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Dynamics of Lunar mantle evolution: exploring the role of compositional buoyancy

E.M. Parmentier Brown University

September 26, 2018

JAXA/NHK

1) hemispheric crustal asymmetry a) mare basalt distribution b) crustal thickness 2) mare basalts: source complementary to anorthositic crust (Eu anomaly) melting at pressure higher than base of crust 3) timing of mare basalt emplacement and evolution from high to low‐Ti 4) mantle structure a) asymmetric distribution of moonquakes b) liquid outer core and weak layer above core/mantle boundary 5) strength and duration of dynamo magnetic field

Global scale characteristics of the Moon

http://curator.jsc.nasa.gov/education/LPETSS/marebasalt.cfm#fig10

Mare basalt ages Clementine Ti‐distribution

Low‐Ti High‐Ti

Multiple saturation (Ol‐Opx) depth

• f lunar ultramafic glasses

Grove and Krawczynski (2009), Elements, 5, 29–34. Khan and Mosegaard (2001) GRL 28 1791‐1794. Warren, (2004) Treatise of Geochemistry.

Eu fractionation between anorthosite and mare basalt source

Lunar Prospector Thorium Distribution

Thorium detected by gamma ray spectroscopy is concentrically distributed around the Imbrum impace basin (formed ~ 3.85 billion years ago – prior to main mare basalt eruption). Suggests that ilmenite bearing KREEP was concentrated in this area which are the later site for mare basalt eruptions.

Lunar crustal structure post Grail Wieczorek et al. Science 2012.

Interior Structure of the Moon

Lognonné et al. (2003) Earth Planet. Sci. Lett. 211, 27‐44. Wieczorek (2009) Elements, 5, 35–40. Weber et al. (2011) Science 331:309‐312 velocity and density models Apollo seismic network

Models matching Grail density, moment, and Love number:

• fluid outer core with radius of 200–380 km
• solid inner core with radius of 0–280 km and mass fraction of 0–1%
• deep mantle zone of low seismic shear velocity
• mass fraction of the combined inner and outer core is ≤1.5%.

Williams, et al. JGR 2014

Schematic structure of solidifying magma ocean

Stratigraphy predicted from ideal fractional solidification

Idealized overturn following fractional solidification

Ringwood and Kesson (1976) Herbert PLPSC (1980) Spera (1992) Hess and Parmentier (1995)

Q=100 kJ/mol Q=200 kJ/mol

viscosity ∝ exp 𝑅 𝑆𝑈

Creep activation energy composition temperature Chemical structure just after overturn

Internal structure 300 Myr model time

Model for evolution of

• verturned mantle

structure (Zhang et al JGR 2013)

One take home message: Low viscosity allows cooling In dense IBC layer due to small scale convection.

1) Deep heat sources more effectively heat interior. 2) Low thermal conductivity (porous) crust raises the temperature at top of mantle. * Heat source distribution and crustal thermal conductivity affect volume evolution

Zhang et al JGR 2013

* Absence of global‐scale thrust faulting (as seen on Mercury) limits the amount of contraction that has occurred.

Solomon GRL (1978)

* Magma ocean solidification would most likely result in expansion of young lunar crust (dikes) * Viscous relaxation of crust would prevent early tectonic features of contraction or expansion from being recorded permanently Elkins‐Tanton & Bercovici (2014)

Volume evolution as expressed in surface tectonic features

Andrews‐Hanna et al. Science 2012; science.1231753

GRAIL gravity field to degree and

• rder 300

* Core dynamo existed on the Moon between at least 4.25 and 3.56 Ga swith surface field intensities reaching ∼70μT. * Paleomagnetic data from mare basalts demonstrate that the surface magnetic field had declined precipitously by 3.19 Ga.

Tikoo, et al. (2017) Sci. Adv. 3.

Paleointensity of the lunar magnetic field as a function of time

Evans et al. (2014) JGR doi:10.1002/2013JE004494.

just after overturn cmb heatflux no water 200‐km water enriched layer 500‐km water enriched layer

Wet layered lunar mantle: affect on temperature/cmb heat flux

no water water enriched layer temperature differnce

Scheinberg, et al., EPSL doi.org/10.1016/j.epsl.2018.04.015 (2018).

