Synthesis of pyrope–grossular garnets: an experimental study at P = 2.5 GPa

Summary ?In order to evaluate the stability of Mg–Ca garnets at P = 2.5 GPa/T = 800–1500 °C, some experiments were performed using both natural and synthetic starting materials. Particular attention was paid to the investigation of the Ca rich side of the pyrope–grossular join and the maximum pyrope content in grossular was found to be about 10 mol.%. The addition of Cr to the system only slightly increases the observed pyrope solubility in grossular whereas Na stabilises melilite and merwinite. Clinopyroxene with a composition lying on the diopside – Ca Tschermak join appears to be the only stable crystalline phase in the middle part of the pyrope–grossular join. Received August 28, 2001; revised version accepted July 23, 2002 Published online February 24, 2003     

An experimental study of Ca-(Fe,Mg) interdiffusion in silicate garnets

Ca-(Fe,Mg) interdiffusion experiments between natural single crystals of grossular (Ca_2.74Mg_0.15 Fe_0.23Al_1.76Cr_0.04Si_3.05O₁₂) and almandine (Ca_0.21Mg_0.40 Fe_2.23Mn_0.13Al_2.00Cr_0.08Si_2.99O₁₂ or Ca_0.43Mg_0.36Fe_2.11 Al_1.95Si_3.04O₁₂), were undertaken at 900–1100 °C and 30 kbar, and pressures of 15.0–32.5 kbar at 1000 °C. Samples were buffered by Fe/FeO in most cases. Diffusion profiles were determined by electron microprobe. Across the experimental couples the interdiffusion coefficients (D˜) were almost independent of composition. The diffusion rates in an unbuffered sample were significantly faster than in buffered samples. The temperature dependence of the D˜ (Ca-Fe,Mg) interdiffusion coefficients may be described by

Cation diffusion in aluminosilicate garnets: experimental determination in pyrope-almandine diffusion couples

Diffusion couples made from homogeneous gem quality natural pyrope and almandine garnets were annealed within graphite capsules under anhydrous conditions at 22–40 kbar, 1057–1400 °C in a piston-cylinder apparatus. The concentration profiles that developed in each couple were modeled to retrieve the self diffusion coefficients [D(I)] of the divalent cations Fe, Mg, Mn and Ca. Because of their usually low concentrations and lack of sufficient compositional change across the interface of the diffusion couples, only a few reliable data can be obtained for D(Ca) and D(Mn) from these experiments. However, nine sets of D(Fe) and D(Mg) data were retrieved in the above P-T range, and cast in the form of Arrhenian relation, D=D ₀exp{−[Q(1 bar)+PΔV ⁺]/RT}. The values of the activation energy (Q) and activation volume (ΔV ⁺) depend on whether f _O2 is constrained by graphite in the system C-O or held constant. For the first case, we have for Fe:Q(1 bar)=65,532±10,111 cal/mol, D ₀=3.50 (±2.30)×10⁻⁵ cm²/s, ΔV ⁺=5.6(±2.9) cm³/mol, and for Mg:Q(1 bar)=60,760±8,257 cal/mol, D ₀=4.66 (±2.48)×10⁻⁵ cm²/s, ΔV ⁺=5.3(±3.0) cm³/mol. Here the ΔV ⁺ values have been taken from Chakraborty and Ganguly (1992). For the condition of constant f _O2, the Q values are ∼9 kcal lower and ΔV ⁺ values are ∼4.9 cm³/mol larger than the above values. Lower temperature extrapolation of the Arrhenian relation for D(Mg) is in good agreement with the Mg tracer diffusion data (D ^* _Mg) of Chakraborty and Rubie (1996) and Cygan and Lasaga (1985) at 1 bar, 750–900 °C, when all data are normalized to the same pressure and to f _O2 defined by graphite in the system C-O. The D ^* _Mg data of Schwandt et al. (1995), on the other hand, are lower by more than an order of magnitude than the low temperature extrapolation of the present data, when all data are normalized to the same pressure and to f _O2 defined by the graphite buffer. Comparison of the D(Fe), D(Mg) and D(Mn) data in the pyrope-almandine diffusion couple with those in the spessartine-almandine diffusion couple of Chakraborty and Ganguly (1992) shows that the self diffusion of Fe and Mn are significantly enhanced with the increase in Mn/Mg ratio; the enhancement effect on D(Mg) is, however, relatively small. Proper application of the self diffusion data to calculate interdiffusion coefficient or D matrix elements for the purpose of modeling of diffusion processes in natural garnets must take into account these compositional effects on D(I) along with the effects of thermodynamic nonideality, f _O2, and pressure. Received: 8 May 1997 / Accepted: 2 October 1997     

