11.4 Lunar geochemistry

Lunar geochemistry offers a unique window into the Moon's formation and evolution. By analyzing lunar samples and comparing them to Earth, scientists gain insights into early Solar System dynamics and planetary differentiation processes.

The Moon's composition, isotopic systems, and geochemical reservoirs provide crucial data for understanding its origin and history. From volatile depletion patterns to evidence of a global magma ocean, lunar geochemistry continues to shape our understanding of planetary formation and evolution.

Composition of lunar materials

• Lunar materials provide crucial insights into the Moon's formation, evolution, and geochemical processes
• Understanding lunar composition informs broader theories in isotope geochemistry and planetary science
• Analyzing lunar samples reveals key differences from Earth's composition, shedding light on early Solar System dynamics

Major element abundances

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• Lunar rocks exhibit higher concentrations of refractory elements (Al, Ca, Ti) compared to Earth
• Silicon and oxygen dominate lunar composition, comprising ~60% of lunar mass
• Iron content varies significantly between lunar mare (~13-17 wt%) and highland regions (~4-7 wt%)
• Magnesium abundance influences mineral assemblages in lunar rocks (olivine, pyroxene)

Trace element distributions

• Rare Earth Elements (REEs) show distinctive patterns in different lunar rock types
• KREEP-rich materials contain elevated concentrations of incompatible elements (K, REE, P, U, Th)
• Siderophile elements (Ir, Os, Pt) depleted in lunar mantle due to core formation
• Lunar anorthosites exhibit positive Eu anomalies, indicating plagioclase accumulation

Volatile element depletion

• Moon shows significant depletion in volatile elements (Na, K, Rb, Cs) relative to Earth
• Volatile/refractory element ratios (K/U, Rb/Sr) lower in lunar rocks compared to terrestrial values
• Depletion pattern consistent with high-temperature formation scenario
• Moderately volatile elements (Zn, Cl, Cu) show variable abundances across lunar samples

Isotopic systems in lunar rocks

• Isotopic compositions of lunar rocks provide critical constraints on Moon's origin and evolution
• Lunar isotope geochemistry reveals similarities and differences with Earth's isotopic signatures
• Studying lunar isotopes helps reconstruct the Moon's thermal and chemical history

Radiogenic isotopes

• Long-lived systems (Rb-Sr, Sm-Nd, Lu-Hf) used to date lunar rocks and determine source characteristics
• Short-lived systems (146Sm-142Nd, 182Hf-182W) constrain timing of lunar formation and differentiation
• U-Pb system in zircons provides precise ages for lunar crustal rocks
• Extinct radionuclides (26Al, 60Fe) offer insights into early Solar System processes

Stable isotopes

• Oxygen isotopes (δ18O) nearly identical between Earth and Moon, suggesting genetic relationship
• Silicon isotopes (δ30Si) show slight differences between lunar and terrestrial samples
• Magnesium isotopes (δ26Mg) used to trace lunar magmatic processes and mantle heterogeneity
• Iron isotopes (δ56Fe) indicate lunar core formation and mare basalt petrogenesis

Noble gas isotopes

• Helium, neon, and argon isotopes in lunar samples reveal solar wind implantation history
• Xenon isotopes provide evidence for early degassing of lunar interior
• Krypton isotopes used to constrain timing of volatile loss during lunar formation
• Cosmogenic noble gas isotopes (21Ne, 38Ar) allow determination of surface exposure ages

Lunar formation theories

• Lunar formation theories aim to explain the Moon's unique geochemical and isotopic characteristics
• Understanding lunar origin crucial for broader planetary formation models in isotope geochemistry
• Geochemical constraints play a key role in evaluating different formation scenarios

Giant impact hypothesis

• Proposes Moon formed from debris ejected after Mars-sized impactor collided with proto-Earth
• Explains lunar mass, angular momentum of Earth-Moon system, and iron depletion in lunar mantle
• Canonical model struggles to account for isotopic similarities between Earth and Moon
• Modified versions (synestia, multiple impact) attempt to reconcile geochemical observations

Isotopic constraints on formation

• Nearly identical oxygen isotope compositions of Earth and Moon support genetic relationship
• Silicon isotopes slightly different between Earth and Moon, challenging complete equilibration
• Titanium isotopes show similarities, suggesting derivation from same reservoir as Earth
• Tungsten isotopes indicate rapid accretion of Moon following giant impact

