Trace elements in anatectic products at the roof of mid-ocean ridge magma chambers: An experimental study

At fast-spreading mid-ocean ridges (MORs), the horizon between the axial melt lens (AML) and the overlying sheeted dikes is characterized by extensive anatectic processes. The heat flux of the AML in combination with hydrothermal fluids from above causes high-grade contact metamorphism, which may result in anatexis of the roof rocks above the AML. The products of this process are silica-rich anatectic melts that have the potential to contaminate MOR basalts and residual hornfels. Here, we simulate the complex igneous and metamorphic processes occurring at the AML roof by hydrous partial melting experiments and provide corresponding trace element partition coefficients between melt and residues, which are useful to quantify those processes. We present trace element patterns from experimental anatectic felsic melts and the related residue produced by hydrous partial melting of various types of AML roof rocks. The starting materials used are sheeted dikes and hornfelses from Hole 1256D drilled by the Integrated Ocean Drilling Program. Results are compared with directly-related natural lithologies (i.e., felsic veins and granoblastic hornfels) from the same site. The trace element contents generally overlap with natural examples and experimental melts produced at low water activity (aH2O < 0.5) can be highly enriched in trace elements despite relatively low SiO2 contents (58.9 to 65.7 wt%). A low aH2O is required to reproduce the low Al2O3 contents observed in natural silica-rich rocks. However, low aH2O implies that the presence of residual amphibole is not required for anatectic processes Even though residual amphibole is often used as an important phase for explaining trace element characteristics in relevant felsic rocks formed at MORs when modeling anatexis. Because amphibole is lacking in any experimental residue, which is in agreement with natural hornfelses from the dike/gabbro transition at Site 1256, we assume that partial melting within the AML roof rocks proceeds without the participation of amphibole as residual phase. We present a comprehensive set of trace element compositions as well as bulk and mineral/melt trace element partition coefficients obtained from our amphibole-free experimental results for different potential protoliths over a large range of temperature and at different aH2Os.