Trace elements play an important role in igneous petrogenesis and can be used to understand magmatic processes. Trace elements are classified based on their behavior into incompatible and compatible elements. Rare earth elements (REE) are especially useful as their near-parallel patterns can indicate crystal fractionation. REE and other trace element analyses help determine the source depth of primary magmas and identify fractionating phases. Trace element discrimination diagrams also allow determination of paleotectonic settings. Overall, trace element studies provide insights into magma differentiation, source characterization, and petrogenetic modeling.

1. ROLE OF TRACE ELEMENTS IN
IGNEOUS PETROGENESIS

2. 1. INTRODUCTION
2. MAGMA – COMPOSION ,ORIGIN AND CLASSIFICATION
3. GEOCHEMISTRY OF MAGMA
4. TRACE ELEMENTS
5. CLASSIFICATION OF TRACE ELEMENTS
6. MECHANISM AND BEHAVIOUR OF TRACE ELEMENTS IN
MAGMATIC PROCESSES
7. ROLE OF REE IN CRYSTAL FRACTIONATION
8. APPLICATION OF TRACE ELEMENTS AND REE IN IGNEOUS
PETROGENESIS
9. CONCLUSION
10. REFERENCES

3. • Molten rock that originates from the partial melting of upper mantle and lower crust
usually at the depth of 10-200km below the surface
• Defined as molten rock material along with the volatile components
• Solidification of magma dose not takes place at definite temperature
• Magma can characterized by
-composition - predominantly siliceous
-temperature - 500o c -1300oc
- mobility, i.e. viscosity
• Viscosity is depends on % of Si contain in the magma, and
accordingly it can be classified on the basis of above characteristics.
Si rich magmas- are acidic, and more viscous that piles at one place,
Si poor magmas- are basic, less viscous, and occupies larger area.

4. Magma = liquid (molten rock) + crystals + dissolved gasses (volatiles)
• As result of melting of crust yield’s most Si rich magmas that also contain
considerable Al, Ca, Na, Fe, Mg, K and several other elements in lesser
quantity.
• Melting of Earth’s upper mantle, which is composed of rocks that contain
mostly ferromagnesian silicates thus magma from this sources contain
comparatively less amount of silica and more iron and magnesium contain

5. • The primary constituent of magma is silica, which varies to distinguish
the magmas and classified as felsic to mafic.
• Most magmas are the part of the ranges from mafic magma to silicic
magma
• Si magmas produce the rock of the granite-rhyolite family which composed
of Quart, K-feldspar, Na-plagioclase, and minor amount of biotite and
amphibolite.
• Mafic magmas produce the rocks of gabbro, basalt family, which composed
of Ca-plagioclase, pyroxene with lesser amount of olivine and lesser or no
quartz.

6.  Major elements:
Comprise most of the rock
Expressed as weight (wt.) % oxides,
each >0.1% SiO2, Al2O3, FeO, MgO, CaO , Na2O, K2O, H2O.
Analysed by XRF, ICP-MS
 Minor elements:
usually 0.1 - 1% ,
TiO2,MnO, P2O5 ,CO2.
 Trace elements:
Present in concentrations <0.1 %
Expressed in ppm or ppb
Analysed by XRF, ICP-MS, INAA
 Volatile elements:
H2O, CO2, SO4
Rare gases: He, Ar, Ne, etc.
Analysed by spectroscopy or mass spectrometry

7. TRACE ELEMENTS IN IGNEOUS PETRO GENESIS
Trace elements are those which occur in very low concentrations in common rocks.
This trace elements generally of less percentage that is less than 0.1% by weight,
hence they are expressed always in PPM.
For ex: chromium = 150PPM
In geochemistry specially a rock chemistry simple analysis is done
in respect of major oxide like K2O , Sio2 and Tio2 etc.
In addition to this trace elements occurs in minor quantity and these
are known as minor elements.
They are basically Ga, Cr, Mo, Li, Ni, Co, Cu, Zr, Y, pt, La, Sr,
Ba, Rb.

8. Contin…..
• Trace elements are observed only in a few minerals. They are useful in identifying the
magmatic differentiation and source of magma.
• Concentration of trace elements varies from rock to rock.
• EX: ultramafic rocks show more Ni and Cr, acidic igneous rock indicates Zr and Rb.
Trace elements during crystallization of magma gets substituted for a
major elements in the structure or they remains in the magma.
• Eg. Potassium never forms its own phase in mid-ocean ridge basalts (MORB), its
concentration rarely exceeding 1500 ppm; but K is certainly not a trace element in
granites.
• Trace elements can be classified as compatible and incompatible.

9. • Incompatible elements :
 K, Rb, Cs, Ta, Nb, U, Th, Y, Hf, Zr, Most have a large ionic radius.
 Do not easily fit into the crystal structure of minerals in the mantle,
 Mantle minerals like olivine, pyroxene, spinel, & garnet do not have
crystallographic sites for large ions.
• Compatible elements :
 Ni, Cr, Co, V, and Sc, which have smaller ionic radii
 Fit easily into the crystal structure of minerals in the mantle.
 crystallographic sites that normally accommodate Mg, and Fe.

10. Based on geochemical characters, trace elements are
classified as
1. Large Ion Lithophile Elements (LILE):-
2. Higher Field Strength Elements
(HFSE):-
3. Transitional Elements
4. R.E.E. (Rare Earth Elements)

11. 1) Large Ion Lithophile Elements (LILE) :-
The ionic radius is large These are incompatible.
They are more concentrated in liquid phase of the magma,
These are found in olivine opx, cpx and garnet largely "incompatible"
particularly with respect to mantle phases (Ol, Opx, Cpx, Gt, .. etc)
Examples: K, Rb, Sr and Ba.
2) Higher Field Strength Elements (HFSE)
These are also concentrated in liquid phase but are compatible
They are seen in sphene, zircon and apatite (SZA)
They are basically Titanium, Th, U, Nb and Hf.

