The Blosseville Coast basalts of East Greenland: Their occurrence,composition and temporal variations

In the system CaO-MgO-A1₂O₃-SiO₂ the tie lines connecting anorthite with other phases are sequentially broken down with increasing pressure according to the following univariant reactions: anorthite+ enstatite_ss+sillimanite pyrope-grossular_ss+quartz (3), anorthite+enstatite_ss pyrope-grossular_ss+diopside_ss+quartz (2), anorthite+pyrope-grossular_ss+ quartz diopside_ss+kyanite (4) and anorthite+diopside_ss grossular-pyrope_ss +kyanite+quartz (8). At 1,200 ° C these reactions occur at 14.5± 0.5, 15.5±0.5, 19.5±0.5 and 26.4±1 kilobar and have positive slopes (dP/dT) of 1±0.5, 2.8±0.5, 13.3±0.5 and 24±2bars/°C respectively. An invariant point involving kyanite rather than sillimanite, occurs at 850 °C±25 °C and 14.5±0.5kbar at the intersection of reactions (3), (2) and (4). Reaction(4) exhibits significant curvature with an increase in dP/dT from 13.3±0.5 to 18.5± 0.5 bars/°C between 1,050° and 850° C. The pressure at which the complete grossular-pyrope join is stable with quartz is estimated at 41 ± 1 kbar at 1,200 ° C. The pressure at which garnet appears according to reaction (2) is lowered by 5 kbar for a composition with anorthite and orthopyroxene (En_0.5Fs_0.5). Enstatite and plagioclase (An_0.5Ab_0.5) first produce garnet at 2 kbar higher pressure than enstatite and pure anorthite (reaction (2)). The calcium content of garnet in various divariant assemblages is relatively insensitive to temperature but very sensitive to pressure, it is therefore a useful geobarometer. At metamorphic temperatures of 700–850 °C pressures of 8–10 kbar are required for the formation of quartz-bearing garnet granulites containing calcic plagioclase and with (Mg/Mg+Fe) bulk = 0.5.     

Experimentelle Untersuchung der Metamorphose von kieselig dolomitischen Sedimenten

The equilibrium conditions of the following diospide forming reactions have been determined: 1 tremolite+3 calcite+2 quartz 5 diopside+3 CO₂+1 H₂O (6) 1 Ca₂Mg₅[(OH)₂Si₈O₂₂]+3 CaCO₃+2 SiO₂ 5 CaMg[Si₂O₆]+3 CO₂+1 H₂O 1 tremolite+3 calcite 4 diopside+1 dolomite+1 CO₂+1 H₂O (7) 1 Ca₂Mg₅[(OH)₂Si₈O₂₂]+3 CaCO₃ 4 CaMg[Si₂O₆]+1 CaMg(CO₃)₂+1 CO₂+1 H₂O 1 dolomite+2 quartz 1 diopside+2 CO₂ (8) 1 CaMg(CO₃)₂+2 SiO₂ 1 CaMg[Si₂O₆]+2 CO₂ The experimentally determined equilibrium data of the heterogeneous bivariant reaction (6) are shown in the temperature- -diagram of Fig. 6. For the total fluid pressures of 500 and 1,000 bars the equilibrium temperatures valid for -values >0.75 were calculated using equilibrium constants derived from experimental equilibrium data at smaller values of , and fugacities of CO₂ and H₂O (see Fig. 7). The equilibrium data of reaction (6) were also calculated thermodynamically for the total fluid pressure of 500 and 1,000 bars (see Fig. 3). The results found by this method agree well with those of the experimental investigation (see Figs. 8 and 9). The equilibrium data of the heterogeneous univariant reaction (8) were calculated thermodynamically only (see Fig. 10), because an experimental determination has not been successful so far. The calculated decrease of the equilibrium temperature of reaction (8) caused by a decrease of the CO₂-concentration and a corresponding increase of the H₂O-concentration is shown in Fig. 11. Combining this curve and the equilibrium curve of reaction (6) determined at a fluid pressure of 1,000 bars, an isobaric invariant point of intersection results [see Fig. 12 and also point (II) in Fig. 2 of Metz and Trommsdorff (1968)]. From this point of intersection two further equilibrium curves radiate, i.e. the isobaric univariant equilibrium curves of the reaction (4) (not investigated in this paper) and (7) (see Fig. 14); this follows from phase theory. The equilibrium data of the heterogeneous bivariant reaction (7) were calculated thermodynamically for the total fluid pressure of 1,000 bars. The calculated -curve, since precisely meeting the isobaric invariant point (II), is very well consistent with the equilibrium data of reactions (6) and (8).During metamorphism of siliceous dolomites diopside is formed almost exclusively by reaction (6). In Fig. 15 the equilibrium data of this reaction are shown in a P _f -temperature-diagram (P _f is the total fluid pressure). In this diagram the maximum temperatures at which diopside is formed correspond to the temperature-maxima of the -equilibrium curves of Fig. 6. The temperature range shown in Fig. 15 covers all equilibrium temperatures for -values from 0.1 to approx. 1.0; the extremely low temperatures of diopside formation valid for <0.1 are not included. Generally, however, such low CO₂ concentrations are not to be expected in the process of diopside formation during the progressive metamorphism of siliceous dolomites, because the formation of tremolite consuming H₂O and liberating CO₂ is the preceeding reaction at lower temperatures. Apart from the conditions of diopside formation, Fig. 15 includes the pressure-temperature data of staurolite formation (Hoschek, 1965–1969; Richardson, 1968; Ganguly and Newton, 1968). The equilibrium data of both reactions show that the formation of staurolite in FeO-rich pelitic sediments coincides with diopside formation in siliceous dolomites only at pressures below approx. 1 kb. This is realized during shallow contact metamorphism; while at higher pressures the formation of diopside takes place at higher temperatures than the formation of staurolite. These statements which are results of equilibrium data determined experimentally very well agree with petrographic observations, e.g. in the contact aureole of Bergell granite, and in the regional metamorphism of the Lepontine Alps (E. Niggli, 1960; E. Niggli and C. E. Niggli, 1965; Trommsdorff, 1966).     

