Chemical composition of siderites and clay minerals from DSDP Holes 70-506G and 70-507B

As soon as they are emplaced on the sea floor, oceanic basalts go through a low-temperature alteration process which produces black halos concentrical with exposed surfaces and cracks, whereas the grey internal parts of the basaltic pieces apparently remain unaltered. This paper reports for the first time the occurrence of authigenic siderite and ankerite in oceanic basalts and more particularly in the grey internal parts of the latter. Small (8-50 µm) crystals of zoned siderite and ankerite have been observed in ten vesicles of two samples recovered from DSDP Holes 506G and 507B drilled south of the Galápagos Spreading Center (GSC). These Fe-carbonates show a large range of chemical composition (FeCO3 = 47-88%; CaCO3 = 5-40%; MgCO3 = 1-20%; MnCO3 = 0-11%). Most of them are Ca-richer than siderite reported in the literature. The chemical composition of the carbonate clearly reflects the fluctuation of the fluid chemical composition during crystallization. Mn and at least part of the Fe are thought to be hydrothermal in origin, whereas Mg and probably Ca were provided by seawater. It is proposed that siderite and ankerite formed at relatively low temperature (<85°C) and is metastable. The alteration of the GSC basalts seems to have proceeded in two stages: during the first, reducing stage, pyrite precipitated from hydrothermal fluids. A little further in the rock, siderite precipitated from the fluid which had already been modified by the formation of pyrite, and thus in a microenvironment where particular conditions prevailed (high P_CO2, increasing p_S**2- or increasing pH or increasing or decreasing pe). During the second, oxidizing, stage of alteration, a seawater-dominated fluid allowed crystallization of mixtures of Fe-rich smectites and micas, and Fe-hydroxides forming the black halos in the external portion of the basalt pieces and locally oxidizing pyrite and siderite in their innermost part. It is shown in this paper that, even at its earliest stage, and at low temperature, alteration of the upper oceanic crust (lavas) involves fluids enriched in Fe and Mn, interpreted to be of hydrothermal origin.

Clay mineralogy of sediments from the Bengal Fan

We have measured the relative abundances of smectite, illite, chlorite, and kaolinite in a composite section of the distal Bengal Fan. Two sources of sediment appear to dominate, a smectite-poor, illite-rich source associated with rapid denudation of the Himalayas and a smectite-rich, illite-poor source probably on the continental margin of the Indian sub-continent. Changes in source appear to be related to uplift in the Himalayas and Tibetan Plateau both directly and through the climatic and oceanographic consequences of uplift.

Thermal diagenesis of clay minerals within volcanogenic material from ODP Site 135-841

Clay minerals are examined in detail in the sediment from the Tonga Trench margin at Site 841 (Leg 135 ODP). The changes in amount and nature of secondary clays with depth provide an alternative explanation for the intensive alteration of volcanogenic material at convergent margins. A characteristic distribution of clay minerals with depth shows four distinct zones unexplainable by simple burial diagenesis processes. These are named the upper, reactive, lower and rhyolitic zones. The reactive zone is intercalated with numerous sills and is characterized by the dominant iron-rich clays such as saponite, corrensite and chlorite associated with analcime. The occurrence of such iron-rich clays, mostly associated with a large amount of analcime, yields chemical and mineralogical evidence for thermal diagenesis. The required heat for the diagenetic process was transferred from recently intruded basaltic andesite sills. In the vicinity of these intrusions, the iron-rich clay minerals may have formed at temperatures up to 200°C. A zoning with respect to clay and zeolite minerals indicates that the influence of the palaeoheat flow decreased with the distance from the intrusion. The formation of interlayered I/S, illite, kaolinite and aluminous chlorite, which are recognized as major secondary minerals within the rhyolitic complex, was mainly controlled by both early diagenesis at moderately elevated temperatures, and since the Eocene by burial diagenesis at low temperatures. The occurrence of a steam zone in an early stage of the intrusion is restricted to Miocene tuffs and has overprinted the early alteration of the volcanogenic material within the tuffs and has changed the originally pristine composition of the pore fluids.

