Showing papers in "European Journal of Mineralogy in 1997"

Nomenclature of amphiboles Report of the Subcommittee on Amphiboles of the International Mineralogical Association Commission on New Minerals and Mineral Names;Nomenclature of amphiboles Report of the Subcommittee on Amphiboles of the International Mineralogical Association Commission on New Minerals and Mineral Names

Abstract: The International Mineralogical Association's approved amphibole nomenclature has been revised to simplify it, make it more consistent with divisions generally at 50%, define prefixes and modifiers more precisely, and include new amphibole species discovered and named since 1978, when the previous scheme was approved. The same reference axes form the basis of the new scheme and most names are little changed, but compound species names like tremolitic hornblende (now magnesiohornblende) are abolished, as are crossite (now glaucophane or ferroglaucophane or magnesioriebeckite or riebeckite), tirodite (now manganocummingtonite), and dannemorite (now manganogrunerite). The 50% rule has been broken only to retain tremolite and actinolite as in the 1978 scheme; the sodic-calcic amphibole range has therefore been expanded. Alkali amphiboles are now sodic amphiboles. The use of hyphens is defined. New amphibole names approved since 1978 include nyboite, leakeite, kornite, ungarettiite, sadanagaite, and cannilloite. All abandoned names are listed. The formulae and source of the amphibole end-member names are listed and procedures outlined to calculate Fe (super 3+) and Fe (super 2+) where not determined by analysis.

Pressure-induced amorphization of minerals; a review

Abstract: A variety of minerals transform to an amorphous material when statically compressed to a few tens of GPa. With quartz and the quartz-type forms of GeO2 and A1PO4 as examples, pressure-induced amorphization is first described with reference to differential stresses, crystalline transformations, compression mechanisms and shearing processes. Less comprehensive information is available for the amorphization of other minerals, but similar features are nevertheless observed for framework silicates, pyroxenes, olivines and hydrous silicates. The differences and similarities between amorphous substances obtained by cooling of liquids, static compression or decompression of crystals, moderate heating of high-pressure minerals at 1 atm, and shock waves are then reviewed. Finally, the thermodynamics of amorphization is discussed and the mechanistic interpretations of amorphization are presented, with special reference to elastic and dynamic instabilities and shearing processes. Key-words: amorphization, high-pressure transformations, densified glasses, diaplectic glasses.

Glaucophane-bearing eclogites in the Tso Morari dome (eastern Ladakh, NW Himalaya)

Abstract: Lenses of glaucophane-bearing eclogites have been discovered within the Cambro-Ordovician metagranitic basement and in the Permian metasedimentary cover of the Tso Morari dome (eastern Ladakh, India). Petrological and thermobarometrical evidence shows that these rocks underwent a high-pressure event (P> 16 kbar, probably near 20±3 kbar) associated with relatively low-temperature conditions (T = 580±60 °C). The beginning of decompression was isothermal down to l 1 ± 2 kbar. Subsequently, temperature increased to 610±70°C under amphibolite-facies conditions. This record of high-pressure and relatively low-temperature metamorphism in the Himalayan belt implies that an early subduction of the whole NW part of the Indian continental margin to a minimum depth of 70+10 km occurred during the India-Asia convergence. Two different units can be distinguished in the Higher Himalaya of Ladakh-Zanskar, recording two successive and distinct orogenic events: (i) the North Himalayan Massif related to the continental subduction and (ii) the High Himalayan Crystalline slab related to the intracontinental subduction.

Reversal of the orthoferrosilite-high-P clinoferrosilite transition; a phase diagram for FeSiO 3 and implications for the mineralogy of the Earth's upper mantle

Abstract: The reconstructive phase transitIOn between orthoferrosilite (OFS) and high-pressure C2/e c1inoferrosilite (HCFS) has been reversed for the FeSi03 composition over the pressure range of 5.0 to 7.5 OPa in a multi-anvil press. The position of the boundary is best fit by: P (OPa) = 0.0045 7(11) T (K) 0.084(153) From this result and ß V298K = -0.90(4) cm3 mol-I from X-ray diffraction, we obtain M298K = -4.11 (2 1) J mol-I K-1 and ßH298K = -76(138) J mol-I for the OFS�HCFS transformation. The OFS-HCFS-LCFS (Iow-P clinoferrosilite) tripie point is located at 820(lO)OC and 4.9(2) OPa from the intersection of the OFS-HCFS boundary with the LCFS-OFS boundary previously reversed by Lindsley (1965). The topology of the FeSi03 phase diagram is, therefore, the same as that for MgSi03, but with the orthorhombic to high-P monoclinic and low-P monoclinic to high-P monoclinic structural transformations occurring at lower pressures. When combined with previous work on the MgSi03 end-member, our results suggest that high-P c1ino­ pyroxene is a stable phase in a pyrolite mantle below = 300 km. However, its occurrence is limited by the progressive dissolution of pyroxene into gamet at higher pressures. High-P C2/e c1inopyroxene should be an even more important phase in the deeper portions of subduction zones where a cooler thermal regime will stabilise this phase at shallower depths. The orthorhombic high-P monoclinic transition in Ca-poor pyroxene, producing a = 3 % density increase, provides a reasonable petrologic explanation for the \"X seismic discon­ tinuity\" (Revenaugh & Jordan, 1991), which lies at a depth of\", 300 km. Key-words: ferrosilite, c1inoferrosilite, phase equilibrium, X-discontinuity, mantle mineralogy.

