Lunar Crustal Composition, Heterogeneity, and Evolution: Implications for Exploration

Lunar Crustal Composition, Heterogeneity, and Evolution: Implications for Exploration

B. Jolliff, R. Korotev, and R. ZeiglerDepartment of Earth & Planetary SciencesWashington University, St. Louis, USA

Workshop on Science Associated with the Lunar Exploration Architecture, Tempe, AZFeb 27-Mar 2, 2007

B. Jolliff, R. Korotev, and R. ZeiglerDepartment of Earth & Planetary SciencesWashington University, St. Louis, USA

Workshop on Science Associated with the Lunar Exploration Architecture, Tempe, AZFeb 27-Mar 2, 2007

Summary of IssuesSummary of Issues

mGEO2 Determine the composition and evolution of the lunar crust and mantle to constrain the origin and evolution of the Moon and other planetary bodies.

mGEO5 Characterize the crustal geology of the Moon to identify the range of geological materials present.

These objectives are not accomplished at a single site and with a one-time investigation.

These require integrated efforts at multiple landing sites and with different kinds of approaches, including field geology, geophysics, and sample investigations (sample return to Earth).

OutlineOutline

1) The View from Apollo

2) Clementine and Lunar Prospector

3) Lunar Meteorites

4) South Pole-Aitken Basin

5) Future Exploration Objectives

6) Implications for a Polar Outpost Architecture

7) Handoff to Larry Nyquist: Isotopes

Major crustal Igneous Rock Suites- Ferroan anorthosite + kin (FAS)- Magnesian norite, troctolite,

dunite, gabbro- KREEP basalt, alkali-

igneous rock types

Volcanic Suites- Basalts- Volcanic Glasses

Impact Breccias- Feldspathic (upper crust)- Mafic (middle to lower crust)

14161,7373

View from Apollo: Rock TypesView from Apollo: Rock Types

Crustal Igneous Rock SuitesCrustal Igneous Rock Suites

Ferroan AnorthositeEarly plagioclase-rich “flotation crust”

Magnesian SuiteTypical plutonic-magmaticfractionation trendsSome had KREEP-enriched parent magmas

Alkali SuiteExtended fractionation (granite, monzogabbro)Possible relationship to Magnesian-Suitethrough fractional crystallization

Anorthositic Suite

Pristine Nonmare Rocks

20

30

40

50

60

70

80

90

100

60 65 70 75 80 85 90 95 100

Plagioclase Anorthite content

Ma

fic

Sili

ca

te M

g/(

Mg

+F

e) Magnesian

Alkali Suite

Ferroan-

Suite

GN

N

Tr

Crustal Rock Ages, Apollo samplesCrustal Rock Ages, Apollo samples

3.63.73.83.94.04.14.24.34.44.54.6

Age (Ga)

Ar-Ar

Rb-Sr

Sm-Nd

Pb-Pb

Magma OceanSolidification

67215

c

6223

6

dunitetroctolitenoritegabbronorite

Alk Anor, NorQMDGran/Fels

FAS

Magnesian Suite

Alkali Suite

Granulite/Granulitic Breccia

|-----------| W(182/184)

urKREEP -------------------------

(mare source regions)

Ap 17Ap 16Ap 15Ap 14

Ferroan Anorthositic Suite: 4.5-4.3 GaMagnesian Suite: 4.5-4.0 GaAlkali Suite: 4.4-3.8 Ga

Depths of Origin: Mostly Shallow CrustDepths of Origin: Mostly Shallow Crust

Ferroan Suite: < 25 kmSp Troctolite: 16-25 kmTroct w/ Chm-Aug-Opxsymplectite: 42-50 km

Alkali suite: very shallow, upper crustal emplacement

Seismic velocities consistent with: anorthositicgabbro-norite in mid tolower crust

Mafic Impact-melt breccias: (Imbrium, Serenitatis)30-50 km, noritic compositions

McCallum and Schwartz, 2001

Crust-mantle DifferentiationCrust-mantle Differentiation

Apollo and Luna Sites cover a small area of the MoonApollo and Luna Sites cover a small area of the Moon

Near Side Far Side

Clementine: Global TopographyClementine: Global Topography

NearSide

FarSide

M. Wieczorek

Global View: Crustal ThicknessGlobal View: Crustal Thickness

NearSide

FarSide

M. Wieczorek

Crust-mantleboundary near Mare Cognitum?

