Crystalline Igneous Rocks Research Articles

The effect of mineralogy and textural characteristics on the strength of crystalline igneous rocks using image-based textural quantification

The attempts made to understand and quantify the relationship between rock texture and engineering properties have been accompanied by some ambiguities and uncertainties, and the majority of the approaches, though focusing on particular rock types, have not been so successful for even the same rock types from other regions. This study, therefore, was aimed to investigate the relationship between the texture and strength properties of a wide range of crystalline igneous rocks along with more aspects of textural analysis. For this purpose, fifteen fresh samples of crystalline igneous rocks belonging to the stone quarries of the NW Iran were collected and subjected to compressive and tensile strength tests (UCS and BTS). Thin sections of the samples were used to prepare the full mosaic image layers in three different optical conditions. About 18,000 mineral grains were manually traced on the processed images and the obtained vector data was used for the quantification of various textural parameters including size metrics, shape descriptors, microfabric indices and mineral content. Different phase considerations (full-phasic, monophasic, multiphasic and phase-ratio) were applied to derive the textural characteristics of the individual and multiple minerals and their proportional ratio. Finally, simple linear regression analysis was performed to evaluate the relationship between the texture and strength properties. The results indicated that the newly developed quartz to feldspar size ratio (QFSR) indicator was the only textural parameter that could be significantly correlated with both compressive and tensile strength in a wide range of rock types. The results also highlighted the fact that the role of quartz and feldspar encountered with different rock failure mechanisms could be different; In fact, the tensile strength was obviously affected by the size and content of K-feldspar, while the compressive strength showed a more complicated mechanism and seemed to be controlled by the interference effects of plagioclase and quartz textural properties. Furthermore, the study indicated that the influence of grain size on the rock strength was more important than the grain shape or mineral content.

Alkali-silica Reaction in Finnish Concrete Structures

Abstract Even though natural phenomena do not abide to borders, Finland has traditionally been considered an alkali-silica reaction (ASR) free country. This is due to exceptional quality of the mostly course crystalline igneous rocks. However, during the last few years dozens of cases of ASR have been reported. The scope of this study was to study the occurrence of ASR, and to find out the initiation time of the reaction in resent investigations of Finnish concrete structures. ASR is found occurring all over Finland. The reacting aggregates consist of rock types, which are considered relatively stable or low reacting in literature.

Poisson's Ratio and Auxetic Properties of Natural Rocks

AbstractHere we provide an appraisal of the Poisson's ratios (υ) for natural elements, common oxides, silicate minerals, and rocks with the purpose of searching for naturally auxetic materials. The Poisson's ratios of equivalently isotropic polycrystalline aggregates were calculated from dynamically measured elastic properties. Alpha‐cristobalite is currently the only known naturally occurring mineral that has exclusively negative υ values at 20–1,500°C. Quartz and potentially berlinite (AlPO4) display auxetic behavior in the vicinity of their α‐β structure transition. None of the crystalline igneous and metamorphic rocks (e.g., amphibolite, gabbro, granite, peridotite, and schist) display auxetic behavior at pressures of >5 MPa and room temperature. Our experimental measurements showed that quartz‐rich sedimentary rocks (i.e., sandstone and siltstone) are most likely to be the only rocks with negative Poisson's ratios at low confining pressures (≤200 MPa) because their main constituent mineral, α‐quartz, already has extremely low Poisson's ratio (υ = 0.08) and they contain microcracks, micropores, and secondary minerals. This finding may provide a new explanation for formation of dome‐and‐basin structures in quartz‐rich sedimentary rocks in response to a horizontal compressional stress in the upper crust.

Studies on Geochemical and Heavy Mineral Characteristics of Sediments of Gosthani River Estuary

The present work deals with the geochemical and heavy mineral characteristics studies of sediments of Gosthani river estuary. The aim of study is to determine the provenance, establish the depositional environment and spatial variability of sediment. From the study it is found that the sediment of Gosthani river contains heavy minerals that consist of Rutile, Garnet, Sillimanite, Staurolite, Monazite, Zercon, Pyroxenes, Epidote, Amphiboles and Opaques minerals. The lesser abundant heavy minerals are amphiboles, zircon, monazite, rutile, staurolite, epidote. These heavy minerals indicate that the sediments were essentially derived from metamorphic provenance varying from medium to high grade and partly from crystalline igneous rocks. Well rounded monazites come from Eastern Ghats. This is based on the occurrence of monazite in pegmatite and charnockites of Eastern Ghats. The various characters of rutile and zircon suggest that each of these minerals has a multisource. According to geochemical studies settling of organic matter is highest in areas where deposition of fine grained sediment takes place. The organic carbon is often a good index for deciphering depositional environment.

