Mantle olivine xenocrysts entrained in Mesozoic basalts from the North China craton：Implication for replacement process of lithospheric mantle

Mesozoic (125 Ma) Fangcheng basalts fromShandong Province contain clearly zoned olivines that arerare in terrestrial samples and provide first evidence for thereplacement of lithospheric mantle from high-Mg peridotitesto Iow-Mg peridotites through peridotite-melt reaction.Zoned olivines have compositions in the core (Mg# = 87.2--90.7) similar to those olivines from the mantle peridotiticxenoliths entrained in Cenozoic basalts from the NorthChina craton and in the rim (Mg# = 76.8--83.9) close to oli-vine phenocrysts of the host basalts (75.7--79.0). Thesecompositional features as well as rounded crystal shapes andsmaller grain sizes (300—800 μm) demonstrate that thesezoned olivines are mantle xenocrysts, i.e. disaggregates ofmantle peridotites. Their core compositions can representthose of olivines of mantle peridotites. The zoned texture ofolivines was formed through rapid reaction between the oli-vine xenocryst and the host basalt. This olivine-basaltic meltreaction could have been ubiquitous in the Mesozoic litho-spheric mantle beneath the North China craton, i.e. an im-portant type of the replacement of lithospheric mantle. Thereaction resulted in the transformation of the Paleozoic re-fractory (high-Mg) peridotites to the late Mesozoic fertile(Iow-Mg) and radiogenic isotope-enriched peridotites, lead-ing to the loss of old lithospheric mantle.     

He and Ar isotope geochemistry of pyroxene megacrysts and mantle xenoliths in Cenozoic basalt from the Changle–Linqu area in western Shandong

We performed a combined study of He–Ar isotopes and element geochemistry for pyroxene megacrysts and mantle xenoliths from Cenozoic basalt in the Changle–Linqu area in western Shandong.The results are used to trace the evolution of subcontinental lithospheric mantle after destruction of the North China Craton.The3He/4He ratios of pyroxene megacrysts,websterite,and dunites are 7.0–7.7 Ra,7.4 Ra,and 7.0–7.7 Ra,respectively.They are related to the origin of host basalts,and are slightly lower than that of mid-ocean ridge basalts(MORB).Their40Ar/36Ar ratios are much lower than that of MORB and close to that of air.The lherzolites and wehrlites represent the fragments of the newly accreted lithospheric mantle,and their REE and trace elements indicate that they experienced melt metasomatism and partial melting.The3He/4He ratios of their olivine are slightly lower than that of MORB,but3He/4He ratios of their clinopyroxene are low(2.3–7.1 Ra)and display an inverse correlation with(La/Yb)N.The40Ar/36Ar ratios of these clinopyroxene are much lower than that of MORB and close to the air ratio.Combining existing studies of petrology,Sr–Nd–Pb isotopes,O isotopes,trace elements,and Mg isotopes,we infer that the juvenile lithospheric mantle in the Changle–Linqu area was metasomatized by oceanic crust-derived melts,which transfer the supracrustal Ar isotope signatures to the mantle sources.The low4He abundance and low3He/4He ratios of clinopyroxene in the lherzolites and wehrlites are ascribed to metasomatism by crustal melts from the subducted Pacific plate.     

Noble gases in corundum megacrysts from the basalts in Changle, Shandong Province, eastern China

This study presents noble gaseous data of the corundum megacrysts from the Cenozoic basalts in Changle, Shandong Province, eastern China. It is known that no noble gaseous data of corundum megacryst have been documented before. The 3He/4He ratios (1.13-7.37 Ra) of the corundums from Changle vary from atmosphere to MORB values; the 20Ne/22Ne (9.67-10.75) and 21Ne/22Ne (0.0280-0.0372) data define two linear trends on Ne three-isotope diagram, respectively, along the MFL and the correlation line between atmosphere and MORB; the 38Ar/36Ar (0.177-0.194) ratios, the 40Ar/36Ar (280.9 -404.2) ratios and the 128-136Xe/132Xe ration with obvious 129Xe excess are generally higher than at-mospheric component, but the 40Ar/36Ar ratios are much closer to atomospheric ratio. The isotopic compositions of noble gases (particularly for He and Ar) of the corundums are similar to those of py-roxene, anorthoclase megacrysts, and mantle-derived xenoliths from this area, and those of man-tle-derived xenoliths from several areas in eastern China. Therefore, the noble gases trapped in the corundums probably are from mantle source, representing a ‘mixed fluid' produced by the interaction between the lithospheric mantle and fluids releasing from the convective plate. Both the noble gas isotopic compositions and the oxygen isotopic compositions of the solid corundums are not the characteristics of crustal source. These suggest that the corundums crystallized from mantle-derived magmas with minimal crustal contamination.     

