Crack Fillings Research Articles

Application of fluid inclusion analysis for buried dissolution predicting in the Tahe Oilfield of Tarim Basin, NW China

This study was undertaken to establish an evaluation method for buried dissolution of carbonate reservoir in the Tahe (塔河) Oilfield. Because of the difficulty in tracing the dissolution in geologic record, the precipitation is taken as a useful indicator to presume the dissolution reversed. The fluid inclusions data is a useful tool to identify the precipitation. On the basis of the principle that the temperature data of fluid inclusions captured in the calcite fillings of caves and cracks can reflect the fluid activity time, a total of 256 fluid inclusions samples in 12 drills were collected and analyzed to study their forming time in buried environment in the Tahe Oilfield, Tarim Basin, NW China. Results show that the temperature data of fluid inclusions captured by Ordovician cave and crack fillings accumulated in six intervals, which indicated six stages of fluid activity in the area of study. Meanwhile, it is clear that a different number of temperature intervals was captured in different wells, representing a different number of precipitation distributed in space. According to this conclusion, the buried precipitation trend in late Himalayan stage (the sixth stage) was supposed in space and its effectiveness has been approved by the chemical thermodynamics method. This method was also used in all of the other five stages of fluid activity and the comprehensive trend of the buried precipitation in space for Ordovician strata was evaluated. It shows that more temperature intervals were captured in the southwestern region (wells S76, S79, S65 and S75) and the eastern region (well S69) in the area of study, indicating intensive precipitation and weak dissolution in this area. This evaluation method for buried dissolution is a new attempt and may have important implications for further petroleum exploration in the Tarim Basin.

New evidence of meteoritic origin of the Tunguska cosmic body

Diamond–lonsdaleite–graphite micro-samples collected from peat after the 1908 catastrophic blast in the Tunguska area were studied with scanning (SEM) and transmission electron (TEM) microscopy, NanoSecondary Ion Mass Spectrometry (NanoSIMS) and Х-ray synchrotron technique. The high-pressure carbon allotropes in the Tunguska samples are being described for the first time and contain inclusions of FeS (troilite), Fe–Ni (taenite), γ-Fe and (FeNi)3P (schreibersite). The samples are nodule-like in shape and consist of 99.5% carbon minerals, e.g. diamond>lonsdaleite>graphite. Micro- and nanoinclusions of troilite (up to 0.5vol%), taenite, γ-iron and schreibersite fill cracks, cleavages and pores in the carbon matrix. Carbon isotope studies from the two analyses of the Tunguska foil showed δ13C=−16.0±1.9‰ and δ13C=−15.2±2.1‰, suggesting δ13C=−15.6±2‰ as an average characteristic of the carbon reservoir. That value is close to δ13C of some extraterrestrial samples. A negligible concentration of Ir and Os in the carbonaceous matrix promotes some controversial interpretation of the origin of the studied materials. Attributing this fact to the primary inhomogeneity, and considering the micro-structural features such as cracks, deformation of the crystal lattices, etc. coupled with high-pressure carbon allotropes association with metals, sulfides and phosphides, and the high ratio of Fe:Ni=22:1 suggest that the studied samples are meteorite micro-remnants.

Microbially induced sedimentary structures from the Mesoproterozoic Huangqikou Formation, Helan Mountain region, northern China

Seven types of microbially induced sedimentary structures (MISS): wrinkle structures, levelled ripple marks, organic carbonaceous laminae, microsequences, polygonal sand cracks, polygonal sand crack fills, and gas domes are documented from the Mesoproterozoic Huangqikou Formation of the Helan Mountain region, western part of North China. Facies analysis suggests that the MISS-hosting rocks were deposited in a supertidal to intertidal setting. Both wrinkle structures and gas domes are morphologically different from those previously documented from other ancient siliciclastic rocks, but probably formed by the same microbially mediated process as those published elsewhere. Other MISS resemble their ancient and modern counterparts in both morphology and size. The presence of carbonaceous laminae, coupled with laser Raman and carbon isotopic data (−25±0.5‰), indicates a biogenic origin of the wrinkle structures. The absence of intercalating mudstone layers as well as different lithologic composition between polygonal sand cracks/crack fills and host rock indicate that both the polygonal sand cracks and polygonal sand crack fills likely resulted from dehydration and desiccation of organic rich microbial mats. The biogenicity of gas domes is supported by the different lithologic composition between infillings and wall.

