基于微量元素岩性地球化学基因的构建与检验

地球化学基因是近年来提出的新技术。在分析地球化学基因构建方法的基础上提出一种基于微量元素的岩性地球化学基因,并将其编号为LG03,其基因链元素序列为Nb→Ti→Zr→Cr→La→V→Pb→Co→U→Ni→Th。分别选择发育在广东连阳花岗岩与佛冈花岗岩的2个花岗岩风化剖面、海南文昌3个玄武岩风化剖面、北京房山1个花岗闪长岩风化剖面、胶东2个花岗岩风化剖面与1个玄武岩风化剖面及1个花岗闪长岩风化剖面共计10个风化剖面对新构建的LG03基因以及以前基于主量和微量元素混合构建的LG01岩性基因进行检验。采用基因相似度≥80%作为样品之间具有相似基因的判别标准来检验地球化学基因在风化过程中的稳定性时,发现基于微量元素的LG03岩性基因与基于主量和微量元素混合的LG01岩性基因在遗传性与继承性方面基本相似,但LG03基因相对于LG01基因更稳定。采用酸性相似度20%和80%作为划分类基性成分、类中性成分和类酸性成分的标准时,发现LG03和LG01岩性基因在成分分类与物源示踪等方面也基本相似。在物源示踪方面LG03基因相对于LG01基因更稳定;在成分分类方面当二者划分结果不一致时应以LG01基因的酸性相似度划分结果为准,可进一步以LG03基因的划分结果加以约束。在风化、搬运与沉积过程中,地质样品的主量成分可能会受到风成沙、生物体等介质的加入而发生明显改变,进而导致LG01基因发生改变。但这些较单一成分物质的加入并不影响相对不活动微量元素之间的相互关系,从而可使LG03基因保持较好的稳定性。因此相对于LG01基因而言,LG03基因由于消除了受风成沙、生物体等介质的影响而可能具有更广泛的应用领域。     

硅酸盐化学风化强度评估的沉积物指标与方法研究进展*

风化作用是源-汇沉积体系中的重要环节,气候、构造、地形、植被和岩性在不同程度上控制着地表硅酸盐化学风化,量化硅酸盐化学风化强度有助于开展全球性的实例研究对比。本研究归纳总结了基于碎屑沉积物的化学风化强度指标,包括岩石学和矿物学指标、元素地球化学指标和非传统稳定同位素指标,并指出了指标在应用中存在的问题。这些指标中:砂质沉积物骨架颗粒组成、粉砂级碎屑的矿物组成和矿物表面结构特征从矿物组成和结构上直观地反映了化学风化强度,运用不多但值得关注;黏土矿物组合和主微量元素指标如CIA、Rb/Sr、α^AlE等在实例研究中运用广泛,同时也显现出易受物源和水动力分选影响等弊端;新近开发的利用锂、硼、钾、镁、硅等同位素示踪化学风化强度的方法具有广阔的应用前景,同位素的分馏机理有待完善。源-汇体系中的其他过程如物源供给、水动力分选、成岩作用、再旋回作用、成土作用、物理淋滤和生物利用等会影响化学风化指标的使用效果,通过多指标的综合运用和相互验证,可以有效提升化学风化强度评估的准确性。     

Late Oligocene to early Miocene humidity change recorded in terrestrial sequences in the Ili Basin (south‐eastern Kazakhstan,Central Asia)

Enhanced aridification of Central Asia driven by the combined effects of orogenic surface uplift, Paratethys retreat, changes in atmospheric moisture transport and global cooling is one of the most prominent Cenozoic climate change events of the Northern Hemisphere. Deciphering regional long‐term patterns of Central Asian hydrology is, therefore, a key element in understanding the role of Northern Hemisphere mid‐latitude drying in the global hydrological system. This study characterizes long‐term palaeoenvironmental conditions between the late Oligocene and early Miocene in south‐eastern Kazakhstan based on stable isotopes, elemental geochemistry and laser ablation uranium–lead geochronology from alluvial, fluvial and pedogenic deposits. Sedimentary facies and geochemical weathering indices suggest an increased surface and groundwater discharge fed by orographically enhanced precipitation in the Tien Shan hinterland. In contrast, pedogenic stable isotope data and elevated rates of magnesium fixation in clay minerals mirror enhanced rates of evaporation in the vadose zone due to protracted aridification. This study posits that pronounced surface uplift of the Tien Shan Mountains during the Oligocene–Miocene transition promoted regionally increased orographic precipitation and the development of fluvial discharge systems.     

