Fluid-rock history of granulite facies humite-marbles from Ambasamudram, southern India.

An extensive humite‐bearing marble horizon within a supracrustal sequence at Ambasamudram, southern India, was studied using petrological and stable isotopic techniques to define its metamorphic history and fluid characteristics. At peak metamorphic temperatures of 775±73°C, based on calcite‐graphite carbon isotope thermometry, the mineral assemblages suggest layer‐by‐layer control of fluid compositions. Clinohumite + calcite‐bearing assemblages suggest X_CO2 < 0.4 (at 700°C and 5 kbar), calcite + forsterite + K‐feldspar‐bearing assemblages suggest X_CO2>0.9 (at 790°C); and local wollastonite + scapolite + grossular‐bearing zones formed at X_CO2 of c. 0.3. Retrograde reaction textures such as scapolite + quartz symplectites after feldspar and calcite and replacement of dolomite + diopside or tremolite+dolomite after calcite+forsterite or calcite+clinohumite are indicative of retrogression under high X_CO2 conditions. Calcite preserves late Proterozoic carbon and oxygen isotopic signatures and the marble lacks evidence for extensive retrograde fluid infiltration, while during prograde metamorphism the possible infiltration of aqueous fluids did not produce significant isotopic resetting. Isotopic zonation of calcite and graphite grains was likely produced by localized CO₂ fluid infiltration during retrogression. Contrary to the widespread occurrence of humite‐marbles related to retrograde aqueous fluid infiltration, the Ambasamudram humite‐marbles record a prograde‐to‐peak metamorphic humite formation and retrogression under conditions of low X_H2O.     

Synthesis and some properties of iron- and vanadium-bearing kyanites, (Al,Fe3+)2SiO5and (Al,V3+)2SiO5

Iron- and vanadium-bearing kyanites have been synthesized at 900 and 1100° C/20 kb in a piston-cylinder apparatus using Mn₂O₃/Mn₃O₄- and MnO/Mn-mixtures, respectively, as oxygen buffers. Solid solubility on the pseudobinary section Al₂SiO₅-Fe₂SiO₅(-V₂SiO₅) of the system Al₂O₃-Fe₂O₃(V₂O₃)-SiO₂ extends up to 6.5 mole% (14mole %) of the theoretical end member FeSiO₅(V₂SiO₅) at 900°C/20 kb. For bulk compositions with higher Fe₂SiO₅ (V₂SiO₅) contents the corundum type phases M₂O₃(M = Fe³⁺, V³⁺) are found to coexist with the Fe³⁺(V³⁺)-saturated kyanite solid solution plus quartz. The extent of solid solubility on the join Al₂SiO₅-Fe₂SiO₅ at 1 100°C was not found to be significantly higher than at 900° C. Microprobe analyses of iron bearing kyanites gave no significant indication of ternary solid solubility in these mixed crystals. Lattice constants a ₀, b ₀, c ₀, and V₀ of the kyanite solid solutions increase with increasing Fe₂SiO₅- and V₂SiO₅-contents proportionally to the ionic radii of Fe³⁺ and V³⁺, respectively, the triclinic angles ,, remain constant. Iron kyanites are light yellowish-green, vanadium kyanites are light green. Iron kyanites, (Al_1.87 Fe _0.13 ³⁺ )SiO₅, were obtained as crystals up to 700 m in length.     

Aragonite crystallization from strained calcite at reduced pressures and its bearing on aragonite in low-grade metamorphism

Aragonite, the dense form of CaCO₃, grew hydrothermally at 100–300° C and dry at 300–400° C at very low pressures from calcite strained by grinding. Nearly complete inversion to aragonite occurred in some runs with Ca-Mg chloride solutions at 0–2.4 kb and 100–200° C on strained calcite having a (10¯14) reflection with a half-width of 0.48° 2 Cu K. A little aragonite grew dry at one atm. from the ground calcite at 300–400° C in a few hrs. Simultaneous shear during recrystallization of calcite in a rotating squeezer resulted in significant aragonite at 300–400° C several kb. below the stability field. No inversion occurred in any ground calcite when previously annealed in CO₂ at 500° C for a few hrs. Thermochemical data show that at least 200 cal/mole of strain energy can be produced in calcite by mild deformation. This much stored energy would lower the pressure requirements of aragonite, relative to the strained calcite by more than 3 kb, and our observation that aragonite growth was faster than strain recovery of calcite indicates that aragonite can grow in nature at reduced pressures from strained calcite.Some experiments were also carried out on highly magnesian calcites with the thought that aragonite might also form at the expense of this metastable material. No aragonite was produced, but the possibility that this mechanism could be operative in nature cannot be discounted.The microtexture of aragonitic deformed marbles from NW Washington (prehnite-pumpellyite facies rocks, courtesy of J. A. Vance) as well as electron probe microanalysis of these rocks indicates that aragonite selectively replaced highly strained calcite. The calcite-aragonite transition is thus a questionable indicator of high-pressure in certain metamorphic rocks.     