Filling of surface cracks in MAX phase ceramics using cathodic electrophoretic deposition

It is well known that surface defects, such as cracks, are electrochemical more active than a smooth surface and usually work like seeds for the deposition processes. The present work used this enhanced activity to locally deposit surface-charged Al2O3 and TiC micro and nano-particles into surface cracks in Ti2AlC substrates. The formed deposit was compact and adhered well to the crack walls. A crack of about 30μm could be filled with alumina particles in 10min when applying 15V and using a 5wt% alumina suspension in ethanol. The deposition of TiC was more challenging due to coarseness of the particles. Grinding these particles to 1–2μm yielded a very stable suspension and an improvement in the crack filling behaviour.

Influence of climate, mineralogy and mineral processing on the weathering behaviour within two, low-sulfide, high-carbonate, gold mine tailings in the Eastern Desert of Egypt

Despite its importance within environmental management strategies, little concern is shown to sulfide oxidation and/or hardpan formation at neutral pH where dry condition prevails. Two gold mine tailings in Egypt, El Sid and Barramiya, were studied for their geochemical/mineralogical properties, and climate influence on hardpan formation. The tailings are characterised by homogeneous silt-sized sediments (>42%), have high carbonate, predominantly as calcite for El Sid and dolomite-ankerite for Barramiya, and low-sulfide contents, chiefly as pyrite, galena and sphalerite for El Sid, and arsenopyrite–pyrite for Barramiya. El Sid is characterised by high average concentrations of Pb (2,758 mg/Kg) and Zn (2,314 mg/Kg), its lower part dominated by mafics, overlaid by granitoids. Barramiya has higher As (average 2,836 mg/Kg) content and represents a mixture of mica-schists/mafics-ultramafics. During field investigations, no hardpans were identified, only bassanite and gypsum were found at the surface of El Sid tailings, forming thin layers and desiccation crack fillings. Column experiments showed a thin crust consisting of gypsum, halite and sodium sulfate formed at the top of the column of El Sid tailings after 2 weeks, this was not recognized in the column from Barramiya. The homogenous thickened tailings deposition in both areas did not favour hardpan formation, since the critical amounts of reacting sulfides were never achieved in individual lamina, due to missing mineral/grain size fractionation. The high-temperature/low-water availability, characteristic for desert climate regions did not allow significant sulfides oxidation. Therefore, both tailings will suffer from continuous erosion and spreading out of contaminants to the environment for a prolonged period of time by sporadic flash floods.

Sedimentology of a wet, pre‐vegetation floodplain assemblage

AbstractDescriptions of fluvial systems operating prior to significant terrestrial macrophyte vegetation have concentrated on assessing the impact of plant evolution on channel style; this is probably in part due to the scarcity of well‐developed floodplain successions in fluvial assemblages of these ages. This study describes wet, pre‐vegetation floodplain deposits and processes observed and inferred from a continuous succession of drill core through an extensive, 1·4 Ga, delta‐top channel–floodplain assemblage forming a portion of the Sibley Group, Ontario, Canada. Sub‐aerial deposits are dominated by flaser, wavy and lenticular bedded, fine‐grained sandstones, siltstones and mudstones, with abundant small mudstone rip‐up clasts. Soft‐sediment deformation of these units is ubiquitous, with loading and injection features being the most prominent. Thicker medium‐grained sandstone beds, representing crevasse splays, commonly have poorly developed protosols in their upper portions. Well‐laminated sediments with wave ripples, and only rarely containing rip‐up clasts and soft‐sediment deformation, were deposited in floodplain ponds. These deposits differ from post‐vegetation floodplain sediments in having: (i) better preservation of layering without rootlet bioturbation; (ii) dominance of rippled sand on the floodplain, probably due to lack of vegetation‐induced baffling; (iii) large‐scale generation of small, intraformational clasts; (iv) desiccation crack fills consisting of peds and locally derived intraclasts, probably delivered during Horton overland flow during rainfall events; (v) ubiquitous soft‐sediment deformation features in sub‐aerial deposits; and (vi) well‐laminated, commonly oxidized sediment that accumulated in floodplain ponds. These attributes are the direct result of the lack of macrophyte vegetation, and produce floodplain assemblages that are distinctly different from those presently forming in similar climatic settings.

