THE PERSISTENCE OF ROCK-FORMING MINERALS IN THE SOIL ENVIRONMENT

It is widely recognized that in most natural settings olivine and glass are among the most rapidly dissolving rock phases at the earth's surface. Despite this, the rates at which these phases weather under field situations are not well-constrained, due in part to the orders of magnitude discrepancy between lab and field weathering rates. Therefore, the length of time that these minerals persist in soils or outcrops at the earth's surface remains enigmatic. In addition, temperature, precipitation, pH, physical weathering rates, and the presence of organic acids can influence relative chemical weathering rates. For example, olivine weathers more rapidly than both clinopyroxene and plagioclase in the presence of abundant organic acids secreted by lichens. Here we compile field weathering rates for olivine and glass and other rock-forming phases, and document the characteristics of soils and exposed outcrops in which these phases remain intact. We have also analyzed weathering profiles formed on basalts: reaction fronts for disappearance of primary minerals vary from a few microns (i.e., Costa Rica) to meters in thickness (i.e., Hawaii). In both of these moist (MAP = 310 cm y-1 and 250 cm y-1 for Costa Rica and Hawaii, respectively) and warm (MAT = 27° C and 16°C for Costa Rica and Hawaii, respectively) climates, base cations and Si are lost from the regolith rapidly. In Costa Rica within a 10 ky interval 80% of these elements are weathered while after 45 ka 75% of the Al remains. Similarly, in the well-studied 4.1 ma “Long-Substate Age Gradient” (LSAG) chronosequence in Hawaii. within a 20 ky interval 90% of base cations and Si are weathered from the soil while after 150 ka 50% of the Al remains. In contrast, during weathering of basalt in Svalbard (a cold and dry climate above the arctic circle), only a hundred micron outer surface of basalt has lost significant glass content. The thickness of the reaction front can be related to rates of weathering and rates of transport of reactants into the unweathered rock.