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.     

Lateral variation of differential stress in the uppermost mantle across the island arc of southwest Japan

The dislocation density and the subgrain size of olivine in peridotite xenoliths in southwest Japan were investigated in order to draw out the lateral variation of the differential stress in the upper mantle of the island arc. Alkali basaltic and andesitic dykes including peridotite xenoliths of Dogo, Kikuma, and Shingu are situated about 200 km behind the Nankai Trough, and those of Oki-Dogo and Takashima located at the portions 400–500 km apart from the trough. The mean dislocation densities of olivine are 2 × 10⁶ cm^?2 for Oki-Dogo, 8 × 10⁶ cm^?2 for Takashima, 1 × 10⁷ cm^?2 for Hamada, 5 × 10⁷ cm^?2 for Aratoyama, 4 × 10⁷ cm^?2 for Kikuma, 3 × 10⁷ cm^?2 for Dogo, and 5 × 10⁶ cm^?2 for Shingu peridotites.It is concluded that the differential stress is high in the uppermost mantle beneath the island arc and low in the back-arc and the mantle wedge behind the plate boundary. The lateral variation of stress may be due to the diapiric upwelling of upper mantle materials under the island arc. The size of the diapir is suggested to be 200 km in width and 60–150 km in depth.     

Deep pooling of low degree melts and volatile fluxes at the 85°E segment of the Gakkel Ridge: Evidence from olivine-hosted melt inclusions and glasses

We present new analyses of volatile, major, and trace elements for a suite of glasses and melt inclusions from the 85°E segment of the ultra-slow spreading Gakkel Ridge. Samples from this segment include limu o pele and glass shards, proposed to result from CO₂-driven explosive activity. The major element and volatile compositions of the melt inclusions are more variable and consistently more primitive than the glass data. CO₂ contents in the melt inclusions extend to higher values (167–1596 ppm) than in the co-existing glasses (187–227 ppm), indicating that the melt inclusions were trapped at greater depths. These melt inclusions record the highest CO₂ melt concentrations observed for a ridge environment. Based on a vapor saturation model, we estimate that the melt inclusions were trapped between seafloor depths (～ 4 km) and ～ 9 km below the seafloor. However, the glasses are all in equilibrium with their eruption depths, which is inconsistent with the rapid magma ascent rates expected for explosive activity. Melting conditions inferred from thermobarometry suggest relatively deep (25–40 km) and cold (1240°–1325 °C) melting conditions, consistent with a thermal structure calculated for the Gakkel Ridge. The water contents and trace element compositions of the melt inclusions and glasses are remarkably homogeneous; this is an unexpected result for ultra-slow spreading ridges, where magma mixing is generally thought to be less efficient based on the assumption that steady-state crustal magma chambers are absent in these environments. All melts can be described by a single liquid line of descent originating from a pooled melt composition that is consistent with the aggregate melt calculated from a geodynamic model for the Gakkel Ridge. These data suggest a model in which deep, low degree melts are efficiently pooled in the upper mantle (9–20 km depth), after which crystallization commences and continues during ascent and eruption. Based on our melting model and the assumption that CO₂ is perfectly incompatible, we show that the highest CO₂ concentrations of the melt inclusions (～ 1600 ppm) are consistent with the calculated CO₂ concentrations of primary undegassed melts. The highest measured CO₂/Nb ratio (443) of Gakkel Ridge melt inclusions predicts a mantle CO₂ content of 134 ppm and would result in a global ridge flux of 2.0 × 10¹² mol CO₂/yr.     

Dissolved carbon dioxide in basaltic glasses: concentrations and speciation

Carbon dioxide dissolved in both synthetic Ca±Mg-bearing silicate glasses and natural basaltic glasses has been characterized using infrared spectroscopy. CO₂ is inferred to be dissolved in these glasses as distorted Ca or Mg carbonate ionic complexes that result in unique infrared absorption bands at 1515 cm⁻¹ and 1435 cm⁻¹. This speciation contrasts with the case of CO₂-bearing sodium aluminosilicate glasses, which contain both dissolved molecular CO₂ and dissolved Na-carbonate ionic-complexes. The difference in speciation in Ca±Mg-bearing melts may result in part from a higher activity of oxygens that react with CO₂ molecules to produce carbonate.Dissolved CO₂ contents of natural basaltic glasses can be determined from the intensities of the carbonate absorption bands at 1515 cm⁻¹ and 1435 cm⁻¹. The uncertainty of the method is estimated to be ± 15% of the amount present. The infrared technique is a powerful tool for the measurement of dissolved CO₂ contents in natural basaltic glasses since it is non-destructive, can be aimed at regions of glass a few tens of microns in size, and can discriminate between dissolved carbonate and carbon present as carbonate alteration, contained in fluid inclusions, or adsorbed on the glass.A set of submarine basaltic glasses dredged from a variety of locations contain 0–400 ppm dissolved CO₂, measured using the infrared technique. These concentrations are lower than most previous reports for similar basaltic glasses. No general relationship is observed between dissolved CO₂ content and depth of magmatic eruption, although some correlation might be present in restricted geographic locales.     

