Geochemistry of late quaternary sediments from Tecocomulco lake, central Mexico: Implication to chemical weathering and provenance

This paper presents the first detailed multi-element geochemical data from the late Quaternary sediments of the Tecocomulco lake basin (central Mexico) and rocks exposed in the basin catchments to understand the extents of chemical weathering and provenance of the siliciclastic fractions. Ternary diagrams of A-CN-K, A-C-N and A-CNK-FM and elemental ratios suggest that most of the lacustrine sediments were derived from mafic volcanic deposits comprising the Chichicuatla and the Apan-Peñon andesites and the Apan-Tezontepec basaltic-andesites. The felsic tephra layers have chemical compositions comparable to the Acoculco volcanic sequences. The calculated indices of chemical weathering such as chemical index of alteration (CIA), plagioclase index of alteration (PIA) and chemical index of weathering (CIW) indicate low to extreme chemical weathering for the lacustrine sediments and low chemical weathering for tephra layers. The varying degree of chemical weathering in lacustrine sediments is related to the fluctuating average annual precipitation during the late Quaternary. However, the low weathering of tephra layers are due to their higher rate of deposition. The dacite-rhyolitic tephra layers of ca. 31,000 ¹⁴C yr BP are relatively more weathered compared to the unweathered rhyolitic tephra of ca. 50,000 ¹⁴C yr BP. This could be due to the rapid deposition of ca. 200 cm of tephra layers during the ca. 50,000 ¹⁴C yr BP volcanic eruption that might have prevented the interaction between tephra layers and weathering agents.     

Long term mobilisation of chemical elements in tephra-rich peat (NE Iceland)

This paper presents geochemical profiles of a tephra-bearing minerotrophic peat column from NE-Iceland obtained using various elemental analyses of the solid phase and the pore water. The influence of tephra grain size, thickness and composition of each tephra on the peat geochemistry was investigated. Interpretations are supported by a statistical approach, in particular by autocorrelation, and by microscopy observations. Minerotrophic peat geochemistry may be strongly dependent upon post-depositional mobilization and possible leaching of elements as demonstrated by Fe and trace metal concentration profiles. Chemical elements, and more specifically potentially harmful metals, can be slowly leached out of volcanic falls during their weathering and re-accumulate downwards. It is emphasised that a tephra deposit can act as an active geochemical barrier, blocking downward elemental movements and leading to the formation of enriched layers. In this study, the formation of poorly amorphous Fe phases above the Hekla 3 tephra is shown. These poorly crystalline Fe phases scavenged Ni.     

华南埃迪卡拉纪陡山沱组物源风化对全球性成磷事件的启示

华南埃迪卡拉纪陡山沱组沉积于原始地球-生命系统向现代地球-生命系统过渡的关键时期,该套地层记录了地质历史时期一次大规模成磷事件,但关于该次成磷事件中磷元素的来源仍存在诸多争议。浅海区磷元素主要来源于由河流搬运的陆表含磷矿物风化产物及上升流作用所带来的其他海域含磷水体,这两种来源可以通过全岩碎屑元素(Al、Fe、Th等)和稀土元素的含量及其分布特征进行区分。本文通过对上扬子地区乡党坪剖面和四斗坪剖面的陡山沱组主、微量元素特征的综合分析,恢复了该地区的物源体系及其风化作用过程。较高的P₂O₅-Al₂O₃、P₂O₅-Fe₂O₃相关性,表明该时期磷来源于陆地风化而非上升流;较低的Th/Co、La/Sc、Th/Cr等比值,TiO₂-Zr相关性以及Th-Hf-Co、La-Th-Sc三角图分布关系,表明研究区内陡山沱组的物源体系主要为玄武岩;较高的化学蚀变指数(CIA)和较低的Sr/Cu值,表明物源区当时...     

