Description: The role of accessory minerals in the incongruent release of Hf and Pb during continental weathering and its implications for the generation of distinct seawater isotope compositions is subject of debate. While it has been suggested that radiogenic Hf and Pb isotope signatures released during the dissolution of rocks are controlled by the relative abundances of minerals with distinct isotope compositions and differences in their resistance to dissolution there has not been a comprehensive experimental investigation of these processes to date. We carried out systematic sequential leaching experiments on fresh and partly weathered granitic rock samples as well as separated zircons from the Central Aar Granite in Switzerland. Combined with major and rare earth element concentrations our new quantitative experimental data reveal systematic preferential release of radiogenic Nd, Hf and Pb isotopes primarily controlled by dissolution characteristics of the host rock's easily dissolvable accessory and major minerals, in particular apatite and sphene, during weak chemical weathering. Moreover, Pb isotope signatures of incipient weathering conditions, contrary to expectations, indicate initial congruent release of Pb from freshly exposed mineral surfaces that becomes subsequently incongruent. During more advanced chemical weathering stages, as well as enhanced physical weathering conditions, the dissolution of major minerals (i.e. feldspars) becomes dominant for Nd and Pb isotope signatures, whereas Hf isotopes are still dominated by contributions from highly radiogenic accessories. Additional leaching experiments of zircon separates were performed to test the specific role of zircons for Hf isotope compositions of riverine runoff. It is demonstrated that zircon is more efficiently dissolved when physical weathering is enhanced. This increased Hf release originating from partial dissolution of zircons, however, is quantitatively not sufficient to explain less radiogenic Hf isotope signatures in seawater during episodes of enhanced mechanical erosion alone. Moreover, the observed addition of Hf from the more congruent dissolution of the zircon-free fractions of the parent rock due to enhanced physical weathering indicate that these minerals also play an important role in controlling Hf isotope signatures released under deglacial conditions.

Description: The Čoka Marin polymetallic (Cu-Au-Ag-Zn-Pb) deposit belongs to the world-class Bor ore district in Serbia and consists of three lens-shaped orebodies hosted by Late Cretaceous andesitic volcanic rocks. Pyrite is the dominant mineral in the deposit and shows a great variety of textures and zoning. It occurs as fine- to coarse-grained crystals and massive, spongy, colloform, framboidal, oolitic, and replacement aggregates. Complex relationships with other minerals and the presence of microscale mineral inclusions and micro- to nanoscale porosity are common features of the pyrite at Čoka Marin. Minor amounts of Cu (up to 7.9 wt.%), Pb (up to 7 wt.%), As (up to 4 wt.%), Ag (up to 1 wt.%), Te (up to 0.07 wt.%), and rarely Sb (up to 0.11 wt.%) were detected in pyrite using an electron microprobe. Zoning and heterogeneity in pyrite is mainly caused by variable copper content. Structurally bound Cu in pyrite from this deposit reported in a previous study was confirmed by our new electron microprobe analyses. Distinct Pb-bearing zones frequently occur in pyrite following oscillatory zoning and crystal shapes. These zones also contain lesser amounts of the other elements mentioned above. Using the focused ion beam (FIB) technique to prepare electron-transparent foils and transmission electron microscopy (TEM), we found that these elements form nanoscale Pb-(±As, Ag, Cu)-bearing inclusions (mosty