Olivine/Fe-metal equilibrium under high pressure: an ATEM investigation

San Carlos olivine samples enclosed in soft iron capsules were annealed in an uniaxial split-sphere apparatus, at pressures ranging from 4.6 to 9.0 GPa and temperature ranging from 1310o to 1595 oC. We estimated the annealing fO₂, theoretically controlled by the olivine/Fe-metal equilibrium, to be 1 to 2 log units above the fO₂ of the iron/wustite buffer. Samples were investigated by analytical transmission electron microscopy (ATEM) in order to verify that olivine and Fe capsule did equilibrate during the annealings. TEM imaging of the olivine bulk shows a and c dislocations confined in the (010) plane, and small (0.5 μm) spatially coupled precipitates of (1) Al-rich spinel and (2) enstatite (volumic proportion of precipitates ≃60 ppm). These coupled precipitates are surrounded by split c dislocation loops. Olivine composition profiles, determined by ATEM near the Fe-capsule/olivine contact, reveal a weak loss of Ni from the olivine matrix toward the capsule, as expected in such reducing conditions. These profiles also reveal a marked incorporation of Fe from the capsule into the olivine matrix. These observations, and their interpretation in terms of olivine point defect chemistry, lead to the following conclusions: (1) the starting olivine contained a high concentration of vacancies on octahedral sites (≥1000 ppm per site); such a high vacancy concentration is expected in San Carlos olivine which equilibrated in nature at relatively high fO₂; (2) the olivine/Fe-metal equilibrium did control fO₂ during the annealings, that resulted in a rapid re-equilibration of olivine at the beginning of the runs to the lower fO₂ imposed by the Fe capsule; this led to a strong decrease of the octahedral vacancy concentration in olivine. (3) Such a fO₂ decrease promoted in olivine the coupled precipitation of both types of Al-rich spinel and enstatite precipitates. These observations show that the use of Fe-capsule in high pressure experiments is an efficient method for controlling fO₂ when studying olivine, and more generally Fe-bearing silicates. Received: 5 November 1997 / Accepted: 5 May 1998     

A study of garnets from eclogite and peridotite xenoliths found in kimberlite

The chemical compositions of garnets from 58 eclogite, 72 peridotite and 4 pyroxenite xenoliths in kimberlites have been estimated from their unit cell edge length and refractive indices. The samples studied were obtained from 17 kimberlite occurrences and include all those of known source which remain in the famous Williams (1932) collection which is stored at the University of Cape Town. Every suitable sample available to the authors has been examined.A gap in the range of garnet volume percentages occurs in the samples studied between approximately 15 and 30%. Garnet peridotites characteristically have <15% garnet and eclogites >30% garnet. Very rare exceptions occur. Our collection contains no eclogites with olivine and only one with orthopyroxene. All but two of the peridotite-pyroxenite group contain orthopyroxene. The garnets from the peridotites and pyroxenites plot on a pyrope-almandine-uvarovite triangle in a narrow band with a remarkably constant almandine/uvarovite ratio. Garnets from the eclogites are plotted on a pyrope-almandine-grossularite triangle and have a wide spread of compositions. These fall into 4 groups viz. eclogite I, eclogite II, kyanite eclogite and corundum eclogite.The reasons for the differences in garnet chemistry are considered and a tentative evolutionary scheme suggested by partial melting of the garnet peridotite which is assumed to occur in the upper mantle. Recent models of upper mantle composition and the genesis of garnet-bearing xenoliths in kimberlite are briefly and critically examined.S.A. UMP Publication No. 9.     

