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Sodic, calcic and subcalcic amphiboles are expressed by the combination of tremolite, edenite, tschermakite, glaucophane, magesioriebeckite and Fe-tremolite. Because such complex compositions of amphiboles provide enough chemical variables to constrain equilibrium conditions of the metabasic system, amphibole composition makes it possible for Gibbs' method to be applied to this system. We made a framework for estimating quantitative P–T paths from compositions of a zoned amphibole in metabasite consisting of amphibole, chlorite, epidote, plagioclase, quartz and water in the system Na2O–CaO–MgO–FeO–Fe2O3–Al2O3–SiO2–H2O. For deriving total derivatives simply, mole fractions of cations in each site were introduced as the variables of amphibole composition. In obtaining non-ideality terms of amphibole activity, we used the regular solution models with 15 Margules parameters derived from published data. Application of the differential thermodynamics to a natural basic schist from the Sanbagawa metamorphic belt in central Shikoku provides both prograde and retrograde paths. The ranges of calculated P–T conditions are in good agreement with the previous study, and calculated compositions of coexisting minerals are consistent with the values from analyses of minerals in the sample. Activity models for amphiboles, especially non-ideality of tremolite–tschermakite (tr–ts) and tremolite–edenite (tr–ed) joins, provide significant effects on the calculated P–T paths. The initial P–T conditions given at the amphibole rim affect the calculated P–T paths slightly, but the shapes and lengths of the paths almost do not change. If compositional changes in each step of the amphibole zoning are small enough, Gibbs' method is able to transform the change of amphibole chemistry into changes of P–T conditions that are independent of the histories of the compositional changes of amphibole. This implies that all amphibole chemistry can be translated into the specific P–T condition, considering the relative compositional change from the reference amphibole, as long as the same mineral assemblage is preserved.

Sodic amphiboles in high pressure and ultra- high pressure (UHP) metamorphic rocks are complex solid solutions in the system Na2O-MgO-Al2O3-SiO2- H2O (NMASH) whose compositions vary with pressure and temperature. We conducted piston-cylinder experi- ments at 20-30 kbar and 700-800 ∞C to investigate the stability and compositional variations of sodic amphi- boles, based on the reaction glaucophanea 2jade- ite+talc, by using the starting assemblage of natural glaucophane, talc and quartz, with synthetic jadeite. A close approach to equilibrium was achieved by performing compositional reversals, by evaluating compositional changes with time, and by suppressing the formation of Na-phyllosilicates. STEM observations show that the abundance of wide-chain structures in the synthetic amphiboles is low. An important feature of sodic amphibole in the NMASH system is that the assemblage jadeite-talc a quartz does not fix its com- position at glaucophane. This is because other amphi- bole species such as cummingtonite (Cm), nyboite (Nyb), Al-Na-cummingtonite (Al-Na-Cm) and sodium anthophyllite (Na-Anth) are also buAered via the model reactions: 3cummingtonite + 4quartz + 4H2Oa 7talc, nyboite + 3quartza 3jadeite + talc, 3Al-Na-cumming- tonite + 11quartz + 2H2Oa 6jadeite + 5talc, and 3 sodium anthophyllite + 13quartz + 4H2Oa 3 jade- itea 7talc. We observed that at all pressures and tem- peratures investigated, the compositions of newly grown amphiboles deviate significantly from stoichiometric glaucophane due to varying substitutions of Al IV for Si, Mg on the M(4) site, and Na on the A-site. The deviation can be described chiefly by two compositional vectors: (Na A Al IV ) (h A Si) (edenite) toward nyboite, and (Na (M4) Al VI ) (Mg (M4) Mg VI ) toward cummington- ite. The extent of nyboite and cummingtonite substitu- tion increases with temperature and decreases with pressure in the experiments. Similar compositional vari- ations occur in sodic amphiboles from UHP rocks. The experimentally calibrated compositional changes therefore may prove useful for thermobarometric applications.

