Pb evolution in the Martian mantle
Pb evolution in the Martian mantle
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DOI：
10.1016/j.epsl.2017.12.039
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发表时间：
2018-03
影响因子：
5.3
通讯作者：
J. Bellucci;A. Nemchin;A. Nemchin;M. Whitehouse;J. Snape;P. Bland;G. Benedix;J. Roszjar
中科院分区：
文献类型：
--
作者：
J. Bellucci;A. Nemchin;A. Nemchin;M. Whitehouse;J. Snape;P. Bland;G. Benedix;J. Roszjar
用二次离子质谱仪(SIMS)测定了1个富集型雪戈特矿、1个中间型雪戈特矿、2个贫化雪岭石和纳赫拉矿的初始铅组成。这些数值，加上之前使用相同的分析方法对三个富集型雪榴石AlH-84001和Chassigny进行的研究的数据，被用来建立一个统一的、内部一致的火星地幔分异历史和火星陨石结晶年龄的模型。硅铝榴石和Nakhla/Chassigny的分异历史完全不同，这与短寿命的放射性成因同位素系统学相吻合。Nakhla/Chassigny的初始铅组成可以通过晚期添加具有原始非放射成因成分的富铅组分得到最好的解释。相比之下，雪尔果特岩群的铅同位素组成表明，火星地幔的演化历史相对简单，可以根据Sm-ND系统的最新结果进行模拟。英云闪长岩与4504 Ma的单一地幔分异事件有关。因此，这里的蛇绿岩铅同位素模式反映了1)前硅酸盐分异(4504 Ma)和2)后硅酸盐分异到喷发年龄(由一致的放射成因等时线年龄确定)的两个阶段的历史。这两个不同生长阶段的μ值(238U/204Pb值)分别为μ_1=1.8，μ_2=1.4~4.7。μ_1值为1.8时，与顽辉岩和普通球粒陨石以及原地球的μ值基本一致，表明这是太阳系内天体的初始值。当与其他源区放射性同位素变量(Sri、γ187Os、ε143ND和ε176Hf)作图时，观测到的地幔μ值的第二阶段地幔演化范围表现出很好的线性相关性(r2>；0.85)，并代表了火星地幔混合端元(亏损、中间、富集型)的光谱。
The initial Pb compositions of one enriched shergottite, one intermediate shergottite, two depleted shergottites, and Nakhla have been measured by Secondary Ion Mass Spectrometry (SIMS). These values, in addition to data from previous studies using an identical analytical method performed on three enriched shergottites, ALH 84001, and Chassigny, are used to construct a unified and internally consistent model for the differentiation history of the Martian mantle and crystallization ages for Martian meteorites. The differentiation history of the shergottites and Nakhla/Chassigny are fundamentally different, which is in agreement with short-lived radiogenic isotope systematics. The initial Pb compositions of Nakhla/Chassigny are best explained by the late addition of a Pb-enriched component with a primitive, non-radiogenic composition. In contrast, the Pb isotopic compositions of the shergottite group indicate a relatively simple evolutionary history of the Martian mantle that can be modeled based on recent results from the Sm–Nd system. The shergottites have been linked to a single mantle differentiation event at 4504 Ma. Thus, the shergottite Pb isotopic model here reflects a two-stage history 1) pre-silicate differentiation (4504 Ma) and 2) post-silicate differentiation to the age of eruption (as determined by concordant radiogenic isochron ages). The μ-values (238 U/204 Pb) obtained for these two different stages of Pb growth are μ 1 of 1.8 and a range of μ 2 from 1.4–4.7, respectively. The μ 1-value of 1.8 is in broad agreement with enstatite and ordinary chondrites and that proposed for proto Earth, suggesting this is the initial μ-value for inner Solar System bodies. When plotted against other source radiogenic isotopic variables (Sr i, γ 187 Os, ε 143 Nd, and ε 176 Hf), the second stage mantle evolution range in observed mantle μ-values display excellent linear correlations (r 2> 0.85) and represent a spectrum of Martian mantle mixing-end members (depleted, intermediate, enriched).