Iron speciation using electron microprobe techniques: application to glassy melt pockets within a spinel lherzolite xenolith

AbstractA quantitative determination of Fe3+/ΣFe ratios using the electron microprobe is presented for glasses. The measurements are based on the shift of the Fe-Lα peak position between Fe3+ and Fe2+, resulting from differences in self-absorption. We present new calibration curves for glasses with Fe contents as low as 2 wt.% Fe.This method was used for glasses from a metasomatized spinel Iherzolite (Massif Central, France) in which highly vesicular glass associated with secondary minerals occurs in reaction pockets around corroded Cr-spinels. The glass was found to be oxidized homogeneously with Fe3+/ΣFe ≈ 0.70 (measured with an uncertainty of ±0.06), in contrast with the value of ∼0.15 that would correspond to the former melt under fo2 conditions inferred from secondary minerals. The glass oxidation probably results from the dissociation of the melt-water content under degassing, prior to local hematite depositions (spinel rims and crack fillings).

Residence of incompatible trace elements in a large spinel lherzolite xenolith from alkali basalt of Shavaryn Tsaram-1 paleovolcano ( western Mongolia)

We have studied a large (12 × 22 × 30 cm) spinel lherzolite xenolith with undeformed margins in alkali basalt (basanite) from the eroded crater of Late Cenozoic Shavaryn Tsaram-1 volcano in western Mongolia. The xenolith was sampled along its median transversal profile, at every 15–20 mm for bulk chemistry of lherzolite and basalt (ICP-MS) and at 4–10 mm for the chemistry of olivine, orthopyroxene, clinopyroxene, and Cr-spinel minerals, and of material filling cracks (LA ICP-MS). Incompatible elements (especially, LREE) are distributed unevenly over the xenolith, both in lherzolite and in its constituent minerals, as well as in crack-filling material, with abnormal LREE enrichment in some specimens. Judging by the measured trace-element spectra compared with the model patterns, incompatible elements reside in different amounts as interstitial impurity in cracks inside and between mineral grains in lherzolite, also being a substitutional impurity in the lherzolite constituent minerals. Experimental acid leaching of specimens from sites of high crack density showed (La/Yb) n ratios in the crack fill to be much higher than in the basalt host and more so in bulk lherzolite (180 against 33 and 1.5–3.6, respectively). The proportional contents of P and Ca in the leaching solution, especially in that from the xenolith’s center mark the presence of an apatite microphase, which can be a LREE repository. The observed patterns of LREE and other incompatible elements in the xenolith and in the host alkali basalt fit a model implying that mobile elements residing as interstitial impurity came with fluids which were released from rising basaltic magma and percolated into the xenolith along cracks.

