镁同位素地球化学研究新进展及其在碳酸岩研究中的应用

镁(Mg)同位素有3个,24Mg、25Mg和26Mg,其中24Mg和26Mg的相对质量差较大,高达8.33％,这种大的相对质量差使地壳活动或其他地质过程中Mg同位素因化学物理条件的变化而发生明显的同位素质量分馏.目前,自然界可观测到的δ26Mg变化范围为?5.60‰～0.92‰,约6.5‰.镁在低温地球化学过程中分馏显...     

锂同位素及其地质应用研究进展

锂同位素示踪是近几年发展起来的一门新兴的稳定同位素地球化学方法,锂有两个稳定同位素：^6Li和^7Li。自在界锂同位素的组成变化很大,其δ^6Li值变化幅度超过60‰,现代大洋水的δ^6Li值为－31．0‰,洋中脊玄武岩（BORB）的δ^6Li值为－4．7‰--3.7‰,由于锂同位素存在大的分馏和不同地质体中在截然不同的δ^6Li值,因此锂同位素地质应用前景十分广泛。目前,锂同位素在研究星云形成过程和宇宙事件,洋壳蚀变和海底热液活动,壳－幔物质循环和板块俯冲作用过程,判断卤水起源和演化等方面的研究中成效显著。     

钙同位素地球化学研究新进展及其在碳酸岩-共生硅酸盐研究中的应用

作为"非传统稳定同位素"家族成员,钙同位素正受到国际地学界日益广泛的关注。钙是主要的造岩元素,也是生物必需的元素。钙在地球各圈层广泛分布,研究钙同位素的地球化学行为将有助于提高我们对各种生物过程和地质过程的认识。钙同位素测定主要采用热电离质谱仪(TIMS)和多接收电感耦合等离子体质谱仪(MC-ICP-MS),分别表示为δ^44/40Ca和δ^44/42Ca。目前自然界可观测到的δ^44/40Ca变化范围为-2.0‰～6.75‰,约8.7‰。本文系统介绍了近年来钙同位素分析中样品溶解、化学分离、质谱测定以及高温地质过程中的钙同位素分馏及其地质应用等方面的研究成果,尤其对钙同位素在碳酸岩-共生硅酸盐岩研究中的意义、钙同位素组成以及取得的主要认识作了较为全面的介绍。阐述了放射成因⁴⁰Ca、部分熔融作用/分离结晶作用、地壳同化作用、古俯冲碳酸盐循环、热液蚀变作用、岩浆起源深度等对碳酸岩、硅酸盐岩的钙同位素组成造成的影响。最后,通过系统对比碳酸岩-共生硅酸盐岩的锂、镁、钙同位素研究成果,认为应该开展多元同位素体系的...     

锂同位素及其在四川甲基卡伟晶岩型锂多金属矿床研究中的应用

锂同位素在地质学、地球化学研究中有着广阔的应用前景。简单介绍了锂同位素研究已取得的进展,并在四川甲基卡伟晶岩型锂多金属矿床研究中开展应用。研究表明,四川甲基卡锂矿床中2件锂辉石的锂含量非常高,分别为33 592×10-6和34 264×10-6,相应的δ7Li值分别为-0.6‰和-0.4‰,平均值为-0.5‰,而二云母花岗岩中黑云母的锂含量为7 350×10-6,δ7Li值为+0.6‰,两者在误差范围内具有非常好的一致性,证明锂辉石来源于二云母花岗岩,这与其他包裹体、同位素地球化学和年代学证据等相一致。     

陕西户县大石沟碳酸岩地球化学特征及其成因意义

户县大石沟碳酸岩,多年来一直被认为是超基性岩—碳酸岩杂岩体的一部分,属岩浆成因。在岩体地质及岩石学研究的基础上,通过微量元素、岩石化学、稀土元素及碳氧同位素分析,认为该岩石属构造—热液成因的交代蚀变碳酸盐岩。     

