Summary of past and current research on subduction throughout geological time and brief review of evidence for the timing of onset of plate tectonics on Earth

1. Tracking subduction through
geological time and dating the
onset of plate tectonics on Earth
Dr. Richard M. Palin
Assistant Professor of Metamorphic Geology
Colorado School of Mines, USA

2. Plate tectonics
> Palin – Metamorphism and Plate Tectonics 2
Stern et al. (2018)
• Formulation of plate tectonic theory
caused a paradigm shift in
understanding how the Earth works
– Why, how, and when did it start?
– What came before?
• Plate boundary zones are areas of
severe hazards, but can also contain
deposits of significant economic value
– Subduction facilitates mass
transfer between the Earth’s
surface and deep interior
• Change in fluxes over time?
• Plate tectonics may be a necessary
driver for the evolution of complex life
– Required for (exo)planet
habitability?

3. What is the problem?
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Palin et al. (2020)

4. Why is it a problem?
• Identifying plate tectonics requires
proving subduction or independent
plate motion and rotation
• Theoretical and empirical data disagree
– Geodynamic numerical modeling
• Subduction or sagduction?
– Isotope/trace element signatures
• Transport of pelagic sediments
into the mantle at 3.5 Ga (e.g.
Blichert-Toft et al., 2015)
– Styles of deformation and
magmatism
• Dome and keel vs. linear arcs
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– Secular distribution of key rock types
• Sheeted dykes in Yellowknife (~2.7 Ga) (e.g. Helmstaedt et al., 1986)

5. Definitions
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Palin et al.
(2020)
• Crust ≠ lithosphere
• A “plate” is a
discrete fragment
of the lithosphere
• LAB variably defined
by change in dominant
mode of heat flow,
chemical composition,
and/or rheology at the
interface
• Lithosphere = static
– Stagnant-lid tectonics, of
which many forms exist
• Lithosphere = mobile
– Mobile-lid tectonics, of which “plate
tectonics” is the only known form

6. Life cycle of a rocky planet
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• Observations of large
and small bodies in our
solar system, and
interpretations made
from numerical
modeling, suggest that
the crusts and mantles
of all silicate planets
follow a predictable
thermo-mechanical
evolution
– Plate tectonics is
not an expected
geodynamic state
– It may, in fact, be
highly unusual and
difficult to ‘create’
Palin et al. (2020)

7. • Tectonic evidence
– Paired metamorphic belts
– Collisional/accretionary
orogens and the
supercontinent cycle
• Geochemical and/or isotopic
evidence
– Trace-element discrimination
– Diamonds and their inclusions
• Modeling
– Petrological and thermo-
mechanical
• Petrological evidence
– Blueschists, (U)HP eclogites
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Geological evidence for operation of plate tectonics
– Ophiolites, andesites and arc/back-arc assemblages Angiboust et al. (2012)

8. Tectonic evidence: Archean terranes
• East Pilbara – dome and keel structures diagnostic of vertical tectonics (RTIs)
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3.53–3.2 Ga
Allwoodetal.(2007)

9. Tectonic evidence: Archean terranes
• West Pilbara – linear features, such as strike-slip faults, and volcanic arc deposits
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3.3–3.05 Ga
Allwoodetal.(2007)

10. Tectonic evidence: Supercontinent cycle
• Global zircon archive
reveals pulses of
growth associated
with magmatism
– Related to
continent
accretion?
– Supercontinents
must form via
lateral assembly
• Hf model ages reveal
age of extraction of
magma from the
mantle
– Formation of
“new” crust
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Roberts and
Spencer (2014)

11. Geochemical/isotopic evidence: diamonds
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Science (2011)

12. Geochemical/isotopic evidence: diamonds
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Science (2019)
• Atmospheric
sulfur began to
enter the mantle
at c. 3 Ga
– Subduction
or something
else?

13. > Palin – Metamorphism and Plate Tectonics 13
Modeling evidence: Thermo-mechanical
Gerya et al. (2008)
• Testing geological/geodynamical
parameter space for conditions that
will allow subduction (e.g. mantle Tp)
Fischer and Gerya (2016)
• How is crustal material transported
into the mantle in a stagnant-lid?

