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Low-temperature hydrous alteration of FeTi oxide-rich ferroan peridotite, Laramie anorthosite complex, Wyoming, produced silician ferrihydrite, cronstedtite, greenalite, hisingerite, and talc. Ferrihydrite occurs as nanocrystals in ~50 nm diameter granules that form monomineralic masses up to 300 μm across. It is inferred to have formed by the replacement of an igneous sulfide such as pyrrhotite. Electron diffraction shows the ferrihydrite to be a 9-line variety. Si-rich cronstedtite formed thin rims around the ferrihydrite, and talc grew patchily around the cronstedtite. Greenalite formed in ~10 μm cracks through all the above minerals and olivine, and hisingerite microveinlets partially replaced olivine. Igneous minerals remaining include olivine Fa46, magnetite, ilmenite, hornblende, biotite, and trace clinopyroxene. Correlations among the constituents of ferrihydrite determined by electron microprobe, including anhydrous totals, indicate progress during the growth of two charge-balanced exchanges involving silica enrichment: an inverse cronstedtite substitution (MgFe2+,Si) (Fe3+Mn3+)–2 and an inverse hydrogarnet substitution SiH–4. The cronstedtite exchange requires charge and size balance across nearest-neighbor T and O crystal sites, suggesting crystal-interior rather than crystal-surface control. Ferrihydrite’s composition reflects time- and space-related variations in the chemical potentials of components in the hydrous fluid at the site of alteration. An upper limit for SiO2 of 14–15 wt%, or ≈1.0 Si per 5-cation formula unit, would seem to correspond to the limit of availability in ferrihydrite of tetrahedral sites open to the entry of Si. Our EPMA data, projected to zero SiO2, indicate an anhydrous total of ≈83 wt% for end-member ferrihydrite, a number that matches the formula: Fe10O15·9H2O. The geochemical properties of Laramie ferrihydrite are shared by some samples of altered chondritic and Martian meteorites. Ferrihydrite on Earth commonly occurs as a surface deposit; unlike the Laramie occurrence, these lack the microspatial coherence of replacements/pseudomorphs to show systematic, structure-related element variations. The superior crystal quality of the Laramie ferrihydrite likely contributed to its unique compositional variability.

In our 2014 paper ([Joy & Evans 2014][1], p. 349–351), we gave reasons for preferring to use an earlier IMA nomenclature scheme (IMA 1997) for the Mg-Fe-Mn amphiboles ([Leake et al . 1997][2], [2003][3]) rather than the latest version, IMA 2012 ([Hawthorne et al . 2012][4]). [Hawthorne et al

We describe a sample of Proterozoic banded iron-formation from the inner contact aureole of the Duluth complex near Babbitt, Minnesota, that contains a compositionally unique Fe-rich monoclinic ferromagnesian amphibole. Its content of T Al (1.3 atoms per 23 anhydrous oxygens) and A Na (0.55) places it in a compositional field (with the label clinoferrogedrite) outside those recognized in the IMA nomenclature for amphiboles. With declining Al, it grades into Al-rich grunerite, and finally to Al-poor grunerite at the crystal margins. While spot-to-spot compositional trends define basic gedrite substitutions, they differ systematically from the well-established anthophyllite-gedrite trends found in amphibolites. They resemble instead two instances of sodic gedrite described in the literature that are enriched in A Na apfu and depleted in octahedral Al. These unusual trends may be the result of very high temperatures of formation, or a high activity ratio of Fe + Mg to Si. Associated minerals in our sample include orthopyroxene Fs 74–76 , olivine Fa 85–91 , almandine, quartz, plagioclase, and accessories graphite, pyrrhotite, ilmenite, biotite, apatite, zircon, monazite, and lollingite. Conditions are estimated to have been ≥800 °C and ≈1.3 kbar, with f O 2 close to FMQ-2 log 10 units. Clinogedrite should be considered extremely rare, but not non-existent. In the monoclinic structure the symmetry of the double Si–O chains limits the uptake of gedritic components to amounts lower than found in the orthorhombic structure, where Al is distributed across three of the four distinct tetrahedral sites. We also present electron microprobe data for a highly aluminous cummingtonite grading compositionally into clinogedrite ( A Na = 0.3, M 2 Al + Fe 3+ + 2Ti = 1.0, T Al = 1.1) that was grown from fused and S-augmented Pinatubo dacite in the laboratory at 780 °C, 2.2 and 3.9 kbar. Growth of this MgFe-clinoamphibole was enabled by the capture of Ca into anhydrite away from potential hornblende.

Rock-forming serpentine minerals form flat, cylindrical, and corrugated crystal microstructures, which reflect energetically efficient layering of alternate tetrahedral and octahedral sheets. Serpentinization of peridotite involves internal buffering of the pore fluid, reduction of oxygen fugacity, and partial oxidation of Fe 2+ to Fe 3+ . Sluggish MgFe diffusion in olivine causes precipitation of magnetite and release of H 2 . The tectonic environment of the serpentinization process dictates the abundance of fluid-mobile elements in serpentinites. Similar enrichment patterns of fluid-mobile elements in mantle-wedge serpentinites and arc magmas suggest a linkage between the dehydration of serpentinite and arc magmatism.

Eruption of the Bishop Tuff magma preserved equilibrium of exchange components and element concentrations among magnetite, ilmenite, biotite, apatite, zircon, and liquid. Orthopyroxene and clinopyroxene were not in exchange equilibrium with the other MgFe-bearing phases, but they appear to have been in equilibrium among themselves. Internally consistent temperatures recorded by the FeTi-oxide, Ti-in-quartz, and Δ 18 O quartz-magnetite thermometers, coupled with evidence for magmatic corrosion of quartz and sanidine, indicate that an initially low- T (≈700 °C), near-H 2 O-saturated, high-SiO 2 rhyolite magma was heated up to ≥800 °C and its crystal cargo partially melted by recharge of hotter melt from below. Oxygen fugacity and compositions of biotite, ilmenite, magnetite, and silicate liquid initially adjusted by internal rearrangement of components and conservation of oxygen. Partial melting of feldspars liberated Sr and Ba back into the melt. Mixing during recharge eventually re-introduced compatible elements (e.g., Mg, Ba, Sr) as well as foreign crystals of euhedral ortho- and clinopyroxene, which evidently never totally re-equilibrated with the rhyolite liquid. Introduction of CO 2 and accompanying reduction in the a H 2 O during recharge raised crystallization temperatures of quartz and sanidine in the rhyolite sufficient to allow marginal regrowth of these phases with enhanced contents Ti, Ba, and Sr.

Outcrops of spinifex-textured metaperidotite in the Cerro del Almirez ultramafic massif, S. Spain, are located a few meters upgrade from a unique eclogite-facies (1.6–1.9 GPa, ≈680 °C), antigorite-out (orthopyroxene-in) isograd. Olivine in the spinifex rock is pleochroic brown in thin section, and microprobe analysis (carefully avoiding accidental inclusions) shows contents of Cr2O3 and TiO2 in the range 0 to 0.4 and 0 to 0.1 weight percent respectively. These amounts of Cr2O3 and TiO2 are matched by olivine in peridotite xenoliths from the mantle, komatiites, and pseudotachylytes. According to laboratory experiments, at 2 GPa and a likely redox state near QFM, these levels of Cr2O3 signify equilibration at temperatures in excess of 1600 °C. Curvature of the elongate olivines, and outcrop-scale alignment of olivine splays suggest a connection with komatiites. Two early studies at Cerro del Almirez emphasized this connection, but all papers from the late-1990s to the present time have advocated a metamorphic origin, analogous to jackstraw-olivine rocks. Some 5 to 6 weight percent or 13 to 15 volume percent of H2O was released at the isograd. We hypothesize that this dehydration water induced embrittlement of chlorite-metaharzburgite, and frictional slip in shear zones raised the temperature by several hundred degrees C above the ambient, more than enough to melt wet or dry ultramafic rock and form spinifex texture on rapid cooling. The amounts of crystallized melt so formed are orders-of-magnitude larger than found in peridotite pseudotachylyte, but are perhaps not totally inconsistent with dehydration-induced seismic events repeated over a protracted period of time during Miocene subduction of an oceanic plate. The transient ultramafic melts yielded on rapid cooling up to 12 cm long olivines and interstitial aggregates of parallel to radiating orthopyroxene prisms, followed by crystallization from glass of assorted eclogite-facies minerals (chlorite, tremolite, talc).

