Geochemical constraints on
mantle structure and
convection
(For example…….)
Trace element fractionation during partial melting
Melts extracted from the mantle rise to the crust, carrying with
them their “enrichment” in incompatible elements.
• Continental crust becomes “incompatible element enriched”.
• Mantle becomes “incompatible element depleted”.
Mantle
Sm
Co
Region of partial melting
Nd
V
Rb
Pb
Sr
Th
Incompatibles
Compatibles
Melting product:
Ni
> Rb/Sr
< Sm/Nd
> U,Th/Pb
U
Cr
Melting residue:
< Rb/Sr
> Sm/Nd
< U,Th/Pb
A simple mantle evolution (example of “petrogenetic
tracing”……
Figure 9.13. Estimated Rb and Sr isotopic evolution of the Earth’s upper mantle, assuming a largescale melting event producing granitic-type continental rocks at 3.0 Ga b.p After Wilson (1989).
Igneous Petrogenesis. Unwin Hyman/Kluwer.
OIB enriched in incompatibles but different OIBs are different
MORB formed from already depleted mantle
Mixing: MORB--OIB--CC??
EM-2 enriched (cont. seds)
Regions depleted in incompatibles have low 87/86 but high 143/144
Apparent age is 2 Gyr
No simple trend -- CC is in middle -- therefore not back-mixing of CC into mantle
Note convergence of HIMU and MORB at FOZO
Many hotspots have limited range of isotopic compositions
EM1 and EM2 are concentrated south of equator -- DUPAL anomaly
Indian ocean MORB is different from Atlantic and Pacific
In 3-D space…. it is a zoo(!) of mantle reservoirs)
(Other) sources of mantle compositional heterogeneity:
Other isotopic systems that contribute to our understanding of mantle
reservoirs and dynamics
Noble gases – e.g., He (isotopes)
Noble gases are inert and volatile
4He
is an alpha particle, produced principally by a-decay of U and Th, enriching
primordial 4He; 3He is largely primordial (constant)
The mantle is continually degassing and He lost (cannot recycle back)
4He enrichment expressed as R = (3He/4He) [unusual in that radiogenic is the
denominator]
Common reference is RA (air) = 1.39 x 10-6
Common assumptions:
•Shallow mantle MORB source is relatively homogeneous and depleted in He
•Deeper mantle has more primordial (high) 3He/4He, but still degassed and less than
primordial (100-200RA) values
•PHEM may be that more primitive reservoir
•Low 3He/4He may be due to recycled crustal U and Th
Kellogg et al – stealth layer
The stealth layer
Generation of piles by convection