大别-苏鲁造山带大理岩碳氧同位素地球化学研究

大别-苏鲁造山带的大理岩分布非常广泛,在三叠纪的陆-陆碰撞中,经受了不同程度的变质作用。本文对分布在大别地区北淮阳、苏家河、桐柏、宿松和苏鲁地区张八岭、五莲和坪上等地区的浅变质大理岩进行了碳氧同位素分析。结果发现,除了张八岭大理岩δ~(13)C值出现明显正异常(1‰～8‰)外,其他地区大理岩的δ~(13)C值均处于0±2‰(PDB)范围之内。所有地区大理岩的δ~(18)O值都有不同程度的降低,最低的δ~(18)O值达到4.5‰(SMOW)。与前人对大别-苏鲁超高压大理岩碳氧同位素研究结果相对比,我们发现大理岩δ~(13)C值分布与岩石是否经过超高压没有联系,而主要反映了其原岩沉积的时代和环境,并且其特征可以与发生在新元古代的冰川事件相关联。区域性的δ~(18)O值降低则说明,大部分岩石都经过了流体交换,并且流体的主要成分是水,含碳很少或者不舍碳,因此流体的来源是大气降水,可能与新元古代冰川溶融有关。     

Boron isotope geochemistry of Paleozoic brachiopod calcite: Implications for a secular change in the boron isotope geochemistry of seawater over the Phanerozoic

Boron isotope composition of marine carbonates has been proposed as a paleo-pH proxy and potential tool to reconstruct atmospheric pCO₂. The precise knowledge of the boron isotopic composition of ancient seawater represents the fundamental prerequisite for any paleo-pH reconstruction. This contribution presents boron isotope values for Silurian to Permian brachiopod calcite that might be used to reconstruct pH or boron isotope composition of past oceans. All brachiopod shells were screened for diagenetic recrystallization by means of cathodoluminescence microscopy, trace element geochemistry (B, Fe, Mn, Sr) as well as SEM. Only nonluminescent shells revealing well-preserved microstructures, high strontium and boron concentrations as well as low iron and manganese contents were accepted for boron isotope analysis. The boron isotope ratios of Silurian, Devonian, Pennsylvanian and Permian brachiopod calcite range from 6.8 to 11.0‰, 7.3 to 14.9‰, 12.4 to 15.8‰ and 10.1 to 11.7‰, respectively. These δ¹¹B values are significantly lower in comparison to δ¹¹B values of modern biogenic carbonates and indicate that the Paleozoic oceans were depleted in ¹¹B by up to 10‰. Box modeling of the boron geochemical cycle suggests that the significant depletion of ¹¹B in the oceanic reservoir may have been initiated by an enhanced continental boron discharge. Our data support the earlier made conclusion that boron isotopes may not be used in the geological past as reliable paleo-pH proxy unless the boron isotopic composition of ancient oceans can be constrained by further studies.     

Carbon,oxygen and strontium isotope geochemistry of Proterozoic carbonate rocks of the Vindhyan Basin,central India

Carbon, oxygen and strontium isotope compositions of carbonate rocks of the Proterozoic Vindhyan Supergroup, central India suggest that they can be correlated with the isotope evolution curves of marine carbonates during the latter Proterozoic. The carbonate rocks of the Lower Vindhyan Supergroup from eastern Son Valley and central Vindhyan sections show δ¹³C values of ∼0‰ (V-PDB) and those from Rajasthan section are enriched up to +2.8‰. In contrast, the carbonate rocks of the Upper Vindhyan succession record both positive and negative shifts in δ¹³C compositions. In the central Vindhyan section, the carbonates exhibit positive δ¹³C values up to +5.7‰ and those from Rajasthan show negative values down to –5.2‰. The δ¹⁸O values of most of the carbonate rocks from the Vindhyan Supergroup show a narrow range between –10 and –5‰ (V-PDB) and are similar to the ‘best preserved’ ¹⁸O compositions of the Proterozoic carbonate rocks. In the central Vindhyan and eastern Son Valley sections, carbonates from the Lower Vindhyan exhibit best-preserved ⁸⁷Sr/⁸⁶Sr compositions of 0.7059±6, which are lower compared to those from Rajasthan (0.7068±4). The carbonates with positive δ¹³C values from Upper Vindhyan are characterized by lower ⁸⁷Sr/⁸⁶Sr values (0.7068±2) than those with negative δ¹³C values (0.7082±6). A comparison of C and Sr isotope data of carbonate rocks of the Vindhyan Supergroup with isotope evolution curves of the latter Proterozoic along with available geochronological data suggest that the Lower Vindhyan sediments were deposited during the Mesoproterozoic Eon and those from the Upper Vindhyan represent a Neoproterozoic interval of deposition.     

