<br><h3> Chapter One </h3> <b>An introduction to magmas and magmatic rocks</b> <p> <p> WHY STUDY MAGMATIC ROCKS? <p> The purpose of this book is to stimulate the reader's interest in magmatic rocks and processes, to develop key skills of describing, classifying and naming such rocks, and to show how much we can learn about igneous processes from careful, informed interpretation of rock textures, mineralogy and geochemistry. The book is aimed primarily at the intermediate-level student of geology who already has a basic knowledge of igneous rocks, but anyone starting from scratch should find that the opening chapter and relevant boxes - together with the Glossary - provide the minimum introduction they require. The emphasis throughout the book will be on practical investigation, mainly by means of the polarizing microscope; basic mineral-identification data have therefore been included to provide - between one set of covers - all that the student needs during a typical igneous practical class. <p> The logical place to begin any 'ig. pet.' course is to ask what purpose the petrologist, geologist or volcanologist hopes to accomplish in studying igneous rocks. Why do we do it? What kinds of things do we hope to learn? What answers are we trying to find? Such questions should always engage the mind of a petrologist who embarks on a petrographic or geochemical study; petrological science has moved on a long way from the early days when merely describing an igneous rock was an end in itself. In real life, a petrologist may study a suite of igneous rocks with one or more objectives in mind, including: <p> understanding eruptive <i>processes</i>; <p> assessing from previously erupted products the <i>hazard</i> presented by a volcano to surrounding communities; <p> investigating <i>magma evolution</i> in a subvolcanic magma chamber; <p> documenting the <i>structure</i> and <i>formation</i> of oceanic or continental crust; <p> inferring past <i>tectonic environments</i> (e.g. mid-ocean ridge, island arc) from the compositions of ancient igneous rocks; <p> understanding the formation of economic <i>mineral deposits</i> associated with igneous rocks. <p> establishing the <i>absolute age</i> of a succession of sedimentary and volcanic rocks (igneous rocks being easier to date isotopically than sedimentary rocks); <p> identifying the <i>source</i> from which a magma has originated, and under what <i>conditions</i> melting occurred (i.e. investigating 'magma genesis'); <p> identifying from erupted magmatic rocks the character and distribution of <i>geochemical domains</i> in the underlying mantle, and their evolution in time. <p> <p> In every such investigation, there is likely to be a role for carefully describing the igneous rocks involved, but the ultimate goal is usually to learn about magmatic processes, or the conditions under which those processes operate. That goal - of studying igneous rocks to learn about process - will come up again and again in this book, because understanding what goes on in magmatic systems is the modern petrologist's principal aim in life. Igneous rocks can tell us not only about processes taking place on the Earth's surface at the present time, but also: <p> about processes that have taken place earlier in Earth history, and <p> about processes that operate in parts of the Earth that are not directly accessible to us, for example in a magma chamber that originally lay 5km below an active volcano (but whose contents - or erupted products - are now exposed at the surface). <p> <p> Today, anyone working with igneous rocks has to apply a range of skills, including the analysis of field relationships, hand-specimen identification in the field, the description and interpretation of thin sections, the allocation of informative rock names, the quantitative interpretation of rock and mineral analyses (often including trace elements and isotope ratios), and the interpretation of experimental equilibria and phase diagrams. This book provides a basic introduction to all but the first of these practical and interpretive skills. The book is not intended to take the place of advanced texts dealing with theories of igneous petrogenesis. <p> The remainder of this chapter is devoted to introducing the basic vocabulary that will be needed for a clear explanation of igneous rocks. <p> <p> WHAT IS 'MAGMA'? <p> Igneous rocks are those that form from molten products of the Earth's interior. Petrologists use two words for molten rock. <b>Magma</b> is the more general term that embraces mixtures of melt and any crystals that may be suspended in it. A good example would be flowing lava which contains <b>crystals</b> suspended in the melt (Fig. 1.1): the term magma refers to the entire assemblage, embracing both solid and liquid states of matter present in the lava. <b>Melt</b>, on the other hand, refers to the molten state on its own, excluding any solid material which might be suspended in or associated with it. The difference becomes clearer if one considers how one would chemically analyse the distinct chemical compositions of the magma and melt, once the lava flow had solidified (Fig. 1.1). The <i>magma</i> composition could be estimated by crushing up a sample of the solidified lava, including both phenocrysts and groundmass (ensuring they are present in representative proportions). Analysing the <i>melt</i> composition, however, would require the groundmass or glassy matrix - the solidified equivalent of the melt between the phenocrysts - to be physically separated out and analysed on its own. <p> In fact, 'magma' may be used in a still broader sense. An ascending magma body, as