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1.
The Southern Marginal Zone of the Limpopo Belt in South Africa is characterised by a granulite and retrograde hydrated granulite terrane. The Southern Marginal Zone is, therefore, perfectly suitable to study fluids during and after granulite facies metamorphism by means of fluid inclusions and equilibrium calculations. Isolated and clustered high-salinity aqueous and CO2(-CH4) fluid inclusions within quartz inclusions in garnet in metapelites demonstrate that these immiscible low H2O activity fluids were present under peak metamorphic conditions (800-850 °C, 7.5-8.5 kbar). The absence of widespread high-temperature metasomatic alteration indicates that the brine fluid was probably only locally present in small quantities. Thermocalc calculations demonstrate that the peak metamorphic mineral assemblage in mafic granulites was in equilibrium with a fluid with a low H2O activity (0.2-0.3). The absence of water in CO2-rich fluid inclusions is due to either observation difficulties or selective water leakage. The density of CO2 inclusions in trails suggests a retrograde P-T path dominated by decompression at T<600 °C. Re-evaluation of previously published data demonstrates that retrograde hydration of the granulites at 600 °C occurred in the presence of H2O and CO2-rich fluids under P-T conditions of 5-6 kbar and ~600 °C. The different compositions of the hydrating fluid suggest more than one fluid source.  相似文献   

2.
Extreme enrichment in H2O, B, P and F is characteristic of many evolved granites and pegmatites. We report experimental phase relations of a synthetic peraluminous pegmatite spiked with P2O5, B2O3 and F (5 wt% of each), Rb2O, Cs2O (1 wt% of each) and Li2O (0.5 wt%). Experiments were carried out in H2O-saturated conditions in cold-seal rapid-quench pressure vessels at 0.1-0.2 GPa. Crystallisation starts at about 820 °C with berlinite and topaz. Quartz appears at 700-750 °C. Topaz is replaced by muscovite at about 600 °C. At near-solidus temperatures (450-500 °C) amblygonite, lacroixite and a Cs-bearing aluminosilicate crystallise. In all charges aluminosilicate melt coexists with low-density hydrous fluid and hydrosaline melt. The latter is strongly enriched in Na3AlF6 and H3BO3 components. Experimental evidence of the liquid immiscibility and mineral reactions documented in our study offers new explanations of many enigmatic features of natural pegmatites.  相似文献   

3.
Tourmaline has been synthesized hydrothermally at 200 MPa between 300 and 700 °C from oxide mixtures with Mg-Al ratios for the end members dravite NaMg3Al6(Si6O18)(BO3)3(OH)3(OH) and Mg-foitite &ding6F;(Mg2Al)Al6 (Si6O18)(BO3)3(OH)3(OH). Six different Na concentrations were investigated to determine the distribution of Na between tourmaline and fluid in the SiO2-saturated system Na2O-MgO-Al2O3-SiO2-B2O3-H2O-HCl. Synthetic tourmaline ranges from X-site vacant (&ding6F;) tourmaline (Mg-foitite) to nearly ideal dravite with Na=0.95 apfu. There are small, but significant, amounts of proton deficiency and negligible tetrahedral Al. Chemical variation is primarily caused by the substitutions Al&ding6F;Mg-1Na-1 and minor AlMg-1H-1. Varying amounts of Na and &ding6F; determine the Mg/Al ratios. Besides tourmaline and quartz, additional Mg-Al phases are chlorite and, at 700 °C, cordierite. Albite is also present at high Na concentrations in the bulk composition. The c dimension of the tourmaline crystals increases with Na in tourmaline. The amount of Na in the X-site depends strongly on the bulk concentration of Na in the system as well as on the temperature. These factors in turn control the phase assemblage and the composition of the fluid phase. For the assemblage tourmaline + quartz + chlorite/cordierite + fluid, a linear relationship exists between Na concentration in the fluid (quenched after the run) and tourmaline with temperature: T °C [ᆭ °C]=(Nafluid/Natur)앾.878-14.692 (r2=0.96). For the assemblage tourmaline + albite + quartz + fluid, it is: T °C [ᆣ °C]=(Nafluid/Natur)욝.813-6.231 (r2=0.95), where Nafluid is the concentration of Na+ in the final fluid (mol/l) and Natur is the number of Na cations in the X-site of tourmaline. The equations are valid in the temperature range of 500-715 °C. Our experiments demonstrate that the occupancy of the X-site in combination with the changing concentrations of Al and Mg can be used to monitor changes in the fluid composition in equilibrium with a growing tourmaline crystal. Currently, this relation can be applied qualitatively to natural tourmaline to explain zoning in Na- and Al/(Al+Mg).  相似文献   

