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Modal vs Norm Mineralogy, Major vs Trace Elements, & Indices- Igneous Petrology #7

Major elements in Igneous Rocks

Igneous rocks that come from a common source (parent magma) can have various compositions due to differentiation

Compatible elements: preferentially incorporated into the crystals from cooling melt

Incompatible elements: Stays in melt (for longer)

The first minerals to crystalize from magic melts are silica poor -→ differentiation causes residual melts (left over melts) to increase in silica content

AKA: silica behaves as an incompatible element

→ We can measure the concentration of other elements against SIO2% to show their concentration change through increased differentiation (Harker diagrams)

Ferromagnesian and calcic plagioclase minerals (CaO, FeO. MgO, TiO2) tend to decrease with increasing SiO2

Al2O3 may also decrease, but not as drastically

Alkalis (K20 and Na2O) increase with increasing silica

Major Element Indices

In addition to Harker diagrams, other chemical variation diagrams can help us reconstruct the differentiation history of magmas.

5 major element indices of differentiation:

Alkali-Lime Index

Iron-Enrichment Index

Aluminum Saturation Index

Alkalinity Index

Feldspathoid Silica-Saturation Index

Alkali-Lime Index

Based on relative abundance of CaO, Na2O, and K2O

If CaO is more abundant, the first feldspar to crystallize will be anorthite (Ca-Spar)

If alkalis are more abundant, the first feldspars will be albite (Na-spar) and orthoclase (K-spar)

Divided into 4 categories of relative Ca and K/Na content:

-Alkalic (<51% SiO2 where CaO= Na2O+K2O)

-Alkalic-calcic (51-56% SiO2)

-Calc-alkalic (56-61% SiO2)

-Calcic (>61% SiO2)

Iron-Enrichment Index

Ferroan trend: rocks undergo Fe enrichment before alkali enrichment

Magnesian trend: rocks undergo minimal Fe enrichment

Reduced melts typically follow a ferroan crystallization trend

Oxidezied melts typically follow magnesian trend

Aluminum Saturation Index

The major hosts of Al in rocks are feldspars

The ASI indicates whether there is an abundance of alkalis (Na and K) or Al

Most mafic rocks have neith excess alkalis or Al, and are therefore, metaluminous

Granitic rocks can be metaluminous, peraluminous (excess Al over alkali), or peralkaline (excess alkalis over Al)

Alkalinity Index (Al)

AL- [Al- alkalis (Na+K)]

Metaluminous: Positve Al

Peralkine: Negative Al

Feldspathoid-Silica-Saturation Index (FSSI)

-Qz excess over leucite and nephline

Positive for quarts saturated rocks

Negative for silica undersaturated rocks

Use of Trace Elements

Trace elements can be extremely useful for identifying the conditions and processes that led to the formation of igneous rocks

Why?→ Trace elements (<0.1%) show such drastic concentration changes in rocks depending on conditions (compared to major elements)

Common trace elements:

Transition metals: Sc, Ti, V, Cr, Mn, Co , Ni, Cu, Zn

Rare Earth Elements (REES): La, Ce, Nd, Sm , Eu….

Others: Cs Rb, Ba, Sr, Y, Zr, Hf, Nb

Trace elements are partitioned between the minerals and melt: Partition coefficient (D)= Ci min/Cimelt (where c is the concentration of the trace element, I, in other mineral (min), over the melt)

Compatible elements concentration in the Mineral (high D, or partition coefficient value)

Incompatible elements concentrate in the melt (Low D, or partition coefficient value)

Phase Diagrams: Minerals Melt at Different Temperatures- Igneous Petrology #5 | GEO GIRL

Magma Viscosity, Volatile Content, Partial Melting, & Mixing- Igneous Petrology #6 | GEO GIRL

Magma Differentiation

Partial melting

-Equilibrium melting; Solid complete melts without melted fraction leaving the system (final liquid= same composition as original solid)

-Fractional melting: partial melt is removed incrementally (final liquid= differentiated)

All partial melts are more felsic that their source rock

Crystallization:

Equilibrium crystallization: crystals don’t leave the system (final sold= same composition as original melt)

Fractional crystallization: crystals are removed (by gravitational settling) causing changes to the remaining liquid’s composition (final solid= differentiated)

Order?→ Bowen!

Assimilation: host rock torn off and incorporated into rising magma

Magma mixing: Magma chambers my mix if they come in contact at depth, and the resulting magma will rise to the surface (or cool near surface)

Magma differentiation can also occur if grains crystallize and settle to the bottom of the chamber, as we saw from fractional crystallization.

Magma Chamber Processes: Fractional Crystallization

Groups: same number of electrons in their outer shell

Periods: same number of shells

Large Ion Lithophile Elements (LILE)

Small charge/ionic radius

High Field Strength Elements (HFSE)

Elements will a small ioinc radius (Z/r) and high charge (and high associated electric field)

Includes all trivalent and tetravelent ions

Goldscmits classification

Igneous Rock Textures & Classification Based On Grain Size & Shape- Igneous Petrology #2 | GEO GIRL

How/why does crystallization begin?

