An introduction to ultrahigh-pressure metamorphism

Abstract Ultrahigh-pressure (UHP) metamorphism refers to mineralogical and structural readjustment of supracrustal protoliths and associated mafic-ultramafic rocks at mantle pressures greater than ∼ 25 kbar (80-90 km). Typical products include metapelite, quartzite, marble, granulite, eclogite, paragneiss and orthogneiss; minor mafic and ultramafic rocks occur as eclogitic-ultramafic layers or blocks of various dimensions within the supracrustal rocks. For appropriate bulk compositions, metamorphism at great depths produces coesite, microdiamond and other characteristic UHP minerals with unusual compositions. Thus far, at least seven coesite-bearing eclogitic terranes and three diamond-bearing UHP regions have been documented. All lie within major continental collision belts in Eurasia, have similar supracrustal protoliths and metamorphic assemblages, occur in long, discontinuous belts that may extend several hundred kilometers or more, and typically are associated with contemporaneous high-P blueschist belts. This paper defines the P-T regimes of UHP metamorphism and describes mineralogical, petrological and tectonic characteristics for a few representative UHP terranes including the western gneiss region of Norway, the Dora Maira massif of the western Alps, the Dabie Mountains and the Su-Lu region of east-central China, and the Kokchetav massif of the former USSR. Prograde P-T paths for coesite-bearing eclogites require abnormally low geothermal gradients (approximately 7°C/km) that can be accomplished only by subduction of cold, oceanic crust-capped lithosphere ± pelagic sediments or an old, cold continent. The preservation of coesite inclusions in garnet, zircon, omphacite, kyanite and epidote, and microdiamond inclusions in garnet and zircon during exhumation of an UHP terrane requires either an extraordinarily fast rate of denudation (up to 10 cm/year) or continuous refrigeration in an extensional regime (retreating subduction zone).     

Ultrahigh-pressure metamorphic records hidden in zircons from amphibolites in Sulu Terrane, eastern China

Abstract The amphibolites occur sporadically as thin layers and blocks throughout the Sulu Terrane, eastern China. All analyzed amphibolite from outcrop and drill cores from prepilot drill hole CCSD‐PP1 and CCSD‐PP2, Chinese Continental Scientific Drilling Project in the Sulu Terrane, are retrograded eclogites overprinted by amphibolite‐facies retrograde metamorphism, with characteristic mineral assemblages of amphibole + plagioclase + epidote ± quartz ± biotite ± ilmenite ± titanite. However, coesite and coesite‐bearing ultrahigh‐pressure (UHP) mineral assemblages are identified by Raman spectroscopy and electron microprobe analysis as inclusions in zircons separated from these amphibolites. In general, coesite and other UHP mineral inclusions are preserved in the cores and mantles of zircons, whereas quartz inclusions occur in the rims of the same zircons. The UHP mineral assemblages consist mainly of coesite + garnet + omphacite + rutile, coesite + garnet + omphacite, coesite + garnet + omphacite + phengite + rutile + apatite, coesite + omphacite + rutile and coesite + magnesite. Compositions of analyzed mineral inclusions are very similar to those of matrix minerals from Sulu eclogites. These UHP mineral inclusion assemblages yield temperatures of 631–780°C and pressures of ≥2.8 × 10³ MPa, representing the P–T conditions of peak metamorphism of these rocks, which are consistent with those (T = 642–726°C; P ≥ 2.8 × 10³ MPa) deduced from adjacent eclogites. These data indicate that the amphibolites are the retrogressive products of UHP eclogites.     

