共查询到20条相似文献，搜索用时 171 毫秒
1.
在大西洋洋中脊北纬14°97′附近贫岩浆型慢速扩张脊区段的洋底岩石调查中,拖网取样及洋底钻探均发现蛇纹石化地幔岩石中有脉状纯橄岩。纯橄岩中的铬铁矿粒度以毫米级为主,个别颗粒直径接近厘米级别,成分上显示中等程度的铬值与镁值(Cr#=52.2～56.1;Mg#=44.6～51.5)较高的TiO2含量(0.60%～0.91%)。铬铁矿内部包裹体中大量出现韭闪石+金云母组合,并发现自形磷灰石包裹体。这些特征与“俯冲带型铬铁矿”的成分特征及矿物包裹体组合非常类似。本文的样品观察说明在洋中脊下方的熔体通道内,高度演化的熔体可以形成小体积的富水岩浆并参与铬铁矿的形成。拖网调查与钻探研究证实,研究区的纯橄岩、异剥橄榄岩、富橄榄石橄长岩、辉石岩、辉长岩及橄榄辉长岩等均以脉体形式产于难熔的大洋岩石圈橄榄岩质地幔中。这些脉状岩石共同记录了洋中脊玄武质熔体向上迁移过程中与围岩方辉橄榄岩间的熔-岩反应,属于“交代橄榄岩”的范畴,揭示了慢速扩张洋脊之下熔体迁移诱发的复杂熔-岩相互作用。 相似文献
2.
洋脊俯冲及其在新疆阿尔泰地区存在的可能证据 总被引：5，自引：1，他引：4
本文综述了洋脊俯冲的研究历史与现状,论述了与洋脊俯冲相关的各种构造运动、岩浆活动以及成矿特点。同时,系统总结了阿尔泰南缘已报道的可能与洋脊俯冲有关的证据,如埃达克岩和富铌玄武岩、玻安岩、苦橄岩、A型花岗岩、双峰式火山岩、阿拉斯加型杂岩、酸性岩墙群等特殊的岩石类型,以及可能与洋脊俯冲相关的变质变形作用,认为洋脊俯冲模式可以合理的解释阿尔泰南缘的这些岩石组合以及变质变形作用。此外,分析了晚古生代阿尔泰南缘洋脊俯冲的时空分布,阐述了洋脊俯冲对中亚造山带陆壳增生及成矿的意义。 相似文献
3.
苦橄岩是一种高温下形成的超镁铁质熔岩,形成温度一般高出地幔部分熔融而成的玄武岩300~400℃,可以为判断热点提供依据。该文以GEOROC和PetDB两个地质数据库为基础,通过对其中新生代苦橄岩年龄及构造环境数据的统计分析,以期查明新生代苦橄岩的时空分布特征和厘定出可能的高温热点分布情况。研究表明,新生代有苦橄岩产出的火山活动自中新世开始变得活跃,在更新世和全新世达到顶峰。同时,苦橄岩产出环境多样且具有演变特征,中新世前以CFB和洋岛环境为主,中新世后转为岛弧和洋岛环境为主,更新世和全新世则主要为洋岛环境,总体上来看新生代苦橄岩主要产于大洋环境,并以洋岛环境为主。另外,根据苦橄岩的出露特征可得出现代高温热点可能分布在夏威夷群岛、科隆群岛(厄瓜多尔)、冰岛、加那利群岛、欧洲造山带德国境内的埃费尔山地区、红海洋脊、东非裂谷处、法属留尼汪岛、中国东南沿海广东省境内以及堪察加岛弧10个地区。 相似文献
4.
熊庆 《矿物岩石地球化学通报》2021,40(5):前插1-前插3,999-1011
大洋岩石圈形成演化的关键过程是熔体如何从软流圈地幔中抽取、迁移汇集并最终形成洋壳.蛇绿岩可能为揭示上述过程提供关键对象.前人研究发现,熔体在地幔中主要通过网脉状纯橄岩通道发生迁移汇聚,这种地幔通道与壳幔过渡带和洋壳内的岩浆体系共同构成大洋扩张中心的巨型熔体抽取系统.地幔深部来源的熔体在通过纯橄岩通道时,会发生复杂的熔岩相互作用,使得熔体和地幔的组成均发生变化,形成大洋岩石圈壳-幔组成解耦现象.在不同的大洋扩张中心,地幔内熔体迁移方式和过程是多样的,导致大洋岩石圈的结构和组成复杂多变.我国西藏雅江带蛇绿岩形成于多种构造体制(洋中脊、俯冲带等),其地幔内发育较为新鲜的纯橄岩通道样品,如纯橄岩、铬铁矿岩、辉石岩等.这些样品能够为深入研究地幔纯橄岩通道的成因、熔体在通道内的反应迁移过程,以及不同构造体制下大洋岩石圈壳-幔组成解耦与否的原因,提供新的窗口,同时有助于进一步认知大洋扩张中心巨型岩浆-动力系统、壳幔圈层分异和地球物质循环过程. 相似文献
5.
