Clay minerals in sediments of the hydrothermally active southern trough in the Guaymas Basin (Gulf of California)

The results of the study of clay mineral alterations in Upper Pleistocene sediments of the southern trough in the Guaymas Basin (Gulf of California) due to the influence of hydrothermal solutions and heat produced by sill intrusions are discussed. Core samples from DSDP Holes 477 and 477A were taken for the analysis of clay minerals. Application of the method of modeling X-ray diffraction patterns of oriented specimens of the finely dispersed particles made it possible to establish the phase composition of clay minerals, determine their structural parameters, and obtain reliable quantitative estimates of their contents in natural mixtures. The modeling data allowed us to characterize reliably the transformation of clay minerals in sediments of the hydrothermally active southern trough in the Guaymas Basin. In Upper Pleistocene sandy–clayey sediments of the southern trough, changes in the composition of clay minerals occurred under the influence of a long-living hydrothermal system. Its lower part (interval 170.0–257.5 m) with maximum temperatures (~300°C) was marked by the formation of chlorite. Terrigenous clay minerals are not preserved here. Saponite appears at a depth of 248 m in the chlorite formation zone. Higher in the sedimentary section, the interval 146–170 m is also barren of terrigenous clay minerals. Sediments of this interval yielded two newly formed clay minerals (chlorite and illite), which were formed at lower temperatures (above 180°C and below 300°C, approximately up to ~250°C), while the relatively low-temperature upper part (110–146 m) of the hydrothermal system (from ~140°C to ~180°C) includes the mixture of terrigenous and newly formed clay minerals. Terrigenous illite is preserved here. Illitization of the mixed-layer illite–smectite was subjected to illitization. The terrigenous montmorillonite disappeared, and chlorite–smectite with 5–10% of smectite layers were formed. In the upper interval (down to approximately 110 mbsf), the composition of terrigenous clay minerals remains unchanged. They are composed of the predominant mixed-layer illite–smectite and montmorillonite, the subordinate illite, mixed-layer chlorite–smectite with 5% of smectite layers, mixed-layer kaolinite–smectite with 30% of smectite layers, and kaolinite. This composition of clay minerals changed under the influence of sill intrusions into the sedimentary cover at 58–105 m in the section of Hole 477. The most significant changes are noted in the 8-m-thick member above the sill at 50–58 m. The upper part of this interval is barren of the terrigenous mixed-layer illite–smectite, which is replaced by the newly formed trioctahedral smectite (saponite). At the same time, the terrigenous dioctahedral smectite (montmorillonite) is preserved. The composition of terrigenous clay minerals remains unchanged at the top of the unit underlying the sill base.     

Variation of the chemical composition of sediments under middle- and low-temperature hydrothermal conditions in the southern trough of Guaymas Basin,Gulf of California: Communication 2. Results of the study of grain size fractions of sediments

The chemical composition of grain size fractions of the Upper Pleistocene sediments in the southern trough of Guaymas Basin (Gulf of California) is studied based on materials from DSDP Hole 477 (191 m). The sediments are located in the upper part of the main long-lived hydrothermal system. Therefore, they were subjected to long-term reworking by the middle- and low-temperature solutions (100?300°C) and short-term hydrothermal impact during the intrusion of basalt sills. The upper part of the main hydrothermal system is divided into three middle- to low-temperature hydrothermal alteration zones: lower (III), middle (II), and upper (I). In zone III (250?300°C), hydrothermal alterations of sediments are most significant. The coarsegrained fractions are enriched in Fe, S, Ni, Cu, Zn, Mo, Co, Se, Cd, Bi, Pb, and Ag due to the formation of sulfides and precipitation of native metals. The silt fractions are enriched in Be, Sc, Nb, Ta, W, Th, U, Y, and REE. In contrast, the pelite fractions are depleted in most of these elements. The fine-dispersed (<0.001 mm) pelite fractions are appreciably enriched in Mg (due to the formation of the authigenic Mg-chlorite), as well as MnO, Cr, V, Ga, Pb, and Zr. Virtually all grain size fractions are depleted in K₂O, Li, Rb, Cs, and Tl due to the dissolution of the K-bearing terrigenous minerals. In zones II (146?170 m) and I (110?146 m), where the temperature of hydrothermal solutions dropped successively from 250 to 180?195 and 100°C, the sedimentary environment was unfavorable for the concentration of Fe, S, and a large group of ore elements that are typical for sediments in zone III. Intrusion of basalt sills and a short-term hydrothermal impact changed concentrations of several components in the sediments overlying the sill complex: CaO, K₂O, Li, Rb, Cs, and Tl acquired a trend typical of the high-temperature lower zone III, whereas Fe, MgO, P₂O₅, TiO₂, V, Sc, and Y distribution pattern became typical of the low-temperature upper zone I. Significant compositional variations in the grain size fractions of sediments are lacking beneath the sill base.     

