REE in igneous petrogenesis with special reference to Basaltic rocks.

1. Rare Earth Elements(REE) In
Igneous Petrogenesis with
special reference to Basaltic
rocks
By
Amit Kumar Mishra
Centre Of Advanced Study In Geology,
University Of Lucknow

2. The Rare Earth Elements (REE)

3. Lanthanide Contraction-Ionic radii of the REEs from La to
Lu continuously decreases.
Figure : Showing Lanthanide contraction(Taken from Google).

4.  incompatible elements are concentrated in
the melt
(KD or D) « 1
 compatible elements are concentrated in
the solid
KD or D » 1
Partition Coefficient :
Di = [i] mineral / [i]melt

5. Basalt
-Aphanitic (fine-grained) igneous rock
-45-55% silica(SiO2) and where at least 65% of the rock is feldspar in the form
of plagioclase.
- The principal volcanic rock in many mid-oceanic islands
Mid oceanic ridge basalt-
-Mid oceanic ridge basalt (MORB) is a basalt commonly erupted
only at ocean ridges.
- Mid oceanic ridge basalt (MORB) is a tholeiitic basalt commonly erupted only at
ocean ridges and is characteristically low in incompatible elements. In the mid
oceanic ridge basalt the ratio of the LREE / HREE is less than 1,that indicates that
source will be enriched in HREE.

6. MORB are distinguish from OIB based on their comparatively low concentration of
LILE(Large ion lithophile elements including K,Rb,Cs),Light REE that are consider
highly incompatible during melting of mantle mineral assemblages.
Ocean island basalts or OIBs
OIBs are basaltic rocks found on many volcanic islands away from
tectonic plate boundaries typically associated with hot spots.
As with MORB, the dominant magma type for oceanic intraplate
volcanism is basalt, which is commonly called ocean island basalt or OIB.
OIB magmas-
1.Tholeiitic (dominant type).
2.Alkaline series (subordinate).

7. The LIL trace elements (K, Rb, Cs, Ba, Pb2+ and Sr) are incompatible
and are all enriched in OIB magmas with respect to MORBs.
Continental Flood Basalt (CFB)-
On the continents it covers large areas(100-1000 square km).
CFB indicates that it is formed episodically.It requires huge energy.
The lava must be fluid ,because of this it covers large areas.
CFB and OIB are formed within the plate but MORB is formed at margin of the plate.
In the continental flood basalt the ratio of the LREE / HREE
is more than 1,that indicates that source will be enriched in LREE.

8. -
Figure : Showing MORB and OIB (From the winter)

9. Thanking you…