Mineral dissolution at the granite-saprolite interface

摘要

The Rio Icacos watershed in Puerto Rico's Luquillo Experimental Forest is located on the Tertiary Rio Blanco quartz diorite bedrock mantled by 200-800 cm of saprolite. Previous workers have documented large differences in mineralogy and chemistry between the bedrock and the overlying saprolite. Those studies have predominately focused on wholesale changes between the bedrock and the saprolite. Herein we present a detailed examination of the weathering across the bedrock-saprolite interface. At the study site, the bedrock weathers spheroidally, forming corestones surrounded by 20-60 cm of concentric layers of weathering rock. We are exploring the chemistry and mineralogy of this complex interface between the corestones and saprolite at the microscale using inductively coupled plasma-mass spectrometry (ICP-MS), optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and electron probe microanalysis (EPMA) to characterize the chemical and physical processes that propagate the weathering front. Preliminary results from EPMA and SEM indicate that weathering is localized in cracks and along the edges of the weathering rock layers. At the outer edge of the corestone and along cracks within the first spheroidal layer, both plagioclase and hornblende release Al in the presence of water and a hydrous aluminosilicate phase precipitates in the void space. ICP-MS results are used to calculate the open-system mass transport (loss or gain) of the elements with respect to the corestone, accounting for density differences across the sampled section. These calculations show that most of the spheroidal layers have a bulk chemistry similar to the corestone but slightly depleted in most elements, with the largest chemical differences in the layer adjacent to the saprolite. Across the interface zone, the average order of mobility of the cations is Ca > P > Na > Si > Al > Fe > Mg≈Mn > K > Ti.