You, too, can be a geochemist! You will never have to look at another rock or mineral again! Your only tenuous links to reality will be tables of numbers and vials of powder by post!
More seriously, geochemical discriminant diagrams can be useful for classifying glassy or fine-grained volcanics, and also for metamorphosed igneous rocks of all sorts. They can also help you start thinking about why the rocks are chemically the way they are. The diagrams below are those I have used, or tried to use, at one time or another. I had to figure out the discriminant line coordinates so that I could plot my data on them. It is a senseless waste of time for everyone else to have to do the same thing, so I present the coordinates for the benefit of all humanity. Let me know if you find this material useful.
Click on the citation for the discriminant lines shown in the adjacent pictures, click on the small image to get a bigger image. All discriminant line coordinates are given in Excel files with X and Y values, Z if necessary. Please contact me if you find any errors. I can add new discriminant diagrams on request, but please send the discriminant line coordinates and reference, or at least the reference. No guarantees as to speed of posting.
An-Ab-Or diagram for tonalite, trondhmemite, granodiorite, granite.
SiO2 vs. K2O for volcanic rocks.
|LeBas et al., 1986|
SiO2 vs. Na2O+K2O for volcanic rocks.
Q – Af – Pf diagram for classification of felsic and intermediate plutonic rocks.
|Winchester and Floyd, 1977|
Immobile trace elements and SiO2 classification for igneous rocks, especially volcanics.
Rock series discriminants
Alkali-lime index vs. SiO2, separating “normal” calc-alkaline andesites from more and less mature arc andesites.
|Floyd and Winchester, 1975|
Tholeiitic vs. alkaline series in oceanic and continental settings (not arcs).
|Frost and Frost, 2008|
Feldspathic igneous rock series (ferroan, magnesian, aluminous, alkaline, etc.).
|Hansen et al., 2002|
Discriminates between Archean TTG suite and more modern adakites on the one hand, and classical island arcs.
|Hastie et al., 2007|
Arc series: tholeiitic, calc-alkaline, and more alkaline series, and basaltic, andesitic, and rhyolitic compositions.
|Irvine and Baragar, 1971|
Tholeiitic vs. calc-alkaline series discriminants.
SiO2 vs. FeOtotal/MgO discriminant for arc tholeiites vs. arc calc-alkaline series.
|Miyashiro and Shido, 1975|
Discriminants for arc tholeiite vs. arc calc-alkaline series.
|Peccerillo and Taylor, 1976|
SiO2 vs. K2O calc-alkaline vs. tholeiitic series.
Tectonic setting discriminants, mostly basalts
|Cabanis and Lecolle, 1989|
Nb – La – Y discriminant for arc, MORB/WP, and intermediate/contaminated rocks.
|Dilek and Furnes, 2009|
Ti-Zr discriminant for MORB, IAT, and boninites, modified after earlier workers.
|Dilek et al., 2007|
Cr vs Y discriminant for MORB, IAT, and boninites, modified after earlier workers.
|Hollocher et al., 2012|
Discriminants for MORB, ocean island, and a variety of arc-type basalts.
Y – Zr – Nb discriminant for arc, MORB, and within-plate basalts.
P2O5 – MnO – TiO2 discriminant for ocean island, MORB, arc basalts, and boninites.
Ti vs. Cr discriminant for ocean floor basalts vs. low-K arc tholeiites.
Zr – Ti discriminant for MORB, within-plate, and arc lavas. Apparently includes low-Ti lavas which are probably felsic.
Discriminants for mantle melting array vs. arc basalts.
Th – Ta – Yb discriminant for mantle melting array vs. arc basalts.
|Pearce and Cann, 1973|
Immobile element discriminant for arc, within-plate, and ocean floor basalts.
|Pearce and Norry, 1979|
Zr vs. Zr/Y discriminant for arc, ocean ridge, and within-plate basalts. Boninite field added.
Ti – V discriminant diagram for tholeiitic to alkaline basalts, excluding calc-alkaline arc basalts. Boninite added.
Ta – Th – Hf discriminant for arc, MORB, and within-plate basalts.
Tectonic setting discriminants, mostly felsic rocks
Al2O3 vs Yb discriminant for oceanic vs. continental granitoids.
Y/Nb – Rb/Nb and other discriminants for A1 vs. A2-type anorogenic granites.
|Gorton and Schandl, 2000|
Th – Ta – Yb discriminants mostly for felsic to intermediate rocks.
Discriminant diagrams for granitic rocks.
|Whalen et al., 1987|
Discriminates A-type granites from all other granite types.