11:15 AM - 11:30 AM
[PCG18-03] Simulating K-Ar isochron ages of Martian meteorites with EPMA mapping
Keywords:K-Ar, LIBS, Martian meteorites, EPMA
Determination of the absolute ages of planetary surfaces is essential for understanding planetary history. In particular, Martian surfaces have not been dated; there are many important unresolved questions, such as dating the duration of the warm and wet climate and the timing of volcanism resulting from Mars’ interior activity. In fact, the Decadal Survey indicates that a 200-Myr accuracy is needed for a 4 Ga rock to address key questions about the epoch of Martian habitability. Such high-precision dating would require a new measurement method. In fact, a new potassium-argon (K-Ar) dating method based on spot-by-spot age measurements by laser ablation has been developed by different groups (e.g., [1–4]).
Previous studies have theoretically evaluated dating accuracy achievable with the K-Ar isochron method. Bogard [5] simulated K-Ar isochrons to investigate how the range of K concentration in measured spots would influence error in isochron slopes and found that K concentration range must be more than a factor of two to achieve isochron slopes with an error of a few tens of percent. Cho et al. [6] also evaluated the effect of the measurement uncertainties of K and Ar concentrations on age determination errors.
However, these studies did not systematically consider the spot size, distribution, and K concentrations in the minerals of Martian rocks, which primarily control the accuracy of in situ K-Ar isochron dating. For example, the range of K concentrations would vary with actual mineral compositions and the size of a target rock, as well as the laser spot size of the instrument. Furthermore, the achievable accuracy of K-Ar isochron ages depends highly on the age of the target rock, but it has not been evaluated yet. Such an assessment requires a numerical approach and high-resolution element mapping because the number and ages of Martian meteorites are limited.
Thus, in this study, we evaluated the accuracy of dating achievable under various measurement conditions by combining actual K concentration maps and numerical simulations. We generated hypothetical isochrons while changing the parameters such as the precision of K and Ar measurements, the number of isochron data points, differences in mineral distribution, the formation ages of samples, and laser spot size.
The key for accurate simulation is to use realistic K distributions on Martian igneous rock samples. To this end, we measured the K distribution of actual Martian meteorite samples (nakhlite NWA817 and shergottites NWA1068 and Zagami) with EPMA. The accuracy of age determination is statistically calculated by simulating isochrons with multiple measurement conditions, assuming that spot-by-spot laser measurements are randomly conducted on Martian rocks.
Our analysis using the EPMA data indicate that the spot-to-spot variation in K concentration was larger than a factor of seven for NWA1068 (0.06–0.94 wt%; bulk K2O = 0.16 wt% [7]) and NWA817 (0.13–0.97 wt%; bulk K2O = 0.32 wt% [8]) when a laser spot diameter of 500 μm and an ablated depth of 500 μm were assumed. In contrast, Zagami has a much narrower range of K concentrations than the other two, 0.14–0.34 wt% compared to the bulk concentration of 0.14–0.17 wt% [9]. Our isochron simulations using the K distribution of these meteorites show that isochron ages can be measured with the error <20% for rocks with a mineral size distribution similar to NWA817 or NWA1068. For a sample with a Zagami-like lithology, the K concentration range cannot be greater than 4 if the laser spot size is 500 μm, and thus the statistical variability in dating accuracy was large, and the dating accuracy was 20% even in the best data suite. The simulation results obtained from EPMA are consistent with the results of the K concentration measured by the laser-induced breakdown spectroscopy (LIBS) instrument under development (Fig. 1).
Also, our analysis indicates that the age of a 4 Ga rock can be measured with an error of 200 Myr if the measurement precision of K and Ar concentrations of 15 data points is less than ±8% and the laser spot size is 500 um. This means that our simulation constrained conditions for the instrument and the lithology of sample to achieve sufficient accuracy in dating Martian rocks using K-Ar isochron.