Olivine enigma: Why alteration controls the seismic properties of oceanic gabbros

The objective of this study was to determine whether the effects of low Mg content and/or cracks that typically populate olivine grains can explain why P and S wave velocities in samples of oceanic gabbro do not increase with increasing olivine content. The Mg content of olivine in these rocks is known to be appreciably less than Fo90, and optical, SEM, and electron microprobe analyses indicate that the “olivine” is actually an aggregate of olivine and 5–15% crack fill consisting of talc and/or serpentine plus magnetite. Voigt average models show that there is a strong, positive correlation between P wave velocities and Fo90 content but no correlation with Fo73. Because of its Mg content, olivine that is typical of oceanic gabbros does not influence seismic velocities. Adding 11% alteration of olivine to serpentine plus magnetite in the form of crack fill to the Fo73 model yields a nearly exact match between the model and measured P and S wave velocities. The fact that the crack fill consists largely of talc and/or serpentine indicates that these minerals formed by hydrothermal alteration in young, hot (>150°C) lower crust. Hence our results apply to gabbros in situ, as well as to the laboratory samples; seismic velocities in lower oceanic crust produced at high and intermediate spreading rates are controlled principally by the degree of hydrothermal alteration of the gabbros that compose the lower crust.

Embedded Shape‐Memory Alloy Wires for Improved Performance of Self‐Healing Polymers

AbstractWe report the first measurements of self‐healing polymers with embedded shape‐memory alloy (SMA) wires. The addition of SMA wires shows improvements of healed peak fracture loads by up to a factor of 1.6, approaching the performance of the virgin material. Moreover, the repairs can be achieved with reduced amounts of healing agent. The improvements in performance are due to two main effects: (i) crack closure, which reduces the total crack volume and increases the crack fill factor for a given amount of healing agent and (ii) heating of the healing agent during polymerization, which increases the degree of cure of the polymerized healing agent.

Petrology of Martian meteorite Northwest Africa 998

Abstract— Nakhlite Northwest Africa (NWA) 998 is an augite‐rich cumulate igneous rock with mineral compositions and oxygen isotopic composition consistent with an origin on Mars. This 456‐gram, partially fusion‐crusted meteorite consists of (by volume) ∼75% augite (core composition Wo39En39Fs22), ∼9% olivine (Fo35), ∼7% plagioclase (Ab61An35) as anhedra among augite and olivine, ∼3.5% low‐calcium pyroxenes (pigeonite and orthopyroxene) replacing or forming overgrowths on olivine and augite, ∼1% titanomagnetite, and other phases including potassium feldspar, apatite, pyrrhotite, chalcopyrite, ilmenite, and fine‐grained mesostasis material. Minor secondary alteration materials include “iddingsite” associated with olivine (probably Martian), calcite crack fillings, and iron oxide/hydroxide staining (both probably terrestrial). Shock effects are limited to minor cataclasis and twinning in augite. In comparison to other nakhlites, NWA 998 contains more low‐calcium pyroxenes and its plagioclase crystals are blockier. The large size of the intercumulus feldspars and the chemical homogeneity of the olivine imply relatively slow cooling and chemical equilibration in the late‐ and post‐igneous history of this specimen, and mineral thermometers give subsolidus temperatures near 730 °C. Oxidation state was near that of the QFM buffer, from about QFM‐2 in earliest crystallization to near QFM in late crystallization, and to about QFM + 1.5 in some magmatic inclusions. The replacement or overgrowth of olivine by pigeonite and orthopyroxene (with or without titanomagnetite), and the marginal replacement of augite by pigeonite, are interpreted to result from late‐stage reactions with residual melts (consistent with experimental phase equilibrium relationships). Apatite is concentrated in planar zones separating apatite‐free domains, which suggests that residual magma (rich in P and REE) was concentrated in planar (fracture?) zones and possibly migrated through them. Loss of late magma through these zones is consistent with the low bulk REE content of NWA 998 compared with the calculated REE content of its parent magma.

Moisture induced crack filling in barrier coatings containing montmorillonite as an expandable phase

In order to explore the option of healing surface scratches in coatings using an expandable phase, the crack filling behavior of transparent, multi layer coating consisting of a polysiloxane film on top of a thin montmorillonite interlayer was studied. The filling process was monitored using confocal and SEM microscopy. Filling of cracks in the polymeric topcoat occurred primarily within the first two hours of exposure to moisture saturated air and resulted in a good restoration of the surface flatness. Beyond this 2 h healing period, further expansion of the clay layers resulted in a “pushing-up” effect of the swollen clay in proximity of the damaged coating. This paper attempts to quantify the crack closure of the clay-containing coating by monitoring changes in scratch width and area as a function of time.

