3. • PHASE RULE APPLICATION
1) ON THE LIQUIDUS ,PHASES ARE FO+ L AND DEGREE OF FREEDOM AT FIXED PRESSURE IS 1.
FP = 2-2+1
2) AT REACTION POINT, THE PHASES ARE THREE AND THEREFORE FP=0.
• A LIQUID OF BULK COMPOSITION (1) LIES TOWARDS FO SIDE AND IS INITIALLY
POORER IN SILICA THAN ENSTATITE.
• SUCH A LIQUID ON COOLING DOWN AT F BEGINS TO PRODUCE FO CRYSTALS.
• CRYSTALLIZATION OF FO CONTINUES CONTINUES UNTILL THE LIQUID REACHES
REACTION POINT R WHEN THE TEMPERATURE IS 1557 DEGREE CELSIUS.
• AT R, THREE PHASES EXIST , NAMELY FORSTERITE, ENSTATITE AND MELT.
• THE TSC WHICH WAS FO NOW MOVES HORIZONTALLY ALONG 1557 DEGREE CELSIUS
ISOTHERM TOWARDS COMPOSITION 1S.
• THE BULK COMPOSITION ON COMPLETION OF CRYSTALLIZATION AT R COMPRISES
CRYSTALS OF FO + EN AND NO LIQUID IS LEFT OVER TO PROCEED WITH FURTHER
CRYSTALLIZATION ON ENSTATITE LIQUIDUS.
4. • FOR A MELT COMPOSITION AT POINT (2) EQUIVALENT TO BULK MGSIO3(
LOCATED VERTICALLY ABOVE EN), THE LIQUID COOLS UNTILL FO BEGINS
TO CRYSTALLIZE ON THE LIQUIDUS.CRYSTALLIZATION CONTINUES
UNTILL R IS REACHED.
• A LIQUID OF COMPOSITION (3) JUST TO RIGHT OF EN( TOWARDS SIO2
END COMPONENT) BUT TO THE LEFT OF R , WILL HAVE A SLIGHTLY MORE
COMPLEX HISTORY OF CRYSTALLIZATION. THE LIQUID PRODUCES
CRYSTALS OF FO ON LIQUIDUS AND THESE CRYSTALS WILL REACT WITH
THE LIQUID AT INVARIANT POINT R. SOME LIQUID WILL REMAIN EVEN
AFTER COMPLETE CONVERSION OF FO TO EN BUT STILL AWAY FROM THE
INITIAL MELT COMPOSITION.
• NOW SINCE ONE OF PHASES DISAPPEARS(?) , THE VARIANCE IS ENHANCED
AND LIQUID LEAVES INVARIANT POINT AND ENTERS UNIVARIANT
LIQUIDUS CURVE RE.
• THE REST OF CRYSTALLIZATION IS SIMILAR TO BINARY EUTECTIC
RELATIONSHIP,UNTILL EUTECTIC POINT E AT 1543 DEGREE CELSIUS
5. • LIQUID OF COMPOSITION (4) LOCATED ABOVE POINT R WILL PRODUCE FO
CRYSTALS WHICH IS INSTANTLY CONVERTED TO ENSTATITE. COMPOSITION
MOVES ALONG ENSTATITE LIQUIDUS AND FINALLY EUTECTIC CRYSTALLIZATION
OF ENSTATITE AND CRYSTOBALITE WILL OCCUR AT 1543 DEGREE CELSIUS.
• LIQUID OF COMPOSITION (5) TO RIGHT OF POINT E WILL BEHAVE SIMPLY AS A
LIQUID OF BINARY EUTECTIC SYSTEM.THE FINAL PRODUCT IS A MIXTURE OF EN
AND CRYSTOBALITE.
6. LIQUID IMMISCIBILITY
• THE AREA SHOWN “2 LIQUIDS” , SINGLE LIQUID UNMIXES INTO TWO FRACTIONS WITH
CONTRASTING COMPOSITIONS: ONE IS ALMOST PURE SILICA , THE OTHER IS ENRICHED IN
MAGNESIAN COMPONENT.
• EG: IN IRON BEARING SYSTEMS THESE LIQUIDS ON SUPERCOOLING PRODUCE TWO TYPES OF
GLASSES ONE COLOURLESS (SI RICH) AND OTHER BROWN ( IRON RICH) AND DISTINGUISHED IN
THIN SECTION, ONE OCCURING AS DROPLETS OR GLOBULES WITHIN THE OTHER.
• NOW THESE TWO LIQUID FRACTIONS ARE STRUCTURALLY DIFFERENT TOO. PURE SIO2 WILL
HAVE FRAMEWORK SILICATE STRUCTURE ESSENTIALLY TETRAHEDRA OF SI-O-SI BONDS.
• BUT IN LIQUID MAGNESIUM FRACTION, IT BREAKS THE SILICA NETWORK BY FORMATION OF M-
O-M BONDS.
• SUCH DEPOLYMERIZATION IS CHARACTERISTIC OF PYROXENES WHERE SI CHAINS ARE
SEPERATED BY MODIFIER CATION.
7. PETROGENETIC SIGNIFICANCE
• HELPS TO UNDERSTAND THE FRACTIONAL CRYSTALLIZATION BEHAVIOUR OF
MAGMAS.
• SINCE FO HAS A REACTION RELATION WITH THE OTHER END COMPONENT
VIZ.SIO2 , CRYSTALS OF FO ONCE FORMED ARE NOT “MAGMATICALLY DEAD”.(?)
• DEVELOPMENT OF OPX RIM AROUND OLIVINE, ( IN GABBROS AND DOLERITES)
CAN BE INTERPRETED IN TERMS OF REACTION OR PERITECTIC RELATION.WHAT
IT DOES IS AN ARRESTING OF OLIVINE BY THE PROTECTIVE ENSTATITE
SHELL.THUS PREVENTING OLIVINE FROM FURTHER REACTION THEREFORE
RESULTING IN ENRICHMENT OF SI IN RESIDUAL LIQUID. THEREFORE RESULTING
IN DIVERSIFICATION OF MAGMA.
• MAGMAS CORRESPONDING TO PERITECTIC COMPOSITION ARE OFTEN EVIDENT
IN IGNEOUS ROCKS LIKE PERIDOTITE .(?)
• INCONGRUENT MELTING BEHAVIOUR OF ENSTATITE DISAPPEARS AT 5KB
8. THERMAL DIVIDE
• Incongruent melting behaviour of enstatite
disappears at 5 kb pressure and phases in the
system assume the nature of a eutectic rather
than peritectic. Thus two eutectics (E, and E,)
separated by thermal divide develops in the
system SiO ,- Mg,SiO, under high pressure as
diagrammatically represented in Fig. 10.10. Such
a thermal ridge can not be crossed by crystallising
magmas and transition of magma composition
from one to the other, is not possible. Thus
olivine normative basalt is separated from quartz
normative basalt by a thermal divide.
9. THANK YOU.
HAVE A NICE DAY.
REFERENCE: IGNEOUS PETROLOGY BY MIHIR K BOSS
