FeO MgO Al2O3 SiO2 Research Articles

Monazite as a monitor of melting, garnet growth and feldspar recrystallization in continental lower crust

AbstractMonazite is a common accessory phase in felsic granulite ribbon mylonites exposed in the Upper Deck domain of the Athabasca granulite terrane, western Canadian Shield. Field relationships, bulk rock geochemistry and phase equilibria modelling in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 system are consistent with the garnet‐rich rocks representing the residual products of ultrahigh temperature melting of biotite‐bearing paragneisses driven by intraplating of mafic magma in continental lower crust. The c. 2.64–2.61 Ga Y‐rich resorbed monazite cores included in garnet are interpreted as relicts of detrital grains deposited on the Earth's surface after c. 2.61 Ga. Yttrium‐poor monazite domains in garnet are depleted in Sm and Gd and linked to fluid‐absent melting of biotite + plagioclase + quartz ± sillimanite during a prograde loading path from ≤0.8 to ≥1.4 GPa. The c. 2.61–2.55 Ga Y‐depleted, Th‐rich monazite domains crystallized in the presence of garnet + ternary feldspar ± orthopyroxene + peraluminous melt. The c. 2.58–2.52 Ga monazite rims depleted in Th + Ca and enriched in Eu are linked to localized melt extraction synchronous with growth of high‐pressure (HP) grossular‐rich garnet at the expense of plagioclase during crustal thickening, culminating at >950 °C. Re‐heating and dextral transpressive lower crustal reactivation at c. 1.9 Ga resulted in syn‐kinematic growth of (La + Ce)‐enriched monazite and a second generation of garnet, concurrent with recrystallization of feldspar and orthopyroxene at 1.0–1.2 GPa and 600–700 °C. Monazite grains in this study are marked by positive Eu‐anomalies relative to chondrite. A direct link is implied between Y, Sm, Eu and Gd in monazite and two major phases in continental lower crust: garnet and plagioclase. Positive Eu‐anomalies in lower crustal monazite associated with modally abundant garnet appear to be directly related to Eu‐enrichment and depletions of Y, Sm and Gd that are consequences of garnet growth and plagioclase breakdown during HP melting of peraluminous bulk compositions.

Melting of metasomatized peridotite at 4–6 GPa and up to 1200 °C: an experimental approach

The phase assemblages and compositions in a K-bearing lherzolite + H2O system are determined between 4 and 6 GPa and 850–1200 °C, and the melting reactions occurring at subarc depth in subduction zones are constrained. Experiments were performed on a rocking multi-anvil apparatus. The experiments had around 16 wt% water content, and hydrous melt or aqueous fluid was segregated and trapped in a diamond aggregate layer. The compositions of the aqueous fluid and hydrous melt phases were measured using the cryogenic LA-ICP-MS technique. The residual lherzolite consists of olivine, orthopyroxene, clinopyroxene, and garnet, while diamond (C) is assumed to be inert. Hydrous and alkali-rich minerals were absent from the run products due to preferred dissolution of K2O (and Na2O) to the aqueous fluid/hydrous melt phases. The role of phlogopite in melting relations is, thus, controlled by the water content in the system: at the water content of around 16 wt% used here, phlogopite is unstable and thus does not participate in melting reactions. The water-saturated solidus, i.e., the first appearance of hydrous melt in the K–lherzolite composition, is located between 900 and 1000 °C at 4 GPa and between 1000 and 1100 °C at 5 and 6 GPa. Compositional jumps between hydrous melt and aqueous fluid at the solidus include a significant increase in the total dissolved solids load. All melts/fluids are peralkaline and calcium-rich. The melting reactions at the solidus are peritectic, as olivine, clinopyroxene, garnet, and H2O are consumed to generate hydrous melt plus orthopyroxene. Our fluid/melt compositional data demonstrate that the water-saturated hybrid peridotite solidus lies above 1000 °C at depths greater than 150 km and that the second critical endpoint is not reached at 6 GPa for a K2O–Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–Cr2O3(–TiO2) peridotite composition.

