Chemical Geology最新文献_第2页

The chemical index of alteration (CIA) and interpretation of ACNK diagrams 化学蚀变指数（CIA）和 ACNK 图的解释

IF 3.6 2区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Chemical Geology

Pub Date : 2024-11-08 DOI: 10.1016/j.chemgeo.2024.122474

Thomas J. Algeo , Hanlie Hong , Chaowen Wang

The chemical index of alteration (CIA) and ACNK ternary diagrams (i.e., with apices of Al₂O₃, CaO + Na₂O, and K₂O) are among the most widely used tools for evaluation of chemical weathering patterns in soils and sediments. However, incorrect use of these tools has become prevalent in recent literature, necessitating a “corrective” discussion of best practices for the CIA proxy and ACNK diagrams, their implications for the chemical weathering intensity of transported sediments, and the recognition of metasomatic effects in deep-time formations. As shown by the diversity of weathering trends in modern soil profiles, the assumption of an “ideal weathering trend” (IWT) for paleosols that runs parallel to the A-CN axis is generally unjustified. Use of an IWT to infer the presence of excess potassium (K) in a paleosol can lead to incorrect inferences of K-metasomatism and to unwarranted “corrections” of CIA values. Protolith reconstruction from a weathering path requires that the latter form a well-defined linear array in ACNK space that can be accurately projected backward to the feldspar join; projections based on poorly defined data arrays are meaningless. In transported sediments, the interpretation of ACNK data as a weathering proxy is further complicated by factors such as hydraulic sorting, two-component mixing systems, and sediment recycling. In metasedimentary formations, metasomatic alteration commonly disrupts weathering paths in ACNK space accompanied by pronounced shifts toward more K- (or Ca-Na-)rich compositions; however, it is doubtful whether this process can operate on a linear sample array representing a well-defined weathering path and shift all samples in a coordinated manner to produce a new array with an equal degree of alignment but an entirely different slope. Although K-enrichment of Paleoproterozoic paleosols has been widely attributed to K-metasomatism, the potential role of supergene weathering linked to rising atmospheric oxygen levels in promoting pedogenic K retention through early illitization of clay minerals requires consideration. For maximum robustness, studies of weathering in paleosols and paleoformations must combine information for the mobile bases (i.e., Ca-Na-K, in the form of the CIA proxy and ACNK relationships) with compositional data for Fe-Mg, SiO₂, and resistate elements as well as petrographic data documenting mineral phase relationships and sample paragenetic histories. In summary, more nuanced use and interpretation of the CIA proxy and ACNK diagrams are necessary.

化学蚀变指数（CIA）和 ACNK 三元图（即顶点为 Al2O3、CaO + Na2O 和 K2O）是评估土壤和沉积物化学风化模式最广泛使用的工具之一。然而，这些工具的错误使用在最近的文献中非常普遍，因此有必要对 CIA 代用图和 ACNK 图的最佳实践、它们对迁移沉积物化学风化强度的影响以及对深时地层中的元气效应的认识进行 "纠正性 "讨论。正如现代土壤剖面中风化趋势的多样性所显示的那样，假设古溶胶的 "理想风化趋势"（IWT）与 A-CN 轴平行一般是不合理的。使用 IWT 来推断古溶岩中是否存在过量的钾（K），可能会导致错误的钾金属化推断，并对 CIA 值进行不必要的 "修正"。根据风化路径重建原岩要求后者在 ACNK 空间中形成一个定义明确的线性阵列，可以准确地向后投影到长石连接处；基于定义不清的数据阵列的投影是没有意义的。在运移沉积物中，由于水力分选、双组分混合系统和沉积物循环等因素，将 ACNK 数据解释为风化替代物变得更加复杂。在新元古代地层中，元气蚀变通常会扰乱 ACNK 空间的风化路径，并伴随着向富含更多 K（或 Ca-Na）成分的明显偏移；然而，这一过程能否在代表明确风化路径的线性样本阵列上运行，并以协调的方式移动所有样本，从而产生一个具有相同排列程度但斜率完全不同的新阵列，这一点值得怀疑。虽然古近纪古溶胶的钾富集被广泛归因于钾-金属化作用，但还需要考虑与大气氧含量上升有关的超基因风化作用的潜在作用，即通过粘土矿物的早期灰化来促进成土钾的保留。为了获得最大的稳健性，古溶胶和古地貌中的风化作用研究必须将移动碱基信息（即以 CIA 代用和 ACNK 关系为形式的 Ca-Na-K）与 Fe-Mg、SiO2 和抗蚀元素的成分数据以及记录矿物相关系和样品成因历史的岩石学数据结合起来。总之，有必要对 CIA 代用图和 ACNK 图进行更细致的使用和解释。

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引用次数: 0

Trace metal evolution of the Late Cretaceous Ocean 晚白垩世海洋的痕量金属演变

IF 3.6 2区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Chemical Geology

Pub Date : 2024-11-07 DOI: 10.1016/j.chemgeo.2024.122477

Mingzhao Sun , Corey Archer , Florian Scholz , Tim Sweere , Derek Vance

The Cenomanian-Turonian boundary (Late Cretaceous) witnessed the last spectacular manifestation of Mesozoic Anoxic Events (OAE 2,

\sim

94 Ma), marked by a prominent carbon isotope excursion (CIE) and burial of organic-matter-rich sediments under high atmospheric CO₂ concentrations. But the Late Cretaceous generally was a time of profound environmental change. OAE 2 was preceded by other CIEs, including the Mid-Cenomanian Event (MCE), and was punctuated by a short re‑oxygenation and cooling event (the Plenus Cold Event, PCE). Extensive previous studies, including many trace metal studies, have focused on OAE 2, but there is still debate concerning the degree of drawdown of oceanic trace metal reservoirs during OAE 2, whether this drawdown is global or local, its causes and consequences for ocean ecology. Here, we present records of eight trace metals, over about 5 Myr of the Late Cretaceous, from the Tarfaya Basin in the proto-North Atlantic. The long records from a core preserving a continuous sedimentary succession allow us to set changes occurring across OAE 2 in the broader context of Late Cretaceous, including the lead up to OAE 2. Moreover, the multiple trace metal dataset allows us to broadly investigate the oceanographic setting in the context of recent studies of multiple trace metals in modern organic-rich sediments aimed at refining the proxies.

