Major and minor elements in hydrothermal and pelagic sediments at DSDP Leg 70 Holes

Interaction between young basaltic crust and seawater near the oceanic speading centers is one of the important processes affecting the chemical composition of the oceanic layer. The formation of metalliferous hydrothermal sediments results from this interaction. The importance of the interaction between seawater and basalt in determining the chemical composition of pore waters from sediments is well known. The influence of mineral solutions derived from this interaction on ocean water composition and the significant flux of some elements (e.g., Mn) are reported by Lyle (1976), Bogdanov et al. (1979), and others. Metal-rich sediments found in active zones of the ocean basins illustrate the influence of seawater-basalt interaction and its effect on the sedimentary cover in such areas. The role of hydrothermal activity and seawater circulation in basalts with regard to global geochemistry cycles has recently been demonstrated by Edmond, Measures, McDuff, McDuff et al. (1979), and Edmond, Measures, Mangum (1979). In the area of the Galapagos Spreading Center the interaction of sediments and solutions derived from interaction of seawater and basalt has resulted in the formation of hydrothermal mounds. The mounds are composed of manganese crusts and green clay interbedded and mixed with pelagic nannofossil ooze. These mounds are observed only in areas characterized by high heat flow (Honnorez, et al., 1981) and high hydrothermal activity.

Analyses of lake La Thuile sediment core

Lake La Thuile, in the Northern French Prealps (874 m a.s.l.), provides an 18 m long sedimentary sequence spanning the entire Lateglacial/Holocene period. The high resolution multi-proxy (sedimentological, palynological, geochemical) analysis of the uppermost 6.2 meters reveals the Holocene dynamics of erosion in the catchment in response to landscape modifications. The mountain belt is at relevant altitude to study past human activities and the watershed is sufficiently disconnected from large valleys to capture a local sedimentary signal. From 12,000 to 10,000 cal. BP (10 to 8 ka cal. BC), the onset of hardwood species triggered a drop in erosion following the Lateglacial/Holocene transition. From 10,000 to 4500 cal. BP (8 to 2.5 ka cal. BC), the forest became denser and favored slope stabilization while erosion processes were very weak. A first erosive phase was initiated at ca . 4500 cal. BP without evidence of human presence in the catchment. Then, the forest declined at approximately 3000 cal. BP, suggesting the first human influence on the landscape. Two other erosive phases are related to anthropic activities: approximately 2500 cal. BP (550 cal. BC) during the Roman period and after 1600 cal. BP (350 cal. AD) with a substantial accentuation in the Middle Ages. In contrast, the lower erosion produced during the Little Ice Age, when climate deteriorations are generally considered to result in an increased erosion signal in this region, suggests that anthropic activities dominated the erosive processes and completely masked the natural effects of climate on erosion in the late Holocene.

Major and trace element analyses from DSDP Hole 26-253 at the Ninetyeast Ridge in the Indian Ocean

A basaltic sequence of Eocene submarine-erupted pyroclastic sediments totals at least 388 m at DSDP Site 253 on the Ninetyeast Ridge. These fossiliferous hyaloclastic sediments have been erupted and fragmented by explosive volcanism (hydroexplosions) in shallow water. The occurrence of interbedded basaltic ash-fall tuffs within the younger horizons of the hyaloclastic sequence marks the emergence of some Ninetyeast Ridge volcanic vents above sea level. Considerable textural variation allows subdivision of the sequence into six informal lithostratigraphic units. Hydrothermal and diagenetic alteration has caused the complete replacement of all original glass by smectites, and the introduction of abundant zeolite and calcite cements. The major and trace element contents of the hyaloclastites vary due to the alteration, and the admixture of biogenous calcite. On a calcium carbonate-free basis systematic variations are recognisable. Mg, Ni, Cr and Cu are enriched, and Li and Zn depleted in the three older units relative to the younger three. The chemical variability is reflected by the development of saponite in the older part of the sequence and montmorillonite in the younger; and by the presence of a quartz-normative basalt flow occurring in Unit II, in contrast to the Mg-rich highly olivine-normative basalt at the base of the sequence. The younger and older parts of the sequence therefore appear to have been derived from magmas of different chemistry. The sequence, like other basaltic rocks recovered from the Ninetyeast Ridge, is enriched in the light relative to the heavy rare earth elements (REE) although the REE contents vary unsystematically with depth, probably because of the high-temperature subaqueous alteration and the presence of biogenous calcite. This REE data indicates that the Ninetyeast Ridge volcanism was different from that which produces mid-ocean ridge basalts.