6.
青藏高原东南木里地区二叠纪苦橄岩的Os-Sr-Nd同位素地球化学研究 总被引：3，自引：2，他引：1
本文报道了在青藏高原东南木里地区发现的二叠纪苦橄岩和与其共生玄武岩的主微量元素地球化学特征以及Os-Sr-Nd同位素组成。苦橄岩和与其共生玄武岩受地壳混染作用影响较小。根据苦橄岩的Ti/Y比值和初始的Os同位素组成,将木里苦橄岩分为两类:高Ti/Y型苦橄岩和低Ti/Y型苦橄岩,其中高Ti/Y型苦橄岩具有高的γ_(Os)= 5.3～ 10.7和ε_(Nd)= 5.9～ 6.4,与全球典型洋岛玄武岩的Os和Nd同位素组成接近,代表了地幔柱源区的同位素特征;而低Ti/Y型苦橄岩具有低的γ_(Os)=-4.1～ 1.2和ε_(Nd)= 3.2～ 5.0,可能表明受到了SCLM(大陆岩石圈地幔)源区物质的混染。与其共生的玄武岩具有低的γ_(Os)=-3.5～-1.6和ε_(Nd)=-0.6～ 0.7,表明其来自于不同于低Ti/Y型苦橄岩也有异于高Ti/Y型苦橄岩的地幔源区,但是也可能受到了SCLM物质的混染。基于Nd-Os同位素的地幔柱与SCLM的二端元混合模型显示:低Ti/Y型苦橄岩可能是SCLM物质组分与地幔柱起源的苦橄质原始岩浆混合形成的;与苦橄岩共生的玄武岩可能是由地幔柱来源的玄武质岩浆与SCLM小比例熔融的熔体混合形成的。 相似文献
7.
近期对科马提岩以及许多大火成岩省中的苦橄岩进行的水含量分析以及地幔潜能温度的研究表明,无论是太古宙还是显生宙的大火成岩省的形成都和含水的地幔柱有关。晚二叠纪的峨眉山大火成岩省（ELIP）位于扬子克拉通西缘,目前主流观点认为其是由地幔柱形成的。前人根据大量的岩石地球化学工作将ELIP分为西区、中区和东区;证明了位于西区的大理、宾川的苦橄岩和玄武岩地幔源区的水含量高于2500×10-6。然而对于其他区域玄武岩源区的含水性还不清楚。文章以位于中区的二滩剖面底部高钛型玄武岩为研究对象,采用单斜辉石斑晶反演的方法研究恢复了其原始岩浆的水含量。结果表明,单斜辉石斑晶水含量范围为76×10-6 ~424×10-6, 对应的平衡熔体水含量为3.01 wt%。在考虑分离结晶影响后,恢复的原始岩浆水含量达到2.71±0.95 wt%。该水含量略低于大理苦橄岩水含量,与宾川苦橄岩相当。而计算的地幔源区水含量最低估计为1357×10-6,该值低于大理、宾川苦橄岩的源区水含量,但仍显著高于正常洋中脊玄武岩和洋岛玄武岩源区。ELIP中不同区域的苦橄岩和玄武岩都存在高水含量,这表明在ELIP的形成和演化过程中水都扮演了很重要的角色。 相似文献
8.
