Week 10a – Convergent Margins

Igneous Petrogenesis: Geological Settings for Melting

• Melting occurs in various geological settings:
  • Mid-ocean ridges
  • Continental rifts
  • Island Arcs
  • Active continental margins
  • Back-arc basins
  • Ocean Islands
  • Intraplate hotspot activity, carbonatites, or kimberlites

Convergent Margins: Volcanic Hazards

• Compared to constructive margins (basaltic, submarine, low viscosity, volatile-poor, tholeiitic magma), convergent margins exhibit:
  • Mafic-felsic magmatism
  • Aerial or sub-aerial eruptions
  • Low to very high viscosity
  • Volatile-rich, calc-alkaline magma
• Volcanic hazards are prevalent, especially those associated with pyroclastic deposits (dome/column collapse).
• Approximately 10% of the global population resides near hazardous volcanoes.
• Volcanic Explosivity Index (VEI):
  • A logarithmic scale measuring eruptive size (0 = non-explosive, 8 = globally relevant supervolcano).
  • Frequency scales inversely with VEI (fortunately).

Stratovolcanoes: Mount St. Helens

• Cone-shaped volcano in the Cascades (NW USA).
• Produces basalt, andesite, and dacite lavas.
• Pre-1980 eruption, it was the most active volcano in the Cascades.
• Active monitoring:
  • A magnitude 4.1 earthquake occurred on March 15th, 1980.
  • The region was put on alert, minimizing loss of life.

Subduction Zones: Types of Convergent Margins

• Island Arcs:
  • Examples: Aleutians, Kuriles, Marianas, Tonga-Kermadec, etc.
• Continental Arcs:
  • Examples: Cascades, Andes, Central American arc, Aegean, Kamchatka, Indonesia, etc.

Island Arcs vs. Continental Arcs

• Island Arcs:
  • Less crust, unlikely to expose plutonic deep sections (rely on xenoliths).
  • Mostly mafic-intermediate volcanic rocks.
• Continental Arcs:
  • Thick overlying crust, more interaction/contamination.
  • Batholiths commonly exposed by erosion.
  • Mafic – felsic volcanic rocks.
  • Important economic deposits.

Arc Structure

• Trench: exhibits a negative (-ve) gravity anomaly.
• Fore-arc:
  • Benioff zone, positive (+ve) gravity anomaly, low heat flow.
• Arc volcano chain: hot magma, high heat flow.
• Back-arc:
  • Behind the arc volcanoes.
  • Often experiences tectonic extension and magmatism associated with spreading.

Thermal Structure of Subduction Zones

• Cold (old) oceanic crust sinking beneath crust.
• Numerical dynamic simulations calculate temperature distribution.
• Inverted and depressed isotherms in the mantle wedge (cooled by the slab).
• Mantle wedge melting in cold rocks (van Keken et al, 2008).
• Modern subduction zones have different temperatures based on age, angle, and subduction rate (Syracuse et al, 2010).

Water in Subduction Zones

• Oceanic crust carries water into subduction zones (Rüpke et al, 2004).
• Water is stored in hydrous minerals (and pore space) in sediments, mafic crust, and ultramafic cumulates.
• Hydrous phases (amphibole, serpentine, chlorite) are present at the surface in altered crust.
• Different layers can contain 2-3% water (by weight).
• Water is released due to limits of mineral stability.
• The slab doesn’t melt (producing a silicate melt) but dehydrates to produce fluid H2O (Rüpke et al, 2004, Grove et al, 2009).

Hydrous Melting of the Mantle Wedge

• Water influences the melting temperature of mantle peridotite (Grove et al, 2009).
• Process:
  1. Hydration of oceanic crust in hydrothermal processes at MOR.
  2. Further alteration during aging.
  3. Subduction.
  4. Dehydration of hydrous phases.
  5. Upward $H_2O$ transport in wedge.
  6. Hydrous peridotite melting at ~1100°C.
  7. Buoyant silicate melts move upwards.