Anthophyllit und Hornblende in einigen metamorphen Reaktionen

Reactions which occur at the lower boundary of the hornblende-hornfels facies and in the so-called pyroxene-hornfels facies were experimentally investigated for an ultrabasic rock at 500, 1000 and 2000 bars H₂O pressure.The starting material used was a mixture of natural chlorite, talc, tremolite and quartz such that its composition, except for surplus quartz, corresponded to that of an ultrabasic rock. The atomic ratio Fe²⁺+Fe²⁺/Mg+Fe³⁺+Fe³⁺ in the system was 0.16.The lower boundary of the hornblende-hornfels facies was defined by the formation of the orthorhombic amphibole anthophyllite and hornblende according to the following idealized reaction: chlorite+talc+tremolite+quartz hornblende+anthophyllite+H₂O In effect, this reaction consists of the two bivariant reactions: chlorite+tremolite+quartz hornblende+anthophyllite+H₂O talc+chlorite anthophyllite+quartz+H₂OThe equilibrium temperatures obtained for the two reactions in the given system are practically the same and are as follows: 535±10°C at 500 bars H₂O pressure 550±20°C at 1000 bars H₂O pressure 560±10°C at 2000 bars H₂O pressure 580±10°C at 4000 bars H₂O pressureAt 2000 bars and higher temperatures within the hornblende-hornfels facies, anorthite is formed in addition to hornblende and anthophyllite, probably according to the following reaction: hornblende₁+quartz hornblende₂+anthophyllite+anorthite+H₂O; because of the formation of anorthite it is to be expected that the hornblende in this case is poorer in aluminium than the hornblende at 500 and 1000 bars. Winkler (1967) suggests renaming the pyroxene-hornfels facies as K-feldspar-cordierite-hornfels facies which, in turn, is subdivided into a lower-temperature orthoamphibole subfacies without orthopyroxene and a higher-temperature orthopyroxene subfacies without orthoamphibole. The orthopyroxene subfacies itself may in its lower temperature part still carry hornblende which finally disappears in the higher temperature part.The appearance of orthopyroxene characterizes the transition from the orthoamphibole to the orthopyroxene subfacies of the K-feldspar-cordierite hornfels facies. The following reaction takes place at pressures lower than 2000 bars: hornblende₁+anthophyllite hornblende₂+enstatite+anorthite+H₂OSince at 2000 bars an Al-poor hornblende already exists in the hornblende-hornfels facies, it is very likely that here only anthophyllite breaks down to give enstatite+quartz+H₂O.The equilibrium temperatures for these reactions which give rise to enstatite are: 650±10°C at 250 bars H₂O pressure 690±10°C at 500 bars H₂O pressure 715±10°C at 1000 bars H₂O pressure 770±10°C at 2000 bars H₂O pressureOnly after an increase in temperature to about 710°C at 500 bars and about 770°C at 1000 bars does hornblende in the system investigated here break down completely according to the reaction: hornblende = enstatite+anorthite+diopside+H₂OExcept at very small H₂O-pressures (see Fig. 3), there exists, therefore, a region within the orthopyroxene subfacies where hornblende, enstatite and anorthite coexist. As a result we have, as mentioned above, a lower-temperature and a higher-temperature part of the orthopyroxene subfacies, and it is only in the latter part that the parageneses correspond to the pyroxene-hornfels facies as stated by Eskola (1939).Summing up, the starting material consisting of chlorite, talc, tremolite plus quartz remains unchanged in the albite-epidote-hornfels facies; this gives rise in the hornblende-hornfels facies to the paragenesis hornblende+anthophyllite, or — at higher pressures — to hornblende+anthophyllite+anorthite. For the particular composition of the starting material, however, no reactions take place at the transition of the hornblende-hornfels facies to the orthoamphibole subfacies of the K-feldspar-cordierite-hornfels facies as this transition is typified by the breakdown of muscovite in the presence of quartz. However, at the end of the orthoamphibole subfacies the breakdown of anthophyllite, by which orthopyroxene is formed, heralds the onset of the orthopyroxene subfacies. In this subfacies — at greater than about 300 bars — hornblende is still present and coexists with enstatite and anorthite, but with rising temperature hornblende breaks down to give way to the paragenesis enstatite+anorthite+diopside. The experimentally determined parageneses confirm known petrographic occurrences.