4.
Most quantitative studies of flow dynamics in eruptive conduits during volcanic eruptions use a simplified energy equation that ignores either temperature changes, or the thermal effects of gas exsolution. In this paper we assess the effects of those simplifications by analyzing the influence of equilibrium gas exsolution and expansion on final temperatures, velocities, and liquid viscosities of magma-gas mixtures during adiabatic decompression. For a given initial pressure (p1), temperature (T1) and melt composition, the final temperature (Tf) and velocity (umax) will vary depending on the degree to which friction and other irreversible processes reduce mechanical energy within the conduit. The final conditions range between two thermodynamic end members: (1) constant enthalpy (dh=0), in which Tf is maximal and no energy goes into lifting or acceleration; and (2) constant entropy (ds=0), in which Tf is minimal and maximum energy goes into lifting and acceleration. For ds=0, T1=900 °C and p1=200 MPa, a water-saturated albitic melt cools by ~200 °C during decompression, but only about 250 °C of this temperature decrease can be attributed to the energy of gas exsolution per se: the remainder results from expansion of gas that has already exsolved. For the same T1 and p1, and dh=0, Tf is 10-15 °C hotter than T1 but is about 10-25 °C cooler than Tf in similar calculations that ignore the energy of gas exsolution. For ds=0, p1=200 MPa and T1=9,000 °C, assuming that all the enthalpy change of decompression goes into kinetic energy, a water-saturated albitic mixture can theoretically accelerate to ~800 m/s. Similar calculations that ignore gas exsolution (but take into account gas expansion) give velocities about 10-15% higher. For the same T1, pI=200 MPa, and ds=0, the cooling associated with gas expansion and exsolution increases final melt viscosity more than 2.5 orders of magnitude. For dh=0, isenthalpic heating decreases final melt viscosity by about 0.7 orders of magnitude. Thermal effects of gas exsolution are responsible for less than 10% of these viscosity changes. Isenthalpic heating could significantly reduce flow resistance in eruptive conduits if heat generation were concentrated along conduit walls, where shearing is greatest. Isentropic cooling could enhance clast fragmentation in near-surface vents in cases where extremely rapid pressure drops reduce gas temperatures and chill the margins of expanding pyroclasts.  相似文献   

5.
Volume diffusion rates for five rare earth elements (La, Ce, Nd, Dy, and Yb) have been measured in single crystals of natural diopside at pressures of 0.1 MPa to 2.5 GPa and temperatures of 1,050 to1,450 °C. Polished, pre-annealed crystals were coated with a thin film of rare earth element oxides, then held at constant temperature and pressure for times ranging from 20 to 882 h. Diffusion profiles in quenched samples were measured by SIMS (secondary ion mass spectrometry) depth profiling. At 1 atm pressure, with the oxygen fugacity controlled near the quartz-fayalite-magnetite buffer, the following Arrhenius relations were obtained for diffusion normal to (001) (diffusion coefficient D in m2/s): log10DYb=(-4.64ǂ.42)-(411ᆠ kJ/mol/2.303RT); log10DDy=(-3.31ǃ.44)-(461ᆽ kJ/mol/2.303RT); log10DNd=(-2.95DŽ.64)-(496ᇡ kJ/mol/2.303RT); log10DCe=(-4.10ǃ.08)-(463ᆳ kJ/mol/2.303RT); log10DLu=(-4.22DŽ.66)-(466ᇢ kJ/mol/2.303RT). Diffusion rates decrease significantly with increasing ionic radius, with La a factor of ~35 slower than Yb. The relationship between diffusivity and ionic radius is consistent with a model in which elastic strain plays a critical role in governing the motion of an ion through the crystal lattice. Activation volumes for Yb and Ce diffusion, at constant temperature and oxygen fugacity, are 9.0DŽ.0 cm3/mol and 8.9Dž.2 cm3/mol, respectively, corresponding to an order of magnitude decrease in diffusivity as pressure is increased from 0 to 3 GPa at 1,200 °C. Diffusion of Nd is such that grain-scale isotopic equilibrium in the mantle can be achieved in ~1 My under conditions near the peridotite solidus (~1,450 °C at 2.5 GPa). The equilibration time is much longer under P, T conditions of the lithospheric mantle or at the eclogite solidus (~1 Gy at 1.5 GPa and 1,150 °C). Because of the relatively strong decrease in diffusivity with pressure (two orders of magnitude between 2.5 and 15 GPa along an adiabatic temperature gradient), Nd transport in clinopyroxene will be effectively frozen at pressures approaching the transition zone, on time scales less than 100 My. Rare earth element diffusion rates are slow enough that significant disequilibrium uptake of REE by growing clinopyroxene phenocrysts may be preserved under natural conditions of basalt crystallization. The relative abundances and spatial distributions of REE in such crystals may provide a sensitive record of the cooling and crystallization history of the host lava.  相似文献   

