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1.
The partition of Ni between olivine and monosulfide-oxide liquid has been investigated at 1300–1395° C, =10–8-9–10–6.8, and =10–2.0–10–0.9, over the composition range 20–79 mol. % NiS. The product olivine compositions varied from Fo98 to Fo59 and from 0.06 to 3.11 wt% NiO. The metal/sulfur ratio of the sulfide-oxide liquid increases with increase in , decrease in , and increase in NiS content. The Ni/Fe exchange reaction has been perfectly reversed using natural olivine and pure forsterite as starting materials. The FeO and NiO contents of olivine from runs equilibrated at the same and form isobaric distributions with NiS content, which, to a first approximation, are dependent at constant temperature and total pressure on a variable term, –0.5 log ( / ). The Ni/Fe distribution coefficient (K D3) exhibits only a weak decrease from 35 to 29 with increase in from the IW buffer to close to the FMQ buffer. At values higher than FMQ, the sulfide-oxide liquid has the approximate composition (Ni,Fe)3±xS2K D358. The present K D3 vs O/(S+O) data define a trend which extrapolates to K D320 at 10 wt% oxygen in the sulfide-oxide liquid. The compositions of olivine and Ni-Cu sulfides associated with early-magmatic basic rocks and komatiites are consistent, at 1400° C, with a value of -log ( / ) of about 7.7, which is equivalent to 0.0 wt% oxygen in the hypothesized immiscible sulfide-oxide liquid. Therefore, K D3 would not be reduced significantly from the 30 to 35 range for sulfide-oxide liquids with low oxygen contents.  相似文献   

2.
Redox states of lithospheric and asthenospheric upper mantle   总被引：31，自引：7，他引：24  
The oxidation state of lithospheric upper mantle is heterogeneous on a scale of at least four log units. Oxygen fugacities ( ) relative to the FMQ buffer using the olivine-orthopyroxene-spinel equilibrium range from about FMQ-3 to FMQ+1. Isolated samples from cratonic Archaean lithosphere may plot as low as FMQ-5. In shallow Proterozoic and Phanerozoic lithosphere, the relative is predominantly controlled by sliding Fe3+-Fe2+ equilibria. Spinel peridotite xenoliths in continental basalts follow a trend of increasing with increasing refractoriness, to a relative well above graphite stability. This suggests that any relative reduction in lithospheric upper mantle that may occur as a result of stripping lithosphere of its basaltic component is overprinted by later metasomatism and relative oxidation. With increasing pressure and depth in lithosphere, elemental carbon becomes progressively refractory and carbon-bearing equilibria more important for control. The solubility of carbon in H2O-rich fluid (and presumably in H2O-rich small-degree melts) under the P,T conditions of Archaean lithosphere is about an order of magnitude lower than in shallow modern lithosphere, indicating that high-pressure metasomatism may take place under carbon-saturated conditions. The maximum in deep Archaen lithosphere must be constrained by equilibria such as EMOG/D. If the marked chemical depletion and the orthopyroxene-rich nature of Archaean lithospheric xenoliths is caused by carbonatite (as opposed to komatiite) melt segregation, as suggested here, then a realistic lower limit may be given by the H2O +C=CH4+O2 (C-H2O) equilibrium. Below C –H2O a fluid becomes CH4 rather than CO2-bearing and carbonatitic melt presumably unstable. The actual in deep Archaean lithosphere is then a function of the activities of CO2 and MgCO3. Basaltic melts are more oxidized than samples from lithospheric upper mantle. Mid-ocean ridge (MORB) and ocean-island basalts (OIB) range between FMQ-1 (N-MORB) and about FMQ +2 (OIB). The most oxidized basaltic melts are primitive island-arc basalts (IAB) that may fall above FMQ+3. If basalts are accurate probes of their mantle sources, then asthenospheric upper mantle is more oxidized than lithosphere. However, there is a wide range of processes that may alter melt relative to that of the mantle source. These include partial melting, melt segregation, shifts in Fe3+/Fe2+ melt ratios upon decompression, oxygen exchange with ambient mantle during ascent, and low-pressure volatile degassing. Degassing is not very effective in causing large-scale and uniform shifts, while the elimination of buffering equilibria during partial melting is. Upwelling graphite-bearing asthenosphere will decompress along -pressure paths approximately parallel to the graphite saturation surface, involving reduction relative to FMQ. The relative will be constrained to below the CCO equilibrium and will be a function of . Upwelling asthenosphere whose graphite content has been exhausted by partial melting, or melts that have segregated and chemically decoupled from a graphite-bearing residuum will decompress along -decompression paths controlled by continuous Fe3+-Fe2+ solid-melt equilibria. These equilibria will involve increases in relative to the graphite saturation surface and relative to FMQ. Melts that finally segregate from that source and erupt on the earth's surface may then be significantly more oxidized than their mantle sources at depth prior to partial melting. The extent of melt oxidation relative to the mantle source may be directly proportional to the depth of graphite exhaustion in the mantle source.  相似文献   

