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Mafic Rocks

EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 6. Mafic Rocks. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 6. Mid-ocean ridges = adiabatic upwelling, melting due to pressure decrease. 60,000 km long Melt production ≈ 20 km 3 /yr New oceanic lithosphere.

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Mafic Rocks

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  1. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 6 Mafic Rocks

  2. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 6 Mid-ocean ridges = adiabatic upwelling, melting due to pressure decrease 60,000 km long Melt production ≈ 20 km3/yr New oceanic lithosphere

  3. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 6 Hot spots (oceanic islands) Plumes/hot spots = adiabatic upwelling of anomalously hot mantle, melting due to pressure decrease

  4. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 6 Large Igneous Provinces (Continental Flood Basalt Provinces, Oceanic Plateaux)

  5. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 6 Subduction zones Dehydration of minerals in slab

  6. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 6 Importance of basalts • Most voluminous volcanic rock on earth • Occur in nearly all tectonic settings on earth • Also found on other terrestrial bodies (moon, Mars, some meteorites) • Parental to many other types of magmas through crystal fractionation • Sometime contain fragments of deep crust or upper mantle (xenoliths) • Formed by partial melting of the mantle, so there the geochemistry • of basalts can inform us about the composition and evolution • of the mantle

  7. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 6 Naming and mineralogy of basalts • Fine grained, often microscopically fine • Minerals include • Augite (= Ca-rich pyroxene i.e. clinopyroxene) , sometimes pigeonite, enstatite • Calcic plagioclase with mol% anorthite (CaAl2Si2O8) > mol% albite (NaAlSi3O8) • in solid solution (An50-An100)

  8. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 6 Naming and mineralogy of basalts • Fine grained, often microscopically fine • Minerals include • Augite (= Ca-rich pyroxene i.e. clinopyroxene), sometimes pigeonite, enstatite • Calcic plagioclase with mol% anorthite (CaAl2Si2O8) > mol% albite (NaAlSi3O8) • in solid solution (An50-An100) • Olivine (in which case it may be called picrite) – usually >50 mol% Fo • Cr-spinel, titanomagnetite, ilmenite (opaque minerals in thin sections) • Nepheline (NaAlSiO4) • Apatite (Ca5(PO4)3(OH,F,Cl)) • Glassysometimes (eg quenched chilled margins etc) • Mafic rocks (i.e. 50-90 volume% dark minerals) • Basic rocks (45-52 wt% SiO2) in the whole rock analysis

  9. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 6 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, 2012 – Lecture 6 Naming and mineralogy of basalts • Also classified depending on tectonic setting in • which they erupted • Mid-Ocean Ridge Basalt (MORB) • Ocean Island Basalt (OIB) = Intraplate basalt • Continental Flood Basalt (CFB) = Intraplate basalt • Island Arc Basalt (IAB) • Back-arc Basin Basalt (BABB)

  11. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 6 Crystallisation of basalts - textures Vesicles – gas bubbles of H2O, CO2, SO2…. Vesicular basalt

  12. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 6 Upward increase in viscosity in a lava flow

  13. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 6 Glassy basalt Quenching from T≈1200°C by seawater or air, or along the margins of dykes intruding into cooler crustal rocks (chilled margin).

  14. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 6 Aphanitic basalt Crystalline but too small to be seen by naked eye = aphanitic texture

  15. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 6 Porphyritic texture – bimodal size distribution of crystals. Large ones are phenocrysts, and the rest is the Groundmass = two stage cooling history Opposite is aphyric.

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