火成碳酸岩的实验岩石学研究及对地球深部碳循环的意义

火成碳酸岩是地表出露较少的幔源岩石之一。实验岩石学研究表明碳酸盐化的橄榄岩和循环的地壳物质(如碳酸盐化榴辉岩或泥质岩)的低程度(<1%)部分熔融均可以产生碳酸岩质的熔体,其中碳酸盐化泥质岩具有最低的熔融温度且更加富碱质、CO2和不相容元素;富CO2的霞石质等硅酸盐岩浆也可以通过不混溶或分离结晶作用产生碳酸岩,用于解释碳酸岩在空间中常与碱性硅酸岩的共生关系。由于碳酸岩熔体具有极低的粘度和高的活性,形成后在上升过程中会将二辉橄榄岩转变为异剥橄榄岩,是引起地幔交代作用和地幔地球化学不均一性的重要介质之一。实验表明在俯冲作用过程中,大多数的碳酸盐在位于岛弧之下的含水熔融并不分解而是被带入到深部地幔并且稳定存在,含碳地幔的熔融又会形成碳酸岩质的熔体,这说明俯冲循环物质可能对碳酸岩的成因也起着重要的作用。然而,对于碳酸岩的初始熔体成分、岩浆演化、地幔交代作用、成矿特征以及碳从地球深部返回到地表的途径和过程等都存在着很大的争议。我国火成碳酸岩出露相对较多,分布广泛,因此,加强我国碳酸岩以及伴生硅酸岩的成因研究,同时开展与碳酸岩相关的实验岩石学工作,不仅可以检验现有的成因理论,而且有助于提高我国对火成碳酸岩的研究水平;由于其特殊的成因背景,还可为许多存在很大争议的重大地质事件提供新的科学依据。     

中欧华力西褶皱带高压和低压变质矿物组合的时空关系

欧洲华力西期褶皱带出露的结晶基底岩块内的高构造层位上,广泛保存着早华力西期(大于370Ma)高压矿物组合的残余。常可见到超镁铁质石榴橄榄岩/辉石岩和酸性石榴麻粒岩在空间上共存。初始高P-T矿物的稳定性用板块构造观点进行解释,包括钙碱性大陆边缘消亡作用、地幔的岩浆和构造相互作用等。然而,构造和变质作用的相关性表明,目前地壳和地幔的相邻关系是在华力西造山旋回的后期韧性剪切作用形成的。在地壳深部,这些与抬升     

大别-苏鲁造山带橄榄岩记录的碰撞造山过程中地幔楔的地壳交代作用

地壳交代作用是洋壳俯冲带板片-地幔界面的普遍现象,由于地幔楔样品的缺乏,其识别存在困难。而碰撞造山带广泛出露的地幔楔来源的造山带橄榄岩则是理想的研究对象。本文对大别-苏鲁造山带橄榄岩的已有研究成果进行了系统总结。这些成果表明这些橄榄岩在大洋俯冲向大陆碰撞转换的不同阶段经历了多期地壳来源流体的交代作用。地壳交代作用不仅改变了地幔楔橄榄岩的地球化学成分,而且导致了交代矿物的生长以及超镁铁质交代体的形成。这些交代体或作为同碰撞和碰撞后镁铁质火成岩的地幔源区,或将地壳组分传输到深部地幔,或释放流体交代大陆俯冲隧道中的俯冲陆壳。本文对地壳交代作用研究中存在的重要问题和解决思路提出了建议。     

