Molybdenum isotope composition of the upper mantle and its origin:insight from mid-ocean ridge basalt

The molybdenum(Mo)isotope system is pivotal in reconstructing marine redox changes throughout Earth’s history and has emerged as a promising tracer for igneous and metamorphic processes.Understanding its composition and variation across major geochemical reservoirs is essential for its application in investigating high-temperature processes.However,there is debate regarding theδ^(98/95)Mo value of the Earth’s mantle,with estimates ranging from sub-chondritic to super-chondritic values.Recent analyses of global mid-ocean ridge basalt(MORB)glasses revealed significantδ^(98/95)Mo variations attributed to mantle heterogeneity,proposing a two-component mixing model to explain the observed variation.Complementary studies confirmed the sub-chondriticδ^(98/95)Mo of the depleted upper mantle,suggesting remixing of subduction-modified oceanic crust as a plausible mechanism.These findings underscore the role of Mo isotopes as effective tracers for understanding dynamic processes associated with mantle-crustal recycling.

Calcium isotopic signatures of depleted mid-ocean ridge basalts from the northeastern Pacific

A number of high-temperature processes(e.g.,melt-rock reactions,metasomatism,partial melting)can produce signifi cant Ca isotopic fractionation and heterogeneity in the mantle,but the mechanism for such fractionation remains obscure.To investigate the eff ect of mantle partial melting on Ca isotopic fractionation,we reported high-precision Ca isotopic compositions of depleted mid-ocean ridge basalts(MORBs)from the East Pacifi c Rise and Ecuador Rift in the northeastern Pacifi c.Theδ44/40 Ca of these MORB samples exhibit a narrow variation from 0.84‰to 0.88‰with an average of 0.85‰±0.03‰,which are similar to those of reported MORBs(0.83‰±0.11‰)and back-arc basin basalts(BABBs,0.80‰±0.08‰)in literature,but are lower than the estimate value for the bulk silicate Earth(BSE,0.94‰±0.05‰).The lowδ44/40 Ca signatures of MORB samples in this study cannot be caused by fractional crystallization,since intermediate-mafi c diff erentiation has been demonstrated having only limited eff ects on Ca isotopic fractionation.Instead,the off set ofδ44/40 Ca between MORBs and the BSE is most likely produced by mantle partial melting.During this process,the light Ca isotopes are preferentially transferred to the melt,while the heavy ones tend to stay in the residue,which is consistent with the fact thatδ44/40 Ca of melt-depleted peridotites increases with partial melting in literature.The behavior of Ca isotopes during mantle partial melting is closely related to the inter-mineral(Cpx and Opx)Ca isotopic fractionation and melting mineral modes.Mantle partial melting is one of the common processes that can induce lowerδ44/40 Ca values in basalts and Ca isotopic heterogeneity in Earth’s mantle.

A Petrogenetic Model of Basalts from the Northern Central Indian Ridge:3-11°S

Mid-Ocean Ridge Basalts （MORB） from the Northern Central Indian Ridge （NCIR） were recovered between latitudes 3° and 11° S and are olivine tholeiite with higher abundances of K and Rb. They are of typical transitional MORB （T-MORB） variety and appear to have been generated from an enriched-mantle peridotite source. The primitive NCIR MORBs having Mg^# 〉 0.68 are the product of partial melting at an estimated pressure of - 1 GPa. It is inferred that the magma was subsequently modified at a pressure 〉 1 GPa by crystal fractionation and spinel was the first mineral to crystallize followed by separation of relatively Fe-rich olivine with subsequent decrease in pressure. During progressive fractionation at lower pressure （between 1-0.5 GPa）, the bulk composition of the magma became systematically depleted in MgO, and enriched in ∑FeO, TiO2, P2Os and Na20. There was, however, limited gradual depletion in Al2O3 and CaO and concomitant enrichment in K20. With the progressive fractionation these basalts became gradually enriched in V, Co, Y, Zr and to some extent in Sr, and depleted in Ni and Cro In addition, the T_JtEE of the magma also increased with fractionation, without any change in （La/Yb）n value.

