1.4 Bulk Earth composition

Earth's layers tell a story of planetary evolution and geochemical cycling. From the silicate-rich crust to the iron-dominated core, each layer's composition reflects Earth's formation and differentiation processes over billions of years.

Understanding the distribution of major elements like oxygen, silicon, iron, and magnesium is key to explaining Earth's structure. Trace elements and isotopes provide further insights into the planet's formation, age, and ongoing geochemical processes.

Composition of Earth's layers

• Earth's layered structure reflects its formation and differentiation processes over geological time
• Understanding the composition of Earth's layers provides insights into planetary evolution and geochemical cycling
• Geochemical analysis of each layer informs models of Earth's bulk composition and internal dynamics

Crust composition

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• Consists primarily of silicate minerals rich in lighter elements
• Oceanic crust composed mainly of basaltic rocks (~7 km thick)
• Continental crust more felsic, dominated by granitic rocks (~35 km average thickness)
• Major elements include oxygen (46.6%), silicon (27.7%), aluminum (8.1%), and iron (5.0%)
• Trace elements concentrated in the crust due to incompatibility in mantle minerals

Mantle composition

• Predominantly composed of silicate minerals rich in magnesium and iron
• Upper mantle consists of olivine, pyroxenes, and garnet
• Lower mantle contains high-pressure phases like perovskite and magnesiowüstite
• Major elements include oxygen (44.8%), magnesium (22.8%), silicon (21.5%), and iron (5.8%)
• Composition inferred from mantle xenoliths, seismic data, and high-pressure experiments

Core composition

• Divided into liquid outer core and solid inner core
• Primarily composed of iron (~85%) and nickel (~5%)
• Light elements (oxygen, sulfur, silicon) present to account for density deficit
• Composition inferred from seismic data, meteorite studies, and high-pressure experiments
• Temperature at the core-mantle boundary estimated at ~3800 K

Major element abundances

• Major elements constitute the bulk of Earth's mass and define its overall composition
• Understanding major element distribution helps explain Earth's structure and differentiation
• Geochemical models of Earth's composition rely heavily on accurate major element estimates

Oxygen and silicon dominance

• Oxygen most abundant element by mass in the bulk Earth (~30%)
• Silicon second most abundant element (~15%) in the bulk Earth
• Together form the backbone of silicate minerals in crust and mantle
• Oxygen-silicon ratio varies between layers due to differentiation processes
• Abundance reflects the composition of the solar nebula during Earth's formation

Iron and magnesium content

• Iron third most abundant element (~35% in core, ~5% in mantle)
• Magnesium fourth most abundant element (~13% in mantle)
• Iron concentrated in the core due to its siderophile nature
• Magnesium primarily found in mantle minerals (olivine, pyroxenes)
• Fe/Mg ratio in mantle influences its physical properties and melting behavior

Minor elements distribution

• Aluminum concentrated in the crust due to its lithophile nature
• Calcium important in both crustal and mantle minerals
• Sodium and potassium enriched in the continental crust
• Titanium present in accessory minerals throughout the crust and mantle
• Phosphorus distributed between mantle, crust, and potentially the core

Trace element distribution

• Trace elements occur in concentrations less than 0.1% by weight
• Distribution provides insights into Earth's formation and differentiation processes
• Geochemical behavior of trace elements used to track mantle melting and crustal recycling

Lithophile elements

• Prefer to bond with oxygen and concentrate in silicate minerals
• Include rare earth elements (REEs), uranium, thorium, and potassium
• Enriched in the crust relative to the mantle
• Used as tracers for crustal processes and mantle melting
• Some (U, Th, K) produce heat through radioactive decay, influencing Earth's thermal evolution

Siderophile elements

• Have strong affinity for metallic iron
• Include platinum group elements (PGEs), gold, and nickel
• Concentrated in Earth's core during planetary differentiation
• Mantle abundances provide clues about core formation processes
• Some (highly siderophile elements) may have been added by late accretion events

Chalcophile elements

• Tend to combine with sulfur
• Include copper, zinc, lead, and mercury
• Concentrated in sulfide minerals in ore deposits
• Distribution affected by core formation and mantle melting processes
• Some chalcophile elements (Cu, Zn) also show lithophile behavior in Earth's mantle

Isotopic composition

• Isotopes provide crucial information about Earth's age, formation, and evolution
• Variations in isotopic ratios used to trace geochemical processes and reservoirs
• Isotopic studies fundamental to understanding Earth's bulk composition and differentiation