Impact of trace mineral phases on the total solute flux from andesitic volcanics

Recent work on the weathering of high standing islands (HSI’s) of New Zealand (Goldsmith et al., 2008), Dominica (Goldsmith et al., 2010) Martinique and Guadeloupe (Rad et al., 2006) and portions of the Philippines (Schopka et al., 2011) shows weathering rates based on stream water chemistry for areas draining andesitic terrains are comparable to weathering rates determined for basaltic terrains, indicating that andesite weathering might be much more important in drawing down atmospheric CO₂ than previously recognized. While an easily erodible parent material has been largely attributed to sustaining rates at these locations, little is known to known regarding its associated reaction kinetics. We conducted a series of batch dissolution experiments on andesitic material collected from ∼10,000 year old tephra deposits from Dominica to determine the dissolution rate of major and trace mineral phases to better understand geochemical processes controlling weathering flux from these areas. Dissolution experiments were conducted over a range of pH (4 and 7) on bulk samples and mineral separates.The dissolution rates based on Si release from the Dominica tephra bulk samples were similar, and ranged from 0.04 to 0.13 μmole Si/g-day in water, and ∼0.14 to 0.27 μmole Si/g-day in dilute acid (initial pH ∼4). Although the bulk of the ash is predominately composed of vesicular felsic (Na–Al–Si) volcanic glass, reaction rates and stoichiometry indicate ash dissolution is dominated by the reactivity of trace Mg or Ca-bearing silicate phases (olivine, pyroxene or amphiboles) and Ca–phosphate phases (apatite), especially under slightly acidic conditions. Analysis of reacted phases by SEM shows little evidence of alteration of glassy material, whereas surfaces of Ca–Mg inosilicates, olivine and apatite show etched features indicative of dissolution. Results of the dissolution experiments suggest that, although these phases are relatively minor components of the ash, they contribute disproportionately to the overall weathering flux, and their reactivity may be particularly important in areas where physical weathering and erosion are constantly exposing new fresh surfaces available for chemical reaction.     

珠江三角洲晚第四纪风化层稀土元素地球化学特征*

珠江三角洲地区上更新统与全新统之间广泛发育1层杂色黏土,其成因多认为主要是由上更新统沉积物在末次冰盛期暴露于地表风化而成。对取自珠江三角洲3条钻孔（PRD09、PRD16和PRD17）的岩心样品分析表明,受风化作用的影响,其稀土元素含量和分异特征发生了较明显的变化。杂色黏土层的稀土总量大大低于下伏沉积物,而在邻近风化层的下伏沉积物中稀土元素却表现为明显富集,尤其是重稀土元素的富集。风化作用强度较大的PRD09孔和PRD17孔下伏沉积物中的稀土富集程度高于风化作用强度相对较小的PRD16孔。珠江三角洲在末次冰盛期时普遍发育的酸性介质条件,促进了风化层的稀土元素发生溶解和迁移。在风化过程中,由于轻、重稀土元素具有不同的溶解迁移能力和吸附能力,导致杂色黏土层的REE指标值（LREE/HREE、(La/Gd)_N和(La/Yb)_N）高于下伏沉积物。风化过程对Ce、Eu异常有一定的影响,但不十分明显,杂色黏土层的Ce、Eu异常值仅略低于下伏沉积物。     