The Finero peridotite massif is a harzburgite that suffered a dramatic metasomatic enrichment resulting in the pervasive presence of amphibole and phlogopite and in the sporadic occurrence of apatite and carbonate (dolomite)-bearing domains. Pyroxenite (websterite) dykes also contain phlogopite and amphibole, but are rare. Peridotite bulk-rock composition retained highly depleted major element characteristics, but was enriched in K, Rb, Ba, Sr, LREE (light rare earth elements) (LaN/YbN = 8–17) and depleted in Nb. It has high radiogenic Sr (87Sr/86Sr(270) = 0.7055–0.7093), low radiogenic Nd (ɛNd(270) = −1 to −3) and EMII-like Pb isotopes. Two pyroxenite – peridotite sections examined in detail show the virtual absence of major and trace element gradients in the mineral phases. In both rock types, pyroxenes and olivines have the most unfertile major element composition observed in Ivrea peridotites, spinels are the richest in Cr, and amphibole is pargasite. Clinopyroxenes exhibit LREE-enriched patterns (LaN/YbN ∼16), negative Ti and Zr and generally positive Sr anomaly. Amphibole has similar characteristics, except a weak negative Sr anomaly, but incompatible element concentration ∼1.9 (Sr) to ∼7.9 (Ti) times higher than that of coexisting clinopyroxene. Marked geochemical gradients occur toward apatite and carbonate-bearing domains which are randomly distributed in both the sections examined. In these regions, pyroxenes and amphibole (edenite) are lower in mg## and higher in Na2O, and spinels and phlogopite are richer in Cr2O3. Both the mineral assemblage and the incompatible trace element characteristics of the mineral phases recall the typical signatures of “carbonatite” metasomatism (HFSE depletion, Sr, LILE and LREE enrichment). Clinopyroxene has higher REE and Sr concentrations than amphibole (amph/cpxDREE,Sr = 0.7–0.9) and lower Ti and Zr concentrations. It is proposed that the petrographic and geochemical features observed at Finero are consistent with a subduction environment. The lack of chemical gradients between pyroxenite and peridotite is explained by a model where melts derived from an eclogite-facies slab infiltrate the overhanging harzburgitic mantle wedge and, because of the special thermal structure of subduction zones, become heated to the temperature of the peridotite. If the resulting temperature is above that of the incipient melting of the hydrous peridotite system, the slab-derived melt equilibrates with the harzburgite and a crystal mush consisting of harzburgite and a silica saturated, hydrous melt is formed. During cooling, the crystal mush crystallizes producing the observed sequence of mineral phases and their observed chemical characteristics. In this context pyroxenites are regions of higher concentration of the melt in equilibrium with the harzburgite and not passage-ways through which exotic melts percolated. Only negligible chemical gradients can appear as an effect of the crystallization process, which also accounts for the high amphibole/clinopyroxene incompatible trace element ratios. The major element refractory composition is explained by an initially high peridotite/melt ratio. The apatite, carbonate-bearing domains are the result of the presence of some CO2 in the slab-derived melt. The CO2/H2O ratio in the peridotite mush increased by crystallization of hydrous phases (amphibole and phlogopite) locally resulting in the unmixing of a late carbonate fluid. The proposed scenario is consistent with subduction of probably Variscan age and with the occurrence of modal metasomatism before peridotite incorporation in the crust.

Amphibole thermodynamics are approximated with the symmetric formalism (regular solution model for within-site non-ideality and a reciprocal solution model for cross-site-terms) in order to formulate improved thermometers for amphibole-plagioclase assemblages. This approximation provides a convenient framework with which to account for composition-dependence of the ideal (mixing-on-sites) equilibrium constants for the equilibria: For A and B all possible within-site and cross-site interactions among the species □−K−Na−Ca−Mg−Fe2+−Fe3+−Al−Si on the A, M4, M1, M3, M2 and T1 amphibole crystallographic sites were examined. Of the 36 possible interaction energy terms, application of the symmetric formalism results in a dramatic simplification to eight independent parameters. Plagioclase nonideality is modelled using Darken's quadratic formalism. We have supplemented an experimental data set of 92 amphibole-plagioclase pairs with 215 natural pairs from igneous and metamorphic rocks in which the pressure and temperature of equilibration are well constrained. Regression of the combined dataset yields values for the eight interaction parameters as well as for apparent enthalpy, entropy and volume changes for each reaction. These parameters are used to formulate two new thermometers, which perform well (±40°C) in the range 400–1000°C and 1–15 kbar over a broad range of bulk compositions, including tschermakitic amphiboles from garnet amphibolites which caused problems for the simple thermometer of Blundy and Holland (1990). For silica-saturated rocks both thermometers may be applied: in silica-undersaturated rocks or magmas thermometer B alone can be applied. An improved procedure for estimation of ferric iron in calcic amphiboles is presented in the appendix.