Core convection predicts magnitudes of only ~1μT For core radius 200–380 km surface dipole field 100–700 times smaller than at cmb

How to produce such a strong magnetic field at lunar surface?

http://curator.jsc.nasa.gov/education/LPETSS/marebasalt.cfm#fig10

Mare basalt ages

Age of Imbrium impact basin

Lunar Prospector Thorium Distribution

KREEP concentrated on near side prior to eruption of hi‐Ti mare basalts

Th radially distributed around Imbrium impact

Haskin JGR (1998)

Laneuville et al., JGR 2013 Equivalent of 10 km of KREEP basalt placed below a 40 km thick crust (blue), in the lower 20 km of the crust (cross hatch), or redistributed over the entire crust (orange).

Mantle evolution with enriched heat sources in/below the PKT crust

Viscous Rayleigh-Taylor instabilty of a dense fluid layer

Parmentier, Zhong and Zuber 2002

Gravitational instability of a thin chemically dense (ilmenite‐rich) cumulate created during the fractionation of an anorthositic crust. Long wavelength instability needed to explain the hemispheric asymmetry. Spherical harmonic degree 1 evolves to be the fastest growing wavelength if the viscosity of the dense layer is sufficiently low relative to that of the deeper mantle.

Haoyuan Li, et al., submitted JGR (2018)

Dynamic models of the instability of ilmenite‐rich KREEP layer

layer viscosity = 10‐3 layer thickness = 100 km layer viscosity = 10‐4 layer thickness = 100 km

Boukare, et al, (2018) EPSL.

Overturn during solidifcation

solidification rate fast slow

Slow solidification

~ 104 years ~ 108 years

cumulates + retained melt cumulate compaction + buoyant melt migration

Initially molten Moon Modified from: http://www.psrd.hawaii.edu/Mar09/magmaOceanSolidification.html

Hess JGR (1994) Gross et al. ELSL http://dx.doi.org/10.1016/j.epsl.2013.12.006 (2014) Shearer et al. American Mineralogist doi.org/10.2138/am‐2015‐4817 (2015). Prissel et al. American Mineralogist doi.org/10.2138/am‐2016‐5581 (2016).

The ‘enigmatic’ Mg‐suite

Mg‐rich cumulates

Modified from Shearer et al., New Views of the Moon, Ch. 4, 2006.

Melting Mg‐rich cumulates during rapid mantle overturn

Modified from Shearer et al., New Views of the Moon, Ch. 4, 2006.

Melting Mg‐rich cumulates during rapid mantle overturn

Melting/assimilation of Mg‐rich cumulates during overturn generates Mg‐suite

~ 104 years ~ 108 years

cumulates + retained melt cumulate compaction + buoyant melt migration

Initially molten Moon Modified from: http://www.psrd.hawaii.edu/Mar09/magmaOceanSolidification.html

Magma ocean evolution controlled by surface heat flux (conductive lid or atmosphere)

Physical parameters with estimated values 𝑏 cumulate grain size 1 mm 𝜚 melt fraction (at deposition boundary) 50% Δ𝜛 cumulate – melt density difference 300 kg/m3 𝜊 compaction viscosity ( = cumulate viscosity/𝜚) 1017‐1019 Pa‐s 𝜈 melt viscosity 1 – 10 Pa‐s 𝑙 permeability (calculated) 3 x 10‐9 m2 𝑀 compaction length (calculated) 10‐250 km 𝑊 melt velocity (calculated) 0.01‐0.1 km/yr

Fast deposition Slow deposition 𝑙 𝜚 150 𝑏 1 𝜚 𝑊 Δ𝜛𝑕𝑙 𝜈𝜚 𝑀 𝑙 𝜊 𝜈 ⁄

Cumulate Compaction

Shirley, J.Geol. 1986. Both figures at same model times but note difference in vertical scale.

• ver 104 years

compacted layer thickness = 100 ‐ 1000 km

Topography from Mars Orbiter Laser Altimetry Mantle source composition

• f SNC meteorites

1) Mars displays early developed hemispheric asymmetry 2) Martian basaltic meteorite mantle sources lie on mixing line between lunar mafic cumulates and KREEP

Zuber et al. (2000) Science 287, 1788‐1793.

Magma ocean on early Mars?