Transformation of blueschist to greenschist facies rocks as a consequence of fluid infiltration,Sifnos (Cyclades), Greece

The transformation from blueschist to greenschist facies forms a major part of the Alpine regional geodynamic evolution of the Cyclades. The transition in metabasic rocks on Sifnos involves the retrogression of eclogites, blueschists and actinolite-bearing rocks from high-pressure conditions which have been estimated at 460±30° C and 15±3 kb. Petrographically observed parageneses are interpreted by a sequence of hydration and carbonation reactions involving the breakdown of omphacite and glaucophane-bearing assemblages to albite+chlorite±actinolite±calcite assemblages. The retrograde processes are calculated to occur at pressures of 10 to 8 kb during the isothermal uplift of the Sifnos units. Oxygen isotope analyses of different rock types show that interlayered lithologies have experienced a large degree of isotopic equilibration during both metamorphic phases. However, chemically equivalent rocks show systematic increases in ¹⁸O from lowest values (8 to 11 in metabasics) in the unaltered blueschists in the upper stratigraphic levels to higher values (>15 in metabasics) associated with greenschists in the deepest stratigraphic levels. Relict eclogites enclosed within greenschists have the lower ¹⁸O values typical of unaltered blueschist facies rocks. These isotopic gradients and the ¹³C and ¹⁸O compositions of carbonates demonstrate an infiltration mechanism involving the upward movement of ¹⁸O-enriched fluids whose compositions were buffered by exchange with marble units. Calculated minimum fluid/rock ratios for the blueschist-to-greenschist transition decrease from 0.4 in the deepest studied level (Central Sifnos) and 0.2 in the intermediate level (Kamares Bay samples) to an assumed value of zero in unaltered blueschists. These ratios may be lower if recycling of fluids occurred between schists and marbles. Infiltration of fluids became inhibited as the transformation advanced as a consequence of sealing effect of the hydration and carbonation reactions. Although infiltration most probably was a pre-requisite for the regional occurrence of the blueschist-to-greenschist transformation in the Cyclades, the evidence on Sifnos does not suggest the introduction of large quantities of fluid.     

Synthetic polycrystalline aragonite to calcite transformation kinetics: experiments at pressures close to the equilibrium boundary

Summary ¶This study experimentally investigated the transformation kinetics of synthetic polycrystalline aragonite to calcite at four temperature/pressure conditions (330°C/200MPa, 380°C/325MPa, 430°C/580MPa, and 480°C/875MPa), close to the calcite-aragonite equilibrium phase boundary. The extents of transformation measured as a function of time in a synthetic system, using in-situ annealed, high purity samples, are consistent with the kinetic model for grain-boundary nucleation and interface-controlled growth. The growth rates are slightly lower than those previously determined for a natural polycrystalline sample at 330 and 380°C. The activation energy (158kJ/mol) for calcite growth from synthetic samples is lower than that (247kJ/mol) from natural samples, but is close to the previously reported value (163kJ/mol) from a single crystal aragonite. The extrapolation of our experimental data to natural conditions reveals unusually fast transformation rates, in contrast to those of natural samples. The presence of deformational strain, fractures, defects or impurities in natural samples, and other factors may account for the discrepancy. This study suggests that the retrograde metamorphism of aragonite to calcite may proceed in a wide range of rates also depending on other geological factors than temperature and pressure.Received July 15, 2002; revised version accepted May 15, 2003     