Trace metal enrichments in these organic-rich sediments are discussed on three different timescales. Firstly, comparison of these Late Cretaceous sediments with modern organic-rich sediments are consistent with deposition in an open ocean upwelling margin in the Late Cretaceous, very like the modern Peru or Namibian Margin, although the deep proto-North Atlantic was probably partially restricted. Secondly, in common with previous studies, metal/TOC ratios often show sharp drops in the early part of OAE 2. Thirdly, however, this sharp drop occurs within a framework of pseudo-cyclical variations in metal/TOC, with a period of about 143

\pm

19 kyr (1 SD), that is a feature of these long records well before OAE 2, including across the MCE. Different metals respond differently to the perturbation in the early part of OAE 2 itself. Simple mass balance considerations suggest that trace metal drawdown with organic carbon must be at least partially compensated by changes in the rate of chemical weathering on the continents, as previously inferred from Li and Ca isotopes. Moreover, changes in the patterns of variation between different metals, as well as covariation of metal/TOC ratios and Os isotopes, hint at changes in the pattern of chemical weathering, most prominently in the contribution of mafic rocks to the chemical weathering flux.

中生代缺氧事件（OAE 2，∼94 Ma）在仙人-土伦边界（晚白垩世）最后一次出现，其特点是碳同位素显著偏移（CIE）以及富含有机物质的沉积物在大气二氧化碳高浓度下被掩埋。但总体而言，晚白垩世是一个环境发生深刻变化的时期。在 OAE 2 之前还发生过其他 CIE，包括中白垩世事件（MCE），并有一个短暂的复氧和冷却事件（Plenus 冷事件，PCE）。以往的大量研究，包括许多痕量金属研究，都集中于 OAE 2，但关于 OAE 2 期间海洋痕量金属储库的缩减程度、这种缩减是全球性的还是局部性的、其原因以及对海洋生态学的影响等问题仍存在争议。在这里，我们展示了来自原北大西洋塔尔法亚盆地的八种痕量金属在晚白垩世约 5 百万年的记录。来自一个保留了连续沉积演替的岩心的长期记录，使我们能够将整个 OAE 2 发生的变化放在晚白垩世的大背景下进行分析，包括 OAE 2 之前的变化。此外，多种痕量金属数据集使我们能够结合近期对现代富含有机质沉积物中多种痕量金属的研究，对海洋环境进行广泛调查，以完善代用指标。首先，将这些晚白垩世沉积物与现代富含有机质沉积物进行比较，结果表明，晚白垩世沉积物沉积在一个开阔的海洋上升流边缘，非常类似于现代的秘鲁或纳米比亚边缘，尽管原北大西洋深层可能受到部分限制。其次，与之前的研究一样，金属/TOC 比值在 OAE 2 早期经常出现急剧下降。第三，这种急剧下降发生在金属/TOC假周期变化的框架内，周期约为 143 ±19 kyr（1 SD），这是 OAE 2 之前这些长记录的一个特征，包括整个 MCE。不同的金属对 OAE 2 早期的扰动有不同的反应。简单的质量平衡考虑表明，痕量金属随有机碳的减少必须至少部分地被大陆化学风化速率的变化所补偿，正如以前从锂和钙同位素推断的那样。此外，不同金属之间的变化规律，以及金属/有机碳比率和 Os 同位素的共变，都暗示了化学风化模式的变化，其中最突出的是岩浆岩对化学风化通量的贡献。

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引用次数: 0

The effects of carbonate contamination on Ni-Cu-PGE deposit genesis in the Platreef, northern Bushveld Complex: A case study using Niggli numbers 碳酸盐污染对布什维尔德复合体北部 Platreef 镍-铜-金矿床成因的影响：利用尼格里数进行案例研究

IF 3.6 2区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Chemical Geology

Pub Date : 2024-11-06 DOI: 10.1016/j.chemgeo.2024.122481

Erin S. Thompson , David A. Holwell , Iain McDonald , Marc Reichow , Thomas G. Blenkinsop , Hannah S.R. Hughes , Katie McFall , Kate R. Canham , Matthew A. Loader , Lara Du Preez , Kofi Acheampong , Andy Lloyd

Crustal contamination in Ni-Cu-PGE deposit genesis is generally regarded as an essential, or at least highly beneficial, process in triggering sulfide saturation, either through the addition of external sulfur or as a mechanism for increasing oxygen fugacity through the incorporation of volatile-rich lithologies. The Platreef, northern limb of the Bushveld Complex, South Africa, forms one of the world's largest resources of platinum-group elements (PGE), with additional Ni-Cu-Co mineralisation, and represents a unique deposit, intersecting numerous footwall lithologies from the underlying Transvaal Supergroup. In this study, Niggli Numbers, a geochemical tool used to classify rocks on the molecular proportions of their major element geochemistry, are used to examine the degree and styles of contamination in the Platreef at three locations: Tweefontein, Overysel, and a newly drilled deeper section at Sandsloot. The potential impact of these varying contamination styles on PGE-Ni-Cu mineralisation is then discussed. At Tweefontein and Overysel, the highest PGE-Ni-Cu grades are found in largely uncontaminated pyroxenites, exemplified by Niggli c values <20. Where carbonate contamination is strong, and Niggli c exceeds 30, it is not associated with elevated grades. In contrast, at Sandsloot elevated PGE and Ni grades are strongly associated with carbonate contamination, with Niggli c commonly exceeding 20, and Niggli mg exceeding 0.8, indicating dolomitic contamination, in the highest-grade horizons. Although other pre-emplacement models may yet explain the elevated grades observed at Sandsloot, there remains a clear correlation between interactions with the Malmani dolomite and elevated Ni-Cu-PGE contents which warrants further investigation.

