BIOTITE WEATHERING IN A HOT-ARID VERSUS HOT-HUMID CLIMATE

Climate imposes a significant effect on physical and chemical weathering of silicate minerals. In this study, we examine the weathering of biotite hosted in granitoid bedrock and in derivative sediment collected in hot-humid (Guyanes, Puerto Rico) and hot-arid (Anza Borrego Desert, CA) climates. The bedrock in Anza Borrego is tonalite, whereas the bedrock in Puerto Rico is granodiorite. Elemental analyses using the electron microprobe were collected on several spots on biotite grains from two fresh bedrock samples, one weathering rind, and three detrital grains from Anza Borrego, and two bedrock samples from Puerto Rico. The most common trend observed within the Anza Borrego sample set is progressive leaching of potassium, with the highest loss in detrital grains and lowest loss in fresh bedrock. SEM imaging was performed on four detrital biotite grains from the Anza Borrego locality; these analyses reveal significant edge delamination between the tetrahedral and octahedral layers, and lateral retreat of the sheets to produce a terraced geometry of the layers along the C axis of the grains. We hypothesize that this physical weathering of the biotite edges contributes to the chemical weathering evidenced by the loss of interlayer potassium cations. The observed physical weathering may derive in part from thermal expansion associated with the significant seasonal and diurnal temperature variations at the Anza Borrego locality. SEM and microprobe analyses of the Puerto Rico detrital biotite are ongoing to assess whether similar degrees of chemical and physical weathering occur there and the relative importance of precipitation versus temperature extremes in the weathering system.