Electrical conduction in rocks and minerals: Parameters for interpretation

Recent studies of electrical properties have clarified the important parameters governing electrical conductivity in minerals — temperature, oxygen fugacity, stoichiometry, iron content — and in porous rocks — shape and interconnections of fluid-filled pore spaces. These parameters are discussed in terms of: (1) how they contribute to bulk conduction mechanisms within minerals; and (2) how they pertain to the conditions of rocks in situ.     

Slow crack growth in minerals and rocks: Theory and experiments

Strength and mechanical behavior of rocks and minerals are modified by aqueous environments. This results in two effects: mechanical and chemical. The chemical effect is investigated from both a theoretical and an experimental point of view. It is shown that a thermodynamic approach leads to a satisfactory understanding of the chemical effect through an extended griffith concept. Predictions of the model have been tested using slow crack growth experiments. The experiments have been performed with a special Double Torsion apparatus which was built for this purpose. The good agreement observed between theory and experiments suggests that subcritical crack growth in rocks is controlled by adsorption onto the crack tip. This result was previously suggested by other authors (Dunning et al., 1984). However, the important consequences of the model are that (1) there should exist a threshold stress below which subcritical crack growth stops, and this threshold depends on the environment; (2) subcritical crack growth and time-dependent phenomena could take place in the crust in a stress interval which could be as high as 50% of the rupture stress.     

Laboratory measurements of electrical properties of rocks and minerals

In this paper all important results on laboratory measurements of electrical properties of rocks and minerals for the last four years are presented. Although basic results from all over world are reviewed, East European and U.S.S.R. works are described more extensively. Only D.C. conductivity results are considered, however all principal conclusions hold in similar form for both D.C. and A.C. conductivities. At the end the most important areas of further study and measurement are depicted and the main problems of the future are outlined.     

(40)~Ar/(39)~Ar chronology and geochemistry of high-K volcanic rocks in the Mangkang basin, Tibet

Our two newly obtained high-quality 40Ar/39Ar ages suggest that the high-K volcanic rocks of the Lawuxiang Formation in the Mangkang basin, Tibet were formed at 33.5±0.2 Ma. The tracing of elemental and Pb-Sr-Nd isotopic geochemistry indicates that they were derived from an EM2 enriched mantle in continental subduction caused by transpression. Their evidently negative anomalies in HFSEs such as Nb and Ta make clear that there is an input of continental material into the mantle source. The high-K rocks at 33.5±0.2 Ma in the Mangkang basin may temporally, spatially and compositionally compare with the early one of two-pulse high-K rocks in eastern Tibet distinguished by Wang J. H. et al., implying that they were formed in the same tectonic setting.     

Distribution and significance of hydrothermal alteration minerals in the Tuzla hydrothermal system, Çanakkale, Turkey

Hydrothermal alteration zones have been investigated by X-ray diffraction, mineralogical–petrographical techniques, and geochemical analysis. Examination of cores and cuttings from two drill sites, obtained from a depth of about 814–1020 m, show that the hydrothermal minerals occuring in the rock include: K-feldspar, albite, chlorite, alunite, kaolinite, smectite, illite, and opaque minerals.In the studied area, silicified, smectite, illite, alunite, and opal zones have been recognized. These alteration mineral assemblages indicate that there are geothermal fluids, which have temperatures of 150–220°C in the reservoir.The distribution of the hydrothermal minerals shows changes in the chemical composition of the hydrothermal fluid, which are probably due not only to interaction with host rock, but also to dilution of the Na–K–Cl-rich hydrothermal fluid of the deep reservoir by cold sea water at shallow levels. Geochemical analyses of the solid and liquid phases indicate that the hydrothermal fluids of the Tuzla geothermal system are in equilibrium with alteration products.The tectonic structure of the studied area is caused by NW–SE and NE–SW directional forces. The volcanic rocks where hydrothermal zones are observed in the studied area are of Lower–Middle Miocene age comprise latite, andesite, dacite, rhyolite-type lavas, tuff, and ignimbrites.     

