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Post-depositional transformations in sedimentary rocks and implications for paleoenvironmental studies: evidence from the Mesoproterozoic (∼1.1 Ga) of the Taoudeni Basin, Mauritania 毛利塔尼亚Taoudeni盆地中元古代(~ 1.1 Ga)沉积岩的沉积后转化及其古环境研究意义
IF 2.9 3区 地球科学 Q3 GEOSCIENCES, MULTIDISCIPLINARY Pub Date : 2022-09-01 DOI: 10.2475/07.2022.02
Mohamed Ghnahalla, A. El Albani, Ahmed Abd Elmola, O. Bankole, C. Fontaine, Mohamed Salem Sabar, A. Trentesaux, Claude Laforest, A. Meunier, Céline Boissard, Chenyi Tu, T. Lyons
Understanding paleoenvironmental dynamics linked to biological evolution in Earth history is a major goal within the geological community. The difficulty of achieving this goal, at least in part, lies with the secondary transformations experienced by a majority of ancient rocks, especially through metamorphism and hydrothermal activity. The Mesoproterozoic (∼1.1 Ga) shallow-marine deposits from the Taoudeni Basin, Mauritania, have suffered a complex, multiphase tectonic, and thermal evolutionary history. Representative samples from two drill cores (a background site [S2] and a dolerite intrusion-bearing drill core [S1]) from the El Mreiti Group were evaluated for transformations and overprints of original mineralogies and geochemical compositions. Our results show that the drill core hosting the dolerite intrusion (S1) is characterized by a suite of minerals (that is, pyroxene, graphite, pyrrhotite, garnet, zeolite, and authigenic clay minerals) resulting from contact metamorphism and associated hydrothermal activity. However, compared to the S1, the S2 core shows no evidence of post-depositional transformation. The geochemical data obtained from S1 reveal a striking elevation of iron contents likely delivered from the hydrothermal fluids. Moreover, concentrations of redox-sensitive trace elements (molybdenum, uranium, and vanadium) increased dramatically during hydrothermal and metamorphic activity. This study demonstrates that need for caution when assessing paleoenvironmental conditions in ancient sedimentary rocks, particularly for iron and trace metal approaches commonly used in reconstructions of paleo-redox.
了解地球历史上与生物进化相关的古环境动力学是地质学界的一个主要目标。实现这一目标的困难至少在一定程度上在于大多数古代岩石经历的二次转化,特别是通过变质作用和热液活动。毛里塔尼亚陶德尼盆地的中元古代(~1.1 Ga)浅海矿床经历了复杂、多期的构造和热演化历史。对来自El Mreiti群的两个岩芯(一个背景场地[S2]和一个含粗玄岩侵入的岩芯[S1])的代表性样品进行了原始矿物和地球化学成分的转化和叠加评估。我们的研究结果表明,粗玄岩侵入体(S1)的岩芯具有接触变质作用和相关热液活动产生的一系列矿物(即辉石、石墨、磁黄铁矿、石榴石、沸石和自生粘土矿物)的特征。然而,与S1相比,S2岩心没有显示沉积后转变的证据。从S1获得的地球化学数据显示,可能由热液流体输送的铁含量显著升高。此外,在热液和变质活动期间,氧化还原敏感微量元素(钼、铀和钒)的浓度急剧增加。这项研究表明,在评估古代沉积岩的古环境条件时需要谨慎,特别是在重建古氧化还原时常用的铁和微量金属方法。
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引用次数: 1
Determination of an intracontinental transform system along the southern Central Asian orogenic belt in the latest Paleozoic 中亚南部造山带晚古生代陆内转换体系的确定
IF 2.9 3区 地球科学 Q3 GEOSCIENCES, MULTIDISCIPLINARY Pub Date : 2022-09-01 DOI: 10.2475/07.2022.01
Jin Zhang, J. Qu, Beihang Zhang, Heng Zhao, Ronggou Zheng, Jianfeng Liu, Jie Hui, Pengfei Niu, Long Yun, Shuo Zhao, Yiping Zhang
Intracontinental transform structures are important forms of continental deformation, such as the Altyn Tagh fault on Tibetan Plateau. Although many intracontinental transform structures have developed throughout geological history, their identification is relatively difficult due to later deformation and sedimentary covering. Strike-slip faults played an important role in the formation and subsequent transformation of the Central Asian orogenic belt (CAOB). In this study, a group of nearly EW-trending dextral shear zones along the southern CAOB in the Beishan, Alxa, northern margin of the North China Craton and the Great Xing'an Mountains to the east, is reported. Regional strike-slip duplex systems were developed and strongly superimposed on the CAOB in the Beishan and Alxa regions. Meanwhile, to the west of the Beishan, coeval ductile shear zones with the same kinematics also developed along the CAOB. The ages of the shear zones range from 280 Ma to 230 Ma and become younger to the east. This megashear system may also connect with the shortening in the Ural Orogenic belt to the west and the convergence along the eastern margin of the Eurasian continent, which is approximately more than 9000 km long in the Asian continent and consists of an intracontinental transform structure in the central Pangea continent. Further west, the dextral shear system may also connect with the coeval shear zones with the same kinematics along the southern Variscan orogenic belt in Europe and even the South Appalachian Orogenic Belt in the southeastern North America, which we call the Intra-Pangean Megashear (IPM) after Irving (2004). The rotation and approach of the Baltic Craton and Siberian Craton and the northern Pangean lithosphere heated by mantle plumes and its lat.eral (eastward) spreading may have caused the development of the IPM and intracontinental deformation from Pangea B to Pangea A.