锂同位素与大陆风化

气候变化与大气二氧化碳浓度息息相关.大陆岩石圈风化是影响大气二氧化碳浓度的重要过程.通过还原陆壳古风化信息,我们可以有效地了解地球气候条件的演化历史.传统方法上,前人曾使用锶同位素示踪大陆风化,但其解释尚有不足.例如,海水锶同位素比值会受到海洋热液的影响,而河流锶同位素比值则易受风化岩石类型的影响.此外,只有硅酸盐风化被认为在长时间尺度控制着大气碳汇,但锶的碳酸盐风化却与硅酸盐风化很难分辨.因此,我们需要一种更理想的同位素体系作为示踪大陆风化历史的介质.锂,作为微量元素,主要集中在岩石圈的硅酸盐矿物中,在碳酸盐岩含量较少.所以,硅酸盐风化可以使用锂同位素予以记录.同时,锂同位素受生物分馏效应影响较小,可以在海相碳酸盐岩中保存良好.这些优势为海相碳酸盐岩的锂同位素信号示踪大陆风化历史提供了有力支撑,但我们仍需对风化、迁移和结晶等过程的锂同位素地球化学行为有清晰的认识.为此,本文回顾不同储库的锂同位素组成以及各物相间锂元素配分和同位素分馏特征,总结了近年来锂同位素在重建大陆风化历史方面的进展,并详述了有待解决的关键问题.     

锂同位素分馏机制讨论

作为一种新兴的稳定同位素示踪工具, 锂同位素地球化学的研究近年来受到了国际地学界日益广泛的关注.其应用领域涵盖了从地表到地幔的流体与矿物之间的相互作用.在地表风化作用过程中, 轻锂同位素(6Li) 优先进入固体相, 而7Li则进入流体相, 因而地表风化作用淋滤出了岩石中的重锂, 致使河水具有重的锂同位素组成, 河水又将重锂同位素组分补给海洋, 洋壳的低温蚀变作用使得海水的锂同位素组成进一步变重.在俯冲带, 由于俯冲板片释放的流体具有重锂同位素组成的特征, 它们上升并交代上覆的地幔楔和相邻的地幔, 使得地幔楔的锂同位素组成变重.同时, 深俯冲的板片由于脱水而具有较轻的锂同位素组成, 它们在地幔中可能形成一个局部轻锂的地幔储源.影响地幔橄榄岩锂同位素分馏的因素主要有3个方面: 温度、扩散机制以及外来熔体的反应.由于高温下地幔矿物之间的锂同位素分馏很小, 而单纯的扩散分馏机制不能够很好的解释我国华北汉诺坝地区地幔橄榄岩中矿物之间的锂同位素分馏.因此, 具有轻锂同位素组成的熔体与橄榄岩之间的反应是上述现象的一个合理解释.需要指出的是, 在橄榄岩-熔体反应的过程中, 锂同位素的扩散作用也对地幔矿物之间的同位素分馏有一定的贡献.      

过渡族金属元素同位素分析方法及其地质应用

由于同位素分析方法的改进和多接收电感耦合等离子体质谱仪 (MC ICP MS)的应用 ,近年来过渡族金属元素 (Cu ,Zn和Fe)同位素地球化学有了长足进步 ,成为国际地学领域的一个前沿研究方向。Cu同位素在自然界中的变化最大 ,δ65Cu值为 - 3.70‰～ +2 .0 5‰ ;Zn和Fe同位素变化比Cu同位素变化小 ,δ66Zn值为 - 0 .6 4‰～ +1.16‰ ,而δ56Fe值为 - 1.6 2‰～ +0 .91‰。自然界中各种无机过程 (从高温到低温 )和生物有机过程均能使Cu ,Zn和Fe同位素发生分馏。Cu、Zn和Fe在自然界中广泛分布于各类矿物、岩石、流体和生物体中 ,并广泛参与成岩成矿作用、热液活动和生命活动过程。因此 ,这些过渡族金属元素同位素已在陨石和宇宙化学、矿床学 ,海洋学和生物学等领域的研究中取得了显著成效 ,并将成为地球科学中具有巨大应用前景的一种新的地球化学手段。     

锂同位素在环境地球化学研究中的新进展

锂的两个稳定同位素(6Li和7Li)相对质量差较大,因此易产生明显的同位素分馏。业已查明,自然界中δ7Li值的变化在-40‰和 50‰之间。其中较小的δ7Li值见于海相生物碳酸盐样品,较大的δ7Li值见于某些盐湖卤水以及有孔虫的样品。由于明显的同位素分馏和不同地质体中截然不同的δ7Li值,锂同位素应用十分广泛,且在壳-幔演化、陆壳风化、卤水和污染水体示踪等研究领域取得显著成效。     

