宁镇山脉的盖帽碳酸盐岩北大核心CSCD

埃迪卡拉系底部发育一套覆盖在成冰纪冰碛杂砾岩之上的碳酸盐岩地层,被称为"盖帽碳酸盐岩",它是超级温室气候事件的沉积标志层,代表了新元古代"雪球"冰期的结束。通过对宁镇山脉地区新元古代地层的野外调查,在镇江丹阳帽山剖面南沱冰碛杂砾岩层顶部发现厚约4 m的含锰碳酸盐岩。沉积岩石学和稳定碳、氧同位素研究表明,这层碳酸盐岩具有盖帽碳酸盐岩的典型特征,可以与扬子板块其他地区的盖帽碳酸盐岩进行对比。这是宁镇山脉地区存在盖帽碳酸盐岩的首次报道。     

新元古代陡山沱组－灯影组中 C-Sr 同位素演化及其古环境意义

碳酸盐岩中碳同位素组成的演化对于古环境研究重要意义。笔者研究了浙江煤山剖面的新元古代地层陡山沱组和灯影组中δ13C，δ18O，87Sr/86Sr以及微量元素含量的演化。所分析的87Sr/86Sr值都大于0.714，同时大多数Mn/Sr比值在4～20之间。因为Sr在海洋中的存留时间很长（约3Ma），盖帽碳酸盐的87Sr/86Sr可指示冰期海水的Sr同位素组成，而在雪球事件期间87Sr/86Sr不可能这么高。在冰期之后，由于冰期前沉积的大量碳酸盐岩和冰期时喷发的玄武岩与陆壳岩石一起风化，海洋87Sr/86Sr也不会升到如此之高，可以认为所研究地层87Sr/86Sr已被成岩改造。大多数δ13O大于－10，并且δ13O和δ13C之间不存在明显的相关关系，表明大多数样品记录了初始的碳同位素组成。δ13C的演化曲线在碳酸盐盖帽以及陡出沱组－灯影组交界呈现明显的负漂移（－4‰～－3‰），在主要的陡山沱和灯影时期则最高升至＋3‰～＋4‰。碳酸盐盖帽中的δ13C负漂移可能由多种原因造成，例如：因为在雪球事件时海洋中光合作用几乎完全中止，造成海水与海底热液中CO2（约－6‰）的δ13C值趋同；冰期后即使生物产量已经快速恢复，但是极高的碳酸盐沉积速率仍会造成较低的δ13C值；原来甲烷水合物在冰期后中释放出甲烷；冰期后原来分层的海洋发生垂向洋流。目前尚不能唯一地确定碳酸盐盖幔中的δ13C负漂移是由某一个原因造成的，可能是由几个因素共同作用的结果。随着生物产量、大气中CO2含量和洋流都在雪球事件之后逐渐恢复正常，陡山沱组碳酸盐岩中δ13C也逐渐升高到正常的正值。当大气中巨量的CO2通过风化、碳酸盐岩沉积和有机质埋藏等途径被移除以后，气候开始变冷。当气个冷到某个临界范围时，强烈的温盐环流造成的上升洋流把富集12C的深部海水带到表层海水，造成陡山沱组－灯影组交界处的δ13C表现负漂移，同时使大气中的CO2升高到正常水平。灯影时期的气候要比陡山沱稳定。大气和海洋环境在几乎整个灯影时期都维持在适合生态系统的状态中，因面灯影组的δ13C大多为正值。     

