BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//Talks.cam//talks.cam.ac.uk//
X-WR-CALNAME:Talks.cam
BEGIN:VEVENT
SUMMARY:Global MORB Chemistry and Ridge Axial Depth - A New Interpretation
  - Yaoling Niu (University of Durham)
DTSTART:20061018T153000Z
DTEND:20061018T163000Z
UID:TALK5488@talks.cam.ac.uk
CONTACT:4047
DESCRIPTION:The paper by Klein and Langmuir [JGR\, 1987] is a milestone on
  MORB genesis. They showed that MORB chemistry correlates with ridge axial
  depth on a global scale: CaO/Al2O3 and Fe8 (FeO corrected for fractionati
 on to MgO = 8.0 wt%) increase whereas Na8 decreases as the ridge shallows.
  They interpreted such correlations as resulting from varying pressures an
 d degrees of melting caused by mantle potential temperature (TP) variation
  of up to 250°C from beneath cold deep ridges to hot shallow ridges. This
  interpretation is reasonable because a hotter rising mantle begins to mel
 t deeper (high Fe8)\, has a taller melting column\, and melts more (high C
 aO/Al2O3\, low Na8) than a cooler mantle. The validity of this interpretat
 ion depends heavily on Fe8. HIDDEN in this interpretation is the FACT that
  at MgO = 8 wt%\, the inverse Fe8-depth correlation equals a positive Mg#-
 depth correlation. That is\, Mg# decreases from ~ 0.66 at deep ridges (e.g
 .\, Cayman Trough\, or CT\, > 5 km below sea level) to ~ 0.56 at shallow r
 idges (e.g.\, Reykjanes Ridge\, RR\, close to sea level). This means that 
 by using Fe8 (total range: 7 - 11) one examines the progressively more evo
 lved melt from deep ridges to shallow ridges\, which does not tell pressur
 es of melting\, thus provide no TP information.  \n\n \nBy correcting for 
 fractionation to Mg# = 0.72\, one examines largely the mantle signals of M
 ORB melts. In this case\, the range of Fe72 is reduced (7.5 - 8.5)\, and t
 he Fe72-depth correlation essentially disappears. IF one used Fe72 to esti
 mate TP\, then ~ 50°C variation may be reasonable beneath global ridges. 
 That is\, degrees of mantle melting may not vary significantly with varyin
 g ridge depth. However\, significant Na72-depth (+) and Ca72/Al72-depth (-
 ) trends remain. Assuming spreading rate effect is small and melting regio
 n shape effect is averaged out\, then Na72 and Ca72/Al72 largely reflect f
 ertile mantle composition. Deeper ridges are underlined by more fertile ma
 ntle with higher Al2O3 and Na2O that make denser garnet and jadeiite-rich 
 cpx\, thus greater bulk density in the mantle than shallower ridges. In or
 der to explain the > 5 km ridge depth variation\, we can use CT as a refer
 ence point to calculate isostatic compensation depth: DC (km) = 339.82X(-0
 .79355)\, where X is % mantle density reduction. This says that the 5 km e
 levation of RR (vs. CT) results from its sub-ridge mantle density reductio
 n of 0.5% (equivalent to 150°C hotter) with DC = 600 km\, or 1% (~ 300°C
  hotter) with DC = 334 km. Obviously\, density reduction due to variation 
 in composition is more realistic than temperature beneath global ocean rid
 ges.
LOCATION:Marine/Wolfson Building lecture hall
END:VEVENT
END:VCALENDAR