Intercomparison of Tephrochronology Laboratories

Introduction

During 2009-2011, the International focus group on tephrochronology and volcanism (INTAV) of the International Union for Quaternary Research (INQUA) conducting a broad, international intercomparison of laboratories that regularly use electron-beam techniques (both SEM-EDS and electron microprobe) to characterize and identify tephra deposits. The intercomparison addressed multiple goals, especially objective 2, of the INTREPID project - i.e. to advance understanding and efficacy in tephra fingerprinting and correlating and to evaluate and quantify uncertainty in tephrochronology. In an overall sense, the intercomparison was intended to see how well the tephra community was doing, similar to what the radiocarbon community does on a regular basis. By supplying four well-characterized reference glasses to approximately twenty participating labs, this project also widely distributed a uniform set of reference samples that will remain useful well into the future. Procedures for this effort were modeled largely after those of the ongoing G-Probe effort of the International Association of Geoanalysts.

Rationale

In many cases, geochemical differences between tephras can be subtle, requiring high levels of precision and accuracy for successful and correct identification. Fine grain size, instability of glass during analysis (i.e. Na-loss), presence of microcrysts, and sample heterogeneity provide additional challenges. Also, the use of published data is often complicated by small differences between results produced by different laboratories. Analyses of common reference materials are necessary to quantify these differences and thereby provide for more robust tephra correlations. Proficiency testing using common samples is also one of the most effective ways for a laboratory to monitor its performance against both its own expectations and the standards of performance set by other laboratories participating in the same test. Proficiency testing represents, therefore, an 'external' form of quality control that helps to highlight not only reliable determinations made by a laboratory, but also measurements that may be subject to unsuspected bias.

Timeline

1. Sent invitations to participate to potential laboratories - October, 2009
2. Sent each participating lab a single, unpolished mount containing four samples: - October, 2009
   
• rhyolitic obsidian
• secondarily hydrated rhyolitic tephra
• high-sodium intermediate composition tephra
• microcryst-bearing basaltic tephra
  
3. Initial data submissions collected through January, 2010
   
4. Presentation of summary results at the INTAV Japan meeting in May, 2010
5. Additional data submissions including reanalyses under revised procedures collected through January, 2011
6. Results published in a special volume of Quaternary International in December 2011
   

Additional documents

Further information may be found in the following documents:

INTAV_Welcome_and_Instructions.pdf

INTAV_Data_Form.xls

Preliminary results and initial manuscript

Steering Committee:

Dr. Stephen C. Kuehn
Concord University, USA
sckuehn @ concord.edu

Dr. Duane G. Froese
University of Alberta, Canada
duane.froese @ ualberta.ca

Dr. Siwan M. Davies
University of Wales Swansea, UK
siwan.davies @ swansea.ac.uk

Dr. Brent V. Alloway
Victoria University of Wellington, New Zealand
brent.alloway @ vuw.ac.nz

Dr. Phil A.R. Shane
University of Auckland, New Zealand
pa.shane @ auckland.ac.nz