Universität zu Köln

Viehmann, Sebastian ORCID: 0000-0001-6138-5862, Bau, Michael, Hoffmann, J. Elis ORCID: 0000-0001-6670-1393 and Muenker, Carsten (2018). Decoupled Hf and Nd isotopes in suspended particles and in the dissolved load of Late Archean seawater. Chem. Geol., 483. S. 111 - 119. AMSTERDAM: ELSEVIER SCIENCE BV. ISSN 1872-6836

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Abstract

It is generally agreed that decoupling of the Hf and Nd isotope systems in modern aqueous systems is a result of incongruent release of Hf during terrestrial weathering of the continental crust, although the mechanism(s) behind this process are not yet fully understood. We here present Hf-Nd isotope data for the Neoarchean Krivoy Rog Banded Iron Formation (BIFs), Ukraine, and combine observations on modern aqueous environments with those of the early Earth to further evaluate the mechanism(s) behind Hf-Nd isotope decoupling in aqueous systems. The pure Late Archean Krivoy Rog chemical sediment endmember, representing the dissolved pool of ancient seawater, shows decoupled epsilon Nd2.60Ga-epsilon Hf2.60Ga values of -2.3 and + 9.48, respectively, and suggests that decoupled Hf-Nd isotopes had been a global rather than a local phenomenon in Neoarchean seawater. This further reveals that incongruent Hf release via terrestrial weathering and erosion of emerged and evolved continental landmasses were widespread geological processes by Late Archean time. Impure Krivoy Rog BIF samples, composed of a mixture of seawater-derived and detrital Nd and Hf, show systematically more positive epsilon Nd-2.60Ga values, but still reveal decoupled eNd-eHf values relative to an associated schist that plots slightly below the terrestrial array. This suggests that mineral sorting between a zirconbearing sand-sized fraction and fine-grained sediment particles occurred on/in Late Archean continents, rivers and oceans, and had significant impact on the chemical compositions of the suspended and dissolved element loads of Late Archean seawater. Less radiogenic Hf isotope compositions in the Krivoy Rog seawater relative to detritus-contaminated BIFs further suggest a pathway for high-temperature hydrothermal Hf into anoxic Archean seawater, that diluted the even more radiogenic Hf isotopic composition of continental run-off, created by the mineralogical composition of the continental hinterland and the zircon effect. Alternatively, the less pronounced decoupling of eHf-eNd in Late Archean seawater may be related to a shorter residence time of Hf relative to Nd. Furthermore, systematically more positive initial eNd values in detritus-contaminated Archean BIFs relative to respective dissolved seawater loads suggest that weathered and eroded material of (ultra) mafic rock suites had significant impact on the suspended and dissolved fractions in Archean seawater.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Viehmann, Sebastian UNSPECIFIED orcid.org/0000-0001-6138-5862 UNSPECIFIED Bau, Michael UNSPECIFIED UNSPECIFIED UNSPECIFIED Hoffmann, J. Elis UNSPECIFIED orcid.org/0000-0001-6670-1393 UNSPECIFIED Muenker, Carsten UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-189173
DOI:	10.1016/j.chemgeo.2018.01.017
Journal or Publication Title:	Chem. Geol.
Volume:	483
Page Range:	S. 111 - 119
Date:	2018
Publisher:	ELSEVIER SCIENCE BV
Place of Publication:	AMSTERDAM
ISSN:	1872-6836
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language RARE-EARTH-ELEMENTS; BANDED IRON-FORMATIONS; LU-HF; SM-ND; NEODYMIUM ISOTOPES; HAFNIUM; OCEAN; SYSTEMATICS; SUPERGROUP; MANTLE Multiple languages Geochemistry & Geophysics Multiple languages
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/18917