The Existence, Longevity and
Composition of Mantle Plumes
and Hotspot Volcanoes
Mark Jellinek
Dept. Earth and
Ocean Sciences
U. British Columbia
Michael Manga
Dept. Earth and
Planetary Science
U. California,
Berkeley
Earth
Venus
Mars
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Hotspots related to (deep mantle)
plumes from CMB
(e.g. Wilson, 1963; Morgan, 1971; 1981; Richards et al., 1989;
Campbell and Griffiths, 1992; Clouard and Bonneville, 2001;
Courtillot et al., 2003)
Duncan and Richards, 1989
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HS island chain w/monotonic age progression
Flood basalt at start (unless subducted)
High melt production rates
Large axisymmetric swell (strong B-flux)
Significant -DVs in underlying mantle
Large DT ; O(100+) viscosity variations
Long term spatial stability
High 3He / 4He
The story…
1.
Earth-like mantle plumes require large
temperature and viscosity variations in TBL
at CMB.
2. Large temperature and viscosity variations
may require strong mantle cooling due to
plate tectonics.
3. Sources for Pacific and African hotspots
involve dense, low viscosity material that is
composed of solid or partially-molten
silicate and outer core material.
4. Interaction between convection due to core
cooling and dense layer is required for longlived spatially stable mantle plumes in the
Earth, consistent with long-lived hotspots.
5. Earth is …. improbable?
“Earth-like” Plumes vs. Thermals
Plumes
• Large O(100) viscosity variations
• Head / Tail structure
• Tails persist >> 1 rise time.
Thermals
• Small O(1) viscosity variations
• Discrete “heads” +/- transient tails
• Tails persist ≤ 1 rise time.
The simplest model of planetary
mantle convection:
Convection in a fluid with T-dependent
viscosity under conditions of thermal
equilibrium
Heat Out
Heat In
Can Earth-like plumes occur?
Stagnant lid convection
weak cooling = small viscosity variations in hot TBL
l = 106
Ra = 106
lh = 4
Simulations by A. Lenardic
Concepts:
• Flow at high-Ra has 3 layers:
• 2 Thin thermal boundary layers of unequal
thickness; well mixed interior
• Cold TBL is a “rheological boundary layer”
• Stagnant lid part
• Active part
Internal T > Taverage, close to Thot
• Small DT to hot boundary = small (order 1) viscosity
variations in hot TBL.
• Earth-like plumes not possible.
Cold Boundary
Hot Boundary
1 / (1+l-1/6)
Qi ≈ constant in Stag-Lid limit
Isoviscous convection
lh ≈ constant in Stag-Lid limit
Isoviscous convection
Role of subduction: stir in stagnant lid
Strong cooling = large viscosity variations
Ra = 106
l = 106
lh = 4
Subduction and Recycling of the lid
lh = 103
2D Numerical Simulation:
Stir in lithosphere, obtain large viscosity ratio
required for plume formation.
Role of subduction: stir in stagnant lid
Strong cooling = large viscosity variations
Ra = 106
l = 106
Ra = 106, l = 106
lh = 103
Imposed stirring of stagnant lid into interior:
Low viscosity upwellings with large heads and
narrow tails
lh ≈
102
Ra = 1.2 x 106, l ≈ 104
Do large viscosity variations
guarantee plume stability and
hotspot longevity?
• Interactions between low viscosity plumes not
consistent with long-term stability at high Ra.
• Large viscosity variations necessary but
insufficient condition for longevity.
Seismic velocity at the base of the
mantle along with (mostly) Pacific
hotspots
Vs model from Ritsema, 2004
• The base of the mantle is laterally heterogeneous.
• Hotspot positions correlate with low velocity material.
(e.g. Williams et al., 1998)
• Low velocity regions shown are buoyant and likely deep
mantle return flows (e.g. Forte and Mitrovica, 2001)
The base of the mantle is laterally
(chemically) heterogeneous
Chemical heterogeneity in lower mantle:
•Vs and Vb anticorrelated
•Acute (i.e. non-diffusive) lateral and vertical seismic
velocity gradients
ULVZ (5 - 40 km thick):
•Vs and Vp reduced 5-10%, 10-30%
• -DVs /-DVp ≈ 3 to 3.5 / 1
•Monotonic increase in Poisson ratio with depth
•African / Pacific hotspots. Not Iceland.
ULVZ composed of dense material
•Joint analysis: normal mode and free air gravity constraints
(Ishii and Tromp, 1999).
Constraints on ULVZ / Dense Layer properties:
Plausibly a mixture of partial melt and OC material
• Seismolgy
6-30% partial melt within TBL (and / or) Metals from
the outer core
• Geodetic studies
Gravitational and electromagnetic coupling at CMB
• Length of Day (e.g. Holme; Zatman; Domberie)
• Gravitationally-forced nutations (e.g. Buffet)
Metallic conductance in thin layer at CMB
• Geomagnetic / Paleomagnetic studies
• Observations of time-averaged radial field in Pacific:
Link to thermal (electrical?) BC at CMB
• Behavior of non-dipole component of radial field
during reversals
•Metallic conductance in thin ULVZ-like patches
• Geodynamic studies:
• Mantle convection models (theoretical, exp., numerical):
Subduction and mantle stirring, entrainment and
longevity of layer, spatial stability of plumes etc.