Limestone assimilation and the origin of CO2 emissions at the Alban Hills (Central Italy): Constraints from experimental petrology

The Alban Hills volcanic region (20 km south of Rome, in the Roman Province) emitted a large volume of potassic magmas (> 280 km³) during the Quaternary. Chemical interactions between ascending magmas and the ∼ 7000–8000-m-thick sedimentary carbonate basement are documented by abundant high temperature skarn xenoliths in the eruptive products and have been frequently corroborated by geochemical surveys. In this paper we characterize the effect of carbonate assimilation on phase relationships at 200 MPa and 1150–1050 °C by experimental petrology. Calcite and dolomite addition promotes the crystallization of Ca-rich pyroxene and Mg-rich olivine respectively, and addition of both carbonates results in the desilication of the melt. Furthermore, carbonate assimilation liberates a large quantity of CO₂-rich fluid. A comparison of experimental versus natural mineral, glass and bulk rock compositions suggests large variations in the degree of carbonate assimilation for the different Alban Hills eruptions. A maximum of 15 wt.% assimilation is suggested by some melt inclusion and clinopyroxene compositions; however, most of the natural data indicate assimilation of between 3 and 12 wt.% carbonate. Current high CO₂ emissions in this area most likely indicate that such an assimilation process still occurs at depth. We calculate that a magma intruding into the carbonate basement with a rate of ∼ 1 – 2 · 10⁶ m³/year, estimated by geophysical studies, and assimilating 3–12 wt.% of host rocks would release an amount of CO₂ matching the current yearly emissions at the Alban Hills. Our results strongly suggest that current CO₂ emissions in this region are the shallow manifestation of hot mafic magma intrusion in the carbonate-hosted reservoir at 5–6 km depth, with important consequences for the present-day volcanic hazard evaluation in this densely populated and historical area.     

Partial pressure and air–sea CO2 flux in the Aegean Sea during February 2006

Data on the distribution of fCO₂ were obtained during a cruise in the Aegean Sea during February 2006. The fCO₂ of surface water (fCO₂^sw) was lower than the atmospheric fCO₂ (fCO₂^atm) throughout the area surveyed and ΔfCO₂ values varied from ?34 to ?61 μatm. The observed under-saturation suggests that surface waters in the Aegean represent a sink for atmospheric CO₂ during the winter of 2006. Higher fCO₂^sw values were recorded in the ‘less warm’ and ‘less saline’ shallow northernmost part of the Aegean Sea implying that the lower seawater temperature and salinity in this area play a crucial role in the spatial distribution of fCO₂^sw.A first estimate of the magnitude of the air–sea CO₂ exchange and the potential role of the Aegean Sea in the transfer of atmospheric CO₂ was also obtained. The air–sea CO₂ fluxes calculated using different gas transfer formulations showed that during February 2006, the Aegean Sea absorbs atmospheric CO₂ at a rate ranging from ?6.2 to ?11.8 mmol m^?2 d^?1 with the shipboard recorded wind speeds and at almost half rate (?3.5 to ?5.5 mmol m^?2 d^?1) with the monthly mean model-derived wind speed. Compared to recent observations from other temperate continental shelves during winter period, the Aegean Sea acts as a moderate to rather strong sink for atmospheric CO₂.Further investigations, including intensive spatial and temporal high-resolution observations, are necessary to elucidate the role of the Aegean Sea in the process of transfer of atmospheric CO₂ into the deep horizons of the Eastern Mediterranean.     