First-report on Mesozoic eclogite-facies metamorphism preceding Barrovian overprint from the western Rhodope (Chalkidiki, northern Greece)

The Chalkidiki block in Northern Greece represents the southwesternmost piece of the ultrahigh-pressure Rhodope and has played an important role in the evolution of the North Aegean. The eastern part of the Chalkidiki block is a basement complex (Vertiskos Unit) that is made largely of Palaeozoic granitoids and clastic sediments that metamorphosed during the Mesozoic. This basement is traditionally considered as part of the Rhodopean hanging-wall, an assignment mainly supported by the absence of high-pressure mineral indicators and the presence of a regional medium-pressure/medium-temperature amphibolite-facies Barrovian metamorphic imprint. Toward the west, the basement is juxtaposed with meta-sedimentary (Circum-Rhodope belt) and arc units (Chortiatis Magmatic Suite) that carry evidence of a Mesozoic high-pressure/low-temperature event. In this study, garnet–staurolite-mica schists from the eastern part of the basement were examined by means of micro-textures, mineral chemistry and isochemical phase-diagram sections in the system NCKFMASHMn(Ti) [Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–MnO–(TiO2)]. The schists represent former Mesozoic sedimentary sequences deposited on the Palaeozoic basement. We document the presence of a relict eclogite-facies mineral assemblage (garnet+chloritoid+phengite+rutile) in an amphibolite-facies matrix composed of garnet+staurolite+phengite±kyanite. Model results suggest the existence of a high-pressure/medium-temperature metamorphic event (1.9GPa/520°C) that preceded regional re-equilibration at medium-pressure/medium-temperature conditions (1.2GPa/620°C). Clearly, the eastern part of the Chalkidiki block (basement complex) retains memory of an as yet unidentified Mesozoic eclogitic metamorphic event that was largely erased by the later Barrovian overprint. In light of our findings, the basement complex of the Chalkidiki block shares a common tectono-metamorphic evolution with both the high-pressure units to the west, and the high-grade Rhodopean gneisses further to the northeast. Our results are consequential for the geodynamic reconstruction of the Rhodope since they require participation of the Chalkidiki block to the well-established Mesozoic subduction system.

Using calculated chemical potential relationships to account for replacement of kyanite by symplectite in high pressure granulites

AbstractCalculated mineral equilibria are used to account for the formation of sapphirine–plagioclase, spinel–plagioclase and corundum–plagioclase symplectites replacing kyanite in quartz–plagioclase–garnet–kyanite granulite facies gneisses from the Southern Domain of the Athabasca granulite terrane, a segment of the Snowbird tectonic zone in northern Saskatchewan, Canada. Metamorphic conditions of >14 kbar and 800 °C are established for the high pressure, garnet–kyanite assemblage using constraints from P–T pseudosections and Zr‐in‐rutile thermometry. Replacement of kyanite by symplectites reflects the reaction of kyanite with the matrix following near‐isothermal decompression to <10 kbar. The chemical potential gradients developed between the kyanite and the matrix led to diffusion that attempted to flatten the gradients, kyanite persisting as a stable phase while it is consumed by symplectite from its edge. In this local equilibrium model, the mineral and mineral compositional spatial relationships are shown to correspond to paths in μ(Na2O)–μ(CaO)–μ(K2O)–μ(FeO)–μ(MgO) in the model chemical system, Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2 (NCKFMAS), with SiO2 and Al2O3 taken to be completely immobile. The values of μ(Na2O) and μ(CaO) are constrained by fixing P–T conditions and choosing appropriate μ(Na2O) and μ(CaO) values that correspond to the observed plagioclase compositions. μ(FeO)–μ(MgO) diagrams show the corresponding spatial relationships with kyanite and the symplectite phases. These results demonstrate that the replacement of kyanite by sapphirine–plagioclase and spinel–plagioclase appears to be metastable with respect to replacement by corundum–plagioclase. Replacement by corundum–plagioclase does also occur, apparently overprinting pre‐existing symplectite and also kyanite. Ignoring corundum, the resulting diagrams account for the spatial relationships and compositions observed in the spinel–plagioclase and sapphirine–plagioclase symplectites. They are predicted to occur over both a wide range of P–T conditions (6–11 kbar, 650–850 °C) and plagioclase compositions (XAn = 0.5–0.9). The wide range of P–T conditions that may result in identical spatial and compositional relationships suggests that such reaction textures may be of limited use in accurately quantifying the P–T conditions of retrograde metamorphism.