14. Modeling evidence: Petrological
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• Define optimum pressure–temperature conditions for generation of Archean
tonalite–trondhjemite–granodiorite (TTG) magmas
– Different P–T conditions (or apparent geothermal gradients) characterize
different geodynamic environments
– Match major and trace element compositions using source-rock mineralogy

15. Modeling evidence: Petrological
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Palin et al. (2016)

16. • When does the oceanic lithosphere become dense enough to subduct?
– Secular mantle cooling and changes in oceanic lithosphere structure affect
buoyancy and the ability for subduction to spontaneously initiate
• Combine petrology and numerical modeling to investigate!
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Modeling evidence: integrated techniques

17. > Palin – Metamorphism and Plate Tectonics 17
Petrological evidence: ophiolites
• A very contentious topic!
– Problems with identifying ophiolites in the Archean rock record relate to likely
changes in petrology/stratigraphy of oceanic lithosphere through time, poor
exposure, poor preservation, re-working etc.
• Nonetheless, many certified Precambrian examples exist (cf. Furnes et al., 2014)

18. > Palin – Metamorphism and Plate Tectonics 18
Metamorphic rocks
• The metamorphic rock record
can reveal changes in tectonic
processes/environments
– Compile peak pressure–
temperature data through
time (e.g. Brown, 2005)
• Divide rocks by apparent peak
metamorphic gradient (dT/dP)
– Blueschist/eclogite (cold)
– High-P granulite (medium)
– Granulite and UHT (hot)
• Low dT/dP rocks are often
focused upon, as they form in
Phanerozoic subduction zones
– But what about the
Archean?
Palin et al. (2020)

19. > Palin – Metamorphism and Plate Tectonics 19
Petrological consequences of secular cooling
• More radiogenic HPEs and
leftover accretionary heat
• Present day
– Mantle TP = 1,350 °C
– F = 0.08–0.10
– ~7-km-thick crust with
10 wt. % bulk MgO
• Archean/“early Earth”
– Mantle TP >1,600 °C
• (Contentious!!)
– F = 0.25–0.45
– Up to 45-km-thick crust
with >18 wt. % bulk MgO
• Picrites and
komatiites
Palin et al. (2020) with other sources as shown

20. > Palin – Metamorphism and Plate Tectonics 20
So, hotter subduction-zone geotherms?
• “Early Archean”
– >>700 °C/GPa
• “Late Archean”
– >>400 °C/GPa

21. • First-order controls on
subduction zone thermal
structure:
– Convergence rate and
slab age (i.e. original
temperature) (Kirby et
al., 1991: Science)
– Mantle TP, slab dip angle
etc. have less influence
• Archean subduction:
– Thicker and older crust,
and a slightly more
sluggish subduction
velocity (~0.5–7.5 cm/yr)
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Perpetuating a metamorphic myth
Syracuse et al.
(2010) PEPI
“…the tempo of plate tectonics in the past should not have differed greatly from
that at the present…” (Korenaga, 2013: Ann. Rev. Earth Planet. Sci.)

22. Secular compositional change
• Geochemical models of Archean
MORB suggest MgO contents of 11–15
wt% (e.g. Ziaja et al., 2014)
– MgO is not the only variable, but is
the best proxy for a cooling Earth
• Representative-aged mafic rocks with
trace element signatures indicative of
formation in a MOR setting
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After:KellerandSchoene(2011)Nature
Furnesetal.(2014)
GondwanaRes.

23. Structure and composition of oceanic lithosphere
> Palin – Metamorphism and Plate Tectonics 23Palin and Dyck (2018)

24. • What metamorphic rock types would
form in an Archean subduction zone
containing high-Mg basalts?
• Ex. Pet. is a tried-and-tested technique
• Synthetic mixtures of minerals and
fluids (e.g. H2O, CO2), or actual rock
compositions
– Lower crust and upper mantle:
piston-cylinder apparatus
– Lower mantle and core: multi-
anvil or diamond-anvil devices
• Experiments can be performed at
gridded P–T conditions
– Products then analyzed via X-ray
diffraction (XRD) and/or electron
probe microanalysis (EPMA)
Experimental petrology
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25. • Unfortunately, not useful in this case
• Major limitation is time
– Too low-T for reactions to
proceed
• Metastable equilibrium?
• Generally coarse resolution
• Could be increased, but still only
one bulk composition!
– Effects of a minor change (e.g.
1.5 × Al2O3)?
• Expensive to run and maintain
• We need a more general method for
investigating the physico-chemical
behavior of fluids, rocks, and
minerals anywhere within the Earth
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Experimental petrology
Tsunoetal.(2012)Geophys.Res.Lett.