Eogenetic diagenesis of Chinle sandstones,Petrified Forest National Park (Arizona,USA): A record of Late Triassic climate change

Fluvially derived tuffaceous Chinle sandstones from Petrified Forest National Park provide a well‐preserved Late Triassic archive of climate information. Petrographic analysis of 38 Chinle sandstones provides new insight into the depositional history and evolution of palaeoclimate during Chinle deposition. This study focuses on the relationship between climate and meteoric diagenesis as a guide for constraining climate change in western equatorial Pangea during the Late Triassic. Petrographic analysis of Chinle sandstones reveals their wide range of textural attributes, as well as pedogenic and shallow burial diagenetic features that occurred during the Late Triassic. These diverse petrological characteristics are indicative of the evolving Late Triassic climate, when placed into a well‐constrained stratigraphic and geochronological framework. The stratigraphic succession is characterized by variations in the abundance of framework grains, detrital matrix, weathering intensity of feldspar and volcanic rock fragments, and the mineralogy of clay cements. Climate records from Chinle palaeosol geochemistry indicate a progression from wet to dry conditions. This trend is also reflected in the meteoric diagenetic features of Chinle sandstones. During deposition of the lower Chinle, elevated rainfall promoted the weathering of labile volcanic detritus to kaolinite, whereas feldspars (especially plagioclase) were partially or completely dissolved. In the upper Chinle, a trend towards drier conditions favoured the formation of smectite and less feldspar dissolution resulting in a higher abundance of well‐preserved plagioclase grains. Shallow burial meteoric weathering reactions in Chinle sandstones reflect the evolving climate during the Late Triassic.     

Mass balance of major elements in relation to weathering in soils developed on igneous rocks in a semiarid region, northwestern Iran

This study was conducted to evaluate the weathering intensity of the major soils developed on igneous rocks in semiarid region of northwestern Iran.Eight parent materials were selected including monzodiorite,alkali granite,granodiorite,syenite,pyroxene diorite,hornblende andesite,pyroxene andesite,and dacite.Representative soil profiles were described and soil samples were collected and analyzed for selected chemical and physical properties and total concentrations of major elements and Zr,V,Ti and Y.Bulk densities as well as Ti,Zr and V concentrations were used to estimate the strain factors and mass balance equations were used to quantify the net result of pedogenic weathering,i.e.elemental loss and gain.The results of clay content and pedogenic iron variability as well as index of compositional variability(ICV),chemical index of alteration(CIA) and,A-CN-K and MFW ternary plots showed that the soils developed on volcanic rocks(hornblende andesite> pyroxene andesite> dacite) were more weathered than those on the plutonic parent rocks(alkali granite,granodiorite,monzodiorite,syenite,pyroxene diorite).The results of mass balance calculations based on the strain factors revealed that the Ca and Na depleted during weathering progress mostly from plagioclase grains.In the semiarid regions Ca is precipitated as pedogenic calcite in the soil horizons.K and Mg depletion is less than Ca and Na especially in the profiles on the hornblende andesite with the highest clay and LOI content.The results of this study clearly suggest that the behavior of K and Mg during the weathering cannot only be explained by the disintegration of the primary minerals,since they are fixed on the secondary clay minerals.Iron did not change in the soils compared to the parent material and was precipitated as the pedogenic iron and conserved in the soil horizons.Overall,the results on the weathering indicators and major elements mass balance enrichment/depletion in the study area confirmed that the soil profiles developed on volcanic rocks are more weathered than those on the plutonic igneous rocks.     