阿尔泰造山带南缘中泥盆世苦橄岩及其大地构造和岩石学意义 总被引：22，自引：1，他引：21
阿尔泰造山带南缘中泥盆世苦橄岩位于北塔山组地层的下部, 其上依次为玄武岩和安山岩.3种岩性共同的特点是贫钛、富铁, 具Nb和Ta的负异常以及高场强元素的丰度与MORB相当, 具有典型的岛弧火山岩系的特点, 是准噶尔洋板块向南西俯冲的结果.苦橄岩和玄武岩的Zr/Nb和Sm/Nd比值与MORB相当, 表明其源区为亏损的MORB源.然而玄武岩的Ti/V和Zr/Sm比值均高于苦橄岩, 而且玄武岩的稀土元素配分曲线呈平缓型, 而苦橄岩则显示出低的稀土总量以及弱富集轻稀土型, 指示了玄武岩是被从俯冲的洋壳释放的流体交代的含角闪石的尖晶石橄榄岩的地幔源区低程度部分熔融形成的, 苦橄岩则是在高温条件下被流体交代过的石榴石橄榄岩高程度熔融的产物.安山岩则可能是榴辉岩部分熔融形成的. 相似文献
9.
《地质通报》2020,(9)
白云山蛇绿混杂岩带位于内蒙古北山造山带中部,呈北西西向展布,向东延伸至月牙山-洗肠井蛇绿混杂岩带,向西延伸至牛圈子-红柳园蛇绿混杂岩带。白云山蛇绿混杂岩带内发育俯冲期的糜棱面理褶皱、拼贴期逆冲断层系及隆升期走滑断层系3期构造变形,由不同类型的岩块与基质组成,岩块主要包括纯橄岩、辉橄岩、橄辉岩、辉石岩、碳酸盐化超基性岩、辉长岩、玄武岩、斜长花岗岩蛇绿岩岩块及硅质岩、灰岩和砂岩岩块,基质主要为蛇纹岩、绿泥片岩及砂板岩。在蛇绿混杂岩带中部发现保存较完整的洋壳残片,由南向北依次出露堆晶超镁铁质岩、堆晶辉长岩及变质玄武岩。结合大洋中脊玄武岩、洋岛玄武岩及晚寒武世岛弧钙碱性辉长岩的识别,认为白云山蛇绿混杂岩带寒武纪发育MOR型、OIB型、SSZ型等不同构造环境的蛇绿岩岩块,俯冲作用持续到晚志留世。 相似文献
10.
准噶尔北缘中泥盆统北塔山组富辉橄玄岩的岩石成因及其对晚古生代洋脊俯冲的启示 总被引：1，自引：0，他引：1
准噶尔北缘中泥盆统北塔山组富辉橄玄岩是一种富含单斜辉石斑晶的岛弧玄武岩,被认为是洋脊俯冲作用的产物,但具体成因机制并不清楚。本文对老山口地区富辉橄玄岩进行了岩石地球化学研究,同时系统测定了其单斜辉石斑晶的成分及其环带变化规律。结果表明,富辉橄玄岩的相容元素和不相容元素的变化规律具有岩浆混合作用的特征,而透辉石斑晶的反环带则将岩浆混合作用限定于高Mg~#熔浆的上涌过程。富辉橄玄岩极高的相容元素含量将上涌的高Mg~#熔浆限定为苦橄质熔浆,而富辉橄玄岩中顽透辉石是被苦橄质熔体携带而来的斑晶物质。主导老山口富辉橄玄岩形成的岩浆过程是苦橄质熔浆的连续补给过程,而苦橄质熔浆被认为是板片窗处软流圈高程度部分熔融的产物。老山口富辉橄玄岩形成的岩浆过程与洋脊俯冲的板片窗模型吻合,因此老山口富辉橄玄岩可能是洋脊俯冲的产物。 相似文献
11.
12.
Hugh Rollinson 《地学前缘(英文版)》2017,8(6):1253-1262
Oman has two ophiolites – the better known late Cretaceous northern Oman (or Semail) ophiolite and the lesser known and smaller, Jurassic Masirah ophiolite located on the eastern coast of the country adjacent to the Indian Ocean. A number of geological, geochronological and geochemical lines of evidence strongly suggest that the northern Oman ophiolite did not form at a mid-ocean ridge but rather in a supra-subduction zone setting by fast spreading during subduction initiation. In contrast the Masirah ophiolite is structurally part of a series of ophiolite nappes which are rooted in the Indian Ocean floor. There are significant geochemical differences between the Masirah and northern Oman ophiolites and none of the supra-subduction features typical of the northern Oman ophiolite are found at Masirah. Geochemically Masirah is MORB, although in detail it contains both enriched and depleted MORB reflecting a complex source for the lavas and dykes. The enrichment of this source predates the formation of the ophiolite. The condensed crustal section on Masirah (ca. 2 km) contains a very thin gabbro sequence and is thought to reflect its genesis from a cool mantle source associated with the early stages of sea-floor spreading during the early separation of eastern and western Gondwana. These data suggest that the Masirah ophiolite is a suitable analogue for an ophiolite created at a mid-ocean ridge, whereas the northern Oman ophiolite is not. The stratigraphic history of the Masirah ophiolite shows that it remained a part of the oceanic crust for ca. 80 Ma. The chemical variability and enrichment of the Masirah lavas is similar to that found elsewhere in Indian Ocean basalts and may simply reflect a similar provenance rather than a feature fundamental to the formation of the ophiolite. 相似文献
13.
