Constructive Margins and Ocean Crust Generation

Igneous Petrogenesis Geological Settings

• 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

The Mid-Ocean Ridges

• Length: Approximately 60,000 km long.
• Magma Production: Produces > 21 km$^3$ magma per year.
• Spreading Rates: Ranges from 2-180 mm per year.

Crust Thickness

• Oceanic crust exhibits a seismically structured appearance that is similar across the globe.
• Average thickness is about 7km (White et al., 1992).

Ocean Crust Structure

• Raitt (1963): Divided the seismic structure into 3 layers based on seismic velocities.
• Different layers suggest varying mineralogy and density (McClain et al., 2003).
• $V<em>p = \sqrt{ } + 4/3$, $V</em>s = \sqrt{ }$

Sampling Mid-Ocean Ridges

• Challenges: Seafloor sampling is difficult due to the 3-4km water depth.
• Methods:
  • Dredging
  • Gravity coring
  • Trawling
  • Submersible sampling
  • Ocean drilling (from ships)
  • On-land sampling from spreading ridges (e.g., Iceland)
  • Ophiolite studies (e.g. Troodos, Semail, Cornwall).

MORB (Mid-Ocean Ridge Basalt)

• Fine-grained, rapidly cooled basalt.
• Fairly uniform composition.
• Low alkali contents (2-3 wt% Na2O).
• Very low H2O (< 0.2 wt.%).
• Typically olivine tholeiites with normative forsterite & hypersphene.
• Phenocrysts of olivine (olv) and plagioclase (plag).
• Composition lies close to low-pressure cotectics.

Ophiolites

• Basaltic lavas, dykes, and the top 200m of gabbro have a similar composition to MORB (Mg# < 70).
• Layered gabbros have higher Mg# (~ 80), which is close to equilibrium with overlying melts.
• Ultramafic rocks are found beneath.
• Ophiolite velocity profiles match oceanic crust profiles (Christensen & Salisbury, 1975).

MORB Petrogenesis Hypothesis

• MORB is produced by fractional crystallization of primary mantle melt.
• Crystals produced form gabbroic rocks (with olivine, plagioclase, and clinopyroxene).
• Process follows low-pressure cotectics.
• primary melt → fractional crystallization → MORB + gabbro (+ ultramafic).

Melting and Mantle Composition

• What is the average mantle composition?
  • Cosmochemical (Earth is chondritic minus siderophile elements).
  • Geophysical (based on velocity inversions).
  • Xenoliths (lherzolite).
  • Use bases of ophiolites (harzburgite?).
• DMM (Workman & Hart, 2005) [57% olivine, 28% orthopyroxene, 13% clinopyroxene, 2% spinel].
• Mantle composition:
  • SiO2: 44.7
  • TiO2: 0.13
  • Al2O3: 3.98
  • Cr2O3: 1.57
  • MnO: 0.13
  • FeO: 8.18
  • NiO: 0.24
  • MgO: 38.7
  • CaO: 3.17
  • Na2O: 0.13
  • K2O: 0.01
  • P2O5: 0.02
  • Mg#: 89.4

Mantle Olivine

• Mantle olivine (from xenoliths) has Mg# ~ Fo90.
• Magnesium number (Mg#) is calculated as: Mg# = 100 * Mg/(Mg + Fe^{2+})
• Mantle olivine is in equilibrium with Mg# 70 melts: $\frac{Fe^{2+}}{Mg^{2+}} + O<em>{livine} \leftrightarrow \frac{Fe^{2+}}{Mg^{2+}}$, $K</em>D = \frac{ }{} = ~0.3$

Disequilibrium

• Mid-ocean ridge basalts are not in equilibrium with the mantle.
• Some post-melting processes must have occurred.
• This could have happened in the large magma/mush body beneath the MOR.

The Basalt Tetrahedron

• Subtle geochemical differences in basalts change normative mineralogy.
• The basalt tetrahedron helps visualize this.
• The albite plane acts as a thermal divide.

