Granitic Host Rocks Research Articles

THE HYPABYSSAL 5034 KIMBERLITE OF THE GAHCHO KUE CLUSTER, SOUTHEASTERN SLAVE CRATON, NORTHWEST TERRITORIES, CANADA: A GRANITE-CONTAMINATED GROUP-I KIMBERLITE

We present petrographic, mineralogical, geochemical and isotopic data on the 5034 kimberlite of the Cambrian Gahcho Kue kimberlite cluster (Slave craton, northern Canada). This Group-IA kimberlite is a multiphase intrusion composed of the diatreme-facies magmatic and hypabyssal kimberlite, which intruded in at least four magmatic phases. The dominant hypabyssal phase is composed of macrocrystal monticellite – phlogopite – serpentine kimberlite. Occasional samples with diopside ± melilite may reflect local heterogeneities in the bulk composition or separate intrusive phases. Aphanitic monticellite – serpentine kimberlite forms as early and contemporaneous small-volume magmas and is present in autoliths and in cross-cutting dykes. The subsequent explosive magmatic phase formed tuffisitic kimberlite breccia, which in turn is cut by macrocrystal phlogopite – serpentine kimberlite. The phlogopite in all recognized hypabyssal rock-types has distinct compositional trends, but within a given rock-type, it also varies significantly in response to local chemical heterogeneities and to the composition of minerals adjacent to it. The large variability in phlogopite compositions is due to very local diffusion-controlled crystallization, and not to changes in bulk composition of the magma. All varieties of the 5034 kimberlite show strong evidence of contamination by host granitic rocks. Pectolite, diopside, melilite and phlogopite have replaced and crystallized around partly assimilated granitic xenoliths from a hybrid melt enriched in Si and Na. A high activity of Si may be the major factor controlling the presence of melilite in kimberlites. The mineralogical character of the 5034 kimberlite, which is intermediate between Group-I and Group-II kimberlites, can be explained by its strong contamination by granites. The composition of the kimberlite in terms of Sr and Nd isotopes suggests an asthenospheric origin of the melt and allows for incorporation of no more than 8–12% granitic material into the parental magma.

Strontium isotopic characterization of the Palmottu hydrosystem (Finland): water–rock interaction and geochemistry of groundwaters

AbstractThe Palmottu hydrosystem is located in a granitic host rock in southern Finland. Along well‐defined pathways in the fractured crystalline rock, strontium isotopes are used to trace the degree of water–rock interaction (WRI) and mixing processes in groundwaters.The 87Sr/86Sr ratios range between 0.716910 and 0.735606 in the surface waters and between 0.719991 and 0.750787 in the groundwaters, but are between 0.720 and 0.735 in most of the samples. Moreover, the results show a lack of correlation between the water chemistries determining the classification into different water‐types (Na–Cl, Na–SO4, etc.) and the results of the strontium (Sr) contents and Sr isotopic ratios. From a WRI standpoint, this implies that the Sr behaviour is independent of the water chemistry; the occurrence of large 87Sr/86Sr variations is site specific and mainly dependent on the lithology. A model to determine the 87Sr/86Sr ratio of water after interaction with granite was developed. This model is based on the assumption that Sr was derived from three minerals: plagioclase, K‐feldspar and biotite. The results of the calculation indicate that around half of the water analysed within the Palmottu hydrosystem can be explained by the weathering of the granites. However, clearly lower 87Sr/86Sr are observed in waters when compared to the calculated 87Sr/86Sr and other sources of Sr, with low 87Sr/86Sr, rather than the calculated granite–water interaction, which may be suspected.When comparing the 87Sr/86Sr and ion ratios (Ca/Na, Mg/Na, Sr/Na, Cl/Na), the scattering of the data can be explained by the presence of four end‐members: a brine component (low 87Sr/86Sr and Ca/Na ratios…), a deep granitic component (high 87Sr/86Sr ratios and low Ca/Na ratios…), a subsurface component (intermediate 87Sr/86Sr ratios associated with high Ca/Na ratios…) and a surface end‐member:snow and river drainage (low 87Sr/86Sr and low Ca/Na ratios…). These extreme end‐members define a series of WRI‐mixing line within a rather complex hydrosystem.

