西北印度洋中脊玄武岩源区地幔特征

利用全球岩石地球化学数据库(Pet DB)中有关卡尔斯伯格洋脊(CR)、北中印度洋脊(NCIR)及南中印度洋脊(SCIR)玄武岩的微量元素及同位素组成数据,分析了玄武岩的元素地球化学特征及其沿脊轴的变化,旨在探讨玄武岩源区地幔的(不)均一性及岩浆作用过程的差异。初步研究结果表明:CR、NCIR及SCIR玄武岩组成相近,仅在个别脊段表现有微量稀土元素和同位素组成上的差异,玄武岩整体与N-MORB组成特征相近,与先前通常认为的典型印度洋中脊玄武岩不同。玄武质岩浆主要源自尖晶石二辉橄榄岩地幔的熔融,岩浆源区主要由两个地幔端元构成,即以亏损型地幔(DMM)为主(69%),其次为富集型地幔(EMⅡ,27%)。富集组分可能源自古老陆壳物质的混染。自CR经NCIR到SCIR整个印度洋中脊西北分支玄武岩的Sr、Nd及Pb同位素组成表现出均一性,表明岩浆源区地幔组成相近。在SCIR 19°S附近脊段岩浆源区地幔存在有不均一性,有EMⅡ型地幔端元混入的迹象。在CR 3.5°N附近脊段,玄武岩明显富集K、Ba、La及U等微量元素,但由于缺少同位素数据,源区地幔特征有待进一步研究。在上述研究成果的基础上,提出了该区大比例尺的调查填图及密集采样和精细室内分析是CR深入研究的基础,同时加强Sr、Nd、Pb及Re、Os、Be等同位素分析测试,可提供揭示CR地幔不均一性的可靠依据,而厘清印度洋型地幔对CR的影响程度则有助于深入认识地幔不均一性的成因及地幔动力学过程。     

东太平洋海隆1.5°N～1.5°S和南大西洋中脊13.2°S玄武岩物质组成

对东太平洋海隆(EPRⅠ区和Ⅱ区)和南大西洋中脊(SMAR)6个站位新鲜的玄武岩样品进行了岩石学及地球化学研究。结果显示,EPR玄武岩样品可分为辉石玄武岩、气孔玄武岩和玻基玄武岩3种类型,SMAR玄武岩样品主要为辉石玄武岩。EPR和SMAR玄武岩样品的标准矿物组合相同,均出现了石英和紫苏辉石标准矿物,为典型的拉斑玄武岩。EPRⅠ区和SMAR玄武岩表现出轻稀土富集的配分模式,可能受到了富集地幔源区(EMORB)的影响,其玄武岩可能形成于未经历早期熔融事件的富集地幔或部分熔融程度相对较低。EPRⅡ区玄武岩为正常型洋中脊玄武岩(N-MORB),其源区为经历了早期熔融事件的、亏损洋中脊地幔源区(DMM),且源岩部分熔融程度较高。EPRⅠ区与Ⅱ区不同的幔源特征说明东太平洋海隆地幔源区存在不均一性。     

印度洋Carlsberg洋脊玄武岩岩石地球化学特征及其地质意义

本文对采自印度洋Carlsberg脊14个站位的新鲜玄武岩样品进行了常量和微量元素组成分析,旨在研究岩浆源区地幔的性质以及岩浆作用过程。研究结果表明:该区玄武岩为典型的源于亏损型地幔的大洋中脊玄武岩,不同样品经历了不同程度的结晶分异作用,演化过程主要受控于橄榄石的结晶分异作用,部分样品中有单斜辉石结晶分异作用的影响,斜长石的结晶分异作用不显著;玄武岩岩浆来源于亏损型尖晶石二辉橄榄岩地幔的熔融,主微量元素组成中尚未见到富集型组分混入的证据;源区地幔不同比例的熔融作用及其后岩浆演化过程的差异是造成不同样品间地球化学性质差异的主要原因,彼此独立的局部岩浆作用过程是岩浆作用差异的主控制因素。Carlsberg脊玄武岩整体与全球标准大洋中脊玄武岩(N-MORB)平均组分相近,不同脊段间岩浆源区地幔的组成、熔融程度(比例)和熔融深度等无明显差异,这种特征向南直到CIR的北段。     

