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1.
Tracer diffusion coefficients of 153Gd and 152Eu in olivine tholeiite have been determined at temperatures between 1150 and 1440°C. The results are identical for both tracers within experimental error. Between 1440 and 1320°C the diffusion coefficients are given by D(Eu, Gd) = 0.058 exp(?40,600/ RT). Between 1320 and 1210°C, the diffusion coefficients are constant at D = (1.4 ± 0.4) × 10?7 cm2s?1 and between 1210 and 1150°C, the D values drop irregularly to 4 × 10?9 cm2s?1. The liquidus temperature (1270°C) lies within the region of constant D. Such anomalous behavior has not been encountered in previous studies of Ca, Sr, Ba and Co diffusion in basalt. To explain the constant D value near the liquidus, we speculate that the structure of the melt changes as a function of temperature in such a way that the normal temperature dependence of the diffusivity is compensated. For example, the rare earth ions may be displaced from their (high temperature) octahedral coordination sites to other sites where they are more readily dissociated and therefore become progressively more mobile. The behavior below 1210°C may be the result of relatively stable complexes or molecules in the melt or of the formation of a REE bearing crystalline phase that has so far escaped detection. Preliminary results for Eu diffusion in obsidian are D (Eu, 800°C) = 5 × 10?13 cm2 s?1 and D (Eu, 950°C) = 1.5 × 10?11 cm2 s?1. These data are consistent with an activation energy of 59 Kcal mole?1. These low diffusivities indicate that the partitioning of REE in crystallizing intermediate and acidic melts may be controlled by diffusion in the melt rather than equilibrium between the crystal surface and the bulk melt.The diffusion data are applied to partial melting in the mantle, in an attempt to explain how LREE enriched tholeiites may be derived from a LREE depleted mantle source. In this model LREE diffuse from garnet bearing regions that have small melt fractions into garnet free regions that have relatively large melt fractions. REE diffusion is so slow that this process is quantitatively significant only in small partially molten bodies (diameter ～1 km or less) or in larger, but strongly flattened bodies. Internal convective motion during diapiric rise would also increase the efficiency of the process.  相似文献   

2.
U-series radioactive disequilibria in basaltic lavas have been used to infer many important aspects of melt generation and extraction processes in Earth’s mantle and crust, including the porosity of the melting zone, the solid mantle upwelling rate, and the melt transport rate. Most of these inferences have been based on simplified theoretical treatments of the fractionation process, which assume equilibrium partitioning of U-series nuclides among minerals and melt. We have developed a numerical model in which solid-state diffusion controls the exchange of U-series nuclides among multiple minerals and melt. First the initial steady-state distribution of nuclides among the phases, which represents a balance between diffusive fluxes and radioactive production and decay, is calculated. Next, partial melting begins, or a foreign melt is introduced into the system, and nuclides are again redistributed among the phases via diffusion. U-series nuclides can be separated during this stage due to differences in their diffusivity; radium in particular, and possibly protactinium as well, can be strongly fractionated from slower-diffusing thorium and uranium. We show that two distinct processes are not required for the generation of 226Ra and 230Th excesses in mid-ocean ridge basalts, as has been argued previously; instead the observed negative correlations of the (226Ra/230Th) activity ratio with (230Th/238U) and with the extent of trace element enrichment may result from diffusive fractionation of Ra from Th during partial melting of the mantle. Alternatively, the (226Ra/230Th) disequilibrium in mid-ocean ridge basalts may result from diffusive fractionation during shallow-level interaction of mantle melts with gabbroic cumulates, and we show that the results of the interaction have a weak dependence on the age of the cumulate if both plagioclase and clinopyroxene are present.  相似文献   

