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The role of the oxo-component on the compressibility of amphibole was studied by means of high-pressure in situ single-crystal X-ray diffraction on two natural kaersutite megacrysts (samples DL5 and FR12) from alkaline basalts. The oxo-component varies significantly (1.1 and 1.9 apfu in DL5 and FR12, respectively), whereas the cation composition is very similar, apart from the [Fe.sup.3+]/([Fe.sup.2+][Fe.sup.3+]), which is 0.33 in DL5 and ~1 in FR12. The larger oxo-component of FR12 is attributed to the [Fe.sup.2+] + O[H.sup.-] = [Fe.sup.3+] + [O.sup.2-] + 1/2[H.sub.2] substitution. Unit-cell parameters were collected at different pressures up to about 8 GPa. Structural refinements of both samples were performed with data collected at different P up to 6 GPa. Fitting the P-V data to a third-order Birch Murnaghan EoS yielded the following parameters: [K.sub.0] = 94(1) GPa, K' = 6.3(4), and [V.sub.0] = 903.6(2) [[Angstrom].sup.3] for FR12 and [K.sub.0] = 91(2) GPa, K' = 6.2(4), and [V.sub.0] = 914.1(2) [[Angstrom].sup.3] for DL5. The axial moduli of the two amphibole samples were: [K.sub.0a] = 86(3) GPa, [K'.sub.a] = 7(1), and [a.sub.0] = 9.815(2) [Angstrom]; [K.sub.0b] = 115(3) GPa, [K'.sub.b] = 4.8(8), and [b.sub.0] = 18.012(2) [Angstrom]; [K.sub.0c] = 112(5) GPa, [K'.sub.c] = 7(1), and [c.sub.0] = 5.300(1) [Angstrom], for sample FR12 and [K.sub.0a] = 85(3) GPa, [K'.sub.a] = 5(1), and [a.sub.0] = 9.8660(9) [Angstrom]; [K.sub.0b] = 113(2), [K'.sub.b] = 4.4(6), and [b.sub.0] = 18.0548(6) [Angstrom]; [K.sub.0c] = 107(3) GPa, [K'.sub.c] = 7(1), and [c.sub.0] = 5.3185(5) [Angstrom], for sample DL5. This suggests that the compressibility of kaersutite decreases with increasing oxo-component. Structural refinements show that the polyhedral compressibility follows the order A = M4 > M2 > M3 > M1 for DL5 andA = M4 > M2 > M1 > M3 for FR12. The most evident geometrical effect induced by P is the decrease in the bending of the double tetrahedral chain, when adjacent I-beams are pushed against each other. This effect is largest for DL5, which has a larger concavity of the A site, (O7-O7' changes from 3.03 to 2.82 [Angstrom]) compared to the one of FR12, (O7-O7' changes from 2.92 to 2.79 [Angstrom]). This mechanism is confirmed by the evolution of T1-O7-T1 angle (from 135.4[degrees] to 132.5[degrees] in FR12 and from 136.6[degrees] to 132.2[degrees] in DL5). Keywords: Oxo-amphiboles, kaersutite, compressibility, equation of state, high-pressure structure, amphibole DOI: 10.2138/am.2010.3429

The diverse range of calcic-amphibole compositions in eruptive products from the ca. 330 ka Matahina eruption (ca. 160 [km.sup.3] rhyolitic magma) of the Okataina Volcanic Complex, Taupo Volcanic Zone, including crystal-rich basalt-dacite pumice from post-collapse deposits, reveals several pre- and syn-eruption magmatic processes. (1) Amphibole phenocrysts in the basaltic-andesite and andesite crystallized at the highest pressures and temperatures (P to 0.6 [+ or -] 0.06 GPa and T to 950[degrees]C), equivalent to mid-crustal depths (13-22 km). Inter- and intra-crystalline compositions range from Ti-magnesiohastingsite [right arrow] Ti-tschermakite [right arrow] tschermakite [right arrow] magnesiohornblende and some display gradual decreases in T from core to rim, both consistent with magma differentiation by cooling at depth. (2) The largest amphibole crystals from the basaltic-andesite to andesite display several core to rim increases in T (to 70 [degrees]C), indicating that new, hotter magma periodically fluxed the crystal mush. (3) The dominant population of rhyolite amphibole is small and bladed (magnesiohornblende) and crystallized at low P-T conditions (P = 0.3 GPa, T = 765[degrees]C), equivalent to the eruptive P-T conditions. Dacitic and low-silica rhyolitic amphibole (tschermakite-magnesiohornblende) form two distinct populations, which nucleated at two different T (high: 820[degrees]C and low: 750[degrees]C). These compositional variations, governed primarily by differences in T conditions during crystal growth, record the mixing of two distinct amphibole populations that approached a thermal equilibrium at the eruptive temperature. Therefore, the diversity in amphibole compositions can be reconciled as an exchange of crystals + liquid between the basaltic-andesite to dacite from the mid-crust and rhyolite from the upper crust, which quenched against one another, modifying the dacite to low-silica rhyolite compositions as the eruption progressed. Keywords: Amphibole, crystal growth, crystal mush, cummingtonite, geothermometry, geobarometry

