榴辉岩部分熔融过程中的同位素分馏

本文对榴辉岩部分熔融过程中不同同位素体系是否存在分馏这一当前研究热点进行了综述。榴辉岩作为研究洋陆俯冲、超高压变质以及壳幔相互作用的主要岩石类型,其部分熔融与地壳增生、板片折返过程以及俯冲隧道中元素的迁移分配等具有紧密的联系。作为典型的高压-超高压变质岩石,榴辉岩可通过俯冲带将壳源信息携带至地幔深部,影响地幔的化学组成,并可在大洋玄武岩中得以表现。近些年,随着仪器分析技术的发展,实验研究和理论计算均表明榴辉岩部分熔融过程中稳定同位素可以产生显著的分馏。作为常见的放射性成因子体同位素和传统稳定同位素Sr-Nd-Hf-O被广泛应用于源区示踪、岩浆混合以及结晶分异等过程。但目前有研究指出,在非平衡熔融过程中,熔体和源区的Sr-Nd-Hf-O同位素可发生解耦,导致二者的同位素组成不均一。另外,通过研究榴辉岩及其熔融产物的金属稳定同位素特征,发现榴辉岩部分熔融过程中,由于石榴石效应,会造成Ca、Mg、Fe、Li等金属稳定同位素的分馏。因此,当利用稳定同位素示踪榴辉岩熔体的源区时,需要考虑其分馏的影响。

     

地幔中的水与熔融

自二十世纪九十年代起,通过对天然样品的观察、矿物中水的溶解度的实验研究、以及地球物理的观测,人们逐步认识到地球内部以矿物晶格缺陷形式蕴含着至少与现今海洋水量相当的水。地球内部的水除了显著影响矿物的热导率、电导率、流变学、扩散等性质之外,还对不同构造背景下的地幔熔融产生了重要影响。水作为强不相容元素,在地幔发生熔融时会更倾向于进入到熔体相中,从而改变熔体的结构,影响与残留固相之间的平衡。总体上讲,在等温等压条件下,地幔中水的存在可以显著降低其固相线并增大熔融程度;而在绝热减压过程中,水的存在则可以加深初始熔融的深度,增大地幔整体熔融的区间,从而降低平均熔融程度。近年来,越来越多的证据表明,水对于从岩石圈-软流圈边界的低程度熔融到地幔超大规模熔融产物（大火成岩省）的形成都具有重要作用。本文综述了不同构造背景下（大洋中脊、岛弧、地幔过渡带、大火成岩省、大陆板内、洋岛等）水对地幔熔融影响的新进展,并提出现有研究的不足和未来的挑战。

     

Anhydrous partial melting of an iron-rich mantle I: subsolidus phase assemblages and partial melting phase relations at 10 to 30 kbar

Anhydrous partial melting experiments, at 10 to 30 kbar from solidus to near liquidus temperature, have been performed on an iron-rich martian mantle composition, DW. The DW subsolidus assemblage from 5 kbar to at least 24 kbar is a spinel lherzolite. At 25 kbar garnet is stable at the solidus along with spinel. The clinopyroxene stable on the DW solidus at and above 10 kbar is a pigeonitic clinopyroxene. Pigeonitic clinopyroxene is the first phase to melt out of the spinel lherzolite assemblage at less than 20°C above the solidus. Spinel melts out of the assemblage about 50°C above the solidus followed by a 150° to 200°C temperature interval where melts are in equilibrium with orthopyroxene and olivine. The temperature interval over which pigeonitic clinopyroxene melts out of an iron-rich spinel lherzolite assemblage is smaller than the temperature interval over which augite melts out of an iron-poor spinel lherzolite assemblage. The dominant solidus assemblage in the source regions of the Tharsis plateau, and for a large percentage of the martian mantle, is a spinel lherzolite.     

Review of melting experiments on carbonated eclogite and peridotite: insights into mantle metasomatism

Experimental studies on the partial melting of eclogite and peridotite provide important clues on mantle metasomatism. Here, we review results from some of the recent experiments and show that melting of carbonated eclogite and peridotite can produce carbonatitic to carbonated silicate melt, in which carbonates melt preferentially before Ti oxides and silicates. Low-degree melting results in carbonatitic melt coexisting with Ti oxides and silicates. This process also leads to the fractionation between some high-field strength elements (Nb, Ta, Zr, Hf, and HREE) and highly incompatible elements (U and Th) in the melt. When Ti oxides are nearly exhausted in eclogite, extremely high TiO₂ contents (e.g. 19 wt.%) are present in the melt with marked concentration of Nb and Ta. These results help to explain the features of carbonatitic metasomatism and the Nb–Ta spike in oceanic island basalts as identified in experimental studies. These studies also explain the reducing conditions that stabilize diamond in the deep mantle (>150 km) as well as the occurrence of diamond at different depths reported in various studies. Melting in such a reduced mantle can happen through redox reaction between diamond, pyroxene, and olivine, in which the initial liquid is a carbonated silicate melt. However, the theoretical oxygen fugacity (fO₂) in the asthenosphere is much lower than that predicted by the reaction and requires elevated fO₂, which can be caused by the addition of relatively oxidized materials from the lower mantle, deep asthenospheric material, and various recycled components. A combination of these processes generates locally oxidized domains in the deep mantle.     

Intergranular diamonds derived from partial melting of crustal rocks at ultrahigh-pressure metamorphic conditions

By reporting for the first time intergranular diamond in quartz–feldspar (Qtz–Kfs) aggregates, the processes of metamorphic diamond formation have to be reconsidered. Based on their Kfs/Qtz ratio, the texture of these aggregates are proposed to result from ‘granitic’ melt with a calculated composition that corresponds well with that of experimental data for the pelitic system. Taking into account experiments on CO₂ solubility in silicate melt under ultrahigh‐pressure conditions, a granitic melt is further suggested to act as a crystallization medium as well as a transport medium for producing metamorphic diamond.     

Anhydrous partial melting of an iron-rich mantle II: primary melt compositions at 15 kbar

Primary melt and coexisting mineral compositions, at increasing degrees of partial melting at 15 kbar, were determined for an iron-rich martian mantle composition, DW. The composition of primary melts near the solidus was determined with basalt-peridotite sandwich experiments. In order to evaluate the approach of the liquids to equilibrium with a DW mantle assemblage, experiments were also performed to establish the liquidus mineralogy of the primary melts. Primary melt compositions produced from an iron-rich mantle are more picritic than those produced from an iron-poor mantle. By increasing the iron content of a model mantle composition (decreasing the mg#, where mg# = atomic [Mg/(Mg+Fe²⁺)^*100]), picritic and komatiitic magmas result at lower percentages of melting and at temperatures closer to the solidus than in an iron-poor mantle. Terrestrial iron-rich primitive volcanics may be the partial melting products of iron-rich, mg# 80, source regions. The DW partial melting results support the conclusion of previous authors that the parent magmas of the SNC (shergottites, nakhlites, chassignites) meteorites were derived from a source region that had been previously depleted in an aluminous phase.     

Experimental constraints on major and trace element partitioning during partial melting of eclogite

Isobaric partial melting experiments were performed on an Fe-free synthetic composition to simulate partial melting of subducted oceanic crust. Nominally anhydrous experiments at 3.0 GPa yielded melts in equilibrium with garnet (13 to 16 mol.% grossular) and aluminous clinopyroxene (14 to 16 wt.% Al₂O₃). Melt compositions show decreasing Si and alkalis and increasing Ca, Mg, and Ti contents with increasing temperatures. Experiments at 1200 and 1300°C were rutile saturated, whereas experiments at 1400°C contained no residual rutile. We argue that during the initial stages of subduction, accessory rutile is likely to be stable in subsolidus eclogites of average midocean ridge basalt composition and that only large degrees of partial melting will eradicate rutile from an eclogitic source. At 3 GPa, any eclogites with a bulk TiO₂ content of ≥1.5 wt.% rutile will produce rutile-saturated partial melts, except at very high degrees of melting. At higher pressures, all bulk Ti may dissolve in clinopyroxene and garnet, leaving no accessory rutile.Trace element partition coefficients for 24 trace elements between clinopyroxene, garnet, and melt were determined by secondary-ion mass spectrometry analysis of experimental run products at 1400°C and 3 GPa. Partition coefficients for the rare earth elements agree well with previous studies and have been evaluated using the lattice strain model. Partitioning data for high-field strength elements indicate complementary D_Zr/D_Hf for clinopyroxene and garnet. Partial melting of an eclogitic component of different modal compositions may therefore explain both subchondritic and superchondritic Zr/Hf ratios. Superchondritic Zr/Hf has recently been observed in some ocean island basalts (OIB), and this may be taken as further evidence for components of recycled oceanic crust in OIB. The data also indicate slight Nb/Ta fractionation during partial melting of bimineralic eclogite, which is not, however, sufficient to explain some recently observed Nb/Ta fractionation in island arc rocks. Accessory rutile, however, can explain such fractionation.     

