Breakdown Of Amphibole Research Articles

Low-rhenium molybdenite by metamorphism in northern Sweden: Recognition, genesis, and global implications

Re–Os dating of molybdenite is an accurate means to date intrusions and intrusion-related ore deposits using the model age or isochron approach. But, molybdenite has a new niche in the greenschist- to granulite-facies metamorphic environment. Re–Os ages for metamorphic molybdenite may be used to construct regional metamorphic histories. Age significance and accuracy are established by analyzing multiple molybdenite separates extracted from single, petrographically-characterized molybdenite occurrences. In this study, twelve geologically distinct molybdenite-bearing samples from two small Mo districts in northern Sweden trace a 150 m.y. Paleoproterozoic Svecofennian metamorphic history from ∼1900 to 1750 Ma. These data reveal a little-known, widespread and protracted, Late Svecofennian anatexis in northern Sweden. The Kåtaberget Mo–(Cu, F) deposit is located in the Moskosel granite batholith north of the economically-renown Skellefte district. Four different molybdenite samples from outcrop at Kåtaberget indicate an intrusion age of 1895 ± 6 Ma with the formation of later pegmatite–aplite at 1875 ± 6 Ma. The Allebuoda (Björntjärn) and Munka Mo–(W) deposits in the Rappen district are represented by three outcrop and five drill core samples of molybdenite-bearing aplite–pegmatite–granite. These two deposits were previously described as intrusion-related Climax-type Mo mineralization. Re–Os ages for molybdenites from these deposits range from 1865 to 1750 Ma and, significantly, Re concentrations are markedly low, extending to the sub-ppm level. Age agreement within the deposits is conspicuously lacking, whereas, with one exception, age agreement within any single sample (geologic occurrence), as established by analysis of additional molybdenite separates, is very good. These data, together with fundamental geologic observations discussed in this paper, suggest that Mo–(W) mineralization in northern Sweden is not intrusion-related, but the local product of episodic melting of Archean–Paleoproterozoic supracrustal gneisses related to the Svecofennian orogeny. Petrographic traverses across the boundary between widespread, foliation-parallel units of aplitic to pegmatitic pink granite and hosting biotite gneiss directly capture the process of ore formation. Dehydration breakdown of zircon-rich biotite aligned with the foliation in the gneiss is accompanied by formation of new pristine, post-deformational biotite plus sulfides, oxides, hydrothermal zircon and fluorite, all associated with microcline-dominant leucosomes. This process has profound implication for the traditional leucogranite, intrusion-related genesis attributed to the broad classification of Mo–W–Sn–base and precious metal mineralization (e.g., South Mountain Batholith, Nova Scotia; Okiep, Namaqualand, South Africa; Mactung, Yukon; Pogo–Liese, Tintina, Alaska; Carajás and Goiás–Rio Tocantins, Brazil; New England Batholith, NSW, Australia; Bergslagen, Sweden; Nevoria, Western Australia; Alpeinerscharte, Austria; Erzgebirge, Germany; Sardinia–Corsica Batholith). In addition to biotite, metallogenic contributions (e.g., Mo, W, Sn, U, Bi, Cu, Pb, Zn, Fe, Ni, Co, Au, Ag, Te, As, Sb, REE) in various combinations may also be controlled by breakdown of amphibole. In effect, the trace element composition of dehydrating or recrystallizing components in a gneissic rock essentially defines the local and district metallogenic suite. In the absence of focusing structures (e.g., shear zones, sheeted vein development), this process will generally form small and disconnected subeconomic deposits with erratic and unpredictable grades. Low Re content in associated molybdenite is a key indicator for a subeconomic origin by local melting of biotite gneiss (Mo–W) or muscovite schist (Sn–W).

Magma mixing during the 2001 event at Mount Etna (Italy): Effects on the eruptive dynamics

During the 2001 eruptive episode three different magmas were erupted on the southern flank of Mount Etna volcano from distinct vent systems. Major and minor element chemistry of rocks and minerals shows that mixing occurred, and that the mixed magma was erupted during the last eruptive phases. The space–time integrated analysis of the eruption, supported by geophysical data, together with major and trace element bulk chemistry (XRF, ICP-MS) and major and trace mineral chemistry (EPMA, LAM ICP-MS), support the following model: 1) trachybasaltic magma rises through a NNW–SSE trending structure, connected to the main open conduit system; 2) ascent of an amphibole-bearing trachybasaltic magma from a 6 km deep eccentric reservoir through newly open N–S trending fractures; 3) just a few days following the eruption onset the two tectonic systems intersect at the Laghetto area; 4) at the Laghetto vent a mixed magma is erupted. Mixing occurred between the amphibole-bearing trachybasaltic magma and an inferred deep more basic end-member. The most relevant aspect in the eruptive dynamics is that the eruption of the mixed magma at the Laghetto vent was highly explosive due to volatile content in the magma. The gas phase formed, mainly because of the decreased volatile solubility due to rapid fractures opening and increased T, related to mixing, and partially because of the amphibole breakdown.

