Comparative studies of the kinetic parameters of various algaenans and kerogens via open-system pyrolyses

Kinetic parameters were determined for the first time, via open-system pyrolyses, on algaenans (highly resistant biomacromolecules that are selectively preserved during kerogen formation) isolated from extant microalgae. Parallel studies were also carried out on 10 kerogens exhibiting, with one exception, a low level of maturity. These kerogens included samples chiefly derived from the selective preservation of the above algaenans and samples mainly, or almost exclusively, derived from the “natural vulcanization” pathway. Important differences in activation energy (E_a) distributions were observed between the four algaenans investigated and correlated with their chemical structures. The kerogens predominantly derived from algaenan-selective preservation (Pula alginite, NE 70 and BJ 248 Torbanites, Rundle Oil Shale) also exhibited pronounced differences in E_a distributions. These distributions provided: (i) information on the diversity of the source materials; and (ii) reflected the occurrence of important differences in chemical structures and thermal behaviour between three of the tested kerogens, even though they are all classified as low maturity type I. The Kimmeridge Clay samples and the Lorca Oil Shale showed broad E_a distributions shifted to low energies when compared with the above algaenans and kerogens. Such shifts reflect an important (or even almost exclusive for some of these kerogens) contribution of materials originating from sulphur incorporation into various lipids during early diagenesis. Finally, the kinetic data derived for the nine low maturity fossil samples were extrapolated to a very low, geological heating rate of 3°C Ma⁻¹ and the generation rate curves and cumulative yield curves thus obtained were compared.     

U—Series Dating of Fossil Bones：Results from Chinese Sites and Discussions on Its Reliability

Calculations,according to some open-system models,point out that while a statistically significant discrepancy between the results of two U-series methods,^230Th/^234U and ^227Th/^220Th(or ^231Pa/^235U),attests a relatively recent and important uranium migration,concordant dates cannot guarantee closes-system behavior of sample.The results of 20 fossil bones from 10 Chinese sites,19 of which are determined by two U-series methods,are given,Judging from independent age controls,8 out of the 11 concordant age sets are unacceptable,The results in this paper suggest that uranium may cycle into or out of fossil bones,such geochemical events may take place at any time and no known preserving condition may securely protect them from being affected.So for the sitew we have studied,the U-series dating of fossil bones is of limited reliability.     

Diffusive dehydration and bubble resorption during open-system degassing of rhyolitic melts

Degassing through open paths such as bubble and/or fracture networks is considered to be the principal mode of degassing in silicic magmas. However, its detailed mechanisms remain unclear. To investigate the behavior of bubbles in a hypothetical open-system condition, we performed a series of vesiculation experiments on natural rhyolitic obsidian using a newly designed semipermeable cell, which artificially maintains a pressure difference between its inside and outside. The thick-wall cell maintains a constant volume within the sample chamber, while allowing water vapor to escape the cell during the experimental runs. The cells containing obsidian cores with ca. 0.66 wt.% initial water content were externally heated to 1000 °C for a period of 1–288 h. The run charges generally showed a zonal structure composed of two contrasting regions: a central region within which the bubbles were concentrated (bubble-rich core, BC) and a bubble-free melt region surrounding the BC (bubble-free margin, BFM). With increased heating duration, the thickness of the BFM increased via dissolution of the outermost bubbles in the BC. The water content was nearly uniform throughout the BC, whereas an outward-decreasing gradient was observed in the BFM. We found that diffusive dehydration occurred from the sample surface, and the bubbles were resorbed into the melt. Thus, the BFM–BC boundary moved inwards. These processes were modelled numerically, and the calculation results were in good agreement with the experimental data. If a “lifetime” of open paths is approximated as the relaxation time of a melt in a shallow volcanic environment, then the paths have to be pinched off quickly (1.2 h at maximum pressure difference between the open path and the melt) and thus the thickness of the bubble-free layer reaches at most ~ 0.1 mm. For the formation of bubble-free obsidian layers with a width of a few millimeters, which are often observed in natural obsidian flows, open paths should be maintained for at least a few hundred hours.     

