首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 46 毫秒
1.
Complex multivariant reactions involving Fe-Ti oxide minerals, plagioclase and olivine have produced coronas of biotite, hornblende and garnet between ilmenite and plagioclase in Adirondack olivine metagabbros. Both the biotite (6–10% TiO2) and the hornblende (3–6% TiO2) are exceptionally Titanium-rich. The garnet is nearly identical in composition to the garnet in coronas around olivine in the same rocks. The coronas form in two stages:
  1. Plagioclase+Fe-Ti Oxides+Olivine+water =Hornblende+Spinel+Orthopyroxene±Biotite +more-sodic Plagioclase
  2. Hornblende+Orthopyroxene±Spinel+Plagioclase =Garnet+Clinopyroxene+more-sodic Plagioclase
The Orthopyroxene and part of the clinopyroxene form adjacent to olivine. Both reactions are linked by exchange of Mg2+ and Fe2+ with the reactions forming pyroxene and garnet coronas around olivine in the same rocks. The reactions occur under granulite fades metamorphic conditions, either during isobaric cooling or with increasing pressure at high temperature.  相似文献   

2.
Phase relations of pumpellyite, epidote, lawsonite, CaCO3, paragonite, actinolite, crossite and iron oxide are analysed on an Al-Ca-Fe3+ diagram in which all minerals are projected from quartz, albite or Jadeite, chlorite and fluid. Fe2+ and Mg are treated as a single component because variation in Fe2+/Mg has little effect on the stability of phases on the diagram. Comparison of assemblages in the Franciscan, Shuksan, Sanbagawa, New Caledonia, Southern Italian, and Otago metamorphic terranes reveals several reactions, useful for construction of a petrogenetic grid:
  1. lawsonite+crossite + paragonite = epidote+chlorite + albite + quartz + H2O
  2. lawsonite + crossite = pumpellyite + epidote + chlorite + albite+ quartz + H2O
  3. crossite + pumpellyite + quartz = epidote + actinolite + albite + chlorite + H2O
  4. crossite + epidote + quartz = actinolite + hematite + albite + chlorite + H2O
  5. calcite + epidote + chlorite + quartz = pumpellyite + actinolite + H2O + CO2
  6. pumpellyite + chlorite + quartz = epidote + actinolite + H2O
  相似文献   

3.
The influence of bacteria on recent sediments was first discussed in 1885, whenFischer andGazert were discussing the cycle of substances in the sea as well as in sediments. The influence of bacteria on the cycling of C, N, S, P in recent sediments and the open sea was soon accepted by marine geologists. Nevertheless, only very few experiments have, so far, shown more than qualitative and quantitative data collection in various restricted areas. This is due to the extensive and complicated chain of reactions on the surface of sediments and in the sediment itself. Biologists are asking for the amount of organic and inorganic matter which is reworked and released to the sea. Geologists usually emphasize the amount of substances which are sedimentated. For biologists the sediment is only part of their dominant ecosystem (the sea). While, for geologists the “sea” is only furnishing and influencing their first range system sediment. How much then, are bacteria involved in the slow process of conversion from a recent sediment to sedimentary rocks? Bacteria influence more or less strongly and to a more or less advanced degree of diagenesis:
  1. The organic matter in sediments and the final form in which it is found.
  2. The anions CO3 2?, NO3 ?, OH-, SO4 2?, PO4 3? as well as their intermediate stages and the resulting minerals.
  3. The cations H+, NH4 +, Ca2+, Fe2+, Fe3+, and a series of metals which are dissolved or precipitated by microbial activities as for example Fe, Mn, Cu, Ag, V, Co, Mo, Ni, U, Se, Zn.
  4. The equilibrium of silicium. At least diatoms and radiolarians are precipitating silica, while other reactions which have been proved are not yet shown to influence marine sediments.
  5. pH-values and oxidation-reduction potentials of the sediment.
  6. The composition of interstitial waters.
  7. The surface activity of minerals, since bacteria are growing especially on particle surfaces.
  8. The energy content and temperature of sediments.
  9. The texture of fine grained sediments.
  10. The fossilization of microfauna, macrofauna and trace fossils.
Sedimentology and mineralogy may also influence the bacterial activities and the composition of the microflora within sediments. Methods and problems of sediment microbiology are demonstrated by some investigations in the German Bay (North Sea) in connection with the first German Underwater Station (UWL). Ecological work proves to be difficult in various directions. The main cause of difficulties in microbiological work on sediments are the great variety of different factors influencing the environment (microbial, chemical, physical, mineralogical), the difficulty of taking representative samples, and the small amount of data which has been collected so far.  相似文献   