GAO StephenShangxing LIU KellyHong FU Xiaofei Haider DAHM KONG Fansheng LIU Lin Cory REED SUN Muchen YU Youqiang 《《地质学报》英文版》2019,93(Z2):3-4
The North Qaidam is an Early Paleozoic UHP metamorphic belt located at the north margin of the Tibet plateau. Eclogites in this belt contain both continental‐and oceanic‐type ones. In which, the continental‐type eclogites have protolith ages of 750–850 Ma and WPB or CFB geochemical signatures and are believed to have formed in a continental rift or an incipient oceanic basin setting related to the breakup of the Rodinia supercontinent, their metamorphic ages (421–458 Ma) and P–T paths are comparable to their host gneisses; oceanic‐type eclogites have cumulate gabbro or E‐MORB geochemical signatures, their protolith and metamorphic ages are 510–516 Ma and 425–450 Ma, respectively(Zhang et al., 2008). Therefore, the North Qaidam UHP belt was thought to record the whole Neoprotoerozoic–Paleozoic Wilson cycle (Song et al., 2014). In this study, we reported three new kinds of eclogites: kyanite‐bearing eclogite, lawsonite pseudomorph‐bearing eclogite and double mineral eclogite. They occur as big lentoid blocks in regional granitic gneiss in the western part of the belt. Phase equilibrium modelling and zircon LA‐ICPMS U‐Pb dating show that all these three eclogites experienced a clockwise P–T path with peak metamorphic conditions close to or fall in the coesite stability field, and their peak metamorphic age were around 436‐439 Ma, similar to those continental‐type eclogites in this belt. But their protolith ages are between 1273 and 1070 Ma, and some of them recorded an amphibolite facies metamorphic age of 927 Ma, and geochemical data and zircon Lu‐Hf and O isotope analysis indicate these eclogites have features of present day N‐MORB. Combined with the existing results, we propose that the North Qaidam is a polycyclic composite orogenwhich recorded tectonic evolution of Mesoproterozoic ocean floor spreading, assembly and breakup of Rodinia supercontinent, Early Paleozoic oceanic deep subduction and subsequently continental deep subduction. 相似文献
14.
Sills, pillow lavas and hyaloclastites of the HFSE-rich picrite and related rocks (ankaramite and basanite) occur in the Middle Permian cherts in the Mino Jurassic accretionary complex, southwestern Japan. These rocks show systematic trace element patterns enriched in incompatible elements, which indicate that the associated ankaramite and basanite are formed by the crystal fractionation from the picrite. The presence of the hyaloclastite in the chert sequence covering a large tholeiitic greenstone body indicates that the picrite was produced in an intraoceanic setting in the Middle Permian time subsequent to the extrusion of the voluminous oceanic island tholeiite. The Mino picrites resemble the Siberian meimechite and Polynesian picrites in its HFSE-rich chemical composition. The HFSE enrichment in these picrites cannot be explained by low degree of partial melting of primitive peridotite mantle only, and needs a source material involving recycled oceanic crust (eclogite). The differences in MgO content and in TiO2/Al2O3 and Zr/Y ratios among the HFSE-rich picrites indicate that the melting pressure increases from the Polynesian picrite through Mino picrite to Siberian picrite. This may reflect the increasing thickness of the overlying lithosphere at the time and place of magmatism. The HFSE-rich picrites may be a product of a superplume event. The presence of HFSE-rich picrite in Mino and Siberia indicate that the superplume activities occur in both continental and oceanic settings in the Permian time. 相似文献
15.
