Finlay M' Stuart, Solveigh LassEvans, J' Godfrey Fitton

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High 3He/4He ratios in picritic basalts from
Baffin Island and the role of a mixed reservoir
in mantle plumes

• Finlay M. Stuart, Solveigh Lass-Evans, J. Godfrey
  Fitton  Robert M. EllamNATURE, VOL 424, 3 JULY
  2003 , pages 57-59

3He/4He Ratios meaning, source, implications
Baffin Island Location, tectonic significance
NAVP, Iceland, MAR, and Hotspots
Current arguments
Plumes or Plums??
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3He/4He Ratio What is it?

• 3He is primordial related to either
• original accretion of the Earth from the solar
  nebula, or
• small amounts of interplanetary/cosmogenic dust
  accreted onto the Earths surface (and possibly
  recycled)
• 4He is derived from U and Th decay
• Current atmospheric ratio of 3He/4He (Ra) is 1.38
  x 10-6
• R/Ra is used to indicate isotopic He enrichment
  or depletion
• N-MORB has R/Ra mean values 7.46 1.95 to 9.06
  3.46
• Values of R/Ra 11 are considered evidence for
  plumes
• Hot argument about mantle reservoirs, mixing,
  degassing, and the implications for hotspots Is
  a non-degassed reservoir needed?
• Yes, but not as a primary source reservoir!

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Helium isotopes must be treated carefully!!

• Sample processing - crushing and gas extraction
  from selected olivines
• Accounting for radiogenic 4He related to post
  crystallization decay of U and - determine the U
  and Th content of original source
• Comparing 3He/4He ratios to absolute amount of He
  in the sample
• Comparing He isotopes to other isotopes, LILs,
  and Noble Gases
• e.g. Sr, Nd, Sm, La, Ne, Ar, Kr, Xe, Re and
  Os...
• And proper treatment of the data that avoids
  circular logic

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Baffin Island Where is it?
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Extremely high 3He/4He Ratio
Plots of olivine 3He/ 4He versus 143 Nd/ 144 Nd
and (La/Sm) n for early Palaeocene basalts from
Baffin Island. a, Whole-rock Nd isotopic
composition also shown are the data for a
Palaeogene ankaramite from NW Iceland (square)
and basaltic glass from Loihi seamount
(triangle). b, Whole-rock (La/Sm)
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Why Baffin Island? Tectonic setting!
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What does it mean?

• Depleted, 'regassed' low U/He reservoir
• Low- 87 Sr/ 86 Sr -----gt Partial Melting
• High- 143 Nd/ 144 Nd -----gt Partial Melting
• High- 3He/ 4He basalts -----gt Volatiles
  introduced by convection
• Dominant control on mantle isotopic heterogeneity
  in the mantle is solid-melt distribution
  coefficients
• He is highly incompatible during mantle melting.
• Low Rb/Sr and high Sm/Nd, indicate partial
  melting residue ----gt low- 3He/ 4He, low- 87 Sr/
  86 Sr and high- 143 Nd/ 144 Nd ratios.
• Subduction of interplanetary dust particles
  cannot change He and Ne isotopic composition
  significantly.
• High- 3He/ 4He component is most probably the
  product of mantle mixing.

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Implications Plumes, Mantle Reservoirs,
Heterogeneity, Mixing, etc...
http//www.mantleplumes.org/ Don Anderson and
others...
OR...??