Open University Uranium-Series Laboratory  
  Earth and Environmental Sciences, The Open University, Milton Keynes, UK  

Expanding OUUSL capability
 
River water  
The U-series team at the OU are currently developing a number of methods and techniques that are new to us and will keep OUUSL at the forefront of U-series research. Some of these techniques are being developed in response to requests from the user community (231Protactinium analysis), others for our own research projects (230Thorium analysis of ground-waters) and some because they have applications to areas of NERC science and once developed, may attract new users to the facility (226Radium dating of authigenic carbonates; Hydrothermal systems as possible analogues for radioactive waste) and in others we are re-visiting samples, such as molluscs, that have long been considered as unsuitable for U-series dating.

 
Measurement of 231Pa
 

We are currently developing techniques for the measurement of 231Pa by mass spectrometry in a variety of geological and environmental samples. Our analytical method to purify protactinium from carbonate rocks was adapted from Burnett &Yeh (pers. com.) and Regelous et al., 2004. 231Pa data have a number of applications that contribute to research within the NERC remit. 231Pa dating can be used to confirm ages obtained by 230Th dating techniques, which is crucial if open-system behaviour is expected (Appendix 2a: 26, 34, 35). The (231Pa/230Th) activity ratio of marine sediments can provide important information about lateral transport of particulate material, which is essential for proper quantification of the particulate carbon fluxes to the seafloor (Appendix 2a: 37). 231Pa data are finding applications in coral dating (Mortlock et al., 2005, Edwards et al., 1997). 231Pa data of young volcanic rocks can provide confirmation of conclusions based on 230Th and add an extra dimension because the geochemical characteristics of Pa are subtly different from Th.

 
Pa-Th trace

Traces of Pa and Th on the Nu Instruments MC-ICPMS. The green trace is Faraday collector VII which is one mass unit higher than the blue trace for ion counting channel IC0. The green trace for 232Th coincides with the blue trace for 231Pa, the larger blue peak is 230Th. The 230Th peak is approximately 1000x smaller than the 232Th peak.

The 230Th/231Pa=49.15±0.17 which is in secular equilibrium, as expected for our SPA standard.

 

 

 
In open oceans, 231Pa and 230Th are removed from the water column by a process of reversible scavenging (Moran et al., 2002, Moran et al., 2005) which quickly removes 230Th to the sediment. 231Pa is less efficiently scavenged onto particles than 230Th and is therefore more effectively transported via advection and diffusion before it reaches the ocean sediment. The equilibrium partition coefficients for particulate versus dissolved matter may vary with particle type and the (231Pa / 230Th) activity ratio can be of great use in paleo-oceanographic studies, see: Thomson et al., 2006.
 
The 226Ra/Ba system
 
The U-series sub-system 230Th-226Ra has a half-life of 1602y which makes it highly appropriate for systems with millennial ages. Using TIMS with ion counting we can analyse <4 femtogram 226Ra routinely and this is essential because 226Ra is present at the fg/g level in geological materials. The age of a sample, for instance a young stalagmite, is calculated from the 226Ra/Ba ratio in the sample relative to present day 226Ra/Ba. The uncertainty in 226Ra/Ba ages depends mostly on the assumption that 226Ra/Ba has remained constant. This is a reasonable assumption because Ra and Ba have very similar geochemical characteristics under environmental conditions. TIMS 226Ra dating has never previously been applied to speleothem samples and an application in conjunction with lamina counting that would re-enforce confidence in both methods and confirm the validity of the 226Ra/Ba dating method in young authigenic carbonates in Holocene speleothems, is being developed.
 
The 226Ra/Ba ratio in seawater is constant, and the increase of 226Ra from the decay of 230Th can be used to date, for instance, young molluscs.
 
U-series systematics of ground-water  

We have recently made the first measurements of 232Th in the colloidal fraction of groundwater (Pogge von Strandmann, PhD thesis (2006). Contrary to expectations, these revealed that the proportion of 232Th residing in the colloidal fraction is extremely low (<0.01%), but concentrations of 232Th in the dissolved fraction are high (5.5-34 pmol/l) despite its strong affinity for particles. Indeed, Th and U have similar concentrations in this oxygenated groundwater. A theoretical framework to explain our observations has been constructed using the groundwater chemistry modelling software PHREEQC. This behaviour of Th has major ramifications for fluid flow models, as well as radioactive waste disposal, and we thus envisage that this capability will widen our user base. Moreover, we have recently acquired HPLC instrumentation that will enable us to investigate uranium and thorium speciation in ground-waters, as well as other fluids.

 
Dating molluscs  
Dating of molluscs by 230Th/234U methods is generally considered to be unreliable because U is considered labile in mollusc shells. However, the living habit of the marine rock-boring bivalve Lithophaga at or near the high-water level in limestone cliffs, could make it suitable for U-series dating sea-level high-stands because after emergence, Lithophaga is much less susceptible to leaching by vadose water in essentially impervious limestone. This may provide a new way of establishing chronologies for well constrained paleo-sealevels.
 
Hydrothermal systems and possible analogues for radioactive waste  
Another area where U-series isotope analyses have significant potential is in the investigation of natural hydrothermal systems. High temperature geothermal fields are found at a number of localities in volcanically and seismically active areas on the Earth's surface, and represent 'natural laboratories' where the reactions which take place between high temperature fluids and silicate rock may be investigated at first hand. The behaviour of naturally occurring radioactive elements in high-temperature fluids may provide an analogue for radioactive waste studies, and the measurement of natural short-lived isotopes such as 234U, 231Pa, 230Th, 228Ra and 226Ra allows constraints to be placed on the time scales of fluid movement. Along the Earth's submarine mid-ocean ridges, hydrothermal systems are ubiquitous and they have a profound influence not only on the basalt erupted there but also on the chemistry of the oceans, and of sulphide formation (You and Bickle, 1998). In the active volcanic geothermal fields of the Taupo area in New Zealand, the rates of mass transfer have been investigated by documenting the change in Sr isotopes with height in the altered volcanic rocks, and by dating the different generations of hydrothermal carbonates using U-Th isotopes (Grimes et al., 1998).
 
   

Edwards et al. 1997 Science 236 1547–1553
Eikenberg et al., 2001. Quaternary Science Reviews 20, 1935–1953.
Grimes et al. 1998 J. Volcanol. Geotherm. Res. 81 327–333
Millard and Hedges 1996 Geochim. Cosmochim. Acta 60 2139–2152
Moran et al. 2002 Earth and Planetary Science Letters 203(3–5) 999–1015
Moran et al. 2005 Earth and Planetary Science Letters 234 235– 248
Mortlock et al. 2005 Geochimica et Cosmochimica Acta 69(3) 649–657
Osmond and Cowart 1992 in: Uranium-series Disequilibrium Oxford Sci. Publ. Harmon and Ivanovich eds. 290–333
Pike et al. 2002 Geochimica et Cosmochimica Acta 66 4273–4286
Regelous et al. 2004 Analytical Chemistry 76(13) 3584-3589
Thomson et al. 2006 Earth Planet. Sci. Lett. 242 170-185
van Calsteren and Schwieters 1995 Int. J. Mass Spec. Ion Proc. 146/147 119–129
You and Bickle 1998 Nature 394 668–671

 
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© Peter van Calsteren
Last updated: 23 December, 2011 11:38