Hydrous partial melting in the sheeted dike complex at fast spreading ridges: experimental and natural observations 总被引：5，自引：1，他引：4
Lydéric France Juergen Koepke Benoit Ildefonse Sarah B. Cichy Fabien Deschamps 《Contributions to Mineralogy and Petrology》2010,160(5):683-704
In ophiolites and in present-day oceanic crust formed at fast spreading ridges, oceanic plagiogranites are commonly observed
at, or close to the base of the sheeted dike complex. They can be produced either by differentiation of mafic melts, or by
hydrous partial melting of the hydrothermally altered sheeted dikes. In addition, the hydrothermally altered base of the sheeted
dike complex, which is often infiltrated by plagiogranitic veins, is usually recrystallized into granoblastic dikes that are
commonly interpreted as a result of prograde granulitic metamorphism. To test the anatectic origin of oceanic plagiogranites,
we performed melting experiments on a natural hydrothermally altered dike, under conditions that match those prevailing at
the base of the sheeted dike complex. All generated melts are water saturated, transitional between tholeiitic and calc-alkaline,
and match the compositions of oceanic plagiogranites observed close to the base of the sheeted dike complex. Newly crystallized
clinopyroxene and plagioclase have compositions that are characteristic of the same minerals in granoblastic dikes. Published
silicic melt compositions obtained in classical MORB fractionation experiments also broadly match the compositions of oceanic
plagiogranites; however, the compositions of the coexisting experimental minerals significantly deviate from those of the
granoblastic dikes. Our results demonstrate that hydrous partial melting is a likely common process in the root zone of the
sheeted dike complex, starting at temperatures exceeding 850°C. The newly formed melt can either crystallize to form oceanic
plagiogranites or may be recycled within the melt lens resulting in hybridized and contaminated MORB melts. It represents
the main MORB crustal contamination process. The residue after the partial melting event is represented by the granoblastic
dikes. Our results support a model with a dynamic melt lens that has the potential to trigger hydrous partial melting reactions
in the previously hydrothermally altered sheeted dikes. A new thermometer using the Al content of clinopyroxene is also elaborated. 相似文献
16.
潮汐应力引起地壳岩石的循环应变、累积疲劳损伤,形成疲劳断裂。本文给出疲劳断裂野外宏观地质特征,并对显微构造、小构造、大陆与大洋区域构造、第四纪软岩层中的疲劳断裂实例进行了分析,指出部分石英变形纹就是疲劳纹,以及环太平洋地震带、海底磁异常条带和潮汐作用、疲劳断裂机制的相关性;给出累积损伤-微体积的玄武岩浆上侵-沿疲劳断裂上涌-海底扩张的模式;推测大陆与海洋地壳的疲劳寿命。 相似文献
17.
M. C. Bruce Y. Niu T. A. Harbort R. J. Holcombe 《Australian Journal of Earth Sciences》2013,60(6):1053-1064
Petrological, geochemical and radiogenic isotopic data on ophiolitic‐type rocks from the Marlborough terrane, the largest (～700 km2) ultramafic‐mafic rock association in eastern Australia, argue strongly for a sea‐floor spreading centre origin. Chromium spinel from partially serpentinised mantle harzburgite record average Cr/(Cr + Al) = 0.4 with associated mafic rocks displaying depleted MORB‐like trace‐element characteristics. A Sm/Nd isochron defined by whole‐rock mafic samples yields a crystallisation age of 562 ± 22 Ma (2σ). These rocks are thus amongst the oldest rocks so far identified in the New England Fold Belt and suggest the presence of a late Neoproterozoic ocean basin to the east of the Tasman Line. The next oldest ultramafic rock association dated from the New England Fold Belt is ca530 Ma and is interpreted as backarc in origin. These data suggest that the New England Fold Belt may have developed on oceanic crust, following an oceanward migration of the subduction zone at ca540 Ma as recorded by deformation and metamorphism in the Anakie Inlier. Fragments of late Neoproterozoic oceanic lithosphere were accreted during progressive cratonisation of the east Australian margin. 相似文献
18.