3.
In the Rogers Pass area of British Columbia the almandine garnet isograd results from a reaction of the form: 5.31 ferroan-dolomite+8.75 paragonite+4.80 pyrrhotite+3.57 albite+16.83 quartz+1.97 O2=1.00 garnet+16.44 andesine+1.53 chlorite+2.40 S2+1.90 H2O+10.62 CO2. The coefficients of this reaction are quite sensitive to the Mn content of ferroan-dolomite.Experimental data applied to mineral compositions present at the isograd, permits calculation of two intersecting P, T equilibrium curves. P=29088–39.583 T is obtained for the sub-system paragonite-margarite (solid-solution), plagioclase, quartz, ferroan-dolomite, and P=28.247 T–14126 is obtained for the sub-system epidote, quartz, garnet, plagioclase. These equations yield P=3898 bars and T=638° K (365° C). These values are consistent with the FeS content of sphalerite in the assemblage pyrite, pyrrhotite, sphalerite and with other estimates for the area.At these values of P and T the composition of the fluid phase in equilibrium with graphite in the system C-O-H-S during the formation of garnet is estimated as: bars, bars, bars, bars, bars, bars, bars, bars, , bars, bars.  相似文献   

4.
Ignimbrites from the central North Island consist mainly of glass or its devitrified product (70–95%); their phenocryst mineralogy is varied and includes plag., hyp., ti-mag., ilm., aug., hblende, biot., san., qtz, ol., with accessory apatite, zircon and pyrrhotite. The Fe-Mg minerals can be used to divide the ignimbrites into four groups with hyp.+aug. reflecting high quench temperatures and biot.+hblende +hyp.+aug., low quench temperatures. Oxygen fugacities lie above the QMF buffer curve and even in ignimbrites with low crystal contents the solid phases apparently buffered fO2. Some ignimbrites contain the assemblage actinolite, gedrite, magnetite and hematite, reflecting post-eruption oxidation. The mineralogy also allows estimation of using pyrrhotite and thence , . The assemblage biotite-sanidine can be used to estimate and thence . Water fugacity is calculated in a variety of ways using both biotite and hornblende as well as the combining reaction . It is high and approaches P total in most ignimbrites (~4kb) but is lower in unwelded pumice breccias. Comparison of temperature estimates using mineral geothermometers for the various phenocryst phases suggests that the ignimbrite magmas showed temperature differences of 60–100 °C and pressure differences of several kilobars. Individual magma chambers therefore, would have extended over several kilometres vertically. The chemical potential of water may have been constant through the magma.  相似文献   

5.
Near-liquidus phase relationships of a spinel lherzolite-bearing olivine melilitite from Tasmania were investigated over a P, T range with varying , , and . At 30 kb under MH-buffered conditions, systematic changes of liquidus phases occur with increasing ( = CO2/CO2 +H2O+olivine melilitite). Olivine is the liquidus phase in the presence of H2O alone and is joined by clinopyroxene at low . Increasing eliminates olivine and clinopyroxene becomes the only liquidus phase. Further addition of CO2 brings garnet+orthopyroxene onto the liquidus together with clinopyroxene, which disappears with even higher CO2. The same systematic changes appear to hold at higher and lower pressures also, only that the phase boundaries are shifted to different . The field with olivine- +clinopyroxene becomes stable to higher with lower pressure and approaches most closely the field with garnet+orthopyroxene+clinopyroxene at about 27 kb, 1160 °C, 0.08 and 0.2 (i.e., 6–7% CO2+ 7–8% H2O). Olivine does not coexist with garnet+orthopyroxene+clinopyroxene under these MH-buffered conditions. Lower oxygen fugacities do not increase the stability of olivine to higher and do not change the phase relationships and liquidus temperatures drastically. Thus, it is inferred that olivine melilitite 2927 originates as a 5% melt (inferred from K2 O and P2O5 content) from a pyrolite source at about 27kb, 1160 dg with about 6–7% CO2 and 7–8% H2O dissolved in the melt. The highly undersaturated character of the melt and the inability to find olivine together with garnet and orthopyroxene on the liquidus (in spite of the close approach of the respective liquidus fields) can be explained by reaction relationships of olivine and clinopyroxene with orthopyroxene, garnet and melt in the presence of CO2.  相似文献   

