Hydrogen in acapulcoites and lodranites: A unique source of water for planetesimals in the inner Solar SystemOPEN ACCESS 

A. Stephant, X. Zhao, M. Anand, J. Davidson, C. Carli, T. Cuppone, G. Pratesi, I.A. Franchi

Earth and Planetary Science Letters
Volume 615, 1 August 2023, 118202



  • Partial melting up to 20% does not induce any H isotopic fractionation.
  • Inner Solar System planetesimals have incorporated various amounts of H.
  • Acapulcoite-lodranite record a δD value lower than bulk carbonaceous chondrites.
  • This δD value is similar to most of inner Solar System planetesimal δD value.
  • This δD implies either a H nebular contribution or an incorporation of D-poor ices.”

“Acapulcoites and lodranites are primitive achondrites, sampling a common planetesimal formed a few million years after calcium-aluminium rich inclusions in the inner Solar System, that provides information into melting and differentiation processes in the early inner Solar System. The chemistry and mineralogy of their chondritic parent body lies in between enstatite and ordinary chondrites. As they record a range in planetary differentiation degree, from 1% up to 20% partial melting, with lodranites experiencing the most melt extraction, we investigate (i) the behaviour of hydrogen in terms of abundance and isotopic composition during the early stages of planetary differentiation and (ii) the source(s) of hydrogen for the acapulcoite-lodranite parent body in order to place it in our current understanding of the source(s) of water in the inner Solar System. In this study, we analysed water content and hydrogen isotopic composition of phosphates and nominally anhydrous minerals in a range of acapulcoite and lodranite meteorite samples. While apatites seemed to have recorded a degassing signature, no such variations were observed in the H2O–δD systematics of the nominally anhydrous minerals suggesting that subsequent to their crystallisation, acapulcoites and lodranites experienced minimal modifications to their volatile composition during thermal metamorphism and partial melting. The low abundance of water in acapulcoite and lodranite nominally anhydrous minerals (i.e., average 5.2±1.6 μg/g H2O) suggests that their parent body was much drier than what has been estimated for enstatite and ordinary chondrite parent bodies. We estimated a bulk water content for the acapulcoite-lodranite parent body of 3 to 19 μg/g H2O, similar to the ureilite parent body. The hydrogen isotopic composition of nominally anhydrous minerals in acapulcoites and lodranites (–211±145‰), and in particular for the two falls Acapulco and Lodran (–239±149‰), matches with the hydrogen isotopic composition recorded by nominally anhydrous minerals in ordinary chondrite falls, eucrites, S–type asteroid Itokawa, consistent with a common source of water for the inner Solar System planetesimals, isotopically distinctive to bulk carbonaceous chondrites.”