Timing of thermal metamorphism in CM chondrites: implication for Ryugu
  and Bennu future sample return
  Carbonaceous chondrites are often considered potential contributors of water
and other volatiles to terrestrial planets as most of them contain significant
amounts of hydrous mineral phases. As such, carbonaceous chondrites are
candidate building blocks for Earth, and elucidating their thermal histories is
of direct important for understanding the volatile element history of Earth and
the terrestrial planets. A significant fraction of CM type carbonaceous
chondrites are thermally metamorphosed or heated and have lost part of their
water content. The origin and the timing of such heating events are still
debated, as they could have occurred either in the first Myrs of the Solar
System via short-lived radioactive heating, or later by impact induced heating
and or solar radiation. Since Rb is more volatile than Sr, and some heated CM
chondrites are highly depleted in Rb, a dating system based on the radioactive
decay of 87Rb to 87Sr could be used to date the heating event relating to the
fractionation of Rb and Sr. Here, we have leveraged the 87Rb 87Sr system to
date the heating of five CM chondrites (PCA 02012, PCA 02010, PCA 91008, QUE
93005 and MIL 07675). We find that the heating events of all five meteorites
occurred at least 3 Ga after the formation of the Solar System. Such timing
excludes short-lived radioactive heating as the origin of thermal metamorphism
in these meteorites, and relates such heating events to ages of collisional
families of C-type asteroids.