4.
A generalized reaction is presented to account for garnet formation in a variety of Adirondack metaigneous rocks. This reaction, which is the sum of five partial reactions written in aluminum-fixed frames of reference, is given by: 4(y+1+w)Anorthite+4k(y+1+2w)Olivine +4(1–k)(y+1+2w)Fe-oxide+(8(y+1) –4k(y+1+2w))Orthopyroxene = 2(y+1)Garnet +2(y+1+2w)Clinopyroxene+4wSpinel where y is a function of plagioclase composition, k refers to the relative amounts of olivine and Fe-oxide participating in the reaction, and w is a measure of silicon mobility. When mass balanced for Mg and Fe, this reaction is found to be consistent with analyzed mineral compositions in a wide range of Adirondack metaigneous rocks. The reaction applies equally well whether the garnets were formed directly from the rectants given above or went through an intermadiate stage involving the formation of spinel, orthopyroxene, and clinopyroxene.The actual reactions which have produced garnet in both undersaturated and quartz-bearing rocks are special cases of the above general reaction. The most important special cases appear to be those in which the reactants include either olivine alone (k=1) or Fe-oxide alone (k=0). Silicon is relatively immobile (w =2) in olivine bearing, magnesium-rich rocks (k1), and this correlates with the increased intensity in spinel clouding of plagioclase in these rocks. Silicon mobility apparently increases in the more iron-rich rocks, which also tend to contain clear or lightly clouded plagioclase. In all the rocks studied the most common composition of metamorphic plagioclase is close to An33 (i.e., y=1). Plagioclase of lower anorthite content may be too sodic to participate in garnet formation at the P-T conditions involved.Published by permission of the Director, New York State Museum and Science Service; Journal Series No. 282  相似文献   

5.
The Nevado Coropuna (6400 m/19 500 ft) is the largest and highest volcano of Peru and is situated 150 km NW of the town of Arequipa at a distance of 110 km from the Pacific coast. Results of a thorough petrographic study are presented including microprobe and radiometric measurements.
  1. The constituent rocks building up the Coropuna volcano are lavas and rhyodacitic ash flows intercalated between older and younger lavas at the foot of the cone. The volcanic edifice rests on older ignimbrite sheets (14 m. y.) exposed only in the surrounding valleys.
  2. The lavas are typically latite-andesites which contain some normative quartz in the groundmass. Plagioclase has 37–47% An. The depth of the phenocryst crystallization is calculated at 8–12 km based on the equilibrium between plagioclase, clinopyroxene and groundmass.
  3. The Coropuna volcano has existed since the Late Miocene (5 m. y.). Approximately 2 m. y. ago a catastrophic explosion produced large rhyodacitic ignimbrite deposits around the foot of the mountain. Thereafter the effusion of lavas was dominant through Holocene times with the latest lavas becoming slightly more acidic (62% SiO2).
  4. 30–40 km to the W and SW of the Coropuna some outliers of the coastal batholites are exposed. Both their radiometric age (Cretaceous, 97 m. y.) and their chemical composition are in disagreement with the notion of these granodioritic to gabbroic rocks as the intrusive equivalents of the young volcanics.
  相似文献   

6.
Laihuite reported in the present paper is a new iron silicate mineral found in China with the following characteristics:
  1. This mineral occurs in a metamorphic iron deposit, associated with fayalite, hypersthene, quartz, magnetitc, etc.
  2. The mineral is opaque, black in colour, thickly tabular in shape with luster metallic to sub-metallic, two perfect cleavages and specific gravity of 3.92.
  3. Its main chemical components are Fe and Si with Fe3+>Fe2+. The analysis gave the formula of Fe Fe 1.00 3+ ·Fe 0.58 2+ ·Mg 0.03 2+ ·Si0.96O4.
  4. Its DTA curve shows an exothermic peak at 713°C.
  5. The mineral has its own infrared spectrum distinctive from that of other minerals.
  6. This mineral is of orthorhombic system; space group:C 2h /5 ?P21/c; unit cell:α=5.813ű0.005,b=4.812ű0.005,c=10.211ű0.005,β=90.87°.
  7. The Mössbauer spectrum of this mineral is given, too.
  相似文献   