Fractional crystallization processes of magma beneath the Carlsberg Ridge(57°–65°E)

Fractional crystallization of basaltic magma at variable depths influences strongly the geochemical compositions of mid-ocean ridge basalts(MORBs),especially at slow-spreading mid-ocean ridges.The Carlsberg Ridge is a typical slow-spreading ridge located in the northwestern Indian Ocean.In this study,we conducted petrological,geochemical and modelling studies of MORBs collected along the Carlsberg Ridge from 57°-65°E to understand the fractional crystallization processes of magma and the controls on variations in MORB geochemistry.Our results show that the mantle sources beneath the Carlsberg Ridge are heterogeneous even on the local scale of a segment;such heterogeneity may be ubiquitous beneath the Carlsberg Ridge.Mantle heterogeneity may be caused by the enriched components resulting in the"DUPAL"anomaly,whereas the effect of pyroxenite on mantle heterogeneity is negligible.The parental melts experienced crystallization of olivine,plagioclase and clinopyroxene prior to eruption,which played a significant role in the major and trace element variations in MORBs from the Carlsberg Ridge.The liquid lines of descent(LLDs),deduced from the forward modelling of three parental magma compositions using the Petrolog3 program at pressures between 1 atm and 10 kbar,demonstrate that clinopyroxene joined the olivine and plagioclase cotectic.The over-enrichment in highly incompatible elements relative to LLDs may be caused by the processes of replenishment-tapping-crystallization in magma chambers.The calculated crystallization pressures suggest that parental magmas beneath the Carlsberg Ridge experienced moderateto high-pressure crystallization and that crystallization beneath the slow-spreading Carlsberg Ridge may start at upper mantle depths.

武当山十堰地区二叠纪E-MORB型玄武岩识别及构造意义

大比例尺构造—岩相填图和测试分析研究显示南秦岭武当山并非统一前寒武纪基底组成,在十堰和两郧断裂之间的黄龙—方滩及丹江口银洞山等地区识别和厘定出一套中生代混杂岩带,开展混杂岩带内物质组成和成因研究对正确认识武当山造山作用和区域构造演化有重要意义。本文重点对武当山十堰黄龙—方滩地区混杂岩带内玄武岩块体岩石成因及时代进行研究,岩石地球化学显示玄武岩块体为亚碱性拉斑玄武岩系列,具有中等Ti、Zr、Hf、Nd含量、弱轻稀土富集重稀土亏损的E-MORB(Enriched Mid-ocean ridge basalt,富集型洋中脊玄武岩)特征,其岩浆为尖晶石二辉橄榄岩低压、高度部分熔融形成,源区可能受富集地幔或俯冲作用等影响导致富集高场强不相容元素;锆石U-Pb测年表明其为中二叠世火山作用产物;区域地质特征及岩石成因研究共同指示该套E-MORB形成于洋中脊或附近海山环境。综上,武当山十堰地区中—晚二叠世存在洋盆,该成果为理解十堰黄龙—方滩混杂岩带成因机制和正确认识南秦岭显生宙构造属性提供了基础资料。

Deep Fractionation of Clinopyroxene in the East Pacific Rise 13°N:Evidence from High MgO MORB and Melt Inclusions

Mid-ocean ridge basalts （MORBs） from East Pacific Rise （EPR） 13°N are analysed for major and trace elements, both of which show a continuous evolving trend. Positive MgO-Al2O3 and negative MgO-Sc relationships manifest the cotectic crystallization of plagioclase and olivine, which exist with the presence of plagioclase and olivine phenocrysts and the absence of clinopyroxene phenoerysts. However, the fractionation of clinopyroxene is proven by the positive correlation of MgO and CaO. Thus, MORB samples are believed to show a “clinopyroxene paradox”. The highest magnesium-bearing MORB sample E13-3B （MGO=9.52%） is modelled for isobaric crystallization with COMAGMAT at different pressures. Observed CaO/Al2O3 ratios can be derived from E13-3B only by fractional crystallization at pressure 〉4±1 kbar, which necessitates clinopyroxene crystallization and is not consistent with cotectic crystallization of olivine plus plagioclase in the magma chamber （at pressure -1 kbar）. The initial compositions of the melt inclusions, which could represent potential parental magmas, are reconstructed by correcting for post-entrapment crystallization （PEC）. The simulated crystallization of initial melt inclusions also produce observed CaO/Al2O3 ratios only at 〉4±1 kbar, in which clinopyroxene takes part in crystallization. It is suggested that MORB magmas have experienced clinopyroxene fractionation in the lower crust, in and below the Moho transition zone. The MORB magmas have experienced transition from clinopyroxene＋plagioclase＋olivine crystallization at 〉4±1 kbar to mainly olivine＋plagioclase crystallization at 〈1 kbar, which contributes to the explanation of the “clinopyroxene paradox”.