Unusual occurrences of aegirine-augite,fassaite and melanite in oceanic basalts (DSDP Hole 504B)

DSDP Hole 504B, located on the south flank of the Costa Rica Rift, shows the most complete reference section through the upper oceanic crust; 274.5 m into the sediment and 1075.5 m into the basement have been penetrated.Very unusual secondary parageneses occur in some basaltic massive flows recovered at DSDP Hole 504 B. Replacement of magmatic augite by secondary Ti- and/or Na-rich augite and aegirine-augite was observed in five veins between the 298 and 685 m sub-basement depth; one of these veins also contains melanite (Ti-rich andradite) and another one contains fassaite. These minerals seem independent of, and prior to, other alteration stages.The conditions of formation and origin of the fluids responsible for the crystallization of these unusual minerals are discussed. Several arguments suggest that their origin is not hydrothermal, but possibly deuteric: (1) these minerals probably formed at higher temperatures than hydrothermal minerals from DSDP Hole 504 B (300–400°C for melanite, 800–1000°C (?) for fassaite, >400°C for aegirine-augite); (2) the chemical composition of the studied minerals are not compatible with the chemical composition of hydrothermal fluids responsible for the alteration at DSDP Hole 504 B; (3) the distribution and chemical composition of these minerals seem to be independent of the alteration zonation and thus of depth; and (4) these minerals are anhydrous, in contrast to mostly hydrated minerals from DSDP Hole 504 B known to be hydrothermal in origin. Moreover, since these minerals occur in subophitic or coarse-grained basalts, their crystallization may have been enhanced by the long standing stability of the physico-chemical conditions existing in the veins.     

塞浦路斯特罗多斯型洋壳最上部500米剖面(CY—1钻孔)中的不透明矿物、磁性及其蚀变作用的研究

引言近十年来为了鉴别大洋盆地中线性磁异常模式的起源,对洋壳磁化强度的研究显示了极大的兴趣。目前了解洋壳(尤其在深部)可以通过几种方法,如在推测为构造断崖的附近进行海底拖曳打捞、通过大洋基底钻探     

Hydrothermal alterations of the chemical composition in grain size fractions of sediments in the Guaymas Basin,Gulf of California

The paper presents data on the contents of macro- and microelements (rare earth elements included) determined in grain size fractions of the Upper Pleistocene hydrothermally altered and unaltered sediments from the Guaymas Basin (Gulf of California). Sediments subjected to high-temperature hydrothermal alteration were recovered by DSDP Hole 477A. In the finely dispersed fractions, which are mainly composed of clay minerals, alteration of the chemical composition was provoked by the hydrothermal transformation of terrigenous clay minerals. The concentration of microelements in these fractions takes place primarily at the cost of the hydrothermal finely dispersed ore minerals. Alteration of the chemical composition of the coarse-grained fractions is related to the replacement of clastogenic minerals by the secondary varieties and the formation of new minerals (including ore minerals and native metals) from the solutions. Hydrothermal alterations of the chemical composition of bulk samples depend on the degree of chemical element concentration in fractions and their content in samples.     

Peculiarities of Clay Mineral Formation in Sediments from the Hydrothermal System Center,Hole 858B,Juan de Fuca Ridge

Lithology and Mineral Resources - Clay minerals in Holocene–Pleistocene sediments from Hole 858B DSDP drilled at 20 m from the black smoker in the Dead Dog hydrothermal field, axial valley of...     