6.
Microphenocrystic pyrrhotites were observed in the glassy groundmass of two dacite rocks from Satsuma-Iwojima, southwest Kyushu, Japan. It suggests that the dacite magma was saturated with respect to pyrrhotite at the time of eruption, and thus the sulfur contents in the groundmass can be taken as the solubility of sulfur in the dacite magma. The solubility of sulfur in the dacite rocks thus calculated is 65 to 72 ppm sulfur at the estimated conditions of T=900±50°C, and atm.  相似文献   

7.
The compositions of coexisting pyroxmangites, rhodonites, rhodochrosites and manganese calcites in regional metamorphosed manganese cale-silicate marbles from Val Scerscen and Alagna were analysed by microprobe and permit definition of critical tie lines at metamorphic grades appropriate to temperatures between 400 and 450 °C.Variations in composition of coexisting mineral pairs in one and the same locality are attributed to variations in and not to metamorphic temperatures. From the analysed assemblages isothermal plots (with SiO2 as excess component) were constructed for the system CaO-MnO-SiO2-CO2.  相似文献   

8.
A simple modification was made to the reaction vessel designed by Shaw (1963) in order to use it as a hydrogen sensor instead of a hydrogen source, thereby allowing a continuous record of to be made during an experiment. The advantage of this arrangement is that (reversed) curves for minerals can be generated from a single experimental run. Other applications are possible.This method was successfully used in the determination of the Ni-NiO-H2O equilibrium between 580 ° and 830 °C at 2 kbar fluid pressure. The corrected 1 atm values are in agreement with electrochemical measurements. An Ag70Pd30 hydrogen permeable membrane was used for the experiment and flow rates of hydrogen through the membrane were calculated by monitoring the rate of increase or decrease in pressure with an induction pressure transducer. These flow rates range up to two times greater than those calculated for a pure platinum membrane.  相似文献   

9.
Thermal aureoles surrounding intrusions of the Nain complex, Labrador, contain many unique or unusual mineral assemblages in aluminous gneisses and granulites, ironstones, and ultramafic rocks. Some of the limiting assemblages are (in addition to feldspars±pyrrhotite±ilmenite±graphite ±biotite±magnetite): Ga-Ol-Hy-Sp, Ga-Cd-Hy-Sp, Ga-Hy-Ol-Qz, Cd-Hy-Os-Qz, Ga-Cd-Sp-Si, Cd-Sp-Co-Si, Ga-Ol-Hy-Sp, Ga-Cd-Hy-Sp, Hy-Ol-Qz-Aug, and Ol-Hy-Sp(±Chl±Ca-amph±Aug). On the basis of some of these assemblages are (in addition to feldspars±pyrrhotite±ilmenite±graphite from 3.7–6.6 kbar and temperatures from 645–915 ° C. The paucity of hydrous phases, the preponderance of Ksp-Plag-Qz or even Cd-Ksp-Qz without evidence of a melt at these temperatures, the stable occurrence of osumilite, and the common presence of graphite suggest that was extremely low. The presence of graphite-pyrrhotite, the compositions of ilmenites, the compositions of coexisting Fe-Ti oxides, and other mineralogical data indicate the was relatively low and, despite the wide range in bulk compositions and rock types, may have followed an approximately buffered trend. It is possible that the magmas of the Nain complex may have acted as an external buffer for the aureole rocks. The unique nature of these mineral assemblages appears to be a function of the extremely low .  相似文献   

