Laser Ablation-ICPMS U-Pb Detrital Zircon Geochronology of Shale

Abstract

Heavy (≥ 2.9 g/cc) accessory (usually ≤1% wt.%) minerals such as zircon in shale hold significant, largely untapped potential for understanding the provenance, distribution and diagenesis of subsurface units in exploration and production drilling programs. However they have been much less well studied than heavy minerals in their coarser grained siliciclastic counterparts. This is because they tend to form silt and clay (< 63 micrometers) sized grains that are difficult to separate and concentrate from clay and micaceous minerals using heavy liquids, and analyze for isotopic composition and age. This study explores U-Pb detrital zircon geochronology of shale by laser ablation-ICPMS dating of grains directly in polished thin sections. Zircon grains are identified and imaged for inclusions, fractures, chemical zoning and petrographic relationships using automated, combined BSE-EDX scanning electron microscopy, and analyzed for chemical composition by electron microprobe. Homogeneous domains within grains are then analyzed for U-Pb age by laser ablation-ICPMS. Multiple thin sections of a sample may be analyzed in order to provide a sufficient number of dated zircon grains (75-100) to identify all major and minor (> 5%) age populations. The method has been applied to a sample of Barnett shale (Fort Worth Basin), which was deposited during a ca. 25-m.y. period during the Mississippian (Osagean to Chesterian). All zircon grains are less than 63 micrometers; of those larger than 8 micrometers (which can be dated by laser ablation-ICPMS), ca. 40% are fine to medium silt (8-32 micrometers) and ca. 60% are coarse silt (32-63 micrometers). Detrital zircon is present dominantly as grain fragments, suggesting mechanical fracturing during long-lived or intensive processing during erosion and transport. Some secondary zircon containing inclusions and pores forms as overgrowths on detrital zircon and may represent fluid-altered or -precipitated grains. U-Pb zircon ages of the zircon indicate a provenance dominated by Devonian and Silurian sources with lesser, older sources extending back into the Paleoproterozoic. A small number of the zircon give Mississippian and younger ages that may reflect early and late diagenesis.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016