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1.
The bulk properties and bitumen molecular compositions of a rank-series of 38 humic coals from the New Zealand Coal Band (Cretaceous–Cenozoic) have been analysed to investigate early maturation processes affecting coaly organic matter through diagenesis to moderate catagenesis (Rank(S r) 0.0–11.8, Ro 0.23–0.81%). The samples comprise a relatively restricted range of vitrinite rich coal types formed largely from higher land plant material under relatively oxic conditions, but with a significant contribution from microbial biomass. With increasing rank, total organic carbon contents show a general increase, whereas moisture and asphaltene contents decrease. Bitumen yields also decrease through the stages of diagenesis and early catagenesis (Rank(S r) < 9, Ro < 0.55%), indicating partial loss of initial bitumen during early maturation. Thermal generation of hydrocarbons begins slowly at Rank(S r) 5–6 ( Ro 0.40%) as indicated by the constant occurrence and gradual increase of isoprenoids (e.g., pristane and phytane) and hopanoids in their more mature αβ configuration. This early phase of catagenesis, not previously recognised in New Zealand coals, is followed at Rank(S r) 9 ( Ro 0.55%) by the main catagenesis phase characterised by a more rapid increase in the generation of hydrocarbons, including total n-alkanes, isoprenoids and αβ-hopanes. Changes in the maturity of New Zealand coals can be traced by the Carbon Preference Index and several hopane maturity parameters, including 22 S/(22 S + 22 R), αβ/(αβ + βα) and ββ/(αβ + βα + ββ). 相似文献
2.
Eighty-two palaeomagnetic samples of calcareous and jaspilitic grainstones (iron-formation or ‘taconite’) and chert carbonate were collected from the 1.88-Ga Gunflint Formation at 22 sites in the Thunder Bay area, Ontario. Twenty clasts of Gunflint taconite also were sampled from the basal conglomerate of the overlying Mesoproterozoic Sibley Group. Anisotropy of magnetic susceptibility measurements indicate the Gunflint Formation in the sampling area has not experienced regional dynamic metamorphism. Analyses by variable-field translation balance and X-ray diffraction show that the predominant magnetic mineral is hematite but a small amount of magnetite also is present in some samples. Altogether, 213 Gunflint specimens and 59 Sibley conglomerate specimens were subjected to stepwise thermal demagnetisation and 74 Gunflint specimens to stepwise alternating-frequency demagnetisation. The following components were isolated for the taconites: - • Gunflint magnetite: normal declination D=293.4°, inclination I=30.8°, α95=7.2°, n=21; reverse D=86.7°, I=–54.6°, α95=5.8°, n=29.
- • Gunflint hematite: normal D=243.6°, I=23.6°, α95=6.0°, n=11; reverse D=70.3°, I=–51.4°, α95=3.2°, n=79.
- • Sibley clasts magnetite: normal D=282.7°, I=33.4°, α95=7.6°, n=20.
- • Sibley clasts hematite: normal D=254.5°, I=56.2°, α95=8.4°, n=13; reverse D=110.6°, I=–55.7°, α95=8.3°, n=11.
None of these sets passed the reversal test, with the normal component generally being the shallower. Fold tests were negative or inconclusive and the conglomerate test also was negative. Chert carbonate at one other site appears to have acquired a remanence carried by magnetite ( D=97.3°, I=−78.2°, α95=6.3°, n=18) prior to folding related to Keweenawan (1.1 Ga) Logan diabase sill emplacement. Most of the components we identified match components for Keweenawan sills, volcanic rocks, intrusions and baked contact rocks in the Thunder Bay area, indicating that Keweenawan magmatism caused widespread chemical remagnetisation of the Proterozoic country rock in our sampling area. Although others have argued that asymmetry was a feature of the Keweenawan geomagnetic field, the declinations of our Gunflint and Sibley hematite and magnetite components are different, suggesting that the components were acquired at significantly different times. We conclude that the reversal asymmetry shown by our Gunflint and Sibley data may best be ascribed to apparent polar wander during Keweenawan times. 相似文献
3.
