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1.
Discrimination between primary magmatic biotites, reequilibrated biotites and neoformed biotites 总被引:11,自引:0,他引:11
Hassane Nachit Abderrahmane Ibhi El Hassan Abia Mohcine Ben Ohoud 《Comptes Rendus Geoscience》2005,337(16):367
The ternary diagram TiO2–FeO*–MgO (FeO* = FeO + MnO) is proposed as a quantitative objective tool for distinguishing between primary magmatic biotites and those that are more or less reequilibrated, or possibly neoformed, by or within a hydrothermal fluid. The limit of the domains of the primary magmatic biotites, the reequilibrated biotites and the neoformed biotites were determined on the basis of optical, paragenetic and chemical criteria. To cite this article: H. Nachit et al., C. R. Geoscience 337 (2005). 相似文献
2.
Summary
Quartz-tourmaline vein-hosting rocks of the Okote area belong to the Neoproterozoic Adola Belt. Metasomatic auriferous quartz-tourmaline
veins occur in ductile N–S trending, sinistral shear zones. These veins commonly contain quartz, carbonates, and tourmaline,
with minor pyrite, and accessory chalcopyrite, pyrrhotite, and gold. Tourmaline forms isolated euhedral crystals in the fracture
surfaces of quartz carbonate veins. Many of the tourmaline crystals are optically zoned with a bluish core and a bluish to
brown rim. Electron microprobe analyses show that the tourmalines comprise an intermediate dravite-schorl solid solution with
a mean FeO/(FeO + MgO) = 0.47. Abrupt transitions between the colour zones within single tourmaline crystals are accompanied
by relative variations in the FeO/(FeO + MgO) ratios. The tourmaline separates indicate that the tourmalines contain highly
variable average contents of trace elements. Chondrite-normalized rare earth element (REE) abundances of tourmaline separates
from auriferous veins show LREE-enriched to LREE-depleted patterns with negative to positive Eu anomalies and a flat, near-chondritic
HREE pattern. The auriferous quartz-tourmaline veins have LREE-enriched patterns without a Eu anomaly and a flat HREE pattern,
but tourmaline-free gold-quartz veins have very low REE contents and LREE-depleted patterns also without Eu anomalies. The
FeO/(FeO + MgO) ratios, major and trace element compositions, and the types of wall-rock alteration are used to suggest that
the sources of boron are dominantly metamorphic (dehydration and devolatilization processes), but do not totally exclude the
possibility of a magmatic source. The occurrences of high-grade gold associated with tourmaline make tourmaline a valuable
prospecting guide for hydrothermal gold mineralization in the Adola Belt, southern Ethiopia.
Received November 17, 1999; revised version accepted July 23, 2001 相似文献
3.
Sandow Mark Yidana 《Environmental Geology》2009,57(4):789-796
Surface water resources play a crucial role in the domestic water delivery system in Ghana. In addition, sustainable food
production is based on the quality and quantity of water resources available for irrigation purposes to supplement rain-fed
agricultural activities in the country. The objective of this research was to determine the main controls on the hydrochemistry
of surface water resources in the southern part of Ghana and assess the quality of water from these basins for irrigation
activities in the area. R-mode factor and cluster analyses were applied to 625 data points from 6 river basins in southern Ghana after the data had
been log transformed and standardized for homogeneity. This study finds that surface water chemistry in the south is controlled
by the chemistry of silicate mineral weathering, chemistry of rainfall, fertilizers from agricultural activities in the area,
as well as the weathering of carbonate minerals. A Gibb’s diagram plotted with total dissolved solids (TDS) on the vertical
axis against (Na+ + K+)/(Ca2+ + K+ + Na+) on the horizontal axis indicates that rock weathering plays a significant role in the hydrochemistry. Activity diagrams
for the CaO–Na2O–Al2O–SiO2–H2O and CaO–MgO–Al2O3–SiO2–H2O systems suggest that kaolinite is the most stable clay mineral phase in the system. In addition, an assessment of the irrigation
quality of water from these basins suggests that the basins are largely low sodium—low to medium salinity basins, delivering
water of acceptable quality for irrigation purposes. 相似文献
4.
Anthony C. Withers Marc M. Hirschmann 《Contributions to Mineralogy and Petrology》2008,156(5):595-605
Olivine crystals were grown in the presence of a hydrous silicate fluid during multi-anvil experiments at 8 GPa and 1,000–1,600°C.