地球深部碳循环与碳酸岩成因研究进展

地球深部碳循环是指地球表层的碳在俯冲带进入地幔深部,然后通过岩浆或者脱气作用再把地幔中的碳释放到地球表层系统中的过程。人类对地球深部碳赋存形式和储量、不同储库的交换方式和交换量尚缺乏清晰认识,近年来随着分析技术的发展和研究的深入,深部碳循环的研究日益丰富。本文总结了地幔中碳赋存状态、地球深部碳储量、碳进出地幔方式及通量、俯冲带碳的行为和碳酸岩成因及成矿方面的研究。地幔中碳赋存形式多样且主要受地球深部压力及氧逸度控制。相平衡实验和热动力学计算发现碳酸盐化榴辉岩在300～600 km发生部分熔融,交代地幔橄榄岩形成碳酸盐化地幔橄榄岩。碳酸盐化地幔橄榄岩的熔融又会形成碳酸岩熔体,这说明俯冲再循环物质可能对碳酸岩的成因起重要作用。碳酸岩是研究深部碳循环的良好载体,其源区特征、岩浆演化过程对示踪碳在地幔和地壳过程中的迁移至关重要。虽然深部碳循环在碳赋存形式、碳储量及通量、俯冲带碳的流变行为和碳酸岩成因对深部碳循环的启示方面已经取得了较大的研究进展,但仍有大量的科学问题亟待解决,如:沉积碳酸盐岩再循环进入地球深部后的行为、俯冲带板片流体地球化学行为、俯冲带流体氧逸度特征等,将来有必要重点开展深入研...     

造山带碳酸岩起源与深部碳循环

碳酸岩是揭示地幔地球化学动力学的"探针岩石",迄今有关碳酸岩的研究集中在裂谷环境,而鲜见造山带地区碳酸岩研究的报道。在四川攀西喜马拉雅期造山带、秦岭造山带和华北中央造山带内均有碳酸岩产出,且蕴藏了大型稀土和钼矿床,是研究深俯冲、壳-幔作用和深部碳循环的理想实验室。传统观念认为碳酸岩形成于裂谷环境与地幔柱活动相关,而造山带碳酸岩很可能是陆源富碳沉积物俯冲至地幔低程度熔融的产物。中国造山带内碳酸岩地球化学研究均显示了地壳物质对碳酸岩地幔源区的贡献,这暗示地表碳俯冲至深部地幔,交代地幔发生熔融,这不仅为较还原的地幔源区提供富氧成分,还可以使碳酸岩的母体岩浆更富集稀土,形成稀土矿床。     

大冶-城门山夕卡岩矿床石榴子石和辉石中熔融包裹体成分研究

在长江中下游地区，Fe、Cu(Au)和Au(Cu)夕卡岩矿床广泛分布，用电子探针测量了大冶-城门山一带7个矿床夕卡岩矿物中熔融包裹体里的固相(晶质和非晶质)成分。根据这些分析结果及已发表的熔融包裹体均一温度测量数据，对该地区熔融包裹体的成分特点及其意义以及岩浆夕卡岩形成的物理化学条件进行了研究和探讨。夕卡岩矿物中熔融包裹体主要含两种成分：成分接近辉石的钙、铁、镁硅酸盐相和成分接近石榴子石的钙、铁、铝硅酸盐以及含SiO2等氧化物的碳酸盐相。夕卡岩矿物中硅酸盐熔融包裹体在均一温度和成分方面与含子矿物的流体包裹体和“盐熔”包裹体显著不同。岩浆成因夕卡岩形成温度主要介于900～1150℃之间，其主要成分与其他成因夕卡岩没有明显区别。     

岩浆夕卡岩的地质特征及其形成机理讨论──以铜陵地区为例

铜陵地区岩浆夕卡岩具有如下特征：（１）既可产于有碳酸盐岩的环境,也可赋存于远离碳酸盐岩的辉石二长闪长岩或角岩中;（２）产出形态复杂多样,可呈透镜状、脉状、小岩床、岩瘤和不规则岩枝状或角砾岩筒等等;（３）与围岩界线截然,在易交代的大理岩中也一样。作为其围岩的大理岩可见港湾状熔蚀边及重结晶褪色边,辉石二长闪长岩出现被烘烤褪色边,夕卡岩边缘部出现矿物呈等轴状三联晶嵌生淬火边;（４）夕卡岩常常与辉石二长闪长岩紧密伴生,构成了特殊的“夕卡岩－富碱侵入岩对”;（５）夕卡岩发育有海绵陨铁结构,豆状构造、流动构造和气孔构造;（６）微量元素、稀土元素的特征与相关的火成岩相似;（７）有熔融包裹体,均一温度高达１０００℃以上;（８）其矿物组成、化学成分与接触交代夕卡岩无明显差别。岩浆夕卡岩是深部中酸性硅酸盐岩浆演化过程中形成的夕卡岩浆冷凝结晶的产物。     