Quantification of the factors controlling the presence of excess Ar or He

A quantitative physical model is presented which includes the factors that control the presence, or absence, of internally derived excess ⁴⁰Ar or excess ⁴He in geological systems. In particular, the model incorporates the transport and partitioning properties of the rock surrounding the mineral of thermochronologic interest and illuminates the related effects on the amount of excess ⁴⁰Ar or ⁴He preserved in the system. Modeling of a simplified 1-D rock column bounded by an external sink for ⁴⁰Ar or ⁴He shows that a steady-state excess ⁴⁰Ar or ⁴He profile develops, the magnitude of which is determined by a system parameter called the ‘transmissive timescale’, τ_T. The characteristic time required to reach this steady state depends upon τ_T and the ‘total local sink capacity’, TLSC, wherein the important role of local matrix mineral and fluid phases is incorporated. Together, these two system parameters (τ_T and TLSC) determine the evolution of excess ⁴⁰Ar or ⁴He buildup within a system above the closure temperatures of all minerals involved. An analytical expression for the 1-D system describing the evolution of excess ⁴⁰Ar (or by analogy ⁴He) in a particular potassium-bearing (or U-Th-bearing) mineral located at a distance, L, from an external sink has been derived empirically from model results:

     

Fractional-order derivative and time-dependent viscoelastic behaviour of rocks and minerals

A general constitutive equation for viscoelastic behaviour of rocks and minerals with fractional-order derivative is investigated. This constitutive law is derived based on differential geometry and thermodynamics of rheology, and the fractional order of derivative represents the degree of time delay. Analyzing some laboratory experimental data of high temperature deformation of rocks and minerals such as halite, marble and orthopyroxene, we propose how to determine the orders of fractional derivative for viscoelastic behaviours of rocks and minerals. The order is related to the exponents for the temporal scaling in the relaxation modulus and the stress power-law of strain rate, i.e., the non-Newtonian flow law, and considered as an indicator representing the macroscopic behaviour and microscopic dynamics of rocks.     

K-Ar geochronology of the late cenozoic volcanic rocks of the Cordillera Occidental, southernmost Peru

Twenty-four K-Ar radiometric ages are presented for late Cenozoic continental volcanic rocks of the Cordillera Occidental of southernmost Perú (lat. 16° 57′–17° 36′S). Rhyodacitic ignimbrite eruptions began in this transect during the Late Oligocene and continued episodically through the Miocene. The development of andesitic-dacitic strato volcanoes was initiated in the Pliocene and continues to the present.The earliest ignimbrite flows (25.3–22.7 Ma) are intercalated in the upper, coarsely-elastic member of the Moquegua Formation and demonstrate that this sedimentary unit accumulated in a trough, parallel to Andean tectonic trends, largely in the Oligocene. More voluminous ash-flow eruptions prevailed in the Early Miocene (22.8–17.6 Ma) and formed the extensively preserved Huaylillas Formation. This episode was coeval with a major phase of Andean uplift, and the pyroclastics overlie an erosional surface of regional extent incised into a Paleogene volcano-plutonic arc terrain. An age span of 14.2–8.9 Ma (mid-Late Miocene) is indicated for the younger Chuntacala Formation, which again comprises felsic ignimbrite flows, largely restricted to valleys incised into the pre-Huaylillas Formation lithologies, and, at lower altitudes, an extensive aggradational elastic facies. The youngest areally extensive ignimbrites, constituting the Sencca Formation, were extruded during the Late Miocene.In the earliest Pliocene, the ignimbrites were succeeded by more voluminous calcalkaline, intermediate flows which generated numerous large and small stratovolcanoes; these range in age from 5.3 to 1.6 Ma. Present-day, or Holocene, volcanism is restricted to several large stratovolcanoes which had begun their development during the Pleistocene (by 0.7 Ma).The late Oligocene/Early Miocene (ca. 22–23 Ma) reactivation of the volcanic arc coincided with a comparable increase in magmatic activity throughout much of the Cordilleras Occidental and Oriental of the Central Andes.     

