1. ROLE OF TRACE ELEMENTS IN
PETROGENESIS
GUIDED BY:
Prof. K.N. Prakash Narsimha
PRESENTED BY:
GOKUL ANAND
Mysore University
2. MAGMA
Hot or semi-fluid material below or within the earth crust
from which lava and other igneous rock is formed on
cooling.
Magma often collects in magma chambers that may
feed a volcano or turn into a pluton.
Besides molten rock, magma may also contain
suspended crystals, dissolved gas & sometimes gas
bubbles
Temperatures of most magmas are in the range
700ºC to 1300ºC
3. CLASSIFICATION OF MAGMA
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. GEOCHEMISTRY OF MAGMA
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 are those which occur in very low
concentrations in common rocks (usually < 0.1 % by
weight).
0
50
100
Trace Elements
8. Geochemical Analysis Of Trace Elements Can Be Done By
These
Techniques
X-ray Fluoresence Spectroscopy
(XRF)
Atomic Absorbtion Spectrometry
(AAS)
ICP-OCP-Optimus 5300
Spectrometer
9. Concentrations are commonly expressed in parts per
million(ppm)
For eg: chromium = 150ppm
Trace elements tend to concentrate in fewer
minerals, and are therefore more useful in
formulating models for magmatic differentiation, and
in some cases to predict the source of a particular
magma.
10. The concentration of trace elements will vary with
the rock type; whereas Ni and Cr show higher
concentrations in mafic and ultramafic rocks, Zr
and Rb are more concentrated in acidic rocks.
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
11. TYPES OF TRACE ELEMENTS
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,
13. Large Ion Lithophile Elements
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.
14. Higher Field Strength Elements
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.
15. 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
Eg: Ni, Co, Cr, and Sc.
16. 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.
17. Behaviour Of Trace Elements In Magmatic
Processes
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.
18. Applications of trace elements and Rare Earth Elements to
Igneous Petro genesis
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).
In this case rather than use the absolute concentrations of trace elements
(which may have been affected by such post-magmatic processes as
weathering, alteration or metamorphism),ratios of relatively immobile trace
elements (as these are least affected by post magamatic processes.)
21. 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.
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.
Geochemical analysis of trace elements can be done by XRF,AAS & ICP
techniques.
Conclusion
22. 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
White W.M., 2009, Geochemistry. pp 258-312
en.wikipedia.org/transition_elements
REFERENCES