四川攀枝花铁矿碳酸岩的发现和确认——地质特征、斜锆石U-Pb年龄、C—O同位素组成证据

碳酸岩是一类稀少但具有重要地质意义的一种岩石类型,且碳酸岩也往往构成大火成岩省岩石序列中的重要组成部分。我国西南地区晚二叠世峨眉山大火成岩省举世闻名,但此前尚未有同期的火成碳酸岩报道。本项目组在四川攀枝花地区开展的地质填图过程中,识别出大规模的碳酸岩,其沿北东—南西向延伸约20千米,可能是我国最大的岩浆型碳酸岩。大量的暗色纯橄榄岩、二辉橄榄岩及煌斑岩地幔捕虏体是认定岩浆型碳酸岩最直接、最重要的野外证据。碳酸岩主体为侵入岩,其次为少量的喷出岩、火山角砾岩及潜火山岩,空间上与侵入碳酸岩紧邻。碳酸岩侵入岩主要由方解石、白云石、橄榄石、斜方辉石及其蚀变而成的蛇纹石组成,并可根据矿物粒度、含量、结构、构造及颜色,划分出不同的碳酸岩类型。碳酸岩喷出岩由方解石、白云石、暗色矿物及气孔构成; 潜火山岩由火山角砾、火山岩玻璃及方解石胶结物构成,方解石胶结物的形成可能与岩浆晚期的少量气液活动有关。拉纳箐矿区粗粒白云石碳酸岩中斜锆石以及与碳酸岩共生的石英二长岩中锆石的加权平均年龄分别为263. 5±3. 2 Ma (MSWD=0. 41, n=20)和260. 2±1. 7 Ma (WSWD=0. 30,n=20)。大规模碳酸岩的识别完善了峨眉山大火成岩省岩浆岩序列。碳酸岩的δ13CPDB(-9. 6‰~+1. 3‰)及δ18OSMOW (+12. 9‰~+21. 3‰)值与其它典型碳酸岩的同位素值接近。攀枝花铁矿区碳酸岩与冕宁—德昌新生代稀土矿化碳酸岩的岩性组合、矿物组合及蚀变类型差异显著,反映了二者具有不同的成因。前者与喜马拉雅造山运动有关,后者形成于与峨眉地幔柱有关陆内裂谷环境,可能为深部地幔低部分熔融的产物,是真正意义上“干的”岩浆成因的碳酸岩。     

西藏泽当地幔橄榄岩中的异常矿物及其指示意义

雅鲁藏布江蛇绿岩带内多个地幔橄榄岩体产有金刚石、碳硅石等异常矿物组合,为了进一步探讨这些异常矿物形成的物理化学条件,在前人已有的研究基础上,对泽当地幔岩体中526 kg的方辉橄榄岩样品开展人工重砂矿物学研究工作。研究表明,同雅鲁藏布江蛇绿岩带内的其他岩体相似,泽当地幔橄榄岩也选出了包含金刚石、碳硅石、锆石等30余种矿物。异常矿物组合指示泽当地幔橄榄岩中存在局部的超高压、极还原环境,可能经历了复杂的演化过程:即古老地壳物质通过深俯冲或者折沉作用,进入地幔甚至是地幔过渡带(410~660 km),随后经历了超高压、极还原环境的改造,在后续的地幔柱或地幔对流作用中从洋中脊上升至浅部环境并返回到地壳中。该过程中地幔橄榄岩中的异常矿物组合记录了岩石的演化信息,因此开展地幔橄榄岩中异常矿物组合的精细矿物学研究,对认识壳-幔物质交换以及深部地幔动力学过程都有重要的研究意义。     