Mineral content, major oxides and trace elements concentrations in clay from different IODP Holes of 331-C0013, 331-C0017, East China sea shelf and different Taiwan rivers

The Okinawa Trough (OT) in the East Asian continental margin is characterized by thick terrigenous sediment and ubiquitous volcanic-hydrothermal activities. In this study, the clays collected during IODP Expedition 331 to the middle OT (Iheya North Knoll) were analyzed for mineralogical and geochemical compositions. By comparing with the clays from the East China Sea shelf and surrounding rivers, we examine different clay origins. The hydrothermal field in the mid-OT is dominated by Mg-rich chlorite, while the recharge zone has clay mineral assemblages similar to the shelf and rivers, showing high content of illite, subordinate chlorite and kaolinite and scarce smectite. Compared to the terrigenous clays, the hydrothermal clays in the OT have high concentrations of Mg, Mn and Zr but low Fe, Na, K, Ca, Ba, Sr, P, Sc and Ti, while the hydrothermal clays in the mid-ocean ridge are relatively enriched in Fe and V and depleted in Al, Mg, Zr, Sc and Ti. Different fractionation patterns of rare earth elements also register in the terrigenous and hydrothermal clays, diagnostic of variable clay origins. We infer that the OT hydrothermal clay was primarily formed by the chemical alteration of detrital sediments subject to the hydrothermal fluids. The remarkably different compositions of hydrothermal clays between the sediment-rich back arc basin like OT and the sediment-starved ocean ridge suggest different physical and chemical processes of hydrothermal fluids and fluid-rock/sediment reactions under various geologic settings.

Sedimentary history and chemical characteristics of clay minerals from the Bengal Fan

The purpose of this study is to clarify the sedimentary history and chemical characteristics of clay minerals found in sediments deposited in the distal part of the Bengal Fan since the Himalayas were uplifted 17 m.y. ago. A total of seventy-eight samples were collected from three drilled cores which were to be used for the clay mineral analyses by means of XRD and ATEM. The results obtained from the analyses show that individual clay mineral species in the sediment samples at each site have similar features when the samples are of the same age, whereas these species have different features in samples of differing geological ages. Detrital clay minerals such as illite and chlorite were deposited in greater amounts than kaolinite and smectite during the Early to Middle Miocene. This means that the Himalayan uplift was vigorous at least until the Middle Miocene. In the Pliocene chemical weathering was more prevalent so that instead, in the distal part of the Bengal Fan, kaolinite shows the highest concentrations. This would accord with weaker uplift in the Himalayas. In the Pleistocene period, vigorous Himalayan uplift is characterized by illite-rich sediment in place of kaolinite. In the Holocene, smectite shows the highest concentration in place of the illite and kaolinite which were the predominant clay minerals of the earlier periods. Increasing smectite concentration suggests the Himalayan uplift to have been stable after the Pleistocene period. The smectite analyzed here is found to be dioctahedral Fe-beidellite, and it originated largely from the augite-basalt on the Indian Deccan Traps. The tri-octahedral chlorite is subdivided into three sub-species, an Fe-type, a Mg-type and an intermediate type. The mica clay mineral can be identified as di-octahedral illite which is rich in potassium. The chemical composition and morphology of each clay mineral appears to exhibit no change with burial depth in the sedimentary columns. This implies that there was no systematic transformation of clay minerals with time.

Sedimentary facies, clay mineralogy and geochemistry of the Paleogene sediments of ODP Site 105-647

Claystones immediately overlying the early Eocene age ocean-floor basalt, cored at Ocean Drilling Program (ODP) Site 647, underwent hydrothermal and thermal alterations originating from the basalt, which resulted in changes in both the mineralogical and chemical composition of the sediments. Chlorites and higher magnesium and iron concentrations were found in the lowermost sediment sequence. Upcore, changes in the bulk chemical composition of the sediments become smaller, when compensated for variations in the carbonate content originating from biogenic and authigenic components. Chlorite disappears upcore, but still only part of the swelling clay minerals have survived the thermal influence. Thirty meters above the basalt, the clay mineralogy and chemical composition become uniform throughout the Paleogene section. Iron-rich smectites (i.e., nontronitic types), totally dominate the clay mineral assemblage. Biogenic components, responsible for the dominant part of the calcite and cristobalite contents, vary in amount in the upper part, and so do the authigenic carbonate and sulfide contents. Detrital components, such as kaolinite, illite, quartz, and feldspars, make up a very small proportion of the sediment record. The nontronitic smectites are believed to be authigenic, formed by a supply of iron from the continuous formation of ocean-floor basalt in the ridge area that reacted with the detrital and biogenic silicates and alumina silicates.