Mineralogical and geochemical study of a newly discovered mafic granulite, northwest China: Implications for tectonic evolution of the Altay Orogenic Belt

Abstract A mafic granulite body was newly discovered in the Altay Orogenic Belt, northwest China. The rocks comprise a suite of coarse‐grained and fine‐grained granulites. Orthopyroxenes (hypersthenes) in the rocks have high X_Mg and low Al₂O₃ contents, whereas clinopyroxenes have low TiO₂ and Al₂O₃ contents. Amphiboles and biotites have a high Mg/(Mg + Fe²⁺) ratio and low contents of F and Cl. The peak metamorphic pressure–temperature (P–T) conditions are estimated as 750–780°C and 6–7 kbar, and retrograde P–T conditions are in the range of 590–620°C and 2.3–3.7 kbar, indicating significant decompression. Metamorphic reactions and P–T estimates define a clockwise P–T path. Geochemically, the rocks are high in Mg/(Mg + Fe) and Al₂O₃, depleted in U, Th, K and Rb, and characterized by light rare earth element enrichment and a weak positive Eu anomaly. The Altay mafic granulite shows depleted Nb, P and Ti contents in the mid‐oceanic ridge basalt normalized spider diagram. The geochemical characteristics suggest that the protolith of the Altay mafic granulite was calc‐alkaline basalt and andesite with an island‐arc affinity. The rock has a high ¹⁴³Nd/¹⁴⁴Nd ratio with ?Nd(0) > 0, indicating derivation from a mantle‐depleted source. In the present study, a two‐stage model for the evolution of the Altay mafic granulite is proposed: an early stage in which calc‐alkaline basalt and andesite with island‐arc affinity were subducted into a deeper level of the crust and subjected to granulite‐facies metamorphism generating the mafic granulite, followed by the later stage exhumation of the system into the upper crust by the late Paleozoic thrusting.     

Relocation of aftershocks of the 2001 Bhuj earthquake: A new insight into seismotectonics of the Kachchh seismic zone,Gujarat, India

In view of an anomalous crust–mantle structure beneath the 2001 Bhuj earthquake region, double-difference relocations of 1402 aftershocks of the 2001 Bhuj earthquake were determined, using an improved 1D velocity model constructed from 3D velocity tomograms based on data from 10 to 58 three-component seismograph stations. This clearly delineated four major tectonic features: (i) south-dipping north Wagad fault (NWF), (ii and iii) south-dipping south Wagad faults 1 and 2 (SWF₁, SWF₂), and (iv) a northeast dipping transverse fault (ITF), which is a new find. The relocated aftershocks correlate satisfactorily with the geologically mapped and inferred faults in the epicentral region. The relocated focal depths delineate a marked variation to the tune of 12 km in the brittle–ductile transition depths beneath the central aftershock zone that could be attributed to a lateral variation in crustal composition (more or less mafic) or in the level of fracturing across the fault zone. A fault intersection between the NWF and ITF has been clearly mapped in the 10–20 km depth range beneath the central aftershock zone. It is inferred that large intraplate stresses associated with the fault intersection, deepening of the brittle–ductile transition to a depth of 34 km due to the presence of mafic/ultramafic material in the crust–mantle transition zone, and the presence of aqueous fluids (released during the metamorphic process of eclogitisation of lower crustal olivine-rich rocks) and volatile CO₂ at the hypocentral depths, might have resulted in generating the 2001 Bhuj earthquake sequence covering the entire lower crust.     

Effects of soil compaction,rain exposure and their interaction on soil carbon dioxide emission

Soils release more carbon, primarily as carbon dioxide (CO₂), per annum than current global anthropogenic emissions. Soils emit CO₂ through mineralization and decomposition of organic matter and respiration of roots and soil organisms. Given this, the evaluation of the effects of abiotic factors on microbial activity is of major importance when considering the mitigation of greenhouse gases emissions. Previous studies demonstrate that soil CO₂ emission is significantly affected by temperature and soil water content. A limited number of studies have illustrated the importance of bulk density and soil surface characteristics as a result of exposure to rain on CO₂ emission, however, none examine their relative importance. Therefore, this study investigated the effects of soil compaction and exposure of the soil surface to rainfall and their interaction on CO₂ release. We conducted a factorial laboratory experiment with three soil types after sieving (clay, silt and sand soil), three different bulk densities (1·1 g cm^–3, 1·3 g cm^–3, 1·5 g cm^–3) and three different exposures to rainfall (no rain, 30 minutes and 90 minutes of rainfall). The results demonstrated CO₂ release varied significantly with bulk density, exposure to rain and time. The relationship between rain exposure and CO₂ is positive: CO₂ emission was 53% and 42% greater for the 90 minutes and 30 minutes rainfall exposure, respectively, compared to those not exposed to rain. Bulk density exhibited a negative relationship with CO₂ emission: soil compacted to a bulk density of 1·1 g cm^–3 emitted 32% more CO₂ than soil compacted to 1·5 g cm^–3. Furthermore we found that the magnitude of CO₂ effluxes depended on the interaction of these two abiotic factors. Given these results, understanding the influence of soil compaction and raindrop impact on CO₂ emission could lead to modified soil management practices which promote carbon sequestration. Copyright © 2012 John Wiley & Sons, Ltd.     