新疆阿尔泰山南缘玛音鄂博断裂南侧变质基性岩的发现及其地质意义 总被引：4，自引：0，他引：4
位于额尔齐斯-玛音鄂博大断裂带南侧的阿热勒托别变质基性岩产于下石炭统姜巴斯套组一套细碎屑沉积岩中.岩石组成以变质玄武岩为主, 它们具有相对高的TiO2、MgO和低SiO2、K2O的特点; 在稀土元素组成上, 显示轻稀土弱富集的配分模式, 无明显Eu异常; 微量元素组成显示大离子亲石元素富集, 并具有明显的正Th异常和弱的Nb负异常、高的Nb含量(> 2×10-6) 以及HFSE基本上和MORB相当的特点; 其Zr/Nb比值落在MORB范围之内, 而Ti/V比值略比MORB高, 表明其为MORB源的亏损地幔在相对较低熔融程度下熔融的产物.其εNd(t) 值为+7.40~+8.35, 略低于MORB, 但高于洋岛以及大陆板内玄武岩.因此该变质基性岩总体上兼有火山弧和洋中脊玄武岩特征, 因而其可能形成于弧后环境, 代表了弧后盆地扩张早期的产物. 相似文献
19.
《Gondwana Research》2010,17(3-4):587-608
Plume-related oceanic magmatism form oceanic islands, seamounts and plateaus (hereafter “seamounts” or “paleoseamounts”), which are important features in geological history. The accretion of oceanic seamounts to active continental margins significantly contributed to the formation of the continental crust. This paper reviews occurrences of Late Neoproterozoic–Mesozoic seamounts of the Paleo-Asian and Paleo-Pacific oceans, which are hosted by accretionary complexes (ACs) of Russian Altai, East Kazakhstan, Mongolia, Russian Far East and Japan. The paleoseamounts commonly consist of Ti–LREE–Nb-enriched plume-related basalts (OIB-type or intraplate basalts) capped with massive limestone and associated with other units of oceanic plate stratigraphy (OPS): oceanic floor basalts (MORB), pelagic chert, epiclastic slope facies, etc. The paper presents available geochemical data on the plume-related basalts including the first geochemical data on the Middle Paleozoic OIB-type basalts of the Paleo-Asian Ocean hosted by the Ulaanbaatar AC of Mongolia. An emphasis is made for the structural setting of OPS units, specific geochemical features of intraplate basalts, problems of their identification, and distinguishing from magmatic units of a different origin such as MORB, island-arc and back-arc basalts. Finally, we propose a continuous, though periodical, evolution of the Pacific superplume-related magmatism, which can be more reliably proved by studying Middle Paleozoic OPS units hosted by ACs of Mongolia and Tien Shan, and discuss prospects of future studies. 相似文献
20.
Constancy of Nb/U in the mantle revisited 总被引：5，自引：0，他引：5
Weidong Sun Yanhua Hu Vadim S. Kamenetsky Ming Chen 《Geochimica et cosmochimica acta》2008,72(14):3542-3549
It has long been proposed that MORB and OIB have constant supra-primitive mantle (PM) Nb/U values identical to each other. This fact together with complementary sub-PM values for the continental crust (CC), are taken as fundamental evidence, linking the mantle sources of MORB and OIB to the formation of the CC. Given that plate subduction at convergent margins is the major known process that dramatically fractionates Nb from U, and consequently that subducted oceanic slabs are the main primary carriers of supra-PM Nb/U, a constant supra-PM Nb/U in MORB mantle implies that the mixing of subducted oceanic crust is essentially finished or the newly recycled oceanic crust has Nb/U close to that of the mantle. The similarity between Nb and U as well as the constancy of Nb/U in MORB are revisited here based on MORB glass data obtained using laser ablation ICP-MS. The result shows that Nb/U is not correlated with Nb/Hf, supporting that Nb and U are similarly incompatible. Further investigation shows that Nb is not perfectly identical to, but is faintly more incompatible than U as indicated by the good correlation between log(U) and log(Nb) with a slope of 0.954, very close to 1. Nonetheless, the similarity between Nb and U is high enough, such that the average Nb/U value of MORB glasses should be very close to that of the MORB mantle. By contrast, the difference between Ce and Pb is more obvious. Ce is more incompatible than Pb with a slope of 1.13 in a log(Pb) versus log(Ce) diagram. Therefore, the Ce/Pb of MORB should be a little bit higher than that of the mantle source. The Nb/U value is not as uniform as expected for the similar incompatibility in studied MORB glasses, but varies by a factor of ∼2, suggesting that MORB mantle source is not yet homogenized in term of Nb/U. This indicates that the mixing back of subducted oceanic crust is still an ongoing process, i.e., subducted oceanic crust is recycling back after staying in the lower mantle for billions of years. 相似文献