Composition of basalts from the north part of the Bougault anomaly, Mid-Atlantic Ridge 15°N

A quantitative model of development of magmatic and ore-magmatic systems under crests of mid-ocean ridges is constructed. Correct physical models of melting zone formation in approximation to active spreading, non-stationary dynamics of magma intrusion from a center of generation, filling of magma chambers of various shapes, feeding of fissure-type volcanoes, and retrograde boiling of melts during solidification of intrusive bodies beneath axial zones of spreading in crests of ridges are proposed. Physicochemical and mathematical theories of disintegration of multi-component solutions, growth of liquational drops of ore melts, and sublimation of components from magmatic gases are elaborated. Methods for constructing physically correct models of heat and mass transfer in heterophase media are devised. Modeling of development of magmatic and ore-magmatic systems on the basis of the Usov-Kuznetsov facies method and the Pospelov system approach are advanced. For quantitative models numerical circuits are developed and numerical experiments are carried out.

Geochemistry of basalts from ODP Leg 49, North Atlantic Ocean

The geochemistry of basalts recovered from seven sites in the North Atlantic is described with particular reference to minor elements. Three sites (407, 408, and 409) along the same mantle flow line, transverse to the Reykjanes Ridge at about 63°N, provide information on the composition of basalts erupted over a 34-m.y. interval between 2.3 and 36 m.y. ago. At Site 410, at 45°N, penetration into 10 m.y.-old crust west of the ridge axis permits comparisons with young basalts dredged from the median valley at 45°N. Three sites in the FAMOUS area at about 36°N provided material from very young (1 m.y.) basaltic crust (Site 411), and material to test the geochemical coherence of basalts of different ages (1.5 and 3.5 m.y.) on either side of a fracture zone (Sites 412 and 413). These sites complement earlier data from dredged and drilled sites (Leg 37) in the FAMOUS area. At Site 407, four geochemically distinct basalt units occur, with different normative and rare-earth element (REE) characteristics, and there is a clear correlation with magnetic stratigraphy. Yet there is a remarkable consistency in incompatible element ratios between these units, indicating derivation from an essentially similar mantle source. The basalts from the younger sites, 408 and 409, show a similar range of normative and REE variation, but incompatible element ratios are identical to those at Site 407, indicating that basalts at all three sites were produced from a mantle source which was geochemically relatively uniform. Rare-earth differences between the basalts can be interpreted in terms of variations in the degree and depth of partial melting causing HREE (+Y) retention in the source, although there may be some inter-site differences with respect to REE. A similar picture is presented at 45°N. Apparently a range of tholeiitic, transitional, and alkalic basalts were being erupted 10 m.y. ago, which have almost identical geochemical characteristics to those recently erupted in the median valley at 45°N. Incompatible element ratios are markedly different from those recorded at the Reykjanes Ridge. Basalts recovered from the FAMOUS sites are geochemically similar to previous samples recovered from the FAMOUS area, and their incompatible element ratios are similar, but not identical, to those at 45°N. However, total trace element levels are consistently lower than in 45°N basalts, which might imply smaller degrees of partial melting and/or greater depths of magma generation at 45°N, or higher trace element levels in the mantle source at 45°N. Few of the basalts recovered on Leg 49 have the geochemical characteristics of typical "MORB" (e.g., Nazca Plate, Leg 34). The data strongly support models invoking geochemical inhomogeneity in the source regions of basalts produced at the Mid-Atlantic Ridge. However, the data also introduce an additional time factor into such models and demonstrate the uniformity of the mantle source at a particular ridge sector (over periods in excess of 30 m.y.), while emphasizing the marked differences along the ridge. Mixing models invoking "depleted" and "enriched" mantle sources would seem to be inadequate to account for the observed variations.