As part of a larger on-going research project, we found amphiboles in soils in Libby, Montana, that were derived from both the former vermiculite mine near Libby and other sources. SEM-EDS spectra of the soil amphiboles from these locations are used to ascertain if the amphiboles came from the vermiculite deposit near Libby, Montana, or some other source. This distinction is possible because amphiboles known to originate from the vermiculite deposit all contain Na and K that can be observed in the EDS spectra. We make this statement with confidence because multiple workers have performed EPMA characterization on over 40 amphiboles collected from the deposit and all are found to contain measureable amounts of Na and K. In this brief communication, we show examples from four locations that contain amphiboles consistent in composition and others that have inconsistent composition when compared to reference amphiboles collected from the vermiculite deposit. Also, we demonstrate the presence of amphiboles consistent in composition with amphiboles known to originate from the deposit in sediment that pre-date mining activity. A full-length publication is in preparation detailing the quantity, distribution, and composition of amphiboles in the Libby Valley, if they are consistent in composition with amphiboles known to originate from the deposit or not, and if they pre- or post-date mining operations. Keywords: Amphibole, amphibole asbestos, Libby, Montana, EPA, SEM-EDS

We present in situ microanalyses of [Fe.sup.3]+[summation]Fe in mantle-derived kaersutites as measured by electron probe microanalysis (EMP) based on the 'self absorption induced FeL[alpha] peak shift' method. The EMP results are not in agreement with bulk (wet chemistry) data. The heterogeneities revealed for some kaersutite megacrysts, when comparing bulk and EMP [Fe.sup.3+]/[summation]Fe results, cannot explain the differences with the EMP measurements. It is thus proposed that any EMP overestimation of [Fe.sup.3+]/[summation]Fe results from a beam-induced dehydrogenation and a subsequent oxidation of [Fe.sup.2+] to [Fe.sup.3+] according to the known relation: [Fe.sup.2+] + O[H.sup.-] = [Fe.sup.3+] + [O.sup.2-] + 1/2 [H.sub.2]. To demonstrate this phenomenon, H losses were measured by secondary ion mass spectrometry (SIMS) after EMP irradiation at different beam currents on two amphiboles with 1.1 and 1.7 wt% [H.sub.2]O, respectively. In both amphiboles, H losses were observed under high beam currents (240 and 100 nA). No dehydrogenation is observed under lower beam currents for the 1.1 wt% [H.sub.2]O amphibole, but still occurs, down to at least 50 nA, for the amphibole with the greatest [H.sub.2]O contents. Amphiboles with low [H.sub.2]O contents (below ~0.5 [H.sup.+]) are less affected by beam damage. For amphiboles with higher [H.sub.2]O contents, the electron beam current density should be reduced with consideration given to the resulting high statistical errors. Keywords: [Fe.sup.3+]/[summation]Fe, EMP and SIMS measurements, amphibole, H loss

This paper reports a single-crystal unpolarized-light FTIR study in the OH-stretching region of a suite of well-characterized Ti-rich pargasites from Lherz (French Pyrenees). All amphiboles studied have fairly constant M-site composition, with [sup.[6]][Al.sub.tot] ~0.55 atoms per formula unit (apfu), [sup.[6]]Ti ~0.45 apfu, and [sup.[6]][Fe.sup.3+] ~0.40 apfu. SIMS and SREF data show all samples to have an O3 anion composition of OH [approximately equal to] [O.sup.2-] [approximately equal to] 1.0 apfu, with negligible F. The FTIR spectra show for all samples a broad absorption consisting of several overlapping bands; three main components can be recognized: ~3710, 3686, and 3660 [cm.sup.-1], respectively, with an asymmetric tail extending to lower frequency. Six Gaussian components can be fitted to the spectra; comparison with spectra of both synthetic and natural pargasites allows five of these components to be assigned to local configurations involving OH-[O.sup.2-] at the O3 site, thus showing that coupling with an [O.sup.2-] anion through an A-cation significantly affects band position. Infrared spectroscopy can detect the presence of [O.sup.2-] in amphiboles in chemically favorable cases, i.e., in the absence of F. Moreover, the FTIR spectra show that all octahedral configurations involving [sup.M1][Ti.sup.4+] or [sup.M1][Fe.sup.3+] [sup.M3][Fe.sup.3+] are associated with [O.sup.2-] at both adjacent O3 sites, and that [sup.M3]Al is locally associated with OH, confirming SRO models based on structure refinement results. Keywords: Ti-rich pargasite, Lherz (French Pyrenees), single-crystal FTIR spectroscopy, anion occupancy

