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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 19. Metamorphic Petrology. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 19.

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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 19 Metamorphic Petrology

  2. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 19 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 19 Mafic protoliths at eclogite facies Al2SiO5 Alm+Py garnet + omphacite are defining eclogitic phases TiO2 Omphacite = sodic clinopyroxene consisting mainly of jadeite (NaAlSi2O6) and diopside-hedenbergite (Ca(Mg,Fe)Si2O6) components Mafic bulk compositions

  4. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 19 Eclogite layer in peridotite at Almklovdalen, Norway Diamond-bearing eclogite Xenolith from kimberlite

  5. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 19 Omphacite (omphacitic clinopyroxene) Omphacite = sodic clinopyroxene consisting mainly of jadeite (NaAlSi2O6) and augite (Ca(Mg,Fe)Si2O6) components in approximately equal proportions. Jadeite (NaAlSi2O6) contains single chain SiO44- tetrahedra and AlO6 octahedra, with Na+ cations. The gem “jade” is sometimes jadeite, or sometimes an amphibole called “nephrite”. Omphacite (the characteristic clinopyroxene of the eclogite facies) is a solid solution of jadeite and augite.

  6. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 19 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

  7. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 19 P-T path into eclogite facies

  8. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 19 • Eclogite occurs in • Subducting mafic crust (after dehydration of lower grade facies rocks) • Orogenic zones (crustal thickenning leading to high pressure metamorphism) • Peridotite massifs tectonically emplaced into crust • Lithosphere, particularly the cratonic lithosphere (often associated with diamonds) • Deep convecting mantle (an important component of oceanic volcanism)

  9. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 19 Thermometry (Fe-Mg exchange between garnet and clinopyroxene in eclogite) Mg3Al2Si3O12 + 3CaFeSi2O6 = Fe3Al2Si3O12 + 3CaMgSi2O6 pyrope hedenbergite almandine diopside Continuous exchange reaction of Fe and Mg between clinopyroxene and garnet Reaction is dependent on temperature and garnet grossular content High pressure experiments were used by Ellis and Green (1979) to calibrate these dependencies and develop a “geothermometer” applicable to eclogite assemblages Application – to determine the temperature at which eclogites formed

  10. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 19 Fe-Mg distribution between garnet and clinopyroxene Mg3Al2Si3O12 + 3CaFeSi2O6 = Fe3Al2Si3O12 + 3CaMgSi2O6 pyrope hedenbergite almandine diopside Kd = (mol frac Fe in garnet)*(mol frac Mg in cpx)/(mol frac Mg in garnet)*(mol frac Fe in cpx) = [XFega*XMgcpx]/[XMgga*Xfecpx] Experiments aim to determine the variation in Kd with temperature and other factors such as grossular component in garnet.

  11. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 19 Experimental compositions and assemblages Experiments run at 2.4 – 3.0 GPa and 750-1300°C in piston-cylinder apparatus.

  12. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 19 Experimental results 3.0 GPa, XCaga = 0.2 3.0 GPa, 1200°C Dependence of Kd on grossular mole fraction Dependence of Kd on temperature Statistical fit of all data at all P-T-XCaga leads to algorithm

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