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.


Rock classification

Barker, 1979

An-Ab-Or diagram for tonalite, trondhmemite, granodiorite, granite.

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Ewart, 1982

SiO2 vs. K2O for volcanic rocks.

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LeBas et al., 1986

SiO2 vs. Na2O+K2O for volcanic rocks.

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Streckeisen, 1967

Q - Af - Pf diagram for classification of felsic and intermediate plutonic rocks.

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Winchester and Floyd, 1977

Immobile trace elements and SiO2 classification for igneous rocks, especially volcanics.

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Rock series discriminants

Brown, 1982

Alkali-lime index vs. SiO2, defining calc-alkaline andesites from "normal" arc suites compared to more and less mature arc suites.

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Floyd and Winchester, 1975

Tholeiitic vs. alkaline series in oceanic and continental settings (not arcs).

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Frost and Frost, 2008

Feldspathic igneous rock series (ferroan, magnesian, aluminous, alkaline, etc.).

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Hansen et al., 2002

Discriminates bewteen Archean TTG suite and more modern adakites on the one hand, and classical island arcs.

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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.

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Miyashiro, 1974

SiO2 vs. FeOtotal/MgO discriminant for arc tholeiites vs. arc calc-alkaline series.

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Miyashiro and Shido, 1975

Discriminants for arc tholeiite vs. arc calc-alkaline series.

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Peccerillo and Taylor, 1976

SiO2 vs. K2O calc-alkaline vs. tholeiitic series

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Tectonic setting discriminants, mostly basaltic rocks

Cabanis and Lecolle, 1989

Nb - La - Y discriminant for arc, MORB/WP, and intermediate/contaminated rocks.

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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

       

Meschede, 1986

Y - Zr - Nb discriminant for arc, MORB, and within-plate basalts.

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Mullen, 1983

P2O5 - MnO - TiO2 discriminant for ocean island, MORB, arc basalts, and boninites.

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Pearce, 1975

Ti vs. Cr discriminant for ocean floor basalts vs. low-K arc tholeiites.

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Pearce, 1981

Zr - Ti discriminant for MORB, within-plate, and arc lavas. Apparently includes low-Ti lavas which are probably felsic.

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Pearce, 1982

Discriminants for mantle melting array vs. arc basalts.

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Pearce, 1983

Th - Ta - Yb discriminant for mantle melting array vs. arc basalts.

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Pearce and Cann, 1973

Immobile element discriminant for arc, within-plate, and ocean floor basalts.

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Pearce and Norry, 1979

Zr vs. Zr/Y discriminant for arc, ocean ridge, and within-plate basalts. Boninite added.

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Shervias, 1982

Ti - V discriminant diagram for tholeiitic to alkaline basalts, excluding calc-alkaline arc basalts. Boninite added.

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Wood, 1980

Ta - Th - Hf discriminant for arc, MORB, and within-plate basalts.

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Tectonic setting discriminants, mostly felsic rocks

Arth, 1979

Al2O3 vs Yb discriminant for oceanic vs. continental granitoids.

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Eby, 1992

Y/Nb - Rb/Nb discriminant for A1 vs. A2-type anorogenic granites.

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Gorton and Schandl, 2000

Th - Ta - Yb discriminants mostly for felsic to intermediate rocks.

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Pearce, 1984

Discriminant diagrams for granitic rocks.

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Whalen et al., 1987

Discriminates A-type granites from all other granite types.

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