Ferro- and magnesiocarpholite assemblages as record of high-P, low-T metamorphism in the central Alpujarrides, Betic Cordillera (SE Spain)

Abstract: Ferroand magnesiocarpholite are widespread in the Alpujarride Complex of the Betic Cordillera, in syn-metamorphic quartz veins within highly aluminous Permo-Triassic metapelites. These minerals occur in pyrophyllite-chlorite-, pyrophyllite-chlorite-chloritoid-, kyanite-chloriteand kyanite-chloritoid-chlorite-bear­ ing assemblages. Aragonite occurs also in the ferrocarpholite-pyrophyllite-chlorite-chloritoid assemblages. These assemblages record Alpine (pre-Miocene) prograde metamorphic evolution towards high-pressure, low­ temperature conditions of the blueschist facies. The P-T formation conditions, ca1culated in the reduced FMASH system using a thermodynamical data base and actual compositions of chloritoid, carpholite and chlorite, range from 7 kbar, 300-350°C up to 11 kbar and 400-450°C. These values place important con­ straints on the metamorphic conditions in the Alpujarride Complex, which approach those observed in the un­ derlying Nevado-Filabride Complex. Key-words: ferrocarpholite, magnesiocarpholite, kyanite, high-pressure metamorphism, Betic Chain, Spain.

First evidence for Lower Cretaceous HP/HT-metamorphism in the eastern Rhodope, North Aegean region, North-east Greece

Abstract: In the Upper Unit of the Eastern Rhodope, North Aegean region, a Sm-Nd whole-rock, garnet and clinopyroxene isochron of a garnet-pyroxenite (metagabbro), corresponding to an age of 119.0 ± 3.5 Ma, gives first evidence of a Lower Cretaceous HP-event in this area. The garnet-pyroxenites are characterised by the assemblage hornblende-spine! ± olivine-garnet-clinopyroxene and are part of a metaperidotite complex. This com­ plex experienced HP/HT-metamorphism, together with the hosting metapelites and orthogneisses. In all rock types, this was followed by near isothermal decompression during exhumation of the metamorphie rocks. In contrast to the situation in the host rocks, HP-mineral assemblages of the investigated sampie were not aftected by plastic deformation and retrogression during decompression. It is thus assumed that the isotopic equilibrium attained during HPIHT-metamorphism was not disturbed during later events. In addition, Rb-Sr mica ages indicate differ­ ential uplift and different minimum ages of Alpine HP-metamorphism in quartzofeldspathic rocks in different tectonic units of the Eastem Rhodope. Key-words: geochronology, Sm-Nd, HP/HT-metamorphism, garnet-clinopyroxenite, Rhodope Zone. Geological setting In northern Greece (northeastern Mediter­ ranean Alpine Belt) HP-metamorphic rocks occur in different structural settings. In NW Greece, HP-metamorphic rocks located along the fossil plate boundary between the Adriatic plate and Eurasia show a low dT/dP ratio. Ages of blues­ chist-facies metamorphism generally cluster within the Eocene (Schermer et af., 1990, and I) e-mail: wawrzen@nwz.uni-muenster.de 001:10.1127/ejm/9/3/0659 references therein). To the NE of this suture in NE Greece, an assemblage of fault-bounded metamorphic complexes (Fig. 1) contains pre­ Alpine continental crust, intensely reworked during Alpine metamorphism by plastic deforma­ tion. In this area, Alpine metamorphism also in­ cluded HP-stages. However, HP-metamorphism and subsequent decompression occurred at gen er­ ally higher temperatures, and in quartzo­ feldspathic rocks HP-mineral assemblages are 0935-1221/97/0009-0659 $ 1.50 © 1997 E. Schweizerbart'sche Verlagsbuchhandlung. D-70176 Stuttgart

