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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 12. Andesites Dacites Rhyolites. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 12.

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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 12 Andesites Dacites Rhyolites

  2. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 12 Andesites, dacites and rhyolites Volcanic rocks – varying from “intermediate” to “acid” Andesites – 57- 63 wt% SiO2 (Intermediate); mesocratic Dacites & rhyolites - >63 wt% SiO2 (Acid); leucocratic

  3. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 12 Andesite – plagioclase + mafic minerals (olivine, enstatite, augite, hornblende). Distinctly plagioclase-phyric. Dacite – Na-plagioclase, quartz, alkali- feldspar, minor mafics. Plagioclase> alkali-feldspar. Usually plagioclase-phyric Rhyolite – quartz, alkali-feldspar, glassy or microcrystalline matrix. QAP diagram – mineralogical classification based on vol%

  4. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 12 Fractional crystallisation of basalt  andesite – dacite/rhyolite series • Andesite-dacite-rhyolite are • arbitrary divisions of a continuum • of magma compositions • Often related by fractional • crystallisation from basalts Le Bas et al., 1992; Le Roex et al., 1990; Cole, 1982; Hildreth & Moorbath, 1988

  5. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 12 Basalt – andesite – dacite/rhyolite series – olivine-opx crossover R = reaction point at which Mg-rich olivine reacts with liquid from which it crystallised to form enstatite-rich orthopyroxne

  6. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 12 Mafic phenocrysts in the basalt – hydrous phases biotite, hornblende • Bt, Hb can only Xtallise at T< dehydration curve • Only melts more evolved than basalt have solidus • Ts low enough for these phases to be stable • Therefore hb first occurs in andesites and bi is • normally only found in evolved andesites, dacites • and rhyolites KFe2+3Fe3+AlSi3O10(OH)2 KAlSi3O8 + Fe3O4 + H2O biotite sanidine magnetite vapour

  7. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 12 Eruptive styles of andesites, dacites and rhyolites Andesite lava flows – higher aspect ratio than typical for basalt flows, blocky flows (a’a, never pahoehoe (ropy) style, autobrecciation

  8. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 12 Eruptive styles of andesites, dacites and rhyolites • Lava domes – dacite lavas form thick domes erupted over the volcanic vent as they are • resistant to flowing away from the vent; • exogenous domes (=growing on the outside) • endogenous domes (=growing on the inside)

  9. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 12 Viscosity of lavas • Major effect on how lavas flow is viscosity. • Viscosity of lavas depends on two variables (temperature and composition) • Increasing T decreases viscosity • Increasing melt SiO2 content increases • viscosity Note non-linear scales!

  10. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 12 Viscosity of lavas Mafic melts contain small and simple silicon-oxygen based anionic units SiO44-; Si2O76-, (SiO3)n2n-….etc These molecular anionic species can easily flow past each other resulting in relatively low viscosity More siliceous melts (dacites, rhyolites etc) contain larger more complex 3-dimensional silicate structures, including chains and networks which stiffen the lava and make it more resistant to flow, i.e. more viscous

  11. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 12 Viscosity of lavas – other factors • Dissolved water in a lava tends to lower viscosity as it de-polymerises the melt • Conversely exsolved H2O (vesicles) tends to increase viscosity • Crystals (phenocrysts) increase viscosity • Therefore, during ascent in a volcanic eruption, the combined effects of vesiculation • as volatiles exsolves and crystallisation of minerals leads to increased viscosity. The decrease in temperature contributes as well and can lead to dome formation and • explosive eruption – pyroclastic eruption + 3 H2O Si Si Si Si Si Si Si Si Si Si OH O O O O O OH OH O O + +

  12. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 12 Mt St Helens eruption

  13. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 12 Feldspars

  14. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 12 Feldspar mineral group Occurrence Feldspars occur in most igneous rocks from basic to acid and in many metamorphic rocks So far you have met them in basalts, andesite, dacite and rhyolite. Also present in the plutonic equivalents - diorite, granite and granodiorite. KAlSi3O8 Framework silicate structures Alkali feldspars = (K,Na)AlSi3O8 Plagioclase feldspars = NaAlSi3O8 – CaAl2Si2O8 alkali-feldspar solid solution No naturally occurring feldspars NaAlSi3O8 CaAl2Si2O8 plagioclase solid solution

  15. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 12 Recognising feldspars in volcanic rocks Anorthoclase – perpendicular Multiple twinning

  16. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 12 Recognising feldspars in plutonic rocks • In volcanic rocks phenocrysts are • quenched, preserving their high T • state • In plutonic rocks crystals cool slowly • and can invert to structures that are • stable at lower Ts

  17. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 12 Feldspar structure and stoichiometry • Network silicates in which SiO44- and AlO46- tetrahedra are linked in a 3 dimensional • structure = ratio (Si+Al):O of 2:1; i.e. tetrahedrally co-ordinated cation:oxygen = 2:1 • Ratio of silica to alumina tetrahedra is 1:4 in alkali-feldspars and 1:2 in anorthite • Interstices of the framework of (Si,Al)O4 tetrahedra are occupied by metal cations, • mainly Na+ and K+ in the alkali-feldspars and Na+ and Ca2+ in the plagioclase series • Number of cations per formula unit appropriate to give charge balance NaAlSi3O8 KAlSi3O8 albite orthoclase Alkali feldspars CaAl2Si2O8 anorthite

  18. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 12 Feldspar structure and stoichiometry Polyhedral framework structure of feldspar MT4O8. The light and dark blue tetrahedra are T1 and T2. The purple spheres are the cation sites (K, Na, Ca) http://www.youtube.com/watch?v=FC2TOWKqOtA

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