7.
The Elzevir batholith belongs to a suite of trondhjemitic intrusions emplaced at ca. 1,240 Ma in the Grenville Province of eastern Ontario. New major and trace element data, including REE, combined with isotopic and petrographic data indicate that:
  1. the batholith has calc-alkalic affinities;
  2. the Elzevir parental magma is very similar to that of dacites in the nearby, coeval metavolcanic rocks; the magma formed by partial melting of crustal material at granulite grade;
  3. chemical differences between the plutonic and volcanic rocks can be best explained by accumulation of plagioclase in the plutonic environment;
  4. fractionation was dominated by plagioclase and quartz, with lesser biotite and epidote, and minor zircon and apatite.
It is suggested that melting of sialic crust took place during the ‘docking’ of a partly-evolved, originally ensimatic arc system against the main cratonic mass to the northwest.  相似文献   

8.
The occurrence of talc and tremolite in a temperature gradient was investigated in siliceous calcite-dolomite sediments exposed along a strip in the southeastern part of the Damara Orogen. Five bivariant reactions may lead to the formation of talc and tremolite:
  1. 3 dolomite+4 quartz+1 H2O ? 1 talc+3 calcite+3 CO2
  2. 5 talc+6 calcite+4 quartz ? 1 tremolite+6 CO2+2 H2O
  3. 2 talc+3 calcite ? 1 tremolite+1 dolomite+1 CO2+1 H2O
  4. 5 dolomite+8 quartz+1 H2O ? 1 tremolite+3 calcite+7 CO2
  5. 2 dolomite+1 talc+4 quartz ? 1 tremolite+4 CO2.
The common paragenesis of four mineral assemblages tc+cc+dol+qtz1 and tre+tc+ cc+qtz with increasing temperature over an extended area show that the reactions must have taken place along the equilibrium curve or when fluid pressure is not constant along the equilibrium plane of reactions (1) or (2). The described occurrence of the five mineral assemblage tre+tc+cc+dol+qtz can be stable only on the isobaric intersection point, or when P f is variable on the univariant intersection curve of the equilibrium planes of all five reactions. The genetic relations of the described parageneses are illustrated with the help of a phase diagram. Minimum P-T conditions which prevailed during metamorphism in this part of the Damara Orogen have been estimated to be about 590° C and 5 kb.  相似文献   

9.
The stability relations between cordierite and almandite in rocks, having a composition of CaO poor argillaceous rocks, were experimentally investigated. The starting material consisted of a mixture of chlorite, muscovite, and quartz. Systems with widely varying Fe2+/Fe2++Mg ratios were investigated by using two different chlorites, thuringite or ripidolite, in the starting mixture. Cordierite is formed according to the following reaction: $${\text{Chlorite + muscovite + quartz}} \rightleftharpoons {\text{cordierite + biotite + Al}}_{\text{2}} {\text{SiO}}_{\text{5}} + {\text{H}}_{\text{2}} {\text{O}}$$ . At low pressures this reaction characterizes the facies boundary between the albite-epidotehornfels facies and the hornblende-hornfels facies, at medium pressures the beginning of the cordierite-amphibolite facies. Experiments were carried out reversibly and gave the following equilibrium data: 505±10°C at 500 bars H2O pressure, 513±10°C at 1000 bars H2O pressure, 527±10°C at 2000 bars H2O pressure, and 557±10°C at 4000 bars H2O pressure. These equilibrium data are valid for the Fe-rich starting material, using thuringite as the chlorite, as well as for the Mg-rich starting mixture with ripidolite. At 6000 bars the equilibrium temperature for the Mg-rich mixture is 587±10°C. In the Fe-rich mixture almandite was formed instead of cordierite at 6000 bars. The following reaction was observed: $${\text{Thuringite + muscovite + quartz}} \rightleftharpoons {\text{almandite + biotite + Al}}_{\text{2}} {\text{SiO}}_{\text{5}} {\text{ + H}}_{\text{2}} {\text{O}}$$ . Experiments with the Fe-rich mixture, containing Fe2+/Fe2++Mg in the ratio 8∶10, yielded three stability fields in a P,T-diagram (Fig.1):
  1. Above 600°C/5.25 kb and 700°C/6.5 kb almandite+biotite+Al2SiO5 coexist stably, cordierite being unstable.
  2. The field, in which almandite, biotite and Al2SiO5 are stable together with cordierite, is restricted by two curves, passing through the following points:
    1. 625°C/5.5 kb and 700°C/6.5 kb,
    2. 625°C/5.5 kb and 700°C/4.0 kb.
  3. At conditions below curves 1 and 2b, cordierite, biotite, and Al2SiO5 are formed, but no garnet.
An appreciable MnO-content in the system lowers the pressures needed for the formation of almandite garnet, but the quantitative influence of the spessartite-component on the formation of almandite could not yet be determined. the Mg-rich system with Fe2+/Fe2++Mg=0.4 garnet did not form at pressures up to 7 kb in the temperature range investigated. Experiments at unspecified higher pressures (in a simple squeezer-type apparatus) yielded the reaction: $${\text{Ripidolite + muscovite + quartz}} \rightleftharpoons {\text{almandite + biotite + Al}}_{\text{2}} {\text{SiO}}_{\text{5}} {\text{ + H}}_{\text{2}} {\text{O}}$$ . Further experiments are needed to determine the equilibrium data. The occurence of garnet in metamorphic rocks is discussed in the light of the experimental results.  相似文献   