Experiments were conducted both in a simple system (FeO–MgO–SiO2–H2O) and in a more complex system containing additional elements (CaO–Na2O–Al2O3–Cr2O3–TiO2–FeO–MgO–SiO2–H2O). Silica activity was buffered by the presence of either pyroxene (high a
SiO2) or ferropericlase (low a
SiO2), and was buffered by the presence of Ni + NiO or Fe + FeO, or constrained by the presence of Fe2O3. Raman spectroscopy was used to identify pyroxene polymorphs in the run products. Clinoenstatite was present in the 1,000°C
experiment, and enstatite in experiments at 1,400–1,520°C. The H2O content of olivine was measured using secondary ion mass spectroscopy, and infrared spectroscopy was used to investigate
the nature of hydrous defects. The H2O storage capacity of olivine decreases with increasing temperature at 8 GPa. In contrast to previous experimental results
at ≤2 GPa, no significant effect of varying oxygen fugacity is evident, but H2O storage capacity is enhanced under conditions of low silica activity. No significant growth of low wavenumber (<3,400 cm−1) peaks, generally associated with high at low pressure, was observed in the FTIR spectra of olivine from the high experiments. Our experiments show that previous high pressure H2O storage capacity measurements for olivine synthesized under more oxidizing conditions than the Earth’s mantle are not likely
to be compromised by the of the experiments. However, the considerable effect of temperature on H2O storage capacity in olivine must be taken into account to avoid overestimation of the bulk upper mantle H2O storage capacity. 相似文献
5.
Dexter Perkins Elizabeth Y. Anthony 《Contributions to Mineralogy and Petrology》2011,162(6):1139-1157
In peridotites, olivine, clinopyroxene, and orthopyroxene are complex solid solutions with wide stability fields. Depending
mostly on bulk composition and pressure, these minerals may be accompanied by plagioclase (low pressure), spinel (moderate
pressure), or garnet (high pressure), resulting in 4-phase and rarer 5-phase assemblages. Although a particular mineral assemblage
is stable over a range of P–T, the compositions of the individual minerals vary with changing P–T conditions. Application
of standard geothermobarometers to olivine–clinopyroxene–orthopyroxene–spinel peridotites is problematic. An alternative approach
is to use a bulk rock composition to calculate equilibrium phase diagrams to determine the conditions under which a particular
assemblage is stable. This requires consideration of the 7-component system SiO2–Al2O3–Cr2O3–FeO–MgO–CaO–Na2O, internally consistent thermodynamic data for end members, and reliable mixing models for all mineral solutions. Experimental
studies in simpler systems, and solution models from the literature, permit derivation of multicomponent thermodynamic mixing
models for the key minerals. The models, when applied to xenoliths from Kilbourne Hole, constrain P and T of equilibration
and are less sensitive to mineral compositional variations, or uncertainty in activity models, than standard thermobarometry.
Our modeling provides the first tightly constrained pressure estimates for Kilbourne Hole, placing the xenoliths in the spinel
stability field at depths (30–45 km) that correspond to the uppermost mantle beneath the Rio Grande Rift. The fine-grained
equigranular lherzolite, porphyroclastic lherzolite, and some harzburgite-dunite specimens equilibrated at average conditions
of 11.5 Kbar-930°C, 12 Kbar-990°C, and 13 Kbar-1,080°C, respectively. The mantle beneath the Rio Grande Rift is layered; the
fine-grained equigranular lherzolite derives from relatively shallow depth (35 km average), and the porphyroclastic lherzolite
from slightly deeper levels. Lying 5–10 km beneath both lherzolites, the harzburgite-dunite represents a depth where melt
extraction has significantly altered mantle chemistry and where local thermodynamic equilibrium has not been maintained. 相似文献
6.
The petrology of five phenocryst-poor (2–5%) andesites and dacites, all of which were erupted from different short-lived,
monogenetic vents, is compared to that of phenocryst-rich (10–25%) andesites erupted from the adjacent stratovolcano, Volcán
Tequila, in the Mexican arc. Despite differences in phenocryst abundances, these magmas have comparable phase assemblages
(plagioclase + orthopyroxene + titanomagnetite + ilmenite + apatite ± augite ± hornblende), and similarly wide variations
in phenocryst compositions, coupled to complex zoning patterns. For the phenocryst-poor lavas, equilibrium pairs of two Fe–Ti
oxides lead to a narrow range of calculated temperatures for each sample that range from 934 (±24) to 1,073 (±6)°C and oxygen
fugacities that range from +0.1 to +0.7 log units relative to the Ni–NiO buffer. Application of the plagioclase-liquid hygrometer
to each sample at these calculated temperatures leads to maximum melt water concentrations of 4.6–3.1 wt% during plagioclase
crystallization, indicating that the magmas were fluid saturated at depths ≥6.4–4.5 km. There is a wide, continuous range
in the composition of plagioclase (≤44 mol% An) and orthopyroxene (≤16% Mg#) phenocrysts in each sample, which is consistent
with a loss of dissolved water (≤2.8 wt%) from the melt phase during degassing as the magmas ascended rapidly to the surface.
Evidence is presented that shows the effect of dissolved water is to reduce the activity of MgO relative to FeO in the melt
phase, which indicates that degassing will also affect the Mg# of pyroxene phenocrysts, with higher melt water concentrations
favoring Fe-rich pyroxene. Both plagioclase and orthopyroxene commonly display diffusion-limited growth textures (e.g., skeletal
and hopper crystals, large interior melt hollows, and swallow tails), which are consistent with large undercoolings produced
by degassing-induced crystallization. Therefore, degassing is proposed as a possible cause for the phenocryst compositional
diversity documented in the phenocryst-poor andesite and dacite lavas erupted from peripheral vents, including the coexistence
of normally zoned plagioclase and reversely zoned orthopyroxene. Degassing-induced crystallization may also explain some of
the phenocryst complexity in crystal-rich andesites erupted from large stratovolcanoes, including Volcán Tequila. 相似文献
7.
