Lecture 9. MANTLE MELTING
Magmas are ultimately derived by melting in the mantle, rise by buoyancy.
Review of the geothermal gradient, increase of pressure with depth (3.3 km/kbar)
Why does melting occur?
Ways to make A melt:
- Increase T
- Decrease P - adiabatic upwelling (rising limb of convection cell at MOR)
- Add fluid - this lowers the solidus (see figure below)
Adding a fluid fluxes the mantle and lowers the melting temperature. This now brings the solidus below the temperature of A (below the geotherm)
Where does this occur? At subduction zones (the downgoing slab dehydrates and releases water).
Mobilization of melt in the mantle
Liquid collects in channels. Like river, smaller channels are abandoned for larger ones.
At some point, channels are abandoned for fractures (<~25 km depth)
If lots of melt (i.e. ~50%), the entire mantle can rise as a diapir. Mantle flows as viscous entity.
Composition of the mantle
Ultramafic - rich in olivine and pyroxene, but also does have an aluminous mineral (See Fig 7-7 of text)
Crystal liquid relations during melting: Progression of changing mantle melting proceeds:
- olivine + opx + cpx + spinel or garnet ± phlogopite (original mantle, no melting)
- olivine + opx + cpx: (small degrees of partial melting)
- olivine + opx (a few percent melting)
- olivine (up to 50% melting)
Direct evidence for composition of the mantle: Ultramafic xenoliths
Different "types" depending on nature of aluminous phase and ratio of px to ol
Group I: Gabbros. Cpx, plag, ol (plag is aluminous phase)
Group II: Dunites and wehrlites - ol, cpx (either plag or spinel as aluminous phase)
Group III: Lherzolites & harzburgites - ol, opx, cpx (spinel as aluminous phase)
Group IV: Garnet peridotites/eclogites - ol, px, gt (garnet as aluminous phase)
Thus, increasing depth of formation from I to IV.
In Hawaii, a researcher noticed that different basalt types carried specific xenolith types:
So more silica undersaturated, greater range of inclusion types. What does this tell you about the depth of the source of undersaturated versus tholeiitic magmas?