Alkaline Magmatism Versus Collision Tectonics in the Eastern Ghats Belt, India: Constraints from Structural Studies in the Koraput Complex

Linear domains of deformed alkaline rocks and carbonatites have recently been identified as representing sites of ancient suture zones. In peninsular India, the western margin of the Proterozoic Eastern Ghats Belt (EGB) is characterized by a series of alkaline plutons that are aligned close to the contact with the Archaean Craton. Most of the complexes were deformed and metamorphosed during a subsequent orogenic event. Unlike other plutons in the belt, the alkaline complex at Koraput reportedly escaped deformation and granulite facies metamorphism forming an anomalous entity within the zone. Multiply-deformed country rocks hosting this complex underwent syn-D 1CR granulite facies metamorphism followed by D 2CR thrusting, with pervasive shearing along a NE-SW trending foliation. A second granulite facies event followed localized D 3CR shearing. Within the Koraput Complex, strain partitioning was responsible for preserving igneous textures in the gabbroic core, but aligned magmatic amphibole needles and plagioclase laths occasionally define a S 1AC fabric. Along the margins, S 1AC is rotated parallel to a NE-trending, east-dipping S 2AC fabric in the gabbro, fringing syenodiorite and nepheline syenite bands. Locally, D 3AC shearing follows D 2AC deformation; S 2AC and S 3AC parallel S 2CR and S 3CR in the country rocks. High-grade metamorphism represented by recrystallization of amphibole and plagioclase, and breakdown of amphibole and biotite to garnet, pyroxene and K-feldspar in the complex follows D 3AC. Unlike earlier reports, therefore, the Koraput body is also deformed and metamorphosed. The aligned alkaline complexes in the EGB probably represent deformed alkaline rocks and carbonatites formed by rifting related to an earlier episode of continental break-up that were deformed during subsequent juxtaposition of the EGB with the Archaean Craton. This supports the contention that the western margin of the EGB and its contact with the Archaean Craton is a suture zone related to the Indo-Antarctica collision event.

Mafic and ultramafic enclaves in Ustica Island lavas: Inferences on composition of lower crust and deep magmatic processes

Ustica Island, southern Tyrrhenian Sea, is constituted of Quaternary alkaline volcanics. A variety of enclaves representative of deep to supra-crustal settings were recently found in a hawaiitic lava flow. Enclaves consist of: (i) Ultramafic meta-cumulates, i.e. clinopyroxenites and wherlites characterized by variably deformed porphyroclastic to granoblastic textures. (ii) Mafic cumulates, i.e. gabbros (± amphibole) and troctolites, the first often characterized by frequent amphibole breakdown coronas (olivine + Ti-augite + plagioclase + magnetite + ilmenite + rhönite) in response to an H 2O decrease during the ascent, while the troctolites interpreted as meta-cumulates. (iii) Microsyenites, consist of anorthoclase and Fe-clinopyroxene organized in a granular sub-ipidiomorphic texture. Amphibole is absent in Ustica lavas and is found only in some old, now exposed, sub-intrusive volcanic bodies. This evidence suggests a late appearance of amphibole on the liquidus, at a high crystal content that inhibits further ascent of the magma. The importance of the amphibole as a medium pressure liquidus phase in Ustica mafic magmas is in the bearings on the geochemistry of lavas e.g. in buffering Na and Ti abundances, in trace elements partitioning, etc. Density measurements pointed out higher values for clinopyroxenites (3160 to 3300 kg/m 3) than for gabbros (ca. 2900 kg/m 3). Given the density contrast between enclaves and host lavas (2790 kg/m 3) and assuming appropriate rheological models, we calculated a minimum ascent rate of 0.01 m/s, corresponding to an ascent time in the range of 5–29 days for a depth of entrapment of 25 km.