镍-铜-铂-铜矿床成因中的地壳污染通常被认为是引发硫化物饱和的一个基本过程，或者至少是一个非常有益的过程，这可能是通过增加外部硫，或者是通过富含挥发性岩性增加氧气逸度的一种机制。南非布什维尔德复合体北缘的 Platreef 构成了世界上最大的铂族元素（PGE）资源之一，还有镍-铜-钴矿化物，是一个独特的矿床，与下伏特兰士瓦超群的众多岩性相交。在这项研究中，使用了尼格利数（一种用于根据主要元素地球化学分子比例对岩石进行分类的地球化学工具）来检查 Platreef 三个地点的污染程度和污染类型：Tweefontein、Overysel 和 Sandsloot 新钻探的较深地段。然后讨论了这些不同的污染类型对 PGE-Ni-Cu 矿化的潜在影响。在 Tweefontein 和 Overysel，PGE-Ni-Cu 品位最高的是基本未受污染的辉长岩，例如 Niggli c 值为 <20。在碳酸盐污染严重、Niggli c 值超过 30 的地方，与品位升高无关。相反，在 Sandsloot，PGE 和 Ni 品位的升高与碳酸盐污染密切相关，在最高品位的地层中，Niggli c 通常超过 20，Niggli mg 超过 0.8，表明存在白云质污染。虽然其他成矿前模型也可以解释在 Sandsloot 观察到的品位升高，但与马尔马尼白云岩的相互作用与镍-铜-铂-镍矿石含量升高之间仍存在明显的关联，值得进一步研究。

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引用次数: 0

Provenance and tectonic intricacies revealed by in situ Rb-Sr dating of detrital micas 通过对碎屑岩进行掺铒锶原位定年揭示出产地和构造的复杂性

IF 3.6 2区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Chemical Geology

Pub Date : 2024-11-06 DOI: 10.1016/j.chemgeo.2024.122482

Dheerej H. Sathish Kumar , Hugo K.H. Olierook , Milo Barham , Christopher L. Kirkland , Kai Rankenburg , Noreen J. Evans

The use of detrital minerals to reconstruct sedimentary provenance is subject to a range of biases, which may reduce the value of the geological information they retain. With the advent of in situ Rb-Sr geochronology, rapid analyses of a representative (>100) number of grains can now be performed on radiogenic Sr-bearing minerals to provide more comprehensive knowledge of source-to-sink processes through more diversified mineral analysis. Here, we test the in situ Rb-Sr technique on detrital white mica and biotite from basin margin and axis samples in a simple rift basin (Perth Basin, Australia) for which previously published detrital zircon U-Pb and Hf isotopic data exist. For the basin margin adjacent to Archean basement, which has unimodal detrital zircon ages at c. 2600 Ma, two mica samples reveal a dominant age mode at c. 500 Ma, reflecting thermal resetting in the craton proximal to the Gondwanan orogenic front, and a subordinate portion of Archean mica detritus that can be interpreted as having been distally sourced (>100 km away). Similarly, a basin axis sample yielded minor c. 1200 Ma apparent mica ages, representing distal sources from the Albany-Fraser-Wilkes Orogen, and c. 500 Ma grains that are likely a resetting product. Whilst obscured in the detrital zircon record, mica samples allow quantification of the relative contributions of distal and proximal sources to the basin. Finally, detrital biotite that was (partially) altered to chlorite yielded partially to fully reset ages between c. 500 and 130 Ma, the latter linked to heating from the c. 137–130 Ma Bunbury Basalt. Ultimately, the use of in situ Rb-Sr geochronology from detrital micas reveal previously unrecognized provenance and tectonic information that is critical to understanding the true complexity of ancient geological histories, but which remains obscured in standard detrital zircon U-Pb geochronology.

利用碎屑岩矿物重建沉积产地受到一系列偏差的影响，这可能会降低它们所保留的地质信息的价值。随着原位 Rb-Sr 地质年代学的出现，现在可以对具有代表性（100 个）的含放射性 Sr 的矿物颗粒进行快速分析，通过更多样化的矿物分析，更全面地了解从源到汇的过程。在这里，我们对来自一个简单裂谷盆地（澳大利亚珀斯盆地）的盆地边缘和轴线样本中的碎屑白云母和生物云母进行了原位Rb-Sr技术测试，之前已经公布了这些样本的碎屑锆石U-Pb和Hf同位素数据。对于邻近阿切安基底的盆地边缘，其锆英石碎片的单模式年龄约为 2600 Ma，两个云母样品显示了约 500 Ma 的主要年龄模式，反映了冈瓦纳造山带前沿附近克拉通的热重置，以及可解释为远源（100 公里以外）的阿切安云母碎屑的次要部分。同样，在一个盆地轴样本中也发现了约 1200 Ma 的表观云母年龄，代表了阿尔巴尼-弗雷泽-威尔克斯造山带的远端来源，以及约 500 Ma 的颗粒，很可能是重置产物。云母样品虽然在锆英石记录中被遮挡，但可以量化远源和近源对盆地的相对贡献。最后，被（部分）改变为绿泥石的非晶质生物岩产生了约 500 至 130 Ma 之间的部分至完全重置年龄，后者与约 137-130 Ma 邦伯里玄武岩的加热有关。最终，利用原位掺杂铷-锶的云母地质年代学揭示了以前未认识到的产地和构造信息，这些信息对于了解古代地质历史的真正复杂性至关重要，但在标准的云母锆石U-Pb地质年代学中却被掩盖了。

