Growth of clinopyroxene in a closed system: Relationships between temperature-time paths and zonation

Abstract

Zoned minerals provide a record of the changing conditions in magma chambers and as such they can be used to determine the dynamic behaviour of subvolcanic magmatic systems prior to eruption. However, interpreting the record preserved in the crystals requires a detailed understanding of the mechanisms that control zonation in minerals. The purpose of this study is to determine if a correlation can be made between the morphology and composition of zoned crystals and the temperature-time (T-t) paths of their formation. The main goal is to compare the experimentally developed zonation patterns to natural zonation patterns in clinopyroxene alkaline basaltic lavas in the Eifel volcanic field in Germany. Synthetic crystals were grown from a leucititic melt (WEG 1999) in a fO2 controlled, 1 atm, tube furnace with a Eurotherm temperature controller. A variety of cooling paths were created with short enough durations, and large enough undercoolings, that equilibrium crystal growth (homogeneous crystals) could not occur. In addition to cooling paths, we also performed several experiments with a short heating step to simulate the heating that might take place during an influx of magma. The resulting crystals were imaged with scanning electron microscopy and high resolution backscattered electron images were collected and processed to highlight growth and zonation patterns. Zonation patterns in diopside were created by varying T-t paths under constant fO2. Reverse, normal and sector zonations were produced in closed systems, evidence that reverse zonation does not require additional magmatic fluxes. Constant slow (1°C/h) cooling T-t paths can create complex sector zonations, as well as cellular morphologies on crystal-melt interfaces. Hopper crystals have reverse zonation associated with the inward growth of the core, and also display the complex sector zonation pattern "mountain range zonation". In the current experiments the large undercoolings led to crystal morphologies that are distinct from those in the natural samples. Thus, although it has been possible to induce zonation in a closed system, the goal of correlating natural and experimentally formed textures has not yet been achieved