峡东地区南华纪-寒武纪地层风化过程元素及Sr-Nd同位素演化特征及其地球化学意义

以峡东地区南华纪、震旦纪和寒武纪标准地层泥岩、冰碛泥岩、砂岩、灰岩和白云质灰岩及对应的风化土壤为研究对象, 分析了地层风化成土过程中不同元素的迁移行为, 根据剖面样品的Sr-Nd同位素组成变化, 探讨了其同位素体系的封闭性特征与应用意义.结果表明: (1) 不同岩性基岩在成土过程中的蚀变强度有明显的差异, 在相似地表条件下, 碳酸岩风化剖面的风化程度高于泥质岩和砂岩; (2) 通过对比稳定高场强Ti元素在基岩和风化剖面中的含量变化, 计算出土壤样品在风化过程中体积相对基岩发生的改变量, 进而计算出不同岩性基岩在风化过程中微量元素的绝对含量变化以探讨这些元素的活动规律.结果表明, 灰岩和白云质灰岩的风化剖面元素含量变化明显, 而在泥质岩风化过程中大多数元素保持了相对稳定, 说明沉积岩风化过程中元素的活动性特征明显地受到了原岩矿物组成的制约.风化过程中, 不同性质的元素的活动性差异明显, 其中亲硫元素(Cu、Zn、Pb、Mo) 和大离子亲石元素(Rb、K、Sr、Ba) 在不同岩性的风化剖面中均表现出明显的元素含量变化, 而高场强元素(Zr、Hf、Nb、Ta) 含量则相对稳定; (3) 泥质岩风化形成的土壤层REE含量变化较小, 而碳酸岩风化土壤层REE含量发生了明显下降, 且其风化形成的土壤表现出LREE和HREE相对于MREE的富集.无论是碳酸岩或泥质岩风化形成的土壤, 均出现了明显的Eu负异常和Ce的正异常, 但在其原岩中这些异常并不存在或不明显; (4) 基岩与土壤剖面间Sr同位素组成和Rb/Sr比值存在明显差异, Rb-Sr同位素组成发生了明显的开放.所形成土壤层的Sr同位素组成受到2种因素的约束: 原岩性质和外来组成的Sr同位素比值.因此在总体上, 风化土壤的Sr同位素组成已不能代表基岩的Sr同位素组成; (5) 沉积岩风化过程中, 碳酸岩和泥质岩形成的风化土壤基本保持了原岩的Sm-Nd同位素组成特点, 由其组成所获得的Nd同位素亏损地幔模式年龄等能反映其原岩信息, 而近源沉积形成的砂岩和含砾冰碛泥岩所形成的土壤, 其Nd同位素组成则存在不同程度的改变.      

铂族元素的表生地球化学研究进展

基性-超基性岩及黑色页岩等风化过程中铂族元素发生了活化和迁移。铂族元素在岩体附近土壤、植物和水体等表生介质中的迁移富集特征可以用来寻找岩浆型铂族元素矿床。黑色页岩风化过程中，Os的表生迁移对海洋^187Os／^188Os值上升有重要意义。此外，作为重金属元素，机动车铂族元素催化剂在环境中的释放，也会带来潜在的生态环境和人体健康问题。     

An evaluation of acid deposition on cation leaching and weathering rates of an Andosol and a Cambisol

An evaluation of the response of an Andosol and a Cambisol to acid deposition and weathering rates was studied by using a controlled laboratory leaching experiment. Both soils where derived from mafic parent material, a Histic Andosol from Western Iceland and a Cambisol from North East Scotland. De-ionized water and water acidified with H₂SO₄ (pH 3) was leached through reconstructed soil columns to simulate 34 years of precipitation.Acidic input increased cation leaching and weathering rates in both soil types and reduced pH levels. The Andosol proved generally to have higher weathering rates, leaching potential, ion exchange and acid-buffering capacity as well as maintaining a relatively steady pH despite intense acidic input. This was due to differences in parent material and mineral composition. The Andosol was developed from basaltic volcanic tephra, which had higher dissolution rates due to its amorphous mineral structures. The Cambisol was developed from gabbro with more stable mineral structures.     

黑色页岩风化过程中元素的活动性及参照系的选取初探--以湖南省麻田、桃花江剖面为例

系统考察了湖南省麻田和桃花江两个不同发育程度的下寒武统黑色页岩风化剖面,并提出了元素在理想剖面中的分布模式(概念模型);通过对比分析这两个剖面中元素的分布特征与理想分布模式的关系,并结合元素相关性研究、等质量图解的判别分析、微量元素的赋存状态及地球化学性质的探讨,认为在黑色页岩风化过程中,Ti为不活动元素,可作为参照系来计算其它元素的相对迁移性,Nb、Ta的活动性较小,Zr与Hf之间存在着共迁移现象;研究表明化学风化程度是影响元素活动性的重要因素.     

