Transcript Lecture 12: Surface Processes I: chemical and physical

Lecture 18: Chemical Geodynamics,
or Mantle Blobology
• Questions
– What can geochemistry tell us about the deep interior of
the Earth?
– Is the mantle homogeneous and if not how many
reservoirs are there? How long have they maintained
their separate identities?
– How do we use radiogenic isotope ratios and trace
element ratios in basalts to make such inferences about
the mantle?
• Reading
– Albarède, Chapter 8
1
Summary of Earth Differentiation
(nucleosynthesis, mixing)
Solar Nebula
(volatiles)
(gas-solid equilibria)
(refractories)
(late veneer)
(siderophile &
chalcophile)
Condensation and Accretion
(melting; gravity and geochemical affinity)
(lithophile)
Core
Silicate Earth
(atmophile)
Primitive Atmosphere
(freezing)
Inner
Core
(continuing
cometary
flux?)
Primitive Mantle
Outer
Core
Lower Mantle
(hotspot plumes)
(catastrophic
impact)
(partial melting;
liquid-crystal partitioning)
(?)
(lost due to
impacts)
Upper Mantle
Moon
degassing
Continental Crust
(plate tectonics: partial
melting, recycling)
Oceanic Crust
degassing
Modern Ocean &
Atmosphere
2
Geochemistry and Geodynamics
• A range of models have been proposed…
3
Geophysics
Geochemistry and Geodynamics
• Our only data about the history of
the Earth’s structure is derived
Time Then
Now
from geochemical inference,
Surface
Geochemistry
because geophysics only samples
the present (exception: paleomag)
• However, geochemistry only
samples the surface, so inferences
Depth
about depths within the Earth are
indirect, and must be supplemented
by geological or geophysical
constraints.
Interior
• In some cases, mantle samples are
directly available as xenoliths or
peridotite massifs, but mostly the
mantle delivers its chemical signals
to us in basaltic magmas.
?
4
Geochemistry and Geodynamics
• What information in a basalt can be taken as direct
information about the source region?
– Not major element composition…partial melting and
shallow differentiation both separate major elements
from one another in complicated ways
– Not trace element concentration…even knowing all the
partition coefficients, these are functions of extent and
style of melting as well as source composition
– Stable isotopes, maybe, if high temperature fractionation
is negligible
– Ratios of incompatible trace elements…yes. If both
elements are sufficiently incompatible that they are
quantitatively extracted, then liquid ratio equals source
ratio.
– Ratios of heavy long-lived isotopes…yes. Arguments
based on diffusion strongly suggest that basalts are
produced in isotopic equilibrium with their source.
5
Heterogeneity of Oceanic Basalts
• Observation: while less diverse than
continental rocks, oceanic basalts do
display a significant diversity of
isotopic compositions in 87Sr/86Sr.
• Focus on oceanic basalts because they
are uncontaminated by continents.
MORB = mid-ocean ridge basalt
OIB = ocean island basalt
6
Isotopic Equilibrium and Disequilibrium
• So heterogeneous isotopic compositions come out of the
mantle. What does this mean about the heterogeneity of the
mantle itself?
• The essential argument for isotopic equilibrium between
source and melt was presented by Hofmann and Hart
(1978). Consider two cases:
– (1) The mantle is uniform on a regional scale (10-1000 km3) due to
efficient mechanical stirring, but not in chemical or isotopic
equilibrium on a local (cm) scale due to inefficient diffusion.
• In case (1), isotope heterogeneity in erupted basalts might reflect, for
example, different degrees of melting if radiogenic Sr accumulates in
phlogopite and is contributed to the melt only as phlogopite melts.
– (2) The mantle contains regional inhomogeneities that have
survived the stirring process for long times, but is isotopically
equilibrated by diffusion on a local (100 m?) scale at least during
melting.
• In case (2), isotope heterogeneity in erupted basalts reflects regionalscale difference in their source compositions only
7
•
Isotopic Equilibrium and Disequilibrium
Case (1): Regional homogeneity, local disequilibrium
– http://wwwrses.anu.edu.au/gfd/members/davies/pages/passmovie.html
• Case (2): Regional heterogeneity, local equilibrium
– http://www.gps.caltech.edu/~gurnis/Movies/MPegs/stirring.mpeg
8
Isotopic Equilibrium and Disequilibrium
• Isotope heterogeneity on the meter scale can certainly persist for long
times in the solid state
– Typical diffusion coefficients of trace elements in mantle minerals are of order
10–12 cm2/s at 1200°C.
– Hence typical timescale for diffusion across 1 m distances is t ~ L2/D = 3 x 108
years
– Stirring of viscous fluids stretches and thins heterogeneities but it also takes
many millions of years to thin them to diffusive lengths.
• Isotope heterogeneity on cm scale probably cannot survive a melting
episode
– Typical diffusion coefficients in silicate liquids are of order 10-7 cm2/s at 1200
°C.
– Hence typical transport distance by diffusion is 2 cm per year or 200 m in 10 ka.
– As soon as partial melt fills all the grain boundaries, the distance over which
solid-state diffusion must act drops from the scale of heterogeneity to the size of
a crystal!
• It follows that basalt liquids are expected to have isotope ratios that
are faithful copies of their sources averaged over at least several
meters.
9
Isotopic Equilibrium and Disequilibrium
• We can see evidence of this in the comparison of isotopic
composition between basalts and associated residual
peridotites:
– Basalts are more homogeneous and more radiogenic than peridotite
suites. Taken to imply that Nd (and Os) from a recycled component
was in the source but is not sampled in the residual assemblage.
– Consistent with regional heterogeneity, local homogenization
10
Isotopes in Oceanic Basalts
the “mantle array”
• What then is the interpretation of the
pattern of Sr isotope heterogeneity
among MORB and OIB?
– Sr by itself is very hard to
interpret…we don’t know bulk
earth value because Rb is volatile
on accretion
– Sr and Pb isotope variations do not
correlate in any simple way, which
caused much gnashing of teeth 3040 years ago
• It took the introduction of Nd
isotope data to begin a real debate
between meaningful models
– Sm and Nd are refractory, so we
know CHUR composition and by
inference BSE
– Sr and Nd isotopes in oceanic rocks
do correlate, inversely
– MORB and crust are seen to be
complementary (recall trace
element story from lecture 2), but
the meaning of OIB is ambiguous
11
The Sm-Nd mantle array
• The distribution of OIB data between MORB
and Bulk Silicate Earth is consistent with at
least three models:
The “standard model” -- MORB samples the
upper mantle which is complementary to
continental crust extraction; OIB samples the
lower mantle which is primitive; the mantle
array is the result of mixing between depleted
and primitive.
Or, different parts of the mantle may have been
depleted to various degrees and never
homogenized…this would also generate an
array of data from depleted to primitive, but
with a very different spatial distribution of
mantle reservoirs!
Or, there may be no primitive reservoir
involved at all, and OIB may be mixtures
between depleted MORB mantle and various
enriched components like recycled oceanic
crust or subducted sediment
There might still be a primitive mantle
somewhere, but it might not ever be sampled
by volcanism
Hofmann and White model
12
Isotopic Mass Balance
• Knowing eNd for bulk silicate earth = 0; eNd, [Nd] and mean age of continents;
and eNd for upper mantle, can we distinguish standard and whole-mantle models
by mass balance? Let’s calculate what volume fraction of the whole mantle
must be depleted to balance the continents.
CNd
Crust
Mc
c
f Sm/ Nd c
eNd d
Md
Primitive Mantle
eNd = 0
CNd o
Mp