Petrological feature of spinel lherzolite xenolith from Oki-Dogo Island: An implication for variety of the upper mantle peridotite beneath southwestern Japan

Abstract   Spinel lherzolite is a minor component of the deep-seated xenolith suite in the Oki-Dogo alkaline basalts, whereas other types of ultramafic (e.g. pyroxenite and dunite) and mafic (e.g. granulite and gabbro) xenoliths are abundant. All spinel lherzolite xenoliths have spinel with a low Cr number (Cr#; < 0.26). They are anhydrous and are free of modal metasomatism. Their mineral assemblages and microtextures, combined with the high NiO content in olivine, suggest that they are of residual origin. But the Mg numbers of silicate minerals are lower (e.g. down to Fo₈₆) in some spinel lherzolites than in typical upper mantle residual peridotites. The clinopyroxene in the spinel lherzolite shows U-shaped chondrite-normalized rare-earth element (REE) patterns. The abundance of Fe-rich ultramafic and mafic cumulate xenoliths in Oki-Dogo alkali basalts suggests that the later formation of those Fe-rich cumulates from alkaline magma was the cause of Fe- and light REE (LREE)-enrichment in residual peridotite. The similar REE patterns are observed in spinel peridotite xenoliths from Kurose and also in those from the South-west Japan arc, which are non-metasomatized in terms of major-element chemistry (e.g. Fo > 89), and are rarely associated with Fe-rich cumulus mafic and ultramafic xenoliths. This indicates that the LREE-enrichment in mantle rocks has been more prominent and prevalent than Fe and other major-element enrichment during the metasomatism.     

A new occurrence of retrogressed eclogite from the Sanbagawa belt of southwest Japan and its significance

The present paper reports, for the first time, the occurrence of an omphacite‐bearing mafic schist from the Asemi‐gawa region of the Sanbagawa belt (southwest Japan). The mafic schist occurs as thin layers within pelitic schist of the albite–biotite zone. Omphacite in the mafic schist only occurs as inclusions in garnet, and albite is the major Na phase in the matrix, suggesting that the mafic schist represents highly retrogressed eclogite. Garnet grains in the sample show prograde‐type compositional zoning with no textural or compositional break, and contain mineral inclusions of omphacite, quartz, glaucophane, barroisite/hornblende, epidote and titanite. In addition to the petrographic observations, Raman spectroscopy and focused ion beam system–transmission electron microscope analyses were used for identification of omphacite in the sample. The omphacite in the sample shows a strong Raman peak at 678 cm^?1, and concomitant Raman peaks are all consistent with those of the reference omphacite Raman spectrum. The selected area electron diffraction pattern of the omphacite is compatible with the common P2/n omphacite structure. Quartz inclusions in the mafic schist preserve high residual pressure values of Δω₁ > 8.5 cm^?1, corresponding to the eclogite facies conditions. The combination of Raman geothermobarometries and garnet–clinopyroxene geothermometry gives peak pressure–temperature (P–T) conditions of 1.7–2.0 GPa and 440–540 °C for the mafic schist. The peak P–T values are comparable to those of the schistose eclogitic rocks in other Sanbagawa eclogite units of Shikoku. These findings along with previous age constraints suggest that most of the Sanbagawa schistose eclogites and associated metasedimentary rocks share similar simple P–T histories along the Late Cretaceous subduction zone.     

