Written by Bradley Cave & Darwinaji Subarkah
Geochronology is used as a tool in ore deposit studies to link the age of mineralisation to a particular orogenic/tectonic/metamorphic event (Olierook et al., 2020; Tillberg et al., 2017), refining and redefining the metallurgical model to be used in future exploration campaigns. One of the oldest geochronological methods, and a method that has picked up traction in the recent years is Rb-Sr geochronology. This article is an updated excerpt from an article written in January 2023 that can be found here and included an interesting case study from the McArthur River Zn-Pb-Ag deposit.
How Does Rb-Sr Geochronology Work?
The world was first introduced to the natural radioactivity of 87Rb by Campbell & Wood (1906). Thirty-two years later, Hahn & Walling (1938) pioneered the Rb-Sr dating technique, which has continued to be refined to this day. The Rb-Sr dating technique utilizes the radioactive decay of 87Rb to 87Sr, which has a half-life of 49.61 ± 0.16 Ga (Villa et al., 2015). With reference to the figure below, the rock/mineral of interest has a natural 87Sr/86Sr ratio called the initial value (the straight blue line at the bottom). Over time 87Rb decays to 87Sr, which forms an isochron (the red dotted line in the figure below). The slope of this isochron is time dependent. Older ages will have a steeper slope due to the production of more radiogenic 87Sr, while younger ages will have a shallower slope due to having relatively less radiogenic 87Sr. Thus, the slope of this line allows us to calculate the time since the mineral/rock was last at its initial 87Sr/86Sr ratio. To produce a spread across the isochron and ensure the highest precision possible, analysing different minerals that contain various initial 87Rb/86Sr ratios (spread across the blue line) is preferred. As Rb has a similar size and charge to K, it will tend to be concentrated in K-rich minerals such as biotite, illite, and K-feldspar, therefore possessing relatively high 87Rb/86Sr ratios. As Sr is similar in size and charge to Ca, it will tend to be concentrated in Ca-rich minerals such as anorthite, clinopyroxene, calcite and apatite, and therefore possess relatively low 87Rb/86Sr ratios (see Figure 1). However, minerals such as biotite often contains a natural variation in 87Rb/86Sr ratios and therefore can be used solely to obtain a meaningful Rb-Sr age.
Comparison Between the Old & New Method
One of the main obstacles that must be overcome with Rb-Sr geochronology is the isobaric interface between 87Rb and 87Sr (as these elements have the same mass, it’s quite hard to get the mass spectrometer to tell them apart). The “old method”, or more accurately the traditional method relied on whole-rock samples. This involved first crushing the rock to separate out the various mineral phases, using column ion chromatography to separate the various Rb and Sr phases, then doping the material with an enriched tracer before analysing the material using Thermal Ionization Mass Spectrometry (Nebel, 2015). In short, the old method is laboratory intensive, expensive and results often take months to produce. Moreover, as this method involved dissolving and analysing whole-rock samples, it may also lead to mixed signatures and an overall geologically meaningless age. However, the advantage of this method is that the results are often very precise, containing errors proportional to <0.5-1% of the produced age (5). The new method uses a Laser Ablation system coupled to a Mass Spectrometer with an additional cell that contains N2O gas used to separate 87Sr from 87Rb, and overcoming the isobaric interference problem (6). This approach is as easy as mounting an appropriate sample into a resin block, polishing it, and putting it into the laser. Hundreds of analyses can be performed a day using this method. This technique also preserves the textural composition of your sample, where particular minerals of interest (for example, biotite in a cross-cutting vein) can be targeted for analyses. So although this new method may produce ages that are relatively less precise (errors proportional to 2% to ><0.5-1% of the produced age (Nebel et al., 2011).
The new method uses a Laser Ablation system coupled to a Mass Spectrometer with an additional cell that contains N2O gas used to separate 87Sr from 87Rb and overcoming the isobaric interference problem (Zack and Hogmalm, 2016). This approach is as easy as mounting an appropriate sample into a resin block, polishing it, and putting it into the laser. Hundreds of analyses can be performed a day using this method. This technique also preserves the textural composition of your sample, where minerals of interest (for example, biotite in a cross-cutting vein) can be targeted for analyses. So, although this new method may produce ages that are relatively less precise (errors proportional to 2% to >5% of the obtained age), they are likely to be more accurate (Redaa et al., 2021). In addition, the new method also allows the user to collect trace elements at the same time as Rb-Sr geochronology, which can be used to differentiate potential age populations.
In conclusion, in-situ Rb-Sr geochronology provides less precise data relative to the traditional Rb-Sr method. However, this trade-off is outweighed by the benefits which includes the ability to provide petrological context of the analyses, the ability to analyse trace elements, the ability to run hundreds of analyses per day and the ability to check for inclusions throughout the individual analyses. With these benefits in mind, this technique has the potential to be a game changer in future ore deposit-related studies.
References
Campbell, N.R., Wood, A., 1906. Radioactivity of the alkali metals. Proc Camb Philos Soc 14, 15–21.
Hahn, O., Walling, E., 1938. Über die Möglichkeit geologischer Altersbestimmungen rubidiumhaltiger Mineralien und Gesteine. Z Anorg Allg Chem 236, 78–82.
Nebel, O., 2015. Rb–Sr Dating BT - Encyclopedia of Scientific Dating Methods, in: Jack Rink, W., Thompson, J.W. (Eds.), . Springer Netherlands, Dordrecht, pp. 686–698. https://doi.org/10.1007/978-94-007-6304-3_116
Nebel, O., Scherer, E.E., Mezger, K., 2011. Evaluation of the 87Rb decay constant by age comparison against the U–Pb system. Earth Planet Sci Lett 301, 1–8.
Olierook, H.K.H., Rankenburg, K., Ulrich, S., Kirkland, C.L., Evans, N.J., Brown, S., McInnes, B.I.A., Prent, A., Gillespie, J., McDonald, B., 2020. Resolving multiple geological events using in situ Rb–Sr geochronology: implications for metallogenesis at Tropicana, Western Australia. Geochronology 2, 283–303.
Redaa, A., Farkaš, J., Gilbert, S., Collins, A.S., Wade, B., Löhr, S., Zack, T., Garbe-Schönberg, D., 2021. Assessment of elemental fractionation and matrix effects during in situ Rb–Sr dating of phlogopite by LA-ICP-MS/MS: implications for the accuracy and precision of mineral ages. J Anal At Spectrom 36, 322–344.
Tillberg, M., Drake, H., Zack, T., Hogmalm, J., Åström, M., 2017. In situ Rb-Sr dating of fine-grained vein mineralizations using LA-ICP-MS. Procedia Earth and Planetary Science 17, 464–467.
Villa, I.M., De Bièvre, P., Holden, N.E., Renne, P.R., 2015. IUPAC-IUGS recommendation on the half life of 87Rb. Geochim Cosmochim Acta 164, 382–385.
Zack, T., Hogmalm, K.J., 2016. Laser ablation Rb/Sr dating by online chemical separation of Rb and Sr in an oxygen-filled reaction cell. Chem Geol 437, 120–133.
Li, Chao-Feng, et al. "A rapid single column separation scheme for high-precision Sr–Nd–Pb isotopic analysis in geological samples using thermal ionization mass spectrometry." Analytical Methods 7.11 (2015): 4793-4802.