Heavy Mineral Concentration and its Importance along the Attrai River, Dinajpur, Bangladesh

Heavy minerals investigation along the Attrai river sand of Dinajpur district, Bangladesh consists of biotite, chlorite, epidote, garnet, hornblende, kyanite, monazite, rutile, staurolite, tourmaline and zircon from non-opaque and hematite, ilmenite, limonite and magnetite from opaque fraction where garnet and magnetite claim the highest position in non-opaque and opaque group, respectively. The heavy minerals have a wide range of economic potentialities but the observed percentage of study area is only 1.54% which does not qualify as potential with the other known potential zones in Bangladesh. The investigation shows that, major portion of the clastic concentration likely to have been derived mainly from crystalline igneous and metamorphic rocks and a considerable part might have contributed through older sedimentary province. The orientation pattern of heavy mineral deposit suggests that, the possible sources lie in the west, north-west and in the northern side of Bangladesh.

A newly developed soil abrasion testing method for tunnelling using shield machines

The importance of the effect of ground abrasivity on the performance of a tunnel boring machine (TBM) is important for many tunnelling projects. In recent years, many abrasivity devices have been developed to measure soil abrasivity for specific conditions. In this paper a new device is developed to estimate the abrasivity of the soil for a wide range of site conditions. The device is designed on the basis of simulating the site conditions regarding the ground water, ground stresses and confining pressure of the TBM chamber, and the performance of the rotating cutter-head and pressurized shield tunnelling during its operation. A number of abrasivity tests were carried out on 12 types of crushed rock samples including six sedimentary rocks and six crystalline igneous and metamorphic rocks. The results showed that the abrasivity of the samples increased as equivalent quartz percentages of the sample increased. It was also found that the texture of the samples has a great influence on their abrasivity. To evaluate the performance of the newly developed device, the same samples were tested by other current abrasivity methods including the Cerchar test, Laboratoire Central des Ponts et Chaussées (LCPC) test and soil abrasion test (SAT). Comparisons of the results indicated close correlations, especially with the results for the Cerchar and LCPC devices.

Lithology of the basement underlying the Campi Flegrei caldera: Volcanological and petrological constraints

A geologically reasonable working hypothesis is proposed for the lithology of the basement underlying the Campi Flegrei caldera in the ca. 4–8 km depth range. In most current geophysical modeling, this portion of crust is interpreted as composed of Meso-Cenozoic carbonate rocks, underlain by a ca. 1 km thick sill of partially molten rock, thought to be a main magma reservoir. Shallower magma reservoirs likely occur in the 3–4 km depth range. However, the lack of carbonate lithics in any Campi Flegrei caldera volcanic rocks does not support the hypothesis of a limestone basement. Considering the major caldera-forming eruptions, which generated widespread and voluminous ignimbrites during late Quaternary times, including the Campanian Ignimbrite and Neapolitan Yellow Tuff eruptions, the total volume of trachytic to phonolitic ejected magma is conservatively estimated at not less than 350 km 3. Results of least-squared mass-balance calculations suggest that this evolved magma formed through fractional crystallization from at least 2500 km 3 of parent shoshonitic magma, in turn derived from even more voluminous, more mafic, K-basaltic magma. Calculations suggest that shoshonitic magma, likely emplaced at ca. 8 km depth, must have crystallized about 2100 km 3 of solid material, dominated by alkali-feldspar and plagioclase, with a slightly lower amount of mafic minerals, during its route toward shallower magma reservoirs, before feeding the Campi Flegrei large-volume eruptions. The calculated volume of cumulate material, likely syenitic in composition at least in its upper portions, is more than enough to completely fill the basement volume in the 4–8 km depth range beneath the Campi Flegrei caldera, estimated at ca. 1250 km 3. Thus, it is proposed that the basement underlying the Campi Flegrei caldera below 4 km is composed mostly of crystalline igneous rocks, as for many large calderas worldwide. Syenite sensu lato would meet physical properties requirements for geophysical data interpretations, explain some geochemical and isotopic features of the past 15 ka volcanics, and justify the carbon isotopic composition of fumaroles at the Campi Flegrei caldera. This implies that Meso-Cenozoic limestones, if still present today beneath the Campi Flegrei caldera, no longer constitute significant portions of its basement.