6.
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7.
Water solubility in orthopyroxene   总被引：7，自引：0，他引：7  
The solubility of water in pure enstatite was measured on samples synthesized at 1,100 °C and pressures to 100 kbar. Enstatite crystals were grown under water-saturated conditions from a stoichiometric mixture of high-purity SiO2 and Mg(OH)2. Water contents were calculated from polarized FTIR spectra measured on oriented single crystals. The water solubility in orthoenstatite increases with pressure to 867ᆷ ppm H2O by weight at 75 kbar. At 100 kbar, in the stability field of high-clinoenstatite, a water solubility of 714ᆷ ppm was observed. The water solubility in enstatite at 1,100 °C can be described by the equation cH2O=AfH2O exp(-P(V/RT), where fH2O is water fugacity, A=0.0204 ppm/bar and (V=12.3 cm3/mol. The infrared spectra of the hydrous enstatite crystals show a sharp, intense band at 3,363 cm-1 and a broad, weaker band at 3,064 cm-1. Both bands are strongly polarized parallel c. Most likely, pairs of protons attached to non-bridging oxygen atoms substitute for Mg2+. In order to investigate the effect of chemical impurities on water solubility in enstatite, an additional series of experiments was carried out with gels doped with Al, B, or Li as starting material. Whereas, the presence of Li and B had no detectable effect on water solubility, the addition of about 1 wt% Al2O3 increased water solubility in enstatite from 199 to 1,100 ppm at 1,100°C and 15 kbar. In the infrared spectra of these aluminous samples, additional bands occur in the range from 3,450 to 3,650 cm-1. Similar bands are also observed in natural, aluminous orthopyroxenes and are most likely caused by protons coupled with Al according to the substitution of Al3++H+ for Si4+. A series of hydrous annealing experiments on a natural, gem-quality aluminous enstatite from Tanzania yielded water solubilities generally consistent with the results from the synthetic model systems. The results presented here imply that pure enstatite has a similar storage capacity for water as olivine; however, aluminous orthopyroxenes in the mantle may dissolve much larger amounts of water comparable with the entire mass of the present hydrosphere. Moreover, the mechanism of aluminum substitution in orthopyroxenes, i.e., the distribution of Al between tetrahedral and octahedral sites, may be a potential probe of water fugacity.  相似文献   

8.
The trace-element signature that cordierite (Crd) imparts to silicic magmas was evaluated by experiment using metapelite mineral mixtures to produce cordierite-bearing peraluminous granitic melts at 200 MPa (PH2O), from 700 to 850 °C. Most elemental partition coefficients vary with T. Beryllium is strongly compatible, with DBeCrd/melt values decreasing linearly from 202.0 to 6.7 as T rises from 700 to 850 °C. Manganese is compatible (DMnCrd/melt=7.67 to 1.92 over the same range of T), and shows similar values to those reported for biotite in silicic melts. Incompatible components include Li, Rb, B, F and P, although Cs is nearly compatible in cordierite, especially at higher T (DCsCrd/melt=~0.19 to 0.60) where the large alkalis are better accommodated structurally. Cordierite appears to be the most effective crystalline reservoir of Be and Cs in metapelites and their anatectic melts. Natural data support the hypothesis that Crd, when present in granitic melts, sequesters Be, Cs and, in the absence of garnet, Mn. S-type granitic rocks containing Crd show consistently low Be contents (mean=0.8 ppm Be with an average range of <1 to 1.20) whereas Crd-free granites (e.g., containing accessory garnet) exhibit distinctly higher Be contents (mean=6 ppm Be with an average range of 3 to 12). These values increase further in evolved facies (mean=69 ppm Be with a an average range of 11 to 145) which commonly give rise to beryl-bearing pegmatites. Whole-rock signatures of Be discriminate source environments of silicic magmas at a resolution equal to the boundaries of the cordierite stability field - e.g., at the P-T-X conditions where cordierite gives rise to garnet+aluminum silicate. Cordierite-bearing granitic rocks contain low Cs contents (mean=1.8 ppm Cs) compared to the Crd-free equivalents (mean=18 ppm Cs). Mn contents also correlate with the presence (mean=0.01 wt% MnO) or absence of Crd (mean=0.09 wt% MnO). Depending on its contribution to anatexis, cordierite may either give or take S-type chemical character from granitic liquids, resulting in a distinctive Crd-associated group of S-type elements. This signature is different from that of micas (high Li, F and, to a lesser degree, Be and Mn). Whole-rock compositions of granites, coupled with notable absences of beryl in their associated pegmatites, indicate that a sizable population of S-type granites originated from Crd-bearing sources. The normative Crd component of silicic peraluminous melts is Д wt% to 850 °C. Higher modal contents of cordierite reflect either restite entrainment or peritectic reactions which produce Crd after magma ascent to shallow depths. The distinctive trace-element signature of cordierite now provides improved resolution of the source mineralogy for S-type magmas.  相似文献   

