Sudoite,a rock-forming mineral in Verrucano of the Northern Apennines (Italy) and the sudoite-chloritoid-pyrophyllite assemblage in prograde metamorphism

Sudoite, the di-trioctahedral chlorite with ideal composition (Mg₂Al)(Al₂)(Si₃Al)O₁₀(OH)₈ is a widespread rock-forming mineral in meta-siltstones and psammites of the Verrucano sequence of the Northern Apennines. Sub-ellipsoidal aggregates, probably derived from muscovite clasts, consisting of sudoite, pyrophyllite and muscovite, are common; sudoite may also occur as thin blades in the rock matrix. The co-existence of sudoite, Ferich chloritoid and pyrophyllite, reported here for the first time, has been observed in specimens from the M. Argentario and Monticiano-Roccastrada areas. This three-phase assemblage, diagnostic of a specific metamorphic facies, may be a tool for detailed zonation of low-grade terranes.     

Widespread fluid infiltration during metamorphism of the Witwatersrand goldfields: generation of chloritoid and pyrophyllite

Abstract Chloritoid and pyrophyllite occur together in all major goldfields of the Witwatersrand Basin and are widespread in virtually all rock types of the upper Witwatersrand Supergroup, including metaconglomeratic reefs and altered mafic rocks. Both minerals are particularly characteristic of the pelitic horizons intimately associated with reef packages, but they are also developed locally in the regionally persistent metapelites that have basin-wide extent. Pyrophyllite is particularly common in foliated zones, adjacent to quartz veins, and near unconformably overlying auriferous conglomerates. The wide distribution of chloritoid and pyrophyllite in metapelites of the Witwatersrand Basin is attributed to alteration of chlorite-rich shales, rather than to unusual premetamorphic starting materials. This alteration event involved the redistribution of many elements, with up to 40% volume loss, mainly due to removal of silica. Removal of most of the Mg and some Fe accounts for the stabilization of chloritoid and pyrophyllite. Relatively immobile elements included Al, Ti, Nb, Cr, V, P, La and Ce, whereas Si, Fe, Mn, Zn, Co, Ni, Cu, Mg and Ca were lost, and K, Rb and Ba were introduced by an infiltrating fluid. The alteration event is inferred to have been within the chloritoid and pyrophyllite stability field (and thus syn-metamorphic) as bulk chemical changes in metapelites are from chlorite directly towards chloritoid and then pyrophyllite, rather than to lower grade minerals such as kaolinite. Muscovite–chlorite–chloritoid and muscovite–chloritoid–pyrophyllite assemblages are attributed to fluid buffering along appropriate curves, as their production by metamorphism of lower grade mineral mixes is considered unlikely, based on the present bulk rock compositional data. A metamorphic timing for the alteration accounts for the correlation of strongly foliated areas with greater degrees of inferred alteration. The transitions from chlorite to chloritoid to pyrophyllite define zones of increasing alteration. Widespread infiltration as part of peak metamorphism is suggested by the distribution of chloritoid and pyrophyllite, quartz veining and textures. Fluid:rock ratios calculated from a silica budget in one metapelitic horizon exceed 100:1 over many square kilometres. These values need not imply multi-pass fluid flow, as much of the silica migration may be redistribution on a scale of a few metres, from source rocks into veins. Although infiltration during metamorphism may have affected much of the upper Witwatersrand succession, channelized fluid flow within reef packages, along faults and unconformities and in certain metaconglomerates and metapelites is inferred.     

Chemographic analysis of assemblages involving pyrophyllite, chloritoid, chlorite, kaolinite, kyanite, quartz: application to metapelites in the Witwatersrand goldfields, South Africa