He, Ne and Ar isotope compositions of fluid inclusions in hydrothermal sulfides from the TAG hydrothermal field Mid-Atlantic Ridge

Helium, neon and argon isotope compositions of fluid inclusions have been measured in hydrothermal sulfide samples from the TAG hydrothermal field at the Mid-Atlantic Ridge. Fluid-inclusion³He/⁴He ratios are 2.2—13.3 times the air value (Ra), and with a mean of 7.2 Ra. Comparison with the local vent fluids (³He/⁴He=7.5—8.2 Ra) and mid-ocean ridge basalt values (³He/⁴He=6—11 Ra) shows that the variation range of³He/⁴He ratios from sulfide-hosted fluid inclusions is significantly large. Values for²⁰Ne/²²Ne are from 10.2 to 11.4, which are significantly higher than the atmospheric ratio (9.8). And fluid-inclusion⁴⁰Ar/³⁶Ar ratios range from 287 to 359, which are close to the atmospheric values (295.5). These results indicate that the noble gases of fluid inclusions in hydrothermal sulfides are a mixture of mantle- and seawater-derived noble gases; the partial mantle-derived components of trapped hydrothermal fluids may be from the lower mantle; the helium of fluid inclusions is mainly from upper mantle; and the Ne and Ar components are mainly from seawater.     

Ar / Ar dating of high-pressure rocks from the Ligurian Alps: Evidence for a continuous subduction–exhumation cycle

We present ³⁹Ar–⁴⁰Ar dating of phengite, muscovite and paragonite from a set of mafic and metasedimentary rocks sampled from the high-pressure (HP) metaophiolites of the Voltri Group (Western Alps) and from clasts in the basal layer conglomerates from the Tertiary molasse which overlie the high-pressure basement. The white mica-bearing rocks display peak eclogitic and blueschist-facies parageneses, locally showing complex greenschist-facies replacement textures. The internal discordance of age spectra is proportional to the chemical complexity of the micas. High-Si phengites from eclogite clasts record a ³⁹Ar–⁴⁰Ar age of ca. 49 Ma for the eclogite stage and ca. 43 Ma for the blueschist retrogression; phengites from a blueschist basement sample yield an age of ca. 40 Ma; low-Si muscovite from a metasediment dates the formation of the greenschist paragenesis at ca. 33 Ma. Our data indicate that the analyzed samples reached high-pressure conditions at different times over a time-span of c.a. 10 Ma. Subduction was continuing during exhumation and blueschist retrograde re-equilibration of higher-pressure, eclogite-facies rocks. This process kept the isotherms depressed, allowing the older HP-rocks to escape thermal re-equilibration. Our results, added to literature data, fit a tectonic model of a subduction–exhumation cycle, with different tectonic slices subducted at different times from Early Eocene until the Eocene–Oligocene boundary.     

Ar and Sr isotopes of mantle-derived rocks from the Japanese Islands

⁴⁰Ar-³⁰Ar and Sr isotope analyses have been made for ultramatic nodules in basaltic rocks from Itinome-gata and the Oki-Dōgo Island, northeast and southwest of Japan respectively, in order to examine the state of the upper mantle under the Japanese Islands. ⁴⁰Ar-³⁰Ar analyses have revealed that ultramafic nodules from Oki-Dōgo Island were involved in events which caused Ar redistribution in these rocks and probably were related to their formation not older than several tens of millions of years ago. These nodules are relatively depleted in both Rb and Sr contents together with some other incompatible elements compared to similar nodules from the other regions in the world. This may be typical for ultramafic nodules from the Japanese Islands. Ultramafic nodules show mostly a little higher⁸⁷Sr/⁸⁶Sr ratios (0.704–0.706) than those of host basaltic rocks (0.703–0.705) indicating the accidental origin for these nodules. Present results also suggest both horizontal and vertical inhomogeneities of the uppermost part of the mantle with respect to Sr isotopes on a regional scale. Hence, the ultramafic nodules in volcanic rocks may not always directly represent the upper mantle materials from which the magma is produced at present. Such ultramafic nodules might be relatively old material from the uppermost part of the mantle under the Japanese Islands.     