Spinel pyroxenites occur locally as clasts in polygenic breccias from the Late Cretaceous sedimentary mélanges of the Northern
Apennine (Italy). They are of cumulus origin and formed in the deep crust by early precipitation of clinopyroxene and minor
olivine and late crystallisation of orthopyroxene, spinel, Ti-pargasite and sulphides. Pyroxenites underwent high-temperature
(~850°C) subsolidus re-equilibration and ductile deformation with development of mylonitic bands made of clinopyroxene, orthopyroxene,
Ti-pargasite and spinel. U–Pb geochronology on zircons revealed the occurrence of inherited grains of Early Proterozoic to
Late Devonian age. The inherited zircons are locally rimmed by recrystallised zircon domains. The oldest rims yield a mean
concordia U–Pb age at 306 ± 8 Ma, which is considered to date the emplacement of the pyroxenites, in the framework of the
post-Variscan lithospheric extension. The incompatible element compositions of calculated melts in equilibrium with clinopyroxenes
from the pyroxenites are characterised by Ba, Nb, LREE and Sr enrichment relative to N-MORB. The depleted Nd isotopic signature
of the pyroxenites (initial εNd values of +5.3 to +6.1) may be thus linked to primary magmas produced by low degrees of melting of asthenospheric mantle.
In addition, the pyroxenites locally record the infiltration of plagioclase-saturated hydrous melts, most likely evolved through
fractional crystallisation and enriched in highly incompatible elements, within the clinopyroxene-dominated crystal mush.
A thermal event in Late Permian–Middle Triassic caused the partial resetting of zircon U–Pb system. 相似文献
8.
The influence of water on melting of mantle peridotite 总被引:47,自引:8,他引:39
This experimental study examines the effects of variable concentrations of dissolved H2O on the compositions of silicate melts and their coexisting mineral assemblage of olivine + orthopyroxene ± clinopyroxene ± spinel ± garnet.
Experiments were performed at pressures of 1.2 to 2.0 GPa and temperatures of 1100 to 1345 °C, with up to ∼12 wt% H2O dissolved in the liquid. The effects of increasing the concentration of dissolved H2O on the major element compositions of melts in equilibrium with a spinel lherzolite mineral assemblage are to decrease the
concentrations of SiO2, FeO, MgO, and CaO. The concentration of Al2O3 is unaffected. The lower SiO2 contents of the hydrous melts result from an increase in the activity coefficient for SiO2 with increasing dissolved H2O. The lower concentrations of FeO and MgO result from the lower temperatures at which H2O-bearing melts coexist with mantle minerals as compared to anhydrous melts. These compositional changes produce an elevated
SiO2/(MgO + FeO) ratio in hydrous peridotite partial melts, making them relatively SiO2 rich when compared to anhydrous melts on a volatile-free basis. Hydrous peridotite melting reactions are affected primarily
by the lowered mantle solidus. Temperature-induced compositional variations in coexisting pyroxenes lower the proportion of
clinopyroxene entering the melt relative to orthopyroxene. Isobaric batch melting calculations indicate that fluid-undersaturated
peridotite melting is characterized by significantly lower melt productivity than anhydrous peridotite melting, and that the
peridotite melting process in subduction zones is strongly influenced by the composition of the H2O-rich component introduced into the mantle wedge from the subducted slab.
Received: 7 April 1997 / Accepted: 9 January 1998 相似文献
9.
Calc-silicate granulites from Rayagada, north-central sector of Eastern Ghats granulite belt show a wide range of mineral
assemblages and chemical compositions, which can be grouped as Gr. I (grossular- rich garnet-wollastonite-scapolite-calcite-clinopyroxene),
Gr. II (andradite-rich garnet-scapolite-calcite-clinopyr- oxene), and Gr. III (scapolite-calcite-clinopyroxene-plagioclase)
assemblages. Petrographic features suggest the following several reactions in the CaO–Al2O3–SiO2-vapor system: Mei+4Wo+Cal=3Grs+Qtz +2CO2, Mei+3Wo+2Cal=3Grs+CO2, Mei= 3An+Cal, Wo+CO2=Cal+Qtz, Mei+5Wo =3Grs+2Qtz+CO2, An+Wo=Grs+Qtz, Mei+ 5Cal+3Qtz=3Grs+6CO2, and the following reactions in the CaO–FeO–MgO–Al2O3–SiO2-vapor system: Cpxss+Scp+Wo=Grtss+Qtz+CO2, 4Hd+ 2Cal+O2=2Adr+2Qtz+2CO2, Cpxss+Scp= Grtss+Cal+Qtz. These reactions have been used to estimate peak T-X
CO2 condition for these granulites. A maximum temperature of ∼920 °C has been calculated at an estimated pressure of 9 kbar.