12. 3). Transitional elements
Small ionic radii, and are either bi- or tri-valent .
strongly partitioned in the solid phases that crystallize during
the early stages of magmatic evolution "compatible" with
mantle phases
Ex: Ni, Co, Cr, and Sc.
4) Rare earth elements
• A group of elements comprising the 15 elements from Lanthanum (At. no. 57)
to Lutetium (At. No. 71)
- Yttrium (At. No. 39) and Scandium (At. no. 21) are also sometimes included in
this group.

14. RARE EARTH ELEMENTS
 15 elements from mass 57 to 71 such as : La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb,
Dy, Ho, Er, Tm, Yb, Lu
 Useful because similar in geochemical behaviour
 Trivalent except Eu can be Eu3+ or Eu2+, depending on fO2
 The REE studies have important applications in igneous, sedimentary and metamorphic
petrology.
The low atomic number members of the series are termed The Light Rare Earth
Elements (LREE) i.e. La -Pm.
Those with higher atomic numbers called the heavy rare earths elements (HREE) i.e.
Er-Lu. And the middle members of the group i.e. Sm- Ho are known as the Middle
REE.

15. Behavior of trace elements in various degrees of
melting:
When a mantle rock begins to melt, the incompatible elements will be
ejected preferentially from the solid and enter the liquid.
A low degree melting of a mantle rock will have high concentrations of
incompatible elements.
As melting proceeds the concentration of these incompatible elements will
decrease because
(1) there will be less of them to enter the melt, and
(2) their concentrations will become more and more diluted as other elements
enter the melt. Thus incompatible element concentrations will decrease with
increasing % melting.

16.  When a trace elements has the same charge and an ionic radius similar to
that of the major element, then it being captured in a crystal lattice
containing the major element.
 when a trace element having similar ionic radius but lower the charge than
that of major element ( or same charge but greater radius), it is said to be
admitted in to the crystal lattice containing the major element.
Ex. Ga3+(0.62Ao) is captured in aluminium minerals, and Hf4+(0.78Ao) is
captured in zirconium minerals.
In capture and admission of ion of different charge the charge balance is
maintained by concomitant substitution elsewhere in the crystal lattice.

17. REE IN CRYSTAL FRACTIONATION
• Crystal Fractionation:
A natural mechanism that can remove
crystals from the magma or at least
separate the crystals so that they can no
longer react with the liquid.
• The REE patterns produced by higher
percentages of crystal fractionation show
higher concentrations, yet the patterns remain
nearly parallel to one another. Thus, a suite of
rocks formed as a result of crystal fractionation
should show nearly parallel trends of REE
patterns.

18. • 1- Testing models of magmatic differentiation using
trace elements:
On calculating the concentrations of trace elements remaining in the liquid
determine how much partial melting is needed to produce a specific magma
from a given rock type
• 2- Determination of the depth of generation of a primary
magma:
magmas produced by small degrees of partial melting
-at shallow depths will be depleted in Sr
-from intermediate depths will be depleted in V and Cr,
-from depths > 80 km will be depleted in HREE.

19. 3 - Prediction of the phases fractionating from a magma:
Identification of the phases which have fractionated from a magma undergoing
fractional crystallization.
Separation of:
(a)Plag depletes the remaining melt in Sr and Eu,
(b) Ol depletes it in Ni and Co,
(c) spinels deplete it in V, Cr and possibly Zn,
(d) K-spar in Ba and Rb, ... etc.
4- REE and REE diagrams:
 REE are very useful for petrogenetic interpretations.
 REE diagrams are also useful in identifying which phase or phases fractionate
from a magma,
 In order to identify such phases, it is necessary to know which REE are
preferentially incorporated in which phases.
 REE diagrams are also used to determine the type of basalt.

20. 5- Discriminant diagrams:
Trace elements can also be used to identify the paleotectonic
setting of some volcanic rocks
(i.e. to determine where they were erupted).
(which may have been affected by such post-magmatic
processes as weathering, alteration or metamorphism),

22.  Trace elements are useful in formulating models for magmatic
differentiation, in predicting the source of a particular magma.
 Trace elements occur in very low concentrations in common rocks.
 Large ion lithophile elements (LILE) have large ionic radii, and low
charges.
 High field strength elements (HFSE) excluded from mantle phases and more
concentrated in residual liquids.
 Trace elements are useful for determination of the depth of generation of a
primary magma.
 REE have proven to be very important for petro genetic interpretations.
 REE are widely used in geochemistry to probe into the genesis of rock suites
and unravel petro logical processes.
 During crystal fractionation the ratios of incompatible elements show little
change, and that we can use this factor to distinguish between crystal
fractionation and partial melting.
Conclusion

23. • Brian Mason and Carleton Moore B.,(1982) Principales Of
Geochemistry. pp.75-150
• Gilbert Hanson N.,1980,Rare Earth Elements in Petrogenessis of
igneous systems:Ann.Rev.Eatrh Planet.Sci.v.8, pp.371-406
• James Moneroe S.,and Reed Wicander, 2006, Changing Earth. pp.88-
102
• Joseph Arth G., 1976, Behavior of trace elements during magmatic
processes: U.S. Geol. Survey. v.4,no.1,pp.41-47
• Pichamuthu C.S.,1989, Archaean Geology, pp 75-150
• White W.M., 2009, Geochemistry. pp 258-312.
• Williams Helen, 1997, High Temprature Geochemistry. pp .102-170
• http://earthscience.brookscole.com/ree4e
• http://geology.com
• http://nasa/astrophysicsdatasystem/annualreviews