Cooling of the lower oceanic crust

Abstract

Thermal models of mid-ocean ridges that balance the influx of heat from magmatic sources with the removal of heat by conduction and hydrothermal circulation allow quantification of cooling of young oceanic crust. These models reproduce key observations relating to crustal accretion and hydrothermal cooling at fast-spreading ridges. The rate of cooling is constrained both by the bathymetry of ridge axes and by olivine compositions from ophiolite gabbros. Successful models involve extensive hydrothermal cooling of the lower crust within 20 km of the ridge, with ϳ50-70 kW of hydrothermal cooling for every 1 m of ridge axis at crustal ages of Ͻ0.1-0.4 Ma. These estimates can be used to refine global models of geochemical and thermal fluxes close to spreading ridges. Figure 1. Heat transfer at fast-spreading ridge axis. Upper crust of dikes and lava is filled dark gray, gabbroic lower crust is light gray, and mantle is green. Crustal thickness is 6-7 km. Bathymetric high is above regionwhere magma is supplied from mantle to crust. Shallow melt lens, which seismic reflection studies at East Pacific Rise have found at~1.5 km depth, is filled orange. Lower axial crust contains small melt lenses shown in orange fill. Possible boundary between crust formed in upper melt lens and that formed in lower sills is shown as dashed black line. Principal means of heat transfer at ridge axis are as follows: advection with magma (orange arrows); advection with solid material spreading away from axis (black arrows); advection with hydrothermal fluids moving in crust (blue arrows, with yellow circle where rising with heat); and conduction (red double-headed arrows).

Formation of fast-spreading lower oceanic crust as revealed by a new Mg–REE coupled geospeedometer

“…The inverted cooling rates are compatible with the hydrothermal cooling models of Maclennan et al (2005; Fig. 6d) but about one to two orders of magnitude greater than those derived from the conductive cooling model of Maclennan et al (2005; Fig.…”

“…Many small-scale temperature variations in the deeper section may be attributed to vigorous convection of a large, deepseated cooling magma chamber, which, however, is against the 3-D seismic observations at EPR (e.g., Han et al, 2014). More likely, these small-scale variations indicate accumulation and in situ solidification of replenished melts in small magma bodies (e.g., tens to hundreds of meters) at various depths, as advocated by the multiple-sill model (e.g., Nicolas et al, 1988;Kelemen et al, 1997;Lissenberg et al, 2004;Maclennan et al, 2005;VanTongeren et al, 2008VanTongeren et al, , 2015Natland and Dick, 2009).…”

“…6d) but about one to two orders of magnitude greater than those derived from the conductive cooling model of Maclennan et al (2005; Fig. 6d).…”

“…Many crustal formation models have been proposed based on petrological observations, deformation structures, and geodynamic simulations at various ocean ridge settings and ophiolites (e.g., Nicolas et al, 1988;Phipps Morgan and Chen, 1993;Quick and Denlinger, 1993;Kelemen et al, 1997;Dick et al, 2008). Among these, two end-member models for fast spreading mid-ocean ridges are actively debating whether cumulates of the lower oceanic crust crystalize mainly in a shallow melt lens (e.g., Phipps Morgan and Chen, 1993;Quick and Denlinger, 1993;Coogan et al, 2007;Faak et al, 2015) or in situ solidify largely from magma sills at various depths (e.g., Nicolas et al, 1988;Kelemen et al, 1997;Lissenberg et al, 2004;Maclennan et al, 2005;VanTongeren et al, 2008VanTongeren et al, , 2015Natland and Dick, 2009). The former postulates much slower cooling in the deeper crust (i.e., near-conductive cooling), whereas the latter necessitates efficient heat removal by hydrothermal circulations throughout the entire crust.…”

Expedition 335 summary

“…F17). Were the gabbros, particularly cumulate gabbros, cooled by conduction or hydrothermal fluids (Manning et al, 1996;Maclennan et al, 2005)? Were hydrothermal interactions pervasive or restricted to veins, and did alteration occur at black smoker or higher temperatures (350°-800°C)?…”