The magmatic-hydrothermal transition: a perspective from fluid & melt inclusions and mineral chemistry

Abstract

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 <120 MPa associated with eruption at similar tem- peratures. The compositions of the various inclusion types were analyzed using laser ablation inductively-coupled plasma mass spectrometry. The glassy melt inclusions exhibited a trend of late-stage fractionation, revealed by increasing concentrations of incompatible trace elements including Cs. The economically important metals Cu, Mo, W, and Zn significantly partitioned into the intermediate-density fluid. In particular, Cu concentrations in this fluid in the range of 100-500 ppm agreed well with previous experimental and modeling studies and, therefore, likely represented values expected for primary magmatic fluids. Most elements except Li, As, and B got further enriched in the brine upon boiling, but the calculated partition coefficients have to be regarded with reservation due to common heterogeneous entrapment of brine in the vapor inclusions. The evolution of a cooling upper-crustal magmatic-hydrothermal system was studied based on quartz (texture and trace elements) and fluid and melt inclusions sampled by the small, dacitic Escorial Ignimbrite. This ignimbrite formed part of the Corrida de Cori volcanic field in the Central Andes, at a predominantly compressional convergent margin involving thick conti- nental crust. A range of magmatic to hydrothermal quartz, comprising magmatic phenocrysts from porphyritic clasts, single megacrysts, and microcrystalline quartz from the epithermal environment were sampled from the volcanic conduit region by the ignimbrite. Trace ele- ment compositions of the different quartz types (analyzed by laser ablation inductively-coupled plasma mass spectrometry) depicted an evolution from Ti-rich (magmatic) to variably Al-rich (pegmatite-like) to very Ti-poor (epithermal), also reflected in a positive trend of Al/Ti and Ge/Ti ratios. In particular, these compositions reflected fast growth of the megacrysts in the presence of both fluid and silicate melt. Further insight into the evolution of the magmatic- hydrothermal system was provided by fluid and melt inclusions in the quartz megacrysts, stud- ied with microthermometry and laser ablation inductively-coupled plasma mass spectrometry. The fluids were entrapped at low pressure (supposedly <100 MPa) and included brine, densely packed with salt crystals and other solid phases but no visible liquid at room temperature, low-density vapor, CO2-rich intermediate-density fluid, and rare liquid. Of particular interest were the high Cu concentrations (several percent) in the brine and the widespread immiscibility between silicate melt and brine. Element distribution between the different fluids and silicate melt could not be estimated here, but the results imply that the brine, although likely minor by volume and mass compared to the vapor, may play an important role in pre-concentrating Cu and other metals.