碳酸岩中高普通铅副矿物原位微区U-Th-Pb测年

<正>岩浆碳酸岩是由超过50%的碳酸盐组成的火成岩石。它不仅与地球的深部碳循环紧密相关,而且与多种矿产资源的成矿作用密切相关,具有重要的经济价值。此外,碳酸岩是鲜少暴露到地表保存有地幔信息的岩石,具有广泛的时空分布,被认为是破译地幔地球化学特征强有力的"探针"。碳酸岩通常在时间和空间上与碱性硅酸盐岩相关联,比如,黄长岩,霞石岩,碧玄岩,响岩,粗面岩,金伯利岩和煌斑岩等。然而,人们至今没有完全了解碳酸岩与其伴生硅酸盐岩岩浆的确切成因关系。两者之间     

Calculated silica activities in carbonatite liquids

Carbonatite magmas precipitate silicates, in addition to the abundant carbonates, oxides, and phosphates. Calculated silica activities for equilibria involving silicates and a silica component in magmatic liquids predict specific assemblages for silicate and oxide phases in carbonatites. These assemblages provide tests of alternative sources (carbonatite magma, coeval silicate magma, or older rock) for silicate minerals in carbonatites. Quartz, feldspars, and orthopyroxene are unlikely to be primary magmatic phases in carbonatites, because the silica activity in carbonatite magmas is too low to stabilize these minerals. Zircon and titanite should be unstable relative to baddeleyite and perovskite, respectively, but they do occur in carbonatites. Liquids dominated by carbonate are strongly nonideal with respect to dissolved silica. Consequently, activity coefficients for a silica component in carbonatite liquids are >>1, so that small mole fractions of SiO₂ translate into silica activities sufficient to stabilize phlogopite, clinopyroxene, amphibole, monticellite, and forsterite, among other silicates. Examination of silicate mineral assemblages in carbonatites in the light of silica activity indicates that many carbonatites are contaminated by solid silicate phases from external sources but these xenocrysts can be discriminated from magmatic minerals.     

Trace elements and REE geochemistry of Siriwasan carbonatite,Chhota Udaipur,Gujarat

The Siriwasan carbonatite-sill along with associated alkaline rocks and fenites is located about 10 km north of the well-known Amba Dongar carbonatite-alkaline rocks diatreme, in the Chhota Udaipur carbonatite-alkaline province. Carbonatite has intruded as a sill into the Bagh sandstone and overlying Deccan basalt. This resulted in the formation of carbonatite breccia with enclosed fragments of basement metamorphics, sandstone and fenites in the matrix of ankeritic carbonatite. The most significant are the plugs of sövite with varied mineralogy that include pyroxene, amphibole, apatite, pyrochlore, perovskite and sphene. REE in sövites is related to the content of pyrochlore, perovskite and apatite. The carbon and oxygen isotopic compositions of some sövite samples and an ankeritic carbonatite plot in the “mantle box” pointing to their mantle origin. However, there is also evidence for mixing of the erupting carbonatite magma with the overlying Bagh limestone. The carbonatites of Siriwasan and Amba Dongar have the same Sr and Nd isotopic ratios and radiometric age, suggesting the same magma source. On the basis of available chemical analyses this paper is aimed to give some details of the Siriwasan carbonatites. The carbonatite complex has good potential for an economic mineral deposit but this is the most neglected carbonatite of the Chhota Udaipur province.     

四川牦牛坪稀土矿床萤石Sr、Nd同位素对地幔成矿流体的指示意义

萤石是四川牦牛坪稀土矿床主要的脉石矿物之一，其形成贯穿了整个稀土成矿过程，因此同位素的研究对探讨萤石和稀土成矿流体的来源具有重要的价值。矿区6件萤石样品的Sr、Nd同位素组成没有明显差异，结合围岩（碳酸岩－正长岩，花岗岩）同位素组成特征研究表明，不同颜色、来自不同矿石类型、具有不同REE类型的萤石为同源产物，稀土成矿流体来源于富集地幔，与区内碳酸岩－正长岩岩浆活动密切相关。     

A model for carbonatite hosted REE mineralisation — the Mianning–Dechang REE belt,Western Sichuan Province,China

