60Fe-60Ni Systematics of Chondrules from Primitive Chondritic Meteorites

Klaus Keil recognized the importance of microanalytical isotope analyses for unraveling the complex origins of meteorites. During his time at the University of Hawaii at Manoa (UH), Klaus worked tirelessly to establish the cosmochemistry group and the secondary ion mass spectrometer (SIMS) facility [1-2]. I matriculated at UH in 2009 for my PhD, I was drawn to the prospect of working alongside such accomplished and enthusiastic researchers, access to the SIMS lab, and the group's fascinating work on short-lived radionuclides (SLRs) in meteorites. SLRs are radionuclides for which the original inventory inherited from the interstellar medium has completely decayed away. They are also referred to as extinct radionuclides. SLRs are important tools for constraining the timescales of early Solar System processes linked to the origin and evolution of planetesimals, the building blocks of planets. Klaus’ development of electron probe microanalysis (EPMA) and electron dispersive spectroscopy (EDS) techniques for meteorite analyses paved the way for the coordinated microanalytical geochemical approach necessary for most SIMS analyses. Iron-60 (t1/2=2.6 Ma), a SLR that decays to 60Ni, is of interest for constraining early solar system chronology, identifying the stellar source of short-lived radionuclides, and understanding the astrophysical environment of the Sun’s formation. Previous efforts to constrain the initial solar system (60Fe/56Fe)SS via in situ microanalytical techniques have primarily focused on chondrules from unequilibrated ordinary chondrites (UOCs) [3-4]. In this study, we compare 60Fe-60Ni systematics of chondrules from UOCs and those from CO3 chondrites, potentially more pristine samples [5]. We analyzed a total of 8 chondrules from primitive chondrites, CO3s and UOCs, because they have experienced relatively little thermal metamorphism and little aqueous alteration, which can disturb 60Fe-60Ni systematics [6]. Four chondrules from Krymka (UOC, LL3.2), NWA 8737 (CO3.0), and NWA 7892 (CO3.0) were carefully separated from whole rock samples using ceramic (non-metal) micro-tools. Another four chondrules were identified in thin sections and/or polished epoxy mounts of Kainsaz (CO3.2), DOM 10104 (CO3), Krymka (UOC, LL3.2) and Semarkona (UOC, LL3.00). The chondrules were characterized prior to SIMS analyses using a combination of the scanning electron microscopy (SEM) EDS at UCSC and EPMA at Stanford and UCLA. Backscatter electron images of a subset of our chondrules are provided in Figure 1. The initial 60Fe/56Fe ratio of a chondrule is inferred from an isochron diagram of the Ni-isotopic composition (e.g., 60Ni/62Ni) as a function of the Fe/Ni ratio (e.g., 56Fe/62Ni). For our in situ isotope analyses, we analyzed chondrule olivine and pyroxene using SIMS at UCLA and UH. We used the Hyperion-II oxygen plasma source to sputter our sample surface. We tuned the instrument to analyze Fe and Ni isotopes in peak-hopping mode: iron (56Fe+) and 60Ni+ were counted simultaneously on a multicollector Faraday cup and the monocollection electron multiplier (EM), respectively; 61Ni+ and 62Ni+ were measured on the monocollection EM after switching the magnetic fields. Analyses typically ran for 100 cycles, but some were cut short due to the beam sputtering into a different phase. For our standards, we used San Carlos olivine and synthetic pyroxene. Isotope ratios were calculated from the summed counts of each isotope. We applied a linear mass fractionation correction to determine the excess 60Ni (e.g., Δ60Ni = δ60Ni - 2 × δ61Ni). A relative sensitivity factor was calculated by comparing the elemental Fe/Ni ratios of the standards from EPMA analyses to the ratios measured using SIMS. The weighted linear regression and the mean squared weighted deviation (MSWD) of the isochrons were determined using Isoplot Model 1 fit. Sources of uncertainty are mainly from the low 61Ni counts. Data reduction follows procedures outlined by [3]. Figure 2 shows isochron diagrams for chondrules from CO3 (left) and UOCs (right) analyzed for this study. The 60Ni/62Ni values for chondrules in both chondrite types are unresolved from the terrestrial value (black line in Fig. 2). The intercept of the linear regression is forced through the terrestrial value (black circle). This reduces the uncertainty slightly but does not significantly change the results. An isochron with a slope of 1×10-7 is provided for comparison (gray line in Fig. 2). Chondrule J6 from NWA 8737 (CO3.0) had the highest 56Fe/62Ni values but did not show resolvable excess in 60Ni. Thus, these results do not provide support for excess 60Ni in chondrules as discussed in [3]. Assuming that UOC and CO3 chondrules formed from the same reservoir and at the same time, an initial 60Fe/56Fe ratio of <0.8×10-7 (1-sided upper limit) is inferred from the chondrules analyzed in this study. This assumption is supported by the similar 26Al-26Mg ages of chondrules from UOCs and CO3s (2-3 Ma after CAIs [7]). This relatively low initial ratio is consistent with previous resonant ionization mass spectrometry of a chondrule from Semarkona [4] and consistent with recent work on 60Fe-60Ni systematics of troilite (FeS) in CO3 chondrites [8]. Bulk MC-ICP-MS Fe-Ni analyses of UOC chondrules also infer a low initial 60Fe/56Fe ratio of 1×10-8 [9]. A recent study on the bulk Fe and Ni isotopic composition of iron meteorites estimated the initial 60Fe/56Fe ratio of 6.4×10-7 for the region where carbonaceous chondrites formed [10]. This would correspond to an initial ratio of (3-4)×10-7 for CO3 chondrules, but this is inconsistent with the results presented here. Telus et al. [3] attributed the low inferred initial ratios for UOCs to Fe and Ni mobilization in the chondrites [6]; however, such alteration in CO3 chondrites was found to be negligible. The lack of resolvable excess 60Ni in chondrules from primitive chondrites calls into question the idea of a supernova source for short-lived radionuclides [11, 12]. Backscatter electron microscope images provide examples of chondrules from CO3s and UOCs that were analyzed for this study. The Kainsaz sample is in a polished mount. NWA 8738 – J6 and Krymka – J10 are chondrule separates mounted in epoxy. Dark circular pits are SIMS spot analyses and boxes highlight areas that were targeted for SIMS analyses. Colored dots are linked to symbols on the isochron plots in Figure 2. 60Fe-60Ni isochron diagrams for chondrules from CO3 (left) and UOC (right) that were analyzed for this study. Each color is for a different chondrule. The linear regression is forced through the terrestrial value (black circles and black lines). The slope corresponding to 1×10-7 is provided for comparison (gray lines).