Stable isotopes in Earth

• Include isotopes of elements like oxygen, carbon, and sulfur
• Fractionation occurs during physical and chemical processes
• $δ^18O$ values used to study climate change and water cycles
• $δ^13C$ variations trace carbon cycling between atmosphere, biosphere, and lithosphere
• Sulfur isotopes ($δ^34S$) indicate redox conditions and biological activity

Radiogenic isotopes

• Produced by radioactive decay of parent isotopes
• Include systems like Rb-Sr, Sm-Nd, U-Pb, and Lu-Hf
• Used for geochronology to determine ages of rocks and minerals
• Serve as tracers for mantle and crustal processes
• Isotopic ratios ($^87Sr/^86Sr$, $^143Nd/^144Nd$) help identify different mantle reservoirs

Bulk silicate Earth model

• Represents the composition of Earth excluding the core
• Essential for understanding mantle composition and evolution
• Provides a framework for interpreting geochemical observations

Definition and assumptions

• Assumes homogeneous mixing of mantle and crust after core formation
• Excludes the core due to its inaccessibility for direct sampling
• Based on analyses of mantle-derived rocks and meteorites
• Assumes chondritic ratios for refractory lithophile elements
• Accounts for volatile element depletion relative to chondrites

Compositional estimates

• Major elements: Si (21%), Mg (22%), Fe (6%), Al (2%), Ca (2%), Na (0.3%)
• Trace element abundances normalized to CI chondrites
• REE pattern shows depletion in light REEs relative to heavy REEs
• Highly siderophile elements depleted due to core formation
• Volatile elements (K, Rb, Cs) depleted relative to chondritic abundances

Primitive mantle composition

• Represents the composition of Earth's mantle before crust extraction
• Crucial for understanding Earth's early differentiation and evolution
• Serves as a baseline for studying mantle melting and crustal formation processes

Chondritic Earth hypothesis

• Assumes Earth formed from chondritic material similar to CI carbonaceous chondrites
• Predicts refractory lithophile element ratios in primitive mantle match chondrites
• Supported by similarities in isotopic compositions (O, Cr, Ti) between Earth and chondrites
• Explains observed depletions in siderophile elements due to core formation
• Challenges include explaining Earth's non-chondritic Mg/Si and Al/Si ratios

Non-chondritic models

• Propose Earth formed from material with non-chondritic composition
• Attempt to explain Earth's unique elemental and isotopic signatures
• Enstatite chondrite model suggests formation from reduced, volatile-poor material
• Collisional erosion model proposes loss of early-formed crust during impacts
• Some models invoke mixing of different types of chondritic and non-chondritic materials

Core formation effects

• Core formation significantly influenced Earth's geochemical evolution
• Understanding these effects crucial for interpreting mantle and crustal compositions
• Provides insights into conditions during Earth's early differentiation

Siderophile element depletion

• Siderophile elements strongly partitioned into the core during formation
• Mantle abundances of siderophile elements lower than chondritic values
• Degree of depletion depends on element's metal-silicate partition coefficient
• Moderately siderophile elements (Ni, Co) less depleted than highly siderophile elements (PGEs)
• Excess of highly siderophile elements in mantle suggests late veneer addition after core formation

Light element enrichment

• Core density lower than pure iron-nickel alloy, indicating presence of light elements
• Candidates include sulfur, oxygen, silicon, carbon, and hydrogen
• Light element content estimated at 5-10% by mass
• Partitioning of light elements between core and mantle affects their mantle abundances
• Core composition models must balance geophysical constraints with geochemical observations

Volatile element depletion

• Earth depleted in volatile elements compared to chondritic compositions
• Understanding this depletion crucial for models of Earth's formation and early evolution
• Influences interpretations of bulk Earth composition and volatile delivery mechanisms

Causes of depletion

• Incomplete condensation from the solar nebula during planet formation
• Loss during high-energy impacts during accretion
• Hydrodynamic escape of early atmosphere
• Preferential incorporation of volatiles into the core
• Combination of multiple processes likely responsible for observed depletion pattern

Implications for Earth formation

• Suggests Earth formed primarily from volatile-poor materials in the inner solar system
• Challenges models of late volatile delivery by comets or water-rich asteroids
• Influences estimates of bulk Earth water content and distribution
• Affects models of early atmospheric composition and evolution
• Provides constraints on timing and conditions of Earth's accretion

Compositional evolution

• Earth's composition has changed over geological time due to various processes
• Understanding this evolution crucial for interpreting present-day geochemical observations
• Provides insights into long-term cycling of elements between different reservoirs