Volcaniclastic stratigraphy of the Tiscapa maar crater walls (Managua, Nicaragua): implications for volcanic and seismic hazards and Holocene climate changes

The Tiscapa maar in the center of Managua city formed by a phreatomagmatic eruption <3 ka ago. The eruption excavated a crater deep into the basement exposing a coherent Pleistocene to Holocene volcaniclastic succession that we have divided into four formations. The lowermost, >60 ka old basaltic–andesitic formation F1 comprises mafic ignimbrites and phreatomagmatic tephras derived from the Las Sierras volcanic complex south of Managua. Formation F2 contains the ~60 ka basaltic–andesitic Fontana tephra erupted from the Las Nubes Caldera of the Las Sierras complex 15 km to the S, the 25 ka Upper Apoyo tephra from the Apoyo Caldera 35 km to the SE, and the Lower (~17 ka) and Upper (12.4 ka) Apoyeque tephras from the Chiltepe volcanic complex 15 km to the NW. These tephras are separated by weathering horizons and paleosols indicating dry climatic conditions. Fluvial deposits of a SSW-NNE running paleo-river system build formation F3. The fluvial sediments contain, from bottom to top, scoriae from the ~6 ka basaltic San Antonio tephra, pumice lapilli from the Apoyo and Apoyeque tephras and the 6.1 ka Xiloà tephra, and scoriae derived from the Fontana tephra. The fluvial sediment succession thus reflects progressively deeper carving erosion in the southern highlands (where a large-amplitude regional erosional unconformity exists at the appropriate stratigraphic level) that began after ~6 ka. This suggests that the mid-Holocene tropical high-precipitation climatic phase affected western Nicaragua about a thousand years later than other circum-Caribbean regions. The end of the wet climate phase ~3 ka ago is recorded by a deep weathering zone and paleosol atop formation F3 prior to the Tiscapa eruption. Formation F4 is the Tiscapa tuffring composed of pyroclastic surge and fallout deposits that cover a minimum area of 1.2 km². The 4 × 10⁹ kg of erupted basaltic magma is compositionally and genetically related to the low-Ti basalts of the N–S striking Nejapa-Miraflores volcanic–tectonic alignment 5 km to the West of Tiscapa. Ascent and eruption mode of the Tiscapa magma were controlled by the Tiscapa fault that has a very active seismic history as it achieved 12 m displacement in about 3000 years. Managua city is thus exposed to continued seismic and volcanic risks.     

云南滇池地区风化磷块岩的风化指标研究

长期风化作用使云南滇池地区形成了大面积的风化磷块岩矿石,这种矿石是制取高效磷肥的优质原料。为了圈定这些风化磷块岩矿石,须确定适宜的风化指标。笔者通过风化磷块岩形成过程化学组分变化研究,首次提出将风化指标分为“直接法”和“间接法”两类,进而提出ＣＯ２风化指标为直接法中指标,２Ｐ２Ｏ５＋Ａ．Ｉ＾１）风化指标为间接法中推荐指标。     

火山岩风化过程中难熔元素的活动性研究——以广西凭祥英安岩风化剖面为例

对广西凭祥叫弄英安岩风化剖面中6个难熔元素的活动性进行了研究。结果表明，Ti,Nb和Ta 3种元素在整个风化成土过程中基本上保持了它们的不活动性。Zr,Hf和Th在风化的初期阶段基本上保持不活动，而在风化中期表现出一定的富集，风化高级阶段则有明显的富集。Th的富集可能与风化壳表层（已剥蚀）淋溶出来的Th向下迁移过程中被氧化铁吸附和形成磷酸盐矿物有关，而Zr的富集则可能与淋溶出来的Zr与SiO2或磷酸盐结合形成某种硅酸盐矿物和磷酸盐矿物有关，具体富集原因还需作进一步的研究。     

The Redistribution of Phosphorus in Basic Volcanic Rocks

This work presents data on phosphorus content in the central and peripheral sectors of primary jointing blocks in olivine basalts of the Baikal rift zone (Khamar-Daban Range), as well as basaltic andesite flows and dolerite sills on the Karabakh Highland of the Lesser Caucasus. The P₂O₅ content decrease near cracks in volcanic rocks of the Baikal rift zone is caused by leaching of phosphorus during humid weathering. Estimates of the amount of mobilized phosphorus suggest that alkaline basic volcanic rocks can serve as the continental source for the formation of phosphates.     

