Hilton Connor D, Walker Richard J
Department of Geology, University of Maryland, College Park, Maryland, 20742, USA.
Earth Planet Sci Lett. 2020 Jun 15;540. doi: 10.1016/j.epsl.2020.116248. Epub 2020 Apr 20.
The origin of the IAB main group (MG) iron meteorites is explored through consideration of W isotopic compositions, thermal modeling of Al decay, and mass independent (nucleosynthetic) Mo isotopic compositions of planetesimals formed in the noncarbonaceous (NC) protosolar isotopic reservoir. A refined W model age for the meteorites Campo del Cielo, Canyon Diablo, and Nantan suggests that the IAB-MG parent body underwent some form of metal-silicate segregation as early as 5.3 ± 0.4 Myr after calcium-aluminum rich inclusion (CAI) formation or as late as 13.8 ± 1.4 Myr after CAI formation. If melting of the IAB-MG occurred prior to 7 Myr after CAI formation, it was likely driven by Al decay for a parent body radius >40 km. Otherwise, additional heat from impact is required for melting metal this late in Solar System history. If melting was partially or wholly the result of internal heating, a thermal model of Al decay heat production constrains the accretion age of the IAB-MG parent body to ~1.7 ± 0.4 Myr after CAI formation. If melting was, instead, dominantly caused by impact heating, thermal modeling suggests the parent body accreted more than 2 Myr after CAI formation. Comparison of Mo mass independent isotopic compositions of the IAB-MG to other NC bodies with constrained accretion ages suggests that the Mo isotopic composition of the NC reservoir changed with time, and that the IAB-MG parent body accreted between 2 to 3 Myr after CAI formation, thus requiring an origin by impact. The relationship between nucleosynthetic Mo isotopic compositions and accretion ages of planetesimals from the NC reservoir suggests that isotopic heterogeneity developed from either addition of -process material to, or removal of coupled /process material from the NC reservoir.
通过考虑钨同位素组成、铝衰变的热模型以及在非碳质（NC）原太阳同位素储库中形成的小行星的质量无关（核合成）钼同位素组成，来探索IAB主群（MG）铁陨石的起源。对坎波德尔西耶洛、迪亚布洛峡谷和南丹陨石的精细钨模型年龄表明，IAB-MG母体早在富钙铝包体（CAI）形成后5.3±0.4百万年就经历了某种形式的金属-硅酸盐分离，或者晚至CAI形成后13.8±1.4百万年。如果IAB-MG在CAI形成后7百万年内发生了熔化，对于半径>40 km的母体，其熔化可能是由铝衰变驱动的。否则，在太阳系历史如此后期熔化金属就需要撞击产生的额外热量。如果熔化部分或全部是内部加热的结果，铝衰变产热的热模型将IAB-MG母体的吸积年龄限制在CAI形成后约1.7±0.4百万年。相反，如果熔化主要是由撞击加热引起的，热模型表明母体在CAI形成后超过2百万年吸积。将IAB-MG的钼质量无关同位素组成与其他吸积年龄受限的NC天体进行比较表明，NC储库的钼同位素组成随时间变化，并且IAB-MG母体在CAI形成后2至3百万年之间吸积，因此需要通过撞击起源。来自NC储库的核合成钼同位素组成与小行星吸积年龄之间的关系表明，同位素非均质性是由向NC储库添加-process物质或从NC储库中去除耦合的/process物质而产生的。