Accumulation rate, carbon geochemistry and clay mineralogy of ODP Site 181-1123 sediments, southwest Pacific

Site 1123 is located on the northeastern flank of the Chatham Rise. Sedimentological and clay mineralogical analyses indicate a very fine grained carbonate-rich sediment. Smectite and illite are the main constituents of the clay mineral assemblage. High smectite values in the Eocene decrease in younger sediment sequences. Illite and chlorite concentrations increase in younger sediments with significant steps at 13.5, 9, and 6.4 Ma. The kaolinite content is near the detection limit and not significant. We observed only small fluctuations of the clay mineral composition, which indicates a uniform sedimentation process, probably driven by long-term processes. Good correspondence is shown between increasing illite and chlorite values and the tectonic uplift history of the Southern Alps.

Bulk composition and clay mineralogy at DSDP Legs 56 and 57

Altogether 513 samples from sediments of Cretaceous to Pleistocene age from DSDP Legs 56 and 57 were examined by x-ray methods. The main constituents are clay minerals, quartz, feldspar, opaline silica, and volcanic glass. The sediment composition reflects the position of the sites in relation to the main source area, the Japanese Island Arc. For example, relatively coarse-grained material rich in quartz and feldspar was deposited closest to the islands, whereas finer-grained material rich in clay minerals (mainly smectite and illite, with lesser amounts of kaolinite and chlorite) was deposited farther seaward. Vertical fluctuations in the composition of the sediments show the same trend in all sites and are caused mainly by a fluctuating contribution of biogenic silica with time. A trend reversal in the chlorite/kaolinite ratio at Site 438 supports the conclusion that the subsidence of the Oyashio ancient landmass took place during the middle Miocene. That ratio also indicates a northwest drift in the position of Site 436 by sea floor spreading. Oscillations of the illite/smectite ratio during the Pleistocene at Site 436 show the variations of climate during this period. During early diagenesis potassium is fixed in smectite. With increasing depth of burial a smectite-illite mixed layer is formed, with increasing illite layering. At Sites 434, 440, and 441, stepwise changes confirm intensive tectonic process at the midslope terrace and the lower inner slope of the Japan Trench.

Geochemistry of the Kimmeridge Clay Formation, Dorset Coast

The Kimmeridge Clay Formation (KCF) and its equivalents worldwide represent one of the most prolonged periods of organic carbon accumulation of the Mesozoic. In this study, we use the molybdenum (Mo) stable isotope system in conjunction with a range of trace metal paleoredox proxies to assess how seawater redox varied both locally and globally during the deposition of the KCF. Facies with lower organic carbon contents (TOC 1-7 wt %) were deposited under mildly reducing (suboxic) conditions, while organic-rich facies (TOC >7 wt %) accumulated under more strongly reducing (anoxic or euxinic) local conditions. Trace metal abundances are closely linked to TOC content, suggesting that the intensity of reducing conditions varied repeatedly during the deposition of the KCF and may have been related to orbitally controlled climate changes. Long-term variations in d98/95Mo are associated with the formation of organic-rich intervals and are related to third-order fluctuations in relative sea level. Differences in the mean d98/95Mo composition of the organic-rich intervals suggest that the global distribution of reducing conditions was more extensive during the deposition of the Pectinatites wheatleyensis and lower Pectinatites hudlestoni zones than during the deposition of the upper Pectinatites hudlestoni and Pectinatites pectinatus zones. The global extent of reducing conditions during the Kimmerigidan was greater than today but was less widespread than during the Toarcian (Early Jurassic) oceanic anoxic event. This study also demonstrates that the Mo isotope system in Jurassic seawater responded to changes in redox conditions in a manner consistent with its behavior in present-day sedimentary environments.

