Frontiers of Technology, Exploring
Earth's Seafloor and Beyond
Clive R. Neal
@Prof_Clive_Neal

Plume Volcanism =Planetary Volcanism

Tohoku University Forum for Creativity: International Symposium, Frontier of Understanding Earth's Interior and Dynamics

Earth is Unique
WaterLife
Plate Tectonics

Plume Volcanism on Earth• Transfer of heat across a boundary• Plume rise initiated by subducted material reaching the boundary• Correlated with Large Low-Shear Velocity Provinces (LLSVPs) – thermochemical
pile at CMB?

Large Low-Shear Velocity Provinces
Two large antipodal LLSVPs Africa andPacific (near equator - 180º apart)Dziewonski et al. (2010) EPSL 299, 69-79

Large Igneous Provinces (LIPs)- age span: 16-297 Ma- irregular distribution

Paleogeographic relocation• LIPs cluster near LLSVP margins• long-term stability, dense and hot
Burke & Torsvik, 2004, EPSL; Torsvik et al., 2006, GIJ; Burke et al. 2007,EPSL; Torsvik et al. 2008, EPSL; Torsvik et al. (2010, Nature)Large Low-Shear Velocity Provinces
LLSVPs are compositionally subdivided into
two domains:••

Primordial bottom domain near the core-mantle
boundaryBasaltic shallow domain that extends from 1100 to2300 km depth.

Ballmer et al (2016) Geochem. Geophys. Geosyst., 17, 5056–5077,
doi:10.1002/2016GC006605•

Double-layered piles exist for billions of of years if
density differences of ~100 kg/m3 exist.

•Short-lived ‘‘secondary’’ plumelets rise from LLSVP roofs and entrain basaltic material that has
evolved in the lower mantle.

•Long-lived, vigorous ‘‘primary’’ plumes instead rise from LLSVP margins and entrain a mix ofmaterials, including small fractions of primordial material.

•

Consistent with the locations of hot spots relative to LLSVPs, and address the geochemical andgeochronological record of (oceanic) hot spot volcanism.

Large Low-Shear Velocity Provinces
Possible LLSVP-material
Basalt-rich•

Separated from deep subducted lithosphere
Niu (2018) Geosci. Front. 9, 1265-1278
Fan et al. (2022) G3, 23, e2021GC009879•

Age of ~3-0 Ga

Peridotitic (or komatiitic) with elevated
Fe/Mg-ratio•

Age of >3.0 Ga

Decomposition of Oxidized Mantle
Wang et al. (2021) Nat. Comm. https://doi.org/10.1038/s41467-021-22185-1•Enrichment of Fe3+-rich bridgmanite in a pyrolitic composition can explain the observed features ofthe LLSVPs.

•Presence of Fe3+-rich materials within LLSVPs may have profound effects on the deep reservoirs ofredox-sensitive elements and their isotopes.

Large Low-Shear Velocity Provinces
Possible LLSVP-material
Basalt-rich•

Separated from deep subducted lithosphere
Niu (2018) Geosci. Front. 9, 1265-1278
Fan et al. (2022) G3, 23, e2021GC009879•Age of ~3-0 Ga

Peridotitic (or komatiitic) with elevated
Fe/Mg-ratio•

Age of >3.0 Ga

Primordial Magma Ocean CumulatesKreielkamp et al. (2022) EPSL 579, 117357.
Pachhai et al. (2022) Nat. Geosci. 15, 79-84.
•Assumed to be compositionally dense leftovers fromthe magma-ocean phase that are formed to dense
accumulations (piles) by mantle flow.

Schaefer & Elkins-Tanton (2018) Phil Trans.
Roy. Soc. A 376, 20180109.
•Magma ocean crystallization leads to a massive mantle
overturn that may set up a stably stratified mantle.

•Could lead to significant delays or total prevention ofplate tectonics on some planets.

Solar System Volcanism• Plate Tectonics &subduction-relatedvolcanism is unique toEarth.
Mars – Tharsis
Volcanoes• Hot spot volcanism is seen
throughout the planets of
the inner Solar System.

Venus: linear “pancake” volcanoes
Mercury – Caloris Basin caldera

Earth:
Hawaiian-Emperor• How is plume volcanism
initiated without platetectonics?

Origin of LLSVPs• Suggests subducted material in LLSVPs are a secondary feature of LLSVPs IF
these induce plume volcanism on otherplanetary bodies.
Hypothesis: LLSVPs represent evidence of theprimordial differentiation of terrestrial planetary
bodies through an initial magma ocean stage, andthese are responsible for plume volcanism.
• Cannot be tested on Earth because of its sizeand long-lived heat engine.
• Difficult to test on other planets for the same reason.

The Moon is ideal for testing this hypothesis

Our Moon
•
•
•
•
•

•

Dark area = mare (flood) basalts.

•Mostly on the near side – crust isthinner.

•Derived from a mantle comprisedof magma ocean cumulates.

•Crust = plagioclase flotationcumulates.

