Composition of Fe-Mn minerals from island arcs of the West Pacific

This paper presents materials on the chemical and mineralogical composition of Fe-Mn mineralization in island arcs (Kuril, Nampo, Mariana, New Britain, New Hebrides, and Kermadec) in the western part of the Pacific Ocean. The mineralization was proved to be of hydrothermal and/or hydrogenic genesis. The former is produced by hydrothermal Fe and Mn oxi-hydroxides that cement volcanic-terrigenous material in sediments. Some Fe oxyhydroxides can be derived via the halmyrolysis of volcaniclastic material. Crusts of this stage are characterized by fairly low concentrations of trace and rare elements, and their REE composition is inherited from the volcanic-terrigenous material. The minerals of the Mn oxyhydroxides are todorokite and "Ca-birnessite". The Mn/Fe ratio increases away from the discharge sites of the hydrothermal solutions. The hydrogenic Fe-Mn crusts are characterized by high concentrations of trace and minor elements of both the Mn group (Co, Ni, Tl, and Mo) and the Fe group (REE, Y, and Th). The hydrogenic crusts consist of Fe-vernadite and Mn-feroxyhyte. Some of the hydrothermal crusts originally had a hydrothermal genesis. The first data were obtained on crust B30-72-10 from the Macauley Seamount in the Kermadec island arc, which contained anomalously high concentrations of Co (2587 ppm) and other Mn-related trace elements in the absence of hydrogeneous Fe oxyhydroxides.

Geochemistry of altered smectites

In basalts and volcanogenic sediments from the Indian Ocean, the successive stages of submarine alteration of volcanic rocks and glasses give rise to the incorporation or the relative increase of iron in smectite lattices. During the first stage, the Mg-smectites are the most abundant; they are occasionally associated with Al-smectites. Afterwards, they are gradually replaced by iron-rich smectites. The REE distribution follows the same trend as the mineralogical changes. During the f'trst stage of alteration, REE distribution in clay minerals is the same as in the fresh glasses but, when the iron-rich smectites increase, the Ce has a specific behaviour. The Ce shows a positive anomaly in iron-rich smectites formed early in palagonitized glasses, and a negative one in authigenic smectites formed later from solutions in equilibrium with seawater.

Isotopic and chemical compositions of DSDP site 18-174 sediments

The Astoria submarine fan, located off the coast of Washington and Oregon, has grown throughout the Pleistocene from continental input delivered by the Columbia River drainage system. Enormous floods from the sudden release of glacial lake water occurred periodically during the Pleistocene, carrying vast amounts of sediment to the Pacific Ocean. DSDP site 174, located on the southern distal edge of the Astoria Fan, is composed of 879 m of terrigenous sediments. The section is divided into two major units separated by a distinct seismic discontinuity: an upper, turbidite fan unit (Unit I), and an underlying finer-grained unit (Unit II). Both units have overlapping ranges of Nd and Hf isotope compositions, with the majority of samples having e-Nd values of -7.1 to -15.2 and eHf values -6.2 to -20.0; the most notable exception is the uppermost sample in the section, which is identical to modern Columbia River sediment. Nd depleted mantle model ages for the site range from 2.0 to 1.2 Ga and are consistent with derivation from cratonic Proterozoic source regions, rather than Cenozoic and Mesozoic terranes proximal to the Washington-Oregon coast. The Astoria Fan sediments have significantly less radiogenic Nd (and Hf) isotopic compositions than present day Columbia River sediment (e-Nd=-3 to -4; [Goldstein, S.J., Jacobsen, S.B., 1987. Nd and Sr isotopic systematics of river water suspended material: implications for crustal evolution. Earth. Planet. Sci. Lett. 87, 249-265; doi:10.1016/0012-821X(88)90013-1]), and suggest that outburst flooding, tapping Proterozoic source regions, was the dominant sediment transport mechanism in the genesis and construction of the Astoria Fan. Pb isotopes form a highly linear 207Pb/204Pb - 206Pb/204Pb array, and indicate the sediments are a binary mixture of two disparate sources with isotopic compositions similar to Proterozoic Belt Supergroup metasediments and Columbia River Basalts. The combined major, trace and isotopic data argue that outburst flooding was responsible for depositing the majority (top 630 m) of the sediment in the Astoria Fan.