9.
In the contact aureole of the Oligocene granodiorite of Cima di Vila, granitic pegmatites of Variscan age were strongly deformed during eo‐Alpine regional metamorphism, with local development of ultramylonites. In the ultramylonite matrix, consisting of quartz, plagioclase, muscovite and biotite, microstructures show grain growth of quartz within quartz ribbons, and development of decussate arrangements of mica. These features indicate that dynamic recrystallization related to mylonite development was followed by extensive static growth during contact metamorphism. K‐feldspar porphyroclasts up to 1.5 cm are mantled by myrmekite that forms a continuous corona with thickness of about 1 mm. In both XZ and YZ sections, myrmekite tubules are undeformed, and symmetrically distributed in the corona, and oligoclase‐andesine hosts have random crystallographic orientation. Myrmekite development has been modelled from the P–T–t evolution of the ultramylonites, assuming that the development of the ultramylonites occurred during eo‐Alpine metamorphism at c. 450 °C, 7.5 kbar, followed by contact metamorphism at c. 530 °C, 2.75 kbar. Phase diagram pseudosections calculated from the measured bulk composition of granitic pegmatite protolith indicate that the equilibrium assemblage changes from Qtz–Phe–Ab ± Zo ± Cpx ± Kfs during the ultramylonite stage to Qtz–Pl(An30–40)–Ms–Kfs–Bt(Ann55) during the contact metamorphic stage. The thermodynamic prediction of increasing plagioclase mode and anorthite content, change of white mica composition and growth of biotite, occurring during the end of the heating path, are in agreement with the observed microstructures and analysed phase compositions of ultramylonites. Along with microstructural evidence, this supports the model that K‐feldspar replacement by myrmekite took place under static conditions, and was coeval with the static growth accompanying contact metamorphism. Myrmekite associated with muscovite can develop under prograde (up‐temperature) conditions in granites involved in polymetamorphism.  相似文献   

10.
Secondary Ca-Al silicates are used to constrain the P-T-x conditions of the very early post-magmatic stage of the intermediate to basic Hercynian plutonic complexes of Charroux-Civray (NW Massif Central, France) and Fichtelgebirge (NE Bavaria, Germany). The secondary Ca-Al silicates hydrogarnet, prehnite, pumpellyite, epidote and laumontite form lenses within unaltered or only slightly chloritized biotite. Hydrogarnet as the first occurring Ca-Al silicate phase crystallizes at temperatures above 340 °C. The common paragenesis prehnite + pumpellyite post-dates hydrogarnet and indicates rather narrow ranges of temperature (200-280 °C) and pressure (2-3 kbar). Laumontite is formed at the end of Ca-Al silicate crystallization (180-260 °C, 1-3 kbar), mostly in small fractures in association with prehnite and adularia. The observed crystallization sequence of the Ca-Al silicates and their stabilities define a retrograde alteration path for the plutonic rocks. The Ca-Al silicate assemblage results from an early pervasive alteration of the plutonic rocks by low XCO2 fluids during post-magmatic cooling. Subsolidus cooling starts at about 4 kbar at solidus temperatures as indicated by magmatic epidote stability, hornblende barometry and fluid inclusion data, and continues under slightly decreasing pressure (uplift) down to 2-3 kbar at 200-280 °C (prehnite-pumpellyite paragenesis). This shows that Ca-Al silicate assemblages may be a unique tool to constrain the P-T conditions of the subsolidus cooling of intermediate to basic plutonic bodies.  相似文献   