白云鄂博碳酸岩墙碳氧同位素地球化学

对内蒙古白云鄂博 REE- Fe- Nb矿床周围碳酸岩墙中共存的方解石和白云石进行了 C和 O同位素分析。结果表明，方解石和白云石的δ 13C值变化范围一致，均为－ 3.5‰～－ 7.3‰，落在正常地幔δ 13C值范围 (－ 5‰± 2‰ )内；而它们的δ 18O值可分为两组，第Ⅰ 组为 9.5‰～ 18.0‰，第Ⅱ 组为 20.6‰～ 22.6‰，均远大于正常地幔δ 18O值范围 (5.7‰± 1.0‰ )。第Ⅰ 组低δ 18O值样品中共存白云石与方解石之间的 C和 O同位素分馏均为负值，因此处于热力学不平衡状态，指示它们自形成后受到过后期热液蚀变，与先前的岩石学观察一致。相反，第Ⅱ 组高δ 18O值样品中白云石与方解石之间的 C和 O同位素分馏均为正值，处于热力学平衡状态，指示它们自形成后未受到后期热液蚀变，因此可能沉淀于晚期低温高δ 18O值流体。第Ⅰ 组碳酸岩墙中白云石的 C和 O同位素组成不呈线性分布，指示碳酸岩浆并非由幔源碳酸盐与沉积碳酸盐混合形成。应用水-岩交换模型计算得到，第Ⅰ 组碳酸岩在侵位后经历了碳酸岩浆期后热液的不均一蚀变，蚀变温度约在 220～ 800℃之间，蚀变流体的 CO2/H2O比值较小 (1/500)，但水 /岩比值变化较大 (10～ 400)。由于低温下方解石与热液之间的碳氧同位素交换速率大于白云石，导致这部分碳酸     

Carbon isotope geochemistry of the Precambrian Lomagundi carbonate province,Rhodesia

Carbon isotope measurements carried out on 67 dolomite samples from the Middle Precambrian Lomagundi Group (Rhodesia) have yielded a δ¹³C mean of +8.2 ± 2.6%. vs PDB. With the outcrop of these dolomites extending over a distance of almost 300 km, the Lomagundi dolomite faces is likely to represent the largest isotopically anomalous sedimentary carbonate province ever recorded. It is concluded that the anomalous carbonates formed in a closed basin whose δ¹³C level had been substantially increased as a result of a preferential removal (within sedimentary organics) of the light carbon isotope.     

湖北徐家山锑矿床方解石C、O、Sr同位素地球化学

湖北通山的徐家山锑矿床产在上震旦统陡山沱组和灯影组的海相碳酸盐岩中.对该矿床中成矿前和成矿期方解石进行了较系统的C、O、Sr同位素地球化学研究,结果表明,成矿前方解石具相对较高的δ13CPDB(-0.7‰～ 2.0‰)、δ18OSMOW( 18.6‰～ 19.6‰)和Sr含量(2645-8174 μg/g,平均5656μg/g),及较低的87Sr/86Sr比值(0.7096～0.7097);而成矿期方解石具相对较低的δ13CPDB(-3.9‰～-2.1‰)、δ18OSMOW( 11.5‰～ 15.3‰)、Sr含量(785～2563μg/g,平均1571μg/g),和较高的87Sr/86Sr比值(O.7109～0.7154,平均0.7141).分析认为,成矿前方解石的C、O和绝大部分Sr来源于赋矿围岩--震旦系海相碳酸盐岩;而成矿流体为富H2CO3的溶液,它来自或流经富放射成因87Sr的下伏基底碎屑岩--中元古界冷家溪群浅变质岩;该流体与围岩发生水.岩反应导致成矿期方解石和辉锑矿的沉淀.     