湖北宜昌埃迪卡拉系陡山沱组底部盖帽碳酸盐岩中具极负碳同位素值方解石的U-Pb年龄北大核心CSCD

作为新元古代全球冰期结束的标志,盖帽碳酸盐岩在全球广泛分布。盖帽碳酸盐岩特殊的沉积构造和地球化学特征使其长时间以来成为学界研究新元古代冰期事件的重点关注对象。湖北宜昌三斗坪地区埃迪卡拉系陡山沱组底部盖帽碳酸盐岩中具极负碳同位素值(δ^(13)C最低值小于-40‰,VPDB)方解石的形成与甲烷氧化有关,被认为是甲烷水合物释放导致冰期结束和盖帽碳酸盐岩沉积这一假说的关键证据。但也有学者认为这些具极负碳同位素值方解石是在盖帽碳酸盐岩沉积之后形成的,因此与冰期结束和盖帽碳酸盐岩沉积无关。为探讨盖帽碳酸盐岩中具极负碳同位素值方解石的形成时间,本文利用近年来发展迅速的碳酸盐矿物激光剥蚀电感耦合等离子体质谱(LA-ICP-MS)U-Pb定年技术,分别对湖北宜昌九龙湾剖面陡山沱组底部盖帽碳酸盐岩裂隙和孔洞中的暗色方解石、白云石胶结物、白云石围岩进行了年龄测定。测试结果表明,暗色方解石的形成时间为593.7±8.7 Ma(2σ,n=111,MSWD=3.6),比盖帽白云岩沉积时间晚约40Myr,而白云石胶结物和围岩的分析测试获得了与方解石近似的U-Pb年龄,表明后期富甲烷流体的活动也对裂隙早期胶结物和周围的白云岩造成了影响。     

峡东青林口地区新元古代地层序列及沉积演变

峡东地区的新元古界由于保存良好的“雪球地球”沉积记录及其崩解后的“盖帽碳酸盐岩”和“庙河生物群”等关键地质记录,因而成为国际新元古界研究的热点地区之一.此次报道的青林口剖面是目前在峡东地区发现的出露最为完整的新元古代地层序列,对于揭示这一关键地质时期的重大生物演变和地球环境发展历程具有重要研究价值.重点介绍该剖面的地层序列并简要分析其沉积演变特点.该剖面出露新元古代地层自下而上依次包括莲沱组、南沱组、陡山沱组和灯影组.其中,莲沱组以紫红色砂岩夹页岩为主,由两个沉积旋回组成;南沱组角度不整合在莲沱组之上,以灰绿色杂砾岩为特征,划分为3个岩性段;陡山沱组以“盖帽白云岩”的出现为底界,以黑色硅质页岩的结束为顶界,划分为4个明显的岩性段;灯影组整体具有“两白夹一黑”的特征,自下而上划分为蛤蟆井段、石板滩段和白马沱段.青林口剖面地层出露完整,各组段岩性特征明显,清晰地反映了该区域的沉积环境演变历程.沉积记录显示,本区新元古代中期开始接受沉积,最先沉积陆相莲沱组,经历成冰纪晚期南沱组代表的“雪球”事件后转为海相沉积,埃迪卡拉纪沉积的陡山沱组和灯影组均为浅海开阔碳酸盐相或局限页岩相,成为后生动物起源和宏观藻类分异发展的创新基地.      

重庆秀山南沱冰期后的海陆环境变化

扬子地区发育地层所记录的南沱冰期在时间上与Marinoan冰期相当,被认为是"雪球事件"的产物,受到广泛关注。借助于重庆秀山长河桥剖面的南沱组冰碛岩上覆陡山沱组盖帽白云岩和页岩样品的精细采集,选取冰碛岩之上2.5 m内的地层进行同位素比值和元素含量测试,并尝试性的使用酸不溶物的元素地球化学数据,对冰期后可能出现的环境变化进行了综合分析,结果表明:盖帽白云岩C同位素数据基本符合海水分层混合模式特征;U/Th值也反映出该地区在雪球后经历了由缺氧-贫氧环境向贫氧-氧化环境的迅速转变,可能反映了雪球后缺氧富有机质的深部大洋水随上升洋流上涌并被迅速氧化的过程;强烈的Eu正异常的出现,可能与埃迪卡拉纪海底火山、热液活动频繁出现或近源热液活动相关;盖帽碳酸盐岩样品中出现Ce轻微负异常,表明其形成于弱氧化环境;87Sr/86Sr值和Y/Ho值均呈现伴有大量陆源碎屑物输入的特点;盖帽碳酸盐岩的酸不溶物可以反映当时的大陆风化背景,其化学蚀变指数（CIA）稳定在72左右,说明当时的大陆环境具有温暖湿润的特征,化学风化作用强度中等。综上所述,在南陀冰期结束后,秀山长河桥剖面的沉积环境经历了缺氧-贫氧环境向贫氧-氧化环境的迅速转变,同时,随着上升洋流的出现原有的冰期海水分层被迅速破坏,并可能伴随着地表径流的不断增强,同时热液活动在这一时期也频繁发生。在这一时期,剖面附近的大陆环境也迅速由冰期过渡为温暖湿润的环境。     