Gabbro and eclogite boudins are preserved within the amphibolites of the composite para- and ortho-gneiss Variscan basement of the Savona Crystalline Massif (Ligurian Brianconnais, Italy). Whole rock trace element patterns, low initial [epsilon]Nd (+5.4 to +8.8) data and trace element analyses on relict igneous clinopyroxene revealed that the mafic rocks were derived from depleted mantle melts, which most likely underwent crustal contamination during emplacement. Gabbros have a cumulus origin controlled by clinopyroxene and plagioclase segregation, whereas the eclogites represent evolved melts. U-Pb and trace element micro-analyses on zircons separated from one amphibolitised gabbro and one eclogite help to constrain coeval ages at ~468 Ma for their igneous protoliths. The occurrence of a few inherited zircons confirms the involvement of a crustal component in the petrogenesis of the mafic rocks. In the eclogite, concordant zircon ages younger than the protolith age testify to metamorphic re-crystallisation (or new growth) from about 420 to 305 Ma. Zircon textures and trace element compositions indicate that eclogite facies metamorphism occurred 392-376 Ma ago. The younger zircon portions yielding a mean Concordia age of 333 [+ or -] 7 Ma are related to equilibration or new growth during the post-eclogite, amphibolite-facies equilibration. Keywords Variscan * Eclogite * Gabbro * Zircon geochronology. LA-ICP-MS

Five expanded vermiculite samples known to have originated from the former vermiculite mine near Libby, Montana were analyzed using powder XRD to quantify their amphibole content. An expanded vermiculite with no detectable amphibole based on our XRD methods discussed herein was spiked with 100, 500, 1000, 2500, 5000, 7500, and 10 000 ppm Libby amphiboles to determine the detection limit of XRD and to develop a calibration method. The 110 amphibole peaks were not detectable in the 100 and 500 ppm samples; thus, defining the detection limit. The 110 peak areas for the remaining samples were measured and used to form a calibration curve relating peak area to ppm amphibole. Of the five Libby samples, each had detectable levels of amphibole and their amphibole content ranged from 1171 to 9218 ppm. The XRD method provides a bulk measurement of the total amphibole content. To determine the amount of asbestiform amphiboles, morphological considerations obtained by microscopy techniques are required. From the results of a previous polarized light microscopy study, the five expanded vermiculites in this study would contain from 390 to 3073 ppm asbestiform amphiboles and not be subject to regulation on the basis of the 1% rule used by regulatory agencies in the U.S.A. Keywords: Amphibole, asbestos, powder X-ray diffraction, vermiculite

An experimental study was carried out to investigate the equilibrium between Fe oxy-component and hydroxy-component in Ti-bearing calcic amphiboles, as described in the dehydrogenation/oxidation reaction [Fe.sup.2] + OH = [Fe.sup.3] + [O.sup.2] + 1/2 [H.sub.2], for which the equilibrium constant (K) can be expressed as K = [f.sub.[H.sub.2]] (28.94) [([X.sub.Fe.sup.3+]].sup.2][([X.sub.[??]]).sup.2] / [([X.sub.[Fe.sup.2+]]).sup.2][([X.sub.OH]).sup.2] . [PHI] = [K.sub.x] . [PHI]. where [??] = H-vacancy on the O3 anion position, is the activity coefficient term, and [K.sub.x] represents the thermodynamic mole fraction term (i.e., the K expressed as mole fractions rather than activities). The variation in [K.sub.x] was quantified experimentally by annealing experiments on amphiboles of two different compositions: a mantle-derived kaersutite from Greenland, and a crustal pargasite from the Tschicoma Formation from the Jemez Mountains, New Mexico, volcanic complex. The conditions of the experiments ranged from 700-1000 [degrees]C, 1-10 kbar, and [f.sub.[H.sub.2]], from that of the HM to GM solid buffer assemblages. The results, combined with similar data for a titanian pargasite from Vulcan's Throne, Arizona (Popp et al. 1995a), define the variation in log [K.sub.x] as a function of T, P, and amphibole composition as given by the equation: log [K.sub.x] = 4.23 - 4380/T(K) + {1.37 * [(Ti + [Al.sub.total]apfu)- 2.49]} + {88/T(K) * [P -1 (kbar)]} If the T, P, and amphibole composition are known, values of log [K.sub.x], calculated from the equation predict the equilibrium log[f.sub.[H.sub.2]], of any experiment to within -0.1 to 0.3 log units. It is assumed that a similar uncertainty in log[f.sub.[H.sub.2] would also to apply to the conditions of formation of natural amphiboles in the same composition range. If log [f.sub.[O.sub.2]] at the time of equilibration can be estimated independently for natural samples (e.g., mantle-derived amphiboles), the [H.sub.2]O activity also can be estimated. An alternate approach for estimating [H.sub.2]O activity from amphibole-bearing mantle rocks is to use a variety of [H.sub.2]O-buffering equilibria among end-member components in olivine, two-pyroxenes, amphibole, and other phases: e.g., 2 tr + 2 fo = 5 en + 4 di + 2 [H.sub.2]O. A self-consistent thermodynamic database (THERMOCALC, Holland and Powell 1990) can be used to determine the [a.sub.[H.sup.O.sub.2]] of such univariant [H.sub.2.O]-buffering equilibria as a function of P and T. A mantle amphibole assemblage from Dish Hill (sample DH101-E, McGuire et al. 1991) was used to calculate [a.sub.[H.sup.O.sub.2]] using the two different methods. The mean value of log [a.sub.[H.sup.O.sub.2] determined from seven different dehydration reactions is -1.70, with a 1[sigma] range of [+ or -] 0.50. That range of water activity is in good agreement with the value of log [a.sub.[H.sup.O.sub.2] = -1.90 [+ or -] 0.3 obtained using the dehydrogenation/oxidation equilibrium, along with an estimate of log [f.sub.[O.sub.2]]. The use of xenolith amphiboles to infer values of [a.sub.[H.sup.O.sub.2]] in the mantle requires that the H content of the amphibole does not change significantly during ascent or eruption. Changes in H content have significantly different effects on the dehydration and dehydrogenation equilibria, such that, comparison of the [a.sub.[H.sup.O.sub.2] estimates from the two different methods may permit quantification of H loss. Keywords: Experimental petrology, amphiboles, redox, fluid phase, activity of [H.sub.2] and [H.sub.2]O, phase equilibria