A T-X
CO2 diagram shows an isobaric cooling from ∼920 °C to ∼815 °C. A range of X
CO2 (0.50 at 920 °C to 0.25 at 815 °C) has been observed for Gr. I calc-silicate granulites based on the reaction sequences including
coronal garnet-forming reactions. This sequence is suggestive of internal fluid buffering rather than external fluid influx
and the differences in X
CO2 conditions has been thought to be due to local buffering of fluid phases. Group II and Gr. III calc-silicate granulites,
on the other hand, exhibit relatively lower temperature conditions.
Received: 11 September 1995/Accepted: 20 June 1996 相似文献
10.
A unique sapphirine + orthopyroxene + quartz granulite from Mt. Riiser-Larsen in the Tula Mountains of Enderby Land, East
Antarctica, preserves two generations of coarse and texturally equilibrated orthopyroxene and sapphirine coexisting with quartz.
Initial subhedral orthopyroxene porphyroblasts retain core compositions enriched in Al2O3 (12.2 ± 0.5 wt%) compared with their rims and finer orthopyroxene (9.6 ± 0.5 wt% Al2O3) that forms granoblastic textures with sapphirine. Sapphirine and quartz also form symplectites on and along cleavage planes
within orthopyroxene. These compositional and textural features are consistent with the reaction [2MgAl2SiO6=Mg2Al4 SiO10 + SiO2] leading to the formation of sapphirine + quartz at the expense of aluminous orthopyroxene. Calculations in the MAS and FMAS
systems and theoretical considerations involving the phases enstatite, sapphirine, sillimanite, quartz and cordierite indicate
that the reaction above progresses from left to right with decreasing temperature in the orthopyroxene + sapphirine + quartz
field, at pressures of ca. 8–10 kbar. The temperature difference required to account for the ca. 2.5–3 wt% decrease in Al2O3 in orthopyroxene is at least 60–80 °C, and implies peak temperatures for the initial assemblage of at least 1120 °C if the
second granoblastic assemblage equilibrated at 1040 °C, the P–T conditions required by the sapphirine + quartz association
and other P–T-sensitive assemblage indicators in the Napier Complex. It is not possible to distinguish whether the two assemblages
are simply related by cooling and re-equilibration or reflect a polyphase evolution involving the superposition of a second
UHT event on an earlier, even higher temperature, UHT metamorphism. Preliminary thermodynamic modelling of the reaction above
incorporating the observed range in orthopyroxene Al2O3 zoning indicates that present estimates for the entropy of high-temperature sapphirine are potentially too high by 15–18%
compared with sapphirine entropy estimates that are consistent with MAS system experiments. The Mt. Riiser-Larsen sapphirine–quartz
rocks preserve the first definitive record of regional metamorphic temperatures in excess of 1120 °C in the Napier Complex,
or indeed any UHT granulite terrain worldwide. Similarly high peak temperatures may be retrieved from detailed studies of
sapphirine–quartz granulites from other regions, further expanding the thermal realm of crustal metamorphism, but progress
will critically depend on the experimental acquisition of new entropy data for sapphirine.
Received: 3 September 1998 / Accepted: 8 November 1999 相似文献
11.
An experimental investigation of antigorite dehydration in natural silica-enriched serpentinite 总被引:1,自引:0,他引:1
José Alberto Padrón-Navarta Jörg Hermann Carlos J. Garrido Vicente López Sánchez-Vizcaíno María Teresa Gómez-Pugnaire 《Contributions to Mineralogy and Petrology》2010,159(1):25-42
Piston cylinder experiments were performed to constrain the pressure and temperature conditions for two high-pressure antigorite
dehydration reactions found in silica-enriched serpentinites from Cerro del Almirez (Nevado–Filábride Complex, Betic Cordillera,
southern Spain). At 630–660°C and pressures greater than 1.6 GPa, antigorite first reacts with talc to form orthopyroxene ± chlorite + fluid.
We show that orthopyroxene + antigorite is restricted to high-pressure metamorphism of silica-enriched serpentinite. This
uncommon assemblage is helpful in constraining metamorphic conditions in cold subduction environments, where antigorite serpentinites
have no diagnostic assemblages over a large pressure and temperature range. The second dehydration reaction leads to the breakdown
of antigorite to olivine + orthopyroxene + chlorite + fluid. The maximum stability of antigorite is found at 680°C at 1.9 GPa,
which also corresponds to the maximum pressure limit for tremolite coexisting with olivine + orthopyroxene. The high aluminium
(3.70 wt% Al2O3) and chromium contents (0.59 wt% Cr2O3) of antigorite in the investigated starting material is responsible for the expansion of the serpentinite stability to 60–70°C
higher temperatures at 1.8 GPa than the antigorite stability calculated in the Al-free system. The antigorite from our study
has the highest Al–Cr contents among all experimental studies and therefore likely constraints the maximum stability of antigorite
in natural systems. Comparison of experimental results with olivine–orthopyroxene–chlorite–tremolite assemblages outcropping
in Cerro del Almirez indicates that peak metamorphic conditions were 680–710°C and 1.6–1.9 GPa. 相似文献
12.
