The November 2002 eruption of Piton de la Fournaise,Réunion: tracking the pre-eruptive thermal evolution of magma using melt inclusions

The November 2002 eruption of Piton de la Fournaise in the Indian Ocean was typical of the activity of the volcano from 1999 to 2006 in terms of duration and volume of magma ejected. The first magma erupted was a basaltic liquid with a small proportion of olivine phenocrysts (Fo₈₁) that contain small numbers of melt inclusions. In subsequent flows, olivine crystals were more abundant and richer in Mg (Fo_83–84). These crystals contain numerous melt and fluid inclusions, healed fractures, and dislocation features such as kink bands. The major element composition of melt inclusions in this later olivine (Fo_83–84) is out of equilibrium with that of its host as a result of extensive post-entrapment crystallization and Fe²⁺ loss by diffusion during cooling. Melt inclusions in Fo₈₁ olivine are also chemically out of equilibrium with their hosts but to a lesser degree. Using olivine–melt geothermometry, we determined that melt inclusions in Fo₈₁ olivine were trapped at lower temperature (1,182 ± 1°C) than inclusions in Fo_83–84 olivine (1,199–1,227°C). This methodology was also used to estimate eruption temperatures. The November 2002 melt inclusion compositions suggest that they were at temperatures between 1,070°C and 1,133°C immediately before eruption and quenching. This relatively wide temperature range may reflect the fact that most of the melt inclusions were from olivine in lava samples and therefore likely underwent minor but variable amounts of post-eruptive crystallization and Fe²⁺ loss by diffusion due to their relatively slow cooling on the surface. In contrast, melt inclusions in tephra samples from past major eruptions yielded a narrower range of higher eruption temperatures (1,163–1,181°C). The melt inclusion data presented here and in earlier publications are consistent with a model of magma recharge from depth during major eruptions, followed by storage, cooling, and crystallization at shallow levels prior to expulsion during events similar in magnitude to the relatively small November 2002 eruption.     

Silicate and carbonate melt inclusions associated with diamonds in deeply subducted carbonate rocks

Deeply subducted carbonate rocks from the Kokchetav massif (Northern Kazakhstan) recrystallised within the diamond stability field (P = 4.5–6.0 GPa; T ≈ 1000 °C) and preserve evidence for ultra high-pressure carbonate and silicate melts. The carbonate rocks consist of garnet and K-bearing clinopyroxene embedded in a dolomite or magnesian calcite matrix. Polycrystalline magnesian calcite and polyphase carbonate–silicate inclusions occurring in garnet and clinopyroxene show textural features of former melt inclusions. The trace element composition of such carbonate inclusions is enriched in Ba and light rare earth elements and depleted in heavy rare earth elements with respect to the matrix carbonates providing further evidence that the inclusions represent trapped carbonate melt. Polyphase inclusions in garnet and clinopyroxene within a magnesian calcite marble, consisting mainly of a tight intergrowth of biotite + K-feldspar and biotite + zoisite + titanite, are interpreted to represent two different types of K-rich silicate melts. Both melt types show high contents of large ion lithophile elements but contrasting contents of rare earth elements. The Ca-rich inclusions display high REE contents similar to the carbonate inclusions and show a general trace element characteristic compatible with a hydrous granitic origin. Low SiO₂ content in the silicate melts indicates that they represent residual melts after extensive interaction with carbonates. These observations suggest that hydrous granitic melts derived from the adjacent metapelites reacted with dolomite at ultra high-pressure conditions to form garnet, clinopyroxene – a hydrous carbonate melt – and residual silicate melts. Silicate and carbonate melt inclusions contain diamond, providing evidence that such an interaction promotes diamond growth. The finding of carbonate melts in deeply subducted crust might have important consequences for recycling of trace elements and especially C from the slab to the mantle wedge.     