(Table 1) Chemical composition and bulk densities of dolerites from ODP Hole 504B lower dikes

Chemical interactions between seawater and the oceanic crust have been widely investigated during recent years. However, most of these studies concern the uppermost volcanic part of the crust. The contribution of the underlying sheeted dike complex to the global budget of the oceans is inferred solely from some ophiolite studies and from the 500-m high-level dike section of DSDP/ODP 504B which was drilled in 1981. Hole 504B is the only place where a continuous and long (1260 m) section in the sheeted dike complex has been cored, and it is now regarded as a reference section for the upper oceanic crust. Many petrological and chemical data from these dolerites are available, including the relative proportions of veins, extensively altered adjacent rocks, and less altered 'host-rocks'. For these three reasons, considering the entire dike section penetrated by Hole 504B is a unique chance to study chemical fluxes related to hydrothermal alteration of this part of the oceanic crust. The calculation of any chemical flux implies knowledge of the chemical composition of the fresh precursor (protolith). Previously, mean compositions of glasses (=P1a) or basalts from the Hole 504B volcanics have been used as protoliths. In this paper, we calculate and discuss the use of various protoliths based on dolerites from Hole 504B. We show that the most adequate and realistic protolith is the mean of individual protoliths that we calculated from the acquisition, by automatic mode, of about 1000 microprobe analyses in each thin-section of dolerite from the Hole 504B lower dikes. Consequently, PFm is further used to calculate chemical fluxes in the dike section of Hole 504B. The chemical compositions of the host-rocks adjacent to alteration halos tend to converge to that of PFm with depth, except for Fe2O3t and TiO2. Because the volume percent of alteration halos increases with depth, the total fluxes related to these halos increase with depth. This explains why the mean flux (host-rocks+halos+veins) of the upper dikes is roughly similar to the mean flux of the lower dikes. During the alteration of the entire Hole 504B dike section, the dolerites gained relatively large quantities of Fe2O3t (+4.0 g/100 cm**3) and released much SiO2 (-6.8 g/100 cm**3), CaO (-5.8 g/100 cm**3), and TiO2 (1.6 g/100 cm**3), and minor Al2O3 (-0.7 g/100 cm**3) and MgO (-0.7 g/100 cm**3). We show the importance of the choice of the protolith in the calculation of chemical budget, particularly for elements showing low flux values. In Hole 504B, the Mg uptake by the volcanics during low temperature alteration added to the Mg release by the dikes gives a net flux of -0.07x10**14 g/year. We propose that part of the Mg uptake by the oceanic crust, which is necessary to compensate the rivers input (-1.33x10**14 g/year), occurs in the underlying gabbros and/or in sections which are altered such as Trinity and Troodos ophiolites. Compared with ophiolites, fluxes calculated for elements other than Mg for the entire crust are generally similar (in tendency, if not in absolute value) to that we obtained from Hole 504B.

Geochemistry of ODP Hole 111-504B basalts (Table 2)

Fifty samples of basalt recovered during ODP Leg 111 from the dikes (Layer 2C) of Hole 504B (1350.0-1562.3 m below seafloor) were analyzed by X-ray-fluorescence techniques. All of the samples are highly depleted in magmaphile elements relative to other mid-ocean ridge basalts, with TiO2 = 0.75-1.24 wt%, Na2O = 1.59-2.22 wt%, Zr = 38-64 ppm, Nb = 0.3-1.5 ppm, and Y = 20-30 ppm (for samples containing 0%-2% phenocrysts), but have ratios of highly incompatible elements similar to normal Type I mid-ocean ridge basalts (e.g., Zr/Nb > 30). Abundances of compatible elements are similar to those of typical mid-ocean ridge basalts, with MgO = 7.2-9.2 wt%, Fe2O3* = 9.3-12.5 wt%, Ni = 55-164 ppm, and Cr = 26-388 ppm. Approximately 2% of the samples recovered from the top part of Hole 504B are similar to normal Type I or Type II ocean floor basalts. However, all of the analyzed Leg 111 samples from Hole 504B are depleted basalts. Aphyric dike rocks from Leg 111 are virtually identical to the depleted aphyric samples recovered from the pillow lavas and dikes in the upper 1075 m of Hole 504B during DSDP Legs 69, 70, and 83, with the exception of elements readily altered by seawater (Sr, Rb, and K). These elements reach a maximum in both abundance and variability in the pillow lavas of the upper 571.5 m of Hole 504B and decline to more constant values in the dike system sampled on Legs 83 and 111, apparently as a result of a decrease in porosity and increase in alteration temperatures relative to the pillow lavas. Based on compositional similarities to the vast majority of the pillows and flows, the dikes sampled on Leg 111 appear to be the feeder system for the pillow lavas in the upper part of Hole 504B. The incompatible-element-depleted compositions of the Costa Rica Rift Zone basalts are consistent with multistage melting of a normal mid-ocean ridge source.