Although only Aliv appears in our thermometer expression, this in no way infers that we believe tschermakite (or katophorite, kaersutite and other) substitutions to be unimportant. A careful reading of our earlier papers makes it clear that the reverse is true, e.g. Holland and Richardson 1979; Will et al. 1990a, b; Guiraud et al. 1990. Indeed in B and H (p. 211) we state that “natural and synthetic amphiboles are dominated by a combination of edenite, pargasite, hornblende and hastingsite substitutions”. What we endeavoured to point out was that because of the nature of reaction (1) and the use of mixing-on-sites activity models the extent of Al substitutions on M2 or Na on M4 do not materially influence our expression for K1 and consequently our thermometer. Certainly any sophisticated activity model for amphibole explicitly should account for all cross-site interactions between M-, Tl- and A-site cations, as alluded to by P and S in reference to Docka et al. (1987), as well as O3-site anions. However, such a day currently is well away and mixing-on-sites, with modifications as outlined above and to be detailed in T.J.B. Holland and J.D. Blundy (in preparation), seems rather promising for the purposes of amphibole thermometry. We do not share the rather negative and pessimistic outlook on activity models and thermobarometry implicit in P and S′ comments.

The calibration by Blundy and Holland is not a calibration of the reaction albite + tremolite = edenite + 4quartz, because the AlIV content of amphiboles is a combined result of \(\vec tk - ,{\text{ }}\vec ed - {\text{ and }}\vec pl - \) substitutions.

There is currently a dearth of reliable thermobarometers for many hornblende and plagioclase-bearing rocks such as granitoids and amphibolites. A semi-empirical thermodynamic evaluation of the available experimental data on amphibole+plagioclase assemblages leads to a new thermometer based on the Aliv content of amphibole coexisting with plagioclase in silica saturated rocks. The principal exchange vector in amphiboles as a function of temperature in both the natural and experimental studies is $$\left( {Na\square _{ - 1} } \right)^A \left( {AlSi_{ - 1} } \right)^{T1}$$ . We have analysed the data using 3 different amphibole activity models to calibrate the thermometer reactions 1. $$1. Edenite + 4 Quartz = Tremolite + Albite$$ 2. $$2. Pargasite + 4 Quartz = Hornblende + Albite.$$ The equilibrium relation for both (1) and (2) leads to the proposed new thermometer $$T = \frac{{0.677P - 48.98 + Y}}{{ - 0.0429 - 0.008314 ln K}} and K = \left( {\frac{{Si - 4}}{{8 - Si}}} \right)X_{Ab}^{Plag} ,$$ where Si is the number of atoms per formula unit in amphiboles, with P in kbar and T in K; the term Y represents plagioclase non-ideality, RTlnγab, from Darken's Quadratic formalism (DQF) with Y=0 for X ab>0.5 and Y=-8.06+25.5(1-X ab)2 for X ab

In low grade metabasites the amphibole components tremolite, glaucophane, edenite and tschermakite have their activities controlled by interactions with the excess components albite, clinozoisite, chlorite, quartz and H2O vapor. Three types of reaction are involved, (i) Those in which only components of condensed phases take part: isopleths of equilibrium constant are straight lines in the P-T plane. (ii) Dehydration reactions in which entropy change due to change in Al coordination is of the same sign as that due to dehydration: isopleths of constant K are positive at low pressure and negative at high pressure. (iii) Dehydration reactions in which entropy change due to Al coordination change is opposite in sign to that of dehydration: isopleths of constant K loop in the P-T plane with positive slopes at low and at high pressure. Zonation in naturally occurring amphiboles records the evolution of metamorphic conditions in particular rocks. In an example from the eastern Alps (Austria) early conditions calculated as 15 kb, 200 ° C evolve upgrade to 6 kb, 525 ° C implying concurrent heating and erosion. The record of evolving conditions may span some 30 Ma of geological history.