Multi-azimuth three-component surface seismic modeling for viscoelastic cracked monoclinic media

In this article, a method of high order, staggered grid, finite difference was used to model snapshots of seismic wave propagation and multi-azimuth surface seismic records in viscoelastic cracked monoclinic media. The modeling results clearly show the anisotropy caused by the crack fill material and the viscoelastic wave field through the monoclinic media. Summarizing the regularity of seismic records for different azimuths, this study is a significant attempt to understand the wave fields propagated in real media and to propose a more rational theoretical model for the future.

Calcretes from a Late Quaternary interfluve in the Ganga Plains, India: Carbonate types and isotopic systems in a monsoonal setting

Calcretes are abundant in Late Quaternary channel and floodplain strata of the southern Ganga plains. In a key section at Kalpi, pedogenic carbonates (nodules, rhizoconcretions, and powdery carbonate) are present within aggradational floodplain deposits, where they correspond to relatively high monsoonal precipitation and river discharge. In contrast, groundwater carbonate has cemented degradational surfaces (discontinuities), which correspond with periods of relatively low precipitation. Mixed groundwater and pedogenic calcretes are present in the deposits of small interfluve channels, and reworked nodules line degradational surfaces and locally fill channels. Most carbonates show alpha fabrics that include floating textures, shrinkage crack fills, and grain coatings. The predominance of alpha fabrics is unexpected, but is characteristic of calcretes across dryland and seasonal parts of northern India, where soil formation led to only weakly developed or poorly preserved beta fabrics. Interpretation of δ 13C and δ 18O values of bulk and microdrilled calcrete samples suggests relatively little variation in precipitation and vegetation types through the sampled interval at Kalpi. Floodplain deposits were vegetated with a mixture of C 4 and C 3 plants (predominantly C 4), with a higher proportion of C 3 plants associated with channel deposits. This apparent lack of variation is surprising because the sampled interval represents at least 60,000 years of Marine Isotope Stages 3–5, during which climate models suggest that Asia experienced radical fluctuations in monsoon intensity and precipitation. Some of the apparent lack of variation may be explained by preferential preservation of aggradational strata that represent relatively active monsoonal periods, as well as by the mixing of drier floodplain (C 4) and riparian (C 3) vegetation. However, local departures from the regionally based climate model cannot be ruled out. A modest upsection increase in C 4 plants may represent increased aridity and lower atmospheric CO 2. Isotopic analysis of organic matter from floodplain pedogenic nodules suggests a higher C 3 plant contribution than carbonate-based data would suggest. The preserved organic matter may reflect the annual average biomass in the soil, whereas carbonate formation may have taken place mainly during the drier season when respiration of C 4 plants was more important. In interfluve settings such as Kalpi, seasonality may strongly affect the C 3–C 4 system, with preferential preservation of only part of the biomass.

Application efficiencies and furrow infiltration functions of irrigations in sugar cane in the Ord River Irrigation Area of North Western Australia and the scope for improvement

The introduction of irrigation in the Ord River Irrigation Area (ORIA) in the North West of Western Australia resulted in rising water tables in many parts of the ORIA. Commercial sugar cane production commenced in 1995 in the area, which exacerbated the problem caused by the longer growing and irrigation season and the large irrigation applications. Rising water tables are associated with deep drainage from paddocks and irrigation infrastructure. Improving the irrigation application efficiency (IAE) was identified as a manageable objective for sugar cane growers that is expected to reduce deep drainage. Existing irrigation practices were monitored and a surface irrigation model (SIRMOD) was used to simulate these irrigations. The lack of accurate infiltration parameters normally limits the extrapolation of SIRMOD simulation results to different irrigation scenarios. It was found that the infiltration into the Vertisols of the ORIA could be properly described by simple infiltration functions which were dependent on the soil moisture deficit (SMD) at irrigation as well as the shape of the irrigation furrow. The function described as Instantaneous Crack Fill (ICF) was appropriate for furrows with broad-based ‘U’-shaped furrows while a non-ICF described the infiltration behaviour of deeper ‘V’-shaped furrows. Summary tables were generated to illustrate the relationship between SMD, inflow rate, furrow length and irrigation duration for ‘U’-shape and ‘V’-shape furrows of Vertosols in the ORIA. From the tables it was determined that gains of up to 20% can be achieved in the IAE compared to the current situation.