Susanne M. Straub Ana Lillian Martin-Del Pozzo 《Contributions to Mineralogy and Petrology》2001,140(4):487-510
Tephra lapilli from six explosive eruptions between April 1996 and February 1998 at Popocatepetl volcano (=Popo) in central
Mexico have been studied to investigate the causes of magma diversification in thick-crusted volcanic arcs. The tephra particles
are sparsely porphyritic (≈5 vol%) magnesian andesites (SiO2=58–65 wt%; MgO=2.6–5.9 wt%) that contain phenocrysts of NiO-rich (up to 0.67 wt% NiO) magnesian olivine (Fo89–91 cores) with inclusions of Cr-spinel (cr#=59–70), orthopyroxene (mg#=63–76), clinopyroxene (mg#=68–86), intermediate to sodic
plagioclase (An33–66), and traces of amphibole. Major and trace element systematics indicate magma mixing. The liquid mg#melt ratios inferred from the ferromagnesian phenocrysts suggest the existence of a mafic (mg#melt ≈ 72–76) and an evolved component magma (mg#melt ≈ 35–40). These component magmas form a hybrid magnesian andesite with an intermediate range of mg#melt=50–72. The mafic end member (mg#melt ≈ 72–75) is saturated with olivine and spinel and crystallizes at temperatures ≈1170–1085 °C with oxygen fugacities close
to the fayalite–magnetite–quartz buffer and elevated water contents of several wt% H2O. A likely location of crystallization is at lower crustal levels, possibly at the Moho. Olivine is followed by high-mg#
clinopyroxene which could start to crystallize during magma ascent. At depths of ≈4 to 13 km, the mafic magma mixes with an
evolved composition containing low-mg# clino- and orthopyroxene and plagioclase at a temperature of ≈950 °C. The repetitive
ascent of batches of mafic magmas spaced days to weeks apart implies multiple episodes of crystallization and magma mixing.
The tephra is similar to the Popo magnesian andesites, suggesting similar generic processes for the common lavas of the volcano.
The advantage of the tephra is that it can be used to reconstruct the composition of the mafic magma. Building on the elemental
systematics of the tephra and a comparison to the near-primary basalts from the surrounding monogenetic fields, we infer that
the Popo mafic end member is a magnesian andesite with variable, but high SiO2 contents of ≈55–62 wt% and near-primary characteristics, such as high-mg#melt of 72–75, FeO*/MgO ratios <1 (if extrapolated to an mg#melt of 72–75), and high Ni contents (=200 ppm Ni). This model implies that the typical elemental signature of the Popo andesites,
such as the low CaO, Al2O3, FeO*, high Na2O contents, and the depletion in high-field strength elements (e.g., P, Zr, Ti), are mantle source phenomena. Thus, determining
the elemental budget of the magnesian andesite, as it is prior to the modifications by crustal differentiation, is central
to quantifying the subcrustal mass fluxes beneath Popo.
Received: 13 December 1999 / Accepted: 11 August 2000 相似文献
13.
Amel Derridj Khadidja Ouzegane Jean-Robert Kienast Djelloul Belhaï 《Journal of African Earth Sciences》2003,37(3-4):257
Garnet pyroxenite from high pressure granulite facies occurs with different mineral assemblages which involve garnet, clinopyroxene, orthopyroxene, plagioclase, amphibole and quartz with spinel developing as symplectite with orthopyroxene and plagioclase in large cracks. Three successive parageneses have been identified. The primary assemblage is characterised by the presence of quartz. The second assemblage involves orthopyroxene–plagioclase–hornblende symplectite, and the third assemblage is characterised by the development of spinel in symplectites with orthopyroxene and plagioclase. Using THERMOCALC (V2.7), a quantitative pseudosection in the system CaO–FeO–MgO–Al2O3–SiO2–H2O has been calculated. The assemblage involving quartz developed at high pressure, while the assemblage involving spinel developed at lower pressure. The peak of metamorphism in Tin Begane was calculated at 860 °C and 13.5 kb with aH2O=0.2. These conditions are followed by a decrease of pressure down to 4.8 kb. 相似文献
14.
Geobarometry for Peridotites: Experiments in Simple and Natural Systems from 6 to 10 GPa 总被引:3,自引:0,他引:3
Experiments with peridotite minerals in simple (MgO–Al2O3–SiO2,CaO–MgO–SiO2 and CaO–MgO–Al2O3–SiO2)and natural systems were conducted at 1300–1500°Cand 6–10 GPa using a multi-anvil apparatus. The experimentsin simple systems demonstrated consistency with previous lowerpressure experiments in belt and piston–cylinder set-ups.The analysis of spatial variations in pyroxene compositionswithin experimental samples was used to demonstrate that pressureand temperature variations within the samples were less than0·4 GPa and 50°C. Olivine capsules were used in natural-systemexperiments with two mineral mixtures: SC1 (olivine + high-Alorthopyroxene + high-Al clinopyroxene + spinel) and J4 (olivine+ low-Al orthopyroxene + low-Al clinopyroxene + garnet). Theexperiments produced olivine + orthopyroxene + garnet ±clinopyroxene assemblages, occasionally with magnesite and carbonate-richmelt. Equilibrium compositions were derived by the analysisof grain rims and evaluation of mineral zoning. They were comparedwith our previous experiments with the same starting mixturesat 2·8–6·0 GPa and the results from simplesystems. The compositions of minerals from experiments withnatural mixtures show smooth pressure and temperature dependencesup to a pressure of 8 GPa. The experiments at 9 and 10 GPa producedandradite-rich garnets and pyroxene compositions deviating fromthe trends defined by the lower pressure experiments (e.g. higherAl in orthopyroxene and Ca in clinopyroxene). This discrepancyis attributed to a higher degree of oxidation in the high-pressureexperiments and an orthopyroxene–high-P clinopyroxenephase transition at 9 GPa. Based on new and previous resultsin simple and natural systems, a new version of the Al-in-orthopyroxenebarometer is presented. The new barometer adequately reproducesexperimental pressures up to 8 GPa. KEY WORDS: garnet; mineral equilibrium; multi-anvil apparatus; orthopyroxene; geobarometry 相似文献
15.