软流圈地幔成分不均一性的研究进展与意义

软流圈通常是指位于岩石圈之下、地幔过渡带之上的地幔圈层,可以发生长期缓慢变形,从而可以在全球范围内对流。软流圈是地球上岩浆岩的重要源区,并作为重要的参考体系广泛应用于讨论地球形成演化、壳幔相互作用、地壳生长等一系列重大地质问题。根据大洋玄武岩的研究结果,软流圈地幔被普遍认为具有相对均一的地球化学组成。然而,深海橄榄岩的元素与同位素组成却显示软流圈的成分存在强烈的不均一性。大量的研究显示软流圈中广泛残存有古老的再循环地幔,其来源既包括再循环的俯冲大洋岩石圈地幔,也包括通过不同方式进入软流圈的古老克拉通地幔。蛇绿岩的研究结果表明这种不均一性也同样存在于不同地质时期的软流圈地幔。本文通过总结洋中脊玄武岩、深海橄榄岩和蛇绿岩的相关研究,揭示软流圈地幔中不均一性的组成与来源,并讨论了它们对于软流圈在洋中脊下方熔融过程的影响。

An Intra-Oceanic Subduction System Influenced by Ridge Subduction in the Diyanmiao Subduction Accretionary Complex of the Xar Moron Area,Eastern Margin of the Central Asian Orogenic Belt

This study focuses on the geology,geochemistry,Sr-Nd isotopes and their tectonic settings of three types of basalts in Diyanmiao ophiolite in the Xar Moron area located on the eastern margin of the Central Asian Orogenic Belt.Type I basalts are oceanic tholeiites with a depleted light rare earth element(LREE)pattern,which are similar to the typical N-mid-oceanic ridge basalt(MORB)and suggests that they were formed at a mid-oceanic ridge.The initial 87Sr/86Sr ratios of Type I basalts range from 0.703966 to 0.705276 and theεNd(t)values are from 16.49 to 17.15,indicating that they were derived from a depleted mantle source.Type II basalts belong to the medium-potassium calc-akaline series and have the geochem-ical characteristics of Nb-enriched basalt(NEB)with high Nb content(14.5 ppm)and strong enrichment in LREEs,implying that they were created by the partial melting of mantle wedge peridotite that previously metasomatized by slab melts.Type III basalts are high-Al basalt(HAB)with high-Al contents(Al_(2)0_(3)=16.75 wt.%-18.00 wt.%),distinct Nb depletion and high Th/Yb ratios.Thus they were likely gen-erated in a normal island-arc setting.Therefore,the association of MORB,NEB,and HAB in the study area may be due to the subduction of a mid-oceanic ridge,and the Diyanmiao ophiolite is proposed to be formed in the forearc setting of a mid-oceanic ridge subduction system.

Sharpness of the paired 660-km discontinuity beneath the Izu-Bonin area

The 660-km discontinuity that separates the Earth's upper and lower mantle has primarily been attributed to phase changes in olivine and other minerals.Resolving the sharpness is essential for predicting the composition of the mantle and for understanding its dynamic effects.In this study,we used S-to-P conversions from the 660-km interface,termed S660P,arriving in the P-wave coda from one earthquake in the Izu–Bonin subduction zone recorded by stations in Alaska.The S660P signals were of high quality,providing us an unprecedented opportunity to resolve the sharpness of the discontinuity.Our study demonstrated,based on the impedance contrast given by the IASP91 model,that the discontinuity has a transitional thickness of^5 km.In addition,we observed a prominent arrival right after the S660P,which was best explained by S-to-P conversions from a deeper discontinuity at a depth of^720 km with a transitional thickness of^20 km,termed S720P.The 720-km discontinuity is most likely the result of a phase transition from majoritic garnet to perovskite in the segregated oceanic crust(mainly the mid-oceanic ridge basalt composition)at the uppermost lower mantle beneath this area.The inferred phase changes are also consistent with predictions from mineral physics experiments.

对雅鲁藏布江结合带形成演化的再探讨

通过1∶5万区域地质调查和收集相关资料的综合研究,本文对雅鲁藏布江结合带的形成演化作了进一步的探讨。雅鲁藏布江特提斯洋具有弧后扩张洋盆的性质,在早三叠世至中三叠世中期洋盆初步形成,中三叠世晚期至晚三叠世洋盆全面形成,从早侏罗世至晚白垩世洋盆逐步萎缩,到古新世至始新世关闭。南带的蛇绿岩主要为洋中脊扩张型(MORB型),形成于中三叠世晚期至晚三叠世。北带的蛇绿岩主要为与洋内俯冲相关的俯冲带上盘型(SSZ型),形成于早中侏罗世。带内侏罗纪至白垩纪其他岩浆岩主要为前弧玄武岩类(FAB型)。显示雅鲁藏布江特提斯洋从早侏罗世开始发生了洋内俯冲,并同步向北向冈底斯带之下主动俯冲消减和向南向喜马拉雅地块之下被动俯冲消减,持续发展到晚白垩世,在古新世至始新世俯冲碰撞消亡转化为结合带。

