation from the upper arc crust, or from variations in degrees of partial melting of the source magmas, or from metasomatic fluids. All the glass layers from the rift-flank samples belong to low-K arc-tholeiitic suites. Half of those in the Pliocene succession are exclusively rhyolitic: the others contain minor admixtures of dacite and andesite, or andesite and either basaltic andesite or basalt. In Contrast, the Quaternary (syn-rift) volcaniclastics atop the rift-flank lack basalt and basaltic andesite shards. These youngest sediments of the rift flank show close compositional affinities with five thick layers of coarse, rhyolitic pumice deposits in the basin fill, the two oldest more silicic than the younger ones. The coarse layers, and most thin ash layers that occur in hemipelagites below and intercalated between them, are low-K rhyolites and therefore probably came from sources in the arc. However, several thin rhyolitic ash beds in the hemipelagites are abnormally enriched in potassium and must have been provided by more distal sources, most likely to the west in Japan. Remarkably, the Pliocene-Pleistocene geochemistry of the volcanic front does not appear to have been influenced by the syn-rift basaltic volcanism only a few kilometers away. Rare, thin layers of basaltic ash near the bases of the rift-basin successions are not derived from the arc. They deviate strongly from trends that the arc-derived glasses display on oxide-oxide plots, and show close affinities to the basalts empted all over the Sumisu Rift during rifting. These basalts, and the basaltic ashes in the basal rift-basin fill, arc compositionally similar to those erupted from mature backarc basins elsewhere.

Geochemistry of ash layers from the Bonin Forearc

Many ash-rich layers, varying from a few millimeters to several centimeters thick, were identified in the sedimentary sequences penetrated during Ocean Drilling Program Leg 125 at Sites 782, 784, and 786, located about 400 to 500 km south of Tokyo in the Bonin forearc. The total age range of the ash layers is from Eocene to Pleistocene, although not all sites cover this full span. The ashes consist of vitric, microlite-bearing, and crystal-rich components; the glassy shards are typically highly vesicular, with elongate, flattened bubbles. The dominant crystalline phases are orthopyroxene, clinopyroxene, and plagioclase. The major-element compositions of individual vitric shards collected from selected layers of Holes 782A, 784A, and 786A were determined by electron microprobe analyses; particular care was taken to ensure that the analytical results were not compromised by electron beam damage to the glasses. Compositions range from basalt through andesite and dacite to rhyolite and generally belong to a tholeiitic, low-K suite. There is no indication of any regular secular change during the evolution of the Bonin arc from tholeiitic through calc-alkalic to alkali compositions with time. In Holes 782A and 784A, some high-K rhyolite compositions of late Miocene and Pleistocene age are present. A clear chemical distinction has existed since arc inception between the source(s) of these ashes and the upper mantle source(s) tapped during construction of the igneous basement that formed the forearc.

Geochemistry and mineralogy of ultramafic rocks from conical seamount

The Mariana arc-trench system, the easternmost of a series of backarc basins and intervening remnant arcs that form the eastern edge of the Philippine Sea Plate, is a well-known example of an intraoceanic convergence zone. Its evolution has been studied by numerous investigators over nearly two decades (e.g., Kang, 1971; Uyeda and Kanamori, 1979; LaTraille and Hussong, 1980; Fryer and Hussong, 1981; Mrosowski et al., 1982; Hussong and Uyeda, 1981; Bloomer and Hawkins, 1983; Karig and Ranken, 1983; McCabe and Uyeda, 1983; Hsui and Youngquist, 1985; Fryer and Fryer, 1987; Johnson and Fryer, 1988; Johnson and Fryer, 1989; Johnson et al., 1991). The Mariana forearc has undergone extensive vertical uplift and subsidence in response to seamount collision, to tensional and rotational fracturing associated with adjustments to plate subduction, and to changes in the configuration of the arc (Hussong and Uyeda, 1981; Fryer et al., 1985). Serpentine seamounts, up to 2500 m high and 30 km in diameter, occur in a broad zone along the outer-arc high (Fryer et al., 1985; Fryer and Fryer, 1987). These seamounts may be horsts of serpentinized ultramafic rocks or may have been formed by the extrusion of serpentine muds. Conical Seamount, one of these serpentine seamounts, is located within this broad zone of forearc seamounts, about 80 km from the trench axis, at about 19°30'N. The seamount is approximately 20 km in diameter and rises 1500 m above the surrounding seafloor. Alvin submersible, R/V Sonne bottom photography, seismic reflection, and SeaMARC II studies indicate that the surface of this seamount is composed of unconsolidated serpentine muds that contain clasts of serpentinized ultramafic and metamorphosed mafic rocks, and authigenic carbonate and silicate minerals (Saboda et al., 1987; Haggerty, 1987; Fryer et al., 1990; Saboda, 1991). During Leg 125, three sites were drilled (two flank sites and one summit site) on Conical Seamount to investigate the origin and evolution of the seamount. Site 778 (19°29.93'N, 146°39.94'E) is located in the midflank region of the southern quadrant of Conical Seamount at a depth of 3913.7 meters below sea level (mbsl) (Fig. 2). This site is located in the center of a major region of serpentine flows (Fryer et al., 1985, 1990). Site 779 (19°30.75'N, 146°41.75'E), about 3.5 km northeast of Site 778, is located approximately in the midflank region of the southeast quadrant of Conical Seamount, at a depth of 3947.2 mbsl. This area is mantled by a pelagic sediment cover, overlying exposures of unconsolidated serpentine muds that contain serpentinized clasts of mafic and ultramafic rocks (Fryer et al., 1985, 1990). Site 780 (19°32.5'N, 146°39.2'E) is located on the western side of Conical Seamount near the summit, at a depth of 3083.4 mbsl. This area is only partly sediment covered and lies near active venting fields where chimney structures are forming (Fryer et al., 1990).