Pressure–temperature–time paths of sediment recycling beneath the Tonga–Kermadec arc

The fate of subducted sediment and the extent to which it is dehydrated and/or melted before incorporation into arc lavas has profound implications for the thermo-mechanical nature of the mantle wedge and models for crustal evolution. In order to address these issues, we have undertaken the first measurements of 10Be and light elements in lavas from the Tonga–Kermadec arc and the sediment profile at DSDP site 204 outboard of the trench. The 10Be/ 9Be ratios in the Tonga lavas are lower than predicted from flux models but can be explained if (a) previously estimated sediment contributions are too high by a factor of 2–10, (b) the top 1–22 m of the incoming sediment is accreted, (c) large amounts of sediment erosion are proposed, or (d) the sediment component takes several Myr longer than the subducting plate to reach the magma source region beneath Tonga. The lavas form negative Th/Be–Li/Be arrays that extend from a depleted mantle source composition to lower Th/Be and Li/Be ratios than that of the bulk sediment. Thus, these arrays are not easily explained by bulk sediment addition and, using partition coefficients derived from experiments on the in-coming sediment, we show that they are also unlikely to result from fluid released during dehydration of the sediment (or altered oceanic crust). However, partial melts of the dehydrated sediment residue formed at ∼800 °C during the breakdown of amphibole±plagioclase and in the absence of cordierite have significantly lowered Th/Be ratios. The lava arrays can be successfully modelled as 10–15% partial melts of depleted mantle after it has been enriched by the addition of 0.2–2% of these partial melts. Phase relations suggest that this requires that the top of the subducting crust reaches temperatures of ∼800 °C by the time it attains ∼80 km depth which is in excellent agreement with the results of recent numerical models incorporating a temperature-dependent mantle viscosity. Under these conditions the wet basalt solidus is also crossed yet there is no recognisable eclogitic signal in the lavas suggesting that on-going dehydration or strong thermal gradients in the upper part of the subducting plate inhibit partial melting of the altered oceanic crust.

Experimentally Determined Phase Relations in Hydrous Peridotites to 6{middle dot}5 GPa and their Consequences on the Dynamics of Subduction Zones

Fluid-saturated subsolidus experiments from 2 0 to 6 5 GPa, and from 680 to 800 C have been performed on three model peridotites in the system Na2O–CaO–FeO–MgO–Al2O3–SiO2– H2O (NCFMASH). Amphibole and chlorite coexist up to 2 4 GPa, 700 C. Chlorite persists to 4 2 GPa at 680 C. Starting from 4 8 GPa, 680 C a 10 A phase structure replaces chlorite in all compositions. The 10 A phase structure contains significant Al2O3 (up to 10 53 wt %) deviating from the MgO–SiO2–H2O 10 A phase (MSH 10 A phase). A mixed layered structure (chlorite–MSH 10 A phase) is proposed to account for aluminium observed. In the Tinaquillo lherzolite amphibole breakdown occurs via the reaction

The origin of glasses in hydrous mantle xenoliths from the West Eifel, Germany: incongruent break down of amphibole

Patches of glass with a second generation of small crystals of olivine, clinopyroxene, and spinel are abundant in hydrous peridotite mantle xenoliths with tabular equigranular textures from two maar-type volcanoes, Meerfelder Maar and Dreiser Weiher (West Eifel, Germany). The patches are similar in size to the main phases of the hosting peridotite. Their central part is often occupied by relics of pargasitic amphibole. Mass-balance calculations show that the patches were formed by surface controlled incongruent thermal breakdown of amphibole according to the reaction: amphibole ↔ olivine + clinopyroxene + spinel + melt. Simultaneously with the decomposition of amphibole, small crystals of olivine, clinopyroxene, and spinel grew radially from the patch/peridotite interface toward the centre of the patch. Apart from sector zoning of clinopyroxene, the crystals are virtually homogeneous and are separated from the amphibole by a seam of melt (glass). Secondary olivines reveal higher Mg-numbers, secondary clinopyroxenes higher Cr2O3 concentrations than olivines and clinopyroxenes, respectively, of the host peridotite. The silica contents of melts produced by the above breakdown reaction range from 48 to 52% SiO2 as a function of the composition of the parent amphiboles. Patches surrounded by primary olivines only reveal no reaction with the host peridotite. The variation of SiO2, MgO and CaO in melts from these patches is the result of minor precipitation of olivine and clinopyroxene during fast cooling. If patches are in contact with primary olivine and orthopyroxene, melts are additionally modified by the reaction liquid 1 + orthopyroxene ↔ liquid 2 + olivine + clinopyroxene resulting in more silica-rich compositions between 54 and 58%. For the rare glasses richer in silica, a more complex formation is required. Veinlets along grain boundaries are filled with glasses which are chemically identical to those from nearby patches. This suggests that the veinlets were filled by melts formed by amphibole breakdown during entrainment of the xenoliths to the host magmas.