Differentiation of the Moon

• Lunar differentiation processes shaped the Moon's internal structure and geochemical reservoirs
• Understanding differentiation essential for interpreting isotopic signatures in lunar samples
• Differentiation history informs broader concepts in planetary evolution and isotope geochemistry

Magma ocean concept

• Proposes global-scale melting of lunar exterior following formation
• Explains observed compositional layering and complementary geochemical reservoirs
• Depth estimates range from 400-1000 km based on geochemical and geophysical constraints
• Crystallization sequence determines distribution of elements between crust, mantle, and core

Lunar crust formation

• Plagioclase flotation in magma ocean led to formation of anorthositic crust
• Ferroan anorthosites represent primary crustal rocks, with ages ~4.3-4.5 Ga
• Mg-suite rocks formed later, possibly through serial magmatism or crustal overturn
• KREEP-rich materials concentrated between crust and mantle during late-stage crystallization

Mantle stratification

• Cumulate mantle formed through fractional crystallization of magma ocean
• Olivine-rich lower mantle transitions to pyroxene-rich upper mantle
• Density inversions may have triggered mantle overturn, explaining some geochemical signatures
• Ilmenite-bearing cumulates formed late in crystallization sequence, influencing mare basalt sources

Lunar volcanism

• Lunar volcanism provides insights into the Moon's thermal and chemical evolution
• Volcanic products offer window into lunar interior composition and differentiation processes
• Understanding lunar volcanism crucial for interpreting isotopic signatures in lunar samples

Mare basalt geochemistry

• Mare basalts formed through partial melting of lunar mantle sources
• Exhibit wide range of TiO2 contents, from very low-Ti (1-2 wt%) to high-Ti (>9 wt%) varieties
• REE patterns reflect varying degrees of partial melting and source heterogeneity
• Isotopic compositions (Sr, Nd, Hf) indicate derivation from distinct mantle reservoirs

KREEP-rich materials

• Represent last dregs of lunar magma ocean crystallization
• Enriched in incompatible elements (K, REE, P, U, Th) by factors of 100-1000 relative to bulk Moon
• Found in various lunar rock types, including some mare basalts and highland breccias
• Isotopic signatures of KREEP provide constraints on timing of lunar differentiation

Pyroclastic deposits

• Formed through explosive volcanic eruptions on lunar surface
• Contain volcanic glass beads with distinctive compositions (green, orange, red)
• Provide evidence for volatile-rich regions in lunar interior
• Isotopic analysis of pyroclastic glasses reveals information about lunar mantle heterogeneity

Lunar chronology

• Lunar chronology establishes timeline of major events in Moon's history
• Accurate dating crucial for understanding lunar evolution and broader Solar System processes
• Integrating multiple chronological methods provides robust framework for lunar history

Radiometric dating methods

• U-Pb system in zircons provides precise ages for ancient lunar crustal rocks
• Rb-Sr and Sm-Nd isochrons used to date crystallization of mare basalts and other igneous rocks
• 40Ar-39Ar dating applied to impact melt rocks to constrain timing of major impact events
• Lu-Hf system offers additional constraints on timing of lunar differentiation

Impact flux history

• Early lunar history characterized by high impact rates during Late Heavy Bombardment
• Decline in impact flux over time inferred from dating of impact melt rocks
• Debate ongoing about existence of impact spike around 3.9 Ga (terminal cataclysm hypothesis)
• Recent studies suggest more continuous decline in impact rate rather than discrete spike

Crater counting techniques

• Utilizes relationship between crater size-frequency distribution and surface age
• Calibrated using radiometric ages from Apollo and Luna landing sites
• Allows estimation of relative ages for unsampled lunar regions
• Crater production functions account for changes in impactor flux over time

Lunar water

• Discovery of lunar water revolutionized understanding of Moon's volatile inventory
• Presence of water influences models of lunar formation and evolution
• Water content and isotopic composition provide insights into lunar interior processes

Evidence from Apollo samples

• Apatite grains in lunar rocks contain measurable amounts of water (100-1000 ppm H2O)
• Melt inclusions in olivine from pyroclastic glasses show elevated water contents
• Hydrogen isotopes in lunar minerals indicate presence of indigenous water
• Plagioclase-hosted melt inclusions in ancient anorthosites suggest early water-bearing magmas