Characteristics and geological significance of olivine xenocrysts in Cenozoic volcanic rocks from western Qinling

Cenozoic volcanic rocks from the Haoti, Dangchang County of the western Qinling Mountains, contain a few clearly-zoned olivines. These olivines are relatively big in grain sizes and usually have cracks or broken features. Their cores have similar compositions (Mg#=90.4-91.0) to those for the peridotitic xenoliths entrained in host volcanic rocks and their rims are close to the compositions of olivine phenocrysts (Mg#=85.5-81.9). The CaO contents in these zoned olivines are lower than 0.1%. These features demonstrate that the clearly-zoned olivines are xenocrysts and disaggregated from mantle peridotites. The zoned texture was the result of the interaction between the olivine and host magma. Available data show that the volcanic rocks would have been derived from the mantle source metasomatized by subducted hydrathermally-altered oceanic crust. The formation of these Cenozoic volcanic rocks was perhaps related to the rapid uplift of the Tibetan Plateau.     

Re-Os isotope geochemistry of three Chinese chondrites

Three Chinese ordinary chondrites,including Jilin (H5),Boxian (LL3.8) and Lujiang (LL6),have been studied for their Re and Os abundances and Os isotopic composition in whole-chondrite samples,separated magnetic and nonmagnetic fractions,and nodules. The results indicate that the Re and Os abundances of the whole-chondrite samples are in the ranges of corresponding H-and LL-Groups,respectively. The Re and Os abundances of magnetic fraction from Boxian and Lujiang are within the range of high-Os ⅡAB and ⅢAB irons,whereas those of nonmagnetic fractions of Boxian and Lujiang are lower than the whole-chondrite values. The Re and Os abundances of nodules in Jilin are in the range of the LL-Group. 187Re/188Os and 187Os/188Os ratios of the three whole chondrites are in the range of ordinary chondrites which locate around the isochron of ⅡAB ⅢAB irons. 187Re/188Os and 187Os/188Os ratios of the magnetic and nonmagnetic fractions from Boxian have a larger difference. The nonmag-netic fraction of Lujiang may contain a recent addition of Re,which causes deviation of the 187Re/188Os ratio from the irons isochron. The Re and Os abundances of nodules in Jilin are lower than those of the whole-chondrite,but their 187Os/188Os ratios are higher than that of the whole chondrite.     

Os isotopic evidence for a carbonaceous chondritic mantle source for the Nagqu ophiolite from Tibet and its implications

Early-crystallizing chromian spinel(Cr-spinel) in the Nagqu ophiolite has high Os and low Re contents,and it is resistant to alteration during serpentinization,weathering and metamorphism.The chemical composition of primitive magma is preserved in Cr-spinel,which makes it suitable for determining the initial Os-isotope composition of the mantle source.This study presents Cr-spinel Os isotopes and zircon U-Pb ages for cumulate dunite and gabbro,respectively,in the same cumulate section of the ophiolite at Nagqu in Tibet.The results shed light on the formation and evolution of lithospheric mantle.The Nagqu ophiolite is located in the central part of the Bangong-Nujiang suture zone.It is a remnant of the Neotethyan oceanic crust,and contains cumulate dunite and gabbro.Zircon from the gabbro yielded a weighted mean 206 Pb/238 U age of 183.7±1 Ma.Cr-spinel exhibits Os values of 0.2 to 0.3,suggesting that the mantle source for the dunite is similar to that of carbonaceous chondrites.Thus,the Tibetan lithosphere is primarily a relic of Tethyan oceanic lithosphere,which has formed by the transformation of the normal asthenospheric mantle in the Mesozoic.This is the first study to combine the spinel Os isotopes with accurate zircon U-Pb ages to constrain the geochemical characteristics of the mantle source for the ophiolite.     