Orthorhombic perovskite phases observed in olivine,pyroxene and garnet at high pressures and temperatures

Ferromagnesian silicate olivines, pyroxenes and garnets with Mg/(Mg + Fe)?0.3 (molar) have been found to transform to high-pressure phases characterized by the orthorhombic perovskite structure when compressed to pressures above 250 kbar in a diamond-anvil press and heated to temperatures above 1,000°C with a YAG laser. The zero-pressure density of the perovskite phase of (Mg,Fe)SiO₃ is about 3–4% greater than that of the close-packed oxides, rocksalt plus stishovite. For (Mg,Fe)₂SiO₄ compounds, the perovskite plus rocksalt phase assemblage is 2–3% denser than the mixed oxides. The experimental synthesis of such high-density perovskite phases in olivine, pyroxene and garnet compounds suggests that (Mg,Fe)SiO₃-perovskite is the dominant mineral phase in the earth's lower mantle.     

Crystallization temperature and gas phase composition of alkaline effusives as indicated by primary melt inclusions in the phenocrysts

Minerals formed during magma crystallization trap droplets of melt that are preserved as primary or secondary inclusions. Depending on the rate of cooling, the droplets may solidify as glass, or crystallize. Inclusions may contain one or more bubbles, or none. When inclusions are heated the glass or crystalline material are melted and the inclusion expands, the size of bubbles diminishes, and homogenization of the inclusion occurs. It is possible to observe these transformations by means of high-temperature cameras which permit visual observations to 1600°C and above. The possibility of using the homogenization of inclusions to determine the temperature of formation of the host mineral has been demonstrated experimentally, using inclusions in artificial diopside formed at 1300 ± 10°. Melt inclusions in phenocrysts from nepheline basalt, fergusite porphyry, and tephrite were investigated. In the leucite-bearing rocks leucite crystallized at 1600° or above, and clinopyroxene in the range 1380–1250°. The central part of olivines in nepheline basalt formed at 1290–1270° and the peripheral zones at 1160–1120°; nepheline formed at 1290–1250°; the central part of pyroxenes at 1280–1250° and the peripheral zones at 1160–1120°. These temperatures suggest almost dry magma. Gas from the bubbles of individual inclusions has been analyzed. The predominant gaseous component of the early crystallization stage of the nepheline basalt and fergusite porphry was CO₂, H₂S, SO₂, NH., HCl, HF, and H. comprise less than 5 volume percent except in olivine of olivine basalt in which the total content of these gases was on average 6.22 volume percent, and in leucite of fergusite porphyry in which H₂ was on average 12.7 volume percent. The main gas component in the crystallization of the leucite tephrite were nitrogen and rare gases. Liquid hydrocarbons in the secondary inclusions in pyroxene from nepheline basalt can be accounted for by their assimilation by the magma from enclosing rocks during its rise.     

Shock-produced vapor-grown crystals in the Yanzhuang meteorite

Vapor-grown crystals intimately related to shock metamorphism of meteorites were found in the Yanzhuang (H6) chondrite which had been heavily impacted in the space. These crystals include: (i) subhedral low-Ca pyroxene occurring on the wall of the pores within a silicate melt pocket that experienced a shock temperature higher than 1500°C, (ii)Fe-Ni needle-whiskers (taenite) occurring in the cracks in the partially melted chondritic facies that experienced a shock temperature of 850–1300°C, (iii) troilite with abundant microholes occurring in the cracks in the brecciated facies and the lightly deformed chondritic facies that experienced a shock temperature lower than 850°C. The occurrence and mineralogical features of vapor-grown crystals show that vaporization of minerals could be produced in heavily impacted meteorites and that a small amount of crystals could be depositedin situ from vapor phases. Project supported by the Natural Science Foundation of Guangdong Province.     

