Extreme Zr stable isotope fractionation during magmatic fractional crystallization

Zirconium is a commonly used elemental tracer of silicate differentiation, yet its stable isotope systematics remain poorly known. Accessory phases rich in Zr⁴⁺ such as zircon and baddeleyite may preserve a unique record of Zr isotope behavior in magmatic environments, acting both as potential drivers of isotopic fractionation and recorders of melt compositional evolution. To test this potential, we measured the stable Zr isotope composition of 70 single zircon and baddeleyite crystals from a well-characterized gabbroic igneous cumulate. We show that (i) closed-system magmatic crystallization can fractionate Zr stable isotopes at the >0.5% level, and (ii) zircon and baddeleyite are isotopically heavy relative to the melt from which they crystallize, thus driving chemically differentiated liquids toward isotopically light compositions. Because these effects are contrary to first-order expectations based on mineral-melt bonding environment differences, Zr stable isotope fractionation during zircon crystallization may not solely be a result of closed-system thermodynamic equilibrium

Origin of uranium isotope variations in early solar nebula condensates

High-temperature condensates found in meteorites display uranium isotopic variations (^(235)U/^(238)U), which complicate dating the solar system’s formation and whose origin remains mysterious. It is possible that these variations are due to the decay of the short-lived radionuclide ^(247)Cm (t_(1/2) = 15.6 My) into ^(235)U, but they could also be due to uranium kinetic isotopic fractionation during condensation. We report uranium isotope measurements of meteoritic refractory inclusions that reveal excesses of ^(235)U reaching ~+6% relative to average solar system composition, which can only be due to the decay of ^(247)Cm. This allows us to constrain the ^(247)Cm/^(235)U ratio at solar system formation to (7.0 ± 1.6) × 10^(−5). This value provides new clues on the universality of the nucleosynthetic r-process of rapid neutron capture

Distinct ^(238)U/^(235)U ratios and REE patterns in plutonic and volcanic angrites: Geochronologic implications and evidence for U isotope fractionation during magmatic processes

Angrites are differentiated meteorites that formed between 4 and 11 Myr after Solar Systemformation, when several short-lived nuclides (e.g., ^(26)Al-^(26)Mg, ^(53)Mn-^(53)Cr, ^(182)Hf-^(182)W) were still alive. As such, angrites are prime anchors to tie the relative chronology inferred from these short-lived radionuclides to the absolute Pb-Pb clock. The discovery of variable U isotopic composition (at the sub-permil level) calls for a revision of Pb-Pb ages calculated using an “assumed” constant ^(238)U/^(235)U ratio (i.e., Pb-Pb ages published before 2009–2010). In this paper, we report high-precision U isotope measurement for six angrite samples (NWA 4590, NWA 4801, NWA 6291, Angra dos Reis, D’Orbigny, and Sahara 99555) using multi-collector inductively coupled plasma mass-spectrometry and the IRMM-3636 U double-spike. The age corrections range from −0.17 to −1.20 Myr depending on the samples. After correction, concordance between the revised Pb-Pb and Hf-W and Mn-Cr ages of plutonic and quenched angrites is good, and the initial (^(53)Mn/^(55)Mn)_0 ratio in the Early Solar System (ESS) is recalculated as being (7 ± 1) × 10^(−6) at the formation of the Solar System (the error bar incorporates uncertainty in the absolute age of Calcium, Aluminum-rich inclusions – CAIs). An uncertainty remains as to whether the Al-Mg and Pb-Pb systems agree in large part due to uncertainties in the Pb-Pb age of CAIs. A systematic difference is found in the U isotopic compositions of quenched and plutonic angrites of +0.17‰. A difference is also found between the rare earth element (REE) patterns of these two angrite subgroups. The δ^(238)U values are consistent with fractionationduring magmatic evolution of the angrite parent melt. Stable U isotope fractionation due to a change in the coordination environment of U during incorporation into pyroxene could be responsible for such a fractionation. In this context, Pb-Pb ages derived from pyroxenes fraction should be corrected using the U isotope composition measured in the same pyroxene fraction

Zirconium stable isotope analysis of zircon by MC-ICP-MS: Methods and application to evaluating intra-crystalline zonation in a zircon megacryst

Zirconium (Zr) plays a key role in the development of phases like zircon (ZrSiO₄) and baddeleyite (ZrO₂) in magmatic systems. These minerals are crucial for the study of geologic time and crustal evolution, and their high resistivity to weathering and erosion results in their preservation on timescales of billions of years. Although zircon and baddeleyite may also preserve a robust record of Zr isotope behavior in high-temperature terrestrial environments, little is known about the factors that control Zr isotope partitioning in magmatic systems, the petrogenetic significance of fractionated compositions, or how these variations are recorded in Zr-rich accessory phases. Here, we describe a new analytical protocol for accurately determining the Zr stable isotope composition of zircon by multicollector-inductively coupled plasma-mass spectrometry (MC-ICP-MS), using the double-spike method to correct for procedural and instrumental mass bias. We apply this technique to test whether zircon crystallization in carbonatite magmatic systems is a driver of Zr isotope fractionation by interrogating the internal zonation of a zircon megacryst from the Mud Tank carbonatite (MTUR1). We find the MTUR1 megacryst to lack internal zoning within analytical uncertainties with a mean μ⁹⁴/⁹⁰Zr_(NIST) = −55 ± 28 ppm (2 SD, n = 151), which suggests that zircon crystallization is not a driver of Zr isotope fractionation in carbonatite magmas. This observation is in stark contrast with those made in silicate magmatic systems, raising the possibility that the bonding environment of Zr⁴⁺ ions may be fundamentally different in carbonatite vs. silicate melts. Because of its remarkable homogeneity, the MTUR1 megacryst is an ideal natural reference material for Zr isotopic analysis of zircon using both solution and spatially resolved methods. The reproducibility of a pure Zr solution and our chemically purified zircon fractions indicate that the external reproducibility of our method is on the order of ±28 ppm for μ⁹⁴/⁹⁰Zr, or ±7 ppm per amu, at 95% confidence

