U–Pb and Al–Mg systematics of the ungrouped achondrite Northwest Africa 7325

摘要: Abstract Northwest Africa (NWA) 7325 is a unique ungrouped gabbroic achondrite which has characteristics consistent with possible link to the planet Mercury. In order understand origin of this meteorite and nature its parent body, we have determined crystallisation age using long-lived U–Pb short-lived Al–Mg chronometers. An internal Pb–Pb isochron defined by six acid leached pyroxene fractions yields an 4563.4 ± 2.6 Ma, assuming that 238U/235U ratio for NWA identical bulk Earth Solar System value 137.794. The isotope analyses seven (four plagioclase, one pyroxene, olivine whole rock) define regression line corresponding 26Al/27Al0 = (3.03 ± 0.14) × 10−7 initial δ26Mg∗ 0.093 ± 0.004‰. When anchored D’Orbigny angrite, 26Al/27Al 4563.09 ± 0.26 Ma. 4563.4 ± 2.6 Ma 4563.09 ± 0.26 Ma are in complete agreement, but low U concentrations resulted relatively precision age. observed excess can be explained 27Al/24Mg fractionation subsequent Mg isotopic evolution after planetary differentiation. Furthermore, parental magma most likely formed within 1.72 Ma calcium-aluminium rich inclusion (CAI) formation. near simultaneously quenched angrites number achondrites at ∼4563 Ma, suggesting multitude bodies had differentiated ∼4–5 Myr CAI This ancient may interpreted as argument against originating from Mercury, however it does not completely rule out.