Dating the lower crust by ion microprobe

Ion microprobe UThPb ages of zircons from granulite facies lower crustal xenoliths from north Queensland, Australia, correlate well with the ages of major orogenic episodes manifest at the earth's surface. About half of the xenoliths contain Proterozoic zircons which are similar in age to the episodes of high-grade metamorphism of the older surface rocks. All the xenoliths contain late Paleozoic zircons which show a real 100 Ma range in²⁰⁶Pb²³⁸/U ages (from 320 to 220 Ma), which is attributed to granulite facies metamorphism followed by slow cooling in the deep crust. The Paleozoic zircon ages coincide in time with the prolonged episode of eruption of voluminous felsic ash-flows and intrusion of high-level granites in this region (320-270 Ma). Mineral and melt inclusions in the zircons provide clues to the origin of some of the xenoliths, and coupled with the age information, can be used to infer the geological processes operating in the lower crust. The zircons from two mafic xenoliths contain felsic and intermediate melt inclusions implying at least a two-stage history for these rocks, involving either partial melting of a more felsic protolith or crystal accumulation from an evolved melt. Some of the zircons from the felsic xenoliths contain CO₂-rich fluid inclusions, indicating that those zircons grew during high-grade metamorphism. The isotopic and chemical data for the whole rock xenoliths show that they originate from a segment of the lower crust which is a heterogeneous mixture of supracrustal and mafic, mantle-derived, lithologies. The major orogenic event responsible for the formation of that crust occurred in the late Paleozoic, when Proterozoic supracrustal rocks were emplaced into the lower crust, possibly along thin-skinned thrust slices. This was accompanied by intrusion of high-temperature, mantle-derived melts which caused partial melting of pre-existing crust. The most likely setting for such tectonism is a continental margin subduction zone.     

Crustal seismic structure and depth distribution of earthquakes in the Archean Kuusamo region,Fennoscandian Shield

Two-dimensional crustal velocity models are derived from passive seismic observations for the Archean Karelian bedrock of north-eastern Finland. In addition, an updated Moho depth map is constructed by integrating the results of this study with previous data sets. The structural models image a typical three-layer Archean crust, with thickness varying between 40 and 52 km. P wave velocities within the 12–20 km thick upper crust range from 6.1 to 6.4 km/s. The relatively high velocities are related to layered mafic intrusive and volcanic rocks. The middle crust is a fairly homogeneous layer associated with velocities of 6.5–6.8 km/s. The boundary between middle and lower crust is located at depths between 28 and 38 km. The thickness of the lower crust increases from 5–15 km in the Archean part to 15–22 km in the Archean–Proterozoic transition zone. In the lower crust and uppermost mantle, P wave velocities vary between 6.9–7.3 km/s and 7.9–8.2 km/s. The average V_p/V_s ratio increases from 1.71 in the upper crust to 1.76 in the lower crust.The crust attains its maximum thickness in the south-east, where the Archean crust is both over- and underthrust by the Proterozoic crust. A crustal depression bulging out from that zone to the N–NE towards Kuusamo is linked to a collision between major Archean blocks. Further north, crustal thickening under the Salla and Kittilä greenstone belts is tentatively associated with a NW–SE-oriented collision zone or major shear zone. Elevated Moho beneath the Pudasjärvi block is primarily explained with rift-related extension and crustal thinning at ∼2.4–2.1 Ga.The new crustal velocity models and synthetic waveform modelling are used to outline the thickness of the seismogenic layer beneath the temporary Kuusamo seismic network. Lack of seismic activity within the mafic high-velocity body in the uppermost 8 km of crust and relative abundance of mid-crustal, i.e., 14–30 km deep earthquakes are characteristic features of the Kuusamo seismicity. The upper limit of seismicity is attributed to the excess of strong mafic material in the uppermost crust. Comparison with the rheological profiles of the lithosphere, calculated at nearby locations, indicates that the base of the seismogenic layer correlates best with the onset of brittle to ductile transition at about 30 km depth.We found no evidence on microearthquake activity in the lower crust beneath the Archean Karelian craton. However, a data set of relatively well-constrained events extracted from the regional earthquake catalogue implies a deeper cut-off depth for earthquakes in the Norrbotten tectonic province of northern Sweden.     

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.     