Stability relation of (Mg,Fe)SiO3 garnets, major constituents in the Earth's interior

High-pressure and high temperature experiments at 20 GPa on (Mg,Fe)SiO₃ have revealed stability fields of two types of aluminium-free ferromagnesian garnets; non-cubic garnet and cubic garnet (majorite). Majorite garnet is stable only within a limited compositional variation, 0.2 < Fe/(Mg + Fe)< 0.4, and in the narrow temperature interval of 200°C around 2000°C, while the stability of non-cubic garnet with more iron-deficient compositions persists up to higher temperatures. These two garnets show fractional melting into iron-deficient garnet and iron-rich liquid, and the crystallization field of cubic garnet extends over Fe/(Mg + Fe)= 0.5. The assemblage silicate spinel and stishovite is a low-temperature phase, which also occurs in the iron-rich portion of the MgSiO₃—FeSiO₃ system. The sequence as given by the Fe/(Mg + Fe) value for the coexisting phases with the two garnets at 2000°C and 20 GPa is: silicate modified spinel aluminium-free garnets silicate spinel.Natural majorite in shock-metamorphosed chondrites is clarified to be produced at pressures above 20 GPa and temperatures around 2000°C. Similar shock events may cause the occurrence of non-cubic garnet in iron-deficient meteorites. Non-cubic garnet could be a stable phase in the Earth's mantle if a sufficiently low concentration of aluminium is present in the layer corresponding to the stable pressure range of non-cubic garnet. The chemical differentiation by melting in the deep mantle is also discussed on the basis of the present experimental results and the observed coexistence of majorite garnet with magnesiowüstite in chondrites.     

Geobarometry of ultramafic xenoliths from Loihi Seamount,Hawaii, on the basis of CO2 inclusions in olivine

Abundant fluid inclusions in olivine of dunite xenoliths (～1–3 cm) in basalt dredged from the young Loihi Seamount, 30 km southeast of Hawaii, are evidence for three coexisting immiscible fluid phases—silicate melt (now glass), sulfide melt (now solid), and dense supercritical CO₂ (now liquid + gas)—during growth and later fracturing of some of these olivine crystals. Some olivine xenocrysts, probably from disaggregation of xenoliths, contain similar inclusions.Most of the inclusions (2–10 μm) are on secondary planes, trapped during healing of fractures after the original crystal growth. Some such planes end abruptly within single crystals and are termed pseudosecondary, because they formed during the growth of the host olivine crystals. The “vapor” bubble in a few large (20–60 μm), isolated, and hence primary, silicate melt inclusions is too large to be the result of simple differential shrinkage. Under correct viewing conditions, these bubbles are seen to consist of CO₂ liquid and gas, with an aggregate ? = ～ 0.5–0.75 g cm^?3, and represent trapped globules of dense supercritical CO₂ (i.e., incipient “vesiculation” at depth). Some spinel crystals enclosed within olivine have attached CO₂ blebs. Spherical sulfide blebs having widely variable volume ratios to CO₂ and silicate glass are found in both primary and pseudosecondary inclusions, demonstrating that an immiscible sulfide melt was also present.Assuming olivine growth at ～ 1200°C and hydrostatic pressure from a liquid lava column, extrapolation of CO₂P-V-T data indicates that the primary inclusions were trapped at ～ 220–470 MPa (2200–4700 bars), or ～ 8–17 km depth in basalt magma of ? = 2.7 g cm^?3. Because the temperature cannot change much during the rise to eruption, the range of CO₂ densities reveals the change in pressure from that during original olivine growth to later deformation and rise to eruption on the sea floor. The presence of numerous decrepitated inclusions indicates that the inclusion sample studied is biased by the loss of higher-density inclusions and suggests that some part of these olivine xenoliths formed at greater depths.     