3.
西南天山哈拉达拉岩体的锆石SHRIMP年代学及地球化学研究   总被引：15，自引：6，他引：9  
薛云兴  朱永峰 《岩石学报》2009,25(6):1353-1363
西南天山哈拉达拉侵入体由橄长岩、橄榄辉长岩和辉长岩组成,橄长岩和橄榄辉长岩具有典型的堆晶结构,堆晶矿物以斜长石和橄榄石为主。辉石、角闪石和金云母主要为堆晶间隙矿物。辉长岩发育辉长—辉绿结构。结晶分异作用在岩浆演化过程中起重要作用。对从辉长岩中分选出来的锆石进行的SHRIMP年代学研究表明,辉长岩形成于308.3±1.8Ma （MSWD=0.86,n=15）。哈拉达拉岩体稀土元素配分模式与E-MORB相似,具有高Rb、Cs、Ba及Sr的特点,87Sr/86Sr初始比值0.7040~0.7050。这些特征表明,岩浆源区具有富集地幔的特征(古南天山洋俯冲流体交代形成了富集地幔)。根据平坦的稀土元素配分模式以及Gd、Sm、Nb、Zr等微量元素的地球化学行为判别,岩浆源区岩石为含角闪石的尖晶石二辉橄榄岩。批式熔融模拟计算显示,地幔岩10%~15%的部分熔融能够形成哈拉达拉岩体的母岩浆。母岩浆通过48%~50%的结晶分异作用则能够形成哈拉达拉岩体。早期结晶的橄榄石和斜长石通过堆晶作用形成橄长岩和橄榄辉长岩,剩余岩浆结晶形成辉长岩。  相似文献   

4.
西藏雅鲁藏布江缝合带东段泽当地幔橄榄岩特征及其意义   总被引：1，自引：1，他引：0  
泽当岩体位于雅鲁藏布江缝合带东段,主要由地幔橄榄岩、辉长辉绿岩和基性火山岩等组成。地幔橄榄岩主要为方辉橄榄岩和二辉橄榄岩,有少量透镜状纯橄岩。地幔橄榄岩经历了强烈的塑性变形作用。地幔橄榄岩中橄榄石的Fo值为89.6~91.8,属镁橄榄石;斜方辉石为顽火辉石,En 87.8~90.3;单斜辉石En 44.1~50.0,主要为顽透辉石和透辉石。铬尖晶石的Cr#值(=100×Cr/(Cr+Al))为17.0~93.6,其中,二辉橄榄岩和方辉橄榄岩中的铬尖晶石为富铝型尖晶石,纯橄岩中的铬尖晶石Cr#最高,为富铬型尖晶石。地幔橄榄岩的部分熔融程度为17%~34%,表明泽当地幔橄榄岩可能经历了多阶段的过程。亏损的主量元素组成和低于原始地幔的稀土元素含量(0.15×10-6~0.61×10-6)指示泽当地幔橄榄岩为经历过部分熔融和熔体抽取的亏损残余地幔岩石。REE配分型式为中稀土亏损的"V"型或"U"型,原始地幔标准化元素比值(La/Sm)N为0.5~8.0,表明泽当地幔橄榄岩经历过交代作用。矿物化学与地球化学数据表明泽当地幔橄榄岩形成于MOR环境,后受到SSZ环境的改造。  相似文献   

5.
The upper mantle is widely considered to be heterogeneous, possibly comprising a “marble-cake” mixture of heterogeneous domains in a relatively well-mixed matrix. The extent to which such domains are capable of producing and expelling melts with characteristic geochemical signatures upon partial melting, rather than equilibrating diffusively with surrounding peridotite, is a critical question for the origin of ocean island basalts (OIB) and mantle heterogeneity, but is poorly constrained. Central to this problem is the characteristic length scale of heterogeneous domains. If radiogenic osmium signatures in OIB are derived from discrete domains, then sub-linear correlations between Os isotopes and other geochemical indices, suggesting melt-melt mixing, may be used to constrain the length scales of these domains. These constraints arise because partial melts of geochemically distinct domains must segregate from their sources without significant equilibration with surrounding peridotite. Segregation of partial melts from such domains in upwelling mantle is promoted by compaction of the domain mineral matrix, and must occur faster than diffusive equilibration between the domain and its surroundings. Our calculations show that the diffusive equilibration time depends on the ratios of partition and diffusion coefficients of the partial melt and surrounding peridotite. Comparison of time scales between diffusion and melt segregation shows that segregation is more rapid than diffusive equilibration for Os, Sr, Pb, and Nd isotopes if the body widths are greater than tens of centimeter to several meters, depending on the aspect ratio of the bodies, on the melt fraction at which melt becomes interconnected in the bodies, and on the diffusivity in the solid. However, because Fe-Mg exchange occurs significantly more rapidly than equilibration of these isotopes under solid-state and partially molten conditions, it is possible that some domains can produce melts with Fe/Mg ratios reflecting that of the surrounding mantle but retaining isotopic signatures of heterogeneous domains. Although more refined estimates on the rates of, and controls on, Os mobility are needed, our preliminary analysis shows that heterogeneous domains large enough to remain compositionally distinct in the mantle (as solids) for ∼109 yr in a marble-cake mantle, can produce and expel partial melts faster than they equilibrate with surrounding peridotite.  相似文献   