Differentiated source for basalts:
Olivine + Orthopyroxene early;
Plagioclase Clinopyroxene +Ilmenite later;“KREEP” = last dregs.
Density instability – overturn ofthe cumulate pile.

Cumulate
Overturn
Spera (1992) GCA 56,2253-2266

Our Moon
Cumulate Overturn

Basalts & Volcanic Glasses:oooVery Low-Ti (<1 wt.% TiO2)
Low-Ti (1-5 wt.% TiO2)
High-Ti (>5 wt.% TiO2)

Spera (1992) GCA
56, 2253-2266

Our Moon

Crisium = 3.42 Ga
Nyquist et al., 1978, Mare
Crisium, 631-656

Fecunditatis = 3.62 Ga
Papanastassiou & Wasserburg,1972, EPSL 13, 368-374

Nectaris = 3.89 Ga
Stöffler & Ryder, 2001, Space
Sci. Rev. 96, 9-54

Hiesinger et al. (2011) GSA Spec. Pap. 477, 1-51

Our Moon
Plume activity over the life of the Moon.

Th “hot spot” identified from the Lunar Prospector
mission – does it extend to depth?
Youngest volcanics in the Th hot spot area:
Procellarum KREEP Terrane.
Laneuville et al. (2018) JGR Planets 123, 3144–3166,

Our Moon
Verification needed –Geophysical data

Apollo detected 4 types of seismic events on the Moon.
1) Thermal Moonquakes - Associated with heating and expansion ofthe crust. Lowest magnitude of all moonquakes.
2) Deep Moonquakes - 850-1,000 km. > 7,000 recorded. Originate from“nests” - >300 nests defined from Apollo seismic data to date. Smallmagnitude (< 3). Associated with tidal forces. Predominantly near side.
3) Meteoroid Impacts - >1,700 events representing meteoroid masses
between 0.1 and 100 kg were recorded 1969-1977. Smaller impacts
were too numerous to count.
4) Shallow Moonquakes - some > 5magnitude. Exact locations unknown.
Indirect evidence suggests focal depthsof 50-200 km.

Network too close to detect deepmantle and core structure

Using the Moon to Explore the Origin of LLSVPs:
Lunar LIPs
The Lunar Geophysical Network Mission• High priority science in NASA’s last two decadalsurveys.
• Named medium-class mission in the New Frontiersline ($1-1.5 billion cost cap).
• Planetary Mission Concept Study funded in 2019https://science.nasa.gov/science-pink/s3fspublic/atoms/files/Lunar%20Geophysical%20Network.pdf• Globally distributed network of seismometers, heatflow probes, electromagnetic sounders, laser
retroreflectors

Using the Moon to Explore the Origin of LLSVPs:
Lunar LIPs
Goal: To understand the initial stages of terrestrial planet evolution.
Objectives: Define the interior structure of the Moon.
Constrain the interior and bulk composition of the Moon.
Delineate the vertical and lateral heterogeneities within the interior of theMoon as they relate to surface features and terranes.
Evaluate the current seismo-tectonic activity of the Moon.

Watters et al. (2012) Nature Geosci.. 5, 181-185

Jolliff et al. (2000) J. Geophys. Res. 105, 4197-4216

Using the Moon to Explore the Origin of LLSVPs:
Lunar LIPs
• 4 identical landers
deployed globally.
• Communications
satellite required forthe far-side station.
• Sites chosen tomaximize science
return.
Landing sites for 4 geophysicalstations:
Haviland et al. (2022) Planet. Sci.
Jour. 3:40 (21 pp.)https://doi.org/10.3847/PSJ/ac0f82

Summary• Plume volcanism is the norm on rocky planetary bodies in the Solar System.
• On Earth, plume volcanism is associated with LLSVPs that contain accumulated
subducted material.
• How is plume volcanism initiated on other planetary bodies?
• Ultimate origin of the LLSVPs is due to cumulates from initial planetarydifferentiation via a magma ocean – the Earth story is confused by subsequentaccumulation of subducted material at the CMB.
• Test this on the Moon through the Lunar Geophysical Network mission:• Moon has experience plume activity (flood basalts, Th anomaly)• The lunar interior preserves the magma ocean cumulates from the initialdifferentiation (as see in mare basalt compositions).

Large Low-Shear Velocity Provinces••••LLSVPs contain well-separated, low-velocity conduitsthat extend vertically throughout the lower mantle.
Each LLSVP is composed of a bundle ofthermochemical upwellings probably enriched indenser than average material.
The overall shape of the LLSVPs controlled by thedistribution of subducted slabs.
Position of thermochemical LLSVPs and individual
Davaille & Romanowicz (2020) Tectonics 39, e2020TC006265
upwelling dynamics should be time dependent.
Ultralow velocity zones (ULVZs) cluster along theedges of the LLSVPs.
• (a) ULVZ locates at the far edge of the LLSVP.
• (b) Steepened edge of the LLSVP may develop
due to presence of subducted material.
• ULVZ may be driven towards the interior of theLLSVP, and develop a possible plume.
Sun et al. (2019) GRL 46, 3142-3152