Partial melting and recrystallization of granite and their application to deep disposal of radioactive waste: Part 1—Rationale and partial melting

A high temperature, deep borehole disposal scheme for nuclear waste is outlined. This requires that granite host rock can be partially melted by the heat from the waste then completely recrystallized, both on the time scale of the cooling of the waste. Experimental studies of non-equilibrium partial melting of a typical S-type crustal granite were carried out under the conditions likely to arise during implementation of this scheme ( P=0.15 GPa, T=700–850 °C). The melting relations were determined under H 2O-saturated and H 2O-undersaturated conditions. Melting begins just under 700 °C and the amount of melt increases with temperature and especially H 2O content (up to saturation) such that with no added H 2O only 5% of melt is produced even at 850 °C but the addition of 1% H 2O will generate 40% of melt 50 °C lower at 800 °C. The partial melts formed at high degrees of melting (e.g. H 2O-saturated; T>800 °C) approximate to equilibrium melting but the smaller amounts of melt produced at lower temperatures under H 2O-undersaturated conditions clearly do not. On the basis of these experiments, it is concluded, from the point of view of partial melting, that an S-type granite similar to the one studied would be a suitable host rock for the proposed high temperature, very deep disposal scheme for radioactive waste.

Partial melting and recrystallization of granite and their application to deep disposal of radioactive waste: Part 2—Recrystallization

A previously published scheme for the high temperature disposal of nuclear waste in deep boreholes requires that granite host rock partially melted by the heat from the waste can be completely recrystallized on the time scale of the cooling of the waste. Experimental studies of dynamic recrystallization of a typical S-type crustal granite were therefore undertaken under the pressure (=0.15 GPa) and other conditions likely to arise during implementation of this scheme. Linear cooling experiments at 1 °C/h, 0.1 °C/h and 1 °C/day were carried out over the temperature range 800–560 °C. These demonstrate that the partially melted granite can be fully recrystallized when cooled at 0.1 °C/h or slower. Crystal growth rates for the main phases formed during cooling are linearly proportional to total H 2O content and decrease from alkali feldspar through biotite to quartz. Recrystallizing the partially melted granite at 0.1 °C/h compared with 1 °C/h increases grain size by a factor of ∼2.5. On the basis of partial melting experiments reported previously by us [Lithos (2003) this issue.] for the same granite and the results presented here, it is concluded that an S-type granite similar to the one studied would be a suitable host rock for the proposed high temperature, very deep disposal scheme for radioactive waste.

Granite Recrystallization - The Key to an Alternative Strategy for HLW Disposal?

ABSTRACTAn alternative strategy is proposed for the disposal of spent nuclear fuel (SNF) and other forms of high-level waste (HLW) whereby the integrity of a mined and engineered repository for the bulk of the waste need be preserved for only a few thousand years. This is achieved by separating the particularly problematic components, notably heat generating radionuclides (HGRs) and very long lived radionuclides (VLLRs) from the waste prior to disposal. Such a solution requires a satisfactory means of disposing of the relatively minor amounts of HGRs and VLLRs removed from the waste. This could be by high-temperature very deep disposal (HTVDD) in boreholes in the continental crust [1,2]. However, the viability of HTVDD, and hence the key to the entire strategy, depends on whether sufficient melting of granite host rock can occur at suitable temperatures and whether the melt can be completely recrystallized. The high-temperature, high-pressure experiments reported here demonstrate that granite can be partially melted and completely recrystallized on a time scale of years, as opposed to millennia as widely believed. Furthermore, both can be achieved at temperatures and on a time scale appropriate to the disposal of packages of heat generating HLW. It is therefore concluded that the proposed strategy, which offers, environmental, safety and economic benefits, could be a viable option for a substantial proportion of HLWs.