湖南香花岭锡多金属矿床C、O、Sr同位素地球化学

对湖南香花岭锡多金属矿床成矿早、晚两期方解石进行了系统的C、O、Sr同位素研究。结果表明,成矿早期方解石具有相对较低的δ13CPDB（-5.4‰ ~ -1.4‰）、δ18OSMOW（+6.1‰ ~ +13.9‰）和较高的87Sr/86Sr值（0.7101 ~ 0.7230）;而成矿晚期方解石则具有相对较高的δ13CPDB（+0.2‰ ~ +0.6‰）、δ18OSMOW（+19.4‰ ~ +21.5‰）和较低的87Sr/86Sr值（0.7101）。成矿早期方解石为岩浆热液与海相碳酸盐岩相互作用的产物,成矿流体中的碳源于岩浆碳和海相碳酸盐岩;模拟计算结果显示,成矿早期方解石为成矿溶液发生0.05-0.1摩尔分数的 CO2去气作用的产物。而成矿晚期方解石主要为矿区碳酸盐岩围岩低温淋滤的产物,流体中的碳主要源于矿区碳酸盐岩围岩。成矿流体中Sr源于矿区花岗岩和碳酸盐岩,从成矿早期到晚期,岩浆来源的Sr逐渐减少,而碳酸盐岩围岩提供的Sr比例不断增大。     

Dynamics of the Galapagos hotspot from helium isotope geochemistry

We have measured the isotopes of He, Sr, Nd and Pb in a number of lava flows from the Galapagos Archipelago; the main goal is to use magmatic helium as a tracer of plume influence in the western volcanoes. Because the Galapagos lava flows are so well preserved, it is also possible to measure surface exposure ages using in situ cosmic-ray-produced ³He. The exposure ages range from <0.1 to 580 Ka, are consistent with other chronological constraints, and provide the first direct dating of these lava flows. The new age data demonstrate the utility of the technique in this important age range and show that the western Galapagos volcanoes have been erupting distinct compositions simultaneously for the last ∼10 Ka. The magmatic ³He/⁴He ratios range from 6.9 to 27 times atmospheric (R_a), with the highest values found on the islands of Isabela (16.8 R_a for Vulcan Sierra Negra) and Fernandina (23 to 27 R_a). Values from Santa Cruz are close to typical mid-ocean ridge basalt values (MORB, of ∼9 R_a) and Pinta has a ³He/⁴He ratio lower than MORB (6.9 R_a). These data confirm that the plume is centered beneath Fernandina which is the most active volcano in the archipelago and is at the leading edge of plate motion. The data are consistent with previous isotopic studies, confirming extensive contributions from depleted asthenospheric or lithospheric mantle sources, especially to the central islands. The most striking aspect of the helium isotopic data is that the ³He/⁴He ratios decrease systematically in all directions from Fernandina. This spatial variability is assumed to reflect the contribution of the purest plume component to Fernandina magmatism, and shows that helium is a sensitive indicator of plume influence. The highest ³He/⁴He ratios are found in volcanoes with lowest Na₂O(8) and FeO(8), which may relate to source composition as well as degree and depth of melting. An excellent correlation is observed between ³He/⁴He and Nb/La, suggesting that the Galapagos plume source is characterized by high concentrations of Nb (and Ta). The major and trace element correlations demonstrate that helium is controlled by silicate melting and source variations rather than degassing and/or metasomatic processes. Although lavas with radiogenic isotopic compositions tend to have higher ³He/⁴He, the island-wide isotopic variability cannot be explained by simple two components mixing alone. The preferred model to explain the isotopic data includes a heterogeneous plume, centered at Fernandina, which undergoes polybaric melting, and spatial divergence and mixing with asthenospheric material at shallower depths. The unique regional pattern of the helium isotopic data suggests that helium is extracted more efficiently than other elements during the early stages of melting in the ascending plume.     