陆内转换构造是大陆变形的重要形式,如青藏高原阿尔金塔格断裂。尽管在整个地质历史上发育了许多陆内转换构造,但由于后期的变形和沉积覆盖,它们的识别相对困难。走滑断裂在中亚造山带的形成和演化过程中发挥了重要作用。本文报道了阿拉善北山、华北克拉通北缘和大兴安岭东部沿南CAOB的一组近ew向右旋剪切带。在北山和阿拉善地区,区域走滑复式系统发育,并与CAOB有强烈的叠加作用。与此同时,在北山以西,沿CAOB也发育了具有相同运动规律的同时期韧性剪切带。切变带的年龄在280 ~ 230 Ma之间,往东变小。这一巨型盆地系统还可能与乌拉尔造山带向西的缩短和欧亚大陆东缘的辐合有关,该盆地在亚洲大陆长约9000多km,构成盘古大陆中部的陆内转换构造。再往西,右旋剪切系统还可能与欧洲南瓦利斯坎造山带甚至北美东南部南阿巴拉契亚造山带的同时期剪切带有相同的运动学联系,我们以Irving(2004)的名称将其称为泛海内大剪切带(IPM)。波罗的海克拉通和西伯利亚克拉通的旋转和接近,以及地幔柱加热的北盘古岩石圈及其后期。总体(向东)扩张可能导致了泛大陆B到泛大陆A间IPM的发育和陆内变形。
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引用次数: 6
The LSU campus mounds, with construction beginning at ∼11,000 BP, are the oldest known extant man-made structures in the Americas 路易斯安那州立大学校园的土堆始于约11000 BP,是美洲现存最古老的人造建筑
IF 2.9 3区 地球科学 Q3 GEOSCIENCES, MULTIDISCIPLINARY Pub Date : 2022-06-01 DOI: 10.2475/06.2022.02
B. Ellwood, S. Warny, Rebecca A. Hackworth, Suzanne H. Ellwood, Jonathan H. Tomkin, S. Bentley, D. Braud, G. Clayton
Radiocarbon dating of the two LSU Campus Mounds (16EBR6) indicates that the construction of one, Mound B, began at ∼11,000 BP, making Mound B the oldest known and intact manmade structure in the Americas. The age analyses presented here are based on thirty one 14C dates. The older (deeper) parts of both of the LSU Campus Mounds contain many thin, burned ash lenses, suggesting that the Mounds may have been used for ceremonial or cremation purposes. These ash layers are composed mainly of phytoliths, bio-silicate (SiO2) structural compounds in plants that remained after burning of these plants. Analysis of the abundant ash lenses indicates that the plants burned were mainly C4 hydrophilic grasses that are dominated by 90-98% reed and cane plants. The ash layers also contain microscopic fragments of burned, large mammal osteons (bone). The layers of reed and cane phytoliths, containing very small numbers of osteons, are indicative of very hot fires. This finding supports the argument that the fires were used for ceremonials or cremations. No ash beds later than 5,000 BP are known from either LSU Campus Mound A or B, although at ∼800 calBP, a wooden post (now charcoal) was planted and burned on the top of Mound B. It appears that construction of Mound B began during the climate amelioration that followed the Younger Dryas climate event, which ended at ∼11,700 BP. Construction of Mound A appears to have begun at ∼9,500 calBP. Building of the LSU Campus Mounds shows a hiatus when climate deteriorated during the 8200 Climate Event, which defined the end of the Holocene Greenlandian Stage and the beginning of the Northgrippian Stage. Construction began again at ∼7,500 BP, when both mounds continued construction until ∼6,000 BP, with one apparently anomalous date in Mound A at ∼5,100 calBP.