Reactions (such as crystallization must have a G<0 to occur spontaneously

What is G? → Gibbs free energy (calculated by Enthalpy (H) and Entropy (S)

What is enthalpy? → The heat stored in the bonds of a substance

Because bond strengths vary, heat can either be released or consumed due to a chemical reaction

When change in enthalpy (Delta H) is negative (heat was released), the reaction is exothermic and can occur spontaneously

But there is another component to this equation!

What us S? → Entropy! → what is entropy? → measure of disorder/randomness

Example: elements in gases have higher entropies

If S increases during a reaction, the reaction will have an easier time proceeding

However, if it decreases the reaction may not proceed.

Example: If deltaS is so low that G becomes positive, even if delta H is negative, the reaction will not occur spontaneously

Equation:

So, if H decrease (negative) and S increases (+), G will be <0 (negative, and proceed spontaneously

So is crystallization a spontaneous process?

It involves building bonds (releasing hear, AKA: Negative H)= exothermic and spontaneous.

But what about S? Entropy decreases during crystallization because the atoms are becoming more orederd, but this is typically outweighed by the H decrease

However, even when crystallization reaction has negative G value, there may be an energy barrier to overcome

Igneous Petrology Series: Lesson 2 - Element compatibility & partition coefficients

Bulk distribution Coeffiction

Element compatibility:

In geological systems, element compatibility categories elements based on their behavior during processes such as cstyallisation and melting. It essentially represents how readily a trace elemt sil substitute for a major element in a mineral

A trace element can sub for a major element when its ionic radius and valence state are similar to that of a major elemt. For ex. Eu2+ is similar to Ca2+ and so can replace that ca in plagioclase felspar

An incompatible element generally refers to an element with D value below 1. During melting, an incompatbile leemt will be the first to partition into melt phase, and last to partition into sold during crystallization

A compatible eleemt refers to cement with D values over 1. Higher amounts o fmeltin will be require to libearte a compatible ement fro the solid phase into the melt, and during crystallization nis is one of the fisr elements to become a solid

This behavior generally means that crust is relatively enriched in incompatible elemtns realtve to the mantle and vice versa for compatible elements

Igneous Petrology Series: Lesson 3 - Gibbs Phase Rule

Identifies the degrees of freedom in a system using the number of components and the number of phases

In more simple words, it tells use how many intensive variables needed to define a system

Number of components (denoted as C) = number of components existing in the system

Number of phaseses (denoted as P)= number of phases existing in the system

Parameters:

Intensive

Variables that don’t scale with a system

AKA parameters that are not proportional to the system

Good examples are temperature and pressure. Just because a system is bigger to smaller, doesn’t mean the temperature has to be!

Extensive:

Variables that don’t scale with a system

AKA parameters that are proportional to the system

Good examples are volume and mass. The bigger the system is, the bigger these values are

Geochemical Data Series: Lesson 1 - Major, minor, and trace elements

Common Diagrams: AFM

Toleiitc rocks are sub-alkaline rocks that crystallize from reduced magmas. In this trend, Fe generally initially increases due to the precipitation of Mg-rich minerals

Tholeiitic basalt → ferrobasalt → tholeiitic andesite → dacite

Calc-alkaline rocks are subalkaline rocks that crystalized from oxidized magmas. In this trend, Fe gernally initially decreases due to the precipitation of Fe-rich minerals

Basalt → andesite → dacite → rhyolite

Harker and Fenner Diagrams

Geochemical Data Series: Lesson 2 - Rare earth elements

Rare Earth Elements → A group of (typically) trivalent metals that have similar physical and chemical properties.

Lanthanides → A group of 15 REEs that have atomic numbers from… each have a 5d valence electron

LREE/MREE/HREE→ Light, middle, and heavy rare earth elements

Lanthanide contraction → phenomenon where the ionic radii of the lanthanides decreases with increasing atomic number

Odo-Harkin’s rule → An element with an even atomic number is more abundant that adjacent elements with odd atomic numbers

In geology, we use REE’S to evaluate processes like mantle melting, fractional crystallization, and crustal contamination

All are generally trivalent and considered incomaptbile… but compatibility is a spectrum, and slight difference in partition coefficient can lead tp patterns we interpret in spider diagrams

Why do we normalize?

Bc of the Oddo-Harkin’s effect, you see zig zag pattern.

To counteract, we normalize REE’s (i.e divide our sample REE concentration with REE concentrations of a well-constrianed reservoir).

The most common normalization values include: chrondrites, primitive mantle, upper lower crust, N-MORB.

In reduced environments Eu3+ can become Eu 2+, which is similar in size and charge to Ca2+, and this becomes compatible in Plagioclase feldspar.

Negative anomalies suggest Eu2+ has been removed from our samples either by plagioclase remaining in the source (residuum) or that some plag has crystallized.

Positive anomalies suggest our rock has some accumulation of primary plag