The Witwatersrand goldfields contain abundant assemblages that include pyrophyllite, chloritoid, chlorite, kaolinite and/or kyanite, with quartz. A chemographic analysis of the system Fe(Mg)-Al-Si-O-H involving these minerals yields 22 potential phase diagrams. Using orientation criteria and thermodynamic calculations as further constraints, this list has been reduced to three possible diagrams. New thermodynamic data favour one of these in particular.
This chemographic analysis demonstrates that formation of chloritoid is not restricted to the breakdown reaction of kaolinite plus chlorite in the F(M)ASH system, as stated by previous studies, but could be from pyrophyllite + chlorite → chloritoid + quartz + H₂O.
The metamorphic temperature variation between Witwatersrand goldfields exceeded 65 C, based on chlorite and chloritoid compositions. The lower and upper pressure limits are constrained by the andalusite to kyanite, and the sudoite/chlorite to carpholite boundaries, i.e. 1.5–2.8, and 7 kbar, respectively. The widespread pyrophyllite, chlorite and Fe-chloritoid in all the Witwatersrand goldfields, and the local occurrence of sudoite indicate a consistent low-pressure environment in which Mg-chloritoid would not be stable.     

Sudoite,di/trioctahedral chlorite: A stable low-temperature phase in the system MgO-Al2O3-SiO2-H2O

Sudoite, ideally (Mg₂Al₃)[AlSi₃O₁₀](OH)₈, was synthesized in small quantities from a number of starting materials using seeds of the natural mineral. Because its powder X-ray diffraction pattern is very similar to that of normal, trioctahedral chlorite, a technique based on relative intensities of 001-peaks of the chlorite-type phases was used, in addition to the standard X-ray method, to determine growth or breakdown of sudoite. Seeded runs indicate that sudoite is more stable than at least five alternative mineral assemblages in the system MgO-Al₂O₃-SiO-H₂O below about 370°–390° C at water pressures up to at least 7 kbar. At higher temperatures sudoite decomposes into assemblages of normal chlorite with an Al₂SiO₅-phase and either quartz or pyrophyllite. However, the exact locations of the univariant breakdown curves could not be determined due to very low reaction rates. Schreinemakers analyses indicate that the assemblage sudoite+quartz represents the low-temperature equivalent of the common pair chlorite+pyrophyllite, and that sudoite+quartz is limited to water pressures below about 7 kbar because of its reaction to form the high-pressure phase Mg-carpholite; however, in the absence of quartz, the stability fields of sudoite and of Mg-carpholite overlap at pressures above 7 kbar.These stability data are in general agreement with two well-documented sudoite occurrences in quartz veins cutting highly oxidized, low-pressure manganiferous metapelites, and with one occurrence in a silica-deficient high-pressure metamorphic metabauxite. Sudoites may be more common in low-grade metamorphic rocks than known thus far, but they may not be stable under surface conditions.     

Metamorphism of the Witwatersrand gold fields: conditions during peak metamorphism

Abstract Pelitic assemblages from all major Witwatersrand gold fields record metamorphic conditions of the greenschist facies, with minimal regional grade changes over at least 200 km strike length. Diagnostic metamorphic assemblages are less common in the volumetrically dominant quartzites, the actively-exploited auriferous conglomerates and some of the regionally persistent metapelitic horizons. Bulk rock composition has been a major control on assemblage development.
Key metapelitic assemblages include pyrophyllite, chloritoid, chlorite and muscovite in each gold field, with less common metamorphic biotite. Accessory minerals are pyrite, tourmaline, rutile and zircon. The abundance of chloritoid and pyrophyllite in thin shaly units, together with their minor, but widespread, distribution in quartzites and conglomerates, indicate that metamorphic temperatures reached 350°C ± 50°C in all the gold fields. Pressures are less-well constrained, 1–2 kbar being inferred. Outside the gold fields, higher grades are indicated by andalusite and kyanite near granitoid domes and later intrusions.
The temperatures during peak metamorphism and the abundance of pyrite provide ideal conditions to (re)mobilize gold and may explain its secondary textural features.     

