Sources of ore-forming fluids and metallic materials in the Jinwozi lode gold deposit,eastern Tianshan Mountains of China

This paper presents gas compositions and H-, O-isotope compositions of sulfide- and quartz-hosted fluid inclusions, and S-, Pb-isotope compositions of sulfide separates collected from the principal Stage 2 ores in Veins 3 and 210 of the Jinwozi lode gold deposit, eastern Tianshan Mountains of China. Fluid inclusions trapped in quartz and sphalerite are dominantly primary. H-and O-isotopic compositions of pyrite-hosted fluid inclusions indicate two major contributions to the ore-forming fluid that include the degassed magma and the meteoric-derived but rock ¹⁸O-buffered groundwater. However, H- and O-isotopic compositions of quartz-hosted fluid inclusions essentially suggest the presence of groundwater. Sulfide-hosted fluid inclusions show considerably higher abundances of gaseous species CO₂, N₂, H₂S, etc. than quartz-hosted ones. The linear trends among inclusion gaseous species reflect the mixing tendency between the gas-rich magmatic fluid and the groundwater. The relative enrichment of gaseous species in sulfide-hosted fluid inclusions, coupled with the banded ore structure indicating alternate precipitation of quartz with sulfide minerals, suggests that the magmatic fluid has been inputted to the ore-forming fluid in pulsation. Sulfur and lead isotope compositions of pyrite and galena separates indicate an essential magma derivation for sulfur but the multiple sources for metallic materials from the mantle to the bulk crust.

     

Sources of ore-forming fluids and metallic materials in the Jinwozi lode gold deposit,eastern Tianshan Mountains of China

This paper presents gas compositions and H-, O-isotope compositions of sulfide- and quartz-hosted fluid inclusions, and S-, Pb-isotope compositions of sulfide separates collected from the principal Stage 2 ores in Veins 3 and 210 of the Jinwozi lode gold deposit, eastern Tianshan Mountains of China. Fluid inclusions trapped in quartz and sphalerite are dominantly primary. H-and O-isotopic compositions of pyrite-hosted fluid inclusions indicate two major contributions to the ore-forming fluid that include the degassed magma and the meteoric-derived but rock ¹⁸O-buffered groundwater. However, H- and O-isotopic compositions of quartz-hosted fluid inclusions essentially suggest the presence of groundwater. Sulfide-hosted fluid inclusions show considerably higher abundances of gaseous species CO₂, N₂, H₂S, etc. than quartz-hosted ones. The linear trends among inclusion gaseous species reflect the mixing tendency between the gas-rich magmatic fluid and the groundwater. The relative enrichment of gaseous species in sulfide-hosted fluid inclusions, coupled with the banded ore structure indicating alternate precipitation of quartz with sulfide minerals, suggests that the magmatic fluid has been inputted to the ore-forming fluid in pulsation. Sulfur and lead isotope compositions of pyrite and galena separates indicate an essential magma derivation for sulfur but the multiple sources for metallic materials from the mantle to the bulk crust.     

Phenocryst-hosted melt inclusions record stalling of magma during ascent in the conduit and upper magma reservoir prior to vulcanian explosions, Soufrière Hills volcano, Montserrat, West Indies

The mechanics of explosive eruptions influence magma ascent pathways. Vulcanian explosions involve a stop–start mechanism that recurs on various timescales, evacuating the uppermost portions of the conduit. During the repose time between explosions, magma rises from depth and refills the conduit and stalls until the overpressure is sufficient to generate another explosion. We have analyzed major elements, Cl, S, H₂O, and CO₂ in plagioclase-hosted melt inclusions, sampled from pumice erupted during four vulcanian events at Soufrière Hills volcano, Montserrat, to determine melt compositions prior to eruption. Using Fourier transform infrared spectroscopy, we measured values up to 6.7 wt.% H₂O and 80 ppm CO₂. Of 42 melt inclusions, 81 % cluster between 2.8 and 5.4 wt.% H₂O (57 to 173 MPa or 2–7 km), suggesting lower conduit to upper magma reservoir conditions. We propose two models to explain the magmatic conditions prior to eruption. In Model 1, melt inclusions were trapped during crystal growth in magma that was stalled in the lower conduit to upper magma reservoir, and during trapping, the magma was undergoing closed-system degassing with up to 1 wt.% free vapor. This model can explain the melt inclusions with higher H₂O contents since these have sampled the upper parts of the magma reservoir. However, the model cannot explain the melt inclusions with lower H₂O because the timescale for plagioclase crystallization and melt inclusion entrapment is longer than the magma residence time in the conduit. In Model 2, melt inclusions were originally trapped at deeper levels of the magma chamber, but then lost hydrogen by diffusion through the plagioclase host during periodic stalling of the magma in the lower conduit system. In this second scenario, which we favor, the melt inclusions record re-equilibration depths within the lower conduit to upper magma reservoir.     

