Lecture 9

Details

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 EarthÕs 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 Ð silicate liquid +/- crystals +/- dissolved volatiles

            lava Ð silicate liquid +/- crystals (lava loses its volatiles during ascent and eruption on the EarthÕs 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

 

 

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.

 

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 Ð boundary between low density felsic and mafic rocks of the crust and high density ultramafic rocks of the mantle

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

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

            d) 670 km discontinuity Ð 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).  Basalt forms by melting of lherzolite, leaving a residue of harzburgite + dunite.

 

Minor aluminum-bearing phases can tell us a lot about the P-T conditions of the mantle. 

 

Also unique to continental (kimberlite) xenoliths are diamonds É WHY?