¹Samuel Ebert, ¹Addi Bischoff
¹Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany

Al-rich objects (Ca,Al-rich inclusions (CAIs), Al-rich chondrules, Al-rich fragments) occur in all chondrite classes. These objects can be centimeter-sized in CV3 carbonaceous chondrites, but they are generally much smaller in other chondrite groups and classes. Within the ordinary chondrites, most Al-rich objects are chondrules that vary from Ca- to Na-rich. Here, we have investigated the mineralogy and major element chemistry of 32 Na-rich chondrules and 3 Na-rich fragments from 15 different chondrites. Most objects (chondrules and chondrule fragments) are from ordinary chondrites (petrologic types 3.2-3.8), but two of the chondrules are from two CO3 chondrites, and three of the chondrules are from one Rumuruti (R)-chondrite. We found that these Na-rich objects have bulk Na2O-concentrations between 4.3 and 15.2 wt%. Texturally, they typically consist of euhedral to subhedral (often skeletal) mafic minerals (olivine and pyroxenes) embedded within a nepheline-normative, glassy mesostasis, which is brownish in transmitted light. In addition, some chondrules contain euhedral to subhedral spinel. Bulk chondrule compositions show group II, group III, and ultrarefractory rare earth element (REE) patterns similar to those found in CAIs. These results clearly demonstrate that the Na-rich chondrules must have been formed by melting of precursors containing an (ultra)-refractory element-rich component and Na-rich constituents. The Na-rich chondrules showed Sm and Eu anomalies, indicating that they must have formed at low oxygen fugacities. Based on the chemical composition of the Na-rich objects, we can rule out that they were formed as a result of planetary formation due to metasomatic processes or processes related to collisions between molten planetesimals.

Reference
Ebert S, Bischoff A (2016) Genetic relationship between Na-rich chondrules and Ca,Al-rich inclusions? – Formation of Na-rich chondrules by melting of refractory and volatile precursors in the Solar Nebula. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.01.014]
Copyright Elsevier

^1,2Myriam Telus, ²Gary R. Huss, ³Ryan C. Ogliore, ^1,2Kazuhide Nagashima, ³Daryl L. Howard, ⁴Matthew G. Newville, ⁵Andrew G. Tomkins
¹Geology & Geophysics, School of Ocean, Earth Science & Technology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
²Hawai‘i Institute of Geophysics and Planetology, School of Ocean, Earth Science & Technology, University of Hawai‘i at Mānoa, Honolulu, HI 96822, USA
³Australian Synchrotron, Clayton, Victoria 3168, Australia
⁴Center for Advanced Radiation Sources, University of Chicago, Chicago, IL 60637, USA
⁵School of Earth, Atmosphere & Environment, Monash University, Melbourne, Victoria 3800, Australia

The Fe and Ni isotopic composition of ferromagnesian silicates in chondrules from unequilibrated ordinary chondrites (UOCs) have been used to estimate the initial abundance of the short-lived radionuclide, 60Fe, in the early Solar System. However, these estimates vary widely, and there are systematic discrepancies in initial 60Fe/56Fe ratios inferred from in situ and bulk analyses of chondrules. A possible explanation is that the Fe-Ni isotope system in UOC chondrules has not remained closed (a necessary condition for isotopic dating), and Fe and Ni have been redistributed since the chondrules formed. In order to evaluate this, we collected high-spatial-resolution X-ray fluorescence (XRF) maps of UOC chondrules to better understand the distribution and mobility of Fe and Ni at the low metamorphic temperatures of these chondrites. We used synchrotron X-ray-fluorescence microscopy to map the distribution of Fe, Ni and other elements in portions of 71 chondrules from 8 UOCs (types 3.00-3.2). The synchrotron XRF maps show clear enrichment of Fe and/or Ni in fractures ranging down to micrometer scale in chondrules from all UOCs analyzed for this study regardless of petrologic type and regardless of whether fall or find, indicating that there was significant exchange of Fe and Ni between chondrules and matrix and that the Fe-Ni system was not closed. Sixty percent of chondrules in Semarkona (LL3.00) have Fe and Ni enrichment along fractures, while 80-100% of chondrules analyzed from the other UOCs show these enrichments. Mobilization was likely a result of fluid transport of Fe and Ni during aqueous alteration on the parent body and/or during terrestrial weathering. In situ and bulk Fe-Ni analyses that incorporate extraneous Fe and Ni from chondrule fractures will result in lowering the inferred initial 60Fe/56Fe ratios.

Reference
Telus M, Huss GR, Ogliore RC, Nagashima K, Howard DL, Newville MG, Tomkins AG (2016) Mobility of Iron and Nickel at Low Temperatures: Implications for 60Fe-60Ni Systematics of Chondrules from Unequilibrated Ordinary Chondrites. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.11.046]
Copyright Elsevier