The Sr, Nd isotopic composition of lamprophyres in Laowangzhai gold orefield, Yunnan Province

Lamprophyres are widely spread over the Laowangzhai gold orefield in northern Mt. Ailao structure zone, Yunnan Province. Lamprophyres in the orefield are temporally and spatially related to gold mineralization. Lamprophyres in the orefield have anomalous Sr and Nd isotopic compositions, e.g. the 87Sr/ 86Sr ratios (0.706 665-0.707 960, the 87Sr/ 86Sr ratio of sample YLW-44 is 0.709 041) are higher than the modern value of the original mantle (0.704 5), the 143Nd/ 144Nd ratios (0.512 463-0.512 551) are lower than the modern value of the original mantle (0.512 638), and the calculated values ε Sr>0 (28.1-63.7) and ε Nd<0 (-1.34--3.07). The results of investigation indicate that lamprophyres in the Laowangzhai gold orefield with anomalous Sr and Nd isotopic compositions would not be produced as a result of contamination of the primary magma with the isotopic features of MORB by the crust materials high 87Sr/ 86Sr ratios and low 143Nd/ 144Nd ratios in the processes of rising or in the magma chamber, but the result of metasomatism of source mantle.     

Re-Os dating of the Raobazhai ultra mafic massif in North Dabie

The ultramafic massif at Raobazhai in North Dabie is located in the suture zone between the Yangtze craton and North China eraton. The Re-Os isotope compositions of the massif are used to decipher the origin and tectonics of the ultramafic rocks involved in continental subduction and exhumation. Fifteen samples were collected from five drill holes along the main SE-NW axis of the Raobazhai massif. Major and trace element compositions of the samples show linear correlations between MgO, Yb and Al_2O_3. This suggests that the massif experienced partial melting with variable degrees and is from fertile to deplete in basaltic compositions. Nine selected samples were analyzed for Re-Os isotope compositions. Re contents range from 0.004 to 0.376 rig/g, Os contents from 0.695 to 3.761 ng/g, ~(187)Re/~(188)Os ratios from 0.022 to 2.564 and ~(187)Os/~(188)Os ratios from 0.1165 to 0.1306. These indicate that the massif is a piece of continental lithospheric mantle with variable depletion. Using the positive corre     

In situ Re-Os isotope ages of sulfides in Hannuoba peridotitic xenoliths： Significance for the frequently-occurring mantle events beneath the North China Block

In situ Re-Os isotopes of sulfides in peridotitic xenoliths from Cenozoic Hannuoba basalts were analyzed by LAM-MC-ICPMS. The suifides developed two types of occurrences including silicate-enclosed and interstitial. In the enclosed sulfides, 187Os/188Os vary from 0.1124 to 0.1362 and 187Re/188Os from 0.0026 to 1.8027. In the interstitial ones, 187Os/188Os have a range from 0.1174 to 0.1354 and 187Re/188Os from 0.0365 to 1.4469. The oldest age, calculated by TRD for the enclosed sulfides, is 2.1 Ga. An isochron age of 2.3±1.2 Ga is obtained by five grains of enclosed sulfides and primitive mantle. The sulfides used have lower Re-Os isotopic ratio than primitive mantle. Meanwhile, an isochron age of 645±225 Ma is given by all in- terstitial sulfides and the enclosed sulfides with higher Re-Os isotopic ratio due to Re addition after man- tle formation. In addition, the model age of 1.3 Ga recorded by one interstitial sulfide, having similar TDM and TRD, should be meaningful to deep thermal event. The coexistence of different ages, revealed by in situ Re-Os isotope, indicates frequently-occurring mantle events beneath Hannuoba area.     

