The effect of natural weathering on the chemical and isotopic compositions of biotites

The effect of progressive natural weathering on the isotopic (Rb-Sr, K-Ar, δD, δ¹⁸O) and chemical (REE, H₂O⁺) compositions of biotite has been studied on a suite of migmatitic biotites from the Chad Republic. During the early stages of weathering the Rb-Sr system is strongly affected, the hydrogen and oxygen isotope compositions change markedly, the minerals are depleted in light REE, the water content increases by a factor of two, and the K-Ar system is relatively little disturbed. During intensive weathering the K-Ar system is more strongly disturbed than the Rb-Sr system.

Most of the isotopic and chemical modifications take place under nonequilibrium conditions and occur before newly formed kaolinite and/or smectite can be detected. These observations suggest that

• 1.

  (a) “protominerals” may form within the biotite structure during the initial period of weathering, and

• 2.

  (b) only when chemical equilibrium is approached in the weathering profile are new minerals able to form.