Melting relations in the chloride–carbonate–silicate systems at high-pressure and the model for formation of alkalic diamond–forming liquids in the upper mantle

The experiments in the model system CaMgSi₂O₆–(Na₂CO₃, CaCO₃)–KCl are performed at 5 GPa and 1400–1600 °C in order to study the phase relations, including liquid immiscibility, in the chloride–carbonate–silicate systems with application to alkali and chlorine-rich liquids preserved in kimberlitic diamonds. Experiments in the boundary joins of the system demonstrated that both the carbonate–silicate and chloride–carbonate melts are homogeneous; while high-temperature (above 1800 °C) liquid immiscibility is assumed for the chloride–silicate join of the above system. Addition of silicate component into the chloride–carbonate melts and chloride component into the carbonate–silicate melts results in splitting of the homogeneous liquids into the immiscible chloride–carbonate brine and carbonate–silicate melt. Carbonate–silicate and chloride–carbonate branches of the miscibility gap converge within the carbonate-rich region of the system. Regular temperature evolution of the shape and size of the miscibility gap is deduced. With decreasing temperature, the convergence point moves toward more Si-rich compositions, expanding fields of homogeneous chloride–carbonate silica-saturated melts. This effect is governed by the precipitation of the silicate phases even from silica-bearing chloride–carbonate melts. In addition, experiments revealed regular evolution of both Cl-bearing carbonate–silicate melt and Si-bearing chloride–carbonate brine toward the low-temperature chlorine–bearing carbonatitic liquid with decreasing temperature. These trends are similar to the evolution of the melt and brine inclusions in some diamonds from Botswana, Brazil, Canada, and Yakutia, indicating their growth during cooling. The model for interaction of the chloride–carbonate brine with the mantle rocks is developed on the basis of the present experimental data. This model is applied to the chlorine-enriched kimberlites of the Udachnaya–East pipe.     

Co-existing fluid and silicate inclusions in mantle diamond

We document the compositions of co-existing silicate macro-inclusions and fluid micro-inclusions in the fibrous coats of eight coated diamonds from the Panda kimberlite (Canada). The mineral inclusions in the diamond coats come from either the peridotite suite (Cr-pyrope, orthopyroxene, olivine and Cr-diopside) or the eclogite suite (omphacite). Therefore, fibrous diamonds grow in the same paragenetic environments as octahedral diamonds. The inclusions document a more fertile source composition (lower Mg# and higher CaO) than for equivalent phases in octahedral diamonds from Panda and worldwide. However, moderate to high Cr₂O₃ contents in garnet and clinopyroxene inclusions suggest that this apparent fertility is due to a secondary process. Geothermometry of the silicate inclusions yields low equilibration temperatures of 930 to 1010 °C. The co-existing fluid micro-inclusions are dominated by H₂O, carbonate and KCl. Fluid inclusions in both the peridotitic and eclogitic samples fall along linear arrays between Fe–Ca–Mg carbonate and KCl. Inclusions in the one eclogitic sample also contain quartz. We suggest that the diamonds have trapped both metasomatised minerals and the metasomatic fluid, and so provide a snap shot of a metasomatic event in the mantle.     

Megacrysts in the Cenozoic basalt of the Tuoyun Basin,Southwest Tianshan

Abundant megacrysts of clinopyroxene, amphibole, anorthoclase, and phlogopite are found together with deep-seated xenoliths in the Cenozoic basalt of the Tuoyun Basin, Southwest Tianshan. The megacrysts are mainly in the cone sheet formed at the early stage of the volcanic activity. Clinopyrox-ene megacrysts are located in the lower part of the profile, with amphibole and phlogopite megacrysts in the middle part and anorthoclase megacrysts in the upper part. The crystal integrity, absence of de-formation fabric and their relation to the host basalt suggest that they were crystallized from the host magma and quickly transported to the surface. The mineralogical studies imply that the clinopyroxene megacrysts are of Al-augite with higher Al2O3 (>9%). Amphibole megacrysts are kaersutite rich in TiO2 (>4.5%). Sulfide inclusions such as pyrrhotite occur in some clinopyroxene and amphibole megacrysts. Thermodynamic calculations reveal that pyroxene megacrysts formed under the temperature of 1185.85―1199.85℃ and the pressure between 1.53 and 1.64 GPa comparable to the crust-mantle boundary and amphibole megacrysts crystallized under the pressure of around 0.85 GPa, temperature about 1000℃ comparable to the depth of 30 km. Anorthoclase megacrysts crystallized under the pressure between 0.8―1 GPa,temperature about 900℃.The absence of Ti-rich inclusions such as rutile can be considered as an evidence of quick magma ascending. The P-T conditions estimated via py-roxene megacrysts and phenocrysts compose a P-T path with a steep slope. It can be considered as another evidence of quick magma ascending. However, the estimated temperatures for amphibole megacrysts are markedly lower than those for pyroxene megacrysts given the same pressure. It probably shows that the amphiboles have crystallized at the vanguard of magma and under the vola-tile-rich condition. Thus, we can conclude that the Cenozoic basalts are produced in an extensional tectonic setting and the processes governing crystallization and ascending of the megacrysts are very complex.     