it approaches the surface, commonly contains gas bubbles as well as phenocrysts, bubbles formed by gas that has escaped from the melt due to the fall in pressure that accompanies ascent (see Box 1.4). The term 'magma' is generally understood to embrace melt, crystals <i>and</i> any gas bubbles present (Fig. 1.1). Once erupted on the surface, on the other hand, and having lost some of its gas content to the atmosphere, the molten material is more appropriately called 'lava'. Determining a representative chemical analysis of the original magma composition, including the gaseous component, would however be difficult: as the melt solidified and contracted on cooling, the gaseous contents of the vesicles would escape to the atmosphere (and they would in any case be lost during crushing of the rock prior to analysis). Determining the concentrations of these <b>volatile</b> magma constituents - from the solid rock that the magma eventually becomes - therefore requires a different analytical approach that will be discussed later. <p> Magmas are originally formed by melting deep within the Earth (Chapter 2). The initial melting event most commonly takes place in the mantle, though passage of hot magma into or through the continental crust may cause additional melting to occur there as well, adding to the chemical and petrological complexity of continental magmatic rocks. In oceanic and continental areas, mantle-derived magmas are liable to undergo cooling and partial crystallization in storage reservoirs (magma chambers) within the crust (Chapter 3), and such processes widen considerably the diversity of magma compositions that eventually erupt at the surface. <p> <p> THE DIVERSITY OF NATURAL MAGMA COMPOSITIONS <p> What do we mean by magma (or rock) composition? <p> The overall composition of an igneous rock can be expressed in two alternative ways: <p> as a quantitative geochemical analysis, giving the percentage by mass of each of the main <i>chemical</i> constituents (Box 1.1); as a list of the <i>minerals</i> present in the rock as seen under a microscope, perhaps including an estimate - qualitative or quantitative - of their relative proportions. <p> <p> Though correlated, these two forms of analysis are not entirely equivalent in the information they convey. As a quantitative statement of chemical composition that can be plotted on graphs (e.g. Fig. 1.2) and used in calculations, a geochemical analysis provides the more exact information. The <b>bulk analysis</b> (also known as a <b>whole-rock analysis</b>) of a volcanic rock approximates closely - except for volatile components - to the composition of the magma from which it formed, considered at a stage before it had begun to crystallize. Careful analysis of geochemical data can reveal a lot about the <i>source</i> of the melt and the <i>conditions</i> (pressure, depth, extent of melting) under which the melt originally formed. <p> In some circumstances, however, other forms of rock analysis are of more practical use. Geochemical analyses, requiring elaborate laboratory facilities, are not usually available at the field stage of an investigation, when a geologist will normally find mineralogical and textural observations on hand-specimens a more practical way of characterizing, and discriminating between, the different rock types present in the area. Moreover, the occurrence in thin section of certain key indicator minerals - such as quartz, olivine, nepheline, aegirine-augite - provides immediate, key clues about the melt's chemical composition without resorting to the expense of geochemical analysis. The mineralogy of an igneous rock also provides information on <i>post-magmatic processes</i> (weathering, hydrothermal <b>alteration</b>) that may have made its chemical composition unrepresentative of magma composition (Box 1.4). <p> The study of a rock's mineralogical composition and texture - using a polarizing microscope to examine a thin section - is the science called <i>petrography</i>. A petrographic analysis of an igneous rock can range from a simple list of minerals seen (noting the textural relationships between them) to a full quantitative analysis of their relative volumes measured in a thin section. Qualitative petrographic examination is the normal prelude to geochemical analysis: it allows one to screen a suite of samples to eliminate unrepresentative or unsuitable specimens, and thereby avoid the expense of unnecessary chemical analyses. But a petrographic examination tells us a lot more about the rock than just its suitability for geochemical analysis: careful study of the rock's texture provides much information about the <i>eruption and crystallization history</i> of the magma. <p> It follows that a geochemical analysis and a petrographic (mineral-based) analysis give us complementary information about an igneous rock, and neither alone provides a complete understanding of the rock's origin and history. <p> <p> How widely do natural magma compositions vary? <p> Figure 1.2 shows a large number of geochemical analyses of volcanic rocks from various geotectonic environments plotted in a variation diagram. The vertical dimension in this diagram depicts the sum of the [Na.sub.2]O and [K.sub.2]O contents (each, and their sum, expressed in mass per cent [mass %; see footnote], i.e. grams of oxide per 100g of rock) for each sample. The horizontal dimension shows the corresponding Si[O.sub.2] content (also in mass %), and each data point in the graph - that is, each pair of [Na.sub.2]O+[K.sub.2]O and Si[O.sub.2] coordinates represents an individual rock analysis. In such diagrams, the rock analysis is taken to