Fluid inclusion and mineral chemistry data was presented for samples from the type charnockite area near Pallavaram (Tamil Nadu, India). The results indicate the presence of a dense CO2 fluid phase, but the data cannot distinguish between influx of this fluid from elsewhere or localized migration of CO2-rich fluids associated with dehydration melting

Hansen, E.

Hunt, W.

Jacob, S. C.

Morden, K.

Reddi, R.

Tacy, P.

English

NASA Technical Reports Server

N09- 2 2 2 0 9  f - 3  
Evidence for C02-rich fluids in rocks from the "tvDe" charnockite - -  
area near Pallavaram, Tamil Nadu 
E. Hansen , W. Hunt , S. C. Jacob , K. Mordenl, R. Reddi2 , P. 
Tacy 
Authorship in alphabetical order 
61 
1 1 2 
1) Geology Department, Hope College. Holland, Michigan 
2) Department of Applied Geology, University of Madras, Madras 
Charnockitic rocks were first described by Sir Thomas Holland 
(1900) from the hills around the village of Pallavaram south of 
Madras. In this area a series of acid to intermediate magmas 
intruded an interbedded sequence of predominatly pelitic and mafic 
rocks which were latter metamorphosed to high grade (Subramaniam, 
1959). According to Weaver (1980) chemical trends in the 
charnockites indicate a period of metasomatism and partial melting 
immediately preceeding the granulite-facies metamorphism which he 
suggests was due to an influx of C02-rich fluids. On the other 
hand, Bhattacharya and Sen (1986) concluded that no pervasive 
fluid was present during the high-grade metamorphism which 
involved internal buffering of fluids and dehydration melting. 
They based their conclusions largely on calculations of 
metamorphic water activities which are different for different 
rock types and show systematic variations with mineral chemistry. 
We have examined rocks from a quarry southeast of Pallavaram for 
evidence indicating the concentration of carbon-dioxide in the 
metamorphic-fluid phase. 
Charnockitic rocks are the major rock type exposed in the 
quarry. These rocks are cut by coarse-grained dykes and veins 
also made up of dark charnockite. Mafic granulites occur as 
enclaves. One especially large mafic enclave contains light 
colored veins made up of calcite and scapolite with smaller 
amounts of diopside and quartz. Garnets are concentrated at the 
margins of these veins. The dark host rock has a granulitic 
texture and is made up of hornblende, diopside, plagioclase and 
quartz with sporadic garnet. 
Results of EDS electron-microprobe analyses on minerals from 
the mafic host and calcite-bearing veins are given in Table 1. 
W . D . S .  analyses of the scapolite indicate only small amounts (less 
than 0.1 wtX) of sulfur or chlorine. The mineral assemblage at 
the edges of the vein allow us to estimate the C02 concentration 
in the metamorphic fluid using the reaction: 
Calculations using the thermodynamic data of Holland and Powell 
(1985) indicate a nearly pure CO2 fluid under the metamorphic 
temperatures (750-800OC) and pressures (6.5 - 7.5 kbars) obtained 
for the area by Bhattacherya and Sen (1986). 
Carbonic fluid inclusions are abundant in a sample of one of 
the coarse-grained charnockitic veins collected near the mafic 
enclave. The vein is roughly granitic in composition containing 
the assemblage alkali-feldspar, plagioclase, quartz, orthopyroxene 
and opaques. The fluid inclusions occur in planar arrays and are 
hence secondary or pseudosecondary. Melting temperatures obtained 
on these inclusions are all within l0C of the melting temperatures 
of a pure C02 standard. Thus the fluid is nearly pure C02 
although small (up to about 20%) amounts of water may be present 
meionite + 5 calcite+ 3 quartz = 3 grossular + CO2. 
https://ntrs.nasa.gov/search.jsp?R=19890012838 2020-03-20T02:31:48+00:00Z
62 
Evidence for C02-rich Fluids 
Hansen, E., Hunt, W., Jacob, S.C., firden, K. , Reddi, R. , TaCY, p- 
Si02 
A1203 
FeO 
MnO 
MgO 
CaO 
Na20 
Total 
Si 
A 1  
Fe 
Mn 
Mg 
Ca 
Na 
Table 1 - Mineral Compositions 
Garnet 
Host Rock Vein 
37.6 37.7 
21.5 21.0 
27.4 25.9 
1.3 0.8 
2.3 1.5 
10.8 13.3 
2.94 2.99 
1.99 1.96 
1.79 1.72 
0.09 0.06 
0.26 0.18 
0.91 1.13 
Diopside Scapolite Plag 
Host Rock Vein Vein Host Rock 
50.5 48.3 42.9 49.7 
2.3 2.5 30.2 33.1 
16.8 24.1 
8.9 4.5 