[1] Geologic mapping and structural analysis of Baltica basement and overlying thrust nappes have yielded a record of the late exhumation history of high-pressure rocks, where strain partitioning has preserved evidence for interpreting the evolution of late structural features. The earliest of these were extensional detachments juxtaposing eclogite-facies rocks against overlying amphibolite-facies rocks that show no evidence for eclogite-facies metamorphism. These early detachments are strongly overprinted and complexly folded, and they represent a phase of upper crustal extension that was active during continued convergence at deeper levels. Younger more localized mylonite zones formed synchronously with tubular, sheath, isoclinal, tight and open folding that shows a progression from WNW to ENE trends. The earliest mylonite zones, interpreted as originally subhorizontal, range in strike through a 20[degrees] angle from 110[degrees] to 90[degrees]. Later steeply dipping mylonite zones, formed under lower amphibolite-facies conditions, strike 75[degrees] and locally truncate earlier structures. The youngest mylonite zones, formed under lowest amphibolite conditions, strike 50[degrees] and truncate all earlier structures. Folds developed during this progression show the range in orientation from WNW to ENE reflected in the orientations of the mylonite zones that is interpreted to represent progressive evolution during top west shearing. These changes in orientation of the late structural features are interpreted to have been caused by changes in boundary conditions related to transtensional deformation during exhumation. L > S fabrics, absence of axial planar foliation, and chaotic orientations of axial surfaces of granulite to amphibolite-facies folds indicate formation in a constrictional strain field. This is also supported by estimates of the finite strain accumulated at ~780[degrees]C and 45 km and similar observations by previous workers. Assuming a simple monoclinic deformation for transtension, strain estimates and structural measurements indicate apparent transtensional angles of 9-11[degrees] that increased to greater than 20[degrees] and then decreased to less than 20[degrees]. These changes in transtensional angle agree with changes in the orientation of the X-Y plane of strain during exhumation from 45 km to less than ~20 km depth and appear to reflect changes in the boundary conditions of deformation. This upper crustal transtension is interpreted to have occurred during continued sinistral oblique convergence and provides a mechanism for syncollisional exhumation of HP and UHP metamorphic rocks. INDEX TERMS: 5475 Planetology: Solid Surface Planets: Tectonics (8149); 8109 Tectonophysics: Continental tectonics--extensional (0905); 8102 Tectonophysics: Continental contractional orogenic belts; 9335 Information Related to Geographic Region: Europe; KEYWORDS: transtension, exhumation, UHP-metamorphism, Western Gneiss Region, Norway, amphibolite, Caledonides. Citation: Terry, M. P., and P. Robinson, Evolution of amphibolite-facies structural features and boundary conditions for deformation during exhumation of high- and ultrahigh-pressure rocks, Nordoyane, Western Gneiss Region, Norway, Tectonics, 22(4), 1036, doi:10.1029/2001TC001349, 2003.

High-resolution transmission-electron microscopy (HRTEM) and infrared spectroscopy (FTIR) analyses have been done on amphiboles near the join tremolite [[Ca.sub.2][Mg.sub.5][Si.sub.8][O.sub.22][(OH).sub.]2 = TR]-pargasite [Na[Ca.sub.2][Mg.sub.4][Al.sub.3][Si.sub.6][O.sub.22] [(OH).sub.2] = PG] in the ternary system tremolite-pargasite-cummingtonite [[Mg.sub.7][Si.sub.][O.sub.22] [(OH).sub.2] = MC] that were synthesized previously by Sharma and Jenkins (1999). Representative samples across the join were examined in detail by HRTEM to document the presence and concentration of chain multiplicity defects (CMFs). There was relatively little change in the defect density with composition, with the tremolitic sample (TREM 23-13) having the highest defect concentration (6%) and the more PG-rich samples having slightly lower CMF concentrations (4-5%). CMFs with multiplicities of 1,3, 4, 5, and 6 were observed, usually as isolated chains, with the most common being triple-chain slabs. Correction of the bulk composition of the tremolitic amphibole for the presence of these Mg-rich, wide-chain defects reduces the MC content from an apparent value of 8.5 to 4-7.5 mol% MC, depending on which composition is used for the triple-chain defect. The entire amphibole join was examined by FTIR spectroscopy in the OH-stretching region (3000-3800 [cm.sup.-1]) for the purpose of determining the presence of short-range order. A total of 10 component bands were fitted to the spectra across the join. These bands were assigned to specific cation configurations on the basis of earlier studies of the FTIR spectra of chemically simplified amphibole joins pertinent to this study. The extent of short-range order was qualitatively determined by comparing the observed intensities for groups of related bands, corrected for differences in their molar absorptivities, to their calculated intensifies based on random-mixing probabilities. From this exercise, it is observed that the intensities of sodic amphibole configurations are consistently high, tremolite is lowest near the middle of the join, and aluminous amphibole configurations cross over from being higher (at low A1 contents) to being lower (at high A1 contents) than expected near the middle of the join. These differences between observed and predicted band intensities may reflect the presence of deviations in the thermodynamic activities of amphibole components from those predicted on the basis of random-mixing models.

