Terrestrial-like zircon in a clast from an Apollo 14 breccia
Terrestrial-like zircon in a clast from an Apollo 14 breccia
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阿波罗 14 号角砾岩碎屑中的类地锆石
DOI：
10.1016/j.epsl.2019.01.010
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发表时间：
2019
影响因子：
5.3
通讯作者：
Bellucci J
中科院分区：
文献类型：
--
作者：
Bellucci J
阿波罗14321号月球角砾岩中的长石碎屑被解释为火山喷发物，其岩相学和化学特征与通常分配给月球和陆地环境的形成条件一致。这里提出的一个简单的撞击喷出物模型表明，撞击前的深度为30-70公里，即接近月球地壳的底部。二次离子质谱微量元素分析结果表明，从该碎屑中回收的锆石颗粒具有正的Ce/Ce_（13）异常，对应于比月球地幔高+2 ~+4个对数单位的氧逸度，结晶温度为771±88 ~ 810±37° C（2σ），这对于月球岩浆来说是异常低的。此外，石英中的钛和锆石的计算表明结晶压力为6.9±1.2千巴，相当于月球上167±27公里的结晶深度，与喷出物模拟结果相矛盾。这种低T、高fO 2和高P的情况在其他任何月球碎屑中都没有观察到，也不知道是否存在于月球上，而且与在陆地岩浆中发现的情况大致相似。推断这些锆石颗粒和其他辅助矿物在Felsite的母岩浆的类地氧化还原条件与铁金属，大块碎屑地球化学，和钾长石颗粒内的碎屑，所有这些都是一致的月球起源的Pb同位素组成的存在形成对比。氧化还原条件和起源的深度之间的二分法推断锆石成分相比，喷出物建模需要一个多阶段的岩石成因。第二，目前无法解决的假设的起源和历史的碎屑是允许的这些数据。第一种假设是，相对氧化的条件是在月球岩浆中发展起来的，可能是通过分离结晶和富集不相容元素在富含流体的、磷酸盐饱和的岩浆中，在月球地壳的底部形成锆石颗粒和它们的寄主长石。随后的挖掘带来了更典型的月球特征，但保留了锆石和其他一些副矿物的主要化学特征。然而，这一假说未能解释结晶的高P。或者，长英岩及其锆石在地球上的大陆地壳中19±3 km的适度深度结晶，那里氧化，低温，富含流体的条件是常见的。随后，碎屑在一次大撞击中从地球喷出，作为陆地陨石夹带在月球风化层中，有证据表明，在其与撞击喷出物和宿主角砾岩结合的过程中，引入了还原条件。
A felsite clast in lunar breccia Apollo sample 14321, which has been interpreted as Imbrium ejecta, has petrographic and chemical features that are consistent with formation conditions commonly assigned to both lunar and terrestrial environments. A simple model of Imbrium impact ejecta presented here indicates a pre-impact depth of 30–70 km, ie near the base of the lunar crust. Results from Secondary Ion Mass Spectrometry trace element analyses indicate that zircon grains recovered from this clast have positive Ce/Ce⁎ anomalies corresponding to an oxygen fugacity+ 2 to+ 4 log units higher than that of the lunar mantle, with crystallization temperatures of 771±88 to 810±37° C (2σ) that are unusually low for lunar magmas. Additionally, Ti-in-quartz and zircon calculations indicate a pressure of crystallization of 6.9±1.2 kbar, corresponding to a depth of crystallization of 167±27 km on the Moon, contradicting ejecta modelling results. Such low-T, high-fO 2, and high-P have not been observed for any other lunar clasts, are not known to exist on the Moon, and are broadly similar to those found in terrestrial magmas. The terrestrial-like redox conditions inferred for the parental magma of these zircon grains and other accessory minerals in the felsite contrasts with the presence of Fe-metal, bulk clast geochemistry, and the Pb isotope composition of K-feldspar grains within the clast, all of which are consistent with a lunar origin. The dichotomy between redox conditions and the depth of origin inferred from the zircon compositions compared to the ejecta modelling necessitates a multi-stage petrogenesis. Two, currently unresolvable hypotheses for the origin and history of the clast are allowed by these data. The first postulates that the relatively oxidizing conditions were developed in a lunar magma, possibly by fractional crystallization and enrichment of incompatible elements in a fluid-rich, phosphate-saturated magma, at the base of the lunar crust to form the zircon grains and their host felsite. Subsequent excavation by the Imbrium impact introduced more typical lunar features to the clast but preserved primary chemical characteristics in zircon and some other accessory minerals. However, this hypothesis fails to explain the high P of crystallization. Alternatively, the felsite and its zircon crystallized on Earth at a modest depth of 19±3 km in the continental crust where oxidizing, low-T, fluid-rich conditions are common. Subsequently, the clast was ejected from the Earth during a large impact, entrained in the lunar regolith as a terrestrial meteorite with the evidence of reducing conditions introduced during its incorporation into the Imbrium ejecta and host breccia.
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