Calcium isotope ratios of pore fluids and solid phase of organic rich sediments

Pore fluid calcium isotope, calcium concentration and strontium concentration data are used to measure the rates of diagenetic dissolution and precipitation of calcite in deep-sea sediments containing abundant clay and organic material. This type of study of deep-sea sediment diagenesis provides unique information about the ultra-slow chemical reactions that occur in natural marine sediments that affect global geochemical cycles and the preservation of paleo-environmental information in carbonate fossils. For this study, calcium isotope ratios (d44/40Ca) of pore fluid calcium from Ocean Drilling Program (ODP) Sites 984 (North Atlantic) and 1082 (off the coast of West Africa) were measured to augment available pore fluid measurements of calcium and strontium concentration. Both study sites have high sedimentation rates and support quantitative sulfate reduction, methanogenesis and anaerobic methane oxidation. The pattern of change of d44/40Ca of pore fluid calcium versus depth at Sites 984 and 1082 differs markedly from that of previously studied deep-sea Sites like 590B and 807, which are composed of nearly pure carbonate sediment. In the 984 and 1082 pore fluids, d44/40Ca remains elevated near seawater values deep in the sediments, rather than shifting rapidly toward the d44/40Ca of carbonate solids. This observation indicates that the rate of calcite dissolution is far lower than at previously studied carbonate-rich sites. The data are fit using a numerical model, as well as more approximate analytical models, to estimate the rates of carbonate dissolution and precipitation and the relationship of these rates to the abundance of clay and organic material. Our models give mutually consistent results and indicate that calcite dissolution rates at Sites 984 and 1082 are roughly two orders of magnitude lower than at previously studied carbonate-rich sites, and the rate correlates with the abundance of clay. Our calculated rates are conservative for these sites (the actual rates could be significantly slower) because other processes that impact the calcium isotope composition of sedimentary pore fluid have not been included. The results provide direct geochemical evidence for the anecdotal observation that the best-preserved carbonate fossils are often found in clay or organic-rich sedimentary horizons. The results also suggest that the presence of clay minerals has a strong passivating effect on the surfaces of biogenic carbonate minerals, slowing dissolution dramatically even in relation to the already-slow rates typical of carbonate-rich sediments.

Physical properties, grain size distribution and clay mineralogy of ODP Leg 117 sites

The fabric of sediments recovered at sites drilled on the Indus Fan, Owen Ridge, and Oman margin during Ocean Drilling Program Leg 117 was examined by scanning electron microscopy to document changes that accompany sediment burial. Two sediment types were studied: (1) biogenic sediments consisting of a variety of marly nannofossil and nannofossil oozes and chalks and (2) terrigenous sediments consisting of fine-grained turbidites deposited in association with the Indus Fan. Biogenic sediments were examined with samples from the seafloor to depths of 306 m below seafloor (mbsf) on the Owen Ridge (Site 722) and 368 mbsf on the Oman margin (Sites 723 and 728). Over these depth ranges the biogenic sediments are characterized by a random arrangement of microfossils and display little chemical diagenetic alteration. The microfossils are dispersed within a fine-grained matrix that is predominantly microcrystalline carbonate particles on the Owen Ridge and clay and organic matter on the Oman margin. Sediments with abundant siliceous microfossils display distinct, open fabrics with high porosity. Porosity reduction resulting from gravitational compaction appears to be the primary process affecting fabric change in the biogenic sediment sections. Fabric of illite-rich clayey silts and silty claystones from the Indus Fan (Site 720) and Owen Ridge (Sites 722 and 731) was examined for a composite section extending from 45 to 985 mbsf. In this section fabric of the fine-grained turbidites changes from one with small flocculated clay domains, random particle arrangement, and high porosity to a fabric with larger domains, strong preferred particle orientation roughly parallel to bedding, and lower porosity. These changes are accomplished by a growth in domain size, primarily through increasing face-to-face contacts, and by particle reorientation which is characterized by a sharp increase in alignment with bedding between 200 and 400 mbsf. Despite extensive particle reorientation, flocculated clay fabric persists in the deepest samples examined, particularly adjacent to silt grains, and the sediments lack fissility. Fabric changes over the 45-985 mbsf interval occur in response to gravitational compaction. Porosity reduction and development of preferred particle orientation in the Indus Fan and Owen Ridge sections occur at greater depths than outlined in previous fabric models for terrigenous sediments as a consequence of a greater abundance of silt and a greater abundance of illite and chlorite clays.

