The Case for Mantle Plumes
Jon Hronsky
GSA – WA Division Meeting
Feb 5 2008
1
Outline
1. Definition of mantle plumes
2. Seven lines of evidence for mantle plumes
3. Why some people are confused about Mantle
Plumes
2
Definition of Mantle Plumes
•
•
•
The definition of a Mantle Plume that is argued for
here is:
– An active upwelling of buoyant, hot mantle that is
sourced from deep in the mantle and impinges on
the lithosphere with significant geological impact
This is a “sensu-lato” definition and contrasts with
the “sensu-stricto” definition proposed in the past
from classical fluid dynamic concepts
“Plumes” are thermo-chemical entities; they are
probably compositional as well as thermal anomalies
3
The Classical Plume Model:
An Oversimplification
plume head
plateau
older plume
hotspot track
subduction
zone
passive
up flow
MOR
new plume
rising
continental
lithosphere
oceanic
lithosphere
670 seismic
discontinuity
increasing
viscosity
After Geoff Davies (1999)
thermal boundary layer
Core-mantle
boundary
The new view from Global Seismic Tomography
(Li & Romanowicz, 1996)
MORs
EPR= East Pacific
MAR= Mid Atlantic
CBR=Carlsberg
Plates
EA=Eurasian Plate
IN=Indian
PA=Pacific
NA=Nth American
SA= Sth American
AF=African
CO=Cocos
The new view of Plumes
•
Major active mantle upwellings (eg under East Africa) occur on a
much broader scale than envisaged for classical plumes, and at a
global scale seem to be a response to major zones of down-going
slabs; these are commonly referred to as super plumes but are not
plumes in a classic fluid dynamical context
•
Narrow pipes of hot material, consistent with the classic plume
model, do occur (Hawaii and Iceland) but are relatively uncommon
•
Also now recognised that smaller-scale active upwellings may
result from small-scale convective instabilities in upper mantle
and slab delamination
Evidence for Plumes
1. We can actually see them in Seismic Tomography
2. They are the only reasonable way of making Large
Igneous Provinces in anorogenic, intercontinental
settings
3. They are predicted because we have a major
thermal boundary layer at the CMB
4. True Primary Plumes are near stationary relative to
each other
5. The surface position of Plumes correlates well with
seismically-anomalous areas of the CMB
6. We can demonstrate distinctive (ie not DMM) mantle
source chemistry for inferred Plume-related rocks
7. Local examples exist where mantle source
heterogeneities are more important than degree of
extension for melt production
7
Hawaii Plume
Source: Dietmar Muller
Source: Montelli (2004)
8
Some more examples….
Source: Guust Nolet, Princeton Uni Website
9
…and some more
Source: Guust Nolet, Princeton Uni Website
10
Large Igneous Provinces
• Plumes of anomalously hot, upwelling mantle
are the only reasonable way of explaining the
very large size and emplacement rate of Large
Igneous provinces
– In many cases, they are emplaced into
environments undergoing no extension at the
time
– There is no petrological evidence for anomalously
hydrous mantle sources (a possible alternative to
a hot plume is a wet plume) for LIP basalts
compared to MORB basalts
11
Some Recent Continental LIPs
(Condie, 1999)
There must be some fluid dynamical
consequence of the Thermal boundary layer at
the CMB
Lay et al (2008)
13
Lack of Inter-Plume Movement
• Courtillot et al (2003) demonstrate that Primary
Plumes show movement relative to each other
of < 0.5 cm per annum
• This is about an order of magnitude less than
typical plate velocities
• Importantly, their population of Primary Plumes
is independently established by the application
of a series of rigorous tests to discriminate
hotspots associated with primary deep-seated
plumes from those associated with more
shallow mantle features
14
The relationship of Plumes
to the Structure of the CMB
Cold, subduction-related
Down welling belt
Centre of
Superswell
Primary
Plume
Tomographic map of shear wave velocity at 2850km depth (ie CMB). Fast (ie cold)
wave speed anomalies in blue, slow (ie hot) wave-speed anomalies in white.
Green and red dots are possible primary plumes. Source: Courtillot et al (2003)
The Chemical Evidence
• Inferred Plume-derived rocks (OIBs, CFBs)
show chemical evidence for a source region
distinct to the upper mantle source for
MORBs (commonly referred to as DMM)
– Distinctive He4/He3 ratios
– Derivation from the FOZO source
16
Sr-Nd isotopes of oceanic basalts show mixing/unmixing arrays from FOZO.
0.5134
DMM
Pacific MORB
Atlantic MORB
Indian MORB
HIMU OIB
EM1 OIB
EM2 OIB
other OIB
143
Nd/144Nd
"FOZO"
0.5130
HIMU
0.5126
EM2
EM1
0.5122
0.702
0.704
0.706
87
Zhang et al (2008)
Sr/86Sr
0.708
OIB field
Zhang et al (2008)
Localized Mantle-Source Heterogeneity trumps
Degree of Extension in Generating Melt
42
O
Azores
Archipelago
Ridge
O
Bathymetry
40
Latitude
Corvo
Flores
Atlantic
<2000m
Grasiosa
Mid -
2000 - 3000m
~
Sao
Jorge
Faial
Terceira
>3000m
Pico
O
38
~
Sao
Miguel
Schaefer et al (2002)
Note:
Concentration of Basalt
magmatism off
ridge axis
36
Santa Mana
East Azores Fracture Zone
O
O
-32
Schaefer et al (2002)
O
-30
O
-28
Longitude
O
-26
O
-24
Azores Archipelago:
Average isotopic ratios for each island
on NW-SE transect across Plume – most anomalous
compositions correlate with greatest basalt production
Schaefer et al (2002)
Schaefer et al (2002)
Flores
Flores
~Sao
~Sao
Pico Terceira
Pico Terceira
Faial
Faial
Chrondritic Os
0.14
0.13
0.12
0.11
0.7050
Chrondritic Sr
0.7045
0.7040
0.7035
O
-32
O
-30
-28
O
Longitude
O
-26
O
-24
Why some people are confused
about Plumes
• There are two important aspects associated
with plumes that have been confusing to
some workers and been an important factor
in leading to the erroneous “anti-plume”
hypotheses of workers such as Don
Anderson:
– The relationship between upwelling
plumes and lithospheric architecture
– All “hot spots” are not plumes
21
EBINGER AND SLEEP (1998) MODEL FOR THE INTERACTION
OF A MANTLE PLUME WITH TOPOGRAPHY
ON THE BASE OF THE LITHOSPHERE
Lava flows
Ocean
Continental
lithosphere
Plume head
Mantle
Plume
Hot thermal
Core
August 1999
boundary layer
Zone of
melting
A REAL EXAMPLE OF PLUME FOCUSSING
AT A CRATON MARGIN
Figure A: west-east profile through S-wave velocity model from Ritsema et-al. (1998).
Uncertainties in horizontal and vertical dimensions of velocity structures are ~50 and 100km,
respectively. Velocity structure above 100km and below 500km is poorly resolved and therefore is
not shown.
(Nyblade et al., 1996) Geology
JMAH-05
Figure C: Schematic cross section at ~4.5 S showing plume head beneath eastern
margin of Tanzania craton. Question marks beneath eastern and western rifts and at
bottom of plume head Indicate that structures illustrated there are poorly determined.
(Nyblade et al., 1996) Geology
JMAH-07
Three types of Plumes/Hot Spots:
The Model of Courtillot et al (2003)
Primary Plume
Secondary
Plume
Superswell
Tertiary Hotspot