Development of the Hatagawa Fault Zone clarified by geological and geochronological studies

The occurrence of mylonite and cataclasite, mineral assemblages of cataclasite, and the K-Ar ages of surrounding granitic rocks and dikes were studied to examine the possibility that the Hatagawa Fault Zone (HFZ), NE Japan was experienced under the conditions of the brittle-plastic transition. The Hatagawa Fault Zone is divided into three structural settings: mylonite zones with a sinistral sense of shear and a maximum thickness of 1 km, a cataclasite zone with a maximum thickness of about 100 m, and locally and sporadically developed small-scale shear zones. Occurrence of epidote and chlorite, lack of montmorillonite in cataclasite, and the coexistence of cataclasite and limestone mylonite suggest that the cataclasite was deformed at temperatures higher than 220°C. Crush zones in the mylonite near the cataclasite zone were recognized in one outcrop; they have a structure concordant with the surrounding mylonite and some fragments in them are dragged plastically. Granodiorite porphyry dikes near the HFZ intruding into cataclasite and mylonite with a sinistral sense of shear exhibit no deformational features. K-Ar ages of hornblende from host granitic rocks and from one granodiorite porphyry dike are 126 ± 6 to 95.7 ± 4.8 and 98.1 ± 2.5 Ma, respectively. These indicate that the fault activity gradually changed from mylonitization to cataclasis within 28 m.y., and suggest that the HFZ underwent a brittle-plastic transition during its activity.

TOURMALINE IN PETALITE-SUBTYPE GRANITIC PEGMATITES: EVIDENCE OF FRACTIONATION AND CONTAMINATION FROM THE PAKEAGAMA LAKE AND SEPARATION LAKE AREAS OF NORTHWESTERN ONTARIO, CANADA

Tourmaline in petalite-subtype granitic pegmatites from the Pakeagama Lake and Separation Lake areas of northwestern Ontario provides evidence for the compositional evolution of pegmatite-forming melts through magmatic crystallization and pegmatite melt – host-rock interactions. At the former locality, tourmaline from the pegmatite units is essentially magmatic, whereas at the latter locality, tourmaline in many of the Fe-suite pegmatites displays characteristics (such as elevated Mg, Ti and Ca contents) that indicate significant contamination through the incorporation and digestion of components of amphibolite (mafic metavolcanic units) and banded iron-formation (BIF) host-rocks. The presence of tourmaline in mafic host-rocks indicates that cation migration also involved the introduction of B, Li and Al from pegmatite-generated fluids to form metasomatic haloes. The Li + Mn versus Mg + Ti + Ca diagram is introduced to discriminate tourmaline with a dominantly magmatic signature from tourmaline with a signature influenced by pegmatite – host-rock interaction. The behavior of Ca in tourmaline is also shown to be potentially useful in recognizing late-stage Ca-enrichment in pegmatite-forming melts as a result of Ca–F complexing, but can give similar trends to tourmaline that has suffered Ca-metasomatism as a result of interaction with granitic host-rocks. The tourmaline data suggest that at Separation Rapids, pegmatites mainly crystallized in an open system (i.e., pegmatite – host-rock interaction was common), whereas at Pakeagama Lake, the pegmatite units were predominantly unaffected by interaction with host rocks (i.e., they crystallized in a closed system). This has implications for concentrating rare-elements such as Rb and Cs, which otherwise would be dispersed throughout pegmatite and host rocks (as at Separation Lake). The potential for a buried pollucite deposit associated with the SE pegmatite at Pakeagama Lake has increased as a result of these observations, particularly as Cs is elevated in associated K-feldspar and mica, and is also present in a late pollucite-bearing vein.