Potassium enrichment by magma mixing and vapor exsolution: an example in the miocene volcanism of eastern Morocco

Miocene volcanism in eastern Morocco is comprised of potash rich calc-alkalic and alkaline rocks. In the southern part of the area, at the base of the Guilliz massif, basic inclusions are found in latites. From a petrological, geochemical and mineralogical study of both latites and their inclusions, it appear that the inclusions represent a basic liquid quenched in the latitic magma, inside the magma chamber. As a result of the drop in pressure and crystallization, a K-rich vapour phase separated from the inclusion-forming liquid and percolated the latitic magma, increasing the K₂O content of the latter and possibly triggering eruptions. The mixing process between the two magmas seems supported by density and viscosity estimations. Calculations show that for temperatures ranging from 700 °C to 1000 °C and H₂O contents from 1.6 to 5%, the inclusion forming liquid is less dense than the latitic liquid and can therefore ascend into the latitic magma by interface disequilibrium and flotation.     

Sources of ore-forming fluids and metallic materials in the Jinwozi lode gold deposit, eastern Tianshan Mountains of China

This paper presents gas compositions and H-, O-isotope compositions of sulfide- and quartz-hosted fluid inclusions, and S-, Pb-isotope compositions of sulfide separates collected from the principal Stage 2 ores in Veins 3 and 210 of the Jinwozi lode gold deposit, eastern Tianshan Mountains of China. Fluid inclusions trapped in quartz and sphalerite are dominantly primary. H- and O-isotopic compositions of pyrite-hosted fluid inclusions indicate two major contributions to the ore-forming fluid that include the degassed magma and the meteoric-derived but rock 18O-buffered groundwater. However, H- and O-isotopic compositions of quartz-hosted fluid inclusions essentially suggest the presence of groundwater. Sulfide-hosted fluid inclusions show considerably higher abundances of gaseous species CO2, N2, H2S, etc. Than quartz-hosted ones. The linear trends among inclusion gaseous species reflect the mixing tendency between the gas-rich magmatic fluid and the groundwater. The relative enrichment of gaseous species in sulfide-hosted fluid inclusions, coupled with the banded ore structure indicating alternate precipitation of quartz with sulfide minerals, suggests that the magmatic fluid has been inputted to the ore-forming fluid in pulsation. Sulfur and lead isotope compositions of pyrite and galena separates indicate an essential magma derivation for sulfur but the multiple sources for metallic materials from the mantle to the bulk crust.     

The nature and distribution of carbon in submarine basalts and peridotite nodules

Primary carbonaceous material has been identified in submarine basaltic glasses and mantle-derived peridotite nodules from alkali basalts using electron microprobe techniques. In the submarine rocks carbon occurs (1) in quench-produced microcracks in glasses and phenocrysts, (2) in vesicles, where it is preferentially concentrated on the sulfide spherules attached to vesicle walls, and (3) in microcracks and CO₂-rich bubbles in inclusions of glass completely enclosed by phenocrysts. In peridotite nodules carbon exists in intergrain cracks, along grain boundaries, and on the walls of fluid inclusions disposed in two dimensional arrays. The carbonaceous material is believed to consist of a mixture of graphite, other forms of elemental carbon, and possibly small amounts of organic matter.It is suggested that carbon precipitates by disproportionation of CO according to the reaction 2 CO→C+CO₂ and that this reaction is catalyzed by sulfide-oxide surfaces in vesicles. Once deposition has begun, the reaction continues on carbon surfaces as well. Based on the large amounts of condensed carbon observed in some vapor inclusions and the apparent lack of oxidation features associated with them, it is proposed that carbon condensed from a magmatic vapor in which CO was a significant constituent. This implies that oxygen fugacities of undegassed basaltic melts under confining pressures of the shallow crust are typically lower than those of the QFM buffer at equivalent temperatures. This is in agreement with some intrinsic oxygen fugacity measurements on similar undegassed materials.Regardless of the mechanism of its formation, the presence of carbon in CO₂-rich vesicles and inclusions in basaltic glasses and mantle nodules adds uncertainty to estimates of minimum pressures of entrapment based on measurements of fluid densities. Condensed carbon also accounts for some of the carbon isotopic characteristics of these rocks.     