High-Ca,low-alkali carbonatite volcanism at Fort Portal,Uganda

A Quaternary volcanic field at Fort Portal, SW Uganda, contains approximately 50 vents that erupted only carbonatite. The vents are marked by monogenetic tuff cones defining two ENE-trending belts. Lava from a fissure at the west end of the northern belt formed a flow 0.3 km² in area and 1–5 m thick. The lava is vesicular throughout with a scoriaceous top, and probably formed by agglutination of spatter from lava fountains. Phenocrysts are olivine, clinopyroxene, phlogopite, and titanomagnetite enclosing blebs of pyrrhotite. Rims of monticellite, gehlenite, and reinhardbraunsite surround olivine, clinopyroxene and phlogopite, and magnetite is rimmed by spinel. The reaction relations suggest that these phenocryst phases are actually xenocrysts, perhaps from a source similar to that which supplied phlogopite clinopyroxenite xenoliths in the Katwe-Kikorongo volcanic field 75 km SW of Fort Portal. The groundmass of fresh carbonatite lava consists of tabular calcite, spurrite, periclase, hydroxylapatite, perovskite, spinel, pyrrhotite, and barite. The lava was readily altered; where meteoric water had access, spurrite and periclase are lacking, and some calcite is recrystallized. Vesicles in lava and rare dike rocks are partly filled with calcite, followed by jennite and thaumasite. Pyroclastic deposits cover 142 km² and are far more voluminous than lava. Carbonatite ejecta were identical to lava in primary mineralogy, but are much more contaminated by crustal rock fragments and xenocrysts. At Fort Portal, eruption of a CaO-MgO-CO₂-SiO₂-P₂O₅-SO₂-H₂O-F liquid was unaccompanied by that of a more silica-rich or alkali-rich liquid. Alkali-rich carbonatite lavas and pyroclastic deposits have been documented elsewhere in East Africa, and calcite-rich volcanic carbonatites have been attributed to replacement of magmatic alkali carbonates by calcite. However, the alkali-poor volcanic carbonatites at Fort Portal were not formed by leaching of alkalis in meteoric water; tabular calcite is not pseudomorphous after alkali carbonates such as nyerereite. The Fort Portal magma was low in alkalis at the time of eruption.     

Corundum-bearing metasomatic rocks in the Central Pamirs

The Muzkol metamorphic complex in the Central Pamirs contains widespread occurrences of corundum mineralization, sometimes with gem-quality corundum. These occurrences are spatially related to zones of metasomatic alterations in calcite and dolomite marbles and crystalline schists. The calcite marbles contain corundum together with muscovite, scapolite, and biotite; the dolomite marbles contain corundum in association with biotite; and the schists bear this mineral coexisting with biotite and chlorite. All these rocks additionally contain tourmaline, apatite, rutile, and pyrite. The biotite is typically highly aluminous (up to 1.9 f.u. Al), and the scapolite is rich in the marialite end member (60–75 mol %). The crystallization parameters of corundum were estimated using mineral assemblages at T = 600–650°C, P = 4–6 kbar, X _{CO 2} = 0.2–0.5 at elevated alkalinity of the fluid. The Sr concentration in the calcite and dolomite marbles is low (345–460 and 62–110 ppm, respectively), as is typical of recrystallized sedimentary carbonates. The variations in the ⁸⁷Sr/⁸⁶Sr ratio in the calcite and dolomite marbles (0.70852–0.70999 and 0.70902–0.71021, respectively) were controlled by the introduction of radiogenic ⁸⁷Sr during the metasomatic transformations of the rocks. The isotopic-geochemical characteristics obtained for the rocks and the results of numerical simulations of the fluid-rock interactions indicate that the corundum-bearing metasomatic rocks developed after originally sedimentary Phanerozoic carbonate rocks, with the desilication of the terrigenous material contained in them. This process was a manifestation of regional alkaline metasomatism during the closing stages of Alpine metamorphism. In the course of transformations in the carbonate reservoir, the juvenile fluid flow became undersaturated with respect to silica, which was a necessary prerequisite for the formation of corundum.     

Occurrence of Contrasting skarn formations in dolomites of the Traversella deposit (Ivrea,Italy)

