CONTENTSCONTENTS

1. INTRODUCTION2. CHEMISTRY OF REE3. PRESENTING REE DATA4. ROLE AND BEHAVIOUR OF REE IN MAGMATIC

PROCESS5. ROLE OF REE IN CRYSTAL FRACTIONATION6. REE IN PARTIAL MELTING7. APPLICATION OF REE IN IGNEOUS PETROGENESIS8. CONCLUSION9. REFERENCES

INTRODUCTIONINTRODUCTION• What are 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.

• 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.

Magma???...Magma???...

• A molten rock material that originates from the partial melting of upper mantle and lower crust usually at the depth of 10-20 kms below the earth’s surface.

• Mainly composed of silicate and also contain considerable Al, Ca, Na, Fe, Mg, K etc.

SiO2 Al2O3 Fe2O3

FeO MgO CaO Na2o K2O H2O TiO2

59.14 15.14 3.08 3.80 3.49 5.08 3.84 3.13 1.15 1.05

CHEMISTRY OF THE RARE CHEMISTRY OF THE RARE EARTH ELEMENTSEARTH ELEMENTS

• They all form stable 3+ ions of similar size. (Some REE exist in oxidation states other than 3+ but the only ions of geological importance are Ce4+ and Eu2+.)

• Lanthanide contraction.

• Small differences in sizes and behaviour are exploited by a number of petrological processes causing the REE series to become fractionated relative to each other.

• It is this phenomenon which is used in geochemistry to probe into genesis of rock suites and unravel petrological processes.

PRESENTING REE DATAPRESENTING REE DATA• REE concentrations in rocks are usually normalized to a

common reference standard, which most commonly comprises the value for chondritic meteorites. They were chosen because they are thought to be relatively unfractionated samples of the solar system dating from the original nucleosynthesis.

• The REE are normally presented on a concentraion Vs atomic no. diagram on which concentration are normalized to the chondritic reference value.

• The plotted position of Eu lies off the general trend defined by other elements on a REE diagram and this may be defined as Europium anomaly.

– if the plotted composition lies above the general trend then the anomaly is described as positive anomaly and if it lies below the trend then the anomaly is said to be negative.

Europium anomaly may be quantified by comparing the measured concentration (Eu) with an expected conc. Obtained by interpolating between the normalized values of Sm and Gd (Eu*).

Thus , EU/Eu* = Europium anomaly.

> 1.0 indicates +ve anomaly.< 1.0 indicates -ve anomaly.

Chondrite values used in normalizing REE (concentration in ppm)

ROLE AND BEHAVIOUR OF RARE ROLE AND BEHAVIOUR OF RARE EARTH ELEMENTS IN MAGMATIC EARTH ELEMENTS IN MAGMATIC PROCESSPROCESS• Size of REE in LREE are slightly larger than HREE – this difference

allow them to slowly enriched or excluded as they enter melts of various composition.

• Although all REE are incompatible, this smooth change in ionic radii means that there is also a progression in terms of the degree of compatibility.

• The progressive change in the behaviour of the REE with respect to partitioning in garnet makes them one of the most useful tools in igneous geochemistry.

• The charge and the comparatively large radius (Lu 0.85 Å – La 1.06 Å) of the Rare Earth coupled with their general low concentration, suggest that they would show relatively little tendency to replace the major elements during magmatic crystallization, and this is borne out by the concentration of Rare Earth as individual minerals in pegmatite.

Fig : Ionic radii (in picometres, 1×10−12 m) of the lanthanide rare earth elements (3+ state except where noted). Promethium (Pm) has no isotope with a half-life longer than 5 years.In Fig two alternative oxidation states and ionic radii are shown for Eu (Eu2+) and Ce (Ce4+). Alternative oxidation states depending on how oxidising the surrounding mantle conditions are

In highly oxidising conditions Ce3+ can be oxidized to Ce4+. Similarly in very reducing conditions, Eu2+ may form. Eu, when in its 2+ state, substitutes for Ca2+ in plagioclase feldspar more readily than the other REE.

REE are fractionated during magmatic processes and because of their smooth change in ionic radii the rare earths are sensitive indicators of different igneous processes.

REE IN PARTIAL MELTING OF REE IN PARTIAL MELTING OF MAGMA.MAGMA.• The incomplete melting of a rock composed of minerals

with differing melting points. When partial melting occurs, the minerals with higher melting points remain solid while the minerals whose melting points have been reached turn to magma.

• Rare Earth elements are particularly useful in this regard:

• Ionic radii decrease with increasing atomic number. i.e. La is largest, Lu is smallest.

• Thus the degree of incompatibility decreases from La to Lu.

REE concentrations have been normalized by dividing the concentration of each element by the concentration found in chondritic meteorites.

REE IN CRYSTAL REE IN CRYSTAL FRACTIONATIONFRACTIONATION

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.

• 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.

APPLICATION OF REE IN APPLICATION OF REE IN IGNEOUS PETROGENESIS.IGNEOUS PETROGENESIS.• REE diagrams are commonly used to analyze igneous

petrogenesis.• Lanthanide concentrations and distributions in lavas are

important indicators of source materials and melting process.

• Basalts of oceanic islands have very different lanthanide distributions from those of ocean floor basalts in which most are alkali-olivine basalts or olivine tholeiites with lanthanide distributions showing strong relative enrichment with decreasing atomic number.

• REE can be used to distinguish between high pressure and low pressure sources of mantle-derived melts.

CONCLUSIONCONCLUSION• REE is a group of elements comprises of 15 elements

from La to Lu.• REE have proven to be very important for petrogenetic

interpretations.• REE are widely used in geochemistry to probe into the

genesis of rock suites and unravel petrological 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.

• REE diagrams are also useful in identifying which phase or phases fractionate from a magma.

REFERENCESREFERENCES• C.S. PICHAMUTHU (1985), ARCHEAN GEOLOGY,

Oxford & IBH publishing co., Pp-96 to 109.• Brian Mason and Carleton B. Moore (1982), Principales Of

Geochemistry, Fourth Edition, Pp.130 to 131.• Hugh Rollinson, Using geochemical data: evaluation,

presentation & interpretation, Pp-102 to 142.• John D. Winter (2012), Principles of Igneous and

Metamorphic petrology, second edition, Pp. 163 to 169.• http://www.geology.com• http://nasa/astrophysicsdatasystem/annualreviews• http://wikipidia.com