Jiddat al Harasis 556: A howardite impact melt breccia with an H chondrite component


*Corresponding author. E-mail: emilie.janots@ujf-grenoble.fr


Abstract– A petrographic and geochemical study was undertaken to characterize Jiddat al Harasis (JaH) 556, a howardite find from the Sultanate of Oman. JaH 556 is a polymict impact melt breccia containing highly shocked clasts, including mosaicized olivine and recrystallized plagioclase, set in a finely recrystallized vesicular matrix (grain diameter <5–10 μm). Plagioclase (An76–92) and clinopyroxene (En48–62Wo7–15) are associated with orthopyroxene and olivine clasts like in a howardite. JaH 556 oxygen isotope data indicate that it has an anomalous bulk-rock composition as howardite, resulting from a mixture between HED material and at least one second reservoir characterized by a higher Δ17O. The bulk meteorite has a composition consistent with howardites, but it is enriched in siderophile elements (Ni = 3940 and Co = 159 ppm) arguing for a chondritic material as second reservoir. This is independently confirmed by the occurrence of chondrule relics composed of olivine (Fo56–80), orthopyroxene (En79Wo2), and plagioclase (An61–66). Based on oxygen isotopic signature, siderophile composition, and chondrule core Mg number (Fo80 and En79Wo2), it is proposed that JaH 556 is a howardite containing approximately 20% H chondrite material. This percentage is high compared with that observed petrographically, likely because chondritic material dissolved in the impact melt. This conclusion is supported by the observed reaction of orthopyroxene to olivine, which is consistent with a re-equilibration in a Si-undersaturated melt. JaH 556’s unique composition enlarges the spectrum of howardite-analogs to be expected on the surface of 4 Vesta. Our data demonstrate that oxygen isotopic anomalies can be produced by a mixture of indigenous and impactor materials and must be interpreted with extreme caution within the HED group.