长江三峡地区成冰纪-埃迪卡拉纪转换时期微量元素和稀土元素地球化学特征

通过微量元素和稀土元素地球化学分析,对长江三峡地区陡山沱组层型剖面--田家园子剖面成冰系南沱组顶部和埃迪卡拉系陡山沱组下部102个岩石样品进行了地球化学研究。重点分析了氧化还原敏感元素(Zn,Co,U,Mo,Ni,V)的富集特征,并探讨其可能成因机制以及三峡地区成冰纪-埃迪卡拉纪转换时期的水体特征。结果表明:在南沱组顶部仅Zn和Co富集;在盖帽白云岩下部,氧化还原敏感元素均富集,而在盖帽白云岩上部,除Zn和V外,其他氧化还原敏感元素均亏损;在陡山沱组Ⅱ段下部,氧化还原敏感元素由最初的亏损,逐渐变为较稳定的富集。在陡山沱组下部,出现2次明显的富集峰值,分别出现在剖面的0.4m处(盖帽白云岩中间)和6.5m处(陡山沱组Ⅱ段下部)。整个剖面大部分样品具有Eu的轻微正异常(Eu/Eu^*＜1.6), 而在剖面0.4m和6.5m处,Eu具有明显的正异常,结合稀土配分类型、Y/Ho值、La异常、Ce异常等指标,推测这2次异常均可能受到深海热液流的影响,而缺氧海水的上涌造成水体缺氧,导致这些元素出现富集峰值。U/Th、V/(V+Ni)以及稀土元素指标综合指示,三峡地区南沱组顶部冰碛岩应为氧化环境下的沉积物;随着冰川消融,冰融淡水注入古海洋,陡山沱组盖帽碳酸盐的沉积受冰融淡水的影响,深部缺氧海水的上涌使沉积水体经历氧化-缺氧-氧化的转变,海水的分层性较强;而陡山沱组Ⅱ段下部沉积环境以分层性较弱的弱氧化环境为主。     

贵州震旦系陡山沱组磷块岩成磷作用及与新元古代末期氧化事件(NOE)的耦合*

新元古代末期历经“雪球地球”和大气、海洋增氧事件后,发生了全球性成磷事件,贵州省震旦系陡山沱组大规模磷矿床沉积是本次成磷事件的典型代表。然而现阶段对贵州陡山沱组磷块岩成磷作用机制研究存在诸多争议,成磷事件与同期古海洋环境转变之间的关联研究也较为薄弱。作者以贵州省瓮安、遵义和丹寨地区陡山沱组原生磷块岩为研究对象开展的沉积学、岩石学、矿物学和地球化学特征研究表明: 陡山沱早期成磷作用被限制在浅水海岸,瓮安地区A矿层球粒磷块岩中大量的含铁自生矿物黄铁矿、海绿石以及无Ce负异常指示的贫氧沉积水体环境均表明,Fe-氧化还原泵成磷模式在富磷过程中发挥了重要作用;陡山沱晚期磷块岩分布扩散至较深水陆棚—斜坡沉积相区,磷块岩与富有机质岩层共生,矿物晶体形态特征与矿石内富含的大量生物化石均表明,有机质沉降聚磷作用和生物成磷作用促使磷块岩大量沉积,较明显的Ce负异常值也指示了海水氧气含量的提升。成磷作用模式的转变和磷块岩分布的扩展是对海洋增氧事件的沉积响应,同时造成的多细胞动物演化也影响了深部水体的氧化还原状态,进而反映了贵州省陡山沱组磷块岩大规模沉积与新元古代末期氧化事件(NOE)密切的耦合关系。     

