The heat capacity of synthetic anhydrous Mg and Fe cordierite

The heat capacity of synthetic anhydrous Mg and Fe cordierite was measured by differential scanning calorimetry (DSC). Mg cordierite was synthesized from a glass at high temperatures and shows an ordered structural state. Fe cordierite was synthesized hydrothermally and dehydrated under reducing conditions to obtain H₂O-free material. IR spectroscopy was used to ascertain the lack of H₂O in both phases. The molar volume of both phases was measured using powder X-ray diffraction giving 23.316 (7) J/bar for Mg cordierite and 23.672 (6) J/bar for Fe cordierite. DSC measurements between 300 and 950 K were made following the procedure of Bosenick et al. (1996). The data show a precision of about 1% in the case of Mg cordierite and 1.5% for Fe cordierite. Fitted C_p polynomials in J/mol/K are:

The thermodynamic properties of H2O in magnesium and iron cordierite

 The equilibrium water content of cordierite has been measured for 31 samples synthesized at pressures of 1000 and 2000 bars and temperatures from 600 to 750° C using the cold-seal hydrothermal technique. Ten data points are presented for pure magnesian cordierite, 11 data points for intermediate iron/magnesium ratios from 0.25 to 0.65 and 10 data points for pure iron cordierite. By representing the contribution of H₂O to the heat capacity of cordierite as steam at the same temperature and pressure, it is possible to calculate a standard enthalpy and entropy of reaction at 298.18° K and 1 bar for, (Mg,Fe)₂Al₄Si₅O₁₈+H₂O ⇄ (Fe,Mg)₂Al₄Si₅O₁₈.H₂O Combining the 31 new data points with 89 previously published experimental measurements gives: ΔH ^° _r =–37141±3520 J and ΔS ^° _r =–99.2±4 J/degree. This enthalpy of reaction is within experimental uncertainty of calorimetric data. The enthalpy and entropy of hydration derived separately for magnesian cordierite (–34400±3016 J, –96.5±3.4 J/degree) and iron cordierite (–39613±2475, –99.5±2.5 J/degree) cannot be distinguished within the present experimental uncertainty. The water content as a function of temperature, T(K), and water fugacity, f(bars), is given by n _H2O=1/[1+1/(K ⋅ f _H2O)] where the equilibrium constant for the hydration reaction as written above is, ln K=4466.4/T–11.906 with the standard state for H₂O as the gas at 1 bar and T, and for cordierite components, the hydrous and anhydrous endmembers at P and T. Received: 2 August 1994/Accepted: 7 February 1996     

Partial melting of metagreywackes, Part II. Compositions of minerals and melts

A series of experiments on the fluid-absent melting of a quartz-rich aluminous metagreywacke has been carried out. In this paper, we report the chemical composition of the phases present in the experimental charges as determined by electron microprobe. This analytical work includes biotite, plagioclase, orthopyroxene, garnet, cordierite, hercynite, staurolite, gedrite, oxide, and glass, over the range 100–1000 MPa, 780–1025 °C. Biotites are Na- and Mg-rich, with Ti contents increasing with temperature. The compositions of plagioclase range from An₁₇ to An₃₅, with a significant orthoclase component, and are always different from the starting minerals. At high temperature, plagioclase crystals correspond to ternary feldspars with Or contents in the range 11–20 mol%. Garnets are almandine pyrope grossular spessartine solid solutions, with a regular and significant increase of the grossular content with pressure. All glasses are silicic (SiO₂ = 67.6–74.4 wt%), peraluminous, and leucocratic (FeO + MgO = 0.9–2.9 wt%), with a bulk composition close to that of peraluminous leucogranites, even for degrees of melting as high as 60 vol.%. With increasing pressure, SiO₂ contents decrease while K₂O increases. At any pressure, the melt compositions are more potassic than the water-saturated granitic minima. The H₂O contents estimated by mass balance are in the range 2.5–5.6 wt%. These values are higher than those predicted by thermodynamic models. Modal compositions were estimated by mass balance calculations and by image processing of the SEM photographs. The positions of the 20 to 70% isotects (curves of equal proportion of melt) have been located in the pressure-temperature space between 100 MPa and 1000 MPa. With increasing pressure, the isotects shift toward lower temperature between 100 and 200 MPa, then bend back toward higher temperature. The melting interval increases with pressure; the difference in temperature between the 20% and the 70% isotects is 40 °C at 100 MPa, and 150 °C at 800 MPa. The position of the isotects is interpreted in terms of both the solubility of water in the melt and the nature of the reactions involved in the melting process. A comparison with other partial melting experiments suggests that pelites are the most fertile source rocks above 800 MPa. The difference in fertility between pelites and greywackes decreases with decreasing pressure. A review of the glass compositions obtained in experimental studies demonstrates that partial melting of fertile rock types in the crust (greywackes, pelites, or orthogneisses) produces only peraluminous leucogranites. More mafic granitic compositions such as the various types of calk-alkaline rocks, or mafic S-type rocks, have never been obtained during partial melting experiments. Thus, only peraluminous leucogranites may correspond to liquids directly formed by partial melting of metasediments. Other types of granites involve other components or processes, such as restite unmixing from the source region, and/or interaction with mafic mantle-derived materials. Received: 11 July 1995 / Accepted: 27 February 1997     

