Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download 1. Partial Melting
Document related concepts
Transcript
Classification of Igneous Rocks • Felsic • Basic • Amygdaloidal • Mafic • Undersaturated/oversaturated 1. Chemical Composition • Intermediate • Volcanic • Ultrabasic • Hypabyssal 2. Colour • Ultramafic • Plutonic • Acidic 3. Texture • Vesicular • Saturated 4. Mineralogy • Porphyritic • Equigranular • Flow banding Olivine Quartz Feldspar Muscovite Mica Biotite mica Augite Hornblende • Crystalline • Interlocking crystals • Randomly orientated • Euhedral/Anhedral/ Subhedral Chemical Composition 1. Acidic >65% SiO2 2. Intermediate 3. Basic 4. Ultrabasic 65% - 52% SiO2 52% - 45% SiO2 <45% SiO2 Colour 1. Felsic - light 2. Intermediate 3. Mafic 4. Ultramafic - medium - dark - very dark Texture Crystalline, interlocking crystals & randomly orientated Grain size Equigranular or porphyritic Vesicular, flow banding, amygdaloidal Mineralogy - Quartz SiO2 (K, Na) AlSiO - Orthoclase feldspar Na AlSiO to CaAlSiO K AlSiO - Plagioclase feldspar - Muscovite mica Felsic Minerals Mineralogy (Mg, Fe) SiO - Olivine (Mg, Fe, Ca) SiO - Augite Ca (Mg, Fe) SiO - Hornblende K (Mg, Fe) AlSiO - Biotite mica Mafic Minerals How Do Igneous Rocks Form? Step 1: MELTING rocks to form MAGMA Step 2: COOL magma so CRYSTALLISATION can take place to form solid rock What causes melting? 1. What state is material of mafic composition under following conditions: • O.1 Mpa & 1100°C Partially molten • 1000 Mpa & 1100°C Solid • 400 Mpa & 1150°C Partially molten • 200 Mpa & 1250°C Liquid What causes melting? 2. Keeping the pressure constant at atmospheric pressure (0.1 Mpa): • at what temperature does mafic mantle material begin to melt? 1075°C • at what temperature does it become completely molten? 1200°C What causes melting? 3. What would happen if you took a piece of mafic mantle material at 1000 Mpa & 1100°C & gradually decreased the pressure to 0.1 Mpa without changing the temperature? Melting This is known as Decompression Melting. What causes melting? 1. What would happen if you took a dry piece of mafic mantle material at 1000 Mpa & 1100°C and (without changing P or T) added sufficient water to make conditions saturated? Liquid What affect does the presence of water have on the melting point of mafic mantle material? Water lowers the melting temperatures. This is known as Hydration Melting How Do Igneous Rocks Form? Step 1: Step 2: MELTING rocks to form MAGMA • temperature • pressure • water vapour content COOL magma so CRYSTALLISATION can take place to form solid rock • contact with air (extrusive rocks) • contact with surrounding country rock (intrusive rocks) • contact with water (extrusive rocks) Where is magma formed? Normal situation around the globe. Geotherm is lower than the solidus curve so rocks do not begin to melt (i.e. no partial melting occurs). 1. Rift Valleys. At constructive plate margins the lithosphere is being pulled apart, causing it to stretch and thin. The ductile and mobile (but solid) asthenosphere can then rise to fill the gap. As the asthenosphere is now nearer to the surface it is under less pressure and partially melt due to decompression melting. The solid peridotite partially melts to form basaltic magma which erupts at the surface in an effusive manner. 2. Mid-Oceanic Ridges. At constructive plate margins the lithosphere is being pulled apart, causing it to stretch and thin. The ductile and mobile (but solid) asthenosphere can then rise to fill the gap. As the asthenosphere is now nearer to the surface it is under less pressure and partially melt due to decompression melting. The solid peridotite partially melts to form basaltic magma which erupts at the surface in an effusive manner. 