New geochemical data on cosmic spherules (187 major element, 76 trace element, and 10 oxygen isotope compositions) and 273 analyses from the literature were used to assess the chemical diversity observed among glass cosmic spherules with chondritic composition. Three chemical groups of glass spherules are identified: normal chondritic spherules, CAT-like spherules (where CAT refers to Ca-Al-Ti-rich spherules), and high Ca-Al spherules. The transition from normal to high Ca-Al spherules occurs through a progressive enrichment in refractory major elements (on average from 2.3 wt.% to 7.0 wt.% for CaO, 2.8 wt.% to 7.2 wt.% for Al(2)O(3), and 0.14 wt.% to 0.31 wt.% for TiO(2)) and refractory trace elements (from 6.2 mu g/g to 19.3 mu g/g for Zr and 1.6CI-4.3CI for Rare Earth Elements-REEs) relative to moderately refractory elements (Mg, Si) and volatile elements (Rb, Na, Zn, Pb). Based on a comparison with experimental works from the literature, these chemical groups are thought to record progressive heating and evaporation during atmospheric entry. The evaporative mass losses evaluated for the high Ca-Al group (80-90%) supersede those of the CAT spherules which up to now have been considered as the most heated class of stony cosmic spherules. However, glass cosmic spherules still retain isotopic and elemental evidence of their source and precursor mineralogy. Four out of the 10 normal and high Ca-Al spherules analysed for oxygen isotopes are related to ordinary chondrites (delta(18)O = 13.2-17.3 parts per thousand and delta(17)O = 7.6-9.2 parts per thousand). They are systematically enriched in Ni and Co (Ni = 24-500 mu g/g) with respect to spherules related to carbonaceous chondrites (Ni < 1.2 mu g/g, delta(18)O = 13.1-28.0 parts per thousand and delta(17)O = 5.1-14.0 parts per thousand). REE abundances in cosmic spherules, which are not fractionated according to parent body or atmospheric entry heating, can then be used to unravel the precursor mineralogy. Spherules with flat REE pattern close to unity when normalized to CI are the most abundant in our dataset (54%) and likely derive from homogeneous, fine-grained chondritic precursors. Other REE patterns fall into no more than five categories, a surprising reproducibility in view of the mineralogical heterogeneity of chondritic lithologies at the micrometeorite scale.
Major, trace element and oxygen isotope study of glass cosmic spherules of chondritic composition: the record of their source material and atmospheric entry heating
FOLCO, LUIGI;
2011-01-01
Abstract
New geochemical data on cosmic spherules (187 major element, 76 trace element, and 10 oxygen isotope compositions) and 273 analyses from the literature were used to assess the chemical diversity observed among glass cosmic spherules with chondritic composition. Three chemical groups of glass spherules are identified: normal chondritic spherules, CAT-like spherules (where CAT refers to Ca-Al-Ti-rich spherules), and high Ca-Al spherules. The transition from normal to high Ca-Al spherules occurs through a progressive enrichment in refractory major elements (on average from 2.3 wt.% to 7.0 wt.% for CaO, 2.8 wt.% to 7.2 wt.% for Al(2)O(3), and 0.14 wt.% to 0.31 wt.% for TiO(2)) and refractory trace elements (from 6.2 mu g/g to 19.3 mu g/g for Zr and 1.6CI-4.3CI for Rare Earth Elements-REEs) relative to moderately refractory elements (Mg, Si) and volatile elements (Rb, Na, Zn, Pb). Based on a comparison with experimental works from the literature, these chemical groups are thought to record progressive heating and evaporation during atmospheric entry. The evaporative mass losses evaluated for the high Ca-Al group (80-90%) supersede those of the CAT spherules which up to now have been considered as the most heated class of stony cosmic spherules. However, glass cosmic spherules still retain isotopic and elemental evidence of their source and precursor mineralogy. Four out of the 10 normal and high Ca-Al spherules analysed for oxygen isotopes are related to ordinary chondrites (delta(18)O = 13.2-17.3 parts per thousand and delta(17)O = 7.6-9.2 parts per thousand). They are systematically enriched in Ni and Co (Ni = 24-500 mu g/g) with respect to spherules related to carbonaceous chondrites (Ni < 1.2 mu g/g, delta(18)O = 13.1-28.0 parts per thousand and delta(17)O = 5.1-14.0 parts per thousand). REE abundances in cosmic spherules, which are not fractionated according to parent body or atmospheric entry heating, can then be used to unravel the precursor mineralogy. Spherules with flat REE pattern close to unity when normalized to CI are the most abundant in our dataset (54%) and likely derive from homogeneous, fine-grained chondritic precursors. Other REE patterns fall into no more than five categories, a surprising reproducibility in view of the mineralogical heterogeneity of chondritic lithologies at the micrometeorite scale.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