Properties of thermally metamorphosed coal from Tanjung Enim Area, South Sumatra Basin, Indonesia with special reference to the coalification path of macerals

Abstract Thermally metamorphosed Tertiary age coals from Tanjung Enim in South Sumatra Basin have been investigated by means of petrographic, mineralogical and chemical analyses. These coals were influenced by heat from an andesitic igneous intrusion. The original coal outside the metamorphosed zone is characterized by high moisture content (4.13–11.25 wt.%) and volatile matter content (> 40 wt.%, daf), as well as less than 80 wt.% (daf) carbon and low vitrinite reflectance (VR max = 0.52–0.76%). Those coals are of subbituminous and high volatile bituminous rank. In contrast the thermally metamorphosed coals are of medium-volatile bituminous to meta-anthracite rank and characterized by low moisture content (only 80 wt.%, daf) and vitrinite reflectance (VR max = 1.87–6.20%). All the studied coals have a low mineral matter content, except for those which are highly metamorphosed, due to the formation of new minerals. The coalification path of each maceral shows that vitrinite, liptinite and inertinite reflectance converge in a transition zone at VR max of around 1.5%. Significant decrease of volatile matter occurs in the zone between 0.5% and 2.0% VR max . A sharp bend occurs at VR max between 2.0% and 2.5%. Above 2.5%, the volatile matter decreases only very slightly. Between VR r = 0.5% and 2.0%, the carbon content of the coals is ascending drastically. Above 2.5% VR r , the carbon content becomes relatively stable (around 95 wt.%, daf). Vitrinite is the most abundant maceral in low rank coal (69.6–86.2 vol.%). Liptinite and inertinite are minor constituents. In the high rank coal, the thermally altered vitrinite composes 82.4–93.8 vol.%. Mosaic structures can be recognized as groundmasss and crack fillings. The most common minerals found are carbonates, pyrite or marcasite and clay minerals. The latter consist of kaolinite in low rank coal and illite and rectorite in high rank coal. Change of functional groups with rank increase is reflected most of all by the increase of the ratio of aromatic C–H to aliphatic C–H absorbances based on FTIR analysis. The Oxygen Index values of all studied coals are low (OI 2 /g TOC) and the high rank coals have a lower Hydrogen Index ( T max increases with maturity (420–440 °C for low rank coals and 475–551 °C for high rank coals). Based on the above data, it was calculated that the temperature of contact metamorphism reached 700–750 °C in the most metamorphosed coal.

Age and Origin of Albian Glauconites and Associated Clay Minerals Inferred from a Detailed Geochemical Analysis

