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

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3He/4He Ratios: meaning, source, implications. Baffin Island: Location, tectonic significance ... Tectonic setting! Depleted, 'regassed' low U/He reservoir ... – PowerPoint PPT presentation

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


1
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. Ellam NATURE, 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??
2
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!

3
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

4
Baffin Island Where is it?
5
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)
6
Why Baffin Island? Tectonic setting!
7
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.

8
Implications Plumes, Mantle Reservoirs,
Heterogeneity, Mixing, etc...
http//www.mantleplumes.org/ Don Anderson and
others...
OR...??
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