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引用次数: 0

Intra-grain variability of hydrogen and trace elements in rutile 金红石中氢和微量元素的晶粒内变化

IF 3.6 2区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Chemical Geology

Pub Date : 2024-11-05 DOI: 10.1016/j.chemgeo.2024.122480

Mona Lueder , Jörg Hermann , Renée Tamblyn , Daniela Rubatto , Pierre Lanari , Thorsten Andreas Markmann

Rutile is a commonly used petrogenetic indicator mineral to determine metamorphic temperatures, ages, host−/source lithologies, geochemical reservoirs, and subduction conditions. However, intra grain variabilities of trace elements in rutile are rarely considered. We performed trace element and hydrogen mapping of rutile to assess zoning and diffusion in natural rutile from various lithologies. Trace element and hydrogen show distinct zoning patterns, with mostly regular zoning in rutile from pegmatites and low-T hydrothermal clefts, and typically irregular zoning in rutile from high-P veins and metamorphic rocks. Whereas no clear patterns of trace element correlations can be identified, hydrogen, tri-, tetra- and pentavalent cations can show the same zoning patterns within single rutile grains, despite different substitution mechanisms. This indicates that hydrogen and trace element incorporation is externally controlled by availability and diffusivity of hydrogen and trace elements within the rock matrix, as well as rutile growth rates. H₂O mapping reveals that hydrogen is retained in rutile at temperatures of up to ∼650 °C. Coupled substitution of hydrogen with divalent and trivalent cations requires coupled diffusion processes for charge balance if hydrogen is diffusively re-equilibrated. Slow diffusion rates and thus relatively high temperatures for diffusive closure in rutile lead to retention of primary hydrogen and trace element zoning. At high-T conditions of >650 °C, diffusive re-equilibration of all trace elements can be observed. Complex zoning patterns of Zr in rutile show that Zr incorporation into rutile is not purely temperature dependent. In this study, Zr-undersaturation can lead to inaccurate Zr-in-rutile temperatures of up to ∼35 °C difference from peak formation temperatures within single rutile grains and might be a useful tool to evaluate rutile growth conditions. Niobium and Ta are highly zoned in rutile, leading to extremely variable Nb/Ta ratios within single rutile grains that cannot be reconstructed from single spot analyses. Overall, mapping offers a novel and promising tool to understanding trace element behavior in rutile.

金红石是一种常用的岩石成因指示矿物，用于确定变质温度、年龄、主岩/源岩性、地球化学储层和俯冲条件。然而，人们很少考虑金红石中微量元素的晶粒内变异。我们对金红石进行了微量元素和氢元素绘图，以评估来自不同岩性的天然金红石中的分带和扩散情况。微量元素和氢元素显示出明显的分带模式，伟晶岩和低T热液裂隙中的金红石大多具有规则的分带，而高P矿脉和变质岩中的金红石则具有典型的不规则分带。虽然无法确定微量元素的明确关联模式，但氢、三价、四价和五价阳离子在单个金红石晶粒中可以显示出相同的分带模式，尽管取代机制不同。这表明，氢和微量元素的掺入受岩石基质中氢和微量元素的可用性和扩散性以及金红石生长速度的外部控制。氢氧化物分布图显示，氢在温度高达 ∼650 ℃时仍保留在金红石中。氢与二价和三价阳离子的耦合置换需要耦合扩散过程，如果氢是扩散再平衡的，则需要电荷平衡。金红石中的扩散速率较慢，因此扩散闭合的温度相对较高，这导致了原氢和微量元素分区的保留。在 650 °C 的高温条件下，可以观察到所有微量元素的扩散再平衡。金红石中 Zr 的复杂分带模式表明，金红石中 Zr 的掺入并不完全取决于温度。在这项研究中，Zr过饱和可导致金红石中的Zr温度不准确，与单个金红石晶粒内的峰值形成温度相差高达 ∼ 35 °C，这可能是评估金红石生长条件的有用工具。铌和钽在金红石中高度分区，导致单个金红石晶粒内的铌/钽比率变化极大，无法通过单点分析进行重建。总之，制图为了解金红石中微量元素的行为提供了一种新颖而有前途的工具。

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引用次数: 0

Pyrite geochemistry and its implications for ore formation in reduced copper skarn systems: A case study of the Saibo deposit, Northwest China 黄铁矿地球化学及其对还原铜矽卡岩系统矿石形成的影响：中国西北赛博矿床案例研究

IF 3.6 2区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Chemical Geology

Pub Date : 2024-11-03 DOI: 10.1016/j.chemgeo.2024.122478

Qing-Fei Sun , Ju-Xing Tang , Shun-Da Li , Qiu-Ming Pei , Xin-Hao Sun , Chuan Chen , Ling-Ling Gao , Xiao-Fei Du , Jin-Ling Xie

