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Petrology and geochemistry of the unbrecciated achondrite Northwest Africa 1240 (NWA 1240): an HED parent body impact melt |
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| Authors: | JA Barrat A Jambon M Bohn J Blichert-Toft V Sautter C Göpel Ph Gillet O Boudouma F Keller |
| Institution: | 1 CNRS UMR 6112 (Géodynamique et Planétologie) and Université d’Angers, 2 bd Lavoisier, 49045 Angers Cedex, France 2 Laboratoire Magie (CNRS UMR 7047), Université Pierre et Marie Curie, case 110, 4 place Jussieu, 75252 Paris Cedex 05, France 3 Ifremer-Centre de Brest, (CNRS-UMR 6538), BP70, 29280 Plouzané Cedex, France 4 Laboratoire des Sciences de la Terre, CNRS UMR 5570, Ecole Normale Supérieure de Lyon, 46 Allée d’Italie, 69364 Lyon Cedex 7, France 5 Muséum National d’Histoire Naturelle, Laboratoire de Minéralogie, CNRS FR 32, 61 rue Buffon, 75005 Paris, France 6 Institut de Physique du Globe de Paris, Laboratoire de Géochimie et Cosmochimie (CNRS-UMR 7579), 4 place Jussieu, 75252 Paris Cedex 05, France 7 Université Pierre et Marie Curie, U.F.R. Sciences de la Terre, case 110, 4 place Jussieu, 75252 Paris Cedex 05, France 8 CNRS UMR 5025-UJF, Maison des Géosciences, 1381 rue de la Piscine, 38400 Saint Martin d’Hères, France |
| Abstract: | NWA 1240 is an unusual eucrite recently recovered in Morocco as a single stone of 98 g. It is an unbrecciated greenish-brown rock nearly devoid of fusion crust. It displays porphyritic texture consisting of skeletal hollow low-Ca pyroxene phenocrysts set in a variolitic (fan-spherulitic) mesostasis of fine elongate pyroxene and plagioclase crystals. Minor phases are skeletal chromite, iron, silica, troilite, ilmenite and minute amounts of phosphate and fayalite. Pyroxenes are unequilibrated and show one of the widest ranges of composition so far described for a eucrite, from En76.0Wo1.9Fs22.1 to compositions nearly devoid of Mg (unusual ferrosilite and Fe-augite symplectites and possibly pyroxferroite). Plagioclase crystals contain significant amounts of Fe and Mg, which are possibly controlled by the Ca(Mg,Fe2+)Si3O8 plagioclase component.To discuss the potential effects of hot-desert weathering on NWA 1240, we have analyzed a series of Saharan eucrites (Agoult, Aoufous, Igdi, Smara, NWA 047 and NWA 049) and large aliquots (0.39 to 2.8 g) of eucrite falls (Bereba, Bouvante, Jonzac, Juvinas and Serra de Magé). These results indicate that among the elements we have determined, Pb, Ba and Sr are the most sensitive indicators of Saharan weathering.The bulk composition of NWA 1240 has been determined for 45 elements by ICP-AES and ICP-MS. The data show that the meteorite is not significantly weathered: its Pb concentration is very low; Ba and Sr concentrations are not anomalously high; the Th/U and Hf/Sm ratios are chondritic (Th/U = 3.65, Hf/Sm = 0.74). NWA 1240 is rich in MgO (10.4 wt%) and Cr2O3 (0.71 wt%), and displays striking similarities with cumulate eucrites, such as having similar incompatible trace element patterns and a significant positive Eu anomaly (Eu/Eu* = 1.37). The combination of fast cooling and cumulate eucrite-dominated composition suggests that NWA 1240 is not an igneous rock but rather an impact melt. |
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