10.
To investigate the point defect chemistry and the kinetic properties of manganese olivine Mn2SiO4, electrical conductivity () of single crystals was measured along either the [100] or the [010] direction. The experiments were carried out at temperatures T=850–1200 °C and oxygen fugacities atm under both Mn oxide (MO) buffered and MnSiO3 (MS) buffered conditions. Under the same thermodynamic conditions, charge transport along [100] is 2.5–3.0 times faster than along [010]. At high oxygen fugacities, the electrical conductivity of samples buffered against MS is 1.6 times larger than that of samples buffered against MO; while at low oxygen fugacities, the electrical conductivity is nearly identical for the two buffer cases. The dependencies of electrical conductivity on oxygen fugacity and temperature are essentially the same for conduction along the [100] and [010] directions, as well as for samples coexisting with a solid-state buffer of either MO or MS. Hence, it is proposed that the same conduction mechanisms operate for samples of either orientation in contact with either solid-state buffer.The electrical conductivity data lie on concave upward curves on a log-log plot of vs , giving rise to two regimes with different oxygen fugacity exponents. In the low- regime , the exponent, m, is 0, the MnSiO3-activity exponent, q, is 0, and the activation energy, Q, is 45 kJ/mol. In the high regime 10^{ - 7} {\text{atm}}} \right)$$ " align="middle" border="0"> , m=1/6, q=1/4–1/3, and Q=45 and 200 kJ/mol for T<1100 °c=" and=">T>1100 °C, respectively.  相似文献   

11.
Recycling as outlined by Garrels and Mackenzie (1971) is the basic process responsible for the formation, preservation, destruction and composition of sedimentary masses during geologic history. However, several sedimentological and geochemical trends are difficult to reconcile with a simple recycling model and would therefore require an evolutionary approach superimposed on the general recycling. These evolutionary trends are observable on an integrated time scale of the order of 108–109 years. The author believes that the majority of the observed “apparent” sedimentological and geochemical trends might be compatible with two to three variables, such as the evolution of the crust, and possibly .  相似文献   

12.
Iron chlorites with compositions intermediate between the two end-members daphnite (Fe5Al2Si3O10(OH)8) and pseudothuringite (Fe4Al4Si2O10(OH)8) were synthesized from mixtures of reagent chemicals. The polymorph with a 7 Å basal spacing initially crystallized from these mixtures at 300 °C and 2 kb after two weeks. Conversion to a 14 Å chlorite required a further 6 weeks at 550 °C. Shorter conversion times were required at higher water pressures. The products contained up to 20% impurities.The maximum equilibrium decomposition temperature for iron chlorite, approximately 550 °C at 2kb, is at an between assemblages (1) and (2) listed below. Synthetic iron chlorite will break down by various reactions with variable P, T, and fugacity of oxygen. For the composition FeAlSi = 523, the sequence of high temperature breakdown products with increasing traversing the magnetite field for P total = =2kb is: (1) corierite+ fayalite+hercynite; (2) cordierite+fay alite+magnetite; (3) cordierite+magnetite+quartz; (4) magnetite+mullite+quartz. Almandine should replace cordierite in assemblages (1) and (2) but it did not nucleate. The significance of the relationship between iron cordierite and almandine in this system is discussed.At water pressures from 4 to 8.5 kb and at the nickel-bunsite buffer, iron chlorite+quartz break down to iron gedrite+magnetite with temperature 550 to 640 °C along the curve. At temperatures 50 °C greater and along a parallel curve, almandine replaces iron gedrite. For on this buffer curve, almandine is unstable below approximately 4 kb for temperatures to approximately 750 °C.  相似文献   

13.
A thermodynamic model is developed that describes the compositional variation of coexisting olivine, orthopyroxene, and ferrite (Fe3O4-MgFe2O4) as a function of and T. The ferrite phase has a cation distribution which varies from nearly inverse to nearly random with increasing T and is described with a model in which the number of sites per formula unit on which mixing occurs varies from 1.67 to 2.0. Given this model and the equilibrium phase composition data for coexisting olivine and ferrite at 1,300° C of Jamieson and Roeder (1984), the ferrite solution is described to an excellent approximation by a symmetric regular solution model with W ft =+14.0 ±0.3 kJ/mole. Orthopyroxene and olivine non-ideality are also considered. The T-dependence of the equilibrium constant for the oxidation reaction 6Fs+2Mt=6Fa+O2 and the two Fe/Mg exchange reactions between olivine-ferrite and olivine-orthopyroxene, are used to calculate the compositional variation of coexisting phases as a function of and T. The results are summarized on an isobaric (1 bar) –1/Tplot with the compositional variation of olivine, ferrite, and orthopyroxene shown by sets of isopleths. The ferrite isopleths intersect those of olivine and orthopyroxene at sufficiently high angles for this assemblage to serve as a sensitive geothermometer and oxygen-barometer. The model is applied to orthopyroxene-ferrite symplectite in coronas around olivine in a metamorphosed gabbro, to olivine-hosted orthopyroxene-ferrite symplectite in unmetamorphosed gabbros and norites and to olivine-hosted orthopyroxene-ferrite symplectites developed within the rims of lherzolite xenoliths.  相似文献   

