Lecture Thirteen - Igneous and metamorphic geology II Flashcards
In which settings can the same kind of magma form?
Basalts can form:
In Island arcs (IAB) - subduction of oceanic crust beneath other oceanic crust).
Mid ocean ridges (MORB) – spreading centres.
Above mantle plumes (OIB).
The trace element characteristics of each kind of basaltic magma are distinctive.
Describe partial melting of the mantle.
The solid (upper) mantle consists of Lherzolite (type of peridotite) – rich in olivine and pyroxene - ULTRAMAFIC.
We know this from mantle xenoliths (solid chunks of the mantle brought to the surface in volcanic eruptions).
When the mantle melts, it produces melts with a BASALTIC composition - mafic magma.
This is because silica (SiO2) in mantle rocks prefers to be in melts (incompatible) rather than stay in the solid non‐ melted residual mantle.
If erupted, this magma is a basalt (fine‐grained, extrusive) or a gabbro (coarse‐grained, intrusive) if cooled in the crust.
Different types of magmas are distinguished based on their chemistry (SiO2 vs. other major oxides) and/or mineralogy.
Draw out the diagram of rocks formed in different settings (TAS diagram of volcanic rocks).
Describe partial melting of the crust.
Melting of the crust can be caused by heat and/or magma input from the mantle OR no effect from the mantle (e.g., crustal thickening).
Most crustal melting occurs in the mid‐lower continental crust. Why is it more difficult to melt the oceanic crust? Becasue of its chemical composition, the minerals in these rocks and the rocks them selves have higher boiling temperatures.
The same methods of melting the mantle apply to melting of the Earth’s crust.
However, the starting composition is now different and has different proportions of minerals/elements. The crust is either:
1) Oceanic crust (mafic), or
2) Continental crust (intermediate‐felsic)
This means in general, melts produced will have a higher SiO2 content than their starting crustal compositions.
Thus, melting of the oceanic crust can yield melts with intermediate compositions (andesite if extrusive; diorite if intrusive).
Melting of the continental crust melts with felsic compositions (dacite – if extrusive, granodiorite – if intrusive).
Partial melting of the crust produces Migmatites (part igneous, part metamorphosed).
How does magmatic evolution work with regards to density?
1) Density.
Magmas/melts are nearly always less dense than what is melting to procude them.
This creates a natural buoyancy or an upwards momentum to most magmas -> they will rise though the mantle and/or crust.
Mantle‐derived basalts will tend to rise “quickly” (hot, low density) through the mantle.
The weight of overlying rock – “lithostatic pressure” can also facilitate upward movement (like stepping on wet sand barefoot and the water squeezes up between your toes).
How does magmatic evolution work with regards to temperature?
When melts are produced, they are above their liquidus (100% liquid).
When they start to cool, crystals form.
These crystals correspond to Bowen’s reaction series (although this is very idealised).
Crystal fractionation:
In cooling magmas there is a large gap between the liquidus (when the first crystals/minerals form) and the solidus (when a magma becomes 100% solid rock).
E.g., liquidus for basaltic magmas >1300°C; solidus ~1100°C.
Over this range, different minerals form and at different proportions.
It is important to realise that this occurs gradually.
In nearly all cases, the minerals that are forming are a different composition to the magma they are crystallising from.
1) E.g., a magma contains a 50:50 ratio of each component
2) Olivine crystals start forming (75% yellow MgO; 25% red SiO2)
–> What happens to the magma composition?
3) With increasing olivine crystallisation, the remaining magma get progressively richer in SiO2, and progressively poorer in MgO.
What happens to the crystals as they form out of magma?
Crystal setting or rising:
This helps to ‘speed’ up magmatic evolution by further separating (fractionating) minerals formed from the magma.
This is because the minerals forming have a different density to the magma they are crystallising from.
Crystals forming have different densities.
Apart from density, factors suchs as magma viscosity, time and the shape of the magma resivoir also affect how quickly and effectively the crystal settling occurs.
What is viscosity?
Measure of the ‘gooeyness’ or how runny a fluid is.
Scientifically, it is the resistance of a fluid to flow (i.e. how much energy is required to make the magma start flowing).
What are igneous rocks classified based on?
Their chemistry (usually SiO2 content, also SiO2 vs other oxides).
Their mineralogy (minerals present and their proportions).
Their textures:
Course grained = intrusive (slow cooled, plutonic).
vs.
Fine grained = extrusive (fast cooled, volcanic).
What are the different names for rocks of the same composition, cooled at different rates?
How can you get a different magma composition at volcano (basaltic or rhyolitic) or in an intrusion (gabbro or granite)?
1) Different source regions.
* Mantle melting tends to produce basaltic melts * Oceanic crust melting produces basaltic to andesite melts • Continental crust melting produces andesite to rhyolite melts (complex due to heterogeneity in the continental crust)
2) Crystal fractionation.
3) Magma mixing.
Mixing between two or more magma batches derived from different sources.
4) Crustal assimilation.
Reaction between hot magma and country rock changes chemistry of magma (assimilation).