Mercury in the Sedimentary Cover and Basalts of the Basement in the Recent Hydrothermal Activity Area,Middle Valley,Juan de Fuca Ridge

The content and distribution of mercury in Holocene–Upper Pleistocene turbidites, hemipelagic sediments intercalating therein, as well as basement basalts are studied. Samples of sediments were taken from the core of Holes 858A, 858B, 858C, 858D, and 858F. Basalt samples were taken from Holes 858F and 858G drilled during Leg 139 ODP in the Middle Valley (Juan de Fuca Ridge) in the Dead Dog hydrothermal field with a high heat flow (4–20 W/m²) and numerous vents with temperature ranging from 234 to 276°C. Samples of sediments and basalts with the background Hg content were taken from the core of Holes 855A, 855C, and 855D are located beyond the hydrothermal system in the base of the fault scarp on the eastern Middle Valley. In rocks, the content of Hg and its occurrence form were determined by the atomic absorption spectrometry with thermal atomization method; the chemical composition, by the XFA and ICP-MS methods. Sections of the sedimentary cover and basalt basement are marked by an alternation of “layer cake” type units with low and high contents of Hg. Mercury occurs in rocks in the physically adsorbed and mineral forms. The Hg concentration in some parts of the sedimentary section is anomalously high: up to 9696 ppb in Hole 858B and 7260 ppb in Hole 858C. In metalliferous sediments, the Hg content is 3130 ppb. Its maximum content (up to 23200 ppb) is recorded in basalts.     

The sedimentary cycle of the boron isotopes

Marine sediments contain two isotopically distinct components: a non-desorbable fraction with δ¹¹B between −4.3 and +2.8 per mil (n = 10) and a desorbable component with δ¹¹B between 13.9 and 15.8 per mil (n = 6). The adsorption coefficient, K, for the uptake of B from seawater by fluvial suspended material (Mississippi) has an experimentally determined value of 1.54 ± 0.05. The associated isotope fractionation factor, α, is 0.974 ± 0.003. Empirical values for K, determined from the analysis of marine sediments (n = 6), were between 3 and 4. The amount of B adsorbed onto fluvial suspended material during estuarine mixing is about 9 × 10⁹ moles/yr. Based on pore water data, adsorption experiments and comparative size fraction analyses (suspended sediments from the Mississippi River vs. bottom deposits from the Delta) there is no evidence for the incorporation of B into detrital sediments at low temperature. However, the high B content of Bauer Deep metalliferous sediments must be due to incorporation of seawater B during formation of authigenic silicates. The δ¹¹B of these minerals is 2 ± 3 per mil.Hydrothermally altered sediments from DSDP Hole 477, Guaymas Basin, Gulf of California have variable B contents, from 33 ppm, similar to the unaltered detritus, to 1.2 ppm in the recrystallized material. The latter has δ¹¹B as low as −9.0 per mil, substantially lower than the unaltered material. Hydrothermal solutions collected from vents adjacent to the hole have boron contents elevated by about a factor of 4 relative to seawater and δ¹¹B between 16.5 and 23.2 per mil. The B mobilized from the sediments is isotopically fractionated with the fluids being preferentially enriched in the heavy isotope.Analysis of an oxisol profile developed over granite in the Guyana Shield showed that boron is partially mobilized during weathering with an isotopic offset of 2.5 per mil between bedrock and soil.The observed enrichment of B in shales relative to igneous rocks does not occur during weathering or during exposure to seawater at low temperature. Incorporation occurs only during burial diagenesis at temperatures greater than about 60°C. However, the enrichments cannot be attained during a single cycle of primary weathering and burial but must be cumulative over many cycles.     

Biometry of the Late Cretaceous Arkhangelskiella group: ecophenotypes controlled by nutrient flux