Mineralogy and temporal relations of coexisting authigenic minerals in altered silicic tuffs and their utility as potential low-temperature dateable minerals

Coexisting fine-grained (0.1–20 μm) authigenic silicate minerals separated from altered tuffs in Miocene and Plio-Pleistocene lacustrine deposits were characterized petrographically and using X-ray powder diffraction. The authigenic minerals are dominated by clinoptilolite, erionite, phillipsite, K-feldspar, silica, calcite, smectite, and randomly interstratified illite/smectite. Minor accessories of opal-CT, cristobalite, and barite are present with the major alteration minerals. Authigenic minerals from altered tuffs were dated using the K/Ar method to evaluate the utility of these minerals for determining the time of alteration in low-temperature diagenetic environments. The eruption ages of some of these zeolite-rich tuffs were determined using the ⁴⁰Ar/³⁹Ar method on single sanidine and plagioclase minerals. The K/Ar isotopic ages of the fine-grained K-feldspar show minimal variation compared with results from the clinoptilolite separates. The isotopic ages from the authigenic K-feldspar (15-13.8 Ma) and some of the zeolites (16.-6.7 Ma) are similar to the eruption ages of the tuffs and indicate early alteration. Despite their open-framework structure, zeolites apparently can retain part or all of their radiogenic argon under favorable conditions (e.g., saturated environment). How much of the radiogenic argon is retained is estimated from the isotopic ages of other coexisting secondary minerals that are commonly dated by the K/Ar method. Although zeolite isotopic ages should be interpreted with caution, they may be useful to constrain temporal relations of low-temperature diagenetic processes when used in conjunction with other dateable minerals.     

White mica K-Ar ages of blueschist-facies rocks from the Piemonte 'calc-schists' of the western Italian Alps

Abstract Fifty calc-schists have been systematically collected from the Piemonte zone of the western Italian Alps and examined in terms of petrology, X-ray powder diffractometer (XRD) analysis of carbonaceous materials, and K-Ar ages of white mica separates. The petrological study and XRD analysis of carbonaceous materials have shown that calc-schists have suffered blueschist-facies metamorphism in the subduction zone of the convergent margin between the Apulian (African) continental and Tethyan oceanic plates. The metamorphic sequence is divided into three mineral zones based on increasing metamorphic temperature: chlorite (lower than 300°C), chloritoid, and rutile (higher than 450°C). The chlorite zone has dispersed ages of white mica separates, ranging from 115 to 44 Ma, whereas the rutile zone has a comparatively uniform age distribution from 60 to 40 Ma. The chloritoid zone has an intermediate age variation. The large variation in the chlorite zone is attributed to mixing of variable amounts of detrital mica derived from older high temperature metamorphic rocks in the separates, which have not been completely reset during Alpine metamorphism. The uniform age (average ca 50 Ma) in the rutile zone is the cooling age of blueschist-facies calc-schists, which have been episodically exhumed at the collision event of the European and Apulian continents in the Paleocene-Eocene.     