Mass transfer and reaction paths in alteration zones around carbonate veins in the Nishisonogi Metamorphic Rocks, southwest Japan

This paper describes local mass transfer and fluid-rock interaction in epidote-blueschist subfacies basic schist around retrograde dolomite + calcite veins in the Nishisonogi Metamorphic Rocks, southwest Japan. The veins have alteration zones on both sides. The mineralogical and chemical changes during alteration were documented along two sample traverses normal to the veins. Approaching the veins, the peak metamorphic assemblage (winchite + epidote + chlorite + calcite + albite + quartz) has been modified by progressive breakdown of amphibole, epidote, and calcite to illite and dolomite. Mass-balance calculations using the isocon method reveal that CO 2 and K were added to the basic schists, and Ca was removed during alteration. The mass balance does not hold for these components between the vein + alteration zone and protolith as a whole. These findings suggest that the vein formation and metasomatism represent a process driven by fluid flow and long-distance element transport. Possible reaction paths between the vein-fluids and basic schists were estimated from the progressive change of mineral assemblages in the alteration zones. The phase relations on ionic activity diagrams indicate an increase in log f CO 2 and decreases in log( a Ca 2+ /σ Ca 2+ a 2 H + ) and log( a Mg 2+ /σ Mg 2+ a 2 H + ) with progress of the alteration. Although a change in log( a K + /σ K + a 2 H + ) is poorly constrained on the activity diagrams, the mass balance suggests a concentration gradient of K across the alteration zones. Hence, the reaction paths and mass balance calculations suggest that a combination of the concentration gradients of CO 2 , Ca, Mg, and K produced by the vein-fluids drove the alteration process.

Density contrast of fluid inclusions associated with melt (glass) from two distinct suites of mantle peridotites from the West Eifel, Germany: Implications for melt origin

CO 2 -rich fluid inclusions associated with silicate melts (glass) covering a wide variety of densities from 0.13 to 1.17 g/cm 3 are present in two suites of genetically different mantle peridotites from the West Eifel, Germany. The two suites consist of low temperature hydrous peridotites (la), which contain amphibole, and high temperature dry peridotites (lb), respectively. None of the inclusions is primary. Highest fluid densities of 1.17 g/cm 3 from the high temperature dry suite corresponding to a maximum pressure of close to 1.5 GPa and a depth of about 47 km compared to 1.05 g/cm 3 and a maximum pressure of 0.99 GPa, equivalent to a depth of 31 km, for the amphibole-bearing hydrous suite. Except for similar silica contents (49 to 59 wt. % SiO 2 ), melts from both suites are of different major element composition and origin. Small proportions of melt (1 vol.%) along grain boundaries in the lb suite were derived from the Quaternary alkali basalt lavas from the West Eifel which infiltrated the peridotites at mantle depths considerably greater than 47 km and were chemically modified by reaction with orthopyroxene. Melts in the la suite result from the breakdown of amphibole. Blebs of melt (glass) with microcrysts of clinopyroxene, olivine and spinel replace former amphibole. The wide variety of homogenization temperatures of fluid inclusions associated with the melts in the la suite is believed to be due to re-equilibration at crustal levels shallower than 31 km. This is consistent with the view that the hydrous peridotites were sampled from immediately below the Moho.

Density contrast of fluid inclusions associated with melt (glass) from two distinct suites of mantle peridotites from the West Eifel, Germany

CO 2 -rich fluid inclusions associated with silicate melts (glass) covering a wide variety of densities from 0.13 to 1.17 g/cm 3 are present in two suites of genetically different mantle peridotites from the West Eifel, Germany. The two suites consist of low temperature hydrous peridotites (la), which contain amphibole, and high temperature dry peridotites (lb), respectively. None of the inclusions is primary. Highest fluid densities of 1.17 g/cm 3 from the high temperature dry suite corresponding to a maximum pressure of close to 1.5 GPa and a depth of about 47 km compared to 1.05 g/cm 3 and a maximum pressure of 0.99 GPa, equivalent to a depth of 31 km, for the amphibole-bearing hydrous suite. Except for similar silica contents (49 to 59 wt. % SiO 2 ), melts from both suites are of different major element composition and origin. Small proportions of melt (1 vol.%) along grain boundaries in the lb suite were derived from the Quaternary alkali basalt lavas from the West Eifel which infiltrated the peridotites at mantle depths considerably greater than 47 km and were chemically modified by reaction with orthopyroxene. Melts in the la suite result from the breakdown of amphibole. Blebs of melt (glass) with microcrysts of clinopyroxene, olivine and spinel replace former amphibole. The wide variety of homogenization temperatures of fluid inclusions associated with the melts in the la suite is believed to be due to re-equilibration at crustal levels shallower than 31 km. This is consistent with the view that the hydrous peridotites were sampled from immediately below the Moho.

