Geochemistry (2): Major and trace elements
I. Major elements
Are commonly measured as wt.% oxides.
Sometimes more convenient to see them as cationic proportions: equivalent to a
mineral formula!
Major elements contents reflect the mineralogy of the rock. Cf norms.
A. Bivariate plots and their interpretation; differenciation
• Harker diagrams: SiO2 vs. oxide.
• The meaning of geochemical trends: can be interpreted as magmatic
“evolution” from “primitive” to “differenciated” rocks. More or less implicitly
assumes fractional crystallization.
• The nature of the phases crystallizing can be inferred from the shape of the
trends. Ex.: decreasing Fe, Mg = precipitation of mafic minerals.
B. Magmatic series
Magmatic series: reflect first order differences between rock groups.
• TAS diagram separates alkali and sub-alkali series
• Sub-alkali series are further separated on the basis of their Fe-Mg contens
(AFM diagram) into tholeitic and calc-alkaline
In addition, important role of the relative proportions of Al2O3 and CaO-Na2O-K2O
• A>CNK: Peraluminous rocks. Have Al-rich minerals such as biotite,
muscovite, garnet, cordierite…
• A<CNK:
o .. and A>NK: Metaluminous. No particular minerals, mafics are
pyroxene, amphibole, biotite
o .. and A<NK: peralkaline rocks. Alklai-rich minerals such as alkali
amphiboles and pyroxenes.
1. Tholeitic series
Fe-rich, alkali poor.
Metaluminous
Px/Hb/Bt-bearing basalts, andesites, dacites, rhyolites (BADR)
Tholeitic series are common in oceanic ridges, intraplate-volcanoes ± convergent
margins. They correspond to melting by decrease of pressure.
2. Calc-alkaline series
Moderately alkaline, more magnesian
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