Metasomatic columns hosted in dolomitic marbles in the thermal aureole of the Traversella monzodiorite (Ivrea, Italy) differ by their mineralogy and/or mineral composition. Three groups have been distinguished. In group A, the first zone always contains forsterite-calcite and the second zone contains clinopyroxene. The last zone is made of wollastonite (A1), andradite-rich garnet (A 2) or grossular-rich garnet (A 3). In group B, tremolite instead of forsterite occurs in the first two zones. In group C columns, there are only two zones and clinohumite or chondrodite characterize the first zone.Field, petrographic and chemical data demonstrate that the occurrence of contrasted mineral zonations in the same protolith has not been induced by local heterogeneities in the dolomitic marbles. The presence of tremolite in group B instead of forsterite is due to the lower temperature prevailing in the external part of the contact aureole. Chemical data as well as - diagrams suggest that the columns in groups A and C were formed through the interaction of the dolomitic marbles with fluids with different _{Fe 2}O₃, _{Al 2 O 3} and _F. A fluid with low _{Al 2 O 3} and _{Fe 2}O₃ is assumed for the metasomatic column with wollastonite (A 1) and different Al₂O₃/Fe₂O₃ values in the fluid are responsible for the formation of columns A 2 and A 3. The stability of clinohumite or chondrodite in group C columns has been induced by a higher _F of the fluid than in the other groups. The presence during the prograde metasomatic stage of distinct fluids differing by their _{Al 2 O 3}/_{Fe 2}O₃ (columns A 2 and A 3) is likely to have been induced by variations in the source composition. Moreover, the low A1 and Fe content in column A 1 could be due to the reaction of the dolomitic marble with a fluid previously depleted in these components during percolation and reaction with the country rocks.As shown by isocon diagrams and Gresens's equation, skarn formation has locally induced both mass and volume change. The volume decreased in the wollastonite zone of A 1 (19%) and in the Fo-Cc zone of A2 (17%). Mass is always increased except in the wollastonite of A 1 where it is decreased. Mass balance of major elements has shown that Ca is likely to be a perfectly inert component and that Si is always strongly increased. Moreover, Fe is increased in A 2 and A 3 whereas Al is only increased in A 3. Mg is leached in the internal part of column Al.Quantification of chemical potential diagrams using different values of _{Al 2 O 3 Fe 2 O 3}, X _F and T suggests that the successive zones observed in all types of columns can be obtained along an increase of _{SiO 2} towards the inner zones. A simultaneous decrease in _MgO is inferred for group A columns.     

An occurrence of ardennite in quartz veins in piemontite schist,Western Otago,New Zealand

Summary Ardennite of complex composition: (Mn²⁺ _3.488Ca_0.509Ba_0.002)₌₄(Mg_0.916916 Fe³⁺ _0.165 Mn³⁺ _0.099Cu_0.033Ni_0.009Zn_0.006 Ti_0.008Al_4.764)₌₆(As⁵⁺ _0.823V⁵⁺ _0.022P_0.005B_0.069Al_0.042Si_5.039)₌₆O_21.81(OH)_6.17 occurs in crack-seal quartz veins in quartz-albite-piemontite-spessartine-phengitehematite-chlorite-rutile-tourmaline ± calcite schist of the Haast Schist Group near Arrow Junction, western Otago, New Zealand. The Mn²⁺/Mn³⁺-ratio is sensitive to calculations and to accuracy of analyses. Boron is detected in ardennite for the first time. Other properties include = 1.734(3), = 1.735(3), = 1.751(3), 2V_Z = 30(2)°;a = 8.721(1),b = 5.816(1),c = 18.545(3) Å,V = 940.7(2) ų. Associated mineral phases are spessartine, hematite, piemontite containing 0.7% SrO and 0.06% PbO, and phengite. Later-stage vein minerals comprise chlorite, albite, and manganoan calcite which were deposited under less highly oxidizing conditions. Digenite with minor intergrown covellite occurs in small amount with manganoan calcite and quartz in a cross-cutting late-stage veina chalcopyrite and native copper occur in other late-stage veins. Arsenic and other components of the ardennite and associated minerals are derived from highly oxidized ferromanganese oxide- and hydroxide-bearing siliceous pelagic sediments that formed the protolith for the piemontite schist. The veins formed at a relatively early stage after metamorphism peaked in the chlorite zone of the greenschist facies under conditions that have been estimated at about 4.5 kbar, 390 °C.

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Vorkommen von Ardennit in Quarzgängen aus Piemontit-Schiefern, West-Otago, Neuseeland