Former idioblastic garnet crystals of an Archaean granulite with Mg/(Mg+Fe+Mn)-ratios (= M) near 0.40 were converted to symplectitic pseudomorphs consisting mainly of cordierite (M=0.61), orthopyroxene (M=0.40), and a relic garnet with a new composition (M=0.18), during a static metamorphism which is related in time to the Vredefort event. On the basis of experimental data for the continuous reaction garnet+quartz=orthopyroxene+cordierite the conditions of metamorphism were near 5 kb, 700 °C. Orthopyroxenes crystallized initially as minute, myrmekitic grains with metastable excess Al contents up to 13 mol% Al2O3 and, through grain growth and Al exsolution, transformed into dense aggregates of coarser crystals with equilibrated compositions near 3 mol% Al2O3. In the absence of free silica hercynitic spinel appears as an additional phase that coexists with cordierite, orthopyroxene, and garnet of more magnesian compositions than in the case of silica saturation.Pseudotachylite veins crosscutting the hornfelsed granulite consist of pyroxene of variable Al contents, two feldspars, opaques, and quartz, and are finely recrystallized. There is textural and mineral chemical evidence from both the Al-contents of the pyroxenes and the Mg/Fe distribution among the phases adjacent to the pseudotachylite that these veins were emplaced late during the static metamorphism, which was probably caused by a local, deep-seated magma diapir. On this basis, the meteorite impact hypothesis for the origin of the Vredefort Structure is considered less likely than a mechanism of internal origin which can be made responsible for the close link in time and space between static metamorphism and the high strain rate deformation that produced the pseudotachylite. 相似文献
16.
Summary ?The Betroka sinistral shear belt is a major geotectonic unit in the Precambrian of southern Madagascar. It consists of migmatitic
paragneiss commonly interlayered with phlogopite-bearing diopsidite, phlogopite-humite-diopside-spinel marble, sillimanite-garnet
quartzite and syn-tectonic S-type leucogranite. H?gbomite occurs sporadically in the migmatitic paragneiss in patches of magnetite
with hercynite, and at the border of magnetite where it is in contact with hercynite, rare ilmenite, rutile and cordierite,
which contains a network of chlorite, pyrophyllite and rare corundum/diaspore. XMg = Mg/(Mg + Fe) decreases as follows: Crd > Bt > Chl > H?g > Hc. The textural relations suggest the following h?gbomite-participating
reactions:
Ti-bearing hercynite ↠ hercynite + h?gbomite (intergrown/exsolution lamellae)
ilmenite + cordierite ↠ hercynite + h?gbomite + rutile + chlorite/pyrophyllite
h?gbomite ↠ hercynite + ilmenite + corundum
The chemical composition of h?gbomite varies substantially from grain to grain in individual samples and from sample to sample,
this variation being highly dependent on the associated minerals. There is a weak zoning from core to rim in individual grains
intergrown with hercynite and also in grains at the margin of hercynite, but this zoning is overprinted by zones formed at
grain rims depending on the surrounding phases. In contact with hercynite, h?gbomite has FeO (total Fe as FeO) 27.1–28.5 wt.%,
and MgO 4.5–5.8 wt.%, and in contact with magnetite FeO 24.9–26.5 wt.%, and MgO 6.0–8.5 wt.% and the core contents are within
these values. TiO27.5–4.0 wt.% and Al2O362.0–59.0 wt.% show zonations with increase from core to rim. Estimated P-T conditions are 6.0 ± 1.0 kbar and 700 ± 100 °C
reached during a peak metamorphic stage of the Pan-African orogeny. However, the presence of diaspore with exsolved hercynite-magnetite
indicates extreme retrograde metamorphism in the decompressional central part of this shear belt of southern Madagascar.