密闭体系与开放体系模拟实验结果的比较研究及其意义

利用广泛应用的高压釜和Rock Eval热解实验技术，对松辽盆地3个有机质样品同时进行了密闭条件下的加水恒温热解实验和开放条件下的恒速升温热解实验。利用后一实验数据所标定得到的化学动力学模型计算了与高压釜相同实验条件下的有机质产油、产气率。结果表明，在线计量的恒速升温Rock Eval实验方法由于不损失C6—C13的轻质烃组分，在计量液态油的产量方面较需要抽提、恒重的高压釜实验方法更为准确。因此为解决热模拟实验中难以计量、但对成烃评价有重要意义的C6—C13组分的计量问题提供了一条有效的途经。这可能也表明，先由实验数据建立有关的化学动力学模型，之后由它来进一步计算有机质的成烃率，不仅是可行的，而且应该更为准确。     

The Role of Open-System Processes in the Development of Silicic Magma Chambers: a Chemical and Isotopic Investigation of the Fogo A Trachyte Deposit, Sao Miguel, Azores

The processes operating in the development of chemical zonationin silicic magma chambers have been addressed with a Sr–Nd–Pb–Hf–Thisotope study of the chemically zoned trachyte pumice depositof the Fogo A eruption, Fogo volcano, Azores. Sr isotopic variationis observed in whole rocks, glass separates and sanidine phenocrysts(whole-rock ⁸⁷Sr/⁸⁶Sr: 0·7049–0·7061; glass⁸⁷Sr/⁸⁶Sr: 0·7048–0·7052; sanidine ⁸⁷Sr/⁸⁶Sr:0·7048–0·7062). Thorium isotopic variationis observed in glass separates, with (²³⁰Th/²³²Th)_o rangingfrom 0·8737 to 0·8841, and exhibiting a negativecorrelation with Sr isotopes. The Nd, Pb and Hf isotopic compositionsof the whole-rock trachytic pumices are invariant and indistinguishablefrom basalts flanking the volcano. The Sr isotope variationsin the whole rocks are proposed to be the result of three distinctprocesses: contamination of the Fogo A magma by assimilationof radiogenic seawater-altered syenite wall rock, to explainthe Sr and Th isotopic compositions of the glass separates;incorporation of xenocrysts into the trachytic magma, requiredto explain the range in feldspar Sr isotopic compositions; andpost-eruptive surface alteration. This study emphasizes theimportance of determining the isotopic composition of glassand mineral separates rather than whole rocks when pre-eruptivemagmatic processes are being investigated. KEY WORDS: Azores; open-system processes; Sr isotopes; trachytic pumices; zoned magma chambers     

开放系统下冻胀引起地表隆起变形预测的解析方法

开放系统下土体冻胀引起土工结构变形致使其服役特性面临严峻挑战。基于土体冻胀非线性弹性力学模型, 推导了计算半空间无限体中土单元在自重下冻胀的数学表达式, 建立了水平场地土冻胀模拟方法, 提出了考虑上部压载效应与位移边界变化的场地表面隆起位移及土体应力与应变的计算方法。在此基础上, 利用开放系统下粉质黏土逐级降温冻胀室内试验, 验证了冻胀引起地表隆起变形预测解析方法的可靠性。经验证, 该变形预测解析方法可应用于寒区场地冻胀量的评估, 进而为开放系统下土工结构物变形预测提供科学参考。     

A study of the kinetic parameters of individual macerals from Upper Permian coals in South China via open-system pyrolysis

A unique Upper Permian coal, Leping coal, is widely distributed in South China. The coal samples studied in the paper were collected from two mines in the Shuicheng coalfield of Guizhou Province, southwest China. The geochemical works including coal petrography, maceral content, Rock–Eval pyrolysis, and kinetic modelling of hydrocarbon-generating have been carried out on whole coal and individual macerals. The higher contents of volatile matter, elemental hydrogen, and tar yield, and the high hydrocarbon generation potential of the Leping coals are attributed to their high content of “barkinite”, a special liptinite maceral.The hydrocarbon generation potential of “barkinite” (S₂=287 mg/g, hydrogen index (HI)=491 mg/g TOC) is greater than that of vitrinite (S₂=180 mg/g, HI=249 mg/g TOC), and much higher than that of fusinite (S₂=24 mg/g, HI=35 mg/g TOC). At the same experimental conditions, “barkinite” has a higher threshold and a narrower “oil window” than those of vitrinite and fusinite, and consequently, can generate more hydrocarbons in higher coalification temperature and shorter geological duration. Data from the activation energy distributions indicate that “barkinite” has a more homogenous chemical structure than that of vitrinite and fusinite. The above-mentioned characteristics are extremely important for exploring hydrocarbon derived from the Leping coals in South China.     

