Chemical composition of basalts and basaltic glasses from the Sierra Leone Fracture Zone region

Major and trace element (including REE) geochemistry of basalts and chilled basaltic glasses from the MAR axial zone in the vicinity of the Sierra Leone FZ (5-7°10'N) has been studied. Associations of basalts of various compositions with particular ocean-floor geological structural features have been analyzed as well. Three basaltic varieties have been discriminated. Almost ubiquitous are high-Mg basalts (Variety 1) that are derivatives of N-MORB tholeiitic melts and that are produced in the axial zone of spreading. Variety 2 is alkaline basalts widespread on the southwestern flank of the MAR crest zone in the Sierra Leone region, likely generated through deep mantle melting under plume impact. Variety 3 is basalts derivative from T- and P-MORB-like tholeiitic melts and originating through addition of deeper mantle material to depleted upper mantle melts. Magma generation parameters, as calculated from chilled glass compositions, are different for depleted tholeiites (44-55 km, 1320-1370°C) and enriched tholeiites (45-78 km, 1330-1450°C). Mantle plume impact is shown to affect not only tholeiitic basalt compositions but also magma generation conditions in the axial spreading zone, resulting in higher Ti and Na concentrations in melts parental to rift-related basalts occurring near the plume. T- and P-MORBs are also developed near areas where mantle plumes are localized. High-Mg basalts are shown to come in several types with distinctive Ti and Na contents. Nearly every single MAR segment (bounded by sinistral strike slips and the Bogdanov Fracture Zone) is featured by its own basalt type suggesting that it has formed above an asthenospheric diapir with its unique magma generation conditions. These conditions are time variable. Likely causes of temporal and spatial instability of the mantle upwelling beneath this portion of the MAR are singular tectonic processes and plume activity. In sulfide-bearing rift morphostructures (so-called "Ore area'' and the Markov Basin), basalts make up highly evolved suites generated through olivine and plagioclase fractionation, which is suggestive of relatively long-lived magma chambers beneath the sulfide-bearing rift morphostructures. Functioning of these chambers is a combined effect of singular geodynamic regime and plume activity. In these chambers melts undergo deep differentiation leading to progressively increasing concentration of sulfide phase, eventually to be supplied to the hydrothermal plumbing system.

Geochemistry of the sheeted dike complex at Pito Deep

Alteration of sheeted dikes exposed along submarine escarpments at the Pito Deep Rift (NE edge of the Easter microplate) provides constraints on the crustal component of axial hydrothermal systems at fast spreading mid-ocean ridges. Samples from vertical transects through the upper crust constrain the temporal and spatial scales of hydrothermal fluid flow and fluid-rock reaction. The dikes are relatively fresh (average extent of alteration is 27%), with the extent of alteration ranging from 0 to >80%. Alteration is heterogeneous on scales of tens to hundreds of meters and displays few systematic spatial trends. Background alteration is amphibole-dominated, with chlorite-rich dikes sporadically distributed throughout the dike complex, indicating that peak temperatures ranged from <300°C to >450°C and did not vary systematically with depth. Dikes locally show substantial metal mobility, with Zn and Cu depletion and Mn enrichment. Amphibole and chlorite fill fractures throughout the dike complex, whereas quartz-filled fractures and faults are only locally present. Regional variability in alteration characteristics is found on a scale of <1-2 km, illustrating the diversity of fluid-rock interaction that can be expected in fast spreading crust. We propose that much of the alteration in sheeted dike complexes develops within broad, hot upwelling zones, as the inferred conditions of alteration cannot be achieved in downwelling zones, particularly in the shallow dikes. Migration of circulating cells along rides axes and local evolution of fluid compositions produce sections of the upper crust with a distinctive character of alteration, on a scale of <1-2 km and <5-20 ka.