Chemical composition of interstitial solutions from sediments of DSDP Legs 6-8, 11, 12, 14, 15, and 22

Through the Deep Sea Drilling Project samples of interstitial solutions of deeply buried marine sediments throughout the World Ocean have been obtained and analyzed. The studies have shown that in all but the most slowly deposited sediments pore fluids exhibit changes in composition upon burial. These changes can be grouped into a few consistent patterns that facilitate identification of the diagenetic reactions occurring in the sediments. Pelagic clays and slowly deposited (<1 cm/1000 yr) biogenic sediments are the only types that exhibit little evidence of reaction in the pore waters. In most biogenic sediments sea water undergoes considerable alteration. In sediments deposited at rates up to a few cm/1000 yr the changes chiefly involve gains of Ca(2+) and Sr(2+) and losses of Mg(2+) which balance the Ca(2+) enrichment. The Ca-Mg substitution may often reach 30 mM/kg while Sr(2+) may be enriched 15-fold over sea water. These changes reflect recrystallization of biogenic calcite and the substitution of Mg(2+) for Ca(2+) during this reaction. The Ca-Mg-carbonate formed is most likely a dolomitic phase. A related but more complex pattern is found in carbonate sediments deposited at somewhat greater rates. Ca(2+) and Sr(2+) enrichment is again characteristic, but Mg(2+) losses exceed Ca(2+) gains with the excess being balanced by SO4(post staggered 2-) losses. The data indicate that the reactions are similar to those noted above, except that the Ca(2+) released is not kept in solution but is precipitated by the HCO3(post staggered -) produced in SO4(post staggered 2-) reduction. In both these types of pore waters Na(+) is usually conservative, but K(+) depletions are frequent. In several partly consolidated sediment sections approaching igneous basement contact, very marked interstitial calcium enrichment has been found (to 5.5 g/kg). These phenomena are marked by pronounced depletion in Na(+), Si and CO2, and slight enhancement in Cl(-). The changes are attributed to exchange of Na(+) for Ca(2+) in silicate minerals forming from submarine weathering of igneous rocks such as basalts. Water is also consumed in these reactions, accounting for minor increases in total interstitial salinity. Terrigenous, organic-rich sediments deposited rapidly along continental margins also exhibit significant evidences of alteration. Microbial reactions involving organic matter lead to complete removal of SO4(post staggered 2-), strong HCO3(post staggered -) enrichment, formation of NH4(post staggered +), and methane synthesis from H2 and CO2 once SO4(post staggered 2-) is eliminated. K+ and often Na+ (slightly) are depleted in the interstitial waters. Ca(2+) depletion may occur owing to precipitation of CaCO3. In most cases interstitial Cl- remains relatively constant, but increases are noted over evaporitic strata, and decreases in interstitial Cl- are observed in some sediments adjacent to continents.

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.

(Table 2) Oxygen, carbon, and strontium isotopic composition of carbonate-brucite material from the Lost City hydrothermal field

The isotopic (dD, d18O, d13C, and 87Sr/86Sr) and geochemical characteristics of hydrothermal solutions from the Mid-Atlantic Ridge and the material of brucite-carbonate chimneys at the Lost City hydrothermal field at 30°N, MAR, were examined to assay the role of the major factors controlling the genesis of the fluid and hydrothermal chimneys of the Lost City field. The values of dD and d18O in fluid samples indicates that solutions at the Lost City field were produced during the serpentinization of basement ultramafic rocks at temperatures higher than 200°C and at relatively low fluid/rock ratios (<1). The active role of serpentinization processes in the genesis of the Lost City fluid also follows from the results of the electron-microscopic studying of the material of hydrothermal chimneys at this field. The isotopic (d18O, d13C, and 87Sr/86Sr) and geochemical (Sr/Ca and REE) signatures indicate that, before its submarine discharging at the Lost City field, the fluid filtered through already cold altered outer zones of the Atlantis Massif and cooled via conductive heat loss. During this stage, the fluid could partly dissolve previously deposited carbonates in veins cutting serpentinite at the upper levels of the Atlantis Massif and the carbonate cement of sedimentary breccias underlying the hydrothermal chimneys. Because of this, the age of modern hydrothermal activity at the Lost City field can be much younger than 25 ka.

