Fluid inclusions in granulite from Yanzihe

Fluid inclusions are moderately abundant in garnet, quartz blebs in garnet, and highly abundant in matrix quartz in granulite from Yanzihe. Three generations of fluid inclusions were distinguished in the granulite based on textural criteria:

Generation 1: H2O-CO2 and carbonic inclusions in quartz blebs in garnet occur isolated or scattered in groups and clusters, and have rounded or negative crystal shapes. They are interpreted as formed early (primary). In most groups or clusters, H2O-CO2 and carbonic inclusions occur together, without evidence of age difference. This suggests simultaneous

trapping of the two fluids, most probably during primary growth of the host quartz blebs.

Most of the carbonic inclusions are monophase inclusions at room temperature; only few inclusions with a vapor bubble have been observed. According to Roedder (1984), apparently monophase CO2-dominated inclusions may contain up to 10% H2O, which coats the walls of the inclusions as a thin film. However, clathrate melting was not observed in these inclusions.

The H2O-CO2 inclusions are usually 2-phase, liquid H2O and liquid CO2, at room temperature. These H2O-CO2 inclusions have fill degrees of H2O ranging from 10 to 80% in volume.

Generation 2: H2O-CO2 and carbonic inclusions (no visible water) were observed in garnet and in matrix quartz. They are isolated or randomly distributed inclusions and assumed primary. Like the fluid inclusions in quartz blebs, they are mostly monophase liquid-CO2 or two-phase H2O + CO2 inclusions, however, H2O occurs less than 10% in volume in the 2-phase inclusions. The morphologies are generally irregular because most of them have been strongly modified (Fig. 6.10A). Most of the carbonic inclusions contain fine-grained solids in addition to the fluid, indicating reactions between garnet and the inclusion fluids. The close textural relationship between H2O-CO2 inclusions and carbonic inclusions and their coexistence in garnet and in matrix quartz suggest that the fluid inclusions may have been trapped simultaneously during the growth of garnet and matrix quartz.

Generation 3: Secondary H2O-CO2 and carbonic inclusions in matrix quartz. These fluid inclusions are bound to healed microfractures, which normally cross-cut quartz grain boundaries (Fig. 6.10B). They have roundish or negative crystal morphologies. The H2O-CO2

inclusions have fill degrees of water ranging from 10 to 90% in volume. Textures suggest that the two fluids were trapped at the same time.

A B

Fig. 6.10. Photomicrographs of fluid inclusions in the granulite from Yanzihe. (A) Carbonic inclusions in garnet, note the inclusions have been strongly modified; (B) Secondary carbonic inclusions in matrix quartz.

Microthermometric results for the fluid inclusions in granulite are shown in Fig. 6.11.

Most inclusions homogenize to the liquid phase; homogenization to the vapor was also rarely observed. The carbonic inclusions in quartz blebs in garnet show CO2 melting temperatures from -58 to -59 °C and homogenization temperatures of CO2 (to liquid) between 11 and 16

°C, whereas H2O-CO2 inclusions have CO2 melting temperatures from -57.5 to -58 °C and homogenization temperatures from 23 to 24 °C. Different melting and homogenization temperatures between the 2 types of fluid inclusions may be due to the forming of clathrate in the H2O-CO2 inclusions. Carbonic fluid inclusions in garnet display melting temperatures of -58.5 to -59.5 °C and homogenization temperatures of -17 to 20 °C. Primary inclusions in matrix quartz show CO2 melting temperatures between -58 and -59 °C and homogenization temperatures between 6 and 20 °C. Secondary inclusions in matrix quartz have CO2 melting temperatures of -57 to -58 °C and CO2 homogenization temperatures of 17 to 27 °C. Some secondary H2O-CO2 inclusions in matrix quartz display ice melting temperatures around 0 °C, indicating a pure-water system besides the CO2-rich phase. The melting temperatures of all fluid inclusions are indicative of the presence of minor N2 and/or CH4 addition to CO2. However, it is evident that the fluid inclusions contain more N2 and/or CH4 compared to the secondary fluid inclusions.

Unlike isochores for inclusions in eclogite, isochores for CO2-H2O inclusions in garnet and matrix quartz are conformable to peak-metamorphic conditions, whereas isochores for CO2 inclusions in garnet and matrix quartz falls 7 kb short of the estimated peak metamorphic conditions. As mentioned previously, however, these two types of inclusions were trapped simultaneously without any relative age difference. This disagreement may suggest that some of the volatile components are selectively consumed by reactions between the fluid in inclusions and garnet. The presence of solid phases in the carbonic inclusions in garnet provides support to this suggestion. The texturally earliest inclusions are CO2 and H2O + CO2

inclusions in quartz blebs in garnet, which are assumed to be trapped during prograde metamorphism. Their highest-density isochore correlates to 6 kbar at 600 °C.

-20 -10 0 10 20 30

-61 -60 -59T_mf(°C)-58 -57 -56 Th(°C)

Inclusions in Qtz blebs Inclusions in matrix Qtz Inclusions in Grt Trails in matrix Qtz

Fluid inclusions studies show that the fluids in the granulite from Yanzihe are dominated by CO2 with minor N2 and/or CH4. The fluids evolved from relatively water-rich during prograde metamorphism towards relatively CO2-rich solutions during peak metamorphism. During the retrograd metamorphism, the fluids became again water-rich but with less N2 and/or CH4 compared to the prograde - and peak-metamorphism.

Fig.6.11. Microthermomtric results for carbonic inclusions in the Yanzihe granulite. The low melting temperatures are the indicative of the presence of additional N₂ and/or CH₄.