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Generation of halogen compounds by pyrolysis, molecule identification by mass spectrometer, and reactivity of fluorine and iodine at cryogenic temperature - dihalomethylenes
One of the most interesting initial results of the characterization of the asteroid Ryugu regolith material returned by the Hayabusa2 spacecraft is the presence of a significant quantity of less-aqueously altered CI material, tentatively called “CI2”. This lithology is characterized by an approximately ten-fold increase in the amount of an hydrous ferromagnesian silicates over what is found n CI1 chondrites, with these being principally olivine, low- and high-calcium pyroxene. Remnant CAI phases are found as well as probable chondrule fragments. Magnetite, pyrrhotite and hydroxyapatite are abundant. The main carbonate is calcite. Matrix is mainly a mixture of saponite and serpentine phases. Assuming that this lithology should be a better representative of the more “primordial” mineralogy of early nebular solids than more processed CI1 material, we have been re-examining CI lithologies found as separate meteorites as well as clasts in other meteorites, seeking similar CI2 material. The Kaidun breccia is notorious for including almost every type of meteorite class except CR2 (which is ironic considering that Kaidun is still officially classified as CR2). We report here the discovery of a probable CI2 lithology in one carbonaceous chondrite-rich Kaidun sample.
The Tagish Lake meteorite is a rare discovery in the meteorite world. Tentatively classified as an ungrouped type 2 carbonaceous chondrite, it is the first known CI2 meteorite [1, 2, 3]. Tagish Lake is the first meteorite to exhibit a reflectance spectrum showing the red color characteristic of the D- and P-type asteroids that populate the outer main-belt [2], although many interplanetary dust particles collected from the Earth s stratosphere exhibit a similar spectrum [4]. Several large pieces of the Tagish Lake meteorite were recovered on Jan. 18, 2000. We obtained two samples of the Tagish Lake meteorite, both collected on April 24, 2000, designated MG02 and MG03, from A. Hildebrand (Univ. of Calgary) and P. Brown (Univ. of Western Ontario). Smaller fragments collected in May 2000 were purchased from the The Meteorite Market (samples designated "MM"). We have reported preliminary results from infrared and X-ray Absorption Near-edge Structure (XANES) spectroscopy on the MM samples [5]. We report evidence to suggest that there was terrestrial contamination of the meteorite, even in fragments in contact with the lake ice for only a few months .
CI chondrites are thought to approximate the bulk solar system composition since they closely match the composition of the solar photosphere. Thus, chemical differences between a planetary object and the CI composition are interpreted to result from fractionations of a CI starting composition. This interpretation is often made despite the secondary mineralogy of CI chondrites, which resulted from low-T aqueous alteration on the parent asteroid(s). Prevalent alteration and the relatively large uncertainties in the photospheric abundances (approx. +/-5-10%) permit chemical fractionation of CI chondrites from the bulk solar system, if primary chondrules and/or CAIs have been altered beyond recognition. Isolated olivine and pyroxene grains that range from approx. 5 microns to several hundred microns have been reported in CI chondrites, and acid residues of Orgueil were found to contain refractory oxides with oxygen isotopic compositions matching CAIs. However, the only CAI found to be unambiguously preserved in a CI chondrite was identified in Ivuna. The Ivuna CAI's primary mineralogy, small size (approx.170 microns), and fine-grained igneous texture classify it as a compact type A. Aqueous alteration infiltrated large portions of the CAI, but other regions remain pristine. The major primary phases are melilite (Ak 14-36 ), grossmanite (up to 20.8 wt.% TiO 2 ), and spinel. Both melilite and grossmanite have igneous textures and zoning patterns. An accretionary rim consists primarily of olivine (Fa 2-17 ) and low-Ca pyroxene (Fs 2-10 ), which could be either surviving CI2 material or a third lithology.
Samples of C-type asteroid (162173) Ryugu returned by the Hayabusa2 mission of JAXA [1-3] have been found to be mineralogically, chemically, and isotopically similar to CI carbonaceous chondrites [3-6]. The rare CI meteorites have bulk chemical compositions closely matching the solar photosphere in all but the most volatile elements, and so are considered to represent the starting composition of the solar system. Paradoxically, however, all known CI have been extensively altered by aqueous fluids (i.e., they are CI1), so that their primary mineralogy, textures, and oxygen isotope compositions have been obscured. Thus, one of the most exciting discoveries from Ryugu samples is that they contain “less-altered” (CI2) areas, which have substantial abundances of anhydrous silicates (olivine and pyroxene) and Ca-Al-rich phases that may be remnants of primordial CI3 materials [4-11]. This discovery has motivated new investigations of CIs that show higher abundances of anhydrous silicate grains than previously recognized [9,12,13]. Such studies are an important step in the search for unaltered CI-like material, a so-called “holy grail” of meteoriticists [14]. To further this goal, we are studying Ryugu samples in the context of a previously unrecognized, potentially Ryugu-like, population of C2 materials that occur as xenoliths in polymict ureilites.