Geochemistry of Miocene "blue tuff" samples

Distinctive, massive to stratified, pale blue volcaniclastics, initially referred to as the "blue tuff," were encountered at all four sites drilled during ODP Leg 127 in the Japan Sea. Detailed vertical sequence analysis, plagioclase chemistry, plagioclase 87Sr/86Sr isotopic composition, and 40Ar/39Ar age dating indicate that thick sequences of the blue tuff are not genetically related. Blue tuffs at Hole 794B were apparently deposited by density flows at ambient temperature. Deposition was penecontemporaneous with a large submarine phreatomagmatic eruption at 14.9 Ma in bathyal or deeper water depths. The blue tuffs at this location comprise mostly reworked hydroclastic glass shards and lesser amounts of plagioclase crystals. Pyrogenic plagioclase has an average An mole% of 18±3. Comparison of blue tuff plagioclase compositions with the composition of plagioclase from acoustic basement at Site 794 suggests that these rocks are not genetically related. As such, the extrapolation of sediment accumulation rate data in conjunction with this more precise age for the blue tuff corroborates previous minimum age estimates of 16.2 Ma for acoustic basement at Site 794. Blue tuffs at Hole 796B were probably deposited at ambient temperatures by downslope slumping and density flow of reworked pyrogenic debris. This debris includes abundant bubble wall glass shards and plagioclase crystals, with variable admixture of volcanic lithic and intrabasinal fragments. Pyrogenic fragments were produced by subaerial or shallow submarine, magmatic eruptions dated at 7.6 Ma. Blue tuffs contain a heterogeneous mixture of unrelated fragments including a mixed population of plagioclase crystals. The average An mole% of the predominant, probable comagmatic, plagioclase population is 30±4. The two sequences of blue tuff studied are distinct in age, mineral composition, and the eruptive origin of pyroclastic fragments. Preliminary 87Sr/86Sr isotopic compositions of plagioclase, however, indicates that blue tuffs at both locations are the product of typical, subduction-related island arc magmatism. Based on the results of this study, there is no justification for stratigraphic correlation of widespread, Miocene, blue to blue-gray bentonitic tuff and tuffaceous sandstones nor the interpretation that these strata are indicative of regional, explosive submarine volcanism genetically related to rifting and formation of the Japan Sea. Rather, these reworked pyroclastic strata of intermediate composition were deposited over a protracted 6-8 m.y. period in association with widespread, subduction-related submarine to subaerial volcanism in the Japan Sea backarc basin.

(Table 1) Oxygen and carbon isotope compositions of altered carbonates from DSDP Hole 6-53 from the western Pacific Ocean

In the lower part of DSDP core 53.0, partly recrystallized carbonate sediments and well recrystallized limestone breccias of Oligo-Miocene age are associated with altered volcanic flows, lithified tuffs, and tuff breccias, suggesting that carbonate alteration was the result of thermal metamorphism. However, the oxygen isotope compositions of these carbonates (-3.4 to +0.6 per mil rel. PDB) are not compatible with recrystallization and isotope exchange with sea water at high temperatures. Evaluating the effects of the composition of the water which exchanged with the carbonates and of carbonate-water isotope exchange in closed systems yields the following approximate maximum temperature of recrystallization: limestone breccias, 100°C; calcite veins rimming breccia clasts, 30°C; and unconsolidated sediments overlying the breccias, 20°C. Therefore, the volcanics at site 53.0 must have been emplaced into the primary carbonate sediments at relatively low temperatures. Subsequent carbonate alteration was probably a consequence of chemical changes in ambient pore waters resulting from the submarine weathering of volcanic material.