Natural Metapelite Research Articles

Iterative thermodynamic modelling—Part 2: Tracing equilibrium relationships between minerals in metamorphic rocks

AbstractUnderstanding equilibrium relationships between minerals is a fundamental goal of metamorphic petrology, especially for the quantification of metamorphic conditions recorded by crustal rocks. Information on pressure (P) and temperature (T) can be derived from mineral compositions based on the a priori assumption that frozen‐in assemblages reflect chemical equilibrium at the time of formation. While the stable coexistence of minerals is commonly assessed from a combination of microstructural criteria and interpretation of phase diagrams, there is no robust modelling strategy available so far for the unambiguous recognition of minerals that grew at different conditions. This study uses iterative thermodynamic models and the computer program Bingo‐Antidote to propose a general strategy for assessing the stable coexistence of minerals and therefore for tracing multiple partial equilibrium relationships preserved in metapelites to reconstruct their P–T evolutions. First the principles of this approach are elaborated in a theoretical example. Application to a metapelite from the Western Central Alps reveals a detailed, clockwise P–T path with at least four different equilibration stages including a poorly defined peak pressure at 1–1.1 GPa, ~560°C, followed by peak temperatures of ~620°C at 0.9 GPa and a late amphibolite stage at ~580°C, 0.65 GPa. The investigation of the natural metapelite via iterative thermodynamic models (ITM) allows the observed mineral formation sequence to be combined with a higher accuracy with variable mineral compositions and results in a detailed P–T path. The metastable perseverance of one or more mineral phases can be statistically inferred via ITM. In the investigated metapelite, only ~50 vol.% of the rock volume equilibrated at peak temperature conditions of 620°C. In addition, ITM enables a more detailed investigation of partial re‐equilibration by diffusion during subsequent metamorphic stages, showing that prograde garnet growth could have been modified during subsequent 10 Ma of amphibolite facies metamorphism in the Central Alps.

Calibration of the biotite-muscovite geobarometer for metapelitic assemblages devoid of garnet or plagioclase

Determination of metamorphic pressure (P) – temperature (T) conditions of metapelitic assemblages sometimes suffers from low-CaO content of garnet and/or plagioclase, thus the garnet-plagioclase-related geobarometers lose their usage under such circumstances. Therefore, calibration of garnet-plagioclase-absent geobarometers is very useful. In this paper, the biotite-muscovite geobarometer was empirically calibrated under P-T conditions of 1–12 kbar and 470–760 °C using 270 natural metapelite samples. About 86.5% of the total calibrant samples yielded similar biotite-muscovite pressure estimates to the well-calibrated garnet-Al2SiO5-plagioclase-quartz (GASP) geobarometer within error of ±1.5 kbar. The total error of the biotite-muscovite geobarometer is estimated to be less than ~±1.6 kbar. The applicability of the biotite-muscovite geobarometer has been certified through application to natural metapelite samples coupled with the Ti-in-biotite geothermometer. When garnet and/or plagioclase are absent in metapelitic assemblages such as two-mica schist / gneiss, this geobarometer may play its unique and irreplaceable role. An electronic spreadsheet (Table S3) is deposited at the journal's website for iterative application of the biotite-muscovite geobarometer in combination with the Ti-in-biotite geothermometer.

Calibration of the garnet–biotite–Al2SiO5–quartz geobarometer for metapelites

AbstractA garnet–biotite–Al2SiO5–quartz (GBAQ) geobarometer was empirically calibrated using more than 700 natural metapelites with a broad compositional range of garnet and biotite under P–T conditions of 450–950°C and 1–17 kbar. In the calibration, activity models of garnet and biotite identical to those in the garnet–biotite (GB) geothermometer of Holdaway [American Mineralogist 2000, 85:881–892] were used. Therefore, the GBAQ geobarometer and the GB geothermometer can be simultaneously applied to iteratively estimate metamorphic P–T conditions. Successful applications of the GBAQ geobarometer to natural metapelites certify its validity. Most importantly, when plagioclase is absent or CaO components in garnet and/or plagioclase are deficient, this geobarometer may prove useful for estimating metamorphic pressures. The random error of the present GBAQ geobarometer is inferred to be around ±1.8 kbar. An electronic spreadsheet is available as Table S4 to apply the GBAQ geobarometer in combination with the GB geothermometer.

