Natural and experimental constraints on formation of the continental crust based on niobium–tantalum fractionation

Fractionation between Nb and Ta, elements generally regarded as geochemical ‘identical twins’, is a key to deciphering the formation of the continental crust (CC). Here we show that Nb/Ta of rutile grains in eclogitic rocks from the Chinese Continental Scientific Drilling (CCSD) project are remarkably heterogeneous but overall subchondritic at core depths of 100–700 m, and are less variable and mainly suprachondritic at core depths of 700–3025 m, indicating clear Nb/Ta fractionation across a subducted slab. To understand the potential mechanism of Nb/Ta fractionation within the subducted plate, we analysed by laser ablation ICPMS a thermal migration experiment in which a wet andesite was placed in a large thermal gradient (300°C/cm with ends ranging from 950–350°C) at 0.5Gpa. Results show that Nb, Ta and Ti, driven by the thermal gradient, preferentially migrate by diffusion through supercritical fluids into the cooler end of the experiment (at 650–350°C). Due to contrasting Nb and Ta thermal migration patterns, dramatic fractionation between Nb, Ta, and Ti took place in the cooler end. Experimental results are consistent with the measured Nb, Ta in rutile from CCSD drillhole samples. We consider that major fractionation between Nb, Ta must occur before rutile appears, most likely during the prograde blueschist to amphibole–eclogite transformation, when Ti is also mobile. Before rutile appears, partitioning between Ti‐rich dominant minerals such as amphiboles and fluids in the hotter region where dehydration preferentially occurs, produces Nb–Ta–Ti‐rich fluids with subchondritic Nb/Ta, and dehydration residues with suprachondritic Nb/Ta. Meanwhile, owing to evolution of the thermal gradient within the subducting slab, thermal migration of Nb, Ta, and Ti in aqueous fluids result in Nb, Ta, and Ti enrichment in the cooler region and depletion in the hotter region. As a result of high‐pressure metamorphism, hydrous rutile‐rich eclogites with overall subchondritic Nb/Ta form in the cooler region, whereas relatively anhydrous rutile‐poor eclogites with suprachondritic Nb/Ta form in the hotter region. Subsequently, partial melting of hydrous rutile‐rich eclogites with initial subchondritic Nb/Ta at deeper levels transfers overall subchondritic Nb/Ta coupled with Nb, Ta, and Ti depletion characteristics to the CC, leaving dry rutile‐poor eclogites with suprachondritic Nb/Ta and rutile‐rich residual eclogites with overall, heterogeneous subchondritic Nb/Ta as a complementary reservoir to the CC.     

Making continental crust through slab melting: Constraints from niobium-tantalum fractionation in UHP metamorphic rutile

The formation of the continental crust (CC) is one of the most important processes in the evolution of the silicate Earth. Exactly how the CC formed is the subject of ongoing debate that focuses on its subchondritic Nb/Ta ratio. Nb and Ta are “geochemical identical twins,” so they usually do not fractionate from each other. Here, we show that rutile grains from hydrous rutile-bearing eclogitic layers recovered from drillcores in the Dabie-Sulu ultrahigh pressure terrain have highly variable Nb/Ta values (ranging from 5.4 to 29.1, with an average of 9.8 ± 0.6), indicating major fractionation of Nb and Ta most likely occurred during blueschist to amphibole-eclogite transformation in the absence of rutile. It is suggested that the released fluids with subchondritic Nb/Ta were transported to, and retained by, hydrous rutile-bearing eclogite in colder regions, resulting in suprachondritic Nb/Ta ratios for drier eclogite in hotter regions. Further dehydration of hydrous rutile-bearing eclogites cannot transfer the fractionated Nb/Ta values to the CC due to the low solubility of Nb and Ta in fluids in the presence of rutile, while dehydration-melting results in a major component of the CC, the tonalite-trondhjemite-granodiorite (TTG) component, which is responsible for the low Nb/Ta of the CC. Consequently, residual eclogites have variable but overall suprachondritic Nb/Ta.     

