Research project: The Accretion of Lower Oceanic Crust
Currently Active:
Yes
Ocean crust formed at fast spreading rates (8cm/yr full rate) has a relatively simple layered stratigraphy, the upper crust is formed of basaltic lava flows and sheeted dikes (1-2km) and the lower crust is formed of gabbroic plutonic rocks (~5km). Despite representing a large proportion of ocean crust, the formation of the lower ocean crust remains poorly understood. Our current understanding of the lower oceanic crust is based on observation from ophiolites (pieces of ocean crust obducted onto continents) and geophysical observation because of the lack of direct sampling.
Project Overview
Theoretical models of accretion
Two end member theoretical models have been developed based on geophysical and ophiolite observations (Figure 1):
Gabbro glacier model (Henstock et al., 1993., Phipps Morgan & Chen 1993., Quick & Denlinger 1993): All of the lower crust crystallises in a high level melt lens and subsequently subsides downwards and away from the ridge axis.
Multiple sills model (Boudier et al., 1996., Korenaga & Kelemen 1997., MacLeod & Yoauancq 2000): The lower crust forms by the intrusion of multiple melt lenses throughout the lower crust
Alternatively, the lower oceanic crust may form by a combination of these processes.
Testing models of accretion
With suitable samples, the different theoretical models generate testable hypotheses for the accretion of the lower crust. These include addressing the following:
Geohemical variation in bulk rock composition
Strain rate variation with depth
Depth, distribution and magnitude of hydrothermal circulation
Cooling rate through the lower crust
Current work
Using recently recovered samples from ODP/IODP Hole 1256 (Fig 3) and Hess Deep, we are applying two approaches to understand the accretion of the lower oceanic crust:
Combined petrographic, isotope and geochemical study to investigate the magnitude and distribution of hydrothermal circulation.
Trace element partitioning geospeedometry to determine the cooling rate in gabbroic rocks