The continental shelf benthic iron flux and its isotope composition

Benthic iron fluxes from sites along the Oregon-California continental shelf determined using in situ benthic chambers, range from less than 10 μmol m⁻² d⁻¹ to values in excess of ∼300 μmol m⁻² d⁻¹. These fluxes are generally greater than previously published iron fluxes for continental shelves contiguous with the open ocean (as opposed to marginal seas, bays, or estuaries) with the highest fluxes measured in the regions around the high-sediment discharge Eel River and the Umpqua River. These benthic iron fluxes do not covary with organic carbon oxidation rates in any systematic fashion, but rather seem to respond to variations in bottom water oxygen and benthic oxygen demand. We hypothesize that the highest rates of benthic iron efflux are driven, in part, by the greater availability of reactive iron deposited along these river systems as compared to other more typical continental margin settings. Bioirrigation likely plays an important role in the benthic Fe flux in these systems as well. However, the influence of bottom water oxygen concentrations on the iron flux is significant, and there appears to be a threshold in dissolved oxygen (∼60-80 μM), below which sediment-ocean iron exchange is enhanced. The isotope composition of this shelf-derived benthic iron is enriched in the lighter isotopes, and appears to change by ∼3‰ (δ⁵⁶Fe) during the course of a benthic chamber experiment with a mean isotope composition of −2.7 ± 1.1‰ (2 SD, n = 9) by the end of the experiment. This average value is slightly heavier than those from two high benthic Fe flux restricted basins from the California Borderland region where δ⁵⁶Fe is −3.4 ± 0.4‰ (2 SD, n = 3). These light iron isotope compositions support previous ideas, based on sediment porewater analyses, suggesting that sedimentary iron reduction fractionates iron isotopes and produces an isotopically light iron pool that is transferred to the ocean water column. In sum, our data suggest that continental shelves may export a higher efflux of iron than previously hypothesized, with the likelihood that along river-dominated margins, the benthic iron flux could well be orders of magnitude larger than non-river dominated shelves. The close proximity of the continental shelf benthos to the productive surface ocean means that this flux is likely to be essential for maintaining ecosystem micronutrient supply.     

Seismic investigation of crust-mantle transition in continental rift systems - Jordan-Dead Sea rift and Rhinegraben

The recent acquisition of high-quality seismic refraction data in the Jordan—Dead Sea rift and adjacent areas has made possible the investigation of the dynamic properties of seismic P-waves refracted and reflected at the crust—upper mantle boundary.

These waves cause high-amplitude arrivals near the outer cusp of the travel-time curve which are followed by an abrupt decrease in amplitudes at increasing distances beyond the cusp.

It has been shown that such amplitude distributions can only be the result of a smooth rapid increase of velocity with depth. In the case of the Jordan—Dead Sea rift the amplitude distribution indicates the presence of a transition zone between the lower crust and upper mantle in which the velocity increases smoothly. The interpretation of seismic refraction data in the Rhinegraben indicates the existence of a similar transition zone. In both rifts the crust—mantle boundary outside the rift is represented by sharp velocity discontinuity.

The comparison of the velocity structure of the crust—upper mantle boundary suggests that a smooth transition zone at the base of the lower crust is a characteristic property of continental rifts which could be interpreted in terms of crust—mantle interaction.     

Molybdenum isotope composition from Yangtze block continental margin and its indication to organic burial rate

The paper presents the molybdenum isotope data, along with the trace element content, to investigate the geochemical behavior of authigenic Mo during long-term burial in sediments in continental margin settings of the Yangtze block, as well as their indication to the burial of original organic carbon. The burial rate of original organic carbon was estimated on the basis of the amount of sedimentary sulfur (TS content), whilst the carbon loss by aerobic degradation was estimated according to calculated Mn contents. On these points, the original organic carbon flux was calculated, exhibiting a large range of variation (0.17–0.67 mmol/m²/day). The strong correlation between sedimentary Mo isotope values and organic carbon burial rates previously proposed on the basis of the investigations on modern ocean sediments, was also used here to estimate the organic carbon burial rate. The data gained through this model showed that organic carbon burial rates have large variations, ranging from 0.43–2.87 mmol/m²/day. Although the two sets of data gained through different geochemical records in the Yangtze block show a deviation of one order of magnitude, they do display a strong correlation. It is thus tempting to speculate that the Mo isotope signature of sediments may serve as a tracer for the accumulation rate of original organic carbon in the continental margin sediments. __________ Translated from Earth Science—Journal of China University of Geosciences, 2007, 32(6) [译自: 地球科学—中国地质大学学报]     

Lithium isotope analysis of olivine by SIMS: Calibration of a matrix effect and application to magmatic phenocrysts