Fluids in deeply subducted continental crust: Petrology, mineral chemistry and fluid inclusion of UHP metamorphic veins from the Sulu orogen, eastern China

The complex vein associations hosted in southern Sulu ultrahigh-pressure (UHP) eclogites contain quartz ± omphacite (or jadeite) ± kyanite ± allanite ± zoisite ± rutile ± garnet. These minerals have chemical compositions similar to those of host eclogites. Inclusions of polycrystalline quartz pseudomorphs after coesite were identified in vein allanite and garnet, and coesite inclusions were found in vein zircon. These facts suggest that the veins together with host eclogites have been subjected to synchronous UHP metamorphism. The vein minerals contain relatively high concentrations of rare earth elements (REE), high-field-strength elements (HFSE) and transition metal elements (TME). A kyanite-quartz vein has a whole-rock composition similar to adjacent UHP metamorphic granitic gneisses. Abundant primary multi-solid fluid inclusions trapped within UHP vein minerals contain complex daughter minerals of muscovite, calcite, anhydrite, magnetite, pyrite, apatite, celestite and liquid and gas phase of H₂O with solids up to 30-70% of the inclusion volume. The presence of daughter minerals anhydrite and magnetite indicates the subduction fluids were oxidizing, and provides a possible interpretation for the high oxygen fugacity of subduction zone magmas. These characteristics imply that the UHP vein minerals were crystallized from supercritical silicate-rich aqueous fluids that were in equilibrium with peak-UHP minerals, and that the fluids in deeply subducted continental crust may contain very high concentrations of silicate as well as HREE, HFSE and TME. Such fluids might have resulted in major fractionation between Nb and Ta, i.e. the UHP fluids have subchondritic Nb/Ta values, whereas the host eclogites after extraction of the fluids have suprachondritic Nb/Ta values. Therefore, voluminous residual eclogites with high Nb/Ta ratios may be the complementary suprachondritic reservoir capable of balancing the subchondritic depleted mantle and continental crust reservoirs.     

Zr-in-rutile thermometry of eclogites from the Karakaya Complex in NW Turkey: Implications for rutile growth during subduction zone metamorphism

Eclogites occur as a tectonic slice within a metabasite-phyllite-marble unit of the Karakaya Complex in northwest Turkey. The high-pressure mineral assemblage in eclogite is mainly composed of garnet + omphacite + glaucophane + epidote + quartz. Trace element characteristics of rutile and Zr-in-rutile temperatures were determined for eclogites from the Karakaya Complex. Core-rim analyses of rutile grains yield remarkable trace element zoning with lower contents of Zr, Nb and Ta in the core than in the rim. The variations in Zr, Nb and Ta can be ascribed to growth zoning rather than diffusion effects. The Nb/Ta and Zr/Hf ratios increase with a decrease in Ta and Hf contents, which could be ascribed to the effect of metamorphic dehydration in subduction zones on rutile Nb/Ta differentiation. The rutile grains from eclogites in the Karakaya Complex are dominated by subchondritic Nb/Ta and Zr/Hf ratios. It can be noted that subchondritic Nb/Ta may record rutile growth from local sinks of aqueous fluids from metamorphic dehydration.The Zr contents of all rutile grains range between 81 and 160 ppm with an average of 123 ppm. The Zr-in-rutile thermometry yields temperatures of 559–604 °C with an average temperature of 585 °C for eclogites from the Karakaya Complex. This average temperature suggests growth temperature of rutile before peak pressure during the subduction. However, some rutile grains have higher Zr contents in the outermost rims compared to the core. Zr-in-rutile temperatures of the rims are about 20 °C higher than those of the cores. This suggests that the outermost rims would have grown from a distinct fluid at higher temperatures than that of the cores. Moreover, Zr contents and calculated temperatures in both inclusion rutile and matrix rutile from eclogites are identical, which suggests that eclogites within the Karakaya Complex belong to the same tectonic slice and underwent similar metamorphic evolution.     

Constraints from eclogite and MARID xenoliths on origins of mantle Zr/Hf–Nb/Ta variability

