Apatite as a quantitative tool for tephrochronology and magmatic evolution

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Apatite is a calcium phosphate mineral that crystallises in small quantities from a wide variety of different magmas. It is particularly useful because it incorporates 'volatile' species such as water (H2O), carbon dioxide (CO2), chlorine, fluorine and sulphur, into its mineral structure. Volatiles are dissolved gases, which are an important component of many magmas. During magma ascent underneath a volcano the volatiles form bubbles, which expand and accelerate the magma upwards, fuelling explosive volcanic eruptions. Apatite has the potential to be a particularly reliable recorder of volatile concentrations in the magma before eruption, but in order to exploit the information it contains, we need independent measurements of the 'partition coefficients' for H2O and CO2 - the amount found in the mineral for a given concentration in the melt. In the first part of the project, we will measure these partition coefficients in the laboratory, so that we can infer magmatic volatile contents from their concentrations in apatite. We will also measure partition coefficients for fluorine and chlorine. We will test the partition coefficients by using apatite to infer volatile concentrations in a magmatic system, known as the Laacher See Tuff, that has already been well characterised using alternative methods. This volcanic deposit was produced in a major explosive eruption nearly 13,000 year ago, with ash layers dispersed across central continental Europe up to 1100 km away from the volcanic vent. In the second part of the project, we propose to develop apatite as a tool for correlating ash deposits from past explosive eruptions. Explosive volcanic eruptions inject large quantities of ash into the atmosphere, dispersing it over large areas, sometimes thousands of kilometres from the vent. These deposits are geologically instantaneous, and therefore widely used as time marker horizons for constraining age in studies ranging from past climate reconstructions to archaeology. It is therefore crucial to be able to identify a given eruption very precisely in sites that may be far apart. Usually volcanic glass compositions are used to identify different eruptions, but sometimes the deposits of different eruptions from the same volcano are similar: ash layers from these eruptions cannot be reliably used as marker horizons. We propose to use apatite crystals in place of glass chemistry to correlate between different ash layers. Apatite is an ideal candidate for this because it: (i) is common in a wide range of eruption deposits; (ii) is stable and insensitive to weathering, and (iii) has high concentrations of the elements of interest, which allows very precise measurements of chemical composition. This development would facilitate the identification and correlation of ash layers and hence improve the robustness of correlations that are used in climate reconstructions and other fields. Better identification of ash layers would also help ash dispersal to be mapped and modelled accurately, which has clear implications for understanding the practical and economic outcomes of future explosive eruptions.