Hydrogen Chloride as a Source of Acid Fluids in Parent Bodies of Chondrites

Introduction: Chondrites reveal signs of aqueous alteration on their parent bodies [1-4] that occurred under a broad range of pH. The dominance of secondary Mg-phyllosilicates (saponite, serpentine, chlorite) in pervasively altered CI and CM chondrites [1-4] suggests alkaline conditions during deposition [5,6]. However, localized occurrences of phosphates and carbonates in CM carbonaceous chondrites [7] and several other types of chondrites [e.g., 8] may indicate local aqueous processes at low pH [7]. These data show that acidic conditions might have existed inside chondrules, while alkaline environments could have characterized matrices [7]. Here, we argue that early low-pH fluids in asteroids were present as HCl-rich aqueous solutions that formed through eutectic (~186 K) melting of HCl hydrate(s). Incorporation of chlorine in chondrites: Chlorine in chondrites is present in sodalite, chlorapatite, halite, sylvite, scapolite, lawrencite, and as an admixture in other phases [2,9]. Although sodalite has been modeled as a high-temperature condensate from nebular gas [10,11], Cl-bearing phases in chondrites exist in minerals formed on parent bodies. In particular, a parent body origin of sodalite in Ca-Al rich inclusions (CAIs), amoeboid olivine aggregates, and matrices from CV chondrites is supported by several studies [2,4,12]. Isotopic (Rb/Sr, Cl/Cl, I/Xe) data indicate a young age for sodalite in CAIs [13-16]. Likewise, apatite and halite in chondrites are products of parent body processes [2]. In the Semarkona (LL3.0) matrix, elevated concentrations of Cl in smectite indicate an asteroidal incorporation of Cl [9]. Therefore, Cl could have been incorporated in parent bodies in a simple condensed form. A delivery of Cl as a component of water ice is consistent with the positive correlation between Cl content in chondrites and the degree of aqueous alteration, which reflects the ice/rock ratio in the primordial composition of parent asteroids. In fact, heavily hydrated CI chondrites have the highest Cl content among chondrites. Fig. 1. The stability of solid HCl·3H2O in equilibrium with water ice as a function of temperature and fugacity of HCl. The solid line corresponds to the equilibrium HCl•3H2O(cr) = HCl(gas) + 3H2O(ice, I). The horizontal dashed lines depict a range of pHCl in the solar nebula. HCl·3H2O can form at temperatures below ~160K. HCl condensation at ~140K is more likely because of low total P in a low-T nebula. It is assumed that water ice condensed at higher temperatures (180K-160K at nebular pressures of 10-10 bar, respectively) and controlled pH2O at temperatures of HCl condensation. Thermodynamic data from the FREZCHEM database [18] were used.