The paper describes the unified scaling theory for distribution functions of temporal and spatial characteristics in seismology. It is based on the scaling of seismological characteristics calculated for various energy–spatial–temporal intervals. The common mathematical methods for the scaling of distribution functions are developed. The means to test possibility of such scaling are found as well. The relationship between the unified scaling theory and other present scaling approaches is determined. The theory is applied to two characteristics of different seismoactive regions. The first characteristic is the waiting time between earthquakes Δ T, the second one is a new space parameter Δ Dmin, which is the minimum distance of a current seismic event to the nearest (in space) neighbor in an energy–spatial–temporal interval. The distribution of the characteristics Δ T and Δ Dmin allows estimating the time interval to the next earthquake and the distance of the following earthquake from previous earthquakes. Thus, these characteristics are very important for seismic hazard estimations. Scaling of distributions functions is proven to be successful for Δ Dmin in all energy–spatial–temporal intervals and for Δ T with variations of energy/magnitude range. The distribution function of Δ T for various time domains was stable in only 60% of the cases, and near to unstable for spatial variations. 相似文献
4.
A case study of three springs in Switzerland is used to demonstrate the value of geochemical time-series data as a powerful tool to study the dynamics of groundwater systems. Values of repeatedly measured parameters revealed intermixings of two water types: (a) a 29°C water, circulating to a depth of 1100 m and containing approximately 700 mg/l Ca, 2000 mg/l SO 4, 700 mg/l HCO 3, 20 mg/l of Na and Cl, 6 mg/l Fe, at least 47 mg/l SiO 2, and with an isotopic composition of δD = − 73.0‰ and δ 18 O = −10.9‰, and (b) a 12°C or colder water, shallow, and of a post-1953 age, containing 420 mg/l TDI or less, very low in Na and Cl (4 mg/l or less), isotopic values of δD = −71.0‰ and δ 18 O = −10.5‰ and tritium as in recent (post-bomb) precipitation. 相似文献
5.
Hydrochemical investigations were carried out in Bahar area, Hamadan, western Iran, to assess the chemical composition of groundwater. The area falls in a semi-arid type of climate. In this area, groundwater has been exploited over the past century mainly for irrigation and water supply. A total of 135 representative groundwater samples were collected from different wells to monitor the water chemistry of various ions. Chemical analysis of the groundwater shows that the mean concentration of the cations is of the order Ca 2+>Mg 2+>Na +>K +, while that for anions is SO 42–>HCO 3–>Cl –>NO 3–. Statistical analyses indicate positive correlation between the following pairs of parameters Cl – and Mg 2+ ( r=0.71), Cl – and Na + ( r=0.76), HCO 3– and Na + ( r=0.56), SO 42– and Mg 2+ ( r=0.76), SO 42– and Na + ( r=0.69). Water presents a large spatial variability of the chemical facies (Ca-HCO 3, Ca-SO 4, Mg-HCO 3, Mg-SO 4, Na-HCO 3) which is in relation to their interaction with the geological formations of the basin (carbonates, dolomite and various silicates) and evaporation. The hydrochemical types Ca-HCO 3 and Ca-SO 4 dominate the largest part of the studied area. The dissolution of halite, calcite, dolomite, and gypsum explains part of the contained Na +, Ca 2+, Mg 2+, Cl –, SO 42– and HCO 3–, but other processes, such as cation exchange and weathering of aluminosilicates also contribute to the water composition. 相似文献
6.