From an initial transient-state to a steady-state in metamorphic reactions: An experimental approach in the system dolomite-quartz-H2O

Abstract We carried out hydrothermal experiments in the system dolomite‐quartz‐H₂O to track the temporal change in reaction rates of simultaneous reactions during the development of reaction zones. Two types of configurations for the starting materials were prepared: dolomite single crystals + quartz powder + water and quartz single crystals + dolomite powder + water, both sealed separately in gold capsules. Runs at 0.1GPa and 600°C with cold seal pressure vessels gave the following results. (i) In short duration (45–71 h) runs metastable layer sequences involving wollastonite and talc occur in the reaction zone, whereas they disappear in longer duration (168–336 h) runs. (ii) The layer sequence of the reaction zones in short duration runs differs from place to place on the dolomite crystal even in the same run. (iii) The diversity of layer sequences in the short duration runs merges into a unique layer sequence in the longer duration runs. (iv) The reaction zone develops locally on the dolomite crystal, but no reaction zone was observed on the quartz crystal in any of the runs. The lines of evidence (i)–(iii) show that the system evolves from an initial transient‐ to a steady‐state and that the kinetic effect is important in the development of reaction zones. A steady diffusion model for the unique layer sequence Qtz/Di/Fo + Cal/Dol + Cal/Dol shows that the Dol + Cal layer cannot be formed by diffusion‐controlled process and that the stability of the layer sequence Qtz/Di/Fo + Cal/Dol depends not only on L‐ratios (a = /L_CaOCaO and b = /L_MgOMgO) but also on the relative rate P = (−2ξ₁ − ξ₂)/(–ξ₁ − 2ξ₂) of competing reactions: Dol + 2Qtz = Di + 2CO₂ (ξ₁) and 2Dol + Qtz = Fo + 2Cal + 2CO₂ (ξ₂). For smaller P the stability field will shift to higher values of a and b. The steady diffusion model also shows that the apparent‐non‐reactivity on the quartz surface can be attributed to void formation in a large volume fraction in the diopside layer.     

Fluids in the lithosphere, 1. Experimentally-determined wetting characteristics of CO2H2O fluids and their implications for fluid transport, host-rock physical properties, and fluid inclusion formation

The equilibrium distribution of CO₂H₂O fluids in synthetic rock samples (principally dunite and quartzite) has been characterized by measurements of the dihedral wetting angle, θ, resulting from 5-day annealing periods at 950–1150°C and 1 GPa. For fluids in equilibrium with polycrystalline quartz, θ varies systematically from 57° for pure H₂O to 90° at X_CO2 0.9. Similarly, for San Carlos olivine, θ varies from 65° for pure H₂O to 90° at X_CO2 0.9. The addition of solutes (NaCl, KCl, CaF₂, Na₂CO₃) to H₂O causes a major decrease in θ in the quartz/fluid system (to values as low as 40°), but has no effect on fluid wetting in dunite. Reconnaissance experiments on other mono- and polymineralic aggregates indicate universally high wetting angles (θ 60°) in upper mantle assemblages and for CO₂ in felsic compositions. For diopside + H₂O, θ 80°, with large variation due to crystalline anisotropy. In no case does θ approach 0°, the condition necessary for fluid to be present along all grain boundaries.Because a value of θ greater than 60° precludes the existence of an interconnected fluid phase in a rock, our results have important implications not only for fluid transport but also for the physical properties of the bulk fluid/rock system. Any static fluid present in the upper mantle must exist as isolated pores located primarily at grain corners, and transport can occur only by hydrofracture. In the continental crust, aqueous fluids (especially saline ones) are likely to form an interconnected network along grain edges, thus contributing to high electrical conductivity and allowing the possibility of fluid transport by porous flow or surface energy-driven infiltration.     

Gas contents and CO2 isotope studies on fluid inclusion in ultra-high pressure metamorphic rock from Shuanghe area, Dabie Mountain

Step heating experiments on ultra-high pressure (UHP) mcks from the Dabie Mountain shows a majority of CO₂ in fluid inclusion (excluding H₂O); CO is also a significant component, with a small content of N₂ and CH₄. Carbon isotopic composition of CO₂ in fluid of metamorphic climax stage (-25%0- -30%0) is different from that of mantle carbon, indicating that UHP rocks did not experience obvious transformation by mantle fluids despite their subduction depth. CO₂ was derived from carbon matter in the pmtoliths of UHP rocks in a relatively confined system, showing that the UHP rocks subsided quickly and uplifted quickly from the mantle. Current organization: Research Institute of Petroleum Exploration and Development, Beijing 100083, China.     

