Sample Preparation

Rock samples for analysis are disaggregated by manual sledging or a hydraulic press, pulverized in a Shatterbox®, and sieved to yield monominerallic, sand-sized fragments. Heavy-mineral concentrates are obtained using magnetic as well as high-density liquid separation. In the case of clay-rich tuffs (bentonites), the sample is soaked in water and liquefied in a blender, and undergoes gradual clay disintegration and removal in a sonic dismembrator device (Hoke et al. 2014).

After the initial processing, magnetic minerals are removed using a Frantz magnetic separator. Zircon and other dense accessory minerals are then isolated from the sample with high-density liquids, such as methylene iodide.

Zircon from a Permo-Triassic tuff sample from the Penglaitan Section in South China

The final separation involves hand-picking desired mineral grains under a binocular microscope based on morphology, inclusions, and color. For example, the most desirable zircons targeted for ash bed chronostratigraphy have high aspect ratios, are euhedral and┬á with pyramidal ends, and preserve colorless to light-brown melt inclusions parallel to the crystallographic ‘c’ axis. Zircons with these properties are most likely to be autocrysts derived from the melt close to the time of ash eruption, and therefore are the most useful for determining the depositional age of the ash bed in question.