A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feeding system

 This work presents the results of a microthermometric and EPMA-SIMS study of melt inclusions in phenocrysts of rocks of the shoshonitic eruptive complex of Vulcano (Aeolian Islands, Italy). Different primitive magmas related to two different evolutionary series, an older one (50–25 ka) and a younger one (15 ka to 1890 A.D.), were identified as melt inclusions in olivine Fo_88–91 crystals. Both are characterized by high Ca/Al ratio and present very similar Rb/Sr, B/Be and patterns of trace elements, with Nb and Ti anomalies typical of a subduction zone. The two basalts present the same temperature of crystallization (1180±20  °C) and similar volatile abundances. The H₂O, S and Cl contents are relatively high, whereas magmatic CO₂ concentrations are very low, probably due to CO₂ loss before low-pressure crystallization and entrapment of melt inclusions. The mineral chemistry of the basaltic assemblages and the high Ca/Al ratio of melt inclusions indicate an origin from a depleted, metasomatized clinopyroxene-rich peridotitic mantle. The younger primitive melt is characterized with respect to the older one by higher K₂O and incompatible element abundances, by lower Zr/Nb and La/Nb, and by higher Ba/Rb and LREE enrichment. A different degree of partial melting of the same source can explain the chemical differences between the two magmas. However, some anomalies in Sr, Rb and K contents suggest either a slightly different source for the two magmas or differing extents of crustal contamination. Low-pressure degassing and cooling of the basaltic magmas produce shoshonitic liquids. The melt inclusions indicate evolutionary paths via fractional crystallization, leading to trachytic compositions during the older activity and to rhyolitic compositions during the recent one. The bulk-rock compositions record a more complex history than do the melt inclusions, due to the syneruptive mixing processes commonly affecting the magmas erupted at Vulcano. The composition and temperature data on melt inclusions suggest that in the older period of activity several shallow magmatic reservoirs existed; in the younger one a relatively homogeneous feeding system is active. The shallow magmatic reservoir feeding the recent eruptive activity probably has a vertical configuration, with basaltic magma in the deeper zones and differentiated magmas in shallower, low-volume, dike-like reservoirs. Received: 11 March 1998 / Accepted: 14 July 1998     

Ore-forming fluids in the dongping gold deposit, northwestern Hebei Province

The Dongping gold deposit is contained within an inner contacting zone of the Hercynian Shuiquangou alkali syenite. The ores occur as veins or as replacement bodies. Fluid inclusion observation shows that in early and main mineralizing stages inclusion types are gas and gas-liquid inclusions, respectively. Gas inclusions occur in isolation in vein quartz, their homog-enization temperature is 372-306°C, and salinity 3.7-1.0 wt% NaCl. Gas-liquid inclusions occur in clusters or healed fractures but do not cut quartz boundary with homogenization temperature 342-267°C and salinity 1.9-0.8 wt% NaCl. Stable isotope measurements show that at main gold mineralization, hydrogen and oxygen isotopic ratios of the mineralizing fluids are -70.8‰-108.4‰ and 2.44‰-4.05‰, respectively. Primary ore fluids in Dongping are higher temperature and lower salinity NaCl-CO₂-H₂O fluids, and come from Yanshanian granitic magmatism. Fluid im-miscibility, fluid-rock interactions and meteoric water adding were main reasons for gold deposition.     

