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Chapter 11 Spring 2010 - Chapter 11 Diversification of...

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Unformatted text preview: Chapter 11: Diversification of Magmas Magmatic Differentiation q q q q q Any process by which a magma is able to diversify and produce a magma or rock of different composition Partial Melting Crystal Liquid Fractionation Magma Mixing Assimilation Magmatic Differentiation q Two essential processes 1. Creates a compositional difference in one or more phases 2. Preserves the chemical difference by segregating (or fractionating) the chemically distinct portions Generating Magma - Partial Melting Separation of a partially melted liquid from the solid residue Causes differentiation Effects of removing liquid at various stages of melting q Eutectic systems 3 3 3 First melt always = eutectic composition Major element composition of eutectic melt is constant until one of the source mineral phases is consumed (trace elements differ) Once a phase is consumed, the next increment of melt will be different X and T Partial Melting q q Separation of a partially melted liquid from the solid residue requires a critical melt % Sufficient melt must be produced for it to 3 Form a continuous, interconnected film 3 Have enough interior volume that it is not all of it is adsorbed to the crystal surfaces Partial Melting The ability to form an interconnected film is dependent upon the dihedral angle ( ) a property of the melt A.Small amount of melt B.More melt with a small wetting angle - moves C.More melt with a large wetting angle - stays q Crystal-Liquid Fractionation Dominant mechanism by which most magmas, once formed, q Dominant mechanism by which most magmas, once formed, differentiate? Processes: q Gravity settling give cumulate textures q Modified by compaction or Filter pressing q Flow Segregation q Vapor Transport q Late-stage fractional crystallization q Liquid Immiscibility q Compositional Convection Differentiation q Diffusion In Situ Differentiation Gravity settling 3 The differential motion of crystals and liquid under the influence of gravity due to their differences in density Cool point a olivine layer at base of pluton if first olivine sinks Next get ol+cpx layer finally get ol+cpx+plag 3 3 3 Cumulate texture: Mutually touching phenocrysts with interstitial crystallized residual melt Flow segregation crystals are concentrated in center of the flow Figures 11-4 and 11-5 Drever and Johnston (1958). Royal Soc. Edinburgh Trans., 63, 459-499. Volatile Transport Albite w/ Xw = 0.5 Mostly melted Vapor released by heating of hydrated or carbonated wall rocks As a volatile-bearing (but undersaturated) magma rises and pressure is reduced, the magma may eventually become saturated in the vapor, and a free vapor phase will be released pegmatite Albite w/ Xw = 0.5 Fully melted Albite w/ Xw = 0.5 Melt saturated Albite w/ Xw = 0.5 Melt xlizes and water released Figure 7-22. From Burnham and Davis (1974). A J Sci., 274, 902-940. Late-stage fractional crystallization q q q q Fractional crystallization enriches late melt in incompatible, LIL, and non-lithophile elements Many concentrate further in the vapor Get q a silicate-saturated vapor + q a vapor-saturated late derivative silicate liquid Vapor and melt escape along fractures as dikes 3 Silicate melt quartz and feldspar small dikes of aplite 3 Vapor phase dikes or pods of pegmatite Late-stage fractional crystallization 3 3 Concentrate incompatible elements Complex: varied mineralogy v May display concentric zonation Figure 11-6 Sections of three zoned fluid-phase deposits (not at the same scale). a. Miarolitic pod in granite (several cm across). b. Asymmetric zoned pegmatite dike with aplitic base (several tens of cm across). c. Asymmetric zoned pegmatite with granitoid outer portion (several meters across). From Jahns and Burnham (1969). Econ. Geol., 64, 843-864. Geol., 8 cm tourmaline crystals from pegmatite 5 mm gold from a hydrothermal deposit Liquid Immiscibility q Example: Liquid immiscibility in the Fo-SiO2 system Figure 6-12. Isobaric T-X phase diagram of the system Fo-Silica at 0.1 MPa. After Bowen and Anderson (1914) and Grieg (1927). Amer. J. Sci. In Situ Differentiation Processes q Diffusion In-Situ Differentiation 3 3 In-situ: crystals don't sink/move Typically involves v Thermal diffusion within magma chamber h Heavy elements/molecules migrate toward the colder end and lighter ones to the hotter end of the gradient In-Situ Diffusion Differentiation Walker and DeLong (1982) subjected two basalts to thermal gradients of nearly 50oC/mm (!) Found that: q Samples reached a steady state in a few days q Heavier elements cooler end and the lighter hot end q The chemical concentration is similar to that expected from fractional crystallization Figure 7-4. After Walker, D. C. and S. E. DeLong (1982). Contrib. Mineral. Petrol., 79, 231-240. Compositional Convection Differentiation Due to thermal gradients in the magma chamber Figure 11-11. Schematic section through a rhyolitic magma chamber undergoing convection-aided in-situ differentiation. After Hildreth (1979). Geol. Soc. Amer. Special Paper, 180, 43-75. Compositional Convection Differentiation Langmuir Model q q Thermal gradient at wall and cap variation in % crystallized Compositional convection evolved magmas from boundary layer to cap (or mix into interior) Figure 11-12 Formation of boundary layers along the walls and top of a magma chamber. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall Magma Mixing q q Two magmas of different composition blend to form a single magma Generally not to completion may have `blebs' of mafic magma in felsic magma How Magmas Evlove: Magma Mixing Comingled basalt-Rhyolite Mt. McLoughlin, Oregon Figure 11-8 From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall Basalt pillows accumulating at the bottom of a in granitic magma chamber, Vinalhaven Island, Maine Assimilation q Incorporation of wall rocks (diffusion, xenoliths) Assimilation q q q Need magma to be super heated in order to give up large amounts of heat Hotter (mafic) magmas will melt cooler (felsic) wall rock Processes: 3 3 3 Melting limited evidence Dissolution evidence rare Reaction ionic exchange of wall rock with magma favored because of chemical gradients ...
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