伊吾Eucrite陨石的岩石学、矿物学特征及其成因

Howardite-Eucrite-Diogenite(HED)族陨石是地球上目前发现数量最多的无球粒陨石类型,已发现的HED族陨石主要来自于南极和非洲西北部的沙漠,本研究的HED族陨石国际命名为Yiwu 001,发现于中国新疆维吾尔自治区伊吾县境内。系统的矿物学和岩石学研究显示,该陨石属于单矿物碎屑角砾岩Eucrite,主要矿物为易变辉石(38%)、普通辉石(8%)和斜长石(46%),副矿物主要为二氧化硅、陨硫铁、铬铁矿、钛铁矿。根据主要矿物的粒径,对该陨石开展了区域划分:粗粒、细粒和极细粒,辉石对应的粒径均值分别为550μm、100μm和60μm。粗粒和部分细粒区域具次辉绿结构,细粒和极细粒区域发育麻粒结构。陨石中细粒玄武岩包裹着以椭球体为主的粗粒和拉长的极细粒玄武岩,极细粒玄武岩呈定向构造。Yiwu 001陨石原岩可能为内含粗粒玄武岩捕虏体的玄武岩,定向构造指示可能发生过近静态的部分熔融。该陨石在晚期经历了复杂的热变质作用,为5型变质类型,主要表现在出溶辉石条带、麻粒结构和三联点结构。     

钙长辉长无球粒陨石NWA 11592的岩石矿物学特征及富铁橄榄石脉成因研究

灶神星是太阳系最大的硅酸盐质小行星,它的岩石样品(HED族陨石)是目前太阳系最古老的岩浆岩之一,很有可能记录了类地行星最早期地质流体活动的重要信息。本次工作首次在一块非角砾岩平衡型的Eucrite NWA 11592中发现了较粗长的富铁橄榄石脉。通过对NWA 11592陨石进行详细的岩相学和地球化学特征研究,将NWA 11592陨石划为玄武质非角砾岩型Eucrite,冲击变质程度至少为S4,热变质程度为6型。NWA 11592中富铁橄榄石脉最有可能为含H₂O流体沿裂隙的沉淀产物,反应系统应是开放且较为氧化的(f_O2达IW+0.9),且应发生在灶神星浅层地壳> 4 km的埋藏深度。含H₂O流体渗透作用除更多地发生在灶神星表面或近地表外,还有可能发生在灶神星内部的浅层地壳中。     

Differentiation and magmatic history of Vesta: Constraints from HED meteorites and Dawn spacecraft data

Quantifying the amounts of various igneous lithologies in Vesta’s crust allows the estimation of petrologic ratios that describe the asteroid’s global differentiation and subsequent magmatic history. The eucrite:diogenite (Euc:Diog) ratio measures the relative proportions of mafic and ultramafic components. The intrusive:extrusive (I:E) ratio assesses the effectiveness of magma ascent and eruption. We estimate these ratios by counting numbers and masses of eucrites, cumulate eucrites, and diogenites in the world’s meteorite collections, and by calculating their proportions as components of crustal polymict breccias (howardites) using chemical mixing diagrams and petrologic mapping of multiple thin sections. The latter two methods yield a Euc:Diog ratio of ∼2:1, although meteorite numbers and masses give slightly higher ratios. Surface lithologic maps compiled from spectra of Dawn spacecraft instruments (VIR and GRaND) yield Euc:Diog ratios that bracket estimates of Euc:Diog from the meteorites. The I:E ratios from HEDs lie between 0.5–2.1:1, due to uncertainties in identifying cumulate eucrite. Gravity mapping of Vesta by the Dawn spacecraft supports the existence of diogenite plutons in the crust. Quantifying the proportion of high-density diogenitic crust in the gravity map yields I:E ratios of 0.8-1:2:1, values which are bracketed by calculations based on HEDs. The I:E ratio for Vesta is lower than for Earth and Mars, consistent with physical modeling of asteroid-size bodies. Nevertheless, it indicates a significant role for pluton emplacement during the formation of Vesta’s crust. These results are inconsistent with simple differentiation models that produce the crust by crystallization of a global magma ocean, unless residual melts are extracted into crustal magma chambers.     

Unique mineral assemblages of shock-induced titanium-rich melt pockets in eucrite Northwest Africa 8003

Shock-induced Ti-rich melt pockets in a basaltic eucrite Northwest Africa (NWA) 8003 were studied using scanning and transmission electron microscopy. Unique mineral assemblages consisting of clinopyroxene, ilmenite, vestaite, corundum, and kyanite are observed. Among them, vestaite and corundum in NWA 8003 are first reported to occur in eucrite meteorites. Petrographic and chemical evidences indicate that the Ti-rich melt pockets have formed by in-situ melting of ilmenite, plagioclase, pyroxene, and possibly minor silica and apatite nearby. The temperature rise and melting were caused by the high shock impedance contrast at interfaces between ilmenite and other phases with a distinctly lower density. Crystallization pressure, temperature and cooling time of the Ti-rich melt pockets in NWA 8003 are constrained to be ˜0.9–˜10 GPa, ˜1300–˜1730 °C, and < 1 ms (5–50 μm in size), respectively.     

Evaluation of a curve-fitting method for diffuse reflectance spectra in the UV–Visible–NIR wavelength region

The Modified Gaussian Method (MGM) proposed by Sunshine et al. [Sunshine, J.M., Pieters, C.M., Pratt, S.F., 1990. Deconvolution of mineral absorption bands: an improved approach. Journal of Geophysical Research 95, 6955–6966.] is generally used to decompose spectra in the UV–Visible–NIR wavelength region into the characteristic absorption bands of minerals. Here, we compare the optimized results obtained using different curve-fitting methods for this spectrum. The result obtained using the Gaussian function for the absorption band shows a better fit than that obtained using the Lorentzian function. The background continuum of a quadratic polynomial for the wavenumber provides a better result than does the linear function for the wavenumber. We successfully decomposed the spectra of ordinary chondrites and eucrites into the absorption bands of olivine and pyroxene. The wavelength positions of these absorption bands are broadly consistent with the Fe contents of olivine and pyroxene. Although the present results are derived from a limited number of spectra, they are of use in terms of the decomposition of diffuse reflectance spectra.