Petrology and Geochemistry of Early Cretaceous Bimodal Continental Flood Volcanism of the NW Etendeka, Namibia. Part 2: Characteristics and Petrogenesis of the High-Ti Latite and High-Ti and Low-Ti Voluminous Quartz Latite Eruptives

As a result of their relative concentration towards the respectiveAtlantic margins, the silicic eruptives of the Paraná(Brazil)–Etendeka large igneous province are disproportionatelyabundant in the Etendeka of Namibia. The NW Etendeka silicicunits, dated at     

Open-system processes in the genesis of silica-oversaturated alkaline rocks of the Rallier-du-Baty Peninsula, Kerguelen Archipelago (Indian Ocean)

The Rallier-du-Baty Peninsula forms the southwestern part of the Kerguelen Archipelago (Indian Ocean), whose magmatic activity is related to the long-lived 115-Ma Kerguelen plume. The peninsula is mostly made of alkaline rocks constituting two well-defined ring complexes. This paper focuses on the northern ring complex, which is not yet known. Recent field studies have revealed seven discrete syenitic ring dykes ranging in age from 6.2 to 4.9 Ma, and two later volcanic systems. ⁴⁰Ar/³⁹Ar dating of a trachytic ignimbrite linked to the Dôme Carva volcano complex yields an age of 26±3 Ka. This represents the last major eruptive event on the Kerguelen Archipelago. The volcanism is bimodal with trachybasalts and trachyandesites constituting the mafic lavas and trachytes and rhyolites constituting the felsic lavas. The volume of erupted felsic magma is by far the larger, and is represented by abundant pyroclastic deposits and lava flows. Boulders of plutonic rocks are found to the northwest of Dôme Carva, and represent intermediate rocks (i.e. monzogabbros and monzonites) that are not present at the surface. Basic rocks are mostly trachybasalts and trachyandesites, while true basalts are scarce. Their mineralogy consists chiefly of plagioclase, olivine, diopside and oxides. Sieve-textured plagioclase is common, as well as corroded olivine and diopside phenocrysts. Peralkaline commenditic trachytes are the most abundant type of acid volcanic rocks. They consist of abundant sanidine, augite and magnetite phenocrysts and interstitial quartz, aegerinic pyroxenes and Na-amphiboles. Ring dykes of quartz-poor alkali feldspar syenites display the same mineralogy, except hornblende is common and replaces diopside. Hornblende is particularly abundant in intermediate monzogabbros. Major and trace element variations of volcanic rocks emphasise the predominant role of fractional crystallisation with a general decrease of MgO, CaO, P₂O₅, TiO₂, FeO, Ba, Sr and Ni from basic to felsic rocks. However, the scattering of the data from the basic rocks indicates that other processes have operated. The overall evolution from trachyte to rhyolite is in agreement with the fractionation of sanidine as the major control. An increase of incompatible elements from trachyte to rhyolite is observed. The felsic lavas display an increase of ⁸⁷Sr/⁸⁶Sr_(i) without any significant variations in the Nd isotopic composition. The genesis of the basic rocks is complex and reflects concomitant processes of fractional crystallisation, mixing between different basic magmas and probable assimilation of Ba-rich oceanic crust. Major and trace element modelling confirms the possibility of producing the trachytes through continuous differentiation from a basaltic alkaline parent. Discrepancies observed for some trace elements can be explained by the crystallisation of amphibole at an intermediate stage of magma evolution. The overall evolution from trachyte to rhyolite is thought to be controlled by crystal fractionation. High ⁸⁷Sr/⁸⁶Sr_(i) of the trachytes is interpreted to reflect interaction with an ocean-derived component, probably during assimilation of hydrothermally altered oceanic crust. Boulders of amphibole-bearing monzonites and monzogabbros found to the northwest of Dôme Carva are thought to represent intermediate magma composition that formed at depths but did not erupt.     

开放体系下有机质与有机碳、氢、氮损失动力学研究

本文对Ⅰ、Ⅱ、Ⅲ型干酪根的有机质、有机碳、氢、氮的损失进行了动力学研究,获得了Ⅰ、Ⅱ、Ⅲ型干酪根的总量、有机碳、氢以及氮质量损失动力学参数。此动力学参数可用于烃源灶的有机质总量,有机碳、氢、氮质量的恢复研究。在此基础上,结合专用Kinetics 软件,将此动力学参数应用于干酪根有机碳丰度、氢／碳和氮／碳原子比恢复系数计算。结果表明,在Ro介于0.47%～3.05%之间Ⅰ型干酪根有机碳丰度,碳、氢、氮总丰度,氢／碳和氮／碳原子比的恢复系数依次为1～1.89、1～1.96、1～2.13和1～0.56;Ⅱ型干酪根依次为0.98～1.05、1～1.06、1～1.89和1～0.66。Ⅲ型干酪根依次为1～0.82、1～0.83、1～1.59和1～0.56。因此在对高成熟—过成熟干酪根进行生烃评价时,Ⅰ、Ⅲ型干酪根残余有机碳丰度需要进行恢复,而Ⅱ型干酪根残余有机碳丰度不需要进行恢复。3种类型干酪根的H/C原子比均需要进行恢复。本文建立的Ⅰ、Ⅱ、Ⅲ型干酪根恢复动力学可用于计算开放体系热演化过程中残留干酪根或有机碳总量,在一定程度上弥补了以往着重考虑生出烃类演化的不足之处。