8.1 Mantle isotope reservoirs
5 min read•august 21, 2024
Earth's mantle holds secrets of our planet's history, revealed through isotope geochemistry. By studying distinct mantle reservoirs with unique isotopic signatures, scientists can reconstruct Earth's past and understand ongoing processes deep beneath our feet.
Mantle reservoirs range from depleted to enriched, each telling a different story. The forms most of the upper mantle, while enriched sources and the HIMU component provide clues about recycled crustal materials and ancient oceanic crust.
Mantle isotope reservoirs overview
- Isotope geochemistry provides crucial insights into the composition and evolution of Earth's mantle
- Mantle reservoirs represent distinct geochemical domains with unique isotopic signatures
- Understanding mantle reservoirs helps reconstruct Earth's geologic history and ongoing geodynamic processes
Isotopic composition of mantle
Bulk silicate earth model
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- Represents the primordial composition of Earth's mantle before differentiation
- Based on chondritic meteorite compositions adjusted for volatile loss during accretion
- Serves as a reference point for comparing isotopic variations in mantle-derived rocks
- Key isotope ratios include , , and
Chondritic uniform reservoir
- Hypothetical mantle reservoir with isotopic composition matching undifferentiated chondritic meteorites
- Assumes Earth formed from chondritic material and maintained its initial composition
- Used as a baseline for interpreting mantle isotopic heterogeneity
- Deviations from CHUR indicate fractionation processes or mixing of different mantle components
Major mantle reservoirs
Depleted MORB mantle
- Source of mid-ocean ridge basalts (MORB) characterized by depletion in incompatible elements
- Exhibits low and high ratios compared to bulk Earth
- Formed through extraction of continental crust and oceanic crust over geologic time
- Comprises the upper mantle and represents the largest volumetric reservoir
Enriched mantle sources
- Mantle domains enriched in incompatible elements relative to depleted MORB mantle
- Characterized by higher and lower ratios
- Include EM1 ( 1) and EM2 (enriched mantle 2) components
- EM1 associated with recycled oceanic crust and sediments
- EM2 linked to subducted continental material or metasomatized lithosphere
HIMU mantle component
- High-μ (μ = ) mantle reservoir with distinct isotopic composition
- Characterized by extremely high ratios
- Thought to originate from recycled oceanic crust altered by seawater
- Found in some ocean island basalts (St. Helena, Cook-Austral Islands)
Isotopic systems in mantle
Sr-Nd isotope systematics
- Coupled behavior of Sr and Nd isotopes due to similar geochemical properties
- increases over time due to radioactive decay of
- increases over time due to radioactive decay of
- Mantle array defines inverse correlation between Sr and Nd isotope ratios
- plots at low Sr, high Nd end of array
- Enriched sources plot at high Sr, low Nd end of array
Pb isotope systematics
- Three : , , and
- Produced by decay of U and Th isotopes with different half-lives
- Pb isotope ratios plotted on uranogenic ( vs ) and thorogenic ( vs ) diagrams
- Northern Hemisphere Reference Line (NHRL) represents average mantle Pb isotope evolution
- HIMU sources plot above NHRL, while enriched sources plot below
Hf isotope systematics
- ratio increases over time due to decay of
- Hf isotopes often coupled with Nd isotopes due to similar behavior during mantle melting
- Epsilon Hf (εHf) notation used to express deviation from chondritic composition
- Depleted mantle characterized by high εHf values, enriched sources by low εHf values
Mantle heterogeneity
Causes of isotopic variations
- Partial melting and melt extraction create
- of oceanic crust and sediments introduces heterogeneities
- Recycling of delaminated continental lithosphere
- Metasomatism by small-degree melts or fluids
- Preservation of primordial mantle domains
Mixing and assimilation processes
- Binary mixing between end-member components produces curved mixing lines on isotope diagrams
- Assimilation of crustal material by mantle-derived magmas
- Magma chamber processes can homogenize or enhance isotopic variations
- Diffusive re-equilibration in the mantle can erase small-scale heterogeneities over time