Copper skarns are the main source of Cu worldwide, and they are classified into two types according to their redox state: oxidized and reduced. Numerous studies have investigated the mineralization processes of oxidized Cu skarn deposits in terms of pyrite trace element compositions. However, it remains unclear how the compositional and micro-textural features of pyrite are linked to specific physicochemical processes in reduced Cu skarn systems. The Saibo deposit in the northern part of the western Tianshan orogenic belt is a typical reduced Cu skarn deposit, characterized by abundant Ti-rich garnet, arsenopyrite, pyrrhotite, and CH₄-rich inclusions. In this study, we examined pyrite geochemical and micro-textural features and related these features to Cu-forming processes at Saibo. Copper skarn mineralization was recognized to occur in the contact zone between the granodiorite porphyry and the Kusongmuqieke Group limestone as veins, bands, massive, and disseminations. Five stages were identified: (I) prograde skarn, (II) retrograde skarn, (III) quartz–pyrite–arsenopyrite (Py1), (IV) quartz–polymetallic sulfides (Py2), and (V) quart–calcite–polymetallic sulfides (Py3).

Pb, Sb, Ag, Cu, Zn, and Bi were typically present in the host pyrite as micro- and nanoscale mineral inclusions, whereas Co, Ni, Se, Te, and As were mostly present as lattice substitutions for S and Fe. In situ δ³⁴S values of Py1 to Py3b for the Cu skarn ore at Saibo defined a narrow range from 4.86 ‰ to 7.67 ‰, which was well consistent with the published S values of sulfide separates (0.2 to 9.1 ‰). These values are similar to those of Middle–Late Devonian granitoids, such as the Lamasu plagiogranite, and different from those of sediments. This indicates that the dominant source of sulfides in the Saibo deposit is the Middle–Late Devonian granodiorite porphyry. The overall increasing concentrations of As, Sb, Te, Pb, Au, and Bi, and decreasing concentrations of Co, Ni, and Se from Py1 to Py3b indicate that prior to Cu precipitation, ore-forming fluids were in low oxygen fugacity conditions that were associated with fluid-rock interactions and fluid boiling. Furthermore, the main controlling factors during Cu precipitation were sulfur loss, the increase in pH, and the decrease in temperatures of ore-forming fluids, which could be attributed to fluid-rock interactions, later-stage fluid boiling, and mixing with meteoric water, respectively. The PLS-DA results show that pyrite composition varies following skarn redox state. Pyrite from reduced skarns have in common high concentrations of Au, Bi, and Ni, while those from oxidized have in common high As, Sb, and Pb. Pyrite trace element composition is effective in discriminating different redox states of skarns.

铜矽卡岩是全球铜的主要来源，根据其氧化还原状态可分为两种类型：氧化型和还原型。许多研究从黄铁矿微量元素组成的角度研究了氧化型铜矽卡岩矿床的成矿过程。然而，黄铁矿的组成和微观纹理特征如何与还原型铜矽卡岩系统中的特定物理化学过程相联系，目前仍不清楚。位于天山西部造山带北部的赛博矿床是一个典型的还原型铜矽卡岩矿床，具有丰富的富钛石榴石、砷黄铁矿、黄铁矿和富含CH4的包裹体。在这项研究中，我们考察了黄铁矿地球化学和微观纹理特征，并将这些特征与赛博的成铜过程联系起来。铜矽卡岩矿化被认为以脉状、带状、块状和散布状出现在花岗斑岩和库松穆基克组石灰岩之间的接触带。确定了五个阶段(I)顺生矽卡岩，(II)逆生矽卡岩，(III)石英-黄铁矿-黄铜矿（Py1），(IV)石英-多金属硫化物（Py2），(V)石英-方解石-多金属硫化物（Py3）。铅、锑、银、铜、锌和铋通常以微尺度和纳米尺度矿物包裹体的形式存在于主黄铁矿中，而钴、镍、硒、碲和砷则主要以晶格替代S和F的形式存在。赛博铜矽卡岩矿石的 Py1 至 Py3b 的原位 δ34S 值范围很窄，从 4.86 ‰ 到 7.67 ‰，这与已公布的硫化物分离物的 S 值（0.2 至 9.1 ‰）完全一致。这些数值与拉马苏长花岗岩等中-晚泥盆世花岗岩的数值相似，而与沉积物的数值不同。这表明赛博矿床硫化物的主要来源是中-晚泥盆世的花岗闪长斑岩。从Py1到Py3b，As、Sb、Te、Pb、Au和Bi的浓度总体呈上升趋势，而Co、Ni和Se的浓度则呈下降趋势，这表明在铜沉淀之前，成矿流体处于低氧富集状态，与流体-岩石相互作用和流体沸腾有关。此外，铜沉淀过程中的主要控制因素是硫的损失、pH 值的升高以及成矿流体温度的降低，这可能分别归因于流体-岩石相互作用、后期流体沸腾以及与陨水的混合。PLS-DA 结果表明，黄铁矿成分随矽卡岩氧化还原状态而变化。还原矽卡岩中的黄铁矿普遍含有高浓度的金、铋和镍，而氧化矽卡岩中的黄铁矿则普遍含有高浓度的砷、锑和铅。黄铁矿微量元素组成可有效区分矽卡岩的不同氧化还原状态。

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引用次数: 0

Fore-arc metasomatism by hybrid slab fluids during subduction initiation: Sr–Mg–Ca isotopes of rodingite, western Yarlung Zangbo suture zone 俯冲起始期混合板块流体的弧前变质作用：雅鲁藏布江西部缝合带罗定岩的钍镁钙同位素

IF 3.6 2区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Chemical Geology

Pub Date : 2024-11-01 DOI: 10.1016/j.chemgeo.2024.122473

Gui-Fang Ou , Song-Jie Wang , Hao Deng , Wen-Yong Duan , Xu-Ping Li , Hans-Peter Schertl