The pressure and temperature conditions and timing of glass formation in mantle-derived xenoliths from Baarley, West Eifel, Germany: the case for amphibole breakdown, lava infiltration and mineral - melt reaction

Mantle-derived xenoliths from Baarley in the Quaternary West Eifel volcanic field contain six distinct varieties of glass in veins, selvages and pools. 1) Silica-undersaturated glass rich in zoned clinopyroxene microlites that forms jackets around and veins within the xenoliths. This glass is compositionally similar to groundmass glass in the host basanite. 2) Silica-undersaturated alkaline glass that contains microlites of Cr-diopside, olivine and spinel associated with amphibole in peridotites. This glass locally contains corroded primary spinel and phlogopite. 3) Silica-undersaturated glass associated with diopside, spinel ± olivine and rhonite microlites in partly to completely broken down amphibole grains in clinopyroxenites. 4) Silica-undersaturated to silica-saturated, potassic glass in microlite-rich fringes around phlogopite grains in peridotite. 5) Silica-undersaturated potassic glass in glimmerite xenoliths. 6) Silica-rich glass around partly dissolved orthopyroxene crystals in peridotites.

Magma Ascent Rates from Mineral Reaction Rims and Extension to Teaching About Volcanic Hazards

An ongoing challenge in curriculum development for modern, process-oriented courses in mineralogy and petrology is to design exercises that promote understanding of the mechanisms and rates of fundamental mineral processes. We have developed an exercise wherein students determine magma ascent rates for recent eruptions of Mount St. Helens. The ascent rates are determined from analysis of the thickness of amphibole breakdown rims and comparison to experimental calibration of the reaction kinetics. As an extension activity, the results are incorporated into a discussion of volcanic hazards planning for the Cascades Range, Pacific Northwest, United States. This exercise requires no special equipment and is easily adapted to introductory courses in earth and environmental sciences without compromising the overall learning objectives.

K-feldspar-quartz and K-feldspar-plagioclase phase boundary interactions in garnet-orthopyroxene gneiss's from the Val Strona di Omegna, Ivrea-Verbano Zone, northern Italy

A detailed study based on textural observations combined with microanalysis [back scattered electron imaging (BSE) and electron microprobe analysis (EMPA)] and microstructural data transmission electron microscopy (TEM) has been made of K-feldspar micro-veins along quartz–plagioclase phase and plagioclase–plagioclase grain boundaries in granulite facies, orthopyroxene–garnet-bearing gneiss's (700–825 °C, 6–8 kbar) from the Val Strona di Omegna, Ivrea–Verbano Zone, northern Italy. The K-feldspar micro-veins are commonly associated with quartz and plagioclase and are not found in quartz absent regions of the thin section. This association appears to represent a localised reaction texture resulting from a common high grade dehydration reaction, namely: amphibole + quartz = orthopyroxene + clinopyroxene + plagioclase + K-feldspar + H2O, which occurred during the granulite facies metamorphism of these rocks. There are a number of lines of evidence for this. These include abundant Ti-rich biotite, which was apparently stable during granulite facies metamorphism, and total lack of amphibole, which apparently was not. Disorder between Al and Si in the K-feldspar indicates crystallisation at temperatures >500 °C. Myrmekite and albitic rim intergrowths in the K-feldspar along the K-feldspar–plagioclase interface could only have formed at temperatures >500–600 °C. Symplectic intergrowths of albite and Ca-rich plagioclase between these albitic rim intergrowths and plagioclase suggest a high temperature grain boundary reaction, which most likely occurred at the start of decompression in conjunction with a fluid phase. Relatively high dislocation densities (>2 × 109 to 3 × 109/cm2) in the K-feldspar suggest plastic deformation at temperatures >500 °C. We propose that this plastic deformation is linked with the extensional tectonic environment present during the mafic underplating event responsible for the granulite facies metamorphism in these rocks. Lastly, apparently active garnet grain rims associated with side inclusions of K-feldspar and quartz and an exterior K-feldspar micro-vein indicate equilibrium temperatures within 20–30 °C of the peak metamorphic temperatures estimated for the sample (770 °C). Contact between these K-feldspar micro-veins and Fe-Mg silicate minerals, such as garnet, orthopyroxene, clinopyroxene or biotite along the interface, is observed to be very clean with no signs of melt textures or alteration to sheet silicates. This lends support to the idea that these micro-veins did not originate from a melt and, if fluid induced, that the water activity of these fluids must have been relatively low. All of these lines of evidence point to a high grade origin for the K-feldspar micro-veins and support the hypothesis that they formed during the granulite facies metamorphism of the metabasite layers in an extensional tectonic environment as the consequence of localised dehydration reactions involving the breakdown of amphibole in the presence of quartz to orthopyroxene, clinopyroxene, plagioclase, K-feldspar and H2O. It is proposed that the dehydration of the metabasite layers to an orthopyroxene–garnet-bearing gneiss over a 4-km traverse in the upper Val Strona during granulite facies metamorphism was a metasomatic event initiated by the presence of a high-grade, low H2O activity fluid (most likely a NaCl–KCl supercritical brine), related to the magmatic underplating event responsible for the Mafic Formation; and that this dehydration event did not involve partial melting.