Cordierite as an indicator of thermodynamic conditions of petrogenesis

A refined thermodynamic model of H₂O and CO₂ bearing cordierite based on recent data on volatile incorporation into cordierite (Thompson et al. in Contrib Mineral Petrol 142:107–118, 2001; Harley and Carrington in J Petrol 42:1595–1620, 2001) reflects non-ideality of channel H₂O and CO₂ mixing. The dependence of cordierite H₂O and CO₂ contents on P, T and equilibrium fluid composition has been calculated for the range 600–800°C and 200–800 MPa. It has been used for establishing thermodynamic conditions of cordierite formation and the following retrograde P–T paths of cordierite rocks from many localities. Estimates of the H₂O and CO₂ activities have shown that cordierites in granites, pegmatites and high-pressure granulites were formed in fluid-saturated conditions and wide range of H₂O/CO₂ relations. Very low cordierite H₂O contents in many migmatites may be caused not only by fluid-undersaturated conditions at rock formation and H₂O leakage on retrograde P–T paths but also by the presence of additional volatile components like CH₄ and N₂. The pressure dependence of cordierite-bearing mineral equilibria on fluid H₂O/CO₂ relations has been evaluated.     

Identification and implications of fight hydrocarbon fluid inclusions from the Proterozoic Bidjovagge gold-copper deposit, Finnmark, Norway

Carbonic fluid inclusions were observed in quartz-bearing veins at the Proterozoic Bidjovagge AuCu deposit within the Kautokeino greenstone belt in Norway, where mineralization occurred in oxidation zones of graphitic schists. A primary fluid inclusion zonation was observed with C0₂-rich fluid inclusions in the structural footwall of the deposit, and CH₄-rich inclusions within the ore zone in the oxidation zone. Microthermometry of the primary hydrocarbon inclusions revealed 2 groups; (1) a group which homogenized between −125°C and the critical temperature of CH₄ (−82.1°C), which indicated the presence of pure CH₄, and (2) a group which homogenized between the critical temperature of CH₄ and −42°C, which indicated the presence CH₄ and higher hydrocarbons (HHC). Raman microprobe analyses of the first group confirmed the presence of CH₄. The second inclusion group were fluorescent, and Raman spectra clearly displayed CH₄,C₂H₆, and rarer C₃H₈ peaks. A typical feature of the Raman spectra were elevated baselines at the hydrocarbon peaks. Carbon peaks were also usually detected in each inclusion by Raman analysis. Bulk gas chromatography analyses of samples containing the first group (CH₄) indicated the presence of CH₄ and low concentrations of C₂H₆ and C₃H₈. Gas chromatography analyses of samples containing the second group (CH₄ and higher hydrocarbons) confirmed the presence of CH₄, and higher hydrocarbons such as C₂H₆ and C₃H₈ and also butanes. Based on the spacial zonation of hydrocarbons and the estimated PT conditions of 300 to 375°C and 2 to 4 kbars, the authors suggest an abiotic origin for the hydrocarbons. It is suggested that the hydrothermal fluids oxidized the graphitic schist, precipitated Cu and Au and formed light gas hydrocarbons.     

Organic inclusions as an indicator of oil/gas potential assessment of carbonate reservoir beds

