Geochemistry of coexisting hornblende and biotite from the Ambalavayal granite,Kerala

Major and trace element geochemistry of coexisting hornblendes and biotites from the Ambalavayal granite, northern Kerala, are presented. The hornblendes correspond to edenitic composition, whereas the biotites correspond to annite. The hornblendes typically show high Al₂O₃ contents (9·69–11·89%) comparable with those from anorogenic granites. The biotites are characteristically low Mg-type, similar to those reported from alkaline rocks. The distribution coefficients calculated for all the major and trace elements are presented and an evaluation of the nature of variation indicate near-chemical equilibrium conditions during the crystallization of the two minerals. The hornblende-biotite tie lines in the Fe³⁺?Fe²⁺?Mg compositional triangle, lie parallel to those of buffered biotites, indicating crystallization in an environment closed to oxygen and well above the Ni?NiO buffer. It is inferred that thefH₂O increased towards the residual stage andfO₂ values were high, in the range of 10^?15 bars.     

Petrology and geochemistry of calc-alkaline volcanic and subvolcanic rocks,Dalli porphyry copper–gold deposit,Markazi Province,Iran

Early Miocene igneous rocks associated with the Dalli porphyry ore body are exposed within the Urumieh-Dokhtar Magmatic Arc (UDMA). The Dalli porphyry Cu–Au deposit is hosted by subduction-related subvolcanic plutons with chemical composition from diorite to granodiorite, which intruded andesitic and dacitic volcanic rocks and a variety of sedimentary sequences. ⁴⁰Ar/³⁹Ar age data indicate a minimum emplacement age of ～21 million years for a potasically altered porphyritic diorite that hosts the porphyry system. The deposit has a proven reserve of 8 million tonnes of rock containing 0.75 g/t Au and 0.5% Cu. Chondrite-normalized rare earth element (REE) patterns for the subvolcanic rocks are characterized by light REE enrichments [(La/Sm) _n ?=?2.57–6.40] and flat to gently upward-sloping profiles from middle to heavy REEs [(Dy/Yb) _n ?=?0.99–2.78; (Gd/Yb) _n ?=?1.37–3.54], with no significant Eu anomalies. These characteristics are generated by the fractionation of amphibole and the suppression of plagioclase crystallization from hydrous calc-alkaline magmas. In normalized multi-element diagrams, all analysed rocks are characterized by enrichments in large ion lithophile elements and depletions in high field strength elements, and display typical features of subduction-related calc-alkaline magmas. We used igneous mineral compositions to constrain the conditions of crystallization and emplacement. Biotite compositions plot above the nickel–nickel oxide (NNO) buffer and close to oxygen fugacity values defined by the hematite–magnetite (HM) buffer, indicating oxidizing conditions during crystallization. Assuming a minimum crystallization temperature of 775°C, the oxygen (fO₂) and water (fH₂O) fugacities are estimated to be 10^?10.3 bars (～ΔNNO+4) and ≤748 bars, respectively, during the crystallization of biotite phenocrysts. The temperature and pressure conditions, estimated from temperature–corrected Al-in-hornblende barometry and amphibole-plagioclase thermometry, suggest that the hornblende phenocrysts in Dalli rocks crystallized at around 780 ± 20°C and 3.8 ± 0.4 kbar. An alternative method using the calcic amphibole thermobarometer indicates that the Dalli magmas were, on average, characterized by an H₂O content of 4.3 wt.%, a relatively high oxygen fugacity of 10^?11.0 bars (ΔNNO+1.3), and a hornblende phenocryst crystallization temperature of 880 ± 68°C and pressure of 2.6 ± 1.7 kbar.     

A Precambrian fayalite granite from the south coast of Western Australia

A Precambrian fayalite granite outcropping at Lower King, near Albany, Western Australia, is interpreted as a high-Fe²⁺/(Fe²⁺ + Mg) analogue of charnockite. Calculation of the original titanomagnetite composition from analytical data on ilmenomagnetite ‘exsolution’ intergrowths suggests initial crystallisation of opaque oxides at about 940°C and 10^?12 bars f_o2. This result indicates a magmatic origin for the rock. Other determinable points on the T-f_o2 cooling curve of the fayalite granite pluton include crystallisation of biotite at roughly 800–820°C and 10^?14.5 bars f_o2, and final equilibration of opaque oxides below 550°C and 10^?23 bars f_o2. Mineralogical data on nearby granulite facies country rocks suggest a regional total pressure of roughly 5 kb, and hence the depth of pluton emplacement was probably around 18–19 km. Thus the Lower King fayalite granite is believed to have crystallised from water-deficient, high-T melt or partial melt generated, possibly from metasedimentary rocks, deep in the crust under granulite facies conditions.     

