In-Class Activity

GEO285 - Petrology In-class Activity 1 Magmatic Differentiation

In the last chapter we learned that we can create tholeiitic basalts and alkaline basalts from a homogeneous mantle. It is not possible, however, to produce all igneous rock types (e.g. andesites, dacites, rhyolites, etc.) by simple partial melting of the mantle. So, how can we make so many different types of igneous rocks? We do this by magmatic differentiation - any process by which magma diversifies to produce magma (or rock) of a different composition. There are four general types of magmatic differentiation: fractional crystallization, liquid immiscibility, magma mixing, and assimilation. Fractional crystallization, the dominant processes by which most magma differentiates, can occur by gravity settling, filter pressing, and flow segregation.

Today we will examine these processes in more detail using various items found around the house and the lab. In the groups you have established for the class project, you will use these random objects to design a mechanism for explaining one of the magmatic differentiation processes. Each process is described for you below and explained in more detail in your textbook (pages 204 – 218). Pay attention to the information that is provided below (particularly textures that form as a result of each process). It is important information that should be part of your explanation. You should also be prepared to answer questions about the mechanism you are describing either from the class or myself, so make sure you fully understand how the process works. You have the rest of today’s class period to prepare your explanation of the differentiation process assigned to you. We will present these during the next class period.

Group 1. Fractional crystallization: Gravity Settling During fractional crystallization, a chemically-distinct solid is created and segregated from the rest of the system (i.e. the liquid). The most common way that crystals are segregated from the melt is by gravity settling. This occurs when crystals separate from the liquid due to differences in density. Higher density minerals, like olivine, form first and sink to the bottom of the magma chamber. Lighter minerals, like plagioclase, form later and rise up through the magma, accumulating at the top of the magma chamber. The order of crystallization typically follows Bowen’s reaction series. Accumulation of crystals due to gravity settling results in a cumulate texture (See Chapter 3, page 42 for a description of this texture).

Group 2. Fractional crystallization: Filter pressing (Compaction) Filter pressing occurs in the “crystals mush” that accumulates as a result of gravity settling (see above section). Although crystals are physically separated from the liquid, intercumulus liquid may still be present between the crystals at abundances up to 60 vol%. As minerals crystalize out of the magma and accumulate at the bottom of the magma chamber, the overlying weight forces intercumulus fluid out of the spaces between the cumulate crystals, thus separating the liquid from the solid.

Group 3. Fractional crystallization: Flow Segregation Flow segregation occurs when crystal-rich magma flows in a laminar fashion near the walls of a magma body. The motion of the magma as it flows past the chamber wall creates a differential pressure that forces the magma to flow around the phenocrysts (see Fig. 11.4 on page 207). The pressure exerted forces crystal grains to concentrate away from the walls of the chamber, resulting in size distributions and textures such as those shown in Fig. 11.5. This concentration of crystals due to flow segregation is most apparent in elongated magma conduits, such as dikes and sills. GEO285 - Petrology In-class Activity 2 Group 4. Fractional crystallization: In-situ Crystallization Because a magma body is hot and the country rock which surrounds it is expected to be much cooler, heat will move outward away from the magma. Thus, the walls of the magma body will be coolest, and crystallization would be expected to take place first in this cooler portion of the magma near the walls. The magma would then be expected to crystallize from the walls inward. Just like in gravity settling, the first layer of crystals precipitated will still be in contact with the liquid, but will eventually become buried by later crystals and effectively be removed from contact with the liquid. The gradient in the extent of crystallization produces a corresponding gradient in the liquid composition, so the entire compositional spectrum of the liquid line of descent is present simultaneously (Fig. 11.12).

Group 5. Liquid immiscibility Liquid immiscibility is a type of magmatic differentiation in which a single magma separates into two unmixable phases, forming two different magma (and rock) compositions. This occurs in three natural magmatic systems, the most well-known of which is Fe-rich tholeiitic basalts. In the later stages of fractionation of these basalts, a granitic melt (>75 wt% SiO2) separates from a basaltic melt (~40 wt%

SiO2). The granitic melt has a lower density than the basaltic melt, so it will rise and collect near the top of the magma chamber. Both liquids typically become trapped between crystals that have already formed. Droplets of both immiscible liquids have been observed in the glassy matrices of some Hawaiian basalts.

Group 6. Magma Mixing Magma mixing is a little like liquid immiscibility in reverse. Here, two magmas of differing compositions come in contact to form a new, hybrid magma. This often occurs when a differentiating magma chamber is replenished from below with a more primitive parental magma. The degree to which magma mixes depends on the properties of the two magmas (e.g. temperature, composition, density, volatile content, and viscosity). Magmas with similar physical properties mix well, making it is difficult to recognize that mixing has occurred. Magmas with contrasting properties have very limited mixing that is observable in rocks. This is often indicated by commingled swirls of magma (Fig. 11.8) or pillow-like blobs (as in the case of basaltic magma injected into granitic magma – Fig. 11.9).

Group 7. Assimilation Assimilation is the incorporation (by melting) of chemical constituents from the walls of a magma chamber into the magma itself. This process may be capable of significantly altering the composition of magma. The degree to which magma can assimilate a country rock is limited by the heat available in the magma. For example, 1000 J of energy is required to raise the temperature of 1 gram of granitic rock from an ambient temperature of 200°C to its melting point (800°C). The only energy that can be supplied from magma is its own heat of crystallization. In the example above, 2.5 g of magma would have to crystallize in order to assimilate 1 g of granite. It is theoretically possible for a magma to assimilate 40% of its weight in country rock. Because assimilation cannot occur without the heat supplied from crystallization, it is often referred to as AFC – Assimilation and Fractional Crystallization