Story
In situ U Pb chronology and chemistry of zirconolite in the andesitic meteorite Erg Chech 002
Key takeaway
Researchers studied a rare meteorite fragment, finding that its mineral composition provides a precise timeline of its formation and evolution billions of years ago. This gives new insights into the early history of our solar system.
Quick Explainer
The core idea of this study is to use the rare mineral zirconolite found in the andesitic meteorite Erg Chech 002 as a tool to better understand the early history of the Solar System. Zirconolite is a unique accessory mineral that can potentially serve as a U-Pb chronometer, providing insights into the timing of events that shaped this ancient asteroidal crust. The researchers analyzed the in situ occurrence, chemistry, and isotopic composition of the zirconolite grains, aiming to resolve discrepancies between different radiometric dating methods and build a more consistent early Solar System chronology. This approach is noteworthy as it leverages the highly radiogenic nature of zirconolite to potentially provide a new high-precision dating technique for early planetary processes.
Deep Dive
In situ U–Pb chronology and chemistry of zirconolite in the andesitic meteorite Erg Chech 002
Overview
This study presents the in situ occurrence, U–Pb chronology, and chemistry of zirconolite in the andesitic meteorite Erg Chech 002 (EC 002). Zirconolite is a rare accessory mineral found in some asteroidal meteorites that can potentially serve as a U–Pb chronometer for early Solar System events.
Context
- EC 002 represents one of the oldest known asteroidal crusts, with previously reported high-precision 207Pb/206Pb ages ranging from 4565.6 to 4566.2 Ma.
- The age of EC 002 is controversial, with discrepancies between short-lived radiometric ages (e.g., Al-Mg, Mn-Cr) and the high-precision Pb-Pb ages.
- Understanding the age and thermal history of EC 002 is crucial for building a consistent chronology of the early Solar System.
Methodology
- Prepared three polished thin sections of a 7.3 g stone of EC 002.
- Used electron probe microanalysis (FE-EPMA) to identify and characterize the zirconolite grains.
- Determined Pb isotope compositions of the zirconolite grains using a NanoSIMS ion microprobe.
- Analyzed the rare earth element (REE) abundances in the zirconolite grains using NanoSIMS.
Results
- Five zirconolite grains were identified in the EC 002 thin sections, occurring along with albitic plagioclase and often associated with silica.
- The zirconolite grains showed significant compositional deviations from the ideal zirconolite stoichiometry, with substantial amounts of MgO, Al2O3, SiO2, Cr2O3, FeO, and REEs.
- The zirconolite had highly radiogenic Pb isotope compositions, yielding a weighted average 207Pb/206Pb of 0.6212 ± 0.0019 (2σ), corresponding to an age of 4557.9 ± 4.3 Ma.
- The REE patterns of the EC 002 zirconolite were similar to those of terrestrial metamorphic zirconolite.
Interpretation
- The young 207Pb/206Pb age of the EC 002 zirconolite, compared to the previously reported older ages for pyroxene and whole-rock samples, is interpreted to reflect the timing of a shock metamorphism event on the parent asteroid.
- The close association of zirconolite with silica in EC 002 and other extraterrestrial samples suggests that zirconolite can crystallize from silica-oversaturated magmas at high temperatures (>1200°C).
- The zirconolite age discrepancy implies that even a small amount of metamorphic zirconolite could significantly affect the high-precision 207Pb/206Pb ages obtained by acid leaching of pyroxene and whole-rock samples, potentially biasing the ages toward older values.
Limitations and Uncertainties
- The possibility of uncorrected instrumental mass bias in the NanoSIMS Pb isotope analysis of the zirconolite cannot be excluded, which could potentially account for the age discrepancy.
- The lack of a zirconolite standard for U abundance determination and Pb/U fractionation correction limits the authors' ability to fully evaluate the potential analytical artifacts.
Implications and Future Work
- This study suggests that zirconolite could be more ubiquitous in alkali-silica-rich and rapidly cooled asteroidal rocks, and may serve as an important in situ U–Pb chronometer for the early Solar System.
- To fulfill the potential of zirconolite as a chronometer, establishing a zirconolite standard for rigorous evaluation of Pb isotopic mass bias and Pb/U fractionation correction is a necessary next step.