ALBERT

Notes: Abstract Three Al-Cr exchange isotherms between Zn(Al, Cr)2O4 spinel and (Al, Cr)2O3 corundum crystalline solutions have been studied experimentally at 900°, 1100°, and 1300° C, at a total pressure of 25 kbar. Employing data on the equation of state of corundum (Chatterjee et al. 1982), the experimental results were evaluated thermodynamically. It was found that the thermodynamic mixing properties of Zn(Al, Cr)2O4 spinels are best described in terms of a symmetric Margules equation. The T- and P- dependence of the Margules Parameter, W G Sp , and of ΔG* of the exchange reaction, 1/2 ZnAl2O4 + 1/2 Cr2O3 = 1/2 ZnCr2O4+1/2 A12O3, are found to be ΔG *=1493−2.869·T+0.0081·P and W G Sp (J/mol)=23456+0.0386·P, with T given in K and P in bar.

Notes: Abstract The system MgO-Al2O3-SiO2(MAS) comprises 88–90% of the bulk composition of an average peridotite. The MAS ternary is thus a suitable starting point for exploring peridotite phase relations in multicomponent natural systems. The basic MAS phase relations may be treated in terms of the reactions (see list of symbols etc). (1) py (in Gt)=en (in Opx)+mats (in Opx), (2) en (in Opx)+sp (in Sp)=mats (in Opx)+fo (in Ol), and (3) py (in Gt)+fo (in Ol)=en (in Opx)+sp (in Sp). Extensive reversed phase equilibria data on these three reactions by Danckwerth and Newton (1978), Perkins et al. (1981), and Gasparik and Newton (1984) employing identical experimental methods in the same laboratory have been used by us to deduce the following internally consistent thermodynamic data applying the technique of linear programming:ΔH 298(1) 0 = 2536 J, ΔS 298(1) 0 =− 6.064 J/K;ΔH 298(2) 0 = 29435 J, ΔS 298(2) 0 = 8.323 J/K; andΔH 298(3) 0 =−26899 J, ΔS 298(3) 0 =−14.388 J/K.These data are also found to be consistent with results of calorimetry. Figure 2 shows the calculated phase relations based on our thermodynamic data; they are consistent with the phase equilibria experiments. Successful extension of the MAS phase relations to multicomponent peridotites pivots on the extent to which the effects of the “non-ternary” (i.e. other than MAS) components can be quantitatively handled. Particularly hazardous in this context is Cr2O3, although it barely makes up 0.2 to 0.5 wt% of such rocks. This is because Cr+3 fractionates extremely strongly into Sp. This study focuses on the peridotite phase relations in the MgO-Al2O3-SiO2-Cr2O3 (MASCr) quaternary. Thermodynamic calculations of the MASCr phase relations have been accomplished by using ΔH 298 0 and ΔS 298 0 values for the reactions (1) through (3) indicated above, in conjunction with data on thermodynamic mixing properties of a) binary Sp (sp-pc) crystalline solution (Oka et al. 1984), b) ternary Opx (en-mats-mcts) crystalline solution (this study), and c) binary Gt (py-kn) crystalline solution (this study). The results are shown in P-T projections (Figs. 3a and b) and isobaric-isothermal sections of MASCr in a projection through the component fo onto the SiO2-Al2O3-Cr2O3 ternary (Figs. 4a and b). The most important results of this work may be summarized as follows: 1. With increasing incorporation of Cr+3 into Sp and Gt, the X mats isopleths of the reactions (1) and (2) are shifted to higher temperatures (Fig. 3a); simultaneously, the spinel-peridotite to garnet-peridotite phase transition is moved to higher pressures (Fig. 3b). 2. At identical P and T, the X mats values of Opx coexisting in equilibrium with Ol and Sp is strongly dependent upon the X pc value in the latter phase (Figs. 4a and b). Accurate correction for the composition of Sp is, therefore, a necessary precondition for geothermometry of the spinelperidotites. 3. The discrepant temperatures reported by Sachtleben und Seck (1981, Fig. 5) from the spinel-peridotites of the Eifel area (systematically too high temperatures as a function of X pc in Sp) are demonstrated to be the result of ignoring the nonideality in the chromian spinels.