The role of eclogite in the mantle heterogeneity at Cape Verde

The Cape Verde hotspot, like many other Ocean Island Basalt provinces, demonstrates isotopic heterogeneity on a 100–200 km scale. The heterogeneity is represented by the appearance of an EM1-like component at several of the southern islands and with a HIMU-like component present throughout the archipelago. Where the EM1-like component is absent, a local DMM-like component replaces the EM1-like component. Various source lithologies, including peridotite, pyroxenite and eclogite have been suggested to contribute to generation of these heterogeneities; however, attempts to quantify such contributions have been limited. We apply the minor elements in olivine approach (Sobolev et al. in Nature 434:590–597, 2005; Science, doi: 10.1126/science.1138113, 2007), to determine and quantify the contributions of peridotite, pyroxenite and eclogite melts to the mantle heterogeneity observed at Cape Verde. Cores of olivine phenocrysts of the Cape Verde volcanics have low Mn/FeO and low Ni*FeO/MgO that deviate from the negative trend of the global array. The global array is defined by mixing between peridotite and pyroxenite, whereas the Cape Verde volcanics indicate contribution of an additional eclogite source. Eclogite melts escape reaction with peridotite either by efficient extraction in an area of poor mantle flow or by reaction of eclogite melts with peridotite, whereby an abundance of eclogite can seal off the melt from further reaction. Temporal trends of decreasing Mn/FeO indicate that the supply of eclogite melts is increasing. Modelling suggests the local DMM-like end-member is formed from a relatively peridotite-rich melt, while the EM1-like end-member has a closer affinity to a mixed peridotite–pyroxenite–eclogite melt. Notably the HIMU-like component ranges from pyroxenite–peridotite-rich melt to one with up to 77 % eclogite melt as a function of time, implying that sealing of melt pathways is becoming more effective.     

Diamond formation during metasomatism of mantle eclogite by chloride-carbonate melt

A xenolith of bimineralic eclogite from the Udachnaya kimberlite pipe provides a snapshot of interaction between mantle rocks and diamond-forming fluids/melts. The major-element composition of the eclogite is similar to that of N-MORB and/or oceanic gabbros, but its trace-element pattern shows the effects of mantle metasomatism, which resulted in diamond formation. The diamonds are clustered in alteration veins that crosscut primary garnet and clinopyroxene. The diamonds contain microinclusions of a fluid/melt dominated by carbonate and KCl. Compared to the worldwide dataset, the microinclusions in these diamonds fall in middle of the range between saline fluids and low-Mg carbonatitic melts. The fluid/melt acted as a metasomatic agent that percolated through ancient eclogitic rocks stored in the mantle. This interaction is consistent with calculated partition coefficients between the rock-forming minerals and diamond-forming fluid/melt, which are similar to experimentally-determined values. Some differences between the calculated and experimental values may be due to the low contents of water and silicates in the chloride-carbonate melt observed in this study, and in particular its high contents of K and LILE. The lack of nitrogen aggregation in the diamonds implies that the diamond-forming metasomatism took place shortly before the eruption of the kimberlite, and that the microinclusions thus represent saline carbonate-rich fluids circulating in the basement of lithospheric mantle (150–170 km depth).     

