Weathered biotites from granitoids: the fractionation of REE, Th and transition elements and the role of accessory and secondary phases.

The distribution of REE, Th, and transition elements in three suites of weathered biotites from tonalite, granite and granodiorite parent rocks, provides information on the fractionation processes affecting these elements during weathering. LREE and Th are mineralogically controlled by monazite. In the early stage of weathering, dominated by formation of vermiculite, monazite is retained as inclusion in the host weathered biotite, and LREE and Th exibhit an enrichment, > 100%, relative to fresh mica. When biotite evolves toward kaolinite, it suffers a pervasive deformation with exfoliation and opening of the mica layers inducing the splitting of monazite. Less than 50% of the original LREE and Th contents are preserved in the weathered mica. HREE are not controlled by primary accessory phases and, in the early stage of weathering, suffer a depletion likely related to stabilization of the elements in aqueous solution as carbonate complexes. During kaolinitization, a HREE enrichment relative to LREE is observed, possibly controlled by the solubility product of REE-hydroxides. A positive Ce-anomaly is recognized in the most kaolinitized biotite. The fractionation of cerium relative the other REE is due to the Ce3+ -> Ce4+ change, yielding to the precipitation of cerianite. Some REE patterns of weathered biotites show a positive Eu anomaly, possibly related to low Eu2+ -> Eu3+ transformation rates. Biotite weathering leads to remobilization of transition elements and causes depletion in V, Cr, Cu and Zn by expulsion, like many six-fold coordinated elements, from octahedral sites. On the other hand, Pb is notably enriched relative to the parent and its content is likely controlled by iron oxi-hydroxide and phosphate minerals trapping lead released from feldspar dissolution.