Possible reservoirs of radioactivity in the deep mantle
Possible reservoirs of radioactivity in the deep mantle
Possible reservoirs of radioactivity in the deep mantle
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<strong>Possible</strong> <strong>reservoirs</strong> <strong>of</strong> <strong>radioactivity</strong><br />
<strong>in</strong> <strong>the</strong> <strong>deep</strong> <strong>mantle</strong><br />
Ed Garnero<br />
School <strong>of</strong> Earth and Space Exploration<br />
Arizona State University
Outl<strong>in</strong>e<br />
Brief overview: motivation for <strong>in</strong>vestigat<strong>in</strong>g<br />
<strong>in</strong>teriors; how seismology helps<br />
Large Low Shear Velocity Prov<strong>in</strong>ces (LLSVPs) at<br />
base <strong>of</strong> <strong>mantle</strong>: large-to-small scales<br />
Dynamical framework: how to build and ma<strong>in</strong>ta<strong>in</strong><br />
an LLSVP, and chemically sample it via plumes<br />
LLSVPs as <strong>reservoirs</strong>
Mercury<br />
The ma<strong>in</strong> feature <strong>of</strong> planetary <strong>in</strong>teriors:<br />
stratification<br />
Europa<br />
Mars<br />
Earth<br />
Venus
Convergence DEATH <strong>of</strong> plates Divergence BIRTH<br />
<strong>of</strong> plates<br />
http://www.ngdc.noaa.gov/mgg/image/relief_slides1.html
To expla<strong>in</strong> surface observables: many possibilities<br />
[Alberede, Anderson, Davies, Hager, Jell<strong>in</strong>ek, Kellogg, Manga, Tackley, van der Hilst, Wysession, Geo101 books…]
Internal structure and circulation play a fundamental<br />
role <strong>in</strong> <strong>the</strong> nature and evolution <strong>of</strong> many phenomena<br />
Plate tectonic driv<strong>in</strong>g force<br />
Plumes & result<strong>in</strong>g volcanism<br />
Subduction and <strong>the</strong> fate <strong>of</strong> slabs<br />
Plate reorganization, super-cont<strong>in</strong>ents<br />
Planetary heat budget/transfer<br />
Core processes & magnetic field<br />
Earth’s chemical & water cycles<br />
Mantle outgass<strong>in</strong>g ocean & atmosphere chem.<br />
Mantle <strong>reservoirs</strong>
Tarbuck & Lutgens (Intro geology text)<br />
Study<strong>in</strong>g planetary <strong>in</strong>teriors<br />
Gravity / geoid<br />
Past / present magnetic field<br />
Chemistry <strong>of</strong> lavas, crust, etc.<br />
Current/past plate motions<br />
Seismic imag<strong>in</strong>g<br />
Geodynamic experiments<br />
High pressure m<strong>in</strong>eral physics<br />
Cosmochemistry / meteoritics<br />
Tectonics / geodesy<br />
Magnetotellurics<br />
Geoneutr<strong>in</strong>os
Earth <strong>in</strong> 1-D: The Prelim<strong>in</strong>ary Reference Earth Model (PREM)<br />
Nearly all seismic results are determ<strong>in</strong>ed & presented<br />
relative to an assumed reference structure<br />
Dziewonski and Anderson [PEPI, 1981]
