Geochemistry and mineralogy of sediments from the Atlantis II Fracture Zone

Thirteen sediment samples, including calcareous ooze, sandy clay, volcanic sand, gravel, and volcanic breccia, from Ocean Drilling Program (ODP) Sites 732B, 734B, 734G and Conrad Cruise 27-9, Station 17, were examined. Contents of major and trace elements were determined using XRF or ICP (on samples <0.5 g). Determinations of rare earth elements (REE) were performed using ICP-MS. Mineralogy was determined using XRD. On the basis of the samples studied, the sediments accumulating in the Atlantis II Fracture Zone are characterized by generally high MgO, Cr, and Ni contents compared with other deep-sea sediments. A variety of sources are reflected in the mineralogy and geochemistry of these sediments. Serpentine, brucite, magnetite, and high MgO, Cr, and Ni contents indicate derivation from ultramafic basement. The occurrence of albite, analcime, primary mafic minerals, and smectite/chlorite in some samples, coupled with high SiO2, Al2O3, TiO2, Fe2O3, V, and Y indicate contribution from basaltic basement. A third major sediment source is characterized as biogenic material and is reflected primarily in the presence of carbonate minerals, and high CaO, Sr, Pb, and Zn in certain samples. Kaolinite, illite, quartz, and some chlorite are most likely derived from continental areas or other parts of the ocean by long-distance sediment transport in surface or other ocean currents. Proportions of source materials in the sediments reflect the thickness of the sediment cover, slope of the seafloor, and the nature of and proximity to basement lithologies. REE values are low compared to other deep-sea sediments and indicate no evidence of hydrothermal activity in the Atlantis II Fracture Zone sediments. This is supported by major- and trace-element data.

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.

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).