International audience; Despite claims to the contrary, the compositions of magnetite and ilmenite in the Bishop Tuff correctly record the changing conditions of T and fO2 in the magma reservoir. In relatively reduced (ΔNNO < 1) siliceous magmas (e.g., Bishop Tuff, Taupo units), Ti behaves compatibly (DTi ≈ 2–3.5), leading to a decrease in TiO2 activity in the melt with cooling and fractionation. In contrast, FeTi-oxides are poorer in TiO2 in more oxidized magmas (ΔNNO > 1, e.g., Fish Canyon Tuff, Pinatubo), and the d(aTiO2)/dT slope can be negative. Biotite, FeTi-oxides, liquid, and possibly plagioclase largely maintained equilibrium in the Bishop Tuff magma (unlike the pyroxenes, and cores of quartz, sanidine, and zircon) prior to and during a mixing event triggered by a deeper recharge, which, based on elemental diffusion profiles in minerals, took place at least several decades before eruption. Equilibrating phases and pumice compositions show evolving chemical variations that correlate well with mutually consistent temperatures based on the FeTi-oxides, sanidine-plagioclase, and Δ18O quartz-magnetite pairs. Early Bishop Tuff (EBT) temperatures are lower (700 to ~780 °C) than temperatures (780 to >820 °C) registered in Late Bishop Tuff (LBT), the latter defined here not strictly stratigraphically, but by the presence of orthopyroxene and reverse-zoned rims on quartz and sanidine. The claimed similarity in compositions, Zr-saturation temperatures and thermodynamically calculated temperatures (730–740 °C) between EBT and less evolved LBT reflect the use of glass inclusions in quartz cores in LBT that were inherited from the low-temperature rhyolitic part of the reservoir characteristic of the EBT. LBT temperatures as high as 820 °C, the preservation of orthopyroxene, and the presence of reverse-zoned minerals (quartz, sanidine, zircons) are consistent with magma recharge at the base of the zoned reservoir, heating the cooler rhyolitic melt, partly remelting cumulate mush, and introducing enough CO2 (0.4–1.4 wt%, mostly contained in the exsolved fluid phase) to significantly lower H2O-activity in the system.

Microprobe analyses of antigorite show that (Al+Cr) and inferred Fe 3+ correlate inversely with Si apfu in a Tschermaks substitution. This observation suggests that the uptake of Fe 3+ is not simply related to f O 2 . For Si = 1.95 apfu estimated Fe 3+ = 0.032 apfu (or 0.95 wt% Fe 2 O 3 ). Such estimates of Fe 3+ require high analytical accuracy and precision, and assume a fixed polysomatic formula (e.g., m = 17) and freedom from interlayer sheet-silicate impurities. In many cases the estimates appear to be high. An alternative measure of Fe 3+ is provided by the partitioning of total Fe and Mg between antigorite and olivine in well-equilibrated natural antigorite-olivine-magnetite parageneses. Extrapolation of Nernst and Roozeboom partition plots to Fe-free olivine permits an estimate of the Fe 3+ content of the average antigorite in this paragenesis, namely 0.42 or 0.64 wt% Fe 2 O 3 . The partition estimates are in good agreement with the results of Mossbauer spectroscopy performed here on 14 antigorites from metaperidotites, together with four from the literature. These spectra reveal a range of 0.16 to 1.94 in wt% Fe 2 O 3 in metaperidotite antigorite, with an average of 0.83. In two olivine-bearing rocks, antigorite has Fe 3+ /∑Fe ratios of 0.13 and 0.15, which corresponds to wt% Fe 2 O 3 = 0.47 and 0.54, respectively. Larger amounts of Fe 2 O 3 occur in some, but not all, vein antigorites. The prograde formation of antigorite in serpentinite from lizardite is accompanied by loss of some cronstedtite component and the precipitation of additional magnetite. The Roozeboom Mg/Fe partition plot is concave down rather than up; in other words the partition coefficient K D is a function of the X Mg of olivine. This behavior has been found in other olivine-mineral pairs. It can be interpreted to reflect strongly non-ideal solution behavior of MgFe-olivine at low temperatures, viz. W G ≈ 8.5 kJ assuming a symmetrical solution. MgFe-brucite appears to be similarly non-ideal.

In two samples of antigorite serpentinite from Sasaguri, Kyushu, Japan, we have found trace amounts of a NiFe-rich silicate whose atomic proportions in terms of the ratio of M to T cations match those of the fibrous MgFe-silicate hydrate balangeroite. Balangeroite in serpentinite at the type locality in Italy is not reported to contain Ni, whereas the balangeroite analogue from Japan has very little MgO. One sample from Japan is nickel-rich (up to 39 wt% NiO), and the other Fe-rich (up to 61 wt% total FeO). The Ni-rich sample appears to contain ~2 wt% Cl, and the Fe-rich sample contains up to 2.3 wt% P2O5. A wide miscibility gap in the ternary system Mg-Fe-Ni separates the Italian and Japanese minerals. Anhydrous microprobe totals are low, and we attribute this to a submicroscopic porosity, which is imaged by electron-beam scanning. Owing to its small grain-size (

The serpentinization of peridotite operates according to one or the other, or a combination, of two end-member mechanisms. In low-temperature environments (50–300 °C), where lizardite is the predominant serpentine mineral, olivine is consumed by reaction with H2O but its composition (Mg#) remains unchanged. Mg-rich lizardite, magnetite, and dihydrogen gas (±brucite) are products of the reaction. At higher temperatures (400–600 °C), rates of MgFe diffusion in olivine are orders of magnitude faster, with the result that the growth of Mg-rich antigorite can be accommodated by a compositional adjustment of olivine, eliminating the need to precipitate magnetite and evolve hydrogen. This latter end-member mechanism probably best reflects the situation in the forearc mantle wedge.

We document an example of serpentinization of olivine and orthopyroxene that produced virtually no magnetite, but instead relatively Fe-rich yellow-colored lizardite (X Fe = 0.08 to 0.17), and the native Fe-Ni-Co metals, awaruite and wairauite. Lizardite’s identity was confirmed by micro-Raman spectroscopy, although peaks are broad. Electron microprobe analyses of the lizardite yield a continuous compositional trend of formula contents suggestive of the progressive uptake of Fe 3+ exclusively on M sites, where it is charge balanced by vacancies. Although these observations are unusual, this secondary mineral assemblage can be explained in terms of the likely intensive variables T , f H 2 O , f H 2 , and a SiO 2 attending the alteration. The absence of magnetite in serpentinization does not signify a lack of oxidation. By forming the hydrated phase-component ferri-lizardite instead of magnetite from the fayalite and ferrosilite components, the yield of hydrogen is reduced by two-thirds. The usual inverse correlation of rock density with magnetic susceptibility is unlikely to be the case in this kind of serpentinization.