10.
The petrography, mineral chemistry and petrogenesis of a sample from the Weissenstein eclogite, Bavaria, Germany, has been investigated. The total mineral assemblage comprises garnet, clinopyroxeneI+II, quartz, amphiboleI+II, rutile, phengite, epidote/allanite, plagioclase, biotite, apatite, pumpellyite, titanite (sphene), zircon, alkali feldspar and calcite. Textural observations combined with geothermobarometry (Fe/Mg distribution between clinopyroxene/garnet and phengite/garnet; jadeite-content of omphacite, Si-content of phengite, and An-content of plagioclase) provide indications of two different stages in the metamorphic evolution of the rock. The main phengitequartz-eclogite mineral equilibration occurred at minimum P=13–17kbar, minimum T=620±50° C; the retrograde symplectite stage (clinopyroxeneII, amphiboleII, biotite, plagioclase) occurred at P total between 12 and 8.5 kbar. Reactions of the symplectite stage are:
  1. phengite (core) + Na2Oaq + CaOaq=phengite (rim) + biotite + plagioclase + K2Oaq + H2O
  2. phengite (core) + clinopyroxeneI + Na2Oaq=phengite (rim + biotite + plagioclase + amphiboleII + SiO2 + K2Oaq + CaOaq + H2O
  3. clinopyroxeneI + SiO2 + K2Oaq + H2O=clinopyroxeneII + plagioclase+amphiboleII + Na2Oaq + CaOaq
The phengite decomposition produces H2O, whereas the clinopyroxene decomposition consumes H2O. The estimated P-T-conditions for the Weissenstein eclogite are in the same order of magnitude as those for other eclogite bodies from the Alps and Caledonides believed to be related to subduction processes.  相似文献   

11.
Prior experimental work has shown that in the laboratory the mineralogy of eclogites is sensitive to the ratio of CaO ∶ MgO ∶ FeO and that the reaction pyroxene + kyanite?garnet + quartz proceeds to the right at high pressures in rocks rich in magnesium and to the left in rocks rich in calcium and iron. Typical basalts crystallized at high pressure never contain kyanite. The chemistry and mineralogy of a large number of naturally occurring eclogites show they belong to three classes.
  1. Kyanite-free magmatic eclogites, rich in magnesium, from:
  2. kimberlites
  3. dunites and serpentinites.
  4. Kyanite-bearing eclogites and grosspydites rich in CaO and low in FeO with intermediate MgO from:
  5. kimberlites
  6. gneisses.
  7. Kyanite-free eclogites of metamorphic origin rich in iron with low magnesium and intermediate amounts of calcium from:
  8. glaucophane schists
  9. gneisses.
  相似文献   