New trace-element data of rutile in kimberlite-borne ~1.85 Ga eclogite and pyroxenite xenoliths from the central Slave craton, as well as ~110 Ma MARID xenoliths from the Kaapvaal craton, provide constraints on the origins of lithospheric and sublithospheric mantle variability in high field strength element ratios. Rutiles in eclogites and pyroxenites have Zr/Hf ranging from 20 to 62 and Nb/Ta ranging from 10 to 40. Rutiles in MARID xenoliths have Zr/Hf from 24 to 33 and Nb/Ta from 10 to 41. Calculated whole-rock Zr/Hf is suprachondritic for eclogites with suggested gabbroic protoliths and subchondritic for boninite-like eclogites; the latter is consistent with cpx-controlled depletion in the protolith source. Within each eclogite type, positive correlations of Zr/Hf with La/Lu and negative correlations with Lu/Hf likely reflect fractionation of cpx and/or plagioclase during crystallisation of the protoliths. Zr/Hf–Nb/Ta relationships of some MARID-type rocks, which are products of lithospheric mantle metasomatism, and eclogite xenoliths plot on a silicate differentiation trend, whereas other samples have higher Nb/Ta at a given Zr/Hf. Fractionation of a few percent rutile from an HFSE-rich mafic melt can generate a trend towards strongly increased Nb/Ta at minimally changed Zr/Hf in the residual melt. Superposition of rutile fractionation on the effects of silicate differentiation, which fractionates Zr/Hf more strongly than Nb/Ta, can explain the Zr/Hf–Nb/Ta relationships of most eclogites from the central Slave craton as well as those of MARID rocks, metasomatised peridotites and group II kimberlites. By contrast, Zr/Hf–Nb/Ta relationships suggest that Group I kimberlites are mixtures between depleted peridotite and carbonatite. Thus, high Nb/Ta is a signature of lithospheric processes and may not be important in deeply subducted eclogites that bypass extended residence in the lithosphere. Conversely, considerable primary Zr/Hf variability was inherited by the eclogites, which is indicative of the compositional diversity of ancient subducted oceanic crust, which is expected to have generated substantial heterogeneity in sublithospheric basalt sources.     

金红石榴辉岩——一个可能的超球粒陨石Nb/Ta储库

Nb,Ta的硅酸盐地球质量不平衡问题争论由来已久,备受关注。近年来研究发现,含金红石的榴辉岩Nb/Ta往往高于球粒陨石值(Nb/Ta=17.5),暗示其可能是平衡地球Nb亏损的独立储库。而洋壳玄武岩部分熔融实验表明Ta比Nb更倾向进入金红石晶格,这意味着作为俯冲洋壳部分熔融残留相的榴辉岩Nb/Ta不可能高于原岩。为了解释地质观察和实验结果之间的矛盾,系统分析了中国大陆科学钻探工程(CCSD)主孔、先导孔及附近地表榴辉岩的矿物微量元素。结果发现:榴辉岩中的Nb,Ta主要存在于金红石之中,其他矿物中含量极少;Nb,Ta之间存在着强烈分异(Nb/Ta=5.3～96.2),并总体上具有超球粒陨石的特征;韭闪石和多硅白云母的Nb/Ta平均分别为48.6,21.8,显示了很强的Nb,Ta分异能力;其他矿物如石榴石、绿辉石、绿帘石、磷灰石等的Nb、Ta含量及Nb/Ta都很低,对Nb-Ta分异不造成影响。认为导致Nb-Ta分异的不是金红石,而应出现在洋壳俯冲过程中金红石相出现之前的脱水和部分熔融阶段。富含Ti的角闪石(韭闪石)和白云母可能对Nb-Ta分异起到了决定性的作用。等金红石相出现之后,由于其对Nb,Ta的绝对控制作用,此前阶段的分异结果便被固定在金红石中而继承下来。因此,含金红石的榴辉岩常常表现出超球粒陨石Nb/Ta的特征,与金红石不能有效地分异Na,Ta的实验结果之间并不矛盾。在不均匀的上地幔中含金红石的榴辉岩是可能的超球粒陨石Nb/Ta储库之一。     

Rutile geochemistry and thermometry of eclogites and associated garnet-mica schists in the Biga Peninsula,NW Turkey

In northwest Turkey, high-pressure metamorphic rocks occur as exotic blocks within the Çetmi mélange located on the south of the Biga Peninsula. Rutile chemistry and rutile thermometry obtained from the eclogite and associated garnet-mica schist in the Çetmi mélange indicate significant trace element behaviour of subducted oceanic crust and source-rock lithology of detrital rutiles. Cr and Nb contents in detrital rutile from garnet-mica schist vary from 355 to 1026 μg/g and 323 and 3319 μg/g, respectively. According to the Cr-Nb discrimination diagram, the results show that 85% of the detrital rutiles derived from metapelitic and 15% from metamafic rocks. Temperatures calculated for detrital rutiles and rutiles in eclogite range from 540 °C to 624 °C with an average of 586 °C and 611 °C to 659 °C with an average of 630 °C at P = 2.3 GPa, respectively. The calculated formation temperatures suggest that detrital rutiles are derived from amphibolite- and eclogite-facies metamorphic rocks. Amphibolite-facies rocks of the Kazdağ Massif could be the primary source rocks for the rutiles in the garnet-mica schist from the Çetmi mélange. Nb/Ta ratios of metapelitic and metamafic rutiles fall between 7–24 and 11–25, respectively. Nb/Ta characteristics in detrital rutiles may reflect a change in source-rock lithology. However, Nb/Ta ratios of rutiles in eclogite vary from 9 to 22. The rutile grains from eclogites are dominated by subchondritic Nb/Ta ratios. It can be noted that subchondritic Nb/Ta may record rutile growth from local sinks of aqueous fluids from metamorphic dehydration.     