Mantle plumes and hotspots
Isotopic signatures of plumes
- Often exhibit more enriched isotopic compositions than MORB
- Can sample deep mantle reservoirs not tapped by mid-ocean ridge volcanism
- May preserve signatures of ancient recycled components
- Show temporal and spatial variations in isotopic composition (Hawaiian-Emperor seamount chain)
OIB vs MORB isotope ratios
- Ocean island basalts (OIB) generally more heterogeneous than MORB
- OIB extend to more radiogenic Sr and Pb isotope compositions
- MORB cluster tightly around depleted mantle values
- OIB can sample enriched mantle components (EM1, EM2, HIMU) not present in MORB source
Mantle evolution models
Two-layer mantle model
- Proposes distinct upper and lower mantle reservoirs separated by 660 km discontinuity
- Upper mantle as source of MORB, lower mantle as OIB source
- Explains differences between MORB and OIB isotopic compositions
- Challenged by seismic evidence for whole
Whole mantle convection model
- Assumes entire mantle participates in convection
- Heterogeneities distributed throughout mantle due to incomplete mixing
- Explains presence of subducted slabs in lower mantle
- Requires mechanisms to preserve long-lived isotopic reservoirs in a convecting mantle
Geodynamic implications
Mantle convection patterns
- Isotopic heterogeneities provide tracers for mantle flow
- Large-scale convection cells inferred from distribution of mantle domains
- Small-scale convection in upper mantle influences local isotopic variations
- Plume-driven upwelling brings deep mantle material to surface
Plate tectonics and mantle mixing
- Subduction introduces crustal material into mantle, creating heterogeneities
- Seafloor spreading and mid-ocean ridge volcanism homogenize upper mantle
- Continental breakup and formation may influence mantle isotopic composition
- Mantle wedge above subduction zones shows complex mixing patterns
Analytical techniques
Mass spectrometry methods
- Thermal ionization (TIMS) for high-precision Sr, Nd, and Pb isotope measurements
- Multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) for rapid, high-precision analyses
- Secondary ion mass spectrometry (SIMS) for in-situ measurements of individual minerals
- Noble gas mass spectrometry for He, Ne, and Ar isotope analyses
Sample preparation challenges
- Ultra-clean laboratory conditions required to minimize contamination
- Chemical separation of elements using ion exchange chromatography
- Dissolution of refractory minerals (zircon, garnet) for complete sample digestion
- Correction for isotopic fractionation during analysis using standard bracketing or internal normalization
Applications in geochemistry
Mantle source identification
- Determining provenance of igneous rocks based on isotopic composition
- Mapping spatial distribution of mantle reservoirs beneath continents and oceans
- Tracing mantle flow patterns using isotopic tracers
- Identifying contributions from recycled crustal components in mantle-derived magmas
Age dating of mantle processes
- Model ages calculated from parent-daughter isotope ratios
- Determining timing of mantle depletion events
- Dating of mantle metasomatism using isotopic disequilibrium
- Constraining rates of mantle mixing and homogenization
Key Terms to Review (23)
Clifford H. Langmuir: Clifford H. Langmuir is a prominent geochemist known for his pioneering work in isotope geochemistry, particularly his studies of mantle isotope reservoirs. His contributions have significantly advanced the understanding of how isotopes can be used to trace processes in the Earth's interior, especially in relation to the mantle's composition and dynamics. Langmuir’s work has helped to clarify the role of isotopic signatures in identifying different mantle sources and their evolution over geological time.
Continental crust formation: Continental crust formation refers to the processes that create and evolve the Earth's continental crust, which is thicker and less dense than oceanic crust. This formation involves various geological processes, including magma differentiation, plate tectonics, and sedimentation, leading to the development of stable continental landmasses over geological time. The isotopic compositions of mantle reservoirs play a crucial role in understanding the sources of materials that contribute to the formation and evolution of continental crust.
