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

Date

2011-08-09

Authors

Le Pioufle, Audrey

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Abstract

The objective of this thesis is to calibrate two oxygen barometers for kimberlite magmas in the system CaO-MgO-Al2O3-SiO2-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 1350C and at fO2 from NNO-4.1 to NNO+5.3 (where NNO is the nickel-nickel oxide buffer). The XFeMtc/XFeliq (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 bulk composition, these dependencies are small (0.03) compared to that with fO2. The experimental data were fitted by weigted least square regression to the following relationship: DNNO= (log (0.858(0.021)*XFeliq/XFeMtc-1)-0.139(0.022))/0.193(0.004) (uncertainties at 2 sigma). I apply this oxygen barometer to natural kimberlite 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 number) for kimberlite melts of 0.40-0.90. This range in composition is broader than previous estimates of 'primary' kimberlites, reflecting the diverse mantle sources and processes that occur during generation and ascent of kimberlites. Second, I calibrate a new oxygen barometer for kimberlite magmas based on the V content of monticellite in equilibrium with kimberlite liquids doped with 0.5 wt% V2O5 at 100kPa at 1280 and 1350C and at fO2 from NNO-4.1 to NNO_0.5. The DV Mtc/liq (DV Mtc/liq = V (ppm) in monticellite/V (ppm) in liquid) decreases with increasing fO2. The partitioning data can be fitted to a model consistent with V5+ as the dominant species in the melt phase above NNO whereas V4+ dominates below those conditions in kimberlitic magmas. The total DV Mtc/liq, which embodies both DV3+ Mtc/liq and DV4+ Mtc/liq, shows a very slight temperature and bulk composition dependence. The experimental data can be fitted by weighted least square regression to the following relationship: DNNO= (log(0.354(1.785)*Vliq/VMtc-1)-1.172(2.302))/0.111(0.071) (uncertainties at 2 sigma and V in ppm). In order to apply this oxygen barometer rigorously, the V concentrations of the kimberlite melt coexisting with monticellite need to be constrained. In contrast to the Fe-in-monticellite oxygen barometer for which the concentration of Fe in monticellite was close to that of the whole rock composition, the concentration of V in the bulk rock composition reflects mostly the large accumulation of olivine xenocrysts which contain low V concentrations. For that reason, the V-in-monticellite oxygen barometer cannot be applied to natural kimberlites until we find a way to overcome this problem. The vanadium concentrations of kimberlite melts are likely higher than the V concentrations of the whole rock compositions leading to underestimated fO2 values.

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Keywords

Oxygen Fugacity, Kimberlite, experimental petrology, monticellite, iron, partitioning, vanadium

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