Notes: Zusammenfassung Studien in den verschiedensten Gebieten mit Regionalmetamorphose zeigen immer wieder, daß die metamorphe Kristallisation mit den Durchbewegungen beginnt und sie meist überdauert.Naha (in diesem Heft) beschreibt aus einem polymetamorphen Komplex einen Fall von — bezogen auf die Faltung — synbis posttektonischer Metamorphose, welcher eine regionale Überschiebung folgt. Damit ist aber noch nicht bewiesen, daß alle diese Vorgänge ein und demselben orogenen Zyklus angehören. Eine solche Beweisführung ist unerläßlich, um eine Metamorphose, die vor der Hauptphase der Tektonik stattfindet, glaubhaft erscheinen zu lassen.

Notes: Abstract The metamorphic rocks of Mesozoic age, found sandwiched between the Pennine nappes of Simplon area, were subjected to a detailed structural-petrological investigation with a view to clarify the nature of the Alpine metamorphism. Preliminary structural works demonstrated the existence of two B-axes — B1 and B2 — both being younger than the regional folding and thrusting movements. Of these two, B1 is restricted to the northernmost belt of the present area and has been imprinted uniformly on the older crystalline rocks of the Aar-Gotthard massif as well as on their Mesozoic sedimentary cover. The B1-axis, with its usually steeply plunging habit, can be traced southwards upto the northernmost fringe of the Pennine Mesozoic metamorphites. The subhorizontal to weakly plunging B2-lineation overprints B1 in the northernmost sector of the region under consideration. Further southwards, it gains rapidly in prominence and becomes the sole linear fabric element. For the most part, B2 coincides roughly with the axes of the regional large scale folds B0; but at places the two may deviate from each other appreciably. The overall behaviour of the B-axes is to be seen in the structural map, in which only the statistical maximum of numerous measurements from each locality has been plotted. Zones of progressive regional metamorphism could be roughly delineated over the whole region. The metamorphic grade rises from low greenschist facies in the NW to middle almandine amphibolite facies to the SE. Although the delineation of the metamorphic facies boundaries is only tentative and it is to be expected that future work would refine them, it is firmly believed that the outline of the pattern would not be necessarily modified thereby. By correlating the individual zones of metamorphic facies of underground exposures of Simplon tunnel with the corresponding ones on the facies map, it was possible to determine approximately the lie of the facies boundary surfaces. They transgress cleanly the regional large scale structures in three dimensions in space, proving thereby that the metamorphism took place after the regional folding and thrusting movements died off. Ancillary evidences on this point were derived through a systematic study of oriented thin sections and intensive petrofabric analysis. It was established that the metamorphic recrystallisation and the post-thrusting penetrative movements leading to the origin of the secondary planar and linear fabric elements went on hand in hand, with the former process ultimately outlasting the latter. In fact, the recrystallisation continued long afterwards under a completely static condition and almost all the porphyroblasts were found to have grown at this stage. The regional metamorphism of this area is therefore independent of the penetrative movements due to Alpine orogenesis and owes its origin to the late plutonic actions. Comparison with other areas shows that the Simplon area could be regarded simply as a prototype of the usual case of regional metamorphism. It is concluded that the regional metamorphism in its ideal form is a complex overprinting of an area of previous dislocation metamorphism by a subsequent phase of plutonic metamorphism. Although deformation does not appear to have played an essential rÔle in bringing about appreciable amount of recrystallisation in regional metamorphic rocks, it ought to be regarded as a conditio sine qua non for the same. Indeed, dislocation metamorphism seems to have paved the path to be trodden over later by plutonic metamorphism — the result being crystalline schists, the products of the so called regional metamorphism. The time-honoured term “regional metamorphism” has got today the sole advantage of being universally understood.