Core-mantle interactions: Core-mantle interactions refer to the complex processes and exchanges between the Earth's outer core and the mantle, which play a significant role in shaping the planet's geodynamics and geochemical cycles. These interactions influence heat transfer, magnetic field generation, and the overall composition of mantle materials through the movement of elements and isotopes between the two layers. Understanding these interactions is key to grasping how Earth's internal processes affect surface phenomena and contribute to the evolution of mantle isotope reservoirs.
Depleted mantle: The depleted mantle refers to a portion of the Earth's mantle that has undergone significant extraction of certain elements, especially incompatible elements like lithium, rubidium, and potassium, leaving it enriched in compatible elements such as magnesium and iron. This depletion occurs due to processes like partial melting, which leads to the formation of magmas that extract these elements from the mantle, resulting in a composition distinct from the more primitive, undepleted mantle material.
Depleted morb mantle: The depleted MORB (Mid-Ocean Ridge Basalt) mantle refers to a specific composition of the Earth's upper mantle that has undergone partial melting, resulting in a reduction of certain trace elements and isotopes. This mantle is crucial in the formation of oceanic crust at mid-ocean ridges, characterized by its depleted isotopic signatures, which reflects its origins from the recycling of oceanic crust and mantle processes. Understanding the depleted MORB mantle helps in revealing the complex dynamics of mantle reservoirs and their contributions to global geochemical cycles.
Enriched mantle: The enriched mantle refers to a portion of the Earth's mantle that has a higher concentration of certain elements, such as incompatible elements, compared to the more primitive or depleted mantle. This enrichment occurs due to processes like partial melting and the addition of materials from subducted oceanic crust, leading to variations in isotope ratios that can be used to trace geochemical processes and mantle dynamics.
Geochemical Mapping: Geochemical mapping is the systematic collection and analysis of geochemical data over a defined area to create spatial representations of various chemical elements and isotopes in the Earth's crust. This process helps in identifying geological features, understanding mineral resources, and assessing environmental impacts by illustrating the distribution of geochemical elements and isotopes in the subsurface. It plays a crucial role in revealing the composition and behavior of mantle isotope reservoirs, which can provide insights into Earth's formation and tectonic processes.
Hf isotope systematics: Hf isotope systematics refers to the study of the distribution and ratios of hafnium isotopes in geological materials, which helps in understanding processes related to the Earth's mantle and crust. This systematics is crucial for deciphering the age, formation, and evolution of mantle reservoirs, as well as tracing geochemical processes that influence the composition of magmas and their source regions.
Himu mantle component: The himu mantle component refers to a specific type of mantle material characterized by high levels of uranium (U) and thorium (Th) relative to lead (Pb), as well as distinct isotopic signatures. This component is important for understanding the geochemical and isotopic evolution of the Earth's mantle, especially in relation to ocean island basalts and hotspot volcanism, as it provides insights into the sources of magma and the processes occurring within the mantle.
Ion microprobe analysis: Ion microprobe analysis is a high-precision technique used to analyze the isotopic composition of solid materials at a microscopic scale. It involves bombarding a sample with a focused ion beam, which then ejects secondary ions that are collected and analyzed to determine elemental and isotopic ratios. This method is particularly valuable for studying mantle isotope reservoirs as it allows scientists to obtain detailed isotopic data from small samples, providing insights into the processes and history of the Earth's mantle.
Isotopic Fractionation: Isotopic fractionation is the process by which different isotopes of an element are separated or partitioned due to physical or chemical processes, leading to variations in their abundance. This phenomenon is crucial for understanding how isotopes behave in various geological and biological contexts, as it can influence measurements in atomic structure, isotope notation, and radiometric dating methods.
