The accretion of lower oceanic crust

The formation of new ocean lithosphere at mid-ocean ridges is a fundamental component of the plate tectonic cycle, and through hydrothermal interactions with seawater is a major control on the composition of the oceans, ocean crust, and upper mantle. Two complementary approaches are used to investigate the thermal implications of endmember theoretical models that describe the accretion of the lower oceanic crust at fast spreading rates. The first approach uses the record of hydrothermal alteration of the ocean crust, including Sr and O-isotopes, to investigate and quantify the role of hydrothermal circulation during the accretion of the ocean crust. The second method uses diffusion based geospeedometry techniques to determine cooling rates in the ocean crust. Samples from two locations of ocean crust formed at fast spreading rates at the East Pacific Rise are used in these investigations, ODP Hole 1256D and Hess Deep. Hole 1256D provides the first intact sampling of a complete section of upper oceanic crust formed at a fast spreading rate and recovered the first in situ sampling of the dike/-gabbro boundary. Hess Deep is a tectonic window where the westward propagation of the Cocos-Naza Ridge has rifted ocean crust formed at the EPR and exposed the lower ocean crust at the seafloor.The whole rock profile for Hole 1256D reveals Sr isotopes in the volcanic sequence to be only slightly shifted from primary MORB values (0.70284-0.703814 compared to 1256 MORB of 0.70283). In contrast, Sr isotopes in the sheeted dike complex (0.70294-0.70536) are strongly elevated towards hydrothermal fluid compositions (0.70505-0.70525). Rocks of the plutonic complex are characterised by elevated Sr ratios along igneous contacts (up to 0.70524) but only limited increases in Sr isotopes relative to MORB in the centres of the gabbro bodies (0.70290-0.70396). The complementary oxygen isotope profile records the downwards transition from low temperature to high temperature hydrothermal alteration but contains small scale variation associated with changes in secondary mineral abundances and local fluid/rock ratios. Both the detailed Sr and O isotope profiles document the importance of dike margins and other igneous contacts as focussed pathways for fluid flow through the crust. The time-integrated fluid flux required to cause the observed Sr isotope profile through the sheeted dike complex is 2.0 - 2.6 x 106 kg/m2 and is consistent with fluid fluxes calculated for other crustal locations (e.g, Hole 504B, Pito Deep, Hess Deep). The heat flux required to sustain this fluid flux is equivalent to half of the latent heat released during the crystallisation of the lower ocean crust. At Hole 1256D the removal of heat by hydrothermal fluids was effcient and demonstrates that the fluid flux in the sheeted dikes must have removed some portion of the heat flux out of the lower ocean crust. In order to remove all of the latent heat of crystallisation from the lower crust, there must be significant hydrothermal circulation in the lower ocean crust.