Biomarkers and compound-specific stable carbon isotope of n-alkanes in crude oils from Eastern Llanos Basin,Colombia

Representative samples of crude oils from Cusiana, Cupiagua, Apiay, Castilla and Chichimene fields in the Eastern Llanos Basin of Colombia were analyzed to determine its compound-specific stable carbon isotope composition (CSIA) using gas chromatography–isotopic ratio–mass spectrometry (GC–IRMS). GC–IRMS analyses of n-alkanes allowed differentiating between Cretaceous and Cretaceous/Tertiary oil samples. Cretaceous sourced samples have δ¹³C-enriched values than Cretaceous/Tertiary sourced samples; the heavier isotope composition of these samples is due to their major terrigenous organic matter input. Their isotope distribution patterns suggest significant algal and/or bacterial contribution (marine origin). The analysis of the n-alkane fractions by GC–IRMS confirms that the organic matter has marine origin in those samples from Cusiana, Cupiagua and Apiay while Castilla and Chichimene have marine origin with terrestrial inputs. The results were confirmed by gas chromatography/FID and gas chromatography/mass spectrometry (GC/MS). Basic geochemical composition show that samples from Cupiagua/Cusiana fields and Apiay/Castilla/Chichimene fields in the Llanos basin, Colombia present different characteristics reflecting a specific for each depositional environment.     

Sulphur isotope geochemistry of carbonatites

Sulphur isotopic data for sulphides and barite from several carbonatites (Mountain Pass, Oka, Magnet Cove, Bearpaw Mountains, Phalabora) show that individual carbonatites have different mean sulphide or barite isotopic compositions which deviate from the meteoritic mean $\text{δ}{}^{34}\text{S(0‰)}$.Classification of carbonatites in terms of T,?O₂ and pH during formation of the sulphur-bearing assemblages indicates that with decreasing T and increasing relative ?O₂ the mean δ³⁴S sulphide becomes increasing negative relative to the mean magma δ³⁴S. Only barite-free high temperature carbonatites (Phalabora) in which the mean δ^34S sulphide approaches the mean magmaδ³⁴S as a consequence of the paucity of oxidized anionic sulphur species in the magma can be used to directly estimate the mean isotopic composition of the source material.Barites from the Mountain Pass carbonatite show an increase in δ³⁴S with sequence of intrusion of the carbonatite units; dolomitic carbonatite (mean δ³⁴S, + 5.4‰), calcitic carbonatite (+ 4.8%.), silicified carbonatite (+ 6.9‰), tabular carbonatite dikes (+ 8.7‰), mineralized shear zones (+ 9.5‰). Within each of these units a spread of 6.8%. is evident. Isotopic trends in this low temperature (300°C) carbonatite are evaluated by treating the system as a hydrothermal fluid. The observed isotopic variations can be explained by removal of large amounts of sulphur from a fluid whose mean δ³⁴S is 0 to + 1‰     

煤成甲烷、乙烷碳同位素动力学研究和应用--以鄂尔多斯盆地上古生界煤成气为例

同位素动力学模型的发展和应用使天然气的研究工作进入了一个动态的研究阶段。迄今为止,大多数研究工作集中在甲烷的碳同位素动力学模拟。而少有地球化学家对乙烷及其他重烃气组分的碳同位素组成的动力学模拟加以重视。本文在甲烷碳同位素动力学模型基础上,编制了限定体系下乙烷的同位素动力学模拟软件。并在热解模拟实验的基础上对鄂尔多斯盆地上古生界煤成气甲烷、乙烷的同位素演化进行了模拟,获得长期累积甲烷、乙烷的同位素值分别为δ^1C1=-33．46‰3、δ^13C2=-23．1‰,与上古生界储层中天然气的组分特征（δ^13C1=-32．95％。、δ^13C2=-23．41‰）基本一致,与下古生界天然气的同位素组成存在着明显的差别,说明上古生界煤与上古生界天然气存在着源—气的对应关系;而对下古生界天然气来说,上古生界煤系不是主要的气源岩。     

