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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.