12.
Basalts from DSDP Site 417 (109 Ma) exhibit the effects of several stages of alteration reflecting the evolution of seawater-derived solution compositions and control by the structure and permeability of the crust. Characteristic secondary mineral assemblages occur in often superimposed alteration zones within individual basalt fragments. By combining bulk rock and single phase chemical analyses with detailed mineralogic and petrographic studies, chemical changes have been determined for most of the alteration stages identified in the basalts.
  1. Minor amounts of saponite, chlorite, and pyrite formed locally in coarse grained portions of massive units, possibly at high temperatures during initial cooling of the basalts. No chemical changes could be determined for this stage.
  2. Possible mixing of cooled hydrothermal fluids with seawater resulted in the formation of celadonite-nontronite and Fe-hydroxide-rich black halos around cracks and pillow rims. Gains of K, Rb, H2O, increase of Fe3+/FeT, and possibly some losses of Ca and Mg occurred during this stage.
  3. Extensive circulation of oxygenated seawater resulted in the formation of various smectites, K-feldspar, and Fe-hydroxides in brown and light grey alteration zones around formerly exposed surfaces. K, Rb, H2O, and occasionally P were added to the rocks, Fe3+/FeT increased, and Ca, Mg, Si and occasionally Al and Na were lost.
  4. Anoxic alteration occurred during reaction of basalt with seawater at low water-rock ratios, or with seawater that had previously reacted with basalt. Saponite-rich dark grey alteration zones formed which exhibit very little chemical change: generally only slight increases in Fe3+/FeT and H2O occurred.
  5. Zeolites and calcite formed from seawater-derived fluids modified by previous reactions with basalt. Chemical changes involved increases of Ca, Na, H2O, and CO2 in the rocks.
  6. A late stage of anoxic conditions resulted in the formation of minor amounts of Mn-calcites and secondary sulfides in previously oxidized rocks. No chemical changes were determined for this stage.
Recognition of such alteration sequences is important in understanding the evolution of submarine hydrothermal systems and in interpreting chemical exchange due to seawater-basalt reactions.  相似文献   

13.
Detailed analysis of textural and chemical criteria in rocks of the anorthosite-charnockite suite of the Adirondack Highlands suggests that development of garnet in silica-saturated rocks of the suite occurs according to the reaction: $$\begin{gathered} {\text{Anorthite}} {\text{Orthopyroxene}} {\text{Quartz}} \hfill \\ {\text{2CaAl}}_{\text{2}} {\text{Si}}_{\text{2}} {\text{O}}_{\text{8}} + (6 - \alpha )({\text{Fe,Mg}}){\text{SiO}}_{\text{3}} + \alpha {\text{Fe - Oxide + (}}\alpha {\text{ - 2)SiO}}_{\text{2}} \hfill \\ {\text{Garnet}} {\text{Clinopyroxene}} \hfill \\ = {\text{Ca(Fe,Mg)}}_{\text{5}} {\text{Al}}_{\text{4}} {\text{Si}}_{\text{6}} {\text{O}}_{{\text{24}}} + {\text{Ca(Fe,Mg)Si}}_{\text{2}} {\text{O}}_{\text{6}} \hfill \\ \end{gathered} $$ , where α is a function of the distribution of Fe and Mg between the several coexisting ferromagnesian phases. Depending upon the relative amounts of Fe and Mg present, quartz may be either a reactant or a product. Using an aluminum-fixed reference frame, this reaction can be restated in terms of a set of balanced partial reactions describing the processes occurring in spatially separated domains within the rock. The fact that garnet invariably replaces plagioclase as opposed to the other reactant phases indicates that the aluminum-fixed model is valid as a first approximation. This reaction is univariant and produces unzoned garnet. It differs from a similar equation proposed by de Waard (1965) for the origin of garnet in Adirondack metabasic rocks, i.e. 6 Orthopyroxene+2 Anorthite = Clinopyroxene+Garnet+2 Quartz, the principle difference being that iron oxides (ilmenite and/or magnetite) are essential reactant phases in the present reactions. The product assemblage (garnet+clinopyroxene+plagioclase ± orthopyroxene ± quartz) is characteristic of the clinopyroxene-almandine subfacies of the granulite facies.  相似文献   