路易斯安那州立大学两个校区丘(16EBR6)的放射性碳年代测定表明,其中一个丘B的建造始于约11000 BP,使丘B成为美洲已知最古老、完整的人造结构。这里给出的年龄分析基于31个14C日期。路易斯安那州立大学校园土丘的较老(较深)部分都有许多薄的、燃烧过的灰烬透镜,这表明土丘可能用于仪式或火葬目的。这些灰层主要由植硅体组成,植物中的生物硅酸盐(SiO2)结构化合物在这些植物燃烧后残留。对大量灰透镜体的分析表明,被烧毁的植物主要是C4亲水草,其中90%至98%为芦苇和甘蔗植物。火山灰层还含有被烧伤的大型哺乳动物骨(骨)的微小碎片。芦苇和甘蔗植硅体层含有非常少量的骨素,表明存在非常热的火灾。这一发现支持了火被用于仪式或火葬的说法。路易斯安那州立大学A校区或B校区均未发现超过5000 BP的火山灰床,尽管在~800 calBP时,在B校区顶部种植并焚烧了一根木桩(现在的木炭)。B校区的建设似乎始于Younger Dryas气候事件后的气候改善期间,该事件于~11700 BP结束。丘A的建造似乎已经在~9500 calBP开始。8200年气候事件期间,当气候恶化时,路易斯安那州立大学校园丘的建设出现了中断,该事件定义了全新世格陵兰阶段的结束和北格里普阶的开始。施工在~7500 BP再次开始,当时两个土堆都在继续施工,直到~6000 BP,其中一个明显异常的日期是在~5100 calBP的土堆A。
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引用次数: 1
Effect of source compositions on adakitic features: A case study from the Buya granite, in western Kunlun, NW China 源成分对阿达基特岩特征的影响——以西昆仑布亚花岗岩为例
IF 2.9 3区 地球科学 Q3 GEOSCIENCES, MULTIDISCIPLINARY Pub Date : 2022-06-01 DOI: 10.2475/06.2022.03
Peng Wang, Guochun Zhao, Qian Liu, Jinlong Yao, Yigui Han, Jianhua Li
Characterized by high Sr/Y (> 40) and La/Yb (> 20) ratios, the adakitic features are, generally, attributed to a garnet-bearing residue. However, adakitic features may be inherited from an adakitic source. In this contribution, we take the Buya granite as an example to demonstrate the effect of source on adakitic features through petrology, zircon U-Pb geochronology and Lu-Hf isotopes, whole-rock geochemistry, and Sr-Nd-Pb isotopes. The 445 Ma Buya granite contains many coeval microgranular magmatic enclaves (MMEs), and some late fine-grained granitic dikes. The host granite and the MMEs display sub-parallel to parallel trace-element patterns characterized by enrichments in LREE and LILE, and depletions in HREE and HFSE, typical of adakitc features. Importantly, the two lithologies possess comparable zircon Hf and whole rock Sr-Nd-Pb isotopic compositions that plot along a mantle array. The MMEs are interpreted as cognate fragments of early-formed crystals and the parental magma was sourced from overriding mantle wedge metasomatized by fluids derived from the subducted Proto-Tethys slab. Comparatively, the dikes, with positive εNd(t) (∼2) values and lower (87Sr/86Sr)t (∼0.705), were products of the partial melting of a plagioclase-rich crustal source. Although the host granite and the MMEs possess adakitic features, the latter have higher La/Yb ratios and relatively lower Sr/Y ratios, which are inconsistent with garnet effect because of similarly high DY/DSr (∼1900) and high DYb/DLa ratios (∼2300) for garnet/melt. Instead, this feature is attributed to the fractional crystallization of hornblende and allanite. Therefore, the host granite and the MMEs may inherit adakitic features from the source. Likewise, the adakitic features of the dikes are attributed to a plagioclase-rich source due to the enrichment in Sr element. The early fractional crystallization of amphibole and biotite in the MMEs can elevate Zr/Sm and lower Nb/Ta ratios, respectively, in the residual liquid to form evolved magma similar to adakites and TTGs. The Buya granite displays an arc affinity, demonstrating the northward subduction of the Proto-Tethys oceanic plate.