Activity-composition relationship in Tschermak's substituted Fe biotites at 700°C, 2 kbar

The reaction-displacement technique was applied to the end-member reaction annite = sanidine + magnetite + H₂ in order to determine the activity of the annite component (a _Ann) in iron biotites with variable degrees of the Tschermak's substitution (^[6]Fe + ^[4]Si = ^[6]Al + ^[4]Al). Based on the simplified relation a _Ann = f _{H 2}/foH₂ (foH₂ = hydrogen fugacity of the end-member reaction at P, T), two types of experiments were performed at 700°C / 2 kbar: Type I used Fe-Al biotites of known starting composition together with sanidine + magnetite + H₂O. This assemblage was exposed to various f _{H 2} conditions (f _{H 2} < foH₂) produced in the pressure vessel either by using different ratios of water/oil as pressure medium (f _{H 2} in this case was measured by the hydrogen sensor technique), or by the Ni′NiO buffer. The composition of the Fe-Al biotites changed through incorporation or release of the annite component in response to the externally imposed f _{H 2}. By using opposite biotite starting compositions, the equilibrium composition as a function of f _H2 was bracketed. For type II, f _{H 2} in equilibrium with a specific combination of fine-grained Fe-Al biotite (+ sanidine + magnetite + H₂O) was measured internally by application of the hydrogen sensor technique. Both type I and type II experiments yield consistent results demonstrating that a fine-grained assemblage of Fe-Al biotite (+ sanidine + magnetite + H₂O) is able to act as a sliding-scale buffer. The final chemical composition of the Fe-Al biotite after the experiments was determined by electron microprobe and Mössbauer spectroscopy. The ^[4]Al and ^[6]Al in the biotites are coupled according to the Tschermak's substitution. In the tetrahedral sheet 0.1 Al-atoms per formula unit are present in excess to the amount required to balance ^[6]Al, and all Fe-Al biotites contain 8–10% Fe³⁺. Therefore, they are not members of the pure annite - siderophyllite join, but have an almost constant amount (15 Mol%) of two additional Fe³⁺-bearing components (ferri-siderophyllite and a vacancy end-member). The volume - composition relationship obtained does not indicate excess molar volumes of mixing for the annite (Ann) - siderophyllite (Sid) binary. The data are consistent with a molar volume of annite of 15.46 ± 0.02 Jbar^–1 and of 15.06 ± 0.02 Jbar^–1 for siderophyllite. The experimentally determined activity - composition relation shows that biotites on the join annite - siderophyllite deviate negatively from ideality. A symmetric interaction parameter W_AnnSid is sufficient to represent the data within error. This was constrained as: W _AnnSid = –29 ± 4 kJmol^–1. This is in contradiction to empirical interaction parameters derived from natural assemblages for this binary that predict positive deviation from ideality. Reasons for this discrepancy are discussed.     

Genesis of Recent silicic magmatism in the Medicine Lake Highland,California: evidence from cognate inclusions found at Little Glass Mountain

We use granular inclusions and phenocrysts in the Little Glass Mountain rhyolite flows to estimate temperature, pressure and the fugacities of O₂, H₂ and H₂O. The compositions of magnetite-ilmenite are used to estimate temperature and oxygen fugacity. Fugacities of H₂ and H₂O are estimated from the compositions of associated biotite-sanidine-magnetite. P_Total depends on the compositions of magnetite -ferrosilite-silica. Lastly, hydrothermal experiments were conducted at the estimated T, P and f_O2 to establish the beginning of melting of the most evolved of the inclusions in CO₂-H₂O fluids.The data suggest that the most evolved inclusions formed at ～ 830°C, a total pressure of 5200 bars, f_O2 of 10^?13 and P_H2O ～ 1000 bars. Of these variables total pressure is most difficult to estimate accurately. The values of T, P etc., previously stated produce a maximum estimate of the depth of equilibration between host magma and the inclusions whereas, assuming P_H2O = P_Total yields a minimum estimate. The physical conditions together with texture suggest a plutonic origin at a minimum depth of 3.4 km but no deeper than 15–18 km beneath the Medicine Lake Highland.The composition and mineralogy suggest that the rhyolite was derived from the dacite by crystal fractionation. The relation between dacite and associated basaltic or andesitic rocks is uncertain. The ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios (essentially 0.7040 for both inclusions and lavas) do not require involvement of crustal rocks. A source in the uppermost mantle or lower lithosphere is considered most probable for the parental liquid which gave rise to the dacite.     

