Lecture 19 - Pyroxene phase relations and geothermometry...

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Pyroxene phase relations and geothermometry Examine phase relations of the pyroxenes Fo-SiO2 system En-Diopside system breakdown of jadeite Use of pyroxenes in geothermometry estimation of equilibration temperatures Use of pyroxenes in geobarometry estimation of equilibration pressures
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Pyroxene end members Naturally occurring end members diopside, hedenbergite etc Three additional end members do not occur in nature Ca-tschermaks molecule – CaAl 2 SiO 6 Mg-tschermaks molecule – MgAl 2 SiO 6 Ferri-teschermaks molecules – CaFe 3+ AlSiO 6
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Additional end members Important in substitution of Al into the pyroxene structure can occur in both 4- and 6-fold coordination high T and low P favours Si in T-sites high pressure favours increased coordination number Al tends to be in 6-fold coordination almost no 4-fold coordinated Al
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Phase relations of orthopyroxene Presence of a peritectic (reaction point) early formed olivine reacts with melt forms orthopyroxene often the opx occurs as rims around the olivine Opx can also form as a primary phase
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The system Enstatite - Diopside Important in understanding exsolution presence of pigeonite in quickly cooled rocks metastable phase restricted range of T-x stability – 1270-1420°C narrow range of composition
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The system Enstatite - Diopside Pigeonite can coexist with orthoenstatite Diopside Forsterite + liquid depends on bulk composition Orthoenstatite, pigeonite and diopside can only coexist at an invariant point (F=c-p+1, since system is isobaric)
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The system Enstatite - Diopside Invariant point pigeonite pinchout point below this point pigeonite is no longer stable As Fe content of the system increases the temperature of the invariant point decreases still pigeonite is ONLY STABLE at high temperature should invert to Oen + Di Oen host with Di lamellae
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This note was uploaded on 03/19/2012 for the course GEOLOGY 6 taught by Professor Rm during the Spring '12 term at UNAM MX.

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Lecture 19 - Pyroxene phase relations and geothermometry...

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