Microbial, redox and organic characteristics of compacted clay-based buffer after 6.5 years of burial at AECL's Underground Research Laboratory

Many countries are considering options for long-term management of nuclear waste. One common aspect among deep geological disposal options in granitic host rock is the use of clay-based buffer materials to limit radionuclide migration in case of container failure. The isothermal test (ITT) involved placing ∼2.4 m 3 of clay-based buffer in a borehole at the 240 m level of AECL's Underground Research Laboratory to study the response of buffer to resaturation by groundwater over a 6.5-year period. Results are reported here on measurements taken at the end of the test for microbial, redox and organic characterization of the buffer. Results from enumerations and biomass determinations suggested that the viable population of cells in the buffer was several orders of magnitude larger than could be cultured. It is postulated that, due to the constrictive and nutrient-poor buffer environment, viable and active cells became stressed during burial and lost activity and culturability but not viability. Culturable microbial populations at interfaces in the ITT were about an order of magnitude larger than in comparable bulk buffer samples, suggesting that interfaces may be preferred sites for microbial activity and transport. The presence of culturable SO 4-reducing bacteria and an increase in solid sulphide concentrations in the buffer suggested SO 4 reduction, which appeared to be very variable locally. Only about 0.02–0.5% of SO 4 was converted to sulphide, suggesting that SO 4 reduction was not (yet) a dominant process. No methanogens could be enumerated from the ITT, and phospholipid fatty acid (PLFA) profiles did not suggest their presence. Gas analysis of samples recovered from the ITT suggested some reduction in O 2 near the top of the experiment, but deeper samples did not show a significant decrease in O 2 and had only a small increase in CH 4 and H 2 levels. This suggested that microbial processes were depressed in the buffer but may have been more active near the concrete/buffer interface. The suggestion of low microbial activity in the buffer was corroborated by the results from the PLFA analysis, which indicated low biomass turnover rates and starvation biomarkers. The combination of enumerations, PLFA and gas analysis results suggested that no significant evolution towards reducing conditions occurred during the duration of the ITT. Fulvic acids made up the largest fraction of water-leachable humic substances but accounted for only about 2% of the total C inventory of the buffer material. The complexing capacity of these humic substances, based on carboxylic functional groups, ranged from 24 to 32 meq/g dissolved organic C. This may provide buffer porewater with considerable complexing capacity for radionuclides.

Uranium/Thorium isotope evidence for ground-water history in the Eastern Desert of Egypt

The ground-water in the surficial aquifer of the central Eastern Desert of Egypt has a relatively long residence time and is essentially in chemical equilibrium with its host rocks. The content of major elements and uranium suggest that granites have been the principal sources of dissolved ions even where sedimentary rocks are present. A leaching study of uranium and thorium isotopes in seven granitic host rocks reveals a history of change in the aquifer. Lightly adsorbed U has a high234U/238U activity ratio similar to the contemporary associated ground-water. However, much of the uranium in the granites is present as acid-leachable adsorbates with a low activity ratio signature, indicative of an earlier period of aggressive leaching and uranium mobility. Using whole rock disequilibrium dating methods we estimate that the time of U mobility occurred 80,000 to 100,000 years ago during the Saharan II pluvial period.

The development of contrasting structures during the cooling and crystallisation of a syn-kinematic pluton

Structural elements of a syn-kinematic pluton in the Pilbara Craton of Western Australia reflect evolving rheology during progressive crystallisation, representing a transition from magmatic, through high-temperature sub-solidus to low-temperature deformation. Magmatic elements include: (1) NNE-trending, sub-vertical, kilometre-scale folds (outlined by xenolith trains) that have a magmatic axial planar foliation defined by aligned K-feldspar, (2) cross-cutting, ENE-trending, sub-vertical shear zones intruded by numerous ≤metre-scale granite sheets, and (3) a narrow, concordant magmatic K-feldspar foliation in the granite host-rock adjacent to the shear zones. Microstructures, such as fractured feldspars healed by quartz that is continuous with matrix grains, further suggest that deformation occurred while the pluton was magmatic. High-temperature structural elements include ‘checkerboard’ sub-grain boundaries in quartz, whereas deformation at lower temperatures is indicated by kinked feldspar, recrystallised microcline and undulose quartz. A NNE-trending, sub-vertical sub-solidus foliation, defined by elongate quartz aggregates, overprints all the above mentioned features. Generation of these sequential structural elements correspond to the transition from Newtonian-fluid, through Bingham-type, to Newtonian-solid behaviour as the crystal fraction of the felsic magma increased. The orientation of the structures is consistent with a regional ESE–WNW shortening direction throughout magma crystallisation. This study indicates how evolving magma rheology and regional strain regimes control the micro- to macro-structural elements of a syn-tectonic pluton.

