¹Edward A.Cloutis, ²Vishnu Reddy, ³David T.Blewett
Icarus 311, 384-393 Link to Article [https://doi.org/10.1016/j.icarus.2018.04.027]
¹Department of Geography, University of Winnipeg, 515 Portage Avenue, Winnipeg, MB R3B 2E9, Canada
²Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA
³Planetary Exploration Group, Johns Hopkins Applied Physics Laboratory, Laurel, MD, USA
Copyright Elsevier

We have measured reflectance spectra (0.35–25.0 µm) of different size powders of the ungrouped achondrite NWA 7325 in order to facilitate spectroscopic identification of its parent body. Previous work has suggested that the meteorite may have come from the planet Mercury based on its oxidation state. The 0.35–2.5 µm reflectance spectra of NWA 7325 exhibit absorption bands that can be attributed to the presence of chromium-bearing diopside and possibly to Ca-rich plagioclase. Spectral evidence for olivine is generally lacking, likely due to interference from stronger diopside absorption bands. With increasing grain size, albedo decreases while absorption band depths increase. The absorption bands are unique in the sense that they allow for the identification of the Cr-rich diopside in NWA 7325. The mid-infrared spectra are similar to those measured by previous investigators, and enable detection of the major silicates in NWA 7325, including more robust identification of olivine and plagioclase feldspar. We find no spectroscopic or compositional evidence supporting a link to Mercury as a possible parent body, even accounting for plausible spectrum-altering processes. In terms of a link to an asteroidal parent body, the most confident link would be made based on the unique Cr-diopside-related absorption bands in the 0.65, 1.05, and 2.3 µm regions. At present, the closest spectral match we have found is with asteroid 10,537 (1991 RY16).

¹Yves Marrocchi, ¹Johan Villeneuve, ²Valentina Batanova, ¹Laurette Piani, ³Emmanuel Jacquet
Earth and Planetary Science Letters 496, 132-141 Link to Article [https://doi.org/10.1016/j.epsl.2018.05.042]
¹CRPG, CNRS, Université de Lorraine, UMR 7358, Vandoeuvre-lès-Nancy, 54501, France
²Université Grenoble Alpes, ISTerre, CNRS, UMR 5275, Grenoble, F-38000, France
³IMPMC, CNRS & Muséum national d’Histoire naturelle, UMR 7590, CP52, 57 rue Cuvier, 75005 Paris, France
Copyright Elsevier

FeO-poor (type I) porphyritic chondrules formed by incomplete melting of solid dust precursors via a yet-elusive mechanism. Two settings are generally considered for their formation: (i) a nebular setting where primordial solids were melted, e.g. by shock waves propagating through the gas and (ii) a collisional planetary setting. Here we report a method combining high-current electron microprobe X-ray mapping and quantitative measurements to determine the chemical characteristics of relict olivine grains inherited from chondrule precursors. We find that these olivine crystals are Ca–Al–Ti-poor relative to host olivine crystals. Their variable Δ17Δ17O, even in individual chondrule, is inconsistent with derivation from planetary interiors as previously argued from 120 ° triple junctions also exhibited by the chondrules studied herein. This indicates that chondrule precursors correspond to solid nebular condensates formed under changing physical conditions.
We propose that porphyritic chondrules formed during gas-assisted melting of nebular condensates comprising relict olivine grains with varying Δ17Δ17O values and Ca–Al–Ti-rich minerals such as those observed within amoeboid olivine aggregates. Incomplete melting of chondrule precursors produced Ca–Al–Ti-rich melts (CAT-melts), allowing subsequent crystallization of Ca–Al–Ti-rich host olivine crystals via epitaxial growth on relict olivine grains. Incoming MgO and SiO from the gas phase induced (i) the dilution of CAT-melts, as attested by the positive Al–Ti correlation observed in chondrule olivine crystals, and (ii) buffering of the O-isotope compositions of chondrules, as recorded by the constant Δ17Δ17O values of host olivine grains. The O-isotopic compositions of host olivine grains are chondrule-specific, suggesting that chondrules formed in an array of environments of the protoplanetary disk with different Δ17Δ17O values, possibly due to variable solid/gas mixing ratios.