Sandstone degradation: an experimental study of accelerated weathering

The investigated rock material belongs to the group of upper cretaceous quader sandstones which are very important construction stones in Poland and Germany. The mineral composition of the rocks is relatively uniform??they are quartzose sandstones. The most important feature is their good workability and comparatively high weathering resistance. Still, regardless of the apparent resemblance, the observations carried on buildings and monuments show differences in weathering processes. The undertaken test was an attempt to verify the proposed before rock classification, performed on the basis of parameterization of porosimetric cumulative intrusion volume curves. The aim of the experiment was the evaluation of the sandstone petrography (including the structure, texture and porosity of the rock) influence on the weathering process. The modelling of accelerated weathering was conducted in Chamber for Ageing Acceleration, where some weather circumstances were simulated (insolation, rain and frost). As the result of the weathering experiment it could be stated that the dominant mechanism of the sandstones deterioration was granular disintegration and weight loss as a consequence of sample destruction. The most important factor influencing rock deterioration is rock texture, especially the character of grain contacts. The effective porosity is a requisite of potential for the stone to take in and hold water, and hence of resistance to weathering. In case of silica-cemented sandstones, the deciding criterion influencing weathering resistance is pore structure. In case of sandstones with clay cement, the most important is mineral composition of the rock.     

The carbon dioxide solubility in alkali basalts: an experimental study

Experiments were conducted to determine CO₂ solubilities in alkali basalts from Vesuvius, Etna and Stromboli volcanoes. The basaltic melts were equilibrated with nearly pure CO₂ at 1,200°C under oxidizing conditions and at pressures ranging from 269 to 2,060 bars. CO₂ solubility was determined by FTIR measurements. The results show that alkalis have a strong effect on the CO₂ solubility and confirm and refine the relationship between the compositional parameter Π devised by Dixon (Am Mineral 82:368–378, 1997) and the CO₂ solubility. A general thermodynamic model for CO₂ solubility in basaltic melts is defined for pressures up to 2 kbars. Based on the assumption that O²⁻ and CO₃²⁻ mix ideally, we have:

On the formation and growth of faults: an experimental study

By using an experimental geometry, which includes a slot loaded to produce a mode 3 shear stress concentration, shear fractures have been grown in two fine-grained rock types. The deformation is accomplished by initially generating an array of oblique, mainly tensile, cracks at the tip of the slot, which are subsequently linked to form a rupture zone. This suggests a more general model for the formation of brittle shear rupture. A through-going shear surface will form if a damage zone is concentrated enough in the appropriate orientation. This may occur at the edges of an existing shear strain discontinuity, such as a fault, particularly in a mode 3 displacement field. Thus under some conditions faults may grow or link up as true macroscopic shear ruptures. This may be part of a solution to the apparent paradox of the existence of long faults. We suggest features that would indicate this sequence of formation in the field and review some geological examples.     

Oxygen isotope fractionation between calcite and tremolite: an experimental study

