Fe XVIII EMISSION LINES IN SOLAR X-RAY SPECTRA

We have calculated intensity ratios for emission lines of Fexviii in the 13–94 Å wavelength range at electron temperatures characteristic of the solar corona, T _e = 2–10 x 10⁶ K. Our model ion includes data for transitions among the 2s ²2p ⁵ , 2s2p ⁶, 2s ²2p ⁴3l, and 2s2p ⁵3l (l = s, p, and d) states. Test calculations which omit the 2s2p ⁵3l levels show that cascades from these are important. We compare our results with observed ratios determined from four solar X-ray instruments, a rocket-borne spectrograph, and spectrometers on the P78–1, OV1–17 and Solar Maximum Mission (SMM) satellites. In addition, we have generated synthetic spectra which we compare directly with flare observations from SMM. Agreement between theory and observation is generally quite good, with differences that are mostly less than 30%, providing limited support for the accuracy of the atomic physics data used in our calculations. However, large discrepancies are found for ratios involving the 2s ²2p ^{5 2}P_3/2- 2s2p ^{6 2}S line at 93.84 Å, which currently remain unexplained. Our analysis indicates that the FeXVIII feature at 15.83 Å is the 2s ²2p ^{5 2}P_3/2 - 2s ²2p ⁴(³P)3s ⁴P_3/2 transition, rather than 2s ²2p ^{5 2}P_3/2 - 2s ²2p ⁴(³P)3s ²P_3/2, as suggested by some authors.     

Variable Variscan thermal rejuvenation in the St Malo region, Cadomian Orogen, France: evidence from 40Ar/39Ar mineral ages

Abstract The St Malo region in north-west France contains migmatites and anatectic granites derived by partial melting of metasedimentary protoliths during Cadomian orogenesis at c. 540 Ma. Previously reported Rb–Sr model ages for muscovite and biotite range from c. 550 to c. 300 Ma, and suggest variable resetting of mineral isotopic systems. These rocks display microscopic evidence for variably intense Cadomian intracrystalline plastic strain but record no obvious evidence of penetrative Palaeozoic regional deformation. ⁴⁰Ar/³⁹Ar mineral ages have been determined to evaluate better the extent, timing and significance of Palaeozoic overprinting. Eleven muscovite concentrates and one whole-rock phyllite have been prepared from various units exposed in the St Malo and adjacent Mancellian regions. In the Mancellian region, muscovite from two facies of the Bonnemain Granite Complex record ⁴⁰Ar/³⁹Ar plateau ages of c. 527 and 521 Ma. An internally discordant ⁴⁰Ar/³⁹Ar release spectrum characterizes muscovite from protomylonitic granite within the Cadomian Alexain-Deux Evailles-Izé Granite Complex, and probably records the effects of Variscan displacement along the North Armorican Shear Zone. Muscovite concentrates from anatectic granite and from Cadomian mylonites along ductile shear zones within the north-western sector of the St Malo region exhibit internally discordant ⁴⁰Ar/³⁹Ar release spectra which suggest variable and partial late Palaeozoic rejuvenation. By contrast, muscovite concentrates from samples of variably mylonitic Brioverian metasedimentary rocks exposed within the south-eastern sector of the St Malo region display internally concordant apparent age spectra which define plateaux of 326–320 Ma. A whole-rock phyllite sample from Brioverian metasedimentary rocks exposed along the eastern boundary of the St Malo region displays an internally discordant argon release pattern which is interpreted to reflect the effects of a partial late Palaeozoic thermal overprint. Muscovite from the Plélan granite, part of the Variscan Plélan-Bobital Granite Complex, yields a ⁴⁰Ar/³⁹Ar plateau age of c. 307 Ma. The ⁴⁰Ar/³⁹Ar results indicate that Cadomian rocks of the St Malo region have undergone a widespread and variable Palaeozoic (Carboniferous) rejuvenation of intracrystalline argon systems which apparently did not affect the Mancellian region. This rejuvenation was not accompanied by penetrative regional deformation, and was probably of a static thermal–hydrothermal origin. The heat source for rejuvenation was probably either the result of heating during Variscan extension or advection from Variscan granites which are argued to underlie the St Malo region.     