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Für die Förderung dieser Arbeit danken wir der Deutschen Forschungsgemeinschaft vielmals. Der Dank von Choudhuri gilt dem Akademischen Auslandsamt der Universität Göttingen für ein Stipendium, das ihm den Abschluß seiner Studien an der Universität Göttingen ermöglichte.     

Pyroxmangite: Stability in H2O-CO2 mixtures at a total pressure of 2000 bars

The curve of the reaction: 1 Rhodochrosite + 1 Quartz Pyroxmangite was determined experimentally using gas mixtures with different CO₂H₂O ratios at a total pressure of 2 kb. In pure CO₂, the equilibrium temperature is 508±2° C. By plotting the equilibrium data in terms of InX_CO2 versus reciprocal temperature, a H of reaction of 51.7 kcal was obtained. This plot shows a straight-line relationship, indicating internal consistency of the data, and is in good agreement with previous findings that CO₂ and H₂O mix ideally within the investigated temperature range (400–600° C). When applied to pyroxmangites formed during moderate greenschist facies metamorphism, these data indicate a high acitivity of H₂O.     

Experimental investigation of the metamorphism of siliceous dolomites III. Equilibrium data for the reaction: 1 Tremolite + 11 dolomite ⇌ 8 forsterite + 13 calcite + 9 CO2 +1H2O for the total pressures of 3,000 and 5,000 bars

The temperature-X _{CO 2}-equilibrium data for the reaction 1 tremolite + 11 dolomite 8 forsterite + 13 calcite + 9 CO₂ +1H₂O have been determined at total pressures (P _{CO 2} + P _H2_O) of 3,000 and 5,000 bars. The results are shown in Figure 2 along with the data for the total pressure of 1,000 bars (Metz, 1967).The MgCO₃ contents of the magnesian-calcites formed during the experiments agree very well with the calcite-dolomite-solvus which can be recalculated from Equation (1) and the activity coefficients for MgCO₃ in magnesiancalcite as given by Gordon and Greenwood (1970).If the T-X _{CO 2}-equilibrium data are calculated from the equilibrium constant as given by Skippen (1974), assuming ideal mixing of CO₂ and H₂O, good agreement is achieved for the total pressure of 1,000 bars (see Figs. 4 and 5). At a total pressure of 3,000 bars, however, the calculated equilibrium temperatures are about 40 ° C below the experimentally determined values (see Fig. 6). This difference increases up to 70 ° C for a total pressure of 5,000 bars (see Fig. 7).From the experimentally determined equilibrium conditions of the assemblage: tremolite + dolomite + forsterite + magnesian calcite (see Fig. 8) the pressure of metamorphism can be estimated if the temperature is determined by the MgCO₃-content of the magnesian-calcite from the calcite-dolomite solvus. However, when using the data of Figure 8, attention has to be drawn to the limiting condition of X _{CO 2}0.2.Simplified reaction equation not considering solid solution in the carbonates     

