Geochemistry of Hydrothermal Veins Containing Zirconolite and Betafite at Adamello, Italy
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]. Similarly, the behavior of Zr in natural systems is of interest because Zr is geochemically very similar to Hf, a neutron absorber added to certain nuclear waste forms, e.g., pyrochlorebased ceramics for the immobilization of excess weapons plutonium [2]. The behavior of REE, Th and U in aqueous fluids is fairly well known at ambient temperatures, and some models exist to predict transport and speciation at elevated temperatures [3-5]. To assess the validity of such thermodynamic models, however, it is necessary to study natural systems where high-temperature transport of actinides and REE is documented. Such an example is provided by hydrothermal veins at Adamello (Italy), a fossil hydrothermal system, where actinides and REE have been precipitated from a fluid and subsequently immobilized by zirconolite, betafite and other minerals (for formulae, see Table I). Even though the Adamello veins were formed at conditions that are different from those expected in a final repository (particularly in regard to temperature and fluid composition), they provide valuable insights into the behavior of actinides and REE in fluids that migrated through the Earth’s crust. EXPERIMENTAL PROCEDURES Petrographic observations were made by optical and scanning electron microscopy on polished thin sections. To obtain whole-rock chemical analyses, cores of approximately 5 g mass were drilled from the vein zones. The host-rock samples weighed about 100 g each. All samples were crushed and subsequently ground into a fine powder using either a ball mill or an agate mortar. The whole-rock compositions of the samples were determined with a variety of methods [6, 7]: X-ray fluorescence (XRF) analysis and instrumental neutron activation analysis (INAA) were used for solid samples, whereas atomic absorption spectroscopy (AAS), isotope dilution (ID), inductively coupled plasma atomic emission spectroscopy (ICP-AES) and mass spectrometry (ICP-MS), UV-visible (UV-Vis) and alpha spectroscopy were applied to determine the composition of the samples after dissolution. The concentration of fluorine was determined by ion selective electrode potentiometry, that of CO2 by coulometric titration, and that of sulfur by LECO-infrared spectroscopy. The ferrous iron content was determined by a colorimetric method. GEOLOGICAL BACKGROUND The Adamello batholith, located in northern Italy, is the largest igneous complex of Tertiary age in the Alps, and it is composed of several distinct plutons, which were intruded into the Southern Alpine basement and its sedimentary cover between 42 and 30 Ma [8]. Hydrothermal veins containing zirconolite and betafite occur in Triassic graphite-bearing, pure dolomite marbles at the contact with a major tonalite intrusion (Re di Castello massif). The hydrothermal veins are highly enriched in a number of elements, including Ti, Zr, Nb, REE, Th and U. The veins are characterized by four distinct mineral zones (for formulae, see Table I): (1) (2) (3) (4)
Forsterite+calcite zone (oldest) Pargasite zone consisting of pargasite+calci
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