Ultrahigh-pressure metamorphism and decompressional P-T paths of eclogites and country rocks from Weihai, eastern China

Abstract Altered quartz-rich and nearly quartz-free eclogitic rocks and completely retrograde quartz-rich garnet amphibolites occur as blocks or lenses in gneisses at Weihai, northeastern tip of the Sulu ultrahigh-P belt. Eclogitic rocks with assemblage garnet ± clinopyroxene ± coesite + rutile have experienced three-stage metamorphic events including ultrahigh-pressure eclogite, granulite and amphibolite facies. Granulite metamorphic event is characterized by formation of the hypersthene + salite + plagioclase ± hornblende corona between garnet and quartz + clinopyroxene. P-T conditions for the three-stage recrystallization sequence are 840 ± 50°C, >28 kbar, about 760±50°C, 9 kbar, and ~650°C, <8 kbar respectively. Most country rock gneisses contain dominant amphibolite-facies assemblages; some garnet-bearing clinopyroxene gneisses recrystallized under granulite-facies conditions at about 740±50°C and 8.5 kbar; similar to granulite-facies retrograde metamorphism of the enclosed eclogitic blocks. Minor cale-silicate lenses within gneisses containing an assemblage grossular + salite + titanite + quartz with secondary zoisite and plagioclase may have formed within a large pressure range of 14-35 kbar. Eclogitic boudins and quartzo-feldspathic country rocks may have experienced coeval in situ UHP and subsequent retrograde metamorphism. The established nearly isothermal decompression P-T path suggests that this area may represent the interior portion of a relatively large subducted sialic block. The recognized UHP terrane may extend eastward across the Yellow Sea to the Korean Peninsula.     

Petrology and retrograde P-T path for eclogites of the Maksyutov Complex, Southern Ural Mountains, Russia

Abstract The Maksyutov Complex, situated in the southern Ural Mountains of Russia, is the first location where quartz aggregates within garnets exhibiting radial fractures were identified as coesite pseudomorphs (Chesnokov & Popov 1965). The complex consists of two tectonic units: a structurally lower eclogite-bearing schist unit and an overlying meta-ophiolite unit. Both units show evidence for multiple stages of metamorphism and deformation. The high-pressure metamorphism of the eclogite-bearing schist unit, discussed in this report, is suspected to be related to a collision between the Russian platform and a fragment of the Siberian continent during the early Cambrian. At least three stages of metamorphism (M_1-3) and two stages of deformation (S₁ and S₂) were observed in thin sections: M₁) garnet (Alm_55-60, Prp_22-28, Grs_16-20) + omphacite (Jd_46-56) + phengite (Si ≅ 3.5) + rutile; M₂) garnet + glaucophane ± lawsonite + white mica; and M₃) epidote + chlorite ± albite ± actinolite + white mica. Observed mineral parageneses define a retrograde P-T path for the eclogite. Mineral assemblages within the most representative eclogite from the lower unit of the Maksyutov Complex indicate minimum peak pressures of 15 kbar at temperatures of approximately 600°C. If the presence of coesite pseudomorph is confirmed, the peak ultrahigh-pressure metamorphism may be as high as 27 kbar at 615°C.     