Early Paleozoic tectonic evolution of the North Qinling orogenic belt: Evidence from geochemistry, phase equilibrium modeling and geochronology of metamorphosed mafic rocks from the Songshugou ophiolite

The Shangdan suture zone (SSZ) is the main collisional boundary between the North China Craton and the South China Craton, along which discontinuous Paleozoic ophiolites and subduction–accretion related volcanic arc assemblages occur. Here we report the petrology, geochemistry, geochronology and phase equilibria modeling of garnet amphibolite from the Songshugou ophiolite which is one of the largest ophiolite outcrops in the northern side of the SSZ. From petrological studies, we identify: (1) prograde stage, defined by garnet+clinopyroxene+calcic amphibole+ilmenite+rutile+epidote+plagioclase+quartz; (2) peak stage with garnet+clinopyroxene+ilmenite+rutile+quartz; and (3) retrograde stage with amphibole+plagioclase+titanite+ilmenite. Our pseudosection analysis defines stability of the peak assemblage at 750–850°C, 15–19kbar and traces a clockwise P–T path in the system Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCFMASHTO), suggesting high pressure (HP) metamorphism. Subsequently, the rocks experienced rapid decompression and cooling. LA-ICP-MS U-Pb analyses of zircons from the garnet amphibolite yield a weighted mean 206Pb/238U age of 515±12Ma. This Early Paleozoic metamorphic age represents the emplacement time of the Songshugou ophiolite, and suggests that the HP metamorphism is possibly related to the northward deep subduction of the Shangdan oceanic crust in Early Paleozoic.

Early Neoproterozoic granulite facies metamorphism of mafic dykes from the Vestfold Block, east Antarctica

AbstractProterozoic mafic dykes from the southwestern Vestfold Block experienced heterogeneous granulite facies metamorphism, characterized by spotted or fractured garnet‐bearing aggregates in garnet‐absent groundmass. The garnet‐absent groundmass typically preserves an ophitic texture composed of lathy plagioclase, intergranular clinopyroxene and Fe–Ti oxides. Garnet‐bearing domains consist mainly of a metamorphic assemblage of garnet, clinopyroxene, orthopyroxene, hornblende, biotite, plagioclase, K‐feldspar, quartz and Fe–Ti oxides. Chemical compositions and textural relationships suggest that these metamorphic minerals reached local equilibrium in the centre of the garnet‐bearing domains. Pseudosection calculations in the model system NCFMASHTO (Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3) yield P–T estimates of 820–870 °C and 8.4–9.7 kbar. Ion microprobe U–Pb zircon dating reveals that the NW‐ and N‐trending mafic dykes were emplaced at 1764 ± 25 and 1232 ± 12 Ma, respectively, whereas their metamorphic ages cluster between 957 ± 7 and 938 ± 9 Ma. The identification of granulite facies mineral inclusions in metamorphic zircon domains is also consistent with early Neoproterozoic metamorphism. Therefore, the southwestern margin of the Vestfold Block is inferred to have been buried to depths of ~30–35 km beneath the Rayner orogen during the late stage of the late Mesoproterozoic/early Neoproterozoic collision between the Indian craton and east Antarctica (i.e. the Lambert Terrane or the Ruker craton including the Lambert Terrane). The lack of penetrative deformation and intensive fluid–rock interaction in the rigid Vestfold Block prevented the nucleation and growth of garnet and resulted in the heterogeneous granulite facies metamorphism of the mafic dykes.