147
f Sm / Nd 
j

147
Sm/ 144Nd j
Sm/ Nd CHUR
144
j
 Mc  Md  M p  Mo
j j
o o
M
C

M
CNd
 Nd
Depleted Mantle
CNd d
M
j
 1 e Nd
j j
j
M
C
e
 Nd Nd  0
j j
j
M
C
f
 Nd Sm / Nd  0
 143 Nd /144Nd 

j
4

  143

1

10
 Nd / 144NdCHUR



13
Isotopic Mass Balance
•
For times short compared to the half-life of 147Sm,
143Nd  143Nd  147Sm   t
143Nd  147Sm 
147
 1  143   143  147t
143   143   143  e
 Nd t  Nd o  Nd t
 Nd o  Nd t

•
Or, in epsilon notation, with initial eNd = 0,
e Nd  fSm / Nd Qt
•
•

IF:
Q
10 4 147 147 Sm /144Nd CHUR

143
Nd/ Nd CHUR
144
– There are only three reservoirs: c, d, and p (and p is primitive)
– We know the Sm/Nd ratio of the crust, [Nd] of the crust, and the eNd of depleted mantle
THEN we get a relationship between the age T of crust formation and the ratio of the
masses of crust and depleted mantle:
c
c
c
 CNd
QfSm
M d CNd
/ Nd

