Earth Science 232 Petrography

Transcription

1 Earth Science 232 Petrography Course notes by Shaun Frape and Alec Blyth Winter 2002

2 1 Petrology - Introduction Some Definitions Petra Greek for rock Logos Greek for disclosure or explanation Petrology -The branch of geology dealing with the origin, occurrence, structure, and history of rocks, especially igneous and sedimentary rocks. Igneous rock: a rock that solidifies from molten or partially molten material (ie magma). Sedimentary rock: a rock that results from consolidation of loose sediment or chemicals precipitating from solution at or near the earth s surface; or organic rock consisting of the secretions or remains of plants and animals. Metamorphic rock: rock derived from pre-existing rocks by mineralogical, chemical or structural changes (especially in the solid-state). Note: Some borderline situations and rock types do exist. They are not common compared to the overall percentage of rocks in existence eg (1) volcanic tuffs, igneous or sedimentary, (2) serpentinite Mg 3 Si 2 O 5 (OH) 4 from low T seafloor alteration, igneous or metamorphic.

3 Classification can be aided using Table (1) Outcrop Characteristics (2) Textures (general) (3) Examples of Minerals

4 Abundance of Different Rock Types 3 There are several schemes and classifications. (1) The abundance of the 3 rock groups on the continents Sedimentary ~66% Igneous + Metamorphic ~34% (bulk is igneous) (2) If we consider the ocean sediments then some of the schemes have sedimentary rocks as high as 80%. (3) Note on Figure 1-1 the relationship with time. As you may have learned in Stratigraphy, there are very large differences in time due to erosion or nondeposition.

5 Igneous Petrology 4 Petrology - is one of the key courses for a geologist - you must be able to interpret the rocks crystallization and chemical history and therefore have to familiarize yourself with the Tools of Petrology. Tools - [A] Petrography Petrography is the microscopic identification and interrelationships of mineral grains in the fabric of a rock. Eg: When you finish studying igneous rocks this term you will develop the feeling that most hand specimens and slides are made up of 2 basic types of mineral assemblages. 1) Quartz, K-Feldspars, Na-Plagioclase and Micas or 2) Ca-Plagioclase, Pyroxenes, Amphiboles The former are usually light in colour while the latter are dark.

6 Tools - [B] Phase Diagrams 5 Igneous Rocks are rocks that solidify from molten or partially molten material ie, from magma. Therefore, a very important tool is experimental and observational High Temperature Geochemistry and the use of Phase Diagrams. In this course we will examine most of the major igneous rock forming mineral assemblages using these two tools. As a brief start, lets examine something you should be familiar with. In the 1920 s N.L. Bowen noted that under a variety of changing temperature conditions, a set group of minerals always crystallized in the same order from a melt. (P50-54 Jackson)

7 6 What is important here is to look at the details and interrelationships of the mineral families. Eg: (a) Why are Olivine and the Plagioclase Group of minerals similar in behavior [Solid Solution Series]? (b) Why are they Different? We will examine each of the systems, examining the controls to the formation of mineral phases Magma Differentiation (1) Fractional Crystallization The Role of Gravity and Density (a) Basaltic magma has density of 2.7 g/cm 3 while early crystals of Olivine or Pyroxene are 3.3 g/cm 3 (i) In large chambers these sink and are removed (ii) Magma becomes lighter; chemical components are missing (iii) Light components like leucite (2.5 g/cm 3 ) float

8 Magma Differentiation - cont. 7 (b) Magma changes composition with crystallization and individual mineral phases change composition with time as early ingredients are used up. We use binary phase diagrams to trace these changes (2) Filter Differentiation or Filter Press Action Flowing magma enters fractures. Crystals are blocked while the crystal-free magma continues. Crystals settling rearrange and tend to expel free liquid (filter press). Less dense felsic/granitic magmas often separate and float to the top of the chamber, cool and form a cap. (3) Flow Differentiation In flowing moving pipes, early formed crystals collect and orient in the middle or center of the pipe- like stirring beaker. (4) Zoning Incomplete reactions - lack of diffusion. Cores or crystals hive higher temperature or different compositions than the rims. Use up certain ingredients first, then can only make residual minerals.

9 Magma Differentiation - cont. 8 (5) Liquid Immiscibility - eg Oil & Water Cool a magma and some ingredients separate eg - sulphide melt -carbonatites separate from mafic alkalines - felsic blobs in ultramafics Immiscibility appears to need : (6) Volatiles high K; low H 2 O; P under 4 Kbars; < 55 % SiO 2 and high Si/Al Late-stage differentiation mechanism by gaseous transfer, secondary boiling (flashing). If gas rises it changes the residual melt composition. Some elements concentrate in the gas phase (eg: Na, Cl, F, Mn, Ti, Rb, La, Ce, Y, Zr, U) and will enrich upward along with SiO 2 in the magma chamber. Others elements prefer the melt (eg: Ca, Mg, Cr, Fe, P,K,B) and will be enriched downward in the chamber. Eg: Na depletion due to gas stripping in deeper Andesitic magmas. We can analyze elements and use this to tell magma depth conditions.

10 Magma Differentiation - cont. 9 (7) Assimilation - Melting and fractional crystallization of host rocks. - Contaminants (8) Magma mixing - Diffusional process

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