Amphiboles were hydrothermally synthesized at 500[degrees]C and 4 kbar in the system [Li.sub.2]O-[Na.sub.2]O-FeO-[Fe.sub.2] [O.sub.3]-Si[O.sub.2]-[H.sub.2]O, with nominal compositions along the riebeckite [[][Na.sub.2][Fe.sup.2+.sub.3][Fe.sup.3+.sub.2]+[Si.sub.8] [O.sub.22] ([OH).sub.2]]-ferri-clinoferroholmquistite [[][Li.sub.2] [Fe.sup.2+.sub.3][Fe.sup.3.sub.2]+[Si.sub.8][O.sub.22]([OH).sub.2]] join, where the exchange vector is Na[Li.sub.-1] at the B-site. Experimental products were characterized by powder XRD and SEM-EDAX, confirming very high amphibole yield along the join (>95%, plus minor quartz). The XRD patterns can be indexed in C2/m, and the refined cell parameters show linear variation as a function of composition. For the [sup.B]Li end-member, the IR spectrum shows a single sharp main band centered at 3614 [cm.sup.-1], which is assigned to the FeFeFe-O[H-.sup.A][] configuration. With increasing [sup.B]Na in the mineral, this band broadens and shifts 4 [cm.sup.-1] to higher frequencies. This effect can be attributed to the change in M4 site occupancy. Minor [sup.A]Na (partial solid-solution toward arfvedsonite) is also observed with increasing sodium in the system. Mossbauer spectroscopy confirms the cation distribution provided by IR data, and shows that a small, but significant amount of [Fe.sup.2+] occurs at M4 along the join. Infrared spectroscopy shows that the Li end-member has a very ordered structure, whereas intermediate compositions show local heterogeneities associated with the presence of two different B sites occupied by Na or Li.

Sodic, calcic and subcalcic amphiboles are expressed by the combination of tremolite, edenite, tschermakite, glaucophane, magesioriebeckite and Fetremolite. 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 [Na.sub.2]O-CaO-MgO-FeO-[Fe.sub.2] [O.sub.3]-[Al.sub.2][O.sub.3]-Si[O.sub.2]-[H.sub.2]O. For deriving total derivatives simply, mole fractions of cations in each site were introduced as the variables of amphibole composition. In obtaining nonideality 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.

A petrogenetic model involving two hydrous minerals is developed in a simple binary system to clarify the nature of melting reactions in presence or absence of a fluid phase. This model shows the changes in shape and location of the solidus in P-T diagrams as a function of the bulk [H.sub.2]O content. The role of divariant equilibria on the beginning and progression of melting is emphasized. In agreement with this model, a generalized scheme for partial melting of metasediments is proposed involving muscovite and biotite as hydrous phases. A P-T projection, a liquidus diagram and its isobaric isothermal sections are combined in a KN(FM)ASH system. Then, CaO is added as a component and it is demonstrated that the anorthite and grossular components in plagioclase and garnet, respectively, play an important role in the melting of biotite + plagioclase with increasing pressure. The reactions involving these components have large [DELTA]V, low dP/dT slopes and generate S-shaped solidi. This study shows remarkable analogies in melting behaviour of metapelites, metagreywackes and metabasalts, including the presence of different hydrous minerals at the wet solidus. At intermediate pressures, muscovite (phengite) coexists with biotite in metapelites, and epidote (zoisite) coexists with amphibole in metabasalts. Both pairs of hydrous phases display analogous features: a single volumetrically important hydrous phase (amphibole or biotite) is stable at low pressures, two main hydrous phases are present at intermediate pressures, whereas a single hydrous phase occurs again at higher pressures (zoisite or phengite).Intermediate compositions such as greywackes or andesites may have both pairs of hydrous phases. In most cases, the beginning of fluid-absent melting at pressures above approximately 10 kbar can be attributed to reactions involving muscovite/phengite or epidote/zoisite, two minerals that melt and disappear 150 to 200 [degrees]C lower than biotite or amphibole alone, respectively. Rocks that underwent high-pressure metamorphism at moderate temperatures are excellent illustrations of the progressive replacement of a hydrous phase (e.g., biotite) by others (phengite + zoisite), and of the major role of anorthite and grossular components in plagioclase and garnet for phase changes and location of the solidus.