Trace-element geochemistry of individual glass shards of the Old Crow tephra and the age of the Delta glaciation, central Alaska

Two widespread tephra deposits constrain the age of the Delta Glaciation in central Alaska. The Old Crow tephra (ca. 140,000 ± 10,000 yr), identified by electron microprobe and ion microprobe analyses of individual glass shards, overlies an outwash terrace coeval with the Delta glaciation. The Sheep Creek tephra (ca. 190,000 yr) is reworked in alluvium of Delta age. The upper and lower limiting tephra dates indicate that the Delta glaciation occurred during marine oxygen isotope stage 6. We hypothesize that glaciers in the Delta River Valley reached their maximum Pleistocene extent during this cold interval because of significant mid-Pleistocene tectonic uplift of the east-central Alaska Range.     

Marine tephrochronology of the Mt. Edgecumbe Volcanic Field, Southeast Alaska, USA

The Mt. Edgecumbe Volcanic Field (MEVF), located on Kruzof Island near Sitka Sound in southeast Alaska, experienced a large multiple-stage eruption during the last glacial maximum (LGM)-Holocene transition that generated a regionally extensive series of compositionally similar rhyolite tephra horizons and a single well-dated dacite (MEd) tephra. Marine sediment cores collected from adjacent basins to the MEVF contain both tephra-fall and pyroclastic flow deposits that consist primarily of rhyolitic tephra and a minor dacitic tephra unit. The recovered dacite tephra correlates with the MEd tephra, whereas many of the rhyolitic tephras correlate with published MEVF rhyolites. Correlations were based on age constraints and major oxide compositions of glass shards. In addition to LGM-Holocene macroscopic tephra units, four marine cryptotephras were also identified. Three of these units appear to be derived from mid-Holocene MEVF activity, while the youngest cryptotephra corresponds well with the White River Ash eruption at ∼ 1147 cal yr BP. Furthermore, the sedimentology of the Sitka Sound marine core EW0408-40JC and high-resolution SWATH bathymetry both suggest that extensive pyroclastic flow deposits associated with the activity that generated the MEd tephra underlie Sitka Sound, and that any future MEVF activity may pose significant risk to local population centers.     

渝东北黑色页岩元素迁移特征及化学风化程度

黑色页岩是富含有机质和硫化矿物的特殊沉积岩,但人们对其风化过程的元素活动性及矿物风化机制关注较少.为探讨不同地形位置的黑色页岩化学风化过程,采集了渝东北城口某山脊 (A)、近山顶 (B) 和沟谷 (C) 的下寒武统水井沱组黑色页岩风化剖面岩样,利用XRD、XRF和化学分析手段对采集样品的矿物成分、主量元素进行测定分析.元素和矿物的质量迁移系数 (τ) 和质量迁移通量 (M_{j, flux}) 的计算结果表明,黑色页岩风化过程中Ca、Mg和Na元素具有明显的贫化现象,近地表处存在Al元素的富集现象;矿物成分方面,黄铁矿和有机质氧化后形成的酸性水环境,造成方解石、白云石、斜长石等不稳定矿物溶解,并生成含水石膏、铁质氧化物、黏土矿物等次生矿物.不同赋存位置的黑色页岩风化程度有所差异,Na/K-CIA、K/Ca*-Al/Na、A-CN-K和A-CNK-FM图解显示:A剖面处于脱Ca过程的初级风化阶段,B剖面处于脱Ca、Na初期的初等-中等风化阶段,C剖面已发生脱Ca、Na过程,并伴随脱Si作用的中等-强烈风化阶段,结合不同风化指数 (如:CIA、CIW、PIA、MWPI等),得出各剖面的化学风化强弱程度依次为C＞B＞A.      

