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SUMMARY:Ni partitioning between olivine and silicate melts\, and insights 
 into Hawaiian petrogenesis - Andrew Matzen\, University of Oxford
DTSTART:20131112T163000Z
DTEND:20131112T173000Z
UID:TALK47291@talks.cam.ac.uk
CONTACT:John Maclennan
DESCRIPTION:Mantle melting that produces ocean island basalts takes place 
 at temperatures and pressures significantly higher than the conditions at 
 which they erupt or are intruded in the crust/shallow upper mantle. To the
  degree that the olivine-liquid nickel partition coefficient\, D_Ni \, dep
 ends on temperature and pressure\, it is important that models used to des
 cribe Ni partitioning during mantle melting include data from experiments 
 at elevated temperature and pressure. Available data on Ni partitioning is
  dominated by 1-atm experiments in which temperature and liquid compositio
 n are highly correlated\, making it difficult to separate the effects of t
 hese variables on the observed variations in D_Ni based on 1-atm experimen
 ts alone. \n\nWe conducted experiments on a mixture of MORB and olivine at
  1 atm (1400°C) and 1-3 GPa (1450-1550°C). We present data from a series
  of reversed experiments where temperature and pressure were increased in 
 such a way that the liquid composition remained approximately constant (Mg
 O about 17 wt%)\, effectively isolating the effects of temperature and pre
 ssure from those of liquid composition on D_Ni. The resulting partition co
 efficient decreases from ~5 to 3.8 (by wt) as the temperature increases fr
 om 1400 to 1550°C\, and is fit well by a simple thermodynamic expression 
 where D_Ni is a function of temperature\, olivine\, and liquid composition
 .\n\nBased on a simple model in which partial melting to produce a primary
  liquid and subsequent crystallization of that liquid take place at two di
 fferent pressures and temperatures\, the temperature dependence of D_Ni pr
 edicts the crystallization of low-pressure olivine phenocrysts with higher
  NiO contents than the olivines in the source region. Observed NiO concent
 rations of magnesian olivine phenocrysts from Mauna Loa\, Mauna Kea and Ki
 lauea are consistent with derivation from primary partial melts produced a
 t the base of the lithosphere from peridotites whose olivines have a distr
 ibution of NiO concentrations equal to that observed in spinel peridotites
 . Although we cannot rule out alternative hypotheses for producing the hig
 h-Ni olivines observed in Hawaii and elsewhere\, these processes or materi
 als are unnecessary to account for NiO enrichments in olivine. 
LOCATION:Harker 1 seminar room\, Department of Earth Sciences
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