Asteroid (101955) 1999 RQ36: Spectroscopy from 0.4 to 2.4 μm and meteorite analogs

We present reflectance spectra from 0.4 to 2.4 μm of Asteroid (101955) 1999 RQ36, the target of the OSIRIS-REx spacecraft mission. The visible spectral data were obtained at the McDonald Observatory 2.1-m telescope with the ES2 spectrograph. The infrared spectral data were obtained at the NASA Infrared Telescope Facility using the SpeX instrument. The average visible spectrum is combined with the average near-infrared wavelength spectrum to form a composite spectrum. We use three methods to constrain the compositional information in the composite spectrum of Asteroid (101955) 1999 RQ36 (hereafter RQ36). First, we perform a least-squares search for meteorite spectral analogs using 15,000 spectra from the RELAB database. Three most likely meteorite analogs are proposed based on the least-squares search. Next, six spectral parameters are measured for RQ36 and their values are compared with the ranges in parameter values of the carbonaceous chondrite meteorite classes. A most likely meteorite analog group is proposed based on the depth of overlap in parameter values. The results of the least-squares search and the parametric comparisons point to CIs and/or CMs as the most likely meteorite analogs for RQ36, and COs and CHs as the least likely. RQ36 has a spectrally “blue” continuum slope that is also observed in carbonaceous chondrites containing magnetite. We speculate that RQ36 is composed of a “CM1”-like material. Finally, we compare RQ36 to other B-type asteroids measured by Clark et al. (Clark, B.E. et al. [2010]. J. Geophys. Res. 115, E06005). The results of this comparison are inconclusive. RQ36 is comparable to Themis spectral properties in terms of its albedo, visible spectrum, and near-infrared spectrum from 1.1 to 1.45 μm. However, RQ36 is more similar to Pallas in terms of its near-infrared spectrum from 1.6 to 2.3 μm. Thus it is possible that B-type asteroids form a spectral continuum and that RQ36 is a transitional object, spectrally intermediate between the two end-members. This is particularly interesting because Asteroid 24 Themis was recently discovered to have H₂O ice on the surface (Rivkin, A., Emery, J. [2010]. Nature 464, 1322–1323; Campins, H. et al. [2010a]. Nature 464, 1320–1321).     

阿尔金索尔库里地区元古代流纹岩锆石SHRIMP U-Pb定年及其地质意义

阿尔金索尔库里北山彩虹沟塔什达坂群卓阿布拉克组流纹岩锆石SHRIMP U-Pb 年龄为920±20Ma (MSWD=1.50),代表流纹岩的喷出时间。岩石的SiO₂介于68.33%~70.60%,K₂O>Na₂O,K₂O/Na₂O平均值6.42,属弱过铝质(ACNK平均为1.25)钙碱性系列(σ平均为2.76),为一套典型的壳源流纹岩岩石系列。稀土元素含量高(平均为211.3×10^-6),轻稀土富集(L/H平均为6.57),Eu亏损中等(δEu平均为0.30)。微量元素以富集Rb、Th、K、Zr,而相对亏损Ba、Sr、Eu、Ti、Nb、Ta为特征。岩石微量及稀土元素具有典型的板内火山岩特征,而岩石显著的低Sr特征(25.0×10^-6~65.3×10^-6,平均值为36.8×10^-6))表明其并非源自加厚的地壳,而是起源于斜长石稳定的正常地壳。阿尔金流纹岩是由进入上地壳高位岩浆房的拉斑玄武岩浆的底侵作用直接使上地壳部分熔融,形成酸性火山岩的原始岩浆。该岩浆体系沿区域断裂构造体系上升,并经历了较强的结晶分异和演化,最终形成阿尔金索尔库里北山流纹岩系列。     

Testing the preservation potential of early diagenetic dolomites as geochemical archives

Early marine diagenetic dolomite is a rather thermodynamically-stable carbonate phase and has potential to act as an archive of marine porewater properties. However, the variety of early to late diagenetic dolomite phases that can coexist within a single sample can result in extensive complexity. Here, the archive potential of early marine dolomites exposed to extreme post-depositional processes is tested using various types of analyses, including: petrography, fluid inclusion data, stable δ¹³C and δ¹⁸O isotopes, ⁸⁷Sr/⁸⁶Sr ratios, and U-Pb age dating of various dolomite phases. In this example, a Triassic carbonate platform was dissected and overprinted (diagenetic temperatures of 50 to 430°C) in a strike-slip zone in Southern Spain. Eight episodes of dolomitization, a dolostone cataclasite and late stage meteoric/vadose cementation were recognized. The following processes were found to be diagenetically relevant: (i) protolith deposition and fabric-preservation, and marine dolomitization of precursor aragonite and calcite during the Middle–Late Triassic; (ii) intermediate burial and formation of zebra saddle dolomite and precipitation of various dolomite cements in a Proto-Atlantic opening stress regime (T ca 250°C) during the Early–Middle Jurassic; (iii) dolomite cement precipitation during early Alpine tectonism, rapid burial to ca 15 km, and high-grade anchizone overprint during Alpine tectonic evolution in the Early Eocene to Early Miocene; (iv) brecciation of dolostones to cataclasite during the onset of the Carboneras Fault Zone activity during the Middle Miocene; and (v) late-stage regression and subsequent meteoric overprint. Data shown here document that, under favourable conditions, early diagenetic marine dolomites and their archive data may resist petrographic and geochemical resetting over time intervals of 10⁸ or more years. Evidence for this preservation includes preserved Late Triassic seawater δ¹³C_DIC values and primary fluid inclusion data. Data also indicate that oversimplified statements based on bulk data from other petrographically-complex dolomite archives must be considered with caution.