Geochemistry of subducted volcaniclastic sediments from Mariana arc magmas

The phase relations of natural volcaniclastic sediments from the west Pacific Ocean were investigated experimentally at conditions of 3-6 GPa and 800-900 °C with 10 wt.% added H2O (in addition to ~ 10 wt.% structurally-bound H2O) to induce hydrous melting. Volcaniclastic sediments are shown to produce a sub-solidus assemblage of garnet, clinopyroxene, biotite, quartz/coesite and the accessory phases rutile ± Fe-Ti oxide ± apatite ± monazite ± zircon. Hydrous melt appears at temperatures exceeding 800-850 °C, irrespective of pressure. The melt-producing reaction consumes clinopyroxene, biotite and quartz/coesite and produces orthopyroxene. These phase relations differ from those of pelagic clays and K-bearing mid ocean ridge basalts (e.g. altered oceanic crust) that contain phengite, rather than biotite, as a sub-solidus phase. Despite their relatively high melt productivity, the wet solidus for volcaniclastic sediments is found to be higher (825-850 °C) than other marine sediments (700-750 °C) at 3 GPa. This trend is reversed at high-pressure conditions (6 GPa) where the biotite melting reaction occurs at lower temperatures (800-850 °C) than the phengite melting reaction (900-1000 °C). Trace element data was obtained from the 3 GPa run products, showing that partial melts are depleted in heavy rare earth elements (REE) and high field strength elements (HFSE), due to the presence of residual garnet and rutile, and are enriched in large ion lithophile elements (LILE), except for Sr and Ba. This is in contrast to previous experimental studies on pelagic sediments at sub-arc depths, where Sr and Ba are among the most enriched trace elements in glasses. This behavior can be partly attributed to the presence of residual apatite, which also host some light REE in our supra-solidus residues. Our new experimental results account for a wide range of trace element and U-series geochemical features of the sedimentary component of the Mariana arc magmas, including imparting a substantial Nb anomaly to melts from an anomaly-free protolith.

Chemical composition of rocks and minerals from the Karymsky volcanic center, Eastern Kamchatka

The powerful eruption in the Akademii Nauk caldera on January 2, 1996 marked a new activity phase of the Karymsky volcano and became a noticeable event in the history of modern volcanism in Kamchatka. The paper reports data obtained by studying more than 200 glassy melt inclusions in phenocrysts of olivine (Fo82-72), plagioclase (An92-73), and clinopyroxene (Mg# 83-70) in basalts of the 1996 eruption. The data were used to estimate composition of the parental melt and physicochemical parameters of the magma evolution. According to our data, the parental melt corresponded to low magnesium, high aluminum basalt (SiO2 = 50.2%, MgO = 5.6%, Al2O3 = 17%) of the mildly potassium type (K2O = 0.56%) and contained much dissolved volatile components (H2O = 2.8%, S = 0.17%, and Cl = 0.11%). Melt inclusions in the minerals are similar in chemical composition, a fact testifying that the minerals crystallized simultaneously with one another. Their crystallization started at pressure ~1.5 kbar, proceeded within a narrow temperature range of 1040+/-20°C, and continued until near-surface pressure ~100 bar was reached. Degree of crystallization of the parental melt during its eruption was close to 55%. Massive crystallization was triggered by H2O degassing under pressure <1 kbar. Magma degassing in an open system resulted in escape of 82% H2O, 93% S, and 24% Cl (of their initial contents in the parental melt) to the fluid phase. Release of volatile compounds to the atmosphere during the eruption that lasted for 18 h was estimated as 1.7x10**6 t H2O, 1.4x10**5 t S, and 1.5x10**4 t Cl. Concentrations of most incompatible trace elements in the melt inclusions are close to those in the rocks and to the expected fractional differentiation trend. Melt inclusions in plagioclase were found to be selectively enriched in Li. The Li-enriched plagioclase with melt inclusions thought to originate from cumulate layers in the feeding system beneath Karymsky volcano, in which plagioclase interacted with Li-rich melts/brines and was subsequently entrapped and entrained by the magma during the 1996 eruption.

Fe-Mn nodules and host sediments from the Gulf of Finland

Distributions of major and trace elements in ferromanganese nodules, which are buried or exposed on the sea floor and in host sediments, were studied in ten concretion/sediment pairs by various physical and chemical methods. It was established that, in addition to Fe and Mn, a limited number of major and trace elements (P, Ca, Sr, Ba, Mo, Co, Zn, Ni, As, Pb, Sb, Tl, U, W, Y, and Ga) is accumulated with variable degree of intensity (relative to sediments) in the nodules. The maximal content of Mn in the nodules is 100 times higher than in the host sediments, whereas for all other elements listed above these ratios vary from more than one to 10-20. Manganese and, to a lesser extent, Ba and Sr are concentrated in the buried concretions. Other elements are primarily concentrated in concretions exposed on the sea floor. The occurrence mode of the concretions and compositional data on interstitial water suggest that metals in the concretions derive from seawater and suspended particulates, in addition to sediments. Burial of concretions in the sediment pile is accompanied by alteration of their composition, accumulation of Mn (relative to Fe), and loss of several associated metals.