Remote sensing observations

• M3 instrument on Chandrayaan-1 detected widespread 3-μm absorption feature indicative of OH/H2O
• LCROSS impact experiment revealed water ice in permanently shadowed regions near lunar poles
• Diviner data shows evidence for surface hydration varying with lunar day/night cycle
• Mini-RF radar observations suggest presence of subsurface ice deposits

Isotopic signatures of water

• D/H ratios in lunar samples show wide range, from Earth-like to highly enriched values
• Some lunar rocks exhibit extremely low δD values, suggesting primitive hydrogen source
• Oxygen isotopes in lunar water similar to terrestrial values, supporting genetic relationship
• Chlorine isotopes in lunar rocks show large fractionations, possibly related to degassing processes

Lunar regolith

• Lunar regolith represents uppermost layer of fragmented rock and dust on Moon's surface
• Regolith formation and evolution processes influence geochemical signatures of lunar samples
• Understanding regolith crucial for interpreting remote sensing data and planning future missions

Space weathering effects

• Micrometeorite impacts and solar wind irradiation alter optical properties of lunar surface materials
• Formation of nanophase iron particles in regolith grains leads to spectral reddening and darkening
• Vapor deposition coatings on regolith particles affect elemental and isotopic compositions
• Agglutinates (glass-bonded aggregates) form through micrometeorite impacts, concentrating certain elements

Solar wind implantation

• Noble gases (He, Ne, Ar) implanted in lunar regolith grains by solar wind
• Hydrogen from solar wind contributes to surficial OH/H2O detected by remote sensing
• Nitrogen isotopes in lunar regolith show evidence of solar wind implantation
• Carbon in lunar soils primarily derived from solar wind, with distinct isotopic signature

Micrometeorite impacts

• Constant bombardment by micrometeorites contributes to regolith gardening and mixing
• Micrometeorite flux estimated from studies of lunar soil grains and impact pits
• Impacts create glassy spherules and agglutinates, altering regolith composition
• Siderophile element enrichments in lunar soils attributed to micrometeorite contributions

Geochemical reservoirs

• Lunar geochemical reservoirs reflect differentiation processes and subsequent evolution
• Understanding reservoir compositions crucial for interpreting isotopic signatures in lunar samples
• Geochemical heterogeneity in lunar interior influences composition of volcanic products

Lunar highlands composition

• Dominated by anorthositic rocks formed through plagioclase flotation in magma ocean
• Ferroan anorthosites represent primary crustal rocks, with high Al2O3 and CaO contents
• Mg-suite rocks (norites, troctolites, dunites) form minor component of highlands crust
• KREEP-rich materials concentrated in Procellarum KREEP Terrane on lunar nearside

Mare basalt source regions

• Heterogeneous mantle sources reflect cumulate layering and possible overturn
• Low-Ti basalt sources generally deeper than high-Ti sources
• Isotopic variations (Sr, Nd, Hf) indicate distinct mantle reservoirs with variable KREEP contributions
• Pyroxene-rich cumulates play important role in mare basalt petrogenesis

Lunar core composition

• Geophysical data suggest small core (~1-3% of lunar mass)
• Likely composed primarily of Fe with some Ni and S
• Tungsten isotopes indicate rapid core formation following Moon's accretion
• Magnetic field measurements suggest early lunar dynamo, implying partially liquid core

Comparative planetology

• Comparing Moon's geochemistry to other planetary bodies provides broader context
• Similarities and differences between Moon and other objects inform theories of planetary formation
• Lunar studies contribute to understanding of early Solar System processes and terrestrial planet evolution

Moon vs Earth geochemistry

• Oxygen isotopes nearly identical, suggesting genetic relationship or similar source materials
• Moon depleted in volatile elements relative to Earth, consistent with high-temperature formation
• Siderophile element abundances in lunar mantle lower than Earth's due to more efficient core formation
• Lunar mantle more reduced than Earth's, influencing mineral assemblages and element partitioning

Moon vs other terrestrial bodies

• Mercury shows extreme depletion in volatiles, even compared to Moon
• Venus lacks satellite, possibly due to different impact history or formation conditions
• Mars exhibits greater volatile retention than Moon, with evidence for past surface water
• Vesta (asteroid) shows some geochemical similarities to Moon, including global differentiation