H2OCO2 fluids supplied in alpine-type mantle peridotites: electron petrology of relic fluid inclusions in olivines

Fluids supplied in alpine-type mantle peridotites and trapped as fluid inclusions in olivines have been fixed by low-temperature reactions, and theirCO₂/H₂O ratios can be deduced from the minerals in the inclusions. Relic fluid inclusions were commonly observed by the optical microscope in olivines from almost all examined solid intrusive ultramafic complexes (Papua, Oman, Troodos and eleven alpine-type complexes of Japan). Such complexes were emplaced into the crust in a solid state. Electron microscopic studies of olivines from three complexes, Higashiakaishi, Horoman and Iwanai-dake, showed that relic fluid inclusions in these olivines have distinctive mineral parageneses: serpentine + magnesite + talc, serpentine + magnesite + brucite, and serpentine + brucite, respectively, depending on theCO₂/(H₂O+CO₂) ratio of the trapped fluid.It is deduced that the fluids had been supplied to peridotites, at least partly, but almost wholly in some case, when the peridotites were still hot, probably at the upper mantle for the following reasons: (1) the curved surfaces along which the inclusions are distributed are cut by post-emplacement serpentine veins; (2) for the Higashiakaishi dunite, the relic fluid inclusions are exclusively found in porphyroclast olivines and are totally absent in matrix olivines recrystallized during the Sanbagawa metamorphism.Recent models on the derivation of ophiolitic or some alpine-type peridotites favor the island-arc or fore-arc settings. Dehydration of the descending oceanic slab may supply H₂OCO₂ vapor to the overlying mantle wedge. Fluid inclusions trapped in such mantle wedge may abound in H₂O component. H₂O-bearing fluid inclusions may, therefore, be important H₂O containers in the upper mantle, especially near the edge of the mantle wedge above downgoing oceanic slabs.     

Kyanite-anthophyllite schist and southwest extension of the Dabie Mountains ultrahigh- to high-pressure belt

Abstract Kyanite-anthophyllite schist preserves the first record of high pressure in the amphibolite-facies unit of the SW Dabie Mountains, whereas ultrahigh- and high-pressure (UHP and HP) metamorphism has been well documented by the occurrence of coesite, diamond and mafic eclogite in the SE Dabie Mountains. Textural evidence indicates that minerals of the kyanite-anthophyllite schist formed mainly in two stages: (i) garnet + kyanite + antho-phyllite + rutile formed at pressure in excess of 1.2 GPa at T < 650°C; (ii) cordierite±staurolite formed by reaction of anthophyllite + kyanite at P < 0.5 GPa, T∼530°C. Plagioclase and ilmenite replaced garnet and rutile respectively during decompression. In a still later stage, secondary biotite recrystallized, accompanied by sillimanite replacing kyanite, and spinel replacing staurolite. The P-T information suggests that the amphibolite unit in the SW Dabie Mountains is part of the Triassic collision belt between the Sino-Korean and Yangtze cratons. The P-T paths of the UHP eclogite in the eastern Dabie Mountains and the HP kyanite-anthophyllite schist in the SW Dabie Mountains show similar decompression and equivalent late stage Barrovian-style metamorphism. Emplacement of voluminous granitoid at middle crustal levels between 134–118 Ma contributed to the development of the Barrovian-type metamorphism in the Dabie Mountains.     

Metamorphic petrology of the Barchi–Kol metabasites, western Kokchetav ultrahigh-pressure–high-pressure massif, northern Kazakhstan

Abstract In the Barchi–Kol area, located at the westernmost part of the Kokchetav ultrahigh pressure (UHP) to high-pressure (HP) massif, northern Kazakhstan, metabasites from the epidote amphibolite (EA) facies to the coesite eclogite (CEC) facies are exposed. Based on the equilibrium mineral assemblages, the Barchi–Kol area is divided into four zones: A, B, C and D. Zone A is characterized by the assemblage: epidote + hornblende + plagioclase + quartz, with minor garnet. Zone B is characterized by the assemblage: garnet + hornblende + plagioclase + quartz + zoisite. Zone C is defined by the appearance of sodic–augite, with typical assemblage: garnet + sodic–augite + tschermakite–pargasite + quartz ± plagioclase ± epidote/clinozoisite. Zone D is characterized by the typical eclogite assemblage: garnet + omphacite + quartz + rutile, with minor phengite and zoisite. Inclusions of quartz pseudomorph after coesite were identified in several samples of zone D. Chemical compositions of rock-forming minerals of each zone were analyzed and reactions between each zone were estimated. Metamorphic P-T conditions of each zone were estimated using several geothermobarometers as 8.6 ± 0.5 kbar, 500 ± 30 °C for zone A; 11.7 ± 0.5 kbar, 700 ± 30 °C for zone B; 12–14 kbar, 700–815 °C for zone C; and 27–40 kbar, 700–825 °C for zone D.     

