Volatiles (CO2, S, F, Cl, Br) in the dike-gabbro transition zone at IODP Hole 1256D: Magmatic imprint versus hydrothermal influence at fast-spreading mid-ocean ridge

Abstract

The dike-gabbro transition zone of mid-ocean ridges (MORs) is a critical site for the accretion of oceanic crusts as it is the main zone of interactions between the hydrothermal and magmatic systems. In this study, volatiles contents of CO2, S, F, Cl and Br for a variety of lithologies from the dike-gabbro transition zone of an intact oceanic crust were investigated in order to examine the magmatic imprint versus hydrothermal influence. The studied samples include plutonic rocks (gabbros, diorites and tonalites), hornfelses (i.e. granoblastic dikes), an amphibole-rich vein and albitites. These rocks were recently sampled by the IODP (Integrated Ocean Drilling Program) at Site 1256, representing parts of an oceanic crust that formed at the fast-spreading ridge of the East Pacific Rise.The bulk CO2 contents (500–10,000ppm) show no lithological dependence and are similar to those of fresh lavas. Highly variable bulk S concentrations (3–900ppm) show significant depletions compared to undegassed MORB melts, which might be caused by exsolution of magmatic fluids during crystallization. The plutonic samples have Cl concentrations of 500–1000ppm, remarkably higher than the hornfelses (200–600ppm Cl) and erupted lavas (average ~200ppm Cl). Except for the albitites that contain very low F (20–30ppm) and the hydrothermal amphibole-rich vein that contains extremely high F (~1000ppm), the other lithologies have similar F concentrations within 100–350ppm. The Br concentrations of different lithologies are largely overlapping within 0.5–3.5ppm.Petrological and mineralogical evidence indicates that bulk F concentrations are dominated by the presence of amphibole and apatite, whereas Cl and Br are mainly hosted in fluid inclusions rather than in hydrous minerals. The variation of F/Cl and Br/Cl ratios may trace the mixing between MORB magmas and seawater-derived fluids, crystallization of apatite and amphibole, and/or extraction of magmatic fluids. Our data support the current S budget of oceanic crust, but indicate that the budgets of C and Cl may have been underestimated.