Nb–Ta fractionation induced by fluid‐rock interaction in subduction‐zones: constraints from UHP eclogite‐ and vein‐hosted rutile from the Dabie orogen,Central‐Eastern China

Niobium and Ta concentrations in ultrahigh‐pressure (UHP) eclogites and rutile from these eclogites and associated high pressure (HP) veins were used to study the behaviour of Nb–Ta during dehydration and fluid‐rock interaction. Samples were collected through a ～2 km profile at the Bixiling complex in the Dabie orogenic belt, Central‐Eastern China. All but one eclogite away from veins (EAVs) display nearly constant Nb/Ta ratios ranging from 16.1 to 19.2, with an average of 16.9 ± 0.8 (2 SE), similar to that of their gabbroic protolith from the Yangtze Block. Nb/Ta ratios of rutile from the EAVs range from 12.7 to 25.3 among different individual grains, with the average values close to those of the corresponding bulk rocks. These observations show that Nb and Ta were not significantly fractionated by prograde metamorphism up to eclogite facies when no significant fluid‐rock interaction occurs. In contrast, Nb/Ta ratios of rutile from eclogites close to veins (ECVs) are highly variable from 17.8 to 49.8, which are systematically higher (by up to 17) than those of rutile from the veins. These observations demonstrate that Nb and Ta were mobilized and fractionated during localized fluid flow and intensive fluid‐rock interaction. This is strongly supported by Nb/Ta zoning patterns in single rutile grains revealed by in situ LA‐ICP‐MS analysis. Ratios of Nb/Ta in the ECV‐hosted rutile decrease gradually from cores towards rims, whereas those in the EAV‐hosted rutile are nearly invariable. Furthermore, the vein rutile shows Nb/Ta zoning patterns that are complementary to those in rutile from their immediate hosts (ECVs), suggesting an internal origin for the vein‐forming fluids. The Nb/Ta ratios of such fluids evolved from low values at the early stage of subduction to higher values at later supercritical conditions with increased temperature and pressure. Quantitative modelling was conducted to constrain the compositional evolution of metamorphic fluids during dehydration and fluid‐rock interaction focusing on Nb–Ta distribution. The modelling results based on our proposed multistage fluid phase evolution path can essentially reproduce the natural observations reported in the present study.     

Nb–Ta fractionation by partial melting at the titanite–rutile transition

During the evolution of the Earth, distinct geochemical reservoirs with different Nb/Ta ratios have developed. Archean granitoids of the tonalite–trondhjemite–granodiorite (TTG) suite, which represent the Earth’s early continental crust, show larger Nb/Ta variations than any other Earth reservoir. This implies that significant Nb–Ta fractionation must have occurred during early crust formation, while the underlying mechanism behind is still unclear. Here, we present a new model on how Nb may be fractionated from Ta during partial melting of subducted oceanic crust. Our data show that Nb/Ta ratios in melts derived from rutile- and titanite-bearing eclogite are largely controlled by the modal relative abundances of rutile and titanite in the source. High modal ratios of titanite over rutile generate melts with very high Nb/Ta (>60), whereas low modal titanite/rutile produces melts with much lower Nb/Ta (≤30). Very low Nb/Ta (<16) occur when all Ti-phases are consumed at very high degrees of melting. As the modal ratio of titanite to rutile is a function of pressure, the Nb/Ta of melts is a function of melting depth. Our new model helps to explain the extreme variation of Nb/Ta observed in many TTGs and thus how Nb and Ta were fractionated during the early evolution of the Earth. Furthermore, the model also indicates that simple one-stage melting models for mafic crust are not sufficient to explain the formation of TTGs.     