A model for the thermodynamic properties of rhombohedral oxide solid solutions in the system Fe2O3-FeTiO3-MgTiO3-MnTiO3 (containing minor amounts of Al2O3) is presented. The model accounts for temperature and compositionally dependent long-range cation-order and the related high to low symmetry structural phase transition. The model is calibrated from the cation-ordering data of Harrison and others (2000; Harrison and Redfern, 2001) and experimental data on Fe+2Ti ⇔ (Fe+3)2 exchange between rhombohedral oxide and spinel from Lattard and others (2005) and Evans and others (2006). Successful calibration require introduction of an energetic contribution attributed to short-range cation-order, which reduces the configurational entropy of the solid solution. The resultant thermodynamic model for the rhombohedral oxides is internally consistent with the model for spinel solid solutions of Sack and Ghiorso (1991a, 1991b) and with the endmember thermodynamic properties database of Berman (1988); a new model equation for the isobaric heat capacity of ulvospinel (cubic Fe2TiO4) is proposed and values of the enthalpy of formation, −1490.417 kJ/mol, and third law entropy, 184.199 J/K-mol, at 298.15 K and 105 Pa are recommended. The new model forms the basis of a revised FeTi-oxide geothermometer/oxygen barometer, which is applied to a newly compiled dataset of natural two oxide pairs from silicic volcanic rocks. Results are compared to previous formulations with the general conclusion that the new model gives a better estimate of oxidation state for magmas that equilibrated under conditions more oxidizing than the nickel-nickel oxide buffer. Estimates of oxygen fugacity are fairly insensitive to analytical uncertainties in oxide compositions. By contrast, temperature estimates are especially sensitive to analytical error and to the abundances of “minor” constituents. Application of the geothermometer to oxide pairs that grew under conditions where the rhombohedral phase was cation disordered (that is high temperature or at oxygen fugacities greater by about one log10 unit than the nickel-nickel oxide buffer) results in an uncertainty due solely to analytical error of at least 50°C and sometimes as high as 100 °C. Temperature estimates from the new geothermometer can be made using either the Fe+2Ti ⇔ (Fe+3)2 exchange or Fe+2 ⇔ Mg exchange between the two oxides. Comparison of the two temperature estimates provides a means of evaluating the internal consistency of coexisting oxide compositions and assessing the extent of disequilibrium. Temperatures calculated from the new model are found to be consistent with experimental phase relations for the stability of cummingtonite in silicic volcanics. Other petrologic constraints on derived temperatures are examined including limits on the width of the miscibility gap and the development of self-reversed remanent magnetization in the rhombohedral series. Software that implements the new thermodynamic model and the two-oxide geothermometer/oxygen barometer is available from http://www.ofm-research.org/.

It is argued that the high-Mg content (mg-number1⁄4 95 3) of the serpentine minerals in serpentinized peridotite is a consequence of the environmental FeMg 1 exchange potential imposed on the system by the abundance of olivine and orthopyroxene. Mass balance in the serpentinization reaction then requires the precipitation of an ironrich mineral that in most cases is magnetite.This causes hydrogen to be evolved in an oxygen-conserved reaction.The low-variance mineral assemblage Olþ SrpþBrcþMag sets the chemical potentials of H2O, SiO2 and O2 internally at an early stage in the process, but the paragenetic assessment of serpentinites is rendered difficult by the variable and usually unknown Fe3þ content of the serpentine minerals, particularly lizardite. Whole-rock analyses of highly to completely serpentinized peridotites reveal Fe/SFe ratios40 4, with an average value (0 69) similar to that of magnetite (0 67). This feature may be attributed to the presence of high-Fe lizardite, as has been found in Mo« ssbauer spectroscopy studies. Electron microprobe and scanning electron microcope analyses in the literature exhibit element trends (e.g. decreasing Si vs SFe a.p.f.u.) for olivinepseudomorph lizardite and, with some exceptions, for bastite lizardite, that show a substitution of the cronstedtite component (Fe charge-balanced onTand M sites). Cronstedtite substitution will be favoured at low temperature and/or low hydrogen fugacity, and in these circumstances less magnetite will be evolved during serpentinization, in some cases none at all. Some bastite lizardites from sea-floor settings show evidence of M-site vacancy substitution of Fe for Fe. In the course of progressive serpentinization, micrometer to millimeter-scale variations in SiO2 potential may well be present, but their influence on Fe in lizardite seems to be limited to a few cases of lizardite associated with orthopyroxene. Chrysotile is on average more Mg-rich and less variable in Fe/Mg ratio than lizardite, facts that may be attributed to the greater Fe content of lizardite. Chrysotile veins provide the best record available to us of the environmental FeMg 1 exchange potential in the pore fluid attending serpentinization.This potential serves as a robust control on serpentine and brucite compositions, although it may fail after olivine and orthopyroxene have been armoured or eliminated, and in more open-system environments (high water/rock ratio) such as on the sea floor or at serpentinite host-rock contacts. The default assumption in microprobe analyses that measured iron is all Fe can lead to inappropriate petrological conclusions in the case of serpentinites.

Reliable characterization of the properties of amphibole end-members is a prerequisite for many important things in mineralogy and petrology, especially for the calculation of phase equilibria that involve amphiboles. Laboratory measurements of the thermochemical and thermophysical properties of amphibole end-members, as well as the experimental reversal of critical reactions, require the availability of adequate amounts of appropriately pure samples. More often than not, nature fails to provide material that fulfills the chemical and physical requirements of phase purity. Synthesis of chosen compositions at high pressure and temperature might seem to be the obvious answer to the problem, but it is now well known that amphiboles are difficult customers in this respect (Hawthorne 1983a, 1995; Graham et al. 1989; Raudsepp et al. 1991; Maresch et al. 1994). In the early days of experimental petrology, in the 1960s and 1970s, phase characterization of products involved optical microscopic measurements and powder XRD, a minimal combination that we now realize was inadequate. Experience has shown that the synthesis of an end-member amphibole commonly yields distinctly less than 100% amphibole, with the result that its composition likely departs significantly from the desired one, often in a reproducible way. In addition, the amphibole may contain unacceptably high concentrations of defects of one sort or another, and, whether or not these drive it off composition, their presence can be energetically detrimental. These deficiencies can be minimized, but not necessarily eliminated, by increasing the pressure, temperature, or duration of synthesis. Heroic efforts have been rewarded in a few cases. For example, by repeated grinding of experimental products, and the use of high pressure and judicious amounts of H2O, Jenkins and Corona (2006) were able to obtain glaucophane very close to its end-member composition, something that numerous previous workers had failed to …

The two rock-forming polymorphs of serpentine Mg3Si2O5(OH)4, lizardite and chrysotile, occur in nature in virtually identical ranges of temperature and pressure, from surficial or near-surficial environments to temperatures perhaps as high as 400°C. Laboratory evidence indicates that lizardite is the more stable at low temperatures, but the difference in their Gibbs free energies is not more than about 2 kJ in the 300-400°C range. Above about 300°C, antigorite + brucite is more stable than both; in other words, chrysotile is nowhere the most stable. The crystal structures of lizardite and chrysotile give rise to contrasting crystallization behaviors and hence modes of occurrence. The hydration of peridotite at low temperature results in the growth of lizardite from olivine, and (commonly topotactically) from chain and sheet silicates, although the MgO-SiO2-H2O (MSH) phase diagram predicts antigorite + talc in bastite. The activity of H2O during serpentinization may be buffered to low values by the solids,...

