Petrogenesis of 3.15 Ga old Banasandra komatiites from the Dharwar craton, India： Implications for early mantle heterogeneity  

作　　者：J.M.Maya Rajneesh Bhutani S.Balakrishnan S.Rajee Sandhya 

机构地区：[1]Department of Earth Sciences,Pondicherry University

出　　处：《Geoscience Frontiers》2017年第3期467-481,共15页地学前缘（英文版）

基　　金：PLANEX(Planetary Exploration) program,Department of Space,India for funding this study(PLANEX Ref.No.5940)

摘　　要：Spinifex-textured, magnesian （MgO 〉25 wt.%） komatiites from Mesoarchean Banasandra greenstone belt of the Sargur Group in the Dharwar craton, India were analysed for major and trace elements and 147,146Sm-143,142Nd systematics to constrain age, petrogenesis and to understand the evolution of Archean mantle. Major and trace element ratios such as CaOJAl203, Al2O3JTiO2, GdJYb, LaJNb and NbJY suggest aluminium undepleted to enriched compositional range for these komatiites. The depth of melting is estimated to be varying from 120 to 240 km and trace-element modelling indicates that the mantle source would have undergone multiple episodes of melting prior to the generation of magmas parental to these komatiites. Ten samples of these komatiites together with the published results of four samples from the same belt yield 147Sm-143Nd isochron age of ca. 3.14 Ga with an initial ENd（t） value of ＋3.5. High precision measurements of 142Nd/144Nd ratios were carried out for six komatiite samples along with standards AMES and La Jolla. All results are within uncertainties of the terrestrial samples. The absence of 142Nd/144Nd anomaly indicates that the source of these komatiites formed after the extinction of 146Sm, i.e. 4.3 Ga ago. In order to evolve to the high eNd（t） value of ＋3.5 by 3.14 Ga the time-integrated ratio of 147Sm/144Nd should be 0.2178 at the minimum. This is higher than the ratios estimated, so far, for mantle during that time. These results indicate at least two events of mantle differentiation starting with the chondritic composition of the mantle. The first event occurred very early at ~ 4.53 Ga to create a global early depleted reservoir with superchondritic Sm/Nd ratio, The source of Isua greenstone rocks with positive ~42Nd anomaly was depleted during a second differentiation within the life time of 146Sm, i.e. prior to 4.46 Ga. The source mantle of the Banasandra komatiite was a result of a differentiation event that occurred after the extinction of the 146Sm, i,e. at 4.3 Ga and prior Spinifex-textured, magnesian （MgO 〉25 wt.%） komatiites from Mesoarchean Banasandra greenstone belt of the Sargur Group in the Dharwar craton, India were analysed for major and trace elements and 147,146Sm-143,142Nd systematics to constrain age, petrogenesis and to understand the evolution of Archean mantle. Major and trace element ratios such as CaOJAl203, Al2O3JTiO2, GdJYb, LaJNb and NbJY suggest aluminium undepleted to enriched compositional range for these komatiites. The depth of melting is estimated to be varying from 120 to 240 km and trace-element modelling indicates that the mantle source would have undergone multiple episodes of melting prior to the generation of magmas parental to these komatiites. Ten samples of these komatiites together with the published results of four samples from the same belt yield 147Sm-143Nd isochron age of ca. 3.14 Ga with an initial ENd（t） value of ＋3.5. High precision measurements of 142Nd/144Nd ratios were carried out for six komatiite samples along with standards AMES and La Jolla. All results are within uncertainties of the terrestrial samples. The absence of 142Nd/144Nd anomaly indicates that the source of these komatiites formed after the extinction of 146Sm, i.e. 4.3 Ga ago. In order to evolve to the high eNd（t） value of ＋3.5 by 3.14 Ga the time-integrated ratio of 147Sm/144Nd should be 0.2178 at the minimum. This is higher than the ratios estimated, so far, for mantle during that time. These results indicate at least two events of mantle differentiation starting with the chondritic composition of the mantle. The first event occurred very early at ~ 4.53 Ga to create a global early depleted reservoir with superchondritic Sm/Nd ratio, The source of Isua greenstone rocks with positive ~42Nd anomaly was depleted during a second differentiation within the life time of 146Sm, i.e. prior to 4.46 Ga. The source mantle of the Banasandra komatiite was a result of a differentiation event that occurred after the extinction of the 146Sm, i,e. at 4.3 Ga and prior

关 键 词：Komatiite Dharwar craton Early mantle differentiation 142Nd/144Nd Sm-Nd geochronology 

分 类 号：P542[天文地球—构造地质学]