Differentiation processes

• Core formation early in Earth's history concentrated siderophile elements
• Mantle melting and crust extraction led to compositional stratification
• Fractional crystallization in magma chambers produced evolved crustal rocks
• Metamorphism altered rock compositions through mineral reactions and fluid interactions
• Subduction recycled crustal material back into the mantle, modifying its composition

Crust-mantle exchange

• Partial melting of mantle produces basaltic magmas, extracting incompatible elements
• Delamination of lower crust returns material to the mantle
• Subduction of oceanic crust and sediments introduces heterogeneities into the mantle
• Fluid and melt migration through the mantle redistributes elements
• Mantle plumes bring deep mantle material to the surface, providing samples of primitive composition

Geochemical reservoirs

• Earth's interior divided into distinct geochemical reservoirs with unique compositions
• Understanding these reservoirs crucial for interpreting mantle heterogeneity
• Provides insights into mantle dynamics and long-term element cycling

Depleted mantle

• Represents mantle that has undergone melt extraction to form oceanic crust
• Characterized by low concentrations of incompatible elements
• Isotopic signatures include low $^87Sr/^86Sr$ and high $^143Nd/^144Nd$ ratios
• Source of mid-ocean ridge basalts (MORBs)
• Occupies upper mantle and portions of the lower mantle

Enriched mantle components

• Represent mantle regions with higher concentrations of incompatible elements
• Include EM1 (enriched mantle 1), EM2 (enriched mantle 2), and HIMU (high μ)
• EM1 possibly related to recycled lower continental crust or pelagic sediments
• EM2 may represent recycled upper continental crust or terrigenous sediments
• HIMU characterized by high U/Pb ratios, possibly from recycled oceanic crust
• These components contribute to the geochemical diversity of ocean island basalts (OIBs)

Composition vs other planets

• Comparing Earth's composition to other planets provides insights into solar system formation
• Helps identify unique aspects of Earth's composition and evolution
• Crucial for understanding habitability and the potential for life on other planets

Terrestrial planets comparison

• Mercury highly enriched in iron, possibly due to giant impact or solar nebula processes
• Venus similar in size and density to Earth, but with a much thicker atmosphere
• Mars smaller and less dense than Earth, with a thin atmosphere and no active plate tectonics
• Earth unique among terrestrial planets in its abundant surface water and plate tectonic activity
• Differences in volatile content and oxidation state influence planetary differentiation and evolution

Earth-Moon system similarities

• Moon likely formed from Earth's mantle material after a giant impact
• Isotopic compositions of Earth and Moon nearly identical for many elements (O, Cr, Ti)
• Moon more depleted in volatile elements compared to Earth
• Lunar highlands crust enriched in aluminum, similar to Earth's continental crust
• Differences in iron content between Earth and Moon constrain models of lunar formation

Analytical techniques

• Various methods used to study Earth's composition and structure
• Combination of direct sampling, remote sensing, and laboratory experiments
• Continuous improvement in analytical precision and accuracy enhances our understanding

Seismic studies

• Use seismic waves to probe Earth's internal structure
• P-wave and S-wave velocities provide information on composition and physical state
• Seismic tomography reveals 3D structure of mantle convection
• Discontinuities (Moho, 410 km, 660 km) indicate compositional or phase changes
• Free oscillations of Earth constrain bulk properties like density and elastic moduli

Geochemical sampling methods

• Direct sampling of crustal rocks through field collection and drilling
• Mantle xenoliths brought to surface by volcanic eruptions provide samples of upper mantle
• Mid-ocean ridge basalts (MORBs) sample upper mantle composition
• Ocean island basalts (OIBs) may sample deeper mantle reservoirs
• High-precision analytical techniques (ICP-MS, TIMS, SIMS) measure elemental and isotopic compositions

Implications for geodynamics

• Earth's composition strongly influences its dynamic behavior
• Understanding compositional variations crucial for modeling mantle convection and plate tectonics
• Provides insights into Earth's thermal and chemical evolution over geological time

Mantle convection

• Compositional variations create density differences driving convection
• Thermal expansion and phase transitions affect convection patterns
• Viscosity variations due to composition and temperature influence convection vigor
• Melting and melt extraction at mid-ocean ridges drive shallow mantle flow
• Deep mantle plumes may originate from compositionally distinct regions (large low shear velocity provinces)

Plate tectonics drivers

• Slab pull from subducting oceanic lithosphere main driver of plate motions
• Ridge push from cooling and thickening of oceanic lithosphere contributes to plate movement
• Mantle drag influences plate velocities and directions
• Compositional buoyancy of continental lithosphere resists subduction
• Dehydration of subducting slabs influences mantle wedge melting and arc magmatism