Zhihai Zhang

Ph.D. Thesis

Diamond resorption morphology as a fluid proxy in diamond-bearing environments: Constraints from empirical and experimental studies

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Volatiles play significant roles in modifying the composition of sub-continental lithospheric mantle, in the formation, ascent and emplacement of kimberlites, and in diamond preservation. Despite of numerous studies the compositions of C-O-H fluids in the mantle and kimberlites remain uncertain due to limitations of conventional methods (e.g. fluid inclusions). Previous experimental studies have suggested that diamond resorption morphology has a great potential for constraining the composition of diamond etching C-O-H fluids.

My study investigates the internal and external factors on diamond resorption. I examined the morphologies of 733 diamonds from five Ekati kimberlite pipes (Canada), and internal properties (N contents and aggregation states, C isotopic composition, internal structures) of 82 diamonds. The lack of correlation between the internal properties in diamond outer rims and resorption morphologies suggests little effect of internal factors on resorption morphologies. Diamond dissolution experiments were conducted at 1-3 GPa and 1150 – 1400 oC in C-O-H fluids with XCO2 =0 -1, monitored in-situ by synthetic fluid inclusions. Etch pits on {111} faces were measured with an atomic force microscope. The experimental results show that 1) a miscibility gap exists in the CO2-H2O-dominated system at 1250 oC under 1 and 3 GPa, 2) circular pits form on tetrahexahedral faces in pure H2O at 1 GPa buffered by MgO-forsterite, and 3) the bulk composition of C-O-H fluids controls the pressure effects on diamond rounding rates and resorption morphologies, including etch pits on {111} faces.

The combined empirical and experimental studies permit a new classification of diamonds based on resorption morphologies, suggest etch pits forming under a defect-initiated and condition-controlled kink mechanism, and demonstrate the condition-dependent resorption morphology as a robust semi-quantitative proxy of fluid composition in diamond-bearing environments. Application to the Ekati diamonds suggests that either pure CO2 or carbonate melts acted as the metasomatic medium in the lithospheric mantle beneath the central Slave craton, and that the reconstructed pre-eruption column of the Ekati kimberlites show a “sandwich” structure beginning with a CO2–rich fluid zone, followed by a H2O-rich fluid zone and a magma zone at tail.

Keywords: Diamonds, atomic force microscope, mantle metasomatism, Kimberlite, synthetic fluid inclusions, etch pits, piston cylinder apparatus, C isotopes, Nitrogen, Carbon - Isotopes
Pages: 319
Supervisor: Yana Fedortchouk