A thermodynamic model has been developed and calibrated for monoclinic and orthorhombic amphiboles compositionally contained within the Ca-Mg-Fe 2 + amphibole quadrilateral. The model incorporates the energetic consequences of cation ordering of Fe 2 + and Mg over nonequivalent sites in the crystal structure, and accounts for the temperature, pressure, and compositional dependence of the orthorhombic-monoclinic phase transition. Calibration is based on previously published work on the thermodynamic properties of Fe 2 + -Mg amphiboles and tremolite, and experimental cation-ordering data, along with solvus width and tieline orientations of natural coexisting amphiboles in the quadrilateral. Among derived parameters is the enthalpy of formation of end-member ferroactinolite (-10534.966 kJ/mol). A calculated FeMg - 1 isopotential solvus at mid-composition in the quadrangle agrees well with a revised calibration of experimental data from Cameron (1975). The solvus is not strongly asymmetric and narrows with increasing Fe/Mg ratio and temperature. Phase relations along the Mg and Fe sides of the quadrilateral are compared and contrasted. Cummingtonite-anthophyllite phase relations are shown in the T-X F e plane, with and without coexisting calcic amphibole. The breakdown reactions of quadrilateral amphiboles to assemblages of two pyroxenes, olivine, quartz, and H 2 O are depicted as functions of temperature, pressure, composition, H 2 O activity, and oxygen fugacity. Some highlights of their relevance to metamorphic and igneous systems are discussed.

A transmission electron microscopy study reveals the microstructure of polysynthetically twinned, near end-member grunerite from Rockport, Massachusetts. A small percentage of Pnma ferroanthophyllite of similar composition is present as thin slabs intergrown parallel to (100) of grunerite and as individual μ-size crystals. HRTEM imaging shows that grunerite is free of chain-multiplicity faults. The observed microstructures are interpreted as a result of partial transformation of ferroanthophyllite to grunerite, a mechanism that is also supported by microstructures observed in the Pnmn protoferroanthophyllite from Cheyenne, Colorado, with similar composition. This study supports the possibility that grunerite, ferroanthophyllite, and protoferroanthophyllite may all possess true stability fields.

Ten new single-crystal X-ray structure refinements of unheated and heat-treated anthophyllite, new measurements of the optical indicatrix of anthophyllite, and previously published data from Mossbauer spectroscopy of heated anthophyllite, show that temperature-dependent long-range order of Fe2+ and Mg on the M-sites of cummingtonite-grunerite and anthophyllite may be considered identical for the purpose of thermodynamic modeling. The difference in solution properties between the monoclinic and orthorhombic series, as expressed in the composition ( X Fe) dependence of ln K D in natural amphibole pairs, is accomodated through adjustment of an enthalpic term that is independent of order-disorder. End-member thermodynamic properties of cummingtonite and ferroanthophyllite are derived from those already known for anthophyllite and grunerite respectively, using intercrystalline K D data and a fit of the T-X Fe phase loop to two critical field constraints: middle amphibolite-facies amphibolites and upper amphibolite-facies metaperidotites. Amphibolites suggest a transition temperature in the system FMSH at ≈ 555 °C and X Fe ≤ 0.3, whereas metaperidotites suggest a transition temperature of ≈ 650 °C at X Fe ≤ 0.1. Ln K D for Fe-Mg exchange between cummingtonite and anthophyllite passes through zero at X Fe ≈ 0.7, and as a result the T-X Fe phase loop shows a minimum at this composition. Extrapolated end-member transition temperatures are estimated to be ≈800 °C (Mg) and ≈450 °C (Fe). At its breakdown to enstatite + quartz + H2O (790 °C at 5 kbar), anthophyllite is marginally stable with respect to end-member cummingtonite, and the addition of Ca renders the breakdown reaction metastable. A stability field is possible for end-member ferroanthophyllite. Cummingtonite-anthophyllite phase relations mirror those of the analogous clino- and orthopyroxene.

Microprobe analysis of tremolite from “St. Gotthard,” Switzerland, shows that it is close to end-member in composition (Ca = 1.97 apfu), and not hypercalcic (Ca = 2.16 apfu), as a previous wet-chemical analysis suggested. The latter formula, and a corresponding thermodynamic activity (0.67), were used in the experimental database for the extraction of an optimal set of thermodynamic properties for tremolite (Chernosky et al. 1998). We derive a revised enthalpy of formation of tremolite (−12307.9 kJ/mol) based on bracketing experiments for the breakdown reactions of tremolite and tremolite + forsterite, and the assumption of time-averaged effective equilibrium compositions of phases in the experimental charges. Mg-cummingtonite substitution in tremolite is accommodated during free-energy minimization calculations by allowing the stoichiometry of the standard-state reaction to vary with the changing bulk composition of the amphibole. The breakdown temperature of tremolite is lowered with increasing cummingtonite component. Our procedure also allows continuous variation in the equilibrium compositions and activities of pyroxene and amphibole phase-components with P and T .

Nearly 30 years have elapsed since Kretz (1983) provided the mineralogical community with a systematized list of abbreviations for rock-forming minerals and mineral components. Its logic and simplicity have led to broad acceptance among authors and editors who were eager to adopt a widely recognized set of mineral symbols to save space in text, tables, and figures. Few of the nearly 5000 known mineral species occur in nature with a frequency sufficient to earn repeated mention in the geoscience literature and thus qualify for the designation “rock-forming mineral,” but a reasonable selection of the most common and useful rock-forming minerals likely numbers in the several hundreds. The original list by Kretz (1983) contained abbreviations for 193 of these. We propose an expansion to the list initiated by Kretz (1983) (see next page). Modest expansions and revisions were made by Spear …

contents of the plagioclase. Compositional zoning of hornblende is Crystallization experiments were carried out on a representative consistent either with an early crystallization event at ~400 MPa sample of the crystal-rich dacite ejected during the 15 June 1991 and 840–900°C, or with mixing events before eruption. The eruption of Mt Pinatubo, to define the pre-eruption conditions of experiments show that addition of S leads to an increase in the this major volcanic event. Experiments were performed in the mg-number of hornblende in the redox range where pyrrhotite occurs temperature and pressure range of 750–900°C and 220–390 (Ζ NNO+ 1·4) The lack of Mg-rich overgrowth on hornblende MPa, respectively. Redox conditions were varied between those of shows that if sulfur was introduced into the magma in the course the NNO (nickel–nickel oxide) buffer and 2·7 log fO2 units above of its crystallization, such as by volatile infiltration from an (NNO + 2·7). Melt water contents ranged from 3 to 7 wt % underlying, triggering basalt magma, then it must have occurred (H2O saturation). Phase equilibria at 220 MPa reproduce the when the dacite was already oxidized. phase assemblage of the magma only at temperatures below 780°C and water-rich conditions: melt H2O content > 6 wt %, or XH2Ofluid > 0·80. Phase abundances and the compositions of hornblende, plagioclase, and melt indicate that the eruption tapped a magma body that was at a temperature of 760 ± 20°C and

International audience; Explosive eruptions involve mainly silicic magmas in which sulfur solubility and diffusivity are low. This inhibits sulfur exsolution during magma uprise as compared to more mafic magmas such as basalts. Silicic magmas can nevertheless liberate large quantities of sulfur as shown by the monitoring of SO2 in recent explosive silicic eruptions in arc settings, which invariably have displayed an excess of sulfur relative to that calculated from melt degassing. If this excess sulfur is stored in a fluid phase, it implies a strong preference of sulfur for the fluid over the melt under oxidized conditions, with fluid/melt partition coefficients varying between 50 and 2612, depending on melt composition. Experimentally determined sulfur partition coefficients for a dacite bulk composition confirm this trend and show that in volcanic eruptions displaying excess gaseous sulfur, the magmas were probably fluid-saturated at depth. The experiments show that in more reduced silicic magmas, those coexisting only with pyrrhotite, the partition coefficient decreases dramatically to values around 1, because pyrrhotite locks up nearly all the sulfur of the magma. Reevaluation of the sulfur yields of some major historical eruptions in the light of these results shows that for oxidized magmas, the presence of 1-5 wt % fluid may indeed account for the differences observed between the petrologic estimate of the sulfur yield and that constrained from ice core data. Explosive eruptions of very large magnitude but involving reduced and cool silicic magmas, such as the Toba or the Bishop events, release only minor amounts of sulfur and could have consequently negligible long-term (years to centuries) atmospherical effects. This redox control on sulfur release diminishes as the melt composition becomes less silicic and as temperature increases, because both factors favor more efficient melt sulfur degassing owing to the increased diffusivity of sulfur in silicate melts under such conditions.