The Lherz orogenic lherzolite massif (Eastern French Pyrenees) displays one of the best exposures of subcontinental lithospheric mantle containing veins of amphibole pyroxenites and hornblendites. A reappraisal of the petrogenesis of these rocks has been attempted from a comprehensive study of their mutual structural relationships, their petrography and their mineral compositions. Amphibole pyroxenites comprise clinopyroxene, orthopyroxene and spinel as early cumulus phases, with garnet and late-magmatic [K.sub.2]O-poor pargasite replacing clinopyroxene, and subsolidus exsolution products (olivine, spinel II, garnet II, plagioclase). The original magmatic mineralogy and rock compositions were partly obscured by late-intrusive hornblendites and over a few centimetres by vein-wallrock exchange reactions which continued down to subsolidus temperatures for Mg-Fe. Thermobarometric data and liquidus parageneses indicate that amphibole pyroxenites started to crystallize at P [greater than or equal to] 13 kbar and recrystallized at P < 12 kbar. The high [Al.sup.VI]/[Al.sup.IV] ratio (> 1) of clinopyroxenes, the early precipitation of orthopyroxene and the late-magmatic amphibole are arguments for parental melts richer in silica but poorer in water than alkali basalts. Their modelled major element compositions are similar to transitional alkali basalt with about 1-3 wt% [H.sub.2]O. In contrast to amphibole pyroxenites, hornblendites only show kaersutite as liquidus phase, and phlogopite as intercumulus phase. They are interpreted as crystalline segregates from primary basanitic magmas (mg = 0.6; 4-6 wt% [H.sub.2]O). These latter cannot be related to the parental liquids of amphibole pyroxenites by a fractional crystallization process. Rather, basanitic liquids mostly reused pre-existing pyroxenite vein conduits at a higher structural level (P [less than or equal to] 10 kbar). A continuous process of redox melting and/or alkali melt/peridotite interaction in a veined lithospheric mantle is proposed to account for the origin of the Lherz hydrous veins. The transitional basalt composition is interpreted in terms of extensive dissolution of olivine and orthopyroxene from wallrock peridotite by alkaline melts produced at the mechanical boundary layer/thermal boundary layer transition (about 45-50 km deep). Continuous fluid ingress allowed remelting of the deeper veined mantle to produce the basanitic, strongly volatiles enriched, melts that precipitated hornblendites. A similar model could be valid for the few orthopyroxene-rich hydrous pyroxenites described in basalt-hosted mantle xenoliths.

Using the elastic-strain theory, researchers discuss how the idea that amphibole structures are partly composed of rare earth elements (REE) unintentionally bias melt partitioning models. Methods of creating accurate predictive models for mineral/melt partitioning are analyzed.

Calcic amphiboles were synthesized from a natural mid-ocean ridge basalt (MORB) in 39 experiments representing 24 sets of pressure-temperature (P-T) conditions ranging from 650-950 [degrees] C, 0.8-2.2 GPa, at [f.sub.[o.sub.2]] controlled by the fayalite-magnetite-quartz (FMQ) buffer, and [P.sub.aqueous fluid] = [P.sub.total]. Experiments lasted up to 1630 h at low temperatures; in all cases, synthesized hornblendes were coarse-grained (5-7 x 10-15 [[micro]meter]) and chemically homogeneous. Over the investigated pressure range, Ca-amphibole coexisting with phases rich in Al and Ti gradually changes composition from sodic-calcic, Si-rich at low temperatures to calcic, Si-poor at high temperatures: it is barroisite at 650 [degrees] C, edenite at 700 [degrees] C, and pargasite at 800-950 [degrees] C. Electron microprobe data were combined with 41 comparable analyses from the literature for Ca-amphiboles synthesized from MORBs at intermediate [f.sub.[o.sub.2]] in order to erect a petrogenetic grid for the experimental range 0.0-2.2 GPa, 450-1050 [degrees] C. Isopleths for [Al.sub.2][O.sub.3] in Ca-amphibole exhibit markedly negative P-T slopes, indicating increasing [Al.sub.2][O.sub.3] contents with both P and T. In contrast, Ti[O.sub.2] isopleths are nearly independent of P, demonstrating that Ti[O.sub.2] in Ca-amphibole increases almost exclusively as a function of T. For natural metabasaltic assemblages that contain coexisting Al-rich and Ti-rich phases, and closely approached chemical equilibrium under crustal or uppermost mantle conditions, this semiquantitative petrogenetic grid allows the simultaneous assignment of attendant P and T employing Ca-amphibole [Al.sub.2][O.sub.3] and Ti[O.sub.2] contents. However, during slow cooling, natural Ca-amphiboles may exsolve Ti[O.sub.2] as rutile, titanite, and/or ilmenite, but in general do not redistribute [Al.sub.2][O.sub.3], so this thermobarometer must be applied with caution to inhomogeneous specimens.