The Pikes Peak batholith,Colorado front range,and a model for the origin of the gabbro—anorthosite—syenite—potassic granite suite

This study of the Pikes Peak batholith includes the mineralogy and petrology of quartz syenite at West Creek and of fayalite-bearing and fayalite-free biotite granite near Mount Rosa; major element chemistry of the batholith; comparisons with similar postorogenic, intracratonic, sodic to potassic intrusives; and genesis of the batholith.The batholith is elongate in plan, 50 by 100 km, composite, and generally subalkalic. It was emplaced at shallow depth 1,040 m. y. ago, sharply transects its walls and may have breached its roof. Biotite granite and biotite—hornblende granite are predominant; quartz syenite, fayalite granite and riebeckite granite are present in minor amounts.Fayalite-bearing and fayalite-free quartz syenite, fayalite-biotite granite and riebeckite granite show a well-defined sodic differentiation trend; the less sodic fayalite-free granites exhibit a broader compositional range and no sharp trends.Crystallization was largely at P_H2O < P_total; P_H2O approached P_total only at late stages. Aplite residual to fayalite-free biotite granite in the north formed at about 1,500 bars, or 5 km depth. Feldspar assemblages indicate late stages of crystallization at about 720°C. In the south ilmenite and manganian fayalite indicate f_O2 of 10^?17 or 10^?18 bars. Biotite and fayalite compositions and the ‘granite minimum’ imply completion of crystallization at about 700°C and 1,500 bars. Nearby fayalite-free biotite granite crystallized at higher water fugacity.All types of syenite and granite contain 5–6% K₂O through a range of SiO₂ of 63–76%. Average Na₂O percentages in quartz syenite are 6.2, fayalite granite 4.2, and fayalite-free granite 3.3 MgO contents are low, 0.03–0.4%; FeO averages 1.9–2.5%. FeO/Fe₂O₃ ratios are high. Fluorine ranges from 0.3 to 0.6%.The Pikes Peak intrusives are similar in mode of emplacement, composition, and probably genesis to rapakivi intrusives of Finland, the Younger Granites of Nigeria, Cape Ann Granite and Beverly Syenite, Mass., and syenite of Kungnat, Greenland, among others — allowing for different levels of erosion. A suite that includes gabbro or basalt, anorthosite, quartz syenite, fayalite granite, riebeckite granite, and biotite and/or hornblende granites is of worldwide occurrence.A model is proposed in which mantle-derived, convecting alkali olivine basaltic magma first reacts with K₂O-poor lower crust of granulite facies to produce magma of quartz syenitic composition. The syenitic liquid in turn reacts with granodioritic to granitic intermediate crust of amphibolite facies to produce the predominant fayalite-free biotite and biotite-hornblende granites of the batholith. This reaction of magma and roof involves both partial melting and the reconstitution and precipitation of refractory phases, as Bowen proposed. Intermediate liquids include MgO-depleted and Na₂O-enriched gabbro, which precipitated anorthosite, and alkali diorite. The heat source is the basaltic magma; the heat required for partial melting of the roof is supplied largely by heats of crystallization of phases that settle out of the liquid — mostly olivine, clinopyroxene and plagioclase.     

Biotites and associated minerals as markers of magmatic fractionation and deuteric equilibration in granites

Analyses of 80 biotite, alkali feldspar, oligoclase, hornblende, Fe-Ti oxide separates from the coarse-grained granites of a late-hercynian epizonal diapir, the Ploumanac'h complex, Brittany, show that these minerals display a regular concentric cryptic layering related to fractional crystallization. The Ca, Mg, Ba, Sr, Cr, V content of minerals decreases as the Na, Fe, and Rb content increases. Biotites become more dioctahedral towards the outer residual syenogranite, with a correlative K deficiency. Trioctahedral biotites from the inner accumulative monzogranite are secondarily oxidized with a gain of Fe³⁺ and a loss of OH. This alteration id due to the percolation of exsolved fluids rich in H₂O and containing a small amount of CO₂, F, S, Cl. During this autometamorphic stage, trace elements like Rb, Sr are completely redistributed on the scale of hand specimens, with a restricted range of partition coefficients between biotite, perthite and oligoclase. This equilibration occurred at a temperature about 550 ° C and a fluid pressure about 1,000 bars, with f _H2_O probably less than 500 bars. A later stage of fluid circulation along fractures brings up a slight Li metasomatism. Biotites are a sensitive marker of both magmatic and postmagmatic stages of subsolvus or ‘wet’ plutonites.     

