Geochemical Records of Mantle Processes in Mantle Xenoliths from Three Cenozoic Basaltic Volcanoes in Eastern China

Rare earth element (REE) contents, and Sr and Nd isotopic compositions were measured for three suites of mantle xenoliths from the Kuandian, Hannuoba and Huinan volcanoes in the north of the Sino-Korean Platform. From the correlations of Yb contents with Al/Si and Ca/Si ratios, the peridotites are considered to be the residues of partial melting of the primitive mantle. The chondrite-normalized REE compositions are diverse, varying from strongly LREE-depleted to LREE-enriched, with various types of REE patterns. Metasomatic alteration by small-volume silicate melts, of mantle peridotites previously variably depleted due to fractional melting in the spinel peridotite field, can account for the diversity of REE patterns. The Sr/ Ba versus La/Ba correlation indicates that the metasomatic agent was enriched in Ba over Sr and La, suggestive of its volatile-rich signature and an origin by fluid-triggered melting in an ancient subduction zone. The Sr and Nd isotopic compositions of these xenoliths, even from a single locality, vary widely, covering those of Cenozoic basalts in eastern China. The depleted end of the Sr-Nd isotope correlation is characterized by clearly higher 143Nd/144Nd and a broader range of 87Sr/86Sr compared to MORB. The low-143Nd/144Nd and high-87Sr/86Sr data distribution at the other end of data array suggests the existence of two enriched mantle components, like EM1 and EM2. The correlations between 143Nd/l44Nd and 147Sm/N4Nd ratios in these xenoliths suggest at least two mantle metasomatic events, i.e. events at 0.6-1.0 Ga and 280-400 Ma ago.

Recycling process and proto-kimberlite melt metasomatism in the lithosphere-asthenosphere boundary beneath the Amazonian Craton recorded by garnet xenocrysts and mantle xenoliths from the Carolina kimberlite

Here we present new data on the major and trace element compositions of silicate and oxide minerals from mantle xenoliths brought to the surface by the Carolina kimberlite,Pimenta Bueno Kimberlitic Field,which is located on the southwestern border of the Amazonian Craton.We also present Sr-Nd isotopic data of garnet xenocrysts and whole-rocks from the Carolina kimberlite.Mantle xenoliths are mainly clinopyroxenites and garnetites.Some of the clinopyroxenites were classified as GPP–PP–PKP(garnet-phlogopite peridotite,phlogopite-peridotite,phlogopite-K-richterite peridotite)suites,and two clinopyroxenites(eclogites)and two garnetites are relicts of an ancient subducted slab.Temperature and pressure estimates yield 855–1102℃ and 3.6–7.0 GPa,respectively.Clinopyroxenes are enriched in light rare earth elements(LREE)(La_(N)/Yb_(N)=5–62;Ce_(N)/Sm_(N)=1–3;where N=primitive mantle normalized values),they have high Ca/Al ratios(10–410),low to medium Ti/Eu ratios(742–2840),and low Zr/Hf ratios(13–26),which suggest they were formed by metasomatic reactions with CO_(2)-rich silicate melts.Phlogopite with high TiO_(2)(>2.0 wt.%),Al_(2)O_(3)(>12.0 wt.%),and FeOt(5.0–13.0 wt.%)resemble those found in the groundmass of kimberlites,lamproites and lamprophyres.Conversely,phlogopite with low TiO_(2)(<1.0 wt.%)and lower Al_(2)O_(3)(<12.0 wt.%)are similar to those present in GPP-PP-PKP,and in MARID(mica-amphibole-rutile-ilmenite-diopside)and PIC(phlogopite-ilmenite-clinopyorxene)xenoliths.The GPP-PP-PKP suite of xenoliths,together with the clinopyroxene and phlogopite major and trace element signatures suggests that an intense proto-kimberlite melt metasomatism occurred in the deep cratonic lithosphere beneath the Amazonian Craton.The Sr-Nd isotopic ratios of pyrope xenocrysts(G3,G9 and G11)from the Carolina kimberlite are characterized by high ^(143)Nd/^(144)Nd(0.51287–0.51371)and eNd(+4.55 to+20.85)accompanied with enriched ^(87)Sr/^(86)Sr(0.70405–0.71098).These results suggest interaction with a proto-kimberlite melt compositionally similar with worldwide kimberlites.Based on Sr-Nd whole-rock compositions,the Carolina kimberlite has affinity with Group 1 kimberlites.The Sm-Nd isochron age calculated with selected eclogitic garnets yielded an age of 291.9±5.4 Ma(2σ),which represents the cooling age after the proto-kimberlite melt metasomatism.Therefore,we propose that the lithospheric mantle beneath the Amazonian Craton records the Paleozoic subduction with the attachment of an eclogitic slab into the cratonic mantle(garnetites and eclogites);with a later metasomatic event caused by proto-kimberlite melts shortly before the Carolina kimberlite erupted.

