Refined isotopic compositions of K, Ca and a complementary comparison of the 40K-40Ca, 40K-40Ar and 87Rb-87Sr chronometers
Creators: name: Dèzes, Mariia
Publisher: name: Universität Bern
Year: 2016
Subjects: 550 Earth sciences &amp; geology

description: The K-Ar geochronometer and its more sophisticated Ar-Ar version are among the most used methods in geochronology. Recently the EarthTime initiative started the attempt to calibrate the Geochronological Time Scale with a precision of 0.1% by intercalibrating orbital tuning, absolute dating and relative dating. This requires the absolute dating to be of a precision of 0.1%. Such a precision is achievable on modern mass-spectrometers for the measurements of isotope ratios; however the accuracy of the Ar-Ar dating system strongly depends on the accuracy of the constants incorporated into the age equation. Three constants are used for Ar-Ar age calculations: 1) the age of the irradiation flux monitor; 2) the 40K isotopic abundance and 3) the 40K decay constant and its branching ratio. The first one has been an object of calibrations during the last two decades; and an age of some irradiation standards is claimed to be known with a precision of 0.1%. We did not focus on this problem and just briefly discuss the issue of the age of the Fish Canyon Tuff sanidine irradiation standard in Chapter 3. Two other constants are the subject of this study. The natural K isotopic composition has been measured in 1975 with the gravimetrical method, which yielded a high precision for the 41K/39K value, but an insufficient precision for the 40K/39K ratio, which is one of the key values for K-Ar and Ar-Ar dating. The isotopic compositions of two standards NIST SRM 918a and SRM 918b have been measured in the frame of this study and compared with the bulk Earth isotopic composition. I measured K isotopic composition on a thermal ionization mass spectrometer (TIMS Triton Plus) with the application of three different amplifiers, which allowed to acquire a wide dynamic range with high precision. Three measurement techniques have been applied: total evaporation, block total evaporation and conventional block measurements. Due to a high instrumental fractionation, total evaporation measurements did not give consistent results, thus the 40K/39K ratios have been normalized to the fixed and precise 41K/39K ratio of Garner et al. (1975) [Garner E.L. et al. 1975. J. Res. Natl. Bur. Stand. 79A, 713-725]. The resulting best estimate for the 40K/39K ratio is 0.000 125 116 ± 57 (2σ), corresponding to an isotopic abundance 40K/K = (1.1668 ± 8) × 104. This value is identical with previous estimations, but the uncertainty is five times better, which brings the uncertainty of 40K isotopic abundance from 0.35% to the desired level below 0.1%. 40K decay constants are more complicated to measure. Radioactive 40K decays to 40Ca and to 40Ar; the decay to Ar presumably has three paths, two of which can be measured and one that is calculated theoretically. Therefore the uncertainty on the 40K decay constants is relatively high, which limits the accuracy of the Ar-Ar dating tool. In this work the estimation of the 40K decay constants is approached by intercalibration of K-Ca and Ar-Ar with U-Pb and Rb-Sr ages of well-characterized magmatic samples with simple geological histories. This task itself relied on two subtasks: 1) developing a high precision K-Ca dating method and 2) dating samples with a well-known geological history, using the K-Ca, Ar-Ar and Rb-Sr methods, and comparing the obtained ages with published U-Pb reference ages. High precision K-Ca dating relies upon high precision Ca measurements. A unique high precision method for the simultaneous measurements of the full range of Ca isotopes was developed, which makes use of a TIMS Triton Plus especially designed for Ca measurements. This allowed to measure 40Ca/44Ca ratios with an outstanding reproducibility of 0.06‰. The method was first used to compare the internal consistency of published Ca isotopic compositions. The isotopic compositions of two Ca standards, NIST SRM 915a and SRM 915b, were found to be identical within uncertainty and in line with published values. Small samples measured in the frame of this study (&lt;1μg) suffered from an intense instrumental fractionation, which cannot anymore be sufficiently corrected with a simple exponential law. Therefore the use of an additional term for the exponential fractionation correction, which eliminates the offset due to an insufficient instrumental fractionation correction, is suggested and applied here. Finally K-Ca combined with Rb-Sr and Ar-Ar dating was performed on four samples. Potential samples for intercalibration should be of considerable age with a “point-like” geological history which limits the choice to well described and already dated samples. Quantitative element profiles and semi-quantitative mapping by electron microprobe were essential to ensure that our samples did not suffer from recrystallization or any other post-emplacement processes. For the Archean Siilinjärvi carbonatite Rb-Sr and low precision Ar-Ar and K-Ca dating indicate the occurrence of a post-emplacement metamorphic event at 1869 Ma. The other three samples appear to have a “point-like” geological history. A sample from the Triassic Bolgokhtokh intrusion, Kotuy, Russia, was dated with the Ar-Ar system and yielded an 225 Ma age, however microscopic investigations showed a very prominent zonation, which prevented the use of this sample for the decay constant intercalibration. The Rubikon lepidolite, Namibia, yielded an 505 Ma Rb-Sr age consistent with a previously published U-Pb age. The low precision K-Ca age we obtained did not allow us, so far, to use this sample for the 40K decay intercalibration. The fourth sample from the Phalaborwa carbonatite complex not only shows a “point-like” geological history, but also yielded high precision Rb-Sr and K-Ca ages. The Rb-Sr age for this sample is coincident with previously reported U-Pb ages for this complex and K-Ca as well as Ar-Ar ages are ca. 1% younger. Results from this last sample allowed us to assess the 40K total decay constant. Within a reported range of the 40K branching ratios the most suitable decay constant is the one published by Min at al. (2000) [Min, K., Mundil, R., Renne, P.R., Ludwig, K.R., 2000. Geochim. Cosmochim. Acta 64, 73-98]. This constant is lower than the currently recommended value and has been previously reported to be the best-fit constant also for Ar-Ar and U-Pb ages intercalibrations. Thus this work presents a newly determined 40K/K ratio with a precision of 0.07%, as well as a narrowed down range of plausible 40K decay constants, and confirms the best fit total decay constant for 40K geochronometers. It also describes high precision static Ca measurements, recommends the most consistent Ca isotopic composition, determines isotopic compositions of two widely used Ca standards and finally proposes an improved fractionation correction for small Ca samples.