⚛️Isotope Geochemistry Unit 8 – Mantle and crustal evolution

Key Concepts and Terminology

• Isotopes are atoms of the same element with different numbers of neutrons in their nuclei
• Radiogenic isotopes are produced by radioactive decay of parent isotopes over time
• Stable isotopes do not undergo radioactive decay and their abundances remain constant
• Fractionation is the partitioning of isotopes between different phases or reservoirs due to physical or chemical processes
• Reservoirs are distinct parts of the Earth system with characteristic isotopic compositions (mantle, crust, atmosphere)
• Differentiation is the process by which the Earth's interior separates into distinct layers with different compositions
• Partial melting is the process by which a portion of a solid rock melts, producing a magma with a different composition than the original rock
  • Occurs when the temperature of a rock rises above its solidus temperature but below its liquidus temperature
• Recycling refers to the process by which material from the Earth's surface is returned to the mantle through subduction or delamination

Geological Timescale and Earth's Formation

• The Earth formed approximately 4.54 billion years ago from the accretion of dust and gas in the early solar system
• The Hadean Eon (4.54-4.0 Ga) was characterized by a molten surface, frequent impacts, and the formation of the Moon
• The Archean Eon (4.0-2.5 Ga) saw the development of the first continents, the emergence of life, and the formation of the Earth's core and mantle
• The Proterozoic Eon (2.5-0.54 Ga) was marked by the formation of supercontinents, the rise of atmospheric oxygen, and the evolution of eukaryotic life
• The Phanerozoic Eon (0.54 Ga-present) is divided into the Paleozoic, Mesozoic, and Cenozoic Eras, each characterized by distinct life forms and geological events
• The Earth's layered structure (core, mantle, crust) was established within the first few hundred million years of its formation
• Isotopic evidence suggests that the Earth's mantle has remained largely undifferentiated since the Archean, with only minor additions from the crust

Composition of Earth's Mantle and Crust

• The Earth's mantle makes up ~84% of its volume and is composed primarily of silicate minerals (olivine, pyroxene, garnet)
• The upper mantle extends from the base of the crust to a depth of ~410 km and is the source region for most magmas
• The lower mantle extends from ~660 km to the core-mantle boundary at ~2900 km and is composed of higher-pressure mineral phases (bridgmanite, ferropericlase)
• The Earth's crust makes up <1% of its volume but contains a wide range of rock types and compositions
• The oceanic crust is thin (~7 km), dense, and composed primarily of basaltic rocks formed at mid-ocean ridges
• The continental crust is thick (up to 70 km), less dense, and composed of a wide range of igneous, metamorphic, and sedimentary rocks
• The composition of the mantle and crust can be inferred from the study of mantle xenoliths, ophiolites, and exposed sections of the lower crust

Isotopic Systems in Mantle and Crustal Studies

• Radiogenic isotope systems (Rb-Sr, Sm-Nd, U-Pb, Lu-Hf) are used to trace the age and origin of mantle and crustal rocks
  • The parent isotopes (Rb, Sm, U, Lu) decay to daughter isotopes (Sr, Nd, Pb, Hf) at known rates, allowing for age determination
  • The initial isotopic composition of a rock reflects the composition of its source region at the time of formation
• Stable isotope systems (O, S, Fe) are used to study fractionation processes and the interaction between different reservoirs
  • Oxygen isotopes ($^{18}O/^{16}O$) can distinguish between mantle-derived and crustal-derived rocks and fluids
  • Sulfur isotopes ($^{34}S/^{32}S$) can trace the origin and evolution of sulfide minerals and the redox state of magmas
• Noble gas isotopes (He, Ne, Ar) are used to study the degassing history of the Earth and the contribution of primordial volatiles to the mantle
• Short-lived isotope systems ($^{26}Al-^{26}Mg$, $^{182}Hf-^{182}W$) provide constraints on the timing of early Earth differentiation events

