5:15 PM - 7:15 PM
[SMP27-P03] Complementary Hadean reservoirs preserved in felsic and sedimentary rocks from the Dharwar Craton, India
Keywords:Archean cratons, Hadean mantle, Continental crust, Indian cratons, Detrital zircon, 142Nd isotopes
Continental crust may have been produced on Earth throughout its history. However, the composition of the oldest crust and possible changes in composition and production rates with time, particularly on early Earth, are not well-constrained. The rare occurrence of ancient crust can be interpreted to be due to new continental crust produced in the Hadean being either destroyed or recycled at dramatic rates [1]]2] or not exposed. Metasedimentary units in Archean greenstone belts surrounding old TTG suites in a craton can provide information regarding the chemical evolution of their precursor sources. To reconstruct the formation of ancient continental crust in the Dharwar Craton [3][4], we studied the chemical and isotopic composition of detrital zircon grains from metasedimentary units in the western Dharwar Craton, India, to look for evidence for Hadean mantle extraction.
Detrital zircon grains from four different greenstone belts have REE patterns similar to those of continental zircon [5]. The oldest zircon in the Ghattihosahalli and Shimoga greenstone belts formed at 3510±46 Ma and 3467±18 Ma with initial εHf values of -17.3±0.9 and -11.6±1.5, respectively. The depleted mantle model ages of the zircon grains give evidence for the presence of a mafic crust (176Lu/177Hf ~0.2) that was potentially extracted from the mantle at ~4.4 Ga. A compilation of initial εHf values from detrital zircon grains in Indian cratons (ranging in age from 4.0 to 2.0 Ga) provides evidence for Hf isotope signatures that indicate mixing of potential source materials after ~3.8-3.7 Ga [6]. These precursor materials of the granitoids in the craton could have undergone mixing with undifferentiated material of non-chondritic composition [4]. Hadean Hf-model ages of ~4.4 Ga preserved in a small group of 3.6-3.5 Ga old rocks ([3] and this study) underwent weathering or destruction with time and the potential zircon grains stored in the sediments. This time range for crustal extraction from the Hadean mantle is supported by the 146,147Sm-142,143Nd isotope systematics of granitoids and TTGs from the Dharwar Craton that yield a differentiation age of 4.38-0.48+0.12 Ga and 4.45-0.18+0.08 Ga respectively [7]. It is also evident that coherent crustal material representing a Hadean mafic crustal signature is not preserved in the craton, most likely due to later reprocessing. However, the recorded Hadean differentiation and crust formation is still preserved in the Hf and Nd isotope composition of different rock suites and in the oldest zircon grains from metasedimentary rocks.
[1] Bédard (2018) Geoscience Frontiers 9, 19-49
[2] Korenaga (2018) Philosophical Transactions of the Royal Society A 376, 20170408
[3] Ranjan et al. (2022) Precambrian Research 371, 106559
[4] Ravindran et al. (2023) Chemical Geology 615, 121196
[5] Grimes et al. (2015) Contributions to Mineralogy and Petrology 170, 46
[6] Ranjan et al. (2020) Earth and Planetary Science Letters 538, 116206
[7] Ravindran et al. (2024) G-cubed 25, e2024GC011834
Detrital zircon grains from four different greenstone belts have REE patterns similar to those of continental zircon [5]. The oldest zircon in the Ghattihosahalli and Shimoga greenstone belts formed at 3510±46 Ma and 3467±18 Ma with initial εHf values of -17.3±0.9 and -11.6±1.5, respectively. The depleted mantle model ages of the zircon grains give evidence for the presence of a mafic crust (176Lu/177Hf ~0.2) that was potentially extracted from the mantle at ~4.4 Ga. A compilation of initial εHf values from detrital zircon grains in Indian cratons (ranging in age from 4.0 to 2.0 Ga) provides evidence for Hf isotope signatures that indicate mixing of potential source materials after ~3.8-3.7 Ga [6]. These precursor materials of the granitoids in the craton could have undergone mixing with undifferentiated material of non-chondritic composition [4]. Hadean Hf-model ages of ~4.4 Ga preserved in a small group of 3.6-3.5 Ga old rocks ([3] and this study) underwent weathering or destruction with time and the potential zircon grains stored in the sediments. This time range for crustal extraction from the Hadean mantle is supported by the 146,147Sm-142,143Nd isotope systematics of granitoids and TTGs from the Dharwar Craton that yield a differentiation age of 4.38-0.48+0.12 Ga and 4.45-0.18+0.08 Ga respectively [7]. It is also evident that coherent crustal material representing a Hadean mafic crustal signature is not preserved in the craton, most likely due to later reprocessing. However, the recorded Hadean differentiation and crust formation is still preserved in the Hf and Nd isotope composition of different rock suites and in the oldest zircon grains from metasedimentary rocks.
[1] Bédard (2018) Geoscience Frontiers 9, 19-49
[2] Korenaga (2018) Philosophical Transactions of the Royal Society A 376, 20170408
[3] Ranjan et al. (2022) Precambrian Research 371, 106559
[4] Ravindran et al. (2023) Chemical Geology 615, 121196
[5] Grimes et al. (2015) Contributions to Mineralogy and Petrology 170, 46
[6] Ranjan et al. (2020) Earth and Planetary Science Letters 538, 116206
[7] Ravindran et al. (2024) G-cubed 25, e2024GC011834