Oxygen isotope partitioning between calcite and tremolite was experimentally calibrated in the presence of small amounts of a supercritical CO₂–H₂O fluid at temperatures from 520 to 680° C and pressures from 3 to 10 kbar. The experiments were carried out within the stability field of the calcite-tremolite assemblage based on phase equilibrium relationships in the system CaO–MgO–SiO₂–CO₂–H₂O, so that decomposition of calcite and tremolite was avoided under the experimental conditions. Appropriate proportions of carbon dioxide to water were used to meet this requirement. Large weight ratios of mineral to fluid were employed in order to make the isotopic exchange between calcite and tremolite in the presence of a fluid close to that without fluid. The data processing method for isotopic exchange in a three-phase system has been applied to extrapolate partial equilibrium data to equilibrium values. The determined fractionation factors between calcite (Cc) and tremolite (Tr) are expressed as:10³1n _Cc-Tr=3.80 × 10⁶/T ²-1.67By combining the present data with the experimental calibrations of Clayton et al. (1989) on the calcite-quartz system, we obtain the fractionation for the quartztremolite system: 10³1n _Qz-Tr=4.18 × 10⁶/T ²-1.67Our experimental calibrations are in good agreement with the theoretical calculations of Hoffbauer et al. (1994) and the empirical estimates of Bottinga and Javoy (1975) based on isotopic data from naturall assemblages. At 700 C good agreement also exists between our experimental data and theoretical values calculated by Zheng (1993b). With decreasing temperature, however, an increasing difference between these data appears.Retrograde isotopic reequilibration by oxygen diffusion may be common for amphibole relative to diopside in metamorphic rocks. However, isotopic equilibrium in amphibole can be preserved in cases of rapid cooling.     

Conditions of magmatic crystallization of Na-bearing majoritic garnets in the earth mantle: Evidence from experimental and natural data

Results of experimental study at 7.0–8.5 GPa and 1300–1900°C of the systems pyrope Mg₃Al₂Si₃O₁₂ (Prp)-Na₂MgSi₅O₁₂ (NaGrt) modeling solid solutions of Na-bearing garnets, Prp-jadeite NaAlSi₂O₆ (Jd) in a simplified mode demonstrating melting relations of Na-rich eclogite, and Prp-Na₂CO₃ are presented. Prp-Na₂MgSi₅O₁₂ join is a pseudobinary that results from the decomposition of NaGrt on to coesite and Na-pyroxene. Synthesized garnets are characterized by Na admixture (>0.32 wt % Na₂O) and excess Si (3.05–3.15 f.u.). Maximal Na₂O concentrations (1.5 wt % Na₂O) are reached on the solidus of the system at 8.5 GPa. Clear correlation between Na and Si was established in synthesized garnets; this provides evidence for heterovalent isomorphism of the Mg + Al → Na + Si type with the appearance of Na₂MgSi₅O₁₂ component as a mechanism of such garnet formation. The Prp-Jd join is also pseudobinary that results from the formation of two series of solid solutions: (1) garnet (Prp-NaGrt-majorite) and (2) pyroxene (Jd-clinoenstatite-Eskola molecule), and the appearance of kyanite at the solidus of the system, where garnets with the highest Na₂O contents (>0.8 wt %) are formed. In spite of quite a wide field of garnet crystallization (20–100 mol % Prp), garnets with significant sodium concentration (>0.3 wt % Na₂O) are formed in a Jd-rich part of the system (20–50 mol % Prp). In the Prp-Na₂CO₃ system at 8.5 GPa garnet crystallizes in a wide range of starting compositions as a liquidus mineral containing up to 0.9 wt % Na₂O. Our experiments demonstrate that melt alkalinity, as well as PT-parameters control the crystallization of Na-bearing majoritic garnets. The results obtained provide evidence for the fact that the majority of natural diamonds with inclusions of Na-bearing majoritic garnets containing <0.4 wt % Na₂O were formed in alkaline silicate (carbonate-silicate) melts at a pressure of <7 GPa. Only a small portion of garnets with higher sodium concentrations (>1 wt % Na₂O) could be formed at a pressure of >8.5 GPa. 1 This article was translated by the authors.     

Strainburst characteristics under bolt support conditions: an experimental study

Natural Hazards - Strainburst, defined as a burst that occurs on the periphery of tunnels, is always associated with intact and hard brittle rocks and high geo-stress. In practical engineering, the...     