Microscopic deformation of the Neoarchean oceanic lithospheric mantle: evidence from the Zunhua Neoarchean ophiolitic mélange, North China Craton

The microstructures of the mantle tectonites in the Zunhua Neoarchean ophiolitic mélange are mainly coarse-mosaic structures with locally interstitial impregnated melts. Olivine and orthopyroxene occur as residual porphyroclastic blasts, dynamic recrystallization neoblasts or elongated to be tabular. The podiform chromitites are mostly strongly deformed with the development of pull-apart structure. These microstructures are typical high-temperature plastic deformation in oceanic upper mantle resulting from ocean-floor spreading. Besides the high-temperature plastic deformation, undeformed magmatic intrusions such as undeformed podiform chromitite, dunite and pyroxenite intrusions are also preserved in the mantle tectonite. Structures of high-temperature plastic deformation and intensive magmatic activity prove that the Zunhua ophiolite was formed under fast spreading oceanic ridge, similar to the Oman ophiolite. And the microstructures of country rocks, such as quartz ribbon, core and mantle structure, dynamic recrystallization and mica-fish stucture, etc., were formed during the uplift of ophiolitic mélange from the mid-crust to the upper-crust associated with continental collision.     

科马提岩和豆荚状铬铁矿的Os同位素组成及其成因意义

在分析科马提岩和豆荚状铬铁矿Os同位素特征的基础上,着重对比了新太古代华北遵化豆荚状铬铁矿与现代豆荚状铬铁矿和同时期世界典型地区科马提岩的Os同位素组成.结果表明,遵化豆荚状铬铁矿Os同位素组成与现代豆荚状铬铁矿类似,具有球粒陨石性质;而同时期的科马提岩的Os同位素相对球粒陨石富集.这种差异说明二者形成于不同的地幔源区,可能反映了新太古代地幔柱和板块构造并存的动力学特征.     

Re-Os isotopic constraint to the age of in komatiites in the Neoarchean Guyang greenstone belt, North China Craton

The 2.5 Ga Guyang greenstone belt is a major lithological unit in the northwestern part of the North China Craton. Komatiites have recently been identified to occur within the lower part of the meta-volcanic sedimentary sequence of the greenstone belt. The biggest komatiite near Guyang has been mapped out, which is 500 m long and 50 m wide lenticular block. Re-Os isotopic compositions of 9 samples were analyzed to date the komatiites. The Os contents are from 0.88 to 2.63 ppb, identical with typical komatiites, and slightly lower than the normal mantle. The ¹⁸⁷Os/¹⁸⁸Os ratios are from 0.1115 to 0.1197, which are lower enough to exclude the origin of Phanerozoic magma as widely developed in adjacent areas. The calculated Re depletion model ages (T _RD) are from 1346 to 2454 Ma, among them the oldest age of 2454 Ma gives the minimum evaluation for the formation age of the Guyang komatiite. Therefore, the komatiites are a part of the Guyang greenstone belt, indicating high degree melting of mantle during ～2.5 Ga crustal growth event.     

Osmium isotope determination on mantle-derived peridotite xenoliths from Panshishan with N-TIMS

The¹⁸⁷Os/¹⁸⁸Os ratios of spinel lherzolite xenoliths from Panshishan determined with N-TIMS are lower than that of the primitive mantle, which shows depleted mantle characteristics. Their positive correlation with Al₂O₃/MgO suggests that the geochemical behavior of trace elements Re and Os is similar to that of Al₂O₃ and MgO respectively during the magmatic evolution and Re-Os isotopic system is largely immune to mantle metasomatism. A model age of 2.8–3.4 Ga obtained by¹⁸⁷Os/¹⁸⁸Os-Al₂O₃/ MgO correlation might represent the homogeneous age of the mantle lithosphere beneath the area.     