11.
SEIFERT  F. 《Journal of Petrology》1970,11(1):73-100
The equilibrium temperatures of the reaction muscovite+chlorite+quartz= cordierite+phlogopite+H2O (1) in the pure system K2O—MgO—Al2O2—SiO2—H2Owere found to be 495±10°C at 1 kb PH2O; 525±10°Cat 2 kb; 610±15°C at 5 kb; 635±10°C at6 kb. From intersection of this curve with the lower temperaturestability limit of cordierite close to 645°C, 6.5 kb PH2O,a reaction cordieritc+muscovite = phlogopite+aluminum silicate+quartz+H2O(2) is generated which has a negative slope and passes throughthe points 645°C, 6.5 kb PH2O and 700°C, 5 kb PH2O.On the high-pressure side of this reaction curve cordieriteis restricted to K2O—poor bulk compositions. Application of the experimentally determined phase relationsto more complex natural pelitic rocks suggests that reaction(1) represents maximum temperatures for the disappearance ofchlorite from pelitic assemblages containing muscovite and quartz,whereas reaction (2) gives maximum water pressures for the disappearanceof cordierite from these rocks.  相似文献   

12.
Intrusion-hosted, low sulfide, sheeted vein systems are common within many plutons and stocks of the middle Cretaceous Tombstone-Tungsten magmatic belt, Yukon Territory, and host significant gold mineralization. Fluid inclusion characteristics of five such systems, namely Emerald Lake, Dublin Gulch, Scheelite Dome, Mike Lake, and MacTung, constrain the vein-forming fluid composition, formation temperatures and pressures, hydrothermal fluid processes, and potential fluid sources. The veins contain a wide range of fluid inclusion types. Ubiquitous type 1A and 1B inclusions are low salinity (1A: XNaCl<0.02; 1B: XNaCl<0.03), CO2-rich (1A: XCO2=0.18-1.00; 1B: XCO2=0.02-0.33). Laser Raman studies indicate that type 1A and 1B inclusions commonly contain minor CH4 (XCH4<0.09) and N2 (XN2<0.12). Type 2 inclusions are H2O-rich (XH2O=0.94-0.99), of low to moderate salinity (XNaCl=0.01-0.06), and were common at Emerald Lake and Dublin Gulch, in addition to localized type 3A halite-bearing inclusions (XNaCl=0.12-0.16). Both inclusion types post-dated the CO2-rich inclusions. Sheeted veins in the Mike Lake pluton contained coexisting type 1A and 3A and 3B (halite + sylvite) inclusions. Type 1A inclusions in all studied systems homogenized between 208 and 362 °C, type 1B ranged between 205 and 329 °C, and type 2 between 154 and 261 °C. Type 3A and 3B inclusions homogenized between 217 and 355 °C. A predominantly magmatic source for both CO2-rich and saline H2O-rich fluids is favored, with variations in trapping pressure (<1 kbar at Mike Lake, >1 kbar at Emerald Lake and Dublin Gulch, and >2 kbar at MacTung and Scheelite Dome) that potentially control fluid composition and evolution. A variety of fluid processes may have been responsible for gold precipitation including immiscibility and/or release of an evolving magmatic fluid.  相似文献   

13.
New major and trace element data are reported for a suite of basalts dredge sampled from the southern MAR (40.6-52.5°S) during cruise 9309 of the R/V Maurice Ewing and cruise 32 of the S.A. Agulhas (51-54.5°S). Samples range from aphyric to moderately porphyritic with plagioclase and olivine the dominant phenocryst and microphenocryst phases. Clinopyroxene occurs rarely as a phenocryst phase. Bulk rock and quench glass samples have major element compositions ranging from moderately primitive (Mg#=68.5) to evolved (Mg#=41.1), with a comparable range in incompatible (Zr=54-232 ppm; Nb=0.8-21 ppm) and highly compatible trace element (Ni=51-289 ppm; Cr=39-949 ppm) abundances. Incompatible trace element ratios (e.g., Zr/Nb=5.9-69; Y/Nb=0.88-29; (La/Sm)N=0.48-2.9) indicate both enriched and depleted MORB occurring along this section of the MAR, agreeing with along-ridge radiogenic isotope ratio variations delineating compositional influences of the off-axis Discovery, and a so-called LOMU-component, and ridge-centered Shona mantle plumes (Douglass et al. 1999; Douglass and Schilling 2000). Major- and trace-element variations, particularly decreasing CaO/Al2O3 ratios and relatively constant Sc abundances with decreasing Mg# suggest crystallization of olivine, plagioclase, and clinopyroxene in most of these basalts prior to eruption. Liquid-lines-of-descent, derived from forward modeling of various reasonable parental magma compositions at pressures between 1 atm-8 kbar, confirm clinopyroxene as an early crystallizing phase, along with olivine and plagioclase, and its pivotal role in the compositional evolution of these basalts. Pressures deduced using algorithms of Grove et al. 1992 and Michael and Cornell 1998 indicate crystallization beneath ridge segments unaffected by mantle plumes occurred predominately at depths of 3-6 kbar (10-20 km). Crystallization beneath ridge segments affected by the Discovery and Shona mantle plumes occurred over a larger pressure range (1 atm-~7 kbar), but predominantly at crustal depths (1 atm-3 kbar), possibly due to more constant magma fluxes and higher temperatures in the crust and lithosphere in these regions favoring the existence and longevity of subaxial magma chambers.  相似文献   