Trace-element compositions of single fluid inclusions in the Kofu granite, Japan: Implications for compositions of granite-derived fluids

Fluid inclusions in quartz from miarolitic cavities, pegmatites, and quartz veins in Miocene biotite-granite plutons, Kofu, Japan, were analyzed by particle-induced X-ray emission to examine chemistries and behaviors of granite-derived fluids in island-arc granite. Most inclusions are aqueous two-phase inclusions, and halite-bearing polyphase inclusions are also observed in quartz veins in the upper part of the plutons. From element contents of fluid inclusions in the miarolitic cavities, the original fluid released from the granite plutons during solidification is inferred to have concentrations of Mn, Fe, Cu, Zn, Ge, Br, Rb, Pb, and Ba of several tens to hundreds of parts per million by weight (ppm) and a salinity of about 10 wt% NaCl equivalent. We estimated the formation conditions of the fluid to have been at about 1.3–1.9 kb and 530–600°C on the basis of the homogenization temperatures of the inclusions and the solidification conditions of the plutons. The polyphase inclusions probably originated from hypersaline fluid by boiling of part of the released fluid during its ascent in the plutons. The polyphase inclusions contain several hundreds to tens of thousands of ppm of Fe and Mn, and tens to several hundreds of ppm of Cu, Zn, Br, Rb, and Pb. The salinities are about 35 wt% NaCl equivalent. Compositional variations in two-phase inclusions from the miarolitic cavities and quartz veins are primarily explained by mineral precipitation with dilution by surface water exerting a secondary influence. Thus, chemistries and behaviors of the granite-derived fluids in the plutons can be explained by mineral precipitation, boiling, and dilution of the originally released fluid.     

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.     

Fluid inclusion from drill hole DW-5, Hohi geothermal area, Japan: Evidence of boiling and procedure for estimating CO2 content

Fluid inclusion studies have been used to derive a model for fluid evolution in the Hohi geothermal area, Japan. Six types of fluid inclusions are found in quartz obtained from the drill core of DW-5 hole. They are: (I) primary liquid-rich with evidence of boiling; (II) primary liquid-rich without evidence of boiling; (III) primary vapor-rich (assumed to have been formed by boiling); (IV) secondary liquid-rich with evidence of boiling; (V) secondary liquid-rich without evidence of boiling; (VI) secondary vapor-rich (assumed to have been formed by boiling). Homogenization temperatures (T_h) range between 196 and 347°C and the final melting point of ice (T_m) between −0.2 and −4.3°C. The CO₂ content was estimated semiquantitatively to be between 0 and 0.39 wt. % based on the bubble behavior on crushing. NaCl equivalent solid solute salinity of fluid inclusions was determined as being between 0 and 6.8 wt. % after minor correction for CO₂ content.Fluid inclusions in quartz provide a record of geothermal activity of early boiling and later cooling. The CO₂ contents and homogenization temperatures of fluid inclusions with evidence of boiling generally increase with depth; these changes, and NaCl equivalent solid solute salinity of the fluid can be explained by an adiabatic boiling model for a CO₂-bearing low-salinity fluid. Some high-salinity inclusions without CO₂ are presumed to have formed by a local boiling process due to a temperature increase or a pressure decrease. The liquid-rich primary and secondary inclusions without evidence of boiling formed during the cooling process. The salinity and CO₂ content of these inclusions are lower than those in the boiling fluid at the early stage, probably as a result of admixture with groundwater.     