Organic inclusions could be formed at the stages of either primary or secondary migration of hydrocarbons so long as mineral crystallization or recrystallization takes place in the sediments, presenting a direct indicator of oil/gas evolution, migration and abundance. Based on the study of organic inclusions in carbonate-type reservoir beds of commercial importance from North China, Xingjing, North Jiangsu, Jianghan, Sichuan and Guizhou in China, many inclusion parameters for oil/gas potential assessment of carbonate reservoir beds are summarized in this paper, including: 1) Types of organic inclusion: Commercially important oil beds are characterized by inclusions consisting of either pure liquid hydrocarbons or liquid plus minor gaseous hydrocarbons, while commercially important gas reservoirs are characterized by inclusions consisting of either pure gaseous hydrocarbons or gas plus minor liquid hydrocarbons. 2) Quantity of organic inclusions: The number of organic inclusions in commercially important oil/gas reservoirs is over 60% of the total inclusion percentage. 3) Temperature of saline inclusions: The homogenization temperatures of contemporaneous saline inclusions in oil reservoirs range from 91–161 °C, while in gas reservoirs from 150–250 °C). 4) Inclusion composition: In commercially important oil reservoirs, C₁/C₂=2−10, C₁/C₃=2−4, C₁/C₄=2−21, (C₂−C₄)/(C₁−C₄)(%)>20, (CH₄+CO+H₂)/CO₂ (molecules/g)=0.5−1.0, and in C₂−C₃−nC₄ triangle diagram there should be an upside-down triangle with the apex within the ellipse, while in commercial gas reservoirs, C₁/C₂=10−35, C₁/C₃=14−82, C₁/C₄=21−200, (C₂−C₄)/(C₁−C₄)(%)<20, (CH₄+CO+H₂)/CO₂>1, and there would be an upright triangle with the apex within the ellipse. The above-mentioned parameters have been used to evaluate a number of other unknown wells or regions and the results are very satisfactory. It is valid to use organic inclusions as an indicator to assess the oil/gas potential during oil/gas exploration and prospecting. This approach is effective, economic, rapid, and easy to popularize.     

The crystal structure of arsentsumebite, Pb2Cu[(As, S)O4]2(OH)

Summary The crystal structure of arsentsumebite, ideally, Pb₂Cu[(As, S)O₄]₂(OH), monoclinic, space group P2₁/m, a = 7.804(8), b = 5.890(6), c = 8.964(8) ?, β = 112.29(6)°, V = 381.2 ?³, Z = 2, d_calc. = 6.481 has been refined to R = 0.053 for 898 unique reflections with I> 2σ(I). Arsentsumebite belongs to the brackebuschite group of lead minerals with the general formula Pb₂ Me(XO₄)₂(Z) where Me = Cu²⁺, Mn²⁺, Zn²⁺, Fe²⁺, Fe³⁺; X = S, Cr, V, As, P; Z = OH, H₂O. Members of this group include tsumebite, Pb₂Cu(SO₄)(PO₄)(OH), vauquelinite, Pb₂Cu(CrO₄)(PO₄)(OH), brackebuschite, Pb₂ (Mn, Fe)(VO₄)₂(OH), arsenbracke buschite, Pb₂(Fe, Zn)(AsO₄)₂(OH, H₂O), fornacite, Pb₂Cu(AsO₄)(CrO₄)(OH), and feinglosite, Pb₂(Zn, Fe)[(As, S)O₄]₂(H₂O). Arsentsumebite and all other group members contain M = M–T chains where M = M means edge-sharing between MO₆ octahedra and M–T represents corner sharing between octahedra and XO₄ tetrahedra. A structural relationship exists to tsumcorite, Pb(Zn, Fe)₂(AsO₄)₂ (OH, H₂O)₂ and tsumcorite-group minerals Me(1)Me(2)₂(XO₄)₂(OH, H₂O)₂. Received June 24, 2000; revised version accepted February 8, 2001     

Near surface manifestation of hydrocarbons in Proterozoic Bhima and Kaladgi Basins: Implications to hydrocarbon resource potential

Reconnaissance surface geochemical survey for adsorbed soil gas analysis conducted in Proterozoic Bhima and Kaladgi Basins, have revealed occurrence of anomalous concentrations of light gaseous hydrocarbons i.e. C₁ to C₄ (CH₄, C₂H₆, C₃H₈, i-C₄H₁₀ and n-C₄H₁₀) in the near surface soils. The concentrations of C₁ and ΣC₂₊(C₂H₆+C₃H₈+ i-C₄H₁₀+ n-C₄H₁₀) in Bhima and Kaladgi Basins are in the range of 1–2594 ppb and 1 to 57 ppb and 1–1142 ppb and 1–490 ppb, respectively. The carbon isotopic data of adsorbed soil gas methane in few selected samples are in the range of −29.9 to −39‰ (PDB). The evaluation of adsorbed soil gas data indicates that all the gas components are cogenetic and hydrocarbon ratios of C₁/(C₂+C₃) < 10 and C₃/C₁*1000 between 60–500 and 20–60 suggest that the adsorbed gases are derived from oil and gas-condensate zones. The carbon isotopic values of methane further support thermogenic origin of these migrated gases. The concentration distribution of C₁ and ΣC₂₊ in the study areas illustrate C₁ and ΣC₂₊ anomalies near Katamadevarhalli, Andola and Talikota in Bhima Basin and near Kaladgi, Lokapur and north of Mudhol in Kaladgi Basin. The hydrocarbon anomalies near the surface coincide with the favourable subsurface structural features and correlate with existing geochemical and geophysical data in these basins suggesting seepage related anomalies.     