Tin-polymetallic greisen-type deposits in the Itu Rapakivi Province and Rondônia Tin Province, Brazil are associated with late-stage rapakivi fluorine-rich peraluminous alkali-feldspar granites. These granites contain topaz and/or muscovite or zinnwaldite and have geochemical characteristics comparable to the low-P sub-type topaz-bearing granites. Stockworks and veins are common in Oriente Novo (Rondônia Tin Province) and Correas (Itu Rapakivi Province) deposits, but in the Santa Bárbara deposit (Rondônia Tin Province) a preserved cupola with associated bed-like greisen is predominant. The contrasting mineralization styles reflect different depths of formation, spatial relationship to tin granites, and different wall rock/fluid proportions. The deposits contain a similar rare-metal suite that includes Sn (±W, ±Ta, ±Nb), and base-metal suite (Zn–Cu–Pb) is present only in Correas deposit. The early fluid inclusions of the Correas and Oriente Novo deposits are (1) low to moderate-salinity (0–19 wt.% NaCl eq.) CO 2-bearing aqueous fluids homogenizing at 245–450 °C, and (2) aqueous solutions with low CO 2, low to moderate salinity (0–14 wt.% NaCl eq.), which homogenize between 100 and 340 °C. In the Santa Bárbara deposit, the early inclusions are represented by (1) low-salinity (5–12 wt.% NaCl eq.) aqueous fluids with variable CO 2 contents, homogenizing at 340 to 390 °C, and (2) low-salinity (0–3 wt.% NaCl eq.) aqueous fluid inclusions, which homogenize at 320–380 °C. Cassiterite, wolframite, columbite–tantalite, scheelite, and sulfide assemblages accompany these fluids. The late fluid in the Oriente Novo and Correas deposit was a low-salinity (0–6 wt.% NaCl eq.) CO 2-free aqueous solution, which homogenizes at (100–260 °C) and characterizes the sulfide–fluorite–sericite association in the Correas deposit. The late fluid in the Santa Bárbara deposit has lower salinity (0–3 wt.% NaCl eq.) and characterizes the late-barren-quartz, muscovite and kaolinite veins. Oxygen isotope thermometry coupled with fluid inclusion data suggest hydrothermal activity at 240–450 °C, and 1.0–2.6 kbar fluid pressure at Correas and Oriente Novo. The hydrogen isotope composition of breccia-greisen, stockwork, and vein fluids ( δ18O quartz from 9.9‰ to 10.9‰, δD H2O from 4.13‰ to 6.95‰) is consistent with a fluid that was in equilibrium with granite at temperatures from 450 to 240 °C. In the Santa Bárbara deposit, the inferred temperatures for quartz-pods and bed-like greisens are much higher (570 and 500 °C, respectively), and that for the cassiterite-quartz-veins is 415 °C. The oxygen and hydrogen isotope composition of greisen and quartz-pods fluids ( δ18O qtz-H2O=5.5–6.1‰) indicate that the fluid equilibrated with the albite granite, consistent with a magmatic origin. The values for mica ( δ18O mica-H2O=3.3–9.8‰) suggest mixing with meteoric water. Late muscovite veins ( δ18O qtz-H2O=−6.4‰) and late quartz ( δ18O mica-H2O=−3.8‰) indicate involvement of a meteoric fluid. Overall, the stable isotope and fluid inclusion data imply three fluid types: (1) an early orthomagmatic fluid, which equilibrated with granite; (2) a mixed orthomagmatic-meteoric fluid; and (3) a late hydrothermal meteoric fluid. The first two were responsible for cassiterite, wolframite, and minor columbite–tantalite precipitation. Change in the redox conditions related to mixing of magmatic and meteoric fluids favored important sulfide mineralization in the Correas deposit. 相似文献
7.
A simple mathematical relation among the dip of isograd surface (θ), dip of isotherm surface (α), temperature gradient ( Tg), pressure gradient ( Pg) and δT/gdP of reaction is given by: Pg is taken to be constant at 0.3 kb/km. Four major theoretical models for shapes and orientations of isograds are formulated on the basis of the parameters α, Tg and δT/δP. This offers a rational basis for studying the geometry of isograds, normal/reverse metamorphism and determination of temperature gradient. Two examples have been studied in the light of these models; in one case the domed shape of the isograd is best explained by a downwardly directed Tg; in the other case, reverse metamorphism is best explained as the result of a horizontal Tg of 18°C/km. 相似文献
8.