Khan et al., 2000

FeO (Lunar Prospector)FeO (Lunar Prospector)

NearSide

FarSide

SPA

Thorium (Lunar Prospector)Thorium (Lunar Prospector)

NearSide

FarSide

Geochemical TerranesGeochemical Terranes

Lunar Terranes: Global FeOLunar Terranes: Global FeO

derived from Clementine UV-VIS

ProcellarumKREEPTerrane

ProcellarumKREEPTerrane

FeldspathicHighlands

Terrane

FeldspathicHighlands

Terrane

Eastern Basin Terrane

Eastern Basin Terrane

SPA

Processing by J. Gillis

Lunar Meteorites: Compositions correspond to specific regions of crustLunar Meteorites: Compositions correspond to specific regions of crust

FeldspathicLunar Meteorites• FeO: 4.7 wt.%• Th: 0.3 ppm• Ferroan

ComponentsFLMs represent FeldspathicHighlandsTerrane

mean of “feldspathic” meteorites: ~ 4.7 wt% FeO, consistent with northern far-side highlands.

R. Korotev, http://epsc.wustl.edu/admin/resources/moon_meteorites.html

After P. D. Spudis

TimingTiming

+ later spikes?

(?)

NWA773 olivine gabbro cumulate, ~ 2.8 Ga crystallization: (Fernandes et al. , MAPS, 2003; Borg et al. , Nature, 2004)

Ap 12 alkali anorthosite rock fragments, 3.1-3.2 Ga (Barra et al.,GCA, 2006)

Restricted to PKT?

Petrogenetic Implications, Global AsymmetryPetrogenetic Implications, Global Asymmetry

Mg-richcumulates

Magnesian-suite Intrusives

Mixing

Sinking ofilmenite,Fe-Px, and KREEP-rich

residuum- localized

overturn

ResidualKREEPpockets

Procellarum“KREEP Terrane”

FeldspathicHighlands Terrane

Asymmetry involves mantle heat sources.

Production of “secondary” crust tied to locus of radioactive heat-producingelements

Cumulatemantle “overturn” may have been localized.

Root cause unknown

- Degree-1 downwelling

- Early Procellarumimpact basin?

Origin of Compositional AsymmetryOrigin of Compositional Asymmetry

I.S. McCallum

A.

Crust

KREEP layer

Ilmenite -rich layer

mantle cumulates

B.

Sinking of ilmenite -rich layer, mixing withmantle cumulates, andmigration of KREEP Plume of mixed layer

C.

KREEP

South Pole-Aitken BasinSouth Pole-Aitken Basin

Topo

AitkenCraterAitkenCrater

South Pole

Largest Impact basin on Moon

~ 2200 km diameter

Age unknown, but from superposition, isoldest of large lunar impact basins.

Exhumed lower crustal rocks

South Pole-Aitken Basin is a window to the lower crustSouth Pole-Aitken Basin is a window to the lower crust

Interior of basin is FeO-rich for the small amount of mare basalt.

Ejecta are “intermediate” in FeO content.

Crust appears to be 10-30 km thick beneath basin interior.

Lunar Prospector Thorium distributionLunar Prospector Thorium distribution

Distribution of Th is strongly localized on the Moon.Procellarum KREEP Terrane may contain as much as 40% of the crust’s thorium, …but only covers ~ 15% of the Moon’s surface.

If thorium enrichment extends to some depth in the PKT, mass balance indicates that the entire crust at depth cannot be similarly enriched. Crustal Thickness is important.

