Apatite as a Monitor of Magmatic Processes and Timescales

Lead Research Organisation: University of Bristol
Department Name: Earth Sciences

Abstract

This study will attempt to show how the remarkable mineral, apatite might help us better understand a range of processes in the magma plumbing systems beneath volcanoes. In particular the timing of events just prior to an eruption which would help to further improve our predictive accuracy, an important goal in terms of responsiveness for evacuation of threatened populations. There is also potential for economic application as some of sub-volcanic trigger processes may also play a role in the mechanism of porphyry copper mineralisation and apatite may help understand how these deposits form.
The role of volatile species such as H2O, CO2, SO2, Cl, & F, continues to emerge as one of the most important areas of research in magma evolution, of particular importance due to the devastation of explosive volcanic eruptions. As magma rise and decompress, the different volatiles exsolve at different rates related to variable solubilities. These volatiles have usually vanished from the system by the time we get to study the rocks.....they are after all, volatile and the cork has been popped. As a result the complex volatile history is notoriously difficult to track, the 'how and when' of volatile escape remaining poorly understood. Traditionally, people have used magmatic melt inclusions captured by growing minerals and theoretically 'isolated' from decompression driven degassing during ascent. However, the trapping and successful quenching of melt inclusions is the exception rather than the rule, and recent research has unraveled many problems with melt inclusions which are not quite as 'isolated' as previously though. They are also only useful if the melt is captured and rapidly quenched by an eruption, not in slow cooled intrusive settings. As a result, the search continues for an alternative, ubiquitous and robust means to monitor volatile activity in magmatic processes - ideally one which records the history of a range of volatiles and their differing behaviours throughout magmatic processes. Increasingly, the answer appears to be the mineral apatite which is often present to record both volcanic and intrusive events.
Apatite is particularly common in the more explosive volatile rich magmas that typify arc magmatism and the plutonic equivalents that occasionally produce ore bodies. It has the general formula Ca5(PO4)3X where X is crystal site that accommodates F-, Cl-, & OH-, but has also been shown to host CO32- and S2- (reduced S). CO32- and SO42- (oxidized S) can also replace PO43- in the main structure. This covers all the major volatile species released by magmas, with S and Cl being particularly important to ore forming processes. Pervious experimental work by ourselves and others has shown how apatite records the melt volatile content in a systematic way, which we are just beginning to apply and model pre-eruptive degassing histories.
But apatite offers even more, in terms of the timescale of events. One of the major challenges in volcanology today is to identify the signals of an imminent eruption on a timescale useful for evacuation and mitigation. This requires an understanding of the timescales of magma fractionation and pre-eruptive volatile build up and release. Newly emerging ideas suggest that eruptions are triggered by catastrophic 'mush destabilisation' which brings together distinct mush, melt and gas components from a vertically extensive mush body. The volatiles in apatite will be reset during these events. One preliminary study made 23 years ago has suggested the re-equilibration of F, Cl and OH will be very rapid. This claim need substantiation and other species adding to the system, to possibly get a range of diffusion rates that probe a range of timescales from pre-eruptive mixing to ore body formation. By experimentally determination of these re-equilibration rates for each volatile, we may have a tool for understanding the volatile history for days, week or months and years, prior to eruption.

Planned Impact

Government agencies that need to manage restless volcanoes where past eruptions are used to predict future events and understanding timescales will greatly help in risk management. Diffusion timescales obtain from erupted apatite will be a powerful tool for them to use.
Mining companies who need to better understand the process of ore formation in order to help increase efficiency of mining and exploration. Apatite may become a key mineral indicator for certain magmatic processes conducive to ore formation
Biomedical companies who use synthetic apatite to create new bone structure and improve teeth enamel.

Nuclear waste burial agencies who need to find robust material to incorporate radioactive waste. The stability of apatite in various environments is important, but this mineral could be used to contain 37Cl in particular and the rate it can 'leak out' is an important consideration.

Publications

10 25 50