The Mianning–Dechang (MD) REE belt of Sichuan, China is one of the most important REE belt in China, which includes Maoniuping, the third largest REE deposit in the world and a series of large to small REE deposits. Mineralization styles varied across the belt, as well as within different parts of the same deposit. Styles include vein-stockworks, pegmatites, breccias and disseminated REE mineralization. Based on geological, geochemical and inclusion studies, this paper proposes a new model for carbonatite hosted REE mineralization. The results show that ore-forming fluid is derived from carbonatite magma, which has high temperature, pressure and density, and is characterized by high K, Na, Ca, Sr, Ba, REE and SO₄ contents. The supercritical ore fluid underwent a distinctive evolution path including phase separation, exolution of sulfate melt and unmixing between aqueous fluid and liquid CO₂. Rapid geochemical evolution of a dense carbonatite fluid causes REE mineralization and associated alteration to occur within or proximal to the source carbonatite. Veins, pegmatites and carbonatite comprise a continuum of mineralization styles. Veins occur in the outer zone of the upper levels of the deposit. Pegmatites occur in the inner zone of upper levels, whereas disseminated REE ore occurs at the base of the carbonatite. High water solubility in the carbonatite magma and low water, high REE in the exsolved ore-forming fluids, imply that a giant carbonatite body and deep magma chamber are not necessary for the formation of giant REE deposits.     

碳酸岩的地质地球化学特征及其大地构造意义

从已知碳酸岩的地质产状、岩石学特征、Nd-Sr-Pb-O-C同位素及痕量元素地球化学特征数据,结合高温高压实验岩石学资料,论述了其地幔源区的物质成分、交代过程软流圈地幔部分熔融机制和碳酸岩岩浆的演化模型。碳酸岩既可以产生于拉张岩石圈构造背景,也能够产生于挤压而派生的引张岩石圈构造背景。前者以产于裂谷环境、与硅酸不饱和过碱性岩构成环状碳酸岩—碱性杂岩为特征,主要由起源于软流圈地幔的霞石质超基性—基性岩浆经液态不混溶作用而形成;后者产于碰撞造山过程中派生的引张岩石圈断裂带,以单一的透镜状、条带状和似层状碳酸岩体为标志,直接由导源岩石圈富集地幔的低程度部分熔融作用而产生的碳酸岩浆侵入或喷发所形成。     

中国东部含金矽卡岩矿床分布规律 与深部地球物理背景研究

在收集整理中国大陆上十几条地学断面、数十条地震剖面、大地电磁剖面、地壳上地幔剪切波资料的基础上,对含金矽卡岩矿床的深部地球物理背景进行了深入的研究,作者认为岩石圈剪切带是控制含金矽卡岩矿床的最主要的深部地球物理背景条件。壳内低速高导层因与地幔流体分布有关,同样对含金矽卡岩矿床有控制作用、大（ 深） 断裂带的交汇部位控制着与矿有关的中酸性侵入岩浆,可以利用这些因素的共同制约作用进一步圈定含金矽卡岩矿床和铜伴生金矽卡岩矿床分布的远景预测区。     

成矿碳酸岩的实验岩石学研究现状与展望

火成碳酸岩及其风化产物是全球战略性关键金属稀土元素(REE)和铌(Nb)的主要来源。因此,对关键金属在火成碳酸岩中的超常富集机理研究具有重要的科学意义。研究表明成矿碳酸岩常常与碱性杂岩体存在密切的时空联系,因而母岩浆应属于碳酸盐化的硅酸盐岩浆,并以霞石岩岩浆为主。针对碳酸岩关键金属矿床的成岩成矿过程,已有实验发现母岩浆在地壳内的演化过程中,既可以通过分离结晶作用,也可以通过液态不混溶作用形成碳酸岩。然而,更加接近自然样品的多组分体系的实验均表明液态不混溶作用总是先于碳酸盐矿物分离结晶作用。因此,液态不混溶作用对关键金属成矿过程有着不可忽视的作用。尽管如此,已有不混溶实验表明当碳酸盐熔体和硅酸盐熔体发生不混溶之后,关键金属REE与Nb总是优先分配到硅酸盐熔体(碱性岩)中,但是在成矿杂岩体中,REE与Nb是高度富集在碳酸岩中。虽然不混溶实验表明REE与Nb在碳酸盐-硅酸盐熔体中的分配系数与含水量有关,即与熔体的聚合程度有关,但是绝大部分成矿碳酸岩成矿过程一般并不富水,所以碳酸岩中REE和Nb等关键金属元素超常富集的机理并不明确。因此未来的研究应重点关注在碳酸岩演化的过程中,除了水以外,其他配体对于关键金属元素在不混溶硅酸盐-碳酸盐熔体之间分配系数是否有影响,从而找到控制碳酸岩中关键金属成矿的关键。     

