Apatite as a quantitative tool for tephrochronology and magmatic evolution

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

Abstract

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.

Planned Impact

The project is focused on developing a new tool for stratigraphic correlation and understanding pre-eruptive magmatic processes and as such, our primary means of communication of our results will be through the primary academic literature. Direct links to specific end users at this stage are tentative. However, we have identified a number of possible end users who may benefit from the research:

- We anticipate that specific end users of our research will include planners and policy makers involved with volcanic hazard and risk assessment and mitigation in central Italy, including the INGV (National Institute of Geophysics and Volcanology, Italy) and volcano observatories. They will benefit because our results will be relevant to understanding the dynamics of explosive volcanic eruptions at Campi Flegrei and similar systems.

- The results may also have indirect relevance for other agencies involved palaeoclimate research, that are investigating the rate of climate change and/or the response of humans and ecosystems to changes in climate. The chronology of many palaeo-enivronmental archives is constrained by tephra correlations. They will benefit through improved stratigraphic correlation.

- Others with wider, indirect interest in the results may include local residents in Italy, who would benefit indirectly from volcano hazard mitigation advice given by authorities, and the general public, who may be interested in general outcomes and future applications of the research. We will use traditional outreach activities to engage with these broader users (see Pathways to Impact).

- The PDRA employed on the project will benefit from training in a wide range of skills, from teaching, presentation and communication skills to experimental petrology, fieldwork and analytical techniques. The PDRA will be well-suited to a any career requiring a numerate employee with good communication skills, or to specific jobs involving microanalysis.

Related Projects

Project Reference Relationship Related To Start End Award Value
NE/K003852/1 07/05/2013 30/09/2013 £198,473
NE/K003852/2 Transfer NE/K003852/1 01/10/2013 31/07/2015 £170,849
 
Description 1. We have developed a well-constrained set of apatite volatile standards for use in various micro-analytical techniques, and a robust protocol for quantifying apatite volatile compositions and relating these to magmatic volatiles.

2. We have performed volatile-melt partitioning experiments between apatite and silicate melts from basaltic andesite to phonolite, and constrained exchange coefficients for OH, F, Cl and C, covering a range of temperature and pressure conditions, for comparison with thermodynamic modelling. We found fundamental changes in partitioning data with changing temperature conditions.

3. We have tested our new partitioning data using apatites from the Laacher See tephra, erupted at ~13 ka, and applied them to investigate pre-eruptive magmatic volatile compositions at the Campi Flegrei magma chamber. Importantly, apatite appears able to provide an otherwise relatively inaccessible record of volatile-undersaturated magma evolution. This is complementary to the information available from melt inclusions and gives new insights into the behaviour of the sub-volcanic magma system. This represents an important new avenue of petrological investigation that can be applied widely at other volcanic centres. At Campi Flegrei, there is no evidence that apatite microphenocrysts have started to re-equilibrate towards compositions that would be consistent with volatile saturation; this indicates that volatile saturation occurred only very shortly before eruption, and probably acted as a trigger by pressurising the magma chamber. At Laacher See, the accumulating crystal mush was dispersed rapidly throughout a heterogeneous melt body, again shortly before eruption. This lindicates that rapid mush reorganisation led to the explosive eruption.

4. We attempted to use apatite to investigate controls on eruption explosivity but found no clear links at Campi Flegrei. In likelihood, explosivity is controlled more by other extrinsic factors than pre-eruptive volatile concentrations or compositions. We have also evaluated the potential benefits and drawbacks of using apatite as a tool for tephrochronology.

5. We have developed a numerical model to explore the quantitative compositional variations in apatite composition during different volcanic scenarios. Using this code we can now successfully recreate observed apatite compositions for several different case studies. The model includes the ability to evaluate sensitivity to starting parameters and produces outputs that match well with independent estimates of starting compositions.
Exploitation Route - Aspects of our findings will be adopted by the volcano observatory staff (INGV) in the Campi Flegrei area and used to inform ongoing hazard assessment programmes in collaboration with local authorities.
- Analytical protocols, standards and partitioning constraints will be used by researchers in ongoing work on magmatic and other apatites. In particular, our finding that apatites can record periods of volatile-undersaturated evolution will be widely applicable to research into other volcanic systems worldwide.
- Our modelling will be of use for interpretation of apatite volatile compositions in rocks related to magmatic-hydrothermal mineralisation including porphyry deposits
Sectors Education,Energy,Environment,Government, Democracy and Justice,Culture, Heritage, Museums and Collections