ABSTRACT Glauconitic are abundant in sandstones and siltstones from Albian and Cenomanian (transition between Lower and Upper Cretaceous) formations of southeastern France. Morphological and crystal-chemical data show that the green grains can be considered as glauconite s.s., with minute amounts of pyrite, dolomite, calcite, and other clay minerals as grain coatings or crack fillings. Glauconite formed early during sedimentation, generally from detrital chlorite, biotite, or clay particles, and were found scattered together with other detrital mineral phases (muscovite, quartz) in the silt, in which porosity was considerably reduced afterwards by four stages of carbonate (calcite, dolomite) cementation. Geochemical data, especially the Fe, U, and Ce contents of the glauconitic grains, strongly suggest increasing oxidizing conditions with time in the sedimentation environment. The detailed geochemical analyses also allow the pristine glauconite to be distinguished from the other clay minerals occurring in the sequence. This enabled K-Ar and Rb-Sr dating of the glauconitic material, which yields identical results of 97.9 ± 0.4 and 97.9 ± 3.5 Ma, respectively. These dates point to a synsedimentary origin for the glauconites during the Late Albian (about 97-98 Ma). The K-Ar data also indicate that the Albian-Cenomanian boundary should be placed at 96.0 ± 1.9 Ma. The similarity between the K-Ar and Rb-Sr ages precludes any further significant pervasive recrystallization of these minerals during burial of the sequence either by heat flux or fluid flow. The K-Ar study of the clay matrix from the same siltstones suggests synsedimentary precipitation of additional clay minerals, probably together with the glauconite grains, such as mixed-layer illite-smectite identified in the clay size fraction of the sedimentary succession.

Syngenetic, thin graphite-rich horizons in lower crustal rocks from the Serre San Bruno, Calabria (Italy), and implications for the nature of high-conducting deep crustal layers

In order to better understand the nature of deep crustal high electrical conductivity, we studied the electrical properties of a tilted section of a former lower continental crust exposed in the Calabrian arc of the Alpine–Apennine mountain system. Geoelectric field measurements and impedance measurements on rock samples showed that these high-grade metamorphic rocks are generally highly resistive as expected for crystalline, electrolytically conducting rocks of low porosity. This holds for graphite-free metabasites as well as for metapelites which generally contain accessory, up to 3% biogenic graphite in the form of isolated grains. Clearly as an exception, a group of thin stratiform black horizons with thicknesses of 1–15 cm within the metapelitic series was detected by means of self-potential (SP) measurements. Rock samples from these horizons exhibit high, quasi-metallic bulk conductivities of up to 50 S/m (0.02 Ωm) in agreeement with up to 20% syngenetic graphite, forming a network of interconnected streaks or crack fillings. The high amount of carbon most probably originates from organic matter of C org-rich black shales. Relative enrichment of the low mobility graphitic matter compared to the carbon content of the assumed protoliths may have been due to pressure solution and partial melting during prograde metamorphism, without major contribution of a fluid phase, resulting in isolated graphite flakes. Although enriched, graphite in this form has little effect on electrical conductivity. For the Calabrian black horizons, microscopic analyses make conceivable that, in a further decisive step, isolated graphite grains were mechanically smeared to continuous pathways during uplift by shearing, producing hereby the observed graphitic network which is needed to generate high conductivity. As C org-rich black shales are common members of sedimentary sequences throughout the earth’s history, good conductors of this type may be expected in the continental crust at any depth depending on tectonic and metamorphic history, with the exception of magmatic protoliths. Regarding the extremely high conductivity of the meta-black shale samples containing syngenetic sheared graphite, a total thickness of a few meters of such rocks is sufficient to explain magnetotelluric high conductivity anomalies in the deep crust.

Low‐temperature alteration of mesozoic oceanic crust, Ocean Drilling Program Leg 185

Basalts drilled from the Mesozoic crust at Ocean Drilling Program (ODP) Sites 801 and 1149 during Leg 185 have been affected by a succession of submarine, variably oxidative alteration processes by cold seawater. The low‐temperature alteration resulted in various combinations of secondary minerals both as replacements of igneous phases, and as void and crack fillings. Mg‐smectite, calcite, celadonite and Fe‐oxyhydroxide are the dominant secondary minerals. Quartz, chalcedony and phillipsite are always very minor phases that fill open spaces. The rocks generally exhibit concentric haloes, 0.5 to 1 cm in thickness, around gray inner cores and usually adjacent to exposed surfaces. Color zonations result from the abundance of various secondary mineral assemblages present in each zone. The green rocks are dominated by celadonitic phyllosilicates, whereas the dusky red haloes are characterized by abundant iddingsite and Fe‐oxyhydroxides replacing the fine clinopyroxene. Finally Mg‐smectite ± calcite, are the most common secondary phases in the inner gray cores. The most unusual effects of the low‐temperature alteration of the study rocks are the pervasive, pale green to gray green color and the relatively abundant dusky red haloes surrounding inner reddish gray cores. Alteration of Mesozoic crust cored during Leg 185 is comparable with low‐temperature alteration of 310 m thick uppermost volcanic section of the 5.9 My old crust of Hole 504B. Except the renewed precipitation of calcite in veins, the aging of oceanic crust does not depend on time probably because it occurs during the first few millions of years after the accretion, and ceases when the crust is sealed off.

