Basalt Formation and Mantle Melting Processes

Posted on Nov 25, 2024 in Geology

Basalts (IC 35-70)

Mode of Occurrence

  • MORB: Tholeiitic
  • Plateau: Tholeiitic dykes, cordon fissure lodes, possibly calc-alkaline
  • OIB: Tholeiitic and calc-alkaline
  • Subduction Zones: Differentiated rocks (possibly with hydrous minerals), tholeiitic calc-alkaline (young arcs). Example: ZVS N = 33-34°30′ NS, hydrated, T = 34°30′-37°NE, 2 pyroxenes + biotite + amphibole + minor olivine. Mocha C NE = 37-41°30′, anhydrous reaction with 2 pyroxenes + contamination. S = 41°30′-46°NE, hydrated with clinopyroxene.

Chemical Characteristics

  • QAP:
  • TAS: 45-52% SiO2, Na2O + K2O ~ 5%, possibly calc-alkaline (nepheline normative) and subalkaline (without nepheline normative), possibly picritic (41-45% SiO2, Na2O + K2O ~ 3%).
  • Mineral Assemblages: Tetrahedron Di-Qz-En-Fo-Ne (Ab between Ne and Qz)
    • Oversaturated tholeiitic (Qz + Hyp)
    • Subsaturated tholeiitic (Hyp)
    • Olivine tholeiite (Ol + Hyp)
    • Olivine basalt (Ol)
    • Alkali basalt (Qz + Ne normative)

Two important planes: Fo-Ab-Di (Ne-Ab-Qz, thermal barrier, tridymite) and Di-Ab-In (Fo-En-Qz, peritectic, critical level of silica saturation).

Calc-alkaline trachyte or rhyolite. Tholeiite: SiO2 45%, SBP ~ 2% higher TiO2, nepheline normative. SiO2 50%, high Al2O3, medium TAS, low TiO2, MgO, FeO, K2O. Early crystallization of titanomagnetite. SiO2 50%, low TiO2, TAS enriched in Fe ~ 2% on average in differentiated rocks.

Mineralogy and Textures

Olivine phenocrysts and microphenocrysts (zoning), titanoaugite, plagioclase phenocrysts (labradorite-andesine), olivine microphenocrysts in microphenocrysts (rare) and alkali feldspar microphenocrysts (rare), differentiated phenocrysts (rare). Accessory minerals: apatite, ilmenite, titanomagnetite. Glass is very rare.

Zoned olivine and pyroxene phenocrysts, possible orthopyroxene and clinopyroxene zoning, bytownite, ferromagnetic plagioclase, Fe-poor glass, titanomagnetite. Disequilibrium textures are rare (corrosion, reaction rims, embayment, sieve texture). Mixed magmas.

Little or no olivine + orthopyroxene + augite – pigeonite + possible hypersthene phenocrysts. Plagioclase An imbalance, labradorite microphenocrysts. No evidence of resorption.

Associated Rock Types

  • With Ca-plagioclase (~20% modal):
    • Nepheline normative ~ 5% (alkali basalt, olivine alkali basalt, picritic alkali basalt)
    • Nepheline ~ 5% normative basanite (Ol ~ 10%) and tephrite (Ol ~ 10%).
  • Without plagioclase:
    • With olivine: nephelinite
    • Without olivine: leucite nephelinite and leucitite
    • Ankaramite (rich in clinopyroxene and olivine phenocrysts)
  • Alkali feldspar trachyte, basaltic trachyandesite

Geotectonic Setting

Plateau, seamount, OIB, hotspots. Basanite and nephelinite to phonolite.

Mantle Melting Percentage

Al2O3 vs. TiO2 Diagram

Dunite, harzburgite, lherzolite, basalt.

Lherzolite Melting

  • Dunite (20-25% fusion rate)
  • Harzburgite (15-20% fusion rate)

Lherzolite Phase Diagram

Geotherm not sufficient to melt due to not intersecting the solidus. Temperature increase by:

  1. Increased mantle temperature (hotspots, localized, high during Archean, poorly differentiated magmas)
  2. Adiabatic ascent of the mantle (decompression melting, 10-30% fusion rate, e.g., divergent margins)
  3. Addition of volatiles (H2O) contained in hydrated minerals.

Peridotite Pressure vs. Temperature Diagram

  • Point A: Oceanic geotherm/hydrated peridotite solidus. H2O fusion without mantle fusion, no H2O available.
  • Point B: Amphibole breakdown in oceanic geotherm, H2O released for fusion.
  • Point C: Phlogopite breakdown in oceanic geotherm, H2O released for fusion.
  • Point D: Amphibole/shield geotherm without fusion, temperature below H2O-saturated solidus, H2O migration.
  • Point E: Shield geotherm/solidus without fusion, H2O migration due to temperature below solidus.

Mantle Composition and Magma Types

Uniform mantle composition leads to tholeiitic and alkaline magmas. Fo-Ne-Qz (Ne-Ab-Qz-En-Fo) ternary system. Eutectic point pressure migrates with:

  • Low pressure: tholeiitic (low H2O), supersaturation in SiO2 (En-Ab-Qz)
  • High pressure: alkali (4-6% H2O), unsaturated in SiO2 (Ne-Ab-Fo)

Subsaturated tholeiites are intermediate. Partial melting of peridotite at increasing pressure leads to more alkaline melts. Higher pressure leads to higher tholeiitic fusion rates.

Residue and Magmas from Pyrolite Melting (Ol + Px)

Depth (km)Fusion Temperature (°C)Magma TypeMineral Assemblage
~3025%Olivine tholeiitePlagioclase + Olivine + Orthopyroxene
30-6020%Alkaline
30-6025%Olivine alkalineOlivine + Clinopyroxene + Plagioclase + Orthopyroxene
6030%Olivine tholeiite
10040%Picrite

Experimental Mantle Melting

Stratified mantle: shallow layer produces tholeiites (depleted), deep layer produces alkali basalts (enriched). Alkaline basalts originate from partial melting of enriched lherzolites.

Characteristics of Primary Magmas from Lherzolite Melting

Mg# = 66-75, ~1000 ppm Cr, Ni ~ 400-500 ppm.

Magma type depends on pressure, fusion rate, and fractional crystallization.

Fractional Crystallization During Ascent

  • Tholeiites: Shallow melting profiles fractionate olivine, deeper profiles fractionate olivine and In-rich minerals, producing picrites during ascent.
  • Alkaline basalts: High-pressure partial melting, fractionate Al-rich phases from tholeiites.
  • Tholeiitic trends: Fractional crystallization at low pressures (1 atm) under anhydrous conditions.
  • Calc-alkaline trends: Fractional crystallization at 5.8 kbar, hydrous conditions, fractional crystallization + magma mixing and crustal assimilation, primary magma differentiation by hydrated peridotite melting.