Previous studies on the coal-bed methane potential of the Zonguldak basin have indicated that the gases are thermogenic and sourced by the coal-bearing Carboniferous units. In this earlier work, the origin of coal-bed gas was only defined according to the molecular composition of gases and to organic geochemical properties of the respective source rocks, since data on isotopic composition of gases were not available. Furthermore, in the western Black Sea region there also exist other source rocks, which may have contributed to the coal-bed gas accumulations. The aim of this study is to determine the origin of coal-bed gas and to try a gas-source rock correlation. For this purpose, the molecular and isotopic compositions of 13 headspace gases from coals and adjacent sediments of two wells in the Amasra region have been analyzed. Total organic carbon (TOC) measurements and Rock-Eval pyrolysis were performed in order to characterize the respective source rocks. Coals and sediments are bearing humic type organic matter, which have hydrogen indices (HI) of up to 300 mgHC/gTOC, indicating a certain content of liptinitic material. The stable carbon isotope ratios ( δ13C) of the kerogen vary from −23.1 to −27.7‰. Air-free calculated gases contain hydrocarbons up to C 5, carbon dioxide (<1%) and a considerable amount of nitrogen (up to 38%). The gaseous hydrocarbons are dominated by methane (>98%). The stable carbon isotope ratios of methane, ethane and propane are defined as δ13C 1: −51.1 to −48.3‰, δ13C 2: −37.9 to −25.3‰, δ13C 3: −26.0 to −19.2 ‰, respectively. The δD 1 values of methane range from −190 to −178‰. According to its isotopic composition, methane is a mixture, partly generated bacterially, partly thermogenic. Molecular and isotopic composition of the gases and organic geochemical properties of possible source rocks indicate that the thermogenic gas generation took place in coals and organic rich shales of the Westphalian-A Kozlu formation. The bacterial input can be related to a primary bacterial methane generation during Carboniferous and/or to a recent secondary bacterial methane generation. However, some peculiarities of respective isotope values of headspace gases can also be related to the desorption process, which took place by sampling. 相似文献
9.
A revised calibration is presented relating the oxygen isotope composition of the aragonite-secreting sclerosponge Ceratoporella nicholsoni, oxygen isotope composition of seawater, and ambient water temperature. This new relationship has been obtained using high-resolution δ 18O data measured in sclerosponges from the Bahamas and Jamaica compared to ambient temperature measurements and δ 18O values of seawater from the two locations, both measured and published. New data improve an existing calibration which was determined using measurements of salinity rather than directly measured δ 18O values of the seawater and was composed of measurements from different species of sclerosponge and other aragonite-secreting organisms. The updated calibration ( n = 12, r2 = 0.95) is: T(°C)=16.1(±3.1)-[6.5(±1.1)](δarag-δsw), | where T is temperature in degrees Celsius, δarag is the δ18O value of aragonite normalized to VPDB, and δsw is the δ18O value of water normalized to VSMOW. This calibration improves accuracy and precision of Caribbean sclerosponges for reconstructions of temperature as well as δ18O values of seawater. 相似文献
10.
Inorganic gases are commonly seen in eastern China and occasionally in southern China from the shallow water columns above hot and cold springs. The gases contain 68% to nearly 100% CO
2, with
δ13C
CO2 and
δ13C
1 values in the range of −1.18‰ to −6.00‰ and −19.48‰ to −24.94‰, respectively. All of the 34 large inorganic CO
2 and one inorganic methane accumulations discovered in China are distributed in eastern parts of the country, from both onshore and continental shelf basins. No commercial inorganic gas accumulation has been found in central and western China. This is a review of the occurrence and geochemical characteristics of inorganic gas accumulations in Chinese sedimentary basins. A detailed study of gas samples collected from four representative inorganic CO
2 pools and one possible inorganic methane pool indicates that inorganic alkane gases typically show
δ13C
1 values greater than −10‰ versus PDB (mostly −30‰), with a positive stable carbon isotope sequence of
δ13C
1 <
δ 13C
2 <
δ13C
3 <
δ 13C
4. In contrast, the
δ13C
1 values of biogenic alkane gases are lighter than −30‰, with a negative isotope sequence (i.e.
δ13C
1 >
δ13C
2 >
δ 13C
3 >
δ13C
4). Inorganic gases also tend to show less negative
δ13C
CO2 values (−10‰) than biogenic gases (<−10‰).
相似文献
11.