Geochronology and excess Ar geochemistry of the Lhotse Nup leucogranite, Nepal Himalaya

⁴⁰Ar/³⁹Ar dating experiments on several coexisting minerals from two close-by leucogranite outcrops near Lhotse Nup glacier (Nepal Himalaya) reveal a complex behaviour. Four biotite and muscovite ages cluster around 15.5 Ma, a lower value than literature Rb/Sr ages on splits of the same four micas, suggesting a discrepancy with the ideal cooling age sequence observed in the Alps.A strongly discordant Ar-Ar spectrum on tourmaline does not allow a chronological interpretation.A potassium feldspar shows a slow-cooling staircase spectrum with a superimposed saddle diagnostic of excess Ar. HF leaching removed excess Ar but caused great perturbations to the minimum step ages, isochron plots, and the release of reactor-produced Ar isotopes. The present data require that the currently fashionable interpretations of feldspar systematics be radically changed.The main chronological conclusions rely on the eight mica ages. Their decrease from 18.2 to 15.3 Ma dates the cooling of the Lhotse Nup leucogranite.     

On water in nominally anhydrous minerals from mantle peridotites and magmatic rocks

Trace amount of water associated with the lattice defects of nominally anhydrous minerals (NAMs) can be measured using Fourier transform infrared spectroscopy (FTIR) and secondary ion mass spectrometry (SIMS). Lots of data on water in NAMs from different lithologies, especially mantle peridotite xenoliths, have been published. The water distribution in olivine from peridotite xenoliths often displays a diffusion profile with high water concentration in the core and low at the rim, which indicates water loss via diffusion during the ascent of host magma. On the other hand, water is homogeneously distributed in pyroxene and its concentration is typically interpreted to represent a mantle value. The water concentration of magma in equilibrium with NAM can be estimated using specific partition coefficient, from which the water content of parental magma and the mantle source can be inferred. The accuracy of this method, however, depends on the selection of appropriate partition coefficient for the system. Using hydrogen isotope compositions and H₂O/Ce ratios of mantle NAMs, water source regions can be traced and water heterogeneity can be mapped in the upper mantle. Water plays an important role in the stability of cratonic mantle. The water contents and vertical distribution patterns can be significantly different among different cratonic mantles, which may result from different geologic activities. However, the mantle-plume interaction may not necessarily result in significant change of water content in cratonic mantle. The estimation of the water content in the upper mantle is still largely based on geochemical models due to the limitations of data on water in mantle NAMs.     

Geochemical magma type discrimination: application to altered and metamorphosed basic igneous rocks

Five minor and trace elements, known to be chemically stable during alteration and metamorphism, have been combined in a set of binary diagrams that distinguish fresh tholeiites from alkali basalts. Of the five elements: Ti, P, Zr, Y, Nb, only P shows slight mobility during metamorphism, which is not sufficient to alter greatly the point distribution on the binary diagrams. Using these stable elements altered basaltic rocks: greenstones, spilites and amphibolites may be distinguished in the same way as fresh basalts, and their original magma may be identified as tholeiitic or alkaline basalt. All five elements are readily and rapidly determined, using XRF, thus this method may be applied as a rapid, easy way of discriminating the magma types of altered basaltic rocks. Using this method it can be demonstrated that alkali basalt magma was produced in minor quantities in the Precambrian.     

Mineralogy and stable isotope geochemistry of hydrothermally altered oceanic rocks

Data for the diffusion of cations in pyroxenes are relevant to a variety of sub-solidus processes including order-disorder and exsolution. Similar data must also be available if the reliability of geobarometers and geothermometers involving pyroxenes is to be assessed. Two types of diffusion experiment have been performed to determine cation diffusion rates in pyroxenes: (1) interdiffusion between single crystals of diopside and polycrystalline sinters enriched in Al and Fe, and (2) interdiffusion between single crystals of diopside and a glass of the same composition which was isotopically enriched in²⁶Mg and⁴³Ca. Following high-temperature annealing for periods up to several hundred hours, analysis of the diffusion couples, using an electron microprobe and an ion microprobe respectively, failed to show any measurable diffusion profiles. From these “null result” experiments the diffusion coefficients (D) for Al and Fe in diopside are estimated to be less than4×10^?14cm²s^?1 at 1200°C, and values ofD for Ca and Mg in diopside are estimated to be less than7 × 10^?14cm²s^?1 at 1250°C. These rates are significantly slower than published tracer-type diffusion data for Ca and Al.A review of studies of order-disorder, microstructural coarsening, and diffusion in pyroxenes suggest that activation energies for cation exchange are typically in excess of 60 kcal mol^?1. Transport rates will be assisted, and activation energies lowered by sample non-stoichiometry, inhomogeneities, high dislocation densities and the presence of water.The collective data for Al, Mg and Ca diffusion in diopside indicate diffusion coefficients? 10^?15cm²s^?1 at 1200°C. A comparison with data for diffusion in garnet, olivine and spinel suggests that pyroxenes may have the highest blocking temperatures.     

