Thermal Evolution of Hydrated Asteroids Inferred from Oxygen Isotopes

Martin Bizzarro,Matthieu Gounelle,Lionel G Vacher,Maxime Piralla,Yves Marrocchi

doi:10.3847/2041-8213/ab3bd0

Abstract

Chondrites are fragments of unmelted asteroids that formed due to gravitational instabilities in turbulent regions of the Solar protoplanetary disk. Hydrated chondrites are common among meteorites, indicating that a substantial fraction of the rocky bodies that formed early in the Solar System accreted water ice grains that subsequently melted due to heat released by the radioactive decay of 26Al. However, the thermal histories of asteroids are still largely unknown whereas it would bring fundamental information on their timing of accretion and their physical characteristics. Here we show that hydrated meteorites (CM chondrites) contain previously uncharacterized calcium carbonates with peculiar oxygen isotopic compositions (Δ17O ≈ -2.5 ‰), which artificially produce the mass-independent trend previously reported for carbonates. Based on these isotopic data, we propose a new model to quantitatively estimate the precipitation temperatures of secondary phases (carbonates and serpentine). It reveals that chondritic secondary phases recorded a gradual increase of the temperature during the extent of aqueous alteration, from –10 °C to maximum 250 °C. We also show that the thermal path of C-type asteroids is independent of the initial oxygen isotopic composition of the primordial water ice grains that they accreted. Our estimated temperatures for hydrated asteroids remain lower than those experienced by other carbonaceous chondrites, providing strong constraints for modelling the formation conditions and size-distribution of water-rich asteroids, especially in anticipation of the return of samples of water-rich asteroids to Earth by the OSIRIS-REx and Hayabusa2 missions.

Highlights

Dark C-type asteroids dominate the main-belt asteroid population and are genetically related to hydrous primitive CI and CM carbonaceous chondrites (Vilas & Gaffey 1989; Hiroi et al 1996; Vilas 1994; Burbine et al 2002; Lauretta et al 2019)
CM chondrites are complex aggregates of high-temperature components formed in the disk and low-temperature secondary minerals formed during subsequent parent-body fluid circulations
In the four CM chondrites surveyed in this work, calcite grains surrounded by Fe-S

Summary

Introduction

Dark C-type asteroids dominate the main-belt asteroid population and are genetically related to hydrous primitive CI and CM carbonaceous chondrites (Vilas & Gaffey 1989; Hiroi et al 1996; Vilas 1994; Burbine et al 2002; Lauretta et al 2019). CM chondrites are the most common water-rich meteorites, and CM-like matter represent an important fraction of exogenic clasts reported in other groups of meteorites, implying that CM parent bodies are widespread in the asteroid belt (Briani et al 2012). CM chondrites are complex aggregates of high-temperature components formed in the disk and low-temperature secondary minerals formed during subsequent parent-body fluid circulations. The latter provide key constraints on the origin of water accreted by asteroids (Vacher et al 2016; Piani et al 2018) as well as their accretion and evolution histories (Young et al 2003; Verdier-Paoletti et al 2017; Vacher et al 2017; Fujiya et al 2015). Determining carbonate precipitation temperatures requires knowledge of the O isotopic compositions of their parental fluids, which itself requires knowledge of the carbonate precipitation temperatures, leading to a seemingly circular problem

Objectives

Methods

Results

Discussion

Conclusion