The quartz-hosted fluid inclusions in all sediment samples are usually very small, generally ranging from 2 to 10 µm in size. A few of them are relative larger, about 15 µm in length.
Three types of fluid inclusions in quartz are observed. The main type is liquid-rich aqueous fluid inclusion (Figs. 2a-2e), which comprises two phases (liquid+vapor H2O) at room temperature with 5%-30% of gas. Vapor-rich aqueous fluid inclusions are also observed in the quartz particles with 50%-90% gas (Figs. 2f-2g); daughter-mineral-bearing aqueous fluid inclusion is rare (Fig. 2h), which consists of H2O liquid, H2O vapor, and a daughter mineral phase.
Figure 2. Microphotographs showing typical quartz inclusions of the Changjiang River Drainage Basin. (a) Primary fluid inclusions with necking, SGJS-01; (b) randomly distributed primary fluid inclusions, YBCJ-01; (c) regionally dispersed primary fluid inclusions, YZD-63; (d) secondary fluid inclusions, YZD-10; (e) banded pseudo-secondary fluid inclusions, HK-01; (f) vapor-rich fluid inclusions, HK-01; (g) vapor-rich fluid inclusions, HK-01; (h) isolated primary fluid inclusions, HK-01.
Most of the fluid inclusions are of regular elliptical, circular, or rectangular shape. As regards arrangement, the fluid inclusions mainly appear in clustered (Fig. 2b), zoned (Fig. 2e), and isolated distributions (Figs. 2a, 2h), of which zoned distribution is the most common. Fluid inclusions in the same zone generally have the same long-axis orientation and similar shapes and sizes. Some of them appears along pseudo-secondary trails (Fig. 2d).
As detailed in Table 1, from the upper to lower reaches of the Changjiang River, there are also some differences in the overall characteristics of the fluid inclusions in the sediments. For the SDJS-01 samples collected from the Shigu area in the upper reaches, irregularly shaped inclusions are bigger, mostly formed by necking, and few secondary inclusions with irregular shapes are present. Most fluid inclusions are distributed in bands, some are massed or isolated. The sizes range of 2-14 μm.
Fluid inclusion characteristics Sample name and location SGJC-1 YBCJ-1 YZD-63 YZD-10 HK-01 Shigu Yibin Yichang Wuhan Chongming Size (μm) 2-14 2-12 2-10 2-12 2-9 Shape Most regular Regularly shaped fluid inclusions predominate Few irregular Arrangement Mostly in bands, some in mass or isolated Mostly in mass and bands, a few isolated Bands, mass and isolated, all occur
Table 1. Morphological characteristics of quartz-hosted fluid inclusions in the Changjiang River Basin
The YBCJ-01 and YZD-63 samples from Yibin and Yichang respectively, were found to have similar fluid inclusion characteristics. Most fluid inclusions are regular and occasionally irregular in shape. The fluid inclusions are mostly banded or massed, a few of them show isolated distribution pattern. The inclusions are smaller than those in SGJS-01 samples.
The YZD-10 samples from Wuhan are similar with HK-01 samples from Chongming in estuary, mainly exhibiting regularly shaped fluid inclusions. Mass, bands and isolated inclusions distribution pattern all occured.
Figure 4. Microphotographs showing typical quartz inclusions in samples YBCJ-01 (a)-(b), YZD-63 (c)-(d), and YZD-10 (e)-(f).
The types of quartz inclusions in the SGJS-01 sediment collected from Shigu, Yunnan Province are shown in Fig. 3. There are five types of fluid inclusions in quartz (Figs. 3a-3e). Type 1 is a primary inclusion in origin, which is characterized by a negative crystal in shape, with a size of 12-18 μm, and contains about 50% of gas (Fig. 3a); Type 2 is a primary inclusion with 3-5 μm in diameter with 10% of gas and regular shape (Fig. 3b). Type 3 is a fluid inclusion with necking. They have 10-18 μm in diameter and contain 10% of gas (Fig. 3c). Type 4 is an isolated vapor-rich fluid inclusion with 80% of gas and 5 μm in diameter (Fig. 3d). Type 5 is a linear distribution of secondary inclusions with 3-5 μm in diameter and less than 10% of gas (Fig. 3e), which is the dominant type of fluid inclusions in the upstream.
The quartz-hosted fluid inclusions in the YBCJ-01, YZD-63 and YZD-10 samples from Yibin, Yichang and Wuhan are different from those in the Shigu sample (SGJS-01). For the primary fluid inclusions, they are isolated and have negative crystal in shape (Figs. 4a, 4b), which are similar to the primary fluid inclusions in the Shigu sample (SGJS-01). In addition, some new types of primary fluid inclusions have been found (Figs. 4c, 4d). They have 10%-50% of gas and are 6-10 μm in diameter. Secondary fluid inclusions with 5-10 μm in diameter also have been found in these samples (Figs. 4e-4f), which are similar to the Shigu sample (Fig. 3e). Compared with Shigu samples, there are more types of quartz-hosted fluid inclusions collected from Yibin, Yichang and Wuhan, and the types of primary fluid inclusions in the Shigu samples can also be found in the middle reaches. Therefore, it can be inferred that the inflow of tributaries from the upstream to the middle reaches brings new types of quartz inclusions, which enrich the types of fluid inclusions in the middle reaches. The middle reaches also receive sediment input from the upstream.