Experimental assessment of the relationships between electrical resistivity, crustal melting and strain localization beneath the Himalayan–Tibetan Belt

Seismic and magnetotelluric field campaigns carried out across the Himalaya and the Tibetan Plateau show mid-crustal low resistivity and low velocity zones. Interpretation of these anomalous observations, either saline fluids or partial melts, is still largely debated partly because experimental data simulating crustal melting under relevant pressure, temperature and water content conditions have not been provided. We report laboratory measurements constraining the resistivity as a function of temperature and the viscosity at 800°C of natural metapelite during partial melting. Dehydration-melting of muscovite is simulated using a Paterson press at 300MPa in the temperature range 600–850°C. Electrical resistivity has been measured from the solid state (<650°C) to 25vol% of partial melt (>800°C) and viscosity has been determined at 800°C. Our results together with recent experimental constraints on seismic properties of partially molten rocks strongly suggest that the electrical and seismic anomalies measured underneath the Himalayan–Tibetan collisional orogen are best explained by partially molten rocks or local accumulation of pure melt bodies. This is also remarkably corroborated by the temperature–depth conditions of crustal partial melting and melt ponding expected from petrological surveys in the Himalayan range. However, our data suggest much higher melt fraction than previously thought and this implies regions in the middle crust having viscosities several orders of magnitude lower than previously assumed. High degree partial melting in the middle crust of the Himalayan–Tibetan orogenic system suggests revision of conceptual models on the development of mountain belts or that geophysical models addressing electrical resistivity at depth must be re-evaluated.

The applicability of the GRIPS geobarometry in metapelitic assemblages

AbstractAlthough the garnet‐rutile‐ilmenite‐plagioclase‐silica (quartz) (GRIPS) geobarometer has been experimentally calibrated and widely applied, its applicability to metapelitic rocks has not yet been discussed carefully. In this paper, this barometer was recalibrated by fitting the available reversed‐phase equilibrium data incorporating different combinations of activity models of garnet, plagioclase and ilmenite. The resultant GRIPS barometer formalisms reproduce the experimental pressures well within ±0.2 kbar. The GRIPS and garnet‐aluminium silicate‐plagioclase‐quartz (GASP) barometer values are concordant within ±1 kbar for pressures above ∼6 kbar for natural metapelites, but the difference of pressure determinations between these two barometers becomes larger when pressure and/or the grossular content of garnet decrease. However, the pressure difference is independent of either temperature, or almandine in garnet, or anorthite in plagioclase, or iron content in ilmenite. After testing and application of the GRIPS barometer to aluminosilicate‐bearing metapelites and metapelitic assemblages within limited geographical areas as well as within contact thermal aureoles, it is concluded that this barometer may be applied to low‐ to high‐grade, medium‐ to high‐pressure metapelites. The application of the GRIPS barometer to metapelites is not advocated in situations where calcium is deficient in garnet ( &lt; 0.05) or plagioclase ( &lt; 0.17), or for pressures below ∼6 kbar.

Synthesis of Staurolite in Melting Experiments of a Natural Metapelite: Consequences for the Phase Relations in Low-Temperature Pelitic Migmatites

We document experiments on a natural metapelite in the range 650–775°C, 6–14 kbar, 10 wt % of added water, and 700–850°C, 4–10 kbar, no added water. Staurolite systematically formed in the fluid-present melting experiments above 675°C, but formed only sporadically in the fluid-absent melting experiments. The analysis of textures, phase assemblages, and variation of phase composition and Fe–Mg partitioning with P and T suggests that supersolidus staurolite formed at (near-) equilibrium during fluid-present melting reactions. The experimental results are used to work out the phase relations in the system K2O–Na2O–FeO–MgO–Al2O3–SiO2–H2O appropriate for initial melting of metapelites at the upper amphibolite facies. The P–T grid developed predicts the existence of a stable P–T field for supersolidus staurolite that should be encountered by aluminous Fe-rich metapelites during fluid-present melting at relatively low temperature and intermediate pressures (675–700°C, 6–10 kbar for XH2O = 1, in the KNFMASH system), but not during fluid-absent melting. The implications of these findings for the scarcity of staurolite in migmatites are discussed.

