Browsing by Author "Gattacceca, Jerome"

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    Description of a very dense meteorite collection area in western Atacama: Insight into the long-term composition of the meteorite flux to Earth
    (2016) Hutzler, Aurore; Gattacceca, Jerome; Rochette, Pierre; Braucher, Regis; Carro, Bertrand; Christensen, Eric J.; Cournede, Cecile; Gounelle, Matthieu; Ouazaa, Nejia Laridhi; Martinez, Rodrigo; Valenzuela, Millarca; Warner, Michael; Bourles, Didier
    We describe the geological, morphological, and climatic settings of two new meteorite collections from Atacama (Chile). The El Medano collection was recovered by systematic on-foot search in El Medano and Caleta el Cobre dense collection areas and is composed of 213 meteorites before pairing, 142 after pairing. The private collection has been recovered by car by three private hunters and consists of 213 meteorites. Similar to other hot desert finds, and contrary to the falls and Antarctica finds, both collections show an overabundance of H chondrites. A recovery density can be calculated only for the El Medano collection and gives 251 and 168 meteorites larger than 10gkm(-2), before and after pairing, respectively. It is by far the densest collection area described in hot deserts. The Atacama Desert is known to have been hyperarid for a long period of time and, based on cosmic-ray exposure ages on the order of 1-10Ma, to have been stable over a period of time of several million years. Such a high meteorite concentration might be explained invoking either a yet unclear concentration mechanism (possibly related to downslope creeping) or a previously underestimated meteorite flux in previous studies or an average terrestrial age over 2Myr. This last hypothesis is supported by the high weathering grade of meteorites and by the common terrestrial fragmentation (with fragments scattered over a few meters) of recovered meteorites.
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    Modification of REE distribution of ordinary chondrites from Atacama (Chile) and Lut (Iran) hot deserts: Insights into the chemical weathering of meteorites
    (2017) Pourkhorsandi, Hamed; D'Orazio, Massimo; Rochette, Pierre; Valenzuela, Millarca; Gattacceca, Jerome; Mirnejad, Hassan; Sutter, Brad; Hutzler, Aurore; Aboulahris, Maria
    The behavior of rare earth elements (REEs) during hot desert weathering of meteorites is investigated. Ordinary chondrites (OCs) from Atacama (Chile) and Lut (Iran) deserts show different variations in REE composition during this process. Inductively coupled plasma-mass spectrometry (ICP-MS) data reveal that hot desert OCs tend to show elevated light REE concentrations, relative to OC falls. Chondrites from Atacama are by far the most enriched in REEs and this enrichment is not necessarily related to their degree of weathering. Positive Ce anomaly of fresh chondrites from Atacama and the successive formation of a negative Ce anomaly with the addition of trivalent REEs are similar to the process reported from Antarctic eucrites. In addition to REEs, Sr and Ba also show different concentrations when comparing OCs from different hot deserts. The stability of Atacama surfaces and the associated old terrestrial ages of meteorites from this region give the samples the necessary time to interact with the terrestrial environment and to be chemically modified. Higher REE contents and LREE-enriched composition are evidence of contamination by terrestrial soil. Despite their low degrees of weathering, special care must be taken into account while working on the REE composition of Atacama meteorites for cosmochemistry applications. In contrast, chondrites from the Lut desert show lower degrees of REE modification, despite significant weathering signed by Sr content. This is explained by the relatively rapid weathering rate of the meteorites occurring in the Lut desert, which hampers the penetration of terrestrial material by forming voluminous Fe oxide/oxyhydroxides shortly after the meteorite fall.
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    The ungrouped chondrite El Medano 301 and its comparison with other reduced ordinary chondrites
    (2017) Pourkhorsandi, Hamed; Gattacceca, Jerome; Devouard, Bertrand; D'Orazio, Massimo; Rochette, Pierre; Beck, Pierre; Sonzogni, Corinne; Valenzuela, Millarca
    El Medano 301 (EM 301) is an ungrouped chondrite with overall texture and trace-element distribution similar to those of ordinary chondrites (OCs), but with silicate (olivine and low-Ca pyroxene) compositions that are more reduced than those in OCs, with average olivine and low-Ca pyroxene of Fa(3.9 +/- 0.3) and Fs(12.8 +/- 4.9), respectively. These values are far lower than the values for OCs and even for chondrites designed as "reduced" chondrites. Low-Ca pyroxene is the dominant mineral phase and shows zoning with higher MgO contents along the crystal rims and cracks (reverse zoning). The Co content of kamacite is also much lower than the concentrations observed in OCs (below detection limit of 0.18 wt% versus 0.44-37 wt%). Oxygen isotopic composition is Delta O-17 =+0.79,+0.78% and slightly different from that of OCs. The lower modal olivine/pyroxene ratio, different Infrared (IR) spectra, lower Co content of kamacite, lower mean FeO contents of olivine and pyroxene, different kamacite texture, and different oxygen-isotopic composition show that EM 301 does not belong to a known OC group. EM 301 shows similarities with chondritic clasts in Cumberland Falls aubrite, and with Northwest Africa 7135 (NWA 7135) and Acfer 370 ungrouped chondrites. However, dissimilar to NWA 7135 and the clasts, it does not contain highly reduced mineral phases like daubreelite.