Rapid determination of lineament and joint densities

Lineament, joint and drainage density values are obtainable from the number of intersections between the mapped lines and a superimposed test grid or array, using the stereological equation where x is the density (length per area) and n′ is the number of intersections per unit length of the test array. Circular test grid must be used in order to eliminate the effect of preferred orientation on the n′-values.     

Allochthonous native gold in piedmont alluvium in the southern West Siberia

The paper presents the results of our investigations carried out to examine the behavior of native gold in the southern West Siberia during the fluvial transport of an auriferous terrigenous material at the mountain-to-plain transition geomorphological zone. The piedmont alluvium accumulates the small- and, to a lesser extent, fine-grained gold transported from the denudation zones by the main mountain-lowland rivers. The background Au content varies from n to 10n mg/m³. High contents (100n mg/m³ to n g/m³) are recorded in some sectors in the upper zones of channel alluvium (spit and shoal facies). Flatness coefficient of particles is an important characteristic of allochthonous native gold in the piedmont alluvium. This property shows direct correlation with the lateral dimension of particles. Composition is an essential typomorphic property of the small- and fine-grained gold in the studied sediments. This statement is primarily valid for the degree of fineness commonly related to the Ag content in gold. In some cases, a significant indicator role can be played by traces of Hg and Cu. For example, Hg is an important indicator for the gold-mercury mineralization, whereas Cu is an essential indicator in the case of gold mineralization associated with rhodingites and high-temperature Cu-Au mineralization. Abundance of the allochthonous native gold in different-age terrigenous sediments testifies to its high stability during the continental sedimentation.     

Analysis of Izmit aftershocks 25 days before the November 12th 1999 Düzce earthquake, Turkey

We investigate spatial clustering of 2414 aftershocks along the Izmit M_w = 7.4 August 17, 1999 earthquake rupture zone. 25 days prior to the Düzce earthquake M_w = 7.2 (November 12, 1999), we analyze two spatial clusters, namely Sakarya (SC) and Karadere–Düzce (KDC). We determine the earthquake frequency–magnitude distribution (b-value) for both clusters. We find two high b-value zones in SC and one high b-value zone in KDC which are in agreement with large coseismic surface displacements along the Izmit rupture. The b-values are significantly lower at the eastern end of the Izmit rupture where the Düzce mainshock occurred. These low b-values at depth are correlated with low postseismic slip rate and positive Coloumb stress change along KDC. Since low b-values are hypothesized with high stress levels, we propose that at the depth of the Düzce hypocenter (12.5 km), earthquakes are triggered at higher stresses compared to shallower crustal earthquake. The decrease in b-value from the Karadere segment towards the Düzce Basin supports this low b-value high stress hypothesis at the eastern end of the Izmit rupture. Consequently, we detect three asperity regions which are correlated with high b-value zones along the Izmit rupture. According to aftershock distribution the half of the Düzce fault segment was active before the 12 November 1999 Düzce mainshock. This part is correlated with low b-values which mean high stress concentration in the Düzce Basin. This high density aftershock activity presumably helped to trigger the Düzce event (M_w = 7.2) after the Izmit M_w 7.4 mainshock.     

A molecular dynamics study of natural organic matter: 1. Lignin, kerogen and soot

Molecular dynamics (MD) simulations were performed on molecular models of a spectrum of natural organic matter (NOM) samples represented by two lignin samples (a softwood lignin and a hardwood lignin), a kerogen (Green River Shale kerogen) and a soot sample (n-hexane soot). Simulated thermodynamic properties of each model, including glass transition temperature (T_g), thermal expansion coefficient (α), density (ρ) and solubility parameter (δ) were compared against experimental data for corresponding samples. Results revealed relatively good agreement for glass transition temperature and solubility parameter for softwood lignin, Green River Shale kerogen and n-hexane soot models. An unexpectedly low solubility parameter for a hardwood lignin model suggests, however, certain model deficiencies in terms of intermolecular interactions. In addition, a lower density for a n-hexane soot model relative to the sample was attributed to the small cluster size and poor parallel stacking of aromatic clusters in the model. Discussion of the results is provided in the context of utilizing thermodynamic properties as constraints for improved structural modeling of NOM.     

