Calcium Isotope Composition Of The Upper Mantle:A Komatiite Perspective

Accurate determination of Ca isotopic composition in the upper mantle is one of the basic works for establishing high temperature Ca isotopic systematics.Previous estimates on Ca isotopic composition of the upper mantle from measurements on mantle peridotite samples and limited by the representativeness of samples,which calls for further works.As a product of high temperature（¹600℃）and high degree partial melting（F=30-50%）of the mantle,Ca isotopic compositions of komatiites should be close to their mantle source,which makes komatiites perfect samples to constrain the Ca isotope composition of the upper mantle.Twelve komatiite samples from four regions（Barberton,Munro,Abitibi,Yilgarn）were selected for this study.They have Al/Ti ranging from 9.5 to 20.5,including two main types of komatiites:Aluminum Deficit Type（ADK）and Aluminum Undeficit Type（AUK）.Ca isotope composition measurements were conducted on a Triton-plus thermal ionization mass spectrometry（TIMS）at the Isotope Geochemistry Laboratory of China University of Geosciences（Beijing）using double-spike technique.The Komatiites have a slightly lowerε^40/44Ca than SRM915a,with an average value of-0.6±0.5ε（2SD）,which supports that the upper mantle is depleted in radiogenic ⁴⁰Ca relative to the reported standard SRM915a.Except for one sample,δ^44/42Ca of the other samples range from 0.39±0.02‰to 0.49±0.02‰（2SE,95 c.i.%）.The sample SD6/400 has the heaviest Ca isotope composition(δ^44/42Ca=0.61±0.02‰)and the highest LOI（⁷.97 wt.%）.Compared with the trend of CaO vs.MgO and Al₂O₃defined by majority samples,the sample SD6/400 has a higher Ca content,and should be significantly affected by external Ca during the later alteration.There is no correlation betweenδ^44/42Ca of the other samples and LOI and Sr/Nd that reflects the alteration intensity,indicating that the later alteration process has limited influence on their Ca isotope compositions.In addition,theδ^44/42Ca of Komatiite samples has no correlation with（Dy/Yb）_N and Nb/Zr,indicating no significant effect of residual garnet and melting extraction on the Ca isotope composition of melt.Based on the previous experimental petrological data and reported fractionation coefficients,the difference between komatiites in this paper and their source is less than 0.02%.Therefore,the averageδ^44/42Ca value of komatiites（0.44±0.02‰,2SE,N=11）obtained in this study can be used to estimate the average Ca isotope composition of the upper mantle.The new estimated value is consistent with the previous ones based on mantle peridotites（0.43‰-0.48‰）within the current analytical error.