Multi-textured garnets from a single growth event: an example from northern Norway

Abstract Textural relationships between porphyroblasts of biotite and garnet in metasediments in the Nordkinn Peninsula area of the Finnmarkian Caledonides of North Norway are apparently complex. There is evidence for two textural zones in both mineral phases and superficially the development of these appears to have overlapped, at least in part, in time and space. This apparently complex porphyroblast growth history can be considerably simplified if only one period of garnet growth occurred and if different inclusion fabrics developed where garnet replaced biotite porphyroblasts and where it overgrew the matrix foliation. The possibility that porphyroblasts with textural evidence for multiphase growth histories actually grew during a single crystallization event is of importance in the interpretation and elucidation of tectonometamorphic relationships.     

Autocyclic behaviour of fan deltas: an analogue experimental study

Fan deltas are excellent recorders of fan‐building processes because of their high sedimentation rate, particularly in tectonically active settings. Although previous research focuses mainly on allogenic controls, there is clear evidence for autogenically produced storage and release of sediment by flume and numerical modelling that demands further definition of characteristics and significance of autogenically forced facies and stratigraphy. Analogue experiments were performed on fan deltas with constant extrinsic variables (discharge, sediment supply, sea‐level and basin relief) to demonstrate that fan‐delta evolution consists of prominent cyclic alternations of channellized flow and sheet flow. The channellized flow is initiated by slope‐induced scouring and subsequent headward erosion to form a channel that connected with the valley, while the removed sediment is deposited in a rapidly prograding delta lobe. The resulting decrease in channel gradient causes a reduction in flow strength, mouth‐bar formation, flow bifurcation and progressive backfilling of the channel. In the final stage of channel filling, sheet flow coexists for a while with channellized flow (semi‐confined flow), although in cycle 1 this phase of semi‐confined flow was absent. Subsequent autocyclic incisions are very similar in morphology and gradient. However, they erode deeper into the delta plain and, as a result, take more time to backfill. The duration of the semi‐confined flow increases with each subsequent cycle. During the period of sheet flow, the delta plain aggrades up to the ‘critical’ gradient required for the initiation of autocyclic incision. This critical gradient is dependent on the sediment transport capacity, defined by the input conditions. These autogenic cycles of erosion and aggradation confirm earlier findings that storage and release of sediment and associated slope variation play an important role in fan‐delta evolution. The erosional surfaces produced by the autocyclic incisions are well‐preserved by the backfilling process in the deposits of the fan deltas. These erosional surfaces can easily be misinterpreted as climate, sea‐level or tectonically produced bounding surfaces.     

Pullout capacity of multi-plate horizontal anchors in sand: an experimental study

Acta Geotechnica - A horizontal anchor is a structural member designed to resist the vertical pullout forces and ensure the stability of structures like the tower foundations, masts and bridges....     

Cation diffusion in aluminosilicate garnets: experimental determination in spessartine-almandine diffusion couples,evaluation of effective binary diffusion coefficients,and applications

