| The Mg isotopic compositions of the Earth's important geological reservoirs are well constrained,and many achievements have been made in the study of fractionation mechanisms in geological processes.Magnesium isotopes have been regarded as a useful tracer of recycled materials in the mantle.Reservoirs related to low-temperature and life-related activities usually exhibit highly variable Mg isotopic compositions,while those related to high-temperature mantle processes show a narrow range of Mg isotopic variations and have Mg isotopic compositions close to the normal mantle value(-0.25±0.07‰).High-temperature geological processes,such as metamorphic dehydration and magma fractionation,commonly cause undetectable Mg isotope fractionation.Therefore,once the surface materials enter the mantle,it will inevitably lead to the local Mg isotopic heterogeneities of the mantle.Subduction zone is the most important area for the recycling of surface materials,and we therefore can study the mechanisms of surface material recycling and Mg isotope fractionation in subduction processes by analyzing Mg isotopes of subduction-related magmatic rocks,such as continental intraplate basalts and arc magmatic rocks.However,the origin of Mg isotope variations of these rocks is an on-going debate.Firstly,basalts of<110 Ma from eastern China generally have isotopically light Mg and heavy Zn isotopic compositions compared with the normal mantle.Most studies suggest such isotopic characteristics were inherited from recycled carbonates carried by the Paleo-Pacific Plate in the basaltic mantle sources.However,carbonate sediments are not the major component of subducted sediments,and the effects of altered oceanic crust and other sediments are rarely discussed.Therefore,the petrogenesis of basalts in eastern China is still controversial.Secondly,the Mg isotopic compositions of arc magmatic rocks are generally heavier than that of the mantle,which are obviously different from those of oceanic basalts.Previous studies suggest that heavy Mg isotope compositions of are lavas may be derived from the subducted sediments or fluids derived from altered oceanic crust in their mantle sources.Considering the complexity of fluid-rock interactions within the mantle wedge,the inversions from compositions of arc magmatic rocks are not convincing.Hence,we have carried out the following studies to solve the above-mentioned scientific issues.To solve the petrogenesis of continental basalts from eastern China,we carried out a combined study of Mg-Zn-Sr-Nd isotopes of 50 Cenozoic(27-1.2 Ma)continental basalt samples from 11 volcanoes in 6 volcanic fields of 3 provinces(Jiangsu,Zhejiang and Fujian Province)in the South China Block,eastern China.The investigated samples are classified into high-and low-CaO basanites,trachybasalts,and tholeiites,and have variable Mg-Zn-Sr-Nd isotopic compositions(?26Mg=-0.29‰-0.46‰,?66Zn=0.27‰? 0.50‰,87Sr/86Sr=0.703233?0.704175,and ?Nd=3.8?8.0),suggesting mantle heterogeneity.Specifically,the high-CaO basanites and some trachybasalts have high Ca/Al and ?66Zn,low Hf/Hf*and ?26Mg,as well as depleted Sr-Nd isotopic compositions,consistent with a carbonated mantle formed by reaction of the mantle with slab-derived carbonate melts.The low-CaO basanites,some trachybasalts,and tholeiites exhibit low Ca/Al and ?26Mg,high Hf/Hf*and ?66Zn,and depleted Sr-Nd isotopic compositions.These characteristics are related to recycled carbonated eclogites,rather thal carbonates,which is consistent with the correlations between ?26Mg,?66Zn,and Zn/Fe values.In addition,previously published data for the late Mesozoic to Cenozoic basalts from eastern China reveal another group of basalts with high Ba/Th ratios and enriched Sr-Nd isotopic compositions,indicative of significant contributions from siliceous sediments.Our study demonstrates that the compositional variations of the late Mesozoic to Cenozoic alkaline and tholeiitic basalts from eastern China reflect variable proportions of oceanic slab components,including carbonate,siliceous sediments,and carbonated eclogites,in their mantle sources.In order to directly study the influence of slab-derived fluid-wedge mantle interaction on Mg isotopes,we selected a set of well-characterized dunite,harzburgite,garnet-poor othorpyroxenite,garnet-rich othorpyroxenite and garnet clinopyroxenite from Maowu in the Dabie orogen for a combined study of Mg and Si isotopes.These ultramafic rocks represent the protoliths and metasomatic products of the sub-arc mantle wedge.The Mg isotopic compositions vary in a large range(?26Mg=-1.91‰?+1.25‰),and have an obvious lithologic correlation.As the protolith of pyroxenite,dunites have the highest MgO contents,and similar Mg isotopic composition to the normal mantle(?26Mg=-0.28‰?0.22‰).The ?26Mg values of garnet-poor orthopyroxenites(-0.12‰?+1.25‰)are significantly higher than those of the normal mantle.Garnet-rich orthopyroxenites have ?26Mg values(-1.91‰?-0.38‰)much lower than those of the normal mantle.Harzburgites have ?26Mg values from 0.02‰ to 0.54‰,laying between those of garnet-poor orthopyroxenites and dunites.These ultramafic rocks show homogeneous Si isotopic compositions(?30Si=-0.32±0.09‰),consistent to the normal mantle within the range of error,even though they exhibit a wide range of Mg isotopic compositions.The Mg isotopic variations can be explained by three metasomatic stages of the sub-arc mantle wedge.In the first stage,garnet clinopyroxenites were formed by intense interaction of dunites with slab-derived,solute-rich supercritical fluids and show high CaO,FeO,TiO2,Al2O3 contents and low MgO contents.Their ?26Mg values are positively and negatively correlated with MgO and CaO contents,respectively,indicating that the Mg isotopic variations are likely controlled by the contributions of Mg-rich carbonates in the metasomatic agents.Two types of orthopyroxenites are the products of the second metasomatic stage.The mineralogical and major and trace element characteristics indicate that garnet was generated in large amounts in garnet-rich orthopyroxenites,while orthopyroxene was produced continuously in this stage.The increase of ?26Mg and MgO in this stage cannot be attributed to the addition of subduction-zone fluids and/or the leaching of fluids.There is no positive or negative correlation between Mg and Si isotopic compositions of bulk rocks,ruling out the effects of soret and chemical diffusion between garnet orthopyroxenites and fluids.The Mg isotopic fractionation between orthopyroxene and garnet(?26Mgopx-Grt=0.83‰?2.58‰)exceeds the theoretical equilibrium inter-mineral Mg isotopic fractionation value(?26Mgopx-pyrope=1.14‰),indicating that inter-mineral disequilibrium might affect the Mg isotopic compositions of orthopyroxenites.The good negative correlation between ?26Mg and Al2O3,FeOT,as well as the high La/Yb ratio of garnet-poor orthopyroxenites indicate that garnet crystallization is likely the main reason for the increase of whole-rock ?26Mg,which is further intensified by the disequilibrium Mg isotope fractionation between garnet and orthopyroxene.The third stage is a cryptic metasomatism stage.Dunites have the highest MgO and the lowest SiO2,Al2O3,CaO and Na2O contents,but are strongly enriched in Rb,Ba,U,and Pb,with very high Ba/Th ratios(309?2043).Previous studies on the mineral inclusion also show that residual metasomatic fluids enriched in fluid mobile elements.We thus infer that no obvious Mg isotopic fractionation occurs at this stage.Consequently,we demonstrate that the isotopically heavy residual fluids of fluid-rock interactions in the deep mantle wedge can migrate upwards to the mantle sources of arc magmas and contribute their heavy Mg isotopic compositions to arc magma.This can be taken as an alternative mechanism for causing the heavy Mg isotopic compositions of arc magmas. |
PDF Full Text Request