Hydrocarbon- and ore-bearing basinal fluids: a possible link between gold mineralization and hydrocarbon accumulation in the Youjiang basin,South China

The Youjiang basin, which flanks the southwest edge of the Yangtze craton in South China, contains many Carlin-type gold deposits and abundant paleo-oil reservoirs. The gold deposits and paleo-oil reservoirs are restricted to the same tectonic units, commonly at the basinal margins and within the intrabasinal isolated platforms and/or bioherms. The gold deposits are hosted by Permian to Triassic carbonate and siliciclastic rocks that typically contain high contents of organic carbon. Paragenetic relationships indicate that most of the deposits exhibit an early stage of barren quartz ± pyrite (stage I), a main stage of auriferous quartz + arsenian pyrite + arsenopyrite + marcasite (stage II), and a late stage of quartz + calcite + realgar ± orpiment ± native arsenic ± stibnite ± cinnabar ± dolomite (stage III). Bitumen in the gold deposits is commonly present as a migrated hydrocarbon product in mineralized host rocks, particularly close to high grade ores, but is absent in barren sedimentary rocks. Bitumen dispersed in the mineralized rocks is closely associated and/or intergrown with the main stage jasperoidal quartz, arsenian pyrite, and arsenopyrite. Bitumen occurring in hydrothermal veins and veinlets is paragenetically associated with stages II and III mineral assemblages. These observations suggest an intimate relationship between bitumen precipitation and gold mineralization. In the paleo-petroleum reservoirs that typically occur in Permian reef limestones, bitumen is most commonly observed in open spaces, either alone or associated with calcite. Where bitumen occurs with calcite, it is typically concentrated along pore/vein centers as well as along the wall of pores and fractures, indicating approximately coeval precipitation. In the gold deposits, aqueous fluid inclusions are dominant in the early stage barren quartz veins (stage I), with a homogenization temperature range typically of 230°C to 270°C and a salinity range of 2.6 to 7.2 wt% NaCl eq. Fluid inclusions in the main and late-stage quartz and calcite are dominated by aqueous inclusions as well as hydrocarbon- and CO₂-rich inclusions. The presence of abundant hydrocarbon fluid inclusions in the gold deposits provides evidence that at least during main periods of the hydrothermal activity responsible for gold mineralization, the ore fluids consisted of an aqueous solution and an immiscible hydrocarbon phase. Aqueous inclusions in the main stage quartz associated with gold mineralization (stage II) typically have a homogenization temperature range of 200–230°C and a modal salinity around 5.3 wt% NaCl eq. Homogenization temperatures and salinities of aqueous inclusions in the late-stage drusy quartz and calcite (stage III) typically range from 120°C to 160°C and from 2.0 to 5.6 wt% NaCl eq., respectively. In the paleo-oil reservoirs, aqueous fluid inclusions with an average homogenization temperature of 80°C are dominant in early diagenetic calcite. Fluid inclusions in late diagenetic pore- and fissure-filling calcite associated with bitumen are dominated by liquid C₂H₆, vapor CH₄, CH₄–H₂O, and aqueous inclusions, with a typical homogenization temperature range of 90°C to 180°C and a salinity range of 2–8 wt% NaCl eq. It is suggested that the hydrocarbons may have been trapped at relatively low temperatures, while the formation of gold deposits could have occurred under a wider and higher range of temperatures. The timing of gold mineralization in the Youjiang basin is still in dispute and a wide range of ages has been reported for individual deposits. Among the limited isotopic data, the Rb–Sr date of 206 ± 12 Ma for Au-bearing hydrothermal sericite at Jinya as well as the Re–Os date of 193 ± 13 Ma on auriferous arsenian pyrite and ⁴⁰Ar/³⁹Ar date of 194.6 ± 2 Ma on vein-filling sericite at Lannigou may provide the most reliable age constraints on gold mineralization. This age range is comparable with the estimated petroleum charging age range of 238–185 Ma and the Sm–Nd date of 182 ± 21 Ma for the pore- and fissure-filling calcite associated with bitumen at the Shitouzhai paleo-oil reservoir, corresponding to the late Indosinian to early Yanshanian orogenies in South China. The close association of Carlin-type gold deposits and paleo-oil reservoirs, the paragenetic coexistence of bitumens with ore-stage minerals, the presence of abundant hydrocarbons in the ore fluids, and the temporal coincidence of gold mineralization and hydrocarbon accumulation all support a coeval model in which the gold originated, migrated, and precipitated along with the hydrocarbons in an immiscible, gold- and hydrocarbon-bearing, basinal fluid system.     