14.
The data obtained can be summed up in 6 points:
  1. The sediment consists of varying amounts of quartz (14–23 weight percent, mean value 18%), aragonite, dolomite, Mg-rich calcite, Mg-poor calcite, Na-rich plagioclase (7–10%, m.v. 8%), chlorite (18–32%, m.v. 22%), kaolinite (4–29%, m.v. 13%), illite and mixedlayered illite-montmorillonite (20–42%, m.v. 34%) and organic matter (0.7–2.5%, m.v. 1.1%).
  2. The composition of the carbonate fraction suggests a detrital origin for the carbonates deposited along the Persian Coast.
  3. The clay mineral distribution follows gradients perpendicular to the long half-axis of the Persian Gulf. This favours detrital origin. The kaolinite distribution suggests an additional detritus from Euphrates and Tigris,
  4. Enrichments of trace elements with regard to mean values in clays (Turekian and Wedepohl, 1961) are estimated for Fe, Co, Mo, Ni, V, Zr and Br; impoverishments are recorded for Cu, Mn, K. The contents of Rb, Ti and Zn comply with mean values of clays. The enrichments distribute as follows: chlorite concentrates Mn and Fe, kaolinite Ti and Zr, illite and mixedlayered illite-montmorillonite Rb, V, M, Zn, Mo and Co.
  5. The high Br-contents (up to 420 ppm) in the sediment give positive correlations with the concentrations of organic carbon. The X-ray analysis of isolated kerogen confirms an enrichment of Br in this fraction of the organic matter.
  6. The distribution of elements in the sediment to a large extent is controlled by a) detritus of Euphrates and Tigris, b) influence of organic matter.
  相似文献   

15.
Samples of micaschists, gneisses, and migmatites from a sequence of metamorphic subfacies of the Gévennes Médianes (Dép. Ardèche, France) are characterized chemically by
  1. a high and only slightly varying Al2O3 content (m.v.=17.57 wt.-%, stand, dev.=1.804, var. coeff.=0.103).
  2. a negligible variation of the molar MgO/MgO+FeO ratio (m.v.=0.504, stand. dev.= 0.066, var. coeff.=0.130), and
  3. an insignificant variation of the molar CaO/CaO + Na2O ratio (m.v.=0.360, stand. dev.=0.014, var. coeff.=0.039).
The Na2O and K2O values of the migmatite samples are respectively 14 and 30% higher than those of the samples of the neighbouring gneisses and schists. The concentrations of all other major elements of the migmatites are within the variation limits determined for the gneisses and schists. Samples displaying initial migmatization have the following petrography:
  1. The light-coloured portions of the migmatites (leueosome) contain alkalifeldspar, plagioclase, and quartz, the dark-coloured ones (melanosome) are always free of alkalifeldspar and contain, besides ferromagnesian constituents, both plagioclase and quartz.
  2. The plagioclase of the melanosome has a remarkably higher An-content (An30–40mol.-%) than that one of the leueosome (An 9–14 mol.-%).
These observations seem to represent petrographic evidence that leueosome and melanosome have been in a restite-melt-relation at a temperature that just reached or slightly exceeded the value necessary to produce the beginning of melting. In addition, major element analyses of separated leueosome portions of samples displaying the petrography mentioned above have been undertaken. Normative Qz ∶ Ab: Or-ratios calculated from these analyses vary within very narrow limits about the average value of Qz ∶ Ab ∶ Or=36 ∶ 31 ∶ 33. Using the diagrams of von Platen (1965) concerning the crystallization of granitic melts the petrographic findings can be verified from analytical results, i.e. the migmatites of the Beaume valley are considered to be products of minimum temperature melting of the gneisses which still occur unaltered in adjoining areas of lower metamorphic grade.  相似文献   

16.
P, T, \(X_{{\text{CO}}_{\text{2}} }\) relations of gehlenite, anorthite, grossularite, wollastonite, corundum and calcite have been determined experimentally at P f =1 and 4 kb. Using synthetic starting minerals the following reactions have been demonstrated reversibly
  1. 2 anorthite+3 calcite=gehlenite+grossularite+3 CO2.
  2. anorthite+corundum+3 calcite=2 gehlenite+3 CO2.
  3. 3anorthite+3 calcite=2 grossularite+corundum+3CO2.
  4. grossularite+2 corundum+3 calcite=3 gehlenite+3 CO2.
  5. anorthite+2 calcite=gehlenite+wollastonite+2CO2.
  6. anorthite+wollastonite+calcite=grossularite+CO2.
  7. grossularite+calcite=gehlenite+2 wollastonite+CO2.
In the T, \(X_{{\text{CO}}_{\text{2}} }\) diagram at P f =1 kb two isobaric invariant points have been located at 770±10°C, \(X_{{\text{CO}}_{\text{2}} }\) =0.27 and at 840±10°C, \(X_{{\text{CO}}_{\text{2}} }\) =0.55. Formation of gehlenite from low temperature assemblages according to (4) and (2) takes place at 1 kb and 715–855° C, \(X_{{\text{CO}}_{\text{2}} }\) =0.1–1.0. In agreement with experimental results the formation of gehlenite in natural metamorphic rocks is restricted to shallow, high temperature contact aureoles.  相似文献   