HFSE systematics of rutile-bearing eclogites: New insights into subduction zone processes and implications for the earth’s HFSE budget

The depleted mantle and the continental crust are generally thought to balance the budget of refractory and lithophile elements of the Bulk Silicate Earth (BSE), resulting in complementary trace element patterns. However, the two high field strength elements (HFSE) niobium and tantalum appear to contradict this mass balance. All reservoirs of the silicate Earth exhibit subchondritic Nb/Ta ratios, possibly as a result of Nb depletion.In this study a series of nineteen orogenic MORB-type eclogites from different localities was analyzed to determine their HFSE concentrations and to contribute to the question of whether subducted oceanic crust could form a hidden reservoir to account for the mass imbalance of Nb/Ta between BSE and the chondritic reservoir. Concentrations of HFSE were analyzed with isotope dilution (ID) techniques. Additionally, LA-ICPMS analyses of clinopyroxene, garnet and rutile have been performed. Rutile is by far the major host for Nb and Ta in all analyzed eclogites. However, many rutiles revealed zoning in Nb/Ta ratios, with cores being higher than rims. Accordingly, in situ analyses of rutiles have to be evaluated carefully and rutile cores do not necessarily reflect a bulk rock Nb and Ta composition, although over 90% of these elements reside in rutile.The HFSE concentration data in bulk rocks show that the orogenic eclogites have subchondritic Nb/Ta ratios and near chondritic Zr/Hf ratios. The investigated eclogites show neither enrichment of Nb compared to similarly incompatible elements (e.g. La), nor fractionation of Nb/Ta ratios relative to MOR-basalts, the likely precursor of these rocks. This indicates that during the conversion of the oceanic crust to eclogites in most cases, (1) HFSE and REE have similar mobility on average, possibly because both element groups remain in the down going slab, and (2) no significant fractionation of Nb/Ta occurs in subducted oceanic crust. With an average Nb/Ta ratio of 14.2 ± 1.4 (2s.e.), the investigated eclogites cannot balance the differences between BSE and chondrite. Additionally, as their average Nb/Ta is indistinguishable from the Nb/Ta of MORB, they are also an unlikely candidate to balance the potentially small differences in Nb/Ta between the continental crust and the mantle.     

金红石-榍石转变过程中元素地球化学行为——以雅鲁藏布江缝合带角闪岩为例

金红石边缘形成榍石冠状边结构在变质中-基性岩中普遍存在,是金红石与退变质流体携带的SiO_2与CaO作用的结果,反应形成的榍石微量元素特征受到金红石和流体的共同影响。雅鲁藏布江缝合带中角闪岩LZ06-04在抬升过程经历近等温降压退变质作用,石榴子石分解导致同一样品中含石榴子石部分与不含石榴子石部分的退变质流体成分的差异。两种流体分别与金红石反应,对应形成的榍石具有相似的Nb、Ta含量和Nb/Ta比值特征,但截然不同的REE特征。榍石的Nb、Ta来源于金红石,残余金红石与含水流体再平衡Nb、Ta的分配系数增大,且D_(Nb)~(Rt/Fluid)≥D_(Ta)~(Rt/Fluid);虽然Nb和Ta在含水流体中都表现为不活动元素,但相对于Nb,Ta在含水流体中活动性较高。榍石的Zr-Hf体系特征受到锆石、石榴子石等矿物的综合影响,并且Zr-Hf在含水流体中表现出比Nb-Ta更高的活动性。榍石的REE特征受流体中REE特征、榍石与流体配分系数以及共生矿物的影响。在岩浆或变质体系,榍石形成过程中,REE富集矿物(如石榴子石、锆石、褐帘石、独居石、磷灰石等)形成或分解将影响榍石的REE分布特征或形成REE环带结构。含水流体中金红石退变质形成榍石反应的进行受流体中TiO_2、CaO和SiO_2活度的影响。因此榍石常见于钙碱性岩浆岩、富Ca基性变质岩和矽卡岩中。流体中CaO活度的变化影响榍石的形成,进而影响Ti、Nb、Ta在流体中的运移能力。俯冲板片产生流体在交代上覆富Ca地幔楔物质过程中形成榍石残留同样可以造成部分熔融体具有亏损HFSE特征。     

Subduction channel fluid-rock interaction:Indications from rutile-quartz veins within eclogite from the Yuka terrane,North Qaidam orogen