Petrogenesis and tectonic setting of bimodal volcanism in the Sakoli Mobile Belt, Central Indian shield

The Sakoli Mobile Belt comprises bimodal volcanic rocks that include metabasalt, rhyolite, tuffs, and epiclastic rocks with metapelites, quartzite, arkose, conglomerate, and banded iron formation (BIF). Mafic volcanic rocks are tholeiitic to quartz‐tholeiitic with normative quartz and hypersthene. SiO₂ shows a large compositional gap between the basic and acidic volcanics, depicting their bimodal nature. Both the volcanics have distinct geochemical trends but display some similarity in terms of enriched light rare earth element–large ion lithophile element characteristics with positive anomalies for U, Pb, and Th and distinct negative anomalies for Nb, P, and Ti. These characteristics are typical of continental rift volcanism. Both the volcanic rocks show strong negative Sr and Eu anomalies indicating fractionation of plagioclases and K‐feldspars, respectively. The high Fe/Mg ratios for the basic rocks indicate their evolved nature. Whole rock Sm–Nd isochrons for the acidic volcanic rocks indicate an age of crystallization for these volcanic rocks at about 1675 ± 180 Ma (initial ¹⁴³Nd/¹⁴⁴Nd = 0.51017 ± 0.00017, mean square weighted deviate [MSWD] = 1.6). The εNd_t (t = 2000 Ma) varies between ?0.19 and +2.22 for the basic volcanic rock and between ?2.85 and ?4.29 for the acidic volcanic rocks. Depleted mantle model ages vary from 2000 to 2275 Ma for the basic and from 2426 to 2777 Ma for the acidic volcanic rocks, respectively. These model ages indicate that protoliths for the acidic volcanic rocks probably had a much longer crustal residence time. Predominantly basaltic magma erupted during the deposition of the Dhabetekri Formation and part of it pooled at crustal or shallower subcrustal levels that probably triggered partial melting to generate the acidic magma. The influence of basic magma on the genesis of acidic magma is indicated by the higher Ni and Cr abundance at the observed silica levels of the acidic magma. A subsequent pulse of basic magma, which became crustally contaminated, erupted as minor component along with the dominantly acidic volcanics during the deposition of the Bhiwapur Formation.     

Fluids in the lithosphere, 2. Experimental constraints on CO2 transport in dunite and quartzite at elevated P-T conditions with implications for mantle and crustal decarbonation processes

In a series of experiments at 0.5–1.3 GPa and 1050–1200°C we have monitored the transport, via crack propagation, of CO₂ into well-annealed olivine and quartz aggregates. The objectives were to determine (1) the extent and rate of fluid penetration; (2) the effect of varying both P-T conditions and microstructure; and (3) the fluid penetration pathways. Experiments on CO₂ penetration into dunite annealed in the absence of MgO indicate rapid and pervasive fluid transport on a grain-dimension scale, but a limited penetration distance ( 1 mm). Additional experiments on dunite annealed in the presence of MgO (either dispersed or present at both ends), however, resulted in CO₂ penetration that was both pervasive on the scale of individual grains and almost always completely through the 5 mm long samples. The abundance of fine (10 μm) grains in the MgO-free dunite, in contrast to the much larger grain sizes of the samples annealed with MgO present, suggests the difference in fluid penetration behavior may arise because the strength variation in dunite scales with the grain size. Effects arising from changes in olivine point defect chemistry, however, are an additional possibility. The response of synthetic quartzite to CO₂ overpressure is distinct from that of dunite: Quartzite experiences rapid and complete penetration of CO₂, via a macroscopically visible system of transgranular fractures, over the range of P-T conditions investigated.The small amount of porosity ( 2–3%) present in most rock samples fabricated for this study, lacks three-dimensional connectivity, thus precluding any enhanced fluid penetration via porous flow. Pores could possibly enhance fluid penetration as the result of a small reduction in resistance to fracture, but the probable abundance of strength-controlling flaws in natural rocks is likely to produce similar behavior.The results of our experiments on olivine and olivine + MgO suggest that the transport of pressurized CO₂ in very olivine-rich mantle environments will be pervasive on the scale of individual grains and its extent may be dependent on rock microstructure and/or crystal chemical effects. Such pervasive fluid transport, perhaps associated with magma decarbonation, may have interesting implications for both magma transport and local LREE enrichment of adjacent mantle wall-rock. The ease with which quartzite is penetrated by CO₂ at the conditions of our experiments underscores the possible role of decarbonation reactions in crustal permeability-enhancement processes.     

