Instrumentation Development forIn Situ⁴⁰Ar/³⁹Ar Planetary Geochronology

Abstract

The chronology of the Solar System, particularly the timing of formation of extra‐terrestrial bodies and their features, is an outstanding problem in planetary science. Although various chronological methods for in situ geochronology have been proposed (e.g., Rb‐Sr, K‐Ar), and even applied (K‐Ar), the reliability, accuracy, and applicability of the 40Ar/39Ar method makes it by far the most desirable chronometer for dating extra‐terrestrial bodies. The method however relies on the neutron irradiation of samples, and thus a neutron source. Herein, we discuss the challenges and feasibility of deploying a passive neutron source to planetary surfaces for the in situ application of the 40Ar/39Ar chronometer. Requirements in generating and shielding neutrons, as well as analysing samples are described, along with an exploration of limitations such as mass, power and cost. Two potential solutions for the in situ extra‐terrestrial deployment of the 40Ar/39Ar method are presented. Although this represents a challenging task, developing the technology to apply the 40Ar/39Ar method on planetary surfaces would represent a major advance towards constraining the timescale of solar system formation and evolution.

A Bayesian approach to the deconvolution of 40Ar/39Ar data from mineral mixtures

“…and Mars Exploration Rover (MER) missions, have pushed the development of an in situ absolute dating package for future rover-based missions (Swindle et al, 2003;Anderson et al, 2012, Cassata, 2014Cohen et al, 2014;Solé, 2014;Cho et al, 2017;Morgan et al, 2017).…”

“…Interpreting these analyses requires a number of assumptions about the ages and distribution of K and Ar in individual components in the mixture. Assumptions about the distribution of Ar could potentially be addressed by putting a 251 Cf source on future rovers, which would be used to irradiate samples with neutrons and allow for in situ 40 Ar/ 39 Ar rather than K-Ar (Morgan et al, 2017). Nonetheless, in situ measurements on future missions will still very likely be applied to mineral mixtures, as mineral separation capabilities on rovers are currently limited to simple processes such as simple sieving (e.g., Mahaffy et al, 2012;Farley et al, 2014).…”

“…Extraterrestrial meteorite samples are another example where 40 Ar/ 39 Ar geochronology of multiple mineral phases is often required due to very small sample sizes, but also desirable because different mineral phases will have different temperature sensitivities and potentially record different thermal events in a meteorite's history (e.g., Cassata et al, 2010;. Similarly, potential in situ rover-based 40 Ar/ 39 Ar geochronology on future missions to Mars, the Moon or other rocky bodies (e.g., Morgan et al, 2017) would require analyses of multiple mineral phases as sample preparation has been limited to simple sieving (Farley et al, 2014).…”

In Situ Geochronology on Mars and the Development of Future Instrumentation

“…Morgan et al (2017) investigated the requirements for an 40 Ar- 39 Ar system to be used for in situ applications. The 40 Ar- 39 Ar method is based on the K-Ar method, where fast neutrons are used to convert some stable 39 K to 39 Ar, and measurement of 39 Ar serves as a proxy for the parent isotope 40 K. After irradiation, incremental heating of samples can be used to interrogate the thermal history and age of the sample.…”

“…The Curiosity drill must be placed against a flat surface and has no backstop mechanism, so irregular float rocks such as Jake_M may be pushed away. Alternative acquisition strategies may help to acquire ages on pieces of igneous rocks that occur as float in the sedimentary systems favored by rover exploration, but a mission to a large, well-characterized igneous province on Mars, carrying one or more geochronology instruments, would be a powerful way to constrain the martian impact flux and anchor the absolute age calibration for Mars (Tanaka et al , 2014; Anderson et al , 2015b; Morgan et al , 2017).…”