High-pressure(HP)or ultrahigh-pressure(UHP)rutile-quartz veins that form at mantle depths due to fluid-rock interaction can be used to trace the properties and behavior of natural fluids in subduction zones.To explore the fluid flow and the associated element mobility during deep subduction and exhumation of the continental crust,we investigated the major and trace elements of Ti-rich minerals.Additionally,U–Pb dating,trace element contents,and Lu–Hf isotopic composition of zircon grains in the UHP eclogite and associated rutile-quartz veins were examined in the North Qaidam UHP metamorphic belt,Yuka terrane.The zircon grains in the rutile-quartz veins have unzoned or weak oscillatory zonings,and show low Th/U ratios,steep chondrite-normalized patterns of heavy rare earth elements(HREEs),and insignificant negative Eu anomalies,indicating their growth in metamorphic fluids.These zircon grains formed in 431
3 Ma,which is consistent with the 432
2 Ma age of the host eclogite.As for the zircons in the rutile-quartz veins,they showed steep HREE patterns on one hand,and were different from the zircons present in the host eclogite on the other.This demonstrates that their formation might have been related to the breakdown of the early stage of garnet,which corresponds to the abundance of fluids during the early exhumation stage.The core-rim profile analyses of rutile recorded a two-stage rutile growth across a large rutile grain;the rutile core has higher Nb,Ta,W,and Zr contents and lower Nb/Ta ratios than the rim,indicating that the rutile domains grew in different metamorphic fluids from the core towards the rim.The significant enrichment of high field strength elements(HFSEs)in the rutile core suggests that the peak fluids have high solubility and transportation capacity of these HFSEs.Furthermore,variations in the Nb vs.Cr trends in rutile indicate a connection of rutile to mafic protolith.The zircon grains from both the rutile-quartz veins and the host eclogite have similar Hf isotopic compositions,indicating that the vein-forming fluids are internally derived from the host eclogite.These fluids accumulated in the subduction channel and were triggered by local dehydration of the deeply subducted eclogite during the early exhumation conditions.     

Mobilization of Ti-Nb-Ta during subduction: Evidence from rutile-bearing dehydration segregations and veins hosted in eclogite, Tianshan, NW China

Field evidence from the western Tianshan subduction complex in northwestern China indicates that the high field strength elements Ti, Nb, and Ta were mobilized and thereby fractionated from Zr and Hf during the dehydration process that transformed blueschist into eclogite. Both a segregation with a depletion halo, thought to represent initial mobilization during dehydration, and a transport vein, indicative of the long distance transport were investigated. In each case, centimeter-sized rutile grains grew as needle-like crystals in the segregation and as prismatic crystals in the vein. Within the host rock of the segregation, the Ti contents of garnet and omphacite, the modal abundances of rutile and titanite and the bulk rock Ti, Nb, and Ta contents decrease towards the segregation. These observations are consistent with transport of Ti, Nb, and Ta from the host rock into the segregation. Textural and geochemical data for the eclogite-facies vein minerals indicate that Ti-Nb-Ta-rich fluids were transported over long-distances (at minimum meter-scale) during fracture-controlled fluid flow. Complex forming ligands (e.g., Na-Si-Al polymers and F⁻) may have enhanced the solubility of Ti, Nb, and Ta in the fluid. Changes in fluid composition (e.g., X_CO2) may both precipitate rutile and fractionate Ti, Nb, and Ta from LILE and REE.     

大陆俯冲带的深部流体及变质化学地球动力学 ——以苏鲁超高压变质带为例

通过苏鲁超高压变质带的岩石学、矿物化学、地球化学和年代学研究,在大陆俯冲带深部流体与变质化学地球动力学方面取得了重要的创新性成果。研究证明大陆俯冲带的深部流体是高氧逸度、富硅酸盐的超临界流体,揭示出超高压变质极端条件下的流体－矿物(岩石)相互作用可以导致不活动元素发生溶解和迁移,可以导致金红石的Nb/Ta之间发生强烈的分异,提出俯冲到地幔深处的超高压榴辉岩是地球内部“隐藏”的超球粒陨石Nb/Ta比值的物质源区,与低球粒陨石Nb/Ta比值的物质源区大陆地壳和亏损地幔在化学成分上形成互补。     

