HR: 0800h
AN: V41A-1426    [Abstracts]
TI: Fluorine and Hydrogen in Mantle Megacrysts
AU: Hervig, R L
EM: hervig@asu.edu
AF: Dept. Geological Sci, Arizona State University, Tempe, AZ 85287-1404 United States
AU: * Bell, D R
EM: david.r.bell@asu.edu
AF: Dept. Geological Sci, Arizona State University, Tempe, AZ 85287-1404 United States
AU: * Bell, D R
EM: david.r.bell@asu.edu
AF: Dept. Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287 United States
AB: Fluorine is the lightest halogen, and has an ionic radius very similar to OH. Based only on this observation, substitution of fluorine into nominally anhydrous minerals (NAMs) might be expected to be similar to that of hydrogen, but there have not been detailed comparisons between OH and F in the same samples. To explore this connection and to look for geochemical relations between these moieties we have examined a suite of megacryst samples from southern African kimberlites previously analyzed for H (Bell et al., 2004). Here we report analyses of F in olivines and pyroxenes by secondary ion mass spectrometry (SIMS). The SIMS analyses were obtained using a negative, primary beam of oxygen ions and detection of negative secondary ions. High mass resolution was used to eliminate hydroxide ions from the fluorine peak at mass/charge = 19. The F contents of olivines and coexisting mica in a wide range of mica peridotites show a wide range in absolute concentrations (30-100 ppm in olivine) but a strong correlation (F(mica/olivine) ~100:1) that helps reinforce a model that F is substituting in the olivine structure and not as an intergrown secondary phase. Earlier geochemical studies of megacryst olivines revealed strong correlations between major and minor elements reflecting fractional crystallization (among other processes). The present analyses show that fluorine in the "Main Silicate Trend" olivines is nearly constant at 50 ppm, while H2O is also constant at ~150 ppm. Forsterite contents decrease from ~89 to 84%, and Na, Al, and Cr decrease linearly with increasing Fe content. Iron-rich "Group 2" olivines vary from Fo84 to Fo80, and show strongly correlated and increasing F with Fe (from 70 to 240ppm F). H2O is variable (250 to 80 ppm) and the minor elements Na, Al and Cr do not vary strongly with Fo content. Inconsistencies in F-H correlations may relate to H loss during ascent because of the expected higher diffusivity of H relative to F. F analyses of clinopyroxene megacrysts are in progress and will help elucidate the connection between F and OH substitution in NAMs. Bell et al., J. Pet. 45 (2004) 1539.
DE: 1000 GEOCHEMISTRY
DE: 1030 Geochemical cycles (0330)
DE: 3600 MINERALOGY AND PETROLOGY
DE: 3900 MINERAL PHYSICS
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005