The evolution of continental roots in numerical thermo-chemical mantle convection models including differentiation by partial melting

Incorporating upper mantle differentiation through decompression melting in a numerical mantle convection model, we demonstrate that a compositionally distinct root consisting of depleted peridotite can grow and remain stable during a long period of secular cooling. Our modeling results show that in a hot convecting mantle partial melting will produce a compositional layering in a relatively short time of about 50 Ma. Due to secular cooling mantle differentiation finally stops before 1 Ga. The resulting continental root remains stable on a billion year time scale due to the combined effects of its intrinsically lower density and temperature-dependent rheology. Two different parameterizations of the melting phase-diagram are used in the models. The results indicate that during the Archaean melting occurred on a significant scale in the deep regions of the upper mantle, at pressures in excess of 15 GPa. The compositional depths of continental roots extend to 400 km depending on the potential temperature and the type of phase-diagram parameterization used in the model. The results reveal a strong correlation between lateral variations of temperature and the thickness of the continental root. This shows that cold regions in cratons are stabilized by a thick depleted root.     

Experimental reactions of amphibolite with boron-bearing aqueous fluids at 200 MPa: implications for tourmaline stability and partial melting in mafic rocks

Reactions between hornblende-plagioclase amphibolite and acidic and alkaline B-bearing aqueous fluids have been investigated by experiments at 475°–600° C and 200 MPa. At 600° C, hornblende+calcic plagioclase react to form tourmaline+danburite+clinopyroxene+quartz in acidic fluids containing 0.5–1.0 wt% B₂O₃.Tourmaline is precipitated directly from acidic fluids, and the reaction is driven by neutralization of fluids by Na±Ca derived from the breakdown of reactant solids. The concentration of B₂O₃ in fluids needed to stabilize tourmaline increases as pH increases (above approximately 6.0), and tourmaline is unstable in alkaline fluids (pH > approximately 6.5–7.0) regardless of B concentration. In addition to acid-base relations, tourmaline stability is favored by comparatively higher activity coefficients for Al species in acidic fluids. The concentrations of Al and Si in fluid increase with alkalinity, with the eventual production of felsic borosilicate melts through partial melting of the plagioclase component of the amphibolite. In seeded experiments, tourmaline also contributes components to melt. Partial melting is evident in the range 500°–525° C at 200 MPa in experiments with 8wt% B₂O₃ in fluid as Na₂B₄O₇. The experimental results are applied primarily to metasomatic reactions between mafic rocks and borate fluids derived from granitic magmas, but tourmaline stability and partial melting in mafic regional metamorphic systems are also discussed briefly.     

Compressional wave velocity of granite and amphibolite up to melting temperatures at 1 GPa

Compressional wave velocities (V_P) at above-solidus temperatures and at 1 GPa were obtained for a granite and amphibolite, which are considered to be major constituents of the continental crust. The temperature variation of velocities showed that the V_P values of granite decreased with rising temperature, but substantially increased beyond the melting temperature (850–900 °C). Such an increase may be caused by the α–β transition of quartz. The velocities of amphibolite decreased linearly with increasing temperature and dropped sharply at temperatures above the solidus (700 °C), indicating that partial melting of amphibolite acts to significantly lower the seismic velocities.     

恒山地区部分熔融地质体填图方法的尝试

五台群地层呈东西向分布在恒山东段中部,其北为高角闪岩相的恒山片麻杂岩,两者岩性呈渐变过渡关系,由此萌发出片麻杂岩是五台群地层部分熔融而来的思路。经过填图实践,总结出地层与片麻岩体之间存在着仍属地层范畴的片麻岩层这一无序岩石地层单位,并建立起它的填图标志。与地层呈渐变关系的片麻岩体外貌复杂、边界不清,不同于具有一定侵入关系、边界明显、成分均一的片麻岩体,据此划分了原地片麻岩体与异地片麻岩体两个系列。     

Partial melting of metavolcanics in amphibolite facies regional metamorphism

Metavolcanic rocks containing low-Ca amphiboles (gedrite, cummingtonite) and biotite can undergo substantial dehydration-melting. This is likely to be most prominent in Barrovian Facies Series (kyanite-sillimanite) and occurs at the same time as widespread metapelite dehydrationmelting. In lower pressure facies series, metavolcanics will be represented by granulites rich in orthopyroxene when dehydration occurs at much lower temperatures than melting. In higher pressure facies series it is not well known whether metavolcanic rocks dehydrate or melt at temperatures lower or similar to that of metapelites.     

