1.3 MORB Petrography: Normative Mineralogy
Normative mineralogy is a theoretical mineralogy calculated on the basis of the major/minor element chemistry of the rock. The calculations generally use element oxides that each make up greater than ~0.1% of the total rock by weight. The CIPW normative calculation is one such technique. This technique recasts the rock chemistry into a set of idealized standard “normative minerals”. The normative mineralogy or “norm” provides a ready means to compare and classify different rocks based on their chemistry, and can be particularly useful in the case where the rocks are very fine-grained or glassy. Note that normative mineralogy does not necessarily correlate to the modal mineralogy, which is the mineral makeup of the rock that is actually observed (typically expressed as the volume %). This is particularly true for glassy rocks, as crystallization was halted before completion, making classification based on observed mineralogy difficult. MORB (and other basalts) can be classified using the calculated CIPW norm and the basalt tetrahedron, a basalt classification scheme based on normative mineralogy (Yoder and Tilley, 1962; Morse, 1980). The basalt tetrahedron (Figure 1.05) is a quaternary system that includes the minerals forsterite (Fo), diopside (Di), nepheline (Ne), and quartz (Qz) as the 4 end-member minerals. Enstatite (En) and Albite (Ab) are intermediate phases in this system. Basalts can be classified into several different groups using this scheme. These classifications primarily relate to the degree of silica (SiO2) saturation in the magma (other classification schemes for basalts use the relative abundance of alkali elements (K, Na), trends of iron enrichment, or other parameters).
The basalt tetrahedron divides basalts into two major classes that can then be further subdivided. The two fundamental divisions are alkalic basalts and tholeiitic basalts. Alkalic basalts are critically undersaturated with respect to silica (nepheline occurs in the normative mineralogy), whereas subalkaline, tholeiitic basalts range from oversaturated (quartz in the norm) with respect to silica to undersaturated (olivine, but no Ne in the norm). Those without quartz in their norm are known as olivine tholeiites or transitional basalts. The differences between alkalic and tholeiitic basalts are at least in part an artifact of the conditions of mantle melting that produced the primary magmas. Typically differentiates of these two basalt types stay within their respective fields on the basalt tetrahedron (i.e. differentiates of alkali basalts continue to plot in the Ne-normative field and differentiates of tholeiites remain in the tholeiite field, though and ol. tholeiite can evolve into a quartz tholeiite). These two magma lineages form two distinctive magma series, each sharing a unique petrogenesis.
Figure 1.05. Basalt tetrahedron. Basalt classification based on normative mineralogy proposed by Yoder and Tilley (1962) and built upon by Morse (1980). Click on the image to see a larger, zoomable version.