This study focuses on the morphometry and taxonomy of the Late Cretaceous coccolith genus Arkhangelskiella. Sixty samples from the Campanian–Maastrichtian interval of DSDP Hole 390A (Blake Nose) were investigated for their contents of Arkhangelskiella spp. In each sample one hundred specimens of Arkhangelskiella spp. were examined by measuring the coccolith length and width, as well as the length and width of the central area. In the samples investigated the Arkhangelskiella group exhibits a large size variation, specimens length varies from 4.95 μm to 14.52 μm. Former taxonomic concepts, based on morphometry, subdivided the Arkhangelskiella group into three species: Arkhangelskiella maastrichtiana, Arkhangelskiella confusa and Arkhangelskiella cymbiformis. Our data show a large variability of the morphometric data (coccolith length, width of the outer rim). There is no indication for three independant species; two of the quoted taxa (1. thick outer rim = Arkhangelskiella maastrichtiana; 2. very thin outer rim = Arkkhangelskiella cymbiformis) seem to be extreme forms of a continuous morphometric lineage. The lower part of the investigated succession (139.92–133.42 mbsf) is dominated by small specimens with an average length of 6.8 μm whereas the upper part (132.86–126.15 mbsf) is characterized by larger specimens (mean 8.7 μm). In DSDP Hole 390A the size increase appears to be very abrupt, within two samples (samples 133.42 mbsf, 132.86 mbsf) the mean size increases by 1.51 μm. Previous morphometric studies of Arkhangelskiella indicate a more continuous size increase throughout the late Campanian–Maastrichtian. The abrupt size increase observed here hints toward a minor hiatus in DSDP Hole 390A separating upper Campanian from lower Maastrichtian sediments. It seems likely that the size increase of Arkhangelskiella reflects changes of various environmental factors like nutrient supply and sea water chemistry (Mg/Ca ratio; Ca concentration). A comparison of morphometric results with previous palaeoecological studies documents a nutrient control for the growth of Arkhangelskiella. Small specimens can be related to more mesotrophic conditions whereas large specimens are linked to oligotrophic surface waters.     

Mechanisms of Mg-phyllosilicate formation in a hydrothermal system at a sedimented ridge (Middle Valley,Juan de Fuca)

We present results of a detailed mineralogical and geochemical study of the progressive hydrothermal alteration of clastic sediments recovered at ODP Site 858 in an area of active hydrothermal venting at the sedimented, axial rift valley of Middle Valley (northern Juan de Fuca Ridge). These results allow a characterization of newly formed phyllosilicates and provide constraints on the mechanisms of clay formation and controls of mineral reactions on the chemical and isotopic composition of hydrothermal fluids. Hydrothermal alteration at Site 858 is characterized by a progressive change in phyllosilicate assemblages with depth. In the immediate vent area, at Hole 858B, detrital layers are intercalated with pure hydrothermal precipitates at the top of the section, with a predominance of hydrothermal phases at depth. Sequentially downhole in Hole 858B, the clay fraction of the pure hydrothermal layers changes from smectite to corrensite to swelling chlorite and finally to chlorite. In three pure hydrothermal layers in the deepest part of Hole 858B, the clay minerals coexist with neoformed quartz. Neoformed and detrital components are clearly distinguished on the basis of morphology, as seen by SEM and TEM, and by their chemical and stable isotope compositions. Corrensite is characterized by a 24?Å stacking sequence and high Si- and Mg-contents, with Fe/(Fe+Mg) ratio of ≈0.08. We propose that corrensite is a unique, possibly metastable, mineralogical phase and was precipitated directly from seawater-dominated hydrothermal fluids. Hydrothermal chlorite in Hole 858B has a stacking sequence of 14?Å with Fe/(Fe+Mg) ratios of ≈0.35. The chemistry and structure of swelling chlorite suggest that it is a corrensite/chlorite mixed-layer phase. The mineralogical zonation in Hole 858B is accompanied by a systematic decrease in δ¹⁸O, reflecting both the high thermal gradients that prevail at Site 858 and extensive sediment-fluid interaction. Precipitation of the Mg-phyllosilicates in the vent region directly controls the chemical and isotopic compositions of the pore fluids. This is particularly evident by decreases in Mg and enrichments in deuterium and salinity in the pore fluids at depths at which corrensite and chlorite are formed. Structural formulae calculated from TEM-EDX analyses were used to construct clay-H₂O oxygen isotope fractionation curves based on oxygen bond models. Our results suggest isotopic disequilibrium conditions for corrensite-quartz and swelling chlorite-quartz precipitation, but yield an equilibrium temperature of 300°?C±30° for chlorite-quartz at 32?m below the surface. This estimate is consistent with independent estimates and indicates steep thermal gradients of 10–11°/m in the vent region.     