Light hydrocarbons geochemistry of surface sediments from Pranhita-Godavari Basin,Andhra Pradesh

A geochemical study of surface sediments from Pranhita-Godavari Basin, Andhra Pradesh, India was carried out using light hydrocarbon compounds to assess the hydrocarbon potential of the basin. Suite of 80 soil samples were collected from the depth of 2.5 m and analyzed for adsorbed light gaseous hydrocarbons namely methane (CH₄), ethane (C₂H₆) and propane (C₃H₈) in Gas chromatograph. Compound specific Carbon isotope ratios for CH₄ and C₂H₆ were also determined using GC-C IRMS (Gas Chromatograph Combustion Isotope Mass Spectrometer). The presence of moderate to low concentrations of methane (C_CH4 C_{CH_4 } : 1 to 138 ppb), ethane (H₄{H_4 }: 1 to 35 ppb) and propane (C_{C3 H8} C_{C_3 H_8 } : 1 to 20 ppb) was measured in the soil samples. Carbon isotopic composition of d¹³ C_CH4 \delta ^{13} C_{CH_4 } ranges between −27.9 to −47.1 ‰ and d¹³ C_{C2 H6} \delta ^{13} C_{C_2 H_6 } ranged between −36.9 to −37.2 ‰ (V-PDB) indicating that these gases are of thermogenic origin. Study of soil samples suggests the area has good potential for hydrocarbon.     

Carbon and hydrogen isotopic compositions of products of open-system catalytic hydrogenation of CO2: Implications for abiogenic hydrocarbons in Earth’s crust

This paper reports the isotope effects in an open-system Fischer-Tropsch type (FTT) synthesis, with implications for the origin of natural abiogenic hydrocarbons. The starting form of carbon was CO₂, with carbon and hydrogen isotopic compositions measured for products of catalytic hydrogenation of CO₂ on iron and cobalt catalysts (FTCO₂-Fe and FTCO₂-Co) at 350 and 245 °C, respectively, and 10 MPa. The carbon isotopic composition of the resulting saturated hydrocarbons (alkanes) as a function of carbon number shows a positive trend for both FTCO₂-Fe and FTCO₂-Co, with a fractionation of 2-4‰ and 3-6‰ between CH₄ and C₂H₆ over the Fe and Co catalysts, respectively. The unsaturated hydrocarbons (alkenes) do not show any trend. A strong kinetic isotope fractionation (>40‰) occurred between CO₂ and CH₄ in both experiments. The hydrogen isotope fractionation between alkanes appeared to be similar to that found in natural (thermogenic and biogenic) gases, with enrichment in deuterium of longer hydrocarbon chains; the dominant H/D fractionation occurred between CH₄ and C₂H₆. Alkenes in the products of the FTCO₂-Fe reaction are enriched in deuterium (∼50‰) and do not show any trend versus carbon number. We suggest that other than FTT reactions or a simple mixing are responsible for the occurrence of the inverse isotopic trends in both δ¹³C and δD found in light hydrocarbons in some terrestrial environments and meteorites.     

Boron in granitic magmas: stability of tourmaline in equilibrium with biotite and cordierite

Experiments at 750 °C, 200 MPa_(H2O), a _(H2O)=1, and fO₂∼Ni-NiO established that the equilibrium among tourmaline, biotite, cordierite, and melt (± spinel, aluminosilicate, or corundum) occurs with ∼2 wt% B₂O₃ in strongly peraluminous melt with an aluminosity, measured by the parameter ASI, of >1.2. The experiments demonstrate the relationship of tourmaline stability to the activity product of the tourmaline components boron and aluminum, which are inversely related to one another. Tourmaline is unstable in metaluminous to mildly peraluminous melts (ASI <1.2) at 750 °C regardless of their boron content. For a given aluminosity, addition of components such as F requires a greater boron content of melt at this equilibrium. The stability of tourmaline increases with decreasing temperatures below 750 °C. At the inception of melting, tourmaline breaks down incongruently to assemblages containing crystalline AFM silicates (biotite, cordierite, garnet, sillimanite), aluminates (spinel, corundum), and B-enriched but Fe-Mg-poor melt. Granitic melts are likely to be undersaturated in tourmaline from the start of their crystallization, and their initial boron contents will be limited by the abundance of tourmaline in their source rocks. Quartzofeldspathic (gneissic, metapelitic) rocks that reached conditions of the granulite facies and still contain (prograde) tourmaline are rare, and probably have never yielded a partial melt. Most leucogranitic magmas will initially crystallize biotite, cordierite, or garnet, but not tourmaline. With crystallization, the Fe-Mg content of melt decreases, and the B₂O₃ content increases until the tourmaline-biotite and/or tourmaline-cordierite (or garnet) equilibria are attained. The B₂O₃ content of melt is buffered as long as these equilibria continue to operate, but low initial Fe-Mg contents of the magmas limit the quantity of boron that can be consumed by these reactions to <1 wt% B₂O₃. Normally, leucogranitic magmas contain insufficient Fe and Mg to conserve all boron as tourmaline and thus lose a large fraction of magmatic boron to wallrocks. Leucogranites and pegmatites with tourmaline as an early and only AFM silicate mineral probably contained >2 wt% B₂O₃ in their bulk magmas. Received: 6 August 1996 / Accepted: 21 July 1997     