We present new experimental data on diffusion of divalent cations in almandine-spessartine diffusion couples in graphite capsules in the P-T range of 14–35 kb, 1100–1200° C. The tracer diffusion coefficients of the major divalent cations, viz. Fe, Mg and Mn, retrieved from the multicomponent diffusion profiles, have been combined with earlier data from our laboratory at 29–43 kb, 1300–1480° C (Loomis et al. 1985) to derive expressions of the P-T dependence of the diffusion coefficients at fO₂ approximately corresponding to that defined by equilibrium in the system graphite-O₂. We review the conditions, discussed earlier by Cooper, under which the flux of a component in a multicomponent system becomes proportional to its concentration gradient (Fickian diffusion), as if the entire solvent matrix behaves as a single component, and also suggest a method of incorporating the thermodynamic effect on diffusion in the same spirit. Regardless of the magnitude or sign of the off-diagonal terms of the D matrix, it is always possible to define an effective binary diffusion coefficient (EBDC) of a component in a semi-infinite multicomponent diffusion-couple experiment such that it has the property of the Fickian diffusion coefficient, provided that there is no inflection on the diffusion profiles. It is shown that the success of Elphick et al. in fitting the experimental diffusion profiles of all components over a limited concentration range by a single diffusion coefficient is due to fortuitous similarity of the EBDCs of the components (Fe, Mg, Mn and Ca) in their diffusion couple experiments. In common metapelitic garnets showing compositional zoning, the EBDCs of the divalent cations do not differ from each other by more than a factor of 2.5. However, the EBDC of a component changes from core to rim by a factor of 3 to 12, depending on the composition. We suggest a method of volume averaging of the EBDC which should prove useful in approximate calculations of diffusion flux during relaxation of compositional zoning. The EBDC of Mn is found to reduce essentially to D _MnMn, the main diagonal term of the D matrix, and consequently can be calculated quite easily. Evaluation of EBDC of Fe, Mg and Mn in garnets from a prograde Barrovian sequence did not reveal any significant dependence on the extent of relaxation of garnet. The diffusion data have been applied to calculate the cooling rate of natural biotite-garnet diffusion couple from eastern Finland and diffusional modification of growth zoning in garnet in early Proterozoic Wopmay orogen, Canada. The results are in good agreement with geochronological and other independent constraints.Symbols and abbreviations a Radius of a spherical garnet crystal - BSE Back-scattered electron imaging - C Column vector of (n-1) independent components - D Diffusion coefficient matrix - D _ij An element of the diffusion matrix on the i th row and j th column - D _* ⁱ Tracer diffusion coefficient of component i - D(i) Effective interdiffusion coefficient (EIC) of various components in a multicomponent solution rich in the component i - D(i-j) Interdiffusion coefficient of components i and j in a binary solution - D _i (EB) Effective binary diffusion coefficient of component i in a multicomponent solution - D _i (EB:Ideal) D _i (EB) under condition of ideal thermodynamic mixing of the diffusing species - D _i (EB:thermo) Thermodynamic component of D _i(EB) - D _O ^A Interdiffusion coefficient at peak temperature T ₀ in the phase A - D ₀ Pre-exponential factor in an Arrhenius relation - EBDC Effective binary diffusion coefficient between a solute and a multicomponent solvent matrix - FEC Fixed edge composition model - EIC Effective interdiffusion coefficient - f _i Fugacity of component i - HM Hematite-magnetite oxygen fugacity buffer - kb Kilobars - P Pressure - Q Activation energy (enthalpy) of diffusion - Extent of relaxation defined as the difference between core and rim compositions normalized to the same difference in the initial zoning profile - R Gas constant - s Cooling rate - T ₀, T _Ch Peak temperature and characteristic temperature, respectively - t Time - VEC Variable edge composition model - V ⁺ Activation volume - W _ij Simple mixture interaction parameter between i and j - W _i(EB) Effective simple mixture interaction parameter of a component i in a multicomponent solution - _ij Margules interaction parameter between i and j - X _i Mole fraction of component i - _i Activity coefficient of component i - A dimensionless variable =D t/a ² - _ij Kronecker delta (i=j, _ij =1; ij, _ij =0) - Z_i Charge on the ion i     

Complex replacement patterns in garnets from Bergen Arcs eclogites: A combined EBSD and analytical TEM study

Eclogite-facies rocks within the Bergen Arcs, western Norway, have formed from granulites along shear zones and fluid pathways. Garnets that were inherited from granulite facies protoliths show different types of replacement patterns due to an incomplete eclogitisation process including concentric rim zoning, zoning along vein fillings and inclusion trails, and zoning bands without inclusions. The interfacial part between the granulitic core and the eclogitic rim of garnet as well as the microstructure of other relevant minerals (omphacite, plagioclase) has been analysed using analytical transmission electron microscopy (ATEM). In garnet, the interface is characterised by gradual changes in composition from X_alm=0.31, X_pyr=0.50 to X_alm=0.54, and X_pyr=0.25 within ≈20 μm and exhibits no distinct change in microstructure. Granulitic plagioclase shows exsolution lamellae of the Bøggild intergrowth. In omphacite, anti-phase domains (APDs) which potentially record the temperature of cation ordering after mineral growth have been observed and their size suggest eclogitisation at 600–700 °C. The electron backscatter diffraction (EBSD) analysis revealed that the lattice orientation of the granulitic feldspar is basically unrelated to tectonic axes whereas newly formed eclogitic minerals omphacite and kyanite show a crystallographic relation to the foliation. In garnet, no change in the basic crystallographic orientation between the eclogitic and granulitic garnet composition was confirmed. However, misorientation analysis suggests a cellular microstructure not more than 1° misorientation in the core of the garnets, which is missing in the eclogitic rim indicating textural equilibration of the latter. The heterogeneous replacement patterns are characteristic for dissolution and re-precipitation reactions in an open system limited to fluid availability. The appearance of the compositional profile in garnet is interpreted as a diffusional re-equilibration step after the time-limited, fluid-mediated eclogitisation event that apparently obscured the initially sharp interface within the further retrograde metamorphic history.     

