Trace element systematics and ~(147)Sm-~(143)Nd and ~(176)Lu-~(176)Hf ages of Larkman Nunatak 06319: Closed-system fractional crystallization of an enriched shergottite magma

摘要

Combined ~(147)Sm-~(143)Nd and ~(176)Lu-~(176)Hf chronology of the martian meteorite Larkman Nunatak (LAR) 06319 indicates an igneous crystallization age of 193±20Ma (2σ weighted mean). The individual ~(147)Sm-~(143)Nd and ~(176)Lu-~(176)Hf internal isochron ages are 183±12Ma and 197±29Ma, respectively, and are concordant with two previously determined ~(147)Sm-~(143)Nd and ~(87)Rb-~(87)Sr internal isochron ages of 190±26Ma and 207±14Ma, respectively (Shih et al., 2009). With respect to the ~(147)Sm-~(143)Nd isotope systematics, maskelynite lies above the isochron defined by primary igneous phases and is therefore not in isotopic equilibrium with the other phases in the rock. Non-isochronous maskelynite is interpreted to result from shock-induced reaction between plagioclase and partial melts of pyroxene and phosphate during transformation to maskelynite, which resulted in it having unsupported ~(143)Nd relative to its measured ~(147)Sm/~(144)Nd ratio. The rare earth element (REE) and high field strength element (HFSE) compositions of major constituent minerals can be modeled as the result of progressive crystallization of a single magma with no addition of secondary components. The concordant ages, combined with igneous textures, mineralogy, and trace element systematics indicate that the weighted average of the radiometric ages records the true crystallization age of this rock. The young igneous age for LAR 06319 and other shergottites are in conflict with models that advocate for circa 4.1Ga crystallization ages of shergottites from Pb isotope compositions, however, they are consistent with updated crater counting statistics indicating that young volcanic activity on Mars is more widespread than previously realized (Neukum et al., 2010).