Thermo-chemical constraints on the interior structure and composition of the lunar mantle

Based on a self-consistent thermodynamic-geophysical approach, we convert the recent seismic models of the lunar mantle to the temperature-depth profiles using Gibbs free energy minimization and petrological constraints in the Na2O–TiO2–CaO–FeO–MgO–Al2O3–SiO2 system. Our calculations are unable to explain the reasonable distribution of temperature for a single homogeneous composition throughout the entire lunar mantle with concentrations of CaO and Al2O3 in the range of 2–6.5%, and FeO content between 8.5% and 13%. The results lend support to the chemically stratified lunar mantle with a change in composition from predominantly pyroxenite upper mantle depleted in Ca and Al to predominantly fertile lower mantle enriched in Ca and Al with larger amounts of garnet. Such a zoned structure places significant constraints on any theory of lunar origin. Unlike the Earth’s mantle, compositional effects play a dominant role in determining the lunar mantle temperatures of the same observational model. Seismically derived temperatures allow us to constrain thermal structure of the lunar mantle and estimate the upper mantle heat flow (3.8–4.7mWm−2), which is not consistent with that found from the Apollo heat flow and thorium abundance measurements. Lower mantle temperatures are well below the probable solidus condition and can be evaluated at the level of 1420–1550°C at the core-mantle boundary without requiring a melt layer. We find that regardless of the composition, the positive S-wave velocity gradient in the lunar mantle leads to a negative temperature gradient, which has no physical basis. The resulting temperature profiles provide an effective independent tool that allows us to discriminate between the available seismic and petrological models.

The effect of Mn on mineral stability in metapelites revisited: new a–x relations for manganese‐bearing minerals

AbstractThe a–x relations recently presented in White et al. (, Journal of Metamorphic Geology, 32, 261–286) are extended to include MnO. This provides a set of internally consistent a–x relations for metapelitic rocks in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 (MnNCKFMASHTO) system. The mixing parameters for the Mn‐bearing minerals were estimated using the micro‐ϕ approach of Powell et al. (, Journal of Metamorphic Geology, 32, 245–260). Then the Mn‐end‐member thermodynamic properties were calibrated using a database of co‐existing minerals involving literature data from rocks and from experiments on natural materials. Mn‐end‐members were calibrated for orthopyroxene, cordierite, staurolite, chloritoid, chlorite, biotite, ilmenite and hematite, assuming known properties for the garnet end‐member spessartine. The addition of MnO to phase diagram calculations results in a marked expansion of the stability of garnet‐bearing assemblages. At greenschist facies conditions garnet stability is extended down temperature. At amphibolite facies conditions, the garnet‐in boundary shifts to lower pressure. While the addition of MnO greatly influences the stability of garnet, it has relatively little effect on the stability of other common metapelitic minerals, with the resultant diagrams being topologically very similar to those calculated without MnO. Furthermore, the addition of MnO in the amounts measured in most metapelites has only a small effect on the mode of garnet, with calculated garnet modes remaining smaller than 1% in the P–T range outside its predicted Mn‐free P–T range.

Erratum to: A thermodynamic model for di-trioctahedral chlorite from experimental and natural data in the system MgO–FeO–Al2O3–SiO2–H2O: applications to P–T sections and geothermometry

Erratum to: A thermodynamic model for di-trioctahedral chlorite from experimental and natural data in the system MgO–FeO–Al2O3–SiO2–H2O: applications to P–T sections and geothermometry

Melting of phase D in the lower mantle and implications for recycling and storage of H2O in the deep mantle

We determined the melting phase relations and conditions of the dense hydrous magnesium silicate phase D (nominally MgSi2O4(OH)2) on composition in MgO–SiO2–H2O (MSH), MgO–Al2O3–SiO2–H2O (MASH), and FeO–MgO–Al2O3–SiO2–H2O (FMASH), and on a mixture of phase D+olivine+enstatite (MSH) at 22–32GPa and 1000–1800°C. Contrasting to previous studies, we performed H2O-undersaturated experiments. Bulk compositions were synthetic mixtures of brucite+silica or brucite+olivine+enstatite on the silica-rich side of the tie-line perovskite–H2O. At 22–24GPa, the maximum thermal stability of phase D is between 1350 and 1400°C in MSH and FMASH, but 1600°C at 24GPa in the Fe-free, Al-bearing bulk composition (MASH). Apparently, addition of Al2O3 increases the stability field of phase D by 200°C, an effect that is counter balanced by addition of FeO. At 32GPa, the stability of phase D (MSH and FMASH) is between 1350 and 1400°C. At 22GPa, phase D melts to a Mg-rich melt coexisting with MgSi-ilmenite+stishovite, whereas at 24–32GPa melt coexists with perovskite and stishovite. Even melts from bulk compositions in the silica-rich part of the MSH system (molar bulk Mg/Si<0.5) are magnesian-rich (Mg/Si molar ratio of 2–5) and are distinct from aqueous fluids and hydrous melts at lower pressures.The temperature stabilities determined in this study indicate that slabs that thermally relax when stagnating on top of the 660-km discontinuity or penetrating into the lower mantle will have their last dense hydrous magnesium silicate phase, i.e., phase D, melting and producing a magnesian and hydrous melt that will rise through the transition zone. Such a melt could be responsible for observed low velocity zones, and may be neutrally buoyant at the 410-km discontinuity and will affect the structure and dynamics of the mantle.