Spatial variation of aftershock activity across the rupture zone of the 17 August 1999 Izmit earthquake, Turkey

We examined the spatial variation in the aftershock activity from the 17 August 1999 Izmit, Turkey earthquake. We found that this aftershock sequence is non-uniform both in space and time, aspects that need to be taken into account in any further statistical analysis. Other aspects of this aftershock sequence are similar to other aftershock sequences, namely low b-values and a high degree of spatial variation. We have detected three zones of relatively high b-values, two of which coincide with asperities revealed by previous slip inversion studies. The third zone with an anomalous b-value is located beyond the fault rupture and indicates a weakened fractured zone in the Yalova-Tuzla area. This b-value analysis provided no evidence for any significant difference that may exist between the two sides of the mainshock fault plane.     

A two-surface thermomechanical plasticity model considering thermal cyclic behavior

In thermal-related engineering such as thermal energy structures and nuclear waste disposal, it is essential to well understand volume change and excess pore water pressure buildup of soils under thermal cycles. However, most existing thermo-mechanical models can merely simulate one heating–cooling cycle and fail in capturing accumulation phenomenon due to multiple thermal cycles. In this study, a two-surface elasto-plastic model considering thermal cyclic behavior is proposed. This model is based on the bounding surface plasticity and progressive plasticity by introducing two yield surfaces and two loading yield limits. A dependency law is proposed by linking two loading yield limits with a thermal accumulation parameter n_c, allowing the thermal cyclic behavior to be taken into account. Parameter n_c controls the evolution rate of the inner loading yield limit approaching the loading yield limit following a thermal loading path. By extending the thermo-hydro-mechanical equations into the elastic–plastic state, the excess pore water pressure buildup of soil due to thermal cycles is also accounted. Then, thermal cycle tests on four fine-grained soils (natural Boom clay, Geneva clay, Bonny silt, and reconstituted Pontida clay) under different OCRs and stresses are simulated and compared. The results show that the proposed model can well describe both strain accumulation phenomenon and excess pore water pressure buildup of fine-grained soils under the effect of thermal cycles.

     

Verification of our previous definition of preferred earthquake nucleation areas in Kanto-Tokai, Japan

We have proposed that points of future initiation of rupture may be mapped, based on minima in local recurrence times, which are equivalent to local maxima in the probability for main shocks to occur. These minima are often controlled by anomalously low b-values (logN = a − bM). Of the Kanto-Tokai area, approximately 12% showed anomalously short recurrence times and was proposed as asperities, based on seismicity up to 1999. During the period 1999–2003.5, about 75% of the earthquakes with M ≥ 3.5 fell into the asperities, earlier defined (for example 19 out of 23 M ≥ 3.8 events). The probability for this to occur by chance is approximately 2 10^− 14. This supports our idea that the most likely volumes to produce main shocks may be mapped by minima in local recurrence times.     

Relative stress variations as determined by b-values from earthquakes in circum-pacific subduction zones

Assuming a relation of “b” to stress state, the possibility of globe-wide stress variation and transmission was investigated. The NOAA earthquake data file served to determine the temporal change in “b” of log N = a − bM from 1963 to 1975.Periods of six to eight years are observed in the b-values (stress pattern) for most circum-Pacific areas (South America, Tonga, Kermadec, New Hebrides, Kamchatka and Eastern Aleutians).In the Kurils, fore- and aftershock sequences of large earthquakes seem to mask any characteristic global pattern that might exist. These sequences exhibit low b-values (high stress) through the time of foreshocks and early stages of aftershocks, followed by rapid increase in b-values (decrease in stress).Use of a worldwide earthquake data file clearly yields less resolution of the temporal “b” variation than the use of local network studies published by other authors.Incidental to the study, 1124 earthquakes of the NOAA data file yield the M_s − m_b relations: M_s = 1.16m_b − 0.835 for 4.5 m_b 6 and: log₁₀M_s = 0.1432m_b − 0.0629 form_b > 6 with correlative coefficients of 0.994 and 0.992 respectively.     