Mantle Differentiation and Evolution

• The Earth's mantle is heterogeneous on various scales, reflecting a complex history of differentiation and mixing
• The depleted mantle is the source of mid-ocean ridge basalts (MORB) and has been depleted in incompatible elements by partial melting over time
• The enriched mantle is the source of ocean island basalts (OIB) and has been enriched in incompatible elements by the recycling of crustal material
• Mantle plumes are upwellings of hot, buoyant material from the deep mantle that can cause intraplate volcanism and the formation of large igneous provinces (LIPs)
• Subduction zones are sites of mantle-crust interaction, where oceanic crust and sediments are recycled back into the mantle, causing chemical and isotopic heterogeneity
• The transition zone (410-660 km depth) may act as a barrier to mantle convection and a reservoir for recycled material
• The lower mantle is generally less heterogeneous than the upper mantle but may contain distinct domains with different compositions and ages

Crustal Growth and Recycling Processes

• The continental crust has grown over time through the addition of mantle-derived magmas and the reworking of older crustal material
• Crustal growth rates were highest in the Archean and have decreased over time, possibly due to the cooling of the Earth's mantle
• Subduction zones are the primary sites of crustal growth, where mantle-derived magmas are added to the crust through arc volcanism and plutonism
• Collisional orogens are sites of crustal thickening and metamorphism, where continental crust is deformed and reworked during the collision of tectonic plates
• Sedimentary processes play a key role in crustal recycling, as weathering and erosion break down crustal rocks and transport them to depositional basins
• Delamination is a process by which dense lower crustal material can detach and sink into the mantle, causing uplift and magmatism in the overlying crust
• The average age of the continental crust is ~2.3 Ga, reflecting a balance between crustal growth and recycling processes over time

Analytical Techniques and Methods

• Mass spectrometry is the primary tool for measuring isotope ratios in mantle and crustal rocks
  • Thermal ionization mass spectrometry (TIMS) is used for high-precision measurements of radiogenic isotopes
  • Multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) is used for rapid, high-precision measurements of stable and radiogenic isotopes
• Laser ablation (LA-ICP-MS) allows for in situ measurements of isotope ratios in individual minerals or zones within a sample
• Secondary ion mass spectrometry (SIMS) is used for high-spatial resolution measurements of isotope ratios in small samples or inclusions
• Sample preparation techniques, such as mineral separation and chemical purification, are critical for accurate and precise isotope measurements
• Data reduction and interpretation involve the use of isotope evolution diagrams, mixing models, and statistical analysis to extract meaningful information from isotope data
• Inter-laboratory calibration and the use of standard reference materials ensure the comparability and accuracy of isotope data across different studies and laboratories

Real-World Applications and Case Studies

• Isotope geochemistry has been used to study the formation and evolution of the Earth's oldest rocks, such as the Acasta Gneiss in Canada (4.0 Ga) and the Isua Supracrustal Belt in Greenland (3.8 Ga)
• The study of mantle xenoliths from kimberlites and basalts has provided insights into the composition and heterogeneity of the subcontinental lithospheric mantle
• Isotopic studies of ophiolites, such as the Samail Ophiolite in Oman, have revealed the processes of oceanic crust formation and hydrothermal alteration
• The Bushveld Complex in South Africa, the largest layered intrusion in the world, has been studied using isotope geochemistry to understand the processes of magma differentiation and the formation of platinum-group element deposits
• Isotopic studies of the Hawai'ian-Emperor Seamount Chain have provided evidence for the long-term evolution and motion of the Hawai'ian mantle plume
• The Himalayan-Tibetan orogen has been studied using isotope geochemistry to understand the processes of crustal thickening, metamorphism, and magmatism during continental collision
• Isotope geochemistry has been applied to the study of ore deposits, such as the Bingham Canyon porphyry copper deposit in Utah, to understand the sources of metals and the timing of mineralization