3- Mixing, assimilation, etc.
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Transcript of 3- Mixing, assimilation, etc.
3- Mixing, assimilation, etc.
Evolution of magmas 3- Mixing, assimilation, etc. Why FC cannot
explain everything?
Need large amounts of cumulate (where??) Difficult in a viscous
magma No good evidence for omnipresent magma chamber Minor(?) magma
differenciation processes
Liquid unmixing Magma mixing Assimilation Liquid Immiscibility
Liquid immiscibility in the Fo-SiO2 system Liquid
1557 Mg2SiO4 20 40 60 80 SiO2 1400 1600 1800 2000 1713 T oC MgSiO2
1543 1470 Enstatite Forsterite + Enstatite Liquid + 1695 Enstatite
+ Tridymite Enstatite + Cristobalite Cristobalite + Liquid Two
Liquids Liquid Several early geologists appealed to liquid
immiscibility as a mechanismfor magmatic differentiation Might be
responsible for the separation of a granitic liquid from anevolving
system (presumably from an initial basaltic parent) Liquid
immiscibility in the Fo-SiO2 system Very limited compositional
range How behave as add more components? Figure Two immiscibility
gaps in the system fayalite-leucite-silica (after Roedder, 1979).
Yoder (ed.), The Evolution of the Igneous Rocks. Princeton
University Press. pp Projected into the simplified system are the
compositions of natural immiscible silicate pair droplets from
interstitial Fe-rich tholeiitic glasses (Philpotts, 1982). Contrib.
Mineral. Petrol., 80, Liquid immiscibility is now widely acceptedas
a phenomenon in natural magmas But the extent of the process, and
itsimportance in generating large bodies or asignificant proportion
of evolved magmaticrocks is still dubious Some Examples Late
silica-rich immiscible droplets in Fe-rich tholeiitic basalts
Sulfide-silicate immiscibility (massive sulfide deposits)
Carbonatite-nephelinite systems Iron-silicates (primitive Earth)
Granites & rhyolites? Bimodal basalt-rhyolite?Unlikely, since
dont correspond to any known experimental gaps Mingling and mixing
Closepet granite (2.5 Ga), South India Closepet granite (2.5 Ga),
South India Closepet granite (2.5 Ga), South India Closepet granite
(2.5 Ga), South India
Low strain High strain Closepet granite (2.5 Ga), South India
Hoedjiespunt granite (540 Ma), Saldanha, RSA Hoedjiespunt granite
(540 Ma), Saldanha, RSA Hoedjiespunt granite (540 Ma), Saldanha,
RSA Closepet granite (2.5 Ga), South India Vredenburg granite (540
Ma), Britannia Bay, RSA Vredenburg granite (540 Ma), Britannia Bay,
RSA Vredenburg granite (540 Ma), Britannia Bay, RSA Vredenburg
granite (540 Ma), Britannia Bay, RSA ~ 1 mm Closepet granite (2.5
Ga), South India Closepet granite (2.5 Ga), South India Closepet
granite (2.5 Ga), South India Closepet granite (2.5 Ga), South
India Assimilation Figure Gradational border zones between
homogeneous igneous rock (light) and country rock (dark). After
Compton (1962), Manual of Field Geology. R. Compton. Possible
reactions Mg2SiO4 + SiO2 = 2MgSiO3
Olivine + Quartz =Orthopyroxene CaMg2Si2O6 + Al2SiO5 = CaAl2Si2O8 +
MgSiO3 Diopside + Sillimanite = Plagioclase + Orthopyroxene
Problems with assimilation
Source of heat? > AFC processes Direct evidences not commonly
found; lot of case interpreted as assimilation could be magma
mixing Generaly invoked to explain geochemical (isotopic)
characteristics with no field evidence