湘南宝山矿床花岗闪长斑岩中黑云母的矿物学特征及其指示意义

湖南宝山矿床处于坪宝矿带的北端,是湘南地区最大的铜多金属矿床。为了进一步探讨矿区内花岗质岩石的形成条件及成矿潜力,文章在详细的野外地质和岩相学观察的基础上,对与成矿密切相关的花岗闪长斑岩中的黑云母进行了详细的矿物化学分析。电子探针分析结果表明:宝山花岗闪长斑岩中的黑云母为铁质黑云母和镁质黑云母,其中,Ti介于0.18~0.30,且Mg/(Mg+Fe2+)值介于0.42~0.58,属于典型的岩浆成因黑云母;黑云母的氧化系数为0.16~0.26,w(Mg O)为8.17%~11.72%,平均9.3%,MF值范围为0.38~0.50,指示其岩体属于壳幔混源型的I型花岗岩;岩体中以黑云母的全铝含量计算的结晶压力为97~174 MPa,相应的结晶深度为3.67~6.57 km,平均深度为5.12 km。其log f(O2)变化范围为-14.5~-12.8,表明黑云母是在较高氧逸度条件下结晶形成的,有利于铜矿的形成。     

Influence of oxygen fugacity and temperature on the redox state of iron in natural silicic aluminosilicate melts

The influence of oxygen fugacity (fO₂) and temperature on the valence and structural state of iron was experimentally studied in glasses quenched from natural aluminosilicate melts of granite and pantellerite compositions exposed to various T-fO₂ conditions (1100–1420°C and 10^?12–10^?0.68 bar) at a total pressure of 1 atm. The quenched glasses were investigated by Mössbauer spectroscopy. It was shown that the effect of oxygen fugacity on the redox state of iron at 1320–1420°C can be described by the equation log(Fe³⁺/Fe²⁺) = k log(fO₂) + q, where k and q are constants depending on melt composition and temperature. The Fe³⁺/Fe²⁺ ratio decreases with decreasing fO₂ (T = const) and increasing temperature (fO₂ = const). The structural state of Fe³⁺ depends on the degree of iron oxidation. With increasing Fe³⁺/Fe²⁺ ≥ 1, the dominant coordination of Fe³⁺ changes from octahedral to tetrahedral. Ferrous iron ions occur in octahedral (and/or five-coordinated) sites independent of Fe³⁺/Fe²⁺.     

Numerical simulation of the origin of red sandstone and Cu accumulation in pore solutions

Interaction of igneous rocks with river (rain) water in the systems granite-water, basalt-water, and dunite-water open with respect to carbon dioxide (PCO₂ = 10^?4, 10^?3, and 10^?2 bar) and oxygen (PO₂ from 10^?81 to 10^?1 bar) is numerically simulated at 25 and 50°C and a mass ratio of water and rock R/W ≤ 10. Equilibrium simulations indicate that, first, the differences in the mineralogical composition of the transformation products of the igneous rocks are insignificant, and second, Cu extraction from minerals of the rocks is optimal at Eh from +200 to ?100 mV. Simulations of the systems with regard for the dissolution rates of minerals indicate that the chemical weathering time of the rocks is few hundred thousands years.     

Oxygen fugacity regime in the upper mantle as a reflection of the chemical differentiation of planetary materials

Oxygen fugacity (fO₂) in the Earth’s mantle has a bearing on the problems of the chemical differentiation of the Earth’s materials and formation of the chemical and phase state of its shells. This paper addresses some problems concerning changes in the redox state of the upper mantle over geologic time and through its depth and the possible influence of fO₂ stratification in the interiors on geochemical processes. Among these problems are the formation of fluids enriched in H₂O, CO₂, CH₄,and H₂; the possible influence of reduced fluid migration from mantle zones with low fO₂ values on reactions in the lithosphere; and the formation of films of silicate liquids with high H₂O and CO₂ contents, which could be responsible for metasomatic transformations in rocks. The formation of a metallic core and accompanying large-scale melting of the silicate part of the Earth are the early mechanisms of the chemical differentiation of the mantle that must have had an effect on the redox state and the composition of volatile components in planetary materials. The molten metallic and silicate phases were prone to gravitational migration, which affected the formation of the metallic core. Volatile components had to be simultaneously formed in the zones of large-scale melting of the early Earth. The composition of these volatiles was largely controlled by the interaction of hydrogen and carbon, the two major gas-forming elements in the mantle, with melt under low fO₂ values. A remarkable feature is that, despite fairly low fO₂ values imposed by the presence of a metallic phase, both reduced (CH₄ and H₂) and oxidized species of hydrogen and carbon (H₂O, OH^? and CO ₃ ^?2 ) are stable in the melt. This peculiarity of carbon and hydrogen dissolution in reduced melts may be crucial for the elucidation of mechanisms for the formation of initial amounts of CO₂ and H₂O connected with incipient melting in the reduced mantle.     

