Cosmogenic Effects On Chromium Isotopes In Meteorites

Short-lived radionuclide systems and nucleosynthetic isotope anomalies are two important topics in cosmochemistry.Short-lived radionuclide systems can provide precise time constraints for planetary evolution in the early solar system,and nucleosynthetic isotope anomalies are very useful for tracing the sources and distributions of nucleosynthetic products in the early solar nebula and understanding the stellar processes that produced the elements.Chromium(Cr)mass-independent isotope systems are widely applied in cosmochemistry.53"Mn-53Cr system(t1/2= 3.7 Myr)is suitable to establish the timescale of volatility-related processes in the first 10 Myr after CAI formation.Because Mn and Cr have relatively high abundances in solar system bodies,the 53Mn-53Cr system can be applied to a wide variety of materials.Previous studies found that meteorites in different groups had systematically 54Cr/52Cr variations,and their 54Cr/52Cr ratios exhibited a range of-2.5?(? is 1 part per 10000 relative to terrestrial standards)at the planetary scale.Thus,they could be used for fingerprinting meteorite classes.However,because samples from the planetary surfaces with no atmospheres(e.g.lunar samples)and meteorites suffered from the long-term bombardment by cosmic rays(especially for the extraterrestrial samples that have long exposure ages and high Fe/Cr ratios),the spallation(e.g.56Fe(n,?)53Cr)and thermal/epithermal neutron capture(e.g.53Cr(n,y)54Cr)reactions will produce cosmogenic nuclides that can significantly modify their original 53Cr/52Cr and 54Cr/52Cr ratios.This effect will influence the application of Cr isotope system in cosmochemistry.Thus,it is necessary to systematically analyze the cosmogenic effects on Cr isotopes of extraterrestrial samples and devise the appropriate method for correcting these effects.Compared with other extraterrestrial samples,iron meteorites are the ideal samples for analyzing the cosmogenic effects on Cr isotopes because of their simple chemical composition(the metal phase dominantly consists of Fe-Ni alloy),high exposure ages,and high Fe/Cr ratios.We systematically analyzed the Cr isotopic composition of 25 samples from nine different chemical groups,including 10 pieces of Carbo(?D)from known sampling locations from a well characterized slice.The results show that the samples display large variations in both ?53Cr and ?54Cr,with ?53Cr ranging from-0.04± 0.44 to +268.29 ± 0.14 and s54Cr varying from +0.28 ± 0.72 to +1053.78 ± 0.72,which are the largest variations reported for iron meteorites.The excesses of ?53Cr and s54Cr exhibit a strong positive correlation(with a best fit line of ?54Cr=(3.90 ± 0.03)×?53Cr),implying the mixing between cosmogenic Cr and native Cr.The variations of Cr isotopic compositions in iron meteorites are influenced by the proportions of cosmogenic Cr to native Cr,which are affected by exposure ages,chemical compositions(Fe-Ni-Cr contents),and shielding conditions.The s53Cr-?54Cr correlation(with a slope of?3.9)in iron meteorites is independent of these factors,which provides an effective method to evaluate the cosmogenic contribution to 53Cr by monitoring the cosmogenic variations in ?54Cr in meteoritic irons.In order to investigate the fluences induced by the above controlling factors on the Cr isotopic compositions in iron meteorites,we updated the numerical model for the production of cosmogenic Cr based on the latest parameters.The modeled slope for cosmogenic ?53Cr-?54Cr correlation is 3.6 ± 0.2.Modeling results revealed that the observed excesses in ?53Cr and ?were positively correlated with the exposure ages and decreased with increasing native Cr contents(exhibiting the dilution effect).The production of cosmogenic Cr is dominated by the spallation reactions on Fe targets in iron meteorites,thus the Cr isotopic compositions are approximately positively correlated with Fe contents.Model simulations for Carbo pieces showed that their Cr isotopic compositions were affected by shielding depth.From the surface to the center of the meteoroid,the Cr isotopic compositions first increased with shielding depths,reached a maximum at the depth of 9-12cm,and then decreased with increasing shielding.The measured results for Carbo pieces are quite different from modelling results,probably reflecting the sensitivity of the Cr isotopic composition to variations in the abundance of native Cr between different subsamples.All the controlling factors can be expressed as a governing equation for cosmogenic Cr.Based on this equation,we can give a first-order constraint for the shielding conditions of the analyzed iron meteorites.Similar to iron meteorites,previous studies found the cosmogenic effects on Cr isotopic compositions of lunar samples,but the production of cosmogenic Cr in lunar samples is more complicated than that in iron meteorites.The mechanism