Phase relationships in Buchan facies series pelitic assemblages: calculations with application to andalusite-staurolite parageneses in the Mount Lofty Ranges,South Australia

Low-pressure, medium- to high-temperature (Buchan-type) regional metamorphism of pelitic rocks in the Mount Lofty Ranges, South Australia, is defined by the development of biotite, staurolite-andalusite, fibrolite, prismatic sillimanite and migmatite zones. K-feldspar makes its first appearance in the prismatic sillimanite zone and here we restrict our discussion to lower grade assemblages containing prograde muscovite, concentrating particularly on well-developed andalusitestaurolite parageneses. In general, the spatial distribution and mineral chemical variation of these assemblages accord with the predictions of petrogenetic grids and P-T and T-X _Fe pseudo-sections constructed from the internally consistent data set of Holland and Powell (1990) in the system KFMASH, assuming a(H₂O) 1, although analysed white mica compositions are systematically more aluminous than predicted. Importantly, the stability ranges of most critical assemblages predicted by these grids and pseudo-sections coincide closely with P-T estimates calculated using the data set of Holland and Powell (1990) from the Mount Lofty Ranges assemblages. With the exception of Mn in garnet and Zn in one staurolite-cordierite-muscovite assemblage non-KFMASH components do not significantly appear to have affected the stability ranges of the observed assemblages. An apparent local reversal in isograd zonation in which andalusite first appears down-grade of staurolite suggests a metamorphic field gradient concave towards the temperature axis and, together with evidence for essentially isobaric heating of individual rocks, is consistent with the exposures representing an oblique profile through a terrain in which heat was dissipated from intrusive bodies at discrete structural levels.Mineral abbreviations used in figures als Al₂SiO₅ phase - bi biotite - chl chlorite - ky kyanite - ph phengite - sill sillimanite - and andalusite - cd cordieritc - gt garnet - mu muscovite - q quartz - st staurolite     

Equilibria within the mineral assemblage quartz + muscovite + biotite + garnet + plagioclase,and implications for the mixing properties of octahedrally-coordinated cations in muscovite and biotite

Interaction parameters derived using empirical calibration methods indicate strong non-ideality in the mixing of octahedrally-coordinated cations in muscovite and biotite. The data set used for calibration comprises mineral compositions from 49 samples containing quartz, muscovite, biotite, garnet, plagioclase and Al₂SiO₅ (kyanite or sillimanite). Pressures and temperatures in the data set were determined through the simultaneous application of geothermometry based on the garnet-biotite FeMg₁ exchange equilibrium and geobarometry based on the anorthite-breakdown equilibrium. Two equilibria yielded simple expressions from which binary interaction parameters for octahedrally-coordinated cations in biotite could be directly determined. A four-component (Fe²⁺, Mg, Al, Ti) regular symmetric mixing model was assumed for biotite. One equilibrium yielded a simple expression from which an interaction parameter for the mixing of the MgAl-celadonite component in muscovite could be directly determined. Two sets of calculations were performed utilizing different calibrations of the garnet-biotite geothermometer and the anothite-breakdown geobarometer and different garnet activity models. Both placed samples within or near the stability field of the Al₂SiO₅ phase present in each sample and both yielded similar values for the interaction parameters within narrow uncertainties, indicating that the values are insensitive to differences in the underlying methods. Using the derived interaction parameters, activity models were formulated for the annite, phlogopite, eastonite, and siderophyllite components of biotite, and for the MgAl-celadonite component of muscovite. These were utilized for the empirical calibration of 45 fluid-independent equilibria involving unique combinations of phase components from the mineral assemblage garnet + plagioclase±biotite±muscovite±quartz. Forty-three of the equilibria may be applied as geobarometers to equilibrium assemblages of quartz + muscovite + biotite + garnet + plagioclase when care is taken to insure that applications are restricted to valid compositional ranges. For these, the calibrations yielded multiple correlation coefficients ranging from 0.953 to 0.998 and standard deviations of the residuals ranging from 597 to 118 bars.     

