Stable Sr Variations in Impactites of Lonar Impact Crater, India: A Terrestrial Analogue for Lunar Crustal Evolution

Impact craters are ubiquitous features on surfaces of planetary bodies in the inner Solar System. Impact cratering exposes subsurface materials, making them valuable for studying subsurface compositions of planetary bodies. The ~1.88 km diameter Lonar crater in India is a simple crater that formed by the impact of a chondritic impactor ~570 ka ago [1,2]. This is a well-preserved crater hosted entirely within the ~66 My old Deccan continental flood basalts, making it an ideal terrestrial analogue for craters on the basaltic surfaces of other planetary bodies like the Moon. We report geochemical and stable (δ⁸⁸Sr) and radiogenic (⁸⁷Sr/⁸⁶Sr) Sr isotopic compositions of six target basalts and nine impact melt breccias sampled from the upper crater wall and the distal ejecta blanket [2,3]. Geochemical measurements were performed using an ICP-MS (Thermo Scientific iCAP RQ), while Sr isotopic compositions were measured using TIMS (Thermo Scientific Triton Plus) at the Centre for Earth Sciences, IISc, Bengaluru. The external reproducibility for δ⁸⁸Sr measurements using an ⁸⁴Sr-⁸⁷Sr double-spike technique [4] was better than 0.033‰ (2SD) based on repeated analyses of NIST SRM-987 Sr standard (n=6).

The δ⁸⁸Sr values of the Lonar crater rocks are the first such values reported for any impact crater; the δ⁸⁸Sr values range from 0.256‰ to 0.305‰ for the target basalts (average = 0.278 ± 0.04‰ (2SD), n = 6) and from 0.113‰ to 0.288‰ for the impact melt breccias. The impactites are categorized into two groups: Group 1 (n=4) with δ⁸⁸Sr values overlapping those of target basalts, and Group 2 (n=5), which exhibits lower δ⁸⁸Sr values relative to the target basalts. The ⁸⁷Sr/⁸⁶Sr ratios of the impactites (0.707519-0.708139) are more radiogenic than the target basalt average of 0.706600 and are consistent with a 3-5 wt% contribution from the underlying granitic basement of Deccan lavas to the impact melt breccias [2,3]. After correcting for the contribution of the basement, the δ⁸⁸Sr values of the impactites were used to model the extent and nature of kinetic isotope fractionation, employing the standard Rayleigh fractionation model using a Monte Carlo simulation. The absence of heavier δ⁸⁸Sr values in the impact melt breccias suggests that Lonar impactites predominantly reflect origin from vapour condensates. The primary vapour originated from complete volatilization of Sr from the target and projectile, yielding a δ⁸⁸Sr similar to Lonar basalts. Group 2 impact melt breccias likely contain a component formed through nearly complete (>99%) Sr condensation within the impact vapour plume. In contrast, Group 1 impact melt breccias may have originated from the impact ejecta blanket, reflecting no evidence of significant evaporative loss.

[1] Fredriksson, K., et al. (1973), Science 180.4088.

[2] Gupta, R. D., et al. (2017), GCA, 215.

[3] Chakrabarti, R., & Basu, A.R. (2006), EPSL 247.3-4.

[4] Ganguly, S., & Chakrabarti, R. (2022), JAAS, 37(10).