Geochemistry of organic matter and elements of black shale during weathering in Northern Guizhou,Southwestern China: Their mobilization and inter-connection

Different from previous studies on effect of weathering upon geochemical variation along a single weathered profile, this paper provides a new methodology validated by comparing a weathered outcrop samples and their stratigraphic counterpart un-weathered core samples in a nearby shallow borehole. This outcrop and borehole penetrated the Ordovician-Silurian Wufeng–Longmaxi shales, located in the same anticline structure in the northern part of Guizhou Province, Southern China. The mineral composition, major, trace and rare earth elements (REEs) composition and Rock-Eval parameters of outcrop and core samples were analyzed and compared. Organic matter (OM) was observed in the microscope and extracted for elements analysis. The results show that short-term weathering still has significant influence on OM, mineral and elemental composition of black shales. The elements composition shows the outcrop profile was moderately weathered. The REEs compositions do not alter much during weathering process and the REEs composition and their relative ratios still are valid for rock origin determination. The OM, mainly composed by graptolite and bitumen, even entering the highly-over thermal maturity, is still sensitive to the weathering with a systematic loss 30–50% of TOC along the outcrop profile, which suggests that the OM consumption is predominantly controlled by weathering duration and the distance from the weathering surface. In turn, OM has significant influence on the trace elements transportation behavior during weathering. Some trace elements associated with the OM such as V, Cr, Th, U, Ni and Co, change significantly in their absolute concentration during weathering, but their relative ratios do not necessarily change too much and might be still reliable proxies for paleo-environmental determination. The mobility of shale minerals during weathering is in the following order: plagioclase?>?potassium feldspar and dolomite >pyrite and OM. Short-term weathering can also result in considerable transportation of elements and significant variation of minerals content in black shale, which may pose potentially high environmental and engineering risk in the regions rich in black shale.     

Radiocarbon Ages and Environments of Deposition of the Wono and Trego Hot Springs Tephra Layers in the Pyramid Lake Subbasin, Nevada

Uncalibrated radiocarbon data from core PLC92B taken from Wizards Cove in the Pyramid Lake subbasin indicate that the Trego Hot Springs and Wono tephra layers were deposited 23,200 ± 300 and 27,300 ± 300¹⁴C yr B.P. (uncorrected for reservoir effect). Sedimentological data from sites in the Pyramid Lake and Smoke Creek–Black Rock Desert subbasins indicate that the Trego Hot Springs tephra layer was deposited during a relatively dry period when Pyramid Lake was at or below its spill point (1177 m) to the Winnemucca Lake subbasin. The Wono tephra layer was deposited when lake depth was controlled by spill across Emerson Pass sill (1207 m) to the Smoke Creek–Black Rock Desert subbasin.¹⁸O data from core PLC92B also support the concept that the Trego Hot Springs tephra fell into a relatively shallow Pyramid Lake and that the Wono tephra fell into a deeper spilling lake.     

Critical metals in the critical zone: controls,resources and future prospectivity of regolith-hosted rare earth elements

The rare earth elements (REE) are a group of 17 metals that include the lanthanides, Sc and Y, which are critical for many modern technologies including consumer electronics, medicine and communication. One of the major controls on the concentrations of the REE in regolith material (including soils) is the abundance of these elements in the parent material. It is known that REE concentrations are largely inherited from the protolith rather than acquired during pedogenic processes but our understanding of how pedogenesis affects fractionation and accumulation of REE to produce potentially economic deposits of these critical metals is limited. This study provides a review of (1) the biogeochemical controls on REE distribution and mobility during pedogenesis and (2) the potential for REE extraction from regolith material. Factors that control mobilisation of REE during weathering include (1) the initial distribution of the REE in protolith minerals and the resistivity of these phases to weathering, (2) adsorption and absorption of REE to Fe- and Mn oxides, clay minerals and organic matter and (3) variations in pH and Eh conditions. We also discuss the relative importance of biogeochemical controls on REE mobility in soils in southern Australia, where REE concentrations are demonstrated to be largely a function of weathering of REE-enriched protoliths, the sorption of REE to weathering products and the accumulation of resistant minerals in soils.     

Weathering indices for rhyolitic tuff and granite in Hong Kong

A series of chemical analyses and dry density tests of rhyolitic tuff and granitic samples have been carried out. The test results have been interpreted using a newly developed theoretical weathering model for calculating a mobility index of major elements in rock and soil. It is found that this index is useful for describing changes of rock and soil during weathering and classifying the degrees of weathering. Based on the theoretical model, a new parameter called the volume index, which combines chemical and physical data, is suggested. This index appears to be a good indicator for classifying the degree of weathering in Hong Kong. A quantitative classification scheme is suggested.