The crystal structures and M-site populations of a series of unheated and heat-treated, near-binary members of the actinolite series were determined from single-crystal X-ray diffraction data. Small, systematic differences in crystal structure (mean and individual bond lengths, octahedral distortions, and Mg-Fe site preferences) between the actinolite and cummingtonite series are documented and rationalized in terms of the bond-valence model. Mg-Fe 21 site preferences among the M1, M2, and M3 sites in the actinolite series are small. Between M1 and M3, site preference (Fe 21 slightly in favor of M1) is independent of temperature. Mg is favored in M2 over M1 in unheated samples (less strongly than in cummingtonite-grunerite), but Fe 21 and Mg are virtually disordered in samples reequilibrated at 700 8C. Site preference between M2 and M3 and possibly also between M1 and M2 appears to undergo a reversal as a result of heat treatment, but consideration of all sources of error renders this conclusion a tentative one, although the observation could be explained in terms of a bond valence deficiency. The ratio of Fe 21 /Mg atoms on M4 cannot be accurately determined, but refinements indicate that, like cummingtonite-grunerite, it is larger than on all other M sites. Expansion of the cell dimensions with an increase in Fe/Mg ratio follows a pattern similar to that of thermal expansion, and a and b are influenced by cummingtonite-grunerite solid solution. The data are consistent with a cell volume of 942.8 A ˚ 3 (or molar volume of 2.8388 J/bar) for end-member ferroactinolite, a first approximation that assumes a reciprocal, quadrilateral DV of zero.

The oxidation state of the 1991 Pinatubo dacite and many other oxidized silicic arc volcanic rocks has been overestimated. We reproduce the compositions of magnetite and ferrian ilmenite found in Pinatubo dacite by doing crystallization experiments at 780 degrees C, 2.2 kbar, and NNO + 1.7 (+ or -0.2) log units. The extra components in igneous ilmenite, notably geikielite, expand the solvus in a way that is not correctly accommodated in current formulations of the iron titanium oxide thermobarometer.

Garnet-bearing ultramafic rocks (GBU) enclosed in high-grade gneisses are known from several parts of the Bohemian Massif. One of these is the high-pressure(HP)-unit 1 in the Erzgebirge Crystalline Complex, which is the subject of the present study. Hitherto, two different models have been put forward to explain the stabilisation of garnet in mantle-derived ultramafic rocks from the Bohemian Massif and their emplacement into the crust. (1) Garnetiferous assemblages were formed in the ultramafics before they came in contact with their crustal host rocks. (2) Garnet was formed in the ultramafics at the expense of spinel due to cooling caused by their tectonic emplacement in the crust. The PT-evolution revealed by the investigated GBU from the Erzgebirge Crystalline Complex, however, requires a third model. The reconstruction of the PT-paths for the Erzgebirge GBU is based on both conventional thermobarometry and phase relations. Thermodynamic calculations allowed the construction of a PT-phase diagram for the system Na2O-CaO-MgO-Al2O3-SiO2-H2O, which is the first quantitative petrogenetic grid in this model system relevant to ultramafic HP-rocks. The grid shows the uni-, di-, and tri-variant assemblages stable in peridotitic rocks at different PT-conditions, providing a tool to constrain PT-paths from the succession of mineral assemblages observed in a rock. The PT-path obtained for the Erzgebirge GBU suggests that the garnet-bearing assemblages formed by HP-metamorphism of spinel peridotite which was emplaced into the crust prior to or during the HP-compressional stage. This model is supported by peak PT-conditions around 900 °C and 30–35 kbar recorded by the ultramafic rocks, which are very similar to those attained in the eclogites sensu stricto which occur in the same tectonic unit (HP-unit 1) and for which an in situ metamorphism has been inferred (Schmadicke et al. 1992). On the other hand, the other two high-pressure units in the Erzgebirge, HP-units 2 and 3, which also contain eclogites sensu stricto but lack peridotites, record lower peak PT-conditions of 650–750 °C/24–26 kbar and 600–650 °C/20–24 kbar, respectively. Postulating an in situ HP-metamorphism for the garnet peridotites as result of continental collision during the Variscan orogeny, a crustal thickness of 90–110 km would be required. A comparison of the distribution of eclogites sensu stricto and mantle-derived rocks from the Bohemian Massif together with their reported PT-conditions reveals a correlation between peak PT-conditions in eclogites sensu stricto and the prevailing assemblages in the ultramafic rocks in the same unit. Furthermore, the Erzgebirge Crystalline Complex and the Snieznik Complex on one hand as well␣as the Granulitgebirge and the Sowie Gory on the other hand are thought to be genetically linked. The garnet peridotites from the Granulitgebirge, the Sowie Gory and the Gfohl unit seem to have experienced peak PT-conditions not recorded by their country rocks with non-eclogitic basic interlayers, inferring a formation of␣HP-assemblages in the ultramafics prior to their emplacement into the crust.

Natural cummingtonite encompasses virtually the entire compositional range from Mg to Fe end-member. It occurs in several metamorphic rock types, but predominantly in amphibolites and metaironstones. Most examples were produced under amphibolite-facies conditions, although the full range for cummingtonite is at least 400-800 'C and < I I 5 kbar. Cummingtonite is also a critical indicator of T, er,o, and fo,in silicic volcanic rocks from shallow magma chambers. The solution properties of cummingtonite reflect the entropic and enthalpic consequences of temperature-dependent, long-range ordering of Mg and Fe2* on the Ml + M3, M} and M4 sites, plus modest site-mismatch energies. Solutions show small positive departure from ideal at 700-800'C and both positive and negative behavior at lower temperatures depending on composition. The quenched state of M-site ordering has an influence on the symmetry of the unit cell (C2/ m vs. P2,/ m) and on the optical indicatrix. The kinetics of diffusion of Fe2* and Mg among sites appears to be site dependent. Magnesio-cummingtonite from 0 to about l0o/o grunerite is less stable than magnesian anthophyllite at metamorphic temperatures of 600-700 'C. The inversion loop in the system FMSH, with anthophyllite as the low-temperature form, probably has a minimum temperature close to the Fe end. The paragenesis of cummingtonite with olivine and quartz (found in metamorphosed iron formation) is confined to pressures below l0 kbar, temperatures below 740 .C, and /o, no more than 0.5 log unit above QFM. The paragenesis of cummingtonite with orthopyroxene and quartz typifies more Mg-rich compositions and generally signifies higher temperatures and in many instances higher pressures, and a temperature maximum (extremum) exists in the isobaric T-Xo,diagram. The composition of cummingtonite together with magnetite, quartz, and HrO fluid is a sensitive indicator of /o,. Cummingtonite must be Mg rich to coexist with hematite. Temperature and fo, determined from iron titanium oxide phenocrysts and measured cummingtonite compositions in rhyolites from the Taupo Zone, New Zealand, agree well with predicted relations. The agreement is less good for dacites from Saint Helens and Pinatubo. With increasing pressure, reactions between cummingtonite and the components of feldspar produce biotite and hornblende and restrict the occurrence of cummingtonite in amphibolites and systems of granitic composition.