Pumpellyites and coexisting minerals in different low-grade metamorphic facies of Liguria, Italy

Pumpellyite from four-phase assemblages (pumpellyite + epidote + prehnite + chlorite; pumpellyite + epidote + actinolite + chlorite; pumpellyite + epidote + Na-amphibole + chlorite, together with common excess phases), considered to be low variance in a CaO-(MgO + FeO)-Al₂O₃-Fe₂O₃ (+Na₂O + SiO₂+ H₂O) system, have been examined in areas which underwent metamorphism in the prehnite-pumpellyite, pumpellyite-actinolite and low-temperature blueschist facies respectively. The analysed mineral assemblages are compared for nearly constant (basaltic) chemical composition at varying metamorphic grade and for varying chemical composition (basic, intermediate, acidic) at constant metamorphic conditions (low-temperature blueschist facies). In the studied mineral assemblages, coexisting phases approached near chemical equilibrium. At constant (basaltic) bulk rock composition the MgO content of pumpellyite increases, and the X_Fe3+ of both pumpellyite and epidote decreases with increasing metamorphic grade, the Fe³⁺ being preferentially concentrated in epidote. Both pumpellyite and epidote compositions vary with the bulk rock composition at isofacial conditions; pumpellyite becomes progressively enriched in Fe and depleted in Mg from basic to intermediate and acidic bulk rock compositions. The compositional comparison of pumpellyites from high-variance (1–3 phases) assemblages in various bulk rock compositions (basic, intermediate, acidic rocks, greywackes, gabbros) shows that the compositional fields of both pumpellyite and epidote are wide and variable, broadly overlapping the compositional effects observed at varying metamorphic grade in low-variance assemblages. The intrinsic stability of both Fe- and Al-rich pumpellyites extends across the complete range of the considered metamorphic conditions. Element partitioning between coexisting phases is the main control on the mineral composition at different P-T conditions.     

Prediction of rock alteration patterns: A potential tool in mineral exploration

Hydrothermal alteration of a quartz‐K‐feldspar rock is simulated numerically by coupling fluid flow and chemical reactions. Introduction of CO₂ gas generates an acidic fluid and produces secondary quartz, muscovite and/or pyrophyllite at constant temperature and pressure of 300°C and 200 MPa. The precipitation and/or dissolution of the secondary minerals is controlled by either mass‐action relations or rate laws. In our simulations the mass of the primary elements are conserved and the mass‐balance equations are solved sequentially using an implicit scheme in a finite‐element code. The pore‐fluid velocity is assumed to be constant. The change of rock volume due to the dissolution or precipitation of the minerals, which is directly related to their molar volume, is taken into account. Feedback into the rock porosity and the reaction rates is included in the model. The model produces zones of pyrophyllite quartz and muscovite due to the dissolution of K‐feldspar. Our model simulates, in a simplified way, the acid‐induced alteration assemblages observed in various guises in many significant mineral deposits. The particular aluminosilicate minerals produced in these experiments are associated with the gold deposits of the Witwatersrand Basin.     

Fractionation of bulk rock composition due to porphyroblast growth: effects on eclogite facies mineral equilibria, Pam Peninsula, New Caledonia

Chemically zoned porphyroblasts in metamorphic rocks indicate that diffusional processes could not maintain equilibrium conditions on a grain scale during porphyroblast growth or establish it afterwards. An effect of this inability to maintain equilibrium is the progressive removal of elements forming garnet cores from any metamorphic reaction that occurs at the porphyroblast boundaries or in the matrix of the rock. To examine this effect on mineral assemblages, the Bence–Albee matrix correction was applied to X‐ray intensity maps collected using eclogite samples from northern New Caledonia in order to determine the chemical composition of all parts of the sample. The manipulation of these element maps allows a quantitative analysis of the fractionation of the bulk rock composition between garnet cores and the matrix. A series of calculated equilibrium‐volume compositions represents the change in matrix chemistry with progressive elemental fractionation as a consequence of prograde garnet growth under high‐P conditions. Pressure–temperature pseudosections are calculated for these compositions, in the CaO–Na₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O system. Assemblages, modal proportions and mineral textures observed in the New Caledonian eclogites can be closely modelled by progressively ‘removing’ elements forming garnet cores from the bulk rock composition. The pseudosections demonstrate how chemical fractionation effects the peak metamorphic assemblage, prograde textures and the development of retrograde assemblages.     