Geochemistry of basalts of the Indian Ocean

Indian Ocean crust formed at Sites 765 and 766 is geochemically comparable to that presently forming in the Red Sea. In both cases, we interpret the crust as reflecting high degrees of mantle melting that are associated with an enhanced thermal gradient below recently rifted continental lithosphere. Asthenospheric melts formed in this environment are rich in CaO and FeO, poor in Na2O and Al2O3, and characterized by depleted rare earth element (REE) profiles ([La/Sm]n approximately 0.5-0.6). Both the Red Sea basalts and the basalts at Sites 765 and 766 are distinct from those erupted at the present Mid-Indian Ocean Ridge. The isotope characteristics of the Site 765 basalts define a geochemical signature similar to that of the present-day Mid-Indian Ocean Ridge basalts (MIORB). The Indian Ocean mantle domain is distinct from that of the Atlantic and Pacific oceans, and this distinction has persisted since Jurassic time, when the Site 765 oceanic crust was formed.

Petrology, mineralogy, and chemistry of basaltic rocks at DSDP Leg 81 Holes

We detail the petrography and mineralogy of 145 basaltic rocks from the top, middle, and base of flow units identified on shipboard along with associated pyroclastic samples. Our account includes representative electron microprobe analyses of primary and secondary minerals; 28 whole-rock major-oxide analyses; 135 whole-rock analyses each for 21 trace elements; 7 whole-rock rare-earth analyses; and 77 whole-rock X-ray-diffraction analyses. These data show generally similar petrography, mineralogy, and chemistry for the basalts from all four sites; they are typically subalkaline and consanguineous with limited evolution along the tholeiite trend. Limited fractionation is indicated by immobile trace elements; some xenocrystic incorporation from more basic material also occurred. Secondary alteration products indicate early subaerial weathering followed by prolonged interaction with seawater, most likely below 150°C at Holes 552, 553A, and 554A. At Hole 555, greenschist alteration affected the deepest rocks (olivine-dolerite) penetrated, at 250-300°C.

Chemical composition of Fe-Mn nodules and host bottom sediments along the submeridional profile across the Brazil Basin

Sedimentation and ore formation were studied in sediments from nine stations located along the 24°W profile in the Brazil Basin of the Atlantic Ocean. Bottom sediments are represented by mio- and hemipelagic muds, which are variably enriched in hydrothermal iron and manganese oxyhydroxides. As compared to bottom sediments from other basins of the Atlantic Ocean, the sediments in study are marked by extremely high manganese contents (up to 1.33%) and maximal enrichment in Ce. It was shown that the positive Ce anomaly is related to REE accumulation on iron oxyhydroxides. Influence of hydrothermal source leads to decrease of Ce anomaly and LREE/HREE ratio. In reduced sediments preservation of positive Ce anomaly and/or its disappearance was observed after iron and manganese reduction. REE contents were determined for the first time in the Ethmodiscus oozes of the Brazil Basin. Ore deposits of the Brazil Basin are represented by ferromanganese crusts and ferromanganese nodules. Judging from contents of iron, manganese, REE, and other trace elements, these formations are ascribed to sedimentation (hydrogenic) deposits. They are characterized by a notable positive Ce anomaly in the REE pattern. Extremely high Ce content (up to 96% of total REE) was discovered for the first time in the buried nodules (Mn/Fe = 0.88).

Description and chemical composition of manganese nodules during R/V "Tyro" cruise APNAP 1 to the Madeira Abyssal Plain in 1986

The porewater and sediment composition of two boxcores and of a small gravity core, taken on a manganese-nodule-covered hill and in the Madeira Abyssal Plain proper respectively, are compared. The pore-water study of the two boxcores indicates that oxic conditions prevail in both cores. In addition, it indicates that no detectable fluxes of Mn or Fe occur from the porewater to the ocean bottom water. Variations in the geochemical composition of the sediments can be explained by fluctuations in the amount of carbonate, which acts as a diluting agent. A clear carbonate minimum is observed at 20-22 cm depth in the two cores. This minimum is likely to be associated with the last glacial period (10-20 kyr B.P.). This association is supported by the sediment accumulation rate of 15 mm/kyr as found by extrapolation from the rate for pelagic sediments in the Madeira Abyssal Plain. The bulk composition of the manganese nodules recovered from the submarine hill is chemically almost identical to the average composition of Atlantic nodules. The trace metal and Rare Earth Elements composition indicate a hydrogenous origin for the manganese nodules of this study. On the basis of the chemical composition, and that of nodules relative to that of the adjacent sediments, an average nodule accretian rate of 2.8-3.3 mm/myr has been calculated. Although the analyses of the entire ferromanganese nodules that have been studied seem to indicate a homogenous composition, internal structures of the nodules reveal great inhomogeneity, both visually and chemically. These fluctuations may be related to variations in the fluxes of Mn and Fe, which in turn could be climate-related.