Phase equilibria modeling of kyanite-bearing eclogitic metapelites in the NCKFMASHTO system from the Egere terrane (Central Hoggar, South Algeria)

Kyanite-bearing metapelites from the Egere terrane (Central Hoggar, South Algeria) have a high-pressure eclogitic facies paragenesis; furthermore, they preserve evidence of partial melting with the development of metamorphic textures involving garnet–kyanite–biotite–white mica–plagioclase–K-feldspar–quartz–rutile and ilmenite. Garnet porphyroblasts in these rocks display a calcium zonation formed during high-P metamorphism followed by decompression. Moreover, the large garnet poikiloblasts reflect complex mineral–melt relationships, and their growth was strongly linked to melt production. In order to give an account of the various textures and explain the evolution of the different parageneses in relation with pressure and temperature, thermodynamic modeling in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO) system is a powerful approach. The intersection of isopleths of grossular content and Fe⁄(Fe + Mg) of core of garnet provides constraints on the P–T conditions of the metamorphic peak. Kyanite-bearing metapelites experienced a clockwise P–T path culminating at about 850 °C and 17–19 kbar. Peak conditions were followed by decompression to about 9 kbar and 750 °C.

Phase equilibria modelling of retrograde amphibole and clinozoisite in mafic eclogite from the Tso Morari massif, northwest India: constraining the P–T–M(H2O) conditions of exhumation

AbstractPhase equilibria modelling of post‐peak metamorphic mineral assemblages in (ultra)high‐P mafic eclogite from the Tso Morari massif, Ladakh Himalaya, northwest India, has provided new insights into the potential behaviour and source of metamorphic fluid during exhumation, and constrained the P–T conditions of hydration. A series of P–M(H2O) pseudosections constructed in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (NCKFMASHTO) system show that a number of petrographically distinct hydration episodes occurred during exhumation from peak P–T conditions (~640 °C, 27–28 kbar), resulting in the formation of abundant compositionally zoned amphibole and minor clinozoisite poikiloblasts at the expense of a peak assemblage dominated by garnet and omphacite. Initial hydration is interpreted to have occurred as a result of the destabilization of talc following isothermal decompression to ~23 kbar, which led to the formation of barroisite–winchite amphibole core domains. An episode of fluid infiltration from an external source at ~19 kbar, with or without syn‐decompressional cooling to ~560 °C, resulted in further barroisitic–winchitic amphibole growth, followed by the formation of clinozoisite poikiloblasts. Continued buoyancy‐driven exhumation to the base of the lower crust is constrained to have taken place with no additional fluid input. A final hydration event is characterized by the formation of magnesiohornblende rims on the barroisite–winchite cores, with the former interpreted to have formed during later prograde overprinting in the middle crust associated with the final stages of exhumation. Notably, the vast majority of externally sourced H2O, comprising just over half of the current bulk rock fluid content, was added during this later hydration event. In a middle crustal setting, this is interpreted as the result of devolatilization reactions occurring in migmatitic host orthogneiss and/or metasedimentary units, or following the crystallization of partial melt.

