Flashcards
Magma Mixing
Cards in this deck (10)
- 1What do two unlike magmas form when they mix?A hybrid daughter magma which is compositionally intermediate between them
- 2Where can we find compelling evidence for magma mixing?Net-veined agmatites, usually of calc-alkaline affinities
Probably represent the first stages of physical mixing - 3What two processes are involved in magma mixing?1. Magma Mingling
2. Magma Blending - 4Magma MinglingThe process in which 2 (or more) magmas of different compositions come together to form layers or blobs of each in the other, but retain their discrete identities so that a heterogenous hybrid is formed
- 5Magma BlendingThe process in which the two magmas progressively
1. exchange atoms between the respective melts by chemical diffusion and
2. exchange of phenocrysts
If these processes are completed, a homogenous (i.e. fully blended) hybrid magma would be formed in which there was a melt of a single composition, and all phenocrysts were in equilibrium with it - 6What factors inhibit blending?1. If the magmas involved are compositionally very different, there could be significant contrasts in temperature, density, and viscosity
2. Diffusion of ions between melts is slow, but especially slow if one magma is very viscous
3. If there are crystals present in the hybrid magma which are not in equilibrium, reactions will be triggered to rectify this by creating crystals which are in equilibrium with the hybrid melt - but this takes time, and might not be completed before the hybrid melt solidifies, i.e. the crystals may be zoned - 7What needs to happen to give blending the best chance to create a homogeneous hybrid magma?If the composition of the magmas involved are basic or picritic (so high temperature, low viscosity, and rapid movement of ions)
AND
if they have only small proportions (a few %) of crystals in suspension - so few crystals needing their solid solution composition to change by reaction - 8Why is it normal to find acid magmas overlying basic magmas?Basic magmas are denser, and it is gravitationally more stable to have the lighter acid magma overlying the basic magma, than to have a homogenous mixture of the two. Even if they were to mix, gravity would tend to separate them
- 9When can layers, lenses, pillow-shaped blocks, or more irregularly shaped bodies in a dissimilar matrix be found in a rock?When initially dissimilar magmas are physically mingled and solidify shortly afterwards
e.g. basic pillows in microgranite in the Austurhorn intrusion, Iceland which have a chilled margin and cauliflower-shaped contacts between the basic and acid rocks (evidence of mingling rather than accidental rock incorporation) - 10What evidence can be found in xenocrysts which show that the rock is not homogenous?1. basaltic rock with quarts xenocrysts
2. rhyolitic rock with Mg-rich olivine xenocrysts
3. plagioclase showing vermicular/sieve-textured/fingerprint shapes +/- and overgrowth zone
4. coroded crystals
5. quartz with a rim of pyroxene or hornblende (ocellar texture)
6. complexely zoned plagioclase with reverse zoning
outer zones of a crystal that cut across inner ones
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- Flashcards on Magma Mixing, created by kean.kim.j on 03/12/2015.
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