巴丹吉林沙漠高大沙山沉积物地球化学元素组成及其环境意义

地球化学元素不仅反映地球表层沉积物的组成特征,并在分析物源、沉积过程及重建古环境等方面有积极意义。通过野外采样、室内实验,对巴丹吉林沙漠伊和吉格德湖、巴丹湖高大沙山区沉积物地球化学特征进行了分析。结果表明：沙山沉积物元素组成特征和含量略有区域差异;与上部陆壳平均化学组成（UCC）相比,常量元素中仅Si富集,其余均亏损;趋向迎风坡上部剖面,Si、K富集程度相对增加,其他常量元素（除Na）亏损程度增加。常量元素间的关系表明SiO₂影响沙山沉积物的地球化学组成及性质;迎风坡底部剖面沉积物呈现出多源性特征,可能是不同外力作用的结果。两湖区沙山的沙源属局部范围混合的结果,就近供应;干旱气候下沙山沉积物缺少较高等化学风化产生的条件,基本处在大陆风化早期阶段的初期。     

DIFFERENTIAL ROCK WEATHERING IN THE 'VALLEY OF THE BOULDERS', KÄRKEVAGGE, SWEDISH LAPLAND

Kärkevagge is an alpine valley in the low arctic of Swedish Lapland. It is named after, and famous for, its large deposit of immense (c. 10–15 m) boulders that almost fill the lower valley. Above the boulder deposit, on the flanks of the valley, are more recent and generally much smaller (c. 1–3 m) individual boulders that have fallen from the valley-wall cliff face, presumably from post-glacial valley-side unloading. Some of these smaller boulders are seemingly fresh and unweathered while others have been reduced to no more than mounds in the tundra. These boulders must be younger than the larger, lower giant boulder deposit, but are not particularly recent rockfalls as they are partially buried in colluvium. Comparisons of mineralogy and chemistry indicate that the possibility exists that the incompetent, 'rotten' rocks, if not considerably older than their competent neighbors, are inherently self-destructive. They have evidence of increased sulfur content, which is a proxy for pyrite, a known weathering accelerant in Kärkevagge.     

贵州水城-纳雍地区峨眉山玄武岩风化壳离子吸附型稀土矿床地质特征及资源潜力

通过对贵州水城—纳雍地区峨眉山玄武岩风化壳离子吸附型稀土矿床的风化壳结构、矿体垂向变化特征的综合研究,总结了矿床特征、风化壳特征、区域成矿条件。风化壳中全风化层是稀土矿富集最好的层位; (La/Yb)N、ΣLREE/ΣHREE比值较高,属于强轻稀土富集类型。严格受母岩、地形、气候、地层、构造等因素共同作用。稀土矿床具备分布面广、埋藏浅、厚度大、品位高等地质特征。可以指导贵州西部地区寻找同类型大型稀土矿产地。     

大兴安岭中南段下二叠统寿山沟组典型泥岩地球化学特征及其地质意义

西乌珠穆沁旗地区下二叠统寿山沟组泥岩为大兴安岭中南段发育的较好烃源岩.通过对寿山沟组泥岩中的主量、微量、稀土元素特征的分析,深入剖析了寿山沟组的物源、构造背景及其风化作用.岩石样品ICV值分布于1.15～1.57,平均值为1.33,认为样品基本未受到二次成岩作用影响.通过A－CN－K三角图判断寿山沟组泥岩受到一定的钾交代作用,经过校正得到CIA_corr值为54.28～70.06,平均值为65.33,表明物源区经历了初等—中等的化学风化作用.通过主量－微量元素多变量物源区环境分析,认为寿山沟组泥质沉积物主要来源于长英质物源区,且混有少量铁镁质火山岩源区物质,其构造背景为大陆岛弧.综合分析认为,大兴安岭中南段下二叠统寿山沟组泥岩的沉积特征归因于古亚洲洋二叠纪初期洋壳的俯冲作用.