Accretion and thermal evolution of iiab and iiiab iron meteorite parent bodies inferred from mn-cr chronometry

Introduction: The IIAB and IIIAB iron meteorites belong to the category of ‘‘magmatic irons’’, and are thought to sample the core of distinct parent bodies that experienced large-scale chemical fractionation, most notably metal-silicate separation. The time of metal core formation in the magmatic iron meteorite parent bodies provides a key time marker for the evolution of early formed planetesimals including the accretion and cooling history of the parent body. The timing and duration of such early Solar System processes, including accretion, differentiation and subsequent cooling, can be investigated using the short-lived 53Mn-53Cr chronometer (t1/2 ≈3.7 Ma). Chromite (FeCr2O4) and daubréelite (FeCr2S4) are the two main carrier phases of Cr in IIAB and IIIAB iron meteorites. Both these minerals have low Mn/Cr ratios (≈0.01) and thus, preserve the Cr isotope composition of their growth environment at the time of isotopic closure, while the ingrowth of radiogenic 53Cr from in-situ decay of 53Mn is negligible. Model ages for chromite and daubréelite in IIAB and IIIAB iron meteorites can be obtained by comparing their Cr-isotopic composition with the Crisotope evolution of its assumed reservoir. In order to systematically resolve the ingrowth of 53Cr over a time span of a very few Myrs, Cr isotope abundances need to be measured with high precision by TIMS. The present study reports chromite and daubréelite model ages for IIAB and IIIAB iron meteorites. The age data provides constraints on the accretion time and evolution of their parent bodies. Methods: Two IIIAB (Cape York and Saint Aubin) and three IIAB (Sikhote Alin, Agoudal and NWA 11420) iron meteorite samples were analyzed in this study. The whole rock fragments were hand crushed using an agate mortar and treated in conc. aqua regia on a hot plate set to 90 °C for 48 hrs. to completely dissolve the metal-sulfide dominated matrix, leaving behind chromite particles as residue. In case of sample NWA 11420, a 16 g rock fragment was treated with conc. aqua regia on a hot plate set to 90 °C for 12 hrs. and the residue was further classified as magnetic and nonmagnetic depending on its magnetic behavior. Daubréelite grains were isolated in the non-magnetic portion and then handpicked. Chromite and daubréelite grains weighing 2-5 mg from all the samples were transferred in a 7 ml vials with 150-250 mg of Ammonium bifluoride (NH4F·HF, Sigma-Aldrich, Trace Metal grade) and completely digested following the protocol described in [1]. In summary, the sample and reagent mixture was thermalized in a convection oven set to 230 °C for 48 hrs. Upon cooling, the mixture was dried twice, first with 2 ml conc. HNO3 and then with 1 ml conc. HNO3 and 2 ml MilliQ® water. Purification of Cr from the dissolved samples follows a combination of three steps of cation-anion exchange chromatography adopted from [2]. The purified Cr was loaded onto outgassed single Re filaments with 1.4 μL alumina doped Si gel activator. The samples were analyzed by Thermo Scientific Triton Plus TIMS instrument at University of Bern. Each sample was measured on 1012 filaments and isotope compositions are reported as parts per 10,000 deviations (ε-notation) from the mean value of the NIST SRM 979 Cr standard measured along with the samples in each session. Model for ε53Cr evolution in chondritic reservoir. The model assumes that chondrites represent an isotopically homogeneous reservoir for Cr and Mn that evolved with a distinct Mn/Cr ratio after primordial nebular Mn/Cr fractionation and isolation of the precursor material of chondrites, terrestrial planets and differentiated planetesimals (parent bodies of iron meteorites) [3]. The evolution of the 53Cr/52Cr isotope composition of the chondritic reservoir through time can be expressed as (Eq. 1):