Earth’s Mantle: A View Through Volcanism’s Window William M. White Dept. of Earth & Atmospheric Sciences Cornell University Ithaca NY USA William M. White Dept. of Earth & Atmospheric Sciences Cornell University Ithaca NY USA

Terrestrial Volcanism Mid-ocean ridges Oceanic Islands/Hot spots Subduction Zones

Terrestrial Volcanism Mid-ocean ridges Oceanic Islands/Hot spots Subduction 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 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.

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.

Hot Spots  Persistent, stationary volcanism over 10 7 to 10 8 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  Chemical compositions provide an instantaneous view of the mantle, but radiogenic isotopes provide a view of the ancient mantle.  Radiogenic isotope ratios, such as 87 Sr/ 86 Sr, 143 Nd/ 144 Nd, 206 Pb/ 204 Pb provide a time-integrated measure of the corresponding parent/daughter ratios – 87 Rb/ 86 Sr, 147 Sm/ 144 Nd, 238 U/ 204 Pb.  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 87 Sr/ 86 Sr, 143 Nd/ 144 Nd, 206 Pb/ 204 Pb provide a time-integrated measure of the corresponding parent/daughter ratios – 87 Rb/ 86 Sr, 147 Sm/ 144 Nd, 238 U/ 204 Pb.  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 Array

The “Mantle Zoo”

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

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

Hofmann & White Hypothesis From Hofmann & White ‘82

A Smoking Gun in Samoa ? From Jackson et al. 2007

Oxygen Isotopes  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.  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 From Eiler et al. ‘97

O, Sr, and Pb isotope covariation in Samoan lavas From Workman et al. 2008

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 Balance  Essentially all the Ar in the atmosphere is 40 Ar, 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 40 Ar, 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

142 Nd: Do we really understand mantle evolution? From Boyet & Carlson ‘05  146 Sm decays to 142 Nd 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 142 Nd /144 Nd of the Earth should be chondritic, i.e., e 142Nd = 0.  Is there a missing ‘early enriched’ reservoir in the deep mantle?  Do we not understand the early solar system?

Summary  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.