The Lodran meteorite and its relationship to the ureilites
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Summary Lodran is a unique meteorite consisting of roughly equal amounts of metal, olivine, and pyroxene with minor amounts of sulphide, chromite, phosphide, chrome-diopside, and a new phase with a composition close to (K,Na)AlSi 5 O 12 . Zähringer reported planetary-type rare gases in both the metal and silicates, suggesting a primitive nature. The pyroxene composition is Fs 13.8 with little variation. Olivine composition averages Fa 12.6 , but varies at least ±20 % both among grains and zoned within single grains; only the Fe-rich olivine is in equilibrium with the pyroxene. The metal probably cooled rapidly (700 K/Myr) at high temperatures and slower (30 K/Myr) at lower temperatures. Two compositional populations of chromite are found. A model for the formation of Lodran includes three steps: Formation of large olivine, pyroxene, and metal grains, with the trapping of small olivine inclusions in pyroxene and pyroxene in olivine. Equilibration and recrystallization of olivine, pyroxene, and metal, loss of alkalis and Ca; this probably occurred in a parent-body setting. And incorporation of reducing materials and mild reheating sufficient to produce the zoning in the olivine but not enough to re-equilibrate the pyroxene. Phase compositions and rare-gas concentrations in ureilites are similar to those in Lodran. In some respects Lodran appears to be a metal-rich ureilite, but the higher Fe/(Fe+Mg) ratios in the latter (Fa 21 olivine) suggest origin on separate parent bodies. The Harvard University meteorite is a mesosiderite and not closely related to Lodran.Keywords:
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Enstatite
Pyroxene
Solidus
Wollastonite
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Abstract We studied 149 pyroxenes from 69 pyroxene‐bearing micrometeorites collected from deep‐sea sediments of the Indian Ocean and South Pole Water Well at Antarctica, Amundsen‐Scott South Pole station. The minor elements in pyroxenes from micrometeorites are present in the ranges as follows: MnO ~0.0–0.4 wt%, Al 2 O 3 ~0.0–1.5 wt%, CaO ~0.0–1.0 wt%, Cr 2 O 3 ~0.3–0.9 wt%, and FeO ~0.5–4 wt%. Their chemical compositions suggest that pyroxene‐bearing micrometeorites are mostly related to precursors from carbonaceous chondrites rather than ordinary chondrites. The Fe/(Fe+Mg) ratio of the pyroxenes and olivines in micrometeorites shows similarities to carbonaceous chondrites with values lying between 0 and 0.2, and those with values beyond this range are dominated by ordinary chondrites. Atmospheric entry of the pyroxene‐bearing micrometeorites is expected to have a relatively low entry velocity of <16 km s −1 and high zenith angle (70–90°) to preserve their chemical compositions. In addition, similarities in the pyroxene and olivine mineralogical compositions between carbonaceous chondrites and cometary particles suggest that dust in the solar system is populated by materials from different sources that are chemically similar to each other. Our results on pyroxene chemical compositions reveal significant differences with those from ordinary chondrites. The narrow range in olivine and pyroxene chemical compositions are similar to those from carbonaceous chondrites, and a small proportion to ordinary chondrites indicates that dust is largely sourced from carbonaceous chondrite‐type bodies.
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The cooling histories of individual meteorites can be empirically reconstructed by using ages from different radioisotopic chronometers with distinct closure temperatures. For a group of meteorites derived from a single parent body such data permit the reconstruction of the cooling history and properties of that body. Particularly suited are H chondrites because precise radiometric ages over a wide range of closure temperatures are available. A thermal evolution model for the H chondrite parent body is constructed by using all H chondrites for which at least three different radiometric ages are available. Several key parameters determining the thermal evolution of the H chondrite parent body and the unknown burial depths of the H chondrites are varied until an optimal fit is obtained. The fit is performed by an 'evolution algorithm'. Empirical data for eight samples are used for which radiometric ages are available for at least three different closure temperatures. A set of parameters for the H chondrite parent body is found that yields excellent agreement (within error bounds) between the thermal evolution model and empirical data of six of the examined eight chondrites. The new thermal model constrains the radius and formation time of the H chondrite parent body (possibly (6) Hebe), the initial burial depths of the individual H chondrites, the average surface temperature of the body, the average initial porosity of the material the body accreted from, and the initial 60Fe content of the H chondrite parent body.
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Abstract Although pyroxene has been detected remotely across the Solar System, limited information is available from infrared remote sensing about the Mg‐Fe composition of pyroxene, and distinguishing between augite (20 < CaSiO 3 < 45) and diopside‐hedenbergite (CaSiO 3 > 45) remains challenging. The characteristics of pyroxene in the intermediate infrared range (4–8 μm), meanwhile, have not been documented. Using reflectance spectra of 72 samples ranging across the pyroxene quadrilateral, we investigate the effect of variations in Mg# (Mg/[Mg + Fe] × 100) and Ca‐content on the positions of strong and well‐defined spectral bands at ∼5.1 and ∼5.3 μm in high‐Ca pyroxene and ∼5.2 in low‐Ca pyroxene. We find that the 5.1, 5.2, and 5.3 μm bands move to shorter wavelengths as Mg# increases, whereas Ca‐content does not significantly affect the positions of these bands, enabling the determination of pyroxene Mg# directly from band positions alone. We also find that the ∼5.1 μm band is significantly more distinctive in diopside‐hedenbergite and the ∼5.3 μm band significantly more so in augite. Therefore, the 5.1, 5.2, and 5.3 μm spectral bands enable discrimination among diopside‐hedenbergite, low‐Ca pyroxene, and augite. Additionally, the 5.1, 5.2, and 5.3 μm bands enable direct determination of Mg# of diopside‐hedenbergite, low‐Ca pyroxene, and augite within ±23, ±10, and ±29 mol% Mg‐Fe, respectively.
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