AFM diagram: tholeiitic and calc-alkaline magmatic series

What the AFM diagram shows

Each sample is plotted by the relative proportions of A = Na₂O + K₂O, F = total iron as FeO and M = MgO. Tholeiitic suites show early iron enrichment towards the F apex, while calc-alkaline suites evolve more directly towards the alkali corner.

The diagram is meant for subalkaline rocks, so samples are usually first checked against the alkaline–subalkaline divide of a TAS diagram.

Data you need and how Kaligeos calculates it

• A: an imported Na₂O + K₂O column if your table has one; otherwise Na₂O + K₂O.
• F: FeO, including total-iron headers such as FeOt or FeO*. If FeO is missing, Kaligeos uses Fe₂O₃ × 0.8998; the two are never added together.
• M: MgO.
• Values are used in wt% and normalized to A + F + M = 100 for plotting.

Plotting an AFM diagram in Kaligeos

• Import CSV or Excel (.xlsx, .xlsm) tables. Kaligeos recognizes common oxide and element headers regardless of case, accents or punctuation, for example FeOt, FeO*, “La ppm” or “La (ppm)”.
• Irvine & Baragar (1971) tholeiitic/calc-alkaline line, with its own colour and width.
• “Plot subalkaline samples only” classifies each sample against the Irvine & Baragar alkaline–subalkaline line in TAS space and hides alkaline ones. Samples outside the SiO₂ range of that line are left unclassified rather than extrapolated.
• Adjustable ticks and field labels that can be dragged into place.
• Group samples with reusable legend schemes, keep several diagrams linked to the same selection and save views inside a .kaligeos project.
• Export the figure as SVG or PDF for papers, theses and posters. Kaligeos Community includes 50 vector exports per account.

Frequently asked questions

Which line does Kaligeos use to separate tholeiitic and calc-alkaline rocks?

The AFM dividing line of Irvine & Baragar (1971), digitized from the original publication.

My table has both FeO and Fe₂O₃. Which one is used?

FeO (or a total-iron column such as FeOt). Fe₂O₃ is converted with the factor 0.8998 only when no FeO column is available.

References

• Irvine, T. N. & Baragar, W. R. A. (1971). A guide to the chemical classification of the common volcanic rocks. Canadian Journal of Earth Sciences 8, 523–548.

Related diagrams

• TAS diagramTotal alkali–silica classification of volcanic rocks (Le Bas et al., 1986).
• Harker diagramsMajor-oxide variation diagrams against SiO₂ in a 3 × 3 matrix.
• Pearce diagramsTh/Yb–Nb/Yb, granite discrimination and other tectonic discrimination diagrams.