 Fig. 1 Simplified thermal gradient caused by heating from below. | Gradients surround us in daily life and are simply defined as a change of a property over distance or time. Whether it's a slope that allows a ball to roll down a hill, or a temperature variation due to heating or cooling, the fundamental concept doesn't shift: some change is occurring over distance and time as long as the two boundaries, for instance the top of the hill and the bottom, are different. Once they are the same, the system is at rest. |
These concepts can also apply to minerals. When a mineral grows in a magma body, its composition resembles that from which it grew, and the conditions under which it grew (temperature, pressure, etc.). When either the magma composition, temperature, or pressure changes, a gradient develops between the mineral and it's surroundings. This causes chemical zonation, similar in some ways to the temperature gradient in a pot during heating water on a stove. Through advancements in microanalytical tools and extensive experimental studies, we now have an idea of how quickly different elements like magnesium, iron, manganese, nickel, hydrogen, etc., diffusive through the crystalline structure of a mineral. With careful application, a scientist attempts to pair timescales of these processes with the physical processes that are occurring within a magmatic system.
Fig 2. On the left is a backscatter electron image of an olivine grain from Volcan Llaima, whereas the right is what this grain looks like in terms of Mg/Fe concentration. The compositional gradient can be easily seen as a color change from the core to the rim, as the olivine is now surrounded by a different compositional external melt.
 Fig. 3 The DIPRA output screen, which allows you to best-fit (least discrepency) your measured data with a modelled slope. DIPRA can be found at this link: http://www.earthobservatory.sg/resources/software | In order to extract timescale information from the olivine minerals from Volcan Llaima (Fig. 2), I have been using DIPRA (Fig. 3), an open access software program created and published by Társilo Girona and Fidel Costa at the Earth Observatory of Singapore. DIPRA is a user-friendly program that allows one to model element diffusion in olivine for five different elements (Mg/Fe, Ni, Ca, Mn). By looking at the timescale information associated with different populations of crystals (ones associated with the intruding magma and ones associated with the resident magma), we are able to start to constrain the processes that occurred leading up to eruption. |
