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Figure F2. Models for the formation of the lower oceanic crust. Note the dramatically different distribution of hydrothermal heat extraction in the two models caused by the difference in where the latent heat of crystallization of the lower crust is released. Figure modified from Coogan (in press). A. A “gabbro-glacier” model (Quick and Denlinger, 1993; Phipps Morgan and Chen, 1993; Henstock et al., 1993) in which the lower oceanic crust crystallizes in a thin magma lens at the base of the sheeted dike complex from which cumulates subside downwards and outwards to form the lower crust. B. A “sheeted sill” model (Bédard et al., 1988; Kelemen et al., 1997) in which the lower oceanic crust forms through the crystallization of multiple sills.

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