HED-like cosmic spherules from the Transantarctic Mountains, Antarctica: Major and trace element abundances and oxygen isotopic compositions

Abstract

Eleven large (∼400–700μm in diameter) differentiated glass cosmic spherules were identified from the Transantarctic Mountain micrometeorite collection on the basis of their high Fe/Mg ratios (from 0.6 to 1.6) and their homogeneous Fe/Mn ratios (31±4). Their oxygen isotope compositions (δ18O values ranging from 14.37‰ to 20.12‰ and Δ17O values ranging from −0.47‰ to −0.65‰) suggest they are derived from a similar parent body, either 4-Vesta or a vestoid with an oxygen isotope composition close to that of 4-Vesta. The ranges in CaO and Al2O3 contents (from 1.6 to 11.6wt.% and from 0.8 to 12.4wt.%, respectively), the average REE contents (1.4–13.7×CI), the REE patterns (flat to strongly depleted in LREE), and the siderophile element contents (0.17–131μg/g for Ni and 1.3–68.1μg/g for Co) result from different mineralogies in the spherule precursors. Type 1 spherules (n=4), with high REE contents and flat patterns, derive from fine-grained precursors with typical eucritic mineralogy. Type 2 spherules (n=4) are strongly depleted in LREE and are related to pigeonite-rich equilibrated eucrite fragments. Type 3 spherules (n=2) have high Fe/Mg ratios (>0.6) relative to chondritic spherules but similar REE patterns (av.REE=1.6–2.2×CI) and siderophile element contents (Ni >17μg/g, Co >50μg/g). We interpret the chemical characteristics of Type 3 spherules by the high proportion of clasts of metal-bearing diogenites in their howardite-like precursors. The combination of elemental and oxygen isotope data confirms that the Fe/Mn ratio and the Co, V and Zn contents are suitable parameters for the identification of the parent body of the differentiated micrometeoroids, provided that the mineralogical control is understood. The relative frequency of HED-like micrometeorites in the Transantarctic Mountain micrometeorite collection is 1.6%. This is consistent with the abundance of eucrite and howardite meteorites (1.8%), which are the main constituents of the regoliths of vestoid asteroids.