Empirical garnet–muscovite geothermometry in metapelites

Two empirical garnet–muscovite geothermometers, assuming no ferric iron (Model A) and 50% ferric iron (Model B) in muscovite, respectively, were calibrated under the physical conditions of P=3.0–14.0 kbar and T=530–700 °C. The input temperatures and pressures were determined by simultaneously applying the garnet–biotite thermometer [Am. Mineral. 85 (2000) 881.] and the GASP geobarometer [Am. Mineral. 86 (2001) 1117.] to natural metapelites. To confirm internal thermodynamic consistency, Holdaway's [Am. Mineral. 85 (2000) 881.] garnet mixing properties were adopted. Muscovite was treated as a symmetric Fe–Mg–Al VI ternary solid solution, and its Margules parameters were derived in this work. The resulting two formulae reproduced the input garnet–biotite temperatures well within ±50 °C, and gave identical results for a great body of natural samples. Moreover, they successfully distinguished the systematic changes of temperatures of different grade rocks from a prograde sequence, inverted metamorphic zone, and thermal contact aureole. Pressure estimation has almost no effect on the two formalisms of the garnet–muscovite geothermometer. Assuming analytical error of ±5% for the relevant components of both garnet and muscovite, the total random uncertainty of the two formulations will generally be within ±5 °C. The two thermometers derived in this work may be used as practical tools to metamorphic pelites under the conditions of 480 to 700 °C, low- to high-pressure, in the composition ranges Xalm=0.51–0.82, Xpyr=0.04–0.22, and Xgros=0.03–0.24 in garnet, and Fe tot=0.03–0.17, and Mg=0.04–0.14 atoms p.f.u. in muscovite.

Isograds and P–T evolution in the eastern Lepontine Alps (Graubünden, Switzerland)

AbstractReactions producing Al‐rich index minerals in the south‐eastern part of the Lepontine Dome (Central Alps, Switzerland) are investigated using mineral distribution maps, microstructural observations and equilibrium phase diagrams. The apparent staurolite mineral zone boundary corresponds to the paragonite breakdown reaction Pg + Grt + Qtz = Pl + Al2O3 + W. Equilibrium phase diagrams show that most natural metapelites do not contain staurolite or alumosilicates as long as univalent cations are predominantly accommodated in white mica. For a wide range of metapelitic compositions the paragonite breakdown releases sufficient Al for the formation of these minerals. Rare occurrences of staurolite and kyanite, north of the formerly mapped mineral zone boundaries, coexist with paragonite and are restricted to extremely Al‐rich bulk compositions. The stable branch of the kyanite‐forming paragonite breakdown reaction above 660 °C yields an additional mapable isograd. The second set of Al‐releasing reactions is biotite‐producing phengite breakdown. However, these reactions are less suitable to produce well defined reaction isograds in the field as they are more continuous and their progress is strongly dependent on bulk composition. Well developed fibrolite in metapelites does not appear until staurolite starts to breakdown. We conclude that amphibolite facies conditions in the study area were attained by decompression, without substantial heating at low pressures.

Effects of pressure and H2O content on the compositions of primary crustal melts

ABSTRACT:Melting experiments with and without added H2O on a model metagreywacke and a natural metapelite demonstrate how pressure and H2O content control the compositions of melts and residual assemblages. Several effects are observed under isothermal conditions. Firstly, the stability field of biotite shrinks with decreasing pressure and with increasing H2O content, whereas that of plagioclase shrinks with increasing pressure and H2O content. Secondly, the ferromagnesian content of melts at the source (i.e. coexisting with their residual assemblages) decreases with decreasing H2O activity. Thirdly, with increasing pressure the Ca/Mg and Ca/Fe ratios of melts decrease relative to those of coexisting garnet. As a consequence, a wide spectrum of melts and crystalline residues can be generated from the same source material. For example, H2O-starved dehydration melting of metagreywacke at low pressure (≤10 kbar) generates K-rich (granitic) melts that coexist with pyroxene- and plagioclase-rich residues, whereas melting of the same material at high pressure (≍15 kbar) and with minor H2O infiltration can generate leucocratic Na-rich and Ca-poor (trondhjemitic) melts that coexist with biotite- and garnet-rich residues. An increased H2O content stabilises orthopyroxene at the expense of garnet + biotite + plagioclase, causing melts to shift towards granodioritic or perhaps tonalitic compositions.