Sublimate deposition from hydrothermal fluids at the Fossa Crater, Vulcano, Italy

Abstract: Sublimates were sampled at the Fossa crater of VuIcano Island (Sicily, Italy) in June and October 199 1. Sampling was carried out by means of silica tubes inserted as deep as possible into those fumarolic vents around which lead and bismuth sulphides and sulphosalts occurred. The main mineralogical phases found within the silica tubes are native sulphur, ammonium tetrafluoroborate, lead and bismuth chlorides, oxychlorides, sulphochlorides, sulphides and sulphosalts. The lead and bismuth mineral assemblage is interpreted as the result of the transport of lead and bismuth as volatile chlorides. Microtextural and microanalytical studies (SEM + EDS) on chloro-sulphides and sulphosalts suggest that the latter were deposited from metastable chlorosulphides through reactions involving gaseous H2S. The absence of chlorosulphides on the ground confirms their metastability. The lack of sulphates in the mineralogical association sampled within the silica tubes is due to the reducing conditions of voIcanic fluids. Key-words: sublimates, sulphosalts, sulpho-chlorides, VuIcano Island.

Thickness of pericline twin walls in anorthoclase: an X-ray diffraction study

Abstract: The thickness of peric\\ine twin walls in disordered feldspar Ab6s0r30Ans has been determined by quantitative measurements of the diffuse X-ray diffraction caused by these walls. The diffuse diffraction appears as streaking between pairs of twin-related Bragg peaks. Comparison of the predictions of simple model calculations with the observed intensity profiles indicates that these walls are'\" 25 A thick at room temperature. On heating, the wall thickness increases, following the predictions of Ginzburg-Landau theory: Woe ITc _ TI-V2. Key-words: alkali feldspar, domain walls, Landau theory, X-ray diffraction.

Investigation of trace-element distribution in detrital quartz by Electron Paramagnetic Resonance (EPR)

Abstract: Paramagnetic defects in natural, irradiated (2 kGy, 60CO, 295 K) and etched (40% HF, 30 min, 295 K) sampies of quartz grains (grain size < 0.16 mm) from quartz-sand deposits in Weferlingen, Haltern and Frechen (Germany) were investigated by EPR at 9.5 GHz and at 20, 70, and 295 K. In all sampies [AI04jO, [TiOJLi+jO, [Ti041H+jO, [Ti04j-, and Fe3+ paramagnetic centres were detected. Generally, y-irradiation caused a transformation of diamagnetic precursor centres into these paramagnetic centres, resulting in an increase of the EPR intensity of up to five times. Sampies from Weferlingen have similar trace-element contents and show only minor differences in the abundance of paramagnetic centres. In contrast, the abundance of paramagnetic centres in quartz from the different deposits may differ significantly, especially Fe3+, [AI04jO and [Ti04/Li+jO centres in the y-irradiated quartz sampIes. The high concentrations of [AI04jO and [Ti04/Li+jO centres in the Weferlingen quartz sampies reflect higher amounts of igneous quartz than in the other deposits investigated. These differences in quartz composition were confirmed by cathodoluminescence microscopy. Etching of quartz sampIes with hydrofluoric acid (ca. 30% loss of mass) followed by further EPR measurement revealed that AI centres generally show a more or less homogeneous distribution pattern, whereas no Fe centres were left in quartz after etching. This indicates that Fe centres are concentrated in the outer rim of the quartz grains. Key-words: quartz, trace-elements, EPR, cathodoluminescence, cIastic sediments.

Xenoliths evidence for a refractory oceanic mantle percolated by basaltic melts beneath the Kerguelen Archipelago

Abstract: Alkali basalts from the Kerguelen islands have entrained many mantle peridotites (harzburgites and dunites) in addition to various other ultramafic and mafic xenoliths, The harzburgites and the dunites were equilibrated in the spinel peridotite stability field (T = 8501 150 °C), They attest to the existence 01' a metasomatized refractory upper mantle beneath the southeastern province of the Kerguelen islands, To date no fertile mantle Iherzolite has been found in this area. The harzburgites can be divided into protogranular Cr-diopside-bearing harzburgites and poikilitic harz­ burgites which contain an interstitial magnesian-augite. They bear the imprint 01' two main processes: (I) high degree 01' partial melting, and (2) interactions with basaltic magmas. The partial melting event is weil estab­ lished by the high refractory indices 01' both whole rocks and minerals (Fo up to 92, Cr-rich spinei, high MgO and low CaO, Ab03, Na20 and total Fe203 contents in bulk-rock analyses). Evidence of the interaction with basaltic melts is apparent in the development of coarse granular to poikilitic textures, U-shaped REE patterns in both types of harzburgites and in the presence 01' Cr-Na-rich magnesian-augite sometimes associated with phlogopite in the poikilitic harzburgites. Moreover, the mg* numbers of minerals and bulk rocks in the poikilitic harzburgites and dunites are systematically lower than those predicted by partial-melting models. These characteristics suggest an interaction between previously depleted harzburgites and LREE-enriched alkali basaItic melts. The dunites are the wall-rocks of magmatic dykes of websterites, clinopyroxenites and hornblendites. They show the same textural (coarse granular to poikilitic textures), chemical (Iow mg* number of mafic phases) and REE patterns as the harzburgites. All these features are consistent with a model whereby the dunites were formed by interactions between formerly depleted harzburgites and basaltic melts of transitional to alkaline affinities propagating within open cracks. Key-words: mantle xenoliths, harzburgites, dunites, partial melting, mantle/melt reactions, Kerguelen islands. 001:1 0.1127/ejm/9/5/1 085 0935-122 1/97/00091085 $ 4.00 (l) 1997 E. Sch\\l..eiLerbart'schc Verlagsbuchhandlung. 0-70176 Stuttgarl

P-V-T measurements of iron silicide (epsilon -FeSi) implications for silicate-metal interactions in the early Earth

Abstract: We have measured the high-pressure high-temperature equation of state of iron silicide (E-FeSi) by synchrotron X-ray diffraction, in-situ in a multi-anvil apparatus. Analysis of the non-hydrostatic stress-free data yielded the following thermoelastic parameters: Vo = 90.39(4) A3, Ko = 172(3) GPa, (dK/dT)p = -0.043(8) GPaK-I, a = 5.1(4) 10-5 K-I with fixed K'o = 4. Discrepancies with previous ultrasonic measurements could be explained by highly anharmonic effects in this compound. These equation-of-state parameters were included in a thermodynamic model of the reaction between Fe and SiOz, showing that the reduction of the silicates by iron presents a maximum at pressures between 8 and 15 GPa, and could be efficient at high temperatures, depending on the values of the activity coefficient of silicon in Fe-Si alloys. Similar reduction reactions in the Earth's complex system would have profound implications for the compositions of mantle and core, as weil as for reactions at the core-mantle boundary. Key-words: high-press ure experiments, equations of state, XRD data, early Earth.

Raman spectra of various types of tourmaline

Abstract: A large number of different tourrnalines are investigated by Raman spectroseopy in the speetral range 150-1550 ern-I. Aeeording to their ehemical eomposition, the studied tourrnalines ean be classified into three main groups: buergerite-sehorl, GI; elbaite-type, G2; and dravite-buergerite-uvite, G3. It is shown that the same classification of tourrnalines ean be established on the basis of their Rarnan spectra. Bands for Glare eentred at about 230 and 670 ern-I, the speetra being eharaeterized by a single peak at 238 ± 2 ern-I and three resolved peaks at 635±3, 674±3 and 697 ±3 ern-I; G2 has a sharp peak at 224±2 and two weH separated peaks, one at 638±3 ern-I and the other higher than 707 ern-I; G3 is eharaeterized by two peaks at 215±3 and 237±3 ern-I, with a srnoothed speetral band eentred at about 670 ern-I. Key-words: tourmaline, Rarnan speetroseopy.

Chemical composition of metapelitic chlorites; X-ray diffraction and optical property approach

Abstract: The crystallographic parameters, intensity relationships of basal ret1ections, and optical properties of chlorites from the low-grade metamorphic metapelites of the Alpujarride Complex (Betic Cordillera, SE Spain) have been compared with EMPA data obtained from areas selected from backscattered electron images. The Rausell-Colom et al. (1991) formula gives the best results for the calculation of Al'v from doO!, corrected according to the Fe content, which may be obtained from the new equation bo = 9.2216 + 0.0293 (Fe+Mn). This relationship gives intermediate results between those obtained by the Brindley (1961) and Von Engelhardt (1942) equations . Poor results are obtained by intensity relationship of basal reflection methods, due to uncertainty in the conversion of intensities to structure factors. Sign of elongation and anomalous interference colours depend exelusively on the Fe/(Fe+Mg) ratio and are independent of the Al'v content. The joint use of these two optical properties allows the elassification of chlorite and the recognition of coarse compositional differences between various chlorites of the same thin section. The proposed methods also allow two of the three mean compositional vectors of chlorite defined by Laird (1988) to be determined. Dioctahedral substitution has been found to be low and fairly constant but significant, with an average value of Loct = 5.9 aJ.u. and a standard deviation of 0.03. Key-words: bo parameter, doOI basal spacing, intensity relationships of basal reflections, electron microprobe, chlorite, Alpujarride Complex.