Experimental determination of the diffusion coefficient for calcium in olivine between 900°C and 1500°C

Experiments have been carried out to determine the temperature, oxygen fugacity (fO₂) and compositional dependence of the tracer diffusion coefficient (D) of calcium in olivine. These data constrain the diffusion coefficient over the temperature range 900 to 1500°C for the three principal crystallographic axes. Well constrained linear relationships between the reciprocal of the absolute temperature and log(D) exist at any given oxygen fugacity. There is a strong dependence of the diffusion coefficient on oxygen fugacity with D ∝ fO₂^(1/3). This makes a knowledge of the T-fO₂ path followed by geological samples a prerequisite for modelling Ca diffusion in olivine. The best fitting preexponential factor (Do) and activation energy (E) to the Arrhenius equation log (D) = log [Do exp(−E/RT)] + 0.31Δ log fO₂ for Ca diffusion in olivine at a given oxygen fugacity (fO₂*) are given by:diffusion along [100]: log [Do (m²/s)] = −10.78 ± 0.43; E = 193 ± 11 kJ/moldiffusion along [010]: log [Do (m²/s)] = −10.46 ± 0.37; E = 201 ± 10 kJ/moldiffusion along [001]: log [Do (m²/s)] = −10.02 ± 0.29; E = 207 ± 8 kJ/molwhere Δ log fO₂ = log[fO₂*] − log[10⁻¹²] with fO₂* in units of bars. There is no measurable compositional dependence of the diffusion coefficient between Fo₈₃ and Fo₉₂. Diffusion in Fo₁₀₀ has a much higher activation energy than in Fe-bearing olivine and has a weaker fO₂ dependence.     

Chemical Composition of Rock-Forming Minerals in Copper–Gold-Bearing Tonalite Porphyries at the Batu Hijau Deposit, Sumbawa Island, Indonesia: Implications for Crystallization Conditions and Fluorine–Chlorine Fugacity

Copper–gold mineralization at the world‐class Batu Hijau porphyry deposit, Sumbawa Island, Indonesia, is closely related to the emplacement of multiple stages of tonalite porphyries. Petrographic examination indicates that at least two texturally distinct types of tonalite porphyries are currently recognized in the deposit, which are designated as “intermediate tonalite” and “young tonalite”. They are mineralogically identical, consisting of phenocrysts of plagioclase, hornblende, quartz, biotite and magnetite ± ilmenite, which are set in a medium‐coarse grained groundmass of plagioclase and quartz. The chemical composition of the rock‐forming minerals, including plagioclase, hornblende, biotite, magnetite and ilmenite in the tonalite porphyries was systematically analyzed by electron microprobe. The chemical data of these minerals were used to constrain the crystallization conditions and fluorine–chlorine fugacity of the corresponding tonalitic magma during its emplacement and crystallization. The crystallization conditions, including temperature (T), pressure (P) and oxygen fugacity (fO₂), were calculated by applying the hornblende–plagioclase and magnetite–ilmenite thermometers and the Al‐in‐hornblende barometer. The thermobarometric data indicate that the tonalite porphyries were emplaced at 764 ± 22°C and 1.5 ± 0.3 × 10⁵ kPa. If the pressure is assumed to be lithostatic, it is interpreted that the rim of hornblende and plagioclase phenocrysts crystallized at depths of approximately 5.5 km. As estimated from magnetite–ilmenite thermometry, the subsolidus conditions of the tonalite intrusion occurred at temperatures of 540–590°C and log fO₂ ranging from ?20 to ?15 (between Ni‐NiO and hematite–magnetite buffers). This occurred at relatively high fO₂ (oxidizing) condition. The fluorine–chlorine fugacity in the magma during crystallization was determined on the basis of the chemical composition of magmatic biotite. The calculation indicates that the fluorine–chlorine fugacity, represented by log (fH₂O)/(fHF) and (fH₂O)/(fHCl) in the corresponding tonalitic magma range from 4.31 to 4.63 and 3.62 to 3.79, respectively. The chlorine fugacity (HCl) to water (H₂O) is relatively higher than the fluorine fugacity (HF to water), reflecting a high activity of chlorine in the tonalitic magma during crystallization. The relatively higher activity of chlorine (rather than fluorine) may indicate the significant role of chloride complexes (CuCl₂^? and AuCl₂^?) in transporting and precipitating copper and gold at the Batu Hijau deposit.     