Adsorption of NO, SO2 and light hydrocarbons on activated Greek brown coals

Twenty-eight samples of peat, peaty lignites and lignites (of both matrix and xylite-rich lithotypes) and subbituminous coals have been physically activated by pyrolysis. The results show that the surface area of the activated coal samples increases substantially and the higher the carbon content of the samples the higher the surface area.The adsorption capacity of the activated coals for NO, SO₂, C₃H₆ and a mixture of light hydrocarbons (CH₄, C₂H₆, C₃H₈ and C₄H₁₀) at various temperatures was measured on selected samples. The result shows a positive correlation between the surface area and the gas adsorption. In contrast, the gas adsorption is inversely correlated with the temperature. The maximum recorded adsorption values are: NO = 8.22 × 10^− 5 mol/g at 35 °C; SO₂ = 38.65 × 10^− 5 mol/g at 60 °C; C₃H₆ = 38.9 × 10^− 5 mol/g at 35 °C; and light hydrocarbons = 19.24 × 10^− 5 mol/g at 35 °C. Adsorption of C₃H₆ cannot be correlated with either NO or SO₂. However, there is a significant positive correlation between NO and SO₂ adsorptions. The long chain hydrocarbons are preferentially adsorbed on activated lignites as compared to the short chain hydrocarbons.The results also suggest a positive correlation between surface area and the content of telohuminite maceral sub-group above the level of 45%.     

Kristiansenite, a new calcium–scandium–tin sorosilicate from granite pegmatite in Tørdal, Telemark, Norway

Summary Kristiansenite occurs as a late hydrothermal mineral in vugs in an amazonite pegmatite at Heftetjern, T?rdal, Telemark, Norway. Tapering crystals, rarely up to 2 mm long, are colourless, white, or slightly yellowish. The mineral has the ideal composition Ca₂ScSn(Si₂O₇)(Si₂O₆OH) and is triclinic C1 with cell parameters a = 10.028(1), b = 8.408(1), c = 13.339(2) ?, α = 90.01(1), β = 109.10(1), γ = 90.00(1)°, V = 1062.7(3) ?³ (Z = 4). It has a monoclinic cell within ∼ 0.1 ? and is polysynthetically twinned on {010} by metric merohedry. The strongest reflections in the X-ray powder pattern are [d in ?, (I _obs), (hkl)]: 5.18 (53) (1–11), 3.146 (100) (004), 3.089 (63) (−222), 2.901 (19) (221), 2.595 (34) (222), 2.142 (17) (−3–31). The Mohs’ hardness is 5?–6; D_calc. = 3.64 g/cm³; only a mean refractive index of 1.74 could be measured. Scandium enrichment in the Heftetjern pegmatite and the crystal chemistry of scandium are briefly discussed. Received April 30, 2001; accepted July 28, 2001     

Multiple phase transitions of leonite-type compounds: optical, calorimetric, and X-ray data

Summary Low-temperature phase transitions of leonite-type compounds, K₂Me²⁺(SO₄)₂ · 4H₂O (Me = Mg, Mn, Fe), are investigated by temperature dependent measurements of single-crystal X-ray reflection intensities and lattice parameters. The transition temperatures and the progress of the transitions are determined by birefringence data and differential scanning calorimetry. The cause for the phase transitions of leonite-type compounds is a dynamic disorder of sulphate groups at room temperature (C2/m), that freezes in to an ordered structure (I2/a) at −4(1) °C in leonite, K₂Mg(SO₄)₂ · 4H₂O. At −153(1) °C the crystal structure switches to another ordered phase (P2₁/a). The Mn analogue shows the same succession with transition temperatures at −68(1) °C and −104(1) °C. The disordered room temperature structure of the isotypic mineral mereiterite, K₂Fe(SO₄)₂ · 4H₂O, transforms directly to the ordered P2₁/a structure at 3(2) °C. Analysis of X-ray intensities and of excess birefringence reveals that the displacive I2/a ⇔ P2₁/a phase transition of leonite and Mn-leonite is first order. According to Landau theory the C2/m ⇔ I2/a (leonite, Mn-leonite) and C2/m ⇔ P2₁/a (mereiterite) order-disorder transitions are almost tricritical. Received March 7, 2001; revised version accepted June 27, 2001     