Petrography, microprobe analyses and isotope composition of ultramafic rock from the northern Haskard Highlands, Shackleton Range, East Antarctica

In the Shackleton Range of East Antarctica, garnet-bearing ultramafic rocks occur as lenses in supracrustal high-grade gneisses. In the presence of olivine, garnet is an unmistakable indicator of eclogite facies metamorphic conditions. The eclogite facies assemblages are only present in ultramafic rocks, particularly in pyroxenites, whereas other lithologies - including metabasites - lack such assemblages. We conclude that under high-temperature conditions, pyroxenites preserve high-pressure assemblages better than isofacial metabasites, provided the pressure is high enough to stabilize garnet-olivine assemblages (i.e. >=18-20 kbar). The Shackleton Range ultramafic rocks experienced a clockwise P-T path and peak conditions of 800-850 °C and 23-25 kbar. These conditions correspond to ~70 km depth of burial and a metamorphic gradient of 11-12 °C/km that is typical of a convergent plate-margin setting. The age of metamorphism is defined by two garnet-whole-rock Sm-Nd isochrons that give ages of 525 ± 5 and 520 ± 14 Ma corresponding to the time of the Pan-African orogeny. These results are evidence of a Pan-African suture zone within the northern Shackleton Range. This suture marks the site of a palaeo-subduction zone that likely continues to the Herbert Mountains, where ophiolitic rocks of Neoproterozoic age testify to an ocean basin that was closed during Pan-African collision. The garnet-bearing ultramafic rocks in the Shackleton Range are the first known example of eclogite facies metamorphism in Antarctica that is related to the collision of East and West Gondwana and the first example of Pan-African eclogite facies ultramafic rocks worldwide. Eclogites in the Lanterman Range of the Transantarctic Mountains formed during subduction of the palaeo-Pacific beneath the East Antarctic craton.

Chemistry of minerals from ODP Leg 127/128 basalts

Suites of basalts drilled during Legs 127 and 128 can be distinguished by their mineral assemblages and compositions of phenocrysts and groundmass phases. An upper suite of plagioclase phyric basaltic sills with a groundmass composed of plagioclase, augite, and magnetite was recovered from Site 794. The upper, evolved part of this suite is highly plagioclase phyric, including calcic plagioclases (~An90). The most primitive, lower part of this upper suite, in addition, contains olivine, but lacks calcic plagioclase. A lower suite at Site 794 is plagioclase and olivine phyric to aphyric basaltic sills and flows with a groundmass of plagioclase, augite, olivine (~Fo75-83), and magnetite. At Site 795, plagioclase and augite phyric basalts and andesites were recovered. The relatively low Ti and Cr contents of augite of these basalts suggest typical arc tholeiitic parental magmas. Two suites of basalt were recovered from Site 797, an upper suite of plagioclase and olivine phyric to aphyric olivine basalts, and a lower suite of evolved plagioclase phyric basaltic sills. The most evolved sills at both sites lack olivine as phenocryst and groundmass phases, while this phase is present in the relatively primitive sills. The olivine-bearing suites contain plagioclase with relatively low potassium content and augite with relatively high sodium content. An exception is the olivine-bearing sills of the upper suite at Site 794 that contains plagioclase with relatively high potassium content similar to the associated olivine-free sills. The olivine-free suites contain plagioclase with high potassium content and augite with low sodium content and have the most evolved compositions of any of the Japan Sea rocks.