Geochemistry of basalts at DSDP Site 89-462

At Site 462 in the Nauru Basin, western Pacific Ocean, 56 lithologic units have been recovered from an extensive flood basalt province. Fossil evidence suggests that the lavas were emplaced during the interval 100-115 Ma, some 30 m.y. after formation of the underlying Jurassic ocean crust. The lithologic units can be broadly divided into three chemical units, the lowermost two of which are chemically monotonous, suggesting rapid eruption of basalt from a compositionally homogeneous magma chamber. All the basalts are hypersthene- (hy-) rich tholeiites, with approximately chondritic La/Sm, La/Yb, Zr/Nb, La/Ta, and Th/Hf ratios. Chemically they resemble, in part, "transitional" mid-ocean ridge basalts (MORB) from areas such as the Reykjanes Ridge, although Rb, Ba, and K contents are very low and similar to those of "normal" MORB. Their 87Sr/86Sr ratios are higher than in N-type MORB (Fujii et al., 1981). The chemistry of the Nauru basalts differs from that of continental flood basalts, which tend to be strongly enriched in large-ion lithophile (LIL) elements, although the extent to which the differences result from sialic contamination or source variability is not clear.

Bulk geochemistry, mass change and alteration intensity of basalts from ODP Leg 187 sites and survey dredges

Shipboard studies during Ocean Drilling Program Leg 187 (Australian Antarctic Discordance, AAD) suggested that there was no discernible coincidence between the interpreted age of rocks recovered and the intensity of alteration observed. Samples from the oldest sites occupied appeared to exhibit the least overall effects of alteration, and the intensity of alteration varied from site to site. Previous investigations of low-temperature alteration in oceanic basement samples have been restricted by the myopic perspective provided by single drill holes or dredge collections. Combining core samples from Leg 187 and dredge samples from the AAD collection at Oregon State University (USA) offers the unique opportunity to investigate mineral and bulk chemical changes attending alteration of basalt over a range of ages from 0 to 28 Ma. Results of this research indicate that there is a general increase in the intensity of alteration as the basalts age and mosve off axis, but that this relationship is somewhat veiled by the dominating control on alteration intensity dictated by variations in permeability.

Geochemistry and isotopic ratios of basalts from ODP Leg 163 sites

Voluminous, subaerial magmatism resulted in the formation of extensive seaward-dipping reflector sequences (SDRS) along the Paleogene Southeast Greenland rifted margin. Drilling during Leg 163 recovered basalts from the SDRS at 66ºN (Site 988) and 63ºN (Sites 989 and 990). The basalt from Site 988 is light rare-earth-element (REE) enriched (La(n)/Yb(n) = 3.4), with epsilon-Nd(t=60) = 5.3, 87Sr/86Sr = 0.7034, and 206Pb/204Pb = 17.98. It is similar to tholeiites recovered from the Irminger Basin during Leg 49 and to light-REE-enriched tholeiites from Iceland. Drilling at Site 989, the innermost of the sites on the 63ºN transect, was proposed to extend recovery of the earliest part of the SDRS initiated during Leg 152. These basalts are, however, younger than those from Site 917 and are compositionally similar to basalts from the more seaward Sites 990 and 915. Many of the basalts from Sites 989 and 990 show evidence of contamination by continental crust (e.g., epsilon-Nd(t=60) extends down to -3.7, 206Pb/204Pb extends down to 15.1). We suggest that the contaminant is a mixture of Archean granulite and amphibolite and that the most contaminated basalts have assimilated ~5% of crust. Uncontaminated basalts are isotopically similar to basalts from Site 918, on the main body of the SDRS, and are light-REE depleted. Consistent with previous models of the development of this margin, we show that at the time of formation of the basalts from Sites 989 and 990 (1) melting was at relatively shallow levels in a fully-fledged rift zone; (2) fragments of continental crust were present in the lithosphere above the zones of melt generation; and (3) the sublithospheric mantle was dominated by a depleted Icelandic plume component.