Shock-induced CO2 loss from CaCO3; implications for early planetary atmospheres

We report new results of shock recovery experiments on single crystal calcite. Recovered samples are subjected to thermogravimetric analysis. This yields the maximum amount of post-shock CO₂, the decarbonization interval, ΔT, and the energy of association (or vaporization), ΔEV, for the removal of remaining CO₂ in shock-loaded calcite. Comparison of post-shock CO₂ with that initially present determines shock-induced CO₂ loss as a function of shock pressure. Incipient to complete CO₂ loss occurs over a pressure range of 10to 70GPa. The latter pressure should be considered a lower bound. Comparable to results on hydrous minerals, ΔT and ΔEV decrease systematically with increasing shock pressure. This indicates that shock loading leads to both the removal of structural volatiles and weakening of bonds between the volatile species and remainder of the crystal lattice.Optical and scanning electron microscopy (SEM) reveal structural changes, which are related to the shock-loading. Comparable to previous findings on shocked antigorite is the occurrence of dark, diffuse areas, which can be resolved as highly vesicular areas as observed with a scanning electron microscope. These areas are interpreted as representing quenched partial melts, into which shock-released CO₂ has been injected.The experimental results are used to place bonds on models of impact production of CO₂ during accretion of the terrestrial planets.     

Stability of forsterite + CO2 and its bearing on the role of CO2 in the mantle

An experimental determination of the reaction MgCO₃ + MgSiO₃ = Mg₂SiO₄ + CO₂ between 20 and 40 kbars and in the range 1000–1500°C yields an average pressure effect on the equilibrium of 44 bars/°C. This result shows that the assemblage forsterite and carbon dioxide is not stable under most pressure and temperature conditions expected in the upper mantle. Hypotheses requiring the presence of free CO₂ in the low-velocity zone, CO₂ as a drive mechanism for kimberlite emplacement, or action of a free CO₂ phase in ultramafic rocks may need considerable revision.     

Effects of elevated CO2 on the reproduction of two calanoid copepods

Some planktonic groups suffer negative effects from ocean acidification (OA), although copepods might be less sensitive. We investigated the effect of predicted CO₂ levels (range 480–750 ppm), on egg production and hatching success of two copepod species, Centropages typicus and Temora longicornis. In these short-term incubations there was no significant effect of high CO₂ on these parameters. Additionally a very high CO₂ treatment, (CO₂ = 9830 ppm), representative of carbon capture and storage scenarios, resulted in a reduction of egg production rate and hatching success of C. typicus, but not T. longicornis. In conclusion, reproduction of C. typicus was more sensitive to acute elevated seawater CO₂ than that of T. longicornis, but neither species was affected by exposure to CO₂ levels predicted for the year 2100. The duration and seasonal timing of exposures to high pCO₂, however, might have a significant effect on the reproduction success of calanoid copepods.     

