Geochemical Indicators of Plate Tectonic Processes in Old Rocks

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Geochemical Indicators of Plate Tectonic
Processes in Old Rocks

• Julian Pearce
• (Cardiff University)

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Bobs Smoking Guns for Archean Subduction (Bluesch
ists, UHP rocks, Ophiolites)
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However, Subduction Fluxes are Forever
Even if the arc is overwritten by collision or
eroded away, inherited subduction signals remain
in the mantle lithosphere and can be reactivated
later
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Finding Evidence of Subduction is the Key to
Knowing when Plate Tectonics started
Pearce Peate (1995)
But present-day subduction is a complicated
processes essentially mantle flow and subduction
input are the geochemical indicators of plate
tectonics, while crustal interactions tend to
mask these indicators. The key to identifying arc
lavas in the Archaean is separating subduction
signals from crustal signals.
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Geochemical Indicators of Plate Tectonic Processes

• Indicators of Plate-driven Mantle Flow
• Indicators of Subduction

At subduction zones, these two processes act
together.
LT
HT
UHT
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Geochemical Tracing of Subduction Input
Progressive subduction leads to sequential
release of LT elements (Rb, Ba etc) HT
elements (LT elements plus L-MREE, Th, P) UHT
elements ( HT elements plus Nb, Ta, Zr, Hf).
LT
HT
UHT
In the Archean, we would expect this sequence to
take place at shallower depths than at present
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Geochemical Tracers for Subduction Input
Shallow subduction components cannot be
investigated in Archaean rocks because of
alteration-sensitivity Deep subduction
components are more robust. Th/Nb (an indicator
of negative Nb anomalies also an effective tracer
of deep subduction input that is robust to upper
amphibolite facies
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Geochemical Tracing of Mantle Flow
Pearce (2005)
Flowing mantle undergoing decompression can
drastically change its chemical composition
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Geochemical Tracing of Mantle Flow
Tonga-LauSystemcollabration w.Pam Kempton,
Jim Gill
Results indicate that mantle entering subduction
systems progressively loses incompatible elements
by melt extraction while flowing to the sub-arc
region
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Geochemical Tracers for Mantle Flow
Isotope ratios reach plateaus, so trace element
ratios are more effective for mapping. Most
effective for subduction systems are VICE/MICE
ratios based on immobile elements
Thus Nb/Yb acts as a Good proxy for mantle
fertility Nb/Yb gradients provide a means of
tracing mantle flow
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Th/Yb-Nb/Yb Fingerprinting

Pearce and Peate, 1995
At the present day, MORB and IOB plot in a well
defined array, along the mantle flow axis
(Nb/Yb) arc lavas are displaced to higher Th/Nb
ratios. The overall dispersion of arc lavas is
parallel to the MORB array indicating the
importance of melt extraction during mantle flow
in magma genesis.
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Th/Yb-Nb/Yb FingerprintingRole of Mantle Flow

HT subduction
two components to a first approximation
melt extraction
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Th/Yb-Nb/Yb FingerprintingInteraction of Mantle
Flow and SZ input

Location of data and shapes of trends indicate
process 1 Add SZ component before melt
extraction 2 Add SZ component during melt
extraction 3. Add SZ component without or after
melt extraction 4. UHT SZ component adds Nb as
well as Th
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1
4
3
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Types of Subduction Zone
North Tonga Oceanic plume-subduction
interaction Cascades Continental
plume-subduction interaction IBM Eocene
Intra-oceanic subduction initiation Japan
Miocene Intra-continental subduction
initiation Various Localities Ridge
subduction Taiwan, SE Indonesia
Syn-collision Mariana Arc Rifting Anatolia
Subduction component reativation plus steady
state subduction
Each type of Subduction Zone shows a different
topology on the Th/Yb-Nb/Yb diagram
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Example W. Pacific Eocene

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Example North Tonga

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Example Kamchatka

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Misinterpretation of Subduction Zones
Amphibolite-facies metamorphism
HT fluids/melts from sediments impregnated and
metamorphosed the lavas. The result is that some
normally-immobile LIL elements (e.g. Th) are
enriched
Broken Hill cores
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Misinterpretation of Subduction Zones granulite
facies metamorphism
David Waters
The lower crust loses a melt fraction (which can
be seen in places escaping from the rock)
leaving a granulite residue This depletes the
residue in LIL elements.
increasing melt depletion
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Misinterpretation of Subduction Zones Crustal
Contamination
The crustal contamination vector is typically
parallel to the UHP subduction vector.
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Crustal Contaminationis also part of continental
arc dispersion
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Proposed Archean Subduction-Related Rocks
Komatiites Boninites BADR volcanic
series Adakite lavas and TTGs Aim is to assess
these subduction signals
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Testing Archean Komatiite Subduction Models
Parman et al. (1997, 2001), following the work of
Grove, controversially argue that Barberton lavas
were wet rather than hot i.e. subduction
related. However, crustal contamination is more
consistent with the data
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Testing Archean Boninite Subduction Models
Dispersion is along a crustal contamination
vector, not mantle flow vector however the Isua
boninites do require source depletion.
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Alternative Model for Archean Boninites and
Related Rocks
Arculus et al. (1992) from IODP Leg 125
Phanerozoic boninites result from shallow, wet
melting (opx ol siliceous melt)
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Alternative Model for Archean Boninites and
Related Rocks
Komatiite
Arculus et al. (1992) from IODP Leg 125
crust
But Archean boninites could be explained by
komatiite-crust interaction
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Testing Archean BADR Subduction Models
The proposed BADR series shows increasing Th/Nb
with increasing silica content and trend parallel
to crustalcontamination trends. Wawa lavas do not
however have an end member in the mantle array a
difficult call.
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Turkish Analogue
Reactivation of sub-arc lithosphere following
collision can be evaluated using Nb anomalies
Post-collision magmas erupted at the site of the
dead arc have subduction signatures
Post-collision magmas erupted where there was no
arc have intraplate signatures ( with crustal
assimilation)
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Testing Archean Adakite/TTG Subduction Models
All agree on melting of mafic material but
how? Flat subduction (lower crust melting) Hot
subduction (slab melting) Delamination (lower
crust melting) Magma chambers (mid-crust melting)
Maybe all four! But subduction not essential.
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Crustal Processing
With no plate tectonics, could Archean volcanic
terranes have undergone intracrustal reprocessing
like present-day Collision Zones, so explaining
subduction-like chemical signatures?
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Conclusions

• Many Archean boninites and other evolved high-Mg
  magmas could be explained by interactions between
  komatiite (and related) magmas and crust rather
  than by subduction.
• Archean basalt-andesite-dacite-rhyolite (BADR)
  series require substantial magma-crust
  interaction and may not all additionally have
  subduction components.
• Adakites could (as is well known) involve melting
  of mafic rocks in the crust as well as in
  subduction zones.
• Unlike modern arc lavas, Archean arc lavas do
  not exhibit a geochemical indication of
  plate-driven flow into and within a mantle wedge.
• If there was subduction in the Archaean, it must
  have involved variable adakitic addition to a
  homogneous mantle
• Personally, I would start subduction around 2.7
  Ga but work still needs to be done separating
  crustal and subduction signals.