A STUDY OF MELT CLASTS IN EUCRITE NORTHWEST AFRICA 11548

Howardites, eucrites and diogenites (HEDs) are part of a meteorite clan thought to originate from the asteroid (4) Vesta [1]. Eucrites likely crystallized as lavas on Vesta’s surface or in shallow dikes and plutons. Hypervelocity collisions and impacts are important solar system processes: their record is preserved as shock metamorphic effects in many meteorites, and result in deformation features including brecciation, melt-pocket formation etc. [2].

Previous HED studies have targeted HED clasts to determine potential resetting timescales. For example, Kennedy et al. [3] studied a number of HED meteorites, revealing ages of 4.5 Ga and between 3.5 – 3.8 Ga. Similar age ranges are observed for the Moon [4]. By contrast, Cartwright et al. [5] revealed significantly broader ages of ~ 2.5 - 4.5 Ga from howardite melt clasts. This suggested a different impact history on Vesta compared to the Moon. Given these findings, melt clasts can provide information for the timing and condition of the impacts that form them.

In this work, we will examine the mineralogy of monomict eucrite Northwest Africa (NWA) 11548 using optical and scanning electron microscopy (SEM) observations. We will describe the melt clasts we have located so far, and will target for further chronometric studies, including Ar-Ar.

Melt clasts often appear dark colored in plane polarized light, but are not always easily distinguishable using optical microscopy. To further assist our search for melt clasts we utilized SEM analyses. The NWA 11548 sections highlight a brecciated eucrite containing lithic and melt breccia clasts. The melt clasts occur as fine-grained (≤ 200 μm) recrystallized materials. However, only a small number of melt clasts have been identified so far (< 10). An impact melt breccia clast ( ~1.5 cm) is of particular interest as it exhibits recrystallized grains along with un-melted debris.

In future work, we plan to employ electron probe microanalysis (EPMA) measurements to study the chemistry of the various melt clast. Moreover, we plan to analyze selected melt clasts using the ⁴⁰Ar/³⁹Ar chronometer in order to study their resetting ages.

Ref: [1] Drake (2001). Met. & Plan. Sci. 36 [2] Jaret et al. (2018). JGR. 123 [3] Kennedy et al. (2013). Geoch. et Cosmoch. Acta 115 [4] Mercer et al. (2015). Science Advances [5] Cartwright et al. (2016) 47^th LPSC #2865.