Mantle convection: Mantle convection is the process by which heat from the Earth's interior causes the mantle to circulate, leading to the movement of tectonic plates on the surface. This process plays a crucial role in transporting heat and materials within the Earth, affecting geological activities such as volcanic eruptions, earthquakes, and the formation of mountain ranges. The dynamics of mantle convection are essential for understanding the composition of mantle reservoirs and how isotopes can trace these processes.
Mantle differentiation: Mantle differentiation refers to the process through which the Earth's mantle separates into distinct layers or reservoirs based on variations in chemical composition and physical properties. This process is crucial for understanding how elements are redistributed in the Earth's interior, influencing the formation of different mantle isotope reservoirs and affecting isotopic systems that help trace the history of the Earth’s formation and evolution.
Mantle plume theory: Mantle plume theory suggests that mantle plumes are localized columns of hot, upwelling material from the deep mantle that can create volcanic activity and contribute to the formation of large igneous provinces. This theory connects to mantle isotope reservoirs, as the composition of these plumes can reveal information about the isotopic signatures of the Earth's mantle and help scientists understand its evolution over time.
Mass spectrometry: Mass spectrometry is an analytical technique used to measure the mass-to-charge ratio of ions, enabling the identification and quantification of different isotopes in a sample. This technique is crucial in isotope geochemistry for analyzing stable and radioactive isotopes, understanding decay processes, and determining isotopic ratios in various materials.
Melting processes: Melting processes refer to the transformation of solid rock into liquid magma due to increased temperature and pressure within the Earth's interior. These processes are crucial in the formation of various mantle isotope reservoirs, as the way rock melts can influence the chemical and isotopic composition of the resulting magma, which in turn affects volcanic activity and the evolution of the Earth’s crust.
Neodymium isotopes: Neodymium isotopes are variants of the element neodymium that differ in the number of neutrons in their atomic nuclei, resulting in distinct mass numbers. These isotopes play a crucial role in isotope geochemistry, particularly in tracing mantle processes and understanding the evolution of the Earth's mantle and crust through their ratios, like the commonly studied Nd-143 and Nd-144 isotopes.
Pb isotope systematics: Pb isotope systematics refers to the study and analysis of lead (Pb) isotopes in geological and environmental samples to understand processes such as magmatic differentiation, crustal evolution, and geochronology. This systematics is crucial for determining the age of rocks, the history of mineral deposits, and the movement of materials through the Earth's mantle and crust.
Radiogenic Isotopes: Radiogenic isotopes are isotopes that are formed through the radioactive decay of parent isotopes. They provide crucial information about geological processes, age dating, and the evolution of the Earth’s crust and mantle over time.
Richard D. H. Wood: Richard D. H. Wood is a notable figure in isotope geochemistry, recognized for his contributions to understanding mantle isotope reservoirs and their implications for geochemical processes in the Earth's interior. His research has focused on how isotopic variations can reveal information about the sources and evolution of mantle materials, aiding in the interpretation of tectonic activity and magmatic processes.
Sr-Nd Isotope Systematics: Sr-Nd isotope systematics refers to the study of isotopic ratios of Strontium (Sr) and Neodymium (Nd) to understand the origins, evolution, and processes of the Earth's mantle and crust. By analyzing these isotopes, scientists can gain insights into the sources of magmas, crustal recycling, and the age of geological materials, connecting them to mantle isotope reservoirs and their dynamics.
Strontium isotopes: Strontium isotopes are variants of the element strontium that have the same number of protons but different numbers of neutrons, leading to different atomic masses. These isotopes, particularly strontium-87 and strontium-86, are important in geochemistry as they provide insights into geological processes, including mantle composition, oceanic crust development, and even forensic investigations.
Subduction: Subduction is the geological process where one tectonic plate moves under another and sinks into the mantle as the plates converge. This process plays a critical role in shaping the Earth's crust, recycling materials, and influencing mantle dynamics, which ultimately affects crustal growth and formation, as well as the evolution of oceanic crust.