以高Sr/Y(>40)和La/Yb(>20)比率为特征的阿达基岩特征通常归因于含石榴石的残留物。然而,阿达基特的特征可能是从阿达基特来源遗传来的。本文以布亚花岗岩为例,通过岩石学、锆石U-Pb地质年代学和Lu-Hf同位素、全岩地球化学和Sr-Nd-Pb同位素,论证了源岩对adakitic特征的影响。445 Ma Buya花岗岩含有许多同时代的细粒岩浆包体(MME)和一些晚期细粒花岗岩脉。寄主花岗岩和MME显示出亚平行至平行的微量元素模式,其特征是LREE和LILE富集,HREE和HFSE贫化,这是典型的阿达基特特征。重要的是,这两种岩性具有相当的锆石Hf和全岩Sr-Nd-Pb同位素组成,沿地幔阵列绘制。MME被解释为早期形成晶体的同源碎片,母岩浆来源于俯冲的原特提斯板块流体交代的覆盖地幔楔。相比之下,具有正εNd(t)(~2)值和较低(87Sr/86Sr)t(~0.705)值的岩脉是富含斜长石的地壳源部分熔融的产物。尽管主花岗岩和MME具有adakitic特征,但后者具有较高的La/Yb比率和相对较低的Sr/Y比率,这与石榴石效应不一致,因为石榴石/熔体的DY/DSr(~1900)和DYb/DLa比率(~2300)同样高。相反,这种特征归因于角闪石和尿囊石的分离结晶。因此,寄主花岗岩和MME可能从源头上继承了adakitic特征。同样,由于Sr元素的富集,岩墙的adakitic特征被归因于富含斜长石的来源。MME中角闪石和黑云母的早期分级结晶可以分别提高残余液体中的Zr/Sm和降低Nb/Ta比率,形成类似于埃达克岩和TTG的演化岩浆。布亚花岗岩表现出弧形亲和性,表明原特提斯洋板块向北俯冲。
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引用次数: 0
A giant sand injection complex: Processes and implications for basin evolution and subsurface fluid flow 一个巨大的注砂复合体:盆地演化和地下流体流动的过程及其意义
IF 2.9 3区 地球科学 Q3 GEOSCIENCES, MULTIDISCIPLINARY Pub Date : 2022-06-01 DOI: 10.2475/06.2022.01
M. Vigorito, A. Hurst, A. Scott, Olivier Stanzione, A. Grippa
Giant sand injection complexes form, intricate, basin-scale fluid plumbing systems and document the remobilisation and intrusion of several tens of cubic kilometres of sand within the shallow crust in stratigraphic units 100's metres thick. This is the first detailed and extensive account of the Panoche Giant Injection Complex (PGIG), a regionally significant outcrop (>300 km2) and part of a larger subsurface development (>4000 km2) identified in boreholes and on seismic reflection data. Magnificent exposure of the PGIC occurs along the north western margin of the San Joaquin Valley and presents the opportunity to examine the regional geological significance of a giant sand injection complex and its origin in the context of a late Cretaceous – early Paleocene forearc basin. Between 25 and 49 km3 of sand were remobilised and injected, at least 0.35 km3 of which extruded onto the paleo-seafloor. Large sandstone intrusions often >10 m thick and laterally extensive on a kilometer scale formed saucer-shaped intrusions, wing-like intrusions and a variety of sill geometries along with volumetrically smaller randomly oriented dikes in a 200–300 m thick interval. Dikes prevail below and above this interval, some reaching the paleo seafloor and extruding sand. Networks of propagating hydrofractures form intensely brecciated host strata, some of which were intruded by sand. All intrusions formed in a single pulsed event in which the most intense hydrofracturing caused by supra-lithostatic fluid pressure occurred approximately 600 to 800 m below the paleo seafloor. A crudely orthogonal arrangement of dikes is preserved with most oriented normal, and less commonly oriented parallel to the oceanic trench associated with the late Mesozoic to early Tertiary North Pacific subduction. Dikes orthogonal to the trench opened against the minimum horizontal stress, which was parallel to the trench. Dikes parallel to the trench opened against the regional maximum horizontal stress along minor faults formed in extension caused by shallow crustal deformation. There is no evidence that compressional tectonics influenced the onset of elevated pore fluid pressure necessary to promote sand injection. However, tectonic compression was responsible for creating the basin physiography that locally increased subsidence and accelerated chemical diagenesis in the basin centre. PGIC outcrop, located along the basin margins, was unlikely to have experienced heating above 70 °C, equivalent about 2 km burial, so the effects of chemical diagenesis in the host strata of the injection complex had negligible potential to evolve significant pore water volume. In a deeper part of the basin approximately 150 km to the south, lateral equivalents of the host strata were subjected to heating >100 °C and would expel significant volumes of water displaced by quartz cementation and clay dehydration that caused lateral pressure transfer to the north and western margin of the basin where the PGIC form
