Pearce diagrams: Th/Yb–Nb/Yb and tectonic discrimination

Nb/Yb vs Th/Yb

In the Th/Yb–Nb/Yb projection introduced by Pearce (2008), basalts that formed without a subduction input fall along a diagonal MORB–OIB array, from depleted N-MORB to enriched OIB. Subduction adds Th but little Nb, so arc-related rocks plot above the array; crustal contamination can produce a similar displacement.

In Kaligeos both axes are logarithmic (Nb/Yb 0.1–100 and Th/Yb 0.01–10). The template draws a MORB–OIB array band, a volcanic-arc array line and N-MORB, E-MORB and OIB reference points; each element can be shown or hidden and its labels edited. Nb, Th and Yb are read in ppm and must be positive.

Other Pearce discrimination diagrams in Kaligeos

• Pearce, Harris & Tindle (1984) granite discrimination: four log–log panels, Rb vs Y + Nb, Nb vs Y, Rb vs Yb + Ta and Ta vs Yb, with VAG, syn-COLG, WPG and ORG fields.
• Pearce & Norry (1979): Zr vs Zr/Y on log–log axes.
• Pearce & Cann (1973): Ti/100–Zr–3Y and Ti/100–Zr–Sr/2 ternary diagrams. Ti is read from a Ti column in ppm.
• Nb/Y vs Zr/Ti after Pearce (1996), with fields after Winchester & Floyd (1977).

Plotting Pearce diagrams 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)”.
• Values below detection such as “<0.5” are imported as half the limit by default; you can choose L/√2, a custom fraction or leave them empty.
• Template labels can be moved and edited, and sample groups keep the same symbols in every diagram.
• 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

Are the tectonic fields definitive?

No discrimination diagram proves a tectonic setting. Use several diagrams, check element mobility and alteration, and interpret the results together with geological context.

Can I add a discrimination diagram that Kaligeos does not include?

Yes. The Custom XY Builder lets you recreate a published Cartesian or ternary diagram with its fields and labels and reuse it as a template.

References

• Pearce, J. A. (2008). Geochemical fingerprinting of oceanic basalts with applications to ophiolite classification and the search for Archean oceanic crust. Lithos 100, 14–48.
• Pearce, J. A., Harris, N. B. W. & Tindle, A. G. (1984). Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. Journal of Petrology 25, 956–983.
• Pearce, J. A. & Norry, M. J. (1979). Petrogenetic implications of Ti, Zr, Y, and Nb variations in volcanic rocks. Contributions to Mineralogy and Petrology 69, 33–47.
• Pearce, J. A. & Cann, J. R. (1973). Tectonic setting of basic volcanic rocks determined using trace element analyses. Earth and Planetary Science Letters 19, 290–300.

Related diagrams

• Spider diagramPrimitive-mantle-normalized multi-element patterns with a configurable element order.
• REE diagramChondrite-normalized rare earth element patterns (Sun & McDonough, 1989).
• Custom XY BuilderRecreate a published Cartesian or ternary diagram and reuse it as a template.