Bottom temperature monitoring in Lake Baikal

We report results of bottom temperature monitoring of 2003–2004 in the deepwater South Baikal basin (Lake Baikal) near active gas-fluid methane vents at lake depths of 1020 and 1350 m. Sediments and water temperatures were measured using an autonomous temperature recorder designed at the Institute of Geophysics (Novosibirsk). Experiments implied short-duration recording and pioneering continuous 350 day-long monitoring near the Staryi vent. Measurements within a 1 m thick layer above and below the bottom showed notable variations in water (up to 0.07 °C) and sediment temperatures and in geothermal gradient. The long temperature records include a relatively steady period (mid-June 2003-early February 2004) with smooth temperature variations (especially in sediments) and two transient unsteady periods. The steady season is the best time for heat flow studies in the South Baikal basin. The 0.04–0.05 °C drop in bottom water temperature during the unsteady periods may result from intrusion of cold surface water. A positive temperature anomaly of ∼0.04 °C recorded in April 2003 may be caused, among other reasons, by active gas venting.     

Diagenesis of late Cenozoic diatomaceous deposits and formation of the bottom simulating reflector in the southern Bering Sea*

Diatom ooze and diatomaceous mudstone overlie terrigenous mudstone beds at Leg 19 Deep Sea Drilling Project sites. The diatomaceous units are 300-725 m thick but most commonly are about 600 m. Diagenesis of diatom frustules follows a predictable series of physical and chemical changes that are related primarily to temperature (depth of burial and local geothermal gradient). During the first 300-400 m of burial frustules are fragmented and undergo mild dissolution. By 600 m dissolution of opal-A (biogenic silica) is widespread. Silica reprecipitates abundantly as inorganic opal-A between 600 and 700 m sub-bottom depth. Inorganic opal-A is rapidly transformed by crystal growth to opal-CT. The result is formation of silica cemented mudstone and porcelanite beds. A regional acoustic reflector (called the bottom-simulating reflector, or BSR) occurs near 600 m depth in the sections. This acoustic event marks the upper surface where silicification (cementation) is active. In Bering Sea deposits, opal-A is transformed to opal-CT at temperatures between 35° and 50°C. This temperature range corresponds to a sub-bottom depth of about 600 m and is the area where silicification is most active. Thus, the BSR represents an isothermal surface; the temperature it records is that required to transform opal-A to opal-CT. Deposition of at least 500 m of diatomaceous sediment was required before the temperature at the base of the diatomaceous section was appropriate (35°-50°C) for silica diagenesis to occur. Accordingly, silica diagenesis did not begin until Pleistocene time. Once silicification began, in response to sediment accumulation during the Quaternary, the diagenetic front (the BSR) moved upsection in pace with the upward migrating thermal boundary. X-ray diffractograms and SEM photographs show three silica phases, biogenic opal-A, inorganic opal-A’, and opal-CT. These have crystallite sizes of 11-16 A, 20-27 A, and 40-81 A, respectively, normal to 101. The d(101) reflection of opal-CT decreases with depth of burial at DSDP Site 192. This occurs by solid-state ordering and requires at least 700 m of burial. Most clinoptilolite in Leg 19 cores forms from the diagenesis of siliceous debris rather than from the alteration of volcanic debris as is commonly reported.     

Variability of the oceanic boundary layer characteristics in the northern Bay of Bengal during MONTBLEX-90

Variability of the ocean surface boundary layer characteristics on daily time-scale is studied utilizing the 3-hourly hydrographic data collected at a stationary location (20°N, 89°E) in the Bay of Bengal during August (18th–31st) and September (9th–19th), 1990 under MONTBLEX-90 field programme. The daily variations of temperature, salinity, σ₀, mixed layer thickness, stability, heat content and rate of change of heat content in the upper 100 m are discussed in relation to prevailing weather (depressions) and hydrographic conditions (influx of fresh water, presence of eddies). The mixed layer thickness is examined through temperatureand σ₀-based criteria considering also the surface salinity in the latter. TheT-based mixed layer thickness is always higher than that of σ₀-based thickness. The rate of change of heat content is also computed up to the depth of 20°C and 14°C isotherms which takes into account the vertical motion and hence divergence. With the development of a low into a deep depression close to the study area, intense upwelling of subsurface cold waters is noticed from 100 m to the bottom of the surface mixed layer (20m) from 18th to 20th August. The upwelling is weakened by 21st August when the depression moved away from the study location. This variation of upwelling is supported by the variation of surface mixed layer thickness, static stability at 30 m depth, heat content in the upper 100 m and the heat content up to the depth of 20°C isotherm from 18th to 21st August. The rate of change of heat content in the upper 100 m and up to the depths of 20°C and 14°C isotherms leads to net heat storage during August and to net heat depletion during September. This together with the net surface heat gain lead to an import (197Wm^?2) and export (233 Wm^?2) of heat during August and September respectively through horizontal advective processes. These advective processes are attributed to the presence and movement of a warm core eddy through the study location.     

