Zn Isotope Analytical Method And Zn Isotopic Fractionation During Mantle Metasomatism

To evaluate whether NIST SRM 683 could be a potential isotopic reference material,its homogeneity of the zinc isotopic composition needs to be carefully examined.We measured the Zn isotopic signature of 84 and 14 micro-drilled sample powders from two metal nuggets.And the ?66Zn value were 0.11 ± 0.02‰ and 0.12 ±0.02‰,respectively,indicating that NIST SRM 683 is a good isotopic reference material with homogeneous Zn isotopes.Besides,we also provide high-precision Zn isotopic data for seventeen rock reference materials.Zinc was purified using AG MP-1M resin and then measured by MC-ICP-MS.Instrumental mass bias was corrected using the "sample-standard bracketing" method and empirical external normalization with Cu doping.After evaluating the potential effects of varying acid mass fractions and different matrices,high-precision Zn isotope data were obtained with an intermediate measurement precision better than ± 0.05‰(?66Zn,2s)over a period of 5 months.The?66Zn values of most of the rock reference materials analyzed in this study were in the range 0.22-0.36‰,except for GSP-2(1.07 ± 0.06‰,n = 12),NOD-A-1(0.9 ± 0.03‰,n = 6)and NOD-P-1(0.78 ± 0.03‰,n = 6).These comprehensive data should serve as reference values for quality assurance and inter-laboratory calibration exercises.To estimate Zn isotopic composition of the bulk silicate earth(BSE),we measured the Zn isotope ratio(2SD = 0.05‰ for ?66Zn)of komatiites,OIBs,and MORBs sourced from different localities,tectonic settings and degree of partial melting.The ?66Zn of komatiites range from 0.19?0.37‰,with an average of 0.27 ±0.14‰(n=8).Though it shows 866Zn variation of 0.18‰ for komatiite samples,no correlation with the age variation has been observed.According to the Zn isotopic analysis,most MORB show similar isotopic characteristics as that published before except two samples,which have much heavier Zn isotopic data(0.44‰ and 0.45‰individually for ?66Zn).Taken their trace element characteristics into consideration,we ascribe to the source effect,probably reflecting the result of partial melting of carbonated mantle.The Zn isotopic compositions for OIB samples are homogeneous,with the average ?66Zn of 0.22±0.04‰.Though komatiite and basalts are the result of different melting degree,but there is no significant difference in Zn isotopic characteristics,which probably leads to the conclusion that no significant Zn isotopic fractionation occur during mantle partial melting.Combining all the komatiites and basalts data in this study with those published before,we estimate the ?66Zn of the BSE to be 0.26±0.09‰.To further understand Zn isotopic fractionation during mantle metasomatism,we also measured Zn isotopic data for Baldissero and Balmuccia(Alps)massive peridotites.These peridotites have underwent complex geological processes,mainly including partial melting,melt refertilization and melt percolation.We observed more than 0.30‰ isotopic fractionation for ?66Zn within these peridotites,and harzburgite and dunite are obviously enriched in heavier Zn isotopes than lherzolites.For lherzolites from each location,variations for ?66Zn are about 0.15‰.Zn isotopic composition for lherzolites from Baldissero show good correlation with the concentration of Zn and Al2O3 and no correlation with Mg#and SiO2.For lherzolites from Balmuccia,no relationship has been observed between ?66Zn with Zn,Al2O3.SiO2 and Mg#.For harzburgite and dunite samples,?66Zn seems to be well related to Zn.Since Zn is not redox-sensitive,it should not produce the significant isotopic fractionation(more than 0.30‰ for ?66Zn).Though sulfide usually have different Zn isotopic data from silicate minerals,its dissolution and re-precipitation will not cause observable fractionation because of low concentration.After taking all possibilities into consideration,we propose that the recognizable Zn isotopic fractionation was probably caused by diffusion during melt percolation and refertilization of mantle peridotites.Driven by the difference in Zn concentration,light Zn isotopes from the peridotite will preferentially diffuse into the melt with lower Zn concentration during mantle-melt reaction,therefore causing the Zn isotopic fractionation.