Authigenic silicate minerals and the magnesium budget in the oceans

Abstract
The relative stabilities of authigenic Fe-rich and Mg-rich components in sheet silicates are considered for reactions of ( a ) clay minerals undergoing anoxic diagenesis; and ( b ) the montmorillonite phase formed by basalt-seawater interaction. For reaction ( a ) coexisting pyrite and a Mg(OH) 2 component of the silicate are close to equilibrium with aMg2+ in pore fluids (Δ G = - 0.29 kcal/mol; - 1.2 kJ/mol) and the excess Mg 2+ present approximates to that added by diffusion from overlying seawater. Reaction ( b ) can be represented by a mixture of ferroan nontronite and talc components and the calculated temperature/stability relationship agrees with mineralogical zoning in the Reykjanes geothermal area. At T = 523 K the Mg-rich component is dominant and aMg2+ is reduced to 1.7 x 10 -5 (cf. 1.35 x 10 -2 in seawater). Data from an experimental study at 473 K give the result a talc ≈ 0.2. Flux calculations give removal rates of Mg from the oceans of 0.06 x stream supply for reaction ( a ) and 0.6-1.7 x stream supply for reaction ( b ).

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