New experimental determination of Li and B partition coefficients during upper mantle partial melting

Despite the growing interest for Li and B as geochemical tracers, especially for material transfer from subducting slabs to overlying peridotites, little is known about the behaviour of these two elements during partial melting of mantle sources. In particular, mineral/melt partition coefficients for B and to a lesser extent Li are still a matter of debate. In this work, we re-equilibrated a synthetic basalt doped with ~10 ppm B and ~6 ppm Li with an olivine powder from a spinel lherzolite xenolith at 1 GPa–1,330°C, and we analyzed Li and B in the run products by secondary ion mass spectrometry (SIMS). In our experiment, B behaved as a highly incompatible element with mineral/melt partition coefficients of the order of 10⁻² (D ^ol/melt = 0.008 (0.004–0.013); D ^opx/melt = 0.024 (0.015–0.033); D ^cpx/melt = 0.041 (0.021–0.061)), and Li as a moderately incompatible element (D ^ol/melt = 0.427 (0.418–0.436); D ^opx/melt = 0.211 (0.167–0.256); D ^cpx/melt = 0.246 (0.229–0.264)). Our partition coefficients for Li are in good agreement with previous determinations. In the case of B, our partition coefficients are equal within error to those reported by Brenan et al. (1998) for all the mineral phases analyzed, but are lower than other coefficients from literature for some of the phases (up to 5 times for cpx). Our measurements complement the data set of Ds for modelling partial melting of the upper mantle and basalt generation, and confirm that, in this context, B is more incompatible than previously anticipated.     

Incomplete reaction of plagioclase in experimental dehydration melting of amphibolite

Dehydration melting of a hornblende‐plagioclase mixture of amphibolitic composition was investigated at 1000°C and at 800 MPa and 1200 MPa. At 1200 MPa the reaction products are garnet, clinopyroxene, melt and relatively Ab‐rich plagioclase (An₄₇). At 800 MPa the products are orthopyroxene, clinopyroxene, magnetite, amphibole (pargasite) and An‐rich plagioclase (An₇₅). The melts are rich in plagioclase components (especially in Ab) and, when compared to tonalites, relatively poor in silica. The grainsize of the starting materials was ≤?5 μm in the 800 MPa and ≤?10 μm in the 1200 MPa runs. All run products show unchanged plagioclase cores, which are the remnants of a very sluggish reaction assumed to be controlled by dissolution/precipitation processes at the plagioclase grain boundaries. The results indicate that only local equilibrium could have been obtained in recent investigations on dehydration melting experiments in plagioclase‐bearing systems. The results also suggest that plagioclase compositions once formed may persist for a very long time, even in hot magma chambers, if the prevailing water activity is low.     

Li abundances in eclogite minerals: a clue to a crustal or mantle origin?

The mineral phases of 33 eclogite and garnet clinopyroxenite samples from various tectonic settings were analysed for Li by secondary ion mass spectrometry (SIMS). In all samples, Li is preferentially incorporated into clinopyroxene (0.4 to 80 µg/g), whereas co-existing garnet contains only minor amounts of Li (0.01 to 3.7 µg/g). When present, glaucophane shows Li abundances which are similar to those of clinopyroxene, but phengite contains significantly less Li than clinopyroxene. Additional phases, such as amphibole, quartz, clinozoisite and kyanite, have low Li concentrations (<1 µg/g). No correlation is apparent between the Li contents and major-element compositions of clinopyroxene or garnet. On the basis of both measured Li concentrations in clinopyroxene and estimated Li abundances in the whole rocks, the investigated samples can be subdivided into high-Li and low-Li groups. These groups coincide with the mode of origin of the rocks. Metabasaltic (metagabbroic) eclogites from high-pressure terranes belong to the high-Li group whereas, except for one eclogite, all kimberlite- and basanite-hosted xenoliths have low Li contents. Samples from eclogites and garnet clinopyroxenites associated with orogenic peridotites fall into both groups. It is suggested that the high-Li eclogites originated from basaltic oceanic crust whereby the notable Li enrichment of some samples was probably caused by low-temperature hydrothermal alteration prior to subduction. Furthermore, the low-Li eclogites and garnet clinopyroxenites may represent high-pressure cumulates from mafic melts percolating through the mantle.