Monticellite chemistry as an oxygen barometer for kimberlitic magmas and estimates of primitive kimberlite magma composition

The objective of this thesis is to calibrate two oxygen barometers for kimberlite magmas in the system CaO-MgO-Al2O3-SiO 2-TiO2-FeO based on the Fe and V content of monticellite, CaMgSiO4, that may be utilized in cases where oxides in olivine phenocrysts and perovskite are absent from a kimberlite pipe. I first calibrate a new oxygen barometer for kimberlite magmas based on the Fe content of monticellite in equilibrium with kimberlite liquids in experiments at 100kPa from 1230 to 1350°C and at fO2 from NNO-4.1 to NNO+5.3 (where NNO is the nickel-nickel oxide buffer). The XFeMtc/XFe liq (where XFeMtc/XFeliq is the ratio of mole fraction of total Fe in monticellite and Fe in liquid) decreases with increasing fO2, consistent with only Fe2+ entering the monticellite structure. Although the XFe in monticellite varies with temperature and composition, these dependencies are small (±0.03) compared to that with fO2. The experimental data were fitted by weighted least square regression to the following relationship: DNNO=log 0.858±0.021 XliqFeXMtcFe -1-0.139± 0.0220.193 ±0.004 (uncertainties at 2σ). I apply this oxygen barometer to natural kimberlites assuming the bulk rock FeO is that of their liquid FeO. Monticellite compositions of five kimberlites from both literature and my own investigations revealed a range in fO2 from NNO-3.5 to NNO+1.7. I finally use my well-defined monticellite-liquid Kd Fe2+-Mg to derive a range of Mg/(Mg+Fe2+) (Mg...