Prof. Vadim Kamenetsky
Institute of Oceanology, Chinese Academy of Sciences
Apr. 24th 15:00
Title: Immiscibility in Silicate Magmas: A Key to Understanding 'Orthomagmatic' Ore Deposits?
Content Abstract:
Volatiles dissolved in magmas play a fundamental role in magmatic evolution across the compositional spectrum from komatiite to granite, and in the formation of magma-related hydrothermal systems. The nature of the phases (melts, vapour- or liquid-rich fluids) that immiscibly derive from magmas during decompression and cooling is important for solving the problems related to the magmatic – hydrothermal transition and origin of ore deposits. Immiscible phase separation is an intrinsically fugitive event, and the melt and fluid inclusions research provide most accurate record of this process. My presentation describes different types and compositions of coexisting immiscible phases represented by silicate, carbonate, chloride, sulphide and oxide magmatic liquids occurring in different magmas. The presentation of methodology and techniques will highlight heating stage experiments with visual control that are a powerful tool for studying in situ separation of immiscible phases within a given inclusion, and melting and crystallisation inside the immiscible phases. Modern microbeam techniques (e.g., electron microprobe, Raman, laser ablation ICPMS, PIXE) are used for identification of mineral constituents and chemical composition of immiscible phases. The methods combined provide constraints on the compositional signature of the immiscibility processes, element partitioning and ultimate fate of volatile and economic elements.
Apr. 25th 10:00
Title: New Models for Kimberlite Parental Melts: Compositions, Temperature, Ascent and Explosion.
Content Abstract:
Existing reconstructions of the kimberlite melt emphasize carbonate-bearing ultramafic compositions with significant amounts of dissolved volatiles CO2 and H2O (10-20 wt%), which are considered to be crucial in reducing the melt's viscosity and enabling its rapid ascent from the mantle. The exsolution of these volatiles from the melt during ascent and emplacement is viewed as being responsible for violent eruption of kimberlite magma, related brecciation of country rocks and fragmentation of the magma. However, our studies of melt inclusions in kimberlites worldwide provide key constraints on compositional properties of the kimberlite melt and suggest that neither CO2 nor H2O in degassed form could be responsible for this explosivity. The alkali-rich carbonatite-chloride composition of the kimberlite primary melts recorded in melt inclusions are essentially anhydrous and CO2 is effectively stored in liquidus carbonates. Such Si-undersaturated compositions are prone to reactions with wall-rock peridotite, followed by carbonate–silicate melt immiscibility at high pressure, and ultimately the CO2 saturation of the silicate melt component. The resultant fluid exsolution lowers magma density and viscosity and may drive crack propagation and emplacement of kimberlite magma with a load of entrained material into the crust. In most cases, massive degassing of H2O and CO2 from the kimberlite magma upon emplacement is unlikely, because the melt is poor in H2O, and CO2 is bonded in the carbonatitic melt.
We propose a new model that explosions in the crust are consequence of detonation of reduced hydrogen species (H2, CH4 and hydrocarbons). On emplacement, the magma releases the dissolved silicate component in the form of olivine and minor phlogopite and monticellite, thus driving the residual melt towards the initial chloride-carbonate composition. This is followed by gravitational separation of silicate solids to the roots of kimberlite dykes, whereas light, low viscosity chloride-carbonate melt is squeezed to the top. Post-magmatic serpentinization of olivine cumulates generates abundant H2 and CH4 that ascent to the carbonate-rich tops, where ‘cold’ explosions and brecciation of already solid magma occur. This unconventional model may explain a pipe-like shape of kimberlite bodies, explosive activity over an extended time span with intermittent phases of quiescence and consolidation, and excavation of rocks from the top down to 100’s m below surface.
Location:
D3 518
