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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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slide1

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

slide2

Earth

Venus

Mars

slide3

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

  • 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
slide4

The story…

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?

slide5

“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.
slide6

Heat Out

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?

slide7

l = 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.
slide8

Cold Boundary

1 / (1+l-1/6)

Qi≈ constant in Stag-Lid limit

Isoviscous convection

lh ≈ constant in Stag-Lid limit

Isoviscous convection

Hot Boundary

slide9

lh= 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.

slide10

lh= 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

slide11

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.
slide12

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)
slide13

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).
slide14

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
slide15

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 %)
slide16

(1840-1980)

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…

slide17

Conductive patches and VGP paths during reversals (Costin and Buffett, 2003)

Indicative of physical properties of ULVZ/dense layer?

slide18

VGP paths from observations

Data from Sediment Cores

VGP paths from Costin and Buffett Model*

*Using same spatial sampling

slide19

MORB

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.

slide20

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)
  • 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.
slide21

Brandon et al., 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.
slide22

MORB

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).
slide23

Heat Out

Heat In

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

slide24

Experimental Apparatus

Dense layer experiments

Two additional parameters:

“Viscosity Ratio”

Sabilizing compositional density difference

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

slide25

Control Experiment:No Dense Layer

Stagnant Lid Convection in the form of thermals

Cold Boundary

Shadowgraph Image

Hot Boundary

slide27

Entrainment from a dense layer:

“Free Slip”, Constant-T Lower BC

“No Slip”, Constant-T Lower BC

  • Topography on the layer.
  • Lateral variations in temperature and viscosity.
slide28

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

slide29

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).
slide30

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?
slide31

Topography can stabilize plumes:

Theory and Experiment

U

z

x

d

µc

h

µ

µd

L

µ

µd

Height of topographyh/d

h/d = C

slide32

How high is the topography?

Theory and Experiment

u’

u’

Ud

h

Ud

µ

d

µd

L

Height of topographyh/d

1/B1/2

slide33

Tendril Thickness

Theory and

Experiment

U

z

x

d

µc

h

µ

r

µd

rc

L

Tendril thickness

Ra

slide34

Entrainment and Plume Spacing?

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

slide35

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.
slide36

B-Flux Constraints

MORB

Good

Medium

Poor

Plume Buoyancy Flux

Entrainment from dense layer and composition of source for volcanics:

3He / 4He: A thermophysical parameter?

slide37

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.
slide38

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?
slide40

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
slide41

Side View

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

slide42

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

slide43

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?
slide44

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

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?

slide45

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”

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