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

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

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


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.


Chondrite-normalized REE patterns (Boynton, 1984)

Allanite

Chevkinite


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.

REFERENCES

Education

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