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

作者：Maya J M; Bhutani Rajneesh^*; Balakrishnan S; Sandhya S Rajee

来源：Geoscience Frontiers, 2017, 8(3): 467-481.

DOI：10.1016/j.gsf.2016.03.007

• Komatiite Dharwar craton
• Early mantle differentiation
• Nd-142/Nd-144
• Sm-Nd geochronology

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 Sm-147,Sm-146-Nd-143,Nd-142 systematics to constrain age, petrogenesis and to understand the evolution of Archean mantle. Major and trace element ratios such as CaO/Al2O3, Al2O3/TiO2, Gd/Yb, La/Nb and Nb/Y 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 Sm-147-Nd-143 isochron age of ca. 3.14 Ga with an initial epsilon(Nd)(t) value of +3.5. High precision measurements of Nd-142/Nd-144 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 Nd-142/Nd-144 anomaly indicates that the source of these komatiites formed after the extinction of Sm-146, i.e. 4.3 Ga ago. In order to evolve to the high epsilon(Nd)(t) value of +3.5 by 3.14 Ga the time-integrated ratio of Sm-147/Nd-144 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 similar to 4.53 Ga to create a global early depleted reservoir with superchondritic Sm/Nd ratio. The source of Isua greenstone rocks with positive Nd-142 anomaly was depleted during a second differentiation within the life time of Sm-146, 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 Sm-146, i.e. at 4.3 Ga and prior to 3.14 Ga. Banasandra komatiites therefore provide evidence for preservation of heterogeneities generated during mantle differentiation at 4.3 Ga.

• 出版日期2017-5