Petrology of Fe-Mg-carpholite-bearing metasediments from NE Oman

Abstract Fe-Mg carpholite occurs in metasediments of tectonically disrupted basement, shelf and foreland basin units that structurally underlie the Semail ophiolite in NE Oman. In the lower grade, structurally higher units, Fe-rich carpholite coexists with paragonite, quartz, illite, kaolinite and chlorite, whereas in deeper units, Fe-Mg carpholite occurs with pyrophyllite, sudoite, phengite and/or chloritoid. Mineral compositions in these units indicate that chlorite is more magnesian than coexisting Fe-Mg carpholite at low temperatures and pressures but, at higher metamorphic grades, X_Mg decreases in the order sudoite > carpholite > chlorite > chloritoid. This suggests a reversal in Fe-Mg partitioning between Fe-Mg carpholite and chlorite at temperatures below or close to those of the breakdown of kaolinite + quartz to pyrophyllite and at X_Mg= 0.35.
Phase relations and mineral equilibria indicate that the P-T conditions of formation of the Fe-Mg-carpholite-bearing rocks of NE Oman range from 280–315° C, 3–6 kbar for the structurally highest units to 325–440° C, 6–9.5 kbar for the deepest units, indicating a systematic down-section increase in metamorphic grade. Textural relations in these rocks, interpreted in the context of pertinent equilibria, are consistent with the clockwise P-T paths previously constrained for these units from petrological studies of interlayered isofacial mafic rocks.     

Jadeite–garnet glaucophane schists in the Bizan area,Sambagawa metamorphic belt,eastern Shikoku,Japan: significance and extent of eclogite facies metamorphism

Eclogite facies metamorphic rocks have been discovered from the Bizan area of eastern Shikoku, Sambagawa metamorphic belt. The eclogitic jadeite–garnet glaucophane schists occur as lenticular or sheet‐like bodies in the pelitic schist matrix, with the peak mineral assemblage of garnet + glaucophane + jadeite + phengite + quartz. The jadeitic clinopyroxene (X_Jd 0.46–0.75) is found exclusively as inclusions in porphyroblastic garnet. The eclogite metamorphism is characterized by prograde development from epidote–blueschist to eclogite facies. Metamorphic P–T conditions estimated using pseudosection modelling are 580–600 °C and 18–20 kbar for eclogite facies. Compared with common mafic eclogites, the jadeite–garnet glaucophane schists have low CaO (4.4–4.5 wt%) and MgO (2.1–2.3 wt%) bulk‐rock compositions. The P–T– pseudosections show that low X_Ca bulk‐rock compositions favour the appearance of jadeite instead of omphacite under eclogite facies conditions. This is a unique example of low X_Ca bulk‐rock composition triggered to form jadeite at eclogite facies conditions. Two significant types of eclogitic metamorphism have been distinguished in the Sambagawa metamorphic belt, that is, a low‐T type and subsequent high‐T type eclogitic metamorphic events. The jadeite–garnet glaucophane schists experienced low‐T type eclogite facies metamorphism, and the P–T path is similar to lawsonite‐bearing eclogites recently reported from the Kotsu area in eastern Shikoku. During subduction of the oceanic plate (Izanagi plate), the hangingwall cooled gradually, and the geothermal gradient along the subduction zone progressively decreased and formed low‐T type eclogitic metamorphic rocks. A subsequent warm subduction event associated with an approaching spreading ridge caused the high‐T type eclogitic metamorphism within a single subduction zone.     

Phase equilibria modelling of blueschist and eclogite from the Sanbagawa metamorphic belt of southwest Japan reveals along‐strike consistency in tectonothermal architecture

The Sanbagawa metamorphic belt of southwest Japan is one of the type localities of subduction‐related high‐P metamorphism. However, variable pressure–temperature (P–T) paths and metabasic assemblages have been reported for eclogite units in the region, leading to uncertainty about the subduction zone paleo‐thermal structure and associated tectonometamorphic conditions. To analyse this variation, phase equilibria modelling was applied to the three main high‐P metabasic rock types documented in the region – glaucophane eclogite, barroisite eclogite and garnet blueschist – with modelling performed over a range of P, T, bulk rock H₂O and bulk rock ferric iron conditions using thermocalc . All samples are calculated to share a common steep prograde P–T path to similar peak conditions of ～16–20 kbar and 560–610 °C. The results establish that regional assemblage variation is systematic, with the alternation in peak amphibole phase due to peak conditions overlapping the glaucophane–barroisite solvus, and bulk composition effects stabilizing blueschist v. eclogite facies assemblages at similar P–T conditions. Furthermore, the results reveal that a steep prograde P–T path is common to all eclogite units in the Sanbagawa belt, indicating that metamorphic conditions were consistent along strike. All localities are compatible with predictions made by a ridge approach model, which attributes eclogite facies metamorphism and exhumation of the Sanbagawa belt to the approach of a spreading ridge.     