14.
Trace element analyses of 1-atm and high-pressure experiments show that in komatiite and peridotite, the olivine (OL)/liquid (L) distribution coefficient for Al2O3 ( ) increases with pressure and temperature. Olivine in equilibrium with liquid accepts as much as 0.2 wt% Al2O3 in solution at 6 GPa. Convergence to equilibrium compositions at this high level is shown by cation diffusion of Al into synthetic forsterite crystals of low-Al contents in the presence of melt. Convergence to low-Al equilibrium compositions at lower P and T is shown by diffusion of Al out of synthetic forsterite with high initial Al content. Isobaric and isothermal experimental data subsets reveal that temperature and pressure variations both have real effects on . Variation in silicate melt composition has no detectable effect on within the limited range of experimentally investigated mixtures. Least-squares regression for 24 experiments, using komatiite and peridotite, performed at 1 atm to 6 GPa and 1300 to 1960°C, gives the best fit equation: Increase in with increasingly higher-pressure melting is consistent with incorporation of a spinel-like component of low molar volume into olivine, although other substitutions possibly involving more complex coupling cannot be ruled out. High P-T ultrabasic melting residues, if pristine, may be recognized by the high calculated from microprobe analyses of Al2O3 concentrations in residual olivines and estimated Al2O3 concentration in the last liquid removed. In general the low levels of Al in natural olivine from mantle xenoliths suggest that pristine residues are rarely recovered.  相似文献   

15.
The phenocryst assemblage of cummingtonite, orthopyroxene, quartz, titanomagnetite and ilmenite in rhyolites of New Zealand has been used to calculate P total and . The values of P total and depend strongly upon whether an ideal mixing, or an ordered, model is used for the solid-solutions, but in both cases P total.The rhyolite magma contained over 9 per cent water (by weight) when the cummingtonite phenocrysts precipitated, and possibly as much as 12 per cent, so that it is surprising that one of these rhyolites is a coherent lava. The calculated values of P total and are very sensitive to uncertainty in both the composition of the solid-solutions and temperature. Calculations show that >0.7–0.8 P total for cummingtonite to precipitate in rhyolites, and that iron-rich olivine and cummingtonite could only exist in rhyolites over a small temperature range at a pressure near 5 kilobars. Hornblende phenocrysts co-existing with fayalitic olivine in rhyolites accordingly have a very low activity of Mg7Si8O22(OH)2.  相似文献   

16.
The Koloula Igneous Complex comprises 26 different intrusive phases that have been divided into two major magmatic episodes — cycle 1 intrusions (4.5 Ma) and cycle 2 intrusions (2.4 to 1.5 Ma). The cycle 2 intrusions are further divided into the Inamumu Zoned Pluton (IZP) which is composed of 6 concentrically disposed quartz diorite and tonalite units; and several satellite intrusions. The IZP is host to porphyry-copper mineralization, whereas the cycle 1, and cycle 2 satellite intrusions are barren. Presently exposed mineralization in the IZP (A system) represents the deeply eroded core of a porphyry copper system, where widely-spaced veinlet alteration envelopes ( 1 mm thick) are separated by large volumes of unaltered rock.Compositional trends in biotites and amphiboles from both individual grains and throughout the differentiation series of the IZP, indicate fluctuating but generally increasing existed through the sequence from early magmatic late magmatic early hydrothermal conditions. In amphiboles, compositional domains (Mg-rich) that are indicative of high are correlated with episodes of fluid exsolution, independent evidence of which is provided by multiple generations of fluid inclusions in quartz phenocrysts. These high domains in amphiboles have higher Si, Mn, and Ca contents, but are depleted in Fe, Ti, Na, K, and Cl relative to the less oxidizing domains. The latter elements are those that are known from veinlet alteration assemblages and fluid-inclusion evidence to have been preferentially partitioned into the co-existing fluid phase (late magmatic hydrothermal solution).By contrast, amphiboles from barren rock types that are slightly older than, and of the same age as the IZP, exhibit restricted compositional ranges, and are more Fe-rich. Some individual grains and two cycle 2 satellite intrusions indicate Fe-enrichment during progressive crystallization. Siliceous deuteric amphiboles are commonly as Mg-rich as the high amphibole domains from the IZP, but are easily distinguished from them by their lack of smooth compositional trends versus Si, and by their highly variable Mg and Fe contents.Biotites from the IZP also indicate progressive oxidation, whereas biotites from the barren rock types show either little compositional variation or progressive Fe-enrichment. Biotites from the barren intrusions are richer in Cl, Li and Rb and poorer in Ba than those of the mineralizing intrusions. fHF was very low ( 0.003 bars) in both barren and mineralizing intrusions. During progressive differentiation, Rb content decreased and Ba content increased in IZP biotites, which is atypical, yet explicable owing to the former presence of a competing fluid phase during biotite crystallization.Because is a function of degree of fluid exsolution, then in igneous systems with sufficient Cu, Cl, and ultimately S, progressively higher should potentially lead to more mineralized intrusions. Higher is reflected by steeper Fe versus Si gradients in amphibole domains. Indeed, such a graph for amphiboles from 5 igneous complexes, indicates that two economically mineralized units produced steeper FeSi than those from weakly mineralized intrusions. Steep FeSi trends that do not continue to amphibole domains more siliceous than Si=7.3 (atoms per 23 oxygens) are unlikely to have resulted from subsolidus crystallization and these intrusions are unlikely to be strongly mineralized.  相似文献   

