Composition of U-Th 'age solutions'

[Dan Condon]

A proposal to develop a series of synthetic U-Th solutions that have isotopic compositions similar to natural materials of a certain age has been submitted to the UK's Natural Environmental Research Council (NERC).  These solutions are synthetic in the sense that these are not simply dissolved natural materials but rather a mixture of pure U and Th (and Pa) in an acid medium.  These 'age solutions' will have their compositions calibrated using U-Th tracers that are themselves calibrated back to SI units via gravimetric principles (e.g., Cheng et al., 2000, Chemical Geology).

Our proposal is to mix and distribute three mixed U-Th solutions and potentially one U-Pa-Th solution with the following compositions:

Solution A.  “Last interglacial coral” - With a ²³⁰Th/U ratio about 0.7 of secular equilibrium; a ≈10% enrichment of ²³⁴U, and a ²³²Th/²³⁰Th atom ratio of ca. 3.  This solution represents a commonly measured sample type with a reasonably large amount of ²³⁰Th.  It will demonstrate the ability of labs to measure Th/U ratios when they are reasonably close to secular equilibrium and when ²³²Th is low.

Solution B. “Last interglacial stalagmite”- Will also have a ²³⁰Th/U ratio about 0.7 of secular equilibrium, but with a 50% enrichment of ²³⁴U, and a ²³²Th/²³⁰Th atom ratio of 5000.  Analytically, the major difference from Solution A is that the higher ²³²Th can cause challenges with tailing, but allows for possible normalizing during dynamic mass spectrometry.  This sample will test the ability of labs to measure samples typical of stalagmites, or dirty carbonates used for isochron approaches.

Solution C. “A young coral” - Will have a ²³⁰Th/U about 0.02 of secular equilibrium, with a ca. 15% enrichment of ²³⁴U and a ²³²Th/²³⁰Th atom ratio of ca. 10.  This sample represents a late Holocene coral and is demanding because of the low concentration of ²³⁰Th.  It tests the ability of laboratories to analyse clean young carbonates.

Solution D. “A young coral with ²³¹Pa” - We will develop a fourth solution for use by the U-Pa community.  This community is smaller (only some 10 laboratories worldwide have this capability at present).  We will use an aliquot of one of the ‘young coral’ U-Th solution and add ²³¹Pa so that ²³⁵U-²³¹Pa and ²³⁸U-²³⁰Th dates are concordant.

Please comment if you have any thoughts on the composition of these ‘age solutions’.