Trace element variations in zoned minerals: a record of geochemical and environmental feedback

点击翻译按钮获取中文摘要

Zoning and layered patterns in rocks and minerals occur on a variety of length- and time-scales in many different environments throughout geologic time. Because minerals take time to grow such patterns must record changes in the environment. Zoned patterns may be interpreted as (1) the product of externally forced processes, for example, where there is evidence of chemical disequilibrium or (2) the product of self organization expressed as differences in growth form or oscillatory zoning patterns (OZP).  Distinguishing the two is important.  While in many instances external forcing has succeeded in explaining a number of patterns such as major element variations in minerals (e.g. garnet), externally driven variations cannot adequately explain the repetitive or periodic character of oscillatory patterns observed on a scale of microns in minerals such as apatite and zircon.   Both types of zoning can occur in the same mineral.   Geochemical self organization in minerals can produce very complex patterns and is essentially a short range process largely influenced by diffusion and mineral structure over the few microns encompassing the growth surface of the mineral.  At this local scale, the periodic or other structured patterns are an expression of reaction coupling and diffusive transport entirely in the absence of large-scale external forcing.  There are now a number of examples of spontaneously generated zoning and layered features in rocks and minerals that arise as the product of coupling and feedback between different systems including zonation in accessory minerals, agate, diagenetic carbonates, biogenic carbonate and cyclic layering in mafic and ultramafic rocks.