Mineralogical Characteristics of the Separated Magnetic Rutile of the Egyptian Black Sands

AbstractThe Egyptian black sands contain several economic minerals, such as ilmenite, magnetite, garnet, zircon, rutile and monazite. During the concentration and separation of a high‐grade rutile concentrate a bulk magnetic fraction is obtained. This fraction is composed mainly of opaques, titanhematite, ilmenite–titanhematite exsolved intergrown grains, magnetic leucoxene in addition to chromite, and magnetic rutile. The magnetic rutile occupies 6 wt.% of the bulk magnetic fraction or approx. 4 wt.% of the original rutile content in the raw sands. Most of magnetic rutile crystals are contaminated with opaque inclusions, staining‐coating and/or composite locked grains. This magnetic rutile has a magnetic range from strongly paramagnetic to very weak paramagnetic. Electron microprobe analysis for twenty‐three magnetic rutile grains identified mineral components of rutile, titanhematite, pseudorutile, leached pseudorutile and ilmenite in decreasing order of abundance. Some other inclusions are also detected in the different magnetic rutile grains. They are most probably garnet, silica, amphibole, ilmenite, feldspar, mica and zircon. The presence of these inclusions reflect the derivation of magnetic rutile of various crystalline igneous and metamorphic rocks. The magnetic susceptibility of magnetic rutile depends on the associated mineral components and their relative volumes in comparison to the rutile mineral component. Magnetic susceptibility of magnetic rutile is also related to both type and size of the associated mineral inclusions. The average chemical composition of the magnetic rutile is 66.34 wt.% TiO2, 21.71 wt.% Fe2O3, 6.39 wt.% SiO2, 1.80 wt.% Al2O3, 1.19 wt.% CaO and 0.10 wt.% Cr2O3. Thus, the contamination of magnetic rutile in the non‐magnetic rutile concentrate would decrease the market value of the rutile concentrate. Alternatively these magnetic rutile grains are recommended to be blended with magnetic leucoxene or some types of ilmenite concentrate to improve the overall marketable specifications especially for both of Ti, Fe and Cr contents.

Application of integrated geophysical techniques for monitoring wells site selection in the White Volta river basin

The CSIR-Water Research Institute has established thirteen (13) borehole monitoring stations in the White Volta River Basin of the three northern regions since 2005. Eight (8) of the monitoring boreholes are located in the Upper East and Upper West Regions, whilst the remaining five(5) are sited in the Northern Region of the basin, including Wa, Tumu, Bawku and Bugya-Pala near Walewale. The objective, among others, is to generate groundwater levels and temperature fluctuations data as well as obtain information on water quality trends for the effective managementof the groundwater resources of the basin. Crystalline igneous and metamorphic rocks as well as consolidated sedimentary formations underlie the basin. The drilling sites were selected using integrated geophysical survey techniques. Inductive electro-magnetic (EM) profiling and electrical resistivity methods including 2-D profiling/sounding and vertical electrical sounding (VES) were used in areas underlain by sandstone, igneous and metamorphic rocks. However, in areas underlain by sedimentary shale and mudstone rocks, where conductivity contrast between their fractured portions and the fresh rock is known to be lacking, only resistivity siting methods were employed. The results of the integrated geophysical methods showed strong correlation indelineating potential anomalies (fractures) in sandstone, igneous and metamorphic rocks, confirming the aquifer depths of existing boreholes in these rocks, which ranged from 15 m to 40 m deep. Unlike crystalline basement rocks (igneous and metamorphic) and sandstone, where lowbedrock resistivity response values (

Kaolinite in the Upper Cretaceous-Paleogene sedimentary facies of Bahariya Oasis, Egypt: Nature and origin