Alkali Basalts

• Richer in alkalis, poorer in silica.
• Are nepheline normative.
• Have olivine as phenocryst and groundmass phase.
• Enriched in incompatible elements.
• Augite is pleochroic (Fe$^{3+}$/$Ti).
• Nepheline usually hidden in groundmass.
• Evolve to basanite, syenite, phonolite.

Tholeiitic Basalts

• Olivine only as a phenocryst.
• Higher silica content.
• Hypersthene or quartz normative.
• Evolve to trachyte or rhyolite.

Depth of Melting

• Phase diagram based on compiled results for melting of aluminous lherzolite (DMM).
• Assume isobaric melting (single pressure).
• Melt compositions lie at the eutectics.

30 kbar Melt Evolution

• High-pressure melts lie close to olivine.
• Primary melt ≠ MORB.
• If moved to lower pressure, eutectics change.
• At 1 atm, the olivine field is much larger.
• 30 kbar melts sit in olivine + liquid (L) field.
• Cooling crystallization order: olivine → olivine + plagioclase → olivine + plagioclase + clinopyroxene.

20 kbar Melt Evolution

• Melts project outside the tholeiite field.
• Lies on the Si-poor side of the critical plane of silica saturation.
• Will evolve to an alkali basalt.
• Cannot form MORB.

< 15 kbar Melt Evolution

• Melts again project inside the tholeiite field.
• Can produce MORBs.
• Some MORB lies close to the plane of silica undersaturation.
• Must be ~ 15 kbar melts (or 25 kbar melts), but melts at lower pressure have Mg# < 89, so cannot be in equilibrium with all mantle rocks.

Ultramafic Cumulates

• Model also explains harzburgite/dunite cumulates.
• When melts move to low pressure, olivine is the first crystallization phase – fractional crystallization makes dunite → the MOHO provides a convenient density barrier.
• Harzburgites (olivine + orthopyroxene) unlikely from low P crystallization as MORB not in equilibrium with orthopyroxene.
• Fractional residue of mantle melting.

MORB Genesis

• Primary MORB melts are either produced at:
  • low pressure (<15 kbar or 45km) or
  • high pressure (> 25 kbar or 75km).
• This assumes that:
  • the phase diagram is correct
  • melting is isobaric
• Details (from more modern experiments) show this picture is sensitive to Na, K, Ti, H and Cr contents.
• Tholeiites formed at large F.
• Tholeiites formed at low pressure.
• (after Kushiro, 2001).

Physical Melting Model

• Mantle adiabat (~ 0.6 °C km$^{-1}$).
• Dry peridotite solidus steeper (~ 5°C km$^{-1}$).
• If no crust, adiabat extends nearer surface – if high enough, decompression melting occurs.
• Upwards flow of mantle beneath ridges allows decompression without conductive heat loss → extensive melting.

Corner Flow Model

• Allows for decompression and melting over a wide depth range.
• Not isobaric.
• Have to aggregate (and mix) melts over the entire melting region (the triangle).
• As crustal spreading rate changes, so does melt production.
• Corner flow model predicts that the melt zone is truncated deeper with decreasing spreading rate.
• Upwelling slower → more time for the melt to cool and solidify.

Melt Focusing

• Melts produced under MOR with corner flow must be focused to the axis (otherwise, they would get stuck).
• Field-scale and numerical modeling confirm this happens (Braun & Kelemen, 2002).

Crustal Thickness and Spreading Rate

• (Bown & White, 1994)
• Global data compilation confirms this.
• More complicated at ultra-slow and ultrafast ridges (Zhou et al., 2020).
• $H = -0.032 * S_r + 8.9$, $R^2 = 0.85$

Reading Resources

• Essential:
  • Chapter 6 & 7 Essentials of Igneous & Metamorphic Petrology (10.1-10.3 inclusive).
• Optional:
  • Chapter 16 (16.2) Principles of Igneous and Metamorphic Petrology.
  • Classic papers by McKenzie & Bickle (1988), Langmuir et al (1992) as well as review papers by Kushiro (2001) and Langmuir & Forsyth (2007) are on the reading list (might be helpful). All on GEOL0011 reading list – linked in Moodle.