Petrology and geochemistry of sedimentary Unit III of ODP Hole 135-840B

The sedimentary sequence recovered at Site 840, on the Tonga frontal-arc platform, is 597.3 m thick and is subdivided into three lithostratigraphic units. The lowermost, late Miocene Unit III is 336.8 m thick and consists of a sequence of volcaniclastic mass-flow deposits (predominantly turbidites) interbedded with pelagic/hemipelagic deposits. Unit III was deposited in the forearc basin of the Lau volcanic arc, probably on a slope dominated by mass flows that built eastward from the ridge front and across the forearc. Upward through the unit a thinning and fining of individual turbidites takes place, interpreted to reflect a reduced sediment supply and a change from large to smaller flows. Decreasing volcanic activity with time is inferred from a decrease in coarse-grained volcaniclastic content in the upper part of the unit. The majority of the turbidites show the typical Bouma-type divisions, although both high- and low-density turbidity currents are inferred. High-density turbidity currents were especially common in the lower part of the unit. Geochemical analyses of detrital glass lie mainly in the low-K tholeiite field with a compositional range from basalt to rhyolite. A coherent igneous trend indicates derivation from a single volcanic source. This source was probably situated on the rifted part of the Lau-Tonga Ridge, within the present Lau backarc basin. The initial opening of the Lau Basin may have been around 6.0 m.y. ago. The onset of more extensive rifting, approximately 5.6 m.y. ago, is reflected in an increase in the silica content of volcanic glass. At the boundary toward Unit II, at approximately 5.25 Ma, an influx of thicker bedded and coarser grained volcaniclastic material is interpreted to reflect increasing volcanism and tectonism during the final breakup of the Lau-Tonga Ridge.

Tab.1: Representative bulk-rock and trace element chemistry

High-pressure/low-temperature metabasites occupy a definite geological position within the structure of the Polar Urals and have a very important bearing on the understanding of the early history of the Ural Mountains. Recently obtained geological, petrographic, geochemical and isotope data allow some conclusions on this history. The metabasites of the Khord"yus and Dzela complexes contain relics of a Neoproterozoic (578 ±8 Ma) oceanic crust. This crust formed part of the base of the early Paleozoic (500 Ma) ensimatic island arc and experienced Ca-Al-Si±Na metasomatism and, probably, partial melting with the formation of boninite melts. However, so far no boninite volcanics have been found. The metabasites at the base of the island arc took part in the collision and as a consequence experienced glaucophane schist and greenschist facies metamorphism during the collision and obduction over the passive Baltic margin 350 ±11 Ma ago.

Geochemical composition and crystallization temperatures of ODP Hole 176-735B gabbroic rocks

The transition from magmatic crystallization to high-temperature metamorphism in deep magma chambers (or lenses) beneath spreading ridges has not been fully described. High-temperature microscopic veins found in olivine gabbros, recovered from Ocean Drilling Program Hole 735B on the Southwest Indian Ridge during Leg 176, yield information on the magmatic-hydrothermal transition beneath spreading ridges. The microscopic veins are composed of high-temperature minerals, (i.e., clinopyroxene, orthopyroxene, brown amphibole, and plagioclase). An important feature of these veins is the 'along-vein variation' in mineralogy, which is correlated with the magmatic minerals that they penetrate. Within grains of magmatic plagioclase, the veins are composed of less calcic plagioclase. In grains of olivine, the veins are composed of orthopyroxene + brown amphibole + plagioclase. In clinopyroxene grains, the veins consist of plagioclase + brown amphibole and are accompanied by an intergrowth of brown amphibole + orthopyroxene. The mode of occurrence of the veins cannot be explained if these veins were crystallized from silicate melts. Consequently, these veins and nearby intergrowths were most likely formed by the reaction of magmatic minerals with fluid phases under the conditions of low fluid/rock ratios. Very similar intergrowths of brown amphibole + orthopyroxene are observed in clinopyroxene grains with 'interfingering' textures. It is believed, in general, that the penetration of seawater does not predate the ductile deformation within Layer 3 gabbros of the slow-spreading ridges. If this is the case, the fluid responsible for the veins did not originate from seawater because the formation of the veins and the interfingering textures preceded ductile deformation and, perhaps, complete solidification of the gabbroic crystal mush. It has been proposed, based on fluid inclusion data, that the exsolution of fluid from the latest-stage magma took place at temperatures >700°C in the slow-spreading Mid-Atlantic Ridge at the Kane Fracture Zone (MARK) area. No obvious mineralogical evidence, however, has been found for these magmatic fluids. The calculated temperatures for the veins and nearby intergrowths found in Hole 735B gabbros are up to 1000°C, and these veins are the most plausible candidate for the mineralogical expression of the migrating magmatic fluids.

