A semiquantitative calculation of the amount of Mg fixation during greenschist-facies metamorphism of a 1-km-thick section of the oceanic crust beneath mid-ocean ridge crests shows that it is of the same order of magnitude as the “excess” (unbalanced) input of this element to the ocean. The estimate is based on a simple model of sea-water circulation through a crack system, infiltration into the rock walls of the cracks, and diffusions of Mg from the open cracks into the sea-water-saturated rock.

The crack spacing must be on the order of metres or less, and the diffusional transport of Mg in addition to infiltration of sea water is essential if the overall process is to remove all the excess Mg influx into the oceans.

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