Chromatographic speciation of Cr(III)-species, inter-species equilibrium isotope fractionation and improved chemical purification strategies for high-precision isotope analysis

Chromatographic purification of chromium (Cr), which is required for high-precision isotope analysis, is complicated by the presence of multiple Cr-species with different effective charges in the acid digested sample aliquots. The differing ion exchange selectivity and sluggish reaction rates of these species can result in incomplete Cr recovery during chromatographic purification. Because of large mass-dependent inter-species isotope fractionation, incomplete recovery can affect the accuracy of high-precision Cr isotope analysis. Here, we demonstrate widely differing cation distribution coefficients of Cr(III)-species (Cr(3+), CrCl(2+) and CrCl(2)(+)) with equilibrium mass-dependent isotope fractionation spanning a range of ~1‰/amu and consistent with theory. The heaviest isotopes partition into Cr(3+), intermediates in CrCl(2+) and the lightest in CrCl(2)(+)/CrCl(3)°. Thus, for a typical reported loss of ~25% Cr (in the form of Cr(3+)) through chromatographic purification, this translates into 185 ppm/amu offset in the stable Cr isotope ratio of the residual sample. Depending on the validity of the mass-bias correction during isotope analysis, this further results in artificial mass-independent effects in the mass-bias corrected (53)Cr/(52)Cr (μ(53) Cr* of 5.2 ppm) and (54)Cr/(52)Cr (μ(54)Cr* of 13.5 ppm) components used to infer chronometric and nucleosynthetic information in meteorites. To mitigate these fractionation effects, we developed strategic chemical sample pre-treatment procedures that ensure high and reproducible Cr recovery. This is achieved either through 1) effective promotion of Cr(3+) by >5 days exposure to HNO(3) —H(2)O(2) solutions at room temperature, resulting in >~98% Cr recovery for most types of sample matrices tested using a cationic chromatographic retention strategy, or 2) formation of Cr(III)-Cl complexes through exposure to concentrated HCl at high temperature (>120 °C) for several hours, resulting in >97.5% Cr recovery using a chromatographic elution strategy that takes advantage of the slow reaction kinetics of de-chlorination of Cr in dilute HCl at room temperature. These procedures significantly improve cation chromatographic purification of Cr over previous methods and allow for high-purity Cr isotope analysis with a total recovery of >95%

Timing and Origin of the Angrite Parent Body Inferred from Cr Isotopes

International audienceAngrite meteorites are some of the oldest materials in the solar system. They provide important information on the earliest evolution of the solar system and accretion timescales of protoplanets. Here, we show that the 54Cr/52Cr ratio is homogeneously distributed among angrite meteorites within 13 parts per million, indicating that precursor materials must have experienced a global-scale melting such as a magma ocean. The 53Cr/52Cr and Mn/Cr ratios are correlated, which is evidence for an initial 53Mn/55Mn ratio of (3.16 ± 0.11) × 10−6 . When anchored to the U-corrected Pb–Pb age for the D’Orbigny angrite, this initial 53Mn/55Mn corresponds to an absolute age of 4563.2 ± 0.3 Ma, i.e., 4.1 ± 0.3 Ma after Ca–Al-rich inclusion-formation. This age is distinct from that of the volatile depletion events dated by the 87Sr/86Sr initial ratio and therefore must correspond to the age of crystallization of the magma ocean and crust formation of the angrite parent body (APB), which can also constrain a slightly bigger size of APB than that of Vesta. Furthermore, this age is similar to those obtained from internal isochrons of the oldest volcanic angrites that cooled rapidly at the surface of the parent body (with ages of 4564 ∼ 4563 Ma), while older than those obtained from plutonic angrites (4561 ∼ 4556 Ma) that cooled down slowly, located deeper within the parent body. This implies that cooling of the APB took at least ∼8 Myr after its differentiation