REE in skarn systems: A LA-ICP-MS study of garnets from the Crown Jewel gold deposit

Metamorphic and magmatic garnets are known to fractionate REE, with generally HREE-enriched patterns, and high Lu/Hf and Sm/Nd ratios, making them very useful as geochemical tracers and in geochronological studies. However, these garnets are typically Al-rich (pyrope, almandine, spessartine, and grossular) and little is known about garnets with a more andraditic (Fe³⁺) composition, as frequently found in skarn systems. This paper presents LA-ICP-MS data for garnets from the Crown Jewel Au-skarn deposit (USA), discusses the factors controlling incorporation of REE into garnets, and strengthens the potential of garnet REE geochemistry as a tool to help understand the evolution of metasomatic fluids.Garnets from the Crown Jewel deposit range from Adr₃₀Grs₇₀ to almost pure andradite (Adr_>99). Fe-rich garnets (Adr_>90) are isotropic, whereas Al-rich garnets deviate from cubic symmetry and are anisotropic, often showing sectorial dodecahedral twinning. All garnets are extremely LILE-depleted, Ta, Hf, and Th and reveal a positive correlation of ΣREE³⁺ with Al content. The Al-rich garnets are relatively enriched in Y, Zr, and Sc and show “typical” HREE-enriched and LREE-depleted patterns with small Eu anomalies. Fe-rich garnets (Adr_>90) have much lower ΣREE and exhibit LREE-enriched and HREE-depleted patterns, with a strong positive Eu anomaly. Incorporation of REE into garnet is in part controlled by its crystal chemistry, with REE³⁺ following a coupled, YAG-type substitution mechanism , whereas Eu²⁺ substitutes for X²⁺ cations. Thermodynamic data (e.g., H_mixing) in grossular-andradite mixtures suggest preferential incorporation of HREE in grossular and LREE in more andraditic compositions.Variations in textural and optical features and in garnet geochemistry are largely controlled by external factors, such as fluid composition, W/R ratios, mineral growth kinetics, and metasomatism dynamics, suggesting an overall system that shifts dynamically between internally and externally buffered fluid chemistry driven by fracturing. Al-rich garnets formed by diffusive metasomatism, at low W/R ratios, from host-rock buffered metasomatic fluids. Fe-rich garnets grow rapidly by advective metasomatism, at higher W/R ratios, from magmatic-derived fluids, consistent with an increase in porosity by fracturing.     

Redox equilibria and the structural role of iron in alumino-silicate melts

The relationship between the redox ratio Fe⁺²/(Fe⁺²+Fe⁺³) and the K₂O/(K₂O + Al₂O₃) ratio (K₂O^*) were experimentally investigated in silicate melts with 78 mol% SiO₂ in the system SiO₂-Al₂O₃-K₂O-FeO-Fe₂O₃, in air at 1,400° C. Quenched glass compositions were analyzed by electron microprobe and wet chemical microtitration techniques. Minimum values of the redox ratio were obtained at K₂O^*0.5. The redox ratio in peralkaline melts (K₂O^*>0.5) increases slightly with K₂O^* whereas this ratio increases dramatically in peraluminous melts (K₂O^*<0.5) as K₂O is replaced by Al₂O₃. These data indicate that all Fe⁺³ (and Al⁺³) occur as tetrahedral species charge balanced with K⁺ in peralkaline melts. In peraluminous melts, Fe⁺³ (and Al⁺³) probably occur as both tetrahedral species using Fe⁺² as a charge-balancing cation and as network-modifying cations associated with non-bridging oxygen.     