1

 o
  o  d T
c
M
CNd  CNd  e Nd
•
•
•
 25.13 Ga -1
The result, for T ~ 2.5 Ga (which we get independently from ƒSm/Ndc and eNdc), is that the
depleted mantle is 0.3 time the mass of the whole mantle.
This fits beautifully with the standard model, since the upper mantle is 1/3 of the mantle.
BUT if there is another large reservoir, namely stored subducted materials, this messes up
the whole calculation. We can easily put enough enriched material with eNd > 0 in this
reservoir that the entire remainder of the mantle would be depleted mantle. So this is
equally consistent with the Hofmann and White model!
14
The Sm-Nd mantle array
• How do we choose between these
models? For starters, get more data and
more isotope systems!
• Problem 1 with standard model: with
more data, we find that OIB extend
beyond primitive mantle (PRIMA)
composition, both to higher 87Sr/86Sr
and lower eNd. Hence they must contain
some enriched material.
• Problem 2 with standard model: the
array is not consistent with twocomponent mixing…the width of the
trend is way outside analytical error and
requires at least two enriched
components.
• Problem 3 with standard model: the
MORB data are spatially organized by
ocean, so the upper mantle is not
homogenous either
• Problem 4 with standard model: add
other isotopes and the binary-ish mantle
array breaks down altogether
15
The mantle isotope zoo
So how many components do you need?
For Sr-Nd-Pb-Pb-Pb space, at least four:
–
–
–
–
DMM = depleted MORB mantle
HIMU = High U/Pb component
EMI = Enriched Mantle I (low Nd)
EMII = Enriched mantle II (high Sr)
If 206Pb, 207Pb, 208Pb are not really
independent, then four end members
to span data in 3-space (Sr-Nd-Pb) is
trivial, but the same components also
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bound data in Hf and Os space.
The Worm-o-gram
How do the four bounding
components mix with one another?
Is there evidence of an “internal”
component, that everything mixes
towards? If so, what is it?
Some authors see mixing towards
particular locations, and argue that
these represent common
components with well-defined
compositions: FOZO, C, PREMA
More on this when we talk about
noble gas systems.
17
An oddity
• The DUPAL (Dupré and Allègre) anomaly: nearly all the isotopically
unusual hotspots are in a well-defined latitude band between 0° and 50°S.
• If this has any geodynamic significance, nobody has figured out what it is!
Getting back to geodynamics...
• So what are DMM, HIMU, EMI, and EMII? Are they well-defined
reservoirs with sensible histories and physical locations in the mantle, or
merely arbitrary points in multi-isotope space?
• Before we can answer that we need to think more about trace elements,
since parent-daughter ratios over time determine the isotope characteristics
of the end members.
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Trace Element Ratios
• Another kind of tracer of mantle sources should be
ratios of incompatible elements in basalts, but one has
to be careful to avoid effects of recent fractionation
• Two cases that do not work: Sm/Nd and Lu/Hf
• Nearly all MORB samples plot above Bulk Earth in Hf and Nd isotopes,
meaning their long term Lu/Hf and Sm/Nd ratios have been higher than
chondritic. But nearly all MORB samples have subchondritic measured Lu/Hf
and Sm/Nd ratios.
• It follows that Lu/Hf and Sm/Nd were fractionated recently (by the melting
process itself), which turns out to requires garnet in the source (P > 2.5 GPa).
19
Trace Element Ratios
• Two that do work, for MORB & OIB melting: Nb/U & Ce/Pb
Nb/U and Ce/Pb in oceanic
basalts do not correlate with
[Nb] and [Ce]. This implies
(1) that the ratio in basalt
does not depend on extent of
melting, and (2) that
depleted and enriched
sources are equal also, so
the ratio in the residue does
not get fractionated. Hence
either the elements have
equal partition coefficients or are both
incompatible enough to be totally extracted.
But the ratio in MORB and OIB is not
chondritic! The continent-forming process did
fractionate these element pairs (either because
arc processes involve oxidizing fluids or because
of very small extents of melting), and crust and
mantle are complementary reservoirs.
But…OIB do not mix towards primitive value, so
there is no evidence here of a primitive reservoir
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sampled by any basaltic magma!
Trace Element Mass Balance
• If we know the Nb/U ratio of the primitive mantle, depleted
mantle, and continental crust, we should be able to calculate the
masses of each of these reservoirs.
• UCCXCC + UDMMXDMM = UBSE
• NbCCXCC + NbDMMXDMM = NbBSE
• XCC + XDMM = 1
->
Nb
 