Porphyroclastic diopside in garnet lherzolite from Alpe Arami, Bellinzona, Switzerland includes optically-visible clinoamphibole lamellae with a composition intermediate between pargasite and edenite. X-ray and electron microscopic observations show that the diopside crystal contains sub-microscopic thin clinoamphibole lamellae parallel to (010), which have coherent interfaces to the host. A kind of planar defect parallel to (010) in clinopyroxene structure, as suggested by Chisholm (1973), is shown here to correspond to intercalation of a 9 A lattice fringe of double-chain structure in the electron micrograph of the diopside. The thin clinoamphibole lamellae are observed to be segregated domains consisting of two and more 9 A fringes. From the chemical characteristics and textural relations of the development of such clinoamphiboles, the chemical change required to form them is considered to have been caused principally by decreasing solubility of atoms such as Na, Al and Cr in clinopyroxene structure during the retrogressive reequilibration. Also, a possibility of finite solid solution of clinoamphibole in clinopyroxene is discussed.

Some 500 microprobe analyses from 43 rock samples, covering the entire range of major and minor intrusive rock-types, define the following ranges of mineral composition: plagioclases (An70-30), alkali feldspars (Or98-20 and Ab90–99), olivines (Fo85-82), clinopyroxenes (aluminous titanaugite through salite and aegirine-augite to acmite), amphiboles (subsilicic kaersutite through pargasite to hastingsite, edenite or katophorite), biotites (titanbiotite to titaniferous manganiferous lepidomelane). Varied discontinuous reaction relationships are evident petrographically between these minerals (e.g. amphibole overgrowths on pyroxenes or biotites), but most appear to reflect reequilibration during slow magmatic cooling, or perhaps local disequilibrium effects, and there is little evidence for significant subsolidus, hydrothermal or deuteric modification of the primary mineralogy.

Equilibria between plagioclase, calcic amphibole and quartz can be described, in part, by the relation among mineral components: NaAlSi3O8+□Ca2Mg5Si8O22(OH)2 = NaCa2Mg5AlSi7O22(OH)2+4SiO2; this relation governs the partitioning of Na between plagioclase and the A-site of coexisting amphibole. Data from natural amphibolites reveal that this partitioning is systematic and sensitive to metamorphic grade. The ideal portion of the equilibrium constant (K id = X Na, A/X□, A · X Ab) derived from natural samples is sensitive to bulk composition, inasmuch as both plagioclase and amphibole are highly non-ideal. Samples from a single outcrop have values ranging from 0.5 (X Ab=0.74) to 4.1 (X Ab=0.10). The continuous reaction, NaAlSi3O8+□Ca2Mg5Si8O22(OH)2 = NaCa2Mg5AlSi7O22(OH)2+4SiO2, proceeds to the right with increasing grade of metamorphism and for a given bulk composition, K id increases with increasing temperature. Two related discontinuous reactions, actinolite+albite=hornblende+oligoclase+quartz and actinolite+oligoclase=hornblende+anorthite+quartz, also proceed to the right with increasing metamorphic grade and result in changes in the topology of a phase diagram that describes the partitioning of Na between plagioclase and amphibole A-site. A Schreinemakers' net is presented that is consistent with natural occurrences. The results of this study should aid in the delineation of metamorphic facies within amphibolites.

Actinolite-actinolitic hornblende and actinolitic hornblende-hornblende pairs are described from gabbroic amphibolites and epidote amphibolites formed by dynamic metamorphism during uplift of gabbroic rooks in the Hidaka Metamorphic Belt, Hokkaido. Electron microprobe analyses indicate that coupled substitutions involved in the transition from actinolite to hornblende are essentially those of edenite and tschermakite-ferritschermakite together with smaller amounts of glaucophane-riebeckite, i.e. AlIV, AlVI, Fe3+, A-site occupancy and NaM4 increase with replacement of Mg by Fe2++ Mn and Si by AlIV.