退水期鄱阳湖薹草(Carex cinerascens)和藜蒿(Artemisia selengensis)洲滩湿地CO2通量变化及其影响因子

于2014年10月到2015年5月鄱阳湖退水期,利用密闭箱—气相色谱法对鄱阳湖北部星子县洲滩两种代表性的植被群落——薹草(Carex cinerascens)和藜蒿(Artemisia selengensis)进行CO_2通量的对比观测,结果表明:薹草和藜蒿湿地的生态系统呼吸具有明显季节变化模式,其最小值均出现在冬季,最大值均出现在春季,平均值分别为3291.80和2581.89mg CO_2/(m~2·h),退水期薹草和藜蒿湿地累积的CO_2通量分别为213.71±2.27和176.39±11.48 t CO_2/hm~2.较高的生物量是薹草湿地CO_2通量高于藜蒿湿地的原因.5 cm土温是影响薹草和藜蒿湿地CO_2通量季节变化最重要的影响因子,藜蒿湿地生态系统呼吸的温度敏感性指数(Q10)高于薹草湿地.水分、植物生物量和湿地CO_2通量之间无显著相关性.     

The join grossularite-calcite through the system CaO—Al2O3—SiO2—CO2 at 30 kilobars: Crystallization range of silicates and carbonates on the liquidus

At 30 kbar, calcite melts congruently at 1615°C, and grossularite melts incongruently to liquid + gehlenite (tentative identification) at 1535°C. The assemblage calcite + grossularite melts at 1450°C to produce liquid + vapor, with piercing point at about 49 wt.% CaCO₃. Vapor phase is present in all hypersolidus phase fields except for those with less than about 7% CaCO₃ or 8% Ca₃Al₂Si₃O₁₂. These results, together with known liquidus data for CaO—SiO₂—CO₂ and inferred results for CaO—Al₂O₃—CO₂ and Al₂O₃—SiO₂—CO₂, permit construction of the position of the CO₂- saturated liquidus surface in the quaternary system, and estimation of the positions of liquidus field boundaries separating some of the primary crystallization fields on this surface. The field of calcite is separated from those for grossularite and quartz by a field boundary with about 50% dissolved CaCO₃. Crystallization paths of silicate liquids in the range Ca₂SiO₄—Ca₃Al₂Si₃O₁₂—SiO₂, with some dissolved CO₂, will terminate at a quaternary eutectic on this field boundary, with the precipitation of calcite together with grossularite and quartz, at a temperature below 1450°C. Addition of Al₂O₃ to CaO—SiO₂—CO₂ in amounts sufficient to stabilize garnet thus causes little change in the general liquidus pattern as far as carbonates and silicates are concerned. With addition of MgO, we anticipate that silicate liquids with dissolved CO₂ will also follow liquidus paths to fields for the precipitation of carbonates; we conclude that similar paths link kimberlite and some carnbonatite magmas.     

Geochemical features of the geothermal CO2-watercarbonate rock system and analysis on its CO2 sources

Taking Huanglong Ravine and Kangding, Sichuan, and Xiage, Zhongdian, Yunnan, as examples, the authors summarize the hydrogeochemical and carbon stable isotopic features of the geothermal CO₂-water-carbonate rock system and analyze the CO₂ sources of the system. It was found that the hydrogeochemical and carbon stable isotopic features of such a system are different from those of shallow CO₂-water-carbonate rock system, which is strongly influenced by biosphere. The former has higher CO₂ partial pressure, and is rich in heavy carbon stable isotope. In addition, such a geothermal system is also different from that developed in igneous rock. The water in the latter system lacks Ca²⁺, and thus, there are few tufa deposits on ground surface, but it is rich in light carbon stable isotope. Further analysis shows that CO₂ of the geothermal CO₂-water-carbonate rock system is a mixture of metamorphic CO₂ and magmatic CO₂.     

