Pd-Ag chronometry of IVA iron meteorites and the crystallization and cooling of a protoplanetary core
Pd-Ag chronometry of IVA iron meteorites and the crystallization and cooling of a protoplanetary core
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IVA 铁陨石的 Pd-Ag 计时以及原行星核心的结晶和冷却
DOI：
10.1016/j.gca.2017.09.009
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发表时间：
2017
影响因子：
5
通讯作者：
Kleine T.
中科院分区：
文献类型：
--
作者：
Matthes M;Fischer-Gödde M;Kruijer T;Kleine T.
为了限制原行星核结晶和冷却所涉及的时间尺度和过程，我们研究了IVA铁陨石Muonion alusta和Gibeon的Pd-Ag同位素体系。用Pd-Ag等时线测得的初始数据为107Pd/108Pd/108Pd/−=10×10~(-5)Pd/108Pd~(-5)。从等时线下方的吉本图上分析的三个金属样品，也显示出明显的宇宙线诱发的中子俘获反应效应，这从吉本样品中196个铂的过量就可见一斑。在对银同位素进行中子俘获效应校正后，Gibeon样品绘制在Muonion alusta等时线上，表明这两个IVA铁具有难以区分的初始107Pd/108Pd。总的来说，Pd-Ag数据表明，在CaI形成后，在Pd-Ag闭合之下的IVA核心在2.9 Ma±100.4 Ma和8.9 Ma±100.6 Ma之间冷却，这一年龄范围反映了太阳系初始107Pd/108Pd比值的不确定性，而这又是Muonion alusta铅-铅年龄不确定性的结果。银同位素数据表明，IVA岩芯最初演化时Pd/Ag略有升高，但某些金属样品测量到的低Ag浓度表明来自银含量低得多的来源，因此Pd/Ag较高。如果IVA铁从最初更富银的核结晶，然后在接近凝固的核的压实过程中提取Fe-S熔体，则这些相反的观察结果可以得到协调。由于银很容易分配到Fe-S熔体中，所以在压制过程中，银从IVA型芯中被去除，导致在IVA型铁的金属样品中观察到很低的银浓度。或者，就在结晶开始之前，银从仍然熔融的金属体中蒸发而损失。Pd-Ag同位素数据表明，∼铁的Pd-Ag闭合温度为900K，冷却温度为&500K/Ma，与金相冷却速度推测的快速冷却一致。综上所述，这些观察结果与IVA铁在一个几乎没有硅酸盐地幔的金属天体中的冷却是一致的。
To constrain the timescales and processes involved in the crystallization and cooling of protoplanetary cores, we examined the Pd-Ag isotope systematics of the IVA iron meteorites Muonionalusta and Gibeon. A Pd-Ag isochron for Muonionalusta provides an initial107Pd/108Pd = (2.57 ± 0.07) × 10−5. The three metal samples analyzed from Gibeon plot below the Muonionalusta isochron, but these samples also show significant effects of cosmic ray-induced neutron capture reactions, as is evident from196Pt excesses in the Gibeon samples. After correction for neutron capture effects on Ag isotopes, the Gibeon samples plot on the Muonionalusta isochron, indicating that these two IVA irons have indistinguishable initial107Pd/108Pd. Collectively, the Pd-Ag data indicate cooling of the IVA core below Pd-Ag closure between 2.9 ± 0.4 Ma and 8.9 ± 0.6 Ma after CAI formation, where this age range reflects uncertainties in the initial107Pd/108Pd ratios of the solar system, which in turn result from uncertainties in the Pb-Pb age of Muonionalusta. The Ag isotopic data indicate that the IVA core initially evolved with a modestly elevated Pd/Ag, but the low Ag concentrations measured for some metal samples indicate derivation from a source with much lower Ag contents and, hence, higher Pd/Ag. These contrasting observations can be reconciled if the IVA irons crystallized from an initially more Ag-rich core, followed by extraction of Fe-S melts during compaction of the nearly solidified core. Owing to its strong tendency to partition into Fe-S melts, Ag was removed from the IVA core during compaction, leading to the very low Ag concentration observed in metal samples of IVA irons. Alternatively, Ag was lost by evaporation from a still molten metallic body just prior to the onset of crystallization. The Pd-Ag isotopic data indicate that Muonionalusta cooled at >500 K/Ma through the Pd-Ag closure temperature of ∼900 K, consistent with the rapid cooling inferred from metallographic cooling rates for IVA irons. Combined, these observations are consistent with cooling of IVA irons in a metallic body with little or no silicate mantle.