Petrological-geophysical models of the internal structure of the lithospheric mantle of the Siberian Craton

Based on the simultaneous inversion of unique ultralong-range seismic profiles Craton, Kimberlite, Meteorite, and Rift, sourced by peaceful nuclear and chemical explosions, and petrological and geochemical data on the composition of xenoliths of garnet peridotite and fertile primitive mantle material, the first reconstruction was obtained for the thermal state and density of the lithospheric mantle of the Siberian craton at depths of 100–300 km accounting for the effects of phase transformation, anharmonicity, and anelasticity. The upper mantle beneath Siberia is characterized by significant variations in seismic velocities, relief of seismic boundaries, degree of layering, and distribution of temperature and density. The mapping of the present-day lateral and vertical variations in the thermal state of the mantle showed that temperatures in the central part of the craton at depths of 100–200 km are somewhat lower than those at the periphery and 300–400°C lower than the mean temperature of tectonically younger mantle surrounding the craton. The temperature profiles derived from the seismic models lie between the 32.5 and 35 mW/m² conductive geotherms, and the mantle heat flow was estimated as 11–17 mW/m². The depth of the base of the cratonic thermal lithosphere (thermal boundary layer) is close to the 1450 ± 100°C isotherm at 300 ± 30 km, which is consistent with published heat flow, thermobarometry, and seismic tomography data. It was shown that the density distribution in the Siberian cratonic mantle cannot be described by a single homogeneous composition, either depleted or enriched. In addition to thermal anomalies, the mantle density heterogeneities must be related to variations in chemical composition with depth. This implies significant fertilization at depths greater than 180–200 km and is compatible with the existence of chemical stratification in the lithospheric mantle of the craton. In the asthenosphere-lithosphere transition zone, the craton root material is not very different in chemical composition, thermal regime, and density from the underlying asthenosphere. It was shown that minor variations in the chemical composition of the cratonic mantle and position of chemical (petrological) boundaries and the lithosphere-asthenosphere boundary cannot be reliably determined from the interpretation of seismic velocity models only.     

Constraining input and output fluxes of the southern-central Chile subduction zone: water,chlorine and sulfur

In this paper, we constrain the input and output fluxes of H₂O, Cl and S into the southern-central Chilean subduction zone (31°S–46°S). We determine the input flux by calculating the amounts of water, chlorine and sulfur that are carried into the subduction zone in subducted sediments, igneous crust and hydrated lithospheric mantle. The applied models take into account that latitudinal variations in the subducting Nazca plate impact the crustal porosity and the degree of upper mantle serpentinization and thus water storage in the crust and mantle. In another step, we constrain the output fluxes of the subduction zone both to the subcontinental lithospheric mantle and to the atmosphere–geosphere–ocean by the combined use of gas flux determinations at the volcanic arc, volume calculations of volcanic rocks and the combination of mineralogical and geothermal models of the subduction zone. The calculations indicate that about 68 Tg/m/Ma of water enters the subduction zone, as averaged over its total length of 1,480 km. The volcanic output on the other hand accounts for 2 Tg/m/Ma or 3 % of that input. We presume that a large fraction of the volatiles that are captured within the subducting sediments (which accounts for roughly one-third of the input) are cycled back into the ocean through the forearc. This assumption is however questioned by the present lack of evidence for major venting systems of the submarine forearc. The largest part of the water that is carried into the subduction zone in the crust and hydrated mantle (accounting for two-thirds of the input) appears to be transported beyond the volcanic arc.     