Alkali olivine basalts from Skye were simultaneously contact metamorphosed by Tertiary gabbro and granite intrusions and altered by the hydrothermal convection system that the plutons induced. Four metamorphic zones were mapped around the plutons. Furthest from the intrusions, in the primary olivine zone, metabasalts are composed of combinations of igneous olivine, augite, plagioclase, titaniferous magnetite, ilmenite, zeolites, gyrolite, sulfides, and chlorite-smectite intergrowths. Closer to the plutons, in the smectite zone, saponite and carbonate appear, primary olivine and gyrolite disappear, and zeolites decrease dramatically in abundance. Still closer to the plutons, in the amphibole zone, actinolite, edenite, chlorite, sphene, epidote, andradite, and quartz appear and saponite and chlorite-smectite intergrowths disappear. Along parts of the contact between gabbro and basalt, in the orthopyroxeneolivine zone, orthopyroxene, metamorphic olivine, and biotite appear and amphibole, chlorite, sphene, epidote, andradite, carbonate, and quartz disappear. Whole-rock chemical data indicate only minor change in the major-element chemical composition of the metabasalts during progressive metamorphism/hydrothermal alteration. Two-pyroxene eothermometry and various mineral-fluid equilibria suggest the range of peak temperatures attained in the metamorphic zones: orthopyroxene-olivine zone, ∼900°1, 030° C; amphibole zone, ∼400°–900° C; smectite and primary olivine zones, +5‰ (SMOW); mineralogically more altered basalts from the smectite zone have whole-rock δ18O=+2 to +5‰; still more mineralogically altered basalts from the amphibole zone (with one exception) have δ18O

The metamorphic evolution of the Western Gneiss Region of Norway (WGR) can be evaluated from examination of eclogites formed as an integral part of the gneiss terrain (ie. in situ). Petrologic data from one of the larger “country-rock” eclogites within the northwestern part of WGR have been used to construct a P, T, t-path for this crustal volume. Textural relations as well as mineral composition and zonation suggest an early period of amphibolite facies conditions, followed by movement along a very steep positive P/T-gradient towards very high pressure (P≥25 kb) and moderate temperature (T≈600° C). Orthopyroxene coexisting with garnet at such conditions was poor in Al (

Heats of mixing of synthetic C2/m fluortremolite-fluoredenite amphiboles measured at 985 K show a systematic deviation from ideal mixing consistent with a subregular solution model. The deviations from ideal mixing are interpreted in terms of Na ordering in the A-site and Na-Al interactions in edenite-poor compositions. Enthalpies of ‘edenite’ substitution reactions in amphiboles and in SiO2-NaAlO2 glasses and framework silicates are comparable. Gibbs free energies of formation of fluortremolite and fluoredenite at 298K are -2,821.07±3.34 kcal mol−1 and -2,889.59±2.40 kcal mol−1 respectively. The former value is in good agreement with values calculated from both F-OH exchange experiments and from a natural fluortremolite-bearing metamorphic rock. Least-squares fitted sub-regular heat-of-mixing parameters are poorly constrained and unrealistically high, but estimated subregular mixing parameters consistent with 95% confidence interval uncertainties in the calorimetric data and with TEM constraints give activity-composition relations in good agreement with the A-site compositions of natural metamorphic and igneous hornblendes. These relations predict unmixing in edenite-rich compositions over a wide range of temperature, but lend no support to the existence of a hornblende-actinolite miscibility gap. Calibration of the reaction tremolite+ albite=edenite+4 quartz as a function ofP,T andX ed amph indicates negativedP/dT slopes and a limited range of X ed amph (0.3 to 0.5) in equilibrium with plagioclase and quartz over a wide range of pressure and temperature, consistent with metamorphic hornblende-plagioclase assemblages. The energetics of this reaction suggest, however, that amphibole-plagioclase disequilibrium may be common.

Calcic amphiboles are ubiquitous in the East Bull Lake anorthosite-gabbro complex, northeastern Ontario. The mode of occurrence suggests the amphiboles replaced clinopyroxene throughout the stratigraphic levels of the pluton, but they are also prolific in the top younger units. Three types of amphiboles, namely (1) tremolite-actinolite, (2) actinolitic hornblende and (3) hornblende, were identified. The composition of the amphiboles shows that they define a continuous series without any obvious compositional-miscibility gap. The analyses further indicated that the amphiboles evolved from actinolite to hornblende through coupled substitution involving edenite and tschermakite end members. Among the three coexisting amphiboles, chlorine is concentrated in hornblende. This is due to preferential location of hornblende along grain boundaries and/or its favourable structure.