Research shows that slowly cooled high-temperature metamorphic and plutonic rocks fail to retain equilibrium oxygen isotope fractionations. A new approach uses models of retrograde diffusion exchange and prograde reaction histories to retrieve equilibrium isotope fractionations from natural rocks. New analyses from 32 natural amphibolite-facies schists are used to demonstrate the viability and limitations of the proposed approach.

Analysis of ungarettiite, a new anhydrous amphibole from the Hoskins mine Grenfell, New South Wales, Australia, by electron microprobe reveals the composition of ungarettiite as Na,Na2(Mn(sub 2)(super 2+)(Mn(sub 3)(super 3+))Si8O22O2. X-ray crystallography of the sample reveals that majority of M1, M2 and M3 sites are occupied by Mn. Bond valence analysis reveals that a divalent anion O2- occupies the O3 site and this is consistent with the electroneutrality of the structural formula.

Basaltic systems are investigated to determine the phase composition and phase relationship through the transformation from feldspar-amphibolite to eclogite. The amphibole crystal-chemistry is used as a monitor of P-T conditions. The study indicates that, at 650 degrees centigrade, the assemblage consisting of amphiboles, plagioclase, epidote and quartz is stable at 0.8 giga pascals. The omphacite substitutes the albite at 1.6 giga pascals to form the composite amphibole, sodium-clinopyroxene, garnet, epidote and quartz. At 2.2 giga pascals, paragonite crystallizes as a minor phase on the albite seeds.

Natural tonalite is investigated to determine the phase relations in the crystals in isothermal section at 650 degrees centigrade under water saturated environments. The pressures varied from 2.5 to 26 kilobars and the oxygen concentration was bufferd to NNO. The phases in the initiating substance were hornblende, biotite, epidote, quartz, orthoclase and plagioclase. The addition of seeds of amphibole with aluminum yielded the amphibole equilibrium and the exchange of vectors caused the various amphibole compositions.

Research describing infiltration of fluid compositions in amphibolite-facies scapolitic calc-silicates is presented. The study focuses on metasediments found in the Mary Kathleen Fold Belt, Australia. These metasediments were characterized by minimal seepage of external fluids in the process of regional metamorphism. It is shown that calc-silicate layers prevented the infiltration of fluids in the H2O-CO2-NaCl-HCl system.

New analyses of rare earth elements (REEs) and abundances of K, Rb, Sr, Ba, Th, U, Ta, and Hf in peridotites from Zabargad Island, Red Sea, indicate that light REE-enriched amphibole peridotites in the northern and central bodies have undergone a complex, multistage, metasomatic history during Red Sea rifting. The metasomatism is partly attributed to the passage of alkalic basalts through the mantle lithosphere, which possibly occurred during the early, continental-rift history of the Red Sea. The metasomatism also results from the interaction of the uppermost lithosphere with hydrous crustal fluids released from thinning continental crust. The last metasomatic event is associated with the development of an oceanic hydrothermal system in the Red Sea, after the continental breakup. The available data also suggest that the Zabargad peridotites represent pieces of pan-African lithospheric mantle (the northern and central bodies) pierced by asthenospheric material (the southern body).

Field, geochronological, and rare earth element (REE) evidence obtained from the Fournier oceanic fragment of the Canadian Appalachians indicates that its plagiogranite component was generated by the anatexis of amphibolite and not by the fractionation of basic magma. We propose that the process occurred in two stages: first, gabbro was plastically deformed at high temperature to form dry, low-angle shear zones that subsequently evolved to amphibolite via the addition of light REE-enriched hydrothermal solutions; second, the amphibolite underwent partial melting during shear to yield a migmatite comprising bands of plagiogranite alternating with amphibolite restite. The plagiogranite locally coalesced to form pods, dikes, and lenses that injected the surrounding undeformed gabbro. We attribute the development of the Fournier plagiogranite to dynamothermal processes occurring in proximity to a spreading center due to asthenosphere-induced shear within ocean layer 3. This serves as an important illustration of the dynamic nature of metamorphism and melting that can occur in the ocean crust.

