Iron in monticellite as an oxygen barometer for kimberlite magmas

Abstract

Monticellite is a common magmatic mineral in the groundmass of kimberlites. A new oxygen barometer for kimberlite magmas is calibrated based on the Fe content of monticellite, CaMgSiO 4 , in equilibrium with kimberlite liquids in experiments at 100 kPa from 1,230 to 1,350°C and at logfO 2 from NNO-4.1 to NNO?5.3 (where NNO is the nickel-nickel oxide buffer). The XFe Mtc /XFe liq was found to decrease with increasing fO 2 , consistent with only Fe 2? entering the monticellite structure. Although the XFein-monticellite varies with temperature and composition, these dependencies are small compared to that with fO 2 . The experimental data were fitted by weighted least square regression to the following relationship:DNNO ¼ log 0:858ðAE0:021Þ X Liq Fe X Mtc Fe h i À0:139ðAE0:022Þ n o 0:193ðAE0:004Þwhere DNNO is the fO 2 relative to that of the Nickel-bunsenite (NNO) buffer and XFe liq /XFe Mtc is the ratio of mole fraction of Fe in liquid and Fe-in-monticellite (uncertainties at 2r). The application of this oxygen barometer to natural kimberlites from both the literature and our own investigations, assuming the bulk rock FeO is that of their liquid FeO, revealed a range in fO 2 from NNO-3.5 to NNO?1.7. A range of Mg/(Mg ? Fe 2? ) (Mg#) for kimberlite melts of 0.46-0.88 was derived from the application of the experimentally determined monticelliteliquid Kd Fe 2? -Mg to natural monticellites. The range in Mg# is broader and less ultramafic than previous estimates of kimberlites, suggesting an evolution under a wide range of petrologic conditions.

Geodynamic and Isotopic Constraints on the Genesis of Kimberlites, Lamproites and Related Magmas From the Finnish Segment of the Karelian Craton

“…We note that Pipe 14 has a poorly constrained f O 2 (−2.3 ± 3.3); removing this data point only strengthens the observed relationship ( R 2 = 0.82, p = 0.01). Although there are some limitations associated with the perovskite and monticellite oxybarometers (Bellis & Canil, 2007; Canil & Bellis, 2007; Le Pioufle and Canil, 2012), the relative differences in f O 2 rather than absolute values are of importance here. Metasomatized mantle lithosphere is considered to be oxidized (e.g., Foley & Fischer, 2017; Foley et al., 2019; Woodland et al., 1996; Yaxley et al., 2017).…”

Oxygen fugacity of Alto Paranaíba kimberlites and diamond instability: Três Ranchos IV and Limeira I intrusions

“…The perovskite oxybarometer from Bellis and Canil (2007) yields ƒO 2 values that range from -3.2 to +2.1 ΔNNO for LM-I kimberlite, while the monticellite oxybarometer of Le Pioufle and Canil (2012) yields ƒO 2 values that range from -4.2 to +2.5 ΔNNO for the same intrusion. This comparison was not possible for the TR-IV, considering the possible monticellites were completely altered in the groundmass.…”

“…Another oxygen fugacity calculation procedure involves the estimation of Fe composition in kimberlite liquid and monticellite. Studies by Le Pioufle and Canil (2012) demonstrated the use of an oxybarometer for kimberlite magmas, based on , Araújo et al 2001, Melluso et al 2008, Felgate 2014…”

“…In which XFe Liq and XFe Mtc correspond to the molar fraction of Fe in the liquid and monticellite, respectively (Le Pioufle and Canil 2012). The global analysis of the matrix of such samples may be considered a good approximation of such liquid, since many xenocrysts are carried by kimberlitic magmas.…”