The Miocene-Pliocene Macusani Volcanics,SE Peru

The Miocene-Pliocene Macusani volcanics, SE Peru, outcrop in three separate tectonic intermontane basins developed on a Paleozoic-Mesozoic volcano-sedimentary sequence. Several ignimbrite sheets are recognized and K-Ar dates record at least semi-continuous volcanic activity from 10 to 4 Ma in the Macusani field. The volcanics in the Macusani basin comprise crystal-rich (45% crystals) ash-flow tuffs and rare obsidians glasses, both with unusual mineralogy, similar to two-mica peraluminous leucogranites. The mineralogical assemblage (quartz, sanidine Or_69–75, plagioclase, biotite, muscovite and andalusite (both coexisting in the entire volcanic field), sillimanite, schörl-rich tourmaline, cordierite-type phases, hercynitic spinel, fluor-apatite, ilmenite, monazite, zircon, niobian-rutile) is essentially constant throughout the entire Macusani field. Two distinct generations of plagioclase are recognized, viz. group I (An_10–20) and group II (An_30–45). Sillimanite forms abundant inclusions in nearly all phases and is earlier than andalusite which occurs as isolated phenocrysts. Biotite (Al-, Ti-, Fe- and F-rich) shows pronounced deficiencies in octahedral cations. Muscovite is also F-rich and displays limited biotitic and celadonitic substitutions. There is no systematic variation in mineral chemistry with stratigraphic position. The mineralogical data provide a basis for distinction between an early magmatic and a main magmatic stage. The early stage corresponds to the magmatic evolution at or near the source region and includes both restites and early phenocrysts. Some biotites (with textures of disequilibrium melting to Fe — Zn spinel), part of the sillimanite, apatite and monazite, possibly some tourmaline and cordierite-type phases are restites. However, the restite content of the magma was low (5 vol. % maximum). The group II plagioclase are interpreted as early phenocrysts. During this stage, temperatures were as high as 800° C, pressure was no more than 5–7.5 kbar, was intermediate between WM and QFM and was low. The biotite melting textures and the coexistence of restites and early phenocrysts imply fast heating rates in the source region. The transition between the early and the main magmatic stage was abrupt (andalusite crystallization in place of sillimanite, group I vs. group II plagioclases) and suggests rapid ascent of the magma from its source region. During the main crystallization stage, temperature was 650° C or lower at a pressure of 1.5–2 kbar. (calculated from equilibrium between muscovite, quartz, sanidine and andalusite) are around 1, suggesting conditions close to H₂O-saturation. f _HF is around 1 bar but the ratios are significantly different between samples. ranges between 138 and 225 bar. This study shows that felsic, strongly peraluminous, leucogranitic magmas having andalusite and muscovite phenocrysts may be generated under H₂O-undersaturated conditions.CRPG Contribution n 769     

Pyroclastic flows and lavas of the Mogan and Fataga formations,Tejeda Volcano,Gran Canaria,Canary Islands: mineral chemistry,intensive parameters,and magma chamber evolution

The Mogan and Fataga formations on the island of Gran Canaria, Canary Islands, represent a sequence of approximately 30 intercalated pyroclastic and lava flows (total volume about 500 km³ dense-rock equivalent) including subalkaline rhyolitic, peralkaline rhyolitic and trachytic pyroclastic flows, nepheline trachyte lavas and a small volume of alkali basaltic lavas and tephra deposits. The eruption of the intermediate to silicic rocks of the Mogan and Fataga formations follows the roughly 4 Ma duration of basaltic shield volcanism. The most common assemblage in the evolved (Mogan and Fataga) rocks is anorthoclase+ edenitic amphibole+ilmenite+magnetite±augite±hypersthene +apatite+pyrrhotite. A few flows also contain plagioclase, biotite, or sphene. Coexisting Fe-Ti oxides yield equilibrium temperatures between 835 and 930° C and log between –11.2 and –12.6. The lowermost pyroclastic flow of the Mogan formation is zoned from a rhyolitic base (848° C) to a basaltic top (931° C). Unit P1 has an oxygen isotope feldspar-magnetite temperature (850° C) very close to its Fe-Ti oxide temperature. One of the youngest Mogan flows is zoned from a comendite (836° C) at the base to a comenditic trachyte (899° C) at the top. The Fataga formation pyroclastic flows show only slight compositional zonation, and one flow has the same Fe-Ti oxide compositions at top and base.Calculations using the reaction 1/3 magnetite+SiO₂ (melt)=ferrosilite+1/6 O₂ indicate total pressures of 1–4 (±3) kb for six of the Mogan flows and one of the Fataga flows. For four of the pyroclastic flows, equilibria involving tremolite-SiO₂-diopside-enstatite-H₂O and phlogopite-SiO₂-sanidine-enstatite-H₂O imply water contents of 0.9 to 2.6 (±0.5) wt% and between 80 and 610 bars, which indicates that magma within the Tejeda reservoir was H₂O-undersaturated throughout the entire history of Mogan to Fataga volcanism. The fluorine contents of amphibole, biotite, and apatite, and chlorine contents of apatite reveal thatf _HF/ andf _HCl/ high compared to most igneous rocks and are consistent with the peralkaline nature of most of the volcanics. Thef _HCl estimate for one flow is 10^–2 to 10^–1 bars andf _HF for six of the flows ranges from about 10^–1 to 6 bars. Pyrrhotite compositions yield estimates for log between –1 and –3, log between –2 and 1.5, and log between 0.5 and 3, which fall in the range of most intermediate to silicic systems. The lack of a systematic trend with time for magma composition, Fe-Ti oxide temperatures, water contents, phenocryst abundances, and ferromagnesian phase composition indicate that the Tejeda magmatic system was open and kept at nearly the same conditions by the periodic addition of more primitive melts.The intensive thermodynamic parameters estimated from coexisting phenocryst equilibria are used to constrain the eruption dynamics based on solution of the conservation equations for a vapor plus pyroclast mixture. The estimates of magma reservoir temperature, pressure, and water concentration, when combined with a one-dimensional fluid dynamical model of a pyroclastic eruption, imply that the velocities of the ash flows at the vent exit were on the order of 100 to 200 m s^–1, and the mass flow rates were about 10⁷ kg s^–1 for an assumed vent radius of 10 m.     

