Njillan Forbes

B.Sc. (Honours) Thesis

EXPERIMENTAL STUDY OF POSITIVE TRIGONS ON DIAMONDS; CONDITIONS OF NEAR-SURFACE DIAMOND RESORPTION WITH APPLICATION TO EMPLACEMENT CONDITIONS OF SNAP LAKE KIMBERLITE.

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Kimberlites are volatile-rich ultrabasic igneous rocks and the primary source rock of diamonds. During ascent in kimberlite magmas and perhaps after emplacement, diamonds undergo resorption by interaction with kimberlitic fluids, yielding a variety of resorption features on diamonds including triangular etch pits (trigons). Positive trigons have been proposed to form as a result of diamond resorption in either oxidizing or reducing conditions depending on the kimberlite pressure, temperature and melt composition, and they are rare on natural diamonds compared pervasive negative trigons. Diamonds from Snap Lake kimberlite dyke in Northwest Territories, Canada have abundant positive trigons. The nature of fluids and emplacement conditions responsible for Snap Lake positive trigons are not well known. Published experimental studies indicate temperature and oxygen fugacity controls on trigon orientation and morphology, although the data is sparse. A better understanding of these controls can help to constrain the conditions that prevailed in kimberlites that host positive trigon-bearing diamonds.

This study focuses on experimental production of positive trigons under variable temperatures and oxygen fugacities, to observe and quantify changes in their morphology. I performed resorption experiments in Na2CO3-NaCl melt at 0.1 MPa (atmospheric pressure), temperatures 700 ºC and 800 ºC (to examine near surface emplacement conditions), and oxygen fugacity in air (log fO2 = -0.68) and in pure CO2 (log fO2 = -2.79 and -2.85). Geometric parameters of positive trigons were examined by conducting quantitative measurements using an atomic force microscope. All except one of the experiments produced positive trigons. Comparison of the results to prior experimental studies of positive trigons confirms different evolution trends for point- and flat-bottomed positive trigons as temperature and oxygen fugacity increases. Comparison of the results to Snap Lake positive trigons helps better understand possible emplacement conditions of Snap Lake kimberlite.

Keywords: Snap Lake kimberlite, positive trigons, diamond resorption, trigon morphology, kimberlitic fluids, experimental petrology.
Pages: 68
Supervisor: Yana Fedortchouk