Lectures 10&11

Igneous Rocks

Introduction

As we learned last time, melting of the mantle usually produced basaltic magmas, compositions that are referred to as mafic because of their high proportions of Mg and Fe. Mantle rocks themselves are called ultramafic.

We also learned that igneous rocks are commonly classifed by as intrusive or extrusive based on their ascent and cooling history (and thus grain size – remember that slow cooling permits extensive crystal growth, and thus intrusive rocks are completely crystalline, often with fairly large crystals). Most of you are probably familiar with the extrusive form of basaltic lava from exposure to the spectacular eruptive activity that has occurred in Hawaii for the past twenty years [DVD examples].

Crystallization of igneous rocks

Last week the simple Fa-Fo phase diagram was introduced to illustrate several points about melting and the composition of mantle melts. As melting and crystallization are reversible processes, we can also use phase diagrams to understand how melts crystallize [EX]. Some generalizations:

1. The compositional paths followed by both the melt and the growing crystal depend on the bulk (initial) composition of the melt.

2. The composition of both the melt and the crystal at any point in the crystallization process also depend on the amount of crystallization (the extent to which the process has been completed).

3. For minerals that exhibit solid solution (olivine, pyroxene, plagioclase), crystallization proceeds in a continuous manner with the composition of the mineral changing along with that of the liquid.

4. For minerals that do not exhibit solid solution (because their crystal structures are different), melting or crystallization will proceed in discrete (discontinuous) steps [Ex: An-Di]. This means that as a melt cools, these minerals will appear suddenly when the melt reaches the appropriate temperature…

Bowen’s reaction series [BRS]

The differences between minerals that crystallize continuously and those that crystallize over discrete temperature intervals was first described by the pioneer of igneous petrology, NL Bowen. Bowen proposed a simple explanation for the crystallization of magma in terms of common minerals that exhibit one type of crystallization or the other … this simple scheme is now known as Bowen’s Reaction Series:

The minerals on the discontinuous side of the series appear abruptly as the magma cools. OR these minerals will also start to melt abruptly in a solid rock as the rock is heated. The melting temperature is that of the eutectic in the An-Di example.

In contrast, the minerals on the continuous side shows the behavior of minerals that crystallize as solid solutions, such as plagioclase [EX]. Note the following features:

• minerals that crystallize (or melt) at high temperatures (olivine, pyroxene) are rich in Mg and Fe and relatively poor in Si; igneous rocks that crystallize these minerals will be mafic in composition (basalt; gabbro)

• minerals that crystallize at the lowest temperatures (quartz, muscovite, alkali feldspar) are rich in Si and Al; rocks that contain these minerals are felsic in composition (granite, rhyolite).

• rocks with intermediate minerals (amphibole, biotite) are intermediate in composition (andesite, dacite, diorite).

Another important observation is that minerals that crystallize at high temperatures show less polymerization of the Si-tetrahedra than those that crystallize at lower temperatures [compare BRS with Table 2.6 in the text].

Structure of silicate liquids

The last observation above leads directly to the subject of the structure of silicate liquids. Yes, these liquids do have structure, which is created largely by varying degrees of bonding among SiO₄ tetrahedra (polymerization). The difference between a silicate liquid and a silicate mineral is that the mineral has a definite structure that is the same throughout (what we refer to as long-range order) while a silicate melt shows different types of polymerization throughout the melt (short-range order).

Additionally, the degree of polymerization of the melt controls the viscosity (stickiness) of the melt. So, mafic magmas that have relatively low Si contents have depolymerized melts that produce depolymerized crystals (olivines with isolated tetrahedral and pyroxenes with single tetrahedral chains); these melts tend to have a high temperature and a low viscosity (that is, they are very fluid, as you have seen on the DVDs]. In contrast, felsic magmas have abundant Si and Al, the melts are highly polymerized, they crystallize sheet and framework silicates (mica, feldspar, quartz) and they are very viscous (sticky). The consequences of these characteristics are twofold:

1) Basaltic magmas erupt easily; eruptions are dominated by fluid lava flows that may travel 10s of kilometers.

2) Rhyolitic magmas often cool and crystallize before reaching the surface, thus forming the large granite intrusions that characterize the Sierra Nevada Mtns. in CA (and the Wallowas). When rhyolites do eruption, those eruptions are typically explosive because the gas bubbles can’t escape from the melt.

Volatile content of melts

Which leads to another topic – volatiles. Volatiles are elements that dissolve in magmas but transform to gas as magma reaches the surface (and thus are depleted in lavas). Examples of important volatiles are H₂O, CO₂, F, Cl, S (as H₂S or SO₂). Note that minerals high on BRS contain no volatiles, while those toward the bottom (amphiboles and micas) contain volatile elements. This tells us that mafic magmas tend to be poor in volatiles (although not always), while felsic magmas tend to be volatile-rich. This is another reason why basaltic magmas tend to erupt passively while felsic magmas tend to erupt more explosively.

Classification of igneous rocks

Igneous rocks are classified on the basis of (1) chemical composition and (2) texture.

Composition is controlled by

composition of parent material

degree of melting of parent material

modification of composition by crystallization and differentiation

Compositional ranges are shown in Box 5.1 of the text. Note in particular variations in Mg & Fe (the mafic components) and Si&Al (the felsic components). Compositional classification may also be done by characteristic minerals found in different rocks, as illustrated in Fig. 5.27 of your text.

Textures are controlled by conditions of crystallization and cooling. By texture we refer primarily to grain size of constituent crystals. Slowly cooled intrusive rocks tend to have larger crystals than rapidly cooled extrusive (volcanic) rocks, which are sometimes quenched so quickly that they are glassy (frozen liquid) rather than crystalline. Combined textural and chemical classifications and characteristics are given in Table 5.3 for the felsic members, in Table 5.4 for the mafic members, and in Table 5.5 for the ultramafic rocks. These classification schemes are illustrated schematically in Fig. 5.28. Not shown here are typical temperature and viscosity ranges, which I show below.