14.
Partial melting and retrogression have been recognized in high-grade metapelites of the Tatra Mountains, Western Carpathians (Slovakia) related to exhumation during Variscan orogeny. Reaction textures and phase equilibria define a clockwise P-T path. The prograde metamorphism from ca 600 °C and 9–10 kbar to >700 °C at 11–12 kbar resulted in muscovite dehydration-melting in the kyanite stability field. Further heating at decreasing pressure led to the dehydration-melting of biotite at >750 °C in the sillimanite stability field. This was followed by nearly isothermal decompression down to 4–5 kbar, producing cordierite and some additional melt. Later nearly isobaric cooling led to melt crystallization and sub-solidus retrogression. CO2-N2 fluids (5–30 mol. % N2) were generated at pressures <6 kbar by interaction between the melt-derived water and graphite at oxidizing conditions.  相似文献   

15.
The enthalpy of drop-solution in molten 2PbO·B2O3 of synthetic and natural lawsonite, CaAl2(Si2O7)(OH)2·H2O, was measured by high-temperature oxide melt calorimetry. The enthalpy of formation determined for the synthetic material is (fHOxides=-168.7Dž.4 kJ mol-1, or (fH0298=-4,872.5dž.0 kJ mol-1. These values are in reasonable agreement with previously published data, although previous calorimetric work yielded slightly more exothermic data and optimisation methods resulted in slightly less exothermic values. The equilibrium conditions for the dehydration of lawsonite to zoisite, kyanite and quartz/coesite at pressures and temperatures up to 5 GPa and 850 °C were determined by piston cylinder experiments. These results, other recent phase equilibrium data, and new calorimetric and thermophysical data for lawsonite and zoisite, Ca2Al3(SiO4)(Si2O7)O(OH), were used to constrain a mathematical programming analysis of the thermodynamic data for these two minerals in the chemical system CaO-Al2O3-SiO2-H2O (CASH). The following data for lawsonite and zoisite were obtained: (fH0298 (lawsonite)=-4,865.68 kJ mol-1 , S0298 (lawsonite)=229.27 J K-1 mol-1 , (fH0298 (zoisite)=-6,888.99 kJ mol-1 , S0298 (zoisite)=297.71 J K-1 mol-1 . Additionally, a recalculation of the bulk modulus of lawsonite yielded K=120.7 GPa, which is in good agreement with recent experimental work.  相似文献   