26. Phase equilibrium modeling
• Almost all metamorphic studies in recent years—whether regionally focused or
process-oriented—have involved calculated phase equilibria of some kind
• Uses thermodynamic principles to predict how minerals, aqueous fluids, and
melts would behave at various physical conditions in different chemical systems
• Intensive (P, T, μ) or extensive variables (S, V, n)
Palin et al. (2016) Geoscience Frontiers
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27. Pseudosections; a new frontier
• Duhem’s theorem
– “The equilibrium state of a closed system
with a known mass is completely defined
by two independent variables”
• Predictive (forward) or descriptive (inverse)
• Derive petrophysical properties
– Vp, Vs, rho, Poisson ratio etc.
Palin et al. (2016) Geoscience Frontiers
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28. Modeling of subduction-zone metamorphism
Palin and White (2016) Nature Geoscience
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29. Phase assemblages along a warm geotherm (350 °C/GPa)
PalinandWhite(2016)NatureGeoscience
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30. Efficiency of H2O transport into the Earth’s interior
• Assume low-grade assemblages in
uppermost subduction channel are at
least minimally fluid saturated
– δ18O zircon data → surface water
at c. 4.4 Ga (Wilde et al., 2001)
– Upper ~2–3 km of crust hydrated
by near-ridge/-trench alteration
• Metamorphosed high-MgO basalt can
hold ~20% more structurally bound
H2O than low-MgO metabasalt can
– Key Chl-out dehydration reaction
occurs later (deeper) in Archean
crust than for modern-day crust
• Volatile recycling on the
early Earth more efficient
than today!
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Palin and White (2016) Nature Geoscience

31. Archean vs. modern-day
subduction-zone facies
Palin and White (2016) Nature Geoscience
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32. Archean vs. modern-day
subduction-zone facies
Palin and White (2016) Nature Geoscience
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33. Identifying metamorphosed high-MgO basic rocks
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34. Identifying metamorphosed high-MgO basic rocks
Have they already been unknowingly documented?
• Moyen et al. (2006; Nature) reported petrological–thermobarometric evidence for
mid-Archean (c. 3.2 Ga) subduction in the Barberton terrane, South Africa
– “…clinopyroxene and quartz…formed [during prograde metamorphism from] a
relatively sodic [clino]amphibole and epidote…”
– Ferro-edenite with Na/(Na+Ca)M4 ~0.14–0.32 (= 0.7–1.1 Na cpfu for 23 O)
• Matches our predictions
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35. • Secular cooling of the Earth requires that the structure, thickness, and chemical
composition of oceanic crust has evolved over time
– Metamorphic and magmatic products of plate-tectonic processes must also have
changed concomitantly (cf. GSF SI; eds. Palin and Spencer, 2018)
• “For understanding early-Earth tectonic processes, it may be better that we forget all
that we have learned about the present day” – Taras Gerya, ETHZ
Uniformitarianism: a double-edged sword
After:Sternetal.(2016)Geology
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36. • Obduction, not subduction; absent from
many Phanerozoic convergent margins;
pre-Neoproterozoic examples reported
• Difficult to preserve during exhumation;
breakdown products resemble
greenschist-facies assemblages
Uniformitarianism: a double-edged sword
After:Sternetal.(2016)Geology
• Related to subducted continental crust;
Mesoarchean (2.87 Ga) non-UHP
subduction-related eclogites (Mints et
al., 2010; Geology)
• Rare, even in Phanerozoic rocks; require
specific fluid–rock interactions to form
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37. Summary: secular trends – the continental crust
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Palin et al. (2020)
• Many independent datasets suggests Late Archean onset of plate tectonics
– Most likely operating at a global scale, as evidenced by the beginning of the
supercontinent cycle
– Change in rate of crustal recycling and increased thickness of juvenile crust

38. Summary: secular trends – the oceanic crust
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Palin et al. (2020)
• Early subduction occurred at a shallow angle, consistent with geochemical data
from TTGs and arc-related volcanostratigraphy in c. 3 Ga terranes
– Eclogitization and slab breakoff transported surface materials into the mantle
– Higher-MgO crust carried more volatiles per unit volume than today

39. Summary: secular trends – the oceanic crust
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Palin et al. (2020)
• Appearance of LT/HP rocks at c. 1.9–2.2 Ga marks the onset of steep subduction
– Gap before re-appearance at c. 0.9 Ga and the preservation of blueschists
• Associated with a period of worldwide tectonic quiescence – the Boring
Billion?

40. Summary: secular trends – global tectonics
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Palin et al. (2020)
• A stagnant-lid environment characterized the pre-c. 3 Ga Earth
– Heat-pipe Earth, drips and plumes, and delamination etc. (“vertical” tectonics)

41. Moving forwards
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• What if the Archean mantle wasn’t as hot as previously supposed? Implications?