The Phase Relations of Aluminous Iron Biotites in the System KAISi3O8-KAISiO4-AI2O3-Fe-O-H

The experimental work on biotites has primarily involved compositionsalong the annite-phlogopite join, but most natural biotitescontain significantly larger amounts of aluminum. At the sametime, the aluminum content of natural biotites varies considerably.The available evidence indicates that these variations in thealuminum content of biotite depend on the conditions of formationand the whole rock chemistry. Experiments on the phase relations of aluminous iron biotitesin the silica deficient system KAlSiO₄-KAlSi₃O₈-Al₂O₃-Fe-O-H(pfluid = 2 kb) indicate that compositions up to Ann₇₅ can besynthesized on the join annite [K₂Fe₆Al₂Si₆O₂₀(OH)₄]-aluminumbiotite [K₂Al₆Al₂Al₆O₂₀(OH)₄]. The aluminous biotites are stableto higher temperatures than annite. An isobaric divariant equilibrium,Bio_ss-Mt_ss-Sa-Lc-V, extends to higher oxygen fugacities fromthe Ann-Mt-Sa-Lc-V curve of Eugster & Wones (1962). Compositioncontours on this surface indicate that both the magnetite andbiotite become more aluminous with increasing temperature and/oroxygen fugacity. The Bio_ss-Mt_ss-Sa-Lc-V reaction surface isterminated by equilibria involving the additional phases muscovite,corundum, and hercynite respectively as the conditions becomemore reducing. At 2 kb fluid pressure; aluminum-rich iron biotiteis stable to 555 °C on the HM buffer, 763 °C on theMt-Hc-Cor buffer, 820 °C on NNO, and about 860 °C onQFM. The data obtained can be applied to a number of biotitesyenites and appears to explain why iron-rich aluminum biotitesoccur in these rocks.     

Manganese pyroxenoids and carbonates: Critical phase relations in metamorphic assemblages from the Alps

The compositions of coexisting pyroxmangites, rhodonites, rhodochrosites and manganese calcites in regional metamorphosed manganese cale-silicate marbles from Val Scerscen and Alagna were analysed by microprobe and permit definition of critical tie lines at metamorphic grades appropriate to temperatures between 400 and 450 °C.Variations in composition of coexisting mineral pairs in one and the same locality are attributed to variations in and not to metamorphic temperatures. From the analysed assemblages isothermal plots (with SiO₂ as excess component) were constructed for the system CaO-MnO-SiO₂-CO₂.     

Activity-composition relationship in Tschermak's substituted Fe biotites at 700°C, 2 kbar

The reaction-displacement technique was applied to the end-member reaction annite = sanidine + magnetite + H₂ in order to determine the activity of the annite component (a _Ann) in iron biotites with variable degrees of the Tschermak's substitution (^[6]Fe + ^[4]Si = ^[6]Al + ^[4]Al). Based on the simplified relation a _Ann = f _{H 2}/foH₂ (foH₂ = hydrogen fugacity of the end-member reaction at P, T), two types of experiments were performed at 700°C / 2 kbar: Type I used Fe-Al biotites of known starting composition together with sanidine + magnetite + H₂O. This assemblage was exposed to various f _{H 2} conditions (f _{H 2} < foH₂) produced in the pressure vessel either by using different ratios of water/oil as pressure medium (f _{H 2} in this case was measured by the hydrogen sensor technique), or by the Ni′NiO buffer. The composition of the Fe-Al biotites changed through incorporation or release of the annite component in response to the externally imposed f _{H 2}. By using opposite biotite starting compositions, the equilibrium composition as a function of f _H2 was bracketed. For type II, f _{H 2} in equilibrium with a specific combination of fine-grained Fe-Al biotite (+ sanidine + magnetite + H₂O) was measured internally by application of the hydrogen sensor technique. Both type I and type II experiments yield consistent results demonstrating that a fine-grained assemblage of Fe-Al biotite (+ sanidine + magnetite + H₂O) is able to act as a sliding-scale buffer. The final chemical composition of the Fe-Al biotite after the experiments was determined by electron microprobe and Mössbauer spectroscopy. The ^[4]Al and ^[6]Al in the biotites are coupled according to the Tschermak's substitution. In the tetrahedral sheet 0.1 Al-atoms per formula unit are present in excess to the amount required to balance ^[6]Al, and all Fe-Al biotites contain 8–10% Fe³⁺. Therefore, they are not members of the pure annite - siderophyllite join, but have an almost constant amount (15 Mol%) of two additional Fe³⁺-bearing components (ferri-siderophyllite and a vacancy end-member). The volume - composition relationship obtained does not indicate excess molar volumes of mixing for the annite (Ann) - siderophyllite (Sid) binary. The data are consistent with a molar volume of annite of 15.46 ± 0.02 Jbar^–1 and of 15.06 ± 0.02 Jbar^–1 for siderophyllite. The experimentally determined activity - composition relation shows that biotites on the join annite - siderophyllite deviate negatively from ideality. A symmetric interaction parameter W_AnnSid is sufficient to represent the data within error. This was constrained as: W _AnnSid = –29 ± 4 kJmol^–1. This is in contradiction to empirical interaction parameters derived from natural assemblages for this binary that predict positive deviation from ideality. Reasons for this discrepancy are discussed.     

Annite stability revised. 1. Hydrogen-sensor data for the reaction annite = sanidine + magnetite + H2

In P - T - logfO₂ space, the stability of annite (ideally KFe ₃ ²⁺ (OH)₂AlSi₃O₁₀) at high fO₂ (low fH₂) is limited by the reaction: annite = sanidine + magnetite + H₂. Using the hydrogen-sensor technique, the equilibrium fH₂ of this reaction was measured between 500 and 800° C at 2.8 kbar in 50° C intervals. Microbrobe analyses of the reacted annite+sanidine+magnetite mixtures show that tetrahedral positions of annite have a lower Si/Al ratio than the ideal value of 3/1. Silicon decreases from 2.9 per formula unit at low temperatures to 2.76 at high temperatures. As determined by Mössbauer spectroscopy in three experimental runs, the Fe³⁺ content of annite in the equilibrium assemblage is 11%±3. A least squares fit to the hydrogensensor data gives H _R ⁰ = 50.269 ± 3.987 kJ and S _R ⁰ = 83.01 ± 4.35 J/K for equilibrium (1). The hydrogene-sensor data are consistent with temperature half brackets determined in the classical way along the nickel-nickel oxide (NNO) and quartz-fayalite-magnetite (QFM) buffers with a mixture of annite+sanidine+magnetite for control. Compared to published oxygen buffer reversals, agreement is only found at high temperature and possible reasons for that discrepancy are discussed. The resulting slope of equilibrium (1) in logfO₂ – T dimensions is considerably steeper than previously determined and between 400 and 800°C only intersects with the QFM buffer curve. Based on the hydrogen-sensor data and on the thermodynamic dataset of Berman (1988, and TWEEQ data base) for sanidine, magnetite and H₂, the deduced standard-state properties of annite are: H _f ⁰ =-5127.376±5.279 kJ and S ⁰=422.84±5.29 J/(mol K). From the recently published unit cell refinements of annites and their Fe³⁺ contents, determined by Mössbauer spectroscopy (Redhammer et al. 1993), the molar volume of pure annite was constrained as 15.568±0.030 J/bar. A revised stability field for annite is presented, calculated between 400 and 800°C.     

The selective replacement of the aluminum silicates by white mica

From considerations of relativeG-T surfaces inferred from publishedP-T data and the occurrence of replacement textures of Al₂SiO₅ polymorphs in rocks, the relative positions of curves representing the following equation in _K+ —T — pH ispace on substituting Al₂SiO₅ different polymorphs are derived.3 Al₂SiO₅ + 3 SiO₂ (quartz) + 2 K⁺ + 3 H₂O 2 KAl₂[AlSi₃O₁₀](OH)₂ (muscovite)+ 2 H⁺. The curves are different because of the differentG-T values for the polymorphs which, in the field, is borne out by the observation that in a rock containing two or three Al₂SiO₅ polymorphs, in nearly all instances only one polymorph is replaced by white mica. Instances of textural relations showing the interpreted selective replacement of one Al₂SiO₅ polymorph by a white mica in the presence of one (or two) other Al₂SiO₅ polymorph(s) are cited both from the literature and various field examples. The selective replacement of kyanite if sillimanite and/or andalusite is/are present, and of andalusite if only sillimanite is present are interpreted to show that generally during the muscovitization reaction, the field of sillimanite in the above reaction (left hand side) at a particular pH (H⁺ concentration) and is larger in _K+ —T space than that of andalusite which in turn is larger than that of kyanite. Theoretically it is shown that variations to this can exist but the field evidence suggests these only occur under rare geological conditions. Although this is not totally conclusive, the selectiveness of the replacement is interpreted to show that the fluid was buffered with respect to K⁺ and H⁺ on or near the curve of the polymorph showing the lowest stability field until that polymorph is totally consumed, after which the fluid composition moves to the next lowest curve for the remaining polymorph(s) present in the rock. The alteration of more than one polymorph by an apparently simultaneous process of alteration is rare and usually occurs at a low grade of metamorphism. This is interpreted to show that the buffering reaction could not keep pace with the influx of fluid and change the composition of this fluid (in most cases).     

The stability and phase relations of iron chlorite below 8.5 kb P_{{\text{H}}_{\text{2}} {\text{O}}}

Iron chlorites with compositions intermediate between the two end-members daphnite (Fe₅Al₂Si₃O₁₀(OH)₈) and pseudothuringite (Fe₄Al₄Si₂O₁₀(OH)₈) were synthesized from mixtures of reagent chemicals. The polymorph with a 7 Å basal spacing initially crystallized from these mixtures at 300 °C and 2 kb after two weeks. Conversion to a 14 Å chlorite required a further 6 weeks at 550 °C. Shorter conversion times were required at higher water pressures. The products contained up to 20% impurities.The maximum equilibrium decomposition temperature for iron chlorite, approximately 550 °C at 2kb, is at an between assemblages (1) and (2) listed below. Synthetic iron chlorite will break down by various reactions with variable P, T, and fugacity of oxygen. For the composition FeAlSi = 523, the sequence of high temperature breakdown products with increasing traversing the magnetite field for P _total = =2kb is: (1) corierite+ fayalite+hercynite; (2) cordierite+fay alite+magnetite; (3) cordierite+magnetite+quartz; (4) magnetite+mullite+quartz. Almandine should replace cordierite in assemblages (1) and (2) but it did not nucleate. The significance of the relationship between iron cordierite and almandine in this system is discussed.At water pressures from 4 to 8.5 kb and at the nickel-bunsite buffer, iron chlorite+quartz break down to iron gedrite+magnetite with temperature 550 to 640 °C along the curve. At temperatures 50 °C greater and along a parallel curve, almandine replaces iron gedrite. For on this buffer curve, almandine is unstable below approximately 4 kb for temperatures to approximately 750 °C.     

The volatile component of quaternary ignimbrite magmas from the North Island,New Zealand

Ignimbrites from the central North Island consist mainly of glass or its devitrified product (70–95%); their phenocryst mineralogy is varied and includes plag., hyp., ti-mag., ilm., aug., hblende, biot., san., qtz, ol., with accessory apatite, zircon and pyrrhotite. The Fe-Mg minerals can be used to divide the ignimbrites into four groups with hyp.+aug. reflecting high quench temperatures and biot.+hblende +hyp.+aug., low quench temperatures. Oxygen fugacities lie above the QMF buffer curve and even in ignimbrites with low crystal contents the solid phases apparently buffered fO₂. Some ignimbrites contain the assemblage actinolite, gedrite, magnetite and hematite, reflecting post-eruption oxidation. The mineralogy also allows estimation of using pyrrhotite and thence , . The assemblage biotite-sanidine can be used to estimate and thence . Water fugacity is calculated in a variety of ways using both biotite and hornblende as well as the combining reaction . It is high and approaches P _total in most ignimbrites (~4kb) but is lower in unwelded pumice breccias. Comparison of temperature estimates using mineral geothermometers for the various phenocryst phases suggests that the ignimbrite magmas showed temperature differences of 60–100 °C and pressure differences of several kilobars. Individual magma chambers therefore, would have extended over several kilometres vertically. The chemical potential of water may have been constant through the magma.     

Empirical calibration of six geobarometers for the mineral assemblage quartz+muscovite+biotite+plagioclase+garnet

Six equilibria among quartz, plagioclase, biotite, muscovite, and garnet were empirically calibrated using mineral composition data from 43 samples having the assemblage quartz+muscovite+biotite+garnet+plagioclase+Al₂SiO₅ (sillimanite or kyanite). Pressures and temperatures in the data set used for calibration were determined through the simultaneous application of garnet-biotite geothermometry and garnet-quartz-plagioclase-Al₂SiO₅ geobarometry. Thermodynamic expressions for four of the six equilibria incorporate interaction parameters that model non-ideality in the mixing of cations in the octahedral sites of both muscovite and biotite. With pressure chosen as the dependent variable, multiple regression was used to solve for unknowns in the equilibrium thermodynamic expressions. The regressions yielded multiple correlation coefficients ranging from 0.983 to 0.999, with corresponding standard deviations of 338 and 92 bars in the residuals. The standard deviations in the residuals may be explained largely or entirely by the propagation of errors associated with electron microprobe analysis. These equilibria enable the determination of pressures from equilibrium assemblages of quartz+garnet+plagioclase+muscovite+biotite, and give results closely comparable to the experimentally calibrated garnet-quartz-plagioclase-Al₂SiO₅ geobarometer. Geobarometric applications should be restricted to rocks in which equilibrium constants and compositional variables fall within the same ranges as those used for calibration.     