Notes: Zusammenfassung Die tektonische Analyse in den mesozoischen Folgen (Trias, Schistes lustrés und Ophiolithe) der penninischen Zone in der Umrahmung des nördlichen Dora-Maira-Massivs ergab drei altersverschiedene B-Achsen: 1. Älteste NNW-SSE verlaufende Achsen südlich und westlich vom Ambinmassiv, 2. regional verbreitete, stark ausgeprägte E-W-Achsen, 3. jüngste, N-S gerichtete, nur lokal auftretende Achsen. Die beiden jungen B-Achsen lassen sich auch ins Altkristallin hinein verfolgen, was dessen alpidische Überprägung bezeugt. Während die älteste B-Achse im Laufe der Prägung des Großbaues bzw. der Deckenbewegung entstanden sein mag, müssen die beiden jüngeren Achsen bei ihrer Prägung den Großbau bereits fertig vorgefunden haben. Das Verhältnis zwischen metamorpher Kristallisation und Durchbewegung spricht gegen die Annahme größerer Zeiträume zwischen der Prägung der zwei älteren Achsen.

Notes: Abstract  Single-phase K-cymrite, K[AlSi3O8]·H2O, has been synthesized in the P-T range 3≤P(GPa)≤4 and 350≤T(°C)≤650, and characterized by a variety of techniques like SEM, FTIR, and 29Si MAS-NMR. Its thermal expansivity and compressibility have been measured up to 375 °C and 6.0 GPa, respectively. Within the uncertainty of the microchemical determination of H2O by Karl-Fischer titration, it invariably contains 1 mol of H2O per mol of KAlSi3O8. Under the SEM, it appears a small idiomorphic prisms. It is optically negative, with n o=1.553(1) and n e=1.521(1). FTIR spectrum identifies the water in its structure as molecular H2O. Its lattice constants are a=5.3348(1) Å, c=7.7057(1) Å, V= 189.924 Å3, the space group being P6/mmm. The 29Si MAS-NMR suggests a weak short-range order of Al and Si in the symmetrically equivalent tetrahedral sites. A Rietveld structure refinement demonstrates that it is isostructural with cymrite (BaAl2Si2O8·H2O), the structure comprising double tetrahedral sheets with H2O molecules residing in their cavities, K serving as an interlayer cation. Whereas cymrite, with its ordered tetrahedral Al/Si distribution, shows a Pm symmetry, the weak short-range Al/Si order in K-cymrite (abbreviated below as KCym) makes it crystallize in the space group P6/mmm. Three reversal experiments on the reaction K[AlSi3O8]·H2O (KCym)=K[AlSi3O8] (Kfs)+H2O, executed in this study, confirm the earlier results of Thompson (1994) and supplement her data. A simultaneous treatment of those reversals, together with the thermodynamic data for Kfs and H2O available in the literature, helps derive the standard enthalpy of formation (−4233±9.4 kJ/mol) and standard entropy (276.3±10.2 J/K·mol) for K-cymrite. The computed phase relations of KCym in the KAlSi3O8-H2O binary are shown in Figure 4 for three different values of aH 2O. Given a 5 °C/km isotherm in a subducting slab of metasediments in a ultra-high-pressure metamorphic environment, KCym will be expected to grow by hydration of Kfs, unless the aH 2O had been substantially less than 0.5. Nevertheless, how far it can survive exhumation of the subducted terrain will depend critically on the rate of uplifting and on the aH 2O prevailing during that process.