湖北峡东地区“盖帽碳酸盐岩”中燧石条带的地球化学特征及其古环境意义

湖北峡东地区新元古代陡山沱组一段的“盖帽碳酸盐岩”中发育有保存良好,与白云岩界线截然,顺层产出的燧石条带.利用硅酸盐的BrF5硅氧同位素分析方法对燧石条带进行测试,得到较高的硅同位素(δ30SiNBS-28为0.4‰～1.7‰)和氧同位素(δ18OV-SMOW为24.‰～26.6‰)组成.燧石条带可能为温暖的滨海相化学沉积产物,氧同位素测温得到的沉积海水温度为35.4～47.7℃.燧石条带的Al/(Fe+Mn+ Al)为0.43～0.50,Si/Al比值为257～269,表明燧石条带形成过程中有少量的陆源物质参与,而较低的∑REE(2.19×10-6～3.59×10-6),Ce正异常(1.20～1.24)和U/Th比值(3.75～14.45)上则明显带有热水沉积的特征.燧石条带的硅质可能主要来自远源火山热液活动,在温暖的滨海环境达到硅质饱和而形成的化学沉积,同时验证了早期研究者认为的“雪球事件”冰后期环境温度曾一度极为温暖的猜想.     

皖南蓝田组碳酸盐岩碳和氧同位素地球化学研究

为了认识新元古代雪球地球事件之后地球表面气候和环境的变化,前人对华南震旦系地层中的陡山沱组和灯影组灰岩进行了大量的古生物地层学和稳定同位素地球化学研究,特别对碳同位素漂移与冰期之间的对应关系提出了很多假说和模型.本文就出露在皖南地区的震旦系蓝田组灰岩进行了方解石和白云石的碳氧同位素分析,分析结果为其沉积环境研究提供了重要的地球化学依据.     

中国三大陆块中—新元古代泥质岩与碳酸盐岩元素地球化学特征

中国三大陆块中—新元古代地层格架中广泛分布低变质程度的泥质岩与碳酸盐岩,其元素地球化学特征为研究地表圈层的历史演化提供了重要的基础资料。本文收集了近十几年国内外已发表的近2000组泥质岩/碳酸盐岩中地球化学数据,包括40种常量、微量元素含量组成,涉及华北陆块长城系、蓟县系、待建系及青白口系,扬子和塔里木陆块南华系、震旦系。结合文献中样品的岩性、实验方法以及地质背景特征,主要得出了以下认识：① 泥质岩中常量元素的平均含量与中国东部未变质泥质岩中的相似,但其整体上受到了新元古代冰期的影响,化学风化作用较弱,易流失的Na、K及Ca元素含量相对较高;碳酸盐岩中的常量元素含量组成与中国东部未变质富泥碳酸盐岩对比,具有少量的陆源物质输入且以白云岩为主,Mg含量较高。② 碳酸盐岩中较高的Mn/Sr比值应与成岩蚀变作用有关,虽然硅酸盐矿物少于泥质岩,但Sc、REE+Y以及氧化还原敏感元素更加富集,Sc和REE+Y元素与陆源碎屑的混染作用有关,而后者与氧化的海洋环境有关。③ 中元古代早期,华北沉积物中记录了Ce元素负异常以及氧化还原敏感元素含量的增高,响应了一次浅海的氧化事件,并促使了生物的演化。④ 进入新元古代,“雪球地球”事件被塔里木陆块泥质岩大范围变化的化学蚀变指数所记录,伴随着间冰期及冰期后海洋的增氧,扬子陆块新元古代沉积物部分微量元素含量又一次增加,对应着伊迪卡拉生物群的出现。     

Rare earth element geochemistry of Late Devonian reefal carbonates, Canning Basin, Western Australia: confirmation of a seawater REE proxy in ancient limestones

