TY - JOUR
T1 - Isotopic fractionation of iron, potassium, and oxygen in stony cosmic spherules
T2 - Implications for heating histories and sources
AU - Taylor, Susan
AU - O'D. Alexander, C. M.
AU - Delaney, J.
AU - Ma, P.
AU - Herzog, G. F.
AU - Engrand, C.
N1 - Funding Information:
We thank NSF for funding the collection of the micrometeorites under grants 9316715 and 9909694 (Dr. Julie Palais, program manager). This work was supported in part by NASA grant NAQG5-11719 (GFH). Dr. Theodoros Ntaflos at the University of Vienna is thanked for his help with the electron microprobe analyses. Suggestions from Dr. Matthew Genge, two anonymous reviewers, and associate editor Dr. Monica Grady, all of whom we thank, greatly improved the paper.
PY - 2005/5/15
Y1 - 2005/5/15
N2 - Atmospheric heating alters the compositions and textures of micrometeorites. To understand the changes and to test a proposed relationship between a micrometeorite's petrographic texture and its degree of heating, we made elemental and multiple isotope analyses of stony cosmic spherules (sCS) collected from the South Pole Water well. Specifically, we analyzed the elemental compositions of 94 sCS and the isotopic ratios of Fe, K and O, on 43, 12 and 8 of these sCS, respectively. Our results show that sCS classified as strongly heated generally have lower concentrations of volatile and moderately volatile elements. Of the 43 spherules analyzed for Fe isotopes, only 5 have δ57 Fe>5‰. In contrast, enrichment of 41K is pervasive (δ41K<0 in all 12 spherules analyzed) and large (up to 183‰). The determination of K isotope abundances in sCS may therefore be useful in deciphering thermal histories. Three of the eight sCS analyzed for O isotopes are mass fractionated with δ18O <30‰. We attribute two of these three δ18O enrichments to evaporative losses of oxygen in the atmosphere and the third to the presence in the parent material of an exotic phase, perhaps a sulfate or a carbonate. The K isotope and O isotope data are broadly consistent with the proposed textural classification. Because most spherules were not heated enough to fractionate Al, Mg, or Si, we compared the measured Mg/Al and Si/Al ratios directly to those of conventional meteorites and their matrices. ∼30% of the sCS have compositions outside the range defined by the bulk and the matrix compositions of known meteorite groups but consistent with those of pyroxene- and olivine-rich materials and may be samples of chondrules. The other 70% have Mg/Al and Si/Al ratios similar to those of CI, CM, CO, and CV chondrites. Natural variability of the Mg/Al and Si/Al ratios precludes the assignment of an individual sCS to a particular class of C-chondrite. Copyight
AB - Atmospheric heating alters the compositions and textures of micrometeorites. To understand the changes and to test a proposed relationship between a micrometeorite's petrographic texture and its degree of heating, we made elemental and multiple isotope analyses of stony cosmic spherules (sCS) collected from the South Pole Water well. Specifically, we analyzed the elemental compositions of 94 sCS and the isotopic ratios of Fe, K and O, on 43, 12 and 8 of these sCS, respectively. Our results show that sCS classified as strongly heated generally have lower concentrations of volatile and moderately volatile elements. Of the 43 spherules analyzed for Fe isotopes, only 5 have δ57 Fe>5‰. In contrast, enrichment of 41K is pervasive (δ41K<0 in all 12 spherules analyzed) and large (up to 183‰). The determination of K isotope abundances in sCS may therefore be useful in deciphering thermal histories. Three of the eight sCS analyzed for O isotopes are mass fractionated with δ18O <30‰. We attribute two of these three δ18O enrichments to evaporative losses of oxygen in the atmosphere and the third to the presence in the parent material of an exotic phase, perhaps a sulfate or a carbonate. The K isotope and O isotope data are broadly consistent with the proposed textural classification. Because most spherules were not heated enough to fractionate Al, Mg, or Si, we compared the measured Mg/Al and Si/Al ratios directly to those of conventional meteorites and their matrices. ∼30% of the sCS have compositions outside the range defined by the bulk and the matrix compositions of known meteorite groups but consistent with those of pyroxene- and olivine-rich materials and may be samples of chondrules. The other 70% have Mg/Al and Si/Al ratios similar to those of CI, CM, CO, and CV chondrites. Natural variability of the Mg/Al and Si/Al ratios precludes the assignment of an individual sCS to a particular class of C-chondrite. Copyight
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U2 - 10.1016/j.gca.2004.11.027
DO - 10.1016/j.gca.2004.11.027
M3 - Article
AN - SCOPUS:19044363754
SN - 0016-7037
VL - 69
SP - 2647
EP - 2662
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
IS - 10
ER -