Subduction zone metasomatism is critical for Earth's material exchanges, yet the details of slab dehydration, particularly deserpentinization beneath fore-arcs, remain poorly understood. Here, we present Sr–Mg–Ca isotopic data for Purang rodingites from the western Yarlung Zangbo suture zone (YZSZ), a remnant of the Neo-Tethys Ocean that was subducted during the collision between the Indian and Eurasian Plates. The rodingites, occurring as centimeter- to meter-sized veins and blocks in serpentinized harburgite, are dominated by amphibolite- to greenschist-facies minerals like tremolite, magnesiohornblende, and chlorite. Their cumulate textures and rare earth element patterns resemble troctolite or gabbronorite, presumably formed beneath a seafloor spreading center. The rodingites are enriched in large ion lithophile elements and depleted in high field strength elements, with higher Sr/Nb and Ba/La and lower Nb/U and Ce/Pb ratios than mid-ocean ridge basalts (MORB) and rodingites formed through seawater or serpentinizing fluid alteration of MORB-like protoliths. They also exhibit higher initial ⁸⁷Sr/⁸⁶Sr ratios (0.7067–0.7075) and elevated δ²⁶Mg values (−0.22 ± 0.07 ‰ to −0.13 ± 0.02 ‰) compared to unaltered oceanic basalts, while their δ^44/40Ca values (0.72 ± 0.02 ‰ to 0.87 ± 0.03 ‰) remain similar to MORB. These signatures point to a metasomatic process beyond seafloor alteration, suggesting modification of a MOR-derived protolith in the fore-arc mantle at slab depths of <40 km, driven by Sr- and Mg-rich fluids from clay-rich sediments and serpentinized mantle. The measured Sr, Mg and Ca isotope compositions can be reproduced by mixing a MORB-like protolith with hybrid fluids derived from varying proportions of sedimentary clays and serpentinized peridotite. Combined with previous studies on YZSZ metamorphic soles, we propose that this fore-arc metasomatism occurred during the early Cretaceous, concurrent with the incipient subduction of the Neo-Tethys oceanic rocks. These results highlight the significance of deserpentinization at shallow fore-arc mantle settings during subduction initiation, suggesting that slab dehydration is more complex than previously recognized.

俯冲带变质作用对地球的物质交换至关重要，然而人们对板块脱水的细节，特别是前弧下的脱蛇绿岩化仍然知之甚少。这里，我们展示了雅鲁藏布江西缝合带（YZSZ）的普朗罗丁岩的钍镁钙同位素数据，该缝合带是印度板块和欧亚板块碰撞期间俯冲的新特提斯洋的残余。罗丁岩以厘米到米级大小的矿脉和矿块形式出现在蛇纹石化的哈勃岩中，以透闪石、镁角闪石和绿帘石等闪长岩至绿泥石成因的矿物为主。它们的积层纹理和稀土元素形态类似于特罗托石或嘎布罗托石，可能形成于海底扩张中心之下。与大洋中脊玄武岩（MORB）和通过海水或蛇纹岩化流体蚀变 MORB 样原岩形成的焙烧岩相比，焙烧岩富含大离子亲岩元素，而贫含高场强元素，Sr/Nb 和 Ba/La 比值较高，Nb/U 和 Ce/Pb 比值较低。与未改变的大洋玄武岩相比，它们还表现出较高的初始 87Sr/86Sr 比值（0.7067-0.7075）和较高的 δ26Mg 值（-0.22 ± 0.07 ‰ 至 -0.13 ± 0.02 ‰），而它们的 δ44/40Ca 值（0.72 ± 0.02 ‰ 至 0.87 ± 0.03 ‰）仍与 MORB 相似。这些特征表明，在海底蚀变之外还有一个变质过程，这表明在来自富粘土沉积物和蛇纹岩化地幔的富硒和富镁流体的驱动下，前弧地幔中的MOR衍生原岩在板块深度40千米处发生了改变。将类似于MORB的原岩与来自不同比例的沉积粘土和蛇绿岩化橄榄岩的混合流体混合，可以再现测量到的Sr、Mg和Ca同位素组成。结合之前对YZSZ变质底岩的研究，我们认为这种弧前变质作用发生在白垩纪早期，与新特提斯大洋岩石的初期俯冲同时发生。这些结果突显了在俯冲起始阶段浅前弧地幔环境中脱蛇绿岩化的重要性，表明板块脱水比以前认识到的更为复杂。

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引用次数: 0

Unveiling the paleosalinity constraints on southern peri-Pannonian Lower Miocene lacustrine systems in Serbia and Bosnia and Herzegovina: Lopare (Dinaride Lake System) versus Toplica basin (Serbian Lake System) 揭示塞尔维亚及波斯尼亚和黑塞哥维那围潘诺尼亚地区南部下中新世湖泊系统的古盐度制约因素：洛帕雷（迪纳里德湖泊系统）与托普里卡盆地（塞尔维亚湖泊系统）

IF 3.6 2区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Chemical Geology

Pub Date : 2024-11-01 DOI: 10.1016/j.chemgeo.2024.122475

Nikola Burazer , Aleksandra Šajnović , Darko Spahić , Pavle Tančić , Nenad Grba , Branimir Jovančićević

