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The phenomenon of deficient electron microprobe totals in radiation-damaged and altered zircon

Nasdala, Lutz and Kronz, Andreas and Wirth, Richard and Váczi, Tamas and Pérez-Soba Aguilar, Cecilia and Willner, Arné and Kennedy, Allen K. (2009) The phenomenon of deficient electron microprobe totals in radiation-damaged and altered zircon. Geochimica et cosmochimica acta, 73 . pp. 1637-1650. ISSN 0016-7037

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Abstract

The phenomenon of deficient electron microprobe analyses, with sums of analyzed constituents often below 95 wt%, is
assigned to the analysis of altered, porous minerals. With the example of three zircon populations we show that low totals
are related to textural features (i.e., numerous pores of tens to hundreds of nanometers size) as well as to the chemical composition
(i.e., water content well within the wt% range, which may affect partial sample degradation under the electron beam).
The formation of the spongy texture is explained by the alteration of a previously radiation-damaged and, thus, volumeexpanded
material in a fluid-driven replacement reaction. The smaller volume of the reaction product (crystalline, non volume-
expanded zircon) accounts for the formation of numerous voids and pores, which are perfect candidates for the incorporation
of water. The alteration has also resulted in uptake of non-formula elements such as Al, P, Ca, Fe, Y, and REEs
whereas Si and Zr are depleted. In one case, strong uptake of non-radiogenic Pb in altered zircon was observed. Because porous,
low-total zircon has formed in secondary alteration process, its occurrence can be considered as an indicator of a secondary
alteration history of the host rock. Low-total zircon is easily recognized by very low electron back-scatter
intensities, which are closely related to the two main causes of the analytical shortfall (i.e., water content and porosity)
and often lowered furthermore by the presence of light non-formula elements (especially P and Fe) up to the wt% range.

Item Type:Article
Uncontrolled Keywords:Zircon; Deficient electron microprobe analyses
Subjects:Sciences > Geology > Petrology
Sciences > Geology > Mineralogy
ID Code:17166
Deposited On:21 Nov 2012 11:51
Last Modified:26 Nov 2012 09:03

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