Uranium-series disequilibrium dating of secondary uranium ore from the south Eastern Desert of Egypt

The secondary uranium ore of Um Ara mining area, south Eastern Desert of Egypt, is composed mainly of uranophane and β-uranophane. They occur in the oxidized zone as idiomorphic crystals filling cavities and as coatings on the fracture surfaces of the alkali-feldspar and albitized granites. The activity ratios 234U/ 238U and 230Th/ 234U of the secondary uranium ore and host rocks indicate that there were two main phases of uranium mobility in recent geological time. An earlier precipitation of uranium from solutions forming the secondary ore and a subsequent mobilization resulted in adsorption of uranium to the host granitic rocks. The 230Th/ 234U age of secondary uranium ore varies from (50±10) to (159+69/−45) ka whereas, the age of uranium adsorption to the rocks ranges from (18±6) to (38±5) ka. The time of secondary uranium ore precipitation can be attributed to the Saharan II pluvial period which prevailed in Egypt during humid oxygen isotope climatic stage 5. The later uranium mobility and subsequent adsorption by the rocks took place during Kubbaniyan and Nabtian pluvial periods that coincided with oxygen isotope stages 3 and 1, respectively.

Comparative study of cataclastic rocks from a drill core and outcrops of the Nojima Fault zone on Awaji Island, Japan

Abstract Cataclastic rocks found in the Disaster Prevention Research Institute, Kyoto University (DPRI) 500 m drill core and outcrops along the Nojima Fault zone on Awaji Island, southwest Japan, were examined at mesoscopic and microscopic scales. The damaged zone of this fault in granitic rocks, observed on the southeast side of the fault, is 50–60 m wide and is composed of fractured host rocks and cataclastic rocks including cataclasite, fault breccia, and fault gouge. The fault breccia and gouge of small scales are scattered in the damaged zone. Fault core (zone of extremely concentrated shearing deformation along a fault) consists of fault gouge measuring several tens to approximately 150 mm in width, as recognized both in the drill core and at outcrops of the Nojima Fault along which surface ruptures formed during the 1995 Kobe earthquake. Fault breccia, measuring a few meters wide, has developed pervasively in the damaged zone, just next to the fault core. Pseudotachylyte has been found interlayered with fault gouge within the fault core only at outcrops at Hirabarashi, not in the DPRI 500 m core. Petrological studies and powder X‐ray diffraction analysis show that the pseudotachylyte and fault gouge are composed mainly of fine‐grained angular clasts of the host granitic rocks, suggesting the pseudotachylyte is of ‘crush origin’. Foliated cataclasite is characterized by the preferred orientation of elongated biotite clasts and granular aggregates of quartz and feldspar clasts, and by the development of cataclastic shear bands. Unlike cataclastically deformed quartz and feldspar in the cataclasite, biotite in the foliated cataclasite shows combinations of brittle and plastic deformation, such as biotite ‘fish’, cleavage steps, bending and kinking. These textures suggest that the foliated cataclasite formed at a deeper level than the cataclasite, fault breccia and gouge, possibly before the Quaternary period during which the Nojima Fault has moved as a dextral strike–slip fault with some reverse movement resulting in the uplifting of Awaji Island. Examination of fault rocks from surface outcrops can yield similar results to those obtained from drill cores with regard to the internal structures of a fault zone.