Partition of Ni between olivine and sulfide: equilibria with sulfide-oxide liquids

The partition of Ni between olivine and monosulfide-oxide liquid has been investigated at 1300–1395° C, =10^–8-9–10^–6.8, and =10^–2.0–10^–0.9, over the composition range 20–79 mol. % NiS. The product olivine compositions varied from Fo98 to Fo59 and from 0.06 to 3.11 wt% NiO. The metal/sulfur ratio of the sulfide-oxide liquid increases with increase in , decrease in , and increase in NiS content. The Ni/Fe exchange reaction has been perfectly reversed using natural olivine and pure forsterite as starting materials. The FeO and NiO contents of olivine from runs equilibrated at the same and form isobaric distributions with NiS content, which, to a first approximation, are dependent at constant temperature and total pressure on a variable term, –0.5 log ( / ). The Ni/Fe distribution coefficient (K _D3) exhibits only a weak decrease from 35 to 29 with increase in from the IW buffer to close to the FMQ buffer. At values higher than FMQ, the sulfide-oxide liquid has the approximate composition (Ni,Fe)₃±_xS₂K _D358. The present K _D3 vs O/(S+O) data define a trend which extrapolates to K _D320 at 10 wt% oxygen in the sulfide-oxide liquid. The compositions of olivine and Ni-Cu sulfides associated with early-magmatic basic rocks and komatiites are consistent, at 1400° C, with a value of -log ( / ) of about 7.7, which is equivalent to 0.0 wt% oxygen in the hypothesized immiscible sulfide-oxide liquid. Therefore, K _D3 would not be reduced significantly from the 30 to 35 range for sulfide-oxide liquids with low oxygen contents.     

Calculated phase equilibria for low- and medium-pressure metapelites in the KFMASH and KMnFMASH systems

Petrogenetic grids in the KFMASH and KMnFMASH model systems calculated with the software thermocalc 3.1 are presented for the P–T range 0.5–12 kbar and 450–900 °C, for assemblages involving garnet, muscovite, chloritoid, biotite, chlorite, staurolite, cordierite, spinel, orthopyroxene, K‐feldspar, Al₂SiO₅ phases, quartz, water and melt. Based on calculated compatibility diagrams and P–T and T–M_Mn [Mn/(Mg + Fe + Mn)] pseudosections for different metapelitic bulk compositions, the principal conclusions are that the addition of Mn to the KFMASH system: (i) enhances the stability of garnet, and, to a lesser extent, aluminosilicates; (ii) reduces the stability of staurolite, cordierite and, to a lesser extent, chlorite; and (iii) extends the medium pressure stability of muscovite and the low‐P stability field of K‐feldspar. The influence of Mn on individual mineral stabilities is strongly related to rock composition, in particular, to the relative contents of Al₂O₃ and K₂O. For metapelites of a range of compositions and M_Mn values, P–T pseudosections in the KFMASH system, in most cases, do not adequately predict the mineral assemblages observed in natural assemblages under medium and low‐pressure conditions. In contrast, the P–T pseudosections in the KMnFMASH system generally provide more satisfactory results, suggesting that MnO is one of the non‐KFMASH components that should not be neglected in documenting the phase equilibria of medium‐ and low‐P metapelites.     

A calculated petrogenetic grid for ultramafic rocks in the system CaO-FeO-MgO-Al2O3-SiO2-CO2-H2O at low pressures

Calculated phase equilibria among the minerals amphibole, chlorite, clinopyroxene, orthopyroxene, olivine, dolomite, magnesite, serpentine, brucite, calcite, quartz and fluid are presented for the system CaO–FeO–MgO–Al₂O₃–SiO₂–CO₂–H₂O (CaF-MASCH), with chlorite and H₂O–CO₂ fluid in excess and for a temperature range of 440°C–600°C and low pressures. The minerals chosen in CaFMASCH represent the great majority of phases encountered in metamorphosed ultramafic rocks. The changes in mineral compositions in terms of FeMg_-1 and (Mg, Fe)SiAl_-1Al_-1 are related to variations in the intensive parameters. For example, equilibria at high in the presence of chlorite involve minerals which are relatively aluminous compared with those at low . The calculated invariant, univariant and divariant equilibria are compared with naturally-occurring greenschist and amphibolite facies ultramafic mineral assemblages. The correspondence of sequences of mineral assemblages and the compositions of the minerals in the assemblages is very good.     