Enhanced mantle-to-crust rhenium transfer in undegassed arc magmas

Variations in the 187Os/188Os isotopic signature of mantle and mantle-derived rocks have been thought to provide a powerful chemical tracer of deep Earth structure. Many studies have inferred from such data that a long-lived, high-rhenium component exists in the deep mantle (187Re is the parent isotope decaying to 187Os, with a half-life of approximately 42 billion years), and that this reservoir probably consists of subducted oceanic crust. The interpretation of these isotopic signatures is, however, dependent on accurate estimates of rhenium and osmium concentrations in all of the main geochemical reservoirs, and the crust has generally been considered to be a minor contributor to such global budgets. In contrast, we here present observations of high rhenium concentrations and low Yb/Re ratios in arc-type melt inclusions. These results indicate strong enrichment of rhenium in undegassed arc rocks, and consequently the continental crust, which results in a crustal estimate of 2 p.p.b. rhenium, as compared to previous estimates of 0.4-0.2 p.p.b. (refs 4, 5). Previous determinations of rhenium in arc materials, which were largely measured on subaerially erupted samples, are likely to be in error owing to rhenium loss during degassing. High mantle-to-crust rhenium fluxes, as observed here, require a revaluation of geochemical models based on the 187Re-187Os decay system.     

Re-Os dating of auriferous pyrite from the Zhenyuan super-large gold deposit in Ailaoshan gold belt, Yunnan Province, Southwestern China

Zhenyuan gold deposit is the largest super large gold deposit in the Ailaoshan gold belt,but its precise mineralization age is still lack.Re-Os isotopic age of the auriferous pyrite from the Zhenyuan gold deposit was determined by using a high-resolution inductively coupled plasma mass spectrometry(HR-ICP-MS).The pyrite samples were decomposed in carius tubes,Os was separated by distillation and Re was extracted by acetone,respectively.The results show that the Re-Os isochron age is 229±38 Ma(MSWD=2.0,confidence level is 95%),with an initial 187 Os/188 Os value of 0.68±0.24 and a corresponding Os value of 442±91.The Re/Os ratios of the pyrite vary from 40.8 to 100.5.The data suggest that at least one important ore-forming event was occurred during Indosinian epoch,and the ore-forming materials probably derived from mixed sources of crustal and mantle,while the later dominated.The Ailaoshan composite orogen experienced complicated evolutional processes,including formation of Precambrian-Early Paleozoic basement,subduction and orogenesis of Late Paleozoic era,collisional orogenesis of Late Hercynian-Indosinain epoch,and extensional or striking orogenesis of Yanshainan-Cenozoic epoch.The polycycle tectonic evolution and magmatism in this area caused multi-stage and superposition metallization characteristics of the Zhenyuan gold deposit:the mineralization probably occur mainly during the Indosinian collisional orogeny,while the minor gold ores of altered granite-porphyry and altered lamprophyre may be formed in the late superposition metallization.     

Osmium isotopic constraints on the nature of the DUPAL anomaly from Indian mid-ocean-ridge basalts

The isotopic compositions of mid-ocean-ridge basalts (MORB) from the Indian Ocean have led to the identification of a large-scale isotopic anomaly relative to Pacific and Atlantic ocean MORB. Constraining the origin of this so-called DUPAL anomaly may lead to a better understanding of the genesis of upper-mantle heterogeneity. Previous isotopic studies have proposed recycling of ancient subcontinental lithospheric mantle or sediments with oceanic crust to be responsible for the DUPAL signature. Here we report Os, Pb, Sr and Nd isotopic compositions of Indian MORB from the Central Indian ridge, the Rodriguez triple junction and the South West Indian ridge. All measured samples have higher (187)Os/(188)Os ratios than the depleted upper-mantle value and Pb, Sr and Nd isotopic compositions that imply the involvement of at least two distinct enriched components in the Indian upper-mantle. Using isotopic and geodynamical arguments, we reject both subcontinental lithospheric mantle and recycled sediments with oceanic crust as the cause of the DUPAL anomaly. Instead, we argue that delamination of lower continental crust may explain the DUPAL isotopic signature of Indian MORB.     

Os isotopic composition of the earth’s primitive upper mantle

The osmium isotopic ratio of¹⁸⁷Os/¹⁸⁸Os ranging from 0.126 to 0.127 for the primitive upper mantle (PUM) is obtained according to the correlation between¹⁸⁷Os/¹⁸⁸Os and Al₂O₃ of mantle-derived xenoliths and orogenic peridotites, which consists with Allegre and Luck’s ALM value and similar to Yin’s nuggets uniform reservoir (NUR) value. It shows that the Os isotope ratio is lower than enstatite or ondinary chondrites, similar to carbonaceous chondrites (CV3).