Nd isotopes and water mixing phenomena in groundwaters from palmottu (Finland)

The Palmottu uranium ore deposit, located within a granitic host rock in southern Finland, is an excellent setting for conducting analogue studies to assess radionuclide transport from the uranium deposit fractured crystalline host rock. In this context, Nd isotope ratios are used to establish the degree of water–rock interaction (WRI) and to clarify mixing processes within the groundwaters. Variations in Nd isotopes in selected water types from the Palmottu hydrosystem provide new information on the mixing history.

Mineralogy and genesis of secondary uranium deposits, Um Ara area, south eastern desert, Egypt

Secondary U mineralisation is found in the oxidised zone pervading fractured albitised and alkali-feldspar granites emplaced at the northern boundary of Um Ara Pluton. It occurs as stains along crevices and fracture surfaces and as acicular crystals filling cavities. X-ray diffraction and SEM were used to identify secondary U minerals and the associated alteration products. Uranophane and β-uranophane are the most abundant U minerals, whereas Ca-montmorillonite and illite represent low temperature alteration products of the host granitic rocks. The genesis of secondary U minerals is mainly attributed to the action of oxic groundwater on previously corroded primary U minerals. These secondary U minerals were deposited near the surface from the circulating groundwater by evaporation.

Crystal chemical variations in Li- and Fe-rich micas from Pikes Peak batholith (central Colorado)

The crystal structure and M-site populations of a series of micas-1 M from miarolitic pegmatites that formed within host granitic rocks of the Precambrian, anorogenic Pikes Peak batholith, central Colorado, were determined by single-crystal X-ray diffraction data. Crystals fall in the polylithionite-siderophyllite-annite field, being 0 ≤ Li ≤ 2.82, 0.90 ≤ Fe total ≤ 5.00, 0.26 ≤ [6] Al ≤ 2.23 apfu. Ordering of trivalent cations (mainly Al 3+ ) is revealed in a cis-octahedral site (M2 or M3), which leads to a lowering of the layer symmetry from C 12 /m (1) (siderophyllite and annite crystals) to C 12(1) diperiodic group (lithian siderophyllite and ferroan polylithionite crystals). On the basis of mean bond length, the ordering scheme of octahedral cations is mostly meso-octahedral, whereas the mean electron count at each M site suggests both meso- and hetero-octahedral ordering, the calculated mean atomic numbers being M1 = M3 ≠ M2, M2 = M3 ≠ M1 and M1 ≠ M2 ≠ M3. As the siderophyllite content increases, so do the a , b, and c unit-cell parameters, as well as the refractive indices, primarily n β . The tetrahedral rotation angle, α, is generally small (1.51 ≤ α ≤ 5.04°) and roughly increases with polylithionite content, whereas the basal oxygen out-of-plane tilting, Δz, is sensitive both to octahedral composition and degree of order (0.0 ≤ Δz ≤ 0.009 A for siderophyllite and annite, 0.058 ≤ Δz ≤0.144 A for lithian siderophyllite and ferroan polylithionite crystals).

Invisible gold in ore and mineral concentrates from the Hillgrove gold-antimony deposits, NSW, Australia

Vein-hosted mesothermal stibnite-gold mineralisation at the Hillgrove Au-Sb mine in northeastern New South Wales has a halo of veinlet and disseminated auriferous arsenopyrite and arsenian pyrite in metasedimentary and granitic host rocks. About 50–55% of the gold produced at Hillgrove occurs invisibly in arsenopyrite and pyrite. Gold losses of ∼20% into tailings are due to this mineral chemical factor. From PIXE probe analyses, it has been found that arsenopyrite contains 255–1500 ppm Au and pyrite 24–223 ppm Au, with Au contents of each mineral correlating moderately with As content. Arsenopyrite and pyrite also contain anomalous values of Cu, Ag and Sb, whereas paragenetically later stibnite contains little invisible gold, but minor Fe, As, Ag, Cu and Pb. The precipitation of invisible gold in arsenopyrite and pyrite by a possible (Fe, Au)3+= (As-S)3− substitution mechanism may have been facilitated by rapid, non-equilibrium conditions involving pressure decreases and wall rock reaction (sulphidation, carbonatisation), as a prelude to the main stage of stibnite and gold deposition.

