International Geologiical Congress - Oslo 2008

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PIS-01 General contributions to impact structures

 

Zirconium-bearing phases from suevite-like rocks in the Eyreville-B drill core, Chesapeake Bay impact structure

 

Harvey E. Belkin, U.S. Geological Survey (United States)
I-Ming Chou, U.S. Geological Survey (United States)
J. Wright Horton Jr., U.S. Geological Survey (United States)
 

 

The late Eocene Chesapeake Bay impact structure, buried beneath continental sediments in southeastern Virginia, USA, has been recently drilled with cores collected continuously to a total depth of 1766 m. Examination of core samples has revealed baddeleyite (ZrO2) and zircon (ZrSiO4) and its shocked equivalent, reidite. All the information discussed here has been obtained from thin sections of suevite and intercalated clast-rich impact melt rock in the Eyreville-B core from the interval 1397.2 - 1474.1 m depth. Detailed examination by optical and electron-beam scanning microscopy, cathodoluminescence, electron microprobe analysis, and Raman micro-spectroscopy has confirmed the phase identification and provided textural information. Zircon is a common accessory phase, either detrital or authigenic, in the known target rocks and sediments. A continuous range of zircon textures is observed; (1) seemingly un-shocked, euhedral to subhedral crystals; (2) fractured to highly fractured but unaltered crystals; (3) altered crystals with decomposition zones; severely altered crystals with decomposition zones containing baddeleyite; (4) and severely altered crystals partially transformed to reidite and also with decomposition zones containing baddeleyite. The highly shocked and decomposed zircons are typically in melt or altered melt. The intergrowth of baddeleyite, reidite and parent zircon is on such a fine scale that focused laser beams excite all three phases. The presence of reidite is confirmed by Raman spectroscopy from the Raman lines 825 and 873 cm-1. Baddeleyite, the low P/T polymorph of ZrO2, is verified by wavelength-dispersive X-ray and Raman spectroscopy; we have not observed evidence for other polymorphs of ZrO2. Cathodoluminescence of highly shocked zircon crystals show patchy and discontinuous active areas; the zircon decomposition zones typically are not cathodoluminescent, suggesting a shock and/or thermal effect. A 10 μm, euhedral, completely unaltered and unfractured zircon in glass in proximity to common, severely decomposed zircons in the same glass suggests zircon crystallization from the melt. The wide spectrum of zircon textures observed in a single thin section attests to the rapid mechanical mixing and aggregation of melt with variously shocked fragments. Baddeleyite was observed in two modes; small, sub-micrometer grains formed from the decomposition of zircon and larger, single crystals, 20 to 250 μm in size, either surrounded by melt or in lithic clasts. The larger crystals are fractured and some show patchy cathodoluminescent areas consistent with shock deformation. No known target lithology contains baddeleyite. The occurrence of baddeleyite, a typical phase in mafic rocks, suggests that the large baddeleyite crystals are refractory, residual fragments from an unknown target rock of mafic or intermediate composition.

 

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