The Existence, Longevity and Composition of Mantle Plumes and Hotspot Volcanoes
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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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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 and Hotspot Volcanoes

Venus

Mars


Hotspots related to (deep mantle) plumes from CMB and Hotspot Volcanoes(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

  • 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… and Hotspot Volcanoes

Earth-like mantle plumes require large temperature and viscosity variations in TBL at CMB.

Large temperature and viscosity variations may require strong mantle cooling due to plate tectonics.

Sources for Pacific and African hotspots involve dense, low viscosity material that is composed of solid or partially-molten silicate and outer core material.

Interaction between convection due to core cooling and dense layer is required for long-lived spatially stable mantle plumes in the Earth, consistent with long-lived hotspots.

Earth is …. improbable?


“Earth-like” Plumes vs. Thermals and Hotspot Volcanoes

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.


Heat Out and Hotspot Volcanoes

Heat In

The simplest model of planetary mantle convection:Convection in a fluid with T-dependent viscosity under conditions of thermal equilibrium

Can Earth-like plumes occur?


l and Hotspot Volcanoes = 106

Ra = 106

lh= 4

Simulations by A. Lenardic

Stagnant lid convection

weak cooling = small viscosity variations in hot TBL

  • 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 and Hotspot Volcanoes

1 / (1+l-1/6)

Qi≈ constant in Stag-Lid limit

Isoviscous convection

lh ≈ constant in Stag-Lid limit

Isoviscous convection

Hot Boundary


l and Hotspot Volcanoesh= 4

lh= 103

Role of subduction: stir in stagnant lid

Strong cooling = large viscosity variations

Ra = 106l = 106

Subduction and Recycling of the lid

2D Numerical Simulation:

Stir in lithosphere, obtain large viscosity ratio required for plume formation.


l and Hotspot Volcanoesh= 103

Role of subduction: stir in stagnant lid

Strong cooling = large viscosity variations

Ra = 106l = 106

Ra = 106, l = 106

Imposed stirring of stagnant lid into interior: Low viscosity upwellings with large heads and narrow tails

Ra = 1.2 x 106, l ≈ 104

lh≈ 102


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

Vsmodel 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: heterogeneous

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: heterogeneous

    • 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 %)


(1840-1980) heterogeneous

Bloxham and Jackson, 1992

(0 - 3 kyr) Constable et al., 2000

(0 - 5 Myr) Johnson et al., 2004

Structure of time-averaged (non-dipole) radial field and core-mantle coupling

Indicative of physical properties of ULVZ/dense layer?

  • 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 and Buffett, 2003)

Data from Sediment Cores

VGP paths from Costin and Buffett Model*

*Using same spatial sampling


MORB and Buffett, 2003)

Plume Buoyancy Flux

What is ULVZ?

Geochemical characteristics of plume source: A mix of LM and core material?

Tracer for Silicate component:

High 3He / 4He (“primitive, undegassed” ?) mantle

Most hotspots related to deep mantle plumes have elevated 3He / 4He relative to MORB.


Geochemical characteristics of plume source: and Buffett, 2003)

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)

  • Coupled187Os / 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.


Brandon et al., 2003 and Buffett, 2003)

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

  • Two Observations related to Re-Os systematics(Walker, Brandon and coauthors)

  • Coupled187Os / 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.


MORB and Buffett, 2003)

Plume

Modified from Brandon et al., 1999

Plume Source

MORB source

DePaolo et al., 2002

Tracers for silicate/outer core mixture in source for hotspots overlying ULVZ?

  • 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).


Heat Out and Buffett, 2003)

Heat In

How does a dense, low viscosity layer influence convection from the hot boundary?


Experimental Apparatus and Buffett, 2003)

Dense layer experiments

Two additional parameters:

“Viscosity Ratio”

Sabilizing compositional density difference

Note: free-slip and no-slip bottom boundaries studied


Control Experiment: and Buffett, 2003)No Dense Layer

Stagnant Lid Convection in the form of thermals

Cold Boundary

Shadowgraph Image

Hot Boundary


Entrainment from a dense layer: and Buffett, 2003)

“Free Slip”, Constant-T Lower BC

“No Slip”, Constant-T Lower BC

  • 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: of long-lived conduits

  • 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 of long-lived conduits

  • 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: of long-lived conduits

Theory and Experiment

U

z

x

d

µc

h

µ

µd

L

µ

µd

Height of topographyh/d

h/d = C


How high is the topography? of long-lived conduits

Theory and Experiment

u’

u’

Ud

h

Ud

µ

d

µd

L

Height of topographyh/d

1/B1/2


Tendril Thickness of long-lived conduits

Theory and

Experiment

U

z

x

d

µc

h

µ

r

µd

rc

L

Tendril thickness

Ra


Entrainment and Plume Spacing? of long-lived conduits

Spacing between conduits approximately fixed Hypothesis: Spacing set by 1st R-T instability to TBL


  • Applications to Earth: of long-lived conduits

  • 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.


B-Flux Constraints of long-lived conduits

MORB

Good

Medium

Poor

Plume Buoyancy Flux

Entrainment from dense layer and composition of source for volcanics:

3He / 4He: A thermophysical parameter?


  • Large Temperature differnces: of long-lived conduits

  • 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: of long-lived conduits

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?


Abouchami et al., Nature, 2005 of long-lived conduits

Weis, unpublished


Internal chemical variation in plume conduits and hotspots of long-lived conduits (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


Side View of long-lived conduits

Top View

Ra = 2.4E6 , Viscosity Ratio = 56

Some results (K&M, 2005):

Velocity Ratio = 2.05

Increasing Shear

Velocity Ratio = 0.85

Velocity Ratio = 0.35


  • Some Implications: of long-lived conduits

  • 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: of long-lived conduits

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?


(0 - 5 Myr) Johnson et al., 2004 of long-lived conduits

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?

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 of long-lived conduits

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”


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