4.13 - Composition of the Oceanic Crust

Oceanic crust is created as magma rises to fill the gap between diverging tectonic plates and is consumed in subduction zones. It is geologically young, with a mean age of 60 Ma, and is thin, averaging 6.5 km in thickness. Oceanic crust consists almost exclusively of extrusive basalt and its intrusive equivalents. This chapter focuses on the roughly 1.5–2 km thick ‘volcanic layer’ consisting of lava flows that overlie the feeder dikes that make up the sheeted dike complex. The basalts of the oceanic crust, referred to as mid-ocean ridge basalts (MORB), are dominantly tholeiitic and are, on average, depleted in incompatible trace elements compared to basalts erupted in other tectonic environments. Isotopic compositions and incompatible trace element concentrations and ratios suggest that their depleted character is inherited from their mantle source and that this source varies in composition both locally and on the scale of ocean basins. Their major element chemistry appears to be controlled primarily by the temperature of the underlying mantle, which determines the extent and pressure of melting, and, consequently, the thickness of the oceanic crust and the depth of the ridge axis. Basalts erupted at back-arc spreading centers, called back-arc basin basalts, are compositionally similar to MORB, but have some compositional features suggesting incorporation of one or multiple subduction-related components in their source.

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