Transcript Chemistry of the mantle

Chemistry of the mantle
Physical processes (subduction, convection)
affect the chemistry of the mantle.
Chemical processes occur mainly through
melting. Resulting chemical differences are
acted upon by physical processes.
 Interaction physical-chemical state of the
mantle.
Mantle and crust contain minor or trace
concentrations of virtually all elements.
•Comparison of concentrations shows that much
of the mantle is a residue (extraction of
atmosphere, ocean continental crust) compared
to primitive meteorites.
•Information on time scales is obtained from
radioactive decay.
•Chemical heterogeneities (different magma
sources) are more than 2 Gyears old.
Geochemical picture of the
mantle
1. The mantle is depleted in those elements that
are found to be concentrated in crust,
hydrosphere and atmosphere, relative to the
original composition of the mantle.
2. Some material from crust, hydrosphere and
atmosphere is re-injected into the mantle.
3. The depletion and re-enrichment are not
uniform.
4. The shallowest mantle sampled by MORB, is
the most depleted.
5. OIB, arising from melting of mantle plumes
(deeper), show less depletion and more
variability.
6. Chemical heterogeneities are old (1-2 Gy)
7. All of the mantle that has been sampled has
been modified from its original composition.
8. At least 5 reservoirs: continental crust
(enriched), continental root (depleted),
MORB, OIB and IAB (+water = CC)
Some definitions
Major element composition: Mg-Fe silicates and
lesser amounts of Al and Ca
These elements determine the structure of the
main minerals. Other elements have to fit in.
Less abundant elements: ores (solid solution in
major minerals)
Trace elements (much less than 1 %).
Incompatible (trace) elements tend to go into the
liquid phase.
Mg2+ can easily be replaced Ni2+ in olivine
U4+ and U6+ are much larger and have difficulty
to fit into olivine
Ni is compatible and U is incompatible
If part of the mantle melts, the liquid tends to
remove the incompatible elements.
Elements that tend to partition into a liquid iron
phase are called siderophile.
Elements that tend to partition into sulphide
phases are called chalcophile
Mantel geochemistry largely exploits the
radioactive decay of certain isotopes. The decay
changes the isotope composition of parent and
daughter elements
 Characteristic fingerprints of melts
 Dating
Fundamental relationship: D=D0+P0(1-e-t/t), t is
time and t is T1/2/ln2 where T1/2 is the half life of
the parent
Observations
Trace elements
The incompatible elements are concentrated in
melts (plot above the primitive line)
MORB: if is assumed that the melt is not
modified during ascent, the MORB source can
be inferred.
e.g. 10 % melting and most incompatible
elements go into melt:
VmantleCmantle
= VmeltCmelt
= 0.1 VmantleCmelt
Cmantle=Cmelt/10  MORB source depleted in
incompatible elements
Correction less secure for OIB
5 % melting ?  correction of 20
EM-1, HIMU source enriched with respect to
primitive mantle
Hawaii (more melting ?) close to primitive
BUT all plume sources are less depleted than
MORB.
Continental crust strongly enriched, but difficult
to understand because very heterogeneous.
But remarkable correlation between CC and
MORB source: the enrichments of trace elements
in the CC are to a first approximation
complementary to their depletions in the MORB
source.
Primitive mantle  MORB source + CC ???
Exceptions are Nb, Pb  recycling
Observations
Refractory element isotopes
Melting has no effect on isotope ratios 
direct information on the source.
MORB and OIB show similar variations, but
the MORB signal is muted.
Observations
Noble gas isotopes
Noble gases are useful because they are
unreactive (no recycling into mantle from
atmosphere, no dissolution into melt minerals)
23 different isotopes
e.g. He3 is primordial
He4 from a decay of U and Th
Crust: low He3/He4  high radioactivity in CC
Mantle: high He3/He4  also high radioactivity
means a lot of He3 is still being outgassed.
MORB uniform He3
OIB tap sources with different concentrations of
He3, but always higher than MORB PHEM
Interpretation
•MORB shallow (passive source) depends on
our understanding of the dynamics of plates and
plumes
•OIB deep (active source)  again dynamics
•Sources have different signatures, but no
information on topology of reservoirs
•Ages from isochrones
•Mass balance: MORB 40-94 %, OIB rest or some
primitive mantle