Earth’s Mantle: A View Through

Volcanism’s WindowWilliam M. White

Dept. of Earth & Atmospheric Sciences

Cornell UniversityIthaca NY USA

Terrestrial VolcanismTerrestrial Volcanism

Mid-ocean ridgesMid-ocean ridges

Oceanic Islands/Hot spotsOceanic Islands/Hot spots

Subduction ZonesSubduction Zones

Terrestrial VolcanismTerrestrial Volcanism

Mid-ocean ridgesMid-ocean ridges

Oceanic Islands/Hot spotsOceanic Islands/Hot spots

Subduction ZonesSubduction Zones

Mid-ocean ridges Decompression melting as mantle upwells beneath diverging lithospheric plates.

Subduction Zones Flux melting due to release of water from sinking lithosphere.

Hot Spots Decompression Melting of convection plumes upwelling from deep mantle.

Mid-ocean ridges Decompression melting as mantle upwells beneath diverging lithospheric plates.

Subduction Zones Flux melting due to release of water from sinking lithosphere.

Hot Spots Decompression Melting of convection plumes upwelling from deep mantle.

Mid-Ocean RidgesMid-Ocean Ridges MORB Basalt of uniform composition created by large (10%) extent of melting at shallow depth.

Depleted in incompatible elements (including K, U, Th) - probably as a result of earlier melting episodes.

Isotope ratios indicate ancient depletion.

MORB Basalt of uniform composition created by large (10%) extent of melting at shallow depth.

Depleted in incompatible elements (including K, U, Th) - probably as a result of earlier melting episodes.

Isotope ratios indicate ancient depletion.

Subduction Zones

Subduction Zones

Magmas of variable composition, but with a number of consistent features.

Melts of mantle “contaminated” by fluids released from the subducting plate.

Provide useful indicators of what is being subducted.

Magmas of variable composition, but with a number of consistent features.

Melts of mantle “contaminated” by fluids released from the subducting plate.

Provide useful indicators of what is being subducted.

Hot SpotsHot Spots Persistent, stationary volcanism over 107 to 108 years.

Basalts of variable composition, generally smaller degree, deeper melts than MORB.

Variable trace element and isotopic composition, generally less “depleted”, with some hot spots derived from an incompatible element enriched source.

Persistent, stationary volcanism over 107 to 108 years.

Basalts of variable composition, generally smaller degree, deeper melts than MORB.

Variable trace element and isotopic composition, generally less “depleted”, with some hot spots derived from an incompatible element enriched source.

The Long Lens of Isotope Ratios

The Long Lens of Isotope Ratios

Chemical compositions provide an instantaneous view of the mantle, but radiogenic isotopes provide a view of the ancient mantle.

Radiogenic isotope ratios, such as 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb provide a time-integrated measure of the corresponding parent/daughter ratios – 87Rb/86Sr, 147Sm/144Nd, 238U/204Pb.

Isotope ratios tell us that the mantle has evolved chemically since the Earth formed - that the hot spot and mid-ocean ridge volcanoes tap chemical reservoirs that have remained isolated for long periods.

Chemical compositions provide an instantaneous view of the mantle, but radiogenic isotopes provide a view of the ancient mantle.

Radiogenic isotope ratios, such as 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb provide a time-integrated measure of the corresponding parent/daughter ratios – 87Rb/86Sr, 147Sm/144Nd, 238U/204Pb.

Isotope ratios tell us that the mantle has evolved chemically since the Earth formed - that the hot spot and mid-ocean ridge volcanoes tap chemical reservoirs that have remained isolated for long periods.

The Mantle ArrayThe Mantle Array

The “Mantle Zoo”The “Mantle Zoo”

Age of HeterogeneityAge of Heterogeneity

Pb isotope systematics require mantle heterogeneity be much younger than the Earth

Mantle Processes: Depletion

Mantle Processes: Depletion

Extraction of partial melts to form continental and oceanic crust depleted the mantle in incompatible elements (such as K, U, Th).

Resulting depleted mantle is thought to dominate the upper mantle* and be the source of MORB

Acronym: DUM

Extraction of partial melts to form continental and oceanic crust depleted the mantle in incompatible elements (such as K, U, Th).

Resulting depleted mantle is thought to dominate the upper mantle* and be the source of MORB

Acronym: DUM *the thermodynamics of melting is such that melting, and melt extraction, is probably only possible in the uppermost mantle. Furthermore, the melt-depleted residue has lower density that virgin mantle.

Mantle Processes: Enrichment

Mantle Processes: Enrichment

It is easy to imagine a geologically reasonable depletion process, enrichment has posed more of a dilemma. What geologic process could enrich or re-enrich the mantle in the incompatible elements removed by melting?

The answer is ultimately obvious - you have to introduce melts into the mantle.

But how?

