¹M. E. Varela, ²P. Sylvester, ³F. Brandstätter, ⁴A. Engler
¹Instituto de Ciencias Astronómicas de la Tierra y del Espacio (ICATE), San Juan, Argentina
²Department of Earth Sciences, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada
³Mineralogisch-Petrographische Abteilung, Naturhistorisches Museum, Wien, Austria
⁴Institute of Earth Sciences, Department of Mineralogy and Petrology, University of Graz, Graz, Austria

Sixteen nonporphyritic chondrules and chondrule fragments were studied in polished thin and thick sections in two enstatite chondrites (ECs): twelve objects from unequilibrated EH3 Sahara 97158 and four objects from equilibrated EH4 Indarch. Bulk major element analyses, obtained with electron microprobe analysis (EMPA) and analytical scanning electron microscopy (ASEM), as well as bulk lithophile trace element analyses, determined by laser ablation inductively coupled plasma–mass spectrometry (LA-ICP-MS), show that volatile components (K2O + Na2O versus Al2O3) scatter roughly around the CI line, indicating equilibration with the chondritic reservoir. All lithophile trace element abundances in the chondrules from Sahara 97158 and Indarch are within the range of previous analyses of nonporphyritic chondrules in unequilibrated ordinary chondrites (UOCs). The unfractionated (solar-like) Yb/Ce ratio of the studied objects and the mostly unfractionated refractory lithophile trace element (RLTE) abundance patterns indicate an origin by direct condensation. However, the objects possess subchondritic CaO/Al2O3 ratios; superchondritic (Sahara 97158) and subchondritic (Indarch) Yb/Sc ratios; and chondritic-normalized deficits in Nb, Ti, V, and Mn relative to RLTEs. This suggests a unique nebular process for the origin of these ECs, involving elemental fractionation of the solar gas by the removal of oldhamite, niningerite, and/or another phase prior to chondrule condensation. A layered chondrule in Sahara 97158 is strongly depleted in Nb in the core compared to the rim, suggesting that the solar gas was heterogeneous on the time scales of chondrule formation. Late stage metasomatic events produced the compositional diversity of the studied objects by addition of moderately volatile and volatile elements. In the equilibrated Indarch chondrules, this late process has been further disturbed, possibly by a postaccretional process (diffusion?) that preferentially mobilized Rb with respect to Cs in the studied objects.

Reference
Varela ME, Sylvester P, Brandstätter F, Engler A (2015) Nonporphyritic chondrules and chondrule fragments in enstatite chondrites: Insights into their origin and secondary processing. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12468]

Published by arrangement with John Wiley&Sons

¹M. R. Lee, ²I. MacLaren, ²S. M. L. Andersson, ³A. Kovács, ^1,4T. Tomkinson, ⁴D. F. Mark, ⁵C. L. Smith
¹School of Geographical and Earth Sciences, University of Glasgow, Glasgow, UK
²SUPA, School of Physics and Astronomy, University of Glasgow, Glasgow, UK
³Ernst Ruska-Centrum für Mikroskopie und Spektroskopie mit Elektronen, Forschungszentrum Jülich GmbH, Jülich, Germany
⁴Scottish Universities Environmental Research Centre, East Kilbride, UK
⁵Department of Earth Sciences, Natural History Museum, London, UK

The nakhlite meteorites are clinopyroxenites that are derived from a ~1300 million year old sill or lava flow on Mars. Most members of the group contain veins of iddingsite whose main component is a fine-grained and hydrous Fe- and Mg-rich silicate. Siderite is present in the majority of veins, where it straddles or cross-cuts the Fe-Mg silicate. This carbonate also contains patches of ferric (oxy)hydroxide. Despite 40 years of investigation, the mineralogy and origins of the Fe-Mg silicate is poorly understood, as is the paragenesis of the iddingsite veins. Nanometer-scale analysis of Fe-Mg silicate in the Nakhla meteorite by electron and X-ray imaging and spectroscopy reveals that its principal constituents are nanoparticles of opal-A. This hydrous and amorphous phase precipitated from acidic solutions that had become supersaturated with respect to silica by dissolution of olivine. Each opal-A nanoparticle is enclosed within a ferrihydrite shell that formed by oxidation of iron that had also been liberated from the olivine. Siderite crystallized subsequently and from solutions that were alkaline and reducing, and replaced both the nanoparticles and olivine. The fluids that formed both the opal-A/ferrihydrite and the siderite were sourced from one or more reservoirs in contact with the Martian atmosphere. The last event recorded by the veins was alteration of the carbonate to a ferric (oxy)hydroxide that probably took place on Mars, although a terrestrial origin remains possible. These results support findings from orbiter- and rover-based spectroscopy that opaline silica was a common product of aqueous alteration of the Martian crust.

Reference
Lee MR, MacLaren I, Andersson SML, Kovács A, Tomkinson T, Mark DF, Smith CL (2015) Opal-A in the Nakhla meteorite: A tracer of ephemeral liquid water in the Amazonian crust of Mars. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12471]
Published by Arrangement with John Wiley&Sons