The hydrothermal karstification and its effect on Ordovician carbonate reservoir in Tazhong uplift of Tarim Basin,Northwest China

With a detailed study on petrology, mineralogy and geochemistry of some important Ordovician carbonate well core samples in Tazhong uplift of Tarim Basin, the distinguishing symbols of hydrothermal karstification are first put forward as the phenomena of rock hot depigmentation, hot cataclasm and the appearance of typical hydrothermal minerals such as fluorite, barite, pyrite, quartz and sphalerite. The main homogenization temperatures of primary fluid inclusions in fluorite are from 260 to 310°C, indicating the temperature of hydrothermal fluid. The fluid affected the dissolved rocks and showed typical geochemistry features with low contents of Na and Mg, and high contents of Fe, Mn and Si. The ratio of ³He/⁴He is 0.02R _a, indicating the fluid from the typical continental crust. The hydrothermal fluid karstification pattern may be described as follows: the hot fluid is from the Permian magma, containing dissolving ingredients of CO₂ and H₂S, and shifts along fault, ruptures and unconformity, and dissolves the surrounding carbonates while it flows. The mechanism of hydrothermal karstification is that the mixture of two or more fluids, which have different ion intensity and pH values, becomes a new unsaturated fluid to carbonates. The hydrothermal karstification is an important process to form hypo-dissolved pinholes in Ordovician carbonates of Tazhong uplift of Tarim Basin, and the forming of hydrothermal minerals also has favorable influence on carbonate reservoirs.

     

The hydrothermal karstification and its effect on Ordovician carbonate reservoir in Tazhong uplift of Tarim Basin,Northwest China

With a detailed study on petrology, mineralogy and geochemistry of some important Ordovician carbonate well core samples in Tazhong uplift of Tarim Basin, the distinguishing symbols of hydrothermal karstification are first put forward as the phenomena of rock hot depigmentation, hot cataclasm and the appearance of typical hydrothermal minerals such as fluorite, barite, pyrite, quartz and sphalerite. The main homogenization temperatures of primary fluid inclusions in fluorite are from 260 to 310°C, indicating the temperature of hydrothermal fluid. The fluid affected the dissolved rocks and showed typical geochemistry features with low contents of Na and Mg, and high contents of Fe, Mn and Si. The ratio of ³He/⁴He is 0.02R _a, indicating the fluid from the typical continental crust. The hydrothermal fluid karstification pattern may be described as follows: the hot fluid is from the Permian magma, containing dissolving ingredients of CO₂ and H₂S, and shifts along fault, ruptures and unconformity, and dissolves the surrounding carbonates while it flows. The mechanism of hydrothermal karstification is that the mixture of two or more fluids, which have different ion intensity and pH values, becomes a new unsaturated fluid to carbonates. The hydrothermal karstification is an important process to form hypo-dissolved pinholes in Ordovician carbonates of Tazhong uplift of Tarim Basin, and the forming of hydrothermal minerals also has favorable influence on carbonate reservoirs.     

Shock reequilibration of fluid inclusions in Coconino sandstone from Meteor Crater,Arizona