Microstructural evolution and geologic significance of garnet pyriclasites in the Hoher-Bogen shear zone (Bohemian Massif, Germany)

Ductile extensional movements along the steeply inclined Hoher-Bogen shear zone caused the juxtaposition of Tepla-Barrandian amphibolites, granulites, and metaperidotites against Moldanubian mica schists and paragneisses. Garnet pyriclasites are well preserved within low-strain domains of this shear zone. Their degree of metamorphism is significantly higher than that of the surrounding rocks. Microstructural and mineral chemical data suggest in situ formation of the garnet pyriclasite by dehydration of pyroxene amphibolite at T>750–840°C and P<10–13 kbar including recrystallization-accommodated grain-size reduction of plagioclase and clinopyroxene, nucleation of garnet, and breakdown of amphibole into garnet+clinopyroxene+rutile. Subsequent decompression and retrograde extensional shearing led to the formation of mylonitic epidote amphibolite. The presence of lower crustal and mantle-derived slices within the Hoher-Bogen shear zone supports the view that (a) in Upper Devonian times the Tepla-Barrandian unit was thrust over Moldanubian rocks as a complete crustal unit, and (b) that during the subsequent Lower Carboniferous orogenic collapse, the garnet pyriclasite and metaperidotite were scraped off from the basal parts of the Tepla-Barrandian unit being dragged into the Hoher-Bogen shear zone due to dramatic and large-scale elevator-style movements.

In situ origin for glass in mantle xenoliths from southeastern Australia: insights from trace element compositions of glasses and metasomatic phases

Siliceous, aluminous and alkali-rich glasses, commonly found in patches and veins in spinel peridotite xenoliths, have been attributed to a number of different origins. These include low-degree primary melts of the mantle, exotic metasomatic melts influxing into the lithosphere, or breakdown of amphibole, and other phases during high-temperature transport of the xenoliths to the surface in their host magmas. We present new laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of trace element abundances in glasses, and in metasomatically introduced phases (clinopyrox-ene, amphibole, phlogopite, apatite) from a suite of spinel wehrlite, lherzolite and harzburgite xenoliths from southeastern Australia. The majority of glass compositions are best explained by melting of amphibole (usually complete, as amphibole is now absent from most samples) with varying but significant contributions from partial melting of clinopyroxene. However, some glasses require additional components derived from partial or complete modal melting of phlogopite, or apatite. The data confirm our earlier model, that the glass present in patches in these samples derives from high-temperature, transport-related breakdown of a metasomatic phase assemblage (amphibole + clinopyroxene ± phlogopite =b apatite) present in the xenoliths prior to their entrainment in the host magmas.

Complete miscibility between silicate melts and hydrous fluids in the upper mantle: experimental evidence and geochemical implications

The phase relationships between silicate melts and hydrous fluids were studied by direct visual observation in an externally heated diamond-anvil cell. Complete miscibility of silicate melt and water was observed for a wide range of melt compositions, including nepheline, jadeite, dacite, haplogranite and Ca-bearing granite. Some evidence for complete miscibility was also observed in the system basalt–H 2O. The critical temperatures in all systems decrease rapidly with pressure. At 15 kbar, the critical temperature for nepheline is around 550°C, for jadeite around 800°C and for granitic compositions it is close to 900°C. In general, the critical temperatures appear to increase with silica content in the system. Our results suggest that there is complete miscibility between silicate melts and water in most of the upper mantle, except at very shallow depths. This means that a water-saturated solidus cannot be defined any more in the deeper parts of the upper mantle. Very silica-rich melt inclusions found in spinel lherzolites associated with fluid inclusion are probably the result of the unmixing of a supercritical fluid containing comparable amounts of water and silicate components. The decomposition of amphibole in a subducted slab occurs at conditions where the miscibility gap between fluid and silicate melt is not yet closed, while the decomposition of phengite and lawsonite occurs far beyond the critical curve. Accordingly, the fluids released by the breakdown of these minerals should have very different properties. Highly mobile, hydrous fluids containing little dissolved silicate should be produced by amphibole breakdown, while the decomposition of lawsonite and phengite will lead to much more silicate-rich and less mobile fluid phases.