Dense (2-5%) low viscosity layer beneath deep-mantle
upwellings :
•“Piles” beneath Africa and central Pacific
• Distribution governed by subduction zones
• Geochemical studies
Silicate component of deep mantle plume source
• 3He / 4He in high-Mg OIB lavas?
• others …. ?
Core component (e.g. Walker; Brandon; Humayun)
• Coupled Os isotopic excesses in high-Mg OIB
• Os systematics over large spatial scales
• Fe/Mn in MORB vs high-Mg OIB lavas
Entrainment of ≤ 1% core material
(implies a density increase of a few %)
Structure of time-averaged (non-dipole) radial
field and core-mantle coupling
Indicative of physical properties of ULVZ/dense layer?
(1840-1980)
Bloxham and Jackson, 1992
(0 - 3 kyr) Constable et al., 2000
(0 - 5 Myr) Johnson et al., 2004
3 Observations in Pacific
matter:
• Complicated structure.
Radial field varies with
latitude and longitude.
• Structure persists over
times >> core overturn
• Low radial field and low
secular variation centered
on HI.
Hypothesis derived from
simulation and theory:
Spatial variations in thermal
and/or electrical coupling at
CMB…
Conductive patches and VGP paths during
reversals (Costin and Buffett, 2003)
Indicative of physical properties of ULVZ/dense layer?
VGP paths from observations
Data from Sediment Cores
VGP paths from Costin and Buffett Model*
*Using same spatial sampling
What is ULVZ?
Geochemical characteristics of plume source:
A mix of LM and core material?
I.
Tracer for Silicate component:
High 3He / 4He (“primitive, undegassed” ?) mantle
MORB
Plume Buoyancy Flux
Most hotspots related to deep mantle plumes have
elevated 3He / 4He relative to MORB.
Geochemical characteristics of plume source:
ULVZ a mix of LM and core material?
II. Core component: Siderophile elements
Hawaii, Siberia, Galapagos, S. Africa, NOT Iceland
Two Observations related to Re-Os systematics
(Walker, Brandon and coauthors)
• Coupled 187Os / 188Os, 186Os / 188Os excesses in lavas associated
with Hawaiian, Siberian and Galapagos plumes consistent with
presence/ entrainment of 0.8-1.2% outer core material.
Geochemical characteristics of plume source:
ULVZ a mix of LM and core material?
II. Core component: Siderophile elements
Hawaii, Siberia, Galapagos, S. Africa, NOT Iceland
H,S
G
Brandon et al., 2003
Two Observations related to Re-Os systematics
(Walker, Brandon and coauthors)
• Coupled 187Os / 188Os, 186Os / 188Os excesses in lavas associated
with Hawaiian, Siberian and Galapagos plumes consistent with
presence/ entrainment of 0.8-1.2% outer core material.
• Intersection/convergence: One interpretation is that each linear
array reflects mixing between two distinct Os isotopic
components where a common radiogenic isotopic component is
present in the sources of all of these materials.
• Identical systematics in Siberia, Hawaii and Gorgona
(Galapagos origin?) require a spatially extensive reservoir
consistent with a large, well-mixed outer core.
Tracers for silicate/outer core mixture in
source for hotspots overlying ULVZ?
MORB
Plume
Modified from Brandon et al., 1999
Plume Source
MORB source
DePaolo et al., 2002
• Linear mixing of outer core and LM silicate consistent
with data from Hawaii.
• N.B.: No obvious correlation exists for Icelandic lavas.
No evidence of core material identified (also no ULVZ
sightings).
How does a dense, low viscosity layer
influence convection from the hot
boundary?
Heat Out
Heat In
Experimental Apparatus
Dense layer experiments
Two additional parameters:
“Viscosity Ratio”
Sabilizing compositional
density difference
Note: free-slip and no-slip bottom boundaries studied
Control Experiment:No Dense Layer
Stagnant Lid Convection in the form of
thermals
Cold Boundary
Hot Boundary
Shadowgraph Image
Entrainment from a dense layer:
“Free Slip”, Constant-T Lower BC
“No Slip”, Constant-T Lower BC
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•
Topography on the layer.
Lateral variations in temperature and viscosity.
Entrainment of dense, low viscosity fluid
leads to formation of long-lived conduits
“Free Slip”, Constant-T Lower BC
“No Slip”, Constant-T Lower BC
Thermal
Coupling:
• Initial decline in w following input of dense fluid :
• Fewer new plumes form for the same heat flux.
• w governed by convection in dense layer
• Steady flow into conduits ultimately established (w = 0).
Theoretical Scaling Analyses
Goals:
- Condition for long-term stability of plumes.
- Topography on dense layer.
- Entrainment from dense layer.