Rare-element composition of pyropes and lamproites and ancient dispersion haloes of the southwestern Siberian platform

The problem of heterogeneity of the mantle lithosphere of the southwestern portion of the Siberian Platform has been considered, and the diamond content in potential mother lodes within this area has been estimated based on original geochemical data on the rare-element composition of pyropes from diamondiferous lamproites of the Ingashin field within the Prisayan region and ancient dispersion haloes of minerals accompanying diamonds in the area between the Angara and Uda rivers. Pyropes from lamproites are characterized by low concentrations of Zr (0.18–9.05 ppm), Hf (0.03–0.37 ppm), and rare earth elements (Sm 0.04–0.49, Eu 0.02–0.16, and Dy 0.05–0.96 ppm). Pyropes from the Lower Carboniferous Baeron Formation within the Tangui-Chuksha area are significantly different from pyropes of the Ingashin lamproites in high contents of Zr (30.36–139.23 ppm) and Hf (0.4–2.22 ppm). These pyropes are characterized by elevated concentrations of rare earth elements (Sm 1.34–3.68, Eu 0.53–1.17, and Dy 1.0–2.05 ppm). The distribution patterns of rare incompatible elements in pyropes of the Lower Carboniferous Mura massif within the Mura area manifest even stronger differences with pyropes of the Ingashin lamproites and in many respects with pyropes from Lower Carboniferous sediments of the Baeron Formation within the Tangui-Chuksha area. The results obtained indicate that there is no large-scale regional spreading of pyropes from Mid-Riphean lamproite bodies in the course of washout of these bodies and that the mantle lithosphere in the southwestern portion of the Siberian Platform is laterally heterogeneous in mineralogical-geochemical terms. The chemical composition and the peculiar distribution pattern of rare elements in pyropes from lamproites of the Prisayan region indicate a depleted, primarily lherzolite composition of the upper mantle that was transformed through low-temperature potassium metasomatosis. In terms of the chemical and rare-element compositions, pyropes from Lower Carboniferous sediments of the Tangui-Chuksha and Mura areas belong to a wider range of mantle rocks: depleted peridotites, metasomatic peridotites under low (900–1000°C) and high (>1000°C) temperature conditions, and megacrysts. This suggests that the composition of the lithospheric mantle in this area of the southern portion of the Siberian Platform is characterized by a considerably differentiated stratification of mantle rocks, some of which were credibly formed in the diamond stability field.     

Structure of the low permeable naturally fractured geothermal reservoir at Soultz

The permeability of the granite geothermal reservoir of Soultz is primarily related to major fracture zones, which, in turn, are connected to dense networks of small-scale fractures. The small-scale fractures are nearly vertical and the major direction is about N0°E. This direction differs from that of the Rhine Graben, which is about N20°E to N45°E in northern Alsace. A total of 39 fracture zones, with a general strike of N160°E, have been identified in six wells between 1400 and 5000 m depth. These fracture zones are spatially concentrated in three clusters. The upper cluster at 1800–2000 m True Vertical Depth (TVD) is highly permeable. At 3000–3400 m TVD, the intermediate cluster in composed of a dense network developed in an altered matrix and forms the upper reservoir. In the lower part of the wells, the deeper cluster appears as a fractured reservoir developed within a low permeable matrix. Fracture zones represent a key element to take into account for predicting the geothermal reservoir life time submitted to various thermo-hydromechanical and chemical processes generated by hydraulic or chemical stimulations and by hydraulic circulation tests related to long-term exploitation.     

Pollen and radiolarian records from deep-sea core RC14-103: Climatic reconstructions of northeast Japan and Northwest Pacific for the last 90,000 years