P-T AND COMPOSITIONAL CONTROLS ON HYDRATION AND DEHYDRATION OF OROGENIC LHERZOLITES

The natural processes, examined in upper mantle-derived xenolith suites, orogenic lherzolites and primary magmas, all confirm the presence of significant and variable quantities of water in minerals or melts. It has been well known that the P-T location of the mantle solidus is extremely sensitive to pargasitic amphibole stability (e.g., Green, 1973). It is hardly known, however, how the amphibole acts under the subsolidus mantle conditions. Recently, Niida and Green (1999) discussed on chemical and modal variations of pargasitic amphibole in mantle peridotites. In this paper, crystallization and breakdown reaction of pargasitic amphiboles, generated through the subsolidus P-T regime, will be focused on orogenic lherzolites, to understand P-T controls and bulk-rock composition effects during hydration and dehydration in the upper mantle. DEHYDRATION A significant dehydration of pargasitic amphibole-bearing lherzolites occurs just on the solidus, where the breakdown reaction of amphibole becomes active, producing a melt phase. The amphiboles are all eliminated at the amphibole stability limit, approximately 15°C higher than the solidus. It is evident from melting experiments (Wallace and Green, 1991) that the location of P-T of amphibole breakdown is controlled by the bulk rock chemical composition. The upper stability limit of amphiboles in a fertile peridotite (Hawaiian pyrolite: 1150-1170°C at 25 kbar) is 150°C higher than those in a depleted peridotite (Tinaquillo lherzolite: 1025°C at 20-25 kbar). The P limit of the stability expands slightly in fertile peridotites. We also expect a breakdown reaction of pargasitic amphibole occurring continuously throughout the subsolidus region, based on the phase relationships combined with the modal abundance of coexisting phases (Niida and Green, 1999). In the plagioclase lherzolite field, tremolite component in amphiboles decreases with increasing T and pargasite component increases with increase in both T and P. In the spinel lherzolite field pargasite components is replaced by richterite towards higher P. The P-T effects on amphibole composition are more evident in the garnet lherzolite field, where there is decrease in Tschermak’s component and further increase in richterite. The modal and mineral compositional data suggest that enstatite is decreasing, garnet is increasing, and jadeite solid solution in clinopyroxene is increasing over the same P intervals, suggesting the reaction: 3pa + (CaAl2SiO6)cpx + 10en -> ri + jd + (2gr+3py) + 4fo + 2H2O. The pargasite breakdown also occurs as part of the spinel lherzolite to the garnet lherzolite transition as: 3pa + sp + di + en -> ri + jd + (2gr+3py) + 5fo + 2H2O. The high P limit is expressed as a richterite breakdown reaction, e.g., ri + 2(CaAl2SiO6)cpx -> 2jd + gr + en + 2fo + H2O, or more plausibly to include a pargasitic amphibole breakdown reaction: (3pa+ri) + (5MgAl2SiO6 + 8en) -> 5jd + 7(gr0.33py0.67) + 8fo + 4H2O. HYDRATION When H2O-rich fluids and/or melts migrate through the upper mantle lherzolites, a considerable amount of amphiboles crystallize in the subsolidus P-T regime. The modal proportion and the chemical composition of amphiboles crystallized are strongly controlled by P-T and the bulk lherzolite compositions (Niida and Green, 1999). We infer that a significant hydration of lherzolite occurs in lower T fields and that the modal amphibole increases with decreasing pressure at a constant T. We also infer that the maximum proportion of amphibole attains at P-T close to the transition of the plagioclase lherzolite and the spinel lherzolite. In the case of MORB pyrolite composition, more than 30% pargasitic amphiboles have been confirmed as maximum modal proportion at T=925°C and P=10 kbar, whereas less than 10% as minimum at T=1000°C and P=27 kbar. The Na content of amphibole is positively correlated with P-T, in harmony with the modal variation of amphibole. Again, in a fertile lherzolite, we expect slightly more abundant amphiboles as hydration products. During hydration of mantle lherzolites, pargasitic amphibole act as a tank of H2O and alkali (Na + K). It is plausible, however, that P-T and the bulk rock compositions strictly constrain the capacity of storage in terms of modal proportions.