Geochemistry of S-type granitic rocks from the reversely zoned Castelo Branco pluton (central Portugal)

The zoned pluton from Castelo Branco consists of Variscan peraluminous S-type granitic rocks. A muscovite>biotite granite in the pluton's core is surrounded successively by biotite>muscovite granodiorite, porphyritic biotite>muscovite granodiorite grading to biotite=muscovite granite, and finally by muscovite>biotite granite. ID-TIMS U–Pb ages for zircon and monazite indicate that all phases of the pluton formed at 310 ± 1 Ma. Whole-rock analyses show slight variation in ⁸⁷Sr/⁸⁶Sr_310 Ma between 0.708 and 0.712, Nd_310 Ma values between − 1 and − 4 and δ¹⁸O values between 12.2 and 13.6. These geological, mineralogical, geochemical and isotopic data indicate a crustal origin of the suite, probably from partial melting of heterogeneous Early Paleozoic pelitic country rock. In detail there is evidence for derivation from different sources, but also fractional crystallization linking some of internal plutonic phases. Least-squares analysis of major elements and modelling of trace elements indicate that the porphyritic granodiorite and biotite=muscovite granite were derived from the granodiorite magma by fractional crystallization of plagioclase, quartz, biotite and ilmenite. By contrast variation diagrams of major and trace elements in biotite and muscovite, the behaviours of Ba in microcline and whole-rock δ¹⁸O, the REE patterns of rocks and isotopic data indicate that both muscovite-dominant granites were probably originated by two distinct pulses of granite magma.     

Revised empirical garnet–biotite–muscovite–plagioclase geobarometer in metapelites

The garnet–biotite–muscovite–plagioclase (GBMP) barometer was empirically revised for P–T conditions of 1–14 kbar and 450–840 °C, using 263 metapelitic rock samples from all over the world. This barometer is based on activity models for garnet, biotite and plagioclase identical to those of the well‐calibrated garnet–biotite thermometer and the garnet–aluminosilicate–plagioclase–quartz (GASP) barometer. The GBMP barometer is less temperature dependent than the GASP barometer and can be applied to either Al₂SiO₅‐absent or Al₂SiO₅‐bearing metapelites. The total error of the GBMP barometer is estimated to be about ±1.2 kbar on considering input temperature error and analytical errors of chemical compositions of the phases involved. The random error of the GBMP barometer is evenly distributed with respect to pressure, temperature and mineral composition. Simultaneous application of the GBMP barometer and the garnet–biotite thermometer identifies the correct stability field for Al₂SiO₅‐bearing metapelites. Application of the GBMP barometer to metapelitic rocks within the same geological terranes or thermal contact aureoles yielded similar pressures within error. A spreadsheet for implementing the proposed GBMP geobarometer is supplied on the journal's website.     

The stability of staurolite and chloritoid and their significance in metamorphism of pelitic rocks

The reaction chlorite+muscovite=staurolite+biotite+quartz+vapor has been experimentally determined and reversible equilibrium has been demonstrated. At an oxygen fugacity corresponding to that of the FMQ buffer and using a starting mixture with a Mg/Mg+Fe ratio of 0.4, the equilibrium conditions of the reaction are 565±15°C at 7 kb and 540±15°C at 4 kb. The preliminary maximum stability of staurolite in the presence of quartz, muscovite, and biotite has been established at the following conditions: 675±15°C at 5.5 kb and 575±15°C at 2 kb. The results of both investigations are in good agreement with other experimental data and with petrographical observations. Furthermore, equilibria between minerals in medium-grade pelitic rocks are deduced from theoretical considerations and the effect of T, P _solid, , on some dehydration reactions is discussed.     

The stability of annite+quartz: reversed experimental data for the reaction 2 annite+3 quartz=2 sanidine+3 fayalite +2 H2O

