Search

Eclogite-facies metamorphic veins provide records of fluid flow dynamics during high-pressure metamorphism. Here, we evaluate the inventory of quartz-rich metamorphic veins and segregations in Variscan eclogites from the Münchberg Massif (NE Bavaria) to investigate the fluid origin and the scale of fluid flow. These eclogites experienced peak P-T conditions around 3 GPa and 700°C, and a continental rather than oceanic mafic igneous protolith setting is proposed. Isolated, mm-sized quartz pockets with euhedral high-pressure minerals are common in the Münchberg eclogites, but continuous veins that may have allowed focused fluid flow to beyond specimen scale are rare. Nevertheless, where such veins occur, they can contain eclogite assemblages including minerals such as omphacite, garnet, rutile, kyanite, and phengite. Oxygen isotope thermometry of quartz-garnet, quartz-phengite, and quartz-kyanite pairs yield temperatures around 700°C. δ18O values of quartz (+6.1 to +10.5‰) from almost all the veins are identical to predicted (from mass balance modelling at 700°C based on host rock δ18O values from +4.0 to +7.9‰) equilibrium δ18O values of host rock quartz. The veins and pockets are mostly in equilibrium with their host eclogites regarding texture, mineralogy, mineral chemistry, and oxygen isotopes. They seem to represent former cavities filled by internally-derived fluids, whereas indications for fluid transport beyond specimen scale are rare. The abundance of isolated quartz pockets and scarcity of more extended veins suggest that rather low quantities of fluid were produced. This may be typical for continental eclogites, which are supposed to have experienced little pre-subduction hydration compared to oceanic eclogites.

Fluid-melt partition coefficients (KD) quantify the distribution of elements at the magmatic-hydrothermal transition and describe the potential for selective mobilisation and transport of elements from a solidifying magma into the overlying crust, where – in case of metals – it can ultimately result in the formation of magmatic-hydrothermal ore deposits. For shallow, subvolcanic intrusions, the crystallising magma typically reaches fluid saturation at conditions falling into the 2-phase field of the H2O-NaCl system. In this case, two separate fluid phases, low-density vapour and higher density, saline brine are exsolved. We present brine-melt, vapour-melt, and vapour-brine partition coefficients for over 30 elements, based on LA-ICP-MS individual fluid and melt inclusion measurements from coexisting assemblages in miarolitic quartz from the Torres del Paine intrusive complex in Chilean Patagonia. Calculated KD(brine-melt) span almost 6 orders of magnitude between 0.001 and ≤600; and are highest for Cl-complexing fluid mobile elements such as Pb, Zn, and Ag. In comparison, KD(vapour-melt) are more moderate with values between 1 – 100 for all fluid-mobile elements. Combining these partitioning data with estimated amounts of vapour and brine exsolving at the magmatic-hydrothermal transition, we can project the relative amounts of a given element that are transferred into low-density vapour and higher density brine. For typical magmatic-hydrothermal systems in the upper crust, amounts of exsolved brine are significantly lower than for vapour. This counterbalances the commonly stronger partitioning of fluid-mobile elements into brine, and confirms that both fluid phases are relevant for mobilising and transporting elements in subvolcanic magmatic-hydrothermal systems.

Hydrothermal veins and altered feldspar are evidence for fluid circulation in granitic rocks in the continental crust. The hydrothermal alteration of feldspar affects the deformation behavior of granitoids, especially if it occurs before orogeny. Geochronology can establish the timing of fluid circulation to determine if this fluid-driven alteration plays a role in crustal deformation. Although existing dating techniques cannot be applied to feldspar alteration directly, absolute ages of fluid circulation can be obtained from hydrothermal veins. We combined U-Pb geochronology and hydrogen isotope data (δD) from epidote [Ca2Al2(Al,Fe3+) Si3O12(OH)] to unravel the hydration of post-Variscan granitoids in the Alpine orogen. The recent protocol for epidote U-Pb dating proves for the first time that fluids of meteoric origin infiltrated the granitoids in Permian times by exploiting synrift faults, consistent with the δD values of the epidote-forming fluids. This hydration event caused at least some degree of feldspar hydration and weakening of the granitic rocks ~260 m.y. before their pervasive structural overprint by the Alpine orogeny. The preservation of Permian U-Pb ages despite Alpine orogenic processes confirms epidote as a powerful tool with which to unveil pre-orogenic hydration events in metagranitoids. Our analytical approach broadens insights into the water cycle in the middle continental crust in orogens.