Kun Shen

Fluid Composition and Evolution Attending UHP Metamorphism: Study of Fluid Inclusions from Drill Cores, Southern Sulu Belt, Eastern China

Rocks from the first pre-pilot hole of the Chinese Continental Scientific Drilling Project (CCSDPPH1, 432 m), located in the eastern part of the Dabie-Sulu ultrahigh-pressure (UHP) metamorphic belt, have been subjected to a coesite-eclogite-facies metamorphic event, followed by an amphibolite-facies overprint. Primary fluid inclusions occur in garnet, omphacite, and apatite from eclogite; in kyanite and in topaz from quartzite; and in garnet, epidote, and apatite from paragneiss. Secondary fluid inclusions are present in all lithologies. Fluid inclusions are absent from ultramafic rocks. Based on fluid compositions and textural criteria we distinguished: (1) low-salinity aqueouscarbonic inclusions in topaz from quartzite, which may have originated from a supracrustal protolith; (2) primary CaCl2-NaCl-rich brine inclusions in garnet and in omphacite from eclogite and in kyanite from quartzite, representing UHP metamorphic fluids; (3) high-salinity aqueous-carbonic inclusions in quartz from eclogite and quartzite, representing amphibolite-facies fluids; (4) aqueous fluids of low- and intermediate salinity trapped as primary inclusions in garnet, epidote (or allanite) and apatite from gneiss, or as secondary inclusions, representing amphibolite-facies and later retrograde fluids; (5) carbonic inclusions are distributed along transgranular fractures in quartz from quartzite, and probably represent the latest retrograde fluid. The diversity in fluid inclusion populations and compositions from different vertical depths suggests a closed fluid system without largescale fluid migration during UHP metamorphism. However, the common low- and medium-salinity inclusions in most rock types suggests that a water-dominated fluid from an external source infiltrated into the rock system during amphibolite-facies metamorphism, resulting in extensive retrogression of the UHP rocks.

Unusual high-density and saline aqueous inclusions in ultrahigh pressure metamorphic rocks from Sulu terrane in eastern China

Primary high-density fluid inclusions were identified in garnet from ultrahigh pressure eclogite in the southern part of the Sulu terrane. They occur isolatedly or in cluster together with relatively low-density two-phase inclusions. The eutectic temperature of the inclusions is as low as ≤52°C. A bubble was nucleated in a liquid inclusion during the specific stage of cyclic cooling-heating runs, and the liquid-gas homogenization temperature was measured to be ≤12.5°C. The composition of the inclusions modeled by the system CaCl2-NaCl-H2O, yields the fluid density of 1.27 g/cm that corresponds to a pressure of ca. 2.4 GPa at the temperature of peak eclogite-facies metamorphism, close to the ultrahigh pressure metamorphic conditions. During the exhumation of the eclogite the inclusions reacted with the host mineral, forming hydrous silicate minerals that resulted in lowering of the fluid density and its transformation to multi-phase inclusions.

Fluid Inclusions Associated with Exsolved Quartz Needles in Omphacite of UHP Eclogites, Chinese Continental Scientific Drilling Main Drill Hole

Primary and exsolution fluid inclusions are recognized in omphacite of ultrahigh-pressure (UHP) eclogites from the main hole of the Chinese Continental Scientific Drilling Program, located in the southern Sulu orogenic belt. These oriented fluid inclusions occur as tubes, and coexist with exsolved quartz needles in the cores of host omphacite. Most complex primary fluid inclusions contain a gas bubble, a liquid phase, and one to several solids, such as quartz, halite, calcite, and opaque and unknown minerals, having compositions in the system of NaCl-CaCl2-CO2-H2O-SiO2 with possibly trace Fe and Mg; in contrast, simple fluid inclusions contain a gas, an aqueous liquid, and sometimes a calcite. We suggest that the complex fluid inclusions were trapped during the omphacite growth, whereas the simple aqueous inclusions and quartz needles exsolved from OH--rich supersilicic omphacite during early uplift of the UHP metamorphic rocks. Omphacite is one of the major fluid carriers during subduction of continental crust to mantle depths.

Channelized fluids in subducted continental crust: constraints from δD–δ18O of quartz and fluid inclusions in quartz veins from the Chinese Continental Scientific Drilling Project

Fluid inclusions hosted by quartz veins in high-pressure to ultrahigh-pressure (HP-UHP) metamorphic rocks from the Chinese Continental Scientific Drilling (CCSD) Project main drillhole have low, varied hydrogen isotopic compositions (δD = −97‰ to −69‰). Quartz δ18O values range from −2.5‰ to 9.6‰; fluid inclusions hosted in quartz have correspondingly low δ18O values of −11.66‰ to 0.93‰ (T h = 171.2∼318.8°C). The low δD and δ18O isotopic data indicate that protoliths of some CCSD HP-UHP metamorphic rocks reacted with meteoric water at high latitude near the surface before being subducted to great depth. In addition, the δ18O of the quartz veins and fluid inclusions vary greatly with the drillhole depth. Lower δ18O values occur at depths of ∼900–1000 m and ∼2700 m, whereas higher values characterize rocks at depths of about 1770 m and 4000 m, correlating roughly with those of wall-rock minerals. Given that the peak metamorphic temperature of the Dabie-Sulu UHP metamorphic rocks was about 800°C or higher, much higher than the closure temperature of oxygen isotopes in quartz under wet conditions, such synchronous variations can be explained by re-equilibration. In contrast, δD values of fluid inclusions show a different relationship with depth. This is probably because oxygen is a major element of both fluids and silicates and is much more abundant in the quartz veins and silicate minerals than is hydrogen. The oxygen isotope composition of fluid inclusions is evidently more susceptible to late-stage re-equilibration with silicate minerals than is the hydrogen isotope composition. Therefore, different δD and δ18O patterns imply that dramatic fluid migration occurred, whereas the co-variation of oxygen isotopes in fluid inclusions, quartz veins, and wall-rock minerals can be better interpreted by re-equilibration during exhumation. Quartz veins in the Dabie-Sulu UHP metamorphic terrane are the product of high-Si fluids. Given that channelized fluid migration is much faster than pervasive flow, and that the veins formed through precipitation of quartz from high-Si fluids, the abundant veins indicate significant fluid mobilization and migration within this subducted continental slab. Many mineral reactions can produce high-Si fluids. For UHP metamorphic rocks, major dehydration during subduction occurred when pressure–temperature conditions exceeded the stability of lawsonite. In contrast, for low-temperature eclogites and other HP metamorphic rocks with peak metamorphic P–T conditions within the stability field of lawsonite, dehydration and associated high-Si fluid release may have occurred as hydrous minerals were destabilized at lower pressure during exhumation. Because subduction is a continuous process whereas only a minor fraction of the subducted slabs returns to the surface, dehydration during underflow is more prevalent than exhumation even in subducted continental crust, which is considerably drier than altered oceanic crust.