Fluid– rock interaction in metamorphosed andesitic dikes, Elat Metamorphic Complex, Israel

Cohen, B., Matthews, A., Bar-Matthews, M., Ayalon, A. 2000. Fluid‐rock interaction in metamorphosed andesitic dikes, Elat Metamorphic Complex, Israel. Isr. J. Earth Sci. 49: 239‐252. Basic dikes intruded Precambrian high T/P metamorphic rocks of the Elat Complex, and subsequently were metamorphosed and deformed. This work explores the fluid– rock interaction during metamorphism and deformation through petrographic, mineralogical, and oxygen isotope study of a profile across a 3.9-m-wide andesitic dike and its granite gneiss host rock. Samples from the center of the metamorphosed dike are undeformed, and relic igneous minerals are preserved. However, the dike margins show sheared, foliated textures, characterized by preferred orientation of elongated biotite grains and a matrix of polygonal quartz, feldspar, and amphibole. Correspondingly, the granite gneiss host rock transforms from a weakly deformed rock to a highly lineated, often schistose, rock as the contact with the dike is approached. The textural changes are accompanied by contemporaneous metamorphic mineralogical changes. The most important evidence for fluid infiltration from the schistose dike into the granite gneiss comes from the oxygen isotope studies of quartz and biotite separates. The dike has a uniform δ 18 O quartz value of about 13.5‰ (SMOW). These values are also found in the granite gneiss at the contact before dropping to lower δ 18 O values (12.9‰ at the southern contact; 12.2‰ at the northern contact). The quartz isotopic systematics are mirrored by those of the biotite, with the result that the quartz–biotite oxygen isotope fractionation shows clear metamorphic resetting at ca. 500 °C in the schistose dike and the immediate 0.5–1 m of granite gneiss. The δ 18 O quartz profile at the southern contact was modeled using a one-dimensional transport equation and indicates that the component of fluid infiltration normal to the contact advected 0.60 m into the granite gneiss. Corresponding integrated (total) fluid flux was about 1 m 3 /cm 2 , connected porosity 2 × 10 –5 and integrated time of infiltration about 2000 a. The study suggests a synergetic relationship between water, metamorphism, and deformation. Ductile shearing is promoted by the presence of water (hydraulic weakening of rock to plastic deformation), while at the same time, the deformation increases permeability of the rock to fluid infiltration.

Thermal Considerations in a Very Deep Borehole Nuclear Waste Repository for Synroc

ABSTRACTA study of heat propagation in a very deep borehole radioactive waste repository for Synroc is presented. The deep borehole modeled for the high level wastes is hypothetically sited at approximately 4 km below ground level in a granite host rock. The high level waste containers are placed in the lower section of the borehole to within 1.5 km of the surface and backfilled. A geothermal gradient of 0.03°C/m is assumed. Transient temperature field calculations are carried out for both 20 wt% and 10 wt% high level radioactive waste (HLW) bearing Synroc, for cooling periods between reactor discharge and geological disposal varying from 5 years to 50 years. The temperatures in the borehole repository were found to be very sensitive to the surface cooling period as well as the waste loading. The initial heat input of the waste may be controlled by cooling in an interim storage until the total heat output is reduced to a suitably low level. Therefore, surface cooling of at least 15 years for 20 wt% HLW containing Synroc is necessary for the given deep bore hole configuration to avoid the centerline temperatures that might affect durability of Synroc if ground water obtained access to the waste form. On the other hand the corresponding minimum surface cooling period for 10 wt% HLW containing Synroc is predicted to be about 5 years to produce similar temperature profiles in the borehole to those of 20 wt% HLW bearing Synroc with a 15-year cooling period before disposal. A variation in the borehole diameter has also been performed to elucidate the sensitivity of the temperature field of the repository.