Raman micro-spectroscopy on diamond,graphite and other carbon polymorphs from the ultrahigh-pressure metamorphic Kimi Complex of the Rhodope Metamorphic Province,NE Greece

Raman micro-spectroscopy was applied on carbon inclusions in garnet porphyroblasts from kyanite–biotite–garnet schists of the Rhodope Metamorphic Province (RMP), NE Greece. Diamond and cuboids of poorly to highly ordered graphite were identified either as single phase inclusions or as polyphase inclusions along with CO₂ and/or carbonates (calcite/magnesian calcite). Questionable Raman bands that may be assigned to other C-phases (?nanodiamond/?lonsdaleite/?a different C-polymorph) have been observed. The presence of diamond confirms beyond any doubt the ultrahigh-pressure (UHP) metamorphism reported by Mposkos and Kostopoulos [1] [E. Mposkos, D. Kostopoulos, Diamond, former coesite and supersilisic garnet in metasedimentary rocks from the Greek Rhodope: a new ultrahigh-pressure metamorphic province established, Earth Planet. Sci. Lett. 192 (2001) 497–506] in the RMP. Cuboid graphite showing variable degree of disordering most probably formed after diamond. The possible involvement of CO₂ and or C–O–H fluids in the formation of diamond is discussed.     

Chemical variations in assemblages including epidote and/or chlorite in the fossil hydrothermal system of Saint Martin (Lesser Antilles)

Epidote and/or chlorite are common minerals in the roots of the fossil geothermal system of Saint Martin (Lesser Antilles). They appear in four distinct assemblages: (1) epidote+actinolite+quartz±magnetite near the contact between the tuffaceous host rocks (andesitic modal composition) and the quartz-diorite intrusion of Philipsburg; (2) epidote+chlorite+quartz in host rocks as far as a lateral distance of about 3 km from the intrusion; (3) epidote+chlorite+haematite+quartz locally in iron and manganese rich host rocks; (4) chlorite±phengite±magnetite appearing as late sealing of porosity in fracture-controlled quartz veins with strongly phengitized wall rocks. All these assemblages constitute a large alteration grading from propylitic alteration to thermal metamorphism (actinolite-bearing assemblage).Detailed microprobe studies of epidotes replacing plagioclases and of chlorites replacing glass and mafic minerals reveal notable compositional variations which have been studied with respect to temperature paleogradients (estimated from fluid inclusions study), distance from the thermal source and f_o2 conditions. The mean Ps+Pm [100 × (Fe³⁺ + Mn³⁺)/(Al³⁺ + Fe³⁺ + Mn³⁺)] of epidotes vary from 21 in the presence of magnetite near the intrusion to 32 in haematite-bearing iron and/or manganese volcanic and sedimentary formations. The intra-grain chemical scattering of epidotes increases with increasing distance of the pluton and decreasing temperature of crystallization. All the chlorites coexisting with epidote are Mg-rich (X_Fe<0.50). Their main compositional variation consists in a significant enrichment in magnesium (toward the chlinochlore end member) in presence of haematite. The intra-grain chemical scattering of chlorite (expressed by the aluminium content in the structural formula) increases with increasing distance of the pluton and decreasing temperature of crystallization. Chlorites associated with phengite and magnetite in vein alteration are Fe- and Al-rich. The Mössbauer spectra indicate that the Fe³⁺ content of chlorite varies between 25 and 32% of total Fe in the presence of epidote; the higher content being attained in the presence of haematite. The Fe³⁺ content of chlorite associated with magnetite and phengite is 16% of total Fe. The compositional variations of epidote and/or chlorite of the four distinct assemblages observed at Saint Martin result from the combined effects of f_O2, temperature, and time of heating. The effect of f_O2 is particularly perceptible in the control of the epidote Ps content, of the chlorite X_Fe ratio of Fe³⁺ distribution between coexisting epidotes and chlorites. Despite the fact that it may be partially canceled out by the effect of f_O2, the variation of compositional ranges of both epidotes and chlorites, which increases toward the outer part of the geothermal system in response to the combination of decreasing temperatures and decreasing time of heating of the rocks, suggests that chemical equilibrium has not been attained in the assemblages bearing epidotes and chlorites.     

