Ig Met Pet Lecture Five: Crystal Fractionation and Major Elements

Fractionation

Separation process in which a certain quantity of a mixture is divided into smaller quantities

Crystallization

Formation of a solid from a liquid by rearrangement of atoms, usually in response to decreasing temperature or pressure

Fractional Crystallization

Removal of crystals changing remaining melt composition

What determines which minerals crystallize and in what order during fractional crystallization?

Minerals crystallize from cooling magma in a systematic fashion based on their melting points, following bowen’s reaction series

What are the ways in which crystals are separated from melt in fractional crystallization (needs to be mechanical)?

Gravitational separation, compaction/filter pressing, flow segregation

Gravitational separation

Crystal settling or floating, based on their density in comparison to magma

Why is gravitational separation not always the most effective?

Because viscosity of magma makes it difficult for crystals to move

Compaction/filter pressing

Crystals begin to press on each other due to gravity causing them to compact and push liquid to the top, sometimes driven by gas

In a mostly crystallized magma, which mechanical form of crystal separation is thought to create eruptive magma?

compaction/filter pressing

Flow segregation

Crystals move more slowly than magma around them causing relatively minor separation

How does heat play a part in fractional crystallization?

It is thought that convection plays a part in crystal separation, as magma chambers are hypothesized to be crystal mush

What factors change as a result of fractional crystallization?

Minerals, rock name, chemical composition

What elements do igneous minerals and rocks start from?

The major elements our planet is made from

What are the three categories of elements classified in the chemical composition of igneous rocks by abundance?

Major, minor, and trace

How much of an igneous rock/mineral is made up by a major element?

> 1 wt.%

Common major elements

SiO2, Al2O3, FeO, MgO, CaO, Na2O, K2O, H2O

What percentage of an igneous rock/mineral is made up by a minor element?

0.1-1 wt.%

Common Minor Elements

TiO2, MnO, P2O5, CO2, CrO2, S

What percentage of igneous rocks/minerals is made up by trace elements?

< 0.1 wt.%

Common trace elements

Rare Earth Elements

What are major elements used for and why?

Because of their high abundances, they control mineralogy and crystallization/melt behavior, allowing us to classify rocks and study chemical control of magmas by crystal separation

How are minor elements commonly incorporated into igneous rocks/minerals?

They often substitute for an element in a principal mineral, or are present in small quantities as accessory minerals

How are trace elements present in igneous rocks/minerals?

They are too dilute to form a separate phase, so they substitute in for major and minor elements.

Why are trace elements important to petrogenesis?

Their concentrations, ratios, and distributions are extremely diagnostic

Why are major elements reported as oxides in analysis?

Because of history with wet chemical analysis

Between minerals and melt, what are the two phases that elements partition between?

Compatibility and incompatibility

Compatible elements

Prefer the solid phase in a solid-liquid system

Incompatible elements

Prefer the liquid phase in a solid-liquid system

Bivariate plots

A standard way to examine the major element compositional variability of a dataset, utilizing a chosen oxide to show petrologic variation of the dataset

Harker diagrams

Bivariate plots that use SiO2 on the x-axis

Fenner diagrams

Bivariate plots that use MgO on the x-axis

Assumption of cosanguineouity

Suite of lavas plotted together in a bivariate plot have a common ancestor

Major elements and mass balance

Major element composition of a given rock is a function of the modal abundances of phases that make up the rock

The lever rule

Mathematical representation of mass balance