A new scheme for the very deep geological disposal of high‐level radioactive waste

A case is presented for reconsidering deep borehole disposal for high‐level radioactive waste (HLW) in the light of recent discoveries about the hydrogeology and hydrogeochemistry of the continental crust. A new multi‐barrier concept is proposed for very deep, elevated‐temperature disposal of vitrified HLW, spent fuel and ex‐military and other fissile materials which involves melting and recrystallization of a granitic host rock utilizing the heat from the waste. This scheme is fundamentally different from previously proposed borehole disposal scenarios in its key aspects, particularly in utilizing only partial melting and a surviving container. The perceived advantages of the scheme are discussed, in general and in relation to previous deep disposal ideas, particularly from the standpoint of safety. It is concluded that the scheme offers a potentially safer and environmentally more acceptable option for the permanent disposal of high‐level waste.

Interaction between crustal-derived felsic and mantle-derived mafic magmas in the Oberkirch Pluton (European Variscides, Schwarzwald, Germany)

The composite Oberkirch pluton consists of three compositionally different units of peraluminous biotite granite. The northern unit is relatively mafic (SiO2∼64%) and lacks cordierite. The more felsic central and southern units (SiO2=67.8 to 70.4%) can only be distinguished from each other by the occurrence of cordierite in the former. Mafic microgranular enclaves of variable composition, texture and size occur in each of these units and are concentrated in their central domains. Most abundant are large (dm to m) hornblende-bearing enclaves with dioritic to tonalitic compositions (SiO2=50.8 to 56.3 wt%; Mg#=63 to 41) and fine grained doleritic textures that suggest chilling against the host granite magma. Some of these enclaves are mantled by hybrid zones. Less common are microtonalitic enclaves containing biotite as the only primary mafic phase (SiO2=53.7 to 64.4%) and small hybrid tonalitic to granodioritic enclaves and schlieren. Synplutonic dioritic dikes (up to 6 m thick) with hybrid transition zones to the host granite occur in the southern unit of the pluton. In chemical variation diagrams, samples from unmodified hornblende-bearing mafic enclaves and dikes form continuous trends that are compatible with an origin by fractionation of olivine, clinopyroxene, hornblende and plagioclase. Chemical and initial isotopic signatures (e.g. high Mg#, low Na2O, ɛNd=−1.2 to −5.1, 87Sr/86Sr=0.7055 to 0.7080, δ18O=8.0 to 8.8‰) exclude an origin by partial melting from a mafic meta-igneous source but favour derivation from a heterogeneous enriched lithospheric mantle. Samples from the granitic host rocks do not follow the chemical variation trends defined by the diorites but display large scatter. In addition, their initial isotopic characteristics (ɛNd=−4.5 to −6.8, 87Sr/86Sr=0.7071 to 0.7115, δ18O=9.9 to 11.9‰) show little overlap with those of the diorites. Most probably, the granitic magmas were derived from metapelitic sources characterized by variable amounts of garnet and plagioclase. This is suggested by relatively high molar ratios of Al2O3/(MgO+FeOtot) and K2O/Na2O, in combination with low ratios of CaO/(MgO+FeOtot), variable values of Sr/Nd, Eu/Eu*[=Eucn/(Smcn × Gdcn)0.5] and (Tb/Yb)cn (cn=chondrite-normalized) as well as variable abundances of Sc and Y. Whole-rock initial isotopic signatures of mafic microtonalitic enclaves (ɛNd=−4.6 to −5.2; 87Sr/86Sr=0.7060 to 0.7073; δ18O ∼8.1‰) are similar to those of the low ɛNd diorites. Plagioclase concentrates from a granite sample and a mafic microtonalitic enclave are characterized by initial 87Sr/86Sr ratios that are significantly higher than those of their bulk rock systems suggesting incorporation of high 87Sr/86Sr crustal material into the magmas. Field relationships and petrographic evidence suggest that the Oberkirch pluton originated by at least three pulses of granitic magma containing mafic magma globules. In-situ hybridization between the different magmas was limited. Late injection of dioritic magma into the almost solidified granitic southern unit resulted in the formation of more or less continuous synplutonic dikes surrounded by relatively thin hybrid zones.