Flashcards
Magma Assimilation
Cards in this deck (26)
- 1AssimilationThe incorporation of country rock in magma
The two interact as they try to reach equilibrium with each other, i.e. the rock is trying to become like the solidification products of the magma - 2ContaminationThe magma absorbs part of the rock it passes through
- 3How is bulk assimilation possible?The heat in the system is all provided by the magma
The country rock must be heated past its solidus temperature so that it will begin to melt and change from a solid to a liquid
(The magma must have the same energy as the latent heat of fusion of the country rock) - 4How can the magma give off heat?1. Superheat = if the magma is above its liquidus temperature, it can lose heat by cooling without crystalliastion
2. Latent heat of fusion = if the magma is below its liquidus temperature, it can lose heat by crystallisation - 5What limits the heat that needed to melt the country rock after the superheat is used up?The heat energy provided by the crystallisation of the magma
The crystallising mass would have to be ~2x bigger than the mass being melted - 6How could movement in the magma, or sinking of stoped blocks help to melt the country rock?The xenoliths would be carried into the hotter parts of the magma body
- 7What is the main limitation on the melting of the country rocks by magma?The need for the country rocks to be raised to their melting temperature first
(NOT the inability of the magma to provide heat for melting) - 8Where is contamination melting most likely to happen?In regions where crustal rocks are already beginning to melt before the magma arrives, e.g. at depth the country rocks may be hotter
- 9StopingBlocks of rock under the roof or sides of an intrusion drop into the magma to form xenoliths which incorporate into the magma (a mechanism of contamination)
- 10How does assimilation without melting take place?1. The country rock is brought into equilibrium with the melt
2. The equiliberated xenocrysts are dispersed through the magma, thus losing their separate identity - 11Discontinuous Reaction Series(High T) Olivine >> Orthopyroxene >> Clinopyroxene >> Amphibole >> Biotite (Low T)
- 12Continuous Reaction Series(High T) ANorthite >> Bytownite >> Labradorite >> Andesine >> Oligoclase >> Albite (Low T)
- 13A xenolith crystal may be...1. a phase which the liquid could have been precipitating earlier in its cooling history
2. a phase which the liquid is currently precipitating
3. a phase which the liquid might eventually precipitate at a lower temperature - 14Only a small proportion of the material added to the magma is melt, how would a partial melt phase contaminate magma?Selectively contamination - the constituents with the fastest diffusion rates would contaminate the magma
- 15What will happen to the contaminants if there is no magma movement?The contaminants will remain in the solid phase, and will stay in their original position at the igneous contact as altered wall rock or xenoliths
- 16What is needed for contamination to be effective, and how is this achieved?The contaminants must be dispersed into the igneous body
1. magma flowage
2. disaggregation due to grain boundary melting - 17What components does a body of contaminated igneous rock contain?1. High-melting crystals which have crystallised from the magma during its reaction with the foreign material
2. Low-melting fraction of the original magma and additional assimilated low-melting material
3. Equiliberated xenocrysts dispersed in the igneous body
4. Undigested xenoliths and wall rocks - 18What is the most obvious sign of contamination?1. The presence of xenoliths
(however, their abundance does not necessarily indicate the amount of contamination that took place)
2. Rocks where assimilation is incomplete, and many xenoliths are modified but undigested, so the end-product is more likely to have the composition of a simple matrix - 19What are other sources of xenoliths, other than contamination?1. inclusions of igneous rocks related to their host, e.g. early differentiates from another part of the magma
2. melting residues from the magmatic source region - 20What is the composition of a contaminated igneous rock?1. the original magma
2. completely assimilated xenoliths
3. the melt fraction of partially melted xenoliths
4. constituents gained from the wall rocks or xenoliths during equilibration other than by melting
5. constituents lost to the wall rocks or xenoliths during equilibration - 21What contaminants will modify the magma the most?Those rich in low-melting constituents, e.g agrillaceous rocks, arkoses and greywackes
- 22Why does contamination need to have occurred on a large scale before emplacement, to have a significant impact on magma evolution?A higher proportion of the assimilated material will have lost its identity by dispersion and homogenisation in the magma
- 23Are acid magmas more or less likely to be contaminated than basic magmas?More likely
- 24DigestionInvolves ion exchange, a 2-1ay process between the host rock and the xenolith which occurs when the xenolith is in the solid state and brings the two into equilibrium
Removes any evidence of the rock fragments ever being present - 25What other uses to petrologists have for the term "assimilation"1. Melting of rock fragments
2. Dissolving of fragments in melt
3. Reactions between the melt portion of the magma, and the crystal in the rock fragments - 26Why are textures not diagnostic of magma mixing vs. rock/crystal incorporation?When individual crystals are incorporated in a magma as xenocrysts, they react with the magma to form the same range of textures as in magma mixing, e.g. fingerprint plagioclase, one mineral mantling the other, quartz ocelli, or corroded crystal shapes
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- Flashcards on Magma Assimilation, created by kean.kim.j on 03/12/2015.
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