西南印度洋中脊50°E基性超基性岩石地球化学特征及其成因初探

西南印度洋中脊是典型的慢速扩张洋中脊之一。对采自西南印度洋中脊50°E附近的7件玄武岩和蛇纹石化橄榄岩样品所作的分析表明,基性玄武岩类SiO2含量为43.72%～48.40%,TiO2含量较少,为1.14%～1.52%;MgO含量为5.96%～10.98%;TFe2O3含量为4.55%～5.2%;Mg#值为0.53～0.64,里特曼指数σ为2.34～20.10。微量元素Zr/Nb和Y/Nb比值为显示N-MORB的性质,但是其他微量元素的比值(Ba/Nb,Ba/Th,La/Nb,Nb/U,Nb/Pb)均不显示正常洋中脊玄武岩的特征,微量元素原始地幔标准化蛛网图显示强烈富集K和Pb,亏损Nb,稀土元素显示较为平缓的分配模式。超基性蛇纹石化橄榄岩的主量元素特征为SiO2为38.91～45.49;TiO2含量为0.02～0.28;MgO含量很高,为36.87～40.61,TFe2O3含量为2.82～3.91,Mg#值为0.92～0.94。微量元素中Ni,Cr的含量很高,原始地幔标准化蛛网图显示橄榄岩强烈富集K和Pb,Ba,Th,La,Ce,Ti中等程度富集,而亏损Nb,Sr。稀土元素总量较低,标准化曲线显示轻稀土元素富集模式。结合地球化学特征及前人研究资料分析认为,西南印度洋中脊的基性岩和超基性岩属同源性质,其原始地幔物质可能为部分正常洋中脊亏损地幔混染了陆壳或远洋沉积物的结果。     

大西洋中脊玄武岩岩浆源区性质、潜在温度及其指示意义

洋中脊玄武岩（MORB）的微量元素成分和同位素比值具有变化范围大的特点,这些变化很难简单地用地幔部分熔融和结晶分异等岩浆演化过程来解释。传统观点认为洋中脊玄武岩的地球化学成分的多样性是由其下部地幔成分的大尺度不均一性决定的。这种地幔不均一性则是外来物质的加入造成的,如再循环的地壳物质、下大陆岩石圈、交代的岩石圈和外地核等成分加入到上地幔中。在本研究中,我们对大西洋洋中脊的玄武岩展开研究工作,评估了玄武岩源区的温压条件并综合对比了微量元素和同位素比值。靠近地幔柱的洋中脊玄武岩的地球化学和同位素成分具有较大的变化。地幔柱对洋中脊地区的影响范围可以达到1400公里,但并不是每个地幔柱都能够影响其周围1400km范围内的所有洋中脊脊段。未受地幔柱影响的洋中脊玄武岩成分和地幔潜在温度均没有异常表现。我们认为上述现象是由于地幔柱柱头形状不同造成的。地幔柱的流动形状可以分为管状和饼状两种,饼状地幔柱影响其周围的地幔是没有方向性的,而管状地幔柱对其周围地幔的影响在方向上具有选择性。沿着大西洋中脊的玄武岩的元素和同位素比值变化较大,暗示其源区具有较高的不均一性。我们认为该地区地幔不均一性主要是由于上地幔中加入了俯冲板片和拆沉下地壳造成的。另外,地幔柱的活动也不容忽视,它们影响了其周围部分洋脊段的成分变化。     