A petrological, organic geochemical and geochemical study was performed on coal samples from the Soko Mine, Soko Banja basin, Serbia. Ten coal and two carbonaceous clay samples were collected from fresh, working faces in the underground brown coal mine from different parts of the main coal seam. The Lower Miocene, low-rank coal of the Soko Mine is a typical humic coal with huminite concentrations of up to 76.2 vol.%, liptinite less than 14 vol.% and inertinite less than 11 vol.%. Ulminite is the most abundant maceral with variable amounts of densinite and clay minerals. Sporinite and resinite are the most common macerals of the liptinite group. Inertodetrinite is the most abundant maceral of the inertinite group. The mineral-bituminous groundmass identified in some coal samples, and carbonaceous marly clay, indicate sub-aquatic origin and strong bacterial decomposition. The mean random huminite reflectance (ulminite B) for the main coal seam is 0.40 ± 0.05% Rr, which is typical for an immature to early mature stage of organic matter.The extract yields from the coal of the Soko Banja basin ranges from 9413 to 14,096 ppm, in which alkanes constituted 1.0–20.1%, aromatics 1.3–14.7%, asphaltenes 28.1–76.2% and resins 20.2–43.5%. The saturated hydrocarbon fractions included
n-C
15 to
n-C
32, with an odd carbon number that predominate in almost all the samples. The contents of
n-C
27 and
n-C
29 alkanes are extremely high in some samples, as a contribution of epicuticular waxes from higher plants. Acyclic isoprenoid hydrocarbons are minor constituents in the aliphatic fraction, and the pristane/phytane (Pr/Ph) ratio varies between 0.56 and 3.13, which implies anaerobic to oxic conditions during sedimentation. The most abundant diterpanes were abietane, dehydroabietane and 16α(H)-phyllocladane. In samples from the upper part of the coal seam, diterpanes are the dominant constituents of the alkane fraction. Polycyclic alkanes of the triterpane type are important constituents of alkane fractions. The occurrence of ββ- and αβ-type hopanes from C
27 to C
31, but without C
28, is typical for the Soko Banja coals.The major and trace elements in the coal were analysed using X-ray fluorescence (XRF), and inductively coupled plasma-mass spectrometry (ICP-MS). In comparison with world lignites, using the geometric mean value, the coal from the Soko Banja Basin has a high content of strontium (306.953 mg/kg). Higher values than the world lignites were obtained for Mo (3.614 mg/kg), Ni (8.119 mg/kg), Se (0.884 mg/kg), U (2.642 mg/kg) and W (0.148 mg/kg). Correlation analysis shows inorganic affinity for almost all the major and trace elements, except for S, which has an organic affinity.
相似文献
12.
Carbon isotope and molecular compositions of Mississippian to Upper Cretaceous mud gases have been examined from four depth profiles across the Western Canada Sedimentary Basin (WCSB). The profiles range from the shallow oil sands in the east (
R0 = 0.25) to the very mature sediments in the overthrust zone to the west (
R0 = 2.5). In the undisturbed WCSB,
δ13C
1–
δ13C
2 and
δ13C
2–
δ13C
3 cross-plots show three maturity and alteration trends: (1) pre-Cretaceous gas sourced from type II kerogen; (2) Cretaceous Colorado Group gas; and (3) Lower Cretaceous Mannville Group biodegraded gas. A fourth set of distinctly different maturity trends is recognized for Lower Cretaceous gas sourced from type III kerogen in the disturbed belt of the WCSB. Displacement of these latter maturity trends to high
δ13C
2 values suggests that the sampled gas was trapped after earlier formed gas escaped, probably as a result of overthrusting. Unusually
13C-enriched gas (
δ13C
1 = −34‰,
δ13C
2 = −13‰, and
δ13C
3 = 0‰), from the Gething Formation in the disturbed belt, is the result of late stage gas cracking in a closed system. In general, gas maturity is consistent with the maturity of the host sediments in the WCSB, suggesting that migration and mixing of gases was not pervasive on a broad regional and stratigraphic scale. The ‘Deep Basin’ portion of the WCSB is an exception. Here extensive cross-formational homogenization of gases has occurred, in addition to updip migration along the most permeable stratigraphic units.
相似文献
13.
Systematic geochronologic, geochemical, and Nd isotopic analyses were carried out for an early Paleoproterozoic high-K intrusive complex exposed in southwestern Tarim, NW China. The results provide a better understanding of the Paleoproterozoic tectonic evolution of the Tarim Block. Zircon U–Pb age dating indicates two Paleoproterozoic magmatic episodes occurring at ca. 2.41 Ga and ca. 2.34 Ga respectively, which were followed by a ca. 1.9 Ga metamorphic event. The 2.41 Ga granodiorite–adamellite suite shares characteristics of late to post-orogenic metaluminous A-type granites in its high alkalinity (Na
2O + K
2O = 7.6–9.3%), total REE (410–788 ppm), Zr (370–660 ppm), and Y (21.7–58.4 ppm) contents.