6.
Spongy textures are observed in mantle peridotite xenoliths hosted in Cenozoic kamafugites from the Western Qinling, central China. These textures are mainly developed in clinopyroxenes and spinels, and occur as spongy rims consisting of low-Na clinopyroxene, ilmenite, and bubbles, enclosing nonspongy cores. The ilmenites and bubbles exhibit shapes and sizes that vary with the width of the spongy rims. The spongy-textured minerals preserve primary shapes and well-defined grain boundaries and do not show apparent interaction with contact minerals or observed melts except the subsequent melts forming melt pockets. The xenocrysts display reactive zoning textures with host magmas rather than spongy textures. Compositionally, the spongy rims are enriched in Ca, Ti, and most trace elements, have high Cr#, and are depleted in Na, Al, Fe, AlVI, and AlIV/AlVI compared with the cores. These observations suggest that melts/host magmas did not play any significant role in the formation of the spongy textures. We therefore propose that spongy-textured clinopyroxenes and spinels in Western Qinling peridotite xenoliths developed from a decompression-induced partial melting event prior to formation of melt pockets and xenolith entrainment in host magmas.  相似文献   

7.
通过龙游晚中生代基性岩岩相学观察、全岩主量、微量元素和Sr-Nd同位素及矿物电子探针分析,对岩石成因、岩浆演化和构造环境进行探讨。龙游基性岩岩性为橄榄辉长岩,Sr-Nd同位素显示为幔源特征;主量、微量元素特征显示其经历了以橄榄石、辉石为主的分离结晶作用,且未发生明显的地壳物质混染。橄榄石颗粒具核-边结构,富Mg贫Fe的核部Fo值为90.1～91.8,指示原始岩浆是软流圈地幔,富Fe贫Mg的边部显示橄榄石Fo值为77.4～85.3,且核-边两部分的Fo值相差较大,显示原始橄榄石形成之后受到地幔熔体/流体的交代作用。辉石斑晶大多属于透辉石,在部分斑晶的边部发育少量霓辉石。透辉石斑晶普遍具有核-幔-边结构,从核部到边部的SiO_2含量降低,TiO_2、Al_2O_3含量升高,结晶温度升高,显示原始辉石形成之后受到温度更高的地幔熔体/流体的交代作用。研究认为,古太平洋板块后撤造成岩石圈地幔拉伸作用并形成赣杭构造带深部断裂后,部分软流圈物质受到地幔流体/熔体的交代作用,并沿这些深部断裂向上侵位,经历了以橄榄石和辉石矿物为主的分离结晶作用和微弱的地壳物质混染,最终形成龙游橄榄辉长岩。  相似文献   

8.
Geochemical and isotopic data for Cretaceous mafic rocks (basalt, gabbro, and diorite) from the Lower Yangtze region, northern Yangtze block, constrain the evolution of the lithospheric mantle. The mafic rocks, separated into the northeast and southwest groups, are alkaline and evolved, with low Mg# values (44–58) and variable SiO2 contents (47.6–57.4 wt%). Enriched LREEs, LILEs, and Pb, together with depleted Nb, Zr, and Ti, suggest that the mantle sources were metasomatized by slab-derived fluid/melt. All samples show high radiogenic 207Pb/204Pb(t) (15.41–15.65) and 208Pb/204Pb(t) (37.66–38.51) ratios at given 206Pb/204Pb(t) (17.65–19.00) ratios, consistent with the mantle sources having been metasomatized by ancient slab-derived material. Mafic rocks of the southwest group show enriched Sr–Nd isotopic characteristics, with 87Sr/86Sr(t) ranging from 0.7056 to 0.7071 and εNd(t) ranging from −5.3 to −8.3, indicating an origin from enriched lithospheric mantle. Mafic rocks of the northeast group, which record 87Sr/86Sr(t) ratios of between 0.7044 and 0.7050 and εNd(t) of −2.8 to −0.7, possibly formed by the mixing of melts from isotopically enriched lithospheric mantle and isotopically depleted asthenospheric mantle. Taking into consideration the geochemical and isotopic characteristics of Cretaceous mafic rocks, Cenozoic basalts, and basalt-hosted peridotite xenoliths from the Lower Yangtze region, we propose that an isotopically enriched, subduction-modified lithospheric mantle was replaced by or transformed into an isotopically depleted “oceanic-type” mantle. Such a process appears to have occurred in the eastern North China Craton as well as the eastern Yangtze block, probably in response to subduction of the paleo-Pacific plate beneath East Asia.  相似文献   