Long length scales of element transport during reaction texture development in orthoamphibole-cordierite gneiss: Thor-Odin dome,British Columbia,Canada

First-order factors controlling the textural and chemical evolution of metamorphic rocks are bulk composition and pressure–temperature–time (P–T–t) path. Although it is common to assume that major element bulk composition does not change during regional metamorphism, rocks with reaction textures such as corona structures record evidence for major changes in effective bulk composition (EBC) and therefore provide significant insight into the scale, pathways, and mechanisms of element transport during metamorphism. Quantifying changes in EBC is essential for petrologic applications such as calculation of phase diagrams (pseudosections). The progressive growth of complex corona structures on garnet and Al₂SiO₅ porphyroblasts in orthoamphibole-cordierite gneiss Thor-Odin dome (British Columbia, Canada) reduced the EBC volume of the rock during metamorphism and therefore had a dramatic effect on the evolution of the stable mineral assemblage. These rocks contain a chemical and textural record of metamorphic reactions and preserve 3D networks (reaction pathways) connecting corona structures. These coronal networks record long (>cm) length scales of localized element transport during metamorphism. P–T, T–X, and P–X pseudosections are used to investigate the control of effective bulk composition on phase assemblage evolution. Despite textural complexity and evidence for disequilibrium, mineral assemblages and compositions were successfully modeled and peak metamorphic conditions estimated at 750°C and 9 kbar. These results illustrate how textural and chemical changes during metamorphism can be evaluated using an integrated petrographic and pseudosection approach, highlight the importance of effective bulk composition choice for application of phase equilibria methods in metamorphic rocks, and show how corona structures can be used to understand the scale of compositional change and element transport during metamorphism.     

Progressive evolution of whole‐rock composition during metamorphism revealed by multivariate statistical analyses

The geochemical evolution of metamorphic rocks during subduction‐related metamorphism is described on the basis of multivariate statistical analyses. The studied data set comprises a series of mapped metamorphic rocks collected from the Sanbagawa metamorphic belt in central Shikoku, Japan, where metamorphic conditions range from the pumpellyite–actinolite to epidote–amphibolite facies. Recent progress in computational and information science provides a number of algorithms capable of revealing structures in large data sets. This study applies k‐means cluster analysis (KCA) and non‐negative matrix factorization (NMF) to a series of metapelites, which is the main lithotype of the Sanbagawa metamorphic belt. KCA describes the structures of the high‐dimensional data, while NMF provides end‐member decomposition which can be useful for evaluating the spatial distribution of continuous compositional trends. The analysed data set, derived from previously published work, contains 296 samples for which 14 elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P, Rb, Sr, Zr and Ba) have been analysed. The KCA and NMF analyses indicate five clusters and four end‐members, respectively, successfully explaining compositional variations within the data set. KCA indicates that the chemical compositions of metapelite samples from the western (Besshi) part of the sampled area differ significantly from those in the east (Asemigawa). In the west, clusters show a good correlation with the metamorphic grade. With increasing metamorphic grade, there are decreases in SiO₂ and Na₂O and increases in other components. However, the compositional change with metamorphic grade is less obvious in the eastern area. End‐member decomposition using NMF revealed that the evolutional change of whole‐rock composition, as correlated with metamorphic grade, approximates a stoichiometric increase of a garnet‐like component in the whole‐rock composition, possibly due to the precipitation of garnet and effusion of other components during progressive dehydration. Thermodynamic modelling of the evolution of the whole‐rock composition yielded the following results: (1) the whole‐rock composition at lower metamorphic grade favours the preferential crystallization of garnet under the conditions of the garnet zone, with biotite becoming stable together with garnet in higher‐grade rock compositions under the same P–T conditions; (2) with higher‐grade whole‐rock compositions, more H₂O is retained. These results provide insight into the mechanism suppressing dehydration under high‐P metamorphic conditions. This mechanism should be considered in forward modelling of the fluid cycle in subduction zones, although such a quantitative model has yet to be developed.     

