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

在整个地质时期，岩石和矿物中的分带和层状模式在许多不同的环境中以不同的长度和时间尺度出现。因为矿物的生长需要时间，所以这种模式必须记录环境的变化。分带模式可以解释为(1)外部强迫过程的产物，例如，有证据表明存在化学不平衡，或(2)自组织的产物，表现为生长形式或振荡分带模式(OZP)的差异。区分两者很重要。尽管在许多情况下，外部强迫成功地解释了一些模式，如矿物(如石榴石)中的主要元素变化，外部驱动的变化不能充分解释在磷灰石和锆石等矿物中观察到的微米尺度上的振荡模式的重复或周期性特征。然而，这两种类型的分带可以在同一矿物中发生。矿物中的地球化学自组织可以产生非常复杂的模式，本质上是一个短范围的过程，主要受围绕矿物生长表面的几个微米范围内的扩散和矿物结构的影响。周期性或其他结构模式是完全在没有大规模外部强迫的情况下反应耦合和扩散输送的表现。现在有许多岩石和矿物中自发产生的分带和层状特征的例子，它们是不同系统之间耦合和反馈的产物，包括副矿物中的分带、玛瑙、成岩碳酸盐、生物碳酸盐和镁铁质和超镁铁质岩石中的旋回分层。

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.