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Mafic Rocks

EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 8. Mafic Rocks. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 8. Melting in the Earth - What, Where, Why, and How?. Melting is the single most important fractionation process of the Earth

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Mafic Rocks

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  1. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 8 Mafic Rocks

  2. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 8 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 8 Table of peridotite and basalt compositions

  4. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 8 Spinel peridotite suite from the Balkans basalt Refractory peridotite =harzburgite Refractory peridotite =harzburgite Fertile peridotite lherzolite Fertile peridotite lherzolite

  5. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 8 Refractory peridotite + basalt = fertile (primitive) peridotite Less fertile lherzolite + small % of basalt liquid Low % melting High % melting Fertile peridotite = lherzolite Refractory harzburgite + higher % basalt/picrite liquid Very high % melting Ultra-refractory dunite + picrite/komatiite What causes mantle peridotite to melt?

  6. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 8 Mid-ocean ridges = adiabatic upwelling, melting due to pressure decrease Hawaii Rift MOR 60,000 km long Melt production ≈ 20 km3/yr New oceanic lithosphere

  7. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 8 Hot spots (oceanic islands) Plumes/hot spots = adiabatic upwelling of anomalously hot mantle, melting due to pressure decrease

  8. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 8 Subduction zones Arcs Dehydration of minerals in slab

  9. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 8 Naming and mineralogy of basalts Basalts usually classified on whole rock chemistry rather than minerals present as they are mostly fine-grained, sometimes glassy TAS-diagram (Total Alkali Silicate) Sub-alkali/tholeiitic are equivalent tholeiitic basalts

  10. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 8 Basalt geochemistry • Basalts (and related rocks) – very diverse compositions • Major aim of petrology is to explain this diversity and learn from it

  11. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 8 Diversity of basalt (and related rock) compositions is a consequence of Source and melting variations (composition of source, PT of melting, % of melting, presence of volatiles during melting……) 2. Magma differentiation (fractional crystallisation, crustal assimilation…..) Evolved magmas Fractionated magmas Contaminated magmas etc Primary magmas Source/melting variations Magma differentiation • Fractional crystallisation • Magma mixing • Crustal contamination

  12. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 8 Fractional Crystallisation of magmas • Crystals that separate from a cooling magma in a magma chamber generally have • different compositions to the parental magma (eg Mg-rich basalt) • As crystallisation proceeds, the first mineral crystallising may be joined progressively by • new minerals. • Typically for basalts the sequence of crystallisation is: • oxide mineral (Cr-spinel, ilmenite, magnetite), olivine, clinopyroxene, plagioclase…. • Rocks formed by accumulation of separated crystals are called “cumulates”. Clearly, the • bulk chemical composition of a cumulate is quite different to the parental magma • As the crystals separate (by sinking or floating) the residual magma composition changes

  13. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 8 A simplified analogy for basalt crystallisation – the system Diopside-Anorthite (Di-An; CaMgSi2O6-CaAl2Si2O8) • Fields = • melt only • Di+melt • An+melt • Di+An (no melt) A-E = liquidus D-E = liquidus B-C = solidus Four fields, two liquidii and solidus all meet at a single unique point (E) called the “Eutectic Point”

  14. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 8 Now we add olivine to the mix (as forsterite = Mg2SiO4) Ternary system (An-Di-Fo) Better analogy for basalt crystallisation as it Involves olivine Contours show the liquidus temperatures 4 fields in which different phases crystallise first (i.e. phases are “on the liquidus”) i.e. anorthite, spinel, forsterite, diopside Boundaries between phase field (temperature valleys) are “cotectics”. “E” is the eutectic point, which is at 1270°C in this system

  15. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 8 Now we add olivine to the mix (as forsterite = Mg2SiO4) Imagine a liquid with composition “x” at 1500°C, beginning to cool down First it just cools down to 1430°C where it hits the liquidus surface and crystallised olivine (pure forsterite Mg2SiO4) Removal of olivine from the liquid drives the liquid composition along the arrow directly away from the forsterite corner When the liquid hits the cotectic it begins to crystallise pure diopside as well as forsterite Removal of Fo + Di drives the liquid Towards An along the cotectic When it reaches E (eutectic) anorthite Starts to crystallise as well as Fo+Di.

  16. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 8 Crystallisation sequence of “basaltic” liquid in the An-Di-Fo system is usually (but not always) Melt Melt + forsterite Melt + forsterite + diopside Melt + forsterite + diopside + anorthite Crystallisation sequence of natural basalts is usually (but not always) Melt Melt + olivine Melt + olivine + clinopyroxene Melt + olivine + clinopyroxene + anorthite

  17. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 8 Fractional Crystallisation of magmas

  18. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 8 Fractional Crystallisation of magmas

  19. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 8 Fractional Crystallisation of magmas Variation diagrams – magma evolution at Gough Island, South Atlantic AB Growth and separation of high MgO olivine depletes residual liquid in MgO SiO2 %

  20. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 8 Variation diagrams – magma evolution at Gough Island, South Atlantic Fractional Crystallisation of magmas AB Growth and separation of low Al2O3 and SiO2 olivine enriches residual liquid in Al2O3 and SiO2

  21. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 8 Variation diagrams – magma evolution at Gough Island, South Atlantic Fractional Crystallisation of magmas Growth and separation of High CaO plagioclase and low CaO olivine depletes residual liquid in CaO and enriches it in SiO2 Some high MgO rocks are are cumulates of olivine SiO2 %

  22. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 8 Variation diagrams – magma evolution at Gough Island, South Atlantic Fractional Crystallisation of magmas Growth and separation of high TiO2 ilmenite and/or titanomagnetite depletes residual liquid in TiO2

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