Kaolinites in the sedimentary facies of Bahariya Oasis, Egypt were found in four formations; the Cenomanian clastics of the Bahariya Formation, the Eocene iron ore of the NaqbQazzun Formation, the late Lutetian – Bartonian glauconites of the Hamra Formation and the conglomerates and sandstones of the Oligocene Qatrani Formation. Kaolinites occur as beds, bands, lenses or pockets of white to gray deposits either as single phase or mixed with other clay minerals such as smectite, glauconite and halloysite. Based on their genesis the kaolinites were subdivided into authigenic and detrital kaolinites. Authigenic kaolinites occur as well-developed stacked hexagonal and book-like flakes in the sandstones of Bahariya and Qatrani Formations and rosette-like shaped in the halloysite pockets inside the iron ore of the Naqb-Qazzun Formation. These kaolinites were formed as a result of alteration of Al-rich minerals such as feldspars and glauconite. Detrital kaolinites occur as undefined, deformed and crumbled flakes in the kaolinite beds of the Hamra Formation. They were formed as a result of tropical weathering of pre-existing Al-rich rocks such as the crystalline igneous rocks that are situated south and southeast of the studied localities reflecting prevailing tropical conditions during that time.

Study of mineral water resources from the Eastern Carpathians using stable isotopes

The Eastern Carpathians contain many mineral water springs that feed famous Romanian health resorts such as Borsec, Biborteni and Vatra Dornei. These waters have been used for their different therapeutic effects. In this work, mineral and spring waters from these Romanian regions were investigated by means of chemical and isotopic (deltaD and delta(18)O) analyses in order to understand the recharge mechanisms and also to determine their origins. Most of the investigated springs are of meteoric origin, having the average deuterium content of the local meteoric water. The higher (18)O content with respect to the Meteoric Water Line (MWL) indicated an exchange reaction with crystalline igneous rocks at depth and with other rocks that the water encounters on its journey back to the surface.

Isotopic and petrographic evidence for young Martian basalts

Radiometric age data for shergottites yield ages of 4.0 Ga and 180–575 Ma; the interpretation of these ages has been, and remains, a subject of debate. Here, we present new 39Ar– 40Ar laser probe data on lherzolitic shergottites Allan Hills (ALH) 77005 and Northwest Africa (NWA) 1950. These two meteorites are genetically related, but display very different degrees of shock damage. On a plot of 40Ar/ 36Ar versus 39Ar/ 36Ar, the more strongly shocked ALH 77005 (45–55 GPa) does not yield an array of values indicating an isochron, but the data are highly scattered with the shock melts yielding 40Ar/ 36Ar ratios of 1600–2026. Apparent ages calculated from these extractions range from 374–8183 Ma, with 50% of the data, particularly from the shock melts, yielding impossibly old ages (>4.567 Ga). On the same plot, extractions from igneous minerals in the less shocked NWA 1950 (30–44 GPa) yield a fitted age of 382 ± 36 Ma. Argon extractions from the shock melts are well distinguished from minerals, with the melts exhibiting the highest 40Ar/ 36Ar ratios (1260–1488) and the oldest apparent ages. Laser step heating was also performed on maskelynite separates from NWA 1950 yielding ages of 1000 Ma at the lowest release temperatures, and ages of 360 and 362 Ma at higher temperature steps. Stepped heating data from previous studies have yielded ages of 500 and 700 Ma to 1.7 Ga for ALH 77005 maskelynite separates. If the ages obtained from igneous minerals represent undegassed argon from an ancient (4.0 Ga) rock, then the ages are expected to anticorrelate with the degree of shock heating. The data do not support this inference. Our data support young crystallization ages for minerals and Martian atmosphere as the origin of excess 40Ar in the shock melts. The shock features of shergottites are also reviewed in the context of what is known of the geologic history of the Martian surface through remote observation. The oldest, most heavily cratered surfaces of Mars are thought to be ⩾4.0 Ga; we contend that ancient rocks from Mars (Noachian >3.5 Ga) are likely to record multiple impact events reflecting megaregolith formation and the cumulative effects of erosion and aqueous alteration occurring during or since that era. Young rocks (Late Amazonian, <0.6 Ga) should record a relatively simple history of emplacement and ejection from the near surface. We show that although shergottites are strongly shocked, they are relatively pristine crystalline igneous rocks and not pervasively altered breccias. The petrography of shergottites is at odds with an ancient age interpretation. A model in which young coherent rocks are preferentially sampled by hypervelocity impact because of material strength is considered highly plausible.