Static compression of hydrous silicate melt and the effect of water on planetary differentiation

High pressure experiments using the sink/float method have bracketed the density of hydrous iron-rich ultrabasic silicate melt from 1.35 to 10.0 GPa at temperatures from 1400 to 1860 °C. The silicate melt composition was a 50–50 mixture of natural komatiite and synthetic fayalite. Water was added in the form of brucite Mg(OH)₂ and was present in the experimental run products at 2 wt.% and 5 wt.% levels as confirmed by microprobe analyses of total oxygen. Buoyancy marker spheres were olivines and garnets of known composition and density. The density of the silicate melt with 5 wt.% water at 2 GPa and 1500 °C is 0.192 g cm^? 3 less than the anhydrous form of this melt at the same P and T. This density difference gives a partial molar volume of water in silicate melt of ～ 7 cm³ mol^? 1, which is similar to previous studies at high pressure. The komatiite–fayalite liquids with 0 and 2 wt.% H₂O, have extrapolated density crossovers with equilibrium liquidus olivine at 8 and 9 GPa respectively, but there is no crossover for the liquid with 5 wt.% H₂O. These results are consistent with the hypothesis that dense hydrous melts could be gravitationally stable atop the 410 km discontinuity in the Earth. The results also support the notion that equilibrium liquidus olivine could float in an FeO-rich hydrous martian magma ocean. Extrapolation of the data suggests that FeO-rich hydrous melt could be negatively buoyant in the Earth's D″-region or atop the core–mantle-boundary (CMB), although experiments at higher pressure are needed to confirm this prediction.     

Gas percolation in upper-crustal silicic crystal mushes as a mechanism for upward heat advection and rejuvenation of near-solidus magma bodies

Several crystal-rich, intermediate to silicic magmas erupted at arc volcanoes record a reheating event shortly prior to eruption: they provide evidence for remobilization of crystal mushes by mafic magmas. As hybridization between the mush and the mafic magma is often limited, bulk mixing could not be the dominant process in transferring heat. Conductive heating from a basaltic underplate plays a role, but a few characteristics of these rejuvenated mushes suggest that reheating occurs faster than predicted by conduction.In the upper crust, a process that can transport heat faster than conduction, and still remain chemically nearly imperceptible, is the upward migration of a hot volatile phase (“gas sparging”) that originates in underplated mafic magmas. Using numerical simulations, we quantified the thermal effects of two-phase flow (a silicic melt phase and a H₂O–CO₂ fluid phase) in the pore space of shallow silicic mushes that have reached their rheological lock-up point (i.e., rigid porous medium, crystallinity ≥ 50 vol.%). Results show that the reheating rates are significantly faster than conduction for volatile fluxes > 0.1 m³/m² yr. Considering that volatiles can be rapidly exsolved from the underplated mafic magma, these high fluxes can be promptly reached, leading to fast reheating; sill-like batches of mushes with volumes similar to the 1995–present eruption of the Soufrière Hills (Montserrat, W.I.) can be reheated by a few tens of degrees and remobilized within days to weeks. At these high fluxes, a considerable volume of volatiles is needed (similar to the volume of mush being reheated). Large silicic systems (> 100–1000 km³) require unrealistic amounts of volatiles to be reheated in a continuous, high-flux sparging event. Rejuvenation of batholithic mushes therefore requires multiple sparging episodes separated by periods dominated by near-conductive heat transfer at low-flux sparging (< 0.1 m³/m² yr) and may take up to 100–200 ky.     

由包体推导的河北汉诺坝下地壳-上地幔地温线及其地质意义

通过对采自河北汉诺坝玄武岩中的下地壳和上地幔包体的详细研究 ,建立了本区下地壳—上地幔地温线。该地温线高于大洋地温线和古老地盾地温线 ,接近克拉通边缘的地温线 ,符合该区的大地构造环境。由该地温线建立的下地壳—上地幔地质结构剖面表明 ,该区下地壳主要由不同类型的麻粒岩相岩石组成 ,其化学成分以镁铁质为主 ,深度范围为 2 5～ 4 2km。上地幔由超镁铁质的二辉橄榄岩组成 ,在尖晶石二辉橄榄岩和石榴石二辉橄榄岩之间有一过渡层。由地温线确定的壳幔边界位于 4 2km附近 ,与地震资料确定的莫霍面一致 ,但在壳幔边界之上的下地壳底部有下地壳麻粒岩和超镁铁质岩的互层。这一现象可以解释在下地壳底部常见的层状反射层。该区岩石圈底界大约在 95km ,其下的软流层仍由石榴石二辉橄榄岩组成     

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.     