Constraints on the calculation and use of mineral/melt and two-mineral partition coefficients for Ti (D.sup.Ti) have been derived from current knowledge of the distinct crystal-chemical mechanisms for the incorporation of [Ti.sup.n+] in the amphibole structure as follows: (1) mineral/melt partition coefficients for Ti, and other tetravalent high field-strength elements (HFSE), can be compared only when considering the fraction of [Ti.sup.4+] that enters the same structural site; (2) accurate two-mineral partition coefficients can be obtained only when considering the fraction of [Ti.sup.4+] that is involved in the same crystal-chemical mechanism in the two relevant phases (i.e., [sup.M2][Ti.sup.4+] and [sup.M1][Ti.sup.4+] for amphibole and clinopyroxene, respectively). The complete crystal-chemical characterization of synthetic titanian pargasite and kaersutite and of synthetic richterite (all crystallized under P, T, X, [fo.sub.2] conditions of interest for upper-mantle studies) shows that the site preference of Zr and Hf differs between the two amphibole compositions; these elements are essentially ordered at M2 in pargasite and kaersutite, but preferentially enter M1 in richterite. In the latter case, Ti segregates into the split M1' site with distorted coordination and shorter Ti-O3 distances, whereas Zr and Hf most likely prefer the larger and more regular M 1 site. The observed site preference is strongly controlled by the relative dimensions of the available sites. The crystal-chemical mechanisms that govern the incorporation of octahedral high-charge cations are the local charge balance of [sup.IV]Al (by [R.sup.3,4+] at M2) and of dehydrogenation (by [R.sup.3.4+] at M 1); thus the incorporation of Zr and Hf depends on distinct intensive parameters in the two amphibole compositions. Calculation of partition coefficients and of elastic-site parameters under the assumption that all Ti and other [HFSE.sup.4+] order at the M2 site in amphibole, as is currently done in geochemical studies, is strongly biased. In the presence of significant dehydrogenation, amphibole/melt [D.sub.0] values obtained from modeling based on the elastic-strain theory starting from the more-accurate site populations for Ti may be only 1/4 of those obtained by using the total Ti content, and the derived site parameters E and [r.sub.0] are more consistent with octahedral coordination. This result has important consequences for the prediction of D values under P-T conditions different from those of the experimental work. Applying the above concepts to data from natural assemblages, we obtained a significantly narrower (0.3-2.4 vs. 1.5-8.9) and more reasonable range of variation for amphibole/clinopyroxene [D.sub.Ti]. A relationship between these values for [D.sub.Ti] and pressure is also now apparent.

Obertiite is a new amphibole species from Bellerberg, Laccher See district, Eifel, Germany. It occurs with tridymite, fluororichterite, hematite, futile, aegirine-augite, kinoshitalite, and fluor-apatite in vugs in volcanic rocks, and crystallized from late-stage hydrothermal fluids associated with recent volcanism. Obertiite occurs as pale-pink elongated blades and divergent aggregates. It is brittle, H = 5, [D.sub.calc] = 3.16 g/[cm.sup.3], has a colorless streak, vitreous luster, and does not fluoresce; it has perfect cleavage on {110} and conchoidal fracture. In plane-polarized light, obertiite is slightly pleochroic in shades of pink to red-orange; X ^ a = 2 [degrees] (in [Beta] obtuse), Z = b, Y ^ c = 12 [degrees] (in [Beta] obtuse) with absorption X ~ Y ~ Z. It is biaxial negative, [Alpha] = 1.643(1), [Beta] =1.657(1), [Gamma] = 1.670(3), 2[V.sub.x] = 81(1) [degrees], no dispersion visible. Obertiite is monoclinic, space group C2/m, a = 9.776(2), b = 17.919(3), c = 5.292(1) [Angstrom], [Beta] = 104.05(2) [degrees], V = 899.3(3) [[Angstrom].sup.3], Z = 2. The strongest ten X-ray diffraction lines in the powder pattern are [d(I,hkl)]: 8.414(10,110), 2.705(7,331,151), 3.390(6,131), 4.467(5,040), 3.117(5,310), 2.531(5,202), 3.255(3,240), 2.577(3,061), 2.163(3,171,261), 4.013(2,111). Analysis by a combination of electron microprobe, SIMS, and crystal-structure refinement gives Si[O.sub.2] 54.53, [Al.sub.2][O.sub.3] 0.15, Ti[O.sub.2] 7.75, [Fe.sub.2][O.sub.3] 2.61, [Mn.sub.2][O.sub.3] 3.27, FeO 3.36, ZnO 0.08, MgO 14.13, [Li.sub.2]O 0.05, CaO 0.52, [Na.sub.2]O 9.51, [K.sub.2]O 0.98, F 0.55, [H.sub.2]O 0.20, O [equivalent] F -0.23, Ni, Cr, V, Cl not detected, sum 97.46 wt%. The formula unit, calculated on the basis of 24(O,OH,F) is ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], and is close to the ideal end-member composition Na [Na.sub.2] ([Mg.sub.3][Fe.sup.3+][Ti.sup.4+]) [Si.sub.8] [O.sub.22] [O.sub.2]. The crystal structure of obertiite was refined to an R index of 2.6% using MoK[Alpha] X-ray intensity data. The M1 site is split into two subsites along the b axis, M1 and M1A; the M1 site is occupied by Mg, and M1A is occupied predominantly by [Ti.sup.4+] and [Mn.sup.3+]; M2 is occupied by Mg, [Fe.sup.2+], and [Fe.sup.3+], and M3 is occupied by Mg. Local bond-valence considerations suggest that [O.sup.2-] at O3 is linked to [Ti.sup.4+]Mg or [Mn.sup.3+][Mn.sup.3+] at the adjacent M1 and/or M1A sites, and that OH or F at 03 is linked to MgMg at the adjacent M1 sites.