Received January 15, 1999;/revised version accepted July 6, 1999 相似文献
Zusammenfassung ?H?gbomit in migmatitischem Paragneis von Vohidava in der Betroka Scherzone im südlichen Pr?kambrium von Madagaskar Die sinistrale Betroka Scherzone ist eine ausgepr?gte tektonische Einheit des Pr?kambriums in Süd Madagaskar. Sie besteht aus migmatitischem Paragneis, in dem Phlogopit-führender Diopsidit, Phlogopit-Humit-Diopsid-Spinell-Marmor, Sillimanit-Granat-Quarzit und syntektonischer S-Typ Granit eingelagert sind. Im migmatitischen Paragneis kommen sporadisch H?gbomit/Hercynit Nester im Magnetit vor und am Magnetitrand findet sich H?gbomit im Kontakt mit Hercynit und Cordierit, der mit einem Netzwerk aus Chlorit/Pyrophyllit gefüllt ist, sowie sporadisch mit Korund/Diaspor, Ilmenit und Rutil. XMg = Mg/(Mg + Fe) nimmt in folgender Reihung ab: Crd > Bt > Chl > H?g > Hc. Aus den texturellen Beziehungen werden folgende H?gbomit-partizipierende Reaktionen abgeleitet: Ti-führender Hercynit ↠ Hercynit + H?gbomit (verwachsen/Entmischungslamellen) H?gbomit ↠ Hercynit + Ilmenit + Korund Die chemische Zusammensetzung von H?gbomit variiert betr?chtlich von Korn zu Korn in einer Probe und von Probe zu Probe; wobei die Variation von den Kontaktmineralen abh?ngt. H?gbomit im Hercynit hat eine schwache Zonierung von Kern zum Rand. Im H?gbomit am Hercynitrand ist die Kern-Rand-Zonierung durch die von den Kontaktmineralen abh?ngige Randzusammensetzung überpr?gt. Im Kontakt zum Hercynit hat H?gbomit 27,1–28,5 Gew.% FeO (total Fe als FeO) und 4,5–5,8 Gew.% MgO und im Kontakt zu Magnetit 24,9–26,5 Gew.% FeO und 6,0–8,5 Gew.% MgO, die Kernzusammensetzung liegt zwischen den beiden Randwerten. TiO2nimmt vom Kern zu den R?ndern von 7,5 bis 4,0 Gew.% ab und Al2O3von 62,0 bis 59,0 Gew.%. Die P-T Bedingungen des Metamorphose-Peaks w?hrend der Pan-Afrikanischen Orogenese erreichten 6,0 ± 1,0 Kbar und 700 ± 100 °C. Die sp?te Bildung von Diaspor und die Hercynit-Magnetit-Entmischung weisen auf eine tiefgreifende retrograde Metamorphose im Dekompressions-Zentralbereich der Betroka-Scherzone im südlichen Madagaskar hin.
Received January 15, 1999;/revised version accepted July 6, 1999 相似文献
17.
Compositional variation and primary water contents of differentiated interstitial and included glasses in boninites 总被引:2,自引:0,他引:2
Interstitial glasses and glasses in small inclusions in Mg-rich phenocrysts of 14 boninites from New Caledonia, the Mariana
Trench, Cape Vogel and Chichi-jima were analysed by electron microprobe and the water contents measured in situ by ion microprobe.
The glasses are remarkably fresh and abundant (∼30–90 vol.%), and the phenocrysts are often skeletal with glass inclusions.
Broad-beam analyses (∼1030) of interstitial glasses and ∼180 point analyses of glass inclusions were carried out, as well
as ∼100 hydrogen analyses. Most glasses have low water-free totals, high water contents, very low MgO, and low total iron;
they are almost entirely quartzofeldspathic and with few exceptions (Q+or+ab+ an+C) lies in the range 83–96. The interstitial
glasses from New Caledonia, the Marianas and most of the glasses from Chichi-jima are dacitic, those from Cape Vogel straddle
dacitic and andesitic compositions, whereas the glasses in a highly glassy sample from Chichi-jima are high-Mg andesitic or
boninitic with up ∼9 wt% MgO, and are, with the exception of a few high-Ca boninites from Tonga, the most Mg-rich interstitial
glasses so far described in boninites. Glasses included in orthopyroxene, olivine or clinoenstatite are boninitic or high-Mg
andesitic in the highly glassy rock and dacitic to high-silica dacitic in the others. They are in general slightly more differentiated
than the interstitial glasses, because of more-extensive crystallization on the host crystal in small inclusions. The interstitial
glass compositions show a direct relationship between silica and Al2O3 and, for most glasses, alkalis, and inverse relationships between silica and CaO, FeO and MgO; alkalis and TiO2 show, however, a broad spread in values in glasses from the Marianas and New Caledonia. Included glasses show similar variations.
Water contents in interstitial glasses are ∼2 wt% for the highly glassy high-Mg andesitic glasses from Chichi-jima, ∼5.4 wt%
for the more differentiated andesitic to dacitic glasses from Cape Vogel, and ∼6.7–7.0 wt% in the most differentiated dacitic
ones from the Marianas and New Caledonia. Water contents in glass inclusions in olivine, orthopyroxene and clinoenstatite
are in the range ∼1.9–3.3 wt%. The interstitial glasses are black and not vesicular, showing that the liquids did not reach
supersaturation after eruption on or intrusion near the sea floor, or were insufficiently so to allow nucleation of water
vapour bubbles. The water is inferred to be primary and to increase strongly with crystallization in the residual liquid down
to the glass-transition T.
Received: 19 December 1994 / Accepted: 5 October 1995 相似文献
18.