Microprobe studies of REE-rich accessory minerals: Implications for Skye granite petrogenesis and REE mobility in hydrothermal systems

The REE geochemistry of accessory allanites, sphenes. apatites and zircons from a range of granitic, sedimentary and hydrothermally altered rocks from Skye has been investigated using the electron microprobe. Allanites and sphenes in Skye Tertiary granites are extremely LREE enriched (Ce_N/Y_N= 40–100) and may contain up to 50% of whole rock LREE (La-Nd). These phases are late crystallisation products of redidual magmatic fluids. Earlier-formed apatites (Ce_N/Yb_N = 7.33) and zircons (Ce_N/Yb_N = 0.05) contain insufficient REE to have influenced the REE geochemistry of the Western Red Hills granites by crystal fractionation. However, Y-, Th- and HREE-rich zircons (Ce_N/Yb_N = 0.03–0.12, ΣREE + Y = 16,500–49,500ppm) occur both as detrital grains in Skye Torridonian sediments and in the Coire Uaigneich Granophyre (CUG), suggesting bulk involvement of these sediments in CUG petrogenesis. Hydrothermal allanites in altered Tertiary igneous rocks from Skye are LREE enriched (Ce_N/Y_N = 16–920), whilst allanites formed during alteration of Torridonian arkoses have less fractionated REE patterns (Ce_N/Y_N = 4.4–1.0), as the instability of metamict HREE-rich detrital zircons buffered the hydrothermal fluids in these rocks to more HREE-rich compositions. This buffering indicates that within unveined rocks the scale of REE mobility during hydrothermal alteration was small, even though the occurrence of allanite in hydrothermal veins on Skye suggests that LREE may have been transported for some distance by meteoric-hydrothermal fluids. Zoning of the REE within individual hydrothermal and metamorphic allanites (e.g. coreCe_N/Y_N = 97.56, rim Ce_N/Y_N = 0.22) suggests evolution of their parent geological fluids to more HREE-rich compositions during allanite growth.     

Geochemistry and Ar/Ar geochronology of Miocene volcanic rocks from the Karaburun Peninsula: Implications for amphibole-bearing lithospheric mantle source,Western Anatolia

New geochemical and ⁴⁰Ar/³⁹Ar age data are presented from the Neogene volcanic units of the Karaburun Peninsula, the westernmost part of Western Anatolia. The volcanic rocks in the region are associated with Neogene lacustrine deposition and are characterized by (1) olivine-bearing basaltic-andesites to shoshonites (Karaburun volcanics), high-K calc-alkaline andesites, dacites and latites (Yaylaköy, Arma?anda? and Kocada? volcanics) of ~ 16–18 Ma, and (2) mildly-alkaline basalts (Ovac?k basalt) and rhyolites (Urla volcanics) of ~ 11–12 Ma. The first group of rocks is enriched in LILE and LREE with respect to the HREE and HFSE on N-MORB-normalised REE and multi-element spider diagrams. They are comparable geochemically with volcanic rocks in the surrounding regions such as Chios Island and other localities in Western Anatolia. The Ovac?k basalt is geochemically similar to the first stage early–middle Miocene volcanic rocks but differs from NW Anatolian late Miocene alkali basalts.