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EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11

EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 20. Metamorphic Petrology. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 20.

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EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11

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  1. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 20 Metamorphic Petrology

  2. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 20 Metamorphic facies – metabasites (mafic protoliths) A suite of mineral assemblages found repeatedly in metamorphic terranes of all ages around the world, with a regular relationship between mineral composition and bulk composition. P-T ranges of major metamorphic facies

  3. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 20 Blueschist facies with mafic protoliths Mafic oceanic crust subjected to subduction along a cool geotherm (high pressures, low temperatures) may pass through the “Blueschist Facies”. Characteristic mineral assemblage contains glaucophane (a sodic amphibole) and lawsonite May also contain albite or jadeite + quartz (depending on P-T conditions) and several other phases are possible (clinopyroxene, garnet, phengitic white mica etc) depending on bulk rock composition, P-T etc. Distinct mineralogy from the greenschist facies (albite + chlorite + actinolite).

  4. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 20 Formation of jadeite component in omphacite NaAlSi3O8 = NaAlSi2O6 + SiO2 albite jadeite qz/co (plagioclase) (omphacite) mole fraction of jadeite Breakdown of plagioclase is a critical reaction in formation of eclogite from other facies. Continuous reaction between crystalline solid solutions

  5. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 20 Alkali amphiboles Glaucophane = blue sodic amphibole, therefore double silicate chain structure Na2Mg3Al2(Si8O22)(OH)2 = glaucophane Na2Fe2+3Fe3+2(Si8O22)(OH)2 = riebeckite Na2Fe2+4Fe3+(Si8O22)(OH)2 = arfedsonite Natural glaucophane always has some Fe2+ substitution, but the extent of substitution is limited. Optical properties rather variable, but pale blue colour and pleochroism (PPL) distinctive of glaucophane. Blue alkali amphiboles

  6. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 20 Panoche Pass, Callifornia. The field of view is about 2.5 mm. Abundant blue glaucophane, some showing the classic diamond-shaped amphibole cross section and cleavage. Many small grains of high relief epidote are visible in PPL but get lost in the XPL. The nearly clear matrix material that encloses the glaucophane and epidote is jadeite. It displays low order inteference colors, anomalous in some grains, in XPL.

  7. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 20 ph ep gl qz Glaucophane, phengite, epidote and quartz.

  8. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 20 Lawsonite Double isolated (couplet) silicate, so based on Si2O76- group, like epidote group. Formula = CaAl2(OH)2(Si2O7)H2O and it doesn’t vary much from that!! Optically – 2 perfect cleavages, strong birefringence in XPL, commonly twinned, colour in PPL is colourless to blue-green or yellow/brown, pleochroic. Chains of AlO4(OH)2 octahedra linked by isolated Si2O7 groups Ca cations and H2O molecules sit in the interstices Two types of water in this structure

  9. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 20 Lawsonite is surprisingly dense! CaAl2(OH)2(Si2O7).H2O = CaAl2Si2O8 + 2H2O lawsonite anorthite water Compositional equivalent of anorthite + water Density of lawsonite = 3.1 g/cm3 whereas anorthite = 2.76 g/cm3 May be important in subduction

  10. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 20 musc lawsonite chlorite gl

  11. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 20 Lawsonite – large P-T stability range Lawsonite+H2O MA-expts Series of dehydration breakdown reactions at high temperature with increasing pressure, which might be intersected on hot subduction geotherms But on cooler geotherms lawsonite could transport 11.5 wt% H2O to ≥10 GPa depths (=300km) before dehydration This may trigger H2O-saturated melting or H2O could be incorporated in hydrous aluminosilicates which are stable at these depths and transported even deeper. Pawley 1994 Contrib. Mineral. Petrol.

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