ИСТИНА

A large-scale seawater-derived hydrothermal circulation is often invoked to explain cooling of accreting oceanic crust at spreading ridges (e.g., German & Lin, 2004). However plutonic-level magma-hydrothermal interaction in different spreading environments remains poorly characterized. In slow-spreading ridges hydrothermal fluid flows can be interrelated to detachment faulting, i.e. development of large-offset extensional faults (e.g., McCaig et al., 2007), whereas vent fluid composition shows high H2 concentrations due to seawater interaction with mantle-derived ultramafic rocks (Charlou et al., 2010). A series of detachment faults occurs on the western slope of the Mid-Atlantic rift valley between 12°58oN and 13°35oN, where the footwall outcrops consist of unevenly hydrated mantle-derived harzburgite with minor gabbro bodies. Hydrothermal activity in these structures is manifested by active ultramafic-hosted vent fields including Ashadze at 13°N (Ondréas et al., 2012). Complex mineralogical features of gabbro from the Ashadze detachment footwall suggest a direct magmatic to hydrothermal transition. Gabbro late magmatic assemblages include interstitial titanomagnetite, blebs of titanian hornblende enclosed in augite, and local apatite grain chains with sporadic zircon. Advection of hydrochloride brine, as documented in the apatite-hosted inclusions (30-32 wt.% NaCleq), resulted in mineralogical modifications at 750 to 400°C: abundant development of highly heterogeneous amphibole, decomposition of titanomagnetite due to reducing reactions, and complete re-crystallization of apatite and zircon. For instance, replacement of titanomagnetite by ilmenite in intergrowths with Cl- and Fe2+-rich hornblende occurred at 600-500°C and log fO2 from -20 to -24. Reducing nature and high salinity of the fluid could be assumed from preceeding low-temperature (400°C) seawater-harzburgite interaction within detachment shear zones, since such interaction (serpentinization) consumes 1-2 moles of H2O and produces 0.1-1 mole of H2 per mole of typical peridotite olivine (Andreani et al., 2007; McCollom & Bach, 2009). A similar fluid origin has been suggested for a different slow spreading setting (Pertsev et al., 2015). Cooling of hot melt-bearing gabbroic body by the serpentinization-related fluid results in the formation of transitional (magmatic-hydrothermal) mineral composition. The transitional hornblende shows an enrichment in such residual magmatic components as K2O (from 0.2-0.5 to 0.7-0.8 wt.%) and locally REEs (e.g., Sm from 25-53 to 77 ppm). At the same time it demonstrates a hydrochloride fluid contribution: irregular decrease in Mg# = Mg/(Mg+Fe) atomic ratio from 0.6 down to 0.2 and associated increase in Cl content from 0.03-0.05 up to 1.6 wt.%. Final hydrothermal cooling is reflected in the hornblende to actinolite compositional trend in amphibole, with depletion in K2O to 0.3–0.1 wt. %, increase in Mg# to 0.4–0.5 and decrease in Cl content to 0.4–0.2 wt.%.