17.
Phase relations of biotite and stilpnomelane in the greenschist facies   总被引：1，自引：0，他引：1  
Phase relations of biotite and stilpnomelane and associated silicate minerals have been studied in rocks of the greenschist facies, chiefly from Otago, New Zealand and western Vermont, but also from Scotland, Minnesota-Michigan iron range, and northwest Washington. That stilpnomelane in the greenschicht facies crystallizes initially with nearly all iron in the ferrous state is indicated by chemical analyses, high p-T experiments, and phase relationships. Alteration of stilpnomelane after metamorphism not only oxidizes iron but leaches potassium; corrections for both effects must be made in using analyses of brown stilpnomelane in studies of phase relations. Two discontinuous reactions which produce biotite at the biotite isograd have been identified:
  1. muscovite+stilpnomelane+actinolite→ biotite+chlorite+epidote
  2. chlorite+microcline→ biotite+muscovite. Biotite produced by the first of these reactions has a limited range of variation in Fe/Mg. As grade advances within the biotite zone more magnesian and ferruginous biotites become stable in consequence of the two continuous reactions:
  3. muscovite+actinolite+chlorite→ biotite (Mg-rich)+epidote
  4. muscovite+stilpnomelane→ biotite (Fe-rich)+chlorite.
Stilpnomelane is stable in muscovite-free rocks throughout the biotite zone, and even up to the grade at which hornblende becomes stable. Phengitic muscovite is stable throughout the biotite zone in New Zealand and thus apparently does not contribute to the formation of biotite until a higher grade is reached.  相似文献   

18.
In the Rogaland granulites of Southern Norway, thin anhydrous pyroxene-bearing margins (5–10 mm) are observed mainly in migmatitic banded gneisses at the contact between hornblende-rich metabasites and charnockites. According to field data, the development of these margins post-dates any deformation. Petrographic data show that they are zoned. Three different types have been recognized:
  1. Metabasite/plagioclase + orthopyroxene/plagioclase + clinopyroxene/gneiss
  2. Metabasite/plagioclase + orthopyroxene + clinopyroxene/plagioclase + clinopyroxene/gneiss
  3. Metabasite/plagioclase + orthopyroxene/gneiss
The first zone corresponds to the reaction of amphibole and biotite of the metabasite into pyroxenes. The second zone, which is not present in the third type, developed essentially at the expense of gneiss and the Z1/Z2 boundary is likely to correspond to the original contact between metabasite and gneiss. When the anorthite content of plagioclase and the Fe no. of pyroxenes are strongly different between adjacent metabasite (An38–40 for plagioclase; Fe no. [Fe/(Fe + Mg)], 0.51–0.52 for orthopyroxene) and gneiss (An22–26; Fe no., 0.58–0.59), the solid solution compositions evolve continuously through the margin from the value in the metabasite to that in the gneiss. On the other hand, a margin is also present when plagioclase and pyroxenes have similar compositions in the adjacent rocks implying that reactions can also take place in the absence of contrasted mineral compositions. The continuous change in solid solution composition as well as evidence of transport in both directions indicates diffusion rather than infiltration as the dominant metasomatic mechanism. The small thickness of the margins is also more typical of a diffusion process. Isocon diagrams demonstrate that Al, Ti, and P are perfectly inert components and that no significant mass or volume change occurred during the margin development. Mass balance of this small-scale granulite formation has been estimated with reference to a perfectly inert component (Ti) and assuming that the metabasite bands were compositionally homogeneous. Most of the geochemical variation is mineralogically controlled. Relative to metabasite, Si and Na are increased due to pyroxenes crystallization and to compositional change of plagioclase when it occurs. Potassium decreases because of biotite disappearance. There is no significant variation in U content but Th is slightly decreased. Zirconium and Hf are not affected whereas Nb, Ta and Mn display the largest increase that requires the involvement of a larger volume of metabasite than that observed in the margin. The K/Rb ratio is increased. Fluorine is strongly depleted due to destabilization of amphibole and biotile. The rare-earth element content in margins is either similar to that of the metabasite or intermediate between that of gneiss and metabasite. This last feature is induced by the development of margins at the expense of both adjacent rock types. Saturation surfaces in chemical potential space provide a graphical method for determination of the parameters controlling the diffusion process. In the simplified system CaO?MgO?SiO2?Al2O3?H2O, these chemical potential diagrams show that evolution along a $(\mu )_{H_2 O} $ gradient cannot take into account the three different types of margins. A $(\mu )_{H_2 O} $ gradient is thus not prerequisite to the margin development. On the other hand, the succession of zones observed in the different types of margins can be obtained in a $(\mu )_{CaO} - (\mu )_{MgO} - (\mu )_{SiO_2 } $ diagram. This suggests that the $(\mu )_{SiO_2 } $ gradient existing between the two adjacent rocks controls the margin development in all cases. Moreover, the variable contrast of plagioclase composition between the adjacent rocks is responsible for the presence of one or two pyroxenes is the first zone. The absence of the second clinopyroxene-bearing zone in the third type of margin is likely due to the scarcity of orthopyroxene in the gneiss.  相似文献   