Zusammenfassung Ardennit mit der Zusammensetzung (Mn²⁺ _3.488Ca_0.509Ba_0.002)₌₄(Mg_0.916Fe³⁺ _0.165Mn³⁺ _0.099Cu_0.033Ni_0.009Zn_0.006 Ti_0.008Al_4.764)₌₆(As⁵⁺ _0.823V⁵⁺ _0.022P_0.005B_0.069Al_0.042Si_5.039)₌₆O_21.81(OH)_6.17 tritt in Crack-seal-Quarzgängen in Quarz-Albit-Piemontit-Spessartin-Phengit-Hämatit-Chlorit-Rutil-Turmalin ± Calcit-Schiefern der Haast Schiefer-Gruppe nahe der Arrow Junction, West-Otago, Neuseeland, auf. Die Proportionen von Mn²⁺/Mn³⁺ hängen von der Kalkulation und der Genauigkeit der Analyse ab. Bor wird zum ersten Mal im Ardennit bestimmt. Andere Eigenschaften sind: = 1.734(3), = 1.735(3), = 1.751(3), 2Vz = 30(2)°; a = 8.721(1), b = 5.816(1), c = 18.545(3) Å, V = 940.7(2) Å3. Assoziierte Mineralphasen sind Spessartin, Hämatit, Piemontit, der 0.7% SrO und 0.06% PbO enthält und Phengit. Spät gebildete Gangmineralien, wie Chlorit, Albit und Mn-Calcit, sind unter geringer oxidierenden Bedingungen entstanden. Digenit mit etwas Covellin tritt in kleinen Mengen zusammen mit Mn-Calcit und Quartz in einem querschlägigen Gang auf, Chalcopyrit und gediegenes Kupfer kommen in anderen späten Gängen vor. Arsen und andere Komponenten des Ardennites and der assoziierten Minerale können von hochoxidierten, Fe-Mn-Oxid- und Hydroxyd-führenden, Sireichen, pelagischen Sedimenten hergeleitet werden, die das Ausgangsgestein für den Piemontit darstellen. Die Gänge wurden in einem relativ frühen Stadium, nach dem Metamorphosehöhepunkt, innerhalb der Chloritzone der Grünschiefer-Fazies, unter ungefähr 4.5 kbar und 390°C, gebildet.

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With 4 Figures     

Experiments on the phase transition calcite+wollastonite+ +epidote=grossular-andraditess+CO2+H2O

The reaction 2 epidote+2 calcite+3 wollastonite3 grossular-andradite_ss+ 2 CO₂+1 H₂O has been explored by hydrothermal experiments at a total fluid pressure of 1000 bars. For a grossular-andradite_ss of andradite ₂₅ composition, the isobaric univariant curve passes through the points 458°C: X_CO2=0.00; 521°C: X_CO2=0.026; 523°C: X_CO2=0.052; 526°C: 0.088; 528°C: X_CO2=0.104. This curve intersects the isobaric univariant curve of the reaction calcite+quartz+[H₂O] wollastonite+CO₂+[H₂O] at the isobaric invariant point around 528°C and X_CO2=0.12. At higher values of X_CO2, this reaction is replaced by another one, namely: 2 epidote+5 calcite+3 quartz3 grossular-andradite_ss+5 CO₂+ 1 H₂O. It is demonstrated that both the reactions do actually take place during the metamorphism of calcareous rocks. The petrologic significance of contrasted sequence of reactions within this system observed by various workers is also discussed.     

Transformation kinetics of polycrystalline aragonite to calcite: new experimental data,modelling, and implications

Experimental data are used to model the transformation rate of polycrystalline aragonite (grain diameter 80 m) to calcite. Optimized values for nucleation and growth rates were obtained by numerically fitting the overall transformation rates from 280° to 380°C and 0.10 MPa to an expression for a grain-boundary-nucleated and interface-controlled transformation. The nucleation rate is 4–5 orders of magnitude faster than for calcite nucleated within aragonite grains, and the growing in rate is slower below 300°C than for calcite growing in aragonite single crystals. The activation enthalpy for growth in polycrystalline aggregate is 247kJ/mol compared to 163 kJ/mol for growth in single crystals. Permanent deformation of the phases limits the elastic strain energy due to the 7% volume change and reduces the coherency of the calcite/aragonite interace. Theoretical expressions are used to extrapolate the data for nucleation and growth to other temperatures, and data from 0.10 to 400 MPa are used to evaluate the effect of pressure on the grain-boundary nucleation rate. Because of permanent deformation of the phases, the effective strain energy for nucleation is 0.55 kJ/mol, which is less than a quarter of the value for purely elastic deformation. These data are used to predict the percent transformation for various P-T-t paths; without heating during uplift partial preservation of aragonite in dry blueschist facies rocks can occur if the calcite stability field is entered at 235° C, and the kinetic data are also consistent with published P-T-t paths which include heating during uplift. The predicted percent transformation is relatively insensitive to variations in the initial grain size of the aragonite, but strongly dependent on the effective strain energy.     

