Subduction Genetic Diamond And Its Geochemical Significance

Diamond as an important ultrahigh pressure(UHP)indicator has been discovered in UHP subduction zone around the word.The genesis of subduction zone diamond is one of the frontier research fields of earth science.Although many studies have been made in detail,the formation mechanism of this kind of diamond is still controversial.The discovery of diamonds inclusions in oceanic and continental subduction zones indicates that oceanic and continental crust could carry a certain amount of carbon to subduction to the mantle depth(>120km)to form ultra-high pressure stable minerals(diamonds)stored in the mantle.Meanwhile,mantle rocks which were metamorphosed by subducted melt/fluid would trap diamond and bring it to the surface through the exhumation and eruption.Therefore,the study of the exhumed/erupped mantle rocks not only provides a systematic understanding of the genesis of subduction zone metamorphic diamond,but also provides important information about the properties of the mantle,the deep carbon cycle and the crust-mantle interaction in subduction zones.In this dissertation,diamond-bearing alkaline basalts from the southeast margin of North China Carton and peridotite from Dabie orogen were selected for analysis systematically.Firstly,we present geochemical and geochronological data of the diamond-bearing alkaline basalts from Lan'gan,located in the southeastern margin of the NCC.Significant magmatic and tectonic activities in the Mesozoic were occurred in the eastern part of the North China Craton.Mantle-derived mafic magmatism could provide a unique window into deciphering the lithospheric mantle composition and its evolution.Here Diamonds with the shape of cube and rhombic dodecahedron with diameters of 0.2 to 0.6mm have been selected from Lan'gan basalts.Zircon U-Pb dating yielded an average age of 174±14 Ma,representing the first reported Jurassic basalts in the eastern NCC.The presence of diamond indicates that the basalt magma derived from a deep mantle(>120km),indicating that no significant thinning had occurred in the lithosphere of the north China craton before early Jurassic.Petrological and mineralogical observation revealed that the Lan'gan alkaline basalts show a porphyritic textures,with pyroxene phenocrysts and abundant plagioclase,amphibole and Fe-Ti oxides in matrix.The presence of amphibole indicates that the magma source of the basalts was enriched in water.The major element compositions suggest that the Lan'gan basalt is alkaline basalt.It is enriched in light rare earth elements(LREE)and large ion lithosphile elements(LILE).Sr-Nd-Pb-Hf isotopic compositions(87Sr/86Sr(t)=0.70646?0.70925,?Nd(t)=-2.1?-4.9,206Pb/204Pb(t)=17.1?1 8.1,207Pb/204Pb(t)=15.3?15.6,208Pb/204Pb(t)=37.8?38.7,and zircon ?Hf(t)=-17?-21)are slightly enriched compared to depleted mantle.The presence of primary amphibole indicates that the magma source of the basalts was water enriched.These observations suggest that,the lithospheric mantle of the eastern NCC were significantly refertilized,likely by slab derived fluids/melts from the Paleo-Pacific subduction.Owing to the Paleo-Pacific subduction,the lithospheric mantle of the eastern NCC were reduced in viscosity and intensity,and finally promoted partial melting in a limited scale to generate the investigated alkaline basalts.Hence,the discovery of diamond in the Lan'gan basalts demonstrates that the lithosphere of the NCC remained thick,and that large-scale destruction had not initiated in the early Jurassic beneath this region.M-type orogenic mantle peridotites are commonly metasomatized by subduction zone derived fluid/melt.It would provide an excellent insight into crust-mantle interactions.We present inclusions,geochemical and mineral studies of the diamond-bearing peridotite from Raobazhai(RBZ).Whole rock compositions of the RBZ peridotites have refractory characteristics with high Mg#,Ni content,Mg/Si ratio,and low Al2O3+CaO content.They show depletion in high field strength elements(HFSE)and heavy rare earth elements(HREE)in trace element distribution patterns.These data suggest that the RBZ peridotite is a typical M-type peridotite.The petrographic study of the RBZ peridotite from the Dabei orogenic belt shows that there are abundant types of inclusions in olivine.Combined with in-situ Raman spectroscopy and Linkam600 microcalorimetry,the results show that there are four types of inclusions:1)silicate melt inclusions;2)monophase solid inclusions(magnesite,diamond and pargasite);3)methane(CH4)inclusions;and 4)high-salinity fluid inclusions.The presence of primary silicate melt inclusions and pargasite(hydrous silicate mineral)inclusions suggest that the RBZ peridotite was metamorphosed by hydrous silicate melt.It is the first time that micro-diamond has been discovered in RBZ peridotite.The presence of diamond indicates that the RBZ peridotite experienced ultra-high pressure(UHP)metamorphism and C-bearing fluid could subducted to>120km or more.The results of Raman analysis show that two types of carbon in the diamond have been distinguished sp2 carbon(disordered carbon)and sp3 carbon(ordered carbon),indicating that methane plays an important role in the formation of diamond.Therefore,it was speculated that RBZ microdiamond was crystallized from CH4-rich CHO hydrous silicate melt by methane oxidation reaction under the suitable P-T-fO2 conditions:CH4+4Cr3+?C(diamond)+4Cr2+4H+.With the migration of CHO hydrous silicate melt,the fO2 increased,and the CH4-H2O melt changes to CO2-H2O melt and reacts with peridotite resulting in the formation of magnesite.Both diamond and magnesite are regarded as stable C-bearing minerals at UHP condition and could carry/store C to mantle,little mantle peridotite could exhumated to surface.The exhumated peridotite could provide important information of the deep earth.Diamond,magnesite and CH4 are the major carbon phase in subduction zone.These stable carbon phases can transport and store carbon in the mantle by subduction and play an important role in the deep carbon cycle.Paragsite is a hydrous silicate mineral.High temperature(>1000?)is required for the crystallization of pargasite.It suggests that the heat source would be associate with the asthenosphere upwelling.With the crystallization of paragsite,H2O(OH-,H+)and silicate content of hydrous silicate melt would decrease,resulting in the formation of high salinity fluids.Amphibole which is hydrous mineral show high chlorine(Cl)content.It indicates that the hydrous silicate melt is enriched Cl.These analyses provide direct evidence for the scenario of C-H-O-Cl hydrous silicate melt transport,infiltrate and react with the overlying mantle wedge in the subduction channel surface.Two types of spinel exsolution from RBZ peridotite were found:group-I high Cr spinel exsolution and group-? low Cr spinel exsolution.Compared with high Cr spinel,low Cr spinel has a high Mg2+/Fe2+ratio,indicating that low Cr spinel may be formed due to the sudden increase of oxygen fugacity.With the increase of oxygen fugaciny,more Cr2+in orthopyroxene are oxidized to Cr3+,resulting in the formation of low Cr spinel solution.The formation of low Cr spinel exsolution may be related to asthenosphere upwelling caused by plate fragmentation.Therefore,the sudden change of oxygen fugacity is recorded by spinel exsolution,which could provide an important basis for us to study oxygen fugacity in subduction zone.Through the study of geochemistry,zircon U-Pb chronology and inclusion composition of diamond-bearing samples in oceanic and continental subduction zones,it is found that diamond would crystallize from CHO fluid which was derived from subducted plates in subduction zones.Valence ions act as the role of oxygen fugacity buffer in the formation of diamond.Ultra-high pressure stabilized minerals such as diamond play an important role in the deep carbon cycle.With plate subduction/exhumation,oxygen fugacity changes in the subduction zone and was recorded by inclusions and minerals which contain variable ions.