Rare earth element and yttrium (REE+Y) concentrations were determined in 49 Late Devonian reefal carbonates from the Lennard Shelf, Canning Basin, Western Australia. Shale-normalized (SN) REE+Y patterns of the Late Devonian samples display features consistent with the geochemistry of well-oxygenated, shallow seawater. A variety of different ancient limestone components, including microbialites, some skeletal carbonates (stromatoporoids), and cements, record seawater-like REE+Y signatures. Contamination associated with phosphate, Fe-oxides and shale was tested quantitatively, and can be discounted as the source of the REE+Y patterns. Co-occurring carbonate components that presumably precipitated from the same seawater have different relative REE concentrations, but consistent REE+Y patterns. Clean Devonian early marine cements (n = 3) display REE+Y signatures most like that of modern open ocean seawater and the highest Y/Ho ratios (e.g., 59) and greatest light REE (LREE) depletion (average Nd_SN/Yb_SN = 0.413, SD = 0.076). However, synsedimentary cements have the lowest REE concentrations (e.g., 405 ppb). Non-contaminated Devonian microbialite samples containing a mixture of the calcimicrobe Renalcis and micritic thrombolite aggregates in early marine cement (n = 11) have the highest relative REE concentrations of tested carbonates (average total REE = 11.3 ppm). Stromatoporoid skeletons, unlike modern corals, algae and molluscs, also contain well-developed, seawater-like REE patterns. Samples from an estuarine fringing reef have very different REE+Y patterns with LREE enrichment (Nd_SN/Yb_SN > 1), possibly reflecting inclusion of estuarine colloidal material that contained preferentially scavenged LREE from a nearby riverine input source. Hence, Devonian limestones provide a proxy for marine REE geochemistry and allow the differentiation of co-occurring water masses on the ancient Lennard Shelf. Although appropriate partition coefficients for quantification of Devonian seawater REE concentrations from out data are unknown, hypothetical Devonian Canning Basin seawater REE patterns were obtained with coefficients derived from modern natural proxies and experimental values. Resulting Devonian seawater patterns are slightly enriched in LREE compared to most modern seawaters and suggest higher overall REE concentrations, but are very similar to seawaters from regions with high terrigenous inputs. Our results suggest that most limestones should record important aspects of the REE geochemistry of the waters in which they precipitated, provided they are relatively free of terrigenous contamination and major diagenetic alteration from fluids with high, non-seawater-like REE contents. Hence, we expect that many other ancient limestones will serve as seawater REE proxies, and thereby provide information on paleoceanography, paleogeography and geochemical evolution of the oceans.     

碳酸盐(岩)的稀土元素特征及其古环境指示意义

稀土元素主要通过交代碳酸盐矿物的Ca^{2+0进入碳酸盐格架,所以沉积碳酸盐(岩}的稀土元素特征能够很好的指示沉积流体来源和古环境。常用的稀土元素指标包括稀土元素总量(ΣREE)、稀土元素配分型式、以及La、Ce、Eu、Gd和Y等元素的异常指数。碳酸盐(岩)的稀土元素含量可能受到硅酸盐矿物、Fe-Mn氧化物/氢氧化物和磷酸盐等非碳酸盐组分以及成岩蚀变作用的影响。因此,在分析过程中,我们只有排除这些影响因素,才能用碳酸盐(岩)的稀土元素指标来探讨流体来源和古环境。这要求我们采集新鲜剖面上的样品,并用适当浓度的弱酸进行分步溶样,提取适当的组分,避免样品中的非碳酸盐组分干扰原始沉积组分的稀土元素特征。不同的沉积水体和沉积相下形成的碳酸盐(岩)具有不同的稀土元素特征:从太古宙到全新世的海相碳酸盐(岩)记录了LREE亏损、La正异常和高Y/Ho值的稀土元素特征;海底孔隙水的稀土元素特征则受氧化-还原条件、离子络合形式、孔隙流体来源的制约;热液流体具有LREE富集、Eu正异常的稀土元素特征;河水和湖泊有相对平坦的稀土元素特征。因此,碳酸盐(岩)的稀土元素特征具有重要的古环境指示意义。     

Do Archean chemical sediments record ancient seawater rare earth element patterns?