U
 NbBSE  NbCCXCC
 U DMM BSE
UCC 
Nb 
X
 U DMM CC
• (Nb/U)DMM ~ 47, XCC (relative to whole silicate earth) ~ 0.6%,
UCC = 0.9-1.3 ppm.
• Conclusion: it does not work…something must be missing, because the
continental crust appears to be 0.7 to 1.15% of the crust+depleted mantle
system. Either there is a hidden reservoir of Nb or U somewhere, or some
fraction of the mantle remains primitive and is not sampled by either MORB
or OIB.
• Possible hidden reservoir is again subducted oceanic crust,
perhaps eclogite with rutile to hold a lot of Nb
21
Anomalous fractionations involving continents
• Why are some trace element ratios different in continents
than in mantle, even though basalt genesis does not
fractionate them? Let’s look at Ce/Pb again:
• Which is the anomalous element, Ce or
Pb? In the spidergram, Pb clearly
stands out as high in CC, low
everywhere else.
• Where do the continents get this
signature? Where are continents made?
In island arcs.
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•
•
•
•
Anomalous fractionation
In this case study of the
Aleutians, we know Ce/Pb
ratio and Pb isotope
composition of the North
Pacific sediment and ocean
crust being subducted.
In Pb-isotope space, the arc
lavas appear to get all their Pb
from mixtures of these two
components.
But the lavas are not a simple
mix of MORB and sediment.
The low 207Pb/204Pb
component has a lowered
Ce/Pb ratio…Pb must be
preferentially extracted
(relative to Ce) from the
subducting basalt (but not
from the sediment).
Implication: Pb is mobile in
aqueous fluid, leading to lowCe/Pb arc source and highCe/Pb residual slab.
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Origin of the four mantle components
• DMM is easy…it is ambient upper mantle, depleted ~2
Ga ago by extraction of the continents.
– However, MORB can be polluted by influence of nearby plumes
(Schilling’s effect), so not all MORB plot right at DMM:
Isotopic composition
of mid-Atlantic ridge
samples near the
Azores hotspot…
Begs questions: How
well mixed is DMM
reservoir? Is even
“pure DMM”
recharged with a flux
from somewhere?
How does the upper
mantle stay fertile
over time?
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Origin of the four mantle components
• EMII is almost certainly recycled continental material,
presumably subducted terrigenous sediment.
– Isotopic composition of young pelagic sediment is a pretty good
match for EMII isotopes, but not perfect…sediments must be
aged for a while.
– As we saw, continents (and hence also continent-derived
sediments) have very high Pb concentrations. Hence U/Pb is not
very high and EMII does not evolve to especially enriched
206Pb/204Pb. But Th/U is high (due to scavenging of Th from
seawater), so 208Pb/204Pb increases faster.
– Because Sr/Pb and Nd/Pb ratios are lower than in other
components, mixing arrays towards EMII should be strongly
curve in isotope ratio-ratio space, as observed.
– Even though sediment signature is transferred to arc basalts at
subduction zones, some sediment or some sediment-derived
trace elements must be subducted, to arise elsewhere in OIBs.
25
Origin of the four mantle components
• HIMU is usually attributed to subducted, altered ancient
oceanic crust.
– The preferential extraction of Pb from the basaltic part of slab at
subduction zones leaves a high U/Pb residual component, which
will evolve to high 206Pb/204Pb with time.
– But it is necessary that Rb also be removed relative to Sr during
subduction, or HIMU would have wrong 87Sr/86Sr.
– Note that HIMU-DMM mixing arrays are linear, which implies
Sr/ Pb and Nd/Pb ratios are similar in these end members…a
problem?
– Other authors think HIMU is a component of metasomatically
altered continental lithospheric mantle…no agreement on this.
– Some even think HIMU has high U/Pb because of late
segregation of Pb into the core...
26
Origin of the four mantle components
• EMI is problematic. All kinds of ideas are in play...
– It is close to Bulk Earth, except in eNd. Perhaps it is slightly
modified BSE (modified how? Nobody says).
– It also resembles lower continental crust, from xenoliths and
granulite terranes. Perhaps EMI and EMII are distinguished by
intracontinental differentiation, and EMI is recycled by
delamination whereas EMII is recycled by erosion and
subduction.
– H2O-rich and CO2-rich fluids mobilize trace elements
differently. It is possible that HIMU and EMI could be
complementary products of migration of CO2-rich fluids from
continental lithospheric mantle into lower continental crust.
• Another issue to revisit after we talk noble gases.
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The Upper Mantle as an Open System
• For Pb, we can prove that there is continuing input to the
upper mantle-ocean crust system from some other longlived reservoir, probably the lower mantle. If this input
balances Pb flux to continents at arcs, the upper mantle
might be in steady-state for incompatible elements.
• This argument is based on
Th/U ratios: the continental
crust has a chondritic Th/U
ratio (3.9), but the MORB
source has a much lower Th/U
ratio (2.5).
• If input to upper mantle is
chondritic in Th/U, and output
to continents is chondritic,
upper mantle could be in steady
state, even with a different
Th/U ratio, but this requires a
short residence time of Pb in
upper mantle.
28
Th/U ratios, Th isotopes and Pb isotopes
• Trying to match up Th/U ratio and 208Pb/206Pb composition of
MORBs is a different exercise from the Sm/Nd and Lu/Hf
problem presented above, because we can correct accurately for
effects of melting and there is still a discordance.
• For a source in secular equilibrium, the activity of 230Th is equal
to that of 238U. Hence the (232Th/230Th) activity ratio is a measure
of the Th/U ratio of the source:
 