Early Neogene saline lakes were widely developed across Central Paratethys, particularly across its „Dinaride-Anatolide“ landbridge (area of Dinarides, Balkans). The constraints on Lopare (western part of Jadar block; eastern Bosnia and Herzegovina) and Toplica (Jastrebac Mt., central Serbia) Neogene paleolakes provide a better understanding of deep-time climate evolution. The study focused on early Neogene paleolacustrine configuration, paleogeography, and the factors affecting a considerable paleosalinity increase. The study integrated available records on the predominantly Lower Miocene sedimentary sequence by analyzing and interpreting several paleosalinity markers (inorganic geochemical proxies, mineralogical data). The constraints on paleosalinity markers are afterward coupled with rather complex tectonic inferences. The two wellbores drilled in the Lopare and Toplica basins expose Neogene sections from which the critical paleosalinity markers are extracted (drillhole depths up to 350 and 1000 m, respectively). The mineral searlesite (NaBSi₂O₅(OH)₂), and high B/Ga, markers of alkaline and arid paleoenvironmental lacustrine conditions, suggested intense evaporation (frequent drought periods). The evaporation was associated with paleoclimatic conditions during the Miocene Climatic Optimum. The Early Neogene arid climate and frequent evaporation episodes resulted in significant salinization of the Lower Miocene Lopare lacustrine sequence. The salinity in the Lopare sequence changed its character between brackish and saline, as evidenced by S/TOC and Ca/(Ca + Fe) ratios (TOC—Total Organic Carbon). At the same time, the salinity in the early Toplica basin was significantly lower and characterized by freshwater and brackish environments. The salinity variations between Toplica and Lopare were further exposed by Fe/(Ca + Mg), (Al + Fe)/(Ca + Mg), and C-value, yielding the change in the (paleo)water column. These markers further indicated the influence of warm climate conditions, consistent with the Miocene paleoclimatic record. Regarding paleogeography and Early Neogene tectonics, the results showed that before the Middle Miocene Badenian transgression, the Paratethyan Sea had no interference with the investigated intramontane lacustrine basins.

新近纪早期的盐湖在中帕拉特提斯地区广泛发育，特别是在其 "Dinaride-Anatolide "陆桥（巴尔干半岛的 Dinarides 地区）上。对 Lopare（Jadar 地块西部；波斯尼亚和黑塞哥维那东部）和 Toplica（Jastrebac 山，塞尔维亚中部）新元古代湖泊的研究，有助于更好地了解深时气候的演变。研究的重点是新近纪早期古岩浆构造、古地理以及影响古盐度大幅上升的因素。该研究通过分析和解释若干古盐度标志物（无机地球化学代用指标、矿物学数据），整合了主要为下中新世沉积序列的现有记录。之后，对古盐度标记的限制与相当复杂的构造推断相结合。在洛帕雷盆地和托普利卡盆地钻探的两口井揭露了新近纪断面，从中提取了关键的古盐度标志物（钻孔深度分别达 350 米和 1000 米）。矿物海泡石（NaBSi2O5(OH)2）和高 B/Ga 是碱性和干旱古环境湖沼条件的标志，表明蒸发强烈（干旱期频繁）。蒸发与中新世气候最适宜期的古气候条件有关。新近纪早期的干旱气候和频繁的蒸发导致下中新世 Lopare 湖底岩层严重盐碱化。从 S/TOC 和 Ca/（Ca + Fe）比值（TOC-总有机碳）可以看出，洛帕雷序列的盐度在咸水和盐水之间变化。与此同时，托普利卡盆地早期的盐度明显较低，具有淡水和咸水环境的特征。铁/（钙+镁）、（铝+铁）/（钙+镁）和碳值进一步揭示了托普里卡和洛帕雷之间的盐度变化，从而得出（古）水柱的变化。这些标记进一步表明了温暖气候条件的影响，与中新世古气候记录一致。在古地理和新近纪早期构造方面，研究结果表明，在中新世巴登大断裂之前，帕拉蒂安海与所调查的海内湖沼盆地没有任何关系。

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引用次数: 0

Uranium-series isotopes as tracers of physical and chemical weathering in glacial sediments from Taylor Valley, Antarctica 作为南极洲泰勒谷冰川沉积物物理和化学风化示踪剂的铀系列同位素

IF 3.6 2区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Chemical Geology

Pub Date : 2024-11-01 DOI: 10.1016/j.chemgeo.2024.122463

Graham Harper Edwards , Gavin G. Piccione , Terrence Blackburn , Slawek Tulaczyk

The McMurdo Dry Valleys of Antarctica formed by extensive glacial erosion, yet currently exhibit hyperarid polar conditions canonically associated with limited chemical and physical weathering. Efficient chemical weathering occurs when moisture is available, and polythermal subglacial conditions may accommodate ongoing mechanical weathering and valley incision. Taylor Valley, one of the MDV, hosts several Pleistocene glacial drift deposits that record prior expansions of Taylor Glacier and sediment redistribution, if not sediment production. We present U-series isotopics of fine-grained sediments from these drifts to assess the timescales of physical weathering and subsequent chemical alteration. The isotopes ²³⁸U, ²³⁴U, and ²³⁰Th are sensitive to both chemical and physical fractionation processes in sedimentary systems, including the physical fractionation of daughter isotopes by energetic recoil following radioactive decay. By comparing U-series isotopic measurements with models of U-series response to chemical weathering and physical fractionation processes, we show that Pleistocene drift sediments record histories of significant chemical alteration. However, fine-grained sediments entrained in the basal ice of Taylor Glacier record only minor chemical alteration and U-series fractionation, indicating comparatively recent sediment comminution and active incision of upper Taylor Valley by Taylor Glacier over the Pleistocene. In addition, the results of this study emphasize the utility of U-series isotopes as tracers of chemical and physical weathering in sedimentary and pedogenic systems, with particular sensitivity to radionuclide implantation by

α

-recoil from high-U authigenic phases into lower-U detrital phases. This process has occurred extensively in

> 200

ka drifts but to a lesser degree in younger deposits. U-series

α

-recoil implantation is an important physicochemical process with chronometric implications in other hyperarid and saline sedimentary systems, including analogous Martian environments.