Fluid Evolution and Origins of Iron Oxide Cu-Au Prospects in the Olympic Dam District, Gawler Craton, South Australia

Geologic observations suggest two stages of hydrothermal activity at a number of presently subeconomic iron oxide copper-gold systems in the Olympic Dam district, eastern Gawler craton. They contain high-, and moderate- to low-temperature Fe oxide-rich hydrothermal alteration. The mineral assemblages include magnetite-calc-silicate-alkali feldspar ± Fe-Cu sulfides and hematite-sericite-chlorite-carbonate ± Fe-Cu sulfides ± U, REE minerals. In all documented prospects, the minerals of the hematitic assemblages replace the minerals of the magnetite-rich assemblages. The bulk of the subeconomic Cu-Au mineralization is associated with the hematitic alteration assemblages. Microanalysis by proton ion probe (PIXE) of hypersaline fluid inclusions in magnetite-rich assemblages, however, demonstrates that significant amounts of copper (>500 ppm) were transported by the early-stage high-temperature (>400°C) fluids responsible for the magnetite-rich alteration. These brine inclusions contain multiple solid phases (liquid + vapor + multiple solids) including chalcopyrite in some cases. In comparison, inclusions of the hematitic stage are relatively simple liquid + vapor types, with homogenization temperatures of 200° to 300°C and containing 1 to 8 wt percent NaCl equiv. The Br/Cl ratios of the magnetite-forming fluids measured by PIXE lie beyond the range of typical magmatic and/or mantle values, allowing for the possibility that the fluids originated as brines from a sedimentary basin or the crystalline basement. Sulfur isotope compositions of chalcopyrite and pyrite demonstrate that sulfur in both alteration assemblages was derived either from cooling magmas and/or crystalline igneous rocks carried by relatively oxidized fluids (∑SO42− ≈ ∑ H2S, δ34Ssulfides from −5 to +2‰) or from crustal sedimentary rocks (δ34Ssulfides from +5 to +10‰). Oxygen and hydrogen isotope compositions of waters calculated for minerals of the magnetite-rich assemblage have δ18O values of +7.7 to +12.8 per mil and δD values of −15 to −21 per mil. The only available δ18O and δDfluid values for the hematitic assemblage are +4.7 and −9 per mil, respectively. The isotopic compositions of both fluids, coupled with the available literature data, can be explained in terms of fluid reequilibration with felsic Gawler Range Volcanics or other felsic igneous rocks in the region and with metasedimentary rocks of the Wallaroo Group at low water-to-rock ratios prior to their arrival at the mineralization sites. The lack of significant copper mineralization associated with magnetite-forming fluids that carried copper suggests that there was no effective mechanism of saturation of copper minerals or the quantity of these fluids was not sufficient to produce appreciable copper mineralization. Association of the copper-gold mineralization with the hematitic alteration in the subeconomic prospects can be explained by a two-stage model in which preexisting hydrothermal magnetite with minor associated copper-gold mineralization was flushed by late-stage oxidized brines that had extensively reacted with sedimentary or metamorphic rocks. The reduction of these brines, driven by conversion of magnetite to hematite, resulted in precipitation of copper and gold. The oxidized brines may have contributed additional copper and gold to the system in addition to upgrading preexisting subeconomic Cu-Au mineralization. When compared to published models for the Olympic Dam deposit, the new data for fluids in subeconomic Fe-oxide Cu-Au prospects of the Olympic Dam district indicate the diversity of origins of iron oxide-copper-gold systems, even within the same geologic region.

