Reply to comment on ‘Assessing the implications of K isotope cosmochemistry for evaporation in the preplanetary solar nebula’ by E. Young

Abstract

Humayun offers several criticisms of the analysis of K isotope cosmochemistry by Young. Although Humayun’s comments provide a useful description of diffusion-limited evaporation, they are not relevant to the mechanism for volatile element depletion put forward by Young. The mechanism described by Young is evaporation of a ‘heterogeneous mixture of crystalline solids’. In presenting his model Young made four essential assertions: (1) the truism that the more volatile minerals are composed of the more volatile elements; (2) most rock-forming elements likely entered the early solar nebula in solid form; (3) dust in the early solar system was unlikely to be uniform in composition on the scale of individual grains; and (4) partial evaporation of crystalline solids has little effect on the isotopic composition of the residues. Humayun expresses concerns about the relevance of the last assertion but does so without the vital context provided by assertions (1)–(3). There are two underpinnings to Humayun’s comments. First is the tacit assumption that dust in the early solar system was composed of grains with identical chemical compositions, effectively ignoring Young’s proposed mechanism for volatile element depletion. Second is the notion that the mass fractions used by Young are extensive parameters that have little to do with the original paper by Humayun and Clayton. The first issue of whether or not solids in the early solar system were uniform in composition at the grain scale is open for debate. Young’s model assumes that the grains were not all identical in composition. The second issue of whether or not Humayun and Clayton and Young used comparable methods for depicting depletion of K in solar system bodies should not be a matter for debate because the normalized concentration ratios used by Humayun and Clayton are equivalent to the intensive mass fractions used by Young, as shown again below.