Charge Contrast Imaging of Zircon in Amphibolite Facies Rock of the Central Appalachian Piedmont, SE Pennsylvania
Abstract
Charge contrast imaging using an environmental scanning electron microscope (ESEM) provides high resolution images of internal zoning in zircon crystals in uncoated thin sections. To locate all zircon (and monazite) crystals within a thin section we utilize an automated routine which combines back-scattered imaging and an integrated energy dispersive spectroscopy (EDS) system. The brightest objects in a given field are located and a two-second EDS spectrum is obtained, along with information on size, shape, and location of the bright features. Objects are thus readily sorted by size and individual grains can be selected for further analysis. Differential charging of compositional domains within zircon crystals occurs under low vacuum conditions (70 - 240 Pa, H20) and is imaged using secondary electrons. Here we examine the internal and external morphology of the zircon crystals in upper amphibolite facies rocks from SE Pennsylvania in the central Appalachian Piedmont, to gain insight into the nature of the protolith and to provide context for planned ion-probe analysis. The Chester Park gneiss has been mapped and described as consisting of meta-volcanic, meta-plutonic and/or meta-sedimentary rock. Zircon in the Chester Park gneiss contains rounded, detrital cores, some of which exhibit concentric oscillatory zoning, and euhedral to subhedral metamorphic overgrowths. These observations are consistent with a supra-crustal origin for this unit. Similar zircon is present in the Fairmount member of the Wissahickon Formation, which is present along strike from the Chester Park gneiss. In contrast, zircons present in the Waterworks gneiss, which underlies the Philadelphia Museum of Art, are euhedral and exhibit oscillatory zoning indicating magmatic origin for this rock.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.V33B2216B
- Keywords:
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- 3625 Petrography;
- microstructures;
- and textures;
- 3694 Instruments and techniques