Stable-isotope profiles show that flat-lying marble units acted as impermeable barriers to upward fluid flow in transitional amphibolite-granulite grade rocks of the Kigluaik Mountains, Seward Peninsula, Alaska. The degree of permeability is related to the composition of the marble. The margin of a thick pure dolomite marble chemically reacted with underlying metasyenite (aH2O=0.2) to form a 2 cm boundary layer of calcite + forsterite by introduction of SiO2. No fluid penetrated past this reaction front, although the high temperature of metamorphism (∼800°C) allowed transport of carbon and oxygen isotopes for an additional 2 cm by diffusion through the solid dolomite. A second marble with a higher silica content underwent more decarbonation, which enhanced porosity and lead to a greater extent of isotope transport (2–3 m) in contact with quartzo-feld-spathic gneiss below. An estimate of total fluid flux across the bottom of this marble layer based on the shape of the isotope profile is 1 cm3/cm2 directed down, out of the marble. At two other marble-gneiss contacts steep isotopic gradients coincide with lithologic contacts, indicating very little cross-lithology fluid flow. The extent of diffusional transport of isotopes in the marbles is limited and interpreted as indicating the transient presence of a pore fluid, generated by thermally driven devolatilization reactions. No wholesale pervasive advection of C-O-H fluid occurred across the thick, continuous, marble units near the exposed base of the Kigluaik Group section during the entire regional metamorphic cycle. Activities of pore-fluid species were controlled by internal processes. Movement of volatiles and stable-isotopes between contrasting rock-types was dominantly diffusive. Channelized fluid pathways through the marble units developed during uplift and cooling but were not present during peak metamorphism. Heating of the section occurred by conduction, probably from an underlying magma source, and not by advection of a C-O-H fluid.

Alpine high-pressure metamorphic rocks exposed in the Cyclades were overpinted during their exhumation. We studied the metamorphic evolution of a pervasively overprinted rock sequence, the greenschist unit, exposed in the island of Sifnos. We show that the metamorphic evolution in the Greenschist unit is characterized by the progressive transformation of eclogite-facies rocks through albite-bearing epidote blueschists into greenschists. The evolution of the mineral assemblages during exhumation is shown by a series of chemographic diagrams which are referenced to specific P-T fields in the petrogenetic grid proposed by Evans (1990). P-T conditions corresponding to the major paragenetic groups are estimated also on the basis of mineral chemistry, Fe/Mg exchange thermometry and the use of GEOCALC-PTX and THERMOCALC softwares. A P-T path for the Greenschist unit is presented indicating eclogite-facies conditions of around 480-520 ° C and 12-15 kbar, followed by overprinting in the albite-bearing epidote-blueschist facies at P-T conditions of 8-10 kbar and ca. 480-500 ° C. Pressures of 5.5 kbar and temperatures around 450 ° C are estmated for the subsequent overprint in the greenschist facies. The decompression part of the P-T path is characterized by cooling with no indication of a temperature rise. Thus, the high-pressure metamorphic rocks of Sifnos appears not to follow the thermal evolution path predicted by models which assume whole crust uplift and erosion. On a regional scale the exhumation of the Cycladic blueschist belt is characterized by cooling in the west, but broad clockwise P-T loops with significant heating during decompression are found in the east. We note that cooling affected rocks situated at the deepest levels of the Alpine orogenic wedge (Sifnos, Syros, Tinos) whereas heating has affected rocks exhumed from shallower structural levels (Naxos, Ios)

Grunerite, Fe 7 Si 8 O 22 (OH) 2 , has been synthesized from oxide-gel and crystalline starting materials seeded with natural grunerite in a piston-cylinder apparatus at P=12-20 kbar and T=490-620°C, under the fO 2 of the Ni-NiO, wuestite-magnetite, and iron-wuestite buffers. The reaction curve: grunerite=7 ferrosilite+quartz+H 2 O has been reversed in the 12-20 kbar range on the NNO and IW buffers. Starting materials were synthetic ferrosilite, grunerite, and quartz, and natural grunerite (Fe/(Fe+Mg)=0.89). The reaction curve: 2 grunerite=7 fayalite+9 quartz+2 H 2 O has been reversed at 2 kbar under QFM-conditions in cold-seal apparatus. Starting materials were synthetic fayalite and natural quartz and grunerite (Fe/(Fe+Mg)=0.985)

Epidote phenocrysts (Ps 19-24 ) in Laramide-age, porphyritic dacite dikes in the Front Range of Colorado prove that epidote can crystallize directly from a magma. The dikes are high-K, calc-alkaline dacites and rhyodacites (SiO 2 = 64%-70%). Phenocryst assemblages consist of combinations of epidote, plagioclase (An 53-19 ), biotite [Fe 3+ / Fe total = 0.25; Mg/(Mg + Fe 2+ ) = 0.42-0.55], quartz, aluminous amphibole (Al total = 2.4-3.0 per formula unit), igneous garnet (And 2-6 Gros 13-27 Spess 2-13 Pyr 7-26 Alm 51-60 ), igneous muscovite, and sanidine. Holocrystalline, aphanitic groundmasses consisting mainly of quartz, plagioclase, and alkali feldspar comprise 60%-75% of the dike rocks and require that epidote grew early in the crystallization sequence. Petrographic and microprobe data distinguish three types of magmatic epidote phenocrysts, with different petrogenetic connotations implied by each type. Type I epidote occurs as euhedral phenocrysts, up to 8 mm long, with low content of allanite component ( Thermobarometric calculations provide empirical evidence that epidote is stable in silicic, calc-alkaline magmas at high total pressure, high water content, and high oxygen fugacity. The dacitic magmas originated by partial melting at temperatures above 800 °C. Temperatures (±50 °C) ranged from 800 to 880 °C during early phenocryst growth down to 620 to 670 °C during groundmass crystallization. Phenocrysts formed at pressures between 7.2 ± 1.0 and 12 ± 2.0 kbar. Oxygen fugacity was within the magnetite stability field at two log units (±1.0) below the hematite-magnetite buffer. H 2 O fugacity calculations give values ranging from 3.4 to 16 kbar, depending on how annite activity in biotite is modeled. The occurrences of types I and III epidote combined with geobarometry give a minimum pressure of epidote stability in the dike magmas of 8.0 ± 1.0 kbar. These results agree with experimental evidence and with conditions inferred for the crystallization of magmatic epidote in large, Cordilleran plutons. Phenocrysts in the dikes were preserved in conditions outside their stability limits by rapid emplacement and quenching in shallow-level dikes.

