GEOL 256 Lecture 15: Igneous Major Elements 15
13 Nov 2020
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Lecture 15: Igneous Major Elements 1 (10/12/20)
● classify igneous rocks
○ Binary eutectic phase diagram
■ No solid solution
■ Eutectic: horizontal solidus
● Location where melting takes place when you
have 2 or more different minerals
● Where two minerals melt until one is completely
melted
■ Always melts at solidus
○ Mantle peridotites
■ Progressivey melted
■ Increasing proportions of olivine
■ Degrees of melt
● Lherzolite → harzburgite → dunite
■ Lherzolite
● Mostly olivine with a lot of orthopyroxene and clinopyroxene with some
plagioclase
■ Harzburgite
● Has been melted, dominated by olivine and some orthopyroxene
● Less clinopyroxene and plagioclase melted out
■ Dunite
● >90% olivine, much less pyroxene
○ Between melting and eruption
■ 1. Partial melting
■ 2. Rising basaltic magma
■ 3. Reaches neutral buoyancy
● Pools and stored in magma chamber
● Density is the same as surrounding rock density
● Storage
○ Fractional crystallization
■ Cools and solidifies
○ Crustal assimilation
■ Surrounding crust melts a little, crust added to magma
changes composition
○ Magma mixing
■ Magma rises, meets another pocket, and mixes
■ 4. Eruption
○ Major igneous rock elements
■ Most abundant elements in earths crust
● Excluding Oxygen
■ Silicon, aluminum, calcium, iron, magnesium, sodium, potassium, titanium, manganese,
phosphorus
■ >1% composition in lavas
■ Using element data
● Weight % oxides
● Classifying igneous rocks
● Show fractional crystallization and “genetic” relationships
Document Summary
Lecture 15: igneous major elements 1 (10/12/20) classify igneous rocks. Location where melting takes place when you have 2 or more different minerals. Where two minerals melt until one is completely melted. Mostly olivine with a lot of orthopyroxene and clinopyroxene with some plagioclase. Has been melted, dominated by olivine and some orthopyroxene. Between melting and eruption: partial melting, rising basaltic magma, reaches neutral buoyancy. Density is the same as surrounding rock density. Surrounding crust melts a little, crust added to magma changes composition. Magma rises, meets another pocket, and mixes: eruption. Silicon, aluminum, calcium, iron, magnesium, sodium, potassium, titanium, manganese, phosphorus. X-ray fluorescence (xrf) spectroscopy and electron microprobe analysis (empa) Fluorescence: energy released equals binding energy of outer electron orbital minus the inner. One of the most useful for classifying igneous rocks. Use 3 elements: sodium and potassium (total alkalis) versus silica. Show fractional crystallization and genetic or petrogenetic relationships. Cools and crystallizes = evolved = increase silica.
