Origin of olivine in kimberlite: Phenocryst or impostor?

Section: Discussionmentioning

confidence: 99%

Orthopyroxene survival in deep carbonatite melts: implications for kimberlites

Stone

Luth

2016

“…The LDG kimberlitic olivine data can be divided into two main trends: (1) cores with high NiO (and low CaO) at variable Mg/(Mg + Fe) (Mg#) and (2) rims with decreasing NiO (and increasing CaO) at relatively constant Mg# (arrow in Fig. 5a) (e.g., Fedortchouk and Canil 2004;Brett et al 2009;Bussweiler et al 2015).…”

Section: Mineral Chemistry Of Inclusion Phasesmentioning

confidence: 99%

“…Twelve olivine mineral inclusions were measured in Cr-pyropes from Diavik, one in Cr-pyrope from Point Lake, and one in Cr-diopside from Diavik. Reference data for LDG kimberlite olivines divided into cores and rims are from Brett et al (2009) andBussweiler et al (2015). The arrow in (a) highlights the differentiation trend of phenocrystic rims (see text).…”

Section: Carbonatesmentioning

confidence: 99%

The evolution of calcite-bearing kimberlites by melt-rock reaction: evidence from polymineralic inclusions within clinopyroxene and garnet megacrysts from Lac de Gras kimberlites, Canada

Bussweiler

Stone

Pearson

et al. 2016

inclusion trails surround the inclusions. In Cr-pyropes, the inclusions additionally contain Al-spinel, clinopyroxene, and dolomite. The major and trace element compositions of the inclusion phases are generally consistent with the early stages of kimberlite differentiation trends. Extensive chemical exchange between the host phases and the inclusions is indicated by enrichment of the inclusions in major components of the host crystals, such as Cr 2 O 3 and Al 2 O 3 . This chemical evidence, along with phase equilibria constraints, supports the proposal that the inclusions within Cr-diopside record the decarbonation reaction: dolomitic melt + diopside → forsterite + calcite + CO 2 , yielding the observed inclusion mineralogy and producing associated (CO 2 -rich) fluid inclusions. Our study of polymineralic inclusions in megacrysts provides clear mineralogical and chemical evidence for an origin of kimberlite that involves the reaction of high-pressure dolomitic melt with diopside-bearing mantle assemblages producing a lower-pressure melt that crystallizes a calcite-dominated assemblage in the crust.

“…The wide crystallization ranges of monticellite and perovskite allow for the examination of kimberlite fO 2 along nearly the entire liquid line of descent. Although olivine is the most common mineral in kimberlite, this study focuses on monticellite because it is abundant in kimberlite, and the origin of olivine crystals in these rocks is currently debated (Arndt et al 2010;Brett et al 2009;Fedortchouk and Canil 2004;Patterson et al 2009) and thus ambiguous for studies on their fO 2 .…”

Section: Introductionmentioning

confidence: 99%

Iron in monticellite as an oxygen barometer for kimberlite magmas

Pioufle

Canil

2011

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.