Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
BEG Seminar, November 18, 2005
Petrography Reality and Chemical Diagenesis
Spatial distribution of features arising from chemical diagenesis is foremost a reflection of rate-limiting processes. This is to say that, of all the many chemical processes that may factor into the formation of a particular mineral, it is always the slowest of these that determines the mineral’s characteristic pattern of occurrence. It applies to secondary pores (mineral dissolution) as well. Examples of possible rate-limiting processes include nucleation, precipitation, dissolution, diffusion, and advection.
The efficacy of predictive models of chemical diagenesis depends on identifying the particular rate-limiting processes that apply to individual minerals in the system being modeled. Petrographic evidence concerning mineral localization provides useful clues for this determination and suggests that authigenic minerals can be grouped into two main classes: those that are precipitation controlled and those that are transport controlled.
At the surface and at comparatively shallow depths, petrographic evidence suggests that processes of precipitation and dissolution are not strongly linked, either spatially, temporally, or mechanistically. For example, feldspar dissolution exploits crystal defects, whereas precipitation of clays may be localized at available sites of nucleation. Differences in individual rates among processes sets up an inherent disconnect between many individual reactions responsible for observed compositional and textural features. Thus, rate controls for dissolution and precipitation are not strongly linked to either solubilities or transport controls, and, hence, not to each other. Indeed, halo textures and other manifestations of spatial linkages between dissolution and precipitation are not generally reported at temperatures characteristic of diagenesis.
The diversity of chemical controls operative within mineral assemblages has implications for determination of mass balance. For minerals limited by precipitation rate, data on mineral volume carry no inherent information on elemental sources, their location, or their volume. Simultaneous solution of rate equations for all major co-occurring dissolution and precipitation processes does have the potential for indicating length scales for elemental transfers, however.