Rare earth elements (REE) concentrations of Archean and Proterozoic chemical sediments are commonly used as proxies to study secular trends in the geochemistry of Precambrian seawater. In addition, similarities in the REE signatures of Archean chemical sediments and modern seawater have led researchers to argue that some Archean rocks originated as biochemical precipitates (i.e., microbial carbonates) in shallow marine (e.g., peritidal) environments. However, terrestrial waters, including river water and groundwater, also commonly exhibit REE fractionation patterns that resemble modern seawater. Here, we present the seawater-like REE data for groundwaters from central México as additional evidence that these patterns are not unique to the marine environment. The shale-normalized REE patterns of the groundwaters are compared to those of modern seawater (open ocean and nearshore), Holocene reefal microbial carbonates and corals, and Archean chemical sediments using statistical means (i.e., ANOVA and Wilcoxon analyses) in order to quantify the similarities and/or differences in the REE patterns. Shale-normalized (SN) Ce anomalies and measures of REE fractionation [i.e., (La/Yb)_SN, (Pr/Yb)_SN, (Nd/Yb)_SN, and (Gd/Yb)_SN] of the central México groundwater samples are statistically indistinguishable from those of modern seawater. Moreover, except for differences in the Ce anomalies, which are lacking in Archean chemical sediments, the REE patterns of the central México groundwaters are also statistically similar to REE patterns of Archean chemical sediments, especially those of the 3.45 Ga Strelley Pool Chert. Consequently, we suggest that without additional information, it may be premature to unequivocally conclude that Archean chemical sediments record REE signatures of an Archean ocean.     

Trace element distribution in Neoproterozoic carbonates as palaeoenvironmental indicator

A first study of REE + Y distribution in a variety of Neoproterozoic (Cryogenian and Ediacaran) carbonates from different settings in the Saldania, Gariep, Damara and West Congo Belts in southwestern and central Africa revealed systematic differences that can be explained by varying palaeoenvironmental factors. The majority of samples display relatively unfractionated, flat shale-normalised REE + Y patterns that cannot be ascribed solely to shale contamination but are interpreted as resulting from the incorporation of near-shore colloids, possibly related to Fe-oxihydroxide scavenging. Only few carbonate units yielded trace element distributions that conform to a typical seawater composition. Those carbonates that were affected by stratiform, syn-sedimentary hydrothermal mineralisation are distinguished by Eu anomalies. Considering the similarity in residence time between REE and carbon, the strong influence of river-born particles on the REE + Y distribution in the analysed carbonates casts considerable doubt over the usefulness of these carbonates for stratigraphic correlation of Neoproterozoic sediment successions based on carbon isotopes.     

Application of Rare Earth Elements of the Marine Carbonate Rocks in Paleoenvironmental Researches

The pattern, contents and ratios of Rare Earth Elements (REE) from marine carbonates play an important role in the paleo-environmental researches. As result of the REE's source being variable, which includes marine carbonates, detrital input and diagenesis, the overall assessment for the reliability of REE's data is necessary. Furthermore, appropriate analytical method is vital for the reliable contents of REE. This paper reviewed the geochemical properties, analytical and data processing methods, feasibility verification and paleo-environmental application of carbonates REE. The patterns of REE, which provide theoretical basis for the provenance and depositional environment of carbonates, are various with different sources. Cerium, as a redox sensitive element, is a key proxy for the reconstruction of paleo-redox conditions. There are two available analytical methods, acid-leaching method and direct LA-ICP-MS analytical method, to extract REE of seawater preserved in marine carbonate rocks. The contamination from detritus and diagenetic alteration can be detected by the correlations of various elements or element ratios. The REE of marine carbonate has been well applied to reconstruct the environment changes during the Precambrian, Permian-Triassic transition and Cenozoic.     

华南埃迪卡拉系底部钾质斑脱岩的岩石地球化学特征及其地质意义

我国华南扬子地台不同地区（湖北宜昌、安徽休宁）埃迪卡拉系底部的盖帽碳酸盐岩中均保存有浅色粘土岩层。岩石薄片和扫描电镜观察,以及X射线衍射分析表明,这些粘土岩层以伊利石和伊/蒙混层矿物为主,同时含有港湾状熔蚀石英、高温透长石及岩浆锆石等斑晶矿物。X荧光光谱仪和ICP-MS等离子质谱仪等地球化学元素分析表明,这些粘土岩层具有相似的元素组成和配分模式,以高K₂O、低TiO₂,富含Nb、Ta、Zr、Hf、Th、Y等亲石元素为特征,Al₂O₃/TiO₂、Zr/Hf 和Ti/Th 值均指示了酸性火山灰的性质,同时稀土元素配分模式与花岗岩较为相似。岩石学和地球化学特征表明,这些浅色粘土岩层是火山喷发事件形成的钾质斑脱岩。通过两处剖面的对比,可以将安徽休宁地区的钾质斑脱岩年龄限制在大约635Ma左右。岩浆判别图指示了这些钾质斑脱岩的原始岩浆为高钾亚碱性流纹英安岩;依据微量元素特征和构造环境判别图,初步认为原始岩浆形成于大陆弧环境。这些钾质斑脱岩在区域上的分布,表明我国扬子地台埃迪卡拉纪早期发生了广泛的火山活动,“雪球地球”事件的结束可能是导致这期以钾质斑脱岩为代表的火山喷发事件的诱因。     

