Evidence of Enriched, Hadean Mantle Reservoir from 4.2-4.0 Ga zircon xenocrysts from Paleoarchean TTGs of the Singhbhum Craton, Eastern India

Abstract

Sensitive High-Resolution Ion Microprobe (SHRIMP) U-Pb analyses of zircons from Paleoarchean (~3.4 Ga) tonalite-gneiss called the Older Metamorphic Tonalitic Gneiss (OMTG) from the Champua area of the Singhbhum Craton, India, reveal 4.24-4.03 Ga xenocrystic zircons, suggesting that the OMTG records the hitherto unknown oldest precursor of Hadean age reported in India. Hf isotopic analyses of the Hadean xenocrysts yield unradiogenic 176Hf/177Hfinitial compositions (0.27995 ± 0.0009 to 0.28001 ± 0.0007; ɛHf[t] = −2.5 to −5.2) indicating that an enriched reservoir existed during Hadean eon in the Singhbhum cratonic mantle. Time integrated ɛHf[t] compositional array of the Hadean xenocrysts indicates a mafic protolith with 176Lu/177Hf ratio of ∼0.019 that was reworked during ∼4.2-4.0 Ga. This also suggests that separation of such an enriched reservoir from chondritic mantle took place at 4.5 ± 0.19 Ga. However, more radiogenic yet subchondritic compositions of ∼3.67 Ga (average 176Hf/177Hfinitial 0.28024 ± 0.00007) and ~3.4 Ga zircons (average 176Hf/177Hfinitial = 0.28053 ± 0.00003) from the same OMTG samples and two other Paleoarchean TTGs dated at ~3.4 Ga and ~3.3 Ga (average 176Hf/177Hfinitial is 0.28057 ± 0.00008 and 0.28060 ± 0.00003), respectively, corroborate that the enriched Hadean reservoir subsequently underwent mixing with mantle-derived juvenile magma during the Eo-Paleoarchean.

The Architecture and Evolution of the Singhbhum Craton

“…5). This implies the existence of a separate depleted reservoir during Paleoarchean which was probably complementary to the ancient enriched reservoir inferred by Chaudhuri et al (2018). Therefore, it appears that different sources contributed to different pulses of the OMTG.…”

“…Other samples of the OMTG have negative ε Nd(t) value and are derived by melting of ancient crustal rocks or from a somewhat enriched mantle which were 150-450 million years older than the OMG amphibolite. Chaudhuri et al (2018) reported that the Eoarchean (~3.7 Ga) cores of zircons in the OMTG and the Paleoarchean (~3.6 to ~3.3) zircon grains in the OMTG have negative ε Hf(t) values of -4.7+1.4 to -6.6+1.1 and -0.4+1.2 to -3.7+1.8 respectively (Fig. 5).…”

“…The oldest reported Hadean ages from the SC are the SHRIMP concordant ages (~4.03 to ~4.24 Ga) of xenocrysts of zircons in the OMTG (~3.4 Ga old) from near Champua (Chaudhuri et al, 2018); these have thin rims with oscillatory zoning and the rims have discordant Eoarchean (~3.8 to ~3.9 Ga) ages ( Table 1). The cores of other xenocrysts yield Eoarchean crystallization ages (~3.7 to ~3.6 Ga).…”

“…Other abbreviations are as in the text. Sreenivas et al, 2019; data source: Chaudhuri et al, 2018, Miller et al, 2018, Sreenivas et al, 2019, Pandey et al, 2019, Olierook et al, 2019 no Eu anomaly and a monzogranite even shows positive Eu anomaly ( Fig. 3d).…”

“…More radiogenic yet subchondritic 176 Hf/ 177 Hf values for some other zircon grains are as follows: ~3.6 to ~3.7 Ga old zircon grains with ε Hf(t) = -1.5 to -6.6, ~3.4 Ga old zircons with ε Hf(t) = -0.4 to -3.7 from OMTG, 3.4 and ~3.3 Ga zircons from the Singhbhum Granite with ε Hf(t) = -0.3 to -4.7. These have been interpreted by Chaudhuri et al (2018) to indicate that the enriched Hadean reservoir subsequently underwent mixing with depleted-mantle-derived juvenile magma during Eoarchean to Paleoarchean. Sreenivas et al (2019), based on SHRIMP and LA-MC-ICPMS measurements, reported Eoarchean detrital zircons (3950-3601 Ma crystallization age) from Mesoarchean (~3.0 Ga) Mahagiri Quartzite (Fig.…”