Geochemistry at DSDP Sites 69-505, 69-504 and 70-504

Deep basement penetration during Legs 69 and 70 at Hole 504B in the Panama Basin allowed the recovery of a 561.5-meter sequence of basaltic pillows, thin flows, and breccias interspersed with thick massive flows. The lavas, which are aphyric to moderately plagioclase-olivine-clinopyroxene phyric, are petrologically indistinguishable from typical mid-ocean-ridge basalts (MORB). Some units are distinctive in that they carry accessory chrome-spinel microphenocrysts or emerald green clinopyroxene phenocrysts. Major and trace element analyses were carried out on 67 samples using X-ray fluorescence techniques. The basalts resemble normal MORB in terms of major elements. However, the trace element analyses show that most of the basalts are characterized by very strong depletion in the more incompatible elements compared with, for instance, normal (N type) MORB from the Atlantic at 22°N. Interdigitated with these units are one or two units that have distinctly higher incompatible element concentrations similar to those in basalts of the transitional (T) type from the Reykjanes Ridge (63°N in the Mid-Atlantic Ridge). All the basalts appear to have undergone some high-level crystal fractionation, although this has not proceeded to the extent of yielding ferrobasalts as it has at the adjacent Galapagos Spreading Center or along the East Pacific Rise. The magnetic anomalies are of lower amplitude than in the latter two regions, which suggests that the absence of ferrobasalts may be a general feature of the ocean crust generated at the Costa Rica Rift. The presence of two distinct magma types, one strongly depleted and the other moderately enriched in incompatible elements, suggests that magma chambers at the spreading center are discontinuous rather than continuous and that there is some chemical heterogeneity in the underlying mantle source. Observed variations in incompatible element ratios of basalts from the more depleted group could, however, reflect mixing between these two magma types. In general it would appear that the mantle feeding the Costa Rica Rift is significantly more depleted in incompatible trace elements than that feeding the Mid-Atlantic Ridge.

Iron isotope and chemical compositions of sediments and basalts drilled seaward of the Mariana Trench

The purpose of this work is to study the mobility and budget of Fe isotopes in the oceanic crust and in particular during low-temperature interaction of seawater with oceanic basalt. We carried out this investigation using samples from Ocean Drilling Program (ODP) Site 801C drilled during Leg 129 and Leg 185 in Jurassic Pacific oceanic crust seaward of the Mariana Trench. The site comprises approximately 450 m of sediment overlying a section of 500 m of basalt, which includes intercalated pelagic and chemical sediments in the upper basaltic units and two low-temperature (10-30°C) ocherous Si-Fe hydrothermal deposits. Fe was chemically separated from 70 selected samples, and 57Fe/54Fe ratios were measured by MC-ICP-MS Isoprobe. The isotopic ratios were measured relative to an internal standard solution and are reported relative to the international Fe-standard IRMM-14. Based on duplicate measurements of natural samples, an external precision of 0.2? (2 sigma) has been obtained. The results indicate that the deep-sea sediment section has a restricted range of d57Fe, which is close to the igneous rock value. In contrast, large variations are observed in the basaltic section with positive d57Fe values (up to 2.05?) for highly altered basalts and negative values (down to ?2.49?) for the associated alteration products and hydrothermal deposits. Secondary Fe-minerals, such as Fe-oxyhydroxides or Fe-bearing clays (celadonite and saponite), have highly variable d57Fe values that have been interpreted as resulting from the partial oxidation of Fe(2+) leached during basalt alteration and precipitated as Fe(3+)-rich minerals. In contrast, altered basalts at Site 801C, which are depleted in Fe (up to 80%), display an increase in d57Fe values relative to fresh values, which suggest a preferential leaching of light iron during alteration. The apparent fractionation factor between dissolved Fe(2+) and Fe remaining in the mineral is from 0.5? to 1.3? and may be consistent with a kinetic isotope fractionation where light Fe is stripped from the minerals. Alternatively, the formation of secondary clays minerals, such as celadonite during basalt alteration may incorporate preferentially the heavy Fe isotopes, resulting in the loss of light Fe isotopes in the fluids. Because microbial processes within the oceanic crust are of potential importance in controlling rates of chemical reactions, Fe redox state and Fe-isotope fractionation, we evaluated the possible effect of this deep biosphere on Fe-isotope signatures. The Fe-isotope systematics presented in this study suggest that, even though iron behavior during seafloor weathering may be mediated by microbes, such as iron-oxidizers, d57Fe variations of more than 4? may also be explained by abiotic processes. Further laboratory experiments are now required to distinguish between various processes of Fe-isotope fractionation during seafloor weathering.

