Phase stability fields in calc-alkaline magmatic systems and textural evolution of crystalline aggregates
The focus of the research unit is to improve the tools of diffusion chronometry and reduce the uncertainty on extracted time scales for processes that occur in igneous systems. Diffusion chronometry requires well calibrated diffusion coefficients (see, for example, Projects #1 and #2). However, for a correct quantification of timescales from diffusion profiles, an excellent knowledge of the conditions (e.g., temperature, pressure, oxygen fugacity) at which diffusion occurred is also necessary. A large number of studies of global significance on magma plumbing systems are related to subduction zone environments, and therefore, calc-alkaline systems are the focus of this experimental project. Several aspects need to be addressed for developing robust tools of diffusion chronometry:
Problem 1. Is compositional zoning produced by diffusion? A major challenge for applying diffusion chronometry is to distinguish compositional zoning formed by diffusion process from compositional zoning formed by other igneous processes such as fractional crystallisation or magma mixing (see also Project #3).
Problem 2. What were the magma storage conditions at which concentration profiles in phases of interest were generated? Temperature (T), which is the main parameter that influences the diffusivity of elements in phases, needs to be constrained accurately. Further, to use retrieved timescales to determine ascent rates – a key variable in any magmatic system – it is also necessary to determine the depths of residence of magmas (or pressure, P), and corresponding equilibration / growth of crystals, with high resolution (barometry).
Problem 3. What is the stability and lifetime of phases used for diffusion chronometry? Conventional phase petrology as well as diffusion and other forms of chronometry (e.g. isotopic dating) assume that a mineral crystallizes once its conditions of stability (P, T, water content etc.) are met and kinetic factors for nucleation and growth are conducive. However, the role of mineral surfaces and interfaces in governing textural evolution has not been as much in focus (see Project #5), although processes such as grain recycling and other dissolution / precipitation processes that occur during textural maturation are critical for setting limits on the segments of magmatic history that may be recorded in a crystal.
Detailed knowledge of phase equilibrium relationships obtained from experiments would help to address all of these questions, and are the target of Project #4. Special attention would be paid to sample characterization after the experiments in order to constrain as many variables as possible (e.g. oxidation state, water content).
Investigators
Felix Marxer, Leibniz Universitaet Hannover
Francois Holtz, Leibniz Universitaet Hannover
Sumit Chakraborty, Ruhr-Universitaet Bochum
Renat Almeev, Leibniz Universitaet Hannover