Models of compacted fine-grained soils used as mineral liner for solid waste

To prevent the leakage of pollutant liquids into groundwater and sublayers, the compacted fine-grained soils are commonly utilized as mineral liners or a sealing system constructed under municipal solid waste and other containment hazardous materials. This study presents the correlation equations of the compaction parameters required for construction of a mineral liner system. The determination of the characteristic compaction parameters, maximum dry unit weight (γ _dmax) and optimum water content (w _opt) requires considerable time and great effort. In this study, empirical models are described and examined to find which of the index properties correlate well with the compaction characteristics for estimating γ _dmax and w _opt of fine-grained soils at the standard compactive effort. The compaction data are correlated with different combinations of gravel content (G), sand content (S), fine-grained content (FC = clay + silt), plasticity index (I _p), liquid limit (w _L) and plastic limit (w _P) by performing multilinear regression (MLR) analyses. The obtained correlations with statistical parameters are presented and compared with the previous studies. It is found that the maximum dry unit weight and optimum water content have a considerably good correlation with plastic limit in comparison with liquid limit and plasticity index.     

Relationships between carbon isotope evolution and variation of microbes during the Permian–Triassic transition at Meishan Section, South China

This paper investigates kerogen carbon isotopes, the difference between carbonate and kerogen carbon isotopes (Δ¹³C_carb-kero = δ ¹³C_carb − δ ¹³C_kero) and the difference between carbonate and n-C₁₉ alkane compound-specific carbon isotopes (Δ¹³C_carb-n-C19 = δ ¹³C_carb − δ ¹³C _n-C19) during the Permian–Triassic transition at Meishan, South China. The results show that kerogen carbon isotopes underwent both gradual and sharp shifts in beds 23–25 and 26–29, respectively. The differences between carbonate and organic carbon isotopes, both the Δ¹³C_carb-kero and Δ¹³C_carb-n-C19, which are mainly affected by CO₂-fixing enzyme and pCO₂, oscillated frequently during the Permian–Triassic transition. Both the variations of Δ¹³C_carb-n-C19 and Δ¹³C_carb-kero coupled with the alternation between cyanobacteria and green sulfur bacteria indicated by biomarkers. The episodic low values of Δ¹³C_carb-n-C19 corresponded to episodic blooms of green sulfur bacteria, while the episodic high values of Δ¹³C_carb-n-C19 corresponded to episodic blooms of cyanobacteria. The relationships between the variation of carbon isotopes and biota show that the microbes which flourished after the extinction of macroorganism affected the carbon isotope fractionation greatly. Combining the carbon isotope compositions and the pattern of size variation of the conodont Neogondolella, this paper supposes that anoxia of the photic zone at bed 24 was episodic and it would be caused by the degradation of terrigenous organic matters by sulfate reducing bacteria in the upper water column. Considered together with results from previous research, the high resolution variation of the biogeochemistry presents the sequence of the important geo-events during the Permian–Triassic crisis.     

Asphalts, heavy oils, ozocerite and gases in the Dead Sea Basin

Solid, liquid and gaseous hydrocarbons occur throughout the Dead Sea Basin (Israel and Jordan) both in surface exposures and in drillings. The unaltered asphalts and heavy oils are characterized by very high sulfur content (ca. 11%) with δ³⁴S = +5% and δ¹³C = −28% to −29%, low content of n-paraffins, pristane to phytane ratio of 0.5 and by containing almost exclusively VO-porphyrins. The distribution of n-paraffins in samples from deep sources shows a smooth enveloped miximizing at C_15–20. Surface and shallow samples show clear evidence of biodegradation. The ozokerite, known only from the east side of the basin, is composed primarily of long chain n-paraffins with a maximum at C₃₉. The gases known from the southern margin of the basin are composed mostly of methane.The source for the bitumens is unknown. Two hypotheses are discussed. The first is that the asphalts and heavy oils represent an alteration products of crude oil which migrated into the basin or which might have been generated in the basin itself. The second hypothesis favors an origin from low temperature alteration of organic matter from a thermally immature source.     

Gold in the Brunswick No. 12 volcanogenic massive sulfide deposit,Bathurst Mining Camp,Canada: Evidence from bulk ore analysis and laser ablation ICP─MS data on sulfide phases