9.
《Journal of Structural Geology》2002,24(6-7):1101-1107
Flow laws for high-temperature creep of olivine, plagioclase, and diabase are used to place constraints on the rheology of partially molten lower oceanic crust. This analysis is motivated by the observation of olivine lattice preferred orientations and subgrain microstructures in oceanic gabbros that lack evidence for dislocation creep in coexisting plagioclase and pyroxene. Extrapolation of experimental flow laws indicates that at temperatures above 1100°C and stresses less than 10 MPa, olivine may be the weakest phase in rocks with gabbroic composition. By accounting for variations in the melt fraction (0–10%) and grain size of partially molten plagioclase aggregates we can constrain the rheological conditions where olivine deforms by dislocation creep while plagioclase deforms by diffusion creep. Calculated effective viscosities range from 1015 to 1019 Pa s; based on observations of the geometry of the partially molten zone beneath the East Pacific Rise and the microstructural and experimental constraints we favor a value of ∼1018 Pa s. This value approaches estimates for the viscosity of the upper mantle beneath ridge axes, but is significantly higher than previously suggested for the partially molten lower crust. Such high viscosities are inconsistent with ridge evolution models that require large amounts of lower crustal flow to accommodate melt redistribution. However, the results are compatible with recent models that favor local magma replenishment from the mantle at closely spaced intervals along the spreading center axis in a 2D, ‘sheet-like’ fashion.  相似文献   

10.
Peridotite xenoliths found in Cenozoic alkali basalts of northern Victoria Land, Antarctica, vary from fertile spinel-lherzolite to harzburgite. They often contain glass-bearing pockets formed after primary pyroxenes and spinel. Few samples are composite and consist of depleted spinel lherzolite crosscut by amphibole veins and/or lherzolite in contact with poikilitic wehrlite. Peridotite xenoliths are characterized by negative Al2O3–Mg# and TiO2–Mg# covariations of clino- and orthopyroxenes, low to intermediate HREE concentrations in clinopyroxene, negative Cr–Al trend in spinel, suggesting variable degrees of partial melting. Metasomatic overprint is evidenced by trace element enrichment in clinopyroxene and sporadic increase of Ti–Fetot. Preferential Nb, Zr, Sr enrichments in clinopyroxene associated with high Ti–Fetot contents constrain the metasomatic agent to be an alkaline basic melt. In composite xenoliths, clinopyroxene REE contents increase next to the veins suggesting metasomatic diffusion of incompatible element. Oxygen isotope data indicate disequilibrium conditions among clinopyroxene, olivine and orthopyroxene. The highest δ18O values are observed in minerals of the amphibole-bearing xenolith. The δ18Ocpx correlations with clinopyroxene modal abundance and geochemical parameters (e.g. Mg# and Cr#) suggest a possible influence of partial melting on oxygen isotope composition. Thermobarometric estimates define a geotherm of 80°C/GPa for the refractory lithosphere of NVL, in a pressure range between 1 and 2.5 GPa. Clinopyroxene microlites of melt pockets provide P–T data close to the anhydrous peridotite solidus and confirm that they originated from heating and decompression during transport in the host magma. All these geothermometric data constrain the mantle potential temperature to values of 1250–1350°C, consistent with the occurrence of mantle decompressional melting in a transtensive tectonic regime for the Ross Sea region.  相似文献   