Controls on ostracod valve geochemistry: Part 2. Carbon and oxygen isotope compositions

The stable carbon and oxygen isotope compositions of fossil ostracods are powerful tools to estimate past environmental and climatic conditions. The basis for such interpretations is that the calcite of the valves reflects the isotopic composition of water and its temperature of formation. However, calcite of ostracods is known not to form in isotopic equilibrium with water and different species may have different offsets from inorganic precipitates of calcite formed under the same conditions. To estimate the fractionation during ostracod valve calcification, the oxygen and carbon isotope compositions of 15 species living in Lake Geneva were related to their autoecology and the environmental parameters measured during their growth. The results indicate that: (1) Oxygen isotope fractionation is similar for all species of Candoninae with an enrichment in ¹⁸O of more than 3‰ relative to equilibrium values for inorganic calcite. Oxygen isotope fractionation for Cytheroidea is less discriminative relative to the heavy oxygen, with enrichments in ¹⁸O for these species of 1.7 to 2.3‰. Oxygen isotope fractionations for Cyprididae are in-between those of Candoninae and Cytheroidea. The difference in oxygen isotope fractionation between ostracods and inorganic calcite has been interpreted as resulting from a vital effect. (2) Comparison with previous work suggests that oxygen isotope fractionation may depend on the total and relative ion content of water. (3) Carbon isotope compositions of ostracod valves are generally in equilibrium with DIC. The specimens’ δ¹³C values are mainly controlled by seasonal variations in δ¹³C_DIC of bottom water or variation thereof in sediment pore water. (4) Incomplete valve calcification has an effect on carbon and oxygen isotope compositions of ostracod valves. Preferential incorporation of at the beginning of valve calcification may explain this effect. (5) Results presented here as well as results from synthetic carbonate growth indicate that different growth rates or low pH within the calcification site cannot be the cause of oxygen isotope ‘vital effects’ in ostracods. Two mechanisms that might enrich the ¹⁸O of ostracod valves are deprotonation of that may also contribute to valve calcification, and effects comparable to salt effects with high concentrations of Ca and/or Mg within the calcification site that may also cause a higher temperature dependency of oxygen isotope fractionation.     

汞同位素地球化学概述

汞同位素是一个新兴的地球化学示踪手段。过去十多年来,随着质谱技术的飞跃发展,汞同位素地球化学研究取得了引人注目的进展,主要体现在如下两个方面。(1)实验及理论地球化学研究表明,汞生物地球化学循环的一系列过程都能导致显著的汞同位素质量分馏。此外,汞还是自然界少数存在同位素非质量分馏的金属元素之一。汞同位素非质量分馏对识别某些特殊地球化学过程(如光还原作用、挥发作用等)具有重要指示意义。(2)自然样品的汞同位素测试表明,自然界汞同位素组成(δ202 Hg和Δ199 Hg)变化可达10‰。目前,汞同位素地球化学已被应用于汞污染源示踪、汞生物地球化学过程判别等领域,并有望在不久的将来在汞的大气化学、生物地球化学等领域得到更为广泛的应用。     

Sedimentology and isotope geochemistry of lacustrine carbonates of the Oligocene Campins Basin, north-east Spain