Geochemistry of pelagic and hemipelagic sediments of ODP Leg 126 sites

Chemical analyses were performed on major, minor, and rare-earth elements of pelagic and hemipelagic sediments of the forearc, arc, and backarc sites of the Izu-Bonin Arc, Ocean Drilling Program Leg 126. Analyses of the hemipelagic and pelagic sediments of this area indicate that the chemical composition of this arc is highly affected by the chemical composition of rocks of this arc as a source of sediments. The Oligocene sediments, which are characterized by high MgO contents, reflect the chemical composition of the Paleogene volcanic rocks of the immature arc. Moreover, the late Miocene to Quaternary sediments with low MgO contents are attributed to the composition of the present arc. We also suggest that the sedimentation rates and topography of the sedimentary basin affect the MnO and SiO2 contents of pelagic and hemipelagic sediments.

Trace element geochemistry of zircons frim in situ ocean lithoshere

We characterize the textural and geochemical features of ocean crustal zircon recovered from plagiogranite, evolved gabbro, and metamorphosed ultramafic host-rocks collected along present-day slow and ultraslow spreading mid-ocean ridges (MORs). The geochemistry of 267 zircon grains was measured by sensitive high-resolution ion microprobe-reverse geometry at the USGS-Stanford Ion Microprobe facility. Three types of zircon are recognized based on texture and geochemistry. Most ocean crustal zircons resemble young magmatic zircon from other crustal settings, occurring as pristine, colorless euhedral (Type 1) or subhedral to anhedral (Type 2) grains. In these grains, Hf and most trace elements vary systematically with Ti, typically becoming enriched with falling Ti-in-zircon temperature. Ti-in-zircon temperatures range from 1,040 to 660°C (corrected for a TiO2 ~ 0.7, a SiO2 ~ 1.0, pressure ~ 2 kbar); intra-sample variation is typically ~60-15°C. Decreasing Ti correlates with enrichment in Hf to ~2 wt%, while additional Hf-enrichment occurs at relatively constant temperature. Trends between Ti and U, Y, REE, and Eu/Eu* exhibit a similar inflection, which may denote the onset of eutectic crystallization; the inflection is well-defined by zircons from plagiogranite and implies solidus temperatures of ~680-740°C. A third type of zircon is defined as being porous and colored with chaotic CL zoning, and occurs in ~25% of rock samples studied. These features, along with high measured La, Cl, S, Ca, and Fe, and low (Sm/La)N ratios are suggestive of interaction with aqueous fluids. Non-porous, luminescent CL overgrowth rims on porous grains record uniform temperatures averaging 615 ± 26°C (2SD, n = 7), implying zircon formation below the wet-granite solidus and under water-saturated conditions. Zircon geochemistry reflects, in part, source region; elevated HREE coupled with low U concentrations allow effective discrimination of ~80% of zircon formed at modern MORs from zircon in continental crust. The geochemistry and textural observations reported here serve as an important database for comparison with detrital, xenocrystic, and metamorphosed mafic rock-hosted zircon populations to evaluate provenance.

Composition of rocks, minerals, water, and gases from uplift areas of the Central Atlantic

Original geological, geophysical, lithological, mineralogical data on uplifts of the Central Atlantic are given in the book based on materials of Cruise 1 of the R/V Akademik Nikolaj Strakhov. Geological and geophysical studies include description of the obtained material and analysis of structural and morphological elements of the ocean floor. Results of lithological, petrochemical and geochemical studies were extremely innovative and develop a conceptual model. The latter include studies of petrochemical evolution of tholeiitic alkaline plate volcanism, large-scale hydrothermal transformation of basement rocks - palygorskitization, phosphatization and ferromanganese mineralization. Showing imposition Superposition of hydrogenic alteration on hydrothermally altered rocks and its role in Cenozoic history of sedimentation is shown.