Deformation‐driven, regional‐scale metasomatism in the Hamersley Basin, Western Australia

AbstractMafic volcanic rocks of the Fortescue Group form the lowermost stratigraphic unit of the 100,000 km2 Hamersley Basin on the southern margin of the Archean Pilbara Craton, Western Australia. A regional burial metamorphic gradient extends across the basin from prehnite–pumpellyite facies in the north to greenschist facies in the south. Phase equilibria modelling of mafic rocks with the computer program thermocalc, in subsets of the system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–Fe2O3, successfully reproduces observed metamorphic mineral assemblages, giving conditions of ~210 °C, 2 kbar in the north and 335 °C, 3.2 kbar in the south. Superimposed on this metamorphic gradient, regional‐scale metasomatism in the Fortescue Group progressively produces a suite of prehnite‐bearing and pumpellyite–quartz/epidote–quartz‐dominated assemblages. Further modelling of variably metasomatized samples consistently estimates conditions of 260–280 °C, 2.5–3 kbar across the basin. All modelled samples were likely metasomatized at approximately the same structural level, following regional deformation during the Ophthalmian orogeny. Folding during the Ophthalmian orogeny produced topographic and/or tectonic driving forces for regional‐scale fluid flow, pushing metasomatic fluid northwards across the Hamersley Basin. These new phase equilibria calculations support previous interpretations linking the Ophthalmian orogeny, fluid flow and upgrading of Hamersley iron ore deposits. We propose an extension of this fluid flow to the Fortescue Group, the metasomatism of which may have contributed a source of Fe to the Hamersley iron ore deposits.

Influence of ferric iron on phase equilibria in greenschist facies assemblages: the hematite‐rich metasedimentary rocks from the Monti Pisani (Northern Apennines)

AbstractWe have investigated the effects of different Fe2O3 bulk contents on the calculated phase equilibria of low‐T/intermediate‐P metasedimentary rocks. Thermodynamic modelling within the MnO–Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (MnNKFMASHTO) chemical system of chloritoid‐bearing hematite‐rich metasedimentary rocks from the Variscan basement of the Pisani Mountains (Northern Apennines, Italy) fails to reproduce the observed mineral compositions when the bulk Fe2O3 is determined through titration. The mismatch between observed and computed mineral compositions and assemblage is resolved by tuning the effective ferric iron content by P–XFe2O3 diagrams, obtaining equilibration conditions of 475 °C and 9–10 kbar related to a post‐compressional phase of the Alpine collision. The introduction of ferric iron affects the stability of the main rock‐forming silicates that often yield important thermobaric information. In Fe2O3‐rich compositions, garnet‐ and carpholite‐in curves shift towards higher temperatures with respect to the Fe2O3‐free systems. The presence of a ferric‐iron oxide (hematite) prevents the formation of biotite in the mineral assemblage even at temperatures approaching 550 °C. The use of P–T–XFe2O3 phase diagrams may also provide P–T information in common greenschist facies metasedimentary rocks.

On parameterizing thermodynamic descriptions of minerals for petrological calculations

AbstractA new regularization approach, termed micro‐ϕ, is outlined for parameterizing activity–composition (a–x) relations and other aspects of the thermodynamic descriptions of minerals for petrological calculations. In the context of the symmetric formalism, a formulation of a–x relations that is easily generalizable to multi‐component minerals, parameterization with micro‐ϕ extends from where there are good data available to constrain, for example, interaction energies, to where there are little or no data. This involves decomposing the interaction energies, which are macroscopic between end‐members, into their microscopic components involving interactions between elements on sites. Micro‐ϕ involves heuristics to simplify and approximate these microscopic interaction energies prior to their reassembly as macroscopic interaction energies. Micro‐ϕ also allows parameterization of Fe2+–Mg order–disorder between the sites of minerals. Application of micro‐ϕ is illustrated by making a thermodynamic description of chlorite in FeO–MgO–Al2O3–SiO2–H2O–O, FMASHO. Micro‐ϕ has been developed for use with the significant upgrade of the Holland &amp; Powell internally consistent thermodynamic data set which required the re‐evaluation of a–x relations of several common rock‐forming minerals.

A new thermodynamic model for sapphirine: calculated phase equilibria in K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3

AbstractThe activity–composition (a–x) relations of sapphirine are re‐evaluated in the light of a recent new internally‐consistent data set of phase end‐members for use in phase equilibria modelling, particularly of ultra‐high‐temperature (UHT) rocks. This is achieved with the aid of relatively oxidized sapphirine+quartz‐bearing granulites from Wilson Lake, Canada. Calculated P–T projections and compatibility diagrams in the K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (KFMASHTO) system are used to illustrate sapphirine+quartz‐bearing phase equilibria in the context of UHT metamorphism. These new a–x relations for sapphirine should allow pseudosection thermobarometry in NCKFMASHTO for estimating peak P–T conditions of sapphirine‐bearing rocks.

