Abbas Seyedolali

Search for evidence of impact at the Permian-Triassic boundary in Antarctica and Australia

Life on Earth was almost destroyed some 250 m.y. ago in the most profound of all known mass extinction events. We investigated the possible role of impact by an extraterrestrial bolide through chemical and mineralogical characterization of boundary breccias, search for shocked quartz, and analysis for iridium in Permian-Triassic boundary sections at Graphite Peak and Mount Crean, Antarctica, and Wybung Head, Australia. Thin claystone breccias at the isotopically and paleobotanically defined boundary at all three locations are interpreted as redeposited soil rather than impact ejecta. The breccias at all three locations also yielded shocked quartz, but it is an order of magnitude less abundant (0.2 vol%) and smaller (only as much as 176 micrometers m diameter) than shocked quartz at some Cretaceous-Tertiary boundary sites. Faint iridium “anomalies” were detected (up to 134 pgṁg −1 ). These values are an order of magnitude less than iridium anomalies at some Cretaceous-Tertiary boundary sites. Furthermore, peak iridium values are as much as 1 m below the isotopically and paleobotanically defined boundary. The idea that impact caused the extinctions thus remains to be demonstrated convincingly.

Provenance interpretation of quartz by scanning electron microscope–cathodoluminescence fabric analysis

We used a cathodoluminescence (CL) detector attached to a scanning electron microscope (SEM) to study patterns of variable-intensity CL in quartz grains from a variety of igneous, metamorphic, sedimentary, and shock-deformed (meteorite-impact) rocks. Distinctive fabrics in quartz grains revealed by SEM-imaged differential CL include zoning, healed fractures, complex shears, planar features (shocked quartz), dark CL streaks and patches, indistinct, mottled texture, and nondifferential (low-contrast) CL. Zoning is common in volcanic quartz and some plutonic quartz. Zoned plutonic quartz is distinguished from volcanic quartz by the presence of closed fractures and dark CL streaks and patches. Metamorphic quartz displays either an indistinct, mottled texture, or nearly uniform (nondifferential) CL. Quartz from rocks severely deformed by tectonism displays a complex pattern of multiple, small-scale shears. Quartz from meteorite-impact sites and some system boundaries is characterized by intricate patterns of planar features, presumably created by shock metamorphism. Thus, the SEM-CL fabric-analysis technique provides a rapid method for distinguishing quartz from a variety of source rocks.

Albitization of feldspars in sandstones from the Gohan (Permian) and Donggo (Permo-Triassic) formations, Gohan area, Kangwondo, Korea

Sandstones in the Gohan and Donggo formations in the Gohan area, Korea, contain considerable amounts of feldspars. Electron probe microanalysis (EPMA), backscattered electron microscopy (BSE), cathodoluminescence (CL), and microscopic petrography show that many of these feldspars consist of albite. Most of these albites are characterized by moderately high chemical purity (Ab 97–100) and do not display cathodoluminescence. These characteristics suggest that the albite is probably authigenic, formed during burial diagenesis by alteration of plagioclase and K-feldspar. The presence of abundant albite suggests that the sediments were buried to depths corresponding to the burial temperature that probably exceeds 90°C.

Three-Dimensional, High-Resolution Light Microscopy - A New Geological Tool

The three-dimensional microscope shows promise as a new tool in optical petrography. It has the same magnifying capability as the conventional compound optical microscope, but it has better resolution and increased depth of field. It also offers the ability to view and photograph objects in three dimensions. An object is obliquely illuminated by four light sources in such a way as to produce an enhanced stereoscopic effect. Fine-scale features, such as inclusions in feldspars, microfractures in quartz grains, and micro-scale layers in desert varnish, can be seen with much greater clarity than with the conventional petrographic microscope.