This study examines the effects of natural shock metamorphism on fluid inclusions trapped in porous sedimentary target rocks and compares these results to previous experimental work on single crystal quartz. Samples of shock metamorphosed Coconino sandstone were collected from Barringer Meteorite Crater (Meteor Crater, Arizona) and classified based on their shock features into the six shock stages described by Kieffer [S.W. Kieffer, 1971. Shock metamorphism of the Coconino sandstone at Meteor Crater, Arizona, Journal of Geophysical Research 76, 5449-5473.]. The frequency of two-phase fluid inclusions decreases dramatically from unshocked samples of Coconino sandstone through shock stages 1a, 1b, and 2. No two-phase fluid inclusions were observed in shock stage 3 or 4 samples. However, the total number of grains containing fluid inclusions remains approximately the same for shock stages 1a–2, suggesting that two-phase fluid inclusions reequilibrated during impact to form single-phase inclusions. In shock stages 3 and 4, the total number of inclusions also decreases, indicating that at these higher shock pressures fluid inclusions are destroyed by plastic deformation and phase changes within the host mineral. Entrained quartz grains within a shock stage 5 sample contain two-phase inclusions, emphasizing the short duration of melting associated with the impact and the heterogeneous nature of impact processes. These results are similar to those observed in single-crystal experiments, although inclusions survive to slightly higher shock pressures in samples of naturally shocked Coconino sandstone. Results of this study suggest that the rarity of fluid inclusions in meteorites does not preclude the presence of fluids on meteorite parent bodies. Instead, fluid inclusions trapped during alteration events may have been destroyed due to shock processing. In addition, loss of fluids from inclusion vesicles along fractures and microcracks may lead to shock devolatilization, even in unsaturated target rocks.     

Ore-forming fluid and metallization of the Huanggangliang skarn Fe-Sn deposit, Inner Mongolia

Two kinds of inclusions, fluid-melting inclusion and gas-liquid inclusion, are present in the Huanggangliang deposit in eastern Inner Mongolia. Temperature ranges from 1050°C of fluid-melting inclusion to 150°C of liquid inclusion. Away from intrusion, the inclusions of orebodies intend to be characterized by simpler type, lower temperature and lower salinity, as well as weakened relation to intrusion. The metallization of the Huanggangliang deposit is characterized by multiple activities of ore-forming fluid, multi-source, multi-stage accumulation of ore-forming material, F-rich environment, enrichment of F, organic gas, CO₂ and N₂, and involving of residual magma.     

On the significance of CO2 inclusions in plagioclase microphenocrysts in tholeiite from Moeraki,New Zealand

Liquid plus vapour inclusions of CO₂ are widespread in plagioclase microphenocrysts in small tholeiitic intrusions and tephra of the Moeraki and adjacent areas of northeast Otago, New Zealand. They imply the presence of immiscible CO₂ droplets in the magma at depths of about 7–14 km. Their presence within 5 μm of the edges of microphenocrysts as little as 35 μm thick and 118 μm long indicates minimal feldspar crystal growth during the final ascent and quenching of the magma. Delicate branching clusters of lath-like microphenocrysts escaped disruption during this ascent. Such CO₂ inclusions are a potential source of ‘excess argon’ perturbing K-Ar age determinations. Received March 24, 1993/Accepted September 10, 1993     

Magma mingling in the ∼50 ka Rotoiti eruption from Okataina Volcanic Centre: implications for geochemical diversity and chronology of large volume rhyolites

Examination of glass and crystal chemistry in the Rotoiti Pyroclastics (>100 km³ of magma) demonstrates that compositional diversity was produced by mingling of the main rhyolite magma body with small volumes of other magmas that had been crystallizing in separate stagnant magma chambers. Most (>90%) of the Rotoiti deposits were derived from a low-K₂O, cummingtonite-bearing, rhyolitic magma (T1) discharged throughout the eruption sequence. T1 magma is homogeneous in composition (melt SiO₂=77.80±0.28 wt.%), temperature (766±13 °C) and oxygen fugacity (NNO+0.92±0.09). Most T1 phenocrysts formed in a shallow (∼200 MPa), near water-saturated (a_water=0.8) storage chamber shortly before eruption. Basaltic scoria erupted immediately before the rhyolites, and glass-bearing microdiorite inclusions within the rhyolite deposits, suggest that basalt emplaced on the floor of the chamber drove vigorous convection to produce the well-mixed T1 magma. Lithic lag breccias contain melt-bearing biotite granitoid inclusions that are compositionally distinct from T1 magma. The breccias which overlie the voluminous T1 pyroclastic flow deposits resulted from collapse of the syn-Rotoiti caldera. Post-collapse Rotoiti pumices contain T1 magma mingled with another magma (T2) that is characterized by high-K glass and biotite, and was cooler and less oxidised (712±16 °C; NNO−0.16±0.16). The mingled clasts contain bimodal disequilibrium populations of all crystal phases. The granitoid inclusions and the T2 magma are interpreted as derived from high-K magma bodies of varying ages and states of crystallization, which were adjacent to but not part of the large T1 magma body. We demonstrate that these high-K magmas contaminated the erupting T1 magma on a single pumice clast scale. This contamination could explain the reported wide range of zircon U–Th ages in Rotoiti pumices, rather than slow crystallization of a single large magma body.     

