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Potassium:Absolute Chronologies Calibrated Using Really Accurate TIMS Experiments

Final Report Summary - K:ACCURATE (Potassium:Absolute Chronologies Calibrated Using Really Accurate TIMS Experiments)

K:ACCURATE Final Report

Project Abstract

Accurate and precise geochronology is fundamental to earth sciences, essential for the quantification of rates and durations of geological processes and in unravelling cause and effect relationships. The 40Ar/39Ar technique has the widest temporal range of any geochronometer, can be applied to many K-rich rocks, and is used to solve key questions in earth, life, and planetary sciences. Modern mass spectrometers generate measurements with analytical uncertainties (precision) much smaller than the uncertainty of the fundamental constants required for age calculations (accuracy). Whilst precision is often better than 0.1%, accuracy is limited to ~ 2.5% due to uncertainties in the decay of 40K and absolute ages of mineral standards. Many of these parameters are based on poorly documented studies from the 1960s and 1970s and need to be improved. I aim to improve the accuracy of the 40Ar/39Ar system from ~2.5% to ~0.5%, calibrated against SI standards and independent from other geochronometers. Objectives include: (1) develop an SI traceable isotopic standard for measuring absolute abundances of 40K, (2) measure the absolute amount of 40K in mineral standards, (3) measure the natural relative abundances of K isotopes and recalculate the partial decay constant of 40K to 40Ca, and (4) test improvements in technique accuracy with other geochronometers (e.g. U-Pb and astronomical tuning). The determination of absolute 40K abundances described in this project is complementary to my current project in a Marie Curie ITN that develops a system for measuring absolute abundances of 40Ar. The long overdue measurements for both K and Ar will facilitate the first principles recalibration of the age of 40Ar/39Ar mineral standards, allow a future redetermination of the 40K decay scheme, and improve the accuracy of the 40Ar/39Ar technique. Ultimately, I aim to deliver the tools with which earth scientists will “dissect” deep geological time with hitherto unattainable resolution.


Results

Objective 1: develop an SI traceable isotopic standard for measuring absolute abundances of 40K
An SI traceable standard solution was created at the National Physical Laboratory in Teddington, UK, from NIST SRM 999b. This standard has certified K elemental concentration, as delivered in a powder. In the Mass Metrology Laboratory at NPL, the powder was dissolved into a solution in carefully determined quantities, so the solution has a calibration K concentration. This solution is a key step towards objective 2.

Objective 2: measure the absolute amount of 40K in mineral standards
The amount of 40K in mineral standards will be determined in early 2014, based on measurements of solutions mixed with the standard developed in objective 1. Towards this, we published a paper presenting isotopic data for a new sampling of a mineral standard and have made this sampling available to the geochronological community.

Objective 3: measure the natural relative abundances of K isotopes and recalculate the partial decay constant of 40K to 40Ca
Attempts to determine the isotopic composition of K in natural materials has led to the development of a new method (ICP-MS) for measuring K isotopes (41K/39K). This method allowed us to identify variability in K isotopic compositions in natural materials for the first time. As a result, it has become clear that recalculating the partial decay constant of 40K is not possible, as the activity counting measurements from the 1950s do not indicate what material was used. Thus, further activity counting measurements, and isotopic measurements on that material, are required.

Objective 4: test improvements in technique accuracy with other geochronometers (e.g. U-Pb and astronomical tuning)

The natural variability in K isotopic compositions discovered here is becoming an important new component in accurate 40Ar/39Ar age calculations. Recent calibrations of the 40Ar/39Ar technique were tested as part of an effort to determine the age of the K-T (dinosaur extinction) boundary. This work was published in Science (see “dissemination” section).


Conclusions

The most important discovery made as part of this project is the presence of natural terrestrial variability in stable K isotopic ratios. This had not previously been conclusively shown, and a manuscript is currently being prepared for publication. This variability may shed light on fields of earth and planetary sciences as diverse as pegmatite formation, subduction zone processes, seawater salinity, and the atomic weight of potassium.


Impact

The most wide-scale impact from this work is an indication that the average terrestrial isotopic composition of potassium needs to be revised. The current value is based on a standard that we show to vary from the average terrestrial value by ~0.8‰. This suggests that the atomic weight of potassium is biased by 0.0001 u, which would result in a shift in the last reported decimal place (39.0983 to 39.0982). The isotopic compositions and atomic weights of the elements are used by the scientific and technological communities for a wide range of physical constants and laws, as well as metrological, legal, and international standards. This work may thus be of interest to the International Union of Pure and Applied Chemistry (IUPAC), which determines and publishes isotopic abundances and atomic weights.