Seismic controls on the progress of serpentinization at ultra-slow
spreading ridges
Abstract
Sustained serpentinization of peridotite within the oceanic lithosphere
requires effective supply of water to systems that experience continuous
expansion of the solid volume. Hence, serpentinization preferentially
occurs along ridge axes and in subduction zones where tectonic activity
is intense and fracturing helps generating and sustaining the
permeability required to connect seafloor-near environments to depth.
The slowest mid-oceanic ridges produce little melt leading to
discontinuous magmatic activity with very thin to no crust along most of
the ridge length and up to 8 km thick crust focused around local
magmatic centers. Three types of ultra-slow ridge sections can be
distinguished: i) amagmatic, characterized by scarce basaltic crust and
deep seismic activity, ii) magmatic, characterized by a thin basaltic
crust and intermediate depth seismic activity, and iii) volcanic,
characterized by a thick basaltic crust and shallow seismic activity. At
amagmatic and magmatic ridge types, aseismic zones are identified above
the seismic zone. The lower limit of the aseismic zone along amagmatic
sections is thermally controlled and follows a 400-500˚C isotherm
corresponding to the upper temperature limit for the onset of
serpentinization. This observation suggests that the aseismic zone is
significantly serpentinized with ample supply of water to the
peridotite-serpentine interface. Based on recorded seismic activity, we
estimate the associated rock volume affected by brittle damage for the
different ultra-slow ridge types. We show that damage produced by
seismic activity sustains pervasive serpentinization along amagmatic and
magmatic types, while it is limited in the case of volcanic sections.