16.
The Mallery Lake area contains pristine examples of ancient precious metal-bearing low-sulfidation epithermal deposits. The deposits are hosted by rhyolitic flows of the Early Proterozoic Pitz Formation, but are themselves apparently of Middle Proterozoic age. Gold mineralization occurs in stockwork quartz veins that cut the rhyolites, and highest gold grades (up to 24 g/t over 30 cm) occur in the Chalcedonic Stockwork Zone. Quartz veining occurs in two main types: barren A veins, characterized by fine- to coarse-grained comb quartz, with fluorite, calcite, and/or adularia; and mineralized B veins, characterized by banded chalcedonic silica and fine-grained quartz, locally intergrown with fine-grained gold or electrum. A third type of quartz vein (C), which crosscuts B veins at one locality, is characterized by microcrystalline quartz intergrown with fine-grained hematite and rare electrum. Fluid inclusions in the veins occur in two distinct assemblages. Assemblage 1 inclusions represent a moderate temperature (Th=150 to 220 °C), low salinity (~1 eq. wt% NaCl, with trace CO2), locally boiling fluid; this fluid type is found in both A and B veins and is thought to have been responsible for Au-Ag transport and deposition. Assemblage 2 inclusions represent a lower temperature (Th=90 to 150 °C), high salinity calcic brine (23 to 31 wt% CaCl2-NaCl), which occurs as primary inclusions only in the barren A veins. Assemblage 1 and 2 inclusions occur in alternating quartz growth bands in the A-type veins, where they appear to represent alternating fluxes of dilute fluid and local saline groundwater. No workable primary fluid inclusions were observed in the C veins. The A-vein quartz yields '18O values from 8.3 to 14.5‰ (average=10.9ǃ.7‰ [1C], n=30), whereas '18O values for B-vein quartz range from 11.2 to 14.0‰ (average=13.0ǂ.9‰, n=12). Calculated '18OH2O values for the dilute mineralizing fluid from B veins range from -2.6 to 0.2‰ (average=-0.8ǂ.9‰, n=12) and are consistent with a dominantly meteoric origin. No values could be calculated for the brine, however, because all A-vein quartz samples contain mixed fluid inclusion populations. However, the fact that A-vein quartz samples extend to lower '18O values than the B veins suggests that the brine had a lighter isotopic signature relative to the dilute fluid. Hydrogen isotopic ratios of fluid inclusion waters extracted from eleven quartz samples of both vein types range from 'DFI=-56 to -134‰, but show no particular correlation with vein type. In most respects, the mineralogical and fluid characteristics of the Mallery Lake system are comparable to those of Phanerozoic low-sulfidation deposits, and although the presence of high salinity brines is unusual in such deposits, it is not unknown (e.g., Creede, Colorado). In addition, one of the few other examples of well-preserved, Precambrian, low-sulfidation epithermal deposits, from the Central Pilbara tectonic zone, Australia, contains a similarly bimodal fluid assemblage. The significance of these saline brines is not clear, but from this study we infer that they were not directly involved with Au-Ag transport or deposition.  相似文献   

17.
The compressibility and structure of a 2M1 paragonite with composition [Na0.88K0.10Ca0.01Ba0.01] [Al1.97Ti0.007Fe0.01Mn0.002Mg0.006]Si3.01Al0.99O10OH2 were determined at pressures between 1 bar and 41 kbar, by single crystal X-ray diffraction using a Merrill-Bassett diamond anvil cell. Compressibility turned out to be largely anisotropic, linear compressibility coefficients parallel to the unit cell edges being βa=3.5(1)·10?4, βb=3.6(1)·10?4, βc=8.3(3)·10?4 kbar?1 (βa:βb·βc=1:1028:2.371). The isothermal bulk modulus, calculated as the reciprocal of the mean compressibility of the cell volume, was 650(20) kbar. The main features of the deformation mechanism resulting from structural refinements at pressures of 0.5, 25.4, 40.5 kbar were: –?variation in sheet thickness, showing that compression of the c parameter was mainly due to the interlayer thickness reduction from 3.07 Å at 0.5 kbar to 2.81 Å at 40.5 kbar; –?the compressibility of octahedra was greater than that of tetrahedra, the dimensional misfit between tetrahedral and octahedral sheets increased with P, so that tetrahedral rotation angel α increased from 15° at 0.5 kbar to 21.6° at 40.5 kbar; –?the basal surface corrugation (Δz) of the tetrahedral layer, due to the different dimensions of M1 and M2 octahedra and to the octahedral distortion, decreased with P (Δz=0.19 and 0.12 Å at 0.5 and 40.5 kbar respectively). Comparison of the new data on paragonite with those of a K-muscovite and a Na-rich muscovite (Comodi and Zanazzi 1995) revealed a clear trend toward decreasing of compressibility when Na substitutes for K atoms in the interlayer sites.  相似文献   

18.
The beginning of melting in the system Qz-Or-Ab-An-H2 O was experimentally reversed in the pressure range kbar using starting materials made up of mixtures of quartz and synthetic feldspars. With increasing pressure the melting temperature decreases from 690° C at 2 kbar to 630° C at 17 kbar in the An-free alkalifeldspar granite system Qz-Or-Ab-H2O. In the granite system Qz-Or-Ab-An-H2O the increase of the solidus temperature with increasing An-content is only very small. In comparison to the alkalifeldspar granite system the solidus temperature increases by 3° C (7° C) if albite is replaced by plagioclase An 20 (An 40). The difference between the solidus temperatures of the alkalifeldspar granite system and of quartz — anorthite — sanidine assemblages (system Qz-Or-An-H2O) is approximately 50° C. With increasing water pressures plagioclase and plagioclase-alkalifeldspar assemblages become unstable and are replaced by zoisite+kyanite+quartz and zoisite+muscovite-paragonitess +quartz, respectively. The pressure stability limits of these assemblages are found to lie between 6 and 16 kbar at 600° C. At high water pressures (10–18 kbar) zoisite — muscovite — quartz assemblages are stable up to 700 and 720° C. The solidus curve of this assemblage is 10–20° C above the beginning of melting of sanidine — zoisite — muscovite — quartz mixtures. The amount of water necessary to produce sufficient amounts of melt to change a metamorphic rock into a magmatic looking one is only small. In case of layered migmatites it is shown that 1 % of water (or even less) is sufficient to transform portions of a gneiss into (magmatic looking) leucosomes. High grade metamorphic rocks were probably relatively dry, and anatectic magmas of granitic or granodioritic composition are usually not saturated with water.  相似文献   

