Dissolved oxygen basalt

Since crushed basalt has been recommended as a major backfill component (1), experiments were completed to evaluate the rate of dissolved oxygen consumption and the redox conditions that develop in basalt-water systems under conditions similar to those expected in the near-field environment of a waste package. Two approaches to this problem were used in this study (l)the As(III)/As(V) redox couple as an indirect method of monitoring Eh and (2) the measurement of dissolved oxygen levels in solutions from hydrothermal experiments as a function of time. The first approach involves oxidation state determinations on trace levels of arsenic in solution (4-5) and provides an estimate of redox conditions over restricted intervals of time, depending on reaction rates and sensitivities of the analyses. The arsenic oxidation state approach also provides data at conditions that are more reducing than in solutions with detectable levels of dissolved oxygen. 

Table IV. Dissolved Oxygen Data from 150°C Basalt + Synthetic Grande Ronde Groundwater Experiment (Run D2-16)...

Figure 1. Dissolved oxygen vs time data. The experiments were basalt + synthetic Grande Ronde groundwater (B+SW) and synthetic Grande Ronde groundwater (SW) at 300 bars. Determination of uncertainties for B+SW data points is discussed in Table IV. Uncertainties for SW data were derived from replicable tests.

Dissolved Oxygen. The experimental results demonstrate that in the absence of basalt, DO is maintained at high levels, while in the presence of basalt, oxygen is effectively removed (see Figure 1). Although the ferrous iron content of Umtanum basalt mesostasis is not well known, estimates from bulk ferrous/ferric iron data and from microcharacterization of mesostasis phases (7, 20) indicate that the amount of Fe2+ available for oxygen consumption by mesostasis dissolution is large relative to the amount of DO (>.10 on a mole/mole basis).Thus, the available Fe2+ concentration should remain constant over the duration of the experiments. 

The relationship of the stirring rate in these experiments to the rates of hydrolysis reactions of basalt phases is indicative of surface-reaction controlled dissolution (21). First order kinetics are not inconsistent with certain rate-determining surface processes (22). Approximate first order kinetics with respect to dissolved oxygen concentration have been reported for the oxidation of aqueous ferrous iron (23) and sulfide (24), and in oxygen consumption studies with roll-type uranium deposits(25). 

This immediately leads to the question when and where did the obsidian acquire peroxy entities The parent magma was certainly not so highly oxidized as to dissolve oxygen according to the scheme 02" + 1/2 02 = 022" or X/° X + 1/2 02 X/00 X. Magmas tend to be always reduced, ranging from strongly reduced like basalts Q ) to weakly reduced like the obsidian where the Fe3+/Fe2+ ratio is reportedly of the order of 0.5 (21). ... 

I consider carbon burial in the next section. In this section, I consider the mantle cycle. Mid-oceanic ridges act as oxygen sinks. Reaction with basalt consumes dissolved oxygen and sulfate. Organic carbon in sediments subducts into the mantle. Reduced volcanic gasses vent on land and beneath the ocean. I consider the loss of H2 to space in this section as it too moves a cmstal reactant out of sight. 

Three major types of rocks are found in Earth s crust igneous rocks, formed by solidification of molten rock (e.g., basalt) sedimentary rocks (e.g., sandstone, which is cemented sand), formed by deposition of dissolved or suspended substances from oceans and rivers and metamorphic rocks (e.g., marble), formed by the action of heat and pressure on existing rocks. Figure 18.2 gives the average composition of Earth s crust. The most abundant substances in rocks are silicates, which are composed of silicon, oxygen, and positive metal ions (Section 18.5). The more than 2000 kinds of known minerals fall into a few major classes (Table 18.1). 

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