东太平洋海隆13°N 附近洋中脊玄武岩微量元素特征

对33件采自东太平洋海隆(EPR)13°N附近的玄武岩和火山玻璃样品进行了微量元素分析,以探讨该区域玄武岩的演化过程和物质来源。测试分析结果显示所有样品的微量元素含量比值m(Tb)/m(Lu)(1.74~2.03),m(Sm)/m(Nd)(0.29~0.35)和m(Nd)/m(Y)(0.32~0.48)存在不能忽略的变化,表明它们可能受到了非均一质地幔来源的影响。m(La)/m(Nb),m(La)/m(Sm)和m(La)的线性相关性辨别结果,以及稀土元素分布型式图表现的元素分布特征等均表明东太平洋海隆13°N附近的洋中脊玄武岩可能来自不同的端元组分,也证明研究区域内的玄武岩除N-MORB(常规型MORB)外,还有E-MORB(富集型MORB)。此外,玄武岩m(Ce)/m(Pb)和m(Ce),m(Nb)/m(U)和m(Nb),以及m(Nb)/m(La)和m(La)线性相关性,均显示了这些微量元素比值和微量元素含量的正相关趋势,这可能与双组分地幔熔融有关。m(Nb)/m(Th)和m(Th)线性相关性显示出负相关,显示该区域玄武岩的地幔来源组成可能受到了富集组分的影响。分析显示,样品的Nb*值均大于1,而大部分样品的Ta*值大于1,这表示大部分玄武岩的来源很可能是从俯冲区域循环的物质。     

西南印度洋脊49.6°E热液区热液产物和玄武岩地球化学特征

利用X射线荧光法和ICP-MS等方法对取自超慢速扩张的西南印度洋脊(SWIR) 49.6°E热液区的热液产物和玄武岩样品进行元素地球化学特征分析研究,结果表明:(1)与亏损型洋中脊玄武岩(N-MORB)相比,研究区玄武岩样品的主量元素组成显示其偏碱性,而微量元素对比表明该区玄武岩明显富集Pb元素;(2)对热液产物的综合分析表明这些样品多为Fe-Si-Mn氧羟化物且都为热液来源;(3)热液产物的∑REE含量介于玄武岩和海水之间,经球粒陨石标准化的稀土元素(REE)分布模式均表现出Eu正异常和轻稀土(LREE)富集的特征。另外,本研究还表明,利用玄武岩和热液产物地球化学指标不仅能够模拟出以热液喷口为中心的元素地球化学晕,而且能反映出热液活动的影响范围。     

大洋地幔化学组成不均一性成因研究回顾及展望

认识地幔组成不均一性及其成因对于揭示固体地球的演化规律具有重要意义。简要论述了全球典型大洋玄武岩(洋岛/海山玄武岩(OIB)、洋中脊玄武岩(MORB))源区组成不均一性的化学特征及成因,并分析了国内外对地幔组成不均一性的认识不足之处和原因。30多年以来,玄武岩地球化学研究主要围绕地幔组成端元成分差异性及其成因,包括HIMU(‘μ’=~(238)U/~(204)Pb)、EMI和EMII及FOZO(同位素组成介于HIMU和MORB之间)富集端元,以及DMM亏损地幔端元(包括印度洋型(Indian-type MORB)和太平洋型(Pacific-type MORB)。富集地幔端元通常被认为与板块构造导致的地球化学循环有关,然而,这些端元的成因存在多解性。尽管过去常将亏损地幔作为一个地幔端元,但全球主要地幔库的亏损端元之间的同位素差别也是长期演化的结果,地幔亏损端元组成差异的研究也是至关重要的。地幔端元成因的多解性主要是由于对板块构造导致物质循环的关键环节了解不够,以及对地球早期熔融导致的上地幔亏损过程的认识不足。在总结研究现状和科学问题的基础上,本文指出地幔不均一性成因研究的潜力方向和方法:(1)深化对玄武质洋壳深部地幔压力下的物理化学相变研究,认识再循环洋壳重返浅部地幔的基本理论前提;(2)利用年轻的大陆裂张海盆玄武岩,有效检验大陆富集物质是否拆离进入地幔软流圈;(3)碳酸岩熔体来源及其对碱性玄武岩富集端元组成的贡献;(4)板块俯冲进入地幔过程中化学分异过程。     

