The fragile memory of the early Earth: Decoding the Sm-Nd systematics in Eoarchean Greenland gneisses

Résumé fourni par la source

Microanalytical studies have shown that Archean gneisses, which underwent high-grade metamorphism, record evidence for 147 Sm- 143 Nd isotope disturbance on a mineral scale. This highlights the necessity to assess isotopic system integrity when interpreting Nd isotope data from metamorphosed Archean terranes in the context of crust-mantle evolution and geodynamic processes in the early Earth. Here, we apply an integrated whole-rock analysis and microanalytical approach to two ∼ 3.82 Ga Amitsôq tonalitic gneisses from a low-strain zone in southern West Greenland to test for 147 Sm- 143 Nd isotope resetting and rare earth element (REE) mobility in comparatively less metamorphosed Eoarchean (≥ 3.6 Ga) rocks than those examined in previous studies. Both samples display distinctive whole rock Nd isotope signatures shared by other tonalitic gneisses from Greenland, with strongly positive μ 142 Nd values of + 18 and + 13, as well as superchondritic ε 143 Nd(3.82 Ga) values of + 2.5 and + 2.1. These characteristics have been interpreted to indicate a depleted mantle source that formed during the Hadean (>4.0 Ga). Zircon U-Pb populations indicate magmatic protolith ages at ∼ 3.82 Ga, but near-chondritic zircon ε Hf(3.82 Ga) values do not support derivation from a depleted mantle reservoir for the Lu-Hf system at that time. Allanite in both gneiss samples shows petrographic evidence for partial metamictization and migration of REE into cracks surrounding these grains. In situ 147 Sm- 143 Nd data from allanite and apatite from both samples define linear arrays on an isochron diagram that imply equilibration at ∼ 2.7 Ga and possibly during older metamorphic events. These accessory minerals register the thermal effects of ∼ 2.7 Ga amphibolite facies metamorphism, causing open system processes at the mineral-scale and the breakdown of REE-bearing minerals. Our observations underscore the vulnerability of the 147 Sm- 143 Nd isotope system to disturbance, even in early Archean meta -igneous rocks considered to be well preserved. The positive μ 142 Nd anomalies require Sm-Nd fractionation that also enhances production of radiogenic 143 Nd. Because the Hf isotope signatures of the studied samples are chondritic and show no evidence for large-scale crust extraction, the positive μ 142 Nd values are interpreted as vestiges of early silicate differentiation, potentially linked to Hadean magma-ocean crystallization, rather than to voluminous extraction of continental crust from the ancient mantle. Mineral-scale evidence and poorly defined whole-rock Sm-Nd isochrons from the low-strain zone suggest that post-magmatic disturbance of the 147 Sm- 143 Nd system can amplify Hf-Nd isotope decoupling and be misinterpreted as evidence for extensive crustal extraction

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