Modelling of mineral equilibria in ultrahigh-temperature metamorphic rocks from the Anápolis–Itauçu Complex, central Brazil

A new quantitative approach to constraining mineral equilibria in sapphirine‐bearing ultrahigh‐temperature (UHT) granulites through the use of pseudosections and compatibility diagrams is presented, using a recently published thermodynamic model for sapphirine. The approach is illustrated with an example from an UHT locality in the Anápolis–Itauçu Complex, central Brazil, where modelling of mineral equilibria indicates peak metamorphic conditions of about 9 kbar and 1000 °C. The early formed, coarse‐grained assemblage is garnet–orthopyroxene–sillimanite–quartz, which was subsequently modified following peak conditions. The retrograde pressure–temperature (P–T) path of this locality involves decompression across the FeO–MgO–Al₂O₃–SiO₂ (FMAS) univariant reaction orthopyroxene + sillimanite = garnet + sapphirine + quartz, resulting in the growth of sapphirine–quartz, followed by cooling and recrossing of this reaction. The resulting microstructures are modelled using compatibility diagrams, and pseudosections calculated for specific grain boundaries considered as chemical domains. The sequence of microstructures preserved in the rocks constrains a two‐stage isothermal decompression–isobaric cooling path. The stability of cordierite along the retrograde path is examined using a domainal approach and pseudosections for orthopyroxene–quartz and garnet–quartz grain boundaries. This analysis indicates that the presence or absence of cordierite may be explained by local variation in a_H2O. This study has important implications for thermobarometric studies of UHT granulites, mainly through showing that traditional FMAS petrogenetic grids based on experiments alone may overestimate P–T conditions. Such grids are effectively constant a_H2O sections in FMAS‐H₂O (FMASH), for which the corresponding a_H2O is commonly higher than that experienced by UHT granulites. A corollary of this dependence of mineral equilibria on a_H2O is that local variations in a_H2O may explain the formation of cordierite without significant changes in P–T conditions, particularly without marked decompression.     

Mineral Chemistry and Crystallization Conditions of the Mboutou Layered Gabbro-Syenite-Granite Complex, North Cameroon

The Mboutou complex is one of a line of early Tertiary ringcomplexes which runs from Lake Chad to the Gulf of Guinea, noneof which has hitherto been described in detail. The main rocktypes are layered gabbros and gabbronorites, with minor bodiesof quartz-syenodiorite, quartz-syenite and hypersolvus granite.Feldspars form a continuum with exceptional compositional range,from An₈₅Ab₁₃Or₂ to around An₁Ab₄₆Or₅₅, and form an entirelyhypersolvus sequence with very strong zoning in the syenodiorites.Ca-rich clinopyroxenes (salite and calcic augite) and olivines(Fo_78–62) have restricted range. Orthopyroxene-bearingleucogabbros and syenodiorites contain minor orthopyroxene (En₆₂Fs₃₅Wo₃)and quartz; olivine and orthopyroxene never coexist. In moreevolved rocks amphibole (magnesio-hornblende to ferroedenite)and minor biotite, showing progressive Fe-enrichment, are theonly mafic silicates.Major-element rock chemistry, minor elementsin clinopyroxenes and biotite chemistry show that, notwithstandingits thoroughly anorogenic setting, Mboutou was, at the outset,only very mildly alkaline. Its more evolved members embarkedon a line of evolution with some calc-alkaline characteristics,probably because of ingress of water into residual batches ofmagma, a possibility supported by stable isotope data. Thischange in behaviour corresponded with the sudden appearanceof quartz and orthopyroxene, which was not in equilibrium withclinopyroxene on the two-pyroxene surface. Amphibole then becamethe main mafic silicate with further increase in . The more evolved rocks are relatively highly altered,but Fe-Ti oxide pairs suggest that was maintained near to and above the QFM buffer and the rangeof biotite compositions further suggests crystallization undera regime of decreasing . Biotites maintain alkaline characteristics throughout the sequence. Zoningpatterns in the ternary feldspars in the syenodiorites, andthe hypersolvus character of the final granite, limit maximumvalues of to < 1 kb, and suggest minimum temperatures for the end of crystallizationin the syenodiorites of{small tilde} 850 ?C.     

