Open-system isotopic exchanges

In hydrothermal systems, a simple distillation equation for an isotope ratio /2/i 1 can be worked out if only i2 is easily exchangeable. This is the case of the 180/160 ratio because the bulk of oxygen is the isotope 16 and its concentrations are determined 

We consider a system of total mass M0 made of porous rock (r) wetted by water (w) with respective mass fractions /w and /r which we assume to be constant. The isotope il is assumed to be unaffected by water-rock interaction. Change in the amount of isotope i2 held by the whole system takes place by circulation of a fluid incremental amount dMw which enters the system with a concentration Cin 2 and leaves it with the i2 concentration Cj2 of the interstitial liquid 

Since the difference in 8 values for water and rock is constant, their incremental 

The assumption of low rock porosity is equivalent to /w 0, hence, from equation (1.5.36) 

We assign the superscripts il to 86Sr, i2 to 87Sr, the subscripts r to the hydrothermal rock (hr), w to the hydrothermal solution (hw), and in to inflowing seawater (sw). Since 86Sr and Sr concentrations are nearly exactly proportional quantities, we get 

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Equation (18) illustrates that the measured 5 Fe value for Fe(ll)jq is dependent not only on t Fe(iii)L-Fe(ii)Lj but On the proportion of Fe(III)-LFe(in) in the components that are open to isotopic exchange, which additionally includes Fe(II)-LFe(n) and Fe(ll)a, we will refer to these three components as the exchangeable pool of Fe in the system. We stress that the isotopic mass balance described by Equation (18) assumes that the ligand-bound Fe(lll) component is not sampled in the aqueous phase component, but instead exists as a component that is bormd to the cells. 

A number of papers have explored the use of 5i vs. 5j diagrams for determining the extent of isotope disequilibrium and open system fluid exchange (Gregory and Criss 1986, Gregory and Taylor 1986a,b Criss et al. 1987,... 

Equation (54) applies in a beaker system, where all fluid arrives unreacted and simultaneously equilibrates with the rock within a defined volume. In the isotope literature (e.g. Taylor 1977) this is referred to as closed system isotope exchange. Alternatively, infiltration has been considered an open-system process such that each infinitely small aliquot of fluid equilibrafes and fhen leaves the system. In this case the appropriate relation is... 

We briefly review processes in which isotopic fractionations may be recorded in isotopically distinct reservoirs that are preserved in nature. These concepts have been extensively covered in the H, C, O, and S isotope literature, and we illustrate several examples for the non-traditional stable isotope systems discussed in this volume. One of the simplest processes that produces isotopically distinct reservoirs would be slow reaction of substance A to B, where A and B remain open to complete isotopic exchange during the process. This is commonly referred to as closed system equilibrium, and the changes in isotopic compositions that occur may be defined by the exact relation ... 

The whole-rock samples are not immune from other open-system exchange, however. For example, we do not yet know how aqueous alteration manifests itself on these diagrams because detailed studies of the Mg isotopic effects of water-rock reactions have not been carried out at the time of this writing. 

Gregory, R.T Criss, R.E. 1986. Isotopic exchange in open and closed systems. In Stable isotopes in high temperature geological processes. Reviews in Mineralogy 16, Mineralogical Society of America, Washington, 91-127. 

Gregory, R.T., et al. 1989. Oxygen isotope exchange kinetics of mineral pairs in closed and open systems application to problems of hydrothermal alteration of igneous rocks and Precambrian iron formations. Chemical Geology, 75, 1-42. 

The evolution of the isotopic compositions of carbon-bearing substances in uncontaminated systems where carbon is derived from carbonate minerals and soil CO2 is bounded between two limiting cases (i) open systems, where carbonate reacts with water in contact with a gas phase having a constant Pco, and (ii) closed systems, where the water is isolated from the CO2 reservoir before carbonate dissolution (Deines et al., 1974 Clark and Fritz, 1997). Both of the extremes assume water residence times long enough for significant isotope exchange between the gas and the aqueous phase to take place. 

Magnesium oxide may be expected to function as an active catalyst for CO2 utilization since CO2 is easily chemisorbed on MgO [1-3]. Most works so far reported on this adsorption system were carried out above the room temperature, and recent works have demonstrated the occurrence of oxygen isotope exchanges between adsorbed species and surface 0 [4-6]. However, dynamic behaviors of adsorbed species, especially below the room temperature, are still open to question. 

Sulfate-sulfide isotopic exchange disequilibrium (where sulfate is reduced to sulfide without isotopic exchange but sulfide minerals and H2S isotopically equilibrate) in closed and open isotopic systems result in significantly smaller shifts in isotopic compositions (Figure 1). In addition, the calculated shifts are to higher sulfide compositions, generally consistent with the measured values for seafloor sulfide minerals. Analyses of the natural samples, however, provide no evidence of systematic isotopic differences between pyrite, sphalerite, and chalcopyrite, as predicted by both standard fractionation approaches and these calculated isotopic reaction pathways, possibly due to dynamic variations of mixing and precipitation on a local scale. 

Graham CM, Valley JW, Eiler JM, Wada H (1998) Timescales and mechanisms of fluid infiltration in a marble An ion microprobe study. Contrib Mineral Petrol 132 371-389 Gregory RT, Criss RE (1986) Isotopic exchange in open and closed systems. Rev Mineral 16 91-127 Gregory RT, Taylor HP (1986a) Possible non-equihbrium oxygen isotope effects in mantle nodules, an alternative to the Kyser-O Neil-Camuchael geothermometer. Contrib Mineral Petrol 93 ... 

In a well mixed, open system, free of isotope exchange, the fractionation between coexisting chemical species may be expressed as ... 

Of the three experiments presented here the CRESU technique is certainly the most versatile and easy to use. Also it is not restricted to molecular hydrogen as neutral reactant. However the ion trap allows measurements of extremely slow reactions (down to 10 cm s ) and of radiative association. The static drift tube can be operated at the highest pressures, therefore sometimes reflecting the influence of this parameter. These new methods complement each other and this research field Is still widely open. The study reported here on the reactivity of polar molecules needs to be extended to numerous systems. Much more data concerning radiative association, as well as data on isotope exchanges, tunnelling effects, slow reactions, is necessary for chemistry modeling. 

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