|
Natale Perchiazzi, Universita' di Pisa (Italy)
Luigi Folco, Universita' di Siena (Italy)
Pierre Rochette, CNRS, Université D?Aix-Marseille III (France)
|
|
The most part of the extraterrestrial matter presently captured by the Earth's gravitational field is constituted by micrometeorites, i.e. cosmic dust liberated from several bodies of the solar system [1]. The study of the mineralogical composition of micrometeorites is of fundamental importance to identify their parent body and to estimate their contribution to the global geochemistry of Earth.
In December 2003, the meteorite recovery party of the XIX PNRA expedition collected by magnetic extraction thousands of micrometeorites in the 100-800 μm size range in an aeolian deposit at the top of Frontier Mountain (northern Victoria Land, Antartica), a few Myr old glacially eroded surface. They were found [2] [3] in some micrometeorite "traps", namely joints and decimetric-sized pot-hole cavities of flat, glacially eroded granitic surfaces filled in with fine grained bedrock detritus. During the 2006 PNRA expedition, microtektites were also found in two other glacially eroded granitic summits of the Transantarctic Mountains in northern Victoria Land [4].
Synchrotron X-ray diffraction patterns for eleven microtektites and eleven micrometeorites were collected at BM8 GILDA beamline (ESRF, Grenoble).
Remarkable differences in diffuse scattering from glass were detected in the microtektites patterns, related to the different chemical composition of the spherules. Terrestrial alteration phases were jarosite and calcite, detected in three and only one microtektites respectively.
Four cosmic spherules (CS), two unmelted coarse grained (UC) and five unmelted fine grained micrometeorites (UF) were analysed. A close scrutiny of the low angle region of the patterns did not reveal any presence of phyllosilicates. Terrestrial secondary minerals found were jarosite and gypsum, detected in eight and two patterns, respectively.
Two CS resulted to be made up by magnetite and wuestite, whereas in the two remaining CS olivine, magnetite, and terrestrial jarosite were detected. Moreover, the occurrence of laihunite (defective olivine) a phase structurally related to forsterite, but with a distinctly lower cell volume, can be hypothesized, to be confirmed by SEM/EPMA studies.UC and UF particles resulted to be composed of variable proportions of magnetite, enstatite, olivine. Full profile Rietveld refinements of the diffraction patterns of the micrometeorites were performed with the TOPAS-Academic program. Structural informations for the various phases were got from literature and only cell parameters were refined, together with profile-related parameters and implementing instrumental corrections. Insights on crystal-chemistry of the mineral constituents were gathered and quantitative ratios between mineralogical phases in the particles were ascertained.
References: [1] Love and Brownlee 1993 Science 262, 550-553. [2] Rochette et al. 2005. LPSC XXXVI, #1315. [3] Folco et al. 2006. Meteorit. Planet. Sci. 41:A56. [4] Folco et al. 2007, Geology 36, 291-294.
|