The Schmidt hammer in rock material characterization

The Schmidt hammer provides a quick and inexpensive measure of surface hardness that is widely used for estimating the mechanical properties of rock material. However, a number of issues such as hammer type, normalization of rebound values, specimen dimensions, surface smoothness, weathering and moisture content, and testing, data reduction and analysis procedures continue to influence the consistency and reliability of the Schmidt hammer test results. This paper presents: a) a critical review of these basic issues; and b) the results of tests conducted on granitic rocks of various weathering grades in the light of the conclusions of this review. It was found that a very good correlation exists between L and N hammer rebound values and that both hammers are fairly sensitive to the physical properties, particularly to dry density though less so to effective and total porosities. The N hammer, producing a lesser scatter in the data, proved to be more efficient than the L hammer in predicting uniaxial compressive strength and Young's modulus. The exponential form of the correlation curves was found to reflect microstructural changes during the course of weathering and the differences in the probing scales or mechanisms in the means of measuring these mechanical properties, and could be generalized to other crystalline igneous rocks. The possibility of predicting weathering grades from rebound values was also explored. The changes in the rebound values during multiple impacts at a given point produced a better indication of the weathering grade than a single impact value. It was concluded that increasing the impact energy and plunger tip diameter should significantly reduce the scatter in coarse-grained weathered rocks and hence improve the reliability of the Schmidt hammer as a rock material characterization tool.

The eruption of Soufrière Hills Volcano, Montserrat (1995-1999): overview of scientific results

Abstract The eruption of Soufrière Hills Volcano, Montserrat (1995-1999) has displayed a wide range of volcanic phenomena: growth of an andesitic lava dome, generation of pyroclastic flows by lava dome collapse and by fountain collapse in explosive eruptions, Vulcanian and sub-Plinian explosivity with accompanying tephra fall, entrance of pyroclastic flows into the sea, sector collapse with formation of a debris avalanche and a high-velocity pyroclastic density current, and generation of lahars. New phenomena include: cyclic patterns of ground deformation linked with shallow seismicity and eruptive activity; pyroclastic flows formed by rapid sedimentation from pyroclastic surges; and an unprecedented slow escalation of eruption intensity. Magma pulsations with timescales of hours to years have been recognized. Transitions from extrusive to explosive activity were triggered by major dome collapses. Relationships between magma ascent dynamics and geophysical signals have been elucidated. Ascending water-rich andesitic magma becomes Theologically stiffened by degassing and groundmass crystallization. Large magma overpressures are consequently developed in the upper conduit, causing shallow seismicity, radial patterns of ground deformation, and cyclic pulsations of eruptive activity. Lava dome growth involved heterogeneous deformation with formation of spines and lobes along shear zones. Collapse of the pressurized dome resulted in substantial pyroclastic surges forming above pyroclastic flows. Influx of hydrous mafic magma remobilized hot crystalline igneous rocks at depths of 5 to 6 km to form crystal-rich andesitic magma and triggered the eruption.

A reassessment of the potential for reduction of dissolved CO2 to hydrocarbons during serpentinization of olivine

The concept that aqueous CO2 can be reduced to hydrocarbons abiotically during serpentinization of olivine has become widespread in the earth and planetary sciences. This process has been invoked to explain the occurrence of hydrocarbons in crystalline igneous rocks and proposed as a source of prebiotic organic compounds for the origin of life. We reevaluate this scenario through an experimental study of the reaction of dissolved CO2 in the presence of olivine under hydrothermal conditions (300°C, 350 bar). Reduction of CO2 to formate (HCOO−) was found to proceed rapidly, with H2 generated from hydrothermal alteration of olivine serving as the reductant. The reverse reaction, decomposition of formic acid to CO2 and H2, was also found to proceed rapidly. Although dissolved hydrocarbon concentrations increased throughout the experiments, isotopic labeling of dissolved CO2 with 13C indicated that these compounds were primarily generated from reduced carbon compounds already present in olivine at the beginning of the experiment rather than by reduction of CO2. The only hydrocarbon product from reduction of CO2 observed in the experiments was a small amount of methane (<0.04% conversion of dissolved CO2 in more than 2500 h of heating). Comparison of the reaction products with thermodynamic data indicates that reactions between dissolved CO2 and formate rapidly achieved metastable equilibrium at the experimental conditions, suggesting that similar reactions could control the concentration of formate in geologic fluids. The results indicate that the potential for abiotic formation of hydrocarbons during serpentinization may be much more limited than previously believed, and other mineral catalysts or vapor phase reactions may be required to explain many occurrences of abiotic hydrocarbons in serpentinites and igneous rocks.