 
232


232


Th
Th
238
se 238
 238 



 kTh
238
230
U
232
U 232
U
Th
232Th
Th
• Since MORB has a Th excess due to melting processes, the
measured value is an upper limit for the Th/U of the source. Data:
–
–
–
–
Mid-Atlantic Ridge
East Pacific Rise
Hawaii and Iceland
Tristan da Cunha
kTh= 2.5
kTh = 2.5±0.2
kTh = 3.0
kTh = 3.7
• Here is a trace-element ratio indicator in which hotspots are closer to primitive!
29
Th/U ratios, Th isotopes and Pb isotopes
• The long-term history of Th/U in a source, on the other hand, is
determined from the Pb isotopes (where T is the age of the Earth):




238 T
e
1
Th
Th
Pb*
 238  206
 k Pb

T
U
U
Pb* e 232  1
232
• Data:
–
–
–
–
–
Mid-Atlantic Ridge
East Pacific Rise
Indian Ridges
Hawaii and Iceland
Tristan da Cunha
208
kPb= 3.78±0.07
kPb = 3.73±0.06
kPb = 3.89±0.11
kPb = 3.83±0.04
kPb = 4.17
• Some hotspots have long-term Th/U higher than chondritic!
• SO…the maximum present day Th/U of the MORB source (from
Th isotopes) is much less than the long-term Th/U average
reflected in the Pb isotopes of the same source, and this is not a
recent melting effect.
– The Pb in MORB cannot have been in a low Th/U reservoir for more than
600 Ma…this must be the residence time of Pb in the upper mantle.
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Th/U ratios, Th isotopes and Pb isotopes
• Where is the reservoir from which Pb is input to the upper mantle?
– Upper continental crust has chondritic Th/U and can be recycled by erosion
(hence EMII flavored hotspots), but it has the wrong 207Pb/204Pb ratio, since its
U was fractionated from Pb more than 1 Ga ago when 235U was more
abundant.
– Continental lithospheric mantle might be the reservoir, but this would require
their entire mass to exchange with the upper mantle every few hundred Ma,
which is inconsistent with the long-term stability of cratonic lithosphere.
– That leaves only the lower mantle, which is so big and Pb-rich that over
geologic time only half the mass of the upper mantle would have to be
replaced by lower mantle to give the necessary flux (~10% per Ga).
• Bottom line: the more incompatible the element, the shorter its
residence time in the upper mantle-oceanic crust system (~200 Ma
for the perfectly incompatible element)
– Hence DMM is roughly equal to BSE in Pb isotopes (which are replaced much
faster than 238U decay), but quite different in Nd and Sr isotopes, since these
elements are more compatible (especially in arcs).
– For the most incompatible elements the global system has evolved to a steadystate where output to the continents is balanced by input from lower mantle.
– Convective isolation (layering?) is necessary to explain long-term evolution of
31
components, but it cannot be perfect…it must be leaky.