represent the original magma's composition. This particular plot is known as a 'total-alkalis <i>versus</i> silica' (or 'TAS') diagram and it is widely used for the geochemical classification of volcanic rocks (see Fig. 1.4). <p> The main purpose of showing this diagram here is to illustrate how widely natural silicate magmas can vary in their composition: Si[O.sub.2] contents range from 31% to 76%, and total alkali contents vary from 1% up to 15%. (This range is solely for silicate magmas: if natural <b>carbonatite</b> magmas were considered as well, the compositional range would become still greater.) This wide range of composition can be attributed primarily to four contributions that play a part in magma genesis: <p> source composition and mineralogy (e.g. whether crust or mantle); <p> depth of melting; <p> extent (%) of melting; <p> shallow magma-chamber fractionation processes, such as <b>fractional crystallization</b>. <p> <p> The effects of these factors will be discussed in later sections of the book. The important conclusion to be drawn here is that natural volcanic rock (and magma) compositions lie scattered across a wide range of total alkali-Si[O.sub.2] space, with no obvious internal breaks to divide up them into natural sub-groups. In other words, Nature creates within the Earth a <i>continuum</i> of potential magma compositions, and any categories or subdivisions we choose to erect (e.g. for the purpose of attaching names) are essentially arbitrary and man-made. <p> <p> PARAMETERS USED TO CLASSIFY IGNEOUS ROCKS <p> Unless igneous petrologists are to communicate entirely in numbers, they need a consistent nomenclature that allows this wide compositional spectrum to be sub-divided into smaller fields, to which specific rock names can be applied, just as a state is divided for administrative purposes into named counties and districts. <p> Modern igneous nomenclature rests on three types of observation, each of which may influence the name given to a rock: <p> <i>qualitative</i> petrographic observations (e.g. the presence or absence of quartz); <p> <i>quantitative</i> petrographic data (e.g. the percentage of quartz in the rock); <p> <i>chemical</i> composition (e.g. position in a TAS diagram - Fig. 1.4). <p> <p> These can be illustrated further by considering three elementary ways in which we categorize igneous rocks. <p> Classification by qualitative criteria - grain size <p> Figure 1.3a shows how igneous rocks are divided into coarse-, medium- and fine-grained categories, based on a qualitative (or semi-quantitative) estimate of the average grainsize of the <i>groundmass</i> of the rock (N.B. <i>not</i> on the size of any phenocrysts present). This estimate may be based on hand-specimen observation or, more reliably, on thin section examination. According to the grain-size category in which it falls (fine, medium or coarse), a rock of basaltic mineralogy, for example, would be called a basalt, a dolerite (UK) or diabase (US), or a gabbro. <p> Another example of a qualitative observation used in rock classification is the presence of quartz or nepheline in the rock, indicating whether it is <b>silica-oversaturated</b> or <b>silica-undersaturated</b>. <p> <p> Classification by mineral proportions - colour index <p> Familiar adjectives like 'ultramafic' and 'leucocratic' refer to the relative proportions of dark and light minerals in an igneous rock, where 'dark' and 'light' relate to the appearance of the minerals in hand-specimen, as indicated on the left of Fig. 1.3b. Dark minerals are known alternatively as <b>mafic</b> or <b>ferromagnesian</b> minerals; light minerals are also known as <b>felsic</b> minerals. The percentage of dark minerals is known as the <b>colour index</b> of the rock. <p> Quantitative measurements of mineral proportions in a thin section rely on a technique known as <i>point counting</i>. This entails mounting the thin section of interest in a special device attached to the microscope stage, which allows the slide to be advanced by regular small increments in both <i>x</i> and <i>y</i> directions by pressing relevant buttons. Starting near to one corner of the slide, the operator identifies the mineral under the cross-wires at each point as the thin section is stepped systematically across the stage, recording the number of 'hits' for each mineral present. Having acquired several hundred data points covering a significant area of the thin section, the percentage of each mineral is easily calculated. As the percentages calculated are proportional to the aggregate area of each minerals on the slide surface, such methods determine relative mineral proportions <i>by volume</i>, not by mass. As most dark minerals are significantly denser than the light minerals, this fact introduces a bias that must be borne in mind if mineral proportions determined in this way are compared with geochemical analyses (which are expressed in percentages <i>by mass</i>). <p> Strictly interpreted, all of the descriptors shown in Fig. 1.3b should be based on <i>quantitative</i> mineral proportions determined in this way, which is a time-consuming exercise. For most day-to-day purposes, however, the terms may be applied on the basis of a quick 'eyeball' estimate of dark and light mineral proportions. <p> <i>(Continues...)</i> <p> <!-- copyright notice --> <br></pre> <blockquote><hr noshade size='1'><font size='-2'> Excerpted from <b>Igneous Rocks and Processes</b> by <b>Robin Gill</b> Copyright © 2010 by Blackwell Publishing. Excerpted by permission.<br> All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.<br>Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.