Water-Rich Carbonaceous Chondrites And Ryugu: Exploring A Common Parent Body.pdf

Testing a potential Ryugu connection, the evolution of the temperature at different depths was compared with CI and CM carbonate formation ages and temperatures. The models show that such fits are possible only for formation times of t0≲4 Myr after CAIs. Representative bodies from the favored parameter space for Ryugu’s parent body that satisfy this condition are considered.

For nearly all bodies shown, the CM carbonate and CI dolomite data can be fitted. However, for the first group, and even for radii of R<15 km with any t0, rapid cooling does not allow to fit the breunnerite data. In addition, in most cases the porosity at the fit depth surpasses the CI and CM class average values and only for R=1 km temperature evolution indicates serpentine dehydration.

A potential precursor of Ryugu with R=1.5-3 km and t0=2-3.5 Myr could only be a CM or CI source, if constraints on partial dehydration, CI and CM porosity, and breunnerite formation data are neglected.

Only the late-forming case of R=3 km, t0=3.5 Myr after CAIs agrees with this concept, without being compatible with the breunnerite formation time. Larger and relatively late accreting bodies of the second group allow for fitting the CI breunnerite data in addition to dolomite in some cases.

Here, it is required that after crossing the dolomite data point, a (t,T) curve proceeds through the breunnerite rectangle without surpassing its Tmax, and then leaves it near the lower right corner. This cannot be fulfilled for an early accretion, since temperature curves would either surpass Tmax=423 K, or fall below Tmin=373 K too early.

For an accretion after 3.8 Myr, it is not possible to fit the dolomite formation data. Larger objects, in addition, fail to fit the CI data, as shown for R=50 km. The CM data are fitted best at smaller depths of less than 1 km, while the CI data are fitted typically between a half-depth and the center of a planetesimal, where heat can be retained longer.

In such cases, the (t,T) fit curve crosses the CI dolomite data range, indicating consecutive mineral formation, with dolomite precipitating during the prograde T evolution and breunnerite forming on both the prograde and the retrograde branch of the (t,T) curve at the same depth. None of the low-T bodies of the second group produce dehydrated material or CI and CM porosities at fit depths.

Bodies from the second group are bad candidates for Ryugu's precursors, but some of them provide far better fits to the carbonate formation data and are relatively homogeneous with Tmax≲423 K throughout most of the interior. Bodies with radii of 15≲R≲25 km are more suitable as CI and CM parent bodies.