Phase partitioning, noble gases

PHASE EQUILIBRIUM AND FRACTIONATION OF NOBLE GASES Liquid-gas phase partitioning of noble gases... 

Partition (Distribution) Coefficients In describing the partitioning of a trace element among coexisting phases, we frequently use a partition (distribution) coefficient for a given element, defined as a concentration ratio C2/Cj. Here C is concentration, and the subscripts identify the phases often the normalizing phase is some convenient reservoir, such as a silicate melt, with which several other phases may equilibrate. For noble gases, it is often most convenient to normalize to a gas phase. If the concentrations are expressed in the same units, the distribution coefficient is dimensionless. It is conventional to cite noble gas concentrations in condensed phases in cm3 STP/g, however, and to describe the gas phase by partial... 

A common difficulty in these partition experiments, with the use of either natural or synthesized samples, is in achieving perfect separation of the melt from the crystal phase for determining the noble gas content. Even a very small amount of glass (melt) contamination in crystal would increase the partition coefficient considerably, since noble gasses are much more enriched in glass. To circumvent this difficulty, Broad-hurst et al. (1990, 1992) prepared natural minerals and synthetic silicate melts that... 

Most submarine volcanic rocks contain C02-filled vesicles (bubbles) in glassy margins. Because noble gases in silicate melts partition very effectively into a gas phase (i.e., their solubilities are low), it would be expected that noble gases in submarine volcanics would be found in the bubbles as will be discussed later, this seems indeed to be the case (e.g., Kurz Jenkins, 1981 Marty Ozima, 1986 Sarda Graham, 1990 Graham Sarda, 1991). The popping rocks, so-called because of... 

Abundance patterns. The measured noble gas abundance patterns of MORE and OIB scatter greatly. This is due to alteration as well as fractionation during noble gas partitioning between basaltic melts and a vapor phase that may be then preferentially gained or lost by the sample. However, MO Ne/Ar and Xe/Ar ratios that are greater than the air values are common (Table 2). This pattern was found in a gas-rich MORE... 

The mineral-fluid system and diffusive gradient For diffusion to occur there must also be a chemical gradient. For noble gases in a mineral-fluid system this occurs when the noble gas equilibrium concentration in the mineral phase (Cm) is higher than the equilibrium concentration in the fluid phase (Cf), where the mineral-fluid partition coefficient (Kd) is defined as... 

Noble gas partitioning and solubility fractionation between equilibrated subsurface phases was initially studied by Goryunov and Kozlov (1940) and further studied (Zartman et al. 1961 Bosch and Mazor 1988 Zaikowski and Spangler 1990 Ballentine et al. 1991 Ballentine et al. 1996 Hiyagon and Kennedy 1992 Pinti and Marty 1995 Torgersen and Kennedy 1999 Battani et al. 2000). Recent reviews are by O Nions and Ballentine (1993), Ballentine and O Nions (1994) and Pinti and Marty (2000). 

A simple dynamic model. The maximum magnitude of noble gas fractionation that can occur when two phases have been equilibrated is summarized for Ne/Ar in Figure 7. Although the phase equilibrium model demonstrates the effect of the physical conditions in a system on the limits of noble gas fractionation, the phase equilibrium model represents only one end-member of the processes that may be occurring in a dynamic subsurface fluid environment. To convey some sense of the relevance of the phase equilibrium model in a dynamic system it is useful to consider the extent to which noble gases partition and fractionate between phases when a gas bubble passes through a column of liquid (Ballentine 1991 Fig. 8). 

Figure 8. (A) A water column is divided into fifty equal unit cells and it is assumed there is no liquid or dissolved gas between cells. Each cell originally has the noble gas content of air-equilibrated water and all calculated Ne/Ar ratios are normalized to this value to obtain a fractionation factor F. The column temperature is taken to be 325 K, which for pure water gives Knc = 133245 atm and Kaf= 55389 atm. A gas bubble of constant volume is passed sequentially through the column, equilibrium assumed to occur in each water cell and the Ne and Ar partitioned into the respective gas and water phases (Eqn. 16). The evolution of the Ne/Ar ratio in the gas bubble (bold) and each water phase increment (Faint) is shown for different gas/water volume ratios, Vg/Vi. The gas bubble Ne/Ar ratio approaches the maximum fractionation value predicted for a gas/water phase equilibrium where as Vg/Vi -> 0, F Knc/Kat. The cell Vg/Vi ratio only determines the rate at which this hmit is approached. (B) The same water column with a fixed cell Vg/Vi ratio of 0.01. n subsequent bubbles are passed through the column and the He/Ne distribution between phases calculated at each stage. The gas bubble Ne/Ar ratio evolution for n = 1, 10, 20 and 30 is shown in bold, together with the residual Ne/Ar in the water colunm cells (faint lines). All gas bubbles approach the limit imposed by the phase equilibrium model. The water phase is fractioned in the opposite sense and is fractionated in proportion to the magnitude of gas loss following the Rayleigh fractionation law (Eqn. 24).

Noble gas partitioning between a seawater-derived groundwater and the oil phase at the average Magnus Sandstone aquifer temperature requires a subsurface seawater/oil volume ratio of 110( 40) to account for both the °Ne and Ar concentrations in the central and southern Magnus samples. 

Table 2 quantitatively describes how the noble gases partition between the three phases when a system containing these three phases is in equilibrium in fresh water at 0°C. The numbers represent the amount of the gas found in one phase relative to the other. For example, the first row describes the amount of each gas that would reside in the gaseous bubble phase relative to the liquid phase thus for helium, there would be 106.8 times more helium present in the bubble than in the water. This illustrates the small solubility of helium in water and its strong affinity for the gas phase. Because helium is 1.9 times more soluble in ice than in water, helium partitions less strongly between the bubble and ice phases compared to the partition between the bubble and water phases. The two numbers shown for neon represent the two different estimates for the solubility of neon in the ice phase. One estimate suggests that neon is less soluble in ice than in water, whereas the other suggests that it is more soluble in ice. 

Naturally occurring radionuclides such as radium isotopes and radon-222 have gained popularity as tracers of SGD due to their enrichment in ground-water relative to other sources and their built-in radioactive clocks . The enrichment of these tracers is owed to the fact that the water-sediment ratio in aquifers is usually quite small and that aquifer sediments (and sediments in general) are enriched in many U and Th series isotopes while many of these isotopes are particle reactive and remain bound to the sediments, some like Ra can easily partition into the aqueous phase. Radon-222 (tia = 3.82 days) is the daughter product of Ra (G/2= 1600 years) and a noble gas therefore, it is even more enriched in groundwater than radium. 

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