Accessory minerals importance in granite petrology: a review and case studies

Accessory minerals importance in granite petrology: a review and case studies

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  Accessory minerals importancein granite petrology: a review and case studies. Astrid Siachoque Velandia Phd. Student Research Docente Responsável: Silvio RF Vlach NOVEMBRO, 2016 SEMINÁRIOS GERAIS II
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  INTRODUCTION Petrogenetic studies ofigneous rocks involve determining:  The history of the sources of melts,  The conditions of melting,  The mineralogical and chemical composition of the source during melting,  The extent of the melting processes involved, and  How the melt is modified by assimilation, metasomatism, differentiation, and fluids (Hanson, 1980). In order to evaluate the importance of each, a detailed knowledge of the geochemistry of systems involving fluids, minerals, and melts would be required. Trace elements studies have become a vital part of modern petrology and are more capable of discriminating between petrological processes than are the major elements.
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  TRACE ELEMENTS What isa Trace Element? By definition, a trace element constitute only a small fraction of a system of interest, they provide geochemical and geological information out of proportion to their abundance. Goldschmidt´s Classification 1. Atmophile 2. Lithophile 3. Siderophile 4. Chalcophile Groupings
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  Behavior of thetrace elements 1) Compatible: Elements are concentrated in the solid 2) Incompatible: Elements are concentrated in the melt High field strength (HFS) Large ion lithophile (LILE) Ionic potential > 2.0 Ionic potential < 2.0
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  Trace Elements Distribution Raoult´sLaw Henry´s Law ai = Xi 𝑎𝑖 𝑗 = 𝑘𝑖 𝑗 𝑋𝑖 𝑗 a = activity of the trace element X = host mineral k = Henry´s law constant for trace element i in mineral j Exchange equilibrium of a component a between two phases (solid and liquid) Trace element concentrations are in the Henry’s Law region of concentration, so their activity varies in direct relation to their concentration in the system.
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  Partition coefficients Nernst distributioncoefficient 𝐾𝑑 = 𝐶 𝑖 𝑚𝑖𝑛𝑒𝑟𝑎𝑙 𝐶𝑖 𝑚𝑒𝑙𝑡 Kd » 1 (compatible elements) Kd « 1 (incompatible elements) Physical controls on the value of partition coefficients in mineral/melt system  Composition  Temperature  Pressure  Oxigen activity  Crystal chemistry Includes the Henry’s Law constants for trace element i in the mineral and in the melt and is a function of temperature, pressure and composition of the melt, but is controlled neither by the concentration of the trace element of interest nor by the concentration of other trace elements
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  Geological controls onthe distribution of trace elements 1) Partial Melting a) Batch melting b) Fractional melting 𝐶𝑙 𝐶0 = 1 𝐹 1 − (1 − 𝐹)1/𝐷𝑜 𝐶𝑙 𝐶0 = 1 𝐷0 + 𝐹 1 − 𝐷0 Implies complete equilibration between solid and melt. Only a small amount of liquid is produced and instantly isolated from the source. F = weight fraction of melt produced D0 = bulk distribution coefficient of the original solid CL = concentration of the trace element in the melt C0 = concentration of the trace element in the solid
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  a) Equilibirum Crystallizationb) Fractional crystallization 2) Crystal Fractionation 𝐶𝑙 𝐶0 = 1 𝐷 𝑥 + 1 − 𝑋 Describes complete equilibrium between all solid phases and the melt during crystallization. Describes the extreme case where crystals are effectively removed from the melt the instant they have formed. 𝐶𝑙 𝐶0 = 1 − 𝑋 𝐷−1 X = fraction of material crystallized Dx = bulk distribution coefficient during crystallization CL = concentration of the trace element in the melt C0 = concentration of the trace element in the solid Geological controls on the distribution of trace elements
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  Rare Earth Elements(REE) Light rare earths (LREE) Heavy rare earths (HREE)  Scandium (Sc)  Lanthanum (La)  Cerium (Ce)  Praseodymium (Pr)  Neodymium (Nd)  Samarium (Sm)  Europium (Eu)  Gadolinum (Gd)  Yttrium (Y)  Terbium (Tb)  Dysprosium (Dy)  Holmium (Ho)  Erbium (Er)  Thuluim (Tm)  Ytterbium (Yb)  Lutetium (Lu) Are the most useful of all trace elements and REE studies have important applications in igneous petrology
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  Presenting REE data a)Primitive mantle-normalized patterns b) Chondrite normalized REE patterns Normalized trace element diagrams for A-types granites from Jabel Sayed complex, NE – Saudi Arabia (Moghazi et al. 2015). Normalizing values from Sun and McDonough (1989).
