Lecture 9

Introduction to Igneous Petrology

Introduction

Igneous rocks form by cooling and crystallization of hot silicate liquids. They are usually divided into two groups:

- intrusive rocks (like granite) that cool and crystallize at depth, and

- extrusive rocks (like basalt) that erupt onto the Earth’s surface

Igneous rocks comprise the bulk of oceanic crust and much of the continental crust; the mantle is also made of rocks generally deemed igneous.

Terminology

magma – silicate liquid +/- crystals +/- dissolved volatiles

lava – silicate liquid +/- crystals (lava loses its volatiles during ascent and eruption on the Earth’s surface)

Melting and Crystallization

The mantle is largely solid, thus magma must form by melting of the mantle. There are three main ways to melt the mantle:

1. increase temperature

2. decrease pressure

3. add H₂O

What types of melting occur in which tectonic environments?

1. Divergent boundaries (mid-ocean ridges) produce melts primarily by decompression … these melts are basaltic and either erupt as pillow basalts or sheet flows, or form intrusive dikes, sills, and gabbros.

2. Hot spots (like Hawaii), as their name suggests, produce melts primarily by heating; these melts are also of basaltic composition. Hawaiian basalts erupt primarily as very fluid lava flows that build large shield volcanoes with low slopes.

3. Convergent boundaries (subduction zones) recycle wet sediment and basalt into the mantle, thus melts produced in this environment are created largely by fluxing of the mantle with H₂O. The melts produced are variable in composition, but are generally basalt or andesite.

Melting and phase diagrams

To understand the concept of melting and crystallization of a multicomponent system we need to understand something about phase diagrams. [explained using handouts in class; see also Box 5.5 in your text]. BOTTOM LINE: when you melt a multicomponent system, the initial melt has a different composition than the bulk solid, and that composition will change with increased amounts of melting.

THEREFORE: melt composition depends not only on the bulk composition of the solid but also on the amount of melting!

Petrology of the Mantle

1. Seismology and mineralogy

Seismic studies define changes in the physical properties of mantle rock with depth. Important boundaries include:

a) the Moho – boundary between low density felsic and mafic rocks of the crust and high density ultramafic rocks of the mantle

b) lithosphere-asthenosphere – boundary between the rigid outer layer of the Earth and a mechanically weak layer that acts like a viscous fluid

c) 400 km discontinuity – increase in seismic velocity (density) caused by a change in olivine structure to that of spinel

d) 670 km discontinuity – change in spinel structure of olivine to that of perovskite

2. Composition of the mantle

We have samples of the upper mantle in the form of xenoliths (mantle fragments), dredged and drilled samples from oceanic fracture zones, and ophiolites, slabs of upper mantle thrust onto the edge of continents at old suture zones.

The upper mantle is peridotite (olivine-bearing).

(from Winter, 2001)

Basalt forms by melting of lherzolite, leaving a residue of harzburgite + dunite.

Although olivine and pyroxene are the dominant phases in the mantle, minor aluminum-bearing phases can tell us a lot about the P-T conditions of the mantle. Aluminous phases include plagioclase (low pressure), spinel (intermediate pressure) and garnet (high pressure; diagram from Winter, 2001).

We can use this diagram to explain why plagioclase-bearing xenoliths are common in oceanic regions, and garnet-beraing xenoliths are restricted to beneath the continents (where the pressures are high and the geothermal gradient lower).

Also unique to continental (kimberlite) xenoliths are diamonds … WHY? Formation of diamons from graphite requires high pressures at moderate temperatures, and even higher pressures at high temperatures. The low geotherm under old cratons thus decreases the pressure of this transition, and allows diamonds to be preserved and carried to the surface (diagram from Winter, 2001):