Study on Sulfide-melt Inclusions in Mantle Xenoliths of Xinchang, Zhejiang

In minerals of mantle xenoliths captured within Tertiary alkali-basalt from Xinchang, Zhejiang province, China, many sulfidemelt inclusions were found by the observation of polished thin section. Electron microprobe analysis has been applied to detect the components of sulfide-melt inclusions. The result shows that the sulfide phases of inclusions are mainly pentlandite, and secondarily pyrrho- tite, The molar ratio of Ni to Fe, r_Ni,/r_Fe, of mineral phases in sulfide inclusions is related to olivine contents in host mantle xenoliths. The r_Ni/r_Ni, of sulfides from Xinchang samples has a possitive correlation to r_(Fe+Ni),/r_S. The r_(Fe+Ni),/r_S, becomes higher with the increasing of r_Ni/r_Fe In single sulfide-melt inclusions, r_Ni,/r_Fe, r_(Fe+Ni),/r, and Ni contents increase from the center to edge, reflecting a result of different cooling speed in an inclusion. A comparison between the data from Hannuoba, West Eifel of Germany and Nograd-Gomor of east Europe suggests that the composition of the inclusions is different for each area, which indicated that a regional differentiation of sulfide in mantle fluids.

Interaction between protokimberlite melts and mantle lithosphere:Evidence from mantle xenoliths from the Dalnyaya kimberlite pipe,Yakutia(Russia)

The Dalnyaya kimberlite pipe（Yakutia,Russia） contains mantle peridotite xenoliths（mostly Iherzolites and harzburgites） that show both sheared porphyroclastic（deformed） and coarse granular textures,together with ilmenite and clinopyroxene megacrysts.Deformed peridotites contain high-temperature Fe-rich clinopyroxenes,sometimes associated with picroilmenites,which are products of interaction of the lithospheric mantle with protokimberlite related melts.The orthopyroxene-derived geotherm for the lithospheric mantle beneath Dalnyaya is stepped similar to that beneath the Udachnaya pipe.Coarse granular xenoliths fall on a geotherm of 35 mWm-2 whereas deformed varieties yield a 45 mWm-2）geotherm in the 2-7.5 GPa pressure interval.The chemistry of the constituent minerals including garnet,olivine and clinopyroxene shows trends of increasing Fe~#（=Fe/（Fe＋Mg））with decreasing pressure.This may suggest that the interaction with fractionating protokimberlite melts occurred at different levels.Two major mantle lithologies are distinguished by the trace element patterns of their constituent minerals,determined by LA-ICP-MS.Orthopyroxenes,some clinopyroxenes and rare garnets are depleted in Ba,Sr,HFSE and MREE and represent relic lithospheric mantle.Re-fertilized garnet and clinopyroxene are more enriched.The distribution of trace elements between garnet and clinopyroxene shows that the garnets dissolved primary orthopyroxene and clinopyroxene.Later high temperature clinopyroxenes related to the protokimberlite melts partially dissolved these garnets.Olivines show decreases in Ni and increases in Al,Ca and Ti from Mg-rich varieties to the more Fe-rich,deformed and refertilized ones.Minerals showing higher Fe~#（0.11-0.15） are found within intergrowths of low-Cr ilmenite-clinopyroxene-garnet related to the crystallization of protokimberlite melts in feeder channels.In P-f（O_2） diagrams,garnets and Cr-rich clinopyroxenes indicate reduced conditions at the base of the lithosphere at-5 log units below a FMQ buffer.However,Cr-poor clinopyroxenes,together with ilmenite and some Fe-Ca-rich garnets,demonstrate a more oxidized trend in the lower part of lithosphere at-2 to 0 log units relative to FMQ.Clinopyroxenes from xenoliths in most cases show conditions transitional between those determined for garnets and megacrystalline Cr-poor suite.The relatively low diamond grade of Dalnyaya kimberlites is explained by a high degree of interaction with the oxidized protokimberlite melts,which is greater at the base of the lithosphere.