Origin of natural sulphur-bearing immiscible inclusions and H2S in oolite gas reservoir, Eastern Sichuan

Based on results of microscopic observation and laser Raman analysis about fluid inclusions, multiple special forms of immiscible inclusions that contain sulphur, liquid hydrocarbon, bitumen, etc. were discovered in samples collected from the H₂S gas reservoir-containing carbonates in the Lower Triassic Feixianguan Formation in the Jinzhu-Luojia area, Kai County, Sichuan Province. Based on the lithology and burial history of the strata involved as well as measurement results of homogenization temperature of fluid inclusions, bitumen reflectivity, etc., it is concluded that the H₂S in the gas reservoir resulted from the thermal reaction between hydrocarbons in reservoir and CaSO₄ in the gypsum-bearing dolostone section at the high temperature (140°C–17°C) oil-cracked gas formation stage in Late Cretaceous. Thereafter, research on a great number of immiscible inclusions in the reservoir reveals that elemental sulphur resulted from oxidation of part of the earlier-formed H₂S and further reaction between sulphates, hydrocarbons and H₂S in geological fluids in H₂S-bearing gas reservoir at a temperature of 86°C–89°C and a pressure of 340×10⁵Pa and during the regional uplift stage as characterized by temperature decrease and pressure decrease in Tertiary. Meanwhile, gypsum, anhydrite and calcite formed at this stage would trap particles like elemental sulphur and result in a variety of special forms of immiscible inclusions, and these inclusions would contain information concerning the complexity of the fluids in the reservoir and the origin of H₂S and natural sulphur in the gas reservoir.     

Evolution of fluid geochemistry at the Turrialba volcano (Costa Rica) from 1998 to 2008

Turrialba (10°02′N, 83°45′W) is a 3,349-m high stratovolcano belonging to the Holocene “Cordillera Central” volcanic belt of Costa Rica. The summit consists of three EW-oriented craters (East, Central, and West). Since its last eruptive phase (1864–1866), the Central and West craters have displayed modest fumarolic activity, with outlet temperatures clustering around 90°C. In 2001, seismic swarms, ground deformation, and increasing fumarolic activity occurred. From 2005 to 2008, new fumarolic vents opened between and within the Central and West craters, and along the western and southwestern outer flanks of the volcanic edifice. These physical changes were accompanied by a drastic modification in the gas chemistry that can be divided in three stages: (1) hydrothermal (from 1998 to autumn 2001), characterized by the presence of H₂O, CO₂, H₂S, and, to a very minor extent, HCl and HF; (2) hydrothermal/magmatic (autumn 2001–2007), with the appearance of SO₂ and a significant increase of HCl and HF; and (3) magmatic-dominated (2007–2008), characterized by increased SO₂ content, SO₂/H₂S > 100, and temperatures up to 282°C. Accordingly, gas equilibrium in the CO₂-CH₄-H₂ system suggests a progressive evolution of the deep fluid reservoir toward higher temperatures and more oxidizing conditions. The chemical–physical modifications of Turrialba in the last decade can be interpreted as part of a cyclic mechanism controlling the balance between the hydrothermal and the magmatic systems. Nevertheless, the risk of rejuvenation of the volcanic activity cannot be excluded, and an appropriate seismic, ground deformation, and geochemical monitoring program is highly recommended. Turrialba lies at a distance of 35 and 15 km from San José and Cartago, respectively, the two largest cities in Costa Rica.     

