L4 - Major Element Geochemistry

How are Major Element compositions usually reported?

1. Percent by weight

2. Atomic Proportion

3. Percent by volume

Define a Major Element

Has an abundance >1.0 wt. %

Define a Minor Element

Has an abundance of 0.1-1.0 wt. %

Define a Trace Element

Has an abundance of < 0.1 wt. %

What are 3 uses of Major Elements in Geochemistry?

1. Used to classify rocks

2. Used to study the chemical control of magmas by the separation of crystals

3. Control the mineralogy, crystallization, and melting behavior in igneous systems

How are Minor Elements incorporated into the crystal lattice?

Commonly substitute for an element in a principal mineral

• Example: Mn2+ for Fe2+ and Mg2+ in olivine and pyroxene

Where are you likely to find minor elements within a rock?

Present in small quantities of accessory minerals.

• Zr in Zircon

• P in Apatite

• Ti in titanite, rutile, or ilmenite

• Sr in Plag

How are Trace Elements incorporated into a rock? Why?

Trace Elements substitute for major and minor elements, because they are too dilute to form a separate phase

What is another name for trace elements and why?

Often called Passive Tracers

Their concentration, ratios, and distributions are extremely powerful tools in Earth Science

How are Whole Rock major element concentrations (wt%) typically analyzed?

Analyzed using X-Ray Fluorescence (XRF)

How are mineral major element concentrations typically analyzed?

Analyzed by electron microprobe (EMPA)

What minerals are technically minor in most igneous rocks?

TiO2

MnO

Na2O

K2O

P2O5

What oxide starts a typical major element table? Why?

SiO2 - because it is the most abundant (standard format)

How were most major analyses done until the 1960s?

Major element analyses were determined by wet chemical methods, where ppts (mostly oxides) were formed and then weighed.

Why are Major Elements presented on a table listed as oxides?

Most Cations in rock-forming minerals are bonded to oxygen, so reporting analyses as “oxides“ is reasonable.

Why is it inconvenient to have major elements reported as wt.%?

• Differences in major element abundances can be obscured by atomic masses (inclusion of different atoms)

• Some workers convert wt% to atom % for a more direct comparison

What is 1wt% equivalent to in ug/g?

1wt% = 10,000 ug/g (ppm)

What are the two different names for iron, and what do they represent?

1. Ferric = Fe3+ - Fe2O3

2. Ferrous = Fe2+ - FeO

Represents the two valence states of the major element Iron

What does a higher Ferric/Ferrous ratio represent?

High Fe3+/Fe2+ = Oxidized conditions

What does a lower Ferric/Ferrous ratio represent?

Low Fe3+/Fe2+ = Reduced conditions

One the Major element table, what does H2O+ represent?

• Represents the water in the lattice of silicate minerals

• released above 110 degrees Celsius

One the Major element table, what does H2O- represent?

Water present as dampness in a rock powder and driven off by heating (not important constituent of a rock)

What is Loss-on-Ignition (LOI) mean?

The total volatile content of a rock is determined by ignition at 800-1000 degrees Celsius

• Example: H2O+, CO2, F, Cl, S; can also be, K, Na (if heated for too long)

What is the process of anhydrous renormalization?

1. Subtract LOI (or H2O) from total

2. divide each oxide by the new anhydrous total

3. multiply by 100

How much H2O% is considered serpentine?

12-14 wt.%

Why is it important to do anhydrous renormalization?

For proper comparison of all major elements

• Plot on a TAS Diagram

Image shows plot before normalization

Where are the following located on the TAS Diagram?

1. Parental Magmas

2. Evolved Magmas

3. The trend during increasing crystal fractionation

Where are the following located on the TAS Diagram?

1. Alkaline Rocks

2. Tholeiitic Rocks

What are the trending directions for:

• Decreasing% melt(F)

• Increasing % crystal fractionation

What is a Bivariate Plot?

A standard way to examine the major element compositional variability of a dataset

• similar to Harker Diagrams, but no SiO2 on the x-axis

What is commonly be plotted on the x-axis in mafic rocks?

MgO (wt%)

What effect do analytical methods involving driving off volatiles at 800-1000 degrees Celsius have on iron?

All the Fe2+ is oxidized in the presence of air to Fe3+; therefore, the results will be reported as Fe2O3(T)

• T = total

• FeO from sample can then be determined using a titration method/calculation.

Define Loss-On-Ignition (LOI). What temperature needs to be reached?

LOI - The volatile content of a rock is determined by ignition at 800-1000 degrees Celsius that is expressed on a table.

What are some examples of volatile compounds?

• -H20

• CO2

• F

• Cl

• S

• and can also be Na and K, if heated for too long

What are the added compounds for LOI?

• O2

Allows for oxidation. Example: FeO → Fe2O3

What oxide should be chosen to be placed on the X-axis?

The oxide used to show the variation of a dataset, and therefore usually the oxide with the greatest absolute range.

What part of the rock represents the major element composition?

The glass or a fine-grained groundmass in volcanic rocks

Define a Co-genetic lava suite.

a suite of lavas that have a common primary melt

What is the best way to test a specific hypothesis in co-genetic lava suites?

Bivariate plots

• Can determine melting, fractional crystallization, contamination, and mixing

Describe Fractional Crystallization

• The crystallization and physical removal of different minerals occur as magma cools.

What is the basic principle of mass balance?

“The whole is the sum of its parts“

What are some examples of mass balance?

• The major element composition of a sediment can be calculated from the known composition and abundance of clay and quartz fragments

• The major element composition of basalt can be calculated from the known starting composition and the composition of the minerals that ppts.

What does a wide range of MgO contents in a table suggest?

Suggests the involvement of one (or more) Mg-rich mineral phase(s) in the chemical variability of these lavas.

What does a very low volatile abundance and total close to 100% suggest?

Indicates that the samples are very fresh and do not have to be renormalized

What type of information can linear relationships of different oxides provide?

Provides important information about how different magmas evolved.

What are control lines, and what do they represent?

Control lines - linear arrays present in a bivariate plot that represent the accumulation of a single mineral phenocryst composition that controls the chemistry of the samples.

• can also be considered as “mixing lines“ between the mineral and the magma (in equilibrium with each other)

What explains the difference in Picrite (very Ol-rich rock) and Tholeiitic Basalt (Ol-Poor rock)?

Mixing calculations through Olivine addition can be used to explain the observed variations in the rocks.

• Olivine accumulation occurs through the crystallization and sinking (high density) of olivine at the bottom of the Kilauea Iki Lava Lake. This causes a Ol-rich sample at the bottom and an Ol-poor sample near the top of the lake.

Note: Phenocryst compositions remain unchanged.

What is the “Constant Sum Problem“?

Minor element oxide abundances will strongly depend on major element oxide abundance variations, creating spurious correlations (since the sum will always be ~100%)