Isotopic and textural analysis of giant unmelted micrometeorites – identification of new material from intensely altered 16O-poor water-rich asteroids

Abstract

Bulk oxygen isotope data has the potential to match extraterrestrial samples to parent body sources based on distinctive δ18O and Δ17O ratios. We analysed 10 giant (>500 μm) micrometeorites using combined micro-Computer Tomography (μCT) and O-isotope analysis to pair internal textures to inferred parent body groups. We identify three ordinary chondrite particles (L and LL groups), four from CR chondrites and the first micrometeorite from the enstatite chondrite (EH4) group. In addition, two micrometeorites are from hydrated carbonaceous chondrite parent bodies with 16O-poor isotopic compositions and plot above the terrestrial fractionation line. They experienced intense aqueous alteration, contain pseudomorphic chondrules and are petrographically similar to the CM1/CR1 chondrites. These micrometeorites may be members of the newly established CY chondrites and/or derived from the enigmatic “Group 4” micrometeorite population, previously identified by Yada et al., 2005 [GCA, 69:5789-5804], Suavet et al., 2010 [EPSL, 293:313-320] (and others). One of our 16O-poor micrometeorite plots on the same isotopic trendline as the CO, CM and CY chondrites – “the CM mixing line” (with a slope of ∼0.7 and a δ17O intercept of -4.23‰), this implies a close relationship and potentially a genetic link to these hydrated chondrites. If position along the CM mixing line reflects the amount of 16O-poor (heavy) water-ice accreted onto the parent body at formation, then the CY chondrites and these 16O-poor micrometeorites must have accreted at least as much water-ice as CM chondrites but potentially more. In addition, thermal metamorphism could have played a role in further raising the bulk O-isotope compositions through the preferential loss of isotopically light water during phyllosilicate dehydration. The study of micrometeorites provides insights into asteroid belt diversity through the discovery of material not currently sampled by larger meteorites, perhaps as a result of atmospheric entry biases preventing the survival of large blocks of friable hydrated material.