How do alkaline rocks get so enriched in alkalis?

[subopolois]

How do alkaline rocks get so enriched in alkalis and incompatible trace
elements ?

 

[Xnn]

Rocks melt in the interior of the earth.
Molten alkalies are limited in their ability to incorporate other materials as the crystallize.

 

[geomajor]

The key here is magma differentiation. Alkali-rich rocks are plutonic, meaning they formed within the Earth's interior (i.e. magma chambers) from a magma of a certain composition. There are three basic types of magmas: basaltic, andesitic, and rhyolitic. Most magmas start out as silicic basalts (from melting of the mantle) and over time become progressively more rhyolitic, or alkali-rich. (Though sometimes, a basaltic magma can seep into a more alkali-rich magma chamber and change the composition back.)

This change over time occurs through one or a combination of the following processes known as MAGMA DIFFERENTIATION:

1) Crystal Fractionation
2) Partial Melting (for this you need to understand ternary eutectics)
3) Assimilation
4) Mingling/Mixing

Crystal fractionation is fairly common. What happens is you start out with a basaltic magma. Then you start crystallizing rocks from it (gabbros). As the rocks crystallize, the relative amounts of available chemicals change. So gabbros are silicic, that means silica is being removed from the system (i.e. the magma). Think ratios-- the proportion of silica to alkalis is decreasing. As silica is depleted from the system because minerals rich in Fe, Mg, and SiO2 (according to Bowen's Reaction series) crystallize and form rocks, the relative percentage of alkalis is increasing. Over time, the magma is becoming more alkali-rich. When temperature and pressure conditions reach a certain point, more minerals with an alkaline composition begin to crystallize, forming your "alkali-rich rocks." Now don't get me wrong, minerals with SiO2 still crystallize, and minerals with alkalis can still crystallize BEFORE the magma is "alkali-rich"-- the lines are a little blurry, so consult temperature/pressure conditions.

This process also ties in with weird trace elements. As the other compounds are being depleted, you start getting a higher % of stuff like Ti, popular in minerals like magnetite and ilmenite if I'm not mistaken.

2) Partial Melting-- I understand the process, but I'm don't feel qualified to explain it to someone else since Petrology was a while ago. I suggest looking up lectures online, journal articles, and consulting your professor. But I think I do remember that with partial melting, you get a ton of incompatible trace elements "escaping" when the first few percentages are melted. They're incompatible because they don't "want" to be in the rock, so when it melts, the diffuse out into the liquid quickly so they don't have to be stuck bonded to stuff they don't like. Yeah, layman's terms, but it was the only way it made sense to me.

3) Assimilation-- really intuitive. The liquid magma assimilates something of a different composition, like melting a hunk of limestone shelf or continental crust. This process can also lead to a higher percentage of "strange" trace elements.

4) Mingling/mixing-- happens when 2 magmas are physically trying to get into the same area. You need mingling for mixing to happen, but not vice versa.

Hope this was a bit more helpful!