Alteration of monazite and zircon and lead migration as geochemical tracers of fluid paleocirculations around the Oklo–Okélobondo and Bangombé natural nuclear reaction zones (Franceville basin, Gabon)

Large-scale light rare earth element (LREE), uranium, lead and phosphorus migration has been evidenced in the FA Lower Proterozoic sandstones of the Franceville basin (Gabon) hosting Oklo natural nuclear reaction zones (RZ) in relation with extensive accessory mineral alteration by highly saline diagenetic brines (28.7 wt.% NaCl eq. to 30 wt.% CaCl 2 eq.) at about 140°C and 1 kbar. Monazite is the most severely altered accessory mineral in the coarse-grained sandstones of the basal FA formation. Detrital monazite crystals are altered to Th–OH silicate microcrystalline phase with very low concentrations of U and LREE. The Th/La ratio increase from non-altered (Th/La∼0.27) to altered sandstones (Th/La∼1.14) shows that about 76% of the LREE was leached. This corresponds to the leaching of 2.01×10 9 metric tons at the scale of the FA formation in the Franceville basin. Similarly, the Th/U increase from monazite (Th/U=18.6) to the Th-silicate phase (Th/U=88.7) is interpreted as a result of an alteration by oxidizing brines with leaching of U together with LREE and P. It corresponds to the leaching of 9.06×10 6 metric tons of uranium. This amount of uranium largely exceeds the known uranium reserves from the Franceville basin. In zircon crystals, the cores are generally homogeneous, weakly fractured and well preserved as attested by the Archean ages (2867±24 and 2865±51 Ma) obtained by ionic microprobe analysis on zircon of the FA Formation, respectively, from the marginal and central parts of the basin. Their composition corresponds to the pure end-member (Zr,Hf)(SiO 4), poor in Th and U (Th/U∼1). At the contrary, their rims, which present several growth zones with cracks fillings, are enriched in REE, P, Th and U with higher Th/U ratios (5–10). Both altered monazite and altered zircon contain galena as numerous inclusions in the outer growth zones and as crack fillings. For example, in zircon, the Pb of galena crystals (3–23 wt.%) largely exceeds the amount of Pb (maximum 0.1 wt.%) that would have been produced in situ by radioactive decay in this mineral. Nearly all the lead were introduced into altered zones of accessories. Dissolution of accessory minerals occurred at 2000 Ma, producing a porous and distorted crystal structure which has allowed a later incorporation of Pb. Galena inclusions in altered zircons located in the vicinity of reactor zones have radiogenic lead compositions. Altered zircon rims and galena inclusions in altered zircon located far from reactor zones have non-radiogenic Pb isotopic compositions, confirming the external origin of lead. Pb isotopic evolution models indicate a crystallization age sometime after 1000 Ma, both for galena located close to and far from U mineralizations and reactor zones, which may be synchronous with a regional extension event contemporaneous with intrusion of dolerite dyke swarms, between 1000 and 750 Ma, at the scale of the Franceville basin. The present study also illustrates the different retention capacities of accessory mineral for elements representing analogs of the radiotoxic nuclides in the relatively extreme natural conditions created by the circulation of moderately hot and chloride-rich fluids during the diagenesis of a sedimentary basin.