Post-Entrainment Mineral-Magma Interaction in Mantle Xenoliths from Inner Mongolia,Western North China Craton

In order to distinguish the primary microstructures developed under mantle conditions from the secondary phenomena after xenolith entrainment in the host magma, this study intends to discuss the genesis of spongy, sieve-textured, and reaction rims on mineral grains of mantle xenoliths in the Cenozoic basalts from the western North China craton. The spongy rims on primary clinopyroxene show neither obvious compositional zoning nor preferential development towards the host basalt and probably suggest an origin via partial melting within the lithospheric mantle or pressure release as the xenoliths were carried upwards. The sieve-textured rims on primary spinel show clear chemical zoning with increasing Cr# and decreasing AI towards the host basalt. They are interpreted as the result of partial melting due to heating of the host basaltic magma and decreasing pressure during ascent proc- ess. Post-entrainment reaction mainly generated secondary minerals at contacts between the host ba- saltic melt and xenoliths. The secondary clinopyroxene in reaction rims develops on primary clinopy- roxene and has higher Ti, Ca, and Fe contents and lower Mg# and Si contents than primary one, while the secondary spinel on primary Cr-AI spinel is titanomagnetite. The secondary olivine and clinopy- roxene in the reaction rims on primary orthopyroxene are enriched in Fe, Al, and Ti. The occurrence of reaction rims in mantle xenoliths reflects disequilibrium processes after xenolith entrainment in the basaltic melt. The spongy rims on primary clinopyroxene may not be related to the interaction with thehost basaltic melt, while the sieve-textured rims on primary spinel and reactions rims on primary clinopyroxene, spinel, and orthopyroxene may result from post-entrainment reaction between the host basaltic melt and xenolith minerals.

The Sytykanskaya kimberlite pipe:Evidence from deep-seated xenoliths and xenocrysts for the evolution of the mantle beneath Alakit,Yakutia,Russia

Mantle xenoliths（>150） and concentrates from late autolithic breccia and porphyritic kimberlite from the Sytykanskaya pipe of the Alakit field（Yakutia） were analyzed by EPMA and LAM ICP methods.In P-TX-f（O2） diagrams minerals from xenoliths show widest variations,the trends P-Fe#-CaO,f（O2）for minerals from porphyric kimberlites are more stepped than for xenocrysts from breccia.Ilmenite PTX points mark moving for protokimberlites from the lithosphere base（7.5 GPa） to pyroxenite lens（5-3.5 GPa） accompanied by Cr increase by AFC and creation of two trends P-Fe#OI10-12%and13-15%.The Opx-Gar-based mantle geotherm in Alakit field is close to 35 mW/m2 at 65 GPa and 600 C near Moho was determined.The oxidation state for the megacrystalline ilmenites is lower for the metasomatic associations due to reduction of protokimberlites on peridotites than for uncontaminated varieties at the lithosphere base.Highly inclined linear REE patterns with deep HFSE troughs for the parental melts of clinopyroxene and garnet xenocrysts from breccia were influenced by differentiated protokimberlite.Melts for metasomatic xenoliths reveal less inclined slopes without deep troughs in spider diagrams.Garnets reveal S-shaped REE patterns.The clinopyroxenes from graphite bearing Cr-websterites show inclined and inflected in Gd spectrums with LREE variations due to AFC differentiation.Melts for garnets display less inclined patterns and Ba-Sr troughs but enrichment in Nb-Ta-U.The40Ar/39Ar ages for micas from the Alakit mantle xenoliths for disseminated phlogopites reveal Proterozoic（1154 Ma） age of metasomatism in early Rodinia mantle.Veined glimmerites with richterite- like amphiboles mark1015 Ma plume event in Rodinia mantle.The600-550 Ma stage manifests final Rodinia break-up.The last 385 Ma metasomatism is protokimberlite-related.

Mantle Redox State Evolution in Eastern China and Its Implications

Using the secondary spinel standard, the authors have precisely measured theFe^(3+)/SIGMA Fe values of spinels in mantle xenoliths from Cenozoic basalts in eastern China, andestimated the oxygen fugacities recorded by 63 mantle xenoliths through olivine-orthopyroxene-spineloxygen barometry. The results indicate that the oxygen fugacities of the lithospheric mantle ineastern China are higher in the south than in the north. Among them, the oxygen fugacity of theNorth China craton lithospheric mantle is the lowest, similar to that of the oceanic mantle, whilethat of Northeast and South China are the same as that of the global continental mantle. Thevariations of mantle redox state in eastern China are mainly controlled by the C-O-H fluids derivedfrom the asthenospheric mantle. According to the mantle oxidation state, it can be concluded thatthe C-O-H fluids in the lithospheric mantle of eastern China consist mainly of CO_2 and minor H_2O,but CH_4-rich fluids should come from the asthenosphere where the oxidation state is lower.

