HR: 13:40h
AN: DI43A-01 INVITED    [Abstracts]
TI: CO2 and potassium in the mantle: carbonaceous pelite melts from the trailing edge of a detached slab hybridizing in the mantle to ultrapotassic kamafugite
AU: * Schmidt, M W
EM: max.schmidt@erdw.ethz.ch
AF: ETH Zuerich, Inst. Mineralogy Petrology, Zuerich, 8092, Switzerland
AB: The ultrapotassic magmas from the Intra-Apennine and Roman provinces constitute worldwide endmembers in terms of K₂O/Na₂O, K₂O content and CO₂ degassing, and are associated with carbonatites. Group II kimberlites, which are geochemically similar but less extreme, occur on cratons stable since several 100 Ma. This geotectonic situation appears in strong contrast to the subduction setting of central Italy, where plate convergence has slowed down to less than a few mm/a, the slab now tearing off leading hot asthenospheric mantle to flow in between the trailing slab and the crust. A successful recipe for ultrapotassic magmas requires K/Na fractionation at some previous stage. Melts from fluid-absent melting of carbonaceous pelites at >3 GPa are ultrapottasic phonolites (SiO₂ ~64 wt%) and have K₂O/Na₂O up to 9 because of residual cpx with jadeite₈₀. The effect of CO₂ is to stabilize residual jadeite, to lower SiO₂, and to increase K₂O/Na₂O ratios (as compared to CO₂ free melts with K₂O/Na₂O of 1-3 and SiO₂ = 73-77 wt%). The carbonaceous pelite melts were equilibrated with fertile, refractory but cpx bearing mantle, and wherlite. At sufficient pressures (3.5 GPa) and X_CO2 in the volatile component, hybridization of the carbonaceous pelite melts produces highly subsilicic kamafugites, with K₂O/Na₂O only slightly lowered, and XMg's >0.70 as characteristic of primitive melts. The essential role of CO₂ is to reduce the olivine saturation volume and to shift the olivine-cpx-opx cotectic to lower SiO₂. The Italian kamafugites are ultracalcic (CaO/Al₂O₃ = 1.2-1.4), and although carbonaceous pelite melts have little CaO and 20 wt% Al₂O₃, the assimilation of cpx and production of aluminous opx leads to ultracalcic compositions when equilibrated with refractory peridotite or wherlite. Temperatures necessary for the fluid absent carbonaceous pelite melting are 1050-1150 °C, far above any reasonable subduction geotherm. Hybridization in the mantle requires 1320-1400 °C (at 3.0-3.5 GPa). The proposition is, that the carbonaceous pelite melts only form in thermally relaxing slabs typical for an ending subduction. Slab break off causes inflow of hot asthenospheric mantle, which further heats the trailing edge of the slab. Normally, subduction ends through continental collision at the surface, and the melts from the trailing edge appear to freeze in the relatively cool mantle below the orogen and await the next thermal event in order to form group II kimberlites. In the particular Italian situation, where large scale plate tectonic movements changed the convergence into a strike slip, temperatures of the trailing slab edge and the asthenosphere are apparently sufficient to keep the carbonaceaous slab melts liquid during hydridization in the mantle, leading to the surface kamafugites. The short longevity of the ultrapotassic magmatism (800 ka) collaborates the concept of a one-time event, i.e. the one-time melting of the trailing slab edge during it's break-off.
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3619 Magma genesis and partial melting (1037)
DE: 3630 Experimental mineralogy and petrology
DE: 3641 Extrusive structures and rocks
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting