Igneous Processes at Subduction Zones

Igneous Processes at Subduction Zones

  • Course Information: EEPS 0230 Lecture 11, March 6, 2025

Learning Goals

  • Review the basics of partial melting and fractional crystallization.
  • Introduce the physical parameters of subduction zones.
  • Explore the chemical variability at subduction zones.

Key Concepts in Partial Melting and Fractional Crystallization

  • Trace Element Diagram:
  • Y-Axis: Normalized concentration relative to bulk earth.
  • X-Axis: Elements arranged in order of increasing compatibility.
  • Partial Melting:
  • Involves melting a bulk earth sample which results in:
    • Incompatible elements (D < 0): Enriched in the melt.
    • Compatible elements (D > 0): Depleted in the melt.
  • Residue:
  • The rock that remains after melt extraction is depleted in incompatible elements and enriched in compatible elements.

Chondrites and Element Concentrations

  • Chondrites display a flat line indicating low rare earth element (REE) concentrations and a low ratio of light REE to heavy REE (LREE/HREE).
  • Boron mid-ocean ridge basalts (MORBs) indicate a different elemental pattern indicative of different mantle melting processes.
  • Continental crust shows higher LREE/HREE ratios due to its formation processes.

Effects of Partial Melting

  • The composition of the major elements in the initial melt remains relatively constant until a specific mineral phase is utilized entirely.
  • As more melt is produced, the trace element composition becomes enriched in incompatible elements.

Fractional Crystallization

  • The process whereby magmas differentiate as they crystallize.
  • Bowen's Reaction Series: Describes the sequence of mineral crystallization based on cooling magma.
  • Differentiation Proceso:
  • Parental magma evolves to more felsic (silica-rich) compositions through the crystallization of early-formed high-temperature minerals which accumulate at the bottom of the magma chamber.

Melting Processes

  • Partial Melting: Occurs when minerals at specific junctions melt, leading to a connected melt network.
  • Decompression Melting: Occurs at mid-ocean ridges where the upward movement of mantle rock reduces pressure without heat loss, causing melting.
  • Flux-Induced Melting: Occurs at subduction zones where the addition of water lowers the melting temperature, allowing mantle rocks to melt even if pressure is high.
  • Geotherm: Represents the relationship between temperature and depth that dictates melting conditions in different geological settings.

Subduction Zones Overview

  • Type: Can be continental or oceanic.
  • Physical Parameters: Features include slabs, trenches, fore arcs, arcs, back arcs, and mantle wedges—it encompasses geological processes between tectonic plates.

Volcano Types at Subduction Zones

  • Continental Arcs: Form over subducting oceanic plates resulting in volcanic activity (e.g., Andes).
  • Island Arcs: Form above subducting oceanic plates (e.g., Japan, Aleutians).
  • Geological evidence of past arcs includes exposed plutonic and volcanic forms or current active volcanism.

Chemical Composition of Arc Magma

  • Arc magma is distinct from mid-ocean ridge basalt (MORB) due to variations in mineral composition and melting processes contributing to differences in trace element concentrations.
  • Magma Diversity: Arc magmas range from basalt to andesite and may even include rhyolite, whereas MORB predominantly consists of basalt. Elements such as Barium and Strontium can fingerprint the tectonic setting of a melt.

Summary of Takeaways

  • Elements and compositions vary greatly between continental and oceanic arcs, influencing their geological and volcanic behavior.
  • Understanding melting, crystallization, and geochemical processes at subduction zones is crucial for grasping Earth's geology and volcanism.