| Popis: |
The exsolution of volatile phases from a silicate melt constitutes the magmatic-hydrothermal transition, which plays a vital role in determining volcanic eruption styles or in generating a sus- tained and focused fluid flux to form magmatic-hydrothermal ore deposits. In particular, metal and sulfur transport by magmatic volatiles is an important step in porphyry Cu ore genesis, but the details and relative importance of processes that shape large deposits are still debated. A major impediment in quantitative studies of volatiles is the intricacy of their preservation in the rock record. Here, the composition of magmatic and hydrothermal minerals was investigated as a potential proxy for the volatile record: (1) Plagioclase phenocrysts were analyzed to test if they could inform about the water content of the melt they crystallized from, and (2) the composition of quartz in a crystallization regime changing from magmatic to lower-temperature hydrothermal was examined, analogous to previous studies that focused on the evolution of min- eralized pegmatites. Furthermore, pristine fluid and melt inclusions from natural samples may provide small aliquots of the magmatic-hydrothermal system. Previous studies largely focused on inclusions hosted in quartz veins from ore deposits or in quartz lining miarolitic cavities. These samples, however, likely suffered from re-equilibration to changing conditions, resulting in changes from the modification of water concentrations and small, monovalent cations like Cu to the point of complete replacement by later fluids. Inclusions in rapidly-quenched volcanic ejecta, on the other hand, are more likely to preserve original compositions. Water concentrations in silicate melts are commonly estimated through phase equilibria, associated with an appreciable uncertainty, or by analyzing undegassed glass, which is not al- ways available. The incorporation of more Al than stoichiometrically allowed in plagioclase, a ubiquitous magmatic mineral, was recently hypothesized to scale with the water concentration of the melt. To test if a geo-hygrometer (tool to determine water concentrations in the melt) could be developed, compositions of natural and synthetic plagioclase were carefully estimated by electron-probe micro-analysis. The natural samples comprised unaltered plagioclase crystals in barren magmatic rocks from different geological environments to cover a wide range of water concentrations as well as rocks related to porphyry Cu deposits. The synthetic samples were crystallized in piston cylinder experiments from melts with contrasting water contents at 500 MPa. The incorporation of additional Al in plagioclase was found not to correlate with the water concentration in the melt. Instead, apparent deviations from stoichiometry were demon- strated to represent analytical artifacts primarily related to beam-induced element migration. Moreover, these results also call into question the robustness of plagioclase compositions as an exploration tool for porphyry Cu deposits. The magmatic-hydrothermal system of an upper-crustal magma chamber at the verge of eruption was studied based on the example of inclusions from the caldera-forming Kos Plateau Tuff eruption. This magma chamber formed in the Aegean Arc (Greece), in a broadly exten- sional subduction zone setting involving thin continental crust, and produced eruptible silicic melt. The studied fluid inclusions were entrapped pre- to syn-eruptively in the crystallized rind of the magma chamber, which was disrupted by the eruption to form granitic clasts in the non- welded (i.e. quickly cooled) ignimbrite deposit. Based on petrography and microthermometry, two generations of fluids were distinguished: Initially, the fluid in equilibrium with rhyolitic melt at ca. 700 ◦C and around 150-180 MPa was of intermediate density and relatively low salinity (3-11 wt% NaClequivalent). Texturally less mature inclusions, comprising vapor and high-salinity brine, recorded significant decompression to |