Eine Subfaziesfolge der Grünschieferfazies in den Mittleren Cévennen (Dép. Ardèche) mit Pyrophyllit aufweisenden Mineralparagenesen

In the middle part of the Cévennes, situated at the south-eastern border of the Massif Central, there are outcrops of micaschists, gneisses and granites in which the increase of metamorphic grade was investigated along the river Beaume. This paper presents the results of the petrographic-geochemical analysis of the micaschists of the greenschist facies.Chemical analyses of the micaschist-samples examined petrographically show their nearly constant composition, a fact which e.g., is expressed by similar CaO/Na₂O- and MgO/FeO-proportions. As the average values of the determined oxides approach the respective average values of analyses of graywackes recorded by Pettijohn (1957) the original sediment is likely to have been an argillaceous graywacke deficient in calcite.In enrichments of the phyllosilicates pyrophyllite was detected by X-ray diffraction; its amount is about 2 to 5 weight percent of the rocks.With increasing metamorphic grade the following parageneses were found in the metamorphic zones of the greenschist facies: Zone Ia: Quartz+chlorite+pyrophyllite+muscovite+clinozoisite. Zone Ib: Quartz+chlorite+pyrophyllite+muscovite+biotite+clinozoisite.The absence of biotite in Zone Ia, though the chemism of the rocks is practically the same, is obviously due to the different Al₂O₃-content of the chlorites of Zones Ia and Ib. The chlorite of Zone Ia is more deficient in Al₂O₃ than the one of Zone Ib. With passage from Zone Ia to Zone Ib the position of the tie line between chlorite and muscovite in the ACF-A'FK-diagram changes in such a way that in Zone Ia, because of purely chemical reasons, biotite cannot occur as coexisting mineral.The beginning of Zone II is characterized by the occurence of almandine, rich in spessartine. The following paragenesis is typical of this zone: Quartz+chlorite+pyrophyllite +muscovite+biotite+almandine+clinozoisite.Additionally the micaschists of these three zones display albite, the greater part of which is concentrated in mm-thin layers with associated minor amounts of quartz and micas. Paragonite whose formation by reaction between albite and pyrophyllite is to be expected based on experimental results (Winkler, 1967, p. 95) could not be proved by X-ray diffraction.In Zone III andalusite occurs instead of pyrophyllite. Furthermore, as chlorite and clinozoisite are absent and oligoclase occurs for the first time this zone is regarded as the first subfacies of the almandine-amphibolite-facies. The chemism and the observed mineral parageneses of the subfacies of the almandine-amphibolite-facies will be treated in a separate publication.     

视剖面图温压计研究进展评述

视剖面图温压计是目前定量估算变质温度和压力的主要方法之一。其理论基础为热力学平衡原理与质量守恒定律。利用内恰的热力学数据库和相应的成分-活度模型对特定成分体系进行视剖面图计算,可以正演模拟给定温度和压力条件下的矿物组合、矿物丰度及其成分,与实测岩石对比,可以准确获取岩石的形成温度和压力。在应用视剖面图温压计时,应选择受全岩成分影响小、且受退变质作用影响微弱的矿物成分;对于有成分变化的体系,应结合多种方法恢复有效全岩成分。文中通过视剖面图对KFMASH(K2O-Fe O-Mg O-Al2O3-Si O2-H2O)体系下不同矿物组合内多硅白云母硅含量及其等值线斜率的研究,认为多硅白云母硅含量及其等值线斜率主要由不同矿物组合内多硅白云母参与的契尔马克替换及纯转换端元反应中的主导反应决定,进一步深化了对变质过程中控制矿物成分变化的内部缓冲反应机制的理解,从而也可为选择和应用矿物温压计提供指导。因此,视剖面图方法是目前研究变质岩石形成条件及变质作用精细过程的最佳方法之一。     

Coexistence of lawsonite‐bearing eclogite and blueschist: phase equilibria modelling of Alpine Corsica metabasalts and petrological evolution of subducting slabs

