Investigating magmatic processes in the early Solar System using the Cl isotopic systematics of eucrites
Investigating magmatic processes in the early Solar System using the Cl isotopic systematics of eucrites
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DOI：
10.1016/j.gca.2019.06.024
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发表时间：
2019-12
影响因子：
5
通讯作者：
T. Barrett;J. Barnes;M. Anand;I. Franchi;R. Greenwood;B. Charlier;X. Zhao;F. Moynier;M. Grady
中科院分区：
文献类型：
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作者：
T. Barrett;J. Barnes;M. Anand;I. Franchi;R. Greenwood;B. Charlier;X. Zhao;F. Moynier;M. Grady
一般来说，陆相岩石、火星陨石和球粒陨石的块体和磷灰石氯同位素组成范围相对较窄，δ37Cl(每毫升与标准平均海洋氯的偏差)值在−5.6到+3.8‰之间。然而，月球岩石的体积和磷灰石δ37Cl值变化更大，范围从∼−4到+40‰。由于Howardite-Eucite-Digenite(Hed)陨石是一颗已分化的玄武质小行星(4 Vesta)中最大的一套地壳和次地壳岩石，研究它们的挥发物可能有助于深入了解太阳系早期的行星分异过程。磷灰石的氯丰度范围为∼25~4900ppm，δ37Cl值范围为−3.98~+39.2ppm。磷灰石H2O含量较低的样品通常也富含37Cl，但各样品之间的δ37Cl和δD值之间没有系统的相关性。模拟的瑞利分馏和块体δ66Zn与磷灰石δ37Cl之间的强正相关支持了氯在贫氢环境中以金属氯化物的形式从真核岩浆中脱气的假说。在月球样品中，磷灰石的δ37Cl值与块体La/Yb比值呈正相关。有趣的是，大多数榴辉石与块体La/Yb比值呈负相关。最近，人们建议用同位素光的Cl值来记录初级太阳星云的特征。如果是这种情况，那么在太阳系历史早期快速增长的4个灶神星也可以记录到与这里测量到的最轻的Cl值相对应的原始星云特征。在榴辉石中观察到的氯同位素组成的显著变化可能与金属氯化物的脱气有关。
Generally, terrestrial rocks, martian and chondritic meteorites exhibit a relatively narrow range in bulk and apatite Cl isotope compositions, with δ37Cl (per mil deviation from standard mean ocean chloride) values between −5.6 and +3.8‰. Lunar rocks, however, have more variable bulk and apatite δ37Cl values, ranging from ∼−4 to +40‰. As the Howardite-Eucrite-Diogenite (HED) meteorites represent the largest suite of crustal and sub-crustal rocks available from a differentiated basaltic asteroid (4 Vesta), studying them for their volatiles may provide insights into planetary differentiation processes during the earliest Solar System history.Here the abundance and isotopic composition of Cl in apatite were determined for seven eucrites representing a broad range of textural and petrological characteristics. Apatite Cl abundances range from ∼25 to 4900 ppm and the δ37Cl values range from −3.98 to +39.2‰. Samples with lower apatite H2O contents were typically also enriched in37Cl but no systematic correlation between δ37Cl and δD values was observed across samples. Modelled Rayleigh fractionation and a strong positive correlation between bulk δ66Zn and apatite δ37Cl support the hypothesis that Cl degassed as metal chlorides from eucritic magmas, in a hydrogen-poor environment. In the case of lunar samples, it has been noted that δ37Cl values of apatite positively correlate with bulk La/Yb ratio. Interestingly, most eucrites show a negative correlation with bulk La/Yb ratio. Recently, isotopically light Cl values have been suggested to record the primary solar nebular signature. If this is the case then 4 Vesta, which accreted rapidly and early in Solar System history, could also record this primary nebular signature corresponding to the lightest Cl values measured here. The significant variation in Cl isotope composition observed within the eucrites are likely related to degassing of metal chlorides.