New mineral activity–composition relations for thermodynamic calculations in metapelitic systems

AbstractNew activity–composition (a–x) relations for minerals commonly occurring in metapelites are presented for use with the internally consistent thermodynamic dataset of Holland &amp; Powell (, Journal of Metamorphic Geology, 29, 333–383). The a–x relations include a broader consideration of Fe2O3 in minerals, changes to the formalism of several phases and order–disorder in all ferromagnesian minerals where Fe–Mg mixing occurs on multiple sites. The a–x relations for chlorite, biotite, garnet, chloritoid, staurolite, cordierite, orthopyroxene, muscovite, paragonite and margarite have been substantially reparameterized using the approach outlined in the companion paper in this issue. For the first time, the entire set of a–x relations for the common ferromagnesian minerals in metapelitic rocks is parameterized simultaneously, with attention paid to ensuring that they can be used together to calculate phase diagrams of geologically appropriate topology. The a–x relations developed are for use in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 (NCKFMASHTO) system for both subsolidus and suprasolidus conditions. Petrogenetic grids in KFMASH and KFMASHTO are similar in topology to those produced with earlier end‐member datasets and a–x relations, but with some notable differences. In particular, in subsolidus equilibria, the FeO/(FeO + MgO) of garnet is now greater than in coexisting staurolite, bringing a number of key staurolite‐bearing equilibria into better agreement with inferences from field and petrographic observations. Furthermore, the addition of Fe3+ and Ti to a number of silicate phases allows more plausible equilibria to be calculated in relevant systems. Pseudosections calculated with the new a–x relations are also topologically similar to equivalent diagrams using earlier a–x relations, although with many low variance fields shifting in P–T space to somewhat lower pressure conditions.

A thermodynamic model for di-trioctahedral chlorite from experimental and natural data in the system MgO–FeO–Al2O3–SiO2–H2O: applications to P–T sections and geothermometry

We present a new thermodynamic activity-composition model for di-trioctahedral chlorite in the system FeO–MgO–Al2O3–SiO2–H2O that is based on the Holland–Powell internally consistent thermodynamic data set. The model is formulated in terms of four linearly independent end-members, which are amesite, clinochlore, daphnite and sudoite. These account for the most important crystal-chemical substitutions in chlorite, the Fe–Mg, Tschermak and di-trioctahedral substitution. The ideal part of end-member activities is modeled with a mixing-on-site formalism, and non-ideality is described by a macroscopic symmetric (regular) formalism. The symmetric interaction parameters were calibrated using a set of 271 published chlorite analyses for which robust independent temperature estimates are available. In addition, adjustment of the standard state thermodynamic properties of sudoite was required to accurately reproduce experimental brackets involving sudoite. This new model was tested by calculating representative P–T sections for metasediments at low temperatures (<400 °C), in particular sudoite and chlorite bearing metapelites from Crete. Comparison between the calculated mineral assemblages and field data shows that the new model is able to predict the coexistence of chlorite and sudoite at low metamorphic temperatures. The predicted lower limit of the chloritoid stability field is also in better agreement with petrological observations. For practical applications to metamorphic and hydrothermal environments, two new semi-empirical chlorite geothermometers named Chl(1) and Chl(2) were calibrated based on the chlorite + quartz + water equilibrium (2 clinochlore + 3 sudoite = 4 amesite + 4 H2O + 7 quartz). The Chl(1) thermometer requires knowledge of the (Fe3+/ΣFe) ratio in chlorite and predicts correct temperatures for a range of redox conditions. The Chl(2) geothermometer which assumes that all iron in chlorite is ferrous has been applied to partially recrystallized detrital chlorite from the Zone houillère in the French Western Alps.