In Alpine Corsica (France), deeply subducted metabasalts are well preserved as lawsonite‐bearing eclogite (Law‐Ecl), occurrence of which is restricted to ～10 localities worldwide. The Corsican Law‐Ecl, consisting of omphacite + lawsonite + garnet + phengite + titanite, occurs as both single undeformed metabasaltic pillows surrounded by lawsonite blueschist (Law‐Bs), and carbonate‐bearing eclogitic veins. Law‐Bs are found as variably deformed metabasaltic pillows locally cross‐cut by eclogitic veins and consist of glaucophane + actinolite + lawsonite + garnet + phengite + titanite. Field evidence and microstructures reveal that both Law‐Ecl and Law‐Bs are stable at the metamorphic peak in the lawsonite‐eclogite stability field. Isochemical phase diagrams (pseudosections) calculated for representative Law‐Ecl and Law‐Bs samples indicate that both lithologies equilibrated at the same conditions of ～520 ± 20 °C and 2.3 ± 0.1 GPa. Therefore, the coexistence at the same peak metamorphic conditions of Law‐Ecl and Law‐Bs implies that different portions of deeply subducted oceanic crust may store significantly different H₂O contents, depending on bulk‐rock chemical composition. In addition, thermodynamic modelling of phase equilibria indicates that the occurring progressive dehydration reactions, which are significantly depending on bulk‐rock chemical composition, strongly influence rock densification and eclogite formation in subducting slabs.     

In situ impedance spectroscopy on pyrophyllite and CaCO<Subscript>3</Subscript> at high pressure and temperature: phase transformations and kinetics of atomistic transport

In situ impedance spectroscopy in laboratory experiments at high pressure and temperature can provide crucial quantitative information on properties of rock materials at depth as well as on physical and chemical processes occurring in the deep Earth. We developed an experimental setup for in situ electrical impedance measurements in a piston-cylinder apparatus and applied it to study the kinetics of charge carriers and phase transformations in pyrophyllite and CaCO₃ aggregates. From comparison with previous studies, we found that absolute values of electrical conductivity and pressure–temperature conditions for dehydration reactions in pyrophyllite and phase transformations in CaCO₃ can be accurately determined using our setup. Dehydration of pyrophyllite significantly enhances the transport kinetics and the effect is more pronounced under undrained conditions than under drained conditions. When dehydroxylation and decomposition temperatures for pyrophyllite under undrained and drained conditions are combined, they appear independent of pressure rather than increasing with pressure as previously suggested. Electrical conductivity of CaCO₃ varies with impurity content and grain size, and is most likely controlled by diffusion of oxygen along wet grain boundaries. When applied to the Earth, the results on pyrophyllite suggest that the increase in electrical conductivity in rocks that undergo dehydration should be taken into account in interpreting magnetotelluric surveys of regions with anomalously high conductivity found above subducting plates. The results on CaCO₃ indicate that grain boundary transport controls the electrical conductivity in fine-grained calcite rocks; hence calcite mylonites may be detected using magnetotelluric methods. Order–disorder transformations, such as occurring in calcite, possibly affect the physical properties of rocks (e.g., rheology) by changing the kinetics of atomistic transport processes.     

Modelling phase–assemblage diagrams for magnesian metapelites in the system K2O–FeO–MgO–Al2O3–SiO2–H2O: geodynamic consequences for the Monte Rosa nappe, Western Alps

