Cassata William S, Cohen Benjamin E, Mark Darren F, Trappitsch Reto, Crow Carolyn A, Wimpenny Joshua, Lee Martin R, Smith Caroline L
Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA.
Isotope Geoscience Unit, Scottish Universities Environmental Research Centre, Rankine Avenue, East Kilbride, G75 0QF, UK.
Sci Adv. 2018 May 23;4(5):eaap8306. doi: 10.1126/sciadv.aap8306. eCollection 2018 May.
Martian meteorite Northwest Africa (NWA) 7034 and its paired stones are the only brecciated regolith samples from Mars with compositions that are representative of the average martian crust. These samples therefore provide a unique opportunity to constrain the processes of metamorphism and alteration in the martian crust, which we have investigated via U-Pu/Xe, Ar/Ar, and U-Th-Sm/He chronometry. U-Pu/Xe ages are comparable to previously reported Sm-Nd and U-Pb ages obtained from NWA 7034 and confirm an ancient (>4.3 billion years) age for the source lithology. After almost 3000 million years (Ma) of quiescence, the source terrain experienced several hundred million years of thermal metamorphism recorded by the K-Ar system that appears to have varied both spatially and temporally. Such protracted metamorphism is consistent with plume-related magmatism and suggests that the source terrain covered an areal extent comparable to plume-fed edifices (hundreds of square kilometers). The retention of such expansive, ancient volcanic terrains in the southern highlands over billions of years suggests that formation of the martian crustal dichotomy, a topographic and geophysical divide between the heavily cratered southern highlands and smoother plains of the northern lowlands, likely predates emplacement of the NWA 7034 source terrain-that is, it formed within the first ~100 Ma of planetary formation.
火星陨石西北非洲（NWA）7034及其伴生石块是来自火星的仅有的角砾化风化层样本，其成分代表了火星地壳的平均组成。因此，这些样本提供了一个独特的机会来限制火星地壳中的变质作用和蚀变过程，我们已通过U-Pu/Xe、Ar/Ar和U-Th-Sm/He计时法对其进行了研究。U-Pu/Xe年龄与先前报道的从NWA 7034获得的Sm-Nd和U-Pb年龄相当，并证实了源岩性的古老（>43亿年）年龄。在近30亿年的静止期后，源地形经历了由K-Ar系统记录的数亿年的热变质作用，这种变质作用似乎在空间和时间上都有所变化。这种长期的变质作用与地幔柱相关的岩浆作用一致，并表明源地形覆盖的面积范围与地幔柱供给的构造相当（数百平方公里）。数十亿年来，这种广阔的古老火山地形在南部高地得以保留，这表明火星地壳二分法的形成，即 heavily cratered南部高地和北部低地较平坦平原之间的地形和地球物理分界线，可能早于NWA 7034源地形的就位——也就是说，它形成于行星形成的最初约1亿年内。
