Accessory minerals have an influence on petrogenetic studies far greater than their abundances might suggest. Accessory phases as zircon, monazite, apatite, allanite, rutile, baddeleyite, titanite and xenotime differ widely from one another in structure and composition. This is because they tend to incorporate elements that are important tracers of geochemical processes (rare earth elements) as well as valuable geochronometers (U and/or Th). It is important, therefore, to understand the growth histories of accessory phases through the distribution of trace elements in these minerals. In order to understand the importance of accessory minerals in igneous petrology was considering the chemical properties of trace elements, with particular emphasis on how they behave in nature. It was introduce the quantitative means of describing trace element distribution, the principles of trace element modeling, the use of mineral/melt distribution coefficients and the application of saturation temperaturates in zircon and apatite to determinate the crystallization parameters in the petrogenesis of igneous rocks (principally in granites).

1. 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

2. 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 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.

3. 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. Siderophile
4. Chalcophile
Groupings

4. 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

5. Trace Elements Distribution
Raoult´s Law
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.

6. Partition coefficients
Nernst distribution coefficient
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

7. Geological controls on the 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

8. a) Equilibirum Crystallization b) 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

9. 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

10. 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).

11.  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)

12. APPLICATION OF ACCESSORY MINERALS 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

13. APPLICATION OF ACCESSORY MINERALS 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

14. CASE STUDY

15. 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

16. 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.

17. CASE STUDY: Allanite and Chevkinite in A-type granites of the Graciosa Province
Chondrite-normalized REE patterns (Boynton, 1984)
Allanite
Chevkinite

18. 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

19. Despite their low abundances 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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