References: [1] Cohen+ 2014 Geostand Geoanal Res, 38:421, [2] Devismes+ 2016 Geostand Geoanal Res, 40:517, [3] Cho & Cohen 2018 RCMS, 32:1755, [4] Cattani+ 2019 Chemical Geology 506 1-16, [5] Bogard+ 2009 MaPS, 44(6):905, [6] Cho+ 2016 PSS, 128:14, [7] Barrat+ 2002 Geochimica et Cosmochimica Acta, 66(19):3505, [8] Sautter+ 2002 EPSL 195 223-238, [9] Kong+ 1999 Geochimica et Cosmochimica Acta, 63, 11-12, 1865-1875
Previous studies have theoretically evaluated dating accuracy achievable with the K-Ar isochron method. Bogard [5] simulated K-Ar isochrons to investigate how the range of K concentration in measured spots would influence error in isochron slopes and found that K concentration range must be more than a factor of two to achieve isochron slopes with an error of a few tens of percent. Cho et al. [6] also evaluated the effect of the measurement uncertainties of K and Ar concentrations on age determination errors.
However, these studies did not systematically consider the spot size, distribution, and K concentrations in the minerals of Martian rocks, which primarily control the accuracy of in situ K-Ar isochron dating. For example, the range of K concentrations would vary with actual mineral compositions and the size of a target rock, as well as the laser spot size of the instrument. Furthermore, the achievable accuracy of K-Ar isochron ages depends highly on the age of the target rock, but it has not been evaluated yet. Such an assessment requires a numerical approach and high-resolution element mapping because the number and ages of Martian meteorites are limited.
Thus, in this study, we evaluated the accuracy of dating achievable under various measurement conditions by combining actual K concentration maps and numerical simulations. We generated hypothetical isochrons while changing the parameters such as the precision of K and Ar measurements, the number of isochron data points, differences in mineral distribution, the formation ages of samples, and laser spot size.
The key for accurate simulation is to use realistic K distributions on Martian igneous rock samples. To this end, we measured the K distribution of actual Martian meteorite samples (nakhlite NWA817 and shergottites NWA1068 and Zagami) with EPMA. The accuracy of age determination is statistically calculated by simulating isochrons with multiple measurement conditions, assuming that spot-by-spot laser measurements are randomly conducted on Martian rocks.
Our analysis using the EPMA data indicate that the spot-to-spot variation in K concentration was larger than a factor of seven for NWA1068 (0.06–0.94 wt%; bulk K2O = 0.16 wt% [7]) and NWA817 (0.13–0.97 wt%; bulk K2O = 0.32 wt% [8]) when a laser spot diameter of 500 μm and an ablated depth of 500 μm were assumed. In contrast, Zagami has a much narrower range of K concentrations than the other two, 0.14–0.34 wt% compared to the bulk concentration of 0.14–0.17 wt% [9]. Our isochron simulations using the K distribution of these meteorites show that isochron ages can be measured with the error <20% for rocks with a mineral size distribution similar to NWA817 or NWA1068. For a sample with a Zagami-like lithology, the K concentration range cannot be greater than 4 if the laser spot size is 500 μm, and thus the statistical variability in dating accuracy was large, and the dating accuracy was 20% even in the best data suite. The simulation results obtained from EPMA are consistent with the results of the K concentration measured by the laser-induced breakdown spectroscopy (LIBS) instrument under development (Fig. 1).
Also, our analysis indicates that the age of a 4 Ga rock can be measured with an error of 200 Myr if the measurement precision of K and Ar concentrations of 15 data points is less than ±8% and the laser spot size is 500 um. This means that our simulation constrained conditions for the instrument and the lithology of sample to achieve sufficient accuracy in dating Martian rocks using K-Ar isochron.
References: [1] Cohen+ 2014 Geostand Geoanal Res, 38:421, [2] Devismes+ 2016 Geostand Geoanal Res, 40:517, [3] Cho & Cohen 2018 RCMS, 32:1755, [4] Cattani+ 2019 Chemical Geology 506 1-16, [5] Bogard+ 2009 MaPS, 44(6):905, [6] Cho+ 2016 PSS, 128:14, [7] Barrat+ 2002 Geochimica et Cosmochimica Acta, 66(19):3505, [8] Sautter+ 2002 EPSL 195 223-238, [9] Kong+ 1999 Geochimica et Cosmochimica Acta, 63, 11-12, 1865-1875