Kyzylkumite: a finding in the southern Baikal region, Russia and refinement of its crystal chemical formula

Kyzylkumite has been found in Cr-V-bearing metamorphic rocks of the Sludyanka Complex, Southern Baikal region; it has been identified by X-ray powder diffraction method. This is a late secondary mineral developed after Ti-V-oxides (schreyerite, berdesinskiite) and V-bearing rutile and titanite. Kyzylkumite represents a new structural type with composition Ti₄V ₂ ³⁺ O₁₀(OH)₂ corresponding to octahedral coordination of Ti⁴⁺ and V³⁺. Its unit-cell dimensions are: a = 8.4787(1), b = 4.5624(1), c = 10.0330(1) Å, β = 93.174(1)°. The ideal formula of kyzylkumite Ti₄V ₂ ³⁺ O₁₀(OH)₂ corresponds to composition, wt %: 65.56 TiO₂, 30.75 V₂O₃, 3.69 H₂O. Indeed, the contents (wt %) of these constituents range from 62 to 70 TiO₂ and from 23 to 33 V₂O₃. Variations in contents and the Ti/V value are caused by partial substitution V³⁺ for V⁴⁺, isovalent substitutions Ti⁴⁺ and V³⁺ for V⁴⁺ and Cr³⁺, respectively, and coupled substitution V³⁺ + OH^? ? Ti⁴⁺ + O^2?. Smyslova et al. (1981)—the discovereres of kyzylkumite—assumed its composition to be the same as for schreyerite V ₂ ³⁺ Ti₃O₉ that principally different from kyzylkumite from the Sludyanka Complex. Therefore, re-examination of the kyzylkumite holotype or cotype from its type locality is needed.     

Geochemistry of hybrid granitoid rocks and of their biotites from central northern Portugal and their petrogenesis

In the area of Franzilhal, S. Lourenço and Pombal, central northern Portugal, occur rocks ranging from hornblende-biotite tonalite to muscovite-biotite granite. The distribution of some major and trace elements in the rocks and in the biotite shows an apparent calc-alkalic trend of differentiation. The behaviour of K, Li, Rb, Cs and K/Rb, Cr.10³/Fe³⁺, Ni.10³/Mg, Li.10³/Mg, Cs.10³/K ratios of the rocks and of F of biotite cannot be explained by mineral fractionation, but can be described in terms of hybridization. The tonalite, granodiorites and biotite-muscovite granite are hybrid rocks formed by assimilation of pelitic sediments and dolomitic marbles by the granite magma. The fO₂ values range from 10^?14.8 to 10^?16.3; fO₂ attains higher values during the crystallization of hybrid rocks than in the uncontaminated muscovite-biotite granite (GIII) magma. The fH₂O and PH₂O values generally increase and the fH₂O/fHF ratio decreases in the hybrid rocks from tonalite to biotite-muscovite-bearing granodiorite and is smaller than in the uncontaminated granite magma.     

Crystallization of melilite from CMAS-liquids and the formation of the melilite mantle of Type B1 CAIs: Experimental simulations

