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Ultramafic Rocks

EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 5. Ultramafic Rocks. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 5. Melting in the Earth - What, Where, Why, and How?. Melting is the single most important fractionation process of the Earth

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Ultramafic Rocks

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  1. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 5 Ultramafic Rocks

  2. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 5 Melting in the Earth - What, Where, Why, and How? Melting is the single most important fractionation process of the Earth What type of liquids are produced? What is left behind Fractionation? Where: links of tectonics with magma type rift to ridge ------- arc ------- hot-spot ------- crust Why & How: generation of melt migrationsolidification

  3. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 5 Spinel or garnet peridotite? – Melting relations of peridotite Solidus = the temperature below which a substance is completely solid, at given pressure Melt + residue CaAl2Si3O8 + 2Mg2SiO4 = 2MgSiO3 + CaMgSi2O6 + MgAl2O4 plagioclase olivine opxcpxspinel 4MgSiO3 + MgAl2O4= Mg2SiO4 + Mg3Al2Si3O12 opxspinel olivine garnet How does peridotite in the mantle cross the solidus and melt?

  4. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 5 Geotherms – conductive and adiabatic • Sources of heat in the earth’s interior. • Primordial heat = gravitational energy trapped during planetary formation • Decay of radioactive isotopes (K, Th, U) Adiabatic upwelling = net heat transfer during uplift is zero, but cooling caused by work done on surrounding peridotite

  5. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 5 Phase transitions Melting of peridotite Adiabats

  6. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 5 Melting of peridotite Melt + residue How does the mantle melt? (1) Add heat

  7. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 5 How does the mantle melt? (2) reduce pressure Melting of peridotite Melt + residue How does the mantle melt? (2) Reduce pressure

  8. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 5 Earth bathymetric map

  9. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 5 Hot-spot Mid-ocean ridge Subduction Adiabatic upwelling = net heat transfer during uplift is zero, but cooling caused by work done on surrounding peridotite Plume Dynamic earth High heatflow Rigid, brittle, cold lithosphere = conductive geotherm Soft, plastic, flowing asthenosphere = convective geotherm

  10. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 5 Mid-ocean ridges = adiabatic upwelling, melting due to pressure decrease

  11. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 5 Plumes/hot spots = adiabatic upwelling of anomalously hot mantle, melting due to pressure decrease

  12. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 5 How does the mantle melt? (3) Add water H2O-saturated peridotite solidus

  13. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 5 Subduction zones = H2O-fluxed melting of peridotite Subduction zones – melting of peridotite fluxed by hydrous fluids Dehydration of minerals in slab

  14. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 5 Olivine binary phase diagram – olivine is the most abundant upper mantle mineral and so its melting relations are a very simple model for melting of the mantle Melting is incongruent Melting T (solidus) varies with composition Complete solids solution (ss) between Fo and Fa

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