role of trace elements in petrogenesis

ROLE OF TRACE ELEMENTS IN

PETROGENESIS

GUIDED BY:Prof. K.N. Prakash Narsimha

PRESENTED BY:GOKUL ANAND

Mysore University

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 bubblesTemperatures of most magmas are in the range 700ºC to 1300ºC

CLASSIFICATION OF MAGMAMagma = 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.

Basaltic

Granitic

Andesitic

MAGMA

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

TRACE ELEMENTS

IN IGNEOUS PETROGENESIS

Trace elements are those which occur in very low concentrations in common rocks (usually < 0.1 % by weight).

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Trace Elements

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

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.

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

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, and Fe.

CLASSIFICATION OF TRACE ELEMENTS

Trace Elements

LILE HFSETransition Elements REE

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.

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.

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.

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.

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.

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 type2- 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.

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.

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.)

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

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

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