11.
This study focuses on the origin of the Os isotope heterogeneities and the behaviour of Os and Re during melt percolation and partial melting processes in the mantle sequence of the Troodos Ophiolite Complex. The sequence has been divided into an eastern (Unit 1) and a western part (Unit 2) (Batanova and Sobolev, 2000). Unit 1 consists mainly of spinel-lherzolites and a minor amount of dunites, which are surrounded by cpx-bearing harzburgites. Unit 2 consists of harzburgites, dunites, and contains chromitite deposits.Unit 1 (187Os/188Os: 0.1169 to 0.1366) and Unit 2 (187Os/188Os 0.1235 to 0.1546) peridotites both show large ranges in their Os isotopic composition. Most of the 187Os/188Os ratios of Unit 1 lherzolites and harzburgites are chondritic to subchondritic, and this can be explained by Re depletion during ancient partial melting and melt percolation events. The old Os isotope model ages (>800 Ma) of some peridotites in a young ophiolitic mantle show that ancient Os isotopic heterogeneities can survive in the Earth upper mantle. Most harzburgites and dunites of Unit 2 have suprachondritic 187Os/188Os ratios. This is the result of the addition of radiogenic Os during a younger major melt percolation event, which probably occurred during the formation of the Troodos crust 90 Ma ago.Osmium concentrations tend to decrease from spinel-lherzolites (4.35 ± 0.2 ng/g) to harzburgites (Unit 1: 4.06 ± 1.12 ng/g; Unit 2: 3.46 ± 1.38 ng/g) and dunites (Unit 1: 2.71 ± 0.84 ng/g; Unit 2: 1.85 ± 1.20 ng/g). Therefore, this element does not behave compatibly during melt percolation as it is observed during partial melting, but becomes dissolved and mobilized by the percolating melt. The Os contents and Re/Os ratios in the mantle peridotites can be explained if they represent mixing products of old depleted mantle with cpx- and opx-veins, which are crystallization products of the percolating melt. This mixing occurred during the melting of a continuously fluxed mantle in a supra-subduction zone environment.This study shows that Unit 1 and Unit 2 of the Troodos mantle section have a complex and different evolution. However, the Os isotopic characteristics are consistent with a model where the harzburgites and dunites of both units belong to the same melting regime producing the Troodos oceanic crust.  相似文献   

12.
The Yarlung Zangbo suture zone (YZSZ) in southern Tibet includes the remnants of Neo‐Tethyan oceanic lithosphere and marks a major suture between the Indian plate to the south and the Lhasa terrane of Tibet to the north. The upper mantle section of the Cuobuzha ophiolite in the northern subbelt of the western YZSZ comprises mainly clinopyroxene (cpx)‐rich and depleted harzburgites. Spinels in the cpx‐harzburgites show lower Cr# values (12.6–15.1) than the spinels in the harzburgites (26.1–34.5), and the cpx‐harzburgites display higher heavy rare earth element concentrations than the depleted harzburgites. The harzburgites have subchondritic Os isotopic compositions (0.11624–0.11699), yielding Re‐depletion model ages (TRD) ages from 1.8 to 1.7 Ga, indicating that the Cubuzha mantle underwent at least one ancient melt extraction event ca. 1.8‐1.7Ga; whereas the cpx‐harzburgites have suprachondritic 187Os/188Os ratios (0.12831–0.13125) with higher Re concentrations (0.380–0.575 ppb), indicating subsequent addition of Re following the last partial melting event that occurred during mid‐ocean ridge melt evolution processes. Although these geochemical and isotopic signatures suggest that both peridotite types in the ophiolite represent mid‐oceanic ridge–type upper mantle units, their melt evolution trends reflect different mantle processes. The cpx‐harzburgites formed from low‐degree partial melting of a primitive mantle source, and they were subsequently modified by melt‐rock interactions in a mid‐oceanic ridge environment. The depleted harzburgites, however, were produced by remelting of the cpx‐harzburgites, which later interacted with mid‐oceanic ridge basalt– or island‐arc tholeiite–like melts, possibly in a trench–distal backarc spreading center. Our new isotopic and geochemical data from the Cuobuzha peridotites confirm that the Neo‐Tethyan upper mantle had highly heterogeneous Os isotopic compositions as a result of multiple melt production and melt extraction events during its seafloor spreading evolution.  相似文献   

13.
During partial melting in the earth’s mantle, the noble metals become fractionated. Os, Ir, Ru, and Rh tend to remain in the mantle residue whereas Pt, Pd, and Re behave mildly incompatible and are sequestered to the silicate melt. There is consensus that sulfide plays a role in the fractionation process; the major noble metal repository in the mantle is sulfide, and most primitive mantle melts are sulfide-saturated when they leave their mantle sources. However, with sulfide–silicate partitioning, the fractionation cannot be modeled properly. All sulfide–silicate partition coefficients are so extremely high that a silicate melt segregating from a mantle source with residual sulfide should be largely platinum-group elements free. We offer a physical alternative to sulfide–silicate chemical partitioning and provide a mechanism of generating a noble metal-rich melt from a sulfide-saturated source: Because sulfide is at least partially molten at asthenospheric temperature, it will behave physically incompatible during melt segregation, and a silicate melt segregating from a mantle residue will entrain molten residual sulfide in suspension and incorporate it in the basaltic pool melt. The noble metal abundances of a basalt then become independent of sulfide–silicate chemical partitioning. They reflect the noble metal abundances in the drained sulfide fraction as well as the total amount of sulfide entrained. Contrary to convention, we suggest that a fertile, sulfide-rich mantle source has more potential to generate a noble metal-enriched basaltic melt than a refractory mantle source depleted by previous partial melting events.  相似文献   

