In a paper published in Science Advances, researchers investigated the origins of Earth's volatiles by analyzing the isotopic composition of zinc (Zn) in meteorites. They found that while differentiated planetesimals contributed about 70 % of Earth's mass, they only provided 10 % of its Zn.
Study: Primitive asteroids are a major source of terrestrial volatiles. Image Credit: Paopano/Shutterstock.com
The remaining Zn originated from primitive, unmelted materials. These findings suggest that primitive materials likely played a crucial role in forming the volatile inventories of terrestrial planets.
Related Work
Previous studies explored Earth's building materials using nucleosynthetic isotope anomalies, revealing distinct isotopic differences between meteorites from different Solar System regions. Carbonaceous meteorites, thought to originate beyond Jupiter, were enriched in neutron-rich isotopes, while non-carbonaceous meteorites from the inner Solar System showed the opposite trend.
Chondritic meteorites, which escaped early melting, have been proposed as major contributors to Earth's formation. However, the distribution of volatile elements, essential for life, is still not fully understood concerning Earth's mass and contributions from differentiated meteorites.
Summary of Meteorite Isotope Study
The study involved the analysis of 21 meteorites, categorized into various types such as achondrites, iron meteorites, carbonaceous chondrites (CCs), enstatite chondrites (ECs), ordinary chondrites (OCs), and R chondrites. Samples were prepared using high-purity water and acids for digestion, following established procedures.
Meteorite samples, including reference material Basalt, Columbia River 2 (BCR-2), were crushed and digested in a combination of acids, and the solutions were purified using a three-stage anion exchange process. The remaining aliquots of specific meteorites were stored at Imperial College London.
The isotopic measurements were conducted using a Nu Instruments Nu Plasma II mass spectrometer with a desolvation system, introducing samples at specific flow rates. Data was collected using Faraday cups configured for different Zn isotopes. The isotope ratios were normalized and corrected using standard bracketing techniques, with results reported in ε notation. The 66Zn/67Zn normalization was used for most data, with the values calculated relative to the London Zn standard.
A Monte Carlo simulation of 10,000 trials was used to estimate the Zn isotope composition of various meteorite groups. The simulation included a mass balance model considering different meteorite types and their contributions to Earth’s accretion. The ε64Zn value was selected due to its precision and abundance, and only simulations that matched terrestrial values were considered valid. The model also accounted for possible Zn loss during Earth’s formation.
Multielement mass balance simulations were designed for various elements, focusing on lithophile elements. The model did not include neodymium or potassium due to limited data. The concentrations of Zn and zirconium (Zr) in different meteorite types were allowed to vary, though some uncertainties remained. While the Zn content of angrites could not be directly measured, an estimated ε64Zn value was derived through regression analysis based on correlations with other isotope systems.
Zn Isotope Variability in Meteorites
The study primarily focused on analyzing the mass-independent Zn isotope compositions of meteorites from differentiated planetesimals, specifically those derived from silicate and metal fractions, represented by achondrites and iron meteorites, respectively.
Additional meteorite types, including CCs and non-carbonaceous chondrites (NCCs), were also included. Isotopic measurements were based on the 66Zn/67Zn ratio for internal normalization, though supplementary data using alternative ratios were available. The terrestrial rock standard, BCR-2, showed no significant deviations from the baseline εZn value of 0, while all meteorite samples displayed measurable deviations from the terrestrial standard.
The study's results revealed distinct patterns in Zn isotopes among different meteorite groups. CCs showed negative ε64Zn and positive ε68Zn and ε70Zn values, while NC meteorites exhibited complementary patterns. These observations align with prior Zn isotope studies and similar isotopic systems.
The Zn isotopic compositions for both CC and NC groups were compared, with NC achondrites and iron meteorites largely overlapping with NCCs. The terrestrial sample, however, remained indistinguishable from the Zn standard, which defines εZn = 0.
Further analysis indicated that meteorites such as ureilites and howardite–eucrite–diogenites (HEDs) were more depleted in neutron-rich isotopes (48Ca, 50Ti, 54Cr) than NCCs.
While current data suggest that NC materials might exhibit more homogeneous Zn isotope compositions compared to more refractory elements, future higher-precision Zn isotope data may reveal greater distinctions between NCCs and NC achondrites. Interestingly, enstatite chondrites (ECs) had Zn isotope compositions distinct from the bulk silicate Earth (BSE) and matched ordinary chondrites (OCs), setting them apart from most other nucleosynthetic isotope systems.
Conclusion
To sum up, the findings underscored the complexity of Earth's volatile origins, highlighting the limited contribution of differentiated planetesimals to its Zn inventory. Despite their significant mass contribution, these bodies suffered substantial losses during formation.
The results emphasized the importance of including unmelted primitive materials in terrestrial planet volatile budget models. Future studies were recommended to explore further the relationships between these materials and the overall composition of the Earth.
Journal Reference
Martins, R., et al. (2024). Primitive asteroids are a major source of terrestrial volatiles. Science Advances. DOI:10.1126/sciadv. ado4121, https://www.science.org/doi/10.1126/sciadv.ado4121
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