There are Mantle Plumes originating from the CMB!

• Higher temperatures of the rising plume material should lead to a reduction of the velocity of seismic waves.

• Plumes should be visible as columnar low-velocity anomalies.

Rost et al. 2005

1. Mantle Tomography

Montelli et al. 2004: Finite-Frequency Tomography

Reveals a Variety of Plumes in the

Mantle; Science, vol. 303, 338-343

• Finite-frequency tomography

– Using of short period and long period P-wave data

– Calculation of the fractional perturbation of the compressional velocity

p

p

v

v

• Vertical average of the P velocity anomaly for the lowest part of the mantle (1800-2800 km depth)

p

p

v

v

Montelli et al. 2004

• All anomalies are present between 2800 and 1800 km.

1. There are low velocity zones corresponding to hot spots at the earth surface

2. There are low velocity zones which are not corresponding to hot spots at the earth surface.

Montelli et al. 2004

• Azores and Canary are separated plumes down to 1450 km.

• They merge together, bend in eastward direction and reach the CMB at 30oN and 10oW

• Farther south, the Cape Verde plume joins this superplume at 1900 km depth.

• Ascension and St. Helena merge at about 1000 km depth.

Montelli et al. 2004

Montelli et al. 2004

• The images of Tahiti and Samoa plumes also show low velocity anomalies down to large depth in the mantle (2800 km).

• Cook island merges with Tahiti at about 1450 km depth.

Montelli et al. 2004

Montelli et al. 2004

• The associated anomaly that corresponds with the Hawaii plume is somewhat surprising.

• Expectation: – should go down to the CMB

• but this is not the case

• Because:– Due to the distance of Hawaii from

the circum-Pacific seismicity.

– Seismic rays from the Tonga subduction zone to North American stations pass southeast of the suspected plume location near the CMB

– In this case only a very wide plume would be resolved (radius >300 km).

– In a separate calculation using only low-frequency data (sense a wider region around the ray – Hawaii plume extends down to the CMB

Montelli et al. 2004

Garnero, E. J. (2004): GEOPHYSICS: A New Paradigm for Earth's Core-

Mantle Boundary; Science, Vol. 304. no. 5672, pp. 834

- 836

Rost, S., Garnero, E. J., Williams, Q., & Manga, M. (2005): Seismological

constraints on a possible plume root at the

core−mantle boundary; Nature 435, 666-669

2. Strong Anisotropy regions in the D”-layer below or close to hot spot locations

2. Strong Anisotropy regions in the D”-layer below or close to hot spot locations

• Large T and ρ contrasts

• Discontinuous increase in wave velocity (particular S-waves) along the D”-layer in several region of the earth (Central America) – associated with subduction zones

• In other regions strong reduction of velocity near the top of D” were observed (South Atlantic)

• Magnitude and sign of the velocity discontinuity may act as an information for the degree of partial melting in the D”-layer.

Ritter 1999

• Central America:– the deepest mantle contains a high-velocity D'' reflector– scatters of pockets of ultralow velocities and strong anisotropy.

• Central Pacific (D” underlies surface hotspot volcanism):– D'' anisotropy and ultralow-velocity zones (possible plume genesis)

• South Atlantic and southern Africa:– a large-scale low-velocity structure with sharp edges extends upward into the

lower mantle

2. Strong Anisotropy regions in the D”-layer below or close to hot spot locations

Garnero et al. 2004

2. Strong Anisotropy regions in the D”-layer below or close to hot spot locations

• Observation of strong Anisotropy regions in the D”-layer below or close to hot spot locations.

• Regions with decrease in wave velocity – hotter than those regions with higher velocity

• Those “hot” regions seems to be elevated with respect to the colder regions

Rost et al. 2005

• Discontinuous reflectors

• Vertical boundaries between low velocity structures and neighboring mantle

– Beneath southern Africa features extending up to 1000 km above the CMB

Garnero 2006 IRIS 5-year Proposal

• These features may be related to ultra low velocity zones exhibiting partial melt of the lower mantle.

• These melt bubbles may become instable an could rise upward through the mantle.

Montelli et al. 2004

• Regions of ULVZ material below plumes could be partly molten material and thus the root of plumes originating from the CMB.

• Example central Pacific:– Strong wave velocity

anisotropy at the D” (Fouch et al. 2001)

– strong lateral and depth variation in the D” anisotropy distribution

– Especially beneath Hawaii

Shallow Mantle Plumes

Montelli et al. 2004