Experimental constraints on crystallization differentiation in a deep magma ocean

Major and trace element mineral/melt partition coefficients are presented for phases on the liquidus of fertile peridotite at 23-23.5 GPa and 2300 °C. Partitioning models, based on lattice-strain theory, are developed for cations in the ‘8-fold’ sites of majorite and Mg-perovskite. Composition-dependant partitioning models are made for cations in the 12-fold site of Ca-perovskite based on previously published data. D^min/melt is extremely variable for many elements in Ca-perovskite and highly correlated with certain melt compositional parameters (e.g. CaO and Al₂O₃ contents). The 8-fold sites in Mg-perovskite and majorite generally have ideal site radii between 0.8 and 0.9 Å for trivalent cations, such that among rare-earth-elements (REE) D^min/melt is maximum for Lu. Lighter REE become increasingly incompatible with increasing ionic radii. The 12-fold site in Ca-perovskite is larger and has an ideal trivalent site radius of ∼1.05 Å, such that the middle REE has the maximum D^min/melt. Trivalent cations are generally compatible to highly compatible in Ca-perovskite giving it considerable leverage in crystallization models. Geochemical models based on these phase relations and partitioning results are used to test for evidence in mantle peridotite of preserved signals of crystal differentiation in a deep, Hadean magma ocean.Model compositions for bulk silicate Earth and convecting mantle are constructed and evaluated. The model compositions for primitive convecting mantle yield superchondritic Mg/Si and Ca/Al ratios, although many refractory lithophile element ratios are near chondritic. Major element mass balance calculations effectively preclude a CI-chondritic bulk silicate Earth composition, and the super-chondritic Mg/Si ratio of the mantle is apparently a primary feature. Mass balance calculations indicate that 10-15% crystal fractionation of an assemblage dominated by Mg-perovskite, but with minor amounts of Ca-perovskite and ferropericlase, from a magma ocean with model peridotite-based bulk silicate Earth composition produces a residual magma that resembles closely the convecting mantle.Partition coefficient based crystal fractionation models are developed that track changes in refractory lithophile major and trace element ratios in the residual magma (e.g. convecting mantle). Monomineralic crystallization of majorite or Mg-perovskite is limited to less than 5% before certain ratios fractionate beyond convecting mantle values. Only trace amounts of Ca-perovskite can be tolerated in isolation due to its remarkable ability to fractionate lithophile elements. Indeed, Ca-perovskite is limited to only a few percent in a deep mantle crystal assemblage. Removal from a magma ocean of approximately 13% of a deep mantle assemblage comprised of Mg-perovskite, Ca-perovskite and ferropericlase in the proportions 93:3:4 produces a residual magma with a superchondritic Ca/Al ratio matching that of the model convecting mantle. This amount of crystal separation generates fractionations in other refractory lithophile elements ratios that generally mimic those observed in the convecting mantle. Further, the residual magma is expected to have subchondritic Sm/Nd and Lu/Hf ratios. Modeling shows that up to 15% crystal separation of the deep mantle assemblage from an early magma ocean could have yielded a convecting mantle reservoir with ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf isotopic compositions that remain internal to the array observed for modern oceanic volcanic rocks. If kept in isolation, the residual magma and deep crystal piles would grow model isotopic compositions that are akin to enriched mantle 1 (EM1) and HIMU reservoirs, respectively, in Nd-Hf isotopic space.