Radiometric Dating of Rocky Planetary Bodies with Rb-Sr and LA-ICPMS

Abstract:

Constraining ages of surface features on rocky objects in the Solar System provides fundamental answers about the nature of planetary accretion and differentiation, the flux of impactors through time, including the Late Heavy Bombardment, rates of tectonic and volcanic processes, and temporal context for detected organic compounds. The Sample Analysis at Mars (SAM) instrument on the Curiosity rover provided the first in situ chronology measurement by using the K-Ar system. High-priority targets for future in situ chronometric analysis include Mars, Vesta, Io and the Moon. Of particular astrobiological interest are environments that have experienced post-depositional alteration (such as outgassing or metamorphism), which are open-system events which compromise the application of K-Ar dating.

We report on the capability of a commercial single collector Element 2/New Wave UP213 Laser Ablation-Inductively Coupled Plasma Mass Spectrometer (LA-ICPMS) system to measure Rb and Sr isotopic signatures in geological samples. The target goal is for sufficient precision to meet the 400 Myr (2σ) uncertainty goal identified in the 2015 NASA Technology Roadmap through the construction of a multipoint isochron. We have determined the production rate of isobaric interferences, including double charged REE, Ca dimers/argides, and Zn oxides, and constrained sources of uncertainty as a function of sample chemistry. Multiple data treatment schemes were explored to correct for interferences and mass fractionation effects and maximize precision. We varied laser parameters such as spot size, dwell time, repetition rate, and fluence to find the laser configuration that allowed for the best statistical treatment of the data.

The success of this investigation, coupled with recent advances in developing a miniaturized ICPMS that requires less power, mass, and volume than commercial systems, expands the realm of feasible spaceflight instrumentation by adding an analytical method that requires little sample preparation and no need for a complex vacuum system. With the advent of miniaturized LA-ICPMS technology, evaluating the viability of this technology is necessary for realizing the scope of its use in planetary chronology.