Metamorphic P–T path and tectonic implications of pelitic granulites from the Daqingshan Complex of the Khondalite Belt, North China Craton

Pelitic granulites are cropped out in the Daqingshan Complex of the Khondalite Belt, a Paleoproterozoic tectonic belt in the Western Block of the North China Craton. Petrological studies show that these granulites contain four distinct metamorphic assemblages. The core of a garnet porphyroblast, along with fine-grained inclusions of biotite+plagioclase+K-feldspar+quartz±sillimanite±ilmenite, defines the prograde metamorphic (M1) stage. The peak (M2) assemblage consists of garnet (mantle)+biotite+K-feldspar+plagioclase+quartz±sillimanite±orthopyroxene±ilmenite±magnetite±rutile. Peak metamorphism was followed by a near-isothermal decompression (M3) and the development of coronae of garnet+biotite+cordierite+plagioclase+quartz±K-feldspar±sillimanite±spinel±ilmenite±magnetite (M3) in the Crd–Grt–Bt gneisses during the following garnet breakdown reactions: garnet+sillimanite+melt→cordierite+biotite+Fe-oxide and garnet+melt→biotite+quartz+plagioclase. Retrograde cooling (M4) assemblages are represented by biotite+muscovite+sillimanite+quartz+plagioclase+K-feldspar±ilmenite. Quantitative phase equilibria modeling in the system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 was applied to obtain P–T conditions of <780°C and <9kbar for M1, 840–880°C and 9–11kbar for M2, 800–870°C and 5.0–7.5kbar for M3, and <660°C and 4.1–6.9kbar for M4. The combination of the mineral assemblages, mineral compositions, and metamorphic reaction histories in the Daqingshan pelitic granulites defines a clockwise P–T path that involves periods of near-isothermal decompression and late cooling that followed the peak granulite-facies metamorphism. This result is consistent with the tectonic history of the Daqingshan Complex in the Khondalite Belt, which involved continent–continent subduction and collision followed by exhumation and cooling. This suggests a continent–continent collisional event between the Yinshan and Ordos blocks, which became amalgamated to form the Paleoproterozoic Khondalite Belt in the Western Block of the North China Craton.

Monazite ages and pressure–temperature–time paths from anatectites in the southern Ruby Range, Montana, USA: evidence for delamination, ultramafic magmatism, and rapid uplift at ca. 1780 Ma

The Wolf Creek ultramafic bodies of the southern Ruby Range, southwest Montana, intruded as high-temperature magma during the late stages of an Early Proterozoic orogeny affecting the Great Falls Tectonic Zone of the western Wyoming craton. Field and petrologic evidence indicate high-temperature metamorphism and deformation uniquely affected metapelitic anatectites adjacent to the ultramafic bodies. Leucosomes are mesoperthite and quartz with garnet. Restite contains sillimanite, garnet, quartz, microcline, and biotite that was partially resorbed by the melt-producing reaction. Late cordierite and sillimanite replace garnet in the migmatites and other pelitic gneiss in the area. Petrogenic grids for the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (NCKFMASH) system indicate that conditions during anatexis were 750–800 °C, 600–900 MPa. Anatexis occurred during deformation of metapelites and associated rocks adjacent to the larger ultramafic bodies, but crystallization and cooling occurred after deformation, as indicated by the preservation of igneous textures and mesoperthite with undeformed lamellae in the leucosome. Element mapping of monazite using electron-probe micro-analysis (EPMA) identifies four distinct chemical domains of monazite growth during the orogenic event in addition to relict, Archaean to earliest Proterozoic cores. U–Th–Pb dating of the domains establishes two periods of growth of matrix monazite in the paleosome that indicate fabric-forming prograde metamorphism at 1829 ± 17 and 1805 ± 4 Ma (2σ). Coarser, igneous monazites from the leucosome also contain two distinct chemical zones visible in yttrium maps, indicating growth with garnet and then during garnet resorption as decompression led to replacement of garnet by cordierite. The domain ages cannot be statistically distinguished. Together they yield an age of 1782 ± 9 Ma (2σ). The data imply that regional thickening leading to upper amphibolite facies metamorphism occurred prior to 1800 Ma. Thickening was followed after ∼25 Ma by the intrusion of ultramafic magmas, contact anatexis, and then decompression. This sequence is consistent with mantle delamination after orogenic thickening.