3. Hot Spots. At intra plate locations (away from plate boundaries) the lithosphere is moving slowly (2-10cm/year) over the top of a mantle plume. A mantle plume is an area of extra high heat flow (up to 300°C) rising up through the mantle. It is NOT magma, but hot rocks which rise as they are less dense than the surrounding rocks. As the plume nears the surface it is under less pressure and partially melts due to decompression melting. The solid peridotite partially melts to form basaltic magma which erupts at the surface in an effusive manner. 4. Subduction Zones. At destructive plate margins the lithosphere is being dragged down into the mantle at a subduction zone. The descending oceanic lithosphere takes down water with it, which is released into the overlying mantle at about 100km in depth. This water lowers the melting point of the asthenosphere which partially melts due to hydration melting. The solid peridotite partially melts to form basaltic magma which as it rises to the surface changes its composition to andesitic magma due to assimilation and fractional crystallisation. This magma erupts at the surface in an explosive manner. How Does Magma Migrate Upwards? > 1,000,000 years < 1,000 years SOLID 1. If melt > 5%, more buoyant liquid rises IF weakness available 2. If NO weakness available, needs > 30% melt to force its way upwards SOLID LIQUID Melting begins PARTIAL MELTING How Does Magma Migrate Upwards? PLUTON PLUTON PLUTON BATHOLITH SOLID LIQUID Andean Batholith Belt How Does Magma Migrate Upwards? How Does Magma Migrate Upwards? How Does Magma Migrate Upwards? Earth surface 5km 20km Upper Crust cold, brittle Lower Crust hot, ductile 20km 15km 5km How Does Magma Migrate Upwards? 1. Diapiric Emplacement 2. Dyke Ascent 3. Magmatic Stoping 20km 15km How Do Magmas of Different Composition Form? K 45% 52% SiO2 65% How Do Magmas of Different Composition Form? 1. Partial Melting 2. Fractional Crystallisation 3. Assimilation 4. Mixing of Magmas 5. Underplating 6. Thickening of the Continental Crust 1. Partial Melting ~1200°C Olivine Augite Bowen’s Reaction Series Plagioclase feldspar (Mg Fe) SiO2 Ca Mg Fe SiO2 Hornblende Plagioclase feldspar Ca Mg SiO2 Biotite mica Fe Mg K Al SiO2 Orthoclase feldspar Muscovite mica ~600°C Ca Al SiO2 Quartz SiO2 K Al SiO2 K Al SiO2 Na Al SiO2 2. Fractional Crystallisation M M M 2. Fractional Crystallisation ~1200°C Olivine Augite Plagioclase feldspar (Mg Fe) SiO2 Ca Mg Fe SiO2 Hornblende Plagioclase feldspar Ca Mg SiO2 Biotite mica Fe Mg K Al SiO2 Orthoclase feldspar Muscovite mica ~600°C Ca Al SiO2 Quartz SiO2 K Al SiO2 K Al SiO2 Na Al SiO2 2. Fractional Crystallisation Physical separation of solid part and liquid part of rising pluton. How? 1. Filter pressing 2. Differentiation/Gravity settling 3. Convection within a magma chamber 3. Assimilation 4. Magma Mixing 4. Magma Mixing Mt St Helens Hawaii Mt Pinatubo Iceland 2. Fractional Crystallisation ~1200°C Olivine Augite Plagioclase feldspar (Mg Fe) SiO2 Ca Mg Fe SiO2 Hornblende Plagioclase feldspar Ca Mg SiO2 Biotite mica Fe Mg K Al SiO2 Orthoclase feldspar Muscovite mica ~600°C Ca Al SiO2 Quartz SiO2 K Al SiO2 K Al SiO2 Na Al SiO2 1. Partial Melting ~1200°C Olivine Augite Bowen’s Reaction Series Plagioclase feldspar (Mg Fe) SiO2 Ca Mg Fe SiO2 Hornblende Plagioclase feldspar Ca Mg SiO2 Biotite mica Fe Mg K Al SiO2 Orthoclase feldspar Muscovite mica ~600°C Ca Al SiO2 Quartz SiO2 K Al SiO2 K Al SiO2 Na Al SiO2 Mineral Zoning NaAlSi3O8 to CaAl2Si2O8 Plagioclase crystal Ca-rich Na-rich Mineral Zoning Olivine crystals Fe Mg Zoned olivine (Mg,Fe)2SiO4 Reaction Rim or Corona Structure Olivine Augite Hornblende