Melting relationships in CaO-CO2 and MgO-CO2 to 36 kilobars with comments on CO2 in the mantle

The melting curves of CaCO₃ and MgCO₃ have been extended to pressures of 36 kb by experiments in piston-cylinder apparatus. At 30 kb, the melting temperatures of calcite and magnesite are 1610°C and 1585°C, respectively. New data for the magnesite dissociation reaction permit the location of an invariant point for the assemblage magnesite + periclase + liquid + vapor near 26 kb-1550°C. New data are also presented for the calcite-aragonite transition at 800°C, 950°C and 1100°C. At pressures above 36–50 kb, calcite and magnesite melt at temperatures lower than the solidus of dry mantle peridotite. Natural and experimental evidence suggests that carbon dioxide in the Earth's mantle could be present in a variety of forms: (a) a free vapor phase, (b) vapor dissolved in silicate magma, (c) crystalline carbonate, (d) carbonatite liquid, (e) carbon-bearing silicate analogs, or (f) carbonato-silicates (such as scapolite, spurrite, tilleyite, and related compounds).     

Precise revision of the garnet-muscovite geothermometer

The garnet-muscovite geothermometer was refined through empirical calibration by using natural rocks metamorphosed under the physical conditions of 238—1306 MPa and 490—700℃. Input temperatures and pressures were determined through simultaneously applying the garnet-biotite geothermometer and the garnet-biotite-plagioclase-quartz barometer, assuming that all FeO in muscovite and garnet be ferrous. Garnet was treated as the asymmetric quaternary solid solution, and muscovite as the symmetric binary solid solution. Input muscovite compositions include Fe atoms between 0.03—0.19 and Mg atoms between 0.04—0.16 on the basis of 11 oxygen atoms, and input garnet compositions include spessartine fractions between 0.01—0.289, grossular fractions between 0.028—0.273, and the Fe/Mg ratio between 3.387-18.986. The resulting garnet-muscovite geothermometer reproduces temperatures within (50℃ compared with the garnet-biotite thermometer. Total random error of ±37℃ of the new thermometer may stem from the pressure uncertainty of ±200 MPa, and uncertainties of ±5% of Fe and Mg components in muscovite, and ±5% of Fe, Mg, Mn and Ca components in garnet, altogether. When there exist 10%, 20%, 30%, 40% and 50% Fe3+ in muscovite, respectively, the computed garnet-muscovite temperatures will be 1—6℃, 2—12℃, 3—16℃, 5—24℃ and 7—29℃, respectively, lower than those obtained when assuming that all FeO be ferrous. The new garnet-muscovite geothermometer can efficiently reflect temperature change of typical prograde sequences and contact aureole rocks, and may be applied to low- to high-grade and low- to high-pressure metamorphic rocks.     

Evidence of fluid inclusion for thermal fluid-bearing hydrocarbon movements in Qiongdongnan Basin, South China Sea

Diagenetic research and inclusion observance indicate that there are seven types of inlcusion in the reservoirs in the Qiongdongnan Basin. Based on the fluorescence color, ratio of gas/liquid, formation temperature, salinity and organic component of fluid inclusions, three events of thermal fluid movement were found, and only the second and third events are relative to hydrocarbon migration and accumulation with the temperatures of 140–150°C and 170–190°C., respectively.The mechanism of gas migration in aqueous phase suggests that the discharging site of thermal fluid is the favourable location for natural gas accumulation. Project supported by the Natlonal Natural Science Foundation of China.     

Multi-episode fluid boiling in the Shizishan copper-gold deposit at Tongling, Anhui Province: its bearing on ore formation

The Shizishan copper-gold deposit at Tongling, Anhui Province consists of two magmato-hydrothermal mineralization types: the crypto-explosive breccia type and the skarn type. At least four episodes of boiling occurred to the ore-forming fluids in this deposit. The first episode took place in accompany with the formation of the crypto-explosive breccias. The melt-fluid inclusions giving temperatures above 600℃ and salinities higher than 42% NaCl equiv represent a residual magma related to this episode. The second episode occurred during skarnization, giving fluid temperatures of 422℃-472℃, averaging 458℃, and salinities of 10.2%-45.1% NaCl equiv. The third episode corresponds to the main mineralization stage, i.e., the quartz-sulphide stage. Fluid temperatures of this episode vary in a range of 337℃-439℃ with an average of 390℃, and salinities in a range of 3%-30% NaCl equiv. The forth episode happened at the waning stage of mineralization, giving fluid temperatures below 350℃ with an average of 265℃ and salinities of 2.1%-40.4% NaCl equiv.     