εNd(
t) values for the suite range from − 3.22 to − 4.71 and accordingly the Nd modal ages (T
2DM) vary between 3.05 Ga and 3.17 Ga. Based on geochemical data, the 2.34 Ga suite can be subdivided into two sub-suites, namely A-type and S-type. However, both types have comparable Nd isotope compositions (
εNd(
t) ≈ − 0.41 to − 2.08) and similar narrow T
2DM ranges (2.76–2.91 Ga).Geochemical and Nd isotopic data for the high-K intrusive complex, in conjunction with the regional geological setting, suggest that both the 2.41 Ga suite and the 2.34 Ga A-type sub-suite might have been produced by partial melting of the Archean mafic crust in a continental rift environment. The S-type sub-suite is thought to have formed by partial melting of felsic pelites and/or metagreywackes recycled from Archean crust (TTG?). Gabbro enclaves with positive
εNd(
t) value (2.15) have been found to be intermingling within the 2.34 Ga suite; ca. 2.34–2.36 Ga gabbroic dykes and adamellites have previously been documented in eastern Tarim. These observations indicate that the high-K intrusions may reflect the emergence of depleted mantle upwelling beneath the Tarim Block at that time. We suggest a three-stages model for the Precambrian crustal evolution in the Tarim Block: (1) the formation of proto-crust (TTG) by ca. 2.5 Ga, (2) episodes of felsic magmatism possibly occurring in continental rift environments at ca. 2.41 Ga and ca. 2.34–2.36 Ga, and (3) ca. 1.9 Ga metamorphism that may represent the solidification of the Precambrian basement of the Tarim Block.
相似文献
14.
Sulfur and O isotope analyses of dissolved SO
4 were used to constrain a hydrogeological model for the area overlying the Gorleben–Rambow Salt Structure, Northern Germany. Samples were collected from 80 wells screened at different depth-intervals. The study area consists of a set of two vertically stacked aquifer systems. Generally, the isotope data show a good spatial correlation, outlining well-defined groundwater zones containing SO
4 of characteristic isotopic composition. Highly saline waters from deeper parts of the lower aquifer system are characterized by rather constant SO
4 isotopic compositions, which are typical of Permian Zechstein evaporites (δ
34S=9.6–11.9‰; δ
18O=9.5–12.1‰). Above this is a transition zone containing ground waters of intermediate salinity and slightly higher isotopic values (average δ
34S=16.6‰; δ
18O=15.3‰). The confined groundwater horizon on the top of the lower aquifer system below the low permeable Hamburg Clays is low in total dissolved solids and is characterized by an extreme
34S enrichment (average δ
34S=39.1‰; δ
18O=18.4‰), suggesting that bacterially mediated SO
4 reduction is a dominant geochemical process in this zone. Two areas of distinct isotopic composition can be identified in the shallow ground water horizons of the upper hydrogeological system. Sulfate in groundwaters adjacent to the river Elbe and Löcknitz has a typical meteoric isotopic signature (δ
34S=5.2‰; δ
18O=8.2‰), whereas the central part of the area is characterized by more elevated isotopic ratios (δ
34S=12.7‰; δ
18O=15.6‰). The two major SO
4 pools in the area are represented by Permian seawater SO
4 and a SO
4 of meteoric origin that has been mixed with SO
4 resulting from the oxidation of pyrite. It is suggested that the S-isotope compositions observed reflect the nature of the SO
4 source that have been modified to various extent by bacterial SO
4 reduction. Groundwaters with transitional salinity have resulted from mixing between brines and low-mineralized waters affected by bacterial SO
4 reduction.
相似文献
15.