Deformation of the Subcontinental Lithospheric Mantle in NE China:Constraints from Rheological and Fabric Study of Mantle Peridotite Xenoliths from Jiaohe,Jilin Province

Mantle peridotite xenoliths in Jiaohe City,located near the northern part of the Tan-Lu fault,are key evidence for constraining the nature and evolution of the subcontinental lithospheric mantle(SCLM)of the NE China.Geochemical characteristics of Jiaohe peridotite xenoliths have been well studied,whereas the microstructures and associated fabrics remain poorly known.We report here major element composition of the constituent minerals,P-T conditions,microstructure,lattice preferred orientations(LPOs)of a set of xenoliths having coarse-grained and granuloblastic to porphyroclastic textures.These xenoliths are characterized by forsterite content of 89-91 in olivine.Dislocation microstructures,in olivine crystals revealed by oxidation decoration technique,are characterized by free dislocation,dislocation walls,dislocation loops and subgrains.Microstructures and deformation mechanism maps indicate that dislocation creep is the dominant deformation mechanism of almost anhydrous olivine in the SCLM.In most samples,the observed LPOs of olivine are typical A-type fabric.Stresses measured in the xenoliths using several olivine piezometers are~2.7-8.5 MPa.The equilibration temperature conditions,calculated using several geo-thermometers,indicate the equilibrium temperature condition of peridotites in a range of 891 to 993℃.These results provide rheological constraint on the deformation of the SCLM in Jiaohe.Combined with the data for mantle xenoliths from adjacent regions,a heterogeneous evolution of the lithosphere deformation is inferred at the Jiaohe region.We propose that characteristics of the studied peridotite may be related to the Tan-Lu fault.

Magmatic systems of large continental igneous provinces

Large igneous provinces（LIPs） formed by mantle superplume events have irreversibly changed their composition in the geological evolution of the Earth from high-Mg melts（during Archean and early Paleoproterozoic） to Phanerozoic-type geochemically enriched Fe-Ti basalts and picrites at 2.3 Ga.We propose that this upheaval could be related to the change in the source and nature of the mantle superplumes of different generations.The first generation plumes were derived from the depleted mantle,whereas the second generation（thermochemical） originated from the core-mantle boundary（CMB）.This study mainly focuses on the second（Phanerozoic） type of LIPs,as exemplified by the midPaleoproterozoic Jatulian-Ludicovian LIP in the Fennoscandian Shield,the Permian-Triassic Siberian LIP,and the late Cenozoic flood basalts of Syria.The latter LIP contains mantle xenoliths represented by green and black series.These xenoliths are fragments of cooled upper margins of the mantle plume heads,above zones of adiabatic melting,and provide information about composition of the plume material and processes in the plume head.Based on the previous studies on the composition of the mantle xenoliths in within-plate basalts around the world,it is inferred that the heads of the mantle（thermochemical） plumes are made up of moderately depleted spinel peridotites（mainly lherzolites）and geochemically-enriched intergranular fluid/melt.Further,it is presumed that the plume heads intrude the mafic lower crust and reach up to the bottom of the upper crust at depths-20 km.The generation of two major types of mantle-derived magmas（alkali and tholeiitic basalts） was previously attributed to the processes related to different PT-parameters in the adiabatic melting zone whereas this study relates to the fluid regime in the plume heads.It is also suggested that a newly-formed melt can occur on different sides of a critical plane of silica undersaturation and can acquire either alkalic or tholeiitic composition depending on the concentration and composition of the fluids.The presence of melt-pockets in the peridotite matrix indicates fluid migration to the rocks of cooled upper margin of the plume head from the lower portion.This process causes secondary melting in this zone and the generation of melts of the black series and differentiated trachytic magmas.

Metasomatic origin of garnet xenocrysts from the V.Grib kimberlite pipe,Arkhangelsk region,NW Russia