The non-marine Campins Basin developed in the Oligocene, during a period of early rifting of the Catalan Coastal Ranges. Lacustrine deposits, interbedded between two alluvial units, comprise shallow and deep lacustrine facies. The lower, shallow lacustrine facies are made up of microbialite buildups and thin limestone beds. In the studied area, these facies are overlain by deep lacustrine facies which consist of alternations of several, metre-thick carbonate- and mudstone-dominated intervals. In addition to calcite, which is characteristic of the shallow lacustrine facies, aragonite and abundant dolomite are present in the deep lacustrine facies. This mineralogical change in the sequence reflects an overall increase in the Mg/Ca ratio of the lake waters. The deep lacustrine sequences are interpreted as having formed in a hydrologically closed basin that was subject to changes in the Mg/Ca ratio of the water, probably related to variations in the evaporation/precipitation rate. The sedimentological, mineralogical and isotopic characteristics of the Campins Basin dolomites suggest that, in general, they are primary in origin. The stable isotope data show an approximate covariance between δ¹³C and δ¹⁸O in the lower shallow lacustrine carbonates (calcite) which suggests that they formed during the onset of closure of the lake. The δ¹³C and δ¹⁸O values of the deep lacustrine carbonates display three different clusters that are roughly related to the carbonate mineralogy. Normalisation with respect to calcite of the isotopic compositions of dolomite and aragonite from the deep lacustrine carbonates allows the integration of all these isotope values into one covariant trend. The sequential appearance of different carbonate minerals and the isotopic covariant trend may indicate an overall evaporative concentration of the lake waters. The change in slope of the covariant trend for the isotope values between the shallow and the deep lacustrine carbonates might reflect the change in the waterbody morphology recorded in the basin fill sequence.     

钒同位素地球化学综述

钒是一个重要的第一行过渡族金属元素,由于其独特的多价态性质,且在地球的各个圈层广泛分布,因此钒元素的丰度变化被广泛用来研究和氧化还原反应相关的地球化学过程。随着基于多接收电感耦合等离子体质谱仪(MC-ICP-MS)的同位素分析技术的发展,钒同位素地球化学的研究也取得初步进展。目前的观测已经初步确定硅酸盐地球的钒同位素组成及其和陨石钒同位素组成的差异,这个差异为了解地球最初的物质来源和形成过程提供了新的制约。对高温岩石样品的测量发现钒同位素在岩浆分离结晶过程中存在显著分馏,因此钒同位素是潜在的研究高温岩浆过程的物理化学条件特别是氧化还原状态的新工具。推测低温下同位素的分馏应更显著,因此钒同位素在环境和生物地球化学领域也有望得到更广泛的应用。     

钛同位素地球化学综述

随着分析技术的进步,非传统稳定同位素体系在地球化学、天体化学和生物地球化学等研究领域的应用日益广泛。钛(Ti)是一个非常重要的过渡族金属元素,在地球和其他类地球行星中广泛存在。但是由于Ti是一种难熔的、流体不活动性元素,高温地质过程中Ti同位素分馏很小。人们对Ti同位素体系的地球化学应用的关注相对其他非传统稳定同位非常有限。而近年来,随着化学纯化方案的优化以及双稀释剂方法的改进和仪器质谱性能的提高,Ti同位素组成的高精度测试已经能够实现。天然样品中Ti同位素组成的变化随之得以发现,使得学者们能够利用这一新的稳定同位素体系来解决与高温和低温地球化学相关的问题。很快Ti同位素体系地球化学研究成为当前国际地质学界的前沿研究课题和新的发展方向之一。本文首先在简要介绍Ti元素和Ti同位素体地球化学性质的基础上,介绍了Ti元素化学分离和Ti同位素分析方法。随后笔者总结了已有的不同类型球粒陨石和地球样品的质量相关Ti同位素组成研究结果,对硅酸盐地球的Ti同位素组成做了初步评估。前人对高温地质样品的Ti同位素组成研究初步探明Ti同位素在岩浆演化过程,例如部分熔融和结晶分异等重要地质过程中的分馏行为。笔者在此基础上探讨了结晶分异过程中引起Ti同位素分馏的主要控制因素,指出Ti同位素是潜在的研究岩浆演化过程的新工具。最后笔者探讨了Ti同位素地球化学未来的发展方向,以加速我国在Ti同位素地球化学方面的应用研究。     

Strontium isotope geochemistry of megacrysts and host basalts from southeastern Australia