Applications to Earth (and other planets):
-Long-lived mantle plumes?
- Bumps on ULVZ material? New way to look for
plumes seismically?
-Understand composition of hotspot lavas in
terms of mechanics governing formation of
plumes?
Topography can stabilize plumes:
Theory and Experiment
U
z
x
L
Height of topography
h/d
µ
µd
h/d = C
h
d
µc
µ
µd
How high is the topography?
Theory and Experiment
u’
u’
Ud
µ
µd
h
Ud
d
Height of topography
h/d
L
1/B1/2
Tendril Thickness
U
Theory and
Experiment
z
x
Tendril thickness
L
Ra
h
d
µc
µ
µd
r
rc
Entrainment and Plume Spacing?
Spacing between conduits approximately fixed
Hypothesis: Spacing set by 1st R-T instability to TBL
 l 1 3
L    , C  CB, l , l d 
 Ra 

Applications to Earth:
106 < Rabot < 108
1< B < 2
Longevity
• h/d > 0.7; topography comparable to
TBL thickness
• Plumes expected to be stable for
life of dense layer
Topography on dense layer
• order 40 - 200 km; comparable to
observed 5 - 40 km.
Entrainment
• Low viscosity material enhances
structure due to large DT.
• Influence composition.
Entrainment from dense layer and
composition of source for volcanics:
3He
/ 4He: A thermophysical parameter?
B-Flux Constraints
MORB
Good
Medium
Poor
Plume Buoyancy Flux
Large Temperature differnces:
• Subduction and stirring of lithosphere
Large viscosity variations: Earth-like plumes
• Subduction and stirring of lithosphere
• Entrainment from dense, low viscosity layer
(ULVZ?).
Long-lived plumes and hotspots
• Topography on dense layer comparable to TBL.
Composition of hotspot lavas
• Entrainment from dense layer explains average
composition of melt source.
Moving Forward:
Effect of mantle stirring on longevity and
composition of mantle plumes and hotspots?
Farnetani et al., 2002
Kerr and Meriaux, 2005
Outstanding questions
• How will large-scale mantle flow affect the dynamics of
plume formation in the presence of a dense, low viscosity
layer?
•Low viscosity outer core: Expect negligible shear
stresses at CMB -- patches expected to be a slave to
subduction.
• How will mantle shear influence the dynamics, rise and
composition of plume conduits?
•Azimuthal stirring within the conduit important?
• Thermal entrainment?
• How will plate motions influence the spreading and
composition of plume material ponded beneath the
lithosphere?
Hualalai
Abouchami et al., Nature, 2005
Weis, unpublished
Internal chemical variation in plume conduits and
hotspots (Kerr and Meriaux, 2005):
What matters:
• Shear by mantle flow (cf. Richards and Griffiths, 1988;
1989): ratio of velocity of horizontal mantle motions to
centerline plume rise velocity.
• Viscosity variations across plume conduit.
• RaQ , Aspect ratio of conduit.
• Density and viscosity of tracer ???
Further implications of this work:
• Spreading of plume material beneath lithosphere
• Chemical variations within spreading plume material (e.g
Farnetani and Samuel, 2004)
• (New) Dynamics of plume rise in the presence of both
shear and a lithosphere: Implications for hotspot tracks
predicted from global models and internal chem. variation:
•Thermal entrainment important
• Drag on the lithosphere important
Some results (K&M, 2005):
Side View
Top View
Increasing Shear
Velocity Ratio = 2.05
Velocity Ratio = 0.85
Velocity Ratio = 0.35
Ra = 2.4E6 , Viscosity Ratio = 56
Some Implications:
Azimuthal and/or radial
chemical variations among
hotspot volcanoes:
• Relate length scale of
variation to buoyancy flux
• Diagnostic of structure
and composition of plume
source.
Hotspot tracks and the
dynamics of plume conduits in a
Convecting Mantle … more to
do on this problem
Steinberger et al., 2004
Dense layer at CMB:
Mixture of melt and core material?
Garnero
Constitution and transport properties
• Physical properties of melt phase (ab initio Stixrude,
in progress)
• Distribution of melt across TBL
• Transport properties of outer core material in
silicate melt vs. solid phases?
• Physical and electrical properties of mixture?
•Connectivity of core material in matrix?
• “Robust” geochemical tracers for core component?
Physical processes within dense layer:
• Compaction?
• Internal Convection?
• Thermal, mechanical and electromagnetic coupling to
core and mantle?
Geomag observations and geodynamic models
Is there a direct relationship between patches
of dense layer and the spatial and temporal
structure of the radial geomagnetic field
observed in the central Pacific and Africa?
(0 - 5 Myr) Johnson et al., 2004
Can the structure and secular variation of the
time-averaged field constrain the geometry
and physical properties of such patches as well
as their influence on core cooling and the
geodynamo?
Concluding Remark
Long-lived mantle plumes and hotspots are likely
a direct consequence of the interactions
between plate tectonics, core cooling and dense
low viscosity material within D”