Next generation of seafloor samplers

The University of Washington, Seattle, has successfully completed the first stage of a proposed multiphase program to develop small rock drills as a new generation of seafloor sampling tools. Phase I of this program, a basic 3‐m drill, was funded by the National Science Foundation, the Office of Naval Research, and Washington Sea Grant. The drill was designed and built for the university by Williamson and Associates of Seattle and was successfully deployed, in 2400m of water on the Juan de Fuca Ridge, during August 1990. The present drill can penetrate and sample 3 m of vertically oriented core from crystalline igneous rocks, lithified sediments, and sulfide and carbonate deposits. It is deployable to a depth of 5000 m (limited by the cable and drill weight) by conventional research vessels using the NSF‐standard armored conducting cable. The drill is presently undergoing post‐test modifications at the University of Washington and will be available to the scientific community through a Regional Facility by summer 1991.

On the lognormal distribution of oxides in igneous rocks, using magnetic susceptibility as a proxy for oxide mineral concentration

Trace element concentrations in igneous rocks are frequently lognormally distributed, and Shaw (1961) suggested that minor minerals in igneous rocks might also be lognormally distributed. There is accumulating evidence to show that bulk magnetic susceptibility (BMS) often closely follows a lognormal distribution in fresh igneous rocks but, because BMS is dependent upon oxide-grain size and mineralogy, it is not obvious why this should be so. We have adopted Shaw's simple theoretical model as an argument to account for the lognormal distribution of BMS. The obvious prevailing conditions must be that the original melt was uniform, that pressure, temperature and oxygen fugacity were constant and that the minerals were formed at equilibrium. In addition, because BMS is also a function of grain size and mineralogy, the distribution of these latter parameters must be insensitive to changes in P, T, etc. at equilibrium. It appears that there must be a limit to the lognormal law in trying to apply it to minerals of greater concentration. However, BMS proxies for oxide content, and because it does closely follow the lognormal distribution it thus appears that, in crystalline igneous rocks, the lognormal law is obeyed by minerals of concentrations at least up to a few percent. Knowledge of the mean and variance of such distributions can be useful in a variety of petrological applications, especially in drillcore logging; a simple example is presented.

On the possibility of determining rates of removal of uranium from crystalline igneous rocks using U-series disequilibria—1: a U-leach model, and its applicability to whole-rock data

The removal of U from granitic rocks is characterised by 234U/ 238U≤1 and 230Th/ 234U> 1. It is notable, from weathered Eye-Dashwa lakes (Canada) granite samples and from the published data, that 230Th/ 234U ratios seldom exceed a limiting value of about 2. A U-leach model is discussed in detail which may account for these observations. The model assumes that 230Th is practically immobile and that the removal of U can then be measured against the decay rate of the unsupported 230Th. A property of the model is that the isotope ratios reach constant values reflecting a kind of “secular equilibrium”, and this implies that U is seldom removed faster than the decay of 230Th. If the limited 230Th/ 234U ratios from the data correspond to “secular equilibrium” ratios then this allows simple calculation of U-leach rates from any given subsurface sample. A test for the model using 235U/ 231Pa ratios is suggested. A comparison of leach rates derivable from U-Th-Pb systematics, of Colorado and Wyoming granite samples, with the corresponding estimates from the model shows that the latter are high by a factor of 10–50. Data from rapidly weathered surface rinds suggests that the presence of labile U restricts the application of the model to subsurface, low-weathered zones.

On the possibility of determining rates of removal of uranium from crystalline igneous rocks using U-series disequilibria—2: applicability of a U-leach model to mineral separates

Preliminary U-series data are presented on the disequilibria of mineral separates of surface samples from the Eye-Dashwa lakes granite (Canada). Most analyses of the ferromagnesian, quartz-plus-feldspar, and zircon-plus-sphene grains, show that leaching of U has taken place such that 234U/ 239U≤ 1 and 230Th/ 234U 1. The simple leach model presented in a companion paper was used to explain these characteristic ratios. It is suggested that the leach model may be used on zircon and sphere grains to estimate U-leach rates, and this is relevant to the suggestion of using sphene-like hosts as the primary barrier in the safe disposal of nuclear waste. A full analysis of U mobility within a given rock volume was not possible due to our limited data base and to our inability to monitor labile U directly.