Reaction progress during mylonitization of basaltic dikes along the Särv thrust,Swedish Caledonides

The mylonite zone at the base of the Särv thrust sheet, Swedish Caledonides, contains diabase dikes which record intense deformation and syntectonic greenschist facies metamorphism. An angular shear strain of 100 is calculated for a single dike which can be followed for 50 m in the mylonite zone and abundant centimeter thick greenschist layers imply shear strains in excess of 1000. This extraordinary amount of deformation is comparable to the largest strains attained during experimental superplastic deformation of metals and alloys and, by analogy, suggests that dike deformation was macroscopically superplastic. The progress of five syntectonic reactions was measured as a function of increasing strain for the continuously exposed dike in order to assess the contribution of reactionenhanced ductility and fluid-rock interactions to strain localization along the thrust. Reaction progress calculations suggest that the breakdown of amphibole to form weaker phyllosilicates (which are added to the incompetent matrix fraction) is the important strain softening mechanism below 100. The ultimate tectonite is a stable biotite-epidote schist comprised of a uniformly fine grain size (< 200 m), constant grain shapes and strain free grains. Below 40, metamorphism was isochemical and shear strain was independent of H₂O in the reactions. Petrologic fluid:rock ratios are low and suggest that deformation could have occurred under relatively dry conditions.Deformation micromechanisms were probably dominated by diffusive mass transfer processes throughout the life of the shear zone. The absence of cataclasis and the fine grain size of the protolith basalt suggest that fluids were introduced via grain boundary diffusion. Incongruent pressure solution at low strains and K-metasomatism above 40 also support diffusional flow. Diffusion-accomodated grain boundary sliding is thought to be the dominant micromechanism once the stable biotite-epidote tectonite forms.     

Changes in stable isotope ratios of metapelites and marbles during regional metamorphism,Mount Lofty Ranges,South Australia: implications for crustal scale fluid flow

The Mount Lofty Ranges comprises interlayered marbles, metapsammites, and metapelites that underwent regional metamorphism during the Delamarian Orogeny at 470–515 Ma. Peak metamorphic conditions increased from lowermost biotite grade (350–400°C) to migmatite grade (700°C) over 50–55 km parallel to the lithological strike of the rocks. With increasing metamorphic grade, ¹⁸O values of normal metapelites decrease from 14–16 to as low as 9.0, while ¹⁸O values of calcite in normal marbles decrease from 22–24 to as low as 13.2 These isotopic changes are far greater than can be accounted for by devolatilisation, implying widespread fluid-rock interaction. Contact metamorphism appears not to have affected the terrain, suggesting that fluid flow occurred during regional metamorphism. Down-temperature fluid flow from synmetamorphic granite plutons (¹⁸O=8.4–8.6) that occur at the highest metamorphic grades is unlikely to explain the resetting of oxygen isotopes because: (a) there is a paucity of skarns at granite-metasediment contacts; (b) the marbles generally do not contain low-XCO₂ mineral assemblages; (c) there is insufficient granite to provide the required volumes of water; (d) the marbles and metapelites retain a several permil difference in ¹⁸O values, even at high metamorphic grades. The oxygen isotope resetting may be accounted for by along-strike up-temperature fluid flow during regional metamorphism with time-integrated fluid fluxes of up to 5x10⁹ moles/m² (10⁵ m³/m²). If fluid flow occurred over 10⁵–10⁶ years, estimated intrinsic permeabilities are 10^-20 to 10^-16m². Variations in ¹⁸O at individual outcrops suggest that time-integrated fluid fluxes and intrinsic permeabilities may locally have varied by at least an order of magnitude. A general increase in XCO₂ values of marble assemblages with metamorphic grade is also consistent with the up-temperature fluid-flow model. Fluids in the metapelites may have been derived from these rocks by devolatilisation at low metamorphic grades; however, fluids in the marbles were probably derived in part from the surrounding siliceous rocks. The marble-metapelite boundaries preserve steep gradients in both ¹⁸O and XCO₂ values, suggesting that across-strike fluid fluxes were much lower than those parallel to strike. Up-temperature fluid flow may also have formed orthoamphibole rocks and caused melting of the metapelites at high grades.This paper is a contribution to IGCP Project 304 Lower Crustal Processes     

Heat capacity of minerals in the system Na2O-K2O-CaO-MgO-FeO-Fe2O3-Al2O3-SiO2-TiO2-H2O-CO2: representation,estimation, and high temperature extrapolation