21.9 21.7 20.4 15.1 
1.94 1.92 6.59 2.26 
0.10 0.12 5.47 1.77 
0.54 0.80 
0.52 0.27 
0.90 0.93 3.37 0.74 
0.47 0.19 
as a thin, undetected, immiscible layer against the walls of the 
incgusions. Homogenization temperatures cluster between -$C and 
-18 C and hence have specific volume between 43 and 45 cm /mole. 
Isochores for those two volumes, calculated with the equations of 
Touret and Bottinga (1979) are given in Figure 1. The box in the 
figure outlines the metamorphic conditions for the area deduced by 
Bhattacharya and Sen (1986). Isochores for the denser fluid 
inclusions pass through this box. If small amounts of water are 
present as an immiscible fluid, then the isochores in Figure 1 
should be moved to slightly higher pressures and this would 
increase the overlap with the metamorphic conditions. 
Figure 1 
L b 
+oo 600 800 - 
T empcraturr ('C) 
Evidence for C02-rich Fluids 
Hansen, E., Hunt, w., Jacob, S.W., MOrden, K . ,  Reddi, R., Tacy, P. 63 
The fluid inclusions indicate that a dense COB-rich fluid was 
present at some point in the history of the charnockite. It is 
difficult to see how the mineral assemblages in the charnockite or 
its igneous precursor could have generated CO2, hence it probably 
flowed in from the outside. The densities of these fluids are 
approximately consistent with entrapment at peak metamorphic 
conditions as would be predicted by the COB-influx model of Weaver 
(1980). However, high densities are no guarantee that the fluid 
actually represents the peak metamorphic fluid nor is the presence 
of COB-rich fluids necessarily incompatible with other models of 
granulite-facies metamorphism (Crawford and Hollister, 1986). 
An influx of C02 should lead to "carbonated" mineral 
assemblages in rocks of the appropriate bulk compositions. This 
may be the case with the carbonate-bearing veins in the mafic 
enclave, although it is by no means certain that the veins are 
metasomatic. The sporadic occurrence of garnet in the host rock 
without scapolite or calcite suggests lower C02 fugacities than in 
the veins and hence indicates some heterogenity in the metamorphic 
fluid phase. According to Subramaniam (personal communication, 
1983) wollastonite-bearing veins had also been found in the mafic 
enclaves in this quarry. Wollastonite is not stable in the 
presence of a CO2-rich fluid during granulite facies conditions 
(Valley, 1985) and hence its presence may indicate large 
heterogenites in the metamorphic fluid. Unfortunately, we were 
unable to locate any of the wollastonite-bearing veins and 
have little indication of how they fit into the metamorphic 
history of the area. 
We have evidence that a dense COB-rich fluid was once 
present in the rocks exposed in the type charnockite area around 
Pallavaram. This gives some support to the idea proposed by 
Weaver (1980) that the granulite-facies metamorphism in this area 
was due to the influx of a COB-rich fluid. However, important 
questions still remain about the timing of CO2 influx, the 
pervasiveness of the CO2-rich fluid, and its source. Thus, for 
example, our results are also consistent with a model in which the 
bulk of the granulite-facies metamorphism occurred through 
dehydration melting accompanied or followed by some localized 
migration of CO2-rich fluids. This is a model very similar to the 
one proposed by Bhattacharya and Sen (1986). More information 
will be needed, especially about the thermal history of the area 
and stable isotope compositions, before the role of the COB-rich 
fluid can be resolved. 
References 
Bhattacharya and Sen (1986) Journ. Petrol. 1'7, 1119-1141. 
Crawford, M.L. and Hollister, L.S. (1986) Advances in Physical 
Holland, T.H. (1900) Geol. Survey India Mem. 22, 119-249. 
Holland, T.J.B., and Powell, R., (1985) J. Metamorphic Geol. 3 ,  
Subramaniam, A.P. (1959) Amer. Journ. Sci. 257, 321-353. 
Touret, J. and Bottinga, Y. (1979) Bull. Mineral. lQ2, 577-583 
Valley, J.W. (1985) The Deep Proterozoic Crust in the North 
Weaver, B.L. (1980) Contrib. Mineral Petrol. 71, 271-279. 
Geochemistry. 5 ,  1-35. 
343-320. 
Atlantic Provinces, 217-236. 


Evidence for CO2-rich fluids in rocks from the type charnockite area near Pallavaram, Tamil Nadu

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