洋中脊与地幔柱热点相互作用研究进展

地表热点是认识地幔柱假说以及地幔柱动力学的窗口,发生在洋脊与热点之间的相互作用则是了解地球上两大动力系统(板块构造和地幔柱)的直接联系的最有利地区。研究洋脊-热点之间的相互作用对于揭示地幔动力学、热点附近洋壳构造的演变以及与热点密切相关的洋中脊处的岩浆热液活动具有重要的意义。在肯定地幔柱假说的基础上,对洋脊-地幔柱(热点)的模拟实验以及三大洋中不同扩张脊与热点相互作用的最新研究作了系统的介绍和评述,指出室内模拟实验以及地质学、岩石学、地球化学和地球物理学(特别是高分辨率的地震技术)的结合研究将是本领域研究的发展趋势。     

全球三大洋底高原重力异常与地壳厚度特征及对比研究*

翁通爪哇高原、凯尔盖朗高原与沙茨基海隆是全球三大洋底高原, 是大量岩浆喷发到地表的结果, 火山面积分别达1.90×10⁶、1.25×10⁶、0.53×10⁶km²。本文详细分析了该三大洋底高原的地形、剩余地幔布格重力异常(residual mantle Bouguer anomaly, RMBA)与重力反演的相对地壳厚度, 并结合地质与地球化学特征约束进行对比研究。结果显示, 翁通爪哇高原、凯尔盖朗高原与沙茨基海隆分别高出周围海底约4.3、5、4km, 相应的地幔布格重力异常最大变化值分别为250、330、200mGal, 以及相应的相对地壳厚度变化分别为11、13、9km, 表明形成三大洋底高原的岩浆量远远大于正常洋中脊的岩浆量。此外, 三大洋底高原皆形成于洋中脊附近。Nd、Pb、Hf同位素比值分析表明, 翁通爪哇高原的玄武岩组分为洋岛玄武岩; 凯尔盖朗高原大部分类似于洋岛玄武岩, 并含有洋中脊玄武岩组分; 沙茨基海隆的玄武岩组分主要为东太平洋海隆正常洋中脊玄武岩, 却又存在少量位于全球洋岛玄武岩范围内。这些特征揭示了三大洋底高原可能形成于“地幔柱-洋中脊相互作用”。对此本文提出了两种模式: 一为洋中脊被地幔柱拖拽至其上方; 二为洋中脊之下的软流圈受到地幔柱影响, 从而产生超常熔融与超厚地壳。     

Mantle source heterogeneity and magmatic evolution at Carlsberg Ridge (3.7°N): constrains from elemental and isotopic (Sr,Nd, Pb) data

We present new major element, ICP-MS trace element, and Sr–Nd–Pb isotope data of basalts from four locations along the Carlsberg Ridge (CR), northern Indian Ocean. The basalts are low-K tholeiites with 7.52–9.51 wt% MgO, 49.40–50.60 wt% SiO₂, 0.09–0.27 wt% K₂O, 2.55–2.90 wt% Na₂O, and 0.60–0.68 Mg^#. Trace element contents of the basalts show characteristics similar to those of average normal MORB, such as LREE depleted patterns with (La/Sm)_N ratio of 0.55–0.69; however, some samples are enriched in large-ion lithophile elements such as K and Rb, suggesting probable modification of the mantle source. Poor correlations between the compatible elements [e.g. Ni, Cr, and Sr (related to olivine, clinopyroxene and plagioclase, respectively)] and the incompatible elements (e.g. Zr and Y), and positive correlations in the Zr versus Zr/Y and Nb versus Nb/Y plots suggest a magmatic evolution controlled mainly by mantle melting rather than fractional crystallization. Our results extend the CR basalt range to higher radiogenic Pb isotopes and lower ¹⁴³Nd/¹⁴⁴Nd. These basalts and basalts from the northern Indian Ocean Ridge show lower ¹⁴³Nd/¹⁴⁴Nd and higher ⁸⁷Sr/⁸⁶Sr values than those of the depleted mantle (DM), defining a trend towards pelagic sediment composition. The Pb isotopic ratios of basalts from CR 3–4°N lie along the compositional mixing lines between the DM and the upper continental crust. However, the low radiogenic Pb of basalts from CR 9–10°N lie on the mixing line between the DM and lower continental crust. Since the Pb isotopic ratio of MORB would decrease if the source mantle was contaminated by continental lithospheric mantle, we suggest that CR contains continental lithospheric material, resulting in heterogeneous mantle beneath different ridge segments. The continental lithospheric material was introduced into the asthenosphere before or during the breakup of the Gondwana. These results support the long-term preservation of continental material in the oceanic mantle which would significantly influence the isotopic anomaly of the Indian Ocean MORB.     