Micro- and macro-behaviour of fluid flow through rock fractures: an experimental study

Microscopic and macroscopic behaviour of fluid flow through rough-walled rock fractures was experimentally investigated. Advanced microfluidic technology was introduced to examine the microscopic viscous and inertial effects of water flow through rock fractures in the vicinity of voids under different flow velocities, while the macroscopic behaviour of fracture flow was investigated by carrying out triaxial flow tests through fractured sandstone under confining stresses ranging from 0.5 to 3.0 MPa. The flow tests show that the microscopic inertial forces increase with the flow velocity with significant effects on the local flow pattern near the voids. With the increase in flow velocity, the deviation of the flow trajectories is reduced but small eddies appear inside the cavities. The results of the macroscopic flow tests show that the linear Darcy flow occurs for mated rock fractures due to small aperture, while a nonlinear deviation of the flow occurs at relatively high Reynolds numbers in non-mated rock fracture (Re?>?32). The microscopic experiments suggest that the pressure loss consumed by the eddies inside cavities could contribute to the nonlinear fluid flow behaviour through rock joints. It is found that such nonlinear flow behaviour is best matched with the quadratic-termed Forchheimer equation.     

LA-ICP-MS标准锆石原位微区U-Pb定年及微量元素的分析测定

LA-ICP-MS面扫描分析能直观细致地展示元素在矿物中的分布特征及相互关系,在揭示矿床成因、精细刻画成矿流体演化过程等方面具有显著优势。辽宁弓长岭铁矿床二矿区以产出沉积变质型磁铁矿富矿石而闻名,且富铁矿石的蚀变围岩中大量产出石榴石,其与富矿体成因关系密切。本文以二矿区富铁矿蚀变围岩中的石榴石为研究对象,为明确元素扩散对石榴石元素分布特征的影响,选择两颗大小不同的石榴石（1.5cm×1.5cm和0.6cm×0.7cm）,应用LA-ICP-MS在10~20Hz、20~150μm正方形激光束斑、20~150μm/s扫描速度的条件下,在4h内完成其面扫描分析,并利用无内标法对数据进行半定量校正,详细研究石榴石主量、微量和稀土元素组合及分布特征,进而有效地分析热液流体演化过程和磁铁矿富矿体的成因。LA-ICP-MS面扫描结果揭示了弓长岭厘米级石榴石连续型环带和次厘米级石榴石突变型环带的特征,准确区分了突变环带的位置和界线。分析结果表明,弓长岭二矿区厘米级石榴石中Si、Al、Fe等主量元素成分较为均一,未显示环带特征;而Mg、Mn、Ca、重稀土及Y元素均保留了原始的生长环带,具有重要的成因指示意义。该石榴石从核部到边部,其Mg含量逐渐升高,Mn含量逐渐降低,指示石榴石形成温度从核部到边部逐渐升高;而Ca含量从核部至边部先升高后降低,指示压力先升高再降低,显示进变质成因石榴石的特点。同时,该石榴石δEu值变化规律指示变质热液流体的氧逸度先减小再增大;重稀土和Y元素与Ca元素一致的变化特征表明其分布主要受压力控制。因此,结合前人研究成果综合推测,弓长岭富铁矿蚀变围岩中的石榴石形成于早元古代晚期胶—辽—吉带大陆碰撞造山过程中的进变质作用阶段,在该阶段形成的变质热液流体沿断层运移,对断层两侧的贫铁矿和围岩进行改造,从而形成富铁矿石及蚀变围岩。