of cosmogenic Cr production in lunar samples is still not understood.More importantly,the pre-exposure lunar Cr isotopic compositions can be used to constrain the formation time of the Moon cand the source of the Moon-forming impactor,which will deepen the understanding of the origin of the Moon.Thus,it is necessary to find a proper method to correct the cosmogenic effects on lunar Cr isotopic compositions.We analyzed theCr isotopic compositions of 3 Apollo samples,13 lunar meteorites,and 4 terrestrial samples.Our results showed that lunar samples exhibited obvious variations with?0.4?in ?E53Cr and?1.0? in ?54Cr.Most lunar samples exhibited a robust positive ?54Cr-?53Cr correlation with a slope of?2.7,which was induced by cosmogenic effects.The shallower slope in lunar samples than that in iron meteorites might be caused by differences of the irradiation targets or radiation geometry.Four lunar impact-melt breccias plotted off from the correlation,indicating that these samples might be contaminated by ordinary chondrites(OC)or enstatite chondrites(EC)during later meteorite impact events.The cosmogenic effects on the Cr isotopic compositions of the analyzed lunar samples were corrected using phases with different Fe/Cr ratios,i.e.,spinels and non-spinel minerals(e.g.pyroxene and olivine).The measurements showed that the separates and whole rocks of the three Apollo samples displayed three different correlations between Cr isotopic compositions and Fe/Cr ratios,implying that the production of cosmogenic Cr in lunar samples was also dominated by the spallation reactions on Fe targets.The different slopes of the correlations reflected the different exposure histories of the analyzed samples and the intercepts of the correlations represented the pre-exposure Cr isotopic compositions of Apollo samples.Combined with the ?53Cr-?54Cr correlation defined by non-impact-melt lunar samples,we estimated the pre-exposure lunar ?53Cr = 0.01± 0.03 and ?54Cr = 0.00 ± 0.05.The Cr isotopic compositions of terrestrial samples exhibited slightly positive anomalies(?53Cr=00.04 ± 0.02,?54Cr=0.12 ± 0.03),which might be due to the residuals imparted by the mass-dependent fractionation that followed the linear fractionation law.We estimated the apparent Cr isotopic differences between the Moon and the Earth of?53CrMoon-Earth=-0.03 ± 0.03 and ?54CrMoon-Earth=-0.12 ± 0.06.The upper limits of?53CrMoon-Earth=-0.02 ± 0.03 and ?54CrMoon-Earth=-0.08 ± 0.05 were defined by the spinels,and the lower limits of s53CrMoon-Earcn=-0.06 ± 0.04 and s54CrMoon-Earth=-0.19 ± 0.08 were calculated after correcting for the potential residuals that were induced by mass-dependent fractionations.Based on the estimated terrestrial and pre-exposure lunar Cr isotopic compositions in this study,we estimated the Mn/Cr ratio of the Moon and the composition of the Moon-forming impactor.Combined with the 53Mn-53Cr isochrone defined by most types of the meteorites,we estimated the lunar 55Mn/52Cr ratio of 0.44 ± 0.09,which was identical with that of bulk silicate earth(BSE).With the knowledge of O-Ca-Ti-Cr isotopic compositions in the Earth-Moon system,we constrained the source of the Moon-forming impactor.In the canonical giant impact model,the impactor had ?17O of 0.011 ± 0.005‰,s48Ca of 0.06 ± 0.06,50Ti of-0.05 ± 0.06,?53Cr of-0.01 ± 0.05,and s54Cr of-0.05 ± 0.09,which implied that the composition of the impactor was similar to the precursor material of EH(high-Fe)enstatite chondrites,but the precursor material of the impactor suffered a strong volatilization process in the early stage,resulting in the lower Mn/Cr ratios and ?53Cr values in the impactor than those in EH chondrites.In the high-energy giant impact senario,mixing between the proto-Earth's mantle and the impactor was likely not complete.Our Cr isotopic data indicated that>30%of the proto-Earth's mantle survived the homogenization with the impactor after the Moon-forming giant impact.In this situation,the candidates for the impactor include material with isotopic composition similar to enstatite chondrites and aubrites.To summarize,we systematically analyzed the Cr isotopic compositions of iron meteorites and lunar samples,and found that the variations of Cr isotopic compositions were mainly induced by cosmogenic effects(dominated by spallation reactions on Fe targets).Combined with the updated simulation model for the production of cosmogenic Cr in iron meteorites,we gave the governing equation for cosmogenic Cr.We also promoted two methods for correcting the cosmogenic effects on Cr isotopic compositions of iron meteorites and lunar samples.With the pre-exposure lunar Cr isotopic compositions,we constrained the isotopic composition of the impactor.Our work provides theoretical and experimental basis for the application of high-precision Cr isotopic systems for the extraterrestrial samples with long exposure ages in cosmochemistry.