17.
The nature of the near-liquidus phases for a mantle-derived olivine melilitite composition have been determined at high pressure under dry conditions and with various water contents. Olivine and clinopyroxene occur on or near the liquidus and there are no conditions where orthopyroxene crystallizes in equilibrium with the olivine melilitite. We have determined the effect on the liquidus temperature and liquidus phases of substituting CO2 for H2O on a mole for mole basis at 30 kb, using olivine melilitite + 20 wt% H2O at = 0 and = (CO2)/(H2+CO2) (mole fraction) = 0.25, 0.5, 0.75 and 1.0 (i.e. olivine melilitite + 38 wt% CO2). Experiments were buffered by the MH or NNO buffers. At 30 kb, CO2 is only slightly less soluble than water for <0.5 as judged by the slight increase in liquidus temperature on mole-for-mole substitution of CO2 for H2O and at 30 kb, 1200° C, = = 0.5 the olivine melilitite contains 8.8 wt% H2O and 21 wt% CO2 in solution. For 1 the CO2 saturated liquidus is depressed 70 ° C below the anhydrous liquidus and the magma dissolves approx. 17% CO2 at 30kb, 1400 ° C, 1, 0. Infrared spectra of quenched glasses have absorption bands characteristic of CO 3 = and OH- molecules and no evidence for HCO 3 - . The effect of CO 3 = molecules dissolved in the olivine melilitite at high pressure is to suppress the near-liquidus crystallization of olivine and clinopyroxene and bring orthopyroxene and garnet on to the liquidus. We infer that olivine melilitite magmas may be derived by equilibrium partial melting (<5%) of pyrolite at 30 kb, 1150–1200 ° C, provided that both H2O and CO2 are present in the source region in minor amounts. Preferred conditions are 0< <0.5, 0.5< <1, and at low oxygen fugacities (相似文献   

18.
Two metamorphic isograds cut across graphitic schist near Pecos Baldy, New Mexico. The southern isograd marks the first coexistence of staurolite with biotite, whereas the northern isograd marks the first coexistence of andalusite with biotite. The isograds do not record changes in temperature or pressure. Instead, they record a regional gradient in the composition of the metamorphic fluid phase. Ortega Quartzite, which contains primary hematite, lies immediately north of the graphitic schist. Mineral compositions within the schist change gradually toward the quartzite, reflecting gradients in and . The chemical potential gradients, locally as high as 72 cal/m in and 9 cal/m in , controlled the positions of the two mapped isograds. The staurolite-biotite isograd records where fell below 0.80, at near 10–23 bars; the andalusite-biotite isograd records where fell below 0.25, at near 10–22 bars. Dehydration and oxidation were coupled by graphite-fluid equilibrium.The chemical potential gradients apparently formed during metamorphism, as graphite in schist reacted with hematite in quartzite. Local oxidation of graphite formed CO2 which triggered dehydration reactions along the schistquartzite contact. This process created a C-O-H fluid which infiltrated into overlying rocks. Upward infiltration, local fluid-rock equilibration and additional infiltration proceeded until the composition of the infiltrating fluid evolved to that in equilibrium with the infiltrated rock. This point occurs very close to the staurolite-biotite isograd. Pelitic rocks structurally above this isograd show no petrographic evidence of infiltration, even though calculations indicate that volumetric fluid/rock ratios may have exceeded 15 and thin, rare calc-silicate beds show extensive K-metasomatism and quartz veining.  相似文献   

