ABSTRACT

Diagenetic alteration of biotite and associated formation of interlayer inclusions may record aspects of pore fluid evolution during burial. Analysis of biotites from the Forbes Formation (Cretaceous) of the Sacramento Valley, California indicates that biotite alteration varies systematically with depth. Kaolinite is the most abundant authigenic phase at shallow depths. Transformation of biotite to chlorite becomes the dominant alteration process below 1500 m. Alteration to illite is very rare.

Many of the Forbes biotites show considerable expansion perpendicular to the (001) surface. Inclusions contained within intragranular micropores created by expansion of these biotites include pyrite, Ti oxide and, most prominently, carbonates. Expansion is due to uneven compactional stress and to the growth of lenticular carbonate inclusions between the phyllosilicate sheets. Pyrite and Ti oxide inclusions are relatively shallow burial diagenetic minerals reflecting reducing pore fluid conditions and high Fe⁺⁺ and Ti⁴⁺ activities in intragranular microenvironments during biotite to kaolinite alteration. Pyrite does not form in chlorite layers because chlorite assimilates all the Fe⁺⁺ from the altered parent biotite. Carbonate inclusions form within the biotites in response to surface effects on newly exposed (001) cleavage planes. The surfaces act as a H⁺ sink, increasing pH in surrounding microenvironments and lowering carbonate solubilities. Variation in the composition of carbonate inclusions reflects changing Fe⁺⁺/Ca⁺⁺ activities during biotite alteration and carbonate inclusion precipitation.