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IGNEOUS-LECTURE 5 - Coggle Diagram
IGNEOUS-LECTURE 5
He isotopes
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If mantle has cycled then primordial He is lost and mantle only has 4He and He leaked from undegassed lower mantle
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Small ratio, so expressed compared to atmospheric He
There is some input from primordial source (Tertiary basalt from baffin Island has ratio of 50). This is evidence for involment of lower mantle in OIB
Pb isotopes
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Model breaks down when mantle-derived basalts are plotted: MORB and OIB basalts plot 'in the future'. This means that U got enriched
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Increase in U/Pb due to formation of the continental
crust, but experimental partition coefficients show
that Pb more incompatible, so U/Pb should decrease.
Unusual enrichment of U
Lead paradox
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During subduction, these elements are removed from ocean crust and added to continental crust (Pb is removed, but not U from mantle)
So, cont crust has low U/Pb and upper mantle has high U/Pb
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Implications also for Nb, as it is retained in rutile
OIB have high concentrations of Nb and low Pb, arc rocks have low Nb and high Pb