19.
The Talasea Peninsula is composed of a chain of Quaternary volcanoes whose lavas range from basalt to rhyolite. The peninsula is situated in an orogenic environment and the lavas, while essentially calc-alkaline, show some differences from other orogenic suites on the Pacific rim. The most distinctive feature of the Talasea series is absolute iron enrichment in some lavas. Mineralogically, the andesites are characterized by phenocrysts of plagioclase, orthopyroxene, clinopyroxene and titanomagnetite, while the basalts lack titanomagnetite phenocrysts but contain olivine. The acid rocks have a mineralogy similar to that of the andesites, but also contain quartz, amphibole, biotite and ilmenite. The compositions of coexisting titanomagnetite and ilmenite in the acid lavas indicate equilibration temperatures in the range 920° to 860° C and oxygen fugacities ( ) above those of the fayalitemagnetite-quartz buffer assemblage. The mineralogical evidence supports the hypothesis of a crystal fractionation origin for this series and there is a possibility that the was more or less constant during the early stages of its evolution. The iron enriched lavas may be an offshoot from the main line of descent, resulting from near-surface fractionation, with the dominance of plagioclase in the crystal residuum producing an iron-rich liquid.  相似文献   

20.
The Miocene-Pliocene Macusani volcanics, SE Peru, outcrop in three separate tectonic intermontane basins developed on a Paleozoic-Mesozoic volcano-sedimentary sequence. Several ignimbrite sheets are recognized and K-Ar dates record at least semi-continuous volcanic activity from 10 to 4 Ma in the Macusani field. The volcanics in the Macusani basin comprise crystal-rich (45% crystals) ash-flow tuffs and rare obsidians glasses, both with unusual mineralogy, similar to two-mica peraluminous leucogranites. The mineralogical assemblage (quartz, sanidine Or69–75, plagioclase, biotite, muscovite and andalusite (both coexisting in the entire volcanic field), sillimanite, schörl-rich tourmaline, cordierite-type phases, hercynitic spinel, fluor-apatite, ilmenite, monazite, zircon, niobian-rutile) is essentially constant throughout the entire Macusani field. Two distinct generations of plagioclase are recognized, viz. group I (An10–20) and group II (An30–45). Sillimanite forms abundant inclusions in nearly all phases and is earlier than andalusite which occurs as isolated phenocrysts. Biotite (Al-, Ti-, Fe- and F-rich) shows pronounced deficiencies in octahedral cations. Muscovite is also F-rich and displays limited biotitic and celadonitic substitutions. There is no systematic variation in mineral chemistry with stratigraphic position. The mineralogical data provide a basis for distinction between an early magmatic and a main magmatic stage. The early stage corresponds to the magmatic evolution at or near the source region and includes both restites and early phenocrysts. Some biotites (with textures of disequilibrium melting to Fe — Zn spinel), part of the sillimanite, apatite and monazite, possibly some tourmaline and cordierite-type phases are restites. However, the restite content of the magma was low (5 vol. % maximum). The group II plagioclase are interpreted as early phenocrysts. During this stage, temperatures were as high as 800° C, pressure was no more than 5–7.5 kbar, was intermediate between WM and QFM and was low. The biotite melting textures and the coexistence of restites and early phenocrysts imply fast heating rates in the source region. The transition between the early and the main magmatic stage was abrupt (andalusite crystallization in place of sillimanite, group I vs. group II plagioclases) and suggests rapid ascent of the magma from its source region. During the main crystallization stage, temperature was 650° C or lower at a pressure of 1.5–2 kbar. (calculated from equilibrium between muscovite, quartz, sanidine and andalusite) are around 1, suggesting conditions close to H2O-saturation. f HF is around 1 bar but the ratios are significantly different between samples. ranges between 138 and 225 bar. This study shows that felsic, strongly peraluminous, leucogranitic magmas having andalusite and muscovite phenocrysts may be generated under H2O-undersaturated conditions.CRPG Contribution n 769  相似文献   

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