Pallasites are stony-iron meteorites essentially composed of olivine [(Mg,Fe)2SiO4] with a metallic portion covering one-third of the total volume (Boesenberg et al., 2012) and principally consisting of Fe-Ni metal. The sample studied in the present work is a piece of the Mineo pallasite, named as the place where it fell in the South-Eastern part of Sicily (Italy) in 1826. The sole sample available in the world belongs to the Department of Physics and Geology at the University of Perugia. Preliminary chemical analysis allowed to locate Mineo meteorite in among the Main Group (MG) pallasites. A peculiar compositional variability of olivines was observed in terms of both iron (11-14 wt%) and trace elements. This observation is quite interesting because the composition of olivines is normally rather uniform within an individual pallasite (Buseck & Goldstein,1969; Boesenberg et al., 2012). The study of oxygen isotopes and trace elements is needed to definitely classify the Mineo meteorite. The Mineo pallasite was sampled and the collected fragments were embedded in epoxy resin and polished up to 1 μm. The isotopic oxygen analysis was obtained by means of Time of Flight – Secondary Ion Mass Spectrometry (ToF- SIMS), which is a static technique used for surface analysis. Samples are bombarded with primary ions at typical energies of 10-30 keV. The corresponding secondary ions are than accelerated into a flight tube and their mass is determined measuring their time of flight. Major advantages of this technique are: a) the high lateral resolution; b) only a limited destruction of the sample, i.e., the erosion is very small due to the use of a pulsed beam; c) parallel detection of all secondary ions with one polarity in a single measurement; d) an adequate mass resolution for the separation of major mass interferences (Stephan, 2001). Five measurements were performed on Mineo fragments: 4 points in olivine and 1 in the metal phase. Oxygen isotopic ratios were used to calculate the δ18O values. Four measurements have been done on a sample of the Brenham meteorite, used as internal standard. The ToF-SIMS analysis showed a variability of the oxygen isotopic composition in the Mineo olivines, on average δ18O = 2.3±0.3, in agreement with the previous findings on the chemical composition. Such a value lies in the lower part of the range of variation of MG pallasites and allows us to definitely classify Mineo among the MG pallasites. Boesenberg, J.S., Delaney, G.S., Hewins, R.H.J. (2012): A petrological and chemical reexamination of Main Group pallasite formation. Geochim. Cosmochim. Acta, 89, 134-158. Buseck, P.R. & Goldstein, J.I. (1969): Olivine compositions and cooling rates of pallasitic meteorites. Geol. Soc. Am. Bull., 80, 2141-2158. Stephan, T. (2001): TOF-SIMS in cosmochemistry. Planet. Space Sci., 49, 859-906.
Oxygen isotopic analysis of Mineo (Sicily, Italy) pallasite
Canteri R.;Pepponi G.;
2017-01-01
Abstract
Pallasites are stony-iron meteorites essentially composed of olivine [(Mg,Fe)2SiO4] with a metallic portion covering one-third of the total volume (Boesenberg et al., 2012) and principally consisting of Fe-Ni metal. The sample studied in the present work is a piece of the Mineo pallasite, named as the place where it fell in the South-Eastern part of Sicily (Italy) in 1826. The sole sample available in the world belongs to the Department of Physics and Geology at the University of Perugia. Preliminary chemical analysis allowed to locate Mineo meteorite in among the Main Group (MG) pallasites. A peculiar compositional variability of olivines was observed in terms of both iron (11-14 wt%) and trace elements. This observation is quite interesting because the composition of olivines is normally rather uniform within an individual pallasite (Buseck & Goldstein,1969; Boesenberg et al., 2012). The study of oxygen isotopes and trace elements is needed to definitely classify the Mineo meteorite. The Mineo pallasite was sampled and the collected fragments were embedded in epoxy resin and polished up to 1 μm. The isotopic oxygen analysis was obtained by means of Time of Flight – Secondary Ion Mass Spectrometry (ToF- SIMS), which is a static technique used for surface analysis. Samples are bombarded with primary ions at typical energies of 10-30 keV. The corresponding secondary ions are than accelerated into a flight tube and their mass is determined measuring their time of flight. Major advantages of this technique are: a) the high lateral resolution; b) only a limited destruction of the sample, i.e., the erosion is very small due to the use of a pulsed beam; c) parallel detection of all secondary ions with one polarity in a single measurement; d) an adequate mass resolution for the separation of major mass interferences (Stephan, 2001). Five measurements were performed on Mineo fragments: 4 points in olivine and 1 in the metal phase. Oxygen isotopic ratios were used to calculate the δ18O values. Four measurements have been done on a sample of the Brenham meteorite, used as internal standard. The ToF-SIMS analysis showed a variability of the oxygen isotopic composition in the Mineo olivines, on average δ18O = 2.3±0.3, in agreement with the previous findings on the chemical composition. Such a value lies in the lower part of the range of variation of MG pallasites and allows us to definitely classify Mineo among the MG pallasites. Boesenberg, J.S., Delaney, G.S., Hewins, R.H.J. (2012): A petrological and chemical reexamination of Main Group pallasite formation. Geochim. Cosmochim. Acta, 89, 134-158. Buseck, P.R. & Goldstein, J.I. (1969): Olivine compositions and cooling rates of pallasitic meteorites. Geol. Soc. Am. Bull., 80, 2141-2158. Stephan, T. (2001): TOF-SIMS in cosmochemistry. Planet. Space Sci., 49, 859-906.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