14.
40Ar-39Ar, Sm-Nd, U-Pb, and Lu-Hf isotope data are reported on the gabbro of the Volkovsky Massif, the only massif of the Uralian Platinum Belt wherein economic copper-iron-vanadium and high-grade gold-palladium mineralization is present. The massif is made up of gabbro blocks with concentrically zoned structure and diorite intrusions in its core. In the northeast and southwest, the gabbro is cut by syenite of the Kushva Massif. Gabbro blocks mainly consist of the olivine-anorthite gabbro, while labradorite two- pyroxene gabbro intersects both olivine-anorthite gabbro and Ti-magnetite and copper-PGE mineralization developed in them. The study of both gabbro types by Sm-Nd isochron and U-Pb (SHRIMP II) zircon methods with subsequent REE and Lu-Hf isotope analysis of zircon made it possible to date reliably (428 ± 7 Ma (SHRIMP) and 436 ± 21 Ma (Sm-Nd)) postore labradorite gabbro and, correspondingly, the upper age limit of the mineralization of the Volkovsky Massif. Ore-bearing olivine-anorthite gabbro contain four different-age zircon populations: 2682 ± 37–972 ± 18 Ma, 655 ± 15 to 565 ± 9 Ma; 450 ± 12 Ma, and 343 ± 8 Ma. Hf-Nd isotope systematics showed that zircon with an age of 450 ± 12 Ma presumably marks the formation age of the rocks, the older zircon was trapped, while zircon with an age of 343 ± 8 Ma was formed during low-temperature transformation of the rock and sometimes contains excess radiogenic Hf. Proterozoic xenogenic zircon was inherited from diverse rocks of ancient crust, while the oldest grain with an age of 2065 Ma was possibly formed in a deep mantle source. Vendian zircon was presumably also entrapped, and its morphology and geochemistry point to the crystallization from a basaltic melt. The abundance of pre-Paleozoic zircon in the olivine-anorthite gabbro suggests significant contribution of ancient material in their petrogenesis. This material could serve as source of ore components (metals and sulfur) for unique copper-sulfide gold-PGE mineralization of the Volkovsky Massif.  相似文献   

15.
The Sidingheishan mafic-ultramafic intrusion is located in the eastern part of the Northern Tianshan Mountain, along the southern margin of the Central Asian Orogenic Belt in northern Xinjiang autonomous region of China. The Sidingheishan intrusion is mainly composed of wehrlite, olivine websterite, olivine gabbro, gabbro and hornblende gabbro. At least two pulses of magma were involved in the formation of the intrusion. The first pulse of magma produced an olivine-free unit and the second pulse produced an olivine-bearing unit. The magmas intruded the Devonian granites and granodiorites.An age of 351.4±5.8 Ma(Early Carboniferous) for the Sidingheishan intrusion has been determined by U-Pb SHRIMP analysis of zircon grains separated from the olivine gabbro unit. A U-Pb age of 359.2±6.4 Ma from the gabbro unit has been obtained by LA-ICP-MS. Olivine of the Sidingheishan intrusion reaches 82.52 mole% Fo and 1414 ppm Ni. On the basis of olivine-liquid equilibria, it has been calculated that the MgO and FeO included in the parental magma of a wehrlite sample were approximately10.43 wt% and 13.14 wt%, respectively. The Sidingheishan intrusive rocks are characterized by moderate enrichments in Th and Sm, slight enrichments in light REE, and depletions in Nb, Ta, Zr and Hf. The ε_(Nd)(t) values in the rock units vary from +6.70 to +9.64, and initial ~(87)Sr/~(86)Sr ratios range between 0.7035 and0.7042. Initial ~(206)Pb/~(204)Pb, ~(207)Pb/~(204)Pb and ~(208)Pb/~(204)Pb values fall in the ranges of 17.23-17.91,15.45-15.54 and 37.54-38.09 respectively. These characteristics are collectively similar to the Heishan intrusion and the Early Carboniferous subduction related volcanic rocks in the Santanghu Basin, North Tianshan and Beishan area. The low(La/Gd)_(PM) values between 0.26 and 1.77 indicate that the magma of the Sidingheishan intrusion was most likely derived from a depleted spinel-peridotite mantle.(Th/Nb)_(PM)ratios from 0.59 to 20.25 indicate contamination of the parental magma in the upper crust.Crystallization modeling methods suggest that the parental magma of the Sidingheishan intrusion was generated by flush melting of the asthenosphere and subsequently there was about 10 vol%contamination from a granitic melt. This was followed by about 5 vol% assimilation of upper crustal rocks. Thus, the high-Mg basaltic parental magma of Sidingheishan intrusion is interpreted to have formed from partial melting of the asthenosphere during the break-off of a subducted slab.  相似文献   