High-temperature order-disorder in (Fe0.5Mn0.5)2SiO4 and (Mg0.5Mn0.5)2SiO4 olivines: an in situ neutron diffraction study

Abstract: Time-of-flight neutron powder diffraction has been used to determine the time-temperature dependence of M-site occupancies and crystal structures of MgMnSiO4 and FeMnSiO4 olivines between room temperature and 1000°C. In each sample Mn shows a preference for the M2 site at low temperature, but becomes increasingly disordered between M1 and M2 on increasing temperature. The equilibrium non-convergent disordering behaviour is described by a Landau expression for the free-energy change due to ordering. The kinetics of non-convergent ordering of the M-site cations in these samples have been analysed using a Ginzburg-Landau model, giving an activation energy for Fe-Mn exchange between M1 and M2 of 193 ± 3 kJ/mol and for Mg-Mn exchange of 172 ± 3 kJ/mol. The M-site occupancy at room temperature is shown to be a function of the cooling rate of the crystal, indicating the possibility that (Mg,Fe)2SiO4 olivine might be useful as a geospeedometer for relatively rapid cooling events, such as the cooling of small high-level basaltic intrusions. The relationship between M-site ordering in these Mn-bearing samples and structural parameters such as bond lengths, octahedral bond-angle variance, and cell parameters is described.

Anharmonic properties of Mg2SiO4-forsterite measured from the volume dependence of the Raman spectrum

Abstract: The effect of temperature on the Raman spectrum of Mg2SiO4-forsterite is investigated between 100 and 2000 K. The data, combined with those obtained at high pressure, are used to discuss the intrinsic anharmonicity of the Raman-active modes. Anharmonicity of a mode of frequency v(i) is measured through a parameter (a(i)) = (partial derivative ln v(i)/partial derivative T)(v) calculated from the measured temperature and pressure shifts of the vibrational frequency. In forsterite, these parameters are negative, of the order of -10(-5) K-1. Those related to the internal modes of the SiO4 tetrahedra are significantly different (-0.5 10(-5) K-1) from those of the lattice modes (-3 10(-5) K-1). Because of the large temperature range investigated in this study, the temperature and volume dependencies of the a(i) parameters could be estimated for the first time in a mineral, and are shown to be negligible, at least up to 2000 K. The significant non-zero values of the al parameters combined with their small volume dependencies imply that intrinsic anharmonicity in forsterite contributes significantly to entropies and specific heats, even at high pressures, but has negligible effects on thermal pressure and high-temperature equations of state.

A vestige of very high-pressure (ca. 28 kbar) metamorphism in the Variscan Bohemian Massif, Czech Republic

Abstract: Pyropic garnetite from the Podolsko Complex, south Bohemian Moldanubian Zone, Bohemian Massif, records very-high pressure (28kbar, 830 deg.C)conditions of formation of the initial mineral assemblage. As such it represents the first evidence for very-high pressure metamorphism of continental crustal material in the Moldanubian Zone of the Bohemian Massif. This assemblage was extensively overprinted by complex symplectites, formed during a subsequent medium-pressure high-temperature event.

Partitioning behaviour of Hf and Zr between amphibole, clinopyroxene, garnet and silicate melts at high pressure

Abstract: Partition coefficients CD) have been determined for Hf (mainly) and Zr for amphibole, c1inopyroxene and gamet crystallizing from Zrand Hf-enriched basanite and andesite compositions at 0.5-3 .0 GPa and 1 0001 050·C (basanite) or 900-950·C (andesite). DHf and DZr for amphibole and clinopyroxene decrease with increasing P, and DHf is consistentiy > Dzr; for gamet, however, Dzr> DHf. The pressure effect on D's is most noteworthy between 0.5 and 1 .5 GPa (about a 3-fold decrease from 0.6 1 to 0.20 in DHf for amphibole). This needs to be considered when modelling Zr, Hf behaviour in magmas, especially at crustal depths, although major change in ZrlHf is unlikely to be produced through fractionation of amphibole and clinopyroxene from silicate meits . The Hf partitioning results are linked with established partitioning patterns for REE in c1inopyroxene and gamet crystallizing from a similar basanite, and confirm that the depth of initiation of melting for oceanic basalts may be shallower than originally constrained in the consideration of the "Hf paradox" (Salters & Hart, 1989) . Also the results show that the near-chondritic constancy of Sm/Hf in mantie-derived melts cannot be attributed to a com­ bined role of residual gamet and c1inopyroxene. Key-words: partition coefficients, hafnium, zirconium, experimental petrology, mantie minerals .