It is easy to imagine a geologically reasonable depletion process, enrichment has posed more of a dilemma. What geologic process could enrich or re-enrich the mantle in the incompatible elements removed by melting?

The answer is ultimately obvious - you have to introduce melts into the mantle.

But how?

A Plate Tectonic Solution

A Plate Tectonic Solution

Plate tectonics continually produces melts at mid-ocean ridges and returns them to the mantle at subduction zones.

Hofmann & White Hypothesis

Hofmann & White Hypothesis

From Hofmann & White ‘82

A Smoking Gun in Samoa ?

A Smoking Gun in Samoa ?

From Jackson et al. 2007

Oxygen IsotopesOxygen IsotopesIsotope ratios can also change due to chemical processes.

For most elements, this effect is so small it is almost immeasurable.

It can be large, however, for light elements that form a variety of chemical bonds.O is the best example (also, conveniently, is 50% of most rocks).

Isotope fractionation is inversely proportional to the square of temperature, so large variations in stable isotope ratios can be produced only at or near the surface of the Earth.

Isotope ratios can also change due to chemical processes.

For most elements, this effect is so small it is almost immeasurable.

It can be large, however, for light elements that form a variety of chemical bonds.O is the best example (also, conveniently, is 50% of most rocks).

Isotope fractionation is inversely proportional to the square of temperature, so large variations in stable isotope ratios can be produced only at or near the surface of the Earth.

Oxygen Isotope Variability in OIB

Oxygen Isotope Variability in OIB

From Eiler et al. ‘97

O, Sr, and Pb isotope covariation in Samoan

lavas

O, Sr, and Pb isotope covariation in Samoan

lavas

QuickTime™ and a decompressor

are needed to see this picture.

From Workman et al. 2008

Mass Balance Considerations

Mass Balance Considerations

Balancing continental crust and depleted mantle indicates about 1/4 to 1/2 the mantle is “depleted”.This assumes there are no other significant “enriched” or “depleted” reservoirs in the Earth - a questionable assumption.

Balancing continental crust and depleted mantle indicates about 1/4 to 1/2 the mantle is “depleted”.This assumes there are no other significant “enriched” or “depleted” reservoirs in the Earth - a questionable assumption.

The K-Ar Mass BalanceThe K-Ar Mass Balance Essentially all the Ar in the atmosphere is 40Ar, and essentially all of this was produced by decay of K. It has escaped from the Earth’s interior.

Ar is a gaseous element and will escape to the atmosphere when the mantle melts. Residual mantle should have little Ar.

Only about 1/2 the mantle need be melted and degassed to account for all the Ar in the atmosphere.

Essentially all the Ar in the atmosphere is 40Ar, and essentially all of this was produced by decay of K. It has escaped from the Earth’s interior.

Ar is a gaseous element and will escape to the atmosphere when the mantle melts. Residual mantle should have little Ar.

Only about 1/2 the mantle need be melted and degassed to account for all the Ar in the atmosphere.

Possible Mantle Structures

Possible Mantle Structures

142Nd: Do we really understand mantle

evolution?

142Nd: Do we really understand mantle

evolution?

From Boyet & Carlson ‘05

146Sm decays to 142Nd with a half-life of 103 Ma - would have completely decayed early in Earth’s history.

If the Earth has a chondritic Sm/Nd ratio, the 142Nd/144Nd of the Earth should be chondritic, i.e., e142Nd = 0.

Is there a missing ‘early enriched’ reservoir in the deep mantle?

Do we not understand the early solar system?

146Sm decays to 142Nd with a half-life of 103 Ma - would have completely decayed early in Earth’s history.

If the Earth has a chondritic Sm/Nd ratio, the 142Nd/144Nd of the Earth should be chondritic, i.e., e142Nd = 0.

Is there a missing ‘early enriched’ reservoir in the deep mantle?

Do we not understand the early solar system?

SummarySummary The mantle is chemically heterogeneous, particularly with respect to K, U and Th. Variations in the concentrations of these elements could exceed an order of magnitude.

This heterogeneity results from removal of partial melts and reintroduction of melts through subduction of crust into other parts of the mantle.

Significant uncertainties remain in both the mass fraction of depleted and enriched reservoirs and in their physical location in the mantle.However, the most probable configuration would include a large reservoir in the upper mantle that is depleted in radioactive elements, with various enriched reservoirs in the lower mantle.

The mantle is chemically heterogeneous, particularly with respect to K, U and Th. Variations in the concentrations of these elements could exceed an order of magnitude.

This heterogeneity results from removal of partial melts and reintroduction of melts through subduction of crust into other parts of the mantle.

Significant uncertainties remain in both the mass fraction of depleted and enriched reservoirs and in their physical location in the mantle.However, the most probable configuration would include a large reservoir in the upper mantle that is depleted in radioactive elements, with various enriched reservoirs in the lower mantle.