Phase diagrams have been computed for minerals in the system NCMASH, using the optimised thermodynamic data-base of Berman (1988) augmented by data on glaucophane and pumpellyite, to illustrate the relationships between rocks containing epidote and sodic amphibole and those containing assemblages characteristic of neighbouring metamorphic facies. The stoichiometries of stable bounding equilibria yield a simple list of mineral parageneses that serves to define an epidote-blueschist facies and surrounding metamorphic facies. The calculations suggest that when iron-free the pairs zoisite+glaucophane and clinozoisite+glaucophane do not possess a stability field. The natural Fe-bearing paragenesis of epidote+sodic amphibole occupies a closed field in the PT-diagram, whose size increases dramatically as the ratio Fe3+/Al in the minerals is increased. Where garnet is involved in bounding reactions, such as with respect to eclogite and (garnet-bearing) albite-epidote-amphibolite, the epidote-blueschist field decreases in size as the ratio Fe2+/Mg is increased. The classic blueschist-to-greenschist transition reaction is best developed in rocks with large Fe3+ and Fe2+ contents and is possibly metastable in MgAl-rich rocks. In the latter, a wedge of albite-epidote-(actinolite/barroisite) amphibolite separates the epidote-blueschist from the greenschits paragenesis field. A first occurrence of garnet divides the epidote-blueschist field into a lower-temperature garnet-free zone and a higher-temperature garnet-bearing zone, although the temperatures calculated for this garnet “isograd” seem high. The locations of all the facies boundaries are influenced by H2O-activity; however, even minor amounts of CO2 in the fluid phase (X(CO2) > 0.005) render the epidote+sodic amphibole paragenesis metastable, so that facies boundaries cannot be shifted by varying the CO2-content of the fluid phase. Below 600°C no mutual boundaries exist between the amphibolite facies and the epidote-blueschist and eclogite facies respectively. The calculated low-pressure limit for the eclogite paragenesis pyralspite+omphacite (Jd 50%)+quartz typical metabasite compositions is 12 to 14 kbars, with H2O-activity=0.9 or 1.5 to 2 kbar lower for Jd 30%.

A synthetic, low-melting rhyolite composition containing TiO2 and iron oxide, with further separate additions of MgO, MnO, and MgO + MnO, was used in hydrothermal experiments to crystallize Ilm-Hem and Usp-Mt solid solutions at 800 and 900°C under redox conditions slightly below nickel–nickel oxide (NNO) to $$\approx 3\,\log_{10} f_{{{\text{O}}_{2}}}$$ units above the NNO oxygen buffer. These experiments provide calibration of the FeTi-oxide thermometer + oxygen barometer at conditions of temperature and oxygen fugacity poorly covered by previous equilibrium experiments. Isotherms for our data in Roozeboom diagrams of projected %usp vs. %ilm show a change in slope at ≈ 60% ilm, consistent with the second-order transition from FeTi-ordered Ilm to FeTi-disordered Ilm-Hem. This feature of the system accounts for some, but not all, of the differences from earlier thermodynamic calibrations of the thermobarometer. In rhyolite containing 1.0 wt.% MgO, 0.8 wt.% MnO, or MgO + MnO, Usp-Mt crystallized with up to 14% of aluminate components, and Ilm-Hem crystallized with up to 13% geikielite component and 17% pyrophanite component. Relative to the FeTiAlO system, these components displace the ferrite components in Usp-Mt, and the hematite component in Ilm-Hem. As a result, projected contents of ulvospinel and ilmenite are increased. These changes are attributed to increased non-ideality along joins from end-member hematite and magnetite to their respective Mg- and Mn-bearing titanate and aluminate end-members. The compositional shifts are most pronounced in Ilm-Hem in the range Ilm50–80, a solvus region where the chemical potentials of the hematite and ilmenite components are nearly independent of composition. The solvus gap widens with addition of Mg and even further with Mn. The Bacon–Hirschmann correlation of Mg/Mn in Usp-Mt and coexisting Ilm-Hem is displaced toward increasing Mg/Mn in ilmenite with passage from ordered ilmenite to disordered hematite. Orthopyroxene and biotite crystallized in experiments with added MgO and MgO + MnO; their X Fe varies with $$\log_{10} f_{{{\text{O}}_{2}}}$$ and T consistent with equilibria among ferrosilite, annite, and ferrite components, and the chemical potentials of SiO2 and orthoclase in the liquid. Experimental equilibration rates increased in the order: Opx

Mr. President, Members and Guests: When I received President Barbara Dutrow’s telephone call in Rome, my first reaction was to wonder what on earth were the Selection Committee thinking? I can name dozens of my contemporaries whose scientific accomplishments easily match mine. Certainly, Barb’s call quite took my mind off the business at hand, which was, of all things, amphibole nomenclature. I can only conclude that my supporters must have written extraordinarily eloquent letters on my behalf, and of course I thank them very much for doing so. I humbly and gratefully accept this award. The list of previous recipients of the Roebling Medal is awesome; one finds oneself in illustrious company. What most impressed a non-scientific friend when she checked the MSA web site was seeing the name Linus Pauling on the list of past recipients. Fortunately, she did not notice that only one of the sixty-nine recipients was a woman. Clearly, it’s time for change. I think all of us here really enjoy the science that we do, and wouldn’t have chosen any other vocation. It affords us tremendous satisfaction, lots of travel, is only occasionally tedious, and in practical terms it earns us a living. Not so many other professions offer all this. Most important is the opportunity our science provides to exercise one’s curiosity, and I suppose doing …

The superimposition of Barrovian-type metamorphism on low-pressure contact aureoles in magmatic arcs is subject to the kinetic constraints of temperature and influx of H 2 O and may be cryptically developed or not at all. The spatial distribution of detectable overprint will reflect these kinetic factors rather than the geometry of the imposed load. Many of the features believed indicative of magma loading, including apparent steep baric gradients, may be produced in this way. On the basis of adding areas of cryptic metamorphism to the area of obvious Barrovian overprint on the northeast side of the Mount Stuart batholith, we conclude that loading 2–3 m.y. after emplacement was probably much more extensive (sheetlike) than believed previously.

The field of ultra-high pressure metamorphism (UHPM) emerged only 12 years ago when microscopic relics of coesite were found in crustal rocks that had been metamorphosed during subduction related to continental collision. Coesite-bearing metamorphic rocks were soon found elsewhere, and then metamorphic microdiamonds were discovered. Suddenly, the pressure axis for orogenic metamorphism had expanded from 1.5–2.0 to 3.0–4.0 GPa!

Progressive metamorphism of serpentinite and allied rocks causes systematic changes in the composition of the chrome-spinel phase, when the latter is in equilibrium with chlorite and two magnesium silicate minerals. The stable spinel in antigorite-serpentinites is Al-poor magnetite, Cr-magnetite, or ferrit-chromite, depending on the local CrFe3+ ratio in the rock. With increasing metamorphic grade up to middle amphibolite facies conditions (talc + olivine, or Ca-poor amphibole + olivine stable) more chromiferous spinels (chromites) are encountered, containing modest amounts of Al. Further increase in grade (enstatite + olivine stable) extends the range of possible spinel compositions to green MgAl2O4-rich spinel. The Al-content is governed by P-, T-, and ƒH2O-sensitive equilibria involving chlorite. MgFe2+ ratios in the spinel are a function of Cr, Al and Fe3+ in the spinel, and the ratio MgFe2+ in coexisting silicates. Solid solution in natural chromites on the magnetite-chromite join is complete at ≈500°C, and close to the join chromite-Al spinel (with variable Fe/Mg) it is complete at ≈700°C. The temperature dependence of KD, the olivine-spinel Fe-Mg partition coefficient, is greater than implied by the jackson (1969) geothermometer. A tentative, revised, graphical calibration is offered, based on microprobe-analyzed high-grade metamorphic pairs and pairs from basaltic pumice. This new plot gives results which are broadly consistent with relative temperatures of equilibration inferred from other geothermometers, for alpine peridotites, peridotite nodules and meteorites.