19.
Amphibole thermodynamics are approximated with the symmetric formalism (regular solution model for within-site non-ideality and a reciprocal solution model for cross-site-terms) in order to formulate improved thermometers for amphibole-plagioclase assemblages. This approximation provides a convenient framework with which to account for composition-dependence of the ideal (mixing-on-sites) equilibrium constants for the equilibria:
  1. edenite+4quartz=tremolite+albite
  2. edenite+albite=richterite+anorthite
For A and B all possible within-site and cross-site interactions among the species □?K?Na?Ca?Mg?Fe2+?Fe3+?Al?Si on the A, M4, M1, M3, M2 and T1 amphibole crystallographic sites were examined. Of the 36 possible interaction energy terms, application of the symmetric formalism results in a dramatic simplification to eight independent parameters. Plagioclase nonideality is modelled using Darken's quadratic formalism. We have supplemented an experimental data set of 92 amphibole-plagioclase pairs with 215 natural pairs from igneous and metamorphic rocks in which the pressure and temperature of equilibration are well constrained. Regression of the combined dataset yields values for the eight interaction parameters as well as for apparent enthalpy, entropy and volume changes for each reaction. These parameters are used to formulate two new thermometers, which perform well (±40°C) in the range 400–1000°C and 1–15 kbar over a broad range of bulk compositions, including tschermakitic amphiboles from garnet amphibolites which caused problems for the simple thermometer of Blundy and Holland (1990). For silica-saturated rocks both thermometers may be applied: in silica-undersaturated rocks or magmas thermometer B alone can be applied. An improved procedure for estimation of ferric iron in calcic amphiboles is presented in the appendix.  相似文献   

20.
The following conclusions can be drawn from the work reported in this paper:
  1. Sixteen samples were determined for uranium by spectrophotometric method. The uranium content in the sea floor sediments of the Bohai Gulf ranges from 1.6 to 6.3 ppm, with an average of 4.3 ppm.
  2. Statistical data show close relationship between U concentration and grain size. Relatively larger amount of uranium was found accumulated in mud than in sand. The bulk of uranium is assumed to be derived from terrestrial detrital minerals.
  3. A positive correlation between U and Fe is recognized. Similar relation also can be seen between U and Al. The plot of U concentration vs. Fe is linear, and can be expressed by the linear regression equation:Y=?0.37+1.35X. The plot of U against Al gives an equation ofY=?2.48+1.01X.
  4. The average U/Corg. ratio for these sediments is 7×10?4, and the average ratios of U/P, U/Mn, and U/CaCO3 are 100×10?4, 50×10?4 and 2×10?4, respectively.
  5. Compared with the abundances of other shelf sediments, the average concentration of U in the area under consideration is close to that of sediments on the selves of Japan and the Gulf of Mexico, and the Black Sea. Uranium concentration in the Bohai Gulf sediments is comparable to that of the continental crust, but differs from that of deep-sea clay.
  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号