Emergence and petrology of the Mendocino Ridge

The Mendocino Fracture Zone, a 3,000-km-long transform fault, extends from the San Andreas Fault at Cape Mendocino, California due west into the central Pacific basin. The shallow crest of this fracture zone, known as the Mendocino Ridge, rises to within 1,100 m of the sea surface at 270 km west of the California Coast. Rounded basalt pebbles and cobbles, indicative of a beach environment, are the dominant lithology at two locations on the crest of Mendocino Ridge and a⁴⁰Ar/³⁹ Ar incremental heating age of 11.0 ± 1.0 million years was determined for one of the these cobbles. This basalt must have been erupted on the Gorda Ridge because the crust immediately to the south of the fracture zone is older than 27 Ma. This age also implies that the crest of Mendocino Ridge was at sea level and would have blocked Pacific Ocean eastern boundary currents and affected the climate of the North American continent at some time since the late Miocene. Basalts from the Mendocino Fracture Zone (MFZ) are FeTi basalts similar to those commonly found at intersections of mid-ocean ridges and fracture zones. These basalts are chemically distinct from the nearby Gorda Ridge but they could have been derived from the same mantle source as the Gorda Ridge basalts. The location of the 11 Ma basalt suggests that Mendocino Ridge was transferred from the Gorda Plate to the Pacific Plate and the southern end of Gorda Ridge was truncated by a northward jump in the transform fault of MFZ.     

西南印度洋中脊玄武岩地球化学特征及其对岩浆过程的指示意义

对采自西南印度洋中脊(SWIR)50°E附近5个站位的玄武岩样品进行了岩石学和元素地球化学研究.样品主量元素、TAS分类图解和AFM图解显示,SWIR研究区样品类型主要为低钾拉斑玄武岩.相对原始地幔SWIR区玄武岩具有Ba、Nb、Sr负异常,K表现为正异常.稀土元素分配模式均为左倾型,具有轻微的Eu、Ce正异常;SWI...     

Seismic refraction observations in the Transkei Basin and adjacent areas

Some seismic refraction observations undertaken during the IGY are reported here together with a summary of other refraction studies carried out within the Transkei Basin, the Mozambique Ridge and the South African continental shelf area.A 2.5 km section of Cretaceous and younger rocks is associated with profiles observed on the continental shelf; directly below this group are rocks with velocities in the range 4.0–5.5 km s^-1, probably representatives of the Karroo and Cape supergroups. The basement material velocity variations were from 5.3 to 6.5 with an average of 5.9 km s^-1, and is correlated with granite or Malmesbury Formation plus granite. This crustal structure is similar to that found on the eastern continental shelf of southern South America.The profiles in the Transkei Basin show a thick layer of sediment with velocity range 1.50 to 3.50 km s^-1, underlain by a refracting layer in which the average velocity is 4.5 km s^-1. The velocity of 6.6 km s^-1 obtained for the oceanic layer is similar to the velocities of the crustal layer measured in the Argentine Basin. The mantle velocity (8.1 km s^-1) is consistent with the average mantle velocity for the Indian Ocean but significantly lower than the Pacific Ocean average of 8.20 km s^-1. The depth to Moho is about 12.0 km and the crustal section is typical oceanic. A plate tectonic model of the early opening of the South Atlantic is used to describe the evolution of the Transkei Basin.On the Mozambique Ridge the thin sediments (0.7 km) are underlain by rocks with velocities averaging 5.6 km s^-1. This is more than 1.0 km s^-1 faster than the velocity for layer 2 from the Transkei Basin and the Agulhas Plateau, indicating rocks of a younger age or of a different type. Moreover the crustal section of the Ridge has a thickness in excess of 22 km and is in isostatic equilibrium when compared with the adjacent Transkei Basin and Agulhas Plateau. DSDP site 249, situated on the Ridge, penetrated basalt at a depth of 0.4 km. Whether this is continental or oceanic basalt is not known; when this site 249 basalt was compared to the cored basalts of the adjacent Mozambique Basin, inconclusive results were obtained. The essential constitution of the Mozambique Ridge remains an enigma, but solution of this problem is vital for the proper understanding of the Mesozoic history of this oceanic region.     

