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Apatite is a ubiquitous phase in granite plutons and in most adjacent country rocks, thus contamination of a granite magma with wall-rock material results in two genetic types of apatite in the magma: cognate and foreign. These two textural and chemical varieties of apatite undergo textural and compositional changes to reach physical and chemical equilibrium (perfect assimilation) in the melt. Our experiments replicate the conditions in such contaminated granites. The starting materials consist of a peraluminous synthetic SiO2-Al2O3-Na2O-K2O (SANK 1.3) granite gel with A/NK of 1.3, synthetic F-apatite, synthetic Cl-apatite, and natural Durango apatite. Initial experiments in cold-seal hydrothermal pressure vessels at magmatically realistic temperatures of 750 °C and pressures of 200 MPa produced negligible reactions, even after run times of 2000 h. Instead, we used an argon-pressurized internally heated pressure vessel with a rapid-quench setup at temperatures of 1200 °C, pressure of 200 MPa, and run durations of 192 h. An advantage of this high temperature is that it exceeds the liquidus for quartz and feldspar; therefore, apatite is the only solid phase in the run products. The starting composition of each run was 90 wt% SANK 1.3 granite gel and 10 wt% crushed apatite (consisting of one, two, or three varieties), with and without 4 wt% added H2O. Run products were examined by SEM for texture and by EMPA and LA-ICP-MS for composition. The starting synthetic granite composition contains no Ca, F, Cl, or REEs thus, in every run, apatite was initially undersaturated in the melt. In all experiments, most large apatite grains consisted of anhedral shards with rounded corners, most small apatite grains were round, and a small proportion of apatite grains developed one or more crystal faces. In experiments with two or three apatite compositions, the run-product apatite grains had compositions intermediate between those of the starting-material grains, and they were homogeneous with respect to Cl, and probably F, but not with respect to REEs. The processes to reach textural equilibrium consist of dissolution until the melt is saturated in apatite, followed by Ostwald ripening to eliminate small grains and to develop crystal faces on larger ones. The processes to reach chemical equilibrium consist of dissolution of apatite, diffusion of cations (Ca, P, REE) and anions (F, Cl, OH) through the silicate melt, and solid-state diffusion in the undissolved apatite grains. The halogens approached chemical equilibrium in all experiments, but in the experiments containing Durango apatite, the REEs have not. Models involving radial diffusion into spherical apatite grains at the temperatures of the experiments show complete re-equilibration of the halogens, but changes in the REE concentrations affecting only the outer few micrometers. We conclude that the rate of chemical equilibrium for the halogens is greater than the rate of physical equilibrium for texture, which in turn is greater the rate of chemical equilibrium for REEs. We illustrate these processes with a natural example of contaminated granite from the South Mountain Batholith in Nova Scotia. Given that all granites are contaminated rocks, we propose that future petrogenetic studies focus on developing techniques for a minerals-based quantitative estimation of contamination (QEC). [ABSTRACT FROM AUTHOR]

Since the beginning of the anthropocene, humans have substantially increased inputs of nutrients into stream ecosystems and subsequently changed nutrient limitations and processing. Human activities also modified fine sedimentation patterns, stream morphology, and light conditions, which furthermore influence the functioning of stream ecosystems. This multitude of pressures may control nutrient processing in streams by altering the biological activity of the major stream ecosystem compartments (pelagic, benthic, and hyporheic zone). Still, quantification remains largely unknown for both, the contribution of each compartment to whole stream nutrient uptake and how the mentioned pressures affect those contributions. Beyond, linking reach scale findings to functioning at catchment scale needs to be investigated more intensively.We chose six first and second order streams of the mesoscale Holtemme catchment located in the Harz Mountains and the nearby lowland region (Central Germany). The catchments of three of the streams are dominated by arable farming and forests, respectively. We repeatedly measure in-stream net nutrient uptake (based on nutrient spiralling approach, i.e. longitudinal variation in nutrient concentration along reaches) and ecosystem metabolism (based on continuous oxygen data). Equally, we investigate hydrologic turnover (mass balance approach for gaining/losing of water from/to the groundwater) by applying conservative tracer additions at several stations along the reaches. We perform this assessment in a one-year monitoring to capture different hydrologic regimes. Additionally, we address a range of other hydrological, morphological, and biogeochemical characteristics as well as measurements regarding fine sediment and light.We aim at finding a statistical linkage between the characteristics of the tracer breakthrough curves and hyporheic exchange fluxes. Combining the concept of nutrient spiralling with hydrologic turnover will help to unravel compartmental contributions to whole-stream uptake. We argue that benthic and, especially, hyporheic uptake will be crucial for the total balance and most sensitive to stress such as fine sediment inputs. We anticipate that the concept of hydrologic turnover can be utilized to link the results to the catchment scale owing to its systemic applicability. [ABSTRACT FROM AUTHOR]