南大西洋慢速洋中脊与地幔柱相互作用研究进展与展望

揭示洋中脊与地幔柱(脊-柱)之间的可能联系为认识地球深部物质组成与深部地幔动力学过程提供了重要窗口,也是过去40多年以来固体地球科学研究领域的前沿与热点。在绵延八万多千米的全球洋中脊系统中,部分洋脊片段会受到地幔柱作用不同程度的影响。研究显示,大西洋的形成演化与地幔柱作用之间具有密切联系,尤其在南大西洋的裂解、打开演化过程中,南大西洋中脊系统始终与其周围地幔柱(如圣赫勒拿、阿森松、特里斯坦、高夫、发现等地幔柱)之间具有不同程度的相互作用关系,导致沿脊出露玄武岩在地球化学组成上呈现出明显的不均一性特征。本文在系统性总结脊-柱相互作用研究现状与南大西洋地区地质构造演化特征的基础上,详细阐述了南大西洋中脊13.2°S~24.2°S地区玄武岩的岩石地球化学特征;揭示了南大西洋中脊研究区的岩浆演化、地幔源区性质;指示出圣赫勒拿地幔柱物质向南大西洋中脊系统传播的主要方向;圈定了圣赫勒拿地幔柱对南大西洋中脊系统地幔源区性质在沿脊方向的影响范围(14.2°S~20.4°S);同时推测了南大西洋中脊系统与圣赫勒拿地幔柱之间受地幔柱影响的软流圈地幔物质在大洋岩石圈底部的空间展布。最后本文提出了关于南大西洋地区脊-柱相互作用领域现存的科学问题与未来的研究方向。

Compositional and temperature variations of the Pacific upper mantle since the Cretaceous

The geological evolution of the Earth during the mid-Cretaceous were shown to be anomalous, e.g., the pause of the geomagnetic field, the global sea level rise, and increased intra-plate volcanic activities, which could be attributed to deep mantle processes. As the anomalous volcanic activities occurred mainly in the Cretaceous Pacific, here we use basalt chemical compositions from the oceanic drilling（DSDP/ODP/IODP） sites to investigate their mantle sources and melting conditions. Based on locations relative to the Pacific plateaus, we classified these sites as oceanic plateau basalts, normal mid-ocean ridge basalts, and near-plateau seafloor basalts. This study shows that those normal mid-ocean ridge basalts formed during mid-Cretaceous are broadly similar in average Na8, La/Sm and Sm/Yb ratios and Sr-Nd isotopic compositions to modern Pacific spreading ridge（the East Pacific Rise）. The Ontong Java plateau（125–90 Ma） basalts have distinctly lower Na8 and143Nd/144 Nd, and higher La/Sm and 87Sr/86 Sr than normal seafloor basalts, whereas those for the near-plateau seafloor basalts are similar to the plateau basalts, indicating influences from the Ontong Java mantle source. The super mantle plume activity that might have formed the Ontong Java plateau influenced the mantle source of the simultaneously formed large areas of seafloor basalts. Based on the chemical data from normal seafloor basalts, I propose that the mantle compositions and melting conditions of the normal mid-ocean ridges during the Cretaceous are similar to the fast spreading East Pacific Rise. Slight variations of mid-Cretaceous normal seafloor basalts in melting conditions could be related to the local mantle source and spreading rate.

北祁连西段石鸡河地区火山岩地球化学特征及其动力学意义

石鸡河一带是近年新发现的北祁连山多金属成矿带,气侯和自然条件恶劣,研究程度很低.该区地层岩性为阳起石岩、角岩和细碧岩;岩石地球化学和Nd同位素数据显示岩石来源于亏损地幔,具有N-MORB洋脊玄武岩特征;Sr同位素特征显示地层Sr同位素组成的改变是蚀变引起,而不是由地壳物质加入引起的.Sm-Nd等时线年龄为481±20Ma.提出了石鸡河地区地层形成于早奥陶世,成岩环境为北祁连洋扩张环境,而非区域资料上显示的残留海盆封闭、大陆碰撞造山环境.