Feldspar dissolution rates and clay precipitation in the Navajo aquifer at Black Mesa, Arizona, USA

摘要

This study measured a plagioclase dissolution rate of 10~(-16.3) mol m~(-2) sec~(-1) and a K-feldspar dissolution rate of 10~(-17.7) molm~(-2) sec~(-1) along up to 80km long flow paths within the confined Navajo sandstone aquifer of the Black Mesa of Arizona, with residence times up to 35 Ky. These rates are among the slowest feldspar dissolution rates reported. However, evaluation of previously published water chemistry from other areas suggests these slow feldspar reaction rates are typical of long residence time, regional aquifers. The well constrained physical and chemical conditions of the Navajo aquifer, coupled with detailed FEG-SEM and HRTEM analysis of the compositions and textures of the primary and secondary phases, allowed the evaluation of most commonly proposed mechanisms for limiting feldspar dissolution rates, all of which are found to be inadequate to explain the observations. Rather, it is proposed that clay precipitation kinetics is a critical process in controlling overall feldspar reaction rates in regional aquifer systems.