Isotopic and chemical compositions of lower crust rocks and minerals from ODP Hole 176-735B

The composition of gabbroic rocks from the drill core of Hole 735B (ODP Leg 176) at the 11 Ma Atlantis II bank close to the slow spreading Southwest Indian Ridge (SWIR) has been analyzed for major and trace elements and Sr, Nd and Pb isotopic composition. The samples are thought to represent much of the mineralogical and geochemical variation in a vertical 1-km section (500-1500 m below the sea floor) of the lower ocean crust. Primitive troctolitic gabbros, olivine gabbros and gabbros that have Mg#=84-70, Ca#>61 and low Na# (Na/(Na+Al)) (8-17) are intruded by patches or veins of more evolved FeTi-oxide rich gabbroic and dioritic rocks with Mg# to 20, Ca# to 32, Na#=14-23, TiO2<7 wt.% and FeOtotal<18 wt.%. All rocks are acdcumulates, and incompatible element concentrations are low, e.g. Pb=0.1-0.7 ppm and U

Geochemistry of selected Ferrar samples from Northern Victoria Land and George V Land during GANOVEX VIII

Newly sampled basaltic andesites and andesites from the tholeiitic Ferrar Supergroup of northern Victoria Land and George V Land, Antarctica, are attributed to the known low-Ti and high-Ti series. Aside from known sparsely distributed high-Ti extrusives, a high-Ti sill was found in the Alamein Range outside the Rennick Graben. Low-Ti lavas, sills and dikes display wide petrographical, mineral and geochemical variations, reflecting extensive in-situ differentiation. High-Ti rocks from Litell Rocks are homogeneous with respect to mineralogy and geochemistry, minor deviations are shown by the sampled sill. Chilled margins of low-Ti sills, dikes and lava flows exhibit nearly constant bulk-rock chemistry (mg# ~60) within the studied area. Compared to chilled margins from Tasmanian sills, the striking uniformity of the pre-emplacement chemistry of Ferrar magmas over large distances supports the magma transport model of Elliot et al. (1999, doi:10.1016/S0012-821X(99)00023-0). In the area investigated, compositional variations within the low-Ti series, caused by in-situ differentiation, increase towards the Wilson-Bowers Terrane boundary, possibly displaying the asymmetrical distribution of outcrops over this area. Absence of Ferrar occurrences east of the Bowers Terrane remains a matter of palaeo-geodynamic discussion. Besides, the secondary mineralogy of extrusives from Litell Rocks and Monument Nunataks exhibits noticeable differences, which indicates an elevated thermal gradient in the vicinity of Litell Rocks compared to Monument Nunataks during the Cretaceous.

Chemical and boron isotopic compositions of volcanics from ODP Sites 135-834 to 135-840

The influence of fluid flux on petrogenesis in the Tonga-Kermadec Arc was investigated using ion microprobe measurements of B/Be and boron isotope ratios (11B/10B) to document the source and relative volumes of the fluids released from the subducting oceanic plate. We analyzed young lavas from eight different islands along the Tonga-Kermadec Arc, as well as glass shards in volcanic sediments from Ocean Drilling Program (ODP) Site 840, which record the variations in the chemistry of Tonga magmatism since 7 Ma. B/Be is variable (5.8-122), in young Tonga-Kermadec Arc lavas. In contrast, glass shards from around 3 to 4 Ma old volcanic sediments at Site 840 have the highest B/Be values yet reported for arc lavas (18-607). These values are too high to be related simply to a sediment influence on petrogenesis. Together with very high d11B values (-11.6 to +37.5) for the same shards and lavas these data indicate that most of the B is derived from fluid escaped from the subducting altered Pacific oceanic crust, rather than from sediment. High d11B values also reflect large degrees of isotopic fractionation in this cold fast subduction zone. Lower d11B values noted in the Kermadec Arc (17 to -4.4) are related to the influence of sediment eroded from New Zealand and slower convergence. High fluid flux (B/Be) is synchronous in Tonga and the Marianas at 3 to 4 Ma and may be related to acceleration of the Pacific Plate just prior to this time. The timing of maximum B/Be at 3 to 4 Ma correlates with maximum light rare earth (LREE) and high field strength element depletion. This suggests maximum degrees of partial melting at this time. Although thinning of the arc lithosphere during rifting to form the Lau Basin is expected to influence the arc geochemistry, variable aqueous fluid flux from the subducting plate alone appears capable of explaining boron and other trace element systematics in the Tonga-Kermadec Arc with no indication of slab melting.