巨大的注砂复体形成了复杂的盆地规模的流体管道系统,并记录了在100米厚的地层单元中,浅层地壳中数十立方公里的沙子的重新活化和侵入。这是对Panoche巨型注入复合体(PGIG)的第一次详细和广泛的描述,这是一个区域性重要的露头(bbb300平方公里),也是在钻孔和地震反射数据中发现的更大的地下开发(>4000平方公里)的一部分。PGIC在圣华金河谷西北边缘的壮观暴露,为研究一个巨大的喷砂复体的区域地质意义及其在白垩纪晚期-古新世早期弧前盆地背景下的起源提供了机会。大约有25到49立方千米的沙子被重新活化和注入,其中至少有0.35立方千米的沙子被挤到了古海底。大型砂岩侵入体厚度通常在10 ~ 10 m左右,在1公里范围内横向扩展,形成碟状侵入体、翼状侵入体和各种基底几何形状,并在200 ~ 300 m厚的区间内形成体积较小的随机定向岩脉。堤防在这个间隔的下面和上面普遍存在,有些到达古海底并挤压沙子。扩展的水力裂缝网络形成了强烈角化的储集层,其中一些被砂侵入。所有侵入体都是在单一脉冲事件中形成的,其中由超静岩流体压力引起的最强烈的水力压裂发生在古海底以下约600至800米处。保存了大致正交的岩脉排列,大多数岩脉定向正向,少数岩脉定向平行于中生代晚期至早第三纪北太平洋俯冲形成的海沟。与壕沟正交的堤防在与壕沟平行的最小水平应力下打开。与海沟平行的岩脉沿浅层地壳变形引起的伸展性小断裂逆区域最大水平应力打开。没有证据表明挤压构造影响了促进注砂所需的孔隙流体压力升高的开始。构造挤压作用形成了局部沉降加剧、盆地中心化学成岩作用加速的盆地地貌。PGIC露头位于盆地边缘,不太可能经历超过70°C的加热,相当于约2公里的埋藏,因此化学成岩作用在注入杂岩的宿主地层中产生显著孔隙水体积的潜力可以忽略不计。在盆地南部约150公里处,宿主地层的侧向等效层受到加热至100°C,并会排出大量的水,这些水是由石英胶结和粘土脱水引起的,导致侧向压力转移到PGIC形成的盆地北部和西部边缘。从盆地深处排出的水的总体积估计表明,相当于总岩石体积减少<1%的流体体积将提供足够的流体预算,以流化和注入形成PGIC的沙子。在面积和垂直范围、体积和结构上,PGIC与北海盆地区域发育的第三纪巨型注入系统有很强的相似性。在这两种情况下,区域注砂都与从更快沉降的盆地中心向盆地边缘转移的压力有关。含水流体来源于深水厚碎屑砂岩和蒙脱质泥岩的热驱动化学成岩作用,或来自更深、地层年龄更老的含水层。
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引用次数: 1
Partial molar volumes of metal oxides in silicate melts: Effects of Coulombic interactions 硅酸盐熔体中金属氧化物的偏摩尔体积:库仑相互作用的影响
IF 2.9 3区 地球科学 Q3 GEOSCIENCES, MULTIDISCIPLINARY Pub Date : 2022-05-01 DOI: 10.2475/05.2022.02
H. Nesbitt, P. Richet, G. Bancroft, G. Henderson
Alkaline, alkaline earth, many 3d and most 4f modifier oxides dissolved in siliceous melts mix non-ideally with SiO2 to produce linear, density-compositional mixing trends from which partial molar volumes of modifier oxides (V*) are determined. An analysis of these experimental data reveals that the partial molar volumes of alkali, alkaline earths, most 4f and many 3d modifier oxides partial molar volumes are accurately reproduced by: where FC = (z+•z−)/d2 (Coulomb's Law) and z indicates charge. The bond length ‘d’ is the sum of the radii of the cation (M+, M2+, M3+ or M4+) and oxide ion (O2−) observed in ionic crystals. The coefficients ‘m’ and ‘b’ are 0.325 and 1.38 Å/atom respectively. Partial molar volumes of network-forming oxides also conform to the above equation where ‘m’ = ∼3.25 and ‘b’ is 1.68 Å/atom. Coulomb's force of attraction (FC) is the product of the cationic field strength (z+/d2) and the charge on an anion, where ‘d’ is the distance separating the centers of the two charges. In silicate melts containing modifier cations, apical O atoms of Si tetrahedra are negatively charged and are displaced toward the cations due to Coulombic attraction. The resulting collapse around the cations is referred to as ‘electrostriction’. Partial molar volumes (V*) of modifier oxides are thus composed of two terms, the volume of the polyhedron of the modifier cation (VPoly) and a volume associated with collapse of tetrahedra around the cation (VCol): VCol is negative for all modifier oxides and becomes increasingly negative with increased charge on the cation and with increased coordination number (CN). VPoly is itself composed of two terms, an intrinsic volume (VInt) and an excluded volume (VEx). The intrinsic volume can be calculated using cationic and O2− radii evaluated from ionic crystals. VEx reflects the state of packing around cationic polyhedra. It is equal to 6.83 Å3/atom for all modifier oxides so that the expression for VPoly is: A linear relationship exists between VPoly and VCol which results in the observed linear density-composition trends from which partial molar volumes are determined. In spite of their linearity, these trends are the result of non-ideal mixing of modifier oxide and SiO2 components in siliceous melts. Our finding that tetrahedra collapse around modifier cations differs from the traditional perspective where modifier cations were considered to occupy voids within the silicate network but otherwise had limited effect on melt structure. These results demonstrate that modifier cations affect the network substantially by causing surrounding tetrahedra to rotate, twist, tilt and flex during their collapse toward modifier cations.