Chemical composition of celadonite-glauconite minerals and cristobalite from altered oceanic basalts of DSDP Hole 49-407

Features of associations of newly formed minerals in Oligocene basalts from the Reykjanes Ridge (DSDP Hole 407), which consist in wide development of celadonite-glauconite group minerals, as well as saponite, cristobalite and specific Fe-Si formations is shown. Detailed mineralogical and geochemical characteristics of these newly formed minerals are reported. It is firstly shown that at underwater alteration of basalts not only celadonite can be generated, but also glauconite. Dependence of secondary mineral composition from petrochemical features of original basalts and from their permeability is revealed. It is concluded that complexes of secondary minerals formed during alteration of basalts with similar petrochemical characteristics in the ocean and on the continent are different.

Mineral an chemical composition of basalts from DSDP Sites 417 and 418, West Atlantic Ocean

Major element composition ranges of closely associated basalt glass-whole rock pairs from individual small cooling units approach the total known range of basalt glass and whole rock compositions at IPOD sites 417 and 418. The whole rock samples fall into two groups: one is depleted in MgO and distinctly enriched in plagioclase but has lost some olivine and/or pyroxene relative to its corresponding glass; and the other is enriched in MgO and in phenocrysts of olivine and pyroxene as well as plagioclase compared to its corresponding glass. By analogy with observed phenocryst distributions in lava pillows, tubes, and dikes, and with some theoretical studies, we infer that bulk rock compositions are strongly affected by phenocryst redistribution due to gravity settling, flotation, and dynamic sorting after eruption, although specific models are not well constrained by the one-dimensional geometry of drill core. Compositional trends or groupings in whole rock data resulting from such late-stage processes should not be confused with more fundamental compositional effects produced in deep chambers or during partial melting.

Chemistry of very young basalts from ODP Hole 106-648B

Basalts in Hole 648B, located in the rift valley of the Mid-Atlantic Ridge at 23°N in crust estimated to be less than 100,000 years old, are mainly fresh, but small amounts of secondary phases are found on fracture surfaces and in alteration halos within the rocks. The halos are defined by dark bands 1-4 mm thick that have developed parallel to fracture surfaces or pillow margins and which in some cases have migrated some centimeters into the rock. The dark bands are the principal locus of secondary phases. The secondary phases are olive-green and yellow protoceladonites, of composition and structure intermediate between celadonite and iron-rich saponite, red (Mn-poor) to opaque (Mn-rich) iron oxyhydroxides, mixtures of protoceladonite and iron oxyhydroxide, and rare manganese oxides. These phases occur mainly as linings or fillings of open spaces in the basalt within the dark bands. Sulfides and intersertal glass are the only primary phases that can be seen to have been altered. Where dark bands have migrated into the rock, the rock behind the advancing band is almost devoid of secondary phases, implying redissolution. The potassium and magnesium in the secondary phases could have been supplied from ambient seawater. The aluminum in the protoceladonites must have been derived from local reaction of intergranular glass. The source of iron and silica could have been intergranular glass or low temperature mineralizing solutions of the type responsible for the formation of deposits of manganese oxides and iron oxyhydroxides and silicates on the seafloor.