Chromium Isotopic Constraints on the Origin of the Ureilite Parent Body

International audienceWe report on the mass-independent Cr isotope compositions of 11 main group ureilites and an ureilitic trachyandesite (ALM-A). The Cr-54/Cr-52 ratios for main group ureilites vary from -1.06 0.04 to -0.78 0.05 and averaged at -0.91 0.15 (2SD, N = 18) including the data from literature. We argue that this variation reflects primitive mantle heterogeneities within the ureilite parent body (UPB). As such, this body did not experience a global-scale magma ocean, which is consistent with heterogeneous O isotope in ureilites. Furthermore, the e(54)Cr values, Mn/Cr ratios, C isotope ratios, Mg# values, and Fe/Mn ratios in the olivine cores of ureilites are correlated with each other, which suggests the mixing of ureilite precursors from at least two reservoirs, rather than a smelting process or the oxidation from ice melting. All the ureilite samples (including the ALM-A) fall on a well-defined Mn-53-Cr-53 isochron corresponding to a Mn-53/Mn-55 ratio of (6.02 1.59) x 10(-6), which translates to an age of 4566.7 1.5 Ma (within 2 Ma after calcium-aluminum-rich inclusions; CAIs) when anchored to the U-corrected Pb-Pb age for the D'Orbigny angrite. This old age indicates early partial melting on the UPB, consistent with the early accretion of the UPB (within 1 Ma after CAIs) predicted by thermal modeling. Furthermore, there is a 4 similar to 5 Ma age difference between the external isochron in this study and internal isochron ages for the feldspathic clasts in polymict ureilites, which likely reflects an impact history during the early evolution of the UPB

Evidence for magnesium isotope heterogeneity in the solar protoplanetary disk

With a half-life of 0.73 Myr, the 26Al-to-26Mg decay system is the most widely used short-lived chronometer for understanding the formation and earliest evolution of the solar protoplanetary disk. However, the validity of 26Al–26Mg ages of meteorites and their components relies on the critical assumption that the canonical 26Al/27Al ratio of ∼5 × 10−5 recorded by the oldest dated solids, calcium–aluminium-rich inclusions (CAIs), represents the initial abundance of 26Al for the solar system as a whole. Here, we report high-precision Mg-isotope measurements of inner solar system solids, asteroids, and planets demonstrating the existence of widespread heterogeneity in the mass-independent 26Mg composition (μ26Mg*) of bulk solar system reservoirs with solar or near-solar Al/Mg ratios. This variability may represent heterogeneity in the initial abundance of 26Al across the solar protoplanetary disk at the time of CAI formation and/or Mg-isotope heterogeneity. By comparing the U–Pb and 26Al–26Mg ages of pristine solar system materials, we infer that the bulk of the μ26Mg * variability reflects heterogeneity in the initial abundance of 26Al across the solar protoplanetary disk. We conclude that the canonical value of ∼5 × 10−5 represents the average initial abundance of 26Al only in the CAI-forming region, and that large-scale heterogeneity—perhaps up to 80 % of the canonical value—may have existed throughout the inner solar system. If correct, our interpretation of the Mg-isotope composition of inner solar system objects precludes the use of th

Isotopic evidence for primordial molecular cloud material in metal-rich carbonaceous chondrites

The short-lived (26)Al radionuclide is thought to have been admixed into the initially (26)Al-poor protosolar molecular cloud before or contemporaneously with its collapse. Bulk inner Solar System reservoirs record positively correlated variability in mass-independent (54)Cr and (26)Mg*, the decay product of (26)Al. This correlation is interpreted as reflecting progressive thermal processing of in-falling (26)Al-rich molecular cloud material in the inner Solar System. The thermally unprocessed molecular cloud matter reflecting the nucleosynthetic makeup of the molecular cloud before the last addition of stellar-derived (26)Al has not been identified yet but may be preserved in planetesimals that accreted in the outer Solar System. We show that metal-rich carbonaceous chondrites and their components have a unique isotopic signature extending from an inner Solar System composition toward a (26)Mg*-depleted and (54)Cr-enriched component. This composition is consistent with that expected for thermally unprocessed primordial molecular cloud material before its pollution by stellar-derived (26)Al. The (26)Mg* and (54)Cr compositions of bulk metal-rich chondrites require significant amounts (25–50%) of primordial molecular cloud matter in their precursor material. Given that such high fractions of primordial molecular cloud material are expected to survive only in the outer Solar System, we infer that, similarly to cometary bodies, metal-rich carbonaceous chondrites are samples of planetesimals that accreted beyond the orbits of the gas giants. The lack of evidence for this material in other chondrite groups requires isolation from the outer Solar System, possibly by the opening of disk gaps from the early formation of gas giants