Amplitude<br />
a o<br />
t o<br />
Time
Amplitude<br />
a’<br />
a o<br />
t o<br />
t’<br />
Time
Amplitude<br />
New arrival!<br />
Time
Amplitude<br />
New arrival!<br />
Time
Amplitude<br />
New arrival!<br />
Time
Mike Thorne (Univ. Utah)
Reference<br />
model<br />
ray paths<br />
syn<strong>the</strong>tic<br />
waveforms<br />
• Wave travel times<br />
• Path geometry<br />
• Wave shapes<br />
• Frequency<br />
Everyth<strong>in</strong>g we do depends on our reliance on <strong>the</strong> start<strong>in</strong>g model
Raw data
Distance-stacked data
earthquakes<br />
(sources <strong>of</strong> energy)<br />
sensors<br />
http://web.mst.edu/~sgao/g51/plots/0916_global_seismicity.jpg<br />
http://www.iris.edu
Earth <strong>in</strong> 3-D: Heterogeneity from seismic tomography<br />
DEPTH<br />
Z=50 km<br />
Z=1500 km<br />
Z=2880 km<br />
HETEROGENEITY<br />
stongest<br />
weakest<br />
strong aga<strong>in</strong><br />
[after Ritsema and van Heijst, 2001]
Base <strong>of</strong> Earth’s <strong>mantle</strong><br />
“Large Low Shear Velocity Prov<strong>in</strong>ces”: LLSVPs<br />
Model <strong>of</strong> Jeroen Ritsema
LLSVP marg<strong>in</strong>s<br />
Strongest<br />
lateral<br />
gradients are<br />
at LLSVP edges
Thorne, Garnero, Grand, PEPI., 2004<br />
red: lowest velocities for S20RTS<br />
green: strongest lateral V S gradients
LLSVP<br />
0<br />
0<br />
0<br />
0<br />
0<br />
0<br />
770<br />
820<br />
780<br />
Seismic waves that<br />
propagate near LLSVP<br />
edges are broadened,<br />
sometimes multipulsed<br />
Raypath PREM-predicted<br />
bottom<strong>in</strong>g depth<br />
Zhao et al (2013 <strong>in</strong> prep)
Wave multi-path<strong>in</strong>g and broaden<strong>in</strong>g from<br />
refraction along heterogeneities<br />
Courtesy <strong>of</strong> Chunpeng Zhao
Zhao et al (2013 <strong>in</strong> prep)
Deep <strong>mantle</strong> heterogeneity<br />
Forward<br />
model<strong>in</strong>g<br />
studies f<strong>in</strong>d<br />
“edges” at<br />
LLSVP<br />
boundaries
Large low shear velocity prov<strong>in</strong>ces:<br />
DENSE<br />
Trampert, Deschaps, Resovsky, Yuen (2004)<br />
Also, Ishii & Tromp (2004)
Dense material (red) <strong>in</strong> <strong>the</strong>rmochemical convection calculation<br />
<strong>the</strong>rmochemical convection: <strong>in</strong>itially uniform dense layer at<br />
base <strong>of</strong> <strong>mantle</strong><br />
surface boundary condition: 119 My past plate motions<br />
downwell<strong>in</strong>gs “sweep” dense material <strong>in</strong>to piles<br />
McNamara & Zhong (2005, Nature)
Dense material (red) <strong>in</strong> <strong>the</strong>rmochemical convection calculation<br />
High (blue) and low (red) seismic shear velocity<br />
Strik<strong>in</strong>g agreement<br />
McNamara & Zhong (2005, Nature)<br />
Grand (2002, Phil. Trans. R. Soc. Lond.)