N. I. Chutas E. Bates S. A. Prevec D. S. Coleman A. E. Boudreau 《Contributions to Mineralogy and Petrology》2012,163(4):653-668
Progressive leaching of plagioclase for Sr isotopes and microdrilling for Sr and Pb isotopes from grains of plagioclase and
orthopyroxene from the Critical Zone and the Lower Zone indicates that these minerals are not in isotopic equilibrium. Leaching
suggests Critical Zone plagioclase either lost Rb or had a more radiogenic Sri rim relative to the core, whereas plagioclase from an Upper Zone sample is isotopically homogeneous for Sri. Microdrilling analyses of plagioclase from the Lower and Critical Zones consistently have a higher initial 87Sr/86Sr (Sri) and a less radiogenic modeled 238U/204Pb composition (μ2) than coexisting orthopyroxene. The range of calculated Sri for plagioclase and orthopyroxene is 0.70506–0.70662(34) and 0.70290–0.70654(36), respectively. The average difference in
Sri between mineral pairs was 0.00095. The range of calculated μ2 for plagioclase and orthopyroxene is 9.42–10.30 (average 9.7) and 9.83–15.75 (average 10.1), respectively. The range of measured
208Pb/206Pb for plagioclase and orthopyroxene is 34.757–36.439(33) and 36.669–41.845(85), respectively. One orthopyroxenite without
evidence for more than one population of crystal size distribution, nonetheless had Sri = 0.70654 (36) with calculated μ2 of 10.32 for larger grains as compared with Sri = 0.70290 (32) and calculated μ2 of 9.97 for smaller grain-size fractions. Isotopic results from this study demonstrate that whole-rock isotopic data may
not provide the appropriate level of detail necessary to address some processes in the Bushveld Complex. However, systematic
changes have the potential to elucidate the timing of contamination with regard to other processes (crystal aging, compaction-driven
recrystallization, and mineral exsolution) occurring within a slowly cooled crystal–liquid–vapor mush system. 相似文献
19.
Grain growth kinetics in CaTiO3-perovskite + FeO-wüstite aggregates were studied at the conditions of T = 1223–1623 K, P = 0.1 MPa and P = 200 MPa. Starting samples were fabricated by hot-pressing mechanically mixed powders of CaTiO3 + FeO with FeO = 0%, 1%, 3%, 6%, 10%, 20% and 100% by weight in a gas-medium apparatus at 1323 K and 300 MPa for 5 h. The
increase of grain size (G) of CaTiO3 with time (t) follows a growth law: G
n
−G
n
0 = κ·t(κ=κ0exp(−(Q/RT)). Two grain growth regimes are observed at T < 1523 K and T ≥ 1523 K. For T < 1523 K, the best fits of the data to the growth law yield growth exponents of n = 2.2 ± 0.2, 3.0 ± 0.3 and 3.5 ± 0.3 for samples with FeO = 0%, 3% and 10% respectively. Under these conditions the rate
constants, κ, obey an Arrhenius relation with Q = 206 ± 35 kJ/mol and 385 ± 65 kJ/mol for samples with FeO = 3% and 10%. Grain growth of CaTiO3 becomes sluggish when FeO content exceeds 6%. For T ≥ 1523 K, the best fits of the data to the growth law yield n = 2.5 ± 0.2 for both samples with FeO = 3% and 10%. The activation energies (Q ) were determined as 71 ± 30 kJ/mol and 229 ± 45 kJ/mol for samples with FeO = 3% and 10%, respectively. The TEM observations
show a remarkable difference in the distribution and geometry of FeO below and above 1523 K: nanometer-sized particles of
FeO were observed along CaTiO3 grain boundaries in samples annealed at T < 1523 K. No FeO particles were detected along CaTiO3 grain boundaries in samples annealed at T ≥ 1523 K, but large clusters of FeO particles are observed locally indicating a fast separation of FeO from CaTiO3. Thus we conclude that the slow growth rate of CaTiO3 at T < 1523 K is due to the pinning by FeO particles at grain boundary, and that the change of grain growth kinetics in CaTiO3 at T ≥ 1523 K may relate to the separation of FeO from CaTiO3, which we interpret as due to the phase transformation of CaTiO3 at around 1523 K.
Received: 19 June 1998 / Revised, accepted: 24 March 1999 相似文献
20.
Subsolidus phase equilibria of pyroxenes in the CaOMgOFeOSiO2system have been experimentally determined at 30 and 15 kb,and mainly at 1200 °C. While phase relations at 30 kb and1200 °C are characterized only by orthopyroxene and clinopyroxene,at 15 kb and 1200 °C phase relations are complicated bythe appearance of the two-clinopyroxene miscibility gap. Thestability of Ca-free clinopyroxene was demonstrated at 15 kband 1200 °C and higher temperatures, and at 30 kb and 1450°C. The new experimental data were combined with the data from theliterature to yield a generalized phase equilibrium scheme ofpyroxenes in temperature-pressure-composition space. Thoughthe majority of naturally occuring pyroxenes can be understoodin the light of the scheme, there are a few samples not fullyconsistent with it. These are briefly mentioned. 相似文献