Strontium isotopic data for megacrysts and lavas from six eruptive centers within the Newer Basalts province of southeastern Australia show that megacrysts of clinopyroxene are in isotopic equilibrium with associated basalts, but that megacrysts of kaersutite, ferrokaersutite, orthopyroxene and anorthoclase may exhibit slight disequilibrium with their host basalts. Furthermore, the anorthoclase megacrysts may be either more or less radiogenic than their hosts. The ⁸⁷Sr/⁸⁶Sr ratios for 14 basalts from throughout the province vary from 0.7035 to 0.7045 and it is proposed that anorthoclase, amphibole and orthopyroxene megacrysts which crystallized in isotopic equilibrium with one magma may have been caught up in a pulse of a later magma of a different isotopic composition. The variations in the ⁸⁷Sr/⁸⁶Sr ratios for the basalts are attributed to variations in the isotopic composition of their source regions. Such isotopic heterogeneity is supported by published data for ultramafic xenoliths which occur in the Newer Basalts lavas.     

鄂尔多斯盆地下古生界石灰岩岩相及碳、氧稳定同位素特征

鄂尔多斯盆地下古生界石灰岩的岩相及碳、氧稳定同位素组成变化大,具有多种成因类型。通过薄片鉴定、岩石及矿物微区碳、氧稳定同位素测试以及包裹体测温资料研究,发现正常海水成因的泥晶灰岩及颗粒灰岩具有较高的δ18O、δ13C;受淡水淋滤、溶蚀的颗粒灰岩、角砾泥晶灰岩、豹斑含云灰岩或灰云岩的δ13C随淡水淋滤及溶蚀程度升高而明显降低,但其δ18O变化较小;去膏化成因的巨晶次生灰岩在阴极发光下具环带结构,并具有较低的δ18O、δ13C,去云化成因的中-细晶次生灰岩多呈麦粒或蚕蛹结构,其δ18O、δ13C相对较高;早期及表生淡水方解石胶结物的δ18O、δ13C均为低值,Mn离子含量低,阴极发光暗淡;晚期热液方解石胶结物具异常低的δ18O、δ13C,包裹体均一温度介于100~150℃,富含Mn离子,阴极发光明亮。研究结果表明,岩相特征、碳、氧稳定同位素组成以及包裹体测温数据是确定石灰岩沉积、成岩环境以及成因类型的重要指标。     

Organic geochemistry of the oils from the southern geological Province of Cuba

The aliphatic hydrocarbon composition (acyclic isoprenoids, hopanoids and steroids) of oils from the most productive fields in the southern geological Province of Cuba have been studied. This province is defined by its position with respect to the Cretaceous overthrust belt generated during the formation of oceanic crust along the axis of the proto-Caribbean Basin. The relative abundances of 18α(H)-22,29,30-trisnorneohopane, gammacerane and diasteranes suggest that Pina oils are related to the carbonate oils from the Placetas Unit in the northern province (low T_s/(T_s+T_m) and C_27,29 rr/(rr+sd) ratios). The Cristales and Jatibonico oils exhibit some differentiating features such as higher T_s/(T_s+T_m) and absence of gammacerane. The oils from this province do not exhibit significant differences in either hopane, C₃₂ 22S/(S+R) and C₃₀ αβ/(αβ+βα), or sterane, C₂₉ αα 20S/(S+R), maturity ratios. However, the relative content of 5α(H),14β(H),17β(H)-cholestanes (C₂₉ ββ/(ββ+αα) ratio) indicates that Pina oils are more mature than Cristales and Jatibonico oils. Several of these oils (Cristales, Jatibonico and Pina 26) are heavily biodegraded, lacking n-alkanes, norpristane, pristane and phytane (the two former oils do not contain acyclic isoprenoid hydrocarbons). Other biodegradation products, the 25-norhopanes, are found in all the oils. Their occurrence is probably due to mixing of severely biodegraded oil residues with undegraded crude oils during accumulation in the reservoir.