溶解在硅质熔体中的碱性、碱性土、许多3d和大多数4f改性剂氧化物与SiO2非理想混合,产生线性、密度-成分混合趋势,由此确定改性剂氧化物的偏摩尔体积(V*)。对这些实验数据的分析表明,碱、碱土、大多数4f和许多三维改性氧化物的偏摩尔体积可以通过:精确地再现,其中FC = (z+•z−)/d2(库仑定律),z表示电荷。键长' d '是离子晶体中观察到的阳离子(M+, M2+, M3+或M4+)和氧化离子(O2−)的半径之和。系数m和b分别为0.325和1.38 Å/atom。形成网络的氧化物的偏摩尔体积也符合上述方程,其中' m ' = ~ 3.25, ' b ' = 1.68 Å/原子。库仑引力(FC)是阳离子场强(z+/d2)和阴离子上的电荷的乘积,其中d是两个电荷中心之间的距离。在含有改性阳离子的硅酸盐熔体中,硅四面体的顶端O原子带负电荷,并由于库仑吸引而向阳离子偏移。在阳离子周围产生的坍塌被称为“电致伸缩”。因此,改性剂氧化物的偏摩尔体积(V*)由两项组成,即改性剂阳离子多面体的体积(VPoly)和与阳离子周围四面体坍塌相关的体积(VCol): VCol对所有改性剂氧化物都是负的,并且随着阳离子上电荷的增加和配位数(CN)的增加而变得越来越负。VPoly本身由两项组成,固有体积(VInt)和排除体积(VEx)。本征体积可以通过离子晶体的阳离子半径和O2−半径来计算。VEx反映了阳离子多面体周围的堆积状态。对于所有的改性氧化物都等于6.83 Å3/原子,因此VPoly的表达式为:VPoly和VCol之间存在线性关系,这导致了观察到的线性密度组成趋势,由此确定了部分摩尔体积。尽管它们呈线性,但这些趋势是硅质熔体中改性剂氧化物和SiO2组分不理想混合的结果。我们发现四面体在改性剂阳离子周围坍塌,这与传统观点不同,传统观点认为改性剂阳离子占据硅酸盐网络中的空隙,但对熔体结构的影响有限。这些结果表明,改性剂阳离子通过引起周围四面体在向改性剂阳离子坍塌过程中旋转、扭曲、倾斜和弯曲,从而对网络产生实质性的影响。
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引用次数: 0
Weathering intensity and lithium isotopes: A reactive transport perspective 风化强度和锂同位素:反应迁移的观点
IF 2.9 3区 地球科学 Q3 GEOSCIENCES, MULTIDISCIPLINARY Pub Date : 2022-05-01 DOI: 10.2475/05.2022.01
M. Winnick, J. Druhan, K. Maher
Lithium isotopes have emerged as a powerful tool to probe the response of global weathering to changes in climate. Due to the preferential incorporation of 6Li into clay minerals during chemical weathering, the isotope ratio δ7Li may be used to interrogate the balance of primary mineral dissolution and clay precipitation. This balance has been linked to relative rates of chemical and physical denudation, such that dissolved δ7Li (δ7Lidiss) is highest at moderate weathering intensities when chemical and physical denudation are comparable. However, we argue that current theory linking δ7Li to weathering regimes through fluid travel times are unable to explain observations of low δ7Li and high Li concentrations in rapidly eroding settings. In this study, we re-examine the relationships between δ7Li, Li concentration, and weathering regime by incorporating Li isotopes into simulations of weathering profiles using a reactive transport model (CrunchFlow) that includes advective fluxes of regolith to simulate variable erosion rates in response to uplift. In these simulations, fractionation is implemented through a kinetic fractionation factor during clay precipitation, which allows the δ7Li of dissolved and suspended loads in the model to vary as a function of Li/Al ratios in primary and secondary minerals. When the model is run over a range of infiltration and erosion rates, simulations reproduce observed global patterns of δ7Lidiss and suspended load δ7Li as a function of weathering intensity, controlled primarily by water travel times and mineral residence times in weathered bedrock. We find that reduced water travel times at low weathering intensity, however, are inconsistent with observations of high Li concentrations. As an alternative, we demonstrate how the rapid weathering of soluble, Li-rich minerals such as chlorite under low weathering intensities may resolve this apparent discrepancy between data and theory. We also suggest that observed patterns are consistent with geothermal Li sources under low weathering intensities. This work offers a foundation guiding future studies in testing potential mechanisms underlying global riverine δ7Lidiss.
