Rheasilvia provenance of the Kapoeta howardite inferred from ∼1 Ga40Ar/39Ar feldspar ages

- We dated submillimeter samples from the howardite Kapoeta AMNH4788.
- Separated lithologies give a wide range of 40Ar/39Ar ages: 0.7-4.5 Ga.
- Ages of brecciated regions and glass are averages of older and younger components.
- A pervasive glass vein in Kapoeta AMNH4788 was a source of contact metamorphism.
- Young feldspar ages, ∼1 Ga, may be linked to the Rheasilvia impact event.

We report 40Ar/39Ar ages for several lithological components of the brecciated howardite Kapoeta and compare the ages with results for asteroid 4 Vesta as observed by the Dawn mission. Our Kapoeta sample has an unusual, millimeter wide glass vein that intruded into a complex breccia. The plateau ages of three lithic clasts of basaltic composition that were remote from the glass vein range from 4.2 to 4.5 Ga. Such ages are typical of eucritic material; the oldest reflects early magmatic crystallization (∼4.5 Ga), the younger (4.2-4.5 Ga) reflect magmatism associated with protracted cooling. Samples of the glass vein itself, which include relict grains, give apparent ages between 3.1 and 3.9 Ga as do chips from the matrix. We consider both glass and bulk matrix ages as mixing ages; not marking the time of a single event, but dating regolith activity (<3.1-4.1 Ga).Eight feldspar grains close to the glass vein give markedly younger plateau ages averaging 1.4 Ga. The Ar release spectra for glass vein and breccia subsamples indicate a disturbance in the last 1.4 Ga. Taken together, these younger ages suggest a recent, major thermal event in the history of howardites that has been under-reported - perhaps the impact that formed the Rheasilvia basin on Vesta.