Silicic glasses in hydrous and anhydrous mantle xenoliths from Western Victoria, Australia: at least two different sources

Glasses in a hydrous wehrlite and in anhydrous lherzolites from Western Victoria, Australia, are present as interstitial glasses and secondary glass inclusions. Interconnections between each other, generally observed as thin necks, are still preserved. These petrographic characteristics are suitable for establishing a space–time relationship. Glasses in hydrous and anhydrous xenoliths show continuous chemical trends apparently governed by different processes. Glass patches in the hydrous wehrlite are interpreted as the product of decompressional breakdown of hydrous phases like amphibole and phlogopite. However, abundances of some elements suggest mixing and the involvement of an additional source. After precipitation of secondary phases (e.g., olivine, clinopyroxene and spinel), the brown microlite-free melt migrated and reacted with primary clinopyroxene and in rare cases was trapped as glass inclusions. The observed chemical trend can be explained by crystallisation of secondary phases of the amphibole breakdown melt with addition of an alkali-volatile-rich phase. In the anhydrous lherzolites, petrographic and chemical evidences suggest the existence of two glasses: a silica-rich glass (glass A) and a very silica-rich glass (glass B). The silica-rich glass A (SiO 2: 60–65 wt.%) is interpreted as an initial silicic melt, possibly generated at mantle depths, with a continuous chemical trend toward low-silica glasses (SiO 2: 52 wt.%). This evolution is possibly governed by increasing melt fractions and dissolution of original apatite. Glass inclusions formed by this melt are rich in CO 2 and characterised by a feldspar–diopside–olivine normative composition. Furthermore, in the proximity to orthopyroxene and, due to a later event possibly related to the ascent of the xenolith, the silica-rich glass acquired a very silicic composition (glass B) by reaction with orthopyroxene and crystallisation of microlites [Zinngrebe, E., Foley, S.F., 1995. Metasomatism in mantle xenoliths from Gees, West Eifel, Germany: evidences for the genesis of calc-alkaline glasses and metasomatic Ca-enrichment. Contrib. Mineral. Petrol., 122, 76–96]. The rare glass inclusions formed by this melt are CO 2-free and have a quartz-feldspar normative composition.

Trace element transport during dehydration processes in the subducted oceanic crust: 2. Origin of chemical and physical characteristics in arc magmatism

The trace elements and Sr-Nd-Pb isotope compositions of arc magmas have been calculated from our estimates of: (1) the chemical composition of aqueous fluids emanating from the subducting basaltic oceanic crust, based on experimental data for element mobilization by aqueous fluids during the amphibolite/eclogite transition [1]; (2) the modal and chemical composition of the hydrous peridotite layer, which is formed by addition of the fluid derived from amphibolite dehydration to the base of the mantle wedge and the change in the hydrous mantle mode with increasing pressure and temperature; (3) the chemical composition of the fluid phase released by amphibole breakdown within the down-dragged hydrous peridotite layer, beneath the volcanic front; and (4) the mixing ratio of the this fluid with the pre-existing mantle wedge, as constrained by the H 2O content of arc magmas and appropriate degrees of partial melting. The resulting estimate for the chemical composition of primary arc basalt magma matches well with that of typical oceanic arc basalts. This supports the suggestion that aqueous fluids derived from the subducting crust play an essential role in generating arc magmatism and in producing their distinctive chemistry. Our calculations also indicate that more aqueous fluid enters the magma source region beneath the trench-side volcanic chain from amphibole breakdown than beneath sites of backarc-side volcanism due to phlogopite decomposition. This greater fluid flux beneath the volcanic front is consistent with the observed greater volume and/or number density of volcanoes at the volcanic front than behind the volcanic front — one of the most notable physical features of arc magmatism.

Stability of hydrous phases in subducting oceanic crust

Experiments in the basalt-H 2O system at 600–950°C and 0.8–3.0 GPa, demonstrate that breakdown of amphibole represents the final dehydration of subducting oceanic tholeiite at T≥650°C; the dehydration H 2O occurs as a free fluid or in silicate melt co-existing with an anhydrous eclogite assemblage. In contrast, about 0.5 wt% of H 2O is stored in lawsonite at 600°C, 3.0 GPa. Our results suggest that slab melting occurs at depths shallower than 60 km for subducting young oceanic crust; along a subduction zone with an average thermal gradient higher than 7°C/km, H 2O stored in hydrated low-potassium, metabasaltic layers cannot be subducted to depths greater than 100 km, then released to generate arc magma.