Chemistry of igneous rocks from, the Celebes Sea basement

Major and trace elements, mineral chemistry, and Sr-Nd isotope ratios are reported for representative igneous rocks of Ocean Drilling Program Sites 767 and 770. The basaltic basement underlying middle Eocene radiolarianbearing red clays was reached at 786.7 mbsf and about 421 mbsf at Sites 767 and 770, respectively. At Site 770 the basement was drilled for about 106 m. Eight basaltic units were identified on the basis of mineralogical, petrographical, and geochemical data. They mainly consist of pillow lavas and pillow breccias (Units A, B, D, and H), intercalated with massive amygdaloidal lavas (Units Cl and C2) or relatively thin massive flows (Unit E). Two dolerite sills were also recognized (Units F and G). All the rocks studied show the effect of low-temperature seafloor alteration, causing almost total replacement of olivine and glass. Calcite, clays, and Fe-hydroxides are the most abundant secondary phases. Chemical mobilization due to the alteration processes has been evaluated by comparing elements that are widely considered mobile during halmyrolysis (such as low-field strength elements) with those insensitive to seafloor alteration (such as Nb). In general, MgO is removed and P2O5 occasionally enriched during the alteration of pillow lavas. Ti, Cs, Li, Rb, and K, which are the most sensitive indicators of rock/seawater interaction, are generally enriched. The most crystalline samples appear the least affected by chemical changes. Plagioclase and olivine are continuously present as phenocrysts, and clinopyroxene is confined in the groundmass. Textural and mineralogical features as well as crystallization sequences of Site 770 rocks are, in all, analogous to typical mid-ocean-ridge basalts (MORBs). Relatively high content of compatible trace elements, such as Ni and Cr, indicate that these rocks represent nearly primitive or weakly fractionated MORBs. All the studied rocks are geochemically within the spectrum of normal MORB compositional variation. Their Sr/Nd isotopic ratios plot on the mantle array (87Sr/87Sr 0.70324-0.70348 with 143Nd/144Nd 0.51298-0.51291) outside the field of Atlantic and Pacific MORBs. However, Sr and Nd isotopes are typical of both Indian Ocean MORBs and of some back-arc basalts, such as those of Lau Basin. The mantle source of Celebes basement basalts does not show a detectable influence of a subduction-related component. The geochemical and isotopic data so far obtained on the Celebes basement rocks do not allow a clear discrimination between mid-ocean ridge and back-arc settings.