Image processedby Jeff Gillis

SPA

PKT

FeO-MgO in lunar samplesFeO-MgO in lunar samples

0

5

10

15

20

0 5 10 15 20 25

FeO, wt.%

MgO

, wt.%

Soils

Lun Mets

Mg’ = 66.5

Mg-SuiteRocks

marebasalts

volcanicglass

Lun Mets

Ap Soils

LP GRS data indicate modestly ferroan SPA rocksLP GRS data indicate modestly ferroan SPA rocks

LP GRS Data, June 2002adj: MgO*0.8

0

5

10

15

20

0 5 10 15 20 25

FeO, wt%

MgO

, wt.%

Mg-SuiteRocks

SPA: mean Mg’ ~ 67SPA: mean Mg’ ~ 67

How to Test Crustal ModelsHow to Test Crustal Models

To improve models, need to sample lower crust.

Best approach is to sample South Pole-Aitken Basin.

Better sample mid-crustal levels: Investigate and sample rocks in-place such as central peak complexes.

Sampling South Pole-Aitken BasinSampling South Pole-Aitken Basin

South Pole is at edge of compositionally distinctive SPA basin materials.

SPA Near side

FeO (LP-GRS)

So. Pole at edge of SPA thorium signatureSo. Pole at edge of SPA thorium signature

Possible Exploration TargetsPossible Exploration Targets

Direct samples of in-place igneous rocks in central peaks, uplifted some tens of km.

Copernicus Crater

GruithuisenDomes

LichtenbergCrater

VallisSchroteri

Northern Procellarum: several high-priority targetsNorthern Procellarum: several high-priority targets

Aristarchus Crater & Volcanic ComplexAristarchus Crater & Volcanic Complex

Lunar Prospector Th data indicate extreme lunar magmatic compositions.

0

10

20

30

40

50

60

70

0 5 10 15 20

FeO, wt%

Th

, pp

m (

INA

A)

Granite

AlkaliAnorth

Monzogabbro(QMD)

KREEP Bas/IMB

Aristarchus

0

10

20

30

40

0 5 10 15 20

FeO, wt.% (Clem)

Th

(p

pm

)

Samples

RemoteSensing

Aristarchus: high Th-FeOAristarchus: high Th-FeO

Lunar “Red Spots” – Evolved nonmare volcanism on the MoonLunar “Red Spots” – Evolved nonmare volcanism on the Moon

Hagerty et al., (2006) JGR 111

Hansteen Alpha, Gruithuisen Domes, and Lassell Massif are silicic, nonmare, volcanic constructs, similar to terrestrial rhyolite domes.

Evolved Magmatism, modelEvolved Magmatism, model

Hagerty et al. (2006) JGR 111

Mantle

Potential Exploration TargetsPotential Exploration Targets

1) South Pole-Aitken Basin: major unsampled geochemical “Terrane”- interior of basin lies 500-1000 km from South Pole; key to early lunar crustal evolution and inner solar-system history.

2) Central peaks of several large impact craters in key locations, e.g., Tycho, Copernicus, Tsiolkovsky = direct samples of bedrock in known locations.

3) Anorthositic Highlands far distant from Procellarum KREEP Terrane.

South Polar Outpost site may allow access to rocks to partially meet 1 and 3 (mGEO2, mGEO5).

Potential Exploration TargetsPotential Exploration Targets

Aristarchus region- Locus of igneous and volcanic (pyroclastic)

activity of extreme compositions“Red Spots” and volcanic domes

- A form of lunar volcanism and rock types from evolved magmatic processes

Youngest mare volcanism:- Determine temporal extent of volcanism; - Association to extended intrusive activity;- Constrain planetary cooling history.

South Polar Outpost site only partially meets mGEO5 (determine range of geologic materials).

Determining Crustal thickness is a high priority.Determining Crustal thickness is a high priority.

A key unknown for understanding the Moon’s crust is its thickness. This has implications for:

- bulk composition

- petrogenesis

At present, only relative thickness is known and multiple geophysical crustal-thickness models exist based on seismic, gravity, and topography data.

A global seismic network is needed to constrain these models adequately (mGEO1). This is not met at a South-Pole Outpost site.