南极洲的麦克默多干谷（McMurdo Dry Valleys）是由大量冰川侵蚀形成的，但目前却呈现出与有限的化学和物理风化作用相关的极地超干旱条件。当有水分时，就会发生有效的化学风化，而多热的冰川下条件可能会适应持续的机械风化和山谷侵蚀。泰勒谷（MDV 之一）拥有多处更新世冰川漂移沉积物，它们记录了泰勒冰川之前的扩张和沉积物的重新分布（如果不是沉积物的产生）。我们展示了这些漂流物中细粒沉积物的铀系列同位素，以评估物理风化和随后化学蚀变的时间尺度。同位素 238U、234U 和 230Th 对沉积系统中的化学和物理分馏过程都很敏感，包括放射性衰变后高能反冲对子同位素的物理分馏。通过将 U 系列同位素测量结果与 U 系列对化学风化和物理分馏过程的响应模型进行比较，我们发现更新世漂移沉积物记录了显著的化学蚀变历史。然而，泰勒冰川基底冰层中夹带的细粒沉积物仅记录了轻微的化学蚀变和铀系列分馏，这表明泰勒冰川在更新世期间对上泰勒谷的沉积物粉碎和切蚀活动相对较新。此外，这项研究的结果还强调了铀系列同位素作为沉积和成土系统中化学和物理风化示踪剂的作用，特别是对放射性核素从高铀自成相α-反弹性植入低铀碎屑相的敏感性。这一过程在200ka漂移中广泛发生，但在较年轻的矿床中发生的程度较低。U系列α-反弹性植入是一个重要的物理化学过程，对其他超干旱和盐碱沉积系统，包括类似的火星环境具有年代学意义。

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引用次数: 0

A zircon petrochronologic perspective on raising the central Tibetan crust 从锆石岩石年代学角度看西藏中部地壳的抬升

IF 3.6 2区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Chemical Geology

Pub Date : 2024-10-29 DOI: 10.1016/j.chemgeo.2024.122472

Shu-Hui Ren , Yun-Chuan Zeng , Roberto Weinberg , Ji-Feng Xu , Jian-Lin Chen , Bao-Di Wang , Feng Huang , Xi-Jun Liu , Hong-Xia Yu , Ming-Jian Li

Being the most commonly dated accessory mineral, the geochemical makeup of zircon has increasingly been recognized as a promising indicator of the parent magma composition, providing a means to reconstruct the paleo-crustal thickness of orogenic belts where whole-rock records are limited. However, the robustness of zircon approaches in paleo-crustal thickness reconstruction remains controversial. Here, we report on UPb age, trace element and Hf isotope data for zircon in sedimentary rocks of central Tibet, aiming to reconstruct the central Tibetan paleo-crustal thickness evolution from Meso-Tethyan subduction to the India–Asia collision, and test the results by comparing against geological evidence. The consistent variations of different pressure-sensitive geochemical proxies over time, e.g., (La/Yb)_N, (Sm/Yb)_N, MREE/HREE, Eu anomaly, and Y values, indicate that pressure is the first-order control on zircon compositional changes. Using southern Tibet as a reference and comparing the results from various geochemical proxies, we find that zircon Eu geochemical proxy yields relatively reasonable estimates of paleo-crustal thickness for this orogen, which is thereby employed to explore the evolution of paleo-crustal thickness in central Tibet. A history of multi-stage thickening crust separated by episodes of thinning for central Tibet is revealed by zircon multi-proxy approach, and supported by multiple lines of independent evidence, including whole-rock geochemical proxies, structural geology, low-temperature thermochronology, and paleo-altimetry. We propose that a thick crust similar to the current one was likely formed in central Tibet by the Late Eocene (∼40–35 Ma), predominantly due to crustal shortening as a consequence of the India–Asia collision. Our study strengthens the applicability of zircon petrochronologic data in delineating the paleo-crustal thickness evolution of specific regions when supported by rigorous and comprehensive regional geological evidence.

锆石是最常被测定年代的附属矿物，其地球化学组成被越来越多地认为是母岩浆成分的一个有前途的指标，为重建造山带的古地壳厚度提供了一种手段，因为在造山带，全岩记录是有限的。然而，锆石方法在古地壳厚度重建中的稳健性仍然存在争议。在此，我们报告了西藏中部沉积岩中锆石的UPb年龄、微量元素和Hf同位素数据，旨在重建西藏中部从中伏太古代俯冲到印度-亚洲碰撞的古地壳厚度演化，并通过与地质证据的对比检验结果。不同的压力敏感地球化学代用指标，如(La/Yb)N、(Sm/Yb)N、MREE/HREE、Eu异常和Y值，随时间的一致变化表明压力是锆石成分变化的第一级控制。以西藏南部为参照系，比较各种地球化学代用指标的结果，我们发现锆石Eu地球化学代用指标可以对该造山带的古地壳厚度进行相对合理的估算，并据此探索西藏中部古地壳厚度的演化过程。锆石多代理方法揭示了西藏中部多阶段地壳增厚和减薄的历史，并得到了多种独立证据的支持，包括全岩地球化学代理、构造地质学、低温热时学和古高纬度测量。我们提出，在晚始新世（∼40-35 Ma），西藏中部很可能形成了与现在相似的厚地壳，这主要是由于印度-亚洲碰撞造成的地壳缩短。我们的研究加强了锆石岩石年代学数据在严格和全面的区域地质证据支持下划分特定区域古地壳厚度演变的适用性。

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