Petrology & Petrography of Igneous Rocks
(Geology and Geophysics 234 & 433)
Exam II: Generation and Crystallization of Basaltic
Magma
The following concepts will be covered on Exam II.
Part I. Generation of Basaltic Magma
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Potential source materials within the asthenosphere
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Pyrolite
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Spinel lherzolite
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Critical Melt Fraction
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Dihedral angle
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Batch melting (within the Forsterite-Quartz-Diopside Ternary)
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Effect of pressure on phase relationships within this same
system.
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Decompression Melting
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steady-state convective geotherm
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upwelling asthensophere during extension
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Controls on melt composition
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importance of invariant points "cusps" on the solidus
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the effect of increasing pressure
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changes in mineral assemblage
Part II. Crystallization of Basaltic Magma
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Formation of Large Layered Igneous Complexes
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Cumulate Terminology (cumulus, intercumulus, orthocumulate,
mesocumulate, adcumulate, heteradcumulate,
crescumulate, oikiocryst, chedacryst, poikiolytic, ophitic)
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Origin of Modal Layering
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Origin of Cryptic Layering
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Processes that operate during crystallization of Large Layered
Igneous Complexes
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gravitational settling
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magma replenishment
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assimilation
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effect on the order of crystallization and appearance of
opx and calcic-plagioclase.
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Application of phase diagrams to understanding these processes.
Part III. Compositional Variation in Igneous Rock Suites
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Variation Diagrams
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Fractionation Indices (Why do we use them?)
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Geochemical Coherence (What is meant by this?)
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Inflection Points (What is their significance?)
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Liquid Line-of-Descent
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What does this signify?
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What is meant by "Crystal-liquid fractionation processes
are reproducible at different times".
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Causes of scatter.
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Graphical Solutions to petrographic problems.
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Use of extraction polygons on two-oxide variation diagrams.
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Quantitative mixing calculations for explaining compositional
variation (lever-rule).
Part IV. Trace Element Behavior
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Distribution coefficient (Kd)
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Incompatible element behavior
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Compatible element behavior
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C = Co(1/F)
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Uses in solving petrologic problems
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calculation of percent crystallization
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determination of equilibrium between olivine and melt
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identification of Mg# of primary liquids