摘要
钙长辉长无球粒陨石(Eucrite)是Howardite-Eucrite-Diogenite(HED)族陨石的重要成员,也是研究灶神星壳演化历史的重要对象。本文研究了多个玄武质Eucrite样品中主要的SiO2相——普通石英和鳞石英的成因,进而讨论其对Eucrite陨石热演化的启示。研究对象包括不同冲击程度样品,以探讨陨击过程对SiO2同质多象转变的影响。冲击程度较弱的包括未角砾化样品NWA 3162、NWA 6594和Igdi,冲击程度较强的为单碎屑角砾岩Millbillillie、Camel Donga和NWA 1654。研究结果显示,不同样品中的普通石英和鳞石英各自均具有相似的岩相学和化学成分,但不同冲击程度样品中普通石英产状存在系统差异。结合Eucrite热变质历史,本研究认为普通石英并非来自共生鳞石英的相变,而是形成于更早期高温SiO2相的转变。Eucrite中广泛存在的鳞石英则很可能是普通石英在后期撞击事件中发生部分熔融快速结晶形成。Eucrite中普通石英和鳞石英可能经历的主要形成过程如下:(1)岩浆喷发形成高温SiO2(方石英和/或鳞石英);(2)随后长期热变质中高温SiO2转变形成普通石英,并因体积缩小发育孔洞结构;(3)后期冲击作用再加热,导致普通石英部分熔融形成鳞石英,在高冲击程度的样品中还普遍发育普通石英的羽状裂理。本研究在Eucrite中观察到的普通石英和鳞石英分别形成于不同阶段热事件。大多数Eucrite中存在普通石英和鳞石英共生,表明Eucrite在热变质后普遍受到热扰动,内部微区受热不均一性明显。上述普通石英和鳞石英成因的厘定,为微区或单碎屑矿物同位素年代学定年样品的选择以及年代学结果的地质解释提供了依据。
Eucrite is one of the most important member of the Howardite-Eucrite-Diogenite(HED) meteorites and offers a unique opportunity to investigate thermal history of the Vestan crust. We studied the petrography and mineral chemistry of quartz and tridymite, the two major polymorphs of silica in basaltic Eucrites, aiming to investigate their genesis and its application to thermal history of Eucrites. Three weakly shocked unbrecciated samples(NWA 3162, NWA 6594, Igdi) and three heavily shocked monomict Eucrites(Millbillillie, Camel Donga, NWA 1654)were studied for evaluating the effect of shock reheating on the formation of quartz and tridymite. The results demonstrate that quartz and tridymite from all samples have similar petrographic, textural and geochemical characteristics respectively, whereas the texture of quartz varies among samples with different shock stages. These features, combined with thermal metamorphic history of Eucrites, reveal that quartz was not transformed from coexisting tridymite, but arose much earlier from pre-existing high-temperature polymorph of SiO2. The tridymite now pervasively observed in Eucrites was the product of shock-induced reheating of quartz, followed by rapid cooling. Quartz and tridymite now presented in Eucrites, may have originated through the following processes:(1) eruption of eucritic basaltic magma and crystallization of high-temperature polymorph of SiO2.(2) Transformation of quartz from high-temperature polymorph during intense thermal metamorphism.(3) Impact-induced partial melting of quartz and crystallization of tridymite, accompanied by development of hackle fracture in quartz in intensely-shocked samples. To summarize, quartz and tridymite now widely observed in Eucrites were products of different thermal events. The coexistence of quartz and tridymite in most Eucrites shows that they were disturbed after thermal metamorphism and there was pervasive local thermal heterogeneity induced by impact reheating. The results presented here also have important implications for selection of samples for in-situ or whole-rock isotopic dating, as well as for interpretation of in situ chronological data.
作者
廖世勇
王英
蒋云
李晔
邓志培
Liao Shiyong;Wang Ying;Jiang Yun;Li Ye;Tang Chipui(Purple Mountain Observatory,Chinese Academy of Sciences,Nanjing,Jiangsu 210046,China;CAS Center for Excellence in Comparative Planetology,Hefei,Anhui 230026,China;State Key Laboratory for Lunar and Planetary Sciences,Macao University of Science and Technology,Macao 999078,China)
出处
《地质学报》
EI
CAS
CSCD
北大核心
2021年第9期2909-2919,共11页
Acta Geologica Sinica
基金
国家自然科学基金项目(编号42073060,41973060,41973065,41803051,41873076)
国防科工局民用航天技术预先研究项目(编号D020202,D020302)
中国科学院紫金山天文台小行星基金会联合资助的成果。