EXPERIMENTAL FORMATION OF IGNEOUS ORBICULAR TEXTURES

The orbicular texture found in igneous and metamorphic rocks consists of ring-like shells arranged about a central core. The texture is not commonly found because the conditions that allow for the formation are rare in nature. For this project, we used a cold-seal apparatus that allows for rock powders to be exposed to high temperatures and pressures. The apparatus makes it possible to vary P and T in order to try to recreate the conditions in which orbicule formation can occur. According to the literature, formation may be due to changes in volatile content and rapid fluctuations in pressure and/or temperature. We subjected various rock compositions to rhythmic fluctuations in P and/or T in water-oversaturated systems in an attempt to produce orbicular textures in the laboratory. Experimental runs varied T from 900-650 °C (with P constant at 200 MPa) and varied P from 50-200 MPa (with T constant at 700 °C) and lasted from 5-17 days. Experiments were initially held at 900 °C and 200 MPa, and then we proceeded to fluctuate the temperature and pressure accordingly. To see if orbicule formation is present in the experimental runs, samples are being analyzed by back-scattered electron imagery. Scientists cannot agree on a definite formation process of the texture because of the difficulty of intentionally producing the texture in a laboratory. Some spherical structures have been produced, but it is unclear yet whether they are composed of mineralogically distinct shells.