Volcanic hazards directly affect more than 10% of the world’s population, and identifying eruption frequencies are fundamental to risk management. Yet there are significant gaps in records of prehistoric volcanic cycles due to geochronological limitations. ‘Un-dateable’ regions are manifold, and include the Mexico City basin, Japan, and New Zealand. Hence, there is a clear need for the development and application of innovative dating methodologies as part of integrated research into pre-historic volcanic events.
Luminescence dating may be able to provide ages for a number of eruptions that cannot be dated so far. This technique exploits the ability of quartz, feldspar, and glass to record the amount of radiation to which they have been exposed. If the dose rate of the environment is also calculated, the age since the last resetting event or crystallization is the absorbed dose divided by the dose rate. Luminescence dating spans the key age range (ca. 1,000-300,000 years), and can potentially be applied to any subaerial volcanic rock. Since the publication of a pioneering study on volcanogenic minerals in 1973, direct dating of volcanogenic material has been met with mixed success. Multiple studies have shown that these minerals often suffer from unstable luminescence signals and therefore underestimate eruption ages. Some promising results have been achieved by varying stimulation parameters and detection wavelengths, but contradictory results are common: a technique that produces an accurate age for one sample may not work for a chemically dissimilar crystal of the same mineral. Additionally, physical separation of mineral types may be difficult or impossible, as with microscopic inclusions of feldspar and quartz in glass shards.
Spatially-resolved luminescence (SRL) is the next step in research on dating volcanic ejecta. It measures luminescence by using an ultra-sensitive scientific camera (‘EMCCD’) to record sequential images during stimulation. In this way, many grains can be measured at the same time, then individual emission regions can be identified during data analysis. SRL has three key advantages over standard multigrain or single grain luminescence techniques:
1. Measurement of consolidated samples
2. Simultaneous measurement by heating
3. Discrimination of signals from ‘mixed’ samples
These advantages are directly applicable to volcanic dating. By measuring SRLfrom mixed and consolidated samples, one can bypass mineral separation which has hampered previous studies. Luminescence ages can also be compared with data from other analyses subsequently performed on the same samples, in order to reject signals from minerals known to give erroneous ages. Characterisation of their luminescence properties is a necessary first step in this direction, as a thorough understanding of the possible emission features and their stabilities must guide the formulation of any dating procedure.
Project HYPERLIGHT was designed to develope the following objectives:
1. A widely-applicable multi-method approach for dating volcanic ejecta
2. Flexible software for analysing SRL data
This action coincided with the planned coring of the Lake Chalco Basin (Mexico Citz), which is expected to yield a record of almost 1 million years of volcanic activity. Preliminary coring yielded numerous ash layers, but almost nothing is known about the long term evolution of the volcanoes in this basin. Chronological data on eruptions is of fundamental significance to volcanic hazard planning in the area, and it is expected that the methods developed during this project will be of significant future value to the program.