Reversals for the reaction 2 annite+3 quartz=2 sanidine+3 fayalite+2 H₂O have been experimentally determined in cold-seal pressure vessels at pressures of 2, 3, 4 and 5?kbar, limiting annite +quartz stability towards higher temperatures. The equilibrium passes through the temperature intervals 500–540°?C (2?kbar), 550–570°?C (3?kbar), 570–590°?C (4?kbar) and 590–610°?C (5?kbar). Starting materials for most experiments were mixtures of synthetic annite +fayalite+sanidine+quartz and in some runs annite+quartz alone. Microprobe analyses of the reacted mixtures showed that the annites deviate slightly from their ideal Si/Al ratio (Si per formula unit ranges between 2.85 and 2.92, Al^VI between 0.06 and 0.15). As determined by Mössbauer spectroscopy, the Fe³⁺ content of annite in the assemblage annite+fayalite +sanidine+quartz is around 5–7%. The experimental data were used to extract the thermodynamic standard state enthalpy and entropy of annite as follows: H ⁰ _f,?Ann =?5125.896±8.319 [kJ/mol] and S ⁰ _Ann=432.62±8.89 [J/mol/K] (consistent with the Holland and Powell 1990 data set), and H ⁰ _f,Ann =?5130.971±7.939 [kJ/mol] and S ⁰ _Ann=424.02±8.39 [J/mol/K] (consistent with the TWEEQ data base, Berman 1991). The preceeding values are close to the standard state properties derived from hydrogen sensor data of the redox reaction annite=sanidine+magnetite+H ₂ (Dachs 1994). The experimental half-reversal of Eugster and Wones (1962) on the annite +quartz breakdown reaction could not be reproduced experimentally (formation of annite from sanidine+fayalite+quartz at 540°?C/1.035?kbar/magnetite-iron buffer) and probable reasons for this discrepancy remain unclear. The extracted thermodynamic standard state properties of annite were used to calculate annite and annite+quartz stabilities for pressures between 2 and 5?kbar.     

Synthesis and stability of bastnaesites in a part of the system (Ce,La)-F-H-C-O

Summary Bastnaesites of Ce and La and their OH-analogs were synthesized and their stability relations were determined atPf = 1 kbar andT = 400 to 900°C in a part of the system (Ce,La)-F-H-C-0. The initial fluid compositions were such that and HF/(HF + H₂O) ratios were 0 to 0.172. XRD and IR studies indicate that bastnaesites equilibrated in initial fluids low in HF are all F-enriched. The hydroxylbastnaesite-(La) is stable up to 810°C and the fluorbastnaesite-(La) is stable up to 860°C. Their condensed breakdown products are La₂O₂CO₃ and LaOF, respectively. The stability of Ce bastnaesites is slightly dependent. The hydroxylbastnaesite-(Ce) is stable up to 660°C at the defined by the IQF buffer and up to 640°C by the MH buffer. The fluorbastnaesite-(Ce) is stable up to 800°C at the defined by the IQF and up to 760°C by the MH buffer. The condensed breakdown product for the hydroxyl end-member is simply CeO₂ but for the fluorine one is a combination of CeO₂, CeF₃, and CeOF. Factors, such as OH vs F, , and bulk composition, that affect the stability of individual species are discussed. Petrogenic implications resulting from the present study include that bastnaesites can be stable from hydrothermal to magmatic conditions, that F-enriched species can form in an environment relatively low in F content, and that OH-species are rare and occur only in low-temperature environments essentially devoid of F.

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Synthese und Stabilität von Bastndsil in einem Teil des Systems (Ce,La)-F-H-C-O

Zusammenfassung Ce- und La-Bastnäsite, sowie deren OH-Analoga wurden synthetisiert und ihre Stabilitätsbeziehunger beiP _f = 1 kbar undT = 400 bis 900°C wurden im System (Ce,La)F-H-C-O bestimmt. Die anfänglichen Flüssigkeitszusammensetzungen waren so, daß und die HF/(HF + H₂O)-Verhältnisse 0–0.172 waren. Röntgenpulver- und Ultrarot-Untersuchungen zeigten, daß Bastnäsite, die mit anfänglich HF-armen Flüssigkeiten equilibriert wurden, alle an F angereichert sind. Hydroxilbastndsit-(La) ist bis 810°C und Fluorbastnäsit-(La) bis 860°C stabil. Ihre festen Zersetzungsprodukte sind La₂O₂O₃, bzw. LaOF. Die Stabilität der Ce-Bastnäsite hängt etwas von ab. Hydroxilbastnäsit-(Ce) ist bei des Eisen-Quarz-Fayalit-Puffers bis 660°C stabil und mit Magnetit-Hämatit-Puffer bis 640°C. Das feste Zerfallsprodukt ist für das Hydroxil-Glied nur CeO₂, für das Fluor-Glied eine Mischung aus CeO₂, CeF₃ und CeOF. Faktoren, welche die Stabilität der einzelnen Spezies beeinflussen, werden diskutiert, wie das Verhältnis OH zu F, und die Gesamtzusammensetzung. Petrogenetische Folgerungen aus der vorliegenden Studie schließen ein, daß Bastnäsite von hydrothermalen bis zu magmatischen Bedingungen stabil sein können, daß sich an F angereicherte Glieder in relativ F-armer Umgebung bilden können, und daß OH-Glieder selten sind und nur unter Bildungsbedingungen niedriger Temperatur und weitgehender Abwesenheit von F auftreten.

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With 8 Figures