This paper presents new major and trace element data from 150 garnet xenocrysts from the V.Grib kimberlite pipe located in the central part of the Arkhangelsk diamondiferous province（ADP）.Based on the concentrations of Cr_2O_3,CaO,TiO_2 and rare earth elements（REE） the garnets were divided into seven groups：（1） lherzolitic ＂depleted＂ garnets（ ＂Lz 1＂）,（2） lherzolitic garnets with normal REE patterns（ ＂Lz2＂）,（3） lherzolitic garnets with weakly sinusoidal REE patterns（ ＂Lz 3＂）,（4） lherzolitic garnets with strongly sinusoidal REE patterns（ ＂Lz 4＂）,（5） harzburgitic garnets with sinusoidal REE patterns（ ＂Hz＂）,（6）wehrlitic garnets with weakly sinusoidal REE patterns（ ＂W＂）,（7） garnets of megacryst paragenesis with normal REE patterns（ ＂Meg＂）.Detailed mineralogical and geochemical garnet studies and modeling results suggest several stages of mantle metasomatism influenced by carbonatite and silicate melts.Carbonatitic metasomatism at the first stage resulted in refertilization of the lithospheric mantle,which is evidenced by a nearly vertical CaO-Cr_2O_3 trend from harzburgitic（ ＂Hz＂） to lherzolitic（ ＂Lz 4＂） garnet composition.Harzburgitic garnets（ ＂Hz＂） have probably been formed by interactions between carbonatite melts and exsolved garnets in high-degree melt extraction residues.At the second stage of metasomatism,garnets with weakly sinusoidal REE patterns（ ＂Lz 3＂, ＂W＂） were affected by a silicate melt possessing a REE composition similar to that of ADP alkaline mica-poor picrites.At the last stage,the garnets interacted with basaltic melts,which resulted in the decrease CaO-Cr_2O_3 trend of ＂Lz 2＂ garnet composition.Cr-poor garnets of megacryst paragenesis（ ＂Meg＂） could crystallize directly from the silicate melt which has a REE composition close to that of ADP alkaline mica-poor picrites.P-T estimates of the garnet xenocrysts indicate that the interval of-60-110 km of the lithospheric mantle beneath the V.Grib pipe was predominantly affected by the silicate melts,whereas the lithospheric mantle deeper than150 km was influenced by the carbonatite melts.

Nature and secular evolution of the lithospheric mantle beneath the North China Craton

The Archean mantle lithosphere beneath the North China Craton(NCC)was transformed in the Mesozoic,leading to the craton destruction.Despite the significant breakthroughs in the craton studies,lithospheric transformation mechanisms are yet to be fully understood.Compositional variations of mantle-derived rocks and xenoliths provide insights into the nature of the mantle lithosphere before and after the craton destruction.The Paleozoic lithosphere of the NCC is~200 km thick.It has a refractory mantle with an evolved isotopic signature.The Mesozoic mantle lithosphere was relatively fertile and highly heterogeneous.In the Cenozoic,the lithosphere in the eastern NCC is about 60–80 km thick.It has an oceanic-type mantle that is fertile in composition and depleted in the Sr-Nd isotopic signature.The Central Zone lithosphere is>100 km thick and has a double-layer mantle with an old upper layer and a new lower layer.The Western Block has a lithosphere of~200 km thick.The lithospheric mantle beneath the southern and northern margins and eastern part of the NCC has been transformed significantly by peridotite-melt reactions due to the multiple subductions of adjacent plates since the Paleozoic.Paleo-Pacific subduction and the associated dynamic processes significantly alter the lithosphere based on the distribution of craton destruction.The involved mechanisms include mechanical intrusion of subduction plates,melt/fluid erosion,and local delamination.The lithospheric thinning of~120 km is relevant to the continental extension caused by subduction plate rollback and trench retreat.

Lattice Preferred Orientation, Water Content, and Seismic Anisotropy of Orthopyroxene

Lattice preferred orientation （LPO） and seismic anisotropy of orthopyroxene （enstatite） in mantle xenoliths from Spitsbergen, Svalbard, near the Arctic, are studied. LPOs of enstatite were determined using electron backseattered diffraction （EBSD）. We found four types of LPOs of orthopyroxene and defined them as type-AC, -AB, -BC, and -ABC. Type-AC LPO of orthopyroxene is defined as （100） plane aligned subparallel to foliation and [001] axis aligned subparallel to lineation. Type-AB LPO is defined as （100） plane aligned subparallel to foliation and [010] axis aligned subparallel to linea- tion. Type-BC LPO is defined as （010） plane aligned subparallel to foliation and [001] axis aligned subparallel to lineation. Type-ABC LPO is defined as both （100） and （010） planes aligned subparallel to foliation with a girdle distribution of both [100] and [010] axes normal to lineation and [001] axis aligned subparallel to lineation. We report for the first time the type-AB, -BC, and -ABC LPO of orthopyroxerie. We found that the LPO pattern has a correlation with the content of orthopyroxene in the specimen. Nicolet 6700 FTIR （Fourier transformation infrared） study of enstatite showed that type-AC LPO was observed mostly in the samples of enstatite with low water content. It is found that the strength of the LPO of enstatite decreases with increasing water content and has a correlation with the strength of the LPO of olivine： the stronger the LPO of enstatite, the stronger the LPO of olivine. Seismic anisotropy of enstatite was smaller than that of olivine in the same specimen.