Numerical<br />
geodynamics<br />
Seismic<br />
tomography<br />
Plumes from pile ridges<br />
Garnero, Lay, McNamara (2007, GSA Monograph on Superplumes)
Prediction: plumes from <strong>the</strong>rmochemical pile marg<strong>in</strong>s<br />
Tan, Leng, Zhong, Gurnis (2011, G^3)
Observation: hotspots & reconstructed LIPs from LLSVP marg<strong>in</strong>s<br />
Hotspots<br />
Thorne, Garnero, Grand [2004, PEPI]<br />
dVs: Grand [Phil. Trans. R. Soc., 2002]<br />
Hotspots: Sleep [JGR, 1981]<br />
Also, Wen [2006, EPSL]<br />
Large Igneous Prov<strong>in</strong>ces<br />
Torsvik et al. [Nature 2010]<br />
Torsvik et al [EPSL 2008]<br />
Burke, Ste<strong>in</strong>bergerb, Torsvik, Smethurst [EPSL, 2007]<br />
Ste<strong>in</strong>berger & Torsvik (2012, G^3)
Seismic<br />
Tomography<br />
Geodynamics<br />
calculation<br />
D<br />
dVs<br />
(dVs)<br />
T
Thermochemical pile <strong>in</strong>ternal temperatures<br />
Garnero, Lay, and McNamara. [P4 Monograph, GSA, 2007]
Ultra-Low Velocity Zone (ULVZ)<br />
LARGE seismic velocity<br />
reductions<br />
(up to 10’s %)<br />
THIN layer<strong>in</strong>g<br />
(usually 5-20 km)<br />
AT <strong>the</strong> CMB
Composition<br />
Temperature<br />
BLACK<br />
(<strong>mantle</strong>)<br />
PINK<br />
(LLSVP)<br />
RED<br />
(ULVZ)<br />
BLUE<br />
(cold)<br />
RED<br />
(hot)<br />
BLACK<br />
(hottest)<br />
Whole <strong>mantle</strong> convection, 3 dist<strong>in</strong>ct compositions (<strong>mantle</strong>,<br />
LLSVP, ULVZ), 4 billion years<br />
McNamara et al (2010)
are chemically dist<strong>in</strong>ct<br />
are shaped by subduction downwell<strong>in</strong>gs<br />
may act as <strong>deep</strong> <strong>mantle</strong> chemical<br />
<strong>reservoirs</strong><br />
guide plumes<br />
Not “hidden”…<br />
Key po<strong>in</strong>ts:<br />
large-low shear velocity prov<strong>in</strong>ces<br />
Matt: Collision modelers tell me a dense basal layer can survive moon form<strong>in</strong>g impact<br />
Zhao, Garnero, McNamara (2013, <strong>in</strong> prep)
LLSVP orig<strong>in</strong><br />
• Primordial (early differentiation)<br />
• Basal magma ocean (BMO)<br />
• Build through time (MORB, CMB rxn products)<br />
Labrosse et al. (2007)<br />
Christensen and H<strong>of</strong>mann (1994)
Subduct<strong>in</strong>g MORB<br />
Li and McNamara (2013x)
Subduct<strong>in</strong>g MORB<br />
Key po<strong>in</strong>t:<br />
MORB does not likely form piles, but small amounts<br />
can go <strong>in</strong>, mix, & even come back out <strong>in</strong> plumes<br />
• Material can accumulate beneath plume roots, <strong>the</strong>n become entra<strong>in</strong>ed <strong>in</strong>to pile.<br />
• May be a source <strong>of</strong> observed LLSVP heterogeneity.<br />
• But most MORB is entra<strong>in</strong>ed <strong>in</strong> <strong>mantle</strong> circulation currents.<br />
Li and McNamara (2012, <strong>in</strong> prep)
Hotspots & 3 He/ 4 He ratios<br />
Is entra<strong>in</strong>ment <strong>of</strong> pile material by lower <strong>mantle</strong> plumes<br />
capable <strong>of</strong> produc<strong>in</strong>g both low (MORB-like) and high<br />
(Hotspot-like) 3He/ 4He ratios?<br />
Compilation from USGS Noble Gas database: http://pubs.usgs.gov/ds/2006/202/<br />
Hotspot 3 He/ 4 He ratios are variable <strong>in</strong> time and space<br />
Several Hotspots show MORB-like 3 He/ 4 He ratios<br />
Courtesy Curtis Williams / Allen McNamara
Background material<br />
Pile material<br />
Surface<br />
CMB<br />
Courtesy Curtis Williams / Allen McNamara
3 He/ 4 He (R/RA)<br />