Epidote (Ps21%) crystallized early as elongate phenocrysts in Late Cretaceous rhyodacitic dikes in the vicinity of Ward, Boulder County, Colorado. Other unusual phenocryst phases are garnet (Gr17–24%) and muscovite. In a xenolith containing kyanite, corundum, biotite, and plagioclase, magmatic garnet grew as a rim around xenocrystic pyrope-rich (Py37%) garnet. The xenolith was derived from a granulite-facies zone, not represented at the present-day erosion surface which is composed of upper amphibolite-facies cordierite and sillimanite-bearing gneisses. The dike magmas were fed not from an immediately underlying batholith but from a magma chamber at a depth corresponding to a pressure of 8–13 kilobars. Phenocrysts cystallized in the temperature range 800 to 700° C, under H2O and O2 activities greater than normal for silicic magmas. This occurrence shows convincingly not only that epidote can be magmatic but that it is a possible early-crystallization phase in silicic magmas.

Garnet lherzolite at Cima di Gagnone has chemical and mineralogical properties similar to those of other garnet lherzolites in the lower Pennine Adula/Cima Lunga Nappe (Alpe Arami, Monte Duria). The Cima di Gagnone occurrence encloses mafic boudins that belong to an eclogite-metarodingite suite common in the numerous neighboring ultramafic lenses. The ultramafic rocks at Cima di Gagnone, including the garnet lherzolite, are interpreted as tectonic fragments of an originally larger lherzolite body that underwent at least partial serpentinization prior to regional metamorphism. This lherzolite body cycled through at least three metamorphic facies: greenschist or blue-schist (as antigorite serpentinite) → eclogite (as garnet lherzolite), pre-Alpine or early Alpine → amphibolite facies (as chlorite-enstatite-tremolite peridotite), Lepontine metamorphism. Relics of titanoclinohumite in the garnet peridotite, as also recorded by Mockel near Alpe Arami, are consistent with this metamorphic history, since they indicate a possible connection with Pennine antigorite serpentinites, e.g., Liguria, Piedmont, Zermatt-Saas, Malenco, Pustertal, all of which have widespread titanoclinohumite belonging to the antigorite paragenesis. Estimated pressures in excess of 20 kbar and temperatures of 800°±50°C for the garnet lherzolite assemblage are not inconsistent with conditions inferred for Gagnone and Arami eclogites. These conditions could have been reached during deep subduction zone metamorphism. It is shown by calculation that the effects of Fe and Cr on the location of the garnet lherzolite/spinel lherzolite phase boundary largely counter-balance each other.

The occurrence of critical assemblages among antigorite, diopside, tremolite, forsterite, talc, calcite, dolomite and magnesite in progressively metamorphosed ophicarbonate rocks, together with experimental data, permits the construction of phase diagrams in terms of the variables P, T, and composition of a binary CO2-H2O fluid. Equilibrium constants are given for the 30 equilibria that describe all relations among the above phases. Ophicalcite, ophidolomite, and ophimagnesite assemblages occupy partially overlapping fields in the \(T - X_{{\text{CO}}_{\text{2}} } \) diagram. The upper temperature limit of ophicalcite rocks lies below that of ophidolomite and ophimagnesite. The fluid phase in ophicarbonate rocks has \(X_{{\text{H}}_{\text{2}} {\text{O}}} > 0.8\), and there are indications that \(P_{{\text{H}}_{\text{2}} {\text{O}}} + P_{{\text{CO}}_{\text{2}} } \) during their progressive metamorphism is approximately equal to Ptotal.

Petrographic evidence is presented for the breakdown of titanian hydroxyl-clinohumite to olivine+magnesian ilmenite (or geikielite)±magnetite in the outermost zone of the Bergell aureole in the Malenco Serpentinite, Prov. Sondrio, Italy. The breakdown coincides in the field with the isograd reaction: antigorite+diopside=olivine+tremolite+H2O. It is therefore concluded that this variety of clinohumite is unstable above approximately 520° C at a pressure of 3 kbars. Elsewhere in the Malenco Serpentinite, titanian hydroxyl-clinohumite may be found to have reacted with CO2 to produce antigorite, magnesian ilmenite and magnesite. Titanian hydroxyl-chondrodite was detected in one sample. Under crustal pressures, the stability field of F-free clinohumite is entirely contained inside that of antigorite. The stable occurrence of titanian clinohumite in high-grade metamorphic ultrabasic rocks may be attributed to the substitution of F for OH.

Outside the Bergell tonalite contact aureole, ophicarbonate rocks consist of blocks of antigorite schist embedded in veins of calcite ± tremolite. An antigorite schistosity predates some of these calcite veins. Mono- and bimineralic assemblages occur in reaction zones associated with the veins. Within the aureole, the ophicarbonate veining becomes less distinct and polymineralic assemblages become more frequent. A regular sequence of isobaric univariant assemblages is found, separated by isograds corresponding to isobaric invariant assemblages. In order of increasing grade the invariant assemblages are: antigorite+diopside+olivine+tremolite+calcite antigorite+dolomite+olivine+tremolite+calcite antigorite+olivine+talc+magnesite antigorite+dolomite+olivine+tremolite+talc These assemblages match a previously derived topology in P-T-XCO2 space for the system CaO-MgO-SiO2-H2O-CO2; the field sequence can be used to adjust the relative locations of calculated invariant points with respect to temperature. Isobaric univariant and invariant assemblages are plotted along a profile map to permit direct comparison with the phase diagram. It is inferred that, during the formation of the ophicarbonate veins, calcite precipitated from fluid introduced into the serpentinite. During contact metamorphism, however, the compositions of pore fluids evolved by reaction in the ophicarbonate rocks were largely buffered by the solid phases. This control occurred on a small scale, because there are local variations in the buffering solid assemblages within a centimeter range.

The emplacement of peridotite into continental crust results in changes in its mineralogical, structural, textural, and bulk chemical properties. Accompanying deformation may partially or completely remove from it the evidence of its original stratigraphy and any envelope of contact alteration it might once have possessed. Such rocks have long been known as alpine-type peridotites (Benson 1926) and their field relationships have been well characterized (Wyllie 1967). With advancing metamorphism, many clues as to the original nature of alpine-type peridotites are therefore effaced, but the loss need not be total if the processes involved are understood. To gain this understanding, we need to examine the inter­ relationships between temperature, pressure, stress, mineral transformations and reactions, textures, and diffusion, etc, in a relatively simple chemical system in the earth's crust. It is not the purpose of this review to study the origin of peridotite; melting and fractionation in the upper mantle; upwelling of peridotite at mid-ocean ridges, at fracture zones, or behind island arcs ; the anatomy or significance of ophiolites; or the crystallization of mafic and ultrabasic magma in the crust. The ample literature on these subjects has been well reviewed elsewhere (Wyllie 1967, 1969, 1970, O'Hara 1968, Miyashiro 1975). Further, the emplacement, tectonic environment, and different basic types of alpine peridotite have been extensively discussed in a number of recent papers (e.g. Coleman 1971a, Dewey & Bird 1971, Jackson & Thayer 1972, Moores & MacGregor 1972, Moores 1973). In his textbook on metamorphism, Miyashiro was obliged to comment ( 1973, p. 30): "In contrast to.other classes of metamorphic rocks, however, no detailed petro­ graphic data are available on the progressive changes to take place [ sic] in this (ultrabasic rock) class." It is hoped that, by gathering and reviewing here some of the recent pertinent literature, this data vacuum may at least partially be filled. This lack of systematic information is in curious contrast to the volume of literature on peridotites exotic to their surroundings-xenoliths in kimberlite, alkali basalt, and nephelinite, alpine "cold-slab" peridotites, serpentinites in melange zones, and samples dredged from the ocean floor. However, there do exist studies of ultra basic