Big Questions, Big Debates• Earth’s formation & Composition• What was the Earth made from and what was the process?

• Continental crust formation through time

• How much was created and how much destroyed?
• Why is crust formation (or survival) punctuated in time?
• Why is Archean crust different?
• Why the change in the Neoproterozoic?

• When did modern plate tectonics begin?
• Why did O2 of the Earth’s atmosphere dramatically change in the
Paleoproterozoic and again in the Neoproterozoic?
• What are the LLSVPs at the base of the mantle?
• What are they made of?
• How have they evolved through time?

• Does the deep mantle ultimately control the evolution of the Earth’s surface orvise versa?

Solar System Formation

Walsh et al. (2011)

Grand Tack Model
Carbonaceous chondrites, such as Ryugu, are isotopically distinct, which reflects primary heterogeneity inmaterial that formed the solar system, with the giant planets initially forming a boundary. The Grand Tackmixed CC material inward, supplying much of Earth’s volatiles.

Earth’s Formation• The Earth is more enriched in sprocess isotopes than all otheranalyzed bulk solar system
materials and less like C.
chondrites.
• “Currently, no mixing model canaccount for the observed
composition of the Earth using
known materials” – Metzger etal. Space Sci. Rev., 2020
Possible missing inner solar system component?

Earth’s composition

From Frossart et al. (2022)• 142Nd produced by the extinct radionuclide 146Sm.
• Excess terrestrial 142Nd requires as higher thanchondritic Sm/Nd ratio, implying a depletion in theEarth of refractory incompatible elements.

From Johnston et al. (2022)

Collisional Erosion may be the cause

From Fossart et al. (2022)

This predicts a depletion of the Earth in heat producing elements U, Th, and K relative to chondritic values

Geoneutrinos may help us better constrain
Earth’s composition and heat production

McDonough (unpublished)Crustal Evolution: Hadean• In situ analyses show that zircons
older than 4 Ga have now beenfound on nearly every continent,
yet no >4 Ga crust exists.

• Continental crust formation began inthe Hadean.

• What happened to it?

• Simultaneous in situ determination ofHf isotope ratios and Pb age help
answer this.
• It appears much of it was recycled
into new crust.
• But how much existed and how much
was recycled into the mantle remains
unclear.

Continental crust formation appears to have
happened in pulses –why?

• Zircon ages suggest increasing crustal production over time.
• Does this reflect crust production or merely preservation?

But Hf model ages suggest much of that crust formed
from pre-existing crust; >50% formed in the Archean.

How much continental crust has been
destroyed and recycled into the mantle?

At present, the combined fluxes of sediment subduction, subduction erosion and lower
crustal floundering appear to equal or exceed the rate of new continental crust
production. Was this true in the past?

Archean crust is different.
Why?

Secular distribution of tonalitetrondhjemite-granodiorite suites (TTG)
Condie (2021)

Keller & Schoene (2012)

A Tectonic Transition in the Neoproterozoic?

Brown et al. (2020)

Condie (2021)

When did modern Plate Tectonics Begin?
• “Modern” means sheet-likeupwellings and downwellings.
• There is no consensus, but mostanswers are either Archean orNeoproterozoic.
• Most nevertheless agree crust
has been both created anddestroyed since the Eoarchean.

Palin et al. (2020)

Dramatic change in S isotopes at ~2.4 Ga

Farquhar et al. (2014)

UV photolysis of SO2 ended in the early Proterozoic

Why did O2 of the Earth’s atmosphere dramaticallychange in the Paleoproterozoic and again in theNeoproterozoic?
Boring Billion

Lyons et al. Nature 506, 307-315 (2014)

Atmosphere, Life, and Climate

Boring Billion• Rising d98Mo and isotopes of otherredox-sensitive metals indicate thatthe oceans were beginning to become
oxidizing in the Neoarchean –photosynthesis began in the Archean.
• Oxidation state of the Earth’s surface
remained static until theNeoproterozoic (boring billion).
• First animal fossils appear at the endof the Neoproterozoic when O2 rises
again.
• Both events marked by extreme
glaciations.

Large Low S-wave Velocity Provinces (LLSVPs)
in the Deep Mantle• Sharp boundaries require more thanjust thermal difference.
• Largest structures on the planet:
occupy ~5% of mantle volume (10times bigger than the continents)• Rise as high as 1000 km above thecore-mantle boundary.
• Approximately antipodal.
• What are they?

• Primordial material (e.g., magma oceanresidues)?
• Subducted oceanic crust?
• Something else?

Mantle Plumes rise from LLSVPs

SW French et al. Nature 525, 95-99 (2015) doi:10.1038/nature14876

Many plumes rise from the edges of LLSVPs

Jackson et al. (2021)

Plumes contain material recycled from theEarth’s surface

Correlations between radiogenic and stable isotopes establish the presence of recycled material in the Samoan plume.

Plumes also contain an ancient primitive component
Extinct radionuclides: nuclides that existed in the early Solar System but have long since decayed away:•••129I ⟶ 129Xe t

1/2 = 15.7 Ma
244Pu⟶131-136Xe t
1/2 = 81.2 Ma
146Sm⟶144Nd t
1/2 = 103 Ma•182Hf⟶182W t
1/2 = 8.9 Ma

Can geodynamic models show how plumes
can contain both?

Li & MacNamera (2014)

Does plate tectonics organize LLSVPs or do
LLSVPs organize plate tectonics?

Li & Zhong (2009) and Murphy et al. (2021)
argued that supercontinent cyclescontrol/produce LLSVPs.

Dziewonski et al. (2010), Torsvik et al. (2014) argue
LLSVPs are fixed and have organized mantle flow
throughout the Phanerozoic and probably longer.

Are tectonic transitions, the deep mantle,
atmospheric O2 (and therefore life) linked?
• Tectonism and magmatism supplies
fresh rock for erosion, supplying
critical nutrients, most notably
phosphorus, to the oceans,enabling photosynthesis.
• Phosphorite sediements first appear
in the Paleoproterozoic and then do
not reappear until theNeoproterozoic (Bekker & Holland,2012).
• Many (e.g., O’Niell et al., 2022) argue
for reduced tectonism during theBoring Billion, limiting phosphorus
supply until the breakup of Rhodinia
around 800 Ma.

• Volcanism supplies reduced gases
and minerals to the surface
consuming atmospheric O2.

• Condie et al. (2008, 2022) argue
a Paleoproterozoic magmatic lull
allowed O2 to rise in the GOE.
• O’Neill & Aubach (2022) argue
destabilization of deep oxidizedmantle (formed during Earth’s magma
ocean stage, by disproportionation ofFeO during bridgmanite formation)
carried to the surface by mantleplumes drove the GOE.

Conclusion: The deepand shallow Earth areintimately connected.
• The Earth has evolved in a very non-linear
manner, including dramatic changes at thebeginning and end of the Proterozoic,
although the details are debated.
• The deep mantle contains the largest
structures on Earth, LLSVPs, which generate
mantle plumes.
• Material from the Earth’s surface issubducted into the deep mantle and may
influence or create these structures, which
also contain primordial material.
• Mantle plumes influence plate tectonics,
break-up continents and provide fresh
nutrients to the Earth’s biosphere.
• The deep and shallow Earth are intimately
connected, but the details of theseconnections are still poorly understood atbest.