大别超高压变质过程中流体作用的微量元素效应及其地球化学意义

对大别地区角闪岩相－麻粒岩相和榴辉岩相岩石进行较系统的地球化学研究,探讨了超高压俯冲变质及折返过程中元素的再分配行为,并就其对变质过程中的部分熔融作用和流体交代作用的指示进行了分析。研究表明,大别榴辉岩原岩在其俯冲过程中发生了部分熔融作用,并在其早期脱水和后期退变质作用过程中与变质流体相之间产生了交换。脱水作用使榴辉岩Ｎｂ／Ｔａ比值升高,但后期的退变质作用使榴辉岩与其寄主围岩间的Ｎｂ／Ｔａ比值相对     

Redistribution of HFSE elements during rutile replacement by titanite

Titanite growth at the expense of rutile during retrograde hydration of eclogite into amphibolite is a common phenomenon. We investigated an amphibolite sample from the Tromsø eclogite facies terrain in Northern Norway to gain insight into the trace element distribution between rutile and titanite during incomplete resorption of the rutile by titanite. Patchy compositional zoning of Al, Ti, and F in titanite relates to the presence of a fluid with variable Ti/Al and/or F during its growth. Laser ablation ICP–MS and electron microprobe data for high field strength elements (HFSE: Nb, Zr, Ta, and Hf) of rutile resorbed by titanite indicate a pronounced enrichment of these elements in the rim of a large single rutile crystal (~8 mm) and a systematic decrease towards uniform HFSE contents in the large core. HFSE contents of smaller rutile grains (~0.5 mm) and rutile inclusions (<100 μm) in the titanite overgrowth are similar or higher than in the rims of large rutile crystals. Element profiles from the rim inward demonstrate that HFSE enrichment in rutile is controlled by diffusion. HFSE ratios in diffusion-altered rutile show systematic variations compared with the uniform core composition of the large rutile. Modelling of Zr and Nb diffusion in rutile indicates that diffusion coefficients in rutile in fluid-dominated natural systems must be considerably higher than those determined experimentally at 1 bar in dry systems. Variations of HFSE contents in the newly formed titanite show no systematic spatial distribution. HFSE ratios in titanite and the rims of rutile are different, indicating different solid/fluid distribution coefficients in these minerals. Element fractionation by diffusion into the relict rutile and during fluid-mediated growth of new titanite could substantially change the HFSE budget of these minerals and could affect their use for geochemical tracing and other applications, such as Zr-based geothermobarometry.     

Trace element distribution in Central Dabie eclogites

Coesite-bearing eclogites from Dabieshan (central China) have been studied by ion microprobe to provide information on trace element distributions in meta-basaltic mineral assemblages during high-pressure metamorphism. The primary mineralogy (eclogite facies) appears to have been garnet and omphacite, usually with coesite, phengite and dolomite, together with high-alumina titanite or rutile, or both titanite and rutile; kyanite also occurs occasionally as an apparently primary phase. It is probable that there was some development of quartz, epidote and apatite whilst the rock remained in the eclogite facies. A later amphibolite facies overprint led to partial replacement of some minerals and particularly symplectitic development after omphacite. They vary from very fine-grained dusty-looking to coarser grained Am + Di + Pl symplectites. The eclogite facies minerals show consistent trace element compositions and partition coefficients indicative of mutual equilibrium. Titanite, epidote and apatite all show high concentrations of REE relative to clinopyroxene. The compositions of secondary (amphibolite facies) minerals are clearly controlled by local rather than whole-rock equilibrium, with the composition of amphibole in particular depending on whether it is replacing clinopyroxene or garnet. REE partition coefficients for Cpx/Grt show a dependence on the Ca content of the host phases, with D _REE ^Cpx/Grt decreasing with decreasing D _Ca . This behaviour is very similar to that seen in mantle eclogites, despite differences in estimated temperatures of formation of 650–850 °C (Dabieshan) and 1000–1200 °C (mantle eclogites). With the exception of HREE in garnet, trace elements in the eclogites are strongly distributed in favour of minor or accessory phases. In particular, titanite and rutile strongly concentrate Nb and Zr, whilst LREE–MREE go largely into epidote, titanite and apatite. If these minor/accessory minerals behave in a refractory manner during melting or fluid mobilisation events and do not contribute to the melt/fluid, then the resultant melts and fluids will be strongly depleted in LREE–MREE. Received: 11 February 1999 / Accepted: 31 January 2000     

