Chemical Weathering, First Cycle Quartz Sand, and its bearing on Quartz Arenite

Abstract

A self-consistent set of experimentally determined rates of mineral dissolution (Francke, 2009) has been used to estimate the relative loss of common constituents of plutonic igneous rocks that supply quartz to sands to be lithified as first-cycle quartz arenite. A first-order decay equation is used setting the decay constant of calcite to one, which renders the time-steps of loss dimensionless but keeps the relative loss of each mineral constant. Calculations show that 99.33% of pure calcite would dissolve only after 5 time-steps. On a relative scale, it would take about 1250, 900, and 1040 time-steps respectively to reduce the original compositions of quartz-bearing mafic plutonic, granodioritic, and granitic rocks to leave >95% undissolved quartz as residue that will qualify as quartz-sand, the precursor to first-cycle quartz arenite. These are very large numbers indicating that chemical dissolution alone, as in chemical weathering by itself, is not sufficient to generate first-cycle quartz sand; accompanied mechanical weathering is necessary. Therefore, it is necessary to re-evaluate many explicitly stated inferences of warm, humid climate in provenance studies of first-cycle quartz arenites.