Interdiffusion of H2O and CO2 in metamorphic fluids at ∼490 to 690°C and 1 GPa

Interdiffusion coefficients have been determined for H₂O-CO₂ mixtures by quantifying the flux of CO₂ between two fluid-filled chambers in a specially designed piston-cylinder cell. The two chambers, which are maintained at 1.0 GPa and at temperatures differing by ∼100°C, each contain the X_CO2-buffering assemblage calcite + quartz + wollastonite, in H₂O. The positive dependence of X_CO2 on temperature results in a down-temperature, steady-state flux of CO₂ through a capillary tube that connects the two chambers. This flux drives the wollastonite = calcite + quartz equilibrium to the right in the cooler chamber, producing a measurable amount of calcite that is directly related to CO₂-H₂O interdiffusion rates. Diffusivities calculated from seven experiments range from 1.0 × 10⁻⁸ to 6.1 × 10⁻⁸ m²/s for mean capillary temperatures between ∼490 and 690°C. The data set can be approximated by an Arrhenius-type relation:

     

The reaction enstatitess+sillimanite ⇌ sapphiriness+quartz in the system MgO-Al2O3-SiO2

The univariant pressure temperature curve of the reaction aluminous enstatite solid solution+sillimanitesapphirine solid solution+quartz was determined experimentally in the pressure range between 12 and 20 kb. It is defined by four reversals at 15 kb, 1140°+10°C; at 1190°C, 15.9±1 kb; at 1300°C, 17.2±1 kb; and at 1400°C, 18±1 kb. Among the coexisting phases the Al-content of the enstatites increases strongly, with rising temperatures and pressures, up to values approaching that of pyrope composition, whereas the Al-content of the sapphirine solid solution appears to increase only slightly. Concomitantly, the sillimanites, most probably of invariant composition, exhibit growing Al/Si-disorder. These compositional and structural variations, in addition to the changing stoichiometry of the reaction equation, cause the progressively decreasing positive dP/dT slope of the equilibrium curve. — The assemblage sapphirine + quartz found in natural granulites is indicative of conditions of water pressure much lower than total pressure.     

Miscibility and compatibility of braunite,Mn2+Mn 6 3+ O8/SiO4, in the system Mn-Si-O at 1 atm in air

Experimental investigations between 800 ° to 1,100 ° C yielded no evidence for extensive substitution of Mn²⁺+Si⁴⁺2Mn³⁺ in braunite, leading to a complete solid solution series between partridgeite (Mn₂O₃) and braunites with silica contents up to 40 wt. % as proposed by Muan (1959a, b). In the presence of excess manganese braunite of nearly ideal composition coexists at 800 ° C with partridgeite and at T1,000 ° C with hausmannite (Mn₃O₄). At 800 ° C and 1,000 ° C braunite coexists, in the presence of excess silica, with a SiO₂-polymorph and at 1,100 ° C with rhodonite (MnSiO₃). Quantitative analysis of the X-ray patterns of coexisting cristobalite and braunite confirms a maximum silica-excess in braunite of only about 2 wt.% over the ideal composition, Mn²⁺Mn ₆ ³⁺ SiO₁₂.     

Hydrothermal studies on fluorine metamorphic reactions in siliceous dolomite

Metamorphic fluorine reactions in silica deficient dolomite have been investigated between 300 and 560° C in the pressure range from 700 to 2000 bars.The equilibrium temperature for the reaction: 1 talc + 7 dolomite + 2 fluorite 2 chondrodite + 9 calcite + 5 CO₂ + 1 H₂O was experimentally determined as a function of the CO₂-content of the CO₂-H₂O gas phase for the total pressure of 2000 bars. The results are given as an isobaric equilibrium curve in the -diagram of Fig. 2. The equilibrium data were checked and confirmed by reversed reactions.Under the specified pressure conditions the equilibrium temperatures for the above reaction changes from 380° C for very low -values to approximately 500° C for -values around 0.5, and then remains slightly above 500° C in the -interval between 0.5 and 1.0. The temperature conditions for the formation of chrondrodite from tremolite, dolomite, and fluorite are similar to those for the reaction including talc.     

