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Mafic Rocks

EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 9. Mafic Rocks. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 9. Fractional Crystallisation of magmas. Crystals that separate from a cooling magma in a magma chamber generally have

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Mafic Rocks

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  1. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 9 Mafic Rocks

  2. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 9 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

  3. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 9 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”

  4. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 9 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

  5. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 9 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.

  6. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 9 Basalt plagioclase also contain Na as albite (NaAlSi3O8) Ternary system (Di-Ab-An) 2 binary eutectics (Ab-Di; An-Di) and a solid solution (Ab-An)

  7. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 9 a b Plagb c Plagc d Plagd

  8. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 9 Ca Na Zoned plagioclase crystals in a basalt

  9. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 9 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 + Ca-plagioclase

  10. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 9 Fractional Crystallisation of magmas

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

  12. 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 %

  13. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 9 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

  14. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 9 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 %

  15. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 9 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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