锂同位素已经成为探测全球风化对气候变化反应的有力工具。由于6Li在化学风化过程中优先与粘土矿物结合,因此δ7Li同位素比值可用于考察原生矿物溶解与粘土降水的平衡。这种平衡与化学和物理剥蚀的相对速率有关,因此,当化学和物理剥蚀相当时,溶解的δ7Li (δ7Lidiss)在中等风化强度下最高。然而,我们认为,目前的理论通过流体运动时间将δ7Li与风化机制联系起来,无法解释在快速侵蚀环境中观察到的低δ7Li和高Li浓度。在这项研究中,我们利用反应输运模型(CrunchFlow)将Li同位素纳入风化剖面模拟,重新研究了δ7Li、Li浓度和风化状态之间的关系,该模型包括风化层的平流通量,以模拟响应隆升的可变侵蚀速率。在这些模拟中,分馏是通过粘土沉淀过程中的动力学分馏因子来实现的,这使得模型中溶解和悬浮载荷的δ7Li随原生矿物和次生矿物中Li/Al比值的变化而变化。当模型在一定的入渗和侵蚀速率范围内运行时,模拟再现了观测到的δ7Lidiss和悬浮载荷δ7Li的全球模式,这些模式作为风化强度的函数,主要受风化基岩中的水移动时间和矿物停留时间的控制。然而,我们发现在低风化强度下减少的水行时间与高Li浓度的观测结果不一致。作为一种替代方案,我们证明了在低风化强度下,绿泥石等可溶性富锂矿物的快速风化可以解决数据和理论之间的明显差异。我们还认为观测到的模式与低风化强度下的地热锂源一致。这项工作为未来研究全球河流δ7Lidiss的潜在机制提供了指导基础。
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引用次数: 5
Proterozoic-Mesozoic development of the Quanji block from northern Tibet and the cratonic assembly of eastern Asia 藏北全济地块元古宙-中生代发育与东亚克拉通组合
IF 2.9 3区 地球科学 Q3 GEOSCIENCES, MULTIDISCIPLINARY Pub Date : 2022-05-01 DOI: 10.2475/05.2022.03
Chao Wang, D. Evans, Meng Li, Ji-Heng Zhang, Jian Han, Bin Wen, J. Wang, Junming Zhao
Unraveling the timing, location, and mechanisms of cratonic aggregation in Earth's continental jigsaw puzzle is a key factor for plate tectonic reconstructions. The Quanji Block (QB) is a sliver of anomalously old and well-preserved continental crust embedded within the Paleozoic-Mesozoic tectonic collage of the northeastern Tibetan Plateau and has played a critical role in Proto-/Paleotethys paleogeographic reconstructions. New geological mapping, stratigraphic logging, and geochronological analysis lead to a refined understanding of QB's history from Paleoproterozoic to present. Deposited atop a largely Paleoproterozoic basement, the Quanji Group records rifting and epicratonic cover at 1.7 to 1.6 Ga. The Quanji Group is unconformably overlain by the Xiaogaolu Group, which preserves black shale, Ediacaran-type Charnia, ribbon-shaped fossils and a late Ediacaran glaciation. U-Pb detrital zircon ages from Cambrian Olongbuluke Group marine platform deposits are quite different from ages in underlying units, with a minor component of Neoproterozoic (880–815 Ma) ages. The apparent change in detrital zircon sources coincides with a regionally expressed Great Unconformity during the Precambrian–Cambrian transition. The new stratigraphy and U-Pb geochronology of QB suggest that the late Paleoproterozoic to Cambrian history of QB has a remarkable similarity to that of the southern margin of North China Block (NCB), indicating that the QB has been displaced dextrally from an initial location adjacent to NCB. The transform motion occurred in stages between ca. 350 and 200 Ma, which suggests that transform tectonism appears to be an essential element of any viable model for kinematic development of the Paleo-Tethyan oceanic domains and the ultimate cratonic assembly of eastern Asia.
在地球大陆的拼图中,揭示克拉通聚集的时间、位置和机制是板块构造重建的关键因素。全济地块(QB)是青藏高原东北部古生代—中生代构造拼贴中的一条异常古老且保存完好的大陆地壳,在古地理重建中起着重要作用。新的地质填图、地层测井和地质年代学分析使我们对QB从古元古代至今的历史有了更精确的了解。全济群主要沉积于古元古代基底之上,在1.7 ~ 1.6 Ga时期有裂谷和地壳覆盖。全济群被小高鲁群不整合覆盖,保存有黑色页岩、埃迪卡拉型Charnia、带状化石和晚埃迪卡拉冰期。寒武系奥龙布鲁克群海相台地沉积物U-Pb碎屑锆石年龄与下伏单元年龄差异较大,仅占新元古代(880 ~ 815 Ma)年龄的一小部分。碎屑锆石来源的明显变化与前寒武纪-寒武纪过渡时期区域性表现的大不整合一致。新地层学和U-Pb年代学表明,秦岭晚古元古代至寒武纪的历史与华北地块南缘的历史具有显著的相似性,表明秦岭从与华北地块相邻的初始位置向右偏移。转换运动发生在约350至200 Ma之间的阶段,这表明转换构造运动似乎是古特提斯洋域运动发展和东亚最终克拉通组合的任何可行模型的基本要素。
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引用次数: 2
New Idria serpentinite protrusion, Diablo Range, California: From upper mantle to the surface 加利福尼亚州迪亚布罗山脉新Idria蛇纹岩凸起:从上地幔到地表
IF 2.9 3区 地球科学 Q3 GEOSCIENCES, MULTIDISCIPLINARY Pub Date : 2022-04-01 DOI: 10.2475/04.2022.01
R. Coleman, J. Gooley, R. Gregory, S. Graham
The New Idria serpentinite body in the Coast Ranges of California is a diapir that resulted from the interaction of the migrating Mendocino trench-ridge-transform fault triple junction, transpression, metasomatic fluids, and previously subducted oceanic crust and mantle. Northward propagation of the San Andreas fault progressively eliminated the original subduction zone, allowing seawater to penetrate into the formerly subducting abyssal peridotite mantle, triggering serpentinization. The associated physical changes in density, volume, and strength yielded an expanding, buoyantly rising serpentinite protrusion, facilitated by transpression along the San Andreas fault. Sedimentary facies and intrusion of minor cross cutting syenite and alkali basalt dikes indicate that the serpentinization-driven diapir buoyantly rose and widely breached the surface by ca. 14 Ma, attending migration of the Mendocino Triple Junction past the latitude of New Idria.