To constrain the amount and rate of crustal contamination that is possible in basaltic and jotunitic magma, and to gain an insight into the physical and thermal processes of assimilation in crustal magma chambers, we have modelled published Sr and Nd isotopic data from three layered intrusions. Well-exposed sequences of cumulates with no evidence of magma recharge provide direct records of concurrent assimilation and fractional crystallization (AFC). The key to the modelling is that
F, the mass fraction of magma remaining in the chamber, can be estimated from the thicknesses of the studied cumulate sequences. This allows AFC model curves to be fitted to the isotopic data by varying
r, the ratio of the rate of mass assimilated to the rate of mass crystallized. The results of modelling show that
r is nearly constant in 800 to 2000 m thick sequences of cumulates displaying up-section decreases in anorthite content of plagioclase, increases in whole-rock Sr
0 (initial
87Sr/
86Sr) and decreases in whole-rock
εNd
0 (initial
εNd). The
r-values of the layered sequences range from 0.12 in the Fongen–Hyllingen Intrusion, over 0.20 in the Bjerkreim–Sokndal Intrusion, to 0.27 in the Hasvik Intrusion. The total amount of assimilation, the
bulk crust/
magma ratio, reaches values of 0.08, 0.19 and 0.28 at the level of the most contaminated samples after 60% to 80% crystallisation, whereas the
instantaneous crust/
magma ratio of the most contaminated magmas were respectively 0.14, 0.46, and 0.70, for the three intrusions.Innumerable country rock xenoliths occur in the three layered intrusions and played a crucial role in the assimilation process. The xenoliths spalled off the roofs of the magma chambers during magma emplacement and their initial temperature and composition relate to
r in the intrusions. In the Hasvik Intrusion (
r = 0.27), the initial temperature of the country rocks was 450 °C and the xenoliths were fusible metasediments and therefore produced a high fraction of partial melt that could be assimilated. In the Bjerkreim–Sokndal Intrusion (
r = 0.20), the country rocks were initially at temperatures of 640–880 °C but included both refractory massif-type anorthosite and fusible gneisses. In the Fongen–Hyllingen Intrusion (
r = 0.12), the country rocks were cooler (300 °C) and the xenoliths include refractory metabasalt (dominant) and fusible metapelite. We argue that the refractory metabasalt and anorthosite xenoliths acted mainly as heat sinks, resulting in reduced
r-values in Fongen–Hyllingen and Bjerkreim–Sokndal Intrusions.Heating of refractory and fusible xenoliths, and melting of fusible xenoliths absorbed sensible and latent heat of the magma. Energy-balanced modelling shows that up to 75% of the heat available was absorbed by xenoliths within the magma chambers, promoting higher rates of cooling and crystallisation than would have resulted from loss of heat to the envelope of country rocks alone. The high
r-values reflect the amount of heat absorbed by heating and melting country rock within the magma chambers themselves, and their constancy reflects the ready availability of fusible xenoliths.
相似文献
16.
Carbon isotope composition (
δ13C) in tree-rings has become routinely used in palaeoclimatic research for the assessment of changes in plant water availability in seasonally dry climates. However, the distribution of long tree-ring records around the world is very limited. Alternatively, the original climate signal of wood
δ13C is well preserved in fossil charcoal and, accordingly, charcoal
δ13C can be used to quantify past changes in water availability (e.g. precipitation). We report a case study on spatial palaeoclimate reconstruction which aims to characterize the transition between Bronze and Iron Ages, the so-called Iron Age Cold Epoch (ca. 900–300 BCE), using charcoals of
Quercus ilex/coccifera from a set of 11 contemporary archaeological sites of eastern Spain. Climatic inferences were obtained after calibrating a linear model predicting seasonal precipitation from
δ13C of
Q. ilex wood samples obtained across a rainfall gradient. The best regression model corresponded to September–December (autumn) precipitation (
Paut), in agreement with the fact that
Q. ilex is able to exploit previous-year water reserves thanks to very effective water uptake. Subsequently, we estimated
Paut from the
δ13C of fossil charcoal to infer spatial patterns in water availability. Overall, estimated past
Paut was about 19% higher (296 mm) than present-time values averaged across archaeological sites (249 mm). However, a clear geographic pattern of differences in precipitation could be observed in which the inner continental regions of eastern Spain were characterized by more humid conditions in the past, whereas the coastal strip of the Mediterranean Sea barely differed in past and present
Paut values. The quite uniform distribution of archaeological sites over eastern Spain allowed development of contour maps of absolute and relative (to present) past
Paut using gridded interpolation methods implemented in a GIS, highlighting the potential of this approach for reconstructing high-resolution spatial patterns of past climate.
相似文献
17.