A revised equation is proposed to represent and extrapolate the heat capacity of minerals as a function of temperature: C _P=k₀+k₁ T ^–0.5+k₂ T ^–2+k₃ T ^–3 (where k₁, k₂0).This equation reproduces calorimetric data within the estimated precision of the measurements, and results in residuals for most minerals that are randomly distributed as a function of temperature. Regression residuals are generally slightly greater than those calculated with the five parameter equation proposed by Haas and Fisher (1976), but are significantly lower than those calculated with the three parameter equation of Maier and Kelley (1932).The revised equation ensures that heat capacity approaches the high temperature limit predicted by lattice vibrational theory (C _P=3R+²VT/). For 16 minerals for which and have been measured, the average C _Pat 3,000 K calculated with the theoretically derived equation ranges from 26.8±0.8 to 29.3±1.9 J/(afu·K) (afu = atoms per formula unit), depending on the assumed temperature dependence of . For 91 minerals for which calorimetric data above 400 K are available, the average C _Pat 3,000 K calculated with our equation is 28.3±2.0 J/(afu·K). This agreement suggests that heat capacity extrapolations should be reliable to considerably higher temperatures than those at which calorimetric data are available, so that thermodynamic calculations can be applied with confidence to a variety of high temperature petrologic problems.Available calorimetric data above 250 K are fit with the revised equation, and derived coefficients are presented for 99 minerals of geologic interest. The heat capacity of other minerals can be estimated (generally within 2%) by summation of tabulated oxide component C _Pcoefficients which were obtained by least squares regression of this data base.     

High-temperature enthalpy at the orientational order-disorder transition in calcite: implications for the calcite/aragonite phase equilibrium

The enthalpy of calcite has been measured directly between 973 K and 1325 K by transposed-temperature- drop calorimetry. The excess enthalpy has been analysed in terms of Landau theory for this tricritical phase transition. The zero-point enthalpy and entropy allow estimates of the parameters a and C in the Landau expansion for free energy which expresses excess free energy G as a function of the order parameter Q and temperature T: G 1/2a(T _2c–T)Q ²+1/6CQ ⁶ with a=24 J·K·mol^-1, C = 30 kJ·mol^– T _c = 1260 ±5 K. The entropy of disorder below the transition has been formulated as a function of temperature allowing the calculation of the calcite/aragonite phase boundary when taking this extra entropy into account. There is remarkable agreement between the calculated equilibrium curve and previous experimental observations. The Landau theory predicts behaviour which fully accounts for the change in slope of the calcite/aragonite phase boundary, which is thus wholly due to the R¯3c –R¯3m transition in calcite.     

Humite- and scapolite-bearing assemblages in marbles and calcsilicates of Dronning Maud Land, Antarctica: new data for Gondwana reconstructions

This study investigates marbles and calcsilicates in Central Dronning Maud Land (CDML), East Antarctica. The paleogeographic positioning of CDML as part of Gondwana is still unclear; however, rock types, mineral assemblages, textures and P–T conditions observed in this study are remarkably similar to the Kerala Khondalite Belt in India. The CDML marbles and calcsilicates experienced a Pan-African granulite facies metamorphism at c. 570 Ma and an amphibolite facies retrogression at c. 520 Ma. The highest grade assemblage in marbles is forsterite+spinel+calcite+dolomite, in calcsilicates the assemblages are diopside+spinel, diopside+garnet, scapolite+wollastonite+clinopyroxene±quartz, scapolite±anorthite±calcite+clinopyroxene+wollastonite. These assemblages constrain the peak metamorphic conditions to 830±20 °C, 6.8±0.5 kbar and X _CO2>0.46. During retrogression, highly fluoric humite-group minerals (humite, clinohumite, chondrodite) replaced forsterite, and garnet rims formed at the expense of scapolite during reactions with wollastonite, calcite or clinopyroxene but without involvement of anorthite. Metamorphic conditions were about 650 °C, 4.5±0.7 kbar, 0.2< X _CO2^fluid<0.36, and the co-existence of garnet, clinopyroxene, wollastonite and quartz constrains f_O2 to FMQ-1.5 log units. Mineral textures indicate a very limited influx of H₂O-rich fluid during amphibolite facies retrogression and point to significant variations of fluid composition in mm-sized areas of the rock. Gypsum was observed in two samples; it probably replaced metamorphic anhydrite which appears to have formed under amphibolite facies conditions. The observed extensive anorogenic magmatism (anorthosites, A-type granitoids) and the character of metamorphism between 610 and 510 Ma suggest that the crustal thermal structure was characterized by a long-lived (50–100 Ma) rise of the crustal geotherm probably caused by magmatic underplating.     

