Fresh Crust - The First Direct Evidence of Magmatism on a Carbonaceous Parent-body

It can be easy to forget that meteoritics is still a rather cutting-edge area of study. Many well-established theories in meteoritics and planetary science, such as planetary differentiation, often have components that have never been verified through direct observation. It’s noteworthy then when new research upends long held assumptions that were largely supported by the absence of direct sampling. In this case the assumption was that differentiation only occurred on the non-carbonaceous bodies.

A paper from Aléon, et al, 2020 titled, Alkali magmatism on a carbonaceous chondrite planetesimal, challenges this assumption and sheds new light on the wholesale dynamics that may have been occurring on some of the larger carbonaceous chondrite parent bodies. The paper focuses on a basaltic clast, designated UH154-11, that was found embedded in the matrix of a CR chondrite. Analysis of stable oxygen isotope ratios in the clast plot to the CV-CK clan parent body.  The authors write that “UH154-11 is the first sample from a carbonaceous chondrite breccia that documents partial melting and magmatic activity on a carbonaceous chondrite planetesimal, which was most likely the CV chondrite planetesimal.”

Aléon et al propose two models for the parent body of the UH154-11 clast. Figure 1. represents a CV parent body with a “fully differentiated core”, and Figure 2. represents a CV parent body with only minor “incipient” partial melting.  Both models allow for at least enough melting to account for the petrogenesis of the clast with or without a fully differentiated metal core and allow for a continuously accreting pristine carbonaceous chondritic upper crust.

There have been previous models proposing differentiated bodies with chondritic crusts and metallic cores. However, this is the first direct evidence of differentiation in a carbonaceous parent body. In this case the CV planetesimal seems to have accreted too late to have undergone full differentiation, but early enough to undergo partial melting with temperatures sufficient to at least partially melt chondritic material.