Centre for Star and Planet Formation, Globe Institute, University of Copenhagen, 1350 Copenhagen, Denmark.
The University Museum, The University of Tokyo, 118-0033 Tokyo, Japan.
Proc Natl Acad Sci U S A. 2020 Dec 8;117(49):30973-30979. doi: 10.1073/pnas.2016326117. Epub 2020 Nov 16.
Combining U-Pb ages with Lu-Hf data in zircon provides insights into the magmatic history of rocky planets. The Northwest Africa (NWA) 7034/7533 meteorites are samples of the southern highlands of Mars containing zircon with ages as old as 4476.3 ± 0.9 Ma, interpreted to reflect reworking of the primordial Martian crust by impacts. We extracted a statistically significant zircon population ( = 57) from NWA 7533 that defines a temporal record spanning 4.2 Gyr. Ancient zircons record ages from 4485.5 ± 2.2 Ma to 4331.0 ± 1.4 Ma, defining a bimodal distribution with groupings at 4474 ± 10 Ma and 4442 ± 17 Ma. We interpret these to represent intense bombardment episodes at the planet's surface, possibly triggered by the early migration of gas giant planets. The unradiogenic initial Hf-isotope composition of these zircons establishes that Mars's igneous activity prior to ∼4.3 Ga was limited to impact-related reworking of a chemically enriched, primordial crust. A group of younger detrital zircons record ages from 1548.0 ± 8.8 Ma to 299.5 ± 0.6 Ma. The only plausible sources for these grains are the temporally associated Elysium and Tharsis volcanic provinces that are the expressions of deep-seated mantle plumes. The chondritic-like Hf-isotope compositions of these zircons require the existence of a primitive and convecting mantle reservoir, indicating that Mars has been in a stagnant-lid tectonic regime for most of its history. Our results imply that zircon is ubiquitous on the Martian surface, providing a faithful record of the planet's magmatic history.
锆石的 U-Pb 年龄与 Lu-Hf 数据相结合,为了解岩石行星的岩浆历史提供了线索。西北非(NWA)7034/7533 陨石是火星南部高地的样本,其中的锆石年龄最老可达 4476.3 ± 0.9 Ma,被解释为撞击对原始火星地壳的再作用。我们从 NWA 7533 中提取了一个具有统计学意义的锆石群体( = 57),该群体定义了一个跨越 42 亿年的时间记录。古老的锆石记录的年龄从 4485.5 ± 2.2 Ma 到 4331.0 ± 1.4 Ma,呈双峰分布,分组在 4474 ± 10 Ma 和 4442 ± 17 Ma。我们将其解释为可能是由巨行星早期迁移引发的,代表了行星表面强烈的撞击事件。这些锆石的无放射性初始 Hf 同位素组成表明,火星在约 43 Ga 之前的岩浆活动仅限于受化学富集原始地壳影响的撞击再作用。一组年轻的碎屑锆石记录的年龄从 1548.0 ± 8.8 Ma 到 299.5 ± 0.6 Ma。这些颗粒唯一可能的来源是与之时间相关的埃利西厄姆和塔尔西斯火山省,它们是深部地幔羽流的表现。这些锆石的球粒陨石型 Hf 同位素组成需要存在一个原始且对流的地幔储层,表明火星在其历史的大部分时间里一直处于停滞板块构造状态。我们的研究结果表明,锆石在火星表面普遍存在,为火星的岩浆历史提供了忠实的记录。