Extraterrestrial water in micrometeorites and cosmic spherules from Antarctica: An ion microprobe study

The D/H ratios and water contents were measured by ion microprobe analysis in 52 individual Antarctic micrometeorites (AMMs) and 10 Antarctic cosmic spherules (ACSs) containing nuggets of iron hydroxide (COPS phase). In AMMs, δD values vary from -366 to +249‰ and water contents lie between 0.4-3.7 wt%. The COPS nuggets in cosmic spherules have high water contents (2 to 8 wt%) and exhibit δD values from -144 to +167‰, which is indicative of an extraterrestrial origin of their constituent water. The silicate portion of ACSs also contain extraterrestrial H equivalent to ~0.1 to 1.2 wt% water. Deuterium-exchange experiments were performed with isotopically spiked water. These experiments demonstrate that water in mineral phases of AMMs and ACSs is indigenous and does not result from contamination during residence in Antarctic ice. The frequency distribution of D/H ratios in AMMs allows us to further narrow the relationship between AMMs and carbonaceous chondrites to CM and CI chondrites but contrasts with that of stratospheric interplanetary dust particles (IDPs) of similar sizes (from ~10 to 50 μm). The relatively narrow range of D/H ratios measured in AMMs as well as in ACSs (which are more resistant and thus less susceptible to collection biases) suggests that D-rich IDP-like particles are very rare in our AMMs collections. This indicates that these D-rich grains might constitute a minor fraction of the micrometeorite flux in the interplanetary medium and that possible collection biases in Antarctica would not be responsible for their strong depletion in the AMMs collections.