The 329-Mt Brunswick No. 12 volcanogenic massive sulfide deposit (total resource of 163 Mt at 10.4% Zn, 4.2% Pb, 0.34% Cu, and 115 g/t Ag) is hosted within a Middle Ordovician bimodal volcanic and sedimentary sequence. Massive sulfides are for the most part syngenetic, and the bulk of the sulfide ore occurs as a Zn–Pb-rich banded sulfide facies that forms an intimate relationship with a laterally extensive Algoma-type iron formation and defines the Brunswick Horizon. Zone refining of stratiform sulfides is considered to have resulted in the development of a large replacement-style Cu-rich basal sulfide facies, which is generally confined between the banded sulfide facies and an underlying stringer sulfide zone. Complex polyphase deformation and associated lower- to upper-greenschist facies regional metamorphism is responsible for the present geometry of the deposit. Textural modification has resulted in a general increase in grain size through the development of pyrite and arsenopyrite porphyroblasts, which tend to overprint primary mineral assemblages. Despite the heterogeneous ductile deformation, primary features have locally been preserved, such as fine-grained colloform pyrite and base and precious metal zonation within the Main Zone. Base metal and trace element abundances in massive sulfides from the Brunswick No. 12 deposit indicate two distinct geochemical associations. The basal sulfide facies, characterized by a proximal high-temperature hydrothermal signature (Cu–Co–Bi–Se), contains generally low Au contents averaging 0.39 ppm (n = 34). Conversely, Au is enriched in the banded sulfide facies, averaging 1.1 ppm Au (n = 21), and is associated with an exhalative suite of elements (Zn–Pb–As–Sb–Ag–Sn). Finely laminated sulfide lenses hosted by iron formation at the north end of the Main Zone are further enriched in Au, averaging 1.7 ppm (n = 41) and ranging up to 8.2 ppm. Laser ablation inductively coupled plasma-mass spectrometry (ICP-MS) analyses of pyrite (n = 97) from the north end of the Main Zone average 2.6 ppm Au and range from the detection limit (0.015 ppm) to 21 ppm. Overall, these analyses reveal a distinct Au–Sb–As–Ag–Hg–Mn association within pyrite grains. Gold is strongly enriched in large pseudo-primary masses of pyrite that exhibit relict banding and fine-grained cores; smaller euhedral pyrite porphyroblasts, and euhedral rims of metamorphic origin surrounding the pyrite masses, contain much less Au, Sb, Ag, As, and Sn. Arsenopyrite, occurring chiefly as late porphyroblasts, contains less Au, averaging 1.0 ppm and ranging from the detection limit (0.027 ppm) to 6.9 ppm. Depth profiles for single-spot laser ablation ICP-MS analyses of pyrite and arsenopyrite display uniform values of Au and an absence of discrete microscopic inclusions of Au-bearing minerals, which is consistent with chemically bonded Au in the sulfide structure. The pervasive correlation of Au with Sn in the Zn–Pb-rich banded sulfide facies suggests similar hydrothermal behavior during the waxing stages of deposition on the seafloor. Under high temperature (>350oC) and moderate- to low-pH conditions, Au and Sn in hydrothermal fluids would be transported as chlorocomplexes. An abrupt decrease in temperature and aH₂S, accompanied by an increase in fO₂ and pH during mixing with seawater, would lead to the simultaneous destabilization of both Au and Sn chlorocomplexes. The enrichment of Au in fine-grained laminated sulfides on the periphery of the deposit, accompanied by sporadic occurrences of barite and Fe-poor sphalerite, supports lower hydrothermal fluid temperatures analogous to white smoker activity on the flanks of a large volcanogenic massive sulfide system. In lower temperature (<350oC) and mildly acidic hydrothermal fluids, Au would be transported by thiocomplexes, which exhibit multifunctional (retrograde–prograde) solubility and a capacity to mobilize Au to the outer parts of the sulfide mound. The sluggish nature of this low-temperature venting together with larger variations in ambient fO₂ could lead to a sharp enrichment of Au towards the stratigraphic hanging wall of massive sulfide deposits. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

High temperature structural and thermoelastic behaviour of mantle orthopyroxene: an in situ neutron powder diffraction study