Magma–conduit interaction at Piton de la Fournaise volcano (Réunion Island): a melt and fluid inclusion study

Major-element, Cl, S, F analyses have been performed on a wide selection of melt inclusions trapped in olivine (Fo_81–87) from scoria and crystal-rich lapilli samples of Piton de la Fournaise volcano. As a whole, they display a transitional basaltic composition. The melt inclusions (8–9 wt.% MgO, 0.62–0.73 wt.% K₂O) are in equilibrium with olivines (Fo_81–85) in the samples from the Central Feeding Zone and the South-East Feeding Zone and show a slight alkaline affinity. The melt inclusions in olivines (Fo_85–87) from the North-West Rift (NWR) contain 9.3–9.7 wt.% MgO and 0.54–0.58 wt.% K₂O, with a more tholeiitic tendency. In oceanitic lavas and crystal-rich lapilli, the olivine xenocrysts are recognisable by the presence of one or more secondary shear plane fracture(s) filled up with CO₂ and alkali-rich basaltic melt inclusions. In dunite nodules, olivines present also contain several secondary shear plane fracture(s) filled up with CO₂ and high-SiO₂ melt inclusions. Secondary CO₂-rich fluid inclusions in olivine (Fo_85–87) from the NWR samples indicate PCO₂ up to 500 MPa whereas, PCO₂ ranges from 95 MPa to few tenths of bars in the other samples. Both the primary melt inclusions and the secondary fluid inclusions strongly suggest that the olivine crystallises and accumulates over a wide depth range (15 km). It is envisioned that cumulative pockets with low residual porosity are repeatedly percolated with a CO₂-rich fluid phase, possibly associated with basaltic to SiO₂-rich melts, and are finally disrupted and entrained to the surface when vigorous magma transfer occurs. The SiO₂-rich residual melts in early-formed dunitic or gabbroic bodies may have acted as contaminant agents for the more alkali character of magmas vented through the central feeding system, where a well-developed cumulative system is thought to exist. Finally, the existence of secondary fluid and melt inclusions in olivines implies that the dunitic bodies are weakened on the micrometric scale.     

The before-eruption water content of some high-alumina magmas

An analysis by difference technique yields estimates of H₂O in basaltic and andesitic glasses, which are sufficiently accurate (± 1.4 percent absolute) to be useful. Glass inclusions trapped in large olivine crystals from tephra-rich eruptions have 1 to 5 percent H₂O. The highest H₂O contents are found in basaltic inclusions in magnesium rich olivines from Mount Shasta, California. Andesitic inclusions have less H₂O. It seems probable that tephra-rich high-alumina magmas evolve in a vapor saturated environment at fairly shallow depths (few kilometers). This depth appears to be less for Medicine Lake Highlands than for Mount Shasta. Vapor saturation probably inhibits the rise of magma, thus the initial vapor content of a magma may govern its stagnation level. Volatile-rich parental magmas like Mount Shasta basalt probably tend to stagnate at deeper levels, crystallize early amphibole and produce comparatively calcic differentiates.     

Femtosecond lasers in fluid inclusion analysis: Three-dimensional imaging and determination of inclusion volume in quartz using second harmonic generation microscopy

Optical second harmonic generation microscopy was used to generate three-dimensional images of fluid inclusions in quartz. Because the exciting light was focused with a high numerical aperture microscope objective, phase-matching conditions in the quartz led to the generation of frequency doubled light only at the surfaces of the fluid inclusions and not in the bulk material, providing an axial resolution of approximately 1 μm. The volume of synthetic two-phase water inclusions was determined from second harmonic images. The bubble volume was measured using brightfield microscopy (the sample was heated to ensure a smaller bubble that could take on a spherical shape inside the inclusion). The bulk molar volume was then calculated from the vapor–liquid ratio. The values obtained for the bulk molar volume were found to be in good agreement with values calculated from measurements of the homogenization temperature.