An experimental investigation of texture evolution during continuous cooling

Abstract

Textural evolution during a controlled cooling experiment has been quantified using bulk textural characterizations (mean crystal length, total numbers of crystals and modal olivine content) and crystal size distributions (CSDs). The experiments involved subliquidus melting of an olivine–glass mixture followed by cooling at a constant rate (92 °C h − 1 ) through the crystallization interval. Measured changes in population density, n( L), for small (< 0.3 mm) euhedral olivine crystals are shown to be consistent with a uniform crystal growth rate of 6.0 × 10 − 7 mm s − 1 . The CSD trends for larger (> 0.3 mm) skeletal crystals are broadly consistent with an exponentially increasing nucleation rate and with a uniform growth rate of 2.0 × 10 − 5 mm s − 1 . However, the textural development of these crystals cannot be explained solely in terms of homogeneous nucleation and growth. Additional factors influencing the development of textures in the skeletal zone of the charges include grain impingement and development of skeletal overgrowths on the top of a layer of seed crystals. CSD data is ideal for analyzing texture development because it is sensitive to the detailed structure of the crystal population and can be used to detect variations in behavior between different size classes. Here, population density trends for various crystal size classes are used as a tool for estimating the kinetics of crystal growth and nucleation in a cooling igneous system. This type of analysis can be used to gain insight into crystallization processes from CSDs without relying on assumptions regarding CSD shape. Examples of the use of population density trends in the evaluation of natural systems are provided for the case of crystallization in porphyritic olivine chondrules and in komatiite flows.