Magnesian metamorphic rocks with metapelitic mineral assemblage and composition are of great interest in metamorphic petrology for their ability to constrain P–T conditions in terranes where metamorphism is not easily visible. Phase–assemblage diagrams for natural and model magnesian metapelites in the system KFMASH are presented to document how phase relationships respond to water activity, bulk composition, pressure and temperature. The phase assemblages displayed on these phase diagrams are consistent with natural mineral assemblages occurring in magnesian metapelites. It is shown that the equilibrium assemblages at high pressure conditions are very sensitive to a(H₂O). Specifically, the appearance of the characteristic HP assemblage chloritoid–talc–phengite–quartz (with excess H₂O) in the magnesian metapelites of the Monte Rosa nappe (Western Alps) is due to the reduction of a(H₂O). Furthermore, the mineral assemblages are determined by the whole-rock FeO/(FeO+MgO) ratio and effective Al content X _A as well as P and T. The predicted mineral associations for the low- and high-X _A model bulk compositions of magnesian metapelites at high pressure are not dependent on the X _A variations as they show a similar sequence of mineral assemblages. Above 20 kbar, the prograde sequence of assemblages associated with phengite (with excess SiO₂ and H₂O) for low- and high-X _A bulk compositions of magnesian metapelites is: carpholite–chlorite → chlorite–chloritoid → chloritoid–talc → chloritoid–talc–kyanite → talc–garnet–kyanite → garnet–kyanite ± biotite. At low to medium P–T conditions, a low-X _A stabilises the phengite-bearing assemblages associated with chlorite, chlorite + K-feldspar and chlorite + biotite while a high-X _A results in the chlorite–phengite bearing assemblages associated with pyrophyllite, andalusite, kyanite and carpholite. A high-X _A magnesian metapelite with nearly iron-free content stabilises the talc–kyanite–phengite assemblage at moderate to high P–T conditions. Taking into account the effective bulk composition and a(H₂O) involved in the metamorphic history, the phase–assemblage diagrams presented here may be applied to all magnesian metapelites that have compositions within the system KFMASH and therefore may contribute to gaining insights into the metamorphic evolution of terranes. As an example, the magnesian metapelites of the Monte Rosa nappe have been investigated, and an exhumation path with P–T conditions for the western roof of the Monte Rosa nappe has been derived for the first time. The exhumation shows first a near-isothermal decompression from the Alpine eclogite peak conditions around 24 kbar and 505°C down to approximately 8 kbar and 475°C followed by a second decompression with concomitant cooling.M. Frey: deceased     

有效全岩成分的计算方法及其在变质相平衡模拟中的应用

变质相平衡模拟是变质岩领域近几十年最重要的进展之一，它已经成为确定变质作用P-T-t轨迹和探索变质演化过程的有力工具。变质岩的矿物组合不但与其形成的温度（T）和压力（P）条件有关，而且受控于岩石的全岩成分（X）。但是变质岩通常是不均匀的并且往往保留两期以上的矿物组合，因此计算不同成分域或不同变质演化期次的有效全岩成分是模拟P-T视剖面图的核心问题之一。在中-低温变质岩中，石榴石变斑晶的生长会不断地将其核部成分"冻结"而不参与后续变质反应，这导致根据实测全岩成分计算的P-T视剖面图无法有效地模拟石榴石幔部或边部生长阶段的变质演化过程。"瑞利分馏法"和"球体积法"利用电子探针实测的石榴石成分环带可以模拟计算石榴石各个生长阶段所对应的有效全岩成分，本文推荐使用这两个方法来处理石榴石变斑晶的分馏效应问题。相比较而言，石榴石在高温变质岩中通常无法保留生长阶段的成分环带特征，这是因为石榴石成分在高温条件下会发生扩散再平衡，并同时与多数基质矿物达到热力学平衡，这时一般不需要考虑石榴石的分馏效应。但是高温变质岩通常会发生部分熔融并伴随熔体的迁移，进而改变岩石的有效全岩成分。因此，通过P-T视剖面图模拟熔体迁移前后的变质演化过程需要使用"相平衡法"计算迁移的熔体成分以及熔体迁移前后岩石的有效全岩成分。此外，后成合晶与反应边是变质岩中最常见的退变质反应结构，但是后成合晶或反应边中的矿物之间并未达到热力学平衡。这种情况需要结合岩相学观察和矿物成分，利用最小二乘法确定后成合晶或反应边中发生的平衡反应方程式，进而获取变质反应发生时的有效全岩成分并通过计算P-T视剖面图来估算退变质的温压条件。除此之外，岩石体系中三价铁（Fe₂O₃）和H₂O含量的估算一直以来都是相平衡模拟研究中的难点，本文推荐使用P/T-X（Fe³⁺/Fe^tot，MH₂O）视剖面图来确定这两个组分的含量，这是因为P/T-X图可以估算各个变质演化阶段或特定矿物组合的Fe₂O₃或H₂O含量。