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 is time and 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 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