Die Reaktion 2 Zoisit+1 CO2 ⇌ 3 Anorthit+1 Calcit+1 H2O

The equilibrium conditions of the following reaction 2 zoisite +1 CO₂?3 anorthite+1 calcite+1 H₂O 2 Ca₂Al₃[O/OH/SiO₄/Si₂O₇]+1 CO₂?3 CaAl₂Si₂O₈+1 CaCO₃+1 H₂O have been determined experimentally at total pressures of P _j= 2000 bars, P _f =5000 bars, and P _f =7000 bars. Owing to the vertical position of the equilibrium curves in isobaric T- \(X_{{\text{CO}}_{\text{2}} }\) diagrams, the composition of the binary H₂O-CO₂ fluid phase coexisting with zoisite is independent of temperature in the temperature interval investigated. According to our experiments, orthorhombic zoisite is only stable in equilibrium with a fluid phase at a concentration of CO₂ which is less than, respectively, ca. 2 Mol% CO₂ at P _f =2000 bars, ea. 6 Mol% at P _f =5000 bars, and ca. 10 Mol% at P _f =7000 bars. Thus, the fluid phase coexisting with zoisite is rich in H₂O. While this is independent of temperature the experimental data demonstrate that the influence of pressure cannot be neglected: With increasing pressure the concentration of CO₂ of the fluid phase coexisting with zoisite can rise a little. The position of the reaction studied, which is independent of temperature and exhibits small values of \(X_{{\text{CO}}_{\text{2}} }\) ,leads to two important petrogenetic conclusions:

1. The occurrence of zoisite is an indicator for a CO₂-poor and H₂O-rich fluid composition during metamorphism of marly calcsilicates.
2. If the concentration of CO₂ of the fluid phase coexisting with zoisite exceeds the equilibrium value of \(X_{{\text{CO}}_{\text{2}} }\) calcite+anorthite+H₂O is formed from zoisite+CO₂. Thus, a considerable increase in the anorthite-content of plagioelase is possible.

     

A new sampling technique for fluid phases in hydrothermal experiments applied to the determination of the HF-fugacities of the WFQ-buffer

A new sampling device is described which allows the separation of a fluid phase from the solid reactants in hydrothermal experiments at run conditions. The new method has been tested at temperatures up to 1,100° C at a total vapor pressure of 1 kbar.HF-concentrations of the fluid phase in equilibrium with quartz, fluorite, and wollastonite in the reaction CaF₂+SiO₂+H₂O CaSiO₃+2 HF known as the WFQ-buffer (Munoz and Eugster 1969) have been determined by direct chemical analysis using a fluoride electrode. HF-concentrations measured in the fluid phase range from 39±3 ppm at 510° C to 1,968±98 ppm at 820° C. This is equivalent to fugacities of 0.036 bar at 510° C compared to 0.033 as calculated from thermodynamic data, and 2.238 bar at 820° C compared to 2.142 bar. Equilibrium was reversed by starting out from aqueous HF-solutions in which the HF-concentrations were above or below the equilibrium value.     

The stability of merwinite in the system CaO-MgO-SiO2-H2O-CO2 with CO2-poor fluids

The stability of merwinite (Mw) and its equivalent assemblages, akermanite (Ak)+calcite (Cc), diopside (Di)+calcite, and wollastonite (Wo)+monticellite (Mc)+calcite was determined at T=500–900° C and P _f=0.5–2.0 kbar under H₂O–CO₂ fluid conditions with X _{CO 2}=0.5, 0.1, 0.05, and 0.02. Merwinite is stable at P _f=0.5 kbar with T>700° C and X _{CO 2}<0.1. At P _f=2.0 kbar, the assemblage Di+Cc replaces merwinite at all T and X _{CO 2} conditions. At intermediate P _f=1 kbar, the assemblage Ak+Cc is stable above 707° C and Wo+Mc+Cc is stable below 707° C. The univariant curve for the reaction Di+Cc=Wo+Mc+CO₂ is almost parallel to the T axis and shifts to low P _f with increasing X _{CO 2}, with the assemblage Di+Cc on the high-P _f side. The implications of the experimental results in regard to contact metamorphism of limestone are discussed using the aureole at Crestmore, California as an example.     

Die obere Stabilitätsgrenze von Tremolit bei der Metamorphose von kieseligen Karbonaten

During the metamorphism of siliceous carbonates, decomposition of tremolite yields diopside, enstatite, quartz and H₂O according to the following reaction: 1 tremolite 2 diopside + 3 enstatite + 1 quartz + 1 H₂O.For the application to natural processes, it is of special interest to evaluate the equilibrium temperature of this reaction, as a function of the CO₂-content of the H₂O-CO₂ fluid phase for several total pressures. These values were calculated for the total pressures of 1000 and 2000 bars, usingBoyd's experimentally determined univariant equilibrium data [Fig. 1 (Boyd, 1954 and 1959)]. Curves (a) and (b) of Fig. 3 give the results in a temperature-X _{CO 2}-diagram.The decomposition curves of tremolite intersect the equilibrium curves of other reactions which also take place during metamorphism of siliceous carbonates. If the total pressure can be estimated, these points of intersection together with the appropriate field observations will give information on the temperature and composition of the fluid phase during metamorphism.     

Phase relations of talc and tremolite in metamorphic calcite-dolomite sediments in the southern portion of the Damara Belt (South West Africa)

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.     

Comparison of the high-pressure schist belts of New Caledonia and Sanbagawa,Japan

The high-pressure schist terranes of New Caledonia and Sanbagawa were developed along the oceanic sides of sialic forelands by tectonic burial metamorphism. The parent rocks were chemically similar, as volcanic-sedimentary trough or trench sequences, and metamorphic temperatures in both belts were 250° to 600° C. From phase equilibria curves, total pressures were higher for New Caledonia (6–15 kb) than for Sanbagawa (5–11 kb) and the estimated thermal gradients were 7–10° C/km and 15° C/km respectively.PT paths identify the higher pressure in New Caledonia (P differences 2 kb at 300° C and 4 kb at 550° C) with consequent contrast in progressive regional metamorphic zonation for pelites in the two areas: lawsonite-epidote-omphacite (New Caledonia) and chlorite-garnet-biotite (Sanbagawa). In New Caledonia the Na-amphibole is dominantly glaucophane and Na-pyroxenes associated with quartz are Jadeite (Jd_95–100) and omphacite; in Sanbagawa the amphibole is crossite or riebeckite and the pyroxene is omphacite (Jd₅₀). For both areas, garnet rims show increase in pyrope content with advancing grade, but Sanbagawa garnets are richer in almandine. Progressive assemblages within the two belts can be equated by such reactions as:New Caledonia Sanbagawa glaucophane+paragonite+H₂Oalbite+chlorite+quartz glaucophane+epidote+H₂Oalbite+chlorite+actinolite and the lower pressure Japanese associations appear as retrogressive phases in the New Caledonia epidote and omphacite zones.The contrasts inPT gradient, regional zonation and mineralogy are believed due to differences in the tectonic control of metamorphic burial: for New Caledonia, rapid obduction of an upper sialic plate over an inert oceanic plate and sedimentary trough; and for Sanbagawa, slower subduction of trench sediments beneath a relatively immobile upper plate.     

The upper thermal stability of clinochlore,Mg5Al[AlSi3O10](OH)8, at 10–35 kb P_{{\text{H}}_{\text{2}} {\text{O}}}

Clinochlore, which is, within the limits of error, the thermally most stable member of the Mg-chlorites, breaks down at = P _tot to the assemblage enstatite+forsterite+spinel+H₂O along a univariant curve located at 11 kb, 838 ° C; 15kb, 862 ° C; and 18 kb, 880 ° C (±1 kb ±10 ° C). At water pressures above that of an invariant point at 20.3 kb and 894 ° C involving the phases clinochlore, enstatite, forsterite, spinel, pyrope, and hydrous vapor, clinochlore disintegrates to pyrope+forsterite+spinel+H₂O. The resulting univariant curve has a steep, negative dP/dT slope of –930 bar/ °C at least up to 35 kb.Thus, given the proper chemical environment, Mg-chlorites have the potential of appearing as stable phases within the earth's upper mantle to maximum depths between about 60 and 100 km depending on the prevailing undisturbed geotherm, and to still greater depths in subduction zones. However, unequivocal criteria for mantle derived Mg-chlorites are difficult to find in ultrabasic rocks.     