Geochemistry of Fe-Mn crusts in sediments of the Central Indian Ridge

Layered Fe-Mn crusts from the off-axis region of the first segment of the Central Indian Ridge north of the Rodrigues Triple Junction were studied geochemically and mineralogically. Vernadite (delta-MnO2) is the main mineral oxide phase. 230Thxs and Co concentrations suggest high growth rates of up to 29 mm/Myr and a maximum age of the basal crust layer of 1 Ma. Whereas most of the major and minor elements show concentrations which are typical of hydrogenetic formation, Co, Pb, Ni and Ti concentrations are strikingly lower. Concentrations and distribution of the strictly trivalent rare-earths and yttrium (REY) are typical of hydrogenetic ferromanganese oxide precipitates, but in marked contrast, the crusts are characterized by negative CeSN (shale normalized) anomalies and (Ce/Pr)SN ratios less than unity. Profiles through the crusts reveal only minor variations of the REY distribution and (Ce/Pr)SN ratios range from 0.45 to 0.68 (compared to ratios of up to 2 for typical hydrogenetic crusts from the Central Indian Basin). The apparent bulk partition coefficients between the crusts and seawater suggest that for the strictly trivalent REY the adsorption-desorption equilibrium has been reached. Positive Ce anomalies in the partition coefficient patterns reveal preferential uptake of Ce, but to a lesser extent than in normal hydrogenetic crusts. A new parameter (excess Ce, Cexs) to quantify the degree of decoupling of Ce from REY(III) is established on the basis of partition coefficients. Cexs/Cebulk ratios suggest that the CIR crusts formed by precipitation of Fe-Mn oxides from a hydrothermal plume and that in hydrothermal plumes and normal seawater the enrichment of Ce results from the same oxidative sorption process. The growth rates, calculated with 230Thxs data as well as with the Co formula, are inversely related to Cexs.

Geochemistry at DSDP Leg 70 Holes

A total of 32 holes at five sites near 1°N, 86°W drilled on Deep Sea Drilling Project (DSDP) Leg 70 (November- December 1979) provide unique data on the origin of the hydrothermal mounds on the southern flank of the Galapagos Spreading Center. Hydrothermal sediments, primarily Mn-oxide and nontronite, are restricted to the immediate vicinity of the mounds (< 100 m) and are probably formed by the interaction of upward-percolating hydrothermal solutions with seawater and pelagic sediments above locally permeable zones of ocean crust. Mounds as high as 25 meters form in less than a few hundred thousand years, and geothermal and geochemical gradients indicate that they are actively forming today. The lack of alteration of upper basement rocks directly below the mounds and throughout the Galapagos region indicates that the source of the hydrothermal solutions is deeper in the crust.

Geochemistry of phosphatic crusts from the shelf off Peru

Authigenic phosphorite crusts from the shelf off Peru (9°40°S to 13°30°S) consist of a facies with phosphatic coated grains covered by younger phosphatic laminite. The crusts are composed of carbonate fluorapatite, which probably formed via an amorphous precursor close to the sediment water interface as indicated by low F/P2O5 ratios, Sr and Ca isotopes, as well as rare earth element patterns agreeing with seawater-dominated fluids. Small negative Ce anomalies and U enrichment in the laminite suggest suboxic conditions close to the sediment-water interface during its formation. Increased contents of chalcophilic elements and abundant sulfide minerals in the facies with phosphatic coated grains as well as in the laminite denote sulfate reduction and, consequently, point to episodical development of anoxic conditions during phosphogenesis. The Peruvian phosphorites formed episodically over an extended period of time lasting from Middle Miocene to Pleistocene. Individual phosphatic coated grains show a succession of phosphatic layers with varying contents of organic matter and sulfide-rich phosphatic layers. Coated grains supposedly formed as a result of episodic suspension caused by high turbulence and shifting redox conditions. Episodic anoxia in the pore water induced pyritization in the outermost carbonate fluorapatite layer. Phosphatic coated grains were later transported to the place of crust formation, where subsequent laminite formation was favored under lower energy conditions. A similar succession of phosphatic layers with varying contents of organic matter and sulfide-rich layers in the laminite suggests a formation mechanism analogous to that of individual coated grains.

Mineralogy and geochemistry of sediments of the Cretaceous/Tertiary boundary at DSDP Holes 62-465 and 62-465A

The complete Paleocene section begins with the basal Tertiary Globigerina eugubina Zone. This zone occurs at 465A-3-3, 4 cm to 465A-3-3, 144 cm and belongs to Lithologic Unit I (Site 465 report, this volume), a homogeneous, white, moderately to highly disturbed nannofossil ooze.