Closed-system behavior

Peninsula, Papua New Guinea (after Edwards et al. 1993). Open elhpses are data for samples collected from outcrop closed elhpses are data for samples collected from drill core. All points plot along the initial 5234 j =150 contom, indicating that all samples have maintained a primary marine uranium isotopic composition, consistent with closed-system behavior. Relatively yonng samples such as these are more likely to satisfy the closed-system assumption whereas older corals snch as those depicted in Figure 13 are not as likely to satisfy this assnmption. 

In addition to the assumptions of initial conditions, the validity of U-Pb methodology relies on closed system behavior of U, Pb and intermediate nuclides in the decay chain. Concordance between the two U-series decay chains is most likely to be compromised by Rn loss because Rn is the only gas in the decay chains and has a high diffusivity. Radon-222 in the decay chain has a half life of 3.8 days. This is much longer than the half-life of Rn (3.96 s) in the decay chain. Therefore, partial loss of Rn will give rise to an apparent age younger than the true age, whereas the 207p /235p ... 

The results of dissolved sulfate concentrations are summarized in Figure 4D. Sulfate concentrations are highest in the top of the evaporative panne (core 1) sediments. Sediments of the evaporative panne and tidal creek (cores 1 and 3 respectively) do not reproduce the anticipated closed system behavior for sediments undergoing sulfate reduction, i.e. progressive decrease with depth (12). 

Note that this is similar to the approach adopted earlier with Sm- " Nd upon which many ideas of Hadean mantle depletion, melting processes and early cmst were based (Chase and Patchett, 1988 Galer and Goldstein, 1991). However, the difficulty with insuring closed-system behavior with bulk rock Sm-Nd in metamorphic rocks and achieving a robust age correction of long-lived " Sm over four billion years has meant that this approach is now viewed as suspect (Nagler and Kramers, 1998). The isotopic system... 

Use of the Th and Pa methods assumes closed-system behavior, i.e., no diffusional mobility that would change the slope of the In A versus z plot. Because of the very strong adsorption of thorium and protactinium on solid phases, significant mobility is unlikely, and no evidence for it has been reported. The concordance between sediment accumulation rates from Th and from Pa (Ku, 1976) also argues against significant mobility. 

Combined Pa and Th dating of corals provides perhaps the most rigorous test for closed-system behavior. Because mass spectrometric Pa techniques are still fairly new and since the measurements themselves are not easy, there is still limited data of this sort. In coming years, Pa measurements will play a major role in assessing the accuracy of Th-based chronologies. The few such data sets on reef-building corals include data reported by Edwards et al. (1997), Gallup et al. (2002), Cutler et al. (2003), and Koetsier et al. (1999). 

The issues of initial Th and Pa are ones of accuracy, that is, how close the age determination is to the true age of the sample. A second concern, one that increases with the sample age, is the closed-system behavior of the U-Th-Pa decay chains. In general, older samples are more at risk because of the longer opportunity for diagenetic changes to have taken place. Speleothems with significant porosity are poorer candidates for closed-system behavior than are speleothems with dense fabrics. Fluid inclusions are a ubiquitous source of micron-scale fabric porosity but are generally isolated and thus do not translate to permeability with diagenetic implications. 

In conclusion, fluid-rock ratios should be used for a rough calculation to demonstrate open or closed system behavior. These ratios can yield qualitative information on stable isotope fluid-rock exchange. Their use should be limited to cases where continuum mechanics approaches to stable isotope transport are not applicable. This is often the case when field relations do not provide evidence of the geometry of the flow system. But one should keep in mind while interpreting the data, that the values do not correspond to actual, physical fluid amounts, but just represent a measurement of exchange (reaction) progress. 

The addition of one extra phase, crude oil, to a gas/water system more than doubles the range of fractionation that can occur in any associated gas phase, from between F(Ne/Ar)gas = 1.0 to 2.03 to between F(Ne/Ar)gas = 0.31 to 5.3. In the case of an open system, fractionation in the residual phase will be even more extreme, and reflected in much lower concentrations (Battani et al. 2000). While it is possible to envisage a myriad of different interactions between water, gas and oil phases, depending on the order of interaction, open or closed system behavior and the relative fluid volumes, it is not a... 

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