Challenges of measuring volatiles in enstatite chondrites and evaluating their contribution to earth's volatile inventory

Abstract Measurements of bulk H, N, and C abundances and isotopic compositions were conducted on (metal‐free) aliquots of 12 powdered enstatite chondrite (EC) samples, from both EH and EL chemical groups, and four aubrites. The ECs covered a range of petrologic types, including both falls and finds. To understand the internal H distributions, the H concentrations of individual silicate minerals were analyzed in situ by NanoSIMS in polished thick sections of six of the ECs. Using these data, combined with published data on H carriers and their modal abundances in ECs, we estimate here that the major silicate minerals, matrix, and mesostasis in unequilibrated ECs account for less than 20% of the total H budget. For the metamorphosed E4–6 chondrites, which exhibit a recrystallized matrix and mesostasis, over 99% of the bulk H contents remain unexplained. We attribute the discrepancies between our analyzed bulk H elemental compositions and the bulk H values reconstituted from in situ measurements are attributed to terrestrial contamination, as water adheres to grain boundaries, surfaces, and fractures. Water also reacts with reduced phases, regardless of whether the meteorite is a fall or find. Similar H isotopic compositions for falls and heavily weathered finds are consistent with, but do not require, that the majority of EC H is terrestrial. Attempts to remove organic terrestrial contamination by solvent extractions only served to moderately shift the H, N, and C isotopic compositions. The δD, δ 15 N, and δ 13 C values of EC powders are lighter than BSE, although δD and δ 13 C values partially overlap with atmospheric values. Reconstruction of the H budget of an EL‐like parent body with an onion‐shell structure suggests that such bodies could account for at most a fourth of the bulk silicate Earth's (BSE's) water budget.