Chemical composition of basalts from ODP Leg 115

Leg 115 of the Ocean Drilling Program recovered basalts from four locations along the hotspot track that leads from the Deccan flood basalts in India to Reunion Island in the western Indian Ocean (Sites 706, 707, 713, and 715). The drilled basalts range in age from 35 Ma (Site 706) to 64 Ma (Site 707), and including the Deccan basalts (66 to 68 Ma), Mauritius Island (0.2 to 8 Ma), and Reunion Island (0 to 2 Ma), seven sites are provided for sampling the volcanic record of the 5000-km-long hotspot track. Chemical and age comparisons indicate that Site 707 lavas correlate with basalt units near the top of the Deccan flood basalt sequence. The lavas of Site 715 (55 to 60 Ma) are most similar to the islands of Mauritius and Reunion. Site 713 basalts (48 Ma) are similar to the earliest lavas of the Deccan province, and Site 706 basalts are intermediate in chemistry between those of central Indian spreading-ridge basalts and Reunion. Differences in lava compositions along the hotspot track can be related to variable mixing of plume and asthenospheric mantle, depending on the changing position of spreading-ridge segments and the hotspot during the opening of the Indian Ocean. Alternatively, time-dependent changes in the composition of hotspot melts may be due to a decrease in partial melting of a heterogeneous plume or to intrinsic changes in the composition of material supplied by the plume.

A Two-Step Sol–Gel Method for Synthesis of Nanoporous TiO2

This article reports a study of the effects of synthesis parameters on the preparation and formation of mesoporous titania nanopowders by employing a two-step sol-gel method. These materials displayed crystalline domains characteristic of anatase. The first step of the process involved the hydrolysis of titanium isopropoxide in a basic aqueous solution mediated by neutral surfactant. The solid product obtained from step 1 was then treated in an acidified ethanol solution containing the same titanium precursor to thicken the pore walls. Low pH and higher loading of the Ti precursor in step 2 produced better mesoporosity and crystallinity of titanium dioxide polymorphs. The resultant powder exhibited a high surface area (73.8 m(2)/g) and large pore volume (0.17 cm(3)/g) with uniform mesopores. These materials are envisaged to be used as precursors for mesoporous titania films as a wide band gap semiconductor in dye-sensitized nanocrystalline TiO2 solar cells.

Size-controlled TiO2 nanoparticles on porous hosts for enhanced photocatalytic hydrogen production

Nanocystalline TiO2 particles were successfully synthesized on porous hosts (SBA-15 and ZSM-15) via a sol-gel impregnation method. Resulting nanocomposites were characterized by XRD, TEM, BET surface analysis, Raman and UV-vis diffuse reflectance spectroscopy, and their photocatalytic activity for H2 production evaluated. XRD evidences the formation of anatase nanoparticles over both ZSM-5 and SBA-15 porous supports, with TEM highlighting a strong particle size dependence on titania precursor concentration. Photocatalytic activities of TiO2/ZSM-5 and TiO2/SBA-15 composites were significantly enhanced compared to pure TiO2, owing to the smaller TiO2 particle size and higher surface area of the former. TiO2 loadings over the porous supports and concomitant photocatalytic hydrogen production were optimized with respect to light absorption, available surface reaction sites and particle size. 10%TiO2/ZSM-5 and 20%TiO2/SBA-15 proved the most active photocatalysts, exhibiting extraordinary hydrogen evolution rates of 10,000 and 8800μmolgTiO2 -1 h-1 under full arc, associated with high external quantum efficiencies of 12.6% and 5.4% respectively under 365nm irradiation.