Using modern pollen and radiolarian distributions in sediments from the northwest Pacific and seas adjacent to Japan to interpret floral and faunal changes in core RC14-103 (44°02′N, 152°56′E), we recognize two major responses of the biota of eastern Hokkaido and the northwest Pacific to climatic changes since the last interglaciation. Relatively stable glacial environments (～80,000–20,000 yr B.P.) were basically cold and wet (<4°C and ～1000 mm mean annual temperature and precipitation, respectively) with boreal conijers and tundra/park-tundra on Hokkaido, and cool (<16°C) summer and cold (<1.0°C) winter surface temperatures offshore. Contrasting nonglacial environments (～10,000–4000 yr B.P.) were warm and humid (>8°C and >1200 mm mean annual temperature and precipitation, respectively), supporting climax broadleaf deciduous forest with Quercus and Ulmus/Zelkova, with surface waters in the northwest Pacific characterized by warm (>1.5°C) winter and cold (10.4°–14.3°C) summer temperatures. Climatic evidence from RC14-103 shows a high degree of local and regional variation within the context of global climatic change. Correlative ocean and land records provide the detailed input necessary to assess local/regional responses to variations in other key elements (i.e., solar radiation, monsoonal variations) of the northeast Asian climate system.     

Geologic and metamorphic evolution of the basement complexes in the Kontum Massif,central Vietnam

This paper presents a regional scale observation of metamorphic geology and mineral assemblage variations of Kontum Massif, central Vietnam, supplemented by pressure–temperature estimates and reconnaissance geochronological results. The mineral assemblage variations and thermobarometric results classify the massif into a low- to medium-temperature and relatively high-pressure northern part characterised by kyanite-bearing rocks (570–700 °C at 0.79–0.86 GPa) and a more complex southern part. The southern part can be subdivided into western and eastern regions. The western region shows very high-temperature (> 900 °C) and -pressure conditions characterised by the presence of garnet and orthopyroxene in both mafic and pelitic granulites (900–980 °C at 1.0–1.5 GPa). The eastern region contains widespread medium- to high-temperature and low-pressure rocks, with metamorphic grade increasing from north to south; epidote- or muscovite-bearing gneisses in the north (< 700–740 °C at < 0.50 GPa) to garnet-free mafic and orthopyroxene-free pelitic granulites in the south (790–920 °C at 0.63–0.84 GPa). The Permo-Triassic Sm–Nd ages (247–240 Ma) from high-temperature and -pressure granulites and recent geochronological studies suggest that the south-eastern part of Kontum Massif is composed of a Siluro-Ordovician continental fragment probably showing a low-pressure/temperature continental geothermal gradient derived from the Gondwana era with subsequent Permo-Triassic collision-related high-pressure reactivation zones.     

Chemical and isotopic characterization of water–rock interactions in shales induced by the intrusion of a basaltic dike: A natural analogue for radioactive waste disposal

Disposal of nuclear waste in deep geological formations is expected to induce thermal fluxes for hundreds of years with maximum temperature reaching about 100–150 °C in the nearfield argillaceous environment. The long-term behavior of clays subjected to such thermal gradients needs to be perfectly understood in safety assessment considerations. In this respect, a Toarcian argillaceous unit thermally disturbed by the intrusion of a 1.1-m wide basaltic dike at the Perthus pass (Herault, France), was studied in detail as a natural analogue. The thermal imprint induced by the dike was evaluated by a mineralogical, chemical and K–Ar study of the <2 μm clay fraction of shale samples collected at increasing distance from the basalt. The data suggest that the mineral composition of the shales was not significantly disturbed when the temperature was below 100–150 °C. Closer to the dike at 150–300 °C, changes such as progressive dissolution of chlorite and kaolinite, increased content of the mixed layers illite–smectite with more illite layers, complete decalcification and subsequent increased content of quartz, were found.At the eastern contact with the dike, the mineral and chemical compositions of both the shales and the basalt suggest water–rock interactions subsequent to the intrusion with precipitation of palagonite and renewed but discrete deposition of carbonate. A pencil cleavage developed in the shales during the dike emplacement probably favored water circulation along the contact. Strontium isotopic data suggest that the fluids of probable meteoric origin, reacted with Bathonian and Bajocian limestones before entering the underlying Toarcian shales.By analogy with deep geological radioactive waste repositories, the results report discrete mineralogical variations of the clays when subjected to temperatures of 100–150 °C that are expected in deep storage conditions. Beyond 150 °C, significant mineralogical changes may alter the physical and chemical properties of the shales, especially of the clay fraction. Also, the development of structural discontinuities in the so-called thermally disturbed zone might be of importance as these discontinuities might become zones for preferential fluid circulation. Finally, the study emphasizes the use of Rb–Sr and K–Ar isotopic systems as tracers of local circulating fluids related to low-grade thermal imprints.