OLD HOMESTEAD 001


Howardite
Fragmental breccia
standby for old homestead 001 photo
Found June 1991
31° 10' S., 127° 45' E.

Two specimens with a total weight of 783 g were found on the Nullarbor Plain in Western Australia. The meteorite exhibits a typical howardite composition with a brecciated and compacted mixture of mineral and rock fragments, primarily diogenite and eucrite—with a cumulate/noncumulate eucrite ratio commensurate with that of known eucrites. A possible paired specimen, the ~60 g Mundrabilla 020, has recently been found. It has a pairing score of 76.7 based on the pairing criteria of petrography and geographic proximity.

Studies of orthopyroxene crystals from selected howardites indicate an iron and magnesium concentration in Old Homestead 001 that is consistent with that of the Mg-rich cumulate eucrites (Domeneghetti et al., 2000, 2004). Notably, certain howardites contain clasts of low-Mg, REE-, K-, and P-rich (KREEPy) lithologies representing compositionally evolved rocks (Barrat et al., 2012).

Late-stage cumulate eucrite material in certain howardites has been found to contain the first symplectic assemblages in such meteorites (Patzer and McSween, 2011). The absence of exsolved augite, the Ca-rich composition, and the relatively rapid cooling rate (1.4°C/day) all suggest an origin from a hot source rock that was excavated by impact. This scenario is supported by the high closure temperature of orthopyroxene of 619°C (Domeneghetti et al., 2000), consistent with quenching upon excavation of hot plutonic rock. A more detailed scenario on the formation of the HED clan can be found on the Millbillillie page. The specimen pictured above is a 4.1 g partial slice.