Clay and silica minerals of ODP Site 123-765

Sediments recovered from Site 765 can be divided into seven mineral associations, based on differences in clay mineralogy. These clay mineral associations correlate with the lithologic units and reflect the rift-to-drift history of the passive Australian margin. In general, the Lower to mid-Cretaceous sediments represent altered volcanic material and detrital aluminosilicates that were deposited during the early formation of the Argo Basin. The predominant clay mineral is randomly interstratified illite/smectite (I/S) that contains less than 10% illite layers. The transformation of smectite to illite is suggested by an increase in the percentage of illite layers in the basal sediments (from <10% to 40%) that corresponds to the silica transformation of opal-CT to quartz. This mixed-layered illite/smectite has an average composition of (K0.14 Na0.29 C0.07)(Al0.88 Mg0.43 Fe0.61 Ti0.06)(Si3.88 Al0.12)(O)10(OH)2. The highly smectitic composition of the I/S and its association with bentonite layers and zeolite minerals suggest that much of the I/S was derived from the alteration of volcanic material. The condensed middle to Upper Cretaceous sediments consist of palygorskite and detrital I/S that contains 30% to 60% illite layers. The condensed Paleogene sediments contain no palygorskite and are dominated by detrital clay minerals or by highly smectitic I/S associated with bentonite layers and zeolite minerals. The overlying, rapidly deposited Neogene clayey calcareous turbidites consist of three distinct clay mineral associations. Middle Miocene sediments contain palygorskite, kaolinite, and a tentatively identified mixed-layered illite/smectite/chlorite (I/S/C) or saponite. Upper Miocene sediments contain abundant sepiolite and kaolinite and lesser amounts of detrital I/S. Detrital I/S and kaolinite dominate the clay mineralogy of Pliocene and Pleistocene sediments. The fibrous, magnesium-rich clay minerals sepiolite and palygorskite appear to be authigenic and occur intimately associated with authigenic dolomite. The magnesium required to form these Mg-rich minerals was supplied by diffusion from the overlying seawater, and silica was supplied by the dissolution of associated biogenic silica.

Composition of bentonite-, smectite-rich- and ash beds from DSDP Leg 19 holes

Late Cenozoic ash deposits cored in Deep Sea Drilling Project Leg 19 in the far northwest Pacific and in the Bering Sea have altered to bentonite beds. Some bentonite layers were subsequently replaced by carbonate beds. A significant part of the Neogene volcanic history of land areas adjacent to the far north Pacific is represented by these diagenetic deposits. Bentonite beds are composed of authigenic smectite and minor amounts of clinoptilolite. Authigenic smectite has fewer illite layers than detrital smectite. Opal-A and opal-CT, abundant in Bering Sea sediment, are not found in ash or bentonite layers. The percentage of smectite in the total clay-mineral assemblage of ash beds is greater than that for adjacent terrigenous sediment, but the total amount of clay minerals in ash sequences is less than in surrounding deposits. Morphology of the 17-Å peak of smectite found in ash may represent newly formed, poorly crystalline smectite. Smectite becomes better crystallized as bentonite layers form. The percentage of smectite of the total clay-mineral assemblage in bentonite beds is greater than that in surrounding sediment, and, in contrast to ash beds, the total amount of clay minerals (mostly smectite) in bentonite layers is greater than in adjacent terrigenous sediment. Apparently, silica is not mobilized when volcanic ash layers transform to bentonite beds. Saponite-nontronite varieties of smectite and high Fe/Al and Ti/Al ratios distinguish bentonite beds derived from basaltic parent material from those beds formed from more silicic volcanic ash. These silicic ash beds produce bentonite composed mostly of montmorillonite. The basal sediment section at site 192 is rich with bentonite beds. Smectite in the upper part of this section (Eocene) was formed by low-temperature diagenesis of volcanic debris of intermediate or more silicic composition derived from arc or Pacific volcanoes. In contrast, smectite from the lowest 10 to 20 m of the sedimentary section (Cretaceous) is formed from either low-temperature or hydrothermal alteration of the underlying basaltic basement and associated pyroclastic debris. This near-basement smectite contains Mg and K acquired from sea water and Si, Al, Fe, Ti, and Mn released from the volcanic material.

Clay mineralogy and organic carbon content of sediment core GeoB1401-4

Late Quaternary sediments recovered in a core from the area of the Zaire Fan, Central Africa, were analyzed for clay mineral composition in order to reconstruct fluctuations in the sediment input and freshwater discharge of the Zaire River. Clay mineral assemblages are dominated by kaolinite and smectite, which both originate mainly from the Zaire River and contain only minor contributions of eolian dust. Smectite crystallinity and chemical character of illites (Fe, Mg- or Al-rich) are used to track sediment input from the Zaire River and assess fluctuations in the freshwater discharge. Both parameters record a high-latitude forcing of river runoff at 100 ka periodicities reflecting glacial aridity and increased runoff during interglacials 1, 5 and 7. This signal is also observed in kaolinite/smectite ratios which represent the extension and intensity of the freshwater plume of the Zaire River. Clay mineral proxies reveal that river discharge and associated sediment input fluctuated in tune with precessional cycles of African monsoon intensity. Increased eolian input of kaolinite-rich dust with intensified northeast trades during glacials flattens the precessional signal in kaolinite/smectite ratios.