Experimental Constraints on Partial Melting in the Crust

Vielzeuf and Holloway (1988) experimentally studied the melting of a metapelite under fluidabsent conditions. At 10 kbar, three main stages of melting were distinguished : I) The first melting occurs below 800~ and corresponds to the breakdown of muscovite via reactions such as Ms + Bt + PI + Qtz + V = M + Als;Ms + PI + Qtz= Bt + Als + Kfs + M. The proportion of melt was small (10-15 vol.%). 2) Between 850 and 875~ there was a dramatic increase in the proportion of melt, changing from about 10% to 50-60%. This step corresponded to the breakdown of biotite (Bt + Als + PI + Qtz = Grt + Kfs + M) and was followed by a plateau along which garnet and sillimanite progressively dissolved in the liquid. 3) The last stage represented the melting of garnet and spinel at very high temperatures (above 1100~ In this scheme, the reaction Bt + Als + P1 + Qtz ~ Grt + Kfs + melt (1) plays a major role. Vielzeuf and Holloway (1988) considered that, at 10 kbar, this reaction occurred at about 850-875~ over a small temperature interval. These experiments indicate a very steep slope for that reaction in the range 7 to 12 kbar. Experimental investigation on related lithologies at 10 kbar by Lc Breton and Thompson (1988) indicated that melting began between 780 and 800~ and was extensive at 850~ Patifio-Douce and Johnston (1989) performed experiments on a natural metapelite. They observed biotite, sillimanitc and quartz coexisting with melt at temperatures up to 975~ and a very progressive increase of the proportion of melt. This is significantly different from the findings of Viclzcuf and Holloway but can be explained by the low amount of plagioclase (4 wt%) in PatifioDoucc and Johnston's starting material. Thus, temperatures between 850 and 900~ seem sufficient to produce a significant proportion of liquid in pelites. Such silicate liquids, or at least a proportion of them, are potentially able to segregate from the source, leaving behind a residue of quartz, garnet, sillimanite, feldspar, and futile. Such a rcstitic residue has the characteristic mineral assemblage of silicic aluminous granulites.

Experimental determination of the fluid-absent melting relations in the pelitic system

In order to provide additional constraints on models for partial melting of common metasediments, we have studied experimentally the melting of a natural metapelite under fluid-absent conditions. The starting composition contains quartz, plagioclase, biotite, muscovite, garnet, staurolite, and kyanite. Experiments were done in a halfinch piston-cylinder apparatus at 7, 10, and 12 kbar and at temperatures ranging from 750° to 1250° C. The following reactions account for the mineralogical changes observed at 10 kbar between 750° and 1250° C: Bi+Als+Pl+Q=L+Gt+(Kf), Ky=Sill, Gt+Als=Sp+Q, Gt=L+Sp+Q, and Sp+Q=L+Als. The compositions of the phases (at T>875° C) were determined using an energy-dispersive system on a scanning electron microscope. The relative proportions of melt and crystals were calculated by mass balance and by processing images from the SEM. These constraints, together with other available experimental data, are used to propose a series of P-T, T-XH2O, and liquidus diagrams which represent a model for the fluid-present and fluid-absent melting of metapelites in the range 2–20 kbar and 600°–1250° C. We demonstrate that, even under fluid-absent conditions, a large proportion (≈40%) of S-type granitic liquid is produced within a narrow temperature range (850°–875° C), as a result of the reaction Bi+Als+Pl+Q=L+Gt(+/-Kf). Such liquids, or at least some proportion of them, are likely to segregate from the source, leaving behind a residue composed of quartz, garnet, sillimanite, plagioclase, representing a characteristic assemblage of aluminous granulites. The production of a large amount of melt at around 850° C also has the important effect of buffering the temperature of metamorphism. In a restitic, recycled, lower crust undergoing further metamorphism, temperature may reach values close to 1000° C due to the absence of this buffering effect. Partial melting is the main process leading to intracontinental differentiation. We discuss the crustal cross-section exposed in the North Pyrenean Zone in the context of our experiments and modelling.