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Earth Sciences at Royal Holloway, University of London uses cathodoluminescence to study geological specimens from SE Asia.

Professor Robert Hall leads field based research into the geology of South East Asia and the western Pacific. Samples of sedimentary rocks are brought back to the UK where they are analysed by heavy and  light  mineral  analyses  as  well  as  uranium-lead  (U-Pb)  zircon  geochronology  for  provenance studies  and  pathway  reconstruction.  Furthermore,  magmatic  and  metamorphic  basement  rocks  are dated  to  identify  potential  sources  for  provenance  studies  and  to  improve  the  understanding  of  the tectonic processes in the region. The results are also incorporated in paleogeographic reconstructions of SE Asia. Team member, post-doctoral researcher Juliane Hennig, takes up the story. “A main aspect of our research involves heavy mineral separation using heavy liquids (LST, DIM) or a Wilfley table, as well as a Frantz Isodynamic separator.  This  includes  extraction  of  zircons  which  are  analysed  by  Laser Ablation  Inductively  Coupled  Plasma  Mass  Spectrometry  (LA-ICP-MS)  U-Pb  geochronology  for  age determination. Cathodoluminescence (CL) imaging is an important tool for us to evaluate the internal zircon structure prior to U-Pb analysis.  It  is  used  to  select laser  spot  positions  and  allows  us  to specifically  target  an  area  of  interest  in  the  zircon.  It can also help to detect possible inclusions or cracks. The CL images can reveal core and rim relationships indicating different growth episodes of the zircons which, in combination with age dating, can provide further insights into the history of the rock.  It  can  also  be  used  to  support  possible  interpretations  of  the  age  results,  such  as  oscillatory zoning  which  indicates  a  magmatic  origin,  or  irregular  convolute  structures  of  zircons  or  rims  which may suggest a metamorphic origin”. Furthermore team member Dr Amy Gough uses the CL detector for imaging of quartz. “Differentiation of volcanic and hydrothermal quartz aids the light mineral analysis forsedimentary provenance. It also allows us to determine which grains are detrital and have authigenic overgrowths.  The  CL  detector  can  also  help  to unravel  the  history  of  the  detrital  quartz grains through highlighting both growth zoning and different generations of growth.”Commenting on the choice of detector, Dr Hennig said “The group chose the Centaurus CL detector from Deben based on previous experiences at Birkbeck College/UCL.  It has provided very good results and high quality images from the scanning electron microscope (SEM).” Examples of the work are shown below.  For  further  information,  readers  are  recommend  to  look  at  recent  publications1,2noted in the reference section below. The  Deben  Centaurus  is  capable  of  producing  high  resolution  cathodoluminescent  (CL)  images  of luminescent  materials.  Using  a  user  exchangeable  diamond  turned  reflector  tip,  monochrome  CL images  can  be  easily collected  and fed  back into  the SEM  auxiliary video  input. The photomultiplier may also be exchanged to select a particular wavelength range with sensitivity available from UV to deep IR at 185 nm to 1200 nm. For more details of how Deben helps customers to solve experimental challenges, develop a special configuration orto obtain more information on Deben’s applications and product portfolio, please visit www.deben.co.uk.

A selection of CL images of zircons showing oscillatory zoning, complex internal structures and core/rim relationships. (Images courtesy of Juliane Hennig& Tim Breitfeld, RHUL)

References

1   Webb, M., White, L.T., 2016. Age and nature of Triassic magmatism in the Netoni Intrusive Complex, West Papua, Indonesia. Journal of Asian Earth Sciences, 132, 58-74.(CL images are included in the supplementary files).

2   Hennig, J., Breitfeld, H.T., Hall, R., Nugraha, A.M.S., (in prep.). The Mesozoic tectonomagmatic evolution at the Paleo-Pacific subduction in West Borneo. (currently in review) Attachment A selection of CL images of zircons showing oscillatory zoning, complex internal structures and core/rim relationships.

 

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