Impact of glacial activity on the weathering of Hf isotopes - observations from Southwest Greenland

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Rickli, Jörg, Hindshaw, Ruth S., Leuthold, Julien, Wadham, Jemma L., Burton, Kevin W., Vance, Derek
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2017
• Subjects: Glacial weathering; Greenland; Hf isotopes; Russell and Leverett Glaciers; Ice Sheet
• Online Access: https://hdl.handle.net/1983/5a1d4697-39a4-4e96-976a-44ad51275f8b; https://research-information.bris.ac.uk/en/publications/5a1d4697-39a4-4e96-976a-44ad51275f8b; https://doi.org/10.1016/j.gca.2017.08.005; https://research-information.bris.ac.uk/ws/files/156930326/Full_text_PDF_accepted_author_manuscript_1_.pdf; http://www.scopus.com/inward/record.url?scp=85027555168&partnerID=8YFLogxK

Data for the modern oceans and their authigenic precipitates suggest incongruent release of hafnium (Hf) isotopes by chemical weathering of the continents. The fact that weathering during recent glacial periods is associated with more congruent release of Hf isotopes has led to the hypothesis that the incongruency may be controlled by retention of unradiogenic Hf by zircons, and that glacial grinding enhances release of Hf from zircons. Here we study the relationship between glacial weathering processes and Hf isotope compositions released to rivers fed by land-terminating glaciers of the Greenland Ice Sheet, as well as neighbouring non-glacial streams. The weathered source rocks in the studied area mostly consist of gneisses, but also include amphibolites of the same age (1.9 Ga). Hafnium and neodymium isotope compositions in catchment sediments and in the riverine suspended load are consistent with a predominantly gneissic source containing variable trace amounts of zircon and different abundances of hornblende, garnet and titanite. Glacially sourced rivers and non-glacial streams fed by precipitation and lakes show very unradiogenic Nd isotopic compositions, in a narrow range (ɛ Nd = −42.8 to −37.9). Hafnium isotopes, on the other hand, are much more radiogenic and variable, with ɛ Hf between −18.3 and −0.9 in glacial rivers, and even more radiogenic values of +15.8 to +46.3 in non-glacial streams. Although relatively unradiogenic Hf is released by glacial weathering, glacial rivers actually fall close to the seawater array in Hf-Nd isotope space and are not distinctly unradiogenic. Based on their abundance in rocks and sediments and their isotope compositions, different minerals contribute to the radiogenic Hf in solution with a decreasing relevance from garnet to titanite, hornblende and apatite. Neodymium isotopes preclude a much stronger representation of titanite, hornblende and apatite in solution, such as might result from differences in dissolution rates, than estimated from mineral abundance. The ...