Rare earth elements of seep carbonates: Indication for redox variations and microbiological processes at modern seep sites

At marine seeps, methane is microbially oxidized resulting in the precipitation of carbonates close to the seafloor. Methane oxidation leads to sulfate depletion in sediment pore water, which induces a change in redox conditions. Rare earth element (REE) patterns of authigenic carbonate phases collected from modern seeps of the Gulf of Mexico, the Black Sea, and the Congo Fan were analyzed. Different carbonate minerals including aragonite and calcite with different crystal habits have been selected for analysis. Total REE content (ΣREE) of seep carbonates varies widely, from 0.1 ppm to 42.5 ppm, but a common trend is that the ΣREE in microcrystalline phases is higher than that of the associated later phases including micospar, sparite and blocky cement, suggesting that ΣREE may be a function of diagenesis. The shale-normalized REE patterns of the seep carbonates often show different Ce anomalies even in samples from a specific site, suggesting that the formation conditions of seep carbonates are variable and complex. Overall, our results show that apart from anoxic, oxic conditions are at least temporarily common in seep environments.     

http://www.sciencedirect.com/science/article/pii/S1674987110000125

<正>Carbonatites are commonly related to the accumulation of economically valuable substances such as REE.Cu,and P.The debate over the origin of carbonatites and their relationship to associated silicate rocks has been ongoing for about 45 years.Worldwide,the rocks characteristically display more geochemical enrichments in Ba,Sr and REE than sedimentary carbonate rocks.However,carbonatite's geochemical features are disputed because of secondary mineral effects.Rock-forming carbonates from carbonatites at Qinling.Panxi region,and Bayan Obo in China show REE distribution patterns ranging from LREE enrichment to flat patterns.They are characterized by a Sr content more than 10 times higher than that of secondary carbonates.The coarse- and fine-grained dolomites from Bayan Obo H8 dolomite marbles also show similar high Sr abundance,indicating that they are of igneous origin.Some carbonates in Chinese carbonatites show REE(especially HREE) contents and distribution patterns similar to those of the whole rocks.These intrusive carbonatites display lower platinum group elements and stronger fractionation between Pt and Ir relative to high-Si extrusive carbonatite.This indicates that most intrusive carbonatites may be carbonate cumulates.Maoniuping and Daluxiang in Panxi region are large REE deposits.Hydrothermal fluorite ore veins occur outside of the carbonatite bodies and are emplaced in wallrock syenite.The fiuorite in Maoniuping has Sr and Nd isotopes similar to carbonatite.The Daluxiang fiuorite shows Sr and REE compositions different from those in Maoniuping.The difference is reflected by both the carbonatites and rock-forming carbonates,indicating that REE mineralization is related to carbonatites.The cumulate processes of carbonate minerals make fractionated fluids rich in volatiles and LREE as a result of low partition coefficients for REE between carbonate and carbonatite melt and an increase from LREE to HREE.The carbonatite-derived fluid has interacted with wallrock to form REE ore veins.The amount of carbonatite dykes occurring near the Bayan Obo orebodies may support the same mineralization model,i.e.that fluids evolved from the carbonatite dykes reacted with H8 dolomite marble,and thus the different REE and isotope compositions of coarse- and fine-grained dolomite may be related to reaction processes.     