High-spatial resolution analysis of light element isotope variations by secondary-ion mass spectrometry (SIMS) has numerous applications in geochemistry and cosmochemistry. Recent attention has focused on ⁷Li/⁶Li variations in magmatic phenocrysts to infer the volatile degassing history of their parent magmas, and on minerals from mantle samples to determine source-region processes and the recycling history of mantle reservoirs. In these studies the effect of mineral composition on the ⁷Li/⁶Li ratio measured by SIMS has been considered secondary, and generally disregarded. We show, using a suite of nine olivines analyzed by MC-ICP-MS or TIMS, that there is a substantial effect of composition on the ⁷Li/⁶Li ratio of olivine measured by SIMS. For magnesian olivine (74 < Fo < 94) the effect is a linear function of composition, with δ⁷Li increasing by 1.3‰ for each mole percent decrease in forsterite component. At higher Fe contents, the relationship ceases to be linear. The composition range over which linear behavior is exhibited appears to depend on instrumental conditions. A calibration of this matrix effect over the linear range is presented, assuming the measurement of ⁷Li/⁶Li relative to an olivine standard of known composition. Application of this calibration to a suite of olivines separated from basaltic lavas from Ko'olau, Hawai'i demonstrates that the matrix effect is responsible for a geologically spurious correlation between δ⁷Li and Mg#. However, after correction, the olivines retain evidence of Li isotope heterogeneity, the degree and nature of which differs in each of the four separates studied. These results emphasize the importance of compositional correction for SIMS measurement of δ⁷Li in olivine, particularly in zoned crystals, and support previous conclusions that Li isotope variability in igneous materials is subject to late-stage disturbance. The significant matrix effect demonstrated for olivine suggests that matrix effects in other minerals require further evaluation.     

西秦岭双王金矿床与火成碳酸岩成因关系

西秦岭双王金矿床以大量发育角砾岩型矿石为特征,金与碳酸盐矿物密切共生,在角砾岩之间呈胶结物产出,显示出强烈的幔源特点.长期以来,这种罕见的矿床类型的成因一直困扰着矿床学家.最近,在双王金矿床深部坑道内新揭露的碳酸岩墙,有可能为揭示这一科学难题提供了一个突破口.文章选择碳酸岩和金矿石为研究对象,通过光薄片观察及TIMA扫...     

大陆碰撞成矿理论的创建及应用

本文阐明了大陆碰撞成矿理论的重要性,将大陆碰撞成矿理论的发展史划分为1970年以前的预备期,1971～1990年的孕育期,1991～2000年的诞生期,2001～2010年的成长期和2011年以后的成熟期;指出大陆碰撞成矿理论的核心内容是4个不同尺度(全球构造、造山带、地体、矿床)的碰撞造山流体成矿模式(CMF模式)和4种类型成矿系统的特征及其与其它环境同类成矿系统的对比;介绍了运用大陆碰撞成矿理论指导找矿预测的成功范例.事实证明,中国学者发挥中国碰撞造山带丰富的自然优势,为发展大陆碰撞成矿理论做出了重大贡献.     

裂陷盆地韧性伸展量计算模型及其在渤海湾盆地的应用

裂陷盆地在拉张过程中存在的韧性变形一直没有得到理论界的关注。两种常用的伸展量计算方法("地壳厚度变化计算法"和"断层平衡复原法")的巨大差异是由于盆地的韧性伸展分量导致的。本文建立了一套基于以上两种方法耦合应用的盆地韧性伸展量和断层滑移伸展量计算模型,并应用于渤海湾盆地。经计算,渤海湾盆地的韧性伸展量大约占总伸展量的40%左右,这是用"断层平衡复原法"计算的伸展量总是小于"地壳厚度变化计算法"计算的伸展量的真正原因。渤海湾歧口凹陷的伸展计算结果显示韧性变形速率与断层滑移速率呈正比关系,盆地在裂陷期不仅断层滑移剧烈,韧性变形速率也很大。     

钼的稳定同位素体系及其地质应用

近年来多接收杯电感耦合等离子体质谱仪(MC-ICP-MS)的广泛应用,大大提高了Mo同位素分析方法的精度和效率,使Mo同位素地球化学成为当前地学研究领域中的一个前沿方向.本文综述了Mo稳定同位素的最新研究进展及其地质应用.自然界中的Mo同位素(δ98/95Mo)的一般变化范围是-1.35‰～2.60‰.Mo同位素分馏在充氧环境下取决于Mn氧化物的吸附或共沉淀,贫氧一缺氧环境下受控于水溶液中的[H2S].沉积物中的Mo同位素既能指示古沉积环境的氧化还原条件,也能够指示与之相关的古海洋地理环境,因此,Mo同位素是了解局域至区域沉积环境的氧化还原条件、硫和碳地球化学循环及古海洋化学演化等的强有力工具.随着其分馏机理的进一步阐明和应用范围的拓展,Mo同位素将在地球与环境科学研究中得到广泛的应用.