超高压榴辉岩金红石中高场强元素变化的控制因素及其地球动力学意义

利用LA-ICP-MS对CCSD-MH超高压榴辉岩中金红石进行了详细的原位微区微量元素组成分析.金红石中高场强元素Nb和Ta含量主要受全岩Nb、Ta和TiO₂含量控制, Zr、Hf含量比较稳定基本不受全岩含量影响.粒间金红石中, 同一颗粒金红石核部Zr含量系统高于边部, 而边部则出现了明显的Pb和Sr富集特征.CCSD-MH榴辉岩中金红石与全岩的Nb/Ta比值呈现明显的不一致性.全岩Nb/Ta比值明显低于金红石且与全岩TiO₂含量负相关, 而金红石的Nb/Ta比值与全岩Nb、Ta含量和Nb/Ta比值没有明显的相关关系.金红石和全岩之间非完全耦合的Nb/Ta组成表明, 金红石并非形成于原岩的结晶过程中而是在超高压变质作用过程中形成, 尽管金红石是榴辉岩中Nb、Ta含量的主要载体矿物, 但金红石的Nb/Ta比值并不一定能完全代表全岩的特征, 而与全岩Nb、Ta和TiO₂的含量有关.粒间金红石核部Zr含量所记录的温度与粒径之间具有明显的正相关性, 反映金红石中的Zr在其形成后没有封闭.粒间金红石所表现出的明显的边部富集Pb和Sr的特征, 反映了后期流体活动对金红石组成的影响.这些研究结果为金红石中Zr在高温下的扩散作用和后期流体活动的影响提供了重要证据, 这可能是利用金红石Zr含量地质温度计计算的苏鲁-大别榴辉岩变质温度(598~827℃) 偏低的主要原因.      

Rutile to Titanite Transformation in Eclogites and its Geochemical Consequences: An Example from the Sumdo Eclogite, Tibet

The formation of titanite coronae after rutile is common in retrograde high-to ultrahigh-pressure meta-mafic rocks, which provides a good opportunity to address the geochemical behavior of HFSE in crustal environments. In the Sumdo eclogite, titanite occurs either as a corona around rutile grains or as semi-continuous veins cross-cutting the major foliation, whereas rutile grains occur either as inclusions in garnet or omphacite or as a relict core surrounded by titanite. Textural relationships ...     

Redistribution of trace elements during prograde metamorphism from lawsonite blueschist to eclogite facies; implications for deep subduction-zone processes

The transfer of fluid and elements from subducting crust to the overlying mantle wedge is a fundamental process affecting arc magmatism and the chemical differentiation of the Earth. While the production of fluid by breakdown of hydrous minerals is well understood, the liberation of trace elements remains generally unconstrained. In this paper, we evaluate the behaviour of trace elements during prograde metamorphism and dehydration using samples of high-pressure, low-temperature metamorphic rocks from New Caledonia. Samples examined include mafic and pelitic rock-types that range in grade from lawsonite blueschist to eclogite facies, and represent typical lithologies of subducting crust. Under lawsonite blueschist facies conditions, the low temperatures of metamorphism inhibit equilibrium partitioning between metamorphic minerals and allow for the persistence of igneous and detrital minerals. Despite this, the most important hosts for trace-elements include lawsonite, (REE, Pb, Sr), titanite (REE, Nb, Ta), allanite (LREE, U, Th), phengite (LILE) and zircon (Zr, Hf). At epidote blueschist to eclogite facies conditions, trace-element equilibrium may be attained and epidote (REE, Sr, Th, U, Pb), garnet (HREE), rutile (Nb, Ta), phengite (LILE) and zircon (Zr, Hf) are the major trace-element hosts. Chlorite, albite, amphibole and omphacite contain very low concentrations of the investigated trace elements. The comparison of mineral trace-element data and bulk-rock data at different metamorphic grades indicates that trace elements are not liberated in significant quantities by prograde metamorphism up to eclogite facies. Combining our mineral trace-element data with established phase equilibria, we show that the trace elements considered are retained by newly-formed major and accessory minerals during mineral breakdown reactions to depths of up to 150 km. In contrast, significant volumes of fluid are released by dehydration reactions. Therefore, there is a decoupling of fluid release and trace element release in subducting slabs. We suggest that the flux of trace elements from the slab is not simply linked to mineral breakdown, but results from complex fluid-rock interactions and fluid-assisted partial melting in the slab.Electronic Supplementary Material Supplementary material is available in the online version of this article at http://dx.doi.org/10.1007/s00410-003-0495-5.Editorial responsibility: J. Hoefs