The pre-eruptive volatile contents of recent basaltic and pantelleritic magmas at Pantelleria (Italy)

Pantelleria Island, located in the Sicily Channel Rift Zone (Italy), is the type locality for the peralkaline rhyolitic rocks called pantellerites. In the last 50 ka, after the large Green Tuff caldera-forming eruption, volcanic activity at Pantelleria has consisted of effusive and explosive eruptions mostly vented inside and along the rim of the caldera and producing silicic lava flows, lava domes and poorly dispersed pantelleritic pumice fall deposits. Basaltic cinder cones and lava flows are only present outside the caldera in the NW sector of the island. The most recent basaltic (Cuddie Rosse, ～ 20 ka) and pantelleritic (Cuddia Randazzo and Cuddia del Gallo, ～ 6 ka) pyroclastic products were sampled to investigate magmatic volatile contents through the study of melt inclusions.The melt inclusions in pyroxene and olivine phenocrysts of Cuddie Rosse scoriae have an alkali basalt composition. The dissolved volatiles comprise 0.9–1.6 wt.% H₂O, several hundred ppm of CO₂, 1600–2000 ppm of sulphur and 500–900 ppm of chlorine. The water–carbon dioxide couple gives a confining pressure ～ 2 kbar prior to the eruption. This result indicates that episodes of magma ponding and crystallization occurred in the upper crust prior to eruption. The melt inclusions in feldspar, fayalite and aenigmatite phenocrysts of Cuddia del Gallo and Cuddia Randazzo pumice have a pantelleritic composition (Agpaitic Indices 1.3–2.1), up to 4.4 wt.% H₂O, 8700 ppm Cl, 6000 ppm F, and CO₂ below the detection limit. Sulphur averaging 420 ppm has been measured in Cuddia Randazzo melt inclusions. These data indicate relatively high volatile contents for these low-energy Strombolian-type eruptions. Melt inclusions in Cuddia del Gallo pumice show the most evolved composition (Agpaitic Indices 2–2.1) and the highest volatile content, in agreement with fluid saturation conditions in the magma chamber prior to the eruption. This implies a confining pressure of ～ 1 kbar for the top of the pantelleritic reservoir. The composition of melt inclusions and mineralogical assemblage of Cuddia Randazzo pumice indicate that it has a lower evolutionary degree (Agpaitic Indices 1.3–1.8) and lower pre-eruptive Cl and H₂O contents than Cuddia del Gallo pumice. An increase in pressure due to the exsolution of volatiles in the upper part of the pantelleritic reservoir may have triggered the Cuddia del Gallo explosive eruption. Evidence of widespread pre-eruptive mingling between trachytes and pantellerites suggests that the intrusion of trachytic magma into the pantelleritic reservoir likely played a major role in destabilizing the magma system just prior to the Cuddia Randazzo event.     

Geology of the Grove Mountains in East Antarctica

Grove Mountains consists mainly of a series of high-grade (upper amphibolite to granulite facies) metamorphic rocks, including felsic granulite, granitic gneiss, mafic granulite lenses and charnockite, intruded by late tectonic gneissic granite and post-tectonic granodioritic veins. Geochemical analysis demonstrates that the charnockite, granitic gneiss and granite belonged to aluminous A type plutonic rocks, whereas the felsic and mafic granulite were from supracrustal materials as island-arc, oceanic island and middle oceanic ridge basalt. A few high-strained shear zones disperse in regional stable sub-horizontal foliated metamorphic rocks. Three generations of ductile deformation were identified, in which D₁ is related to the event before Pan-African age, D₂ corresponds to the regional granulite peak metamorphism, whereas D₃ reflects ductile extension in late Pan-African orogenic period. The metamorphic reactions from granitic gneiss indicate a single granulite facies event, but 3 steps from mafic granulite, with P-T condition of M1 800°C, 9.3×10⁵ Pa; M₂ 800–810°C, 6.4 × 10⁵ Pa; and M₃ 650°C have been recognized. The U-Pb age data from representative granitic gneiss indicate (529±14) Ma of peak metamorphism, (534±5) Ma of granite emplacement, and (501±7) Ma of post-tectonic granodioritic veins. All these evidences suggest that a huge Pan-African aged mobile belt exists in the East Antarctic Shield extending from Prydz Bay via Grove Mountains to the southern Prince Charles Mountains. This orogenic belt could be the final suture during the Gondwana Land assemblage.