Mineral inclusions in pyrope crystals from Garnet Ridge, Arizona, USA: implications for processes in the upper mantle

Mineral inclusions in pyrope crystals from Garnet Ridge in the Navajo Volcanic Field on the Colorado Plateau are investigated in this study with emphasis on the oxide minerals. Each pyrope crystal is roughly uniform in composition except for diffusion halos surrounding some inclusions. The pyrope crystals have near constant Ca:Fe:Mg ratios, 0.3 to 5.7 wt% Cr₂O_3, and 20 to 220 ppm H₂O. Thermobarometric calculations show that pyrope crystals with different Cr contents formed at different depths ranging from 50 km (where T ≈ 600 °C and P = 15 kbar) to 95 km (where T ≈ 800 °C and P = 30 kbar) along the local geotherm. In addition to previously reported inclusions of rutile, spinel and ilmenite, we discovered crichtonite series minerals (AM₂₁O₃₈, where A = Sr, Ca, Ba and LREE, and M mainly includes Ti, Cr, Fe and Zr), srilankite (ZrTi₂O₆), and a new oxide mineral, carmichaelite (MO_2−x(OH)_x, where M = Ti, Cr, Fe, Al and Mg). Relatively large rutile inclusions contain a significant Nb (up to 2.7 wt% Nb₂O₅), Cr (up to ∼6 wt% Cr₂O₃), and OH (up to ∼0.9 wt% H₂O). The Cr and OH contents of rutile inclusions are positively related to those of pyrope hosts, respectively. Needle- and blade-like oxide inclusions are commonly preferentially oriented. Composite inclusions consisting mainly of carbonate, amphibole, phlogopite, chlorapatite, spinel and rutile are interpreted to have crystallized from trapped fluid/melt. These minerals in composite inclusions commonly occur at the boundaries between garnet host and large silicate inclusions of peridotitic origin, such as olivine, enstatite and diopside. The Ti-rich oxide minerals may constitute a potential repository for high field strength elements (HFSE), large ion lithophile elements and light rare earth elements (LREE) in the upper mantle. The composite and exotic oxide inclusions strongly suggest an episode of metasomatism in the depleted upper mantle beneath the Colorado Plateau, contemporaneous with the formation of pyrope crystals. Our observations show that mantle metasomatism may deplete HFSE in metasomatic fluids/melts. Such fluids/melts may subsequently contribute substantial trace elements to island arc basalts, providing a possible mechanism for HFSE depletion in these rocks. Received: 20 December 1997 / Accepted: 15 October 1998     

Graphite precipitation in C—O—H fluid inclusions: closed system compositional and density changes,and thermobarometric implications

 Equilibrium C–O–H fluid speciation calculations predict that graphite will precipitate from initially graphite saturated fluid inclusions during cooling and exhumation of metamorphic rocks. In the case that no mass is gained or lost by the inclusions, the original X _O ratio [O/(O+H)] of the fluid phase must be maintained. Given this closed system constraint, the down-temperature progress of graphite precipitation can easily be monitored as a function of the varible X _O, and produces some effects that are of significance to fluid inclusion studies: 1. Variation of the H₂O : CO₂ : CH₄ relationship in the graphite-saturated COH fluid, namely increase of X _H2 ^O and decrease of the carbonic fraction; 2. Decrease of fluid density due to precipitation of graphite, which is denser than the residual fluid; 3. Alteration of the CO₂ : CH₄ ratio of the fluid, depending on the initial O : H ratio of the fluid: for X _O>1/3, fluids increase their CO₂ : CH₄ ratio with decreasing temperature, and vice-versa. This implies that the CO₂ : CH₄ ratio measured at room T will not represent the trapping value, which is in any case closer to unity. As a consequence of density reduction, isochores extrapolated from densities observed at room temperature do not pass through the pressure-temperature conditions at which the inclusion was trapped, with pressure underestimates of up to 2 kbar. Actual P-T trapping conditions are located along the equilibrium “bulk isochore” (curve of constant-X _O, constant-volume) of the fluid. Alteration of the CO₂ : CH₄ ratio is a mechanism by which a CO₂-rich or CH₄-rich carbonic phase can be formed from aqueous fluids that are slightly off the neutral X _O=1/3 value. Subsequent segregation of this phase from the aqueous counterpart may account for the formation of pure CO₂ and CH₄ fluids in the upper crust. Received: 15 March 1995 / Accepted: 1 June 1995     