Carbon isotopic fractionation between CO2 vapour,silicate and carbonate melts: an experimental study to 30 kbar

The carbon isotopic fractionation between CO₂ vapour and sodamelilite (NaCaAlSi₂O₇) melt over a range of pressures and temperatures has been investigated using solid-media piston-cylinder high pressure apparatus. Ag₂C₂O₄ was the source of CO₂ and experimental oxygen fugacity was buffered at hematite-magnetite by the double capsule technique. The abundance and isotopic composition of carbon dissolved in sodamelilite (SM) glass were determined by stepped heating and the ¹³C of coexisting vapour was determined directly by capsule piercing. CO₂ solubility in SM displays a complex behavior with temperature. At pressures up to 10 kbars CO₂ dissolves in SM to form carbonate ion complexes and the solubility data suggest slight negative temperature dependence. Above 20 kbars CO₂ reacts with SM to form immiscible Na-rich silicate and Ca-rich carbonate melts and CO₂ solubility in Na-enriched silicate melt rises with increasing temperature above the liquidus. Measured values for carbon isotopic fractionation between CO₂ vapour and carbonate ions dissoived in sodamelilite melt at 1200°–1400° C and 5–30 kbars average 2.4±0.2, favouring¹³C enrichment in CO₂ vapour. The results are maxima and are independent of pressure and temperature. Similar values of 2 are obtained for the carbon isotopic fractionation between CO₂ vapour and carbonate melts at 1300°–1400° C and 20–30 kbars.     

NaSi⇌CaAl exchange equilibrium between plagioclase and amphibole

The exchange equilibrium between plagioclase and amphibole, 2 albite+tschermakite=2 anorthite+glaucophane, has been calibrated empirically using data from natural amphibolites. The partition coefficient, K _D, for the exchange reaction is (X _an/X _ab)_plag ·(Na, M4/Ca, M4)_amph.. Partitioning is systematic between plagioclase and amphibole in suites collected from single exposures, but the solid solutions are highly non-ideal: values of In K _D range from –3.0 at X _an=0.30 to –1.0 at X _an=0.90 in samples from a single roadcut. Changes in both K _D and the topology of the ternary reciprocal exchange diagram occur with increasing metamorphic grade. Temperature dependence of In K _D is moderate with ¯H35 to 47 kcal at X _an=0.25; pressure dependence is small with ¯V –0.24 cal/bar. Usefulness of this exchange equilibrium as a geothermometer is restricted by uncertainties in the calculation of the amphibole formula from a microprobe analysis, especially with regard to Na, M4 in amphibole, to approximately ±50 ° C.     

Der “Protocalcit” von Gumpoldskirchen (Niederösterreich)

Zusammenfassung In einem in Abbau befindlichen Steinbruch bei Gumpoldskirchen (Niederösterreich) wurde in Fugen und Hohlräumen eines Störungssystems von Hauptdolomit als rezente Verwitterungsbildung des Gesteins ein weißes, gelartiges Mineral aufgefunden, das einen Gehalt von mehr als 90% lose gebundenes Wasser aufweist. An der Luft trocknet die quallige Mineralbildung zu einem papierartigen Häutchen (Bergmilch oder Papierspat) und rekristallisiert unter natürlichen Bedingungen zu erdigen, weißen Krusten (Bergmehl). Die chemische Analyse, die Röntgendiffraktionsanalyse und Elektronenbeugungsaufnahmen zeigen einwandfrei, daß es sich, auch im wasserhaltigen Zustand, um Calciumcarbonat in der Modifikation des Calcites handelt. Die elektronenmikroskopischen Aufnahmen erklären die Eigenschaften dieser Mineralbildung. Im wasserhaltigen Zustand sind wirrgelagerte Calcitfäden zu erkennen, deren Durchmesser ca. 500 Å beträgt. Das Wasser ist im Calcitfadengerüst als grobe Feuchtigkeit gebunden. Für die wasserhaltige Varietät (CaCO₃+xH₂O) wird der Name Protocalcit vorgeschlagen.

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Summary A white gel-like mineral containing more than 90% moisture was found in a quarry near Gumpoldskirchen (Lower Austria). The mineral had been formed recently in gaps and cavities of a fissure system in the Triassic Main Dolomite.Airdried, the jelly mineral turns into a paper-like skin (rockmilk) and can be recrystallised to a white earthy crust (rockflour).The chemical analysis, the X-ray diffraction and electron diffraction investigation show that the mineral is calcite, even it contains high amounts of water. The electronmicroscopical investigations explain the properties of this peculiar formation of the mineral. If the mineral is containing water, the filiform crystallites of calcite 500 Å of diameter are completely disorderd. The water is bound adsorptively in the framework of calcite crystallites. During the loss of water the filiform calcite crystalls are ordered more or less parallel to each other.The name protocalcite is proposed for this variety of calcite, which is containing variable amounts of water (CaCO₃+xH₂O).

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Mit 9 Textabbildungen

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Herrn Professor Dr.F. Machatschki zum 70. Geburtstag gewidmet.     

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