45<br />
40<br />
35<br />
30<br />
25<br />
20<br />
15<br />
10<br />
5<br />
Key po<strong>in</strong>t:<br />
Plumes can irregularly entra<strong>in</strong> pile material,<br />
from zero to maximum<br />
Predicted 3 He/ 4 He ratios <strong>of</strong> a s<strong>in</strong>gle plume<br />
0 100 200 300 400 500 600 700<br />
Time (Myrs)<br />
MORB-like<br />
Courtesy Curtis Williams / Allen McNamara
Some vocabulary review<br />
D” depth shell: lowest 200-300 km <strong>of</strong> <strong>mantle</strong><br />
LLSVP name for <strong>the</strong> distribution <strong>of</strong> ~deg 2 low shear<br />
velocity <strong>in</strong> lowest 100’s km <strong>of</strong> <strong>mantle</strong><br />
Pile geodynamically <strong>in</strong>spired <strong>in</strong>terpretation <strong>of</strong> LLSVP.<br />
Dense. Stable. Usually primordial <strong>in</strong> calculations.<br />
Sometimes “<strong>the</strong>rmochemical pile”<br />
Superplume An <strong>in</strong>terpretation <strong>of</strong> an LLSVP. Sort <strong>of</strong><br />
slang (term used without any <strong>in</strong>tention <strong>of</strong> <strong>the</strong><br />
consequences <strong>of</strong> such a model)
Note on “superplume”<br />
If <strong>the</strong>rmochemical piles leave <strong>the</strong> CMB <strong>in</strong> superplume events,<br />
geodynamic predictions suggest it will be mixed <strong>in</strong>to <strong>the</strong> <strong>mantle</strong><br />
over time (i.e., and not settle back at CMB)<br />
Implication: today’s large <strong>the</strong>rmochemical piles: not superplume<br />
Implication: today’s piles are long-lived (regardless <strong>of</strong> orig<strong>in</strong>)
• Primitive<br />
• Primordial<br />
• Recycled<br />
• BSE<br />
• Isotopic ratio key<br />
Geochemists:
surface<br />
CMB<br />
<strong>mantle</strong><br />
ULVZ ULVZ<br />
outer ? core<br />
LLSVP<br />
reservoir
Dye, S. T. (2012). "Geoneutr<strong>in</strong>os and <strong>the</strong> radioactive power <strong>of</strong> <strong>the</strong> Earth". Rev. Geophys. 50 (3).
surface<br />
CMB<br />
<strong>mantle</strong><br />
ULVZ n e<br />
ULVZ<br />
40 K<br />
232 Th<br />
235,238 U<br />
?<br />
outer ? core<br />
LLSVP<br />
reservoir
surface<br />
CMB<br />
Ocean island basalts should show more evidence<br />
for radiogenic isotopes than mid-ocean ridges<br />
<strong>mantle</strong><br />
ULVZ ULVZ<br />
40 K<br />
232 Th<br />
235,238U<br />
?<br />
outer ? core<br />
LLSVP<br />
reservoir
Tomographically derived LLSVPs (red)<br />
Caribbean<br />
anomaly<br />
Pacific<br />
anomaly<br />
Chemically dist<strong>in</strong>ct,<br />
dense, long-lived,<br />
shaped by<br />
subduction-related<br />
downwell<strong>in</strong>g, guide<br />
plumes.<br />
core<br />
South pole<br />
<strong>mantle</strong><br />
African<br />
anomaly<br />
Long-lived <strong>reservoirs</strong> <strong>of</strong><br />
radiogenic materials?
Acknowledgements:<br />
Chunpeng Zhao<br />
Allen McNamara<br />
M<strong>in</strong>gm<strong>in</strong>g Li
EXTRA
Thermochemical pile temporal evolution<br />
deg 2 deg 1 associated with super cont<strong>in</strong>ent construction/break up cycles?<br />
Li & Zhong (EPSL, 2009)
If S waves graze a low velocity <strong>the</strong>rmochemical pile,<br />
what do we expect?<br />
Temperature field for<br />
<strong>the</strong>rmochemical piles<br />
Shear velocity heterogeneity<br />
from seismic tomography<br />
McNamara & Zhong [Nature, 2005] Ritsema [Sci. Tech, 2000]
Garnero, Kennett, Loper [PEPI, 2005]
http://garnero.asu.edu
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