19.
Metabasaltic rocks in the Klamath Mountains of California with ‘komatiitic’ major element concentrations were investigated in order to elucidate the origin of the magnesian signature. Trace-element concentrations preserve relict igneous trends and suggest that the rocks are not komatitic basalts, but immature arc rocks and within-plate alkalic lavas. Correlation of ‘excess’ MgO with the volume per cent hornblende (±clinopyroxene) suggests that the presence of cumulus phases contributes to the MgO-rich compositions. Early submarine alteration produced regional δ18O values of +10±1.5%° and shifts in Al2O3, Na2O, and K2O concentrations. Regional metamorphic grade in the study area varies from biotite-zone greenschist facies (350–550°C, c. 3 kbar) southward to prehnite–actinolite facies (200–400°C, ≤3 kbar), but little isotopic or elemental change occurred during the regional recrystallization. The greenschist facies assemblage is actinolitic hornblende + phengite + epidote + sodic plagioclase + microcline + chlorite + titanite + hematite + quartz in Ti-poor metabasaltic rocks; in addition to these phases biotite is present in Ti-rich analogues. Lower grade greenstones contain prehnite and more nearly stoichiometric actinolite. The moderate to low pressures of regional metamorphism are compatible with P–T conditions in a magmatic arc. Later contact metamorphism at 2–2.9±0.5 kbar and at peak temperatures approaching 600° C around the English Peak and Russian Peak granodiorites produced 3–4–km-wide aureoles typified by gradual, systematic increases in the pargasite content of amphibole, muscovite content of potassic white mica, and anorthite content of plagioclase compositions. Metasomatism during contact metamorphism produced further increases in bulk-rock δ18OSMOW of as much as +6%°. Thus, the unusually MgO-rich nature of the Sawyers Bar rocks may be attributed at least partly to metasomatism and the presence of magnesian cumulus phases.  相似文献   

20.
Reversals for the reaction 2 annite+3 quartz=2 sanidine+3 fayalite+2 H2O have been experimentally determined in cold-seal pressure vessels at pressures of 2, 3, 4 and 5?kbar, limiting annite +quartz stability towards higher temperatures. The equilibrium passes through the temperature intervals 500–540°?C (2?kbar), 550–570°?C (3?kbar), 570–590°?C (4?kbar) and 590–610°?C (5?kbar). Starting materials for most experiments were mixtures of synthetic annite +fayalite+sanidine+quartz and in some runs annite+quartz alone. Microprobe analyses of the reacted mixtures showed that the annites deviate slightly from their ideal Si/Al ratio (Si per formula unit ranges between 2.85 and 2.92, AlVI between 0.06 and 0.15). As determined by Mössbauer spectroscopy, the Fe3+ content of annite in the assemblage annite+fayalite +sanidine+quartz is around 5–7%. The experimental data were used to extract the thermodynamic standard state enthalpy and entropy of annite as follows: H 0 f,?Ann =?5125.896±8.319 [kJ/mol] and S 0 Ann=432.62±8.89 [J/mol/K] (consistent with the Holland and Powell 1990 data set), and H 0 f,Ann =?5130.971±7.939 [kJ/mol] and S 0 Ann=424.02±8.39 [J/mol/K] (consistent with the TWEEQ data base, Berman 1991). The preceeding values are close to the standard state properties derived from hydrogen sensor data of the redox reaction annite=sanidine+magnetite+H 2 (Dachs 1994). The experimental half-reversal of Eugster and Wones (1962) on the annite +quartz breakdown reaction could not be reproduced experimentally (formation of annite from sanidine+fayalite+quartz at 540°?C/1.035?kbar/magnetite-iron buffer) and probable reasons for this discrepancy remain unclear. The extracted thermodynamic standard state properties of annite were used to calculate annite and annite+quartz stabilities for pressures between 2 and 5?kbar.  相似文献   

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