FURTHER INSIGHTS OF TEMPERATURE-TIME EVENTS ON THE HED PARENT BODY USING U-TH-PB CHRONOLOGY OF ZIRCON-BEARING NORITIC DIOGENITE NORTHWEST AFRICA 10666

Howardite, Eucrite, and Diogenites (HEDs) are a suite of achondrite meteorites believed to be derived from asteroid 4 Vesta and represent the largest suite of achondrites from a single parent body. The diogenites represent the lower crust of the parent body, but their igneous and metamorphic histories are not well understood. NWA 10666, which has been classified as a zircon bearing noritic diogenite was investigated by in situ U-Th-Pb and trace element analyses of trace and major phases, optical light and electron-beam petrologic analyses, and mineral thermometry to better constrain its igneous and metamorphic histories and its geochemical and petrologic relationships with other diogenite, eucrite, and cumulate eucrite specimens. NWA 10666 is moderately shocked, exhibits pyroxene compositions that are more ferroan than most other diogenite specimens, and contains minor and accessory phases: ilmenite, a silica polymorph, Ca-phosphate, and zircon. Laser-ablation ICP-MS analyses of shock-fractured zircon and phosphate at UH yielded 207Pb/206Pb – 204Pb/238U inverse isochron ages of 4542 ±20 Ma (MSWD = 0.3) and 4248 ±25 Ma (MSWD = 1.5), respectively. Trace element analyses of bulk rock indicates a slight positive Eu anomaly (Eu/Eu* = 1.2) and in situ rare-earth element (REE) analyses of pyroxene and plagioclase indicate REE equilibration at 1262 ± 8 ºC (2SD). Fe-Mg exchange between OPX and CPX records an equilibration temperature of 850 ºC. The U-Pb age of the shocked zircon is likely a minimum age for those grains indicating magmatism in the HED parent body was likely coeval with the main phase of eucrite magmatism at ~4555 Ma. Geochemical and petrologic data indicate that NWA 10666 is related to Yamato type-B diogenites, a group that may be compositionally transitional between cumulate eucrites and diogenites. The 4248 ±25 Ma age of Ca-phosphate is older than most Ar-Ar thermochronology that peak between 3.3 and 3.8 Ga. Dates between 4.1 and 4.4 Ga are more common in higher temperature systems than Ar-Ar and may indicate that the Ar-Ar age cluster between 3.3-3.8 may reflect an overall waning impact intensity in the inner Solar System.