Volatile emission during the eruption of Baitoushan Volcano (China/North Korea) ca. 969 AD

3 [magma volume (DRE): 24 ± 5 km³]. The main phase (ca. 95 vol.%) is represented by comenditic tephra deposited dominantly as widespread fallout blankets and proximal ignimbrites. The eruption column is estimated to have reached ca. 25 km and thus entered the stratosphere. A late phase (5 vol.%) is represented by trachyte emplaced chiefly as moderately welded ignimbrites. The comendites contain  ∼ 3, and the trachytes 10–20 vol.% phenocrysts, mainly anorthoclase, hedenbergite, and fayalite. Primary glassy melt inclusions with no signs of leakage were found only in phenocrysts in the comenditic tephra, whereas those in phenocrysts in the trachytes are devitrified. The comendite magma is interpreted to have been generated by fractional crystallization from a trachyte magma represented by melt inclusions in the phenocrysts in the comendite tephra. The mass of volatiles emitted to the atmosphere during the eruption was estimated using the petrologic method. The average H₂O concentration of the comenditic matrix glass is 1.5 wt.% (probably largely secondary) and of the corresponding melt inclusions  ∼ 5.2 wt.%. Melt inclusions in feldspar and quartz present the highest halogen concentrations with a calculated average for chlorine of 4762 ppm and for fluorine of 4294 ppm. The comenditic matrix glasses are represented by a fluorine-rich (3992 ppm F) and fluorine-poor group (2431 ppm F), averaging 3853 ppm for chlorine. Only 20% of all sulfur analyses of the comenditic matrix glasses and melt inclusions are above the detection limit of  ≥ 250 ppm S. The difference between pre- and post-eruptive concentration of H₂O is at least 3.7 ± 0.6 wt.% H₂O taking into consideration re-hydration of the matrix glass and possible leakage of melt inclusions. The difference between pre- and post-eruptive concentrations of the halogens amounts to 909 ± 90 ppm Cl, and 1863 ± 280 ppm and 302 ± 40 ppm F. The difference for S was estimated based on the average of the maximum S concentrations in the melt inclusions (455 ppm S) and the detection limit, resulting in 205 ± 40 ppm S. The calculated mass of volatiles injected into the atmosphere, based on the erupted magma volume and volatile data, is 1796 ± 453 megatons for H₂O, 45 ± 10 megatons for chlorine, 42 ± 11 megatons for fluorine, and 2 ± 0.6 megatons for sulfur. The 969 ± 20 AD eruption of Baitoushan Volcano, one of the largest eruptions of the past 2000 years, is thought to have had a substantial but possibly short-lived effect on climate. Received: 25 July 1998 / Accepted: 8 September 1999     

Determination of paleo-pressure for a natural gas pool formation based on PVT characteristics of fluid inclusions in reservoir rocks

It has been proved to be a difficult problem to determine directly trapping pressure of fluid inclusions. Recently, PVT simulation softwares have been applied to simulating the trapping pressure of petroleum inclusions in reservoir rocks, but the reported methods have many limita-tions in practice. In this paper, a method is suggested to calculating the trapping pressure and temperature of fluid inclusions by combining the isochore equations of a gas-bearing aqueous inclusion with its coeval petroleum inclusions. A case study was conducted by this method for fluid inclusions occurring in the Upper-Paleozoic Shanxi Formation reservoir sandstones from the Ordos Basin. The results show that the trapping pressure of these inclusions ranges from 21 to 32 MPa, which is 6–7 MPa higher than their minimum trapping pressure although the trapping temperature is only 2–3°C higher than the homogenization temperature. The trapping pressure and temperature of the fluid inclusions decrease from southern area to northern area of the basin. The trapping pressure is obviously lower than the state water pressures when the inclusions formed. These data are consistent with the regional geological and geochemical conditions of the basin when the deep basin gas trap formed.