Carbon chemistry of Luna 16 and Luna 20 samples

Luna 16 and Luna 20 samples were analyzed for volatilizable species using vacuum pyrolysis to 1400°C. The major gaseous products evolved (ranging from 10–650 μg/g) were H₂O, CO, CO₂, N₂ and CH₄. Minor components (all < 10 μg/g) included NH₃, HCN, NO, SO₂, H₂S, C₂H₂, C₂H₄, C₂H₆, C₃H₆ and higher hydrocarbons, benzene, toluene, and the polymeric contaminants Teflon^® and silicone oil. The total carbon and nitrogen contents (μg/g) for these sieved samples (< 125 μm) were: Luna 16—C 418, N 134 and Luna 20—C 380, N 80.     

Fluid regime of platinum group elements (PGE) and gold-bearing reef formation in the Dovyren mafic–ultramafic layered complex, eastern Siberia, Russia

The Dovyren layered dunite–troctolite–gabbro massif (northern Transbaikalia region, Russia) contains precious metal mineralization related to sparsely disseminated sulfides (Stillwater type). The distribution of gases trapped in micro-inclusions and intergranular pores of the Dovyren massif has been investigated. This type of study had previously only been undertaken on the traps or peridotite–pyroxenite–norite intrusions hosting copper–nickel sulfide deposits. A novel method of analyzing trapped gases, involving the grinding of samples under high vacuum at room temperature, was employed. A modified gas-chromatography and mass-spectrometry approach was used to analyze the composition of the extracted gases. The concentrations of reduced gases (CH₄ and H₂) are higher in inclusions trapped by silicate minerals, whereas oxidized gases (H₂O, CO₂) are less common. The content of reduced gases (H₂, CH₄, CO), N₂, He, radiogenic Ar, and C₂H₆ increases upward through the layered series of the massif. The distribution of all gases, especially methane and hydrogen, show peak concentrations coincident with the PGE and gold reef type horizons. A correlation of the gas peaks and noble metal contents appears to be related to their geochemical affinities. This conclusion is supported by the experimental modeling. Received: 4 August 1999 / Accepted: 13 January 2000     

Partial fusion of basic granulites at 5 to 15 kbar: implications for the origin of TTG magmas

Partial fusion experiments with basic granulites (S6, S37) believed to represent the lower crust beneath the Eifel region (Germany) were performed at pressures from 5 to 15 kbar. Water-undersaturated experiments were carried out in the presence of 1 wt% H₂O plus 2.44 or 0.81 wt% CO₂ equivalent to mole fractions of H₂O/(H₂O + CO₂) of 0.5 and 0.75, respectively, of the volatile components added. At temperatures from 850 to 1100 °C the weight proportions of melt range from 7 to 30 %. Melt compositions change from trondhjemitic over tonalitic to dioritic with increasing degree of partial melting. Crystalline residua are plagioclase/pyroxene dominated at 5 kbar to garnet/pyroxene dominated at 15␣kbar. Dehydration melting was studied in granulite S35 similar in composition to S6. The magmatic precursors of the granulite xenoliths used in this study had geochemical characteristics of cumulate gabbro (metagabbro S37) and evolved melts (metabasalts S6, S35), respectively. Melts from granulite S37 match the major element compositions of natural trondhjemites and tonalites. At 5 kbar, their Al₂O₃ is relatively low, similar to tonalites from ophiolites. At 15 kbar, Al₂O₃ in the melts is high due to the near absence of plagioclase in the crystalline residua. The Al₂O₃ concentrations in 15 kbar melts from S6 (˜20 wt%) are higher than in natural tonalites. Depth constraints on the formation of tonalitic magmas in the continental crust are provided by REE (rare earth element) patterns of the synthetic melts calculated from the known REE abundances in metagabbro S37 and metabasalt S6 assuming batch melting and using partition coefficients from the literature. The REE patterns of tonalites from active continental margins and Archean trondhjemite-tonalite-granodiorite␣associations low in REE with La_N (chondrite normalised) from 10 to 30 and Yb_N from 1 to 2 are reproduced at pressures of 10 and 12.5 kbar from metagabbro S37 which displays a slightly L(light)REE enriched pattern with La_N = 8 and Yb_N = 3. Natural tonalites with La_N from 30 to 100 require a source richer in REE than granulite S37. At 15 kbar, H(heavy)REE_N in melts from granulite S37 are depressed below the level observed in natural tonalites due to the high proportion of garnet (>30 wt%) in the residue. Melts from metabasalt S6 (enriched in REE with La_N = 38 and Yb_N = 16) do not match the REE characteristics of natural tonalites under any conditions. Received: 1 July 1994 / Accepted: 11 September 1996