Flat rare earth element patterns as an indicator of cumulate processes in the Lesser Qinling carbonatites, China

The Lesser Qinling carbonatite dykes are mainly composed of calcites. They are characterized by unusually high heavy rare earth element concentrations (HREE; e.g. Yb > 30 ppm) and flat to weakly light rare earth element (LREE) enriched chondrite-normalized patterns (La/Yb_n = 1.0–5.5), which is in marked contrast with all other published carbonatite data. The trace element contents of calcite crystals were measured in situ by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Some crystals show reduced LREE from core to rim, whereas their HREE compositions are relatively constant. The total REE contents and chondrite-normalized REE patterns from the cores of carbonate crystals are similar to those of the whole rock. The carbon and oxygen isotopic compositions of calcites fall within the range of primary, mantle-derived carbonatites. The initial Sr isotopic compositions (0.70480–0.70557) of calcites are consistent with an EM1 source or mixing between HIMU and EM1 mantle sources. However these sources cannot produce carbonatite parental magmas with a flat or slightly LREE enrichment pattern by low degrees of partial melting. Analyses of carbonates from other carbonatites show that carbonates have nearly flat REE pattern if they crystallize from a LREE enriched carbonatite melt. This implies that when carbonates crystallize from a carbonatite melt the calcite/melt partition coefficients (D) for HREE are much greater than the D for the LREE. The nearly flat REE patterns of the Lesser Qinling carbonatites can be explained if they are carbonate cumulates that contain little trapped carbonatite melt. Strong enrichment of HREE in the carbonatites may require their derivation by small degrees of melting from a garnet-poor source.     

Rare Earth Element and yttrium compositions of Archean and Paleoproterozoic Fe formations revisited: New perspectives on the significance and mechanisms of deposition

The ocean and atmosphere were largely anoxic in the early Precambrian, resulting in an Fe cycle that was dramatically different than today’s. Extremely Fe-rich sedimentary deposits—i.e., Fe formations—are the most conspicuous manifestation of this distinct Fe cycle. Rare Earth Element (REE) systematics have long been used as a tool to understand the origin of Fe formations and the corresponding chemistry of the ancient ocean. However, many earlier REE studies of Fe formations have drawn ambiguous conclusions, partially due to analytical limitations and sampling from severely altered units. Here, we present new chemical analyses of Fe formation samples from 18 units, ranging in age from ca. 3.0 to 1.8 billion years old (Ga), which allow a reevaluation of the depositional mechanisms and significance of Precambrian Fe formations. There are several temporal trends in our REE and Y dataset that reflect shifts in marine redox conditions. In general, Archean Fe formations do not display significant shale-normalized negative Ce anomalies, and only Fe formations younger than 1.9 Ga display prominent positive Ce anomalies. Low Y/Ho ratios and high shale-normalized light to heavy REE (LREE/HREE) ratios are also present in ca. 1.9 Ga and younger Fe formations but are essentially absent in their Archean counterparts. These marked differences in Paleoproterozoic versus Archean REE + Y patterns can be explained in terms of varying REE cycling in the water column.Similar to modern redox-stratified basins, the REE + Y patterns in late Paleoproterozoic Fe formations record evidence of a shuttle of metal and Ce oxides across the redoxcline from oxic shallow seawater to deeper anoxic waters. Oxide dissolution—mainly of Mn oxides—in an anoxic water column lowers the dissolved Y/Ho ratio, raises the light to heavy REE ratio, and increases the concentration of Ce relative to the neighboring REE (La and Pr). Fe oxides precipitating at or near the chemocline will capture these REE anomalies and thus evidence for this oxide shuttle. In contrast, Archean Fe formations do not display REE + Y patterns indicative of an oxide shuttle, which implies an absence of a distinct Mn redoxcline prior to the rise of atmospheric oxygen in the early Paleoproterozoic. As further evidence for reducing conditions in shallow-water environments of the Archean ocean, REE data for carbonates deposited on shallow-water Archean carbonate platforms that stratigraphically underlie Fe formations also lack negative Ce anomalies. These results question classical models for deposition of Archean Fe formations that invoke oxidation by free oxygen at or above a redoxcline. In contrast, we add to growing evidence that metabolic Fe oxidation is a more likely oxidative mechanism for these Fe formations, implying that the Fe distribution in Archean oceans could have been controlled by microbial Fe uptake rather than the oxidative potential of shallow-marine environments.