🌋Geochemistry Unit 6 – Igneous geochemistry

Key Concepts and Definitions

• Igneous rocks form from the cooling and solidification of magma (molten rock beneath the Earth's surface) or lava (molten rock above the Earth's surface)
• Magma composition influenced by the source material, partial melting, and various differentiation processes (fractional crystallization, assimilation, and magma mixing)
• Fractional crystallization process where minerals crystallize from magma at different temperatures, altering the composition of the remaining melt
• Assimilation incorporates surrounding rock into the magma, modifying its composition
• Magma mixing occurs when two or more magmas with different compositions combine
• Geochemical classification systems categorize igneous rocks based on their chemical composition (TAS diagram, QAPF diagram)
• Trace elements present in minute quantities (<0.1 wt%) provide insights into magmatic processes and source characteristics
• Isotope ratios (radiogenic and stable) used as tracers for magmatic processes, source material, and age determination

Formation of Igneous Rocks

• Igneous rocks form through the cooling and solidification of magma or lava
• Magma generation occurs by partial melting of the mantle or crust due to increased temperature, decreased pressure, or the addition of volatiles (water, carbon dioxide)
• Partial melting produces magmas with compositions distinct from the source material
  • Degree of partial melting influences the magma composition
  • Low degrees of partial melting result in more enriched magmas (higher concentrations of incompatible elements)
• Magma rises through the crust due to its lower density compared to the surrounding rock
• Intrusive (plutonic) igneous rocks form when magma cools and solidifies beneath the Earth's surface (granite, diorite, gabbro)
• Extrusive (volcanic) igneous rocks form when lava cools and solidifies on the Earth's surface (basalt, rhyolite, andesite)
• Cooling rate affects the texture of igneous rocks
  • Slow cooling (intrusive) results in larger crystals (phaneritic texture)
  • Rapid cooling (extrusive) results in smaller crystals or a glassy texture (aphanitic or glassy texture)

Magma Composition and Evolution

• Magma composition depends on the source material and various differentiation processes
• Primary magmas derived from partial melting of the mantle are generally basaltic in composition
• Differentiation processes modify the composition of magma as it evolves
  • Fractional crystallization removes early-forming minerals, enriching the remaining melt in incompatible elements
  • Assimilation incorporates wall rock, potentially altering the magma composition
  • Magma mixing combines magmas of different compositions
• Bowen's reaction series describes the order of mineral crystallization from magma based on temperature and composition
  • Discontinuous series (olivine, pyroxene, amphibole, biotite) and continuous series (plagioclase feldspar)
• Magmatic differentiation can produce a wide range of igneous rock compositions (mafic to felsic)
• Volatiles (water, carbon dioxide) play a crucial role in magma evolution and eruption style
  • Dissolved volatiles lower the magma's viscosity and melting temperature
  • Exsolution of volatiles can trigger explosive volcanic eruptions

Geochemical Classification Systems

• Igneous rocks classified based on their chemical composition and mineral content
• Total Alkali-Silica (TAS) diagram plots the total alkali content (Na2O + K2O) against silica content (SiO2)
  • Subdivides igneous rocks into fields (basalt, andesite, dacite, rhyolite, trachyte, phonolite)
• Quartz-Alkali Feldspar-Plagioclase Feldspar (QAPF) diagram used for the classification of plutonic rocks
  • Based on the relative proportions of quartz (Q), alkali feldspar (A), plagioclase feldspar (P), and feldspathoids (F)
• Normative mineralogy calculated from the chemical composition of the rock
  • CIPW norm calculates the hypothetical mineral assemblage based on the rock's chemistry
• Magmatic series (tholeiitic, calc-alkaline, alkaline) reflect different tectonic settings and magma evolution paths
• Aluminum saturation index (ASI) distinguishes peraluminous, metaluminous, and peralkaline igneous rocks
  • ASI = molar Al2O3 / (CaO + Na2O + K2O)

