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SUMMARY:Molten Earth: Magma in the Deep Mantle - Lars Stixrude\, Universit
 y College London
DTSTART:20090120T163000Z
DTEND:20090120T173000Z
UID:TALK15258@talks.cam.ac.uk
CONTACT:John Maclennan
DESCRIPTION:We discuss recent first principles molecular dynamics simulati
 ons of silicate liquids and their geophysical implications1\,2. We find th
 at the Grüneisen parameter increases rapidly on compression\, yielding li
 quid state isentropes that are much hotter than previously thought: a pote
 ntial temperature of only 2450 K is sufficient to melt the entire mantle\,
  and a magma ocean will begin crystallization at mid-mantle depths\, rathe
 r than at the base of the mantle. Our predicted melting temperature of MgS
 iO3 perovskite at 136 GPa is 5400±600 K\, which if lowered by freezing po
 int depression by 1300 K\, yields a mantle solidus temperature of 4100 K\,
  identical to recent estimates of the temperature at the base of the mantl
 e.  We argue on this basis that partial melting at the base of the present
 -day mantle is plausible.  While MgSiO3 perovskite is denser than the isoc
 hemical liquid\, reasonable values of iron partitioning and bulk iron cont
 ents\, leads to the conclusion that a melt at the base of the multi-compon
 ent mantle would be denser than its surroundings and therefore buoyantly s
 table.  We predict the bulk sound velocity of this melt to be 10.9 km s-1\
 , less than that of ultra-low velocity zones\, and consistent with the exp
 lanation of these regions as being due to partial melting.  We also find t
 hat water and silicate melt are completely miscible over nearly the entire
  mantle pressure regime\, suggesting a mechanism for storing accreted wate
 r at depth early in Earth’s history.\n\nReferences\n1. Stixrude\, L. and
  B. Karki\, Structure and freezing of MgSiO3 liquid in Earth’s lower man
 tle\, Science\, 310\, 297-299\, 2005.\n2. Mookherjee\, M.\, L. Stixrude\, 
 and B. Karki\, Hydrous silicate melt at high pressure\, Nature\, 452\, 983
 -986\, 2008\, doi: 10.1038/nature06918.\n
LOCATION:Harker 1 seminar room\, Department of Earth Sciences
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