Notes: Abstract Internally consistent thermodynamic data, including their uncertainties and correlations, are reported for 22 phases of the quaternary system CaO-Al2O3-SiO2-H2O. These data have been derived by simultaneous evaluation of the appropriate phase properties (PP) and reaction properties (RP) by the novel technique of Bayes estimation (BE). The thermodynamic model used and the theory of BE was expounded in Part I of this paper. Part II is the follow-up study illustrating an application of BE. The input for BE comprised, among others, the a priori values for standard enthalpy of formation of the i-th phase, Δf H i 0 , and its standard entropy, S i 0 , in addition to the reaction reversal constraints for 33 equilibria involving the relevant phases. A total of 269 RP restrictions have been processed, of which 107 turned out to be non-redundant. The refined values for Δf H i 0 and S i 0 obtained by BE, including their 2σ-uncertainties, appear in Table 4; the Appendix reproduces the corresponding correlation matrix. These data permit generation of computed phase diagrams with 2σ-uncertainty envelopes based on conventional error propagation; Fig. 3 depicts such a phase diagram for the system CaO-Al2O3-SiO2. It shows that the refined dataset is capable of yielding phase diagrams with uncertainty envelopes narrow enough to be geologically useful. The results in Table 4 demonstrate that the uncertainties of the prior values for Δf H i Emphasis〉0 , given in Table 1, have decreased by up to an order of magnitude, while those for S i 0 improved by a factor of up to two. For comparison, Table 4 also lists the refined Δf H i 0 and S i 0 data obtained by mathematical programming (MAP), minimizing a quadratic objective function used earlier by Berman (1988). Examples of calculated phase diagrams are given to demonstrate the advantages of BE for deriving internally consistent thermodynamic data. Although P-T curves generated from both MAP and BE databases will pass through the reversal restrictions, BE datasets appear to be better suited for extrapolations beyond the P-T range explored experimentally and for predicting equilibria not constrained by reversals.

Notes: Abstract Modified Redlich-Kwong (MRK) equations of state have been derived for the pure fluid species H2 and H2O by expressing the parametera as a function ofT andP, andb as as a function ofP only. These equations are valid above 0° and 0.01° C, respectively. For H2O, the prediction of volumes is successful not only in the supercritical, but also in the subcritical range. As a result of this, the saturation curve of H2O can be calculated with a maximum deviation of ±1.4 bar in the range 100–350° C. Between 350° C and the critical point (374.15° C), the uncertainty increases somewhat; this is due to a fundamental inadequacy of the Redlich-Kwong equation itself. These equations of state permit extrapolations to pressures of 100 kbar for H2 and at least 200 kbar for H2O and are, therefore, eminently suited for geochemical applications. Formulation of the MRK of the binary H2-H2O mixtures was achieved by assuming the quadratic mixing rule for the parametersa mix andb+mix. To derive the cross coefficients,aH2-H2Oandb H 2-H 2O, adjustable corrective factors ɛ and τ had to be introduced. TheT- andP-dependences of ɛ and τ are based onP-V-T-X H 2 data (Seward and Franck 1981) to 440° C and 2500 bar. The resulting equation of state very satisfactorily reproduces the volumes observed experimentally at various sets ofT,P, andX H 2. At a total pressure of 2 kbar, positive deviation from ideal mixing behaviour is still perceptible at as high a temperature as 1000° C. At some temperature around 380° C, phase separation sets in, an aqueous solution with dissolved H2 coexisting in equilibrium with an H2-rich fluid with dissolved H2O. The computedP-T-X H 2 surface of this two-phase region agrees well with that observed in Seward and Franck's (1981) experiments. An independent proof of the validity of this equation of state is the accuracy with whichH m ex can be predicted. Calorimetric measurements ofH m ex (Smith et al. 1983, Wormald and Colling 1985) compare excellently with those predictions. This is indicative of the fact that theP-V-T-X H 2 measurements by Seward and Franck (1981) are mutually consistent with the calorimetricH m ex data reported by Smith et al. (1983) and by Wormald and Colling (1985). Extrapolation of the equation of state for the H2-H2O mixture is not recommended to pressures far beyond those for whichP-V-T-X H 2 data are available at present. Despite this, it sets the stage for rigorous thermodynamic treatment of some redox equilibria of interest to geochemistry, which was not possible hitherto because of lack of such data.