加利福尼亚海岸山脉的新伊德里亚蛇纹岩体是迁移性门多西诺沟-脊-转换断层三结、逆冲、交代流体和先前俯冲的海洋壳幔相互作用形成的底辟岩。圣安德烈亚斯断层的向北扩展逐渐消除了原来的俯冲带,使海水渗透到以前俯冲的深海橄榄岩地幔中,引发蛇纹岩化。相关的物理变化在密度、体积和强度上产生了一个扩张的、浮力上升的蛇纹岩突出,这是由沿着圣安德烈亚斯断层的挤压推动的。少量横切正长岩和碱性玄武岩岩脉的沉积相和侵入表明,在约14 Ma时,由蛇纹岩作用驱动的底喷岩浮升并广泛突破地表,参与了门多西诺三结带越过新伊德里亚纬度的迁移。
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引用次数: 1
From Gondwana rifting to Alpine orogeny: Detrital zircon geochronologic and provenance signals from the Kopet Dagh Basin (NE Iran) 从冈瓦纳大陆裂谷作用到阿尔卑斯造山运动:来自Kopet-Dagh盆地(伊朗东北部)的碎屑锆石地质年代和物源信号
IF 2.9 3区 地球科学 Q3 GEOSCIENCES, MULTIDISCIPLINARY Pub Date : 2022-04-01 DOI: 10.2475/04.2022.02
A. Mohammadi, J. Ruh, M. Guillong, O. Laurent, L. Aghajari
The Kopet Dagh mountains in NE Iran exhibit a 7-km-thick continuous sedimentary sequence recording detritus from exposed surrounding terranes from the last 175 Ma. This work presents a multi-disciplinary geochronologic and provenance analysis in an attempt to identify and date major geologic events along the northern segment of the Tethys and reconstruct the regional tectonic history from Gondwana-related rifting until the Alpine orogeny. Sandstone framework, heavy mineral analysis, U-Pb dating of detrital zircons, and Hf-isotope ratio measurements on dated zircons from Triassic to Paleocene sandstones indicate three main tectonic events that include Early Silurian intracontinental rifting (opening of Paleo-Tethys), Early Carboniferous rifting of a back-arc basin (Aghdarband Complex), and Late Triassic collisional to post-collisional magmatism (Paleo-Tethys collision). Mineralogical and age peak considerations indicate that detritus was supplied from the south into the extensional Kopet Dagh Basin during Middle Jurassic, while Cretaceous to Paleocene sandstones show signs of increasing recycling.
伊朗东北部的Kopet-Dagh山脉显示了一个7公里厚的连续沉积序列,记录了最近175 Ma暴露的周围地体的碎屑。这项工作提出了一个多学科的地质年代和物源分析,试图确定和确定特提斯北段的主要地质事件的年代,并重建从冈瓦纳大陆相关的裂谷作用到阿尔卑斯造山运动的区域构造历史。砂岩骨架、重矿物分析、碎屑锆石的U-Pb测年以及三叠纪至古新世砂岩测年锆石的Hf同位素比值测量表明了三个主要的构造事件,包括志留系早期陆内裂谷(古特提斯开口)、石炭系早期弧后盆地裂谷(Aghdarband杂岩),以及晚三叠纪碰撞-碰撞后岩浆作用(古特提斯碰撞)。矿物学和年龄峰值考虑表明,碎屑在侏罗纪中期从南部供应到伸展的Kopet-Dagh盆地,而白垩纪至古新世砂岩显示出越来越多的回收迹象。
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引用次数: 2
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