Ig Met Pet Lecture Ten: Other Magmatic Processes

Cumulate Texture

Accumulation of minerals through sinking or floating, often in layers

Zoned Magma Chamber

Mafic minerals sink, while felsic ones float, creating distinct layers within the magma chamber

Pegmatite

Extremely coarse grains of hydrous minerals formed from the accumulation of fluids left over from extensive crystallization

Magma mixing and mingling

Two or more magmas meet and comingle

Magma mixing

When two magmas mix to make a new, homogenous composition somewhere between the two original magma compositions

Magma mingling

Two magma types meet, but not do not mix, resulting in enclaves or inclusions

Why don’t magmas always mix when they meet?

There may not have been enough time between meeting and solidification for homogenization to occur, or the magmas have characteristics that made them immiscible

What does the level of diffusion tell you about time in contact before cooling

The more defuse a boundary between enclave and host roc, the more time in contact before cooling

Magma recharge and rejuvenation

New injection of magma, which is a good way to build a pluton or initiate a volcanic eruption

Magma recharge

Injection where there is a large chemical contrast between magmas

Magma rejuvenation

Injection where there is little chemical contrast between magmas

Crustal assimilation

Incorporation of surrounding crust into the magma

Why does crustal assimilation occur?

As magma cools, it shares its heat with the surrounding rocks, causing them to crack or melt and allowing for mixing/mingling

Why do peridotite xenoliths have such sharp contacts with surrounding magma?

Magma had to ascend and cool extremely quickly to have carried the xenoliths to the surface, as they are much denser and would want to sink

First boiling

When magma ascends to shallower depths, solubility decreases and volitiles exsolve out

Second boiling

Volatile components are concentrated in the melt as early minerals are typically anhydrous, resulting in concentrations exceeding solubility limits

Magma processing zones and their acronyms

Assimilation and fractional crystallization (AFC) and melting-assimilation-storage and homogenization (MASH)

What is the difference between AFC and MASH

AFC is a monogenic system that deals with one event and one magma, while MASH is polygenic and deals with multiple events from multiple injections

What are they hypothesis for how there is space for so much magma (and eventually plutons) in the crust?

Forcible displacement of wall and roof rocks, stoping, extension, incorporation of crustal materials into magma, erosion and expulsion of volcanic materials generated in the crust

Which hypothesis for how there is space for magma in the crust correct?

It is likely a combination of all of them

Normal zoning

High temperature composition to low temperature composition, caused by cooling

Reverse zoning

Low to high temperature composition from heating