Type B CAIs are subdivided into B1s, with well-developed melilite mantles, and B2s, with randomly distributed melilite. Despite intensive study, the origin of the characteristic melilite mantle of the B1s remains unclear. Recently, we proposed that formation of the melilite mantle is caused by depletion of the droplet surface in volatile magnesium and silicon due to higher evaporation rates of volatile species compared to their slow diffusion rates in the melt, thus making possible crystallization of melilite at the edge of the CAI first, followed by its crystallization in the central parts at lower temperatures. Here, we present the results of an experimental study that aimed to reproduce the texture observed in natural Type B CAIs. First, we experimentally determined crystallization temperatures of melilite for three melt compositions, which, combined with literature data, allowed us to find a simple relationship between the melt composition, crystallization temperature, and composition of first crystallizing melilite. Second, we conducted a series of evaporation and cooling experiments exposing CAI-like melts to gas mixtures with different oxygen fugacities (f_O2). Cooling of the molten droplets in gases with logf_O2?IW-4 resulted in crystallization of randomly distributed melilite, while under more reducing conditions, melilite mantles have been formed. Chemical profiles through samples quenched right before melilite started to crystallize showed no chemical gradients in samples exposed to relatively oxidizing gases (logf_O2?IW-4), while the near-surface parts of the samples exposed to very reducing gases (logf_O2?IW-7) were depleted in volatile MgO and SiO₂, and enriched in refractory Al₂O₃. Using these experimental results and the fact that the evaporation rate of magnesium and silicon from CAI-like melts is proportional to , we estimate that Type B1 CAIs could be formed by evaporation of a partially molten precursor in a gas of solar composition with . Type B2 CAIs could form by slower evaporation of the same precursors in the same gas with .     

Die Reaktion 2 Zoisit+1 CO2 ⇌ 3 Anorthit+1 Calcit+1 H2O

The equilibrium conditions of the following reaction 2 zoisite +1 CO₂?3 anorthite+1 calcite+1 H₂O 2 Ca₂Al₃[O/OH/SiO₄/Si₂O₇]+1 CO₂?3 CaAl₂Si₂O₈+1 CaCO₃+1 H₂O have been determined experimentally at total pressures of P _j= 2000 bars, P _f =5000 bars, and P _f =7000 bars. Owing to the vertical position of the equilibrium curves in isobaric T- \(X_{{\text{CO}}_{\text{2}} }\) diagrams, the composition of the binary H₂O-CO₂ fluid phase coexisting with zoisite is independent of temperature in the temperature interval investigated. According to our experiments, orthorhombic zoisite is only stable in equilibrium with a fluid phase at a concentration of CO₂ which is less than, respectively, ca. 2 Mol% CO₂ at P _f =2000 bars, ea. 6 Mol% at P _f =5000 bars, and ca. 10 Mol% at P _f =7000 bars. Thus, the fluid phase coexisting with zoisite is rich in H₂O. While this is independent of temperature the experimental data demonstrate that the influence of pressure cannot be neglected: With increasing pressure the concentration of CO₂ of the fluid phase coexisting with zoisite can rise a little. The position of the reaction studied, which is independent of temperature and exhibits small values of \(X_{{\text{CO}}_{\text{2}} }\) ,leads to two important petrogenetic conclusions:

1. The occurrence of zoisite is an indicator for a CO₂-poor and H₂O-rich fluid composition during metamorphism of marly calcsilicates.
2. If the concentration of CO₂ of the fluid phase coexisting with zoisite exceeds the equilibrium value of \(X_{{\text{CO}}_{\text{2}} }\) calcite+anorthite+H₂O is formed from zoisite+CO₂. Thus, a considerable increase in the anorthite-content of plagioelase is possible.

     

