accessory minerals importance in granite petrology: a review and case studies
TRANSCRIPT
Accessory minerals importance in granite petrology: a review and case studies.
Astrid Siachoque Velandia
Phd. Student Research
Docente Responsável: Silvio RF Vlach
NOVEMBRO, 2016
SEMINÁRIOS GERAIS II
INTRODUCTION
Petrogenetic studies of igneous 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 thegeochemistry of systems involving fluids, minerals, and melts would be required.Trace elements studies have become a vital part of modern petrology and are morecapable of discriminating between petrological processes than are the majorelements.
TRACE ELEMENTS
What is a 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. Siderophile4. Chalcophile
Groupings
Behavior of the trace 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
Trace Elements Distribution
Raoult´s Law
Henry´s Law
ai = Xi
𝑎𝑖𝑗= 𝑘𝑖
𝑗𝑋𝑖𝑗
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.
Partition coefficients
Nernst distribution coefficient
𝐾𝑑 =𝐶 𝑖𝑚𝑖𝑛𝑒𝑟𝑎𝑙
𝐶𝑖𝑚𝑒𝑙𝑡
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
Geological controls on the distribution of trace elements
1) Partial Melting
a) Batch melting b) Fractional melting
𝐶𝑙𝐶0
=1
𝐹1 − (1 − 𝐹)1/𝐷𝑜𝐶𝑙
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
a) Equilibirum Crystallization b) Fractional crystallization
2) Crystal Fractionation
𝐶𝑙𝐶0
= 1 𝐷𝑥 + 1 − 𝑋
Describes completeequilibrium 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.
𝐶𝑙𝐶0
= 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
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
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).
The solubility of 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)
APPLICATION OF ACCESSORY MINERALS TO THE GRANITE PETROLOGY
1) Zircon Saturation Thermometry
ln𝐷𝑍𝑟,𝑧𝑖𝑟𝑐𝑜 𝑛 𝑚𝑒𝑙𝑡 = −3.8 − 0.85 𝑀 − 1 + 12900 𝑇
𝑇𝑍𝑟 =12900
2.95 + 0.85𝑀 + 𝑙𝑛 496000 𝑍𝑟𝑚𝑒𝑙𝑡12
Watson and Harrison (1983)
𝐷𝑍𝑟,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 geothermometerfor melt
APPLICATION OF ACCESSORY MINERALS TO THE GRANITE PETROLOGY
2) Apatite Saturation
SiO2 = is the weight fraction of silicat in the melt
𝐼𝑛𝐷𝑃𝑎𝑝𝑎𝑡𝑖𝑡 𝑒 𝑚𝑒𝑙𝑡
= 8400 + 𝑆𝑖𝑂2 − 0,5 ∙ 2,64𝑥104 𝑇 − 3,1 + 12,4 ∙ 𝑆𝑖𝑂2 − 0,5
(Harrison and Watson, 1984)
𝐷𝑎𝑝𝑎𝑡𝑖𝑒/𝑚𝑒𝑙𝑡 = is the ratio of P concentration (ppm)
in apatite in the satured melt
CASE STUDY: Allanite and 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
CASE STUDY: Allanite and 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.
CASE STUDY: Allanite and Chevkinite in A-type granites of the Graciosa Province
Chondrite-normalized REE patterns (Boynton, 1984)
Allanite
Chevkinite
CASE STUDY: Allanite and 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
Despite their low abundances in crustal rocks accessory minerals are of considerablegeochemical importance because they appear to be key tracers for many geologicalprocesses.
For instance, numerous chemical elements of geological and geochronologicalinterest, such as the rare earth elements (REEs) U, Th, Pb, Ti, Nb, V, and Ta, arecontained in these minerals.
Trace element geochemistry has been of enormous use in understanding theevolution of the Earth. A number of studies have shown that trace elements can beused to great advantage for determining the origin of granitic rocks and are usefulpetrogenetic indicators to unravel complex geologic histories preserved in igneousrocks.
SUMMARY
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 etCosmochimica 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 toMineralogy 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 ofdefect 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 cosmochimicaActa 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 toMineralogy 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. Geochimicaet 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 ofexperimental 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.
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