The temperature induced structural evolution and thermoelastic behaviour of a natural (Pbca) orthopyroxene (Opx), with chemical formula ^M2(Mg_0.856Ca_0.025Fe²⁺ _0.119) ^M1(Mg_0.957Fe²⁺ _0.011Fe³⁺ _0.016Cr_0.011Al_0.005)Al_0.032Si_1.968O₆, from a suite of high pressure ultramafic nodules of mantle origin, have been investigated by in-situ neutron powder diffraction at several temperatures starting from 1,200°C down to 150°C. Unit-cell parameter variations as a function of T show no phase transition within this temperature range. The volume thermal expansion coefficient, α = V ⁻¹(∂V/∂T) _P0, varies linearly with T. The axial thermal expansion coefficients, α_j = l _j⁻¹(∂l _j/∂T)_P0, increase non-linearly with T. The principal Lagrangian unit-strain coefficients (ɛ//a, ɛ//b, ɛ//c), increase continuously with T. However, the orientation of the unit-strain ellipsoid appears to change with T. With decreasing T, the values of the unit-strain coefficients along the b and c axes tend to converge. The orientation at ΔT = 1,080°C is maintained down to the lowest temperature (150°C). The two non-equivalent tetrahedral chains, TA _n OA_3n and TB _n OB_3n , are kinked differently. At room-T, the TB _n OB_3n chain is more strongly kinked by about 23° than the TA _n OA_3n chain. With increasing T, the difference decreases by 3° for the TB _n OB_3n chain. The intersite cation exchange reaction between M1 and M2 (Mg²⁺ and Fe²⁺) shows a slight residual order at 1,200°C followed by reordering with decreasing temperature although seemingly not with a definite progressive trend. At the lowest temperature reached (150°C), reordering has occurred with the same value of partitioning coefficient K _D as that before heating. The absence of the expected phase transition is most likely due to the presence of minor amounts of Fe³⁺, Al, Ca and Cr which must play a crucial role on the thermoelastic behaviour and phase stability fields in natural Opx, with consequent important petrologic and geological implications.     

Environmental control of carbon isotope variations in Pennsylvania black-shale sequences, Midcontinent, U.S.A.

A reversal of the conventional carbon isotope relationship, “terrestrial-lighter-than-marine” organic matter, has been documented for two Pennsylvanian (Desmoinesian) cyclothemic sequence cores from the Midcontinent craton of the central United States. “Deep” water organic-rich phosphatic black shales contain a significant proportion of algal-derived marine organic matter (as indicated by organic petrography, Rock-Eval hydrogen index and ratios) and display the lightest δ¹³C-values (max −27.80‰ for kerogen) while shallower water, more oxic facies (e.g. fossiliferous shales and limestones) contain dominantly terrestrial organic matter and have heavier δ¹³C_kerogen-values (to −22.87‰ for a stratigraphically adjacent coal). δ¹³C-values for extract fractions were relatively homogeneous for the organic-rich black shales with the lightest fraction (often the aromatics) being only 1‰, or less, more negative than the kerogen. Differences between extract fractions and kerogens were much greater for oxic facies and coals (e.g. saturates nearly 5‰ lighter than the kerogen).A proposed depositional model for the black shales calls upon a large influx of nutrients and humic detritus to the marine environment from the laterally adjacent, extremely widespread Pennsylvanian (peat) swamps which were rapidly submerged by transgression of the epicontinental seas. In this setting marine organisms drew upon a CO₂-reservoir which was in a state of disequilibrium with the atmosphere, being affected by isotopically light “recycled-CO₂” derived from the decomposition of peaty material in the water column and possibly from the anoxic diagenesis of organic matter in the sediments.     

Non-aromatic biomarkers associated with a Palaeogene salt formation (Bresse, France)

This study provides information on non-aromatic hydrocarbons contained in thick Palaeogene halite sequences in the Bresse region (France). The presence of almost uniquely biological markers and the absence of thermogenic compounds denote the low maturity of the corresponding samples consistent with Rock-Eval pyrolysis data. The distribution of n-alkanes enables the distinction of three main biological signatures: algal (mode in nC17–nC18); terrestrial (mode in nC29–nC31) and bacterial (mode in nC25–nC26 with no odd-even carbon number predominance). As to the cyclic biomarkers, hopanes and hopenes are largely predominant over steranes and diasterenes in samples having received appreciable amounts of terrestrial inputs (Type B) and conversely for the samples containing high proportions of algal debris (Type A, Type C).     