华北克拉通新太古代板块俯冲作用:来自山西云中山地区变质高镁火成岩组合的证据

在华北克拉通中部的山西云中山地区,新太古代花岗闪长质片麻岩中存在一些超镁铁质岩-镁铁质岩块及由斜长角闪岩、角闪变粒岩、石英岩和石榴夕线黑云片岩等岩石类型构成的变质表壳岩残片,其中的超镁铁质-镁铁质岩、斜长角闪岩和角闪变粒岩构成一套高镁火成岩组合。超镁铁质岩已变质为橄榄绿泥阳起片岩等岩石类型,呈变余斑状结构,橄榄石斑晶仍有保存;岩石SiO_2含量为39.22%～44.99%,Al_2O_3为8.82%～13.47%,Mg O为19.24%～22.13%,Na_2O+K_2O=0.71%～1.11%,CaO为5.75%～8.42%;Al_2O_3/TiO_2=14.8～17.4,CaO/Al_2O_3=0.60～0.84;化学成分上与科马提岩有一定的相似性。与之紧密伴生的斜长角闪岩也具有高镁特征,Mg O含量为11.28%～15.09%,铝、硅和碱质均偏低,具正铕异常,显示堆晶辉长岩的特征。非高镁斜长角闪岩有相对高的铝、硅和碱质,其原岩应为钙碱性玄武岩。角闪变粒岩样品的SiO_2含量为54.21%～55.71%,Al_2O_3为14.24%～15.49%,Mg O为6.26%～8.28%,Fe OT/Mg O=1.11～1.58,高钠低钾,Na_2O+K_2O=3.7%～4.78%,Na_2O/K_2O=5.15%～13.13,Mg#=53.0～61.5,属于高镁安山岩。由超镁铁质质岩-斜长角闪岩-角闪变粒岩构成的变质高镁火山岩组合具有钙碱性系列趋势。超镁铁质岩稀土元素含量总量较低,具有轻稀土富集和重稀土亏损的稀土型式;斜长角闪岩与超镁铁质岩比较,除富集大离子亲石元素和Cr、Ni明显较低外,具有相似的微量元素图谱形态。三种岩石类型在微量元素蛛网图上均显示出Ta、Nb、Ti负异常和Pb正异常。野外产状和岩石地球化学特征表明超镁铁质岩和高镁斜长角闪岩属于阿拉斯加型杂岩体,角闪变粒岩属于赞岐岩质高镁安山岩。在Zr/Nb-Nb/Th和Nb/Y-Zr/Y构造环境判别图解上显示出与俯冲相关的演化趋势,在Hf-Th-Ta、Nb/La-(La/Sm)N和Th/Yb-Nb/Yb图解上也落在岛弧钙碱性岩石区域。以上特征表明高镁火成岩组合形成于与板块俯冲相关的岛弧构造背景。野外地质关系和锆石U-Pb年龄限定高镁火成岩组合形成时代在～2.5Ga。云中山地区阿拉斯加型镁铁质-超镁铁质杂岩与赞岐岩质高镁安山岩共生,表明该地区存在新太古代的板块俯冲作用,为太古宙存在板块构造机制提供了新证据。     

Geochemistry of actinolitic hornblendes from tonalitic rocks,Northern Portugal

Chemical analyses are given for actinolitic hornblendes of tonalitic rocks from the Hercynian belt of Northern Portugal. The distribution of elements between amphibole and co-existing biotite is studied. The composition of the amphiboles is analysed in the light of experimental data on amphiboles and the physical conditions of crystallization inferred from the study of the biotite and rock series. The data on the biotites lead to the definition of a temperature of 800°C for the crystallization of actinolitic hornblendes with Mg/(Mg + Fe) ratios of 0·72-0·61 at pressures of about 3 Kb and fO₂ defined by FMQ.     

Effects of fO2, fS2, temperature, and melt composition on Fe-Ni exchange between olivine and sulfide liquid: implications for natural olivine-sulfide assemblages

The apparent equilibrium constant for the exchange of Fe and Ni between coexisting olivine and sulfide liquid (K_D = (X_NiS/X_FeS)_liquid/(X_NiSi12O2/X_FeSi12O2)_olivine; X_i = mole fraction) has been measured at controlled oxygen and sulfur fugacities (fO₂ = 10^−8.1 to 10⁻¹⁰ and fS₂ = 10^−0.9 to 10^−1.7) over the temperature range 1200 to 1385°C, with 5 to 37 wt% Ni and 7 to 18 wt% Cu in the sulfide liquid. At log fO₂ of −8.7 ± 0.1, and log fS₂ of −0.9 to −1.7, K_D is relatively insensitive to sulfur fugacity, but comparison with previous results shows that K_D increases at very low sulfur fugacities. K_D values show an increase with the nickel content of the sulfide liquid, but this effect is more complex than found previously, and is greatest at log fO₂ of −8.1, lessens with decreasing fO₂, and K_D becomes independent of melt Ni content at log fO₂ ≤ −9.5. The origin of this variation in K_D with fO₂ and fS₂ is most likely the result of nonideal mixing of Fe and Ni species in the sulfide liquid. Such behavior causes activity coefficients to change with either melt oxygen content or metal/sulfur ratio, effects that are well documented for metal-rich sulfide melts.Application of these experimental results to natural samples shows that the relatively large dispersion that exists in K_D values from different olivine + sulfide-saturated rock suites can be interpreted as arising from variations in fO₂, fS₂, and the nickel content of the sulfide liquid. Estimates of fO₂ based on K_D and sulfide melt composition in natural samples yields a range from fayalite-magnetite-quartz (FMQ)-1 to FMQ-2 or lower, which is in good agreement with previous values determined for oceanic basalts that use glass ferric/ferrous ratios. Anomalously high K_D values recorded in some suites, such as Disko Island, probably reflect low fS₂ during sulfide saturation, which is consistent with indications of low fO₂ for those samples. It is concluded that the variation in K_D values from natural samples reflects olivine-sulfide melt equilibrium at conditions within the T-fO₂-fS₂ range of terrestrial mafic magmas.