The solution behavior of H2O in peralkaline aluminosilicate melts at high pressure with implications for properties of hydrous melts

Solubility and solution mechanisms of H₂O in depolymerized melts in the system Na₂O-Al₂O₃-SiO₂ were deduced from spectroscopic data of glasses quenched from melts at 1100 °C at 0.8-2.0 GPa. Data were obtained along a join with fixed nominal NBO/T = 0.5 of the anhydrous materials [Na₂Si₄O₉-Na₂(NaAl)₄O₉] with Al/(Al+Si) = 0.00-0.25. The H₂O solubility was fitted to the expression, X_H2O=0.20+0.0020f_H2O-0.7X_Al+0.9(X_Al)², where X_H2O is the mole fraction of H₂O (calculated with O = 1), f_H2O the fugacity of H₂O, and X_Al = Al/(Al+Si). Partial molar volume of H₂O in the melts, , calculated from the H₂O-solulbility data assuming ideal mixing of melt-H₂O solutions, is 12.5 cm³/mol for Al-free melts and decreases linearly to 8.9 cm³/mol for melts with Al/(Al+Si) ∼ 0.25. However, if recent suggestion that is composition-independent is applied to constrain activity-composition relations of the hydrous melts, the activity coefficient of H₂O, , increases with Al/(Al+Si).Solution mechanisms of H₂O were obtained by combining Raman and ²⁹Si NMR spectroscopic data. Degree of melt depolymerization, NBO/T, increases with H₂O content. The rate of NBO/T-change with H₂O is negatively correlated with H₂O and positively correlated with Al/(Al+Si). The main depolymerization reaction involves breakage of oxygen bridges in Q⁴-species to form Q² species. Steric hindrance appears to restrict bonding of H⁺ with nonbridging oxygen in Q³ species. The presence of Al³⁺ does not affect the water solution mechanisms significantly.     

Gehlenite stability in the system CaO-Al2O3-SiO2-H2O-CO2

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.     

Die Reaktion Phlogopit + Calcit + Quarz = Tremolit + Kalifeldspat + H2O + C2O

Hydrothermal experiments with mixtures of synthetic minerals have shown the reversibility of the reaction 5 phlogopite + 6 calcite + 24 quartz = 3 tremolite + 5 K-feldspar + 2 H₂O + 6 CO₂. In an isobaric T – diagram the equilibrium curve reaches a maximum at = 0,75. The P, T-values for this maximum are: 2 kb-523°; 4 kb-585°; 6 kb-625°; P±5%, T±10° C. These results give a first approximation of the P, T conditions responsible for a similar mineral reaction which has been recorded from natural metamorphic assemblages.

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Herrn Prof. H. G. F. Winkler danke ich für anregende Diskussionen, desgleichen Herrn Dr. D. Puhan für wichtige Hinweise und Mitteilung seiner exp. Daten. Herrn Prof. V. Trommsdorff und Herrn P. H. Thompson bin ich für petrographische Angaben zu Dank verpflichtet. Der Aufbau der Hydrothermalanlage und die Finanzierung der laufenden Untersuchungen wurde aus den Mitteln des Fonds zur Förderung der wissenschaftlichen Forschung ermöglicht. Für diese Unterstützung gilt daher mein besonderer Dank.     

P CO2 in low-grade metamorphism; zeolite,carbonate, clay mineral,prehnite relations in the system CaO-Al2O3-SiO2-CO2-H2O

Equilibria for several reactions in the system CaO-Al₂O₃-SiO₂-CO₂-H₂O have been calculated from the reactions calcite+quartz=wollastonite+CO₂ (5) and calcite+Al₂SiO₅+quartz=anorthite+CO₂ (19) and other published experimental studies of equilibria in the systems Al₂O₃-SiO₂-H₂O and CaO-Al₂O₃-SiO₂-H₂O.The calculations indicate that the reactions laumontite+CO₂=calcite+kaolinite+2 quartz+2H₂O (1) and laumontite+calcite=prehnite+quartz+3H₂O+CO₂ (3) in the system CaO-Al₂O₃-SiO₂-CO₂-H₂O, are in equilibrium with an H₂O-CO₂ fluid phase having -0.0075 for P _fluid=P _total=2000 bars.These calculations limit the stability of zeolite assemblages to low p CO₂.Using the above reactions as model equilibria, several probelms of p CO₂ in low grade metamorphism are discussed. (a) the problem of producing zeolitic minerals from metasedimentary assemblages of carbonate, clay mineral, quartz. (b) the significance of calcite (or aragonite) associated with zeolite (or lawsonite) in low grade metamorphism and hydrothermal alteration. (c) the reaction of zeolites (or lawsonite) with calcite (or aragonite) to produce dense Ca-Al-hydrosilicates (eg. prehnite, zoisite, grossular).