Seasonal variations in pCO2 and its controlling factors in surface seawater of the northern East China Sea

Surface partial pressure of CO₂ (pCO₂), temperature, salinity, nutrients, and chlorophyll a were measured in the East China Sea (ECS; 31°30′–34°00′N to 124°00′–127°30′E) in August 2003 (summer), May 2004 (spring), October 2004 (early fall), and November 2005 (fall). The warm and saline Tsushima Warm Current was observed in the eastern part of the survey area during four cruises, and relatively low salinity waters due to outflow from the Changjiang (Yangtze River) were observed over the western part of the survey area. Surface pCO₂ ranged from 236 to 445 μatm in spring and summer, and from 326 to 517 μatm in fall. Large pCO₂ (values >400 μatm) occurred in the western part of the study area in spring and fall, and in the eastern part in summer. A positive linear correlation existed between surface pCO₂ and temperature in the eastern part of the study area, where the Tsushima Warm Current dominates; this correlation suggests that temperature is the major factor controlling surface pCO₂ distribution in that area. In the western part of the study area, however, the main controlling factor is different and seasonally complex. There is large transport in this region of Changjiang Diluted Water in summer, causing low salinity and low pCO₂ values. The relationship between surface pCO₂ and water stability suggests that the amount of mixing and/or upwelling of CO₂-rich water might be the important process controlling surface pCO₂ levels during spring and fall in this shallow region. Sea–air CO₂ flux, based on the application of a Wanninkhof [1992. Relationship between wind speed and gas exchange over the ocean. Journal of Geophysical Research 97, 7373–7382] formula for gas transfer velocity and a set of monthly averaged satellite wind data, were −5.04±1.59, −2.52±1.81, 1.71±2.87, and 0.39±0.18 mmol m⁻² d⁻¹ in spring, summer, early fall, and fall, respectively, in the northern ECS. The ocean in this study area is therefore a carbon sink in spring and summer, but a weak source or in equilibrium with the atmosphere in fall. If the winter flux value is assumed to have been the mean of autumnal and vernal values, then the northern ECS absorbs about 0.013 Pg C annually. That result suggests that the northern ECS is a net sink for atmospheric CO₂, a result consistent with previous studies.     

Response of needle dark respiration of Pinus koraiensis and Pinus sylvestriformis to elevated CO2 concentrations for four growing seasons’ exposure

The long-term effect of elevated CO2 concentrations on needle dark respiration of two coniferous species-Pinus koraiensis and Pinus sylvestriformis on the Changbai Mountain was investigated using open-top chambers. P. Koraiensis and P. Sylvestriformis were exposed to 700,500μmol·mol-1 CO2 and ambient CO2(approx.350 μmol·mol-1)for four growing seasons. Needle dark respiration was measurd during the second, third and fourth growing seasons' exposure to elevated CO2.The results showed that needle dark respiration rate increased for P. Koraiensis and P. Sylvestriformis grown at elevated CO2 concentrations during the second growing season, could be attributed to the change of carbohydrate and/or nitrogen content of needles. Needle dark respiration of P. Koraiensis was stimulated and that of P. Sylvestriformis was inhibited by elevated CO2 concentrations during the third growing season. Different response of the two tree species to elevated CO2 mainly resulted from the difference in the growth rate. Elevated CO2 concentrations inhibited needle dark respiration of both P. Koraiensis and P. Sylvestriformis during the fourth growing season. There was consistent trend between the short-term effect and the long-term effect of elevated CO2 on needle dark respiration in P. Sylvestriformis during the third growing season by changing measurement CO2 concentrations. However, the short-term effect was different from the long-term effect for P. Koraiensis. Response of dark respiration of P. Koraiensis and P. Sylvestriformis to elevated CO2 concentrations was related to the treatment time of CO2 and the stage of growth and development of plant. The change of dark respiration for the two tree species was determined by the direct effect of CO2 and long-term acclimation. The prediction of the long-term response of needle dark respiration to elevated CO2 concentration based on the short-term response is in dispute.