Derivation of Internally-Consistent Thermodynamic Data by the Technique of Mathematical Programming: a Review with Application the System MgO-SiO2-H2O

The problem of deriving an optimal set of thermodynamic propertiesof minerals from a diverse experimental data base is reviewedand a preferred methodology proposed. Mathematical pro-gramming(MAP)methods extend the linear programming (LIP) approach first presentedby Gordon (1973), and make it possible to account for the typeof information conveyed, and the uncertainties attending bothphase equilibrium data and direct measurements of phase properties.For phase equilibrium data which are (in most cases) characterizedby non-normal error distributions across experimental brackets,the midpoint of a bracket is no more probable than other points,and the data are best treated by considering the inequalityin the change in Gibbs free energy of reaction at each half-bracket.Direct measurements of phase properties can be assumed to haveapproximately normal error distributions, and the MAP techniqueoptimizes agreement with these values by using the principlesof least squares in the definition of an objective function.The structure of this problem, treatment of uncertainties invarious types of experimental data, and method of optimizingfinal solutions are discussed in some detail.The method is appliedto experimental data in the MgO-SiO₂-H₂O system, where inconsistenciesamong the data are resolved and an optimal set of thermodynamicproperties is presented. The derived standard state entropiesand volumes agree with all direct measurements (within theiruncertainties), as do enthalpies of formation from the elementsexcept for those of talc (+16 kJ mol^–1), anthophyllite(+ 14 kJ mol^–1), and brucite (–1 kJ mol^–1).Stable phase relations in the system have the topology predictedby Greenwood (1963, 1971), with quartz- and forsterite-absentinvariant points at 683 ?C-6-4 kb and 797 ?C-12 kb respectively,repeating at 552 ?C-120 b and 550 ?C-55 b. The thermodynamicanalysis indicates little remaining flexibility in the phaserelations, which, when combined with suitable activity modelsfor solid solution, should allow for accurate determinationof the conditions of metamorphism of ultramafic rocks.     

TUNING EFFECTS,LITHOLOGICAL EFFECTS AND DEPOSITIONAL EFFECTS IN THE SEISMIC RESPONSE OF GAS RESERVOIRS*

Synergism of 3-D seismic data, wavelet processing, colour display and interactive interpretation has been exploited in the study of a Gulf of Mexico gas reservoir. Seismic amplitude has been used as a measure of the proportion of a sand/shale reservoir capable of producing gas. This has led to the mapping of net producible thickness of gas sand. The tuning phenomena resulting from geometric effects alone were studied in detail, and tuning curves of various levels of sophistication were used as the basis for amplitude editing. Statistical tuning curves were derived by interactive cross-plotting and deterministic curves by wavelet extraction. Multiple wavelet side lobes cause multiple maxima in the tuning curve. Depositional effects and intrareservoir communication have also been studied by interactive cross-plotting.     

深反射地震揭示喜马拉雅地区地壳上地幔的复杂结构

报告了中、美两国在喜马拉雅山区进行的第一次深反射地震试验的结果．试验剖面南起喜马拉雅山山脊南亚东县的帕里镇，向北穿过喜马拉雅山脊的荡拉，到达康马南的萨马达．剖面长约１００ｋｍ．共中心点（ＣＭＰ）叠加剖面上显示出：１．在地壳中部有一强反射带，向北缓倾斜下去，延长达１００ｋｍ以上．它可能代表了一个活动的逆冲断裂或是一条巨大的拆离带，印度地壳整体或下地壳沿此拆离层俯冲到藏南之下．２．上部地壳的反射很丰富，显示了上地壳存在着大规模的叠瓦状结构．３．下地壳的反射同相轴呈现短而有规律的分布，显示了塑性流变特征．４．在测线南部莫霍反射明显，深度达７２－７５ｋｍ．发现南部有双莫霍层的存在．５．试验中还取得莫霍层下面３２，３８，４８ｓ等双程走时的多条反射，向北倾斜，反射同相轴延续较长，信息丰富，反映了上地幔的成层结构和变形特征．这些结果对印度大陆地壳整体或其下地壳俯冲到藏南特提斯喜马拉雅地壳之下，并导致西藏南端地壳增厚的观点，给予了实质性的支持．