Trace Element Geochemistry

• Trace elements present in low concentrations (<0.1 wt%) in igneous rocks
• Behavior of trace elements controlled by their partitioning between minerals and melt
  • Compatible elements preferentially incorporated into minerals (e.g., Ni in olivine)
  • Incompatible elements preferentially remain in the melt (e.g., Rb, Zr)
• Distribution coefficients (Kd) quantify the partitioning of trace elements between minerals and melt
  • Kd = (concentration in mineral) / (concentration in melt)
• Trace element patterns provide insights into magmatic processes and source characteristics
  • Spider diagrams normalize trace element concentrations to a reference composition (e.g., primitive mantle, chondrite)
  • Anomalies in specific elements indicate particular processes or source characteristics (e.g., Eu anomaly related to plagioclase fractionation)
• Rare Earth Elements (REEs) are a group of trace elements with similar chemical properties
  • Light REEs (LREEs) and Heavy REEs (HREEs) fractionated during magmatic processes
  • REE patterns used to infer magma source and evolution

Isotope Geochemistry in Igneous Systems

• Isotopes are atoms of the same element with different numbers of neutrons
• Radiogenic isotopes produced by the decay of radioactive parent isotopes over time
  • Commonly used radiogenic isotope systems in igneous geochemistry: Rb-Sr, Sm-Nd, U-Pb, K-Ar
  • Provide information on the age and source of igneous rocks
• Stable isotopes do not undergo radioactive decay
  • Commonly used stable isotope systems in igneous geochemistry: O, H, S
  • Provide information on magmatic processes and fluid-rock interactions
• Isotope ratios measured using mass spectrometry techniques
• Initial isotope ratios reflect the composition of the magma source
• Radiogenic isotope ratios change over time due to radioactive decay
  • Isochron dating uses the change in isotope ratios to determine the age of igneous rocks
• Stable isotope fractionation occurs during magmatic processes and fluid-rock interactions
  • Oxygen isotope ratios (δ18O) used to identify crustal contamination or hydrothermal alteration

Analytical Techniques and Tools

• X-ray fluorescence (XRF) spectrometry measures the major and trace element composition of igneous rocks
  • Sample irradiated with high-energy X-rays, causing the emission of characteristic X-rays
• Inductively coupled plasma mass spectrometry (ICP-MS) used for high-precision trace element and isotope analysis
  • Sample ionized in a high-temperature plasma and analyzed based on mass-to-charge ratios
• Electron probe microanalysis (EPMA) provides in-situ chemical analysis of individual minerals
  • Focused electron beam interacts with the sample, generating X-rays characteristic of the mineral composition
• Thermal ionization mass spectrometry (TIMS) used for high-precision isotope ratio measurements
  • Sample thermally ionized and isotopes separated based on their mass-to-charge ratios
• Laser ablation ICP-MS (LA-ICP-MS) allows for high-spatial resolution analysis of trace elements and isotopes in minerals
  • Laser used to ablate small portions of the sample, which are then analyzed by ICP-MS
• Petrographic microscopy used to study the texture, mineralogy, and alteration of igneous rocks in thin sections

Real-World Applications and Case Studies

• Igneous geochemistry applied to understanding the formation and evolution of the Earth's crust and mantle
• Trace element and isotope geochemistry used to identify the source regions of igneous rocks (e.g., mantle vs. crust)
• Geochemical signatures of igneous rocks provide insights into tectonic settings (e.g., mid-ocean ridges, subduction zones, intraplate volcanism)
• Igneous geochemistry used in mineral exploration to identify potential ore deposits
  • Certain trace elements and isotopic signatures can indicate the presence of economically valuable minerals (e.g., Cu, Au, Ni)
• Geochemical monitoring of active volcanoes helps assess the risk of volcanic eruptions
  • Changes in the composition of volcanic gases and fluids can signal impending eruptions
• Igneous geochemistry contributes to the study of planetary evolution and the formation of extraterrestrial bodies
  • Analysis of meteorites and lunar samples provides insights into the early history of the Solar System
• Case study: The Bushveld Igneous Complex in South Africa
  • Largest layered intrusion in the world, rich in platinum group elements (PGEs) and chromium
  • Geochemical studies have revealed the complex magmatic processes and source characteristics that led to the formation of this economically significant deposit
• Case study: The Yellowstone Volcanic System in the United States
  • Trace element and isotope geochemistry used to understand the magmatic plumbing system and potential for future eruptions
  • Geochemical monitoring of hot springs and geysers provides insights into the current state of the volcanic system