Ordos Basin, the second largest sedimentary basin in China, contains enormous natural gas resources. Each of the four giant gas fields discovered so far in this basin (i.e., Sulige, Yulin, Wushenqi and Jingbian) has over 100 billion cubic meters (bcm) or 3.53 trillion cubic feet (tcf) of proven gas reserves. This study examines the stable carbon isotope data of 125 gas samples collected from the four giant gas fields in the Ordos Basin. Source rocks in the Upper Paleozoic coal measures are suggested by the generally high
δ13C values of C
1–C
4 gaseous hydrocarbons in the gases from the Sulige, Yulin and Wushenqi gas fields. While the
δ13C
iC4 value is higher than that of the
δ13C
nC4, the dominant ranges for the
δ13C
1,
δ13C
2, and
δ13C
3 values in these Upper Paleozoic reservoired gases are −34 to −32‰, −27 to −23‰, and −25 to −24‰, respectively. The
δ13C values of methane, benzene and toluene in gases from the Lower Paleozoic reservoirs of the Jingbian field indicate a significant contribution from humic source rocks, as they are similar to those in the Upper Paleozoic reservoirs of the Sulige, Yulin and Wushenqi gas fields. However, the wide variation and reversal in the
δ13C
1,
δ13C
2 and
δ13C
3 values in the Jinbian gases cannot be explained using a single source scenario, thus the gases were likely derived dominantly from the Carboniferous-Permian coal measures with some contribution from the carbonates in the Lower Permian Taiyuan Formation. The gas isotope data and extremely low total organic carbon contents (<0.2% TOC) suggest that the Ordovician Majiagou Formation carbonates are unlikely to be a significant gas source rock, thus almost all of the economic gas accumulations in the Ordos Basin were derived from Upper Paleozoic source rocks.
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18.
The densification curves for the hot-pressing of pure olivine powders were obtained as a function of grain size (5 μ–2000 μ), temperature (1000–1600°C), and compacting stress (166–298 bars). This range of variables was found to straddle two fields of hot-pressing behavior, one dominated by power-law creep, one by Coble creep. The time required to density a powder to 99% of the single crystal density could be represented by the shorter of the two times:
t1 = 2.2 · 103σ−3.4exp
(85,000/RT)t2 = 1.3 · 104σ−1.5(G)+3exp
(85,000/RT) where the compacting stress or pressure, σ, is given in bars and the grain size,
G, in centimeters. It was also possible to estimate the parameters appropriate to Coble creep in a solid polycrystalline aggregate from the hot-pressing data; and these were:
The strain rates computed from this formula are close to those predicted by Stocker and Ashby (1973) and those found by Twiss (1976).
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19.
The thermodynamic stability of selected alkylated, dealkylated and rearranged 17α- and 17β-hopane isomers in the C
27, C
28, C
29, C
30 and C
31 families were calculated using molecular mechanics (MM2) methods and, where possible, calculated equilibrium ratios of certain isomers were compared with observed ratios of isomers in thermally mature crude oil samples. Those calculated and observed ratios having similar values include: (1) the relative distributions among 17β(H)/17α(H) and 21β(H)/21α(H)-hopanes including the absence of the 17β(H),21β(H)- and 17α(H),21α(H)-hopanes; (2) the 22
R/22
S ratios in 30-methyl-17α-hopane and 30-methyl-17β-moretane; (3) the relative distributions among 17α(H)/17β(H)- and 21α(H)/21β(H)-28,30-bisnorhopanes and among 25,28,30-trisnorhopanes, including the relatively greater stability of 17β(H) isomers in contrast to the regular hopane series; and (4) the ratios of 28(18−17
S)
abeo hopanes with respect to their unrearranged counterparts including the C
27 compounds, Ts/Tm.
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20.
From alternating-field and thermal demagnetization studies on two dolerite “Traps” in the Gwalior Series (Central India), dated at 1830 ±200 m.y., three different palaeomagnetic directions could be distinguished. The characteristic magnetization component, which is considered as the primary magnetization, has a mean direction:
D=78°,
I=+34.5°,
α95=5°,
k=369,
N=4 (Pole): 155.5°E19°N,
dp=3°,
dm=5.5°.A comparison of the presented data with other Precambrian and Phanerozoic data from the Indian subcontinent might suggest that the Indian subcontinent underwent a continuous anticlockwise rotational movement during the last 1800 m.y.
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