16.
《International Geology Review》2012,54(15):1721-1745
The Panzhihua layered intrusion hosts a giant V–Ti–iron oxide deposit with ore reserves estimated at 1333 Mt. Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) U–Pb zircon dating of comagmatic anorthosite yields a crystallization age of 259.77 ± 0.79 million years, coeval with the Emeishan flood basalts. Recently, we identified a small wehrlite dike in microgabbroic rocks and marbles. The wehrlite consists of high-Mg olivine phenocrysts with up to 90.44 wt.% Fo. Incompatible element-normalized patterns between bulk wehrlite and clinopyroxenes in gabbro suggest that they are cogenetic. The Panzhihua parental magma is estimated to have been picritic (～10 wt.% FeO and ～16 wt.% MgO), produced by partial fusion of garnet peridotite. Much of the melting occurred in garnet-facies mantle at an initial melting temperature of about 1530°C and pressure of ～3.4 GPa, suggesting involvement of a mantle plume. The degree of partial melting was rather modest and could have been generated by plume–lithosphere interaction or ascending plume-derived melting contaminated by lithospheric mantle. Field relationships show sharp contacts between the massive ores and gabbro, between wehrlite and fine-grained gabbro, and between disseminated ores and gabbro. Considering the entire intrusion, which is locally cut by dikes or veins of anorthosite, together with the occurrence of a breccia made up of gabbro clasts cemented by disseminated ores, we suggest that different types of magmas were generated by liquid differentiation in a deeper-level chamber. This differentiation could have resulted from double-diffusive convection cells, with melt later intruding into a higher-level chamber, rather than by crystal settling or in situ growth on the floor of the intrusion. However, rhythmic layering produced by in situ crystallization only occurs in the middle of the Panzhihua intrusion and was caused by periodic fluctuation in water pressure.  相似文献   

17.
Lithium elemental and isotopic disequilibrium has frequently been observed in the continental and oceanic mantle xenoliths, but its origin remains controversial. Here, we present a combined elemental and Li isotopic study on variably metasomatised peridotite xenoliths entrained in the Cenozoic basalts from Shangzhi in Northeast (NE) China that provides insight into this issue. Li concentration (0.3–2.7 ppm) and δ7Li (mostly 2‰–6‰) in olivine from the Shangzhi peridotites are similar to the normal mantle values and show roughly negative correlations with the indices of melt extraction (such as modal olivine and whole rock MgO). These features are consistent with variable degrees of partial melting. In contrast, clinopyroxene from the Shangzhi xenoliths shows significant Li enrichment (0.9–6.1 ppm) and anomalously light δ7Li (??13.8‰ to 7.7‰) relative to normal mantle values. Such features can be explained by Li diffusion from silicate melts or Li-rich fluids occurring over a very short time (several minutes to several hours). Moreover, the light Li isotopic compositions preserved in some bulk samples also indicate that these percolated melts/fluids have not had enough time to isotopically equilibrate with the bulk peridotite. We thus emphasize that Li isotopic fractionation in the Shangzhi mantle xenoliths is mainly related to Li diffusion from silicate melts or Li-rich fluids that took place shortly before or coincident with their entrainment into the host magmas.  相似文献   