Aftershock sequences in the mid-ocean ridge environment: an analysis using hydroacoustic data

Hydroacoustic data from autonomous arrays and the U.S. Navy's Sound Surveillance System (SOSUS) provide an opportunity to examine the temporal and spatial properties of seismicity along portions of the slow-spreading Mid-Atlantic Ridge (MAR), intermediate-spreading Juan de Fuca Ridge (JdFR) and fast-spreading East Pacific Rise (EPR). Aftershock and foreshock events are selected from the hydroacoustic earthquake catalog using single-link cluster (SLC) analysis, with a combined space–time metric. In the regions examined, hydroacoustic data improve the completeness level of the earthquake catalog by 1.5–2.0 orders of magnitude, allowing the decay constant, p, of the modified Omori law (MOL) to be determined for individual sequences. A non-parametric goodness-of-fit test indicates six of the seven sequences examined are described well by a MOL model. The p-values obtained for individual ridge and transform sequences using hydroacoustic data are larger than that previously estimated from the analysis of a stacked sequence generated from teleseismic data. For three sequences along the Siqueiros, Discovery and western Blanco Transforms, p-values are estimated to be 0.94–1.29. The spatial distribution of aftershocks suggests that the mainshock rupture is constrained by intra-transform spreading centers at these locations. An aftershock sequence following a 7.1M_s thrust event near the northern edge of the Easter Microplate exhibits p=1.02±0.11. Within the sequence, aftershocks are located to the north of a large topographic ridge, which may represent the surface expression of the shallow-dipping fault that ruptured during the mainshock. Two aftershock sequences near 24°25′N and 16°35′N on the MAR exhibit higher p-values, 1.74±0.23 and 2.37±1.65, although the latter estimate is not well constrained because of the small number of aftershocks. Larger p-values along the ridge crest might reflect a hotter thermal regime in this setting. Additional monitoring, however, will be needed to determine if p-value differences between the ridge and transform sequences are robust. A 1999 sequence on the Endeavour segment of the JdFR, which has been correlated with changes in the hydrothermal system, is described poorly by the MOL model. The failure of the MOL model, the anomalously large number of earthquakes within the sequence and absence of a clearly dominant mainshock are inconsistent with aftershock activity and the simple tectonic origin that has been proposed previously for this sequence.     

The nature of crystalline silica from the TAG submarine hydrothermal mound, 26°N Mid Atlantic Ridge

Silica occurs in abundance in a variety of hydrothermal samples from the Trans-Atlantic Geotraverse (TAG) hydrothermal mound, 26°N Mid-Atlantic Ridge. The water content, trace element chemistry, and mineralogy of crystalline silica from 15 different samples have been examined by vibrational spectroscopy and probe microanalysis. The samples are from: shallow subsurface ferric iron oxyhydroxide silica deposits (n=4), a fragment of an active white smoker chimney (n=1), anhydrite bearing hydrothermal breccias (n=2), pyrite silica breccias (n=3), and silicified wall rock breccias (n=5). Length-fast chalcedony occurs in association with variable quantities of ferric iron oxyhydroxides in hydrothermal breccias from the mound flanks, within shallower subsurface chert samples, and within white smoker chimney walls. Samples from the anhydrite zone contain textures which are suggestive of an origin involving replacement of anhydrite. Samples taken from TAG 1 and 5 from below the anhydrite zone contain no chalcedony. Instead they contain subhedral quartz crystals which show oscillatory zoning in aluminium. Two types of crystalline silica namely, type A and type B quartz, are defined on the basis of the infrared spectra in the OH region from 3200 cm⁻¹ to 3600 cm⁻¹. The type A quartz occurs beneath the anhydrite zone at TAG 1 and 5. We propose a model that relates specific varieties of crystalline silica to different thermal and chemical environments within the mound interior. Length-fast chalcedony occurs in an outer low temperature envelope across the top and sides of the mound. The common association between length-fast chalcedony and ferric iron oxyhydroxides suggests that chalcedony crystallization is favoured where catalysis by ferric iron can occur. The apparent suppression of fibrous silica at the expense of single quartz crystals with increasing depth is attributed to differing growth rates and degrees of supersaturation of silica-bearing solutions with increasing temperature within the mound. The transition from type A to type B single crystal growth is interpreted to occur at temperatures approaching ˜360 °C due to decreasing solubility of aluminium in quartz, so that aluminium is rendered unavailable for type A valence compensation. Received: 10 September 1998 / Accepted: 6 July 1999     

On the influence of wavelength-dependent light scattering on the UV-VIS absorption spectra of oxygen-based minerals: a study on silicate glass ceramics as model substances