URL http://community.dur.ac.uk/madeleine.humphreys/Apatite%20for%20eruption/Welcome.html
 
Description - Increasing public engagement with research and related societal issues. One of the key findings from this grant was recently published in Nature Geoscience and received broad attention by online media. To date, the original article has been viewed >1100 times, prompting 16 news articles and posts in scientific blogs and online magazines.
First Year Of Impact 2016
Sector Education,Culture, Heritage, Museums and Collections
Impact Types Cultural,Societal

 
Description NERC Standard grant
Amount £568,788 (GBP)
Funding ID NE/R011389/1 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 07/2018 
End 07/2021
 
Description NERC ion microprobe facility
Amount £17,500 (GBP)
Funding ID IMF521/0514 
Organisation Natural Environment Research Council 
Department NERC Ion Micro-Probe Facility
Sector Academic/University
Country United Kingdom
Start 05/2016 
End 01/2018
 
Description NERC standard grants
Amount £568,788 (GBP)
Funding ID NE/R011389/1 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 04/2018 
End 04/2021
 
Title Apatite EPMA protocol 
Description Improved understanding of susceptibility of apatite to electron microprobe analysis; robust analytical protocol for apatite 
Type Of Material Data analysis technique 
Year Produced 2015 
Provided To Others? Yes  
Impact Improved understanding of susceptibility of apatite to electron microprobe analysis; robust analytical protocol for apatite adopted by our research group in data collection 
 
Description Hannover 
Organisation Gottfried Wilhelm Leibniz Universität Hannover
Country Germany 
Sector Academic/University 
PI Contribution Intellectual and material input into joint development of analytical standards
Collaborator Contribution Access to C/S analysis for joint development of analytical standards
Impact Joint development of analytical standards
Start Year 2014
 
Description INGV 
Organisation National Institute for Geophysics and Volcanology (INGV)
Country Italy 
Sector Public 
PI Contribution Expertise and intellectual input; access to data and interpretations from samples collected jointly.
Collaborator Contribution Expertise and intellectual input; access to field knowledge and assistance with joint sample collection
Impact Stock et al. (2016). doi:10.1038/ngeo2639; Stock et al. (2018) in press
Start Year 2014
 
Description Mike Cassidy IRF 
Organisation University of Oxford
Country United Kingdom 
Sector Academic/University 
PI Contribution Intellectual input, provision of standards, and contribution of analytical expertise for measurement of volatiles in apatite.
Collaborator Contribution Expertise in synthesis of experimental materials
Impact None yet
Start Year 2018
 
Description Misasa manometry 
Organisation Okayama University
Country Japan 
Sector Academic/University 
PI Contribution Collaborative work on measurement of OH in apatite by hydrogen manometry (hosted by University of Okayama, Misasa)
Collaborator Contribution Collaborative work on measurement of OH in apatite by hydrogen manometry (hosted by University of Okayama, Misasa)
Impact None yet
Start Year 2014
 
Description TCEA Wesleyan Greenwood 
Organisation Wesleyan University
Department Planetary Science
Country United States 
Sector Academic/University 
PI Contribution Provision of apatite standards for analysis; compilation of calibration curves
Collaborator Contribution Provision of thermo-chemical elemental analyser (TCEA) data for trace H2O contents of apatite crystals.
Impact Development of precise calibration curves for trace H2O concentrations in apatite standard matierals.
Start Year 2015
 
Description deHoog 
Organisation University of Edinburgh
Department School of Geosciences Edinburgh
Country United Kingdom 
Sector Academic/University 
PI Contribution Intellectual input to joint development of standards and analytical protocol
Collaborator Contribution Intellectual input to joint development of standards and analytical protocol
Impact Development of standards and analytical protocols
Start Year 2014
 
Description Nature Geoscience press release 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Media (as a channel to the public)
Results and Impact Press release for published paper
Year(s) Of Engagement Activity 2016