Phantom zircons provide new insights on the history of one of the oldest banded iron formation (BIF) on Earth (Isua greenstone belt, Greenland)

• Main Authors: Aquila, Quentin, Garçon, Marion, Olivier, Nicolas, Bekker, Andrey
• Other Authors: Laboratoire Magmas et Volcans (LMV), Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Université Clermont Auvergne (UCA)-Observatoire de Physique du Globe de Clermont-Ferrand (OPGC), Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Université Clermont Auvergne (UCA)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Université Clermont Auvergne (UCA), University of California Riverside (UC Riverside), University of California (UC), ERC Starting Grant - GOforISOBIF
• Format: Report
• Language: English
• Published: HAL CCSD 2024
• Subjects: Archean; Hf-Nd isotopic composition; Magnetite; Dating BIF; Seawater signature; Terrigenous contamination; [SDE]Environmental Sciences; Greenland
• Online Access: https://hal.science/hal-04585650; https://hal.science/hal-04585650/document; https://hal.science/hal-04585650/file/HAL_Article_GCA.pdf

Banded Iron Formations (BIFs) offer a wide temporal and spatial window into the Precambrian seawater composition starting with the Eoarchean records. One of the oldest BIFs on Earth is preserved in the northeastern part of the ca. 3.7 Ga Isua Greenstone Belt (Greenland) in the Snowpatch Formation and this BIF unit has been used to generate the IF-G reference material (SARM, Nancy). The IF-G reference material is thought to record a primary seawater signature with no terrigenous contamination. Previous studies on Isua BIF have raised the paradox of the well-preserved, primary Pb isotopic composition along with the reset Nd isotopic composition. This study aims to distinguish primary vs. secondary signals imprinted in diverse mineralogy of the IF-G BIF by combining a petrological approach with trace element and Pb, Nd, and Hf isotopic analyses. Repeated analyses of trace element concentrations revealed a high variability induced by the heterogeneous grain-scale composition of the IF-G reference material. This heterogeneity is most pronounced in LREE, Zr, and Hf contents due to the nugget effect of apatite and zircon micro-grains. Pb isotopic composition of the IF-G reference material yields a well- defined 207Pb-206Pb isochron of 3810 ± 7 Ma (MSWD = 1.06), whereas the 176Lu-177Hf and 147Sm-144Nd isotopic systems are reset by post-depositional processes and/or result from a mixing of non-cogenetic minerals. We suggest that the 3810 Ma 207Pb-206Pb isochron reflects an average age of detrital zircons derived from a continental crust 110 Ma-older than the age of the BIF. The IF-G reference material yields an <Hf(3.7 Ga) > +100, which is unrealistically high for an Eoarchean BIF. Development of secondary apatite might have resulted in reset Hf and Nd isotopic signatures. Our findings signal that caution is required in interpreting records of highly metamorphosed Archean BIFs as seawater REE+Y patterns since chemical sedimentary rocks with Zr content as low as 1 ppm could contain zircon and thus are not entirely ...