 The Mie scattering theory shows that the presence of randomly distributed submicroscopic inclusions with narrow size distribution and a refractive index n _i in a matrix with different refractive index n _m may give rise to a λ-dependent, band-like scattering (e.g., Kortüm 1969). Intensity and spectral position of this scattering depend on a combination of several independent parameters, such as size and number of inclusions, their form and n _i /n _m ratio (Kortüm 1969). Recently, it was confirmed that at a certain inclusion size and n _i /n _m ratio the scattering bands can contribute to the UV-edge in the electronic absorption spectra of garnets, influencing their colour (Khomenko et al. 2001). In natural minerals, however, it is impossible to differentiate between individual influence on scattering of the above mentioned parameters because of complex and interconnected variations in number, size and type of inclusions. Additionally, in most natural minerals variable amounts of transition metal ions are present. They may cause UV-VIS absorption due to ligand to metal charge transfer (LMCT) in the same region where band-like scattering may occur (Khomenko et al. 2001). At least some of these difficulties may be avoided in the case of some glass ceramics where number and size of crystalline microinclusions can be controlled by varying ceramization conditions such that fine-grained homogeneous microstructures are formed (e.g., James 1982; Petzoldt and Pannhorst 1991). Thus, glass ceramics of known composition, containing different amounts of microcrystals of specified size, may serve as unique patterns for the experimental study of effects caused by submicrocrystals on bulk properties of transparent solid materials, such as scattering, UV-VIS transparency and colour. Four types of parent glasses and a series of glass ceramic materials produced from them by heat treatment were investigated using transmission electron microscopy (TEM). These materials were also studied by transmission spectrometry in the range 35 000–20 000 cm⁻¹. Different inclusions, from five to several hundred nm in size, were observed in the glass matrices depending on their compositions and heating history. These inclusions represent two groups: early very small crystals of Ti, Zr oxides and relatively large crystals of stuffed high-quartz type or keatite. The absorption spectra of the glass ceramics show largely varying long-wavelength slopes of the UV absorption. UV-edge intensity correlates mostly with the size of the inclusions and changes drastically when larger keatite-type microcrystals are growing. Small variations in the UV edges also follow the early process of Ti-phase separation and nucleation. This may be explained by Ti depletion from the glass matrix and, thus, by reducing the measured intensity of LMCT in the first co-ordination sphere of Ti⁴⁺ ions. The different yellowish colourations of unheated glasses studied here are caused by this effect, whereas developing several hundred-nm-large keatite crystals leads to a strong scattering effect and a milky colour in glass ceramics. Received: 15 February 2002 / Accepted: 11 November 2002 Acknowledgements M. Garsche and A. Kisljuck generously provided the base glass and glass ceramic samples. F. Galbert and S. Herhing-Aghte, both at the Technical University Berlin, helped with electron microprobe analyses and refractive indices measurements, respectively. The Deutsche Forschungsgemeinschaft, Bonn-Bad Godesberg, provided financial support through grant La324/35. To all these individuals and this institution our sincere thanks are due.     

Principal fault zone width and permeability of the active Neodani fault, Nobi fault system, Southwest Japan

The internal structure and permeability of the Neodani fault, which was last activated at the time of the 1891 Nobi earthquake (M8.0), were examined through field survey and experiments. A new exposure of the fault at a road construction site reveals a highly localized feature of the past fault deformation within a narrow fault core zone. The fault of the area consists of three zone units towards the fault core: (a) protolith rocks; (b) 15 to 30 m of fault breccia, and (c) 200 mm green to black fault gouge. Within the fault breccia zone, cataclastic foliation oblique to the fault has developed in a fine-grained 2-m-wide zone adjacent to the fault. Foliation is defined by subparallel alignment of intact lozenge shaped clasts, or by elongated aggregates of fine-grained chert fragments. The mean angle of 20°, between the foliation and the fault plane suggests that the foliated breccia accommodated a shear strain of γ<5 assuming simple shear for the rotation of the cataclastic foliation. Previous trench surveys have revealed that the fault has undergone at least 70 m of fault displacement within the last 20,000 years in this locality. The observed fault geometry suggests that past fault displacements have been localized into the 200-mm-wide gouge zone. Gas permeability analysis of the gouges gives low values of the order of 10⁻²⁰ m². Water permeability as low as 10⁻²⁰ m² is therefore expected for the fault gouge zone, which is two orders of magnitude lower than the critical permeability suggested for a fault to cause thermal pressurization during a fault slip.