冲绳海槽中部伊平屋脊火山岩地球化学和Sr-Nd-Pb同位素组成：对岩石成因和地幔源区的指示

As an active back-arc basin, the Okinawa Trough is located in the southeastern region of the East China Sea shelf and is strongly influenced by the subduction of the Philippine Sea Plate. Major element, trace element and Sr-NdPb isotopic composition data are presented for volcanic rocks from the Iheya Ridge(IR), the middle Okinawa Trough. The IR rocks record large variations in major elements and range from basalts to rhyolites. Similar trace element distribution characteristics together with small variations in ~(87)Sr/~(86)Sr(0.703 862–0.704 884), ~(144)Nd/~(143)Nd(0.512 763–0.512 880) and Pb isotopic ratios, demonstrate that the IR rocks are derived from a similar magma source. The fractional crystallization of olivine, clinopyroxene, plagioclase, and amphibole, as well as accessory minerals, can reasonably explain the compositional variations of these IR rocks. The simulations suggest that approximately 60% and 75% fractionation of an evolved basaltic magma can produce trace element compositions similar to those of the intermediate rocks and acid rocks, respectively. The analysis of their Sr-Nd-Pb isotopic content ratios suggest that the source of the rocks from the IR is close to the depleted mantle(DM) but extends to the enriched mantle(EMII), indicating that the mantle source of these rocks is a mixture between the DM and EMII end members. The simulations show that the source of the IR volcanic rocks can be best interpreted as the result of the mixing of approximately 0.8%–2.0% subduction sediment components and 98.0%–99.2% mantlederived melts.     

Seamount chains formed near the crest of Juan de Fuca Ridge,northeast Pacific Ocean

Heck and Heckle are seamount chains trending approximately northwest on the western flank of Juan de Fuca Ridge near its northern end. Evidence from magnetic anomalies and from chemistry and relative ages of dredged basalt suggests that the seamounts in these chains are produced near the spreading centre on Juan de Fuca Ridge and do not continue to grow as they are carried away by sea-floor spreading. Their development is possibly related to transverse fractures on Juan de Fuca Ridge resulting from reorientation of the ridge from north to north-northeast which began about 8 m.y. ago, combined with tension in the Pacific Plate. In contrast the Eickelberg Chain to the south may have been produced by a fixed-mantle plume now located near Juan de Fuca Ridge, as suggested by limited basalt geochemistry and by the long and productive life of that chain. The Pratt-Welker Chain may also have been produced by a mantle plume, but most other seamounts on the western flanks of Juan de Fuca and Explorer ridges are thought to have formed at crustal fractures near the spreading centres in the same way as the seamounts of the Heck and Heckle chains.     

Features of the crustal structure of the Arabian Sea

An analysis of the gravity field and geoid heights allowed us to distinguish a third buried basin filled with sediments located in the southwestern part of the sea in the regions adjacent to the Carlsberg Ridge. From the previously known basins, it is separated by saddles. The saddles correspond to a series of faults and are possibly related to the pulse character of the northwestward prograding of the spreading axes of the Carlsberg Ridge. The continental origin of the Laxmi ridge is confirmed. The results of an analysis of the gravity field and its transformants, together with the two-dimensional density modeling, agree with the possibility of the existence of a spreading type of the crust (I) in the region of the Laxmi Basin. An analysis of the geoid height anomalies allows us to suggest that, with respect to the upper layers of the lithosphere, the Laxmi Ridge is not connected with the Chagos-Laccadive Ridge.