laser Rb-SR inductively coupled dual mass spectrometry

 

Rubidium (Rb) is a common element that substitutes easily for potassium (K) in minerals and rocks due to its similar ionic radius and charge. Therefore, Rb is found in many common potassium (K)-bearing minerals including K-feldspar, micas, clay minerals, illite and glauconite. ⁸⁷Rb radioactively decays (via emission of negative beta particle) to ⁸⁷Sr over a half-life of 49.61 ± 0.16 billion years. The common presence of rubidium in many minerals as well as its long half-life make it a useful chronometer for dating ancient rock systems.

The use of the ⁸⁷Rb–⁸⁷Sr system to date minerals spans back to Hahn et al., (1943) pioneering dating of mica and over the last 80 years it has been used widely to date rocks and minerals by chemically separating the rubidium and strontium from the material of interest, then analyzing the isotopic ratios of each element. The age of the rock or mineral is then determined using the isochron method where ⁸⁷Rb/⁸⁶Sr is plotted against ⁸⁷Sr/⁸⁶Sr. With increasing age and radiogenic ingrowth of ⁸⁷Sr, the slope of a line connecting the data becomes steeper and the age since isotopic closure can be calculated.

Description of Analytical Method

The laser-based technique described here uses new generation inductively coupled plasma mass spectrometers (ICP-MS) that have in-built reaction cells and multiple quadrupoles at entry and exit points of the reaction cell. With these instruments, ⁸⁷Sr can be mass shifted to ¹⁰³SrO in the presence of N₂O, while ⁸⁷Rb is still measured on-mass. This setup allows for direct, in-situ Rb-Sr dating of minerals and rocks when coupled to a laser ablation system.

At present, the applicability of nanopowder, glasses and matrix-matched mineral standards with well characterized ages are being assessed and advanced. This effort is moving towards developing an array of mineral isochronous and isochemical standards that will allow accuracy and reliability to be monitored and controlled. Precision in Rb–Sr age is primarily a function of a good spread in ⁸⁷Rb/⁸⁶Sr ratios, the number of data points used to define the regression line, and the errors on each individual analysis; complemented also by analytical uncertainties of single collector ICP MS/MS, which however is theoretically as low as 0.15%, thus about an order of magnitude lower than best achieved uncertainty for in-situ RbSr dating and ‘ages’ of about 1.5%.

A particular strength of the technique is that in addition to obtaining the age of the material in question, you obtain the initial ⁸⁷Sr/⁸⁶Sr ratio that can be used to assess for the amount of radiogenic Sr in the environment in which the mineral grew (and/or closed with respect to Sr diffusion). The laser also allows you to simultaneously analyse for other elements of interest that allow you to triage the data for alternation, environment of formation (e.g. Subarkah et al., 2022) or other factors, such as mineralisation prospectivity (e.g. Subarkah et al., 2023). Other advantages of the in situ technique includes faster analytical time, easier sample preparation, and a spatial control on the analysed samples.

Type of Sample Material/Media (e.g. Minerals, TS/Mount)

Separated minerals can be analysed as well as rock chips. In most cases these will be encased in a 2.5 cm diameter epoxy resin mount. Having a polished flat surface is important, especially if the sample is to be imaged before analysis to map for K-rich phases, or specific textures of interest.

Materials successfully analysed include: muscovite, biotite, phlogopite, illite, glauconite, celadonite, K-feldspar. Techniques such as detrital muscovite and feldspar dating are being developed.

In-situ Rb-Sr dating is capable of analyzing paragenetic phases while maintaining their microscale textural context. It is also possible to date shales and aphanitic volcanic rocks as long as the mineral phases hit by the laser beam have variable amounts of rubidium within them and have maintained closed system behavior since their formation.

MinEx Research

Within MinEx CRC, this research has been progressed through various PhD students and applied in NDI regions. Through pairing this technique with other geochronometers and novel metal isotope proxies, this technique can aid in unravelling the history of region.

Workflows in case study areas that integrate this work is currently being produced.

MinEx CRC Researcher Contact Details