Low water-rock ratio

Sverjensky (1984) calculated the dependency of Eu +/Eu + in hydrothermal solution on /oj (oxygen fugacity), pH and temperature. According to his calculations and assuming temperature, pH and /oj for epidote-stage alteration of basalt and Kuroko ores (Shikazono, 1976), divalent Eu is considered to be dominant in the rocks and hydrothermal solution. Thus, it is reasonable to consider that Eu in the rocks was removed to hydrothermal solution under the relatively reduced condition more easily than the other REE which are all tiivalent state in hydrothermal solution. Thus, it is hkely that Eu is enriched in epidote-rich altered volcanic rocks. Probably Eu was taken up by the rocks from Eu-enriched hydrothermal solution which was generated by seawater-volcanic rock interaction at relatively low water/rock ratio. 

Several factors such as Cl concentration, water/rock ratio and temperature are important in controlling the chemical composition of the hydrothermal solution interacted with the rocks. For example, water/rock ratio affects the alteration mineralogy (Mottl and Holland, 1978 Seyfried and Mottl, 1982 Shikazono, 1984). For example, at low water/rock ratio, epidote is stable, while chlorite at high water/rock ratio (Shikazono, 1984 Shikazono and Kawahata, 1987). 

Wolery (1978) and Reed (1982, 1983) have indicated based on a computer calculation of the change in chemistry of aqueous solution and mineralogy during seawater-rock interactions that epidote is formed under the low water/rock ratio less than ca. 50 by mass. Humphris and Thompson (1978), Stakes and O Nell (1982) and Mottl (1983) have also suggested on the basis of their chemical and oxygen isotopic data of the altered ridge basalts that epidote is formed by seawater-basalt interaction at elevated temperatures (ca. 200-350°C) under the rock-dominated conditions. If epidote can be formed preferentially under such low water/rock ratio, the composition of epidote should be influenced by compositions of the original fresh rocks. 

The effects of laboratory thermal alteration experiments (low water-rock ratio) on the organic matter of composited Cretaceous black shale samples from DSDP Hole 368 are evident in the following parameters (1) for the hydrocarbons—total yield, formation of PAH and thio-PAH compounds, and increase of Pr/Ph and (2) for kerogen—atomic H/C, HI, 6 C, and vitrinite reflectance (cf. Tables 1 and 2). 

Chemical and mineralogical compositions of altered basalt depend oti water/rock ratio (Mottl 1983 Shikazono et al. 1995). For example, epidote forms at low water/rock ratio W/R (<1) (Shikazono 1984), while chlorite is stable at high W/R (1-100) (Shikazono and Kawahata 1987). 

The hydrogeologic properties of rocks have a strong influence on the extent of water/rock reaction. High groundwater-flow velocities usually imply groundwaters that are relatively low in dissolved solids because of short rock-contact times and high water/rock ratios, and conversely. 

The flow system was modeled using (6). Calculation conditions are porosity 0.56, water/rock ratio 96, and fluid velocity 10 [m/s], which are based on experimental condition (except for fluid velocity). No secondary minerals appeared when using the experimental velocity, so a relatively low velocity was adopted in the model. 

Oxygen isotopes. 5 0 in ocean-crust studies is typically dehned as the per mil deviation in 180/1 0 ratio of a rock relative to a standard mean ocean water (S Osmow) and it is widely used to understand ocean-crust alteration processes. Fresh MORB has an S Osmow value of - -5.7%o, and water-rock interaction with seawater (S OsMow = at low temperatures increases the value, while high-temperature alteration decreases it. Muehlenbachs and Clayton (1972) drew attention to this relationship and suggested that hydrothermal alteration of the crust may buffer the oxygen isotopic composition of seawater. Oxygen is the major component in the oceanic crust, and therefore, changes in 5 0 are a rather profound indicator of hydrothermal alteration. 

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