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   The solubilityof the accessory mineral in crustal melts  The equilibrium mineral/liquid partition coefficients for the trace element and isotopes of interest  The dillusivities that govern the rates at which equilibrium will be approached. ACCESSORY PHASE BEHAVIOR “Fundamental accessory-phase parameters” Accessory Mineral? Any mineral in an igneous rock not essential to the naming of the rock (<0.1%) (Harrison and Watson, 1983; Watson, 1980a, 1979a; Watson and Harrison, 1984)
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  APPLICATION OF ACCESSORYMINERALS TO THE GRANITE PETROLOGY 1) Zircon Saturation Thermometry ln 𝐷 𝑍𝑟,𝑧𝑖𝑟𝑐𝑜 𝑛 𝑚𝑒𝑙𝑡 = −3.8 − 0.85 𝑀 − 1 + 12900 𝑇 𝑇𝑍𝑟 = 12900 2.95 + 0.85𝑀 + 𝑙𝑛 496000 𝑍𝑟 𝑚𝑒𝑙𝑡 12 Watson and Harrison (1983) 𝐷 𝑍𝑟,𝑧𝑖𝑟𝑐𝑜𝑛/𝑚𝑒𝑙𝑡 = is the ratio of Zr concentration (ppm) in zircon (476,000 ppm) to that in the satured melt M = concentration of the trace element in the solid T, is in kelvins Rearranging the equation to yield T yields a geothermometer for melt
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  APPLICATION OF ACCESSORYMINERALS TO THE GRANITE PETROLOGY 2) Apatite Saturation SiO2 = is the weight fraction of silicat in the melt 𝐼𝑛𝐷 𝑃 𝑎𝑝𝑎𝑡𝑖𝑡 𝑒 𝑚𝑒𝑙𝑡 = 8400 + 𝑆𝑖𝑂2 − 0,5 ∙ 2,64𝑥104 𝑇 − 3,1 + 12,4 ∙ 𝑆𝑖𝑂2 − 0,5 (Harrison and Watson, 1984) 𝐷 𝑎𝑝𝑎𝑡𝑖𝑒/𝑚𝑒𝑙𝑡 = is the ratio of P concentration (ppm) in apatite in the satured melt
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  CASE STUDY
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  CASE STUDY: Allaniteand Chevkinite in A-type granites of the Graciosa Province BSE images showing the textures and compositional variations in allanite (a to d) and chevkinite (e and f) crystals
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  CASE STUDY: Allaniteand Chevkinite in A-type granites of the Graciosa Province a) Allanite compositions plotted on the REE+Y+Sr+Th versus AlT diagram of Petrík et al. (1995). b) Chevkinite compositions plotted on the FeOT versus CaO diagram. Symbols: open circles, primary chevkinite; open diamonds, post-magmatic altered chevkinite.
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  CASE STUDY: Allaniteand Chevkinite in A-type granites of the Graciosa Province Chondrite-normalized REE patterns (Boynton, 1984) Allanite Chevkinite
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  CASE STUDY: Allaniteand Chevkinite in A-type granites of the Graciosa Province LaN/NdN vs CeN plot CONCLUSIONS Allanite compositions lead to determinated that these rocks are related to extensional or anorogenic tectonic regimes elsewhere is characteristically richer in the ferriallanite molecule, in REE, and in TiO2, and poorer in Al2O3. Chevkinite–(Ce) compositions observed in the Graciosa Province are similar to, but on average richer in Ti than those seen in chevkinite– (Ce) from evolved undersaturated and saturated rocks of alkaline affinity worldwide. All integrated data reveal that allanite and chevkinite are the main LREE reservoirs in rocks of the aluminous and alkaline associations of the Graciosa Province. In addition the composition of the magmas is an important control on the stability fields of these minerals, a fact that is supported by the presence of primary allanite in rocks formed by processes of mixing and mingling of magmas. Allanite Chevkinite
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  Despite their lowabundances in crustal rocks accessory minerals are of considerable geochemical importance because they appear to be key tracers for many geological processes. For instance, numerous chemical elements of geological and geochronological interest, such as the rare earth elements (REEs) U, Th, Pb, Ti, Nb, V, and Ta, are contained in these minerals. Trace element geochemistry has been of enormous use in understanding the evolution of the Earth. A number of studies have shown that trace elements can be used to great advantage for determining the origin of granitic rocks and are useful petrogenetic indicators to unravel complex geologic histories preserved in igneous rocks. SUMMARY
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  Hanson, G.N., 1980.Rare earth elements in petrogenetic studies of igneous systems. Annual Review of Earth and Planetary Sciences 8, 371. Harrison, T.M., Watson, E.B., 1984. The behavior of apatite during crustal anatexis: equilibrium and kinetic considerations. Geochimica et Cosmochimica Acta 48, 1467–1477. Harrison, T.M., Watson, E.B., 1983. Kinetics of zircon dissolution and zirconium diffusion in granitic melts of variable water content. Contributions to Mineralogy and Petrology 84, 66–72. Harrison, W.J., Wood, B.J., 1980. An experimental investigation of the partitioning of REE between garnet and liquid with reference to the role of defect equilibria. Contributions to Mineralogy and Petrology 72, 145–155. Vlach, S.R.F., Gualda, G. a R., 2007. Allanite and chevkinite in A-type granites and syenites of the Graciosa Province, southern Brazil. Lithos 97, 98–121. Watson, E.B., 1980a. Some experimentally determined zircon/liquid partition coefficients for the rare earth elements. Geochimica et cosmochimica Acta 44, 895–897. Watson, E.B., 1980b. Apatite and phosphorus in mantle source regions: an experimental study of apatite/melt equilibria at pressures to 25 kbar. Earth and Planetary Science Letters 51, 322–335. Watson, E.B., 1979a. Zircon saturation in felsic liquids: experimental results and applications to trace element geochemistry. Contributions to Mineralogy and Petrology 70, 407–419. Watson, E.B., 1979b. Apatite saturation in basic to intermediate magmas. Geophysical Research Letters 6, 937–940. Watson, E.B., 1976. Two-liquid partition coefficients: experimental data and geochemical implications. Contributions to Mineralogy and Petrology 56, 119–134. Watson, E.B., Capobianco, C.J., 1981. Phosphorus and the rare earth elements in felsic magmas: an assessment of the role of apatite. Geochimica et Cosmochimica Acta 45, 2349–2358. Watson, E.B., Harrison, T.M., 1984. Accessory minerals and the geochemical evolution of crustal magmatic systems: a summary and prospectus of experimental approaches. Physics of the Earth and Planetary Interiors 35, 19–30. doi:10.1016/0031-9201(84)90031-1 Watson, E.B., Harrison, T.M., 1983. Zircon saturation revisited ’ temperature and composition effects in a variety of crustal magma types 64, 295– 304. REFERENCES