Geochronology and fluid inclusion study using petrography and microthermometry.

1. EFFECT OF FLUIDS ON
MINERAL
GEOCHRONOLOGY
ABHIRUP SAHA
DEPARTMENT OF GEOLOGY
PRESIDENCY UNIVERSITY, KOLKATA

2. METAMORPHIC FLUIDS
AND METASOMATISM
 Fluids can come from meteoric sources, juvenile magmatic sources,
subducted material, trapped sedimentary brines etc.
 Intergranular metamorphic fluids are usually dominated by H2O, but CO2
may also be present in some rocks, as can CH4.
 Virtually all of the intergranular HT-HP fluids that were in equilibrium with
the metamorphic mineral assemblage during peak metamorphic
conditions escaped as pressure was released upon uplift and erosion.
 Metasomatism is defined as metamorphism accompanied by changes in
whole-rock composition. It follows 2 principles: Diffusion and Inflitration.

3.  PRIMARY FLUID INCLUSIONS
 Monophase fluid Inclusion
 Biphase fluid Inclusion
 Multiphase fluid Inclusion
 SECONDARY FLUID INCLUSIONS
 Filling by the later fluids along the fractures
initiated during the mechanical and thermal stress
on the early formed crystal.

5. FLUID INCLUSIONS
There are 5 types of inclusions:
1. W Type – H2O rich
2. C Type – CO2 rich
3. S Type – Halite rich
4. M Type – Methane rich
5. Pure CO2 rich
W type and C type can occur in
both vapour and liquid phase
(VH2O & LH20) and (VCO2 & LC02).

6. EFFECTS OF FLUIDS ON
MINERALS LIKE MONAZITE
AND ZIRCON

7. Effect of Fluids on
MONAZITE
GRAINS.
Partially altered
Monazite grains

8. EFFECT OF ALKALI BEARING FLUID ON
MONAZITE GRAINS.
DISSOLUTION-REPRECIPITATION
Migration of the reaction front
through original monazite
leaving behind ThSiO4 enriched
and Y, Pb, Ca, Nd depleted zones.

9. Effect of Metamorphic Fluids on
ZIRCON GRAINS.
.Zircon shows structural damage due
to diffusion reaction caused by self-irradiation.
.Zircon replacement with undamaged structure by
a coupled dissolution-reprecipitation will produce
similar textures like irregular, curved , inward
penetrating and patchy reaction zones.

12. EFFECT OF METASOMATISM ON
ISOTOPIC AGE DETERMINATION
 Argon isotopic compositions show scatter in the calculated apparent ages
depending on the amount of fluid influx.
 40Ar/39Ar ages in the unmodified cores of all samples yield apparent ages
consistently between 84±3 Ma and 87±4 Ma.
 The calculated ages in the overprinted rims of the weakly deformed
samples are between 70±3 and 86±2 Ma,
 The samples that show an intense deformational overprint associated with
a strong mylonitisation of larger mica grains yield bimodal apparent ages:
Ages around 85±3 Ma for large mica clasts that are surrounded by the
foliation and ages around 65±4 Ma for the smaller mylonitic mica.

13. CONCLUSION
 The diffusion–reaction process causes only partial loss of radiogenic Pb, i.e. the reacted domains retain an isotopic
memory.
 Limited structural recovery in radiation damaged zircon at temperatures as low as 75°C suggests radiogenic Pb can be removed
from radiation-damaged zircon, even under low-temperature weathering conditions.
 observed enrichment, in the wt% range, of “non-formula” elements such as Ca, Al, and Fe.
 During a coupled dissolution–reprecipitation process The enrichment of “non-formula” elements is highly unlikely
to occur because during this process the parent zircon dissolves completely before new crystalline zircon reprecipitates.
 In the common case where a high-temperature magmatic zircon reacts at lower temperatures with an aqueous fluid, a coupled
dissolution–reprecipitation process will produce a zircon that contains, on average, less minor and trace elements than the parent
zircon.
 Domains formed by a coupled dissolution–reprecipitation process should yield U–Pb isotope data that are concordant so long as
they are not affected by a second hydrothermal re-equilibration event and the analysis points do not overlap with adjacent
domains.
 These re-equilibrated zircon domains will very likely remain concordant since a coupled dissolution–reprecipitation process
usually results in a reduction of U and Th contents of the zircon.
 The reaction of zircon with aqueous fluids and melts - These ages likely correspond to the timing of fluid
influx or melt production (anatexis) rather than to the age of peak metamorphic conditions.

14. REFERENCES
 Harlov and Herlington (2010), Partial high-grade alteration of monazite
using alkali-bearing fluids: Experiment and nature.
 Daniel E. Harlov • Richard Wirth • Callum J. Hetherington, Fluid-mediated
partial alteration in monazite: the role of coupled dissolution–reprecipitation
in element redistribution and mass transfer.
 Geochronology and fluid inclusion studies of the Lailisigaoer and Lamasu
porphyry–skarn Cu–Mo deposits in Northwestern Tianshan, China Mingtian
Zhu, Guang Wuc, Hongjing Xie , Jun Liu, Mei Mei.
 Fluid Inclusion Petrography and Microthermometry of Zn and Pb Deposits
of Rajpura-Dariba, Bethumni Belt, Udaipur District (Rajasthan), India
Juned Alam*, Farhat Nasim Siddiquie (Department of Geology, Aligarh
Muslim University, Aligarh, India).

15. THANK YOU