Introducing a comprehensive data reduction algorithm for high-precision U-Th geochronology with isotope dilution MC-ICP-MS

Multi collector inductively coupled plasma mass spectrometry (MC-ICP-MS) is being increasingly utilized for U-Th geochronology of carbonate deposits with comparable precision to thermal ionization mass spectrometry (TIMS) [1, 2]. While attention has been paid to propagation of uncertainties for U-Th-Pb analysis by TIMS and the isochron technique [3,4], a comprehensive data processing scheme is lacking for MC-ICP-MS. To address this need, we have developed an algorithm in Mathematica application to allow for step-by-step monitoring of the data reduction process. The program is flexible and affords the user easy control over input variables. Adjustments for background and spike isotope contributions, abundance sensitivity and instrumental mass bias are implemented through the code, followed by age calculation and propagation of uncertainties with Monte Carlo simulation. A rigorous standard bracketing procedure was adopted using Uranium (CRM-112A) and Th (IRMM-035) standard solutions, doped with IRMM-3636a ^(233)U/^(236)U “double-spike”, to account for deviations of isotope ratios from certificate values and improve accuracy. Following a single U/TEVA extraction chromatography step to separate U from Th, ten replicate ages from a speleothem in Cathedral Cave (CC), Utah showed excellent agreement (R^2 = 0.999) with results previously measured at the University of Minnesota by single collection ICP-MS [5]. The external reproducibility of our analytical technique was evaluated by analyzing six aliquots of an in-house standard, prepared by homogenizing a piece of the CC speleothem, which returned a mean age of 21468±120 y (2SD). A limited amount of the standard powder is available upon request for interlaboratory calibration. We have successfully dated 36 samples from caves in the Bahamas, the Dominican Republic and Iran

Distinct ^(238)U/^(235)U ratios and REE patterns in plutonic and volcanic angrites: Geochronologic implications and evidence for U isotope fractionation during magmatic processes

Angrites are differentiated meteorites that formed between 4 and 11 Myr after Solar Systemformation, when several short-lived nuclides (e.g., ^(26)Al-^(26)Mg, ^(53)Mn-^(53)Cr, ^(182)Hf-^(182)W) were still alive. As such, angrites are prime anchors to tie the relative chronology inferred from these short-lived radionuclides to the absolute Pb-Pb clock. The discovery of variable U isotopic composition (at the sub-permil level) calls for a revision of Pb-Pb ages calculated using an “assumed” constant ^(238)U/^(235)U ratio (i.e., Pb-Pb ages published before 2009–2010). In this paper, we report high-precision U isotope measurement for six angrite samples (NWA 4590, NWA 4801, NWA 6291, Angra dos Reis, D’Orbigny, and Sahara 99555) using multi-collector inductively coupled plasma mass-spectrometry and the IRMM-3636 U double-spike. The age corrections range from −0.17 to −1.20 Myr depending on the samples. After correction, concordance between the revised Pb-Pb and Hf-W and Mn-Cr ages of plutonic and quenched angrites is good, and the initial (^(53)Mn/^(55)Mn)_0 ratio in the Early Solar System (ESS) is recalculated as being (7 ± 1) × 10^(−6) at the formation of the Solar System (the error bar incorporates uncertainty in the absolute age of Calcium, Aluminum-rich inclusions – CAIs). An uncertainty remains as to whether the Al-Mg and Pb-Pb systems agree in large part due to uncertainties in the Pb-Pb age of CAIs. A systematic difference is found in the U isotopic compositions of quenched and plutonic angrites of +0.17‰. A difference is also found between the rare earth element (REE) patterns of these two angrite subgroups. The δ^(238)U values are consistent with fractionationduring magmatic evolution of the angrite parent melt. Stable U isotope fractionation due to a change in the coordination environment of U during incorporation into pyroxene could be responsible for such a fractionation. In this context, Pb-Pb ages derived from pyroxenes fraction should be corrected using the U isotope composition measured in the same pyroxene fraction

Distribution Coefficients of the REEs, Sr, Y, Ba, Th, and U between α-HIBA and AG50W-X8 Resin

Rare-earth elements (REEs) are known for their similar behaviors, which make their purification through chromatographic techniques particularly challenging. The use of α-hydroxyisobutyric acid (α-HIBA) in combination with a cation-exchange resin is perhaps the most widely used chromatographic technique to separate individual REEs from each other. However, only limited REE partition data between α-HIBA and cation resins exist, which makes it challenging to develop and optimize purification techniques using this platform. Here, we report distribution coefficients (K_d) of REEs, as well as Sr, Y, Ba, Th, and U, between α-HIBA at pH = 4.50 and AG50W-X8 cation-exchange resin, obtained by batch equilibration experiments. For all 19 elements, the distribution coefficients decrease with increasing acid concentration. For the REEs, a linear relationship is observed in log–log space between K_d values and α-HIBA molarity. While the K_d values have been calibrated at pH = 4.50, formulas are provided allowing recasting of the K_d values at any pH. To test the accuracy of the data, we compare elution curves simulated using the newly determined distribution coefficients to actual elution curves. The close agreement between simulated and experimental elution curves demonstrates that the distribution coefficients obtained in this study are effective to devise multielement extraction and purification scheme for high-precision elemental and isotopic analyses of REEs for various applications