Zinc isotope analyses of singularly small samples (<5 ng Zn): Investigating chondrule-matrix complementarity in Leoville

Abstract

The potential complementarity between chondrules and matrix of chondrites, the Solar System's building blocks, is still a highly debated subject. Complementary superchrondritic compositions of chondrite matrices and subchondritic chondrules may point to formation of these components within the same reservoir or, alternatively, to mobilization of elements during secondary alteration on chondrite parent bodies. Zinc isotope fractionation through evaporation during chondrule formation may play an important role in identifying complementary relationships between chondrules and matrix and is additionally a mobile element during hydrothermal processes. In an effort to distinguish between primary Zn isotope fractionation during chondrule formation and secondary alteration, we here report the Zn isotope data of five chondrule cores, five corresponding igneous rims and two matrices of the relatively unaltered Leoville CV3.1 chondrite. The detail required for these analyses necessitated the development of an adjusted Zn isotope analyses protocol outlined in this study. This method allows for the measurement of 5 ng Zn fractions, for which we have analyzed the isotope composition with an external reproducibility of 120 ppm. We demonstrate that we measure primary Zn isotope signatures within the sampled fractions of Leoville, which show negative δ 66 Zn values for the chondrule cores (δ 66 Zn = -0.43±0.14 ), more positive values for the igneous rims (δ 66 Zn = -0.01±0.30 ) and chon-dritic values for the matrix (δ 66 Zn = 0.19±0.14 ). In combination with elemental compositions and petrology of these chondrite fractions, we argue that chondrule cores, igneous rims and matrix could have formed within the same reservoir in the protoplanetary disk. The required formation mechanism involves Zn isotope fractionation through sulfide loss during chondrule core formation and concurrent thermal processing of matrix material. Depleted olivine-bearing grains representing this processed matrix would have accreted to the depleted chondrule cores and subsequently reabsorbed material (including 66 Zn-rich) from a complementary volatile-rich gas, thereby forming the igneous rims. This would have allowed the rims to move towards an isotopically chondritic composition, similar to the non-processed matrix in Leoville. We note that Zn isotope analyses of components in other chondrites (f.e., CM, CO, EC) are necessary to identify if this complementarity relationship is generic or unique for each chondrite group. The development of a Zn isotope protocol for singularly small samples is a step forward in that direction.

Imprint of chondrule formation on the K and Rb isotopic compositions of carbonaceous meteorites

“…The concentrations calculated for the chondrule component are lower than the CI-like matrix, consistent with the fact that chondrules were formed at high temperature and were depleted in MVEs. The chondrule component has lighter isotopic compositions than the matrix, consistent with previous bulk chondrule measurements ( 22 , 25 , 45 ). As discussed below, the finding that chondrules have, on average, light isotopic compositions not only for Te and Zn but also for K and Rb is important as it puts new constraints on the thermal history of chondrules.…”

“…As discussed above, the chondrule component is depleted in volatile elements Rb, K, Te, and Zn and has lighter isotopic composition than the matrix (CI). Previous Zn isotope measurements of chondrules from Allende, Mokoia, and Leoville CV chondrites revealed light Zn isotope enrichments in chondrules, which were interpreted to reflect removal of isotopically heavy sulfide from chondrules or their precursors ( 25 , 45 ). This interpretation might also be able to explain the light Te isotopic composition of the chondrule component because chalcophile Te can partition into sulfides.…”

Mare basalt meteorites, magnesian-suite rocks and KREEP reveal loss of zinc during and after lunar formation

“…This purification step was repeated to ensure purification of the Zn fraction. The procedural blank measured with samples was 0.2 ng, and generally represents less than 4% of total measured Zn for samples, which was typically 5-10 ng (van Kooten and Moynier, 2019). Zinc isotopic compositions were measured on the Thermo Fischer Neptune Plus multi collector-inductively coupled plasma-mass spectrometer housed at the Institut de Physique du Globe, Paris.…”

Analytical protocols for Phobos regolith samples returned by the Martian Moons eXploration (MMX) mission

“…Stable isotopic composition of MVE is usually measured by ICP-MS. For the stable isotopic composition of MVE such as Zn, the amount of sample required would vary considerably depending on whether it is chondritic (> 100 ppm) or volatile depleted like lunar basalts (~ 1 ppm) (Paniello et al 2012 ). The most recent analytical development requires ~ 10 ng of Zn to produce high precision Zn isotopic data and ~ 5 ng for slightly lower precision (van Kooten and Moynier 2019 ). This, therefore, represents less than a mg for a chondritic composition and ~ 10 mg for a volatile depleted composition.…”