18.
Analyses of enriched mantle (EM)-basalts, using lithophile element-based isotope systems, have long provided evidence for discrete mantle reservoirs with variable composition. Upon partial melting, the mantle reservoir imparts its isotopic fingerprint upon the partial melt produced. However, it has increasingly been recognised that it may not be simple to delimit these previously well-defined mantle reservoirs; the “mantle zoo” may contain more reservoirs than previously envisaged.Here we demonstrate that a simple model with varying contributions from two populations of compositionally distinct mantle sulphides can readily account for the observed heterogeneities in Os isotope systematics of such basalts without additional mantle reservoirs. Osmium elemental and isotopic analyses of individual sulphide grains separated from spinel lherzolites from Kilbourne Hole, New Mexico, USA demonstrate that two discrete populations of mantle sulphide exist in terms of both Re-Os systematics and textural relationship with co-existing silicates. One population, with a rounded morphology, is preserved in silicate grains and typically possesses high [Os] and low [Re] with unradiogenic, typically sub-chondritic 187Os/188Os attributable to long term isolation in a low-Re environment. By contrast, irregular-shaped sulphides, preserved along silicate grain boundaries, possess low [Os], higher [Re] and a wider range of, but generally supra-chondritic 187Os/188Os ([Os] typically ? 1-2 ppm, 187Os/188Os ? 0.3729; this study). This population is thought to represent metasomatic sulphide.Uncontaminated silicate phases contain negligible Os (<100 ppt) therefore the Os elemental and isotope composition of basalts is dominated by volumetrically insignificant sulphide ([Os] ? 37 ppm; this study). During the early stages of partial melting, supra-chondritic interstitial sulphides are mobilised and incorporated into the melt, adding their radiogenic 187Os/188Os signature. Only when sulphides armoured within silicates are exposed to the melt through continued partial melting will enclosed sulphides add their high [Os] and unradiogenic 187Os/188Os to the aggregate melt. Platinum-group element data for whole rocks are also consistent with this scenario. The sequence of (i) addition of all of the metasomatic sulphide, followed by (ii) the incorporation of small amounts of armoured sulphide can thus account for the range of both [Os] and 187Os/188Os of EM-basalts worldwide without the need for contributions from additional silicate mantle reservoirs.  相似文献   

19.
The Dangqiong ophiolite, the largest in the western segment of the Yarlung-Zangbo Suture Zone(YZSZ)ophiolite belt in southern Tibet, consists of discontinuous mantle peridotite and intrusive mafic rocks. The former is composed dominantly of harzburgite, with minor dunite, locally lherzolite and some dunite containing lenses and veins of chromitite. The latter, mafic dykes(gabbro and diabase dykes), occur mainly in the southern part. This study carried out geochemical analysis on both rocks. The results show that the mantle peridotite has Fo values in olivine from 89.92 to 91.63 and is characterized by low aluminum contents(1.5–4.66 wt%) and high Mg# values(91.06–94.53) of clinopyroxene. Most spinels in the Dangqiong peridotites have typical Mg# values ranging from 61.07 to 72.52, with corresponding Cr# values ranging from 17.67 to 31.66, and have TiO2 contents from 0 to 0.09%, indicating only a low degree of partial melting(10–15%). The olivine-spinel equilibrium and spinel chemistry of the Dangqiong peridotites suggest that they originated deeper mantle(20 kbar). The gabbro dykes show N-MORB-type patterns of REE and trace elements. The presence of amphibole in the Dangqiong gabbro suggests the late-stage alteration of subduction-derived fluids. All the lherzolites and harzburgites in Dangqiong have similar distribution patterns of REE and trace elements, the mineral chemistry in the harzburgites and lherzolites indicates compositions similar to those of abyssal and forearc peridotites, suggesting that the ophiolite in Dangqiong formed in a MOR environment and then was modified by late-stage melts and fluids in a suprasubduction zone(SSZ) setting. This formation process is consistent with that of the Luobusa ophiolite in the eastern Yarlung-Zangbo Suture Zone and Purang ophiolite in the western Yarlung-Zangbo Suture Zone.  相似文献   

20.
The mineralogy and texture of shock-induced melt veinlets and melt pockets in silicate inclusions in the Elga IIE iron meteorite have been studied by reflected-light optical microscopy, EMPA, SEM, Raman spectroscopy and TEM. The results suggest that Elga experienced two discrete impact events. The earlier event involved the collision of a metallic projectile with a silicate target and resulted in partial melting and recrystallization of the silicate material, forming schreibersite and oxide rims between the metal and silicate. The later impact event resulted in melt pockets in the silicate inclusions and was associated with fragmentation, melting, and brecciation of the rims and displacement of some fragments into the melt pockets. These fragments are shown to contain carbon-bearing phases: siderite and amorphous sp 2 carbon, which form carbon–oxide, siderite–oxide, and siderite–schreibersite associations. The fact that the carbon-bearing fragments are spatially constrained to shock breccia and melt zones indicates that these fragments are genetically related to the impact process and that their carbon-bearing phases are of cosmic origin.  相似文献   

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