Sulfuric acid as a cryofluid and oxygen isotope reservoir of planetesimals

Abstract

The Sun exhibits a depletion in $^{17,18}$O relative to $^{16}$O by 6 % compared to the Earth and Moon$^{1}$. The origin of such a non-mass-dependent isotope fractionation has been extensively debated since the three-isotope-analysis$^{2}$ became available in 1970's. Self-shielding$^{3,4}$ of CO molecules against UV photons in the solar system's parent molecular cloud has been suggested as a source of the non-mass-dependent effect, in which a $^{17,18}$O-enriched oxygen was trapped by ice and selectively incorporated as water into planet-forming materials$^{5}$. The truth is that the Earth-Moon and other planetary objects deviate positively from the Sun by ~6 % in their isotopic compositions. A stunning exception is the magnetite/sulfide symplectite found in Acfer 094 meteorite, which shows 24 % enrichment in $^{17,18}$O relative to the Sun$^{6}$. Water does not explain the enrichment this high. Here we show that the SO and SO$_2$ molecules in the molecular cloud, ~106 % enriched in $^{17,18}$O relative to the Sun, evolved through the protoplanetary disk and planetesimal stages to become a sulfuric acid, 24 % enriched in $^{17,18}$O. The sulfuric acid provided a cryofluid environment in the planetesimal and by itself reacted with ferric iron to form an amorphous ferric-hydroxysulfate-hydrate, which eventually decomposed into the symplectite by shock. We indicate that the Acfer-094 symplectite and its progenitor, sulfuric acid, is strongly coupled with the material evolution in the solar system since the days of our molecular cloud.