Evidence for Hydrous Melting

• Some subduction zone magmas contain amphibole - $X<em>2Y</em>5Si<em>8O</em>{22}(OH)_2$. Where X = Ca, Na, Fe, Mg, Mn and Y = Mg, Fe, Al, Ti, Mn, Cr, V, Zn, Li
• Water contents in melt inclusions can contain >4 wt.% water (10 times MORB).
• Convergent margin basalts are $SiO_2$ rich and often quartz normative, unlike most MORB.
• This is explicable by investigating the effect of water on mineral phase relations (Kushiro, 1969).
• Olivine field expands with water (Kushiro, 1969).
• Water saturated peridotite generates silica oversaturated melts (Kushiro, 1969).
• Anhydrous melting (at the same pressure) produces melts that are silica saturated (Kushiro, 1969).
• Evolution of melts in hydrous systems can produce very silicic products (i.e., felsic rocks) (Kushiro, 1969).

Calc-Alkaline Volcanism

• The differentiation trend observed in arc magma compositions differs from oceanic settings.
• Two relevant diagrams:
  • Ferroan vs. magnesian
  • Alkali-FeO-MgO (AFM)
• Constructive margins only generate tholeiitic differentiation trends (Chin et al., 2018).
• Convergent margins also generate calc-alkaline trends (Chin et al., 2018).
• Tholeiitic (TH) & calc-alkaline (CA) trends:
  • Primary melts near F-M edge.
  • As crystallization proceeds, incompatible elements become more enriched in melt (move towards A).
  • TH – plag xlln means large ↑ in F at onset. Late FeTi oxide xlln causes drop.
  • TH move towards F, before turning to A.
  • CA – show little Fe enrichment (Chin et al., 2018).
• Explanations for the calc-alkaline trend:
  • Melt mixing
  • Crustal assimilation
  • Suppression of plagioclase
  • Enhanced FeTi-oxide production (Chin et al., 2018).

Case Study: Tonga - Kermadec – Taupo Volcanic Zone

• Taupo Volcanic Zone (TVZ): continental arc.
• Kermadec trench: oceanic arc.
• Tonga trenches: oceanic arc (near a bend, so more crust).
• Stretches ~2000km N from New Zealand.
• Almost all magmas are quartz normative (wet peridotite melting).
• Opportunity to study the effect of the overlying plate on magmatism.
• Calc-alkaline trend can be explained by:
  • Magma mixing: basalt is the first melt in the trend, rhyolite is last; define a mixing line (join) between them (Ewart et al., 1978).
  • Crustal assimilation: New Zealand (continental arc) = most crust and more SiO2; Kermadec (island arc) = least crust and least SiO2; New Zealand follows calc-alkaline, Kermadec and Tonga follow tholeiite (Ewart et al., 1978).
  • Plagioclase suppression (doesn’t allow buildup of FeO): ocean arc products contain less plagioclase; maybe thicker crust means crystallization at higher pressure – stabilizes amphibole relative to plagioclase (Ewart et al., 1978, Grove et al., 1986).
• FeTi-oxide saturation:
  • Fluid from the slab makes the mantle wedge oxidizing: $H<em>2O \leftrightarrow H</em>2 + O_2$.
  • The oxygen fugacity (the partial pressure of oxygen) increases.
  • This stabilizes Fe3+
  • Fe3+ is incompatible (goes into melt even at low F).
  • Magnetite crystallizes early and stops FeO enrichment.
• Irrespective of mechanism, there is a global correlation between crustal thickness and FeO (Chin et al., 2018).

Reading Resources

• Essential:
  • Chapter 8, Essentials of Igneous & Metamorphic Petrology
• Optional:
  • Chapter 16 (16.3), Principles of Igneous and Metamorphic Petrology
  • Grove & Kinzler (1986) review paper
  • Chin et al (2019) paper – the introduction is a good summary of hypotheses for the calc-alkaline trend