New insights into how the interaction of young basalt with seawater influences the elemental budget of the oceanic crust

Geochemical studies of the fluids filling two of the cased boreholes, 1979 (SE-01) and 2017 (SE-03), by Barbara Kleine and other geochemists at University of Iceland, gives new insights into how the interaction of young basalt with seawater influences the elemental budget of the oceanic crust.

This study focused on investigating the geothermal processes and water-rock interactions that take place during the earliest stages of seawater infiltration into the oceanic crust. Borehole water was sampled from two cased boreholes (SE-1 and SE-3) in 10m depth intervals. This sampling revealed that certain elemental concentrations, those of SiO2, Na, Ca, and Cl, were similar to the corresponding seawater concentrations while the elemental concentrations of B, Mg, F, CO2 and SO4 were depleted. Analysis of the borehole water also revealed differences in concentrations of SiO2, K, Ca, Mg, Fe, Al, F and SO4 between SE-1 and SE-3.

Diagram showing the elemental concentrations in water — Elemental concentrations in water samples as a function of vertical depth from SE-1 and SE-3. The horizontal dotted line represents the average water level in the borehole (Jackson et al., 2019b). The vertical dashed line represents the average concentration of the corresponding element in seawater (Bruland, 1983). DIC = dissolved inorganic carbon

Water samples were obtained from the 1979 borehole (SE-1) with the termination of the SUSTAIN drilling (read more about the SUSTAIN drilling on the Drilling Operations Tab) on September 6th 2016 and September 5th 2017. The samples were taken at 10m depth intervals to evaluate potential in-borehole processes that may interfere with the water’s chemical and isotopic composition. The samples from the lowest depths could be representative of the deep fluids that interact with the seafloor sedimentary host rocks. On July 20th 2018 water samples were taken from borehole SE-3. The process by which the water samples were extracted can be seen below. A stainless steel bailer attached to a slickline was lowered to the sampling depth. 1.5 L was the maximum amount of water sampled at each depth.

Images depicting the water sampling process — Water sampling from boreholes SE-1 and SE-3 at Surtsey in summer 2018 (A–C). (D) Water sampling procedure: (1) the open bailer is lowered into the borehole to the desired sampling depth. Water passes freely through the bailer during the descent, (2) a glass-fiber jar is sent down the wire to mechanically snap shut the bailer at the sampling depth, (3) the sealed bailer is pulled up to the surface, (4) the sampled water is retrieved from the bailer by attaching a sample adapter to the lower end of the bailer, (5) after emptying, the adapter is removed, (6) the bailer is opened and prepared for the retrieval of a subsequent sample.

Analysis of these borehole waters was compared with seafloor alteration in low temperature Mid-Ocean Ridge Recharge Zones (“the area where seawater enters the permeable volcanic rocks of the seafloor, percolates down toward the heat source, and begins to undergo low temperature chemical reactions”) in the ridge flank environments. Modeling showed that water-rock interaction in the range of 50-150°C could serve as a sink of Mg and SO4 derived from seawater while also providing a major source of Ca. These models imply that seafloor alteration in these low-temperature geothermal systems that exist within the oceanic crust, may have large effects of the global elemental budgets of seawater