CHEMISTRY OF IGNEOUS ROCKS

The chemical composition of rocks is determined by analyzing a powder of the rock. This aspect of geology is carried out in ERSC 3P31 - Geochemistry.

Routine geochemical analysis of geologic materials can be carried out using either or a combination of the following two techiques:

1. X-ray Fluoresence Spectroscopy (XRF) to determine both major and trace elements
2. Atomic Absorbtion Spectrometry (AAS) to determine both major and trace elements

The composition of an igneous rock is dependant on:

1. Composition of the source material
2. Depth of melting
3. Tectonic environment where crystallization occurs.
e.g. rifting vs. subduction
4. Secondary alteration

Generally three groups of elements are analysed in Igneous rocks.

Elements Analyzed in Petrogenetic Studies

1. Major Elements

These are the 13 major oxide components which are reported as weight percent (wt%).

Major Elements as Oxides

OXIDE	Range in Normal Igneous Rocks
SiO2	35 - 80 wt%
Al2O3	8 - 22 wt%
TiO2, Fe2O3 (ferric), FeO (ferrous), MnO, MgO,CaO	4 - 30 + Wt%
Na2O	1.5 - 8 + wt%
K2O	0.5 - 8 + wt%
H2O+,-	Varies
P2O5	< 0.15 wt%
CO2	Varies

Because these are reported as a percentage the total should sum to 100 %, ideally, however acceptable totals lie in the range 98.5 to 101 wt%.

Appendix 1 in the text gives the average chemical composition for a variety of Igneous rocks.

2. Minor or Trace Elements

Values for these elements fall in the ppm range and are rarely reported in terms of wt %.

Elements include:

Li, Be, Sc, V, Cr, Co, Ni, Cu, Zn, Ga

Rb, Sr, Y, Zr, Nb

Ba, Pb

plus F, Cl, S.

Theoretically any element can be analyzed for, but you must be aware of the overall composition of the rock, e.g.:

Basalt - analyze for Cr, Ni, Cu, but not Li, Be, Ba as these are not important or neccassary to analyze for as they are not present in detectable amounts.

Granite pegmatite with lepidolite mica, analyze for Li, Be, and Ba but not Cr, Ni, Cu.

3. Rare Earth Elements
(REE or lathanides atomic number 57 to 71), are reported in ppm or mg/g. The REE are important for petrogenetic studies, because as a group the REE behave coherently.