3- Mixing, assimilation, etc.

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Presentation transcript:

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 mechanism for magmatic differentiation Might be responsible for the separation of a granitic liquid from an evolving 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 11-7. Two immiscibility gaps in the system fayalite-leucite-silica (after Roedder, 1979). Yoder (ed.), The Evolution of the Igneous Rocks. Princeton University Press. pp. 15-58. 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, 201-218. Liquid immiscibility is now widely accepted as a phenomenon in natural magmas But the extent of the process, and its importance in generating large bodies or a significant proportion of evolved magmatic rocks 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 don’t 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 4-27. 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…