The type of fluid inclusions in quartz of the sediments (HK-01) collected from the Changjiang Estuary is also different from other sites. Figure 5 shows the types of quartz-hosted fluid inclusions in HK-01 samples. The same types of quartz-hosted fluid inclusions from the upper and middle reaches also have been found in HK-01 samples (Figs. 5a-5d, 5g). In addition, two new types of fluid inclusions are found in the estuarine samples. One has a rod-like shape with 10 μm in diameter and contains less than 10% of gas (Fig. 5e). Another is vapor-rich fluid inclusions with a size of 15 μm and more than 80% of gas (Fig. 5f). It can be inferred that the sediments in estuarine were sourced from the downstream tributaries as well as from the upper and middle reaches.
The statistical characteristics of quartz-hosted fluid inclusions in the Changjiang River sediments are indicated by the fluid inclusion size, number, and gas percentage (Table 1). The upstream sample (SGJS-01) has larger size of 2 to 14 µm. The number of fluid inclusions is in the range of 2-166 per 10-3 mm3, and the gas percentage is 10% to 90%. For the YBCJ-1 sample collected from Yibin, located in the lower part of the upper reaches, the size and the gas percentage of fluid inclusions are similar to those of the SGJS-01, ranging from 2 to 12 μm and 10% to 80%, respectively. The number of fluid inclusions increases obviously from 1 to 270 per 10-3 mm3. This indicates a change in the sediment source from Shigu to Yibin along the mainstream of the Changjiang River. On the Shigu site the sediments most likely have been input from the Jinshajiang River because the Jinshajiang River is the only main river (Fig. 1), whereas on Yibin site the sediments have been input partially from the Yalongjiang, Daduhe, and Minjiang rivers because the quartz there has more fluid inclusions than that on the Shigu site, which may be derived from more sources by tributaries. For the YZD-63 samples collected from Yichang (the middle reaches), the size of fluid inclusions range from 2 to 10 µm with 10%-80% of gas. The fluid inclusion numbers range from 2 to 220 per 10-3 mm3, which are similar to those in the upper reaches. For the YZD-10 sample collected from Wuhan in the middle reaches, the size of fluid inclusions ranges from 2 to 12 µm, but both the number and the gas percentage of fluid inclusions vary. The number range of fluid inclusions is wider from 1 to 308 per 10-3 mm3, and the gas percentages are from 5% to 70%. These features suggest that some sediments may be input from the Hanjiang River Basin. The upper reaches of the Hanjiang River Basin correspond to a loess area with notably different sediment supply. In the HK-01 sample of the Chongming in the estuary, the sizes of fluid inclusions range from 2 to 9 µm with 10%-80% of gas. The fluid inclusion numbers range from 2 to 116 per 10-3 mm3.
The statistical data of number and size of the quartz-hosted fluid inclusions are shown in Table 2 and Fig. 6. There are some similarities in the size and the number of fluid inclusions. The size of quartz-hosted fluid inclusions in the sediments of the Changjiang River Basin is mostly concentrated in 2-5 μm in diameters, and the number is 10-200 per 10-3 mm3. Only a few quartz-hosted fluid inclusions are larger than 5 μm in diameter and more than 200 per 10-3 mm3. However, there are differences in the number and size of fluid inclusions between the upper, middle, and lower reaches of the Changjiang River Basin. The samples from Shigu, Yichang, and Yibin, which are located at the upper and middle reaches of the Changjiang River, exhibit greater size (up to 18 μm) and the number ranges of fluid inclusions (up to 270 per 10-3 mm3), whereas the fluid inclusions in the samples (HK-01) from the Chongming estuary are concentrated in 2-4 μm with 2-116 per 10-3 mm3 of fluid inclusions, indicating that not all the sediments in the upper reaches of the Changjiang River reach the middle and lower reaches and the estuary area, while the sediments in the estuary area mainly receive the input from the sediments in the upper, middle and lower reaches of the Changjiang River.
Sample Size (μm) Number of fluid inclusions per volume (number per 10-3 mm3) Gas percentages SGJS-1 2-14 2-166 10%-80%, 10%-20% mainly, 7% vapor-rich YBCJ-1 2-12 1-270 10%-80%, 5%-20% mainly, 4% vapor-rich YZD-63 2-10 2-220 10%-80%, 10%-20% mainly, 5% vapor-rich YZD-10 2-12 1-308 5%-70%, 10%-20% mainly, 3% vapor-rich HK-01 2-9 2-116 10%-80%, 10%-50% mainly, 4% vapor-rich
Table 2. Statistical characteristics of quartz-hosted fluid inclusions of the Changjiang River Basin
The fluid inclusions in the Changjiang River sediment samples were classified into two genetic types, as shown in Fig. 7.
Figure 7. Genetic type proportions of quartz inclusions in different samples from the Changjiang River Drainage Basin.
The Shigu sample (SGJS-01) contains 14% of primary fluid inclusions. From Shigu to Yibin, the number of primary fluid inclusions decreases significantly to 6%. The YZD-10 samples, collected from Wuhan, the number of primary fluid inclusions is 6%. The estuary sample (HK-01) contains 5% of primary fluid inclusions. The genetic types of sediment samples in Wuhan is similar to the samples in Changjiang Estuary, indicating that the source of sediment in the estuary is quite similar to that in Wuhan. This difference in quartz inclusion type reflects the different sources of sediments in the upper and lower reaches of the Changjiang River.