Geochemical Differentiation of Sedimentary Rocks after B. Mason

Only a few geochemical classification schemes for sedimentary rocks are available, and even less are generally accepted. This is mainly due to their classification based on grain size rather than mineralogy or chemical composition, particularly for siliciclastic sediments.

Besides the widely accepted geochemical sand-shale classification diagram of log(SiO₂/Al₂O₃) vs. log(Fe₂O₃/K₂O) after Herron (1988), there are hardly any other chemical discrimination diagrams for sedimentary rocks.

Although rarely used, another attempt to distinguish sedimentary rocks through their chemical composition is the diagram by B. Mason (1966), and its modified version, e.g. after Hasterok et al. (2018).

The Geochemical Differentiation Diagram for Sedimentary Rocks after Mason (1966)

Brian Mason (1966) developed a ternary diagram for the differentiation of sedimentary rock types. Mason points out that general features of sedimentary rocks [siliciclastics] are: a) K dominates over Na, b) Al is in excess of the 1:1 ratio to alkalies and Ca, c) high Si in sandy rocks and cherts, d) high CaO and MgO in carbonates, and e) Fe is mainly present in the ferric state (Mason, 1966, p.154f.).

The ternary diagram is based on the three components SiO₂, (Al,Fe)₂O₃·xH₂O, and (Ca,Mg)CO₃ (Figure 1).

The diagram differentiates broadly sandstones, argillaceous rocks/shales, from mature soils on one side, and between these siliciclastic and carbonate rocks on the other side.

Like Mason pointed out, this diagram neglects the alkalies. He argues, however, that those are generally low except in some argillaceous rocks. Although the main groups overlap, the proposed diagram, nevertheless, displays some tentative limits enabling an approximate differentiation of sedimentary rock types.

Mason (1966) does not give details on how these parameters are determined. Difficulties may arise when no data for volatiles, i.e., H₂O and/or CO₂ concentrations, are available, as often the case, e.g., with ICP-MS/OES and XRF analyses.

The modified SiO₂ – Al₂O₃+Fe₂O₃ – CaO+MgO diagram

Hasterok et al. (2018) use a modification of Mason’s ternary diagram utilizing SiO₂, Al₂O₃+Fe₂O₃, and CaO+MgO at its apices. To differentiate sedimentary (and metasedimentary) rocks, they apply a 3-step approach.

The 3 steps after Hasterok et al. (2018)

Step 1:

Due to differences in geochemical data sets of major oxides, some reported with and others without volatiles, Hasterok et al. (2018) normalize the following oxides to a volatile-free composition:

SiO₂, TiO₂, Al₂O₃, Cr₂O₃, FeO_T, MgO, MnO, NiO, CaO, Na₂O, K₂O, P₂O₅, and BaO. As they point out, normalizing the data can result in a change of rock type classification, but these changes are regarded as minor.

Step 2:

In Step 2 they utilize the triangular (as in Figure 2 below) to differentiate carbonates and soils from more clastic sediments.

Step 3:

In another step, they combine the results from the triangular and apply the classification diagram after Herron (1988) to further classify the clastic sediments.

Bonus

If you use a ternary diagram built on XY coordinates, e.g., the one I describe in my article ‘how to plot a ternary diagram in Excel’, Figure 3 provides you with the coordinates for the boundaries.

References in this Article

Hasterok, D., Gard, M., and Webb, J. (2018): On the radiogenic heat production of metamorphic, igneous, and sedimentary rocks. Geoscience Frontiers, 9, 1777-1794. [Link]

Herron, M.M. (1988): Geochemical classification of terrigenous sands from shales from core or log data. Journal of Sedimentary Petrology, Vol. 58, No. 5, 820-829. [Link]

Mason, B. (1966): Principles of Geochemistry. John Wiley & Sons, Inc. New York, London, Sydney, 329p.