HR: 1340h
AN: V33A-1163    [Abstracts]
TI: The importance of magnesite and CO2 in the initiation and subsequent evolution of mantle plumes
AU: * Murphy, D T
EM: david.murphy@qut.edu.au
AF: SNRS, QUT, QLD, Brisbane, QLD 4001, Australia
AU: Muhlhaus, H B
EM: muhlhaus@access.edu.au
AF: ESSCC, UQ, QLD, Brisbane, QLD 4076, Australia
AU: Collerson, K D
EM: k.collerson@mailbox.uq.edu.au
AF: ES, UQ, QLD, Brisbane, QLD 4076, Australia
AB: To better understand the nature of the processes during the generation and evolution of mantle plumes it is imperative that geochemical, geophysical and mineral experimental observations are combined to generate realistic computational models. In an axi-symmetrical finite element model we make use of observations that; (1) some mantle plumes can be imaged to D"; (2) mantle plumes have high contents of CO₂ (300 to 1000 ppm; [1]) of which only 12 ppm can partition into olivine [2]; (3) high-pressure experimental studies show that although carbonate is present as a liquid at upper mantle conditions [3], it is stable as a magnesite solid solution (Mg,Ca CO₃) at <14 GPa and throughout the lower mantle [4]. However at D" it disassociates to MgO and CO₂ [5]. CO₂ is ubiquitous in mantle materials. CO₂ is the dominant phase that degasses in purely mantle derived melts both mid ocean ridge basalts and ocean island basalts. That mantle plumes have high CO₂ contents and are derived from the lower mantle requires that magnesite plays an integral role in plume evolution. The stability of magnesite varies dramatically in the Earth from D", where deep sourced plumes originate, to the upper mantle, where generation of basaltic melts occurs. In our simulations we attempt to address the scenario where CO₂-bearing liquids can be generated at the D", can freeze in the lower mantle and again melt as a thermal upwelling passes into the upper mantle. Our computational study focuses specifically on the dynamics of the generation and evolution of a mantle plume containing magnesite in its source at the D". Our initial results centre purely on the affect of plume migration across the stability field of magnesite. This allows investigation of migration and accumulation of carbonate melt both during plume initiation and as plumes reach steady state. Preliminary results provide constraints for the dynamics and interaction of carbonate and silicate melts in mantle plumes. We propose that carbonate melting plays an extremely important role in plume evolution. 1 Trull et al. 1993; EPSL, 118; 43-64 2 Shcheka et al. 2